U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5118

Prepared in cooperation with the Sonoma County Water Agency

Hydrologic and Geochemical Characterization

of the Santa Rosa Plain Watershed,

Sonoma County, California
Cover. View of a vineyard on the Santa Rosa Plain, Sonoma County, California. 

(Photography by Tracy Nishikawa, U.S. Geological Survey.)
Hydrologic and Geochemical 

Characterization of the Santa Rosa Plain 

Watershed, Sonoma County, California

Edited by Tracy Nishikawa

Chapter A

Introduction to the Study Area

By Tracy Nishikawa, Joseph A. Hevesi, Donald S. Sweetkind, and Linda R. Woolfenden

Chapter B

Hydrology of the Santa Rosa Plain Watershed, Sonoma County, 

California

By Donald S. Sweetkind, Joseph A. Hevesi, Tracy Nishikawa, Peter Martin, and 

Christopher D. Farrar

Chapter C

Groundwater Quality and Source and Age of Groundwater in the 

Santa Rosa Plain Watershed, Sonoma County, California

By Peter Martin, Loren F. Metzger, Jill N. Densmore, and Roy A. Schroeder

Chapter D

Conceptual Model of Santa Rosa Plain Watershed Hydrologic 

System

By Tracy Nishikawa, Joseph A. Hevesi, Donald S. Sweetkind, and Peter Martin

Scientific Investigations Report 2013–5118

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Sonoma County Water Agency
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Nishikawa, Tracy, ed., 2013, Hydrologic and geochemical characterization of the Santa Rosa Plain watershed, Sonoma 

County, California: U.S. Geological Survey Scientific Investigations Report 2013–5118, 178 p.
iii

Contents

Acknowledgements...................................................................................................................................xvii

Executive Summary.......................................................................................................................................1

Study Area..............................................................................................................................................1

Geohydrology.........................................................................................................................................1

Surface-Water Hydrology ...................................................................................................................2

Groundwater, Groundwater Movement, and Groundwater Levels..............................................3

Groundwater Quality ............................................................................................................................3

Conceptual Model.................................................................................................................................4

Data Gaps...............................................................................................................................................6

Chapter A

Introduction to the Study Area

Introduction.....................................................................................................................................................7

Previous Work ................................................................................................................................................7

Purpose and Scope .....................................................................................................................................11

Study Area Description...............................................................................................................................11

Physiography.......................................................................................................................................13

Climate..................................................................................................................................................13

Precipitation................................................................................................................................13

Air Temperature .........................................................................................................................14

Soils ......................................................................................................................................................16

Land Use...............................................................................................................................................16

Modern (post-1950) Changes in Land Use.............................................................................16

Hydrologic Effects of Changing Land Use .............................................................................21

References Cited..........................................................................................................................................24
iv

Introduction...................................................................................................................................................27

Geology..........................................................................................................................................................27

Geologic Setting..................................................................................................................................27

Stratigraphic Units..............................................................................................................................29

Basement Rocks ........................................................................................................................29

Tertiary Volcanic Rocks ............................................................................................................29

Tertiary Sedimentary Rocks .....................................................................................................31

Wilson Grove Formation ..................................................................................................31

Petaluma Formation .........................................................................................................32

Glen Ellen Formation ........................................................................................................32

Quaternary Deposits .................................................................................................................32

Faults.....................................................................................................................................................33

Basin Depth and Geometry ...............................................................................................................34

Surface-Water Hydrology ..........................................................................................................................35

Surface-Water Drainage Pattern.....................................................................................................35

Laguna de Santa Rosa Drainage Basin .................................................................................35

Santa Rosa Creek Drainage Basin..........................................................................................38

Mark West Creek Drainage Basin ..........................................................................................38

Water Bodies..............................................................................................................................40

Streamflow ..........................................................................................................................................40

Streamflow Gages .....................................................................................................................40

Correlation Between Drainage Basin Area and Average Discharge ...............................41

Monthly and Seasonal Streamflow ........................................................................................41

Daily Mean Streamflow Records ............................................................................................41

Laguna de Santa Rosa Drainage Basin ........................................................................49

COPE Gage................................................................................................................49

LAGC Gage................................................................................................................49

COLU Gage................................................................................................................49

COLL Gage.................................................................................................................50

LAGS Gage................................................................................................................50

Santa Rosa Creek Drainage Basin.................................................................................50

SRCU Gage................................................................................................................50

BRSH Gage ...............................................................................................................51

MATC Gage...............................................................................................................51

SRCS Gage................................................................................................................51

SRCW Gage ..............................................................................................................52

Mark West Creek Drainage Basin .................................................................................52

MWCW Gage............................................................................................................52

MWCM Gage............................................................................................................53

Extension of the MWCM Streamflow Record.......................................................................53

Contents—Continued

Chapter B

Hydrology of the Santa Rosa Plain Watershed, Sonoma County, California
v

Flow-Duration Analysis.............................................................................................................55

Summary of Streamflow Characteristics...............................................................................55

Flood Control, Flow Diversions, and Drainage Modifications .....................................................58

Flood Control...............................................................................................................................59

Surface-Water Diversions........................................................................................................60

Drainage Modifications ............................................................................................................60

Groundwater Hydrology..............................................................................................................................63

Groundwater Subbasins and Storage Units...................................................................................63

Groundwater Subbasins...........................................................................................................64

Groundwater Storage Units .....................................................................................................64

Definition of Aquifer System .............................................................................................................64

Aquifer Extent and Distribution of Material Properties.......................................................66

Hydrogeologic Characteristics of Aquifers...........................................................................68

Quaternary Alluvial Deposits ..........................................................................................70

Glen Ellen Formation ........................................................................................................70

Wilson Grove Formation ..................................................................................................70

Petaluma Formation .........................................................................................................74

Sonoma Volcanics ............................................................................................................74

Basement Rocks ...............................................................................................................75

Faults and Groundwater Flow..................................................................................................75

Groundwater Recharge and Discharge..........................................................................................75

Groundwater Recharge ............................................................................................................75

Groundwater Discharge ...........................................................................................................76

Groundwater Levels and Movement ...............................................................................................87

Basin-wide Groundwater Levels.............................................................................................87

1951 (Predevelopment) Conditions ...............................................................................87

1974 Conditions .................................................................................................................89

1980 Conditions .................................................................................................................89

1990 Conditions .................................................................................................................90

2001 Conditions .................................................................................................................90

2007 Conditions .................................................................................................................90

Water-level Changes in Monitoring Wells.............................................................................91

Windsor Basin Storage Unit ...........................................................................................91

Cotati Basin Storage Unit ................................................................................................96

Valley Storage Unit...........................................................................................................96

Wilson Grove Storage Unit..............................................................................................96

Uplands Storage Unit .......................................................................................................96

Multiple-Level Monitoring Site.....................................................................................107

Summary......................................................................................................................................................107

References Cited........................................................................................................................................108

Contents—Continued
vi

Introduction.................................................................................................................................................113

Previous Investigations ...................................................................................................................113

Historical Monitoring........................................................................................................................114

Methods of Sample Collection and Analysis ........................................................................................114

Recent Sample Collection and Analysis .......................................................................................114

Historical Sample Collection and Analysis...................................................................................123

Well Construction of Sampled Wells.............................................................................................126

Chemical Character of Groundwater .....................................................................................................126

Major-Ion Composition ....................................................................................................................126

Areal and Depth-Related Patterns........................................................................................135

Uplands (UPL) Storage Unit ..........................................................................................135

Valley (VAL) Storage Unit...............................................................................................135

Windsor Basin (WB) Storage Unit ...............................................................................135

Cotati Basin (CB) Storage Unit .....................................................................................135

Wilson Grove (WG) Storage Unit .................................................................................138

Changes in Chemical Character along Geologic Section B–B’ .......................................138

Geologic Units along Section B–B’..............................................................................138

Geologic Structure along Section B–B’......................................................................138

Groundwater Flow along Section B–B’.......................................................................139

General Chemical Composition ....................................................................................139

Temporal Trends.......................................................................................................................140

Water-Quality Constituents of Potential Concern .......................................................................140

Specific Conductance.............................................................................................................143

Chloride .....................................................................................................................................143

Dissolved Solids.......................................................................................................................143

Nitrate .......................................................................................................................................143

Arsenic.......................................................................................................................................144

Boron..........................................................................................................................................144

Iron .............................................................................................................................................144

Manganese...............................................................................................................................145

Source and Age of Groundwater............................................................................................................145

Oxygen-18 and Deuterium...............................................................................................................145

Background...............................................................................................................................145

Isotopic Composition of Samples..........................................................................................151

Changes in Isotopic Composition along Geologic Section B–B’ .....................................151

Tritium and Carbon-14 ......................................................................................................................156

Background...............................................................................................................................156

Tritium ..............................................................................................................................156

Carbon-14.........................................................................................................................157

Contents—Continued

Chapter C

Groundwater Quality and Source and Age of Groundwater in the Santa Rosa 

Plain Watershed, Sonoma County, California
vii

Contents—Continued

Groundwater Age in the Santa Rosa Plain Watershed.....................................................157

Groundwater Age Along Geologic Section B–B’ ...............................................................160

Summary and Conclusions.......................................................................................................................161

References Cited........................................................................................................................................163

Chapter D

Conceptual Model of Santa Rosa Plain Watershed Hydrologic System

Introduction—Purpose of Conceptualization .......................................................................................167

Boundaries and Boundary Conditions ...................................................................................................168

Region 1: Plant Canopy, Land Surface, and Soil Zone.........................................................................168

Inflows to Region 1 ...........................................................................................................................168

Precipitation..............................................................................................................................168

Irrigation Return Flows ...........................................................................................................170

Groundwater Discharge .........................................................................................................170

Outflows from Region 1....................................................................................................................170

Evapotranspiration ..................................................................................................................170

Surface-Water Runoff.............................................................................................................170

Infiltration and Recharge........................................................................................................171

Storage in Region 1...........................................................................................................................171

Region 2: Surface-Water Zone ................................................................................................................171

Inflows to Region 2 ...........................................................................................................................171

Outflows from Region 2....................................................................................................................171

Region 3: Subsurface Zone ......................................................................................................................172

Unsaturated Zone .............................................................................................................................172

Inflows to the Unsaturated Zone...........................................................................................172

Outflows from the Unsaturated Zone ...................................................................................172

Saturated Zone..................................................................................................................................173

Inflows to the Saturated Zone ...............................................................................................173

Outflows from the Saturated Zone........................................................................................173

Summary of Conceptual Model...............................................................................................................174

Precipitation.......................................................................................................................................174

Streamflow.........................................................................................................................................174

Aquifer System..................................................................................................................................174

Groundwater Flow ............................................................................................................................175

Groundwater Age..............................................................................................................................176

Data Gaps....................................................................................................................................................176

References Cited........................................................................................................................................177

Appendix A. Selected chemical and physical properties and inorganic constituents 

in samples from selected springs, and wells, Santa Rosa Plain Watershed, 

Sonoma County, California, 1947–2010 .....................................................................................178
viii

Figures

Chapter B

Chapter A

1. Map showing location of the Santa Rosa Plain watershed study area within

Sonoma County, California..........................................................................................................8

2. Map showing Santa Rosa Plain watershed boundary with the Mark West Creek,

Santa Rosa Creek, and Laguna de Santa Rosa drainage basins, Sonoma County,

California ........................................................................................................................................9

3. Map showing Santa Rosa Plain watershed boundary with groundwater subbasins,

Sonoma County, California........................................................................................................10

4. Map showing Kadir and McGuire (1987) model domain in relation to study area,

Santa Rosa Plain watershed, Sonoma County, California ...................................................12

5. Bar graph showing total annual precipitation for water-years 1906–2010, California

Data Exchange Center SRO (City of Santa Rosa) climate station, Sonoma County,

California ......................................................................................................................................14

6. Map showing contours of average annual precipitation for 1971–2000, Santa Rosa

Plain watershed, Sonoma County, California.........................................................................15

7. Map showing Soil Survey Geographic (SSURGO) soil thickness, Santa Rosa Plain

watershed, Sonoma County, California...................................................................................17

8. Map showing Soil Survey Geographic (SSURGO) average percentage of clay in

soils, Santa Rosa Plain watershed, Sonoma County, California.........................................18

9. Map showing Soil Survey Geographic (SSURGO) average percentage of sand in

soils, Santa Rosa Plain watershed, Sonoma County, California.........................................19

10. Map showing Soil Survey Geographic (SSURGO) soil hydrologic groups, Santa

Rosa Plain watershed, Sonoma County, California...............................................................20

11. Maps showing land-use, Santa Rosa Plain watershed, Sonoma County, California......22

1. Map showing Santa Rosa Plain watershed, Sonoma County, California..........................28

2. Figure showing interpretive geologic cross sections of the Santa Rosa Plain

watershed, Sonoma County, California...................................................................................30

3. Map showing Santa Rosa Plain watershed boundary and surface-water drainage

basins, Sonoma County, California ..........................................................................................36

4. Map showing major and minor stream channels and stream gage locations within

the Santa Rosa Plain watershed, Sonoma County, California ............................................37

5. Map showing water bodies within the Santa Rosa Plain watershed, Santa Rosa

Plain, Sonoma County, California.............................................................................................39

6. Graphs showing average water year discharge for gages within and neighboring

the Santa Rosa Plain watershed, Sonoma County, California ............................................44

7. Map showing U.S. Geological Survey stream gages within and neighboring the

Santa Rosa Plain watershed, Sonoma County, California ...................................................45

8. Graph showing correlation between drainage basin area and average discharge

for streamflow records within and around the Santa Rosa Plain watershed,

Sonoma County, California........................................................................................................46

9. Graph showing monthly mean discharge for four selected stream gages in the

Santa Rosa Plain watershed, Sonoma County, California, 1999–2010...............................47

10. Graphs showing average monthly discharge for gages having continuous stream

discharge records in the Santa Rosa Plain watershed, Sonoma County, California ......48
ix

11. Graph showing average monthly discharge for gages having seasonal-only

(October–April) records in the Santa Rosa Plain watershed, Sonoma County,

California ......................................................................................................................................49

12. Graph showing daily mean discharge measured at gage 11465680, Laguna de

Santa Rosa near Cotati (LAGC), Sonoma County, California, 1999–2010...........................50

13. Graph showing daily mean discharge measured at gage 11465700, Colgan Creek

near Sebastopol (COLL), Sonoma County, California 1999–2010.........................................51

14. Graph showing daily mean discharge measured at gage 11466200, Laguna de

Santa Rosa near Sebastopol (LAGS), Sonoma County, California, 1999–2011 .................52

15. Graph showing daily mean discharge measured at gage 11466200, Santa Rosa

Creek near Santa Rosa (SRCU), Sonoma County, California, 1959–1970...........................53

16. Graph showing daily mean discharge measured at gage 11466320, Santa Rosa

Creek at Willowside Road (SRCW), Sonoma County, California, 1998–2010 ....................54

17. Graph showing daily mean discharge measured at gage 11466800, Mark West

Creek near Mirabel Heights (MWCM), Sonoma County, California, 2006–2010...............55

18. Graphs showing correlation between seasonal daily streamflow at NAPN and

MWCM gages, along with MOVE.1-estimated streamflow for the Santa Rosa Plain

watershed, Sonoma County, California...................................................................................56

19. Graphs showing extended streamflow record estimated at the lower Mark West

Creek near Mirabel Heights (MWCM) gage ..........................................................................57

20. Graph showing flow duration of daily mean discharge and modeled daily mean

discharge for the lower Mark West Creek near Mirabel Heights (MWCM) gage,

Santa Rosa Plain watershed, Sonoma County, California ...................................................59

21. Map showing location of land parcels where reclaimed water is applied for

irrigation of crops and urban areas, Santa Rosa Plain watershed, Sonoma County,

California ......................................................................................................................................61

22. Graphs showing reclaimed water applied as irrigation in Santa Rosa Plain

watershed, Sonoma County, California, for water-years 1990–2009..................................62

23. Figure showing conceptual diagram of the groundwater system, Santa Rosa Plain

watershed, Sonoma County, California...................................................................................63

24. Map showing groundwater basins, subbasins, and storage units in the Santa Rosa

Plain watershed, Sonoma County, California.........................................................................65

25. Figure showing vertical sections from three-dimensional lithologic model of the

Santa Rosa Plain watershed, Sonoma County, California ...................................................67

26. Figure showing assignment of sediment texture attributes, Santa Rosa Plain

watershed, Sonoma County, California...................................................................................69

27. Graphs showing specific-capacity data for wells in the Santa Rosa Plain

watershed, Sonoma County, California...................................................................................71

28. Map showing locations of wells with pumping tests, Santa Rosa Plain watershed,

Sonoma County, California........................................................................................................73

29. Map showing groundwater-level contours for spring or fall, Santa Rosa Plain

watershed, Sonoma County, California...................................................................................77

30. Map showing current and historical springs in the Santa Rosa Plain watershed,

Sonoma County, California........................................................................................................86

31. Map showing location of public-supply wells in the Santa Rosa Plain watershed,

Sonoma County, California........................................................................................................88

Figures—Continued
x

32. Graph showing total annual pumping from public-supply wells in the Santa Rosa

Plain watershed, Sonoma County, California, 1975–2009.....................................................89

33. Graph showing total annual precipitation for water-years 1906–2010, Santa Rosa Plain 

watershed, Sonoma County, California...................................................................................90

34. Map showing locations of selected hydrograph wells, Santa Rosa Plain watershed,

Sonoma County, California........................................................................................................92

35. Graphs showing water-level data for selected wells in the Windsor Basin storage

unit, 1974–2010, Santa Rosa Plain watershed, Sonoma County, California ......................93

36. Graphs showing water-level data for selected wells in the Cotati Basin storage

unit, 1974–2010, Santa Rosa Plain watershed, Sonoma County, California ......................97

37. Graph showing total annual pumping from city of Rohnert Park production wells,

Santa Rosa Plain watershed, Sonoma County, California, 1973–2009.............................102

38. Graphs showing water-level data for selected wells in the Valley storage unit,

1974–2010, Santa Rosa Plain watershed, Sonoma County, California.............................103

39. Graphs showing water-level data for selected wells in the Wilson Grove storage

unit, 1974–2010, Santa Rosa Plain watershed, Sonoma County, California ....................104

40. Graphs showing water-level data for selected wells in the Uplands storage unit,

1974–2010, Santa Rosa Plain watershed, Sonoma County, California.............................106

41. Graph showing water-level data for 6N/8W-07A4-6, 1977–2007, and daily pumpage

data for 6N/8W-07A2, 2000–2007, Santa Rosa Plain, Sonoma County, California..........107

Figures—Continued

Chapter C

Chapter D

1. Map showing locations of surface-water sites and springs and, wells sampled in

the Santa Rosa Plain watershed, Sonoma County, California, 1947–2010 ......................115

2. Figure showing trilinear diagrams of the most recent, complete sample from

selected surface-water sites, springs, and wells in storage units in the Santa Rosa

Plain watershed, Sonoma County, California, 1947–2010...................................................127

3. Maps showing stiff diagrams showing water-type for selected in the Santa Rosa

Plain watershed, Sonoma County, California.......................................................................132

4. Time-series plots of specific conductance and chloride for selected wells in the

Santa Rosa Plain watershed, Sonoma County, California, 1950–2009.............................141

5. Map showing delta deuterium and oxygen-18 data from selected wells in the Santa

Rosa Plain watershed, Sonoma County, California, 2003–10.............................................146

6. Graph showing relation between delta deuterium and delta oxygen-18 for water

samples from in the Santa Rosa Plain watershed, Sonoma County, California,

2003–10 .......................................................................................................................................152

7. Map showing tritium and carbon-14 values from selected surface-water sites and

wells in the Santa Rosa Plain watershed, Sonoma County, California, 2004–10 ...........158

1. Schematic diagram of the general conceptual model of the Santa Rosa Plain

watershed hydrologic system ................................................................................................167

2. Map showing locations where groundwater in the Santa Rosa Plain watershed

could be in hydraulic communication with neighboring groundwater subbasins,

Santa Rosa Plain watershed, California ...............................................................................169

3. Figure showing conceptual model of the groundwater-flow system, Santa Rosa

Plain watershed, Sonoma County, California.......................................................................175
xi

Tables

1. Population for the cities and township in the Santa Rosa Plain, Sonoma County,

California, 1950, 1960, 1970, 1980, 1990, 2000, and 2010 ........................................................16

2. Land use in Santa Rosa Plain Watershed, Sonoma County, California, 1974, 1979,

1986, 1999, and 2008....................................................................................................................21

1. Description of hydrogeologic units, Santa Rosa Plain watershed, Sonoma County,

California ......................................................................................................................................31

2. Description of gaging stations and streamflow records within and adjacent to the

Santa Rosa Plain watershed, Sonoma County, California ...................................................42

3. Summary statistics for streamflow records within and neighboring Santa Rosa

Plain watershed (SRPW), Sonoma County, California..........................................................43

4. Reported specific-yield values by lithology and geologic formation, Santa Rosa

Plain watershed, Sonoma County, California.........................................................................68

5. Hydraulic properties from aquifer tests, Santa Rosa Plain watershed, Sonoma

County, California........................................................................................................................72

6. Construction information of hydrograph wells, Santa Rosa Plain watershed,

Sonoma County, California........................................................................................................91

1. Construction information for selected wells used to obtain water-quality data in the

Santa Rosa Plain watershed, Sonoma County, California .................................................117

2. Summary of discrete-depth samples from selected wells, Santa Rosa Plain

Watershed, Sonoma County, California, 2006–2010............................................................124

3. Minimum reporting levels and analytical method references for water-quality

analyses performed by non-U.S. Geological Survey laboratories, Santa Rosa Plain

Watershed, Sonoma County, California, 2006–2010............................................................125

4. Summary of groundwater quality for most recent complete sample from selected

wells, classified by storage unit and depth category, Santa Rosa Plain Watershed,

Sonoma County, California, 1947–2010..................................................................................136

5. Summary of isotopic data in samples from selected surface-water sites and wells,

Santa Rosa Plain watershed and neighboring area, Sonoma County, California,

2003–2010 ...................................................................................................................................148

Chapter C

Chapter B

Chapter A
xii

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2

) 

acre 0.004047 square kilometer (km2

)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3

/s)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3

/yr)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Leakance

foot per day per foot [(ft/d)/ft] 1 meter per day per meter

foot per second (ft/s) 0.3048 meter per second (m/s)

foot per day (ft/d) 0.3048 meter per day (m/d)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

cubic foot per day (ft3

/d) 0.02832 cubic meter per day (m3/d)

inch per year per foot [(in/yr)/ft] 83.33 millimeter per year per meter

[(mm/yr)/m]

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

Transmissivity*

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88) and National Geodetic Vertical Datum of 1929 (NGVD29).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Conversion Factors

Inch/Pound to SI
xiii

Altitude, as used in this report, refers to distance above the vertical datum.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3

/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L)
xiv

APHA American Public Health Association

asl above sea level

bls below land surface

CB Cotati Basin

CDPH California Department of Public Health

CDWR California Department of Water Resources

ECON The Environment, Community, and Opportunity Network

ELAP Enviromental Laboratory Accreditation Program (California)

GAMA Groundwater Ambient Monitoring and Assessment

ET evapotranspiration

GIS geographic information system

GMWL global meteoric water line

LLNL Lawrence Livermore National Laboratory

LMWL local meteoric water line

MCL maximum contaminant level (USEPA or California)

NL notification level (California)

NOSAMS National Ocean Sciences Accelerator Mass Spectrometry Facility

NSF North San Francisco

PES PES Environmental, Inc.

SMCL secondary maximum contaminant level

SPR spring site

SRP Santa Rosa Plain

SRPW Santa Rosa Plain Watershed

SWRCB State Water Resources Control Board (California)

SW surface-water site

T/R/S township/range/section

UPL upland

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VAL valley

VSMOW Vienna Standard Mean Ocean Water

WB Windsor Basin

WG Wilson Grove

W well site (groundwater)

As arsenic

12C carbon-12

13C carbon-13

14C carbon-14

DIC dissolved inorganic carbon

DO dissolved oxygen

Abbreviations
xv

Fe iron

1

H hydrogen isotope

2

H or D deuterium

3

H tritium

3

He helium-3

N nitrogen

NO3 nitrate

16O oxygen-16

18O oxygen-18

pmc percent modern carbon

t time

TDS total dissolved solids

TU tritium unit

VOCs volatile organic compounds

δi

E delta notation; the ratio of the heavier isotope (i) to the more common lighter isotope of 

an element (E), relative to a standard reference material, expressed as per mil

‰ per mil (parts per thousand)

Abbreviations—Continued
xvi

7N/7W-19B1

C B A

F G H

L JK

P Q R

6 5 4 3 2 1

7 8 9 10 11 12

18 17 16 15 14 13

19 20 21 22 23 24

30 29 28 26 25

31 32 33 34 35 36

27

R7W R6W

T8N

T9N

T10N

R8W

RANGE

TOWNSHIP

R10W R9W

T7N

T6N

R7W

Well-numbering diagram

SECTION 19

T7N

D

E

M

N

Well-Numbering System

	 Wells are identified and numbered according to their location in the rectangular system for the subdivision of public lands. Identification consists of the township number, north or south; the range number, east or

west; and the section number. Each section is divided into sixteen 40-acre tracts lettered consecutively (except I 

and O), beginning with "A" in the northeast corner of the section and progressing in a sinusoidal manner to "R" 

in the southeast corner. Within the 40-acre tract, wells are sequentially numbered in the order they are inventoried. The final letter refers to the base line and meridian. In California, there are three base lines and meridians; Humboldt (H), Mount Diablo (M), and San Bernardino (S). All wells in the study area are referenced to 

the San Bernardino base line and meridian (S) Well numbers consist of 15 characters and follow the format 

011N001E24Q008S. In this report, well numbers are abbreviated and written 11N/1E-24Q8. Wells in the same 

township and range are referred to only by their section designation, 24Q8. The following diagram shows how the 

number for well 11N/1E-24Q8 is derived.
xvii

Acknowledgements

The authors acknowledge the support of Sonoma County Water Agency. The authors thank 

the city of Santa Rosa, the city of Sebastopol, the city of Rohnert Park, the city of Cotati, the 

town of Windsor, California American Water, and Sonoma County and their respective personnel. The authors also acknowledge the data contributions by the California Department of 

Water Resources and the California Department of Public Health. 

The authors thank the two USGS colleague reviewers; their comments greatly improved 

this report.
Hydrologic and Geochemical Characterization of the Santa 

Rosa Plain Watershed, Sonoma County, California

By Tracy Nishikawa

Executive Summary

The Santa Rosa Plain is home to approximately half 

of the population of Sonoma County, California, and faces 

growth in population and demand for water. Water managers 

are confronted with the challenge of meeting the increasing 

water demand with a combination of water sources, including local groundwater, whose future availability could be 

uncertain. To meet this challenge, water managers are seeking 

to acquire the knowledge and tools needed to understand the 

likely effects of future groundwater development in the Santa 

Rosa Plain and to identify efficient strategies for surface- and

groundwater management that will ensure the long-term 

viability of the water supply. The U.S. Geological Survey, 

in cooperation with the Sonoma County Water Agency and 

other stakeholders in the area (cities of Cotati, Rohnert Park, 

Santa Rosa, and Sebastopol, town of Windsor, Cal-American 

Water Company, and the County of Sonoma), undertook this 

study to characterize the hydrology of the Santa Rosa Plain 

and to develop tools to better understand and manage the 

groundwater system.

The objectives of the study are: (1) to develop an updated 

assessment of the hydrogeology and geochemistry of the Santa 

Rosa Plain; (2) to develop a fully coupled surface-water and 

groundwater-flow model for the Santa Rosa Plain watershed;

and (3) to evaluate the potential hydrologic effects of alternative groundwater-management strategies for the basin. The 

purpose of this report is to describe the surface-water and 

groundwater hydrology, hydrogeology, and water-quality characteristics of the Santa Rosa Plain watershed and to develop a 

conceptual model of the hydrologic system in support of the 

first objective. The results from completing the second and

third objectives will be described in a separate report.

Study Area

The area of interest of this study is referred to herein as 

the Santa Rosa Plain watershed.

• The population centers are the cities of Santa Rosa, 

Rohnert Park, Cotati, Sebastopol, and the town of Windsor.

• The 167,410-acre Santa Rosa Plain watershed area 

includes all of the Mark West Creek watershed 

(161,410 acres), with areas to the northwest and south of 

the Mark West Creek watershed boundary added to include 

most of the Santa Rosa Plain groundwater subbasin as 

defined the California Department of Water Resources.

The Mark West Creek watershed includes three drainage 

basins: Mark West Creek, Santa Rosa Creek, and Laguna 

de Santa Rosa.

• The Santa Rosa Plain watershed includes the Santa Rosa 

Plain and Rincon Valley groundwater subbasins, as well as 

parts of the Wilson Grove Formation Highlands groundwater basin, Kenwood Valley groundwater basin, Healdsburg area groundwater subbasin, and Alexander Valley 

groundwater subbasin.

Geohydrology

The geohydrology of the study area was refined through

the incorporation of recently published studies of subsurface 

geology, geophysics, and surface geologic mapping. The specific goals of this study were to identify the surface and subsurface configuration of the water-bearing units, the textural

variations in these units, the major structures in the study area, 

and the three-dimensional shape of the basins.

Major findings regarding the geohydrology from this

study are the following:

• Four principal aquifer units were defined for the study

area: the Glen Ellen Formation (including Quaternary 

alluvial deposits), Wilson Grove Formation, the Petaluma 

Formation, and the Sonoma Volcanics. The units were 

identified through borings, geophysics, and surface exposures.

• Reported hydraulic-conductivity values for the Glen 

Ellen Formation range from 13 to 23 feet per day. 

Reported specific-yield values for the formation range

from 3 to 7 percent.

• Reported hydraulic-conductivity values for the Wilson 

Grove Formation range from 2 to 65 feet per day. Estimated and reported specific yield and storativity values

for the formation range from 10 to 20 percent and from 

0.00095 to 0.08, respectively.
2 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

• Reported transmissivity values for the Petaluma Formation range from 130 to 1,600 square feet per day.

• Reported transmissivity values for the Sonoma 

Volcanics range from 0.8 to 5,300 square feet per day. 

Reported specific-yield values for the formation range

from 0 to 15 percent.

• Geophysical data indicate that two main groundwater subbasins exist beneath the Santa Rosa Plain; the basins are as 

deep as 4,500 ft below the town of Windsor and as deep as 

10,000 ft beneath Rohnert Park. The two basins are separated by a buried bedrock ridge, the Trenton Ridge, which 

has a minimum depth of about 1,000 ft below land surface.

• Faults could be barriers to groundwater flow in the Santa

Rosa Plain watershed. Recent surface geologic mapping 

and geophysical studies have refined the locations of the

major faults within the Santa Rosa Plain watershed.

• Three-dimensional subsurface models of lithologic variations indicate a west-to-east transition from dominantly 

fine-grained marine sands to heterogeneous continental

sediments interbedded with Sonoma Volcanics. In contrast 

to previous studies, the three-dimensional models of the 

Santa Rosa Plain watershed indicate that the Petaluma 

Formation extends throughout the deeper parts of the 

basins beneath the Santa Rosa Plain.

• The most consistently productive wells in the study area 

extract water from the Wilson Grove Formation. The Glen 

Ellen Formation is heterogeneous and contains substantial 

amounts of clay such that well production from this unit 

is highly variable. Previous reports have reported that the 

Glen Ellen Formation is as thick as 3,000 ft; however, 

recent work, incorporated in this report, indicates that 

the thickness of the formation is variable, but typically 

hundreds of feet. The Petaluma Formation is the deepest 

and thickest aquifer; it is generally less permeable than 

others, but water is produced from sandy horizons within 

the formation.

• For the purposes of this study, five groundwater storage

units were defined on the basis of previous work, hydrogeology, and fault locations.

• The Windsor Basin (WB) storage unit is located north of 

the Trenton Ridge fault, west of the Mayacmas Mountain foothills, and east of the Sebastopol fault.

• The Cotati Basin (CB) storage unit is located south of 

the Trenton Ridge fault, west of the Sonoma Mountain 

foothills, and east of the Sebastopol fault.

• The Wilson Grove (WG) storage unit is located between 

the Mendocino Range and the Sebastopol fault.

• The Valley (VAL) storage unit includes the alluvial fill

of the Rincon, Bennett, and Kenwood valleys.

• The Uplands (UPL) storage unit includes the Mayacmas 

and Sonoma mountains east of the Rodgers Creek fault 

zone but excludes the Valley storage unit.

Surface-Water Hydrology

The surface-water hydrology of the study area was 

evaluated on the basis of streamflow records from 15 U.S.

Geological Survey stream gages. Seven gages within the Santa 

Rosa Plain watershed had records less than 10 years, and the 

remaining gages had records of about 12 years. Records from 

three additional gages in watersheds adjacent to the Santa 

Rosa Plain watershed were also evaluated to extend the record 

of the lower Mark West Creek gage (gage number 11466800, 

Mark West Creek near Mirabel Heights). The length of record 

for the three gages outside of the Santa Rosa Plain watershed 

varied from about 36 to 69 years. The data from the Napa 

River near Napa gage (gage number 11458000) were selected 

for use in extending the Mark West Creek near Mirabel 

Heights record.

Major findings regarding the surface-water hydrology

from this study are the following:

• Highly variable streamflow—Streamflow within the

Santa Rosa Plain watershed is variable with high flows

(7,200 cubic feet per second at Mark West Creek near 

Mirabel Heights) during winter and very low to zero flow

during summer.

• Rapid response to precipitation—Streamflow within the

Santa Rosa Plain watershed is characterized as having a 

rapid response relative to precipitation events.

• Very low summer flows—Baseflow is a minor component

of total mean flow, but it constitutes the only flow during the dry, summer months. Most of the stream reaches 

within the Santa Rosa Plain watershed are intermittent.

• Flow-duration curves—Flow-duration curves were used to

summarize flow conditions at stream gages in and next to

the Santa Rosa Plain watershed. Daily mean discharges of 

600 cubic feet per second and higher occurred 10 percent 

of the time at only one gage within the Santa Rosa Plain 

watershed that had continuous records (11466800, Mark 

West Creek near Mirabel Heights). Daily mean discharges 

of 10 cubic feet per second and less occurred 35 percent 

of the time at all gages with continuous records within the 

Santa Rosa Plain watershed.

• Record extension—Correlations between the Mark West

Creek near Mirabel Heights gage (11466800) and the 

Napa River near Napa gage (expressed as coefficient of

determination, r2) were found to be good (0.59 ≤ r

2 ≤ 0.91).

The maintenance of variance-extension, type 1 (MOVE.1) 

method was used to extend (water-years 1960–2005) 

the record at the Mark West Creek near Mirabel Heights 

gage. Considering the combined record of flood flowrates
Executive Summary 3

between 1959 and 2010 and assuming flood flowrates of

6,000 or 8,000 cubic feet per second, Mark West Creek 

flooded 62 and 25 days, respectively.

• Flooding in the Laguna de Santa Rosa—In the Laguna

de Santa Rosa floodplain, streamflow can be affected

by backwater from the Russian River. Flow can reverse 

during periods of flooding on the Russian River.

Groundwater, Groundwater Movement, and 

Groundwater Levels

The groundwater reservoir in the study area includes the 

saturated sedimentary rocks and sediments underlying the 

floor of Santa Rosa Valley and neighboring lowlands, as well

as the volcanic rocks underlying the mountains to the east of 

the Santa Rosa Valley where such rocks are sufficiently permeable to yield water. Beneath the floor of Santa Rosa Valley, the

principal groundwater reservoir is lithologically heterogeneous 

but consists of one continuous body of saturated material. This 

reservoir is typically in hydraulic communication laterally 

with permeable consolidated rocks that underlie the uplands 

surrounding the Santa Rosa Valley and interfinger with the

basin-fill deposits. Groundwater in the principal reservoir

is contained in the pore spaces of the Quaternary alluvial 

materials and Tertiary sedimentary material, including the 

Glen Ellen Formation, the Wilson Grove Formation, and the 

Petaluma Formation. Groundwater is also contained in locally 

permeable areas within the Sonoma Volcanics. Major findings

regarding groundwater movement and levels from this study 

are the following:

• Significant sources of groundwater recharge in the Santa

Rosa Plain watershed are infiltration of precipitation, infiltration from streams, and irrigation-return flow.

• Significant groundwater sinks are groundwater pumpage,

evapotranspiration from phreatophytes, and baseflow in

streams.

• The water-level contour maps show that, on a larger scale, 

groundwater flows from the Mayacmas and Sonoma

Mountains in the Uplands storage unit westward, toward 

the Laguna de Santa Rosa on the western edge of the Santa 

Rosa Plain, and eastward, from the highlands in the Wilson 

Grove storage unit toward the Laguna de Santa Rosa.

• In general, the water-level contours indicate that Santa 

Rosa Creek gains water east of the Rodgers Creek fault 

zone, and Mark West and Santa Rosa creeks gain water in 

the western part of the Santa Rosa Plain. 

• Water-level contours, based on available data, indicate 

cones of depression in the Cotati/Rohnert Park area and 

north of Sebastopol from Spring 1974 to Fall 2001; however, water levels partially recovered in the Cotati/Rohnert 

Park area by 2007 in response to decreased pumping.

• Groundwater levels in monitoring wells showed response 

to pumping, with variable response with depth.

• Pumping tests published in 1987 to determine if the Sebastopol fault is a barrier to groundwater flow were inconclusive.

Groundwater Quality

Groundwater quality was characterized for the Santa 

Rosa Plain watershed by using analyses for selected physical 

properties and inorganic constituents compiled from previous 

investigations and databases maintained by the U.S. Geological Survey, California Department of Public Health, California 

Department of Water Resources, and public-supply purveyors, 

for the period 1947–2010. These data were used to characterize the areal, vertical, and temporal variations in groundwater 

quality and to identify constituents of potential concern. Stable 

and radioactive isotopes analyzed from groundwater samples 

collected in 2004 as part of the U.S. Geological Survey 

Groundwater Ambient Monitoring and Assessment Program 

or analyzed from surface-water and groundwater samples 

collected from 2006–10 as part of this study, or concurrently 

by private consultants, were used to help identify the recharge 

source and age of groundwater in the study area.

Major findings on water quality from this study are the

following:

• Major ions.

• Samples from springs in the Mayacmas Mountains were 

a mixed cation-bicarbonate type water, with dissolvedsolids concentrations less than about 100 milligrams 

per liter—the lowest dissolved-solids concentrations in

the Santa Rosa Plain watershed. The median dissolvedsolids concentrations of well samples in the Uplands and 

Valley storage units were 330 and 392 milligrams per 

liter, respectively. The higher dissolved-solids values 

compared with springs in hard rock settings reflect

increasing sediment and water interaction and, perhaps, 

anthropogenic effects.

• Most of the well samples from the Windsor Basin and 

Cotati Basin storage units were mixed cation-bicarbonate and sodium-bicarbonate type waters. Data indicated 

limited vertical mixing between upper and lower aquifers.

• Most well samples in the Wilson Grove storage unit 

were calcium-bicarbonate type or mixed cation-bicarbonate type waters with dissolved-solids concentrations 

less than 300 milligrams per liter.

• Data indicated that the Rodgers Creek fault zone 

restricts the lateral movement of water from the Uplands 

and Valley storage units to the Windsor Basin and Cotati 

Basin storage units.
4 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

• Data indicated little mixing across the Sebastopol fault.

• Temporal variation of specific conductance and chloride.

• The specific conductance of water from almost

75 percent of the wells evaluated increased over time, 

with about half increasing by more than 10 percent during the period of record.

• Chloride concentrations increased in about 67 percent 

of the wells evaluated, with just over half increasing by 

more than 10 percent during the period of record.

• The largest increases in specific conductance, chloride

concentration, or both were in wells located in the vicinity of the cities of Rohnert Park and Cotati.

• Concentrations of manganese, iron, and arsenic in 73, 43, 

and 12 percent, respectively, of all samples from wells in 

the study area were greater than State and federal drinking 

water standards prior to any treatment to achieve drinking 

water standards.

• Stable isotopes.

• In general, the isotopic values of samples from the 

Uplands and Valley storage units grouped together in the 

lighter range of all isotopic values, which is consistent 

with groundwater in these storage units being recharged 

by precipitation falling on higher elevations.

• The isotopic values for well samples from the Wilson 

Grove storage unit also grouped together, but were 

within the heavier range of isotopic values from all wells 

in the Santa Rosa Plain watershed, which is consistent 

with groundwater in this storage unit being recharged by 

precipitation falling on lower elevations.

• The isotopic values from wells in the Windsor Basin 

and Cotati Basin storage units spanned the entire range 

of isotopic values in the Santa Rosa Plain watershed, 

indicating a mixture of water sources. Furthermore, data 

indicated that some of the recharge to the Windsor Basin 

and Cotati Basin storage units originates as precipitation 

directly falling on the lower elevations of the Santa Rosa 

Plain.

• The isotopic values of samples collected from a borehole 

in the Petaluma Formation within the Rodgers Creek 

fault zone, on the east side of the Santa Rosa Plain, and 

from a well perforated in the Petaluma Formation near 

the Laguna de Santa Rosa, on the west side of the Santa 

Rosa Plain, were significantly lighter (more negative)

than the other surface-water or well samples. These 

results are consistent with groundwater derived from 

recharge at higher altitudes or under cooler conditions 

than have prevailed in the post-glacial period.

• Age dating.

• Tritium (3H) concentrations were analyzed in 35 samples 

collected from 30 wells and ranged from less than 

0.3 (detection limit of samples collected in 2004) to 

2.4 tritium units.

• About 43 percent of the samples had detectable 3H concentrations, indicating that these samples contain some 

modern water (water recharged since 1952).

• Modern recharge was more prevalent in the shallowwell samples.

• The vertical migration of recharge in the Santa Rosa 

Plain probably is restricted by the presence of low-permeability clay deposits in the Glen Ellen and Petaluma 

Formations.

• Carbon-14 (14C) activities for the 16 well samples analyzed ranged from 1.3 to 84.7 percent modern carbon.

• 14C activities represent uncorrected ages of 1,000 to 

34,000 years before present.

• The deep well samples had uncorrected 14C ages 

of 4,000 years or older, and five of the deep well

samples were 10,000 years or older. These ages are 

consistent with other chemical and isotopic data 

indicating limited vertical mixing.

• The two oldest samples (27,000 and 34,000 years 

before present) were collected from a borehole in 

the Petaluma Formation within the Rodgers Creek 

fault zone on the east side of the Santa Rosa Plain 

and from a well perforated in the Petaluma Formation near the Laguna de Santa Rosa on the west 

side of the Santa Rosa Plain. The old ages of these 

samples, coupled with major-ion data, indicated 

a long groundwater-flowpath passing through the

Cotati Basin storage unit.

Conceptual Model

A conceptual model of the hydrologic system for the 

Santa Rosa Plain watershed was developed (1) to better understand the movement and storage of water in the Santa Rosa 

Plain watershed; (2) to aid in interpretations of field data, such

as water quality, well hydrographs, and stream-gage records; 

and (3) to provide a framework for the development of a 

numerical flow model to help understand the response of the

Santa Rosa Plain watershed to current and historic water uses 

and to predict the response of the hydrologic system to potential future conditions. The conceptual model of the Santa Rosa 

Plain watershed is based on known and estimated physical and 

hydrologic characteristics of the surface-water and groundwater systems and how these characteristics influence the flow

and storage of water in the Santa Rosa Plain watershed. Following Markstrom and others (2008), the hydrologic system 
Executive Summary 5

of the Santa Rosa Plain watershed is conceptualized as having 

three regions: region 1 consists of the plant canopy, the land 

surface, and the soil zone; region 2 consists of streams, lakes, 

and wetlands; and region 3 is the subsurface zone that consists 

of an unsaturated zone and an underlying saturated zone.

The characteristics of the conceptual model for this study 

include the following:

• Boundary conditions—75 percent of the Santa Rosa

Plain watershed boundary is no flow, with the remaining parts allowing hydraulic communication between the 

Santa Rosa Plain watershed and the Kenwood Valley and 

Sonoma Valley, the Cotati-Rohnert Park area and Petaluma 

Valley, the area west of the Sebastopol Fault and Wilson 

Grove Formation Highlands, and the Windsor area and the 

Russian River Basin. The lower boundary of the aquifer 

system is the contact with the low-permeability basement 

rock, which was assumed to be no flow.

• Region 1—Region 1 of the Santa Rosa Plain watershed

includes the plant canopy, the land surface, and the soil 

zone. Region 1 provides the primary link between climatic 

factors and the Santa Rosa Plain watershed hydrologic system. The plant canopy includes natural vegetation, crops 

(for example, row crops, pastures, vineyards, and orchards) 

and landscaped urbanized areas (for example, residential 

areas, parks, and golf courses). The land surface includes 

areas covered by soil; areas with naturally exposed bedrock areas free of soil cover; and areas covered by anthropogenic features such as buildings, roads, and parking lots. 

The soil zone represents the upper unsaturated zone and 

is conceptually defined as the layer extending from the

ground surface to the base of the root zone (Markstrom 

and others, 2008). The soil zone stores and transmits water 

between the atmosphere and the underlying unsaturated 

and saturated zones.

• Inflows to region 1 include precipitation, irrigationreturn flow, surface water, and groundwater discharge.

• Outflows from region 1 include evapotranspiration,

surface-water runoff, and infiltration to the unsaturated

zone.

• Storage components in region 1 include plant canopy, 

depressions in impervious surfaces, depressions in pervious surfaces, and soil-zone storage.

• Region 2—Region 2 of the Santa Rosa Plain watershed

consists of surface-water bodies such as streams, lakes, 

and wetlands. Three major streams drain the Santa Rosa 

Plain watershed: (1) Mark West Creek, (2) Santa Rosa 

Creek, and (3) the Laguna de Santa Rosa. The surfacewater bodies, primarily streams, are the primary storage 

components and flow conduits for surface water.

• Inflows to region 2 include overland flow and interflow

from region 1, groundwater discharge (gaining streams 

include reaches of Mark West and Santa Rosa creeks), 

precipitation, and reclaimed municipal wastewater.

• Outflows include surface-water discharge, streambed

losses (infiltration), evaporation, and diversions (for

example, irrigation).

• The 5-year average surface-water outflow

from the Santa Rosa Plain watershed was 

210,000 acre-feet per year.

• Losing streams include reaches of Mark West and 

Santa Rosa creeks to the west of the Rodgers Creek 

fault zone and in some upstream reaches within the 

uplands.

• Region 3—Region 3 of the Santa Rosa Plain watershed

consists of the subsurface zone, which comprises an 

unsaturated zone and a saturated zone. 

• The areally extensive unsaturated zone connects the soil 

zone to the saturated part of the groundwater system. 

The unsaturated zone is beneath stream channels in parts 

of the Santa Rosa Plain watershed. Percolation through 

the soil zone in region 1 and infiltration through streambeds and lakebeds in region 2 becomes infiltration to

the top of the unsaturated zone and is a function of the 

physical characteristics of the unsaturated zone (vertical 

hydraulic conductivity, thickness, effective porosity, and 

vertical saturation profile).

• Inflows to the unsaturated zone of region 3 include

infiltration from the soil zone, streams, and water

bodies and capillary inflow from the water table.

• More infiltration is expected in areas with

Wilson Grove and Glen Ellen formations 

because of higher permeability.

• Less infiltration is expected in mountains

because of low-permeability Sonoma Volcanics 

and the Franciscan formation.

• Infiltration from stream channels could be

greatest in the transition from the mountains 

to the Santa Rosa Plain.

• Outflows from the unsaturated zone include flows

into the saturated zone (recharge) and transpiration.

• Outflow is expected in the Wilson Grove storage

unit because of permeable unsaturated/saturated 

zone interface.

• Less outflow is expected in the Windsor Basin

and Cotati Basin storage units because of less 

permeable unsaturated/saturated zone interface 

(the clay-rich Petaluma formation underlies the 

Glen Ellen or Quaternary formations).
6 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

• Important hydrologic characteristics of the Santa Rosa 

Plain watershed groundwater-flow system include those 

that determine the ability of the groundwater system to 

transmit water, to store and release water, and to allow 

for vertical passage of water between layers, as well 

as those that control the flow of water across geologic 

or hydrologic boundaries. The movement of infiltrated 

recharge through the saturated groundwater system is 

controlled by topography, aquifer and aquitard properties, the magnitude of natural discharge, and pumping. 

Aquifer and aquitard properties depend on the type 

of sediments and rocks composing the hydrogeologic 

system.

• Groundwater is contained in the pore spaces of the 

Quaternary alluvial materials and Tertiary sedimentary rocks, including the Glen Ellen Formation, the 

Wilson Grove Formation, the Petaluma Formation, 

and Sonoma Volcanics.

• These units, although lithologically heterogeneous, generally form a continuous body of 

saturated material.

• The most permeable formations are the Glen 

Ellen and Wilson Grove formations.

• The Petaluma Formation has low productivity 

but is widely distributed.

• Water-quality data indicated the following:

• The source of natural recharge is precipitation 

falling on the Santa Rosa Plain and on the surrounding mountains.

• There is limited hydraulic communication 

between shallow and deeper aquifer systems

• There is limited groundwater underflow across 

the Rodgers Creek fault zone from the uplands 

to the Santa Rosa Plain.

• The Sebastopol fault could be a partial barrier to 

groundwater flow.

• Outflows from the saturated zone include pumpage, evapotranspiration, discharge to streams and 

lakes, discharge to the soil zone, and underflow to 

surrounding basins.

• Pumping for public supply, agricultural, and 

domestic uses is the largest component of 

groundwater discharge.

• Reported public-supply pumpage ranged from 

3,900 to 10,100 acre-feet per year.

• Estimated agricultural pumpage ranged from 

8,900 to 46,600 acre-feet per year.

• Estimated domestic pumpage ranged from 

11,000 to 23,200 acre-feet per year.

• Estimated total actual evapotranspiration from 

the soil, unsaturated, and saturated zones 

(regions 1 and 3) by a preliminary watershed 

model was about 265,700 acre-feet per year.

• Water-level contours indicated that Santa 

Rosa Creek consistently gains streamflow east 

of Rodgers Creek fault zone.

• Groundwater discharged from springs is a 

source of baseflow for streams or is lost to 

evapotranspiration.

• Underflow to neighboring basins is insignificant.

• Faults can be barriers to groundwater flow.

• Geochemical data indicated that Rodgers Creek 

fault is a barrier to groundwater flow.

• It is unclear whether the Sebastopol fault is a barrier; water quality and reported results are conflicting.

Data Gaps

Analyses of hydrologic, hydrogeologic, and geochemical 

data collected for this study indicated gaps in the data available for the SRPW.

• Lack of depth-dependent water-level and water-quality 

data make it difficult to calibrate a groundwater-flow 

model by aquifer or layer.

• Additional water-quality data are needed to help explain 

variability in observed water-quality data in the Cotati 

Basin storage unit.

• Better estimates, or direct measurements, of agricultural 

and domestic pumpage are required to improve calibration of a numerical groundwater and surface-water flow 

model. The locations of these wells are often unknown or 

unreported; therefore, the locations of these wells should 

be determined.
Chapter A. Introduction to the Study Area

By Tracy Nishikawa, Joseph A. Hevesi, Donald S. Sweetkind, and Linda R. Woolfenden

Introduction

The Santa Rosa Plain (SRP) is home to approximately 

half of the population of Sonoma County, Calif., which has 

over 249,000 residents (California Department of Finance, 

2012b), and is experiencing growth in population and demand 

for water. Water managers face the challenge of meeting the 

increasing water demand with a combination of Russian River 

water—which has uncertainties in its future availability—local

groundwater resources, and extending recycled water and 

other water conservation programs (Sonoma County Water 

Agency, 2012). To meet this challenge, water managers are 

seeking to acquire the knowledge and tools needed to understand the likely effects of future groundwater development 

in the SRP and to identify efficient strategies for surface- and

groundwater management that will ensure the long-term 

viability of the water supply. The U.S. Geological Survey 

(USGS), in cooperation with the Sonoma County Water 

Agency (SCWA) and other stakeholders in the area, undertook this study to characterize the hydrology of the SRP 

and to develop tools to better understand and manage the 

groundwater system.

The SRP is located about 50 miles (mi) north of 

San Francisco, Calif. (fig. 1). The population centers are the 

cities of Santa Rosa, Rohnert Park, Cotati, Sebastopol, and the 

town of Windsor (fig. 2). The study area is a modified form

of the Mark West Creek watershed (also referred to in previous studies as the Laguna de Santa Rosa watershed), which 

includes three surface-water drainage basins: Mark West, 

Santa Rosa, and the Laguna de Santa Rosa (Cal-Atlas, 2007; 

fig. 2). The study-area boundary extends beyond the Mark 

West Creek watershed, along the northwestern and southern 

sections of the boundary, to better represent the complete area 

of the SRP groundwater subbasin as defined by the California

Department of Water Resources (2003; fig. 2). In addition, 

a small area on the west side of the boundary is included to 

account for the entire drainage area upstream of the tributary 

junction with the Russian River. This western addition is not 

included in some published versions of the Mark West Creek 

watershed (Cal-Atlas, 2007). For the purpose of this study, the 

modified form of the Mark West Creek watershed is referred

to as the Santa Rosa Plain watershed (SRPW; fig. 2).

The SRP groundwater subbasin is part of the larger 

Santa Rosa Valley groundwater basin and is the largest 

groundwater subbasin in Sonoma County (California Department of Water Resources, 1980; Sonoma County Water 

Agency, 2006). The 262 square mile (mi2

) SRPW contains the 

SRP groundwater subbasin, Rincon Valley groundwater subbasin, the northern half of the Kenwood Valley groundwater 

basin, eastern portions of the Wilson Grove Formation Highlands groundwater basin, southern portion of the Healdsburg 

area groundwater subbasin, and the southern portion of the 

Alexander Valley groundwater subbasin (California Department of Water Resources, 1980; Sonoma County Water 

Agency, 2006; City of Rohnert Park, 2007; fig. 3).

Previous Work

Cardwell (1958) characterized the hydrogeology of the 

Santa Rosa and Petaluma Valleys, but did not consider the 

hydrogeology of the surrounding mountains. He described the 

geology of the Jurassic and Cretaceous consolidated rocks, the 

Tertiary sedimentary and volcanic rocks, and the late Tertiary 

to Quaternary unconsolidated sediments. The late Tertiary to 

Quaternary unconsolidated sediments (Merced, now Wilson 

Grove, and Glen Ellen Formations), as defined by Cardwell

(1958), were described as the most important water-bearing 

formations; the Tertiary rocks were of secondary importance. 

In addition to the hydrogeology, Cardwell (1958) 

described the groundwater sources and sinks, the water quality, and the potential storage capacity of the groundwater 

subbasins. The primary sources of groundwater recharge were 

rainfall and seepage from streams. The primary groundwater 

sinks were natural discharge (groundwater flow to streams,

springs, evapotranspiration, and adjacent basins) and pumping. In 1949, domestic and agricultural pumping accounted 

for 23 and 73 percent, respectively, of the total pumping in the 

Santa Rosa Valley area (Cardwell, 1958, table 7). In general, 

the groundwater quality met health-based standards for potable use at the time of publication. Cardwell (1958) assumed 

that the zone for available groundwater storage extended from 

10 to 200 ft below land surface (bls). The total estimated available storage in the Santa Rosa Valley area was about 1 million 

acre-feet (acre-ft; Cardwell, 1958, table 12).
8 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Lake

Sonoma

Napa

River

Mendocino

County

Sonoma

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Marin County

Napa County

Lake County

Contra

Costa

 County

128

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121

116

121

128

37 101

101

San

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San

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Santa Rosa Plain

watershed boundary

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Petaluma

Healdsburg

Geyserville

Cloverdale

San

Francisco

0 10 20 30 Miles

0 10 20 30 Kilometers

Mayacmas Mountains

Mendocino Range

Sonoma Mountains

Sonoma Valley

Santa

Rosa

Plain

Alexander

Valley

Petaluma

Valley

Napa Valley

Solano County

Pacific Ocean

IP019208_Figure A01.

R i v e r

N

R u s s i a n

o

r

t h

e

r

n

C

o

a

s

t

R

a

n

g e s

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

C

Area of map

a l i f o r n i a

Pacific Ocean

Northern Coast Ranges

geomorphic province

Santa Rosa Plain

watershed boundary

Figure 1. Location of the Santa Rosa Plain watershed study area within Sonoma County, California.
Previous Work 9

12

29

121

116

128

101 Lynch

Creek

Petaluma River

Moore Creek

Creek

Creek

Conn Creek

Pope Creek

Chileno Creek

Dry Creek

Sonoma Creek

Napa River

Putah Creek

Russian River

Keys Creek

Americano Creek

Stemple Creek

Russian

River

Napa River

Mark

Santa

de

Santa

Laguna

Rosa

Rosa

We st

Sonoma

County

Marin County

Napa County

Lake County

Sonoma

Santa Rosa

Petaluma

Healdsburg

Geyserville

Napa

0

0 5 10 Kilometers

5 10 Miles

122°50’ 122°40’ 122°30’ 122°20’

38°

40’

38°

30’

38°

20’

Surface-water subbasins

Mark West Creek

Laguna de Santa Rosa

Santa Rosa Creek

EXPLANATION

Santa Rosa Plain

 watershed

 model boundary

Major streams

IP019208_Figure A02

Base from U.S. Geological Survey digital data, 1:1,000,000, downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model: sun illumination from northwest at 30 degrees above horizon

California Department of

 Water Resources (CDWR)

 -delineated groundwater

 basin from Bulletin 118 (2003).

Windsor

Sebastopol

Rohnert Park

Cotati

CDWR-delineated

Santa Rosa Valley

groundwater basin

Mt. Hood

Figure 2. Santa Rosa Plain watershed boundary with the Mark West Creek, Santa Rosa Creek, and Laguna de Santa Rosa drainage 

basins, Sonoma County, California.
10 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Gates Canyon

Rodgers Creek fault

thrust fault

Mt. St. John thrust fault 

Petrified Forest thrust zone

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Bennett

Valley

Petaluma

Healdsburg

Area

Alexander

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Rodgers Creek

 fault zone

Santa Rosa Plain

Wilson Grove

Formation Highlands

Rincon Valley

Healdsburg Area

EXPLANATION

Alexander Valley

Kenwood Valley

Petaluma

Santa Rosa Plain

 watershed

 boundary

IP019208_Figure A03

Groundwater basin or subbasins

Fault

Inferred fault

Meacham

Hill

Figure 3. Santa Rosa Plain watershed boundary with groundwater subbasins, Sonoma County, California.
Study Area Description 11

The California Department of Water Resources (CDWR) 

published a series of reports describing the hydrogeology 

of the SRP (Ford, 1975; Herbst and others, 1982; Kadir and 

McGuire, 1987). Ford (1975) presented a general overview 

of the geology, hydrology, water-supply systems, soil types 

associated with septic tanks, available groundwater, and 

water quality of groundwater basins in Sonoma County. Ford 

reported a gross groundwater storage capacity of almost 

10 million acre-ft for the Santa Rosa groundwater basin, which 

was almost an order of magnitude greater than the value estimated by Cardwell (1958). However, the saturated thickness 

used by Ford ranged from 630 to 1,160 feet (ft), compared to 

the 190 ft used by Cardwell (1958). Ford (1975) suggested 

that the Santa Rosa groundwater basin be given a high priority 

for further study.

Herbst and others (1982) carried out a similar study as 

Ford (1975); however, their study was focused on the SRP and 

a link-node type of groundwater-flow model (TRANSCAP;

Miyazaki, 1980) of the subbasin was developed. The total 

available storage capacity of the SRP groundwater subbasin was estimated at about 4.3 million acre-ft, assuming an 

average saturated thickness of 400 ft. Note that the study area 

used by Herbst and others (1982) was smaller than the one 

used by Ford (1975). Total estimated natural recharge from 

1960 to 1975 was 439,200 acre-ft, or about 29,300 acre-ft per 

year (acre-ft/yr), and total estimated groundwater pumpage 

for the period was 445,300 acre-ft, or about 29,700 acre-ft/yr. 

The authors determined that there were insufficient data to

calibrate and verify the model and made suggestions for additional data collection.

Kadir and McGuire (1987) updated the model developed 

by Herbst and others (1982) by using an updated version of 

the link-node type model. The model domain used by Kadir 

and McGuire (1987) is shown in figure 4 overlain on the 

current study area. Kadir and McGuire (1987) treated the 

Sebastopol fault (fig. 3) as a no-flow boundary and the area

west of the fault was not simulated. There were limited data to 

calibrate the model; therefore, the model was calibrated using 

1978–81 water-level data and was verified using 1981–83

data. Calibration hydrographs were presented; however, 

neither a simulated hydrologic budget nor results from 

simulating water-management scenarios were presented.

Kulongoski and others (2010) presented an assessment 

of the groundwater quality of multiple groundwater basins in 

the north San Francisco Bay area, of which one was the SRP 

groundwater subbasin, as part of the USGS component of 

the state Groundwater Monitoring and Assessment (GAMA) 

program. The USGS collected water-quality samples from 

89 wells in 2004, of which 28 were located in the study 

area, and analyzed water-quality data for 2001–04 from the 

California Department of Public Health database. The waterquality samples were analyzed for a wide range of constituents, including volatile organic compounds, pesticides, and 

naturally occurring inorganic constituents. A complete review 

and analysis of the GAMA dataset as applied to the SRPW is 

presented in chapter C of this report.

Purpose and Scope

The USGS, in cooperation with the Sonoma County 

Water Agency, cities of Cotati, Rohnert Park, Santa Rosa, and 

Sebastopol, town of Windsor, Cal-American Water Company, 

and the County of Sonoma, undertook this study to evaluate 

the groundwater resources of the SRP and to develop tools to 

better understand and manage the groundwater system. The 

objectives of the study are (1) to develop an updated assessment of the hydrogeology and geochemistry of the SRP, 

(2) to develop a fully-coupled surface- and groundwater-flow

model for the SRPW, and (3) to evaluate the potential hydrologic effects of alternative groundwater-management strategies for the basin. The purpose of this report is to describe 

the surface- and groundwater hydrology, hydrogeology, and 

water-quality characteristics of the SRPW and to develop a 

conceptual model of the hydrologic system in support of the 

first objective. The results from completing the second and

third objectives will be described in a separate report.

This report comprises four chapters. Chapter A summarizes the purpose and scope of the study, provides a description of the study area, and presents an overview of previous 

work. Chapter B provides a more detailed description of the 

geology and the surface- and groundwater hydrology of the 

study area. The geology section provides a description of the 

geologic setting, stratigraphic units, major faults, and basin 

depth and geometry. The surface-water hydrology section 

provides a detailed description of the hydrography of the 

study area, the characteristics of the main stream channels, 

the processes controlling streamflow in the study area, and the

known characteristics of streamflow in the study area based

on the available records. The groundwater hydrology section 

describes the groundwater subbasins and storage units, aquifer 

system, groundwater recharge and discharge, and groundwater flow. Chapter C focuses on groundwater-quality conditions and the sources and ages of groundwater in the basin. 

Chapter D presents a conceptual model of the SRPW based on 

the data presented in the previous chapters.

The data and interpretive results presented in this 

report form the foundation for the fully-coupled surfaceand groundwater-flow model developed for the study area

(described in a separate report), which provides further 

insights into the overall water budget, potential effects of 

groundwater development, and distribution of groundwater 

recharge and discharge.

Study Area Description

The following description of the study area provides 

an overview of the physiography, climate, hydrography, 

soils, vegetation and land cover, and land uses of the SRPW. 

The description of physiography includes an overview of 

geologic controls on landforms within and surrounding the 

study area. The description of land use provides a brief history 
12 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Lynch Creek

Petaluma River

Moore Creek

Creek

Creek

Conn Creek

Pope Creek

Chileno Creek

Dry Creek

Sonoma Creek

Napa River

Putah Creek

Russian River

Keys Creek

Americano Creek

Stemple

Creek

Russian

River

Napa River

Mark

Santa

de

Santa

Laguna

Rosa

Rosa

We st

Sonoma

County

Marin

County

Napa County

Lake County

12

29

121

116

128

101

Healdsburg fault

Carneros fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Petrified Forest thrust zone

Rodgers Creek fault

Sonoma

Santa Rosa

Petaluma

Healdsburg

Geyserville

Napa

0

0 5 10 Kilometers

5 10 Miles

122°50’ 122°40’ 122°30’ 122°20’

38°

40’

38°

30’

38°

20’

California Department of

 Water Resources-delineated

 Santa Rosa Valley groundwater

 basin from Bulletin 118 (2003).

Other groundwater

 basins

EXPLANATION

Santa Rosa Plain

 watershed

 boundary

Study area from Kadir

 and McGuire (1987).

Major streams

IP019208_Figure A04

Base from U.S. Geological Survey digital data, 1:1,000,000, downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model: sun illumination from northwest at 30 degrees above horizon

Rodgers Creek

 fault zone

Fault

Inferred fault

Windsor

Sebastopol

Rohnert Park

Cotati

Figure 4. Kadir and McGuire (1987) model domain in relation to study area, Santa Rosa Plain watershed, Sonoma County, California.
Study Area Description 13

of land-use changes and includes an overview of the effect 

of land use change on the hydrology of the SRPW, as documented in previous studies. More detail on the study area is 

provided in later chapters. 

Physiography

The SRPW lies within the Coast Range geomorphic 

province (fig. 1) that occupies most of the northwestern edge 

of California and consists of a series of small mountain ranges 

and ridges that trend generally northwest-southeast, subparallel to the Pacific coast line (Jenkins, 1938; California Geological Survey, 2002). The northern portion of the Coast Range 

geomorphic province extend from the San Francisco Bay 

northward to the California-Oregon border.

The principal mountain ranges within and adjoining the 

study area, from the Pacific Ocean coastline to approximately

30 mi inland, include the Mendocino Range, the Sonoma 

Mountains, and the Mayacmas Mountains (fig. 1). All of these 

highlands are of modest relief, generally less than 2,500 feet 

above sea level (ft asl), and most ridge lines are between 500 

and 1,500 ft asl. The Mendocino Range is a large, primarily 

rugged and heavily wooded mountain block that extends over 

200 mi from the San Francisco Bay north to Humboldt Bay. 

Within the study area, however, the Mendocino Range is made 

up of low, rounded hills that range in altitude between 600 

and 1,200 ft asl. The Mendocino Range commonly exhibits 

a knobby, irregular topography that results from erosion of 

the tectonically mixed basement rocks and overlying poorly 

cemented rock units (Graymer and others, 2007). Large and 

small landslides are common to this area (Wentworth and 

others, 1997). The Sonoma Mountains rise from near sea 

level to altitudes of 1,000–2,500 ft asl southeast of Santa 

Rosa. Within the SRPW, the maximum altitude of the Sonoma 

Mountains is 2,452 ft asl and is found along the southeasternmost extent of the study-area boundary. The Mayacmas Mountains are less steep and generally range between 500 and 2,500 

ft asl. The maximum altitude within the SRPW is 2,730 ft asl 

at the summit of Mt. Hood in the Mayacmas Mountains.

The flat-floored, elongate valleys that lie between many

of the ranges of north coastal California have provided land 

for urban and agricultural development. Santa Rosa Valley 

and its eastern neighbors, Sonoma Valley and Napa Valley, 

are three of the largest centers of development (fig. 1). These 

valleys have substantial urban population centers surrounded 

by large tracts of agricultural land and rural populations. 

Each of the valleys forms the lowlands of well-delineated and 

separate watersheds with separate underlying groundwaterflow systems (California Department of Water Resources,

2003). Although the groundwater systems within Sonoma 

Valley are separated from those in the Santa Rosa Valley by a 

groundwater-flow divide, the two valleys are topographically

connected through the narrow, northwest-to-west arcing Kenwood Valley, which extends from the upper end of Sonoma 

Valley into the minor lowlands of Rincon Valley and Bennett 

Valley (fig. 3). Rincon and Bennett valleys lie 1 to 2 mi east 

of Santa Rosa Valley and are mostly separated from it by 

the Sonoma Mountains and a narrow ridge in the Mayacmas 

Mountains. The two valleys occupy an approximately 7-mi 

long northwest-trending structural trough that parallels the 

eastern side of Santa Rosa Valley, but connect to it through a 

narrow gap in the mountains at the eastern side of the city of 

Santa Rosa (fig. 3).

The SRP is a lowland area of about 90 mi2 in a northwest trending structural depression that separates the Mendocino Range to the west from the Sonoma Mountains and 

Mayacmas Mountains to the east (fig. 3). The valley floor

lies mostly between altitudes of about 50 and 150 ft asl. 

The north-northwest to south-southeast axis of the valley 

extends for about 20 mi, from near the Russian River on the 

north to Meacham Hill on the south; the valley width ranges 

mostly from 4 to 7 mi. The floor of the valley is relatively flat

compared to the surrounding mountains, but is not without 

internal topographic features. Most of the valley floor consists

of a low, uneven topography developed on poorly cemented 

sedimentary rocks and weakly compacted sediments underlying the alluvial flood plains, terraces, and fans that have been

deposited by west-flowing intermittent streams (Sowers and

others, 1998).

Climate

The climate for the study area is generally Mediterranean, with cool, wet winters, warm, dry summers, and a 

strong coastal influence on climate that significantly moderates

temperature extremes (Sloop and others, 2009).

Precipitation

The spatial distribution of mean annual precipitation 

is strongly affected by topography and varies considerably 

within the SRPW. Temporal variability in precipitation is primarily controlled by the seasonal pattern of cool, wet winters 

and warm, dry summers. Mean annual precipitation calculated 

for the California Data Exchange Center (CDEC) climate 

station in the city of Santa Rosa (SRO) (California Data 

Exchange Center, 2011), for the period 1906–2010, was 30 in. 

(fig. 5). For the period 1990–2005, about 98 percent of the 

annual precipitation falls from October through May (Sonoma 

County Water Agency, 2006). At the Santa Rosa climate station, the wettest month has been January, with an average precipitation of 6.4 in. for the period 1990–2005 (Sonoma County 

Water Agency, 2006). February and December are the next 

wettest months, with averages of 5.3 and 5.2 in., respectively. 

The months of May through September all have less than 1 in. 

average monthly precipitation; July is the driest month, at 

0.03 in., for the period 1990–2005.

In addition to seasonal variation, there is also significant

year-to-year variability in precipitation due to natural cycles 

and trends in global circulation patterns that strongly affect 

climate for a given year. Wetter-than-normal periods occur 
14 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 5. Total annual precipitation for water-years 1906–2010, California Data Exchange Center SRO (City of Santa Rosa) climate 

station, Sonoma County, California (California Data Exchange Center, 2011).

0 

10 

20 

30 

40 

50 

Long-term mean (29.8 inches)

60 

1906 1908 1910 1912 1914 1916 1918 1920 1922 1924 1926 1928 1930 1932 1934 1936 1938 1940 1942 1944 1946 1948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Water year

Total water-year precipitation, in inches

IP019208 Figure A05

Data missing

when conditions are favorable for establishing a persistent 

storm track centered on the SRPW. These conditions generally 

result in an increase in the frequency and duration of storms. 

For example, wetter than normal conditions prevailed during 

the winter of 2005–06, when a relatively high frequency of 

storms, followed by a relatively large storm event, resulted in 

significant flooding in the SRP on December 21, 2005, through

January 1, 2006. In contrast, circulation patterns can also 

cause a decrease in the frequency of winter storms, resulting in 

drier than normal conditions. Such were the conditions during 

water year 1977, and annual precipitation was only 12.3 in. at 

the CDEC SRO climate station (fig. 5). When there are drierthan-normal winters, the main stream channels will often go 

dry during the summer, although there still can be some flow

downstream of the developed areas as a result of irrigation 

runoff.

Estimates of mean annual precipitation for the period 

1971–2000, obtained using Parameter-elevation Regressions 

on Independent Slopes Model (PRISM; Daly and others, 

2004), were used to indicate the spatial and temporal distribution of precipitation in the SRPW. The PRISM model provides 

an estimate of spatial and temporal variability in precipitation in response to (1) distance from moisture sources, 

(2) average storm track, (3) aspect of land surface in relation 

to storm track, and (4) effect of altitude on adiabatic cooling 

of moisture-laden air masses (Daly and others, 2004). The 

PRISM estimates of average annual precipitation (PRISM 

Climate Group, 2012) for the SRPW include minimum values 

of approximately 30 in. in the central part of the SRP, which 

is in agreement with the measured data, and maximum values 

of more than 50 in. in the Mayacmas and Sonoma Mountains 

(fig. 6).

Air Temperature

Similar to precipitation, the spatial and temporal distributions of air temperature are also strongly affected by topography and season. Mean air temperature between 1990 and 2005 

for a California Irrigation Management Information System 

(CIMIS) climate station near the city of Santa Rosa (CIMIS 

station 83) varied from a minimum of 47°F during January to 

a maximum of 70°F during July, with a mean temperature of 

59°F (Sonoma County Water Agency, 2006). 

Spatial variation of mean monthly maximum and minimum air temperatures for the SRPW is complex. For example, 

the mean minimum air temperature for January is 35°F for the 

lower altitudes in the central and western part of the SRPW 

compared to 40 and 44°F for higher locations in the Mayacmas and Sonoma Mountains, respectively (PRISM Climate 

Group, 2012). In contrast, the mean maximum air temperature 

in January is 52°F for the summit areas in the mountains compared to 56°F in the lowlands throughout the western part of 

the watershed. The mean difference between monthly maximum and minimum air temperatures is approximately 25°F, 

with the largest difference between maximum and minimum 

air temperatures of more than 30°F usually occurring during 

the summer.
Study Area Description 15

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Bennett

Valley

Petaluma

Russian

River

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

EXPLANATION

Santa Rosa Plain

 watershed

 boundary

IP019208_Figure A06_precip contours

30.1 to 33

33.1 to 36

36.1 to 39

39.1 to 42

42.1 to 45

45.1 to 48

48.1 to 51

51.1 to 54

54.1 to 56.9

28.7 to 30

PRISM average annual precipitation, in inches

Figure 6. Contours of average annual precipitation for 1971–2000, Santa Rosa Plain watershed, Sonoma County, California.
16 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Soils

The U.S. Department of Agriculture (2007a) developed 

a spatial database of soils for the entire United States (Soil 

Survey Geographic, or (SSURGO). The SSURGO database 

defines 2,165 separate soil map units within the SRPW. The

soil map units define the spatial distribution of soil types (soil

components) within the study area. According to the SSURGO 

database, the thickness of soils is variable within the SRPW, 

with thinner soils located in the highlands and thicker soils 

located in the basins and valleys (fig. 7). Soil is absent in a few 

isolated locations in the more rugged terrain of the Mayacmas 

Mountains; these locations are dominated by rock outcrops. 

The average soil thickness in the Mayacmas and Sonoma 

Mountains is approximately 1.8 ft. The average soil thickness 

throughout the lowlands of the SRPW is approximately 5 ft. 

The thickest soils, approximately 6 ft and greater, are found 

within the Laguna de Santa Rosa floodplain. Soil thickness is

indicated as zero for areas identified as surface water.

The SSURGO database includes basic soil properties, 

such as soil texture (the percent fraction of sand, silt, and 

clay), porosity, and permeability. In general, soil texture is 

highly variable throughout the SRPW. The percentage of clay 

in soils within the SRPW varies from 0 to 50 percent, with a 

mean of 29 percent (fig. 8). The soils with highest percentage of clay (47–50 percent clay) tend to be located in the 

southern part of the SRP, south of the city of Santa Rosa, 

and throughout the lowest altitudes of the Laguna de Santa 

Rosa floodplain. The soils with the lowest percentage of clay

(0–18 percent) tend to be located along the more rugged areas 

of the uplands, in the northeastern part of the SRPW, and 

along many of the major stream channels, such as Mark West 

Creek and Santa Rosa Creek.

The percentage of sand in soils within the SRPW varies 

from 0 to 98 percent, with a mean of 33 percent (fig. 9). Soils 

with the highest percentage of sand (greater than 53 percent 

sand) tend to be located along the western part of the SRPW, 

west of the Laguna de Santa Rosa channel. They also are 

found along the lower reaches of Santa Rosa Creek and in 

some higher altitude locations in the Sonoma Mountains. Soils 

with a low percentage of sand (less than 25 percent) are found 

throughout the SRPW, including the Laguna de Santa Rosa 

lowlands, areas close to the city of Santa Rosa, and throughout 

a large area in the southern part of the SRPW.

The average porosity of soils is 0.34, with higher porosities mostly in the central parts of the SRP lowlands and lower 

porosity in sandy soils at many locations in the highlands 

throughout the eastern parts of the watershed. The average 

soil-field capacity, a measure of the volume fraction of water

retained by a given soil in response to capillary forces, is 0.12 

for the SRPW. The SSURGO database also includes integrated 

soil properties that indicate how each soil is likely to behave 

under various moisture conditions. An example of such a property is the soil hydrologic group, a standard soil classification

ranging from A to D (U.S. Department of Agriculture, 2007b). 

When thoroughly wet, soil-type A has a low runoff potential, 

type B has a moderately low runoff potential, type C has a 

moderate to high runoff potential, and type D has a high runoff 

potential. The runoff potential is based on a combination of 

soil properties, such as soil thickness, the presence or absence 

of restrictive layers, soil texture, permeability, and the depth 

to the water table. The distribution of soil hydrologic groups 

in the SRPW shows soils with relatively low runoff potential 

along the western boundary (types A and B) and soils with 

high to moderately high runoff potential (types C and D) in the 

southern part of the SRP and throughout the upland areas of 

the Sonoma and Mayacmas Mountains (fig. 10).

Land Use

Modern (post-1950) Changes in Land Use

Historically, the population in the SRPW was mostly 

rural, and agriculture was the main developed land use. In 

1950, the population of the city of Santa Rosa was 17,902. 

At that time, the only other incorporated city was Sebastopol, 

with a population of 2,601 (Cardwell, 1958). The populations of the primary cities and towns in the SRPW increased 

between 1950 and 2010 (table 1), and there were corresponding increases in urban and residential land use (table 2). The 

greatest rate of increase in reported population between 1970 

and 2010 was for the town of Windsor, where the population increased by more than an order of magnitude (table 1; 

California Department of Finance, 2012a and 2012b).

Table 1. Population for the cities and township in the Santa Rosa 

Plain, Sonoma County, California, 1950, 1960, 1970, 1980, 1990, 2000, 

and 2010.

[Abbreviation: NR, not reported]

City or 

township 19501 19601 19701 19801 19901 20001 20102

Santa Rosa 17,902 31,027 50,006 82,658 113,313 147,595 167,302

Rohnert Park NR NR 6,133 22,965 36,326 42,236 40,952

Cotati NR 1,852 1,368 3,346 5,714 6,471 7,258

Sebastopol 2,601 2,694 3,993 5,595 7,004 7,774 7,380

Windsor NR NR 2,359 NR 13,371 22,744 26,751

1

California Department of Finance (2012a).

2

 Estimated population on January 1, 2010, California Department of 

Finance (2012b).
Study Area Description 17

122°50’ 122°45’ 122°40’ 122°35’

38°35’ 38°30’ 38°25’ 38°20’

IP019208_figA07

SSURGO average soil thickness, in feet

EXPLANATION

0.0

0.1 to 1.4

1.5 to 2.2

2.3 to 2.8

2.9 to 3.6

3.7 to 4.1

4.2 to 4.8

4.9 to 5.3

5.4 to 6.7

Santa Rosa Plain

 model boundary

Santa Rosa Plain

 watershed boundary

Figure 7. Soil Survey Geographic (SSURGO) soil thickness, Santa Rosa Plain watershed, Sonoma County, California (U.S. Department 

of Agriculture, 2007a). Soil thickness is indicated as zero for areas identified as water.
18 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 8. Soil Survey Geographic (SSURGO) average percentage of clay in soils, Santa Rosa Plain watershed, Sonoma County, 

California (U.S. Department of Agriculture, 2007a).

Santa Rosa Plain

 model boundary

Santa Rosa Plain

 watershed boundary

EXPLANATION

SSURGO average percentage of clay in soils

0 to 18

19 to 22

23 to 26

27 to 30

31 to 34

35 to 38

39 to 42

43 to 46

47 to 50

122°50’ 122°45’ 122°40’ 122°35’

38°35’ 38°30’ 38°25’ 38°20’

IP019208_figA08
Study Area Description 19

122°50’ 122°45’ 122°40’ 122°35’

38°35’ 38°30’ 38°25’ 38°20’

IP019208_figA09

SSURGO average percentage of sand in soils

0 to 24

25 to 28

29 to 32

33 to 36

37 to 40

41 to 44

45 to 48

49 to 52

53 to 98

EXPLANATION

Santa Rosa Plain

 model boundary

Santa Rosa Plain

 watershed boundary

Figure 9. Soil Survey Geographic (SSURGO) average percentage of sand in soils, Santa Rosa Plain watershed, Sonoma County, 

California (U.S. Department of Agriculture, 2007a).
20 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

SSURGO soil hydrologic group and runoff potential

EXPLANATION

A. Low Other (water)

B. Moderately low

C. Moderate to high

D. High

122°50’ 122°45’ 122°40’ 122°35’

38°35’ 38°30’ 38°25’ 38°20’

IP019208_figA10

Santa Rosa Plain

 model boundary

Santa Rosa Plain

 watershed boundary

Figure 10. Soil Survey Geographic (SSURGO) soil hydrologic groups, Santa Rosa Plain watershed, Sonoma County, California (U.S. 

Department of Agriculture, 2007a).
Study Area Description 21

Land-use data for the SRPW were available for 1974, 

1979, 1986, 1999, and 2008 (California Department of Water 

Resources, 1974, 1979, 1986, and 1999; Sonoma County 

Water Agency, written commun., 2010). The first four land-use

surveys show eight general land-use classifications, including agricultural (irrigated, non-irrigated, and idle); residential, 

commercial, and industrial; suburban-native vegetation or 

agriculture; and native vegetation (fig. 11A–D). The land-use 

data for 2005–08 only distinguished the agricultural (vineyard, 

orchard, row crop, pasture, and unknown) areas from the nonagricultural areas (fig. 11E).

The land-use data indicated minor changes in land use for 

most major categories since the 1970s, but indicated substantial increases in irrigated agricultural land use and decreases 

in non-irrigated agriculture. The land-use data showed that 

native vegetation and riparian areas (including water surfaces) 

changed little over time, constituting 68 percent, 62 percent, 

57 percent, and 61 percent of the total area for 1974, 1979, 

1986, and 1999, respectively (table 2). The combination of 

all agricultural land uses constituted an average of about 

15 percent of the total area and did not change substantially 

over these periods. Similarly, the total agriculture acreage in 

2005–08 was about 25,800, or about 15 percent of the total 

area (County of Sonoma Permit and Resource Management 

Department, 2007); however, irrigated agriculture increased 

steadily from 4 percent of the total area in 1974 to 11 percent 

in 1999 and non-irrigated land use decreased (table 2). 

Between 1974 and 1999, urban and residential land uses 

increased slightly from 17 percent (1974) to 19 percent (1999) 

of the study area.

Hydrologic Effects of Changing Land Use

Although changes in land use during the last few decades 

were modest, more substantial changes in land use during the 

preceding century in the SRPW has resulted in significant

changes to the hydrologic system (Sloop and others, 2009). 

Sloop and others (2009) described the history of important 

anthropogenic changes with relevance to hydrologic conditions. In general, these changes were (1) the conversion of 

native vegetation to grassland and agriculture that started in 

the mid-1800s and (2) the start of rapid urbanization in the 

1940s.

The conversion of land cover from native vegetation 

(perennial bunch grasses and annual forbs) to grassland (for 

ranching) and agriculture has generally resulted in a reduction 

in interception storage capacity, a reduction in transpiration, 

a decrease in permeability, and an increase in propagation of 

the drainage network (Sloop and others, 2009). The combined 

effect of these anthropogenic changes is more runoff generation (compared to unaltered landscapes), including an increase 

in the “flashiness” of streamflow as characterized by a steepening of the streamflow hydrograph (Sloop and others, 2009).

With the onset of rapid population increase and associated urbanization in the 1940s, which continues to present 

day, agricultural land has been converted to urban land uses 

(mostly housing and commercial). With increased urbanization, the percentage of impervious surfaces (rooftops, parking 

lots, roads, among others) has probably increased within the 

SRPW. The hypothesized increased imperviousness could 

have resulted in increased runoff affecting areas within and 

downstream of the more heavily urbanized zones (Christopher 

Delaney, Sonoma County Water Agency, written commun., 

2008). In addition, the increase in impervious area also contributes to the “flashiness” of streamflow (Sloop and others,

2009).

Changing land use could also affect the groundwater 

system. The conversion of land cover from native vegetation 

to agriculture could reduce direct infiltration to the soil zone;

however, an increase in runoff, as described above, could 

result in increased streambed recharge downstream from the 

runoff. If agriculture requires irrigation, then irrigation-return 

flow is another source of water to the soil zone. A change to

increased impervious area could reduce direct infiltration to,

and evapotranspiration from, the soil zone.

Table 2. Land use in Santa Rosa Plain Watershed, Sonoma County, California, 1974, 1979, 1986, 1999, and 2008.

[Bracketed, italicized numbers are a subset of total agriculture and are not included in total area. Abbreviations: Mi2, square miles; —, no data]

Land-use type

Land-use surveys1

1974 1979 1986 1999 2008

Single use Acres Mi2 Acres Mi2 Acres Mi2 Acres Mi2 Acres Mi2

Urban and residential 28,154 44.0 30,919 48.3 31,827 49.7 32,265 50.4 — —

Commercial and industrial 751 1.2 2,075 3.2 2,703 4.2 2,914 4.6 — —

Total agriculture 24,303 38.0 23,295 36.4 25,807 40.3 23,444 36.6 25,782 40.3

[Irrigated] 6,839 10.7 10,262 16.0 11,811 18.5 18,698 29.2 — —

[Non-irrigated] 17,464 27.3 13,033 20.4 13,996 21.9 4,746 7.4 — —

Native vegetation and riparian 112,996 176.6 103,276 161.4 94,591 147.8 101,629 158.8 — —

Other 836 1.3 7,475 11.7 12,112 18.9 6,788 10.6 — —

Total area 167,040 261.1 167,040 261 167,040 260.9 167,040 261.0 — —

1

Modified from California Department of Water Resources, 1974, 1979, 1986, and 1999; County of Sonoma Permit and Resource Management Department;

unpublished crop surveys of Sonoma County, Division of Planning and Local Assistance, Sacramento.
22 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

12

116

101

12

116

101

12

116

101

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

R9W N

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

0

0 5 10 Kilometers

5 10Miles

1974 1979

1986 1999 122°50’ 122°40’

38°

30’

38°

20’

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

122°50’ 122°40’

38°

30’

38°

20’

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

122°50’ 122°40’

38°

30’

38°

20’

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

122°50’ 122°40’

38°

30’

38°

20’

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

EXPLANATION

Land-use classification

Irrigated agriculture

Non-irrigated agriculture

Idle agriculture

Residential, commercial, industrial

Suburban-native vegetation or agriculture

Water

Unknown

Native vegetation

Chapter A_Figure 11abcd

Santa Rosa Plain

 watershed boundary

Selected faults

A B

C D

Figure 11. Land-use maps, Santa Rosa Plain watershed, Sonoma County, California: A, 1974; B, 1979; C, 1986; D, 1999; and E, 2005–08.
Study Area Description 23

S a n t a R o s a P l a i n

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Gates Canyon thrust fault

Mt. St. John thrust fault 

Petrified Forest thrust zone

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

EXPLANATION

Agricultural land-use classification

Vineyard

Unknown

Orchard

Row Crop

Pasture

IP019208_Figure A11e

Santa Rosa Plain

 watershed boundary

River or stream

E

2008

Selected faults

Inferred fault

Rodgers Creek

 fault zone

Figure 11. Continued
24 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

References Cited

California Data Exchange Center, 2011, Station Meta Data: 

Santa Rosa: Sacramento, Calif., State of California, Department of Water Resources, accessed February 28, 2011, at 

URL http://cdec.water.ca.gov/cgi-progs/staMeta?station_

id=SRO

California Department of Finance, 2012a, Population totals 

by township and place for California Counties: 1860 to 

1950: Sacramento, Calif., State of California, accessed 

October 18, 2012, at URL http://www.dof.ca.gov/research/

demographic/reports

California Department of Finance, 2012b, E-4 Historical 

population estimates for cities, counties, and the state, 

2001–2010, with 2000 and 2010 census counts: Sacramento, Calif., State of California, accessed October 18, 

2012, at URL http://www.dof.ca.gov/research/demographic/

reports.

California Department of Water Resources, 1974, 1974 

Sonoma County land use survey data: unpublished data 

located at Division of Integrated Regional Water Management, North-Central Region, West Sacramento, California, 

1:24,000 scale.

California Department of Water Resources, 1979, 1979 

Sonoma County land use survey data: unpublished data 

located at Division of Integrated Regional Water Management, North-Central Region, West Sacramento, California, 

1:24,000 scale.

California Department of Water Resources, 1980, Groundwater basins in California—a report to the Legislature in

Response to Water Code Section 12924: California Department of Water Resources, Bulletin 118-80, 73 p.

California Department of Water Resources, 1986, 1986 

Sonoma County land use survey data: unpublished data 

located at Division of Integrated Regional Water Management, North-Central Region, West Sacramento, California, 

1:24,000 scale.

California Department of Water Resources, 1999, 1999 

Sonoma County land use survey data: Sacramento, Calif., 

State of California, digital map accessed on January 28, 

2011, at http://www.water.ca.gov/landwateruse/lusrvymain.

cfm

California Department of Water Resources, 2003, California’s Groundwater: Bulletin 118–Update 2003: Sacramento, Calif., State of California, 246 p., available online 

at URL http://www.water.ca.gov/groundwater/bulletin118/

update2003.cfm

California Geological Survey, 2002, California Geomorphic 

Provinces Note 36: Sacramento, Calif., State of California, Department of Conservation, accessed November 16, 

2011, at URL http://www.consrv.ca.gov/cgs/information/

publications/cgs_notes/note_36/Documents/note_36.pdf.

California Interagency Watershed Mapping Committee, 2004, 

California Interagency Watershed Map of 1999 (Calwater 

2.2.1): Sacramento, Calif., State of California, California 

Resources Agency, accessed September 7, 2007, at URL

http://atlas.ca.gov/download.html#/casil/inlandWaters

Cardwell, G.T., 1958, Geology and ground water in the Santa 

Rosa and Petaluma areas, Sonoma County, California: U.S. 

Geological Survey Water Supply Paper 1427, 273 p., 5 

plates.

City of Rohnert Park, 2007, 2005 Urban Water Management 

Plan: Rohnert Park, Calif., City of Rohnert Park, 108 p.

County of Sonoma Permit and Resource Management Department, 2007, LanduseByArea1: unpublished data.

Daly, Christopher, Gibson, W.P., Doggett, Matthew, Smith, 

Joseph, and Taylor, George, 2004, Up-to-date monthly climate maps for the conterminous United States: Proceedings 

of the 14th American Meteorological Society Conference on 

Applied Climatology, 84th AMS Annual Meeting Combined 

Preprints, American Meteorological Society, Seattle, Washington, January 13-16, 2004, Paper P5.1, CD-ROM.

Ford, R.S., 1975, Evaluation of ground water resources: 

Sonoma County, volume 1: geologic and hydrologic data: 

California Department of Water Resources, Bulletin 118-4, 

177 p., 1 plate.

Graymer, R.W., Brabb, E.E., Jones, D.J. , J. Barnes, R.S. 

Nicholson, and R.E. Stamski, 2007, Geologic map and map 

database of eastern Sonoma and western Napa counties, 

California: U.S. Geological Survey Scientific Investigations

Map 2956, scale 1:100,000.

Herbst, C.M., Jacinto, D.M., and McGuire, R.A., 1982, Evaluation of ground water resources, Sonoma County, volume 

2: Santa Rosa Plain: California Department of Water 

Resources, Bulletin 118-4, 107 p., 1 plate.

Jenkins, O. P., 1938, Geomorphic Map of California: State of 

California Department of Natural Resources, Bulletin 158, 

Plate 2 .

Kadir, T.N. and McGuire, R.A., 1987, Santa Rosa Plain 

ground water model: California Department of Water 

Resources Central District, 318 p.
References Cited 25

Kulongoski, J.T., Belitz, Kenneth, Landon, M.K., and Farrar, 

Christopher, 2010, Status and understanding of groundwater 

quality in the North San Francisco Bay groundwater basins, 

2004: California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2010-5089,

88 p.

Miyazaki, Brent, 1980, Preliminary investigations of computer 

program TRANSCAP and its application to ground water 

basin studies: California Department of Water Resources, 

Technical Information Record 1388-CD-1.

PRISM Climate Group, 2012, PRISM home page: accessed 

October 18, 2012, at http://www.prism.oregonstate.edu/.

Sloop, Christina, Honton, Joseph, Creager, Clayton, Chen, 

Limin, Andrews, E.S., Bozkurt, Setenay, 2009, Hydrology and sedimentation, Chapter 4, of The Altered Laguna, 

A Conceptual Model of Watershed Stewardship: Laguna 

de Santa Rosa Foundation, p. 63-110, at http://www.

lagunafoundation.org/knowledgebase/?q=node/182.

Sonoma County Water Agency, 2006, 2005 Urban Water Management Plan, Santa Rosa, Calif., 152 p., available at URL 

http://scwatercoalition.org/images/pdf/2005_uwmp_report.

pdf

Sonoma County Water Agency, 2012, Sonoma County water 

agency home page: accessed October 16, 2012, at URL 

http://www.scwa.ca.gov/index.php

Sowers, J.M., Noller, J.S., and Lettis, W.R., 1998, Quaternary 

geology and liquefaction susceptibility, Napa, California 

1:100,000 quadrangle; a digital database: U.S. Geological Survey Open-File Report 98-460, 20 p., 1 sheet, scale 

1:100,000.

U.S. Department of Agriculture, 2007a, Natural Resources 

Conservation Service, Soil Survey Geographic (SSURGO) 

database for Sonoma County, California: National 

Resources Conservation Service, accessed on May 14, 

2009, at URL http://SoilDataMart.nrcs.usda.gov

U.S. Department of Agriculture, 2007b, Natural Resources 

Conservation Service, National Engineering Handbook, Part 

630 Hydrology, Chapter 7 Hydrologic Soil Groups: Washington, DC, 16 p.

Wentworth, C.M., Graham, S.E., Pike, R.J., Beukelman, 

G.S., Ramsey, D.W., and Barron, A.D., 1997, Summary 

distribution of slides and earth flows in the San Francisco

Bay region, California: U.S. Geological Survey Open-File 

Report 97-745C, 10 p.
26 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California
Chapter B. Hydrology of the Santa Rosa Plain Watershed, 

Sonoma County, California

By Donald S. Sweetkind, Joseph A. Hevesi, Tracy Nishikawa, Peter Martin, and Christopher D. Farrar

Introduction

The Santa Rosa Plain watershed (SRPW) is geologically 

and hydrologically complex, with interfingering geologic units

and cross-cutting structures affecting a variety of surface-water 

and groundwater processes (fig. 1). This chapter presents a 

description and analysis of the geology, surface-water hydrology, and groundwater hydrology of the SRPW for the purpose 

of evaluating regional groundwater availability. Aspects of 

the SRPW described in this chapter include the delineation of 

hydrogeologic units based on lithology and hydraulic properties, construction of a detailed three-dimensional hydrogeologic framework, description of the surface-water and groundwater systems, and an analysis of both predevelopment and 

recent groundwater recharge and discharge.

Geology

The geology of the SRPW has been previously described 

in groundwater resource studies of the Santa Rosa Plain (SRP; 

Cardwell, 1958; Ford, 1975; Herbst and others, 1982). Since 

these studies, geologic mapping, geophysical studies, and 

interpretation of borehole data have refined the understanding

of the basin geometry and the identity and location of major 

basin-filling units (McLaughlin and others, 2008; Langenheim and others, 2010; Sweetkind and others, 2010). Threedimensional (3D) lithologic and stratigraphic models of the 

SRPW constructed from borehole data, geologic mapping, and 

geophysical data were used to define the lithologic, stratigraphic, and structural architecture for the region (Sweetkind 

and others, 2010). This chapter summarizes and integrates 

these previous studies to develop a modern understanding 

of the geology of the SRPW as it relates to the groundwater 

resources.

Geologic Setting

The elongate ridges and valleys in the northern Coast 

Ranges are the geomorphic expression of folds and fault slices 

formed in response to compression and lateral transport of 

oceanic sediments and crust that collided with and accreted to 

the western margin of North America during the last several 

million years (Bailey, 1966; Norris and Webb, 1976; Dickinson, 1981). Most of the Coast Ranges are underlain by a 

basement of Mesozoic consolidated to weakly metamorphosed 

sedimentary and crystalline rocks of the Franciscan Complex 

(Fox, 1983), the Great Valley Group (Ingersoll, 1990), and 

Coast Range ophiolite (Hopson and others, 2008). Basement 

rocks are exposed at the surface throughout much of the Coast 

Ranges (Gutierrez and others, 2010) but also underlie valleys 

and coastal plains, where they are buried beneath variably 

thick accumulations of Tertiary marine and continental sediments and volcanic rocks, which, in turn, are overlain by thin 

Quaternary alluvial deposits.

The highlands in the eastern part of the SRPW are 

underlain by various types of Miocene and Pliocene volcanic 

rocks, interbedded in places with the largely non-marine and 

estuarine strata of the Petaluma Formation; both of these units 

unconformably overlie basement rocks (fig. 2). The rocks in 

the western part of the highlands are highly deformed and cut 

by the active, northwest-striking, right-lateral Rodgers Creek 

fault zone (figs. 1 and 2). The central part of the SRPW is 

occupied by the topographically broad, low-lying SRP (fig. 1). 

The southern part of the SRP is covered by Quaternary alluvial 

deposits (fig. 1). The northern part features low, slightly dissected exposures of late Pliocene and Quaternary (Pleistocene 

and Holocene) fluvial, lacustrine, and alluvial-plain deposits 

that have been referred to as the Glen Ellen Formation (Fox, 

1983), along with younger alluvium within stream channels (Graymer and others, 2007; fig. 1). The western edge 

of the SRPW lies within a broad, topographically low area 
28 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

12

116

101

KJf

KJf

Mzv

Mzv

Twg

Qge

Qge

Qge

Qge

Q

Q

Q

Q

Q

Tsv

Tsv

Tsv

TK

Kl

Kl

um

Tsv

Twg

Tsv

Qge

Tp

Tp

Tp

Tp

um Healdsburg fault

Carneros

 fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Mt. St. John

thrust fault 

Gates Canyon thrust

Petrified Forest thrust zone

IP019208_Figure B01

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Geology modified from

Saucedo and others (2000)

Cotati

basin

Windsor

basin

Bennett

Valley

Spring

Lake

reservoir

Creek

A'

B'

A

B

Wilson Grove

Formation

Highlands

Figure 1. Santa Rosa Plain watershed, Sonoma County, California.
Geology 29

EXPLANATION

IP019208_Figure b01b - Geology explanation

Water

Franciscan Complex

Lower Cretaceous marine

Volcanic and metavolcanic

Coastal belt rocks

Tertiary-Cretaceous

Cretaceous

Ultramafic rocks

TK

Kl

um

Mzv

KJf

GEOLOGIC UNIT

Sedimentary and metasedimentary rocks

Plutonic, metavolcanic, and mixed rocks

Mesozoic

Wilson Grove Formation

Quaternary

Pliocene and Miocene

Pliocene and Miocene

Plio-Pleistocene and Pliocene 

Recent alluvium, landslide,

 and sand dune deposits

Sonoma Volcanics

Tp Petaluma Formation

Tv

Sedimentary rocks

Cenozoic

Volcanic rocks

Glen Ellen Formation

Q

Qge

Twg

Santa Rosa Plain watershed

 boundary

Location of deep basins as defined 

 the –14 milligal isostatic gravity

 contour of Langenheim and

 others (2006)

Fault—Dashed where approximately

 located, dotted where concealed

 

Line of geologic section shown on

 figures 2, 25

Wilson Grove Formation

Quaternary

Pliocene and Miocene

Pliocene and Miocene

Plio-Pleistocene and Pliocene 

Recent alluvium, landslide,

 and sand dune deposits

Sonoma Volcanics

Tp Petaluma Formation

Tsv

Sedimentary rocks

Cenozoic

Volcanic rocks

Glen Ellen Formation

Q

Qge

Twg

Rodgers Creek fault zone

A'

A

that is underlain by locally fossiliferous Miocene to Pliocene 

marine sandstone, formerly known as the “Merced” Formation (Cardwell, 1958), and now referred to as the Wilson 

Grove Formation (Fox, 1983). These marine strata dip gently 

northeastward beneath the SRPW and unconformably overlie Mesozoic rocks (figs. 1 and 2). The west and southwest 

sides of the SRPW are bounded by a system of poorly defined

Pliocene and younger normal faults, here generalized as the 

Sebastopol fault (fig. 1).

Stratigraphic Units

The stratigraphic units in the SRPW include Mesozoic 

to Early Tertiary basement rocks, Tertiary sedimentary and 

volcanic rocks, and Quaternary sedimentary deposits (table 1). 

Mesozoic to Early Tertiary rocks of the Franciscan Complex, Great Valley Group, and Coast Range ophiolite make 

up the basement rocks beneath the entire study area. Tertiary 

sedimentary and volcanic rocks overlie the basement and 

are exposed in upland areas around the valleys. Quaternary 

sedimentary deposits underlie the valley floors and form fans

along the valley margins (fig. 1).

Basement Rocks

Pre-Miocene basement includes Cretaceous and older 

rocks of the Franciscan Complex (Kjf; fig. 1), ultramafic rocks

of the Jurassic Coast Range ophiolite (um, fig. 1), and Cretaceous marine sediments of the Great Valley Group (Kl, fig. 1; 

Blake and others, 1984; McLaughlin and Ohlin, 1984; Blake 

and others, 2002). The collective thickness of these basement 

units is unknown, but could be a few tens of thousands of 

feet (Bailey and others, 1964). These rocks are characterized 

by a variety of consolidated rock types, including penetratively sheared shale (melange matrix), graywacke, blocks of 

blueschist, chert, greenstone, thinly interbedded shale and 

sandstone, and mafic to ultramafic ophiolitic rocks. Waterwell drillers typically describe these rocks as either sandstone, 

greywacke, chert, or serpentine (table 1).

Tertiary Volcanic Rocks

The 3-8 million year old Sonoma Volcanics (Wagner and 

others, 2005) dominate the upland areas in the eastern part of 

the SRPW in the Sonoma Mountains and Mayacmas Mountains and are present within the basin fill beneath the Santa

Rosa Plain (fig. 2). These volcanic rocks are well-exposed to 

the east of the Rodgers Creek fault zone and exist as complexly faulted slices along the southwest side of the Rodgers 

Creek fault zone, where they project beneath, and probably 

correlate with, volcanic units in the subsurface of the SRPW 

(McLaughlin and others, 2005; McLaughlin and others, 

2008). A well drilled east of Rohnert Park (6N/8W-13R2) 

encountered Sonoma Volcanics at a depth of 900-ft below land 

surface (bls) and penetrated an additional 430 ft without reaching the underlying bedrock (California Department of Water 

Resources, 1979).

The Sonoma Volcanics include a thick accumulation of 

andesitic and basaltic tuffs containing interbedded lavas and 

volcaniclastic rocks (Wagner and others, 2005; McLaughlin 

and others, 2008). The volcanic units have a wide variety of 

volcanic rock types, including basaltic, andesitic, dacitic and 

rhyodacitic flows, flow breccias, avalanche or talus breccia,

tuff, and several andesitic to rhyodacitic tephra units. Both 

welded and unwelded tuffs are seen in outcrops and probably reflect differences in emplacement style, accumulated

thickness of individual tuff beds, and their location relative 

to the source vent. Many of the units have relatively limited 

lateral extent and appear to have been erupted from local 

volcanic vents. Older volcanics, such as the Tolay and Burdell 

Figure 1. Continued.
30 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California Figure 2. Interpretive geologic cross sections of the Santa Rosa Plain watershed, Sonoma County, California. sac12-0452_Figure B02

B B'

No vertical exaggeration

FEET

SEA

LEVEL

FEET

–2,000–4,000–6,000

SEA

LEVEL

FEET

–2,000–4,000–6,000

A A'

No vertical exaggeration

SEA

LEVEL

FEET

1,000

–2,000–4,000–6,000

SEA

LEVEL

1,000

–2,000–4,000–6,000

Rodgers Creek

fault zone

Rodgers Creek

fault zone

Maacama

fault zone

Petrified Forest

thrust zone Trenton Ridge fault Sebastopol fault 

Sebastopol

fault 

Trenton Ridge

 fault 

Bennett Valley fault zone

Cotati Basin

Geology modified from McLaughlin and others 

(2008) and Sweetkind and others (2010)

Quaternary deposits andGlen Ellen Formation, undividedWilson Grove FormationSonoma VolcanicsPetaluma FormationUndifferentiated basement

EXPLANATION 

Fault, sense of offset not shown

Southwest NortheastSouthwest Northeast Rincon Valley

Mayacmas

Mountains

Mayacmas Mountains Santa Rosa Plain
Geology 31

Table 1. Description of hydrogeologic units, Santa Rosa Plain watershed, Sonoma County, California.

[Abbreviations: >, greater than; —, no data]

Hydrogeologic 

unit

General age 

range1

Examples of mapped 

geologic units2

Estimated 

thickness, 

in feet

Typical 

depositional 

environment3

Typical 

lithologic 

description

Quaternary 

deposits

Quaternary Includes units mapped as younger 

and older alluvium and alluvial 

fan and terrace deposits.

0–550 Stream-channel, 

flood-plain deposits,

older alluvium and 

terrace deposits.

Gravel; sand and gravel; 

sand, clay, and gravel; 

sand and clay.

Glen Ellen 

Formation

Early Pleistocene (?) 

and Pliocene

Includes the Glen Ellen 

Formation, the Huichica 

Formation, and other unnamed 

Tertiary continental deposits.

0–600 Continental, piedmont, 

and valley alluvial 

fans, local lacustrine 

deposits.

Clay and sand, clay 

and gravel, sand, 

sand and gravel, tuff, 

conglomerate.

Wilson Grove 

Formation

Late Pliocene to 

late Miocene

Includes rocks formerly assigned 

to the Merced Formation.

0–2,700 Deep to shallow marine, 

locally transitional 

to continental 

environments.

Sand, sandstone, blue 

sandstone; clay, sand or 

gravel and shells; clay 

and sand.

Volcanic rocks Pliocene and 

Miocene 

Includes Sonoma, Tolay, and 

Burdell Mountain volcanics.

0–3,000 — Basalt, volcanic breccia, 

tuff.

Petaluma 

Formation

Pliocene to late 

Miocene 

Includes the Petaluma Formation. 0–3,000 Fluvial and lacustrine, 

with estuarine and 

transitional marine 

environment.

Clay, clay and sand, shale, 

sand or sandstone.

Basement rocks, 

undifferentiated 

Pre-Miocene; 

predominantly 

Jurassic and 

Cretaceous

Includes the Franciscan Complex, 

the Great Valley Complex, 

including the Coast Range 

ophiolite.

>2,000 — Sandstone, graywacke, 

chert, serpentine.

1

General age ranges from Wagner and Bortungo (1982); Blake and others (2002); Graymer and others (2007).

2

Mapped geologic units from Wagner and Bortungo (1982); Blake and others (2002); Graymer and others (2006 and 2007).

3

Depositional environment listed only for Miocene and younger sedimentary rocks.

Mountain Volcanics (Wagner and others, 2011), are interbedded with the Petaluma or Wilson Grove Formations (section

B–B’ on fig. 2, table 1), whereas the younger parts of the 

Sonoma Volcanics overlie the Petaluma Formation and are 

interbedded with, or underlie, the Pliocene-Pleistocene Glen 

Ellen Formation (Wagner and others, 2005).

The total thickness of Sonoma Volcanics ranges up to at 

least a few thousand feet; however, volcanic-rock thickness 

is highly variable and, in general, water wells drilled in the 

volcanic uplands do not penetrate the entire thickness of the 

formation.

Tertiary Sedimentary Rocks

Three sedimentary formations of late Miocene to Early 

Pleistocene age dominate the basin fill beneath the SRPW: the

Wilson Grove, Petaluma, and Glen Ellen Formations (table 1). 

In places, these three formations interfinger with each other

and with the Sonoma Volcanics, forming a complex geometry. 

The three sedimentary units are described in the following 

sections in terms of their lithologic character, spatial extent, 

thickness, and relation to other units.

Wilson Grove Formation

The late Miocene to late Pliocene Wilson Grove Formation is mostly composed of fine- to medium-grained, thickbedded to massively-bedded, moderate- to well-sorted, uncemented to weakly cemented marine sandstone. The cement 

is calcium carbonate and iron hydroxides. The formation is 

distinctly fossiliferous, containing stringers and lenses of shell 

beds (Powell and others, 2004). The unit also contains pebble 

and gravel stringers, some of which consist almost entirely 

of chert derived from the Franciscan Formation, as well as 

clay lenses and local thin beds of pumiceous tuff (Fox, 1983; 

Blake and others, 2002; Powell and others, 2004). The Wilson 

Grove Formation can be divided into three distinct lithologic 

variants that represent slightly different marine environments 

(Powell and others, 2004): (1) fine-grained, deep-water marine

sandstones most commonly found to the west of the SRPW; 

(2) well-sorted, fine-to medium-grained, shallow-water

marine sandstone that is represented by much of the exposed 

Wilson Grove Formation in the study area, especially north 

of Sebastopol; and (3) medium- to coarse-grained sandstone beds interbedded with pebble conglomerate beds that 
32 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

represent transitional marine/continental settings, commonly 

found south and east of Sebastopol. Along its southeastern 

margin, the Wilson Grove Formation interfingers with the

Petaluma Formation in outcrops near the town of Cotati and at 

Meacham Hill immediately southwest of the SRPW (Powell 

and others, 2004). Wilson Grove Formation marine sandstones 

have been inferred to interfinger with transitional marine and

non-marine deposits of the Petaluma Formation beneath the 

SRPW (McLaughlin and others, 2008; Sweetkind and others, 

2010), but this transition zone is obscured by younger deposits 

(section B–B’ on fig. 2).

In the study area, the maximum thickness of the Wilson 

Grove Formation has been estimated to be about 2,700 ft 

(Powell and others, 2004; table 1). Another study of the Wilson Grove Formation estimated that the total thickness of the 

formation is greater than 6,000 ft (Holland and others, 2009); 

however, this study aggregated measured thickness from multiple locations for portions of the unit deposited over a range 

of time. Exposures in the Wilson Grove Formation in the 

uplands to the west of the SRPW are generally 500 ft thick, 

whereas wells in the western part of the SRPW penetrate as 

much as 1,000 ft of the formation without penetrating the base 

of the unit. The Wilson Grove Formation is exposed over a 

broad area to the west of the SRPW, extending from Petaluma 

in the south to the Russian River on the north and westward 

into the Mendocino Range from the west edge of the SRPW 

boundary (fig. 1).

Petaluma Formation

The late Miocene to Pliocene Petaluma Formation is 

dominated by deposits of moderately to weakly consolidated, 

silty to clayey mudstone with local beds and lenses of poorly 

sorted sandstone and minor beds of nodular limestone and 

conglomerate. The Petaluma Formation can be subdivided into 

lower, middle, and upper members on the basis of detailed 

stratigraphic analysis using the coarser-grained materials 

and fossils (Allen, 2003). In general, the formation coarsens 

from claystone and siltstone at its base to sandstone and 

conglomerate higher up in the section (Holland and others, 

2009). In outcrops near the southern part of the study area, the 

lower member consists of blue-gray, thick- and thin-bedded, 

massive-to-thinly laminated mudstone and very fine-grained

sandstone (Holland and others, 2009). The lower member also 

includes minor interbeds of sandstone, conglomerate, limestone, dolomite, and chert. The middle and upper members are 

coarser-grained than the lower member and contain lenticular 

beds of conglomerate and thick-bedded sandstone.

The Petaluma Formation consists of transitional marine 

and non-marine sediments that were deposited in estuarine, 

lacustrine, and fluvial depositional settings (Allen, 2003;

Powell and others, 2004). The continental fluvial and lacustrine facies extend westward from the Sonoma and Mayacmas 

mountains; they transition to marine facies beneath the SRP 

(McLaughlin and others, 2008; Sweetkind and others, 2010).

The Petaluma Formation is exposed in outcrops along the 

western slopes of the Mayacmas and Sonoma mountains from 

north of Santa Rosa to near Cotati, around the southern margin 

of SRPW, and within Bennett Valley (Clahan and others, 2003; 

McLaughlin and others, 2008; fig. 1). On the basis of geologic 

logs from wells, it is also known to be present under a large 

part of the valley floor of SRP (fig. 2). The formation is at least 

4,000-ft thick in the Petaluma oil field, located 10-mi south of

the study area, and is at least 3,000-ft thick in the study area 

based on outcrops and cuttings from deep, petroleum exploration wells (Morse and Bailey, 1935; Allen, 2003; Powell and 

others, 2004).

Glen Ellen Formation

The Pliocene to early Pleistocene (less than 3.2 million 

years old) Glen Ellen Formation consists of clay-rich stratified deposits of poorly sorted sand, silt, and gravel. Grain-size

grades both laterally and vertically from coarse to fine, commonly over distances of a few tens to a few hundreds of feet 

(Cardwell, 1958). Bedding is thick to massive and often has 

lenticular form. Most of the clasts and probably much of the 

matrix were derived from the Sonoma Volcanics. Cobbles in 

the conglomerates are mostly subangular to rounded and range 

mostly between 3 and 6 in. in diameter (Weaver, 1949). The 

cobbles are mostly of andesitic or basaltic composition; obsidian clasts are one of the hallmark characteristics of this formation (Cardwell, 1958; McLaughlin and others, 2005). The 

sediments making up this formation were probably originally 

deposited as alluvial fans and piedmont. Some of the material 

beneath and next to SRP was probably deposited in lagoons or 

shallow bays and could grade into a marine facies.

The Glen Ellen Formation is exposed near the town of 

Glen Ellen in upper Sonoma Valley and to the north and west 

of Santa Rosa. Within the SRPW, the Glen Ellen Formation rests directly on the basement rocks of the Franciscan 

Complex in places, but more generally overlies the Sonoma 

Volcanics, Wilson Grove, or Petaluma Formations. Along 

the eastern margin of and beneath the SRP, the Glen Ellen 

Formation is overlain by alluvial units of Quaternary age 

(fig. 1). Cardwell (1958) reported the formation to be as thick 

as 3,000 ft beneath the SRP, although no drill-hole or outcrop 

data document such a thickness. Sweetkind and others (2010) 

utilized available drillers’ logs in the SRPW to interpret the 

thickness of the Glen Ellen Formation as highly variable but, 

generally, to be a few hundred feet thick or less; this interpretation is used for this report.

Quaternary Deposits

Quaternary sedimentary deposits recognized and mapped 

within the SRPW include alluvial deposits, terrace deposits, 

near-shore marine and estuarine sediments, colluvium, and 

landslide deposits (Sowers and others, 1998; Graymer and 

others, 2007). Alluvial sediments of Quaternary age have 

been mapped as distinct deposits on the basis of the degree of 
Geology 33

consolidation, cementation, clast size and sorting, and geomorphic expression. The alluvial sediments can be divided into 

older (Pleistocene) and younger (Holocene) deposits, which, 

to some degree, are consistent with the amount of cementation 

and consolidation of the sediments (Cardwell, 1958; Sowers 

and others, 1998). Alluvial fan deposits cover the largest areal 

extent of any of the alluvial sediments in the study area. The 

fan, river terrace, and stream-channel deposits generally consist of heterogeneous mixtures of poorly- to well-sorted sand, 

silt, clay, gravel, cobbles, and boulders in thin to massive, 

interfingering beds of limited lateral extent (tens to hundreds

of feet), often with lenticular form. The deposits near valley 

axes are mostly flat lying, but near valley margins, deposits in

wedge-shaped fans dip toward the valley.

Large volumes of alluvial sediments are present in all 

the main valleys in the study area. The older alluvial deposits 

are as much as 400 ft thick; the younger alluvium is generally 

less than 150 ft thick. Landslide and colluvial deposits form 

isolated patches or thin cover near the flanks of the valleys.

Faults

The largest-offset faults in the study area are the northwest-trending, right-lateral strike-slip faults that are part of the 

San Andreas dextral transform system, including the Rodgers 

Creek fault zone, Maacama fault zone, and Bennett Valley 

fault zone (fig. 1); the Carneros fault zone, important farther to 

the southeast in Sonoma Valley, could extend into the eastern 

part of the study area. These faults often exhibit components of transtensional or transpressional dip-slip movement, 

particularly at fault bends. The major strike-slip faults in the 

study area typically are a complex of steeply dipping, braided 

strands that form zones several hundred to several thousand 

feet wide rather than distinct single planes of weakness. The 

major faults with strike-slip motion can be generally regarded 

as vertical for the purpose of this hydrologic study; this simplification could be less applicable to the Trenton Ridge fault,

which has been interpreted as a reverse fault with a gentle dip 

(Fox, 1983) or steep dip (Williams and others, 2008).

Faults with large components of normal slip are interpreted to bound deep, sedimentary basins beneath the SRPW 

(McLaughlin and others, 2008). Normal faults have north to 

northeast strikes; however, many have curvilinear surface 

expression, such as the Sebastopol fault that bounds the west 

side of the SRPW and the unnamed fault that lies to the east of 

the Sebastopol fault (fig. 1 and section B–B’ on fig. 2). Normal 

faults with dip-slip displacement are common in local areas 

of extension within the overall strike-slip regime. These faults 

often bound pull-apart basins, particularly where fault strands 

overlap and displacement is transferred from one fault strand 

to another. Where faults are at angles to the prevailing stress 

field, or where the principal stresses have rotated through time,

compressional structures can be formed, such as the mostly 

buried Trenton Ridge fault and Petrified Forest thrust zone

(fig. 1 and section A–A’ on fig. 2).

The Rodgers Creek fault zone consists of two segments: 

a northern Healdsburg fault segment and the southern Rodgers 

Creek fault segment, which are separated by the Santa Rosa 

Creek floodplain (Gealey, 1951; Fox, 1983; McLaughlin and

others, 2008). The surface traces of these two segments are 

offset from each other; the traces probably overlap beneath the 

Santa Rosa Creek floodplain (McLaughlin and others, 2008).

The predominant sense of displacement on both fault segments 

is right lateral strike-slip and reflects the same regional pattern

of displacement as on the San Andreas fault system. The fault 

zone is about 0.6 mi wide, and though partially obscured by 

landslides and urban development, the linear topography and 

elongate depressions, some containing wetlands or sag ponds, 

serve to identify the fault-zone location (McLaughlin and others, 2008). Movement along the fault zone has been complex 

with more than one sense of displacement during Pliocene to 

Holocene, dependent on the extant stress field (McLaughlin

and others, 2008). The northern segment of the fault dips 

steeply (greater than 70 degrees) to the northeast on the basis 

of the hypocenters of earthquakes (Wong and Bott, 1995). 

North of Santa Rosa a strong subsurface gravity gradient is 

offset by 1.2 mi from the surface trace of the fault, indicating 

reverse faulting with a down-to-the-northeast sense of offset. 

However, Langenheim and others (2010) suggest the offset 

of the gravity anomaly from the mapped surface trace could 

represent a recent adjustment of the active fault position rather 

than evidence of a through-going thrust fault.

The Maacama fault zone is a northwest striking, steeply 

dipping, right-lateral strike-slip fault that extends across about 

93 mi of northwestern California. In the study area, the Maacama fault zone is located in the Mayacmas Mountains (fig. 1), 

where highly disrupted slivers of Franciscan Complex basement rocks are exposed in a zone up to 1 mi wide. Although 

the predominant sense of displacement along the Maacama 

fault zone is dextral strike-slip, the fault is coincident with an 

elongate gravity low bounded by a strong gravity gradient, 

which is consistent with long-term southwest-side down offset 

(Langenheim and others, 2010).

The Bennett Valley fault zone is a sinuous zone of rightlateral fault strands that extends southeastward about 19 mi 

from the Spring Lake (or Santa Rosa Creek) reservoir into 

Sonoma Valley (fig. 1). Landslides and urbanization obscure 

much of the zone; where exposed, the fault zone is narrow 

(10-ft wide), steeply dipping, and north striking (McLaughlin 

and others, 2008). The Spring Lake reservoir was constructed 

over a marshy wetland area along the fault zone.

The Sebastopol fault is a curved zone of poorly exposed 

faults that generally parallels the contact between Pliocene 

Wilson Grove Formation and Quaternary alluvial deposits and 

the topographic break in slope between the valley floor and the

low hills to the west and south of Sebastopol (figs. 1 and 2). In 

the vicinity of Sebastopol, the fault bends to a more northerly 

trend (Bezore and others, 2003; Clahan and others, 2003; 

Delattre and others, 2008). The Sebastopol fault probably 

consists of a network of subparallel, related, short fault segments; lack of strong geophysical expression indicates that 
34 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

total vertical and horizontal offset on these faults is small (less 

than 1,500 ft; fig. 2; Langenheim and others, 2010). North 

of Sebastopol, near Trenton, trenching for a treated-water 

pipeline exposed the fault and revealed that the main sense of 

displacement is east-side-down normal faulting. There is no 

evidence for any strike-slip displacement along the Sebastopol 

fault (R. McLaughlin, written communication, 2009).

The mountain uplands in the easternmost part of the study 

area are generally the result of uplift associated with young, 

compressional structures that largely post-date the Sonoma 

Volcanics (McLaughlin and others, 2005 and 2008; Graymer 

and others, 2006 and 2007). The Petrified Forest thrust zone

and Gates Canyon thrust fault (McLaughlin and others, 2005) 

form an imbricate zone of south-southwest-directed, northeastdipping reverse faults that truncate at the Maacama fault zone 

(fig. 1). These reverse faults trend northwest-southeast and 

place rocks of the Franciscan Complex above younger, ashflow tuff and basaltic andesite of the Sonoma Volcanics (Petrified Forest thrust zone along section A–A’, fig. 2) (McLaughlin 

and others, 2004). A similarly oriented thrust fault, the Mt. 

St. John thrust (Graymer and others, 2006 and 2007; Langenheim and others, 2010), juxtaposes Franciscan Complex rocks 

against the Sonoma Volcanics near the southern end of the 

Maacama fault (fig. 1).

On the basis of a prominent gravity anomaly, Langenheim and others (2010) extended the Carneros fault northwest 

of its mapped extent to upper Sonoma Valley and the southeast 

tip of the Maacama fault. Within the study area, this northern 

extension of the Carneros fault is buried by younger parts of 

the Sonoma Volcanics and by Franciscan Complex rocks that 

lie above the Mt. St. John thrust. Where exposed to the south, 

the Carneros fault juxtaposes Tertiary marine strata, including 

the Neroly Formation and overlying Sonoma Volcanics on the 

west, with Great Valley Group rocks on the east (Graymer and 

others, 2007; Langenheim and others, 2010). It is possible that 

there is a similar stratigraphic juxtaposition in the subsurface 

in the eastern part of the study area.

Basin Depth and Geometry

Analysis and modeling of gravity data indicate that the 

Tertiary sedimentary and volcanic rocks and Quaternary 

deposits underlying the SRPW conceal a complex basement 

topography developed primarily on rocks of the Franciscan 

Complex (Langenheim and others, 2006; McPhee and others, 

2007; Langenheim and others, 2010). Taking advantage of the 

contrast between dense, pre-Miocene rocks (predominantly 

composed of Mesozoic rocks of the Franciscan Complex and 

the mafic Coast Range ophiolite) and less dense, Tertiary and

Quaternary sedimentary rocks and Pliocene and Miocene volcanic rocks, gravity measurements detected variations in the 

density of rocks in the shallow and middle levels of the crust 

(McPhee and others, 2007). Gravity highs were measured 

over basement rock outcrops, areas of thin, sedimentary-rock 

cover, and over dense lavas; gravity lows were measured over 

areas of thick, low-density sedimentary rock or sediment cover 

(McPhee and others, 2007; Langenheim and others, 2010). 

A surface representing the top of the high-density basement 

was derived from mathematical inversion of regional gravity measurements (Langenheim and others, 2006 and 2010; 

McPhee and others, 2007) constrained by outcrop data and 

well data to assign density values to the formations suspected 

to underlie a particular area (Nettleton, 1976). This surface 

was inferred to represent the elevation of pre-Miocene rocks 

and allowed estimation of the thickness of relatively low-density rocks overlying the basement.

The gravity-mapping technique was used to identify 

major structural features beneath the SRPW that have no clear 

surface manifestation. The resulting model of depth to preMiocene bedrock for the SRPW defines both the overall basin

geometry and the configuration of subbasins that are bounded

by internal faults (McPhee and others, 2007; Langenheim and 

others, 2010). The gravity method allows for good delineation 

of the basin shapes, but is less precise in the thickness or depth 

estimations because the density-depth relation at any particular location can be poorly known. Steep-gravity gradients 

bound many of the basement bedrock highs and lows, which 

is indicative of some degree of fault control. Such a fault is 

shown at depth bounding the southwestern side of the Cotati 

basin on interpretive geologic section B–B’ of figure 2.

The analysis of gravity data reveals two deep, steep-sided 

sedimentary basins: the Windsor basin beneath the northern 

part of the SRP and the Cotati basin beneath the southern 

part, which are separated by a buried bedrock ridge (McPhee 

and others, 2007; Langenheim and others, 2008; fig. 1). The 

Windsor basin is about 5.5 by 7.5 mi in size and is centered 

near the town of Windsor. The thickest exposures of the Glen 

Ellen Formation in the SRPW are observed near this basin 

in the hills that flank the northeast side of the SRPW (fig. 1). 

The basin has a roughly triangular form, bounded by the 

Healdsburg fault segment on the northeast, the Trenton Ridge 

fault to the south, and a zone of poorly exposed normal faults 

on the west. Inversion of gravity data indicates the basin is 

3,000–6,500 ft deep (Langenheim and others, 2008). The 

southern and western margins of the Windsor basin appear to 

have a series of downward steps into the basin (Langenheim 

and others, 2010), indicating that normal faulting played a 

role in basin subsidence. Based on outcrop and well data, the 

deeper parts of the Windsor basin are likely filled with tuff

beds and lavas of the Sonoma Volcanics intercalated with sedimentary units of the Petaluma Formation (McLaughlin and 

others, 2008). Rocks of the Glen Ellen Formation and Quaternary alluvial fan deposits overlie these older rocks.

Analysis of gravity data indicates that the Cotati basin 

in the southern part of the study area is larger and has a more 

complex shape than the Windsor basin (Langenheim and 

others, 2008; fig. 1). The Cotati basin includes two structurally-controlled subbasins with an intervening east-trending 

basement ridge that is less than 4,000 ft bls (Langenheim and 

others, 2010). The deepest part of the Cotati basin is beneath 

Rohnert Park and is modeled as 8,000 to 10,000 ft deep. 
Surface-Water Hydrology 35

Gravity data and lithologic logs from oil exploration boreholes 

indicate the basin is filled with as much as 6,500 ft of Miocene

to Pliocene rocks of the Sonoma Volcanics, Petaluma Formation, and Wilson Grove Formation that are unconformably 

overlain by about 520 ft of Late Pliocene to Quaternary alluvial and lacustrine deposits (McLaughlin and others, 2008).

The Windsor and Cotati basins are separated by a buried 

ridge of relatively elevated bedrock that is uplifted by the 

west-northwest-striking Trenton Ridge fault (fig. 1). The 

Trenton Ridge fault is exposed only in the northwestern part 

of the study area, where it strikes northwest to west-northwest, 

dips to the northeast, and places Franciscan Complex basement rocks over the Pliocene sandstone of the Wilson Grove 

Formation (Delattre and others, 2008). Seismic reflection

surveys and gravity data indicate that basement rocks could be 

as shallow as 1,000 ft bls in the area between the Windsor and 

Cotati basins (Williams and others, 2008). Reflections from

basement rocks or Pliocene to Quaternary sedimentary rocks 

show increasing downwarping with depth across the Trenton 

Ridge fault, indicative of the slow formation of the ridge over 

time (Williams and others, 2008).

To the east of the southern segment of the Rodgers Creek 

fault, gravity data and geologic mapping define a shallow,

sediment-filled, pull-apart basin underlying most of Rincon

Valley and part of Bennett Valley (section B–B’ on fig. 2; 

McLaughlin and others, 2008; Langenheim and others, 2010). 

This small basin is interpreted to be the result of interaction 

between the Rodgers Creek fault and the Maacama fault to the 

northeast (McLaughlin and others, 2008). Gravity data define

a large feature in the bedrock of Bennett Valley (Langenheim 

and others, 2008). The Bennett Valley gravity high extends 

from the Rodgers Creek fault zone on the west to the Bennett 

Valley fault zone on the east. Although relatively high-density 

basement rocks or dense lavas in the Sonoma Volcanics could 

produce the gravity high, neither of these formations crop 

out extensively above or near the gravity high. A few small 

outcrops of serpentine and Franciscan Complex rocks exposed 

in ravines in the bounding fault zones indicate a Mesozoic 

basement could lie at shallow depth in this area, however. Langenheim and others (2008 and 2010) also interpreted magnetic 

data in concert with gravity data to better constrain the formations producing the gravity high and concluded Franciscan 

Complex greenstone or hydrothermally altered ophiolitic rocks 

are the most likely source, not Sonoma Volcanics.

Surface-Water Hydrology

In general, the characteristics of streamflow provide

an indication of the integrated hydrologic response of the 

upstream drainage to precipitation. Available streamflow

records from gages in the SRPW were analyzed to develop a 

better understanding of the seasonal distribution of streamflow, along with the response of streamflow to variability in

climate. The long-term history of streamflow was evaluated

by using an estimated (also referred to as an extended) record 

of streamflow. The analysis of streamflow is used to help

develop the conceptual model of the SRPW hydrologic system 

presented in chapter D of this report.

Surface-Water Drainage Pattern

Throughout much of the Northern California Coast 

Ranges, the surface-water drainage pattern is strongly influenced by geologic structure. Most of the larger streams, such 

as the Russian River, flow parallel to the dominant northwestsoutheast structural trend of folds and faults for much of their 

length. In contrast to this regional drainage pattern, many of 

the streams within the SRPW do not parallel the southeast- to 

northwest-trending geologic and physiographic structure, but 

rather cut across the structural trends, flowing east to west

from the highlands in the Sonoma and Mayacmas Mountains 

in the eastern SRPW. The SRPW is mostly within the middle 

Russian River drainage basin and includes 16 subbasins identified by the California Interagency Watershed Map (CIWM)

of 1999 (Cal-Atlas, 2007). The 16 CIWM subbasins define an

area of 251 square miles (mi2) and are grouped into 3 larger 

drainage-basin areas that are named according to the main 

stream draining the area: Mark West Creek, Santa Rosa Creek, 

and Laguna de Santa Rosa drainage basins (figs. 3 and 4). 

The USGS National Hydrography Dataset (NHD; Simley 

and Carswell, 2009) identifies 28 named streams within the

SRPW. The Sonoma County Water Agency (SCWA) identifies

additional named streams within the SRPW (Sonoma County 

Water Agency, written commun., 2008). The larger streams 

identified by NHD and SCWA are labeled in figure 4 as major 

streams. The three largest stream channels within the SRPW 

are Mark West Creek, Santa Rosa Creek, and the Laguna de 

Santa Rosa. According to both the NHD and SCWA, Mark 

West Creek is the main tributary connecting the SRPW to the 

Russian River (fig. 3). However, the Laguna de Santa Rosa 

stream channel is the primary channel draining the SRP (Sloop 

and others, 2007; Cummings, 2004).

Laguna de Santa Rosa Drainage Basin

The Laguna de Santa Rosa drainage basin is an 88-mi2

area drained by the Laguna de Santa Rosa upstream of the 

Santa Rosa Creek tributary (fig. 3). The basin drains the southern and southwestern areas of the SRPW. The Laguna de Santa 

Rosa is a low-gradient drainage that originates at an altitude 

of 260 ft, west of the city of Cotati and close to the southern 

boundary of the SRPW (fig. 4). Much of the Laguna de Santa 

Rosa upstream of the Mark West Creek confluence is below an

altitude of 50 ft. Santa Rosa Creek, which joins the Laguna de 

Santa Rosa north of the Laguna de Santa Rosa drainage basin, 

is the largest tributary to the Laguna de Santa Rosa. Other 

important tributaries to the Laguna de Santa Rosa include 

Copeland Creek, Crane Creek, Hinebaugh Creek, Five Creek, 

Colgan Creek, Gossage Creek, Washoe Creek, and Roseland 
36 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

PETALUMA VALLEY

KENWOOD

VALLEY

SANTA ROSA PLAIN

RINCON VALLEY

MT. HOOD

Pool Creek

Monroe

Laguna de Santa Rosa

Blucher Creek Crane Creek

Santa Rosa Creek

Porter Creek

Rincon Creek

Van Buren Creek

Humbug

Creek

Mark West

Springs

Upper Windsor Creek

South Fork Matanzas Creek

Santa Rosa Creek Reservoir

Pine Creek

Reservoir

Upper Matanzas Creek

122°50’ 122°45’ 122°40’ 122°35’

Surface-water subbasins

Mark West Creek

Santa Rosa Creek

Laguna de Santa Rosa

EXPLANATION

Groundwater basins and subbasins

Major streams

Surface-water subbasins

 (California Atlas, 2007)

38°

35’

38°

30’

38°

25’

38°

20’

Santa Rosa Plain

 watershed boundary

IP019208_Figure B03

0

0 5 10 Kilometers

5 10 Miles

M e n d o c i n o R a n g e

M a y a c m a s M o u n t a i n s

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

S o n o m a M o u n t a i n s

Figure 3. Santa Rosa Plain watershed boundary and surface-water drainage basins, Sonoma County, California.
Geology 37

Figure 4. Major and minor stream channels and stream gage locations within the Santa Rosa Plain watershed, Sonoma County, California. 

(COPE, Copeland Creek at Rohnert Park; LAGC, Laguna de Santa Rosa near Cotati; COLU, Colgan Creek near Santa Rosa; COLL, Colgan Creek 

near Sebastopol; LAGS, Laguna de Santa Rosa near Sebastopol; LAGG, Laguna de Santa Rosa near Graton; SRCU, Santa Rosa Creek near 

Santa Rosa; SRCM, Santa Rosa Creek at Mission Blvd; BRSH, Brush Creek at Santa Rosa; SRCA, Santa Rosa Creek at Alderbrook Drive; MATC, 

Matanzas Creek at Santa Rosa; SRCS, Santa Rosa Creek at Santa Rosa; SRCW, Santa Rosa Creek at Willowside Road; MWCW, Mark West 

Creek near Windsor; MWCM, Mark West Creek near Mirabel Heights)

Starr

Mark West

Ducker

Leslie

Pool

Abramson

Windsor

Van Buren

Five

Weeks

Crane

Pruit

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Riccas

Wright 

Alpine

Paulin

Olivet

Steele

Todd

Todd

Riebli

SantaRosaCreek

Reiman

Limerick

Creek

Lane Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Badger

Cook

Fulton

Arwine

Gold

PetersonCreek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek Creek

Creek

PoppyCreek

Windmill

Laguna de

Santa 

Rosa

Slusser

Moon

Duer

Melita

Humbug

GumviewCreek

Creek

Airport

Piner

Jamison

Kawana

East

Wikiup

Salt

Golf

Moorland

Grain

Hopper

Bellevue-Wilfred Channel

Hartman

Creek

Wilfred

Woolsey

Linda Lane

Childers

College

Cotati

Shinn

Ewing

Vine Hill

Oakmont

Wasp

Sachett

Mark

West

Irwin

Porter

Santa Rosa Creek

Mill Creek

Creek

Creek Creek

Creek

MarkWest

Blucher Creek

C anfield Creek

Dix on Creek

Gossage Creek

Washo e Creek

Mark West

Creek

BrushCreek

Creek

Rincon

CandyCreek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Creek

Austin

Otis

Creek

Creek

Roseland

Colgan

Springs

Spring Creek

Creek

Dutton ?

Matanzas

Matanzas

Powel

Fork

Copeland

Hinebaugh

SRCW

SRCS

MATC

BRSH

SRCU

LAGS

MWCM

COLL

COLU

LAGC

COPE

MWCW

LAGG SRCA

SRCM

122°50’ 122°45’ 122°40’ 122°35’

38°

35’

38°

30’

38°

25’

38°

20’

Lynch Creek

Americano Creek

Sonoma Creek

Dry Creek

Napa River

Russian River

Surface-water subbasins

Mark West Creek

Santa Rosa Creek

Laguna de Santa Rosa

Major streams

Minor streams

USGS stream gages

USGS stream gages (stage only)

USGS stream gages (seasonal)

Santa Rosa Plain

 watershed boundary

EXPLANATION

IP019208_Figure B04

0

0 5 10 Kilometers

5 10 Miles

S o n o m a M o u n t a i n s

M e n d o c i n o R a n g e

M a y a c m a s M o u n t a i n s

Windsor

Santa

Rosa

Sebastopol

Rohnert

Park

Cotati
38 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Creek (fig. 4). Copeland Creek and Crane Creek have short 

perennial reaches (Simley and Carswell, 2009) draining the 

Sonoma Mountains in the southeastern part of the SRPW 

study area. Copeland Creek is perennial in its upper sections, 

becomes intermittent as it flows westward across the alluvial

fan east of Rohnert Park, and then becomes mostly channelized as it continues flowing westward through Rohnert Park

and the city of Cotati before joining the Laguna de Santa Rosa 

at an altitude of 92 ft.

Crane Creek originates in the northern Sonoma Mountains, at an altitude of 1,535 ft, and flows westward. Crane

Creek joins Hinebaugh Creek within the city of Rohnert Park. 

Hinebaugh Creek is channelized downstream of the confluence

with Crane Creek and joins the Laguna de Santa Rosa at an 

altitude of 85 ft. Washoe Creek is a short, 2 mi-long, perennial 

channel draining the southern-most part of the SRPW, flowing

from south to north. On the western side of the Laguna de 

Santa Rosa subbasin, Blucher Creek is also characterized as a 

perennial stream (Simley and Carswell, 2009).

The “Laguna de Santa Rosa” also refers to the general

area of wetlands, ponds, and vernal pools within the area of 

the 1-percent annual exceedance probability (AEP) floodplain surrounding the main Laguna de Santa Rosa channel 

(Federal Emergency Management Agency, 2002; fig. 5). The 

Laguna de Santa Rosa channel and floodplain together form

a natural overflow basin connecting Santa Rosa Creek, Mark

West Creek, and the smaller creeks in the SRPW with the 

Russian River. The overflow basin, approximately defined by

the 1-percent AEP floodplain, is the second largest freshwater

wetland area in the coastal northern California region and is 

valued as an important ecological resource.

During the dry summer season, the Laguna de Santa 

Rosa overflow basin consists of a winding ribbon of channels

and interconnected wetlands. In addition to the swamp and 

marsh areas connected to the main channel, the area within the 

floodplain includes numerous vernal pools. During the winter

storm season, the channel and wetland areas transform into a 

series of lakes within the floodplain. When flooding occurs in

response to larger storms, the lakes and wetlands coalesce into 

larger water bodies within the floodplain. During the largest

storms, flooding is more extensive and, for short durations

(generally no longer than several days), the water bodies in 

the Laguna de Santa Rosa combine into a nearly continuous 

lake 10 mi long that can be several hundred feet to over a mile 

wide (Cardwell, 1958). In addition, during periods of high 

runoff in the Russian River, directions of flow in the Laguna

de Santa Rosa can be either to or from the Russian River. The 

Laguna de Santa Rosa floodplain acts as a natural regulator of

floods on the lower Russian River by temporarily capturing

and storing up to 79,000 acre-ft of flood water, thus dampening the peak flows in the Russian River downstream of the

Mark West Creek tributary (Sloop and others, 2007). Outflow

from the Laguna de Santa Rosa can be partially or completely 

blocked by backwater effects from the Russian River. As 

the Russian River flooding recedes, the discharge from the

Laguna de Santa Rosa into Mark West Creek can exceed the 

storm runoff generated within the Mark West Creek drainage 

basin because of the additional volume of Russian River water 

included in the discharge.

Santa Rosa Creek Drainage Basin

The Santa Rosa Creek drainage basin is a 77-mi2 drainage area in the central and eastern parts of the SRPW (fig. 3). 

Santa Rosa Creek, the main stream in the Santa Rosa Creek 

drainage basin, is 22 mi long and flows in a westerly direction from drainage divides in the Mayacmas and Sonoma 

Mountains to the confluence with the Laguna de Santa Rosa

(fig. 4). Similar to Mark West Creek, the headwaters of Santa 

Rosa Creek and its major tributaries begin in steep, relatively 

undeveloped mountainous terrain that features natural vegetative cover, including both forest and grassland. The middle 

part of the Santa Rosa Creek drainage includes urbanized 

areas within the city of Santa Rosa and also includes rolling 

hills covered in grass, pasture, and crops. Santa Rosa Creek 

becomes an engineered channel with concrete embankments 

as it passes through the urbanized areas within the city of 

Santa Rosa. In the central part of Santa Rosa, a section of the 

engineered channel is underground. Downstream of Santa 

Rosa, Santa Rosa Creek is a straightened flood-control channel, designed to drain more rapidly across the alluvial basin 

and into the Laguna de Santa Rosa floodplain. The lower part

of the drainage consists mostly of agricultural land. The lowest 

point in the drainage has an altitude of approximately 49 ft at 

the confluence of Santa Rosa Creek with the Laguna de Santa

Rosa. Important tributaries to Santa Rosa Creek include Piner 

Creek, Paulin Creek, Brush Creek, Rincon Creek, and Ducker 

Creek on the north side of the drainage, and Matanzas Creek, 

Spring Creek, and Oakmont Creek on the south side (fig. 4).

The upper sections of Santa Rosa Creek and Matanzas 

Creek are classified as perennial streams (Simley and Carswell, 2009), although, by late summer and fall, flows diminish to less than 2 cubic-feet per second (ft3

/s) throughout much 

of the drainage. Most of Piner Creek, the lower section of 

Brush Creek, and sections of Spring Creek are classified as

reconstructed channels because of a history of channel modifications as a result of agricultural development, urbanization,

and flood control (U.S. Army Corps. of Engineers, 2002). The

upper section of Paulin Creek, all of Rincon Creek, Ducker 

Creek, Spring Creek, Oakmont Creek, and all first-order

streams within the Santa Rosa Creek subbasin are classified as

intermittent (Simley and Carswell, 2009).

Mark West Creek Drainage Basin

The Mark West Creek drainage basin is an 86-mi2 drainage area in the northern part of the SRPW (fig. 3). The basin 

is drained by Mark West Creek, but excludes the Mark West 

Creek drainage area downstream of the Windsor Creek tributary as defined by Cal-Atlas (2007; fig. 4).
Surface-Water Hydrology 39

Mark West Creek

Matanzas Creek

Colgan Creek

Spring Creek

Pool Creek

Windsor Creek

Five Creek

Crane Creek

Pruit Creek

Paulin Creek

Copeland Creek

Irwin Creek

Santa Rosa Creek

Badger Creek

Roseland Creek

Mill Creek

Blucher Creek

Duer Creek

Humbug

Creek

Golf

Creek

Hinebaugh Creek

Fulton Creek

Piner Creek

Porter Creek

Santa Rosa Creek

Copeland Creek

Lake Ilsanjo

Roberts Lake

Brush Creek Reservoir

Santa Rosa Plain

 watershed

 boundary

Fountaingrove 

Lake

Piner Creek

Reservoir Spring Lake

Matanzas

Creek

Reservoir

M e n d o c i n o R a n g e

M a y a c m a s M o u n t a i n s

S o n o m a M o u n t a i n s

0

0 5 10 Kilometers

5 10 Miles

122°50’ 122°45’ 122°40’ 122°35’

38°

35’

38°

30’

38

25’

38°

20’

Surface-water subbasins

Mark West Creek

Santa Rosa Creek

Laguna de Santa Rosa

EXPLANATION

1-percent annual exceedance

 probability floodplain

Major streams

Water body

Laguna de Santa Rosa

IP019208_Figure B05

Floodplain modified from Federal Emergency Management Agency (2008)

Figure 5. Water bodies within the Santa Rosa Plain watershed, Santa Rosa Plain, Sonoma County, California.
40 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Mark West Creek is the main stream in the Mark West 

Creek subbasin. The stream channel is 30 mi long and originates at an altitude of 1,922 ft in the Mayacmas Mountains, 

close to the western-most part of the study area. Mark West 

Creek has a mountainous, relatively undeveloped and forested 

upper drainage. In the lower part of the subbasin, Mark West 

Creek maintains a well-defined channel in mostly agricultural

land.

The main tributaries of Mark West Creek are Windsor 

and Porter Creeks on the north and the Laguna de Santa Rosa 

on south (fig. 4). Smaller tributaries on the south side of the 

main branch include Mill Creek, Weeks Creek, and Van Buren 

Creek. Smaller tributaries on the north side of the main branch 

include Humbug and Leslie Creeks. Other important named 

streams within the Mark West Creek drainage basin include 

Pool and Pruit Creeks, which are tributaries to Windsor Creek.

According to the NHD (Simley and Carswell, 2009), the 

main channel of Mark West Creek is perennial throughout its 

length, with summer flows maintained by numerous springs

in the Mayacmas Mountains and other locations. Most of the 

main channel is in its natural state; anthropogenic (engineered) 

channel modifications have been minor relative to changes

to other channels in the SRPW. Much of the riparian vegetation adjacent to the Mark West Creek channel, as well as the 

channel bed itself, is undeveloped and characteristic of natural 

channel conditions. The Weeks Creek and Humbug Creek 

tributaries are also perennial streams, whereas Windsor Creek, 

Pool Creek, Porter Creek, Mill Creek, and Van Buren Creek 

are generally intermittent streams that become dry during late 

spring to late summer (Simley and Carswell, 2009).

Water Bodies

The SRPW includes a total of 403 permanent and 

semi-permanent water bodies identified on 7.5-minute USGS

topographic maps, composing a total area of 982 acres (Simley and Carswell, 2009; fig. 5). The majority of these water 

bodies have areas less than 10 acres, with a minimum area 

of 0.02 acres and a mean area of 2.4 acres. The water bodies 

include 16 intermittent lakes and ponds (with a total area of 

12 acres), 291 perennial lakes and ponds (with a total area of 

394 acres), 83 reservoirs (with a total area of 315 acres), and 

10 swamp and marsh areas, or wetlands (with a total area of 

260 acres). The largest water bodies are swamp or marsh areas 

(wetlands), with an average area of 26 acres, located mostly 

within the 1-percent AEP floodplain of the Laguna de Santa

Rosa. The largest water body within the SRPW is an unnamed 

swamp/marsh feature of 103 acres located in the main channel 

of the Laguna de Santa Rosa, east of Sebastopol and directly 

upstream of the confluence with Santa Rosa Creek.

The SRPW includes eight named water bodies identified by the NHD (Simley and Carswell, 2009; fig. 5). Four of 

the named water bodies, Brush Creek reservoir, Piner Creek 

reservoir, Matanzas Creek reservoir, and Spring Lake (also 

referred to as Santa Rosa Creek reservoir) are flood-control

facilities (U.S. Army Corps. of Engineers, 2002). Piner Creek 

and Brush Creek reservoirs are mostly empty during summer. Spring Lake and Matanzas Creek reservoirs store water 

throughout the year for recreational purposes and to maintain 

summer flows in Santa Rosa Creek. Lake Ilsanjo (also referred

to as Annadel reservoir), Fountaingrove Lake, Lake Ralphine, 

and Roberts Lake also store water throughout the year primarily for recreational purposes. Spring Lake is the largest named 

water body classified as a lake or reservoir, with an area of

72 acres, followed by Fountaingrove Lake (32 acres), Lake 

Ilsanjo (30 acres), Lake Ralphine (21 acres), Matanzas Creek 

Reservoir (11 acres), and Roberts Lake (5 acres).

Streamflow 

Streamflow records from gages within and adjacent to the

SRPW were analyzed to help characterize the surface-water 

hydrology in the watershed, including correlation of stream 

discharge with drainage area, monthly and seasonal distribution of streamflow, and climatic variability.

Streamflow Gages

Streamflow or stream-stage records are available at 15

USGS streamflow gaging stations within the SRPW (fig. 4, 

table 2). To simplify the identification of gages on maps and in

the text of this report, the following gage codes were defined

for this study (table 2): 

Laguna de Santa Rosa drainage basin

• COPE (11465660, Copeland Creek at Rohnert Park)

• LAGC (11465680, Laguna de Santa Rosa near 

Cotati)

• COLU (11465690, Colgan Creek near Santa Rosa)

• COLL (11465700, Colgan Creek near Sebastopol)

• LAGS (11466200, Laguna de Santa Rosa near 

Sebastopol)

• LAGG (11466500, Laguna de Santa Rosa near 

Graton)

Santa Rosa Creek drainage basin

• SRCU (11465800, Santa Rosa Creek near Santa 

Rosa)

• SRCM (11466050, Santa Rosa Creek at Mission 

Blvd)

• BRSH (11466065, Brush Creek at Santa Rosa)

• SRCA (11466080, Santa Rosa Creek at Alderbrook 

Drive)

• MATC (11466170, Matanzas Creek at Santa Rosa)

• SRCS (11466200, Santa Rosa Creek at Santa Rosa)

• SRCW (11466320, Santa Rosa Creek at Willowside 

Road)

Mark West Creek drainage basin

• MWCW (11465500, Mark West Creek near Windsor)

• MWCM (11466800, Mark West Creek near Mirabel 

Heights)
Surface-Water Hydrology 41

Of the 15 gages, 6 (COLL, SRCU, MWCM, LAGS, 

SRCW, and LAGC) have records that are continuous throughout the entire year, while 5 gages (BRSH, MATC, MWCW, 

COPE, and COLU) are seasonal-only, with a measurement 

period from late fall through early spring (generally from 

October through April) to record winter streamflow. One of the

gages (SRCS) has a combination of continuous and seasonalonly data. Three of the gages (SRCM, SRCA, and LAGG) 

have stage-only records (fig. 4, table 2).

Most streamflow records within the SRPW start in

water-year 1998 or later (table 2). Many of the records are 

short, with an average record length of only 2 to 5 water years 

(table 3). Only four of the stations within the SRPW (COLL, 

LAGC, LAGS, and SRCW) have relatively longer continuous 

streamflow records of 11 to 12 years (fig. 6, table 3). For this 

study, all streamflow records through water year 2010 (ending

September 30, 2010) were analyzed. At the time of this study, 

nine stream gages within the SRPW were still active (table 2).

Correlation Between Drainage Basin Area and 

Average Discharge

Drainage basin area was compared to average streamflow

for each gage with sufficient record in the SRPW and three

gages in the adjacent Sonoma Creek and Napa River drainages (fig. 7). The comparison, using log-log (power function) 

regression, indicated a strongly positive correlation between 

drainage basin area and average streamflow for the 12 gages

within the SRPW (Pearson correlation coefficient, r, of 0.962 

in log-log space) and the 3 gages neighboring the SRPW (r of 

0.998 in log-log space; fig. 8). The similarity of the regressions indicated that the factors controlling the rainfall-runoff 

response are similar for the drainages within and neighboring 

the SRPW. 

Monthly and Seasonal Streamflow

A comparison of monthly mean discharge for water years 

1999 to 2010 for the four gages in the SRPW with relatively 

longer continuous streamflow records of 11 to 12 years

(COLL, LAGS, MWCM, and SRCW) showed characteristic 

seasonal variability with high winter (December–February) 

and low summer (June–August) flows (fig. 9). For all gages, 

streamflow varied by at least two orders of magnitude between

the high winter and the low summer flows. The highest

monthly discharge was measured at the MWCM gage, with 

monthly mean flows of more than 1,200 ft3/s for the months of 

January 2006, March 2006, and February 2008. Winter flows

for three of the gages (MWCM, SRCW, and LAGS) exceeded 

100 ft3

/s for at least 1 month for all water years. For all water 

years, summer flows for all gages were less than 10 ft3/s. 

The gage with the smallest drainage area (COLL) had zero 

measured discharge for 1 to 4 month periods during the summer for all water years. Gage LAGS, on the central part of the 

Laguna de Santa Rosa channel, had zero-flow conditions during 1-month or longer periods in the summer for 2 water years.

Average monthly streamflow records from seven gages

in the SRPW with continuous records (COLL, LAGC, and 

LAGS in the Laguna de Santa Rosa subbasin; SRCU, SRCS, 

and SRCW in the Santa Rosa Creek subbasin; and MWCM in 

the Mark West Creek subbasin) indicated a consistent seasonal 

distribution of streamflow in the SRPW (fig. 10). In general, 

December, January, and February were the months with 

the highest streamflow, followed by March, April, and then

November. Each of the surface-water drainage basins has a 

gage (LAGS, SRCW, and MWCM) that had average monthly 

discharge of at least 120 ft3/s or higher for December, January, 

February, and March. August and September were the driest 

months at all gages, with average monthly discharge less than 

5 ft3/s at all gages. Gage MWCM in the Mark West Creek 

drainage basin, with a drainage area that includes almost the 

entire area of the SRPW, had a maximum average monthly 

discharge of 900 ft3/s for January and a minimum average 

monthly discharge of 2 ft3/s for September (fig. 10C). Gage 

COLL in the Laguna de Santa Rosa drainage basin has one of 

the smallest drainage areas within the SRPW and had a maximum average monthly discharge of only 30 ft3

/s for February and a minimum average monthly discharge was less than 

0.01 ft3/s for September (fig. 10A).

The distribution of average monthly discharge for the 

five seasonal gages was generally consistent with the results

obtained for the seven gages having continuous records within 

the SRPW. In general, the months of December, January, and 

February had the highest mean discharge at the five gages,

with the exception of the COPE gage (fig. 11). The SRCS 

gage had the highest average monthly discharge of 256 ft3/s 

in February. October had the lowest mean discharge (about 

4 ft3/s or less) for all gages, ranging from a minimum of about 

1.2 ft3/s for the BRSH gage to a maximum of about 4 ft3/s for 

the SRCS gage (fig. 11).

Daily Mean Streamflow Records

Six gages within the SRPW (MWCM, SRCW, SRCU, 

LAGS, COLL, and LAGC) had continuous or nearly continuous records of daily mean discharge (table 2). Continuous 

records that span multiple water years are the most valuable 

for analyzing the water budget, for developing statistical 

characterizations of streamflow, and for calibrating hydrologic models. Five of the six gages with continuous records 

(MWCW, SRCW, LAGC, COLL, and LAGS) were still active 

at the time of this study (September 30, 2010), whereas gage 

SRCU has been discontinued.

Of the 15 gages within the SRPW, 5 had seasonal-only 

daily mean discharge records: MWCW, COPE, COLU, BRSH, 

and MATC (fig. 4, table 2). The seasonal records (October 1 

through April 30) provide data for local flood control. A sixth

gage, SRCS, had a record that was partly continuous, but 

mostly seasonal only. Although the seasonal data generally 

are not as useful as the continuous records for analyzing water 

budgets and developing statistical characterizations of streamflow, the data are important for characterizing winter-flow
42 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California Table 2. Description of gaging stations and streamflow records within and adjacent to the Santa Rosa Plain watershed, Sonoma County, California.

[Abbreviations: Blvd, boulevard; Dr., drive; ID, identifier; mi2, square mile; mm/dd/yy, month/day/year; NAD, North American datum; NGVD, National Geodetic Vertical Datum; Rd., road; SRPW, Santa Rosa Plain watershed; St., street; USGS, U.S. Geological Survey; —, no data; °, degree; ’, minute; ”, second]

SRPW

gage

ID

SRPW

gagecode

USGS

gage

ID

USGS

gage

name

North

latitude

NAD 1927

West

longitude

NAD 1927

Gage 

datum

NGVD

1929

(feet)

Drainage

area

(mi2

)

Subbasin

or drainage basin name

Period of record

for this study

(mm/dd/yy)

Gage

status

at end

of study

Measured

parameter

Record

type

Record

affected

by

backwater? Begin End

Laguna de Santa Rosa stream gages

2 COPE 11465660 Copeland Creek at Rohnert Park 38°20’36” 122°42’03” 100.0 5.5 Laguna de Santa 

Rosa

10/01/06 04/30/10 Active Discharge Seasonal No.

3 LAGC 11465680 Laguna de Santa Rosa near 

Cotati

38°21’08” 122°44’35”

— 40.8 Laguna de Santa 

Rosa

11/06/98 09/30/10 Active Discharge Continuous No.

4 COLU 11465690 Colgan Creek near Santa Rosa 38°24’08” 122°43’55”

— 3.4 Laguna de Santa 

Rosa

10/01/06 04/30/10 Active Discharge Seasonal No.

5 COLL 11465700 Colgan Creek near Sebastopol 38°22’25” 122°46’02”

— 6.8 Laguna de Santa 

Rosa

11/07/98 09/30/10 Active Discharge Continuous Yes.

6 LAGS 11465750 Laguna de Santa Rosa near 

Sebastopol

38°25’32” 122°49’41”

— 79.6 Laguna de Santa 

Rosa

11/18/98 09/30/10 Active Discharge Continuous Yes.

15 LAGG 11466500 Laguna de Santa Rosa near 

Graton

38°27’10’’ 122°50’03’’

— 166.0 Mark West Creek 11/09/64 09/30/10 Inactive Stage Periodic Yes.

Santa Rosa Creek stream gages

7 SRCU 11465800 Santa Rosa Creek near Santa 

Rosa

38°27’25” 122°37’50”

— 12.5 Santa Rosa Creek 08/01/59 10/13/70 Inactive Discharge Continuous No.

14 SRCM 11466050 Santa Rosa Creek at Mission 

Blvd

38°27’28” 122°40’16” 220.0

— Santa Rosa Creek 11/01/97 04/30/10 Active Stage Seasonal No.

8 BRSH 11466065 Brush Creek at Santa Rosa 38°27’18” 122°40’45”

— 10.1 Santa Rosa Creek 10/01/05 04/30/10 Inactive Discharge Seasonal No.

13 SRCA 11466080 Santa Rosa Creek at 

Alderbrook Dr.

38°26’58” 122°41’50” 170.0

— Santa Rosa Creek 10/31/97 04/30/10 Inactive Stage Seasonal No.

9 MATC 11466170 Matanzas Creek at Santa Rosa 38°26’20” 122°42’05”

— 21.0 Santa Rosa Creek 10/01/04 04/30/10 Active Discharge Seasonal No.

10 SRCS 11466200 Santa Rosa Creek at Santa Rosa 38°26’12” 122°43’25” 100.0 57.0 Santa Rosa Creek 10/01/39 04/30/10 Active Discharge Mixed No.

11 SRCW 11466320 Santa Rosa Creek at 

Willowside Rd.

38°26’43’’ 122°48’22’’

— 77.6 Santa Rosa Creek 12/09/98 09/30/10 Active Discharge Continuous Yes.

Mark West Creek stream gages

1 MWCW 11465500 Mark West Creek near Windsor 38°30’34” 122°46’07” 140.0 43.0 Mark West Creek 10/01/06 04/30/08 Inactive Discharge Seasonal No.

12 MWCM 11466800 Mark West Creek near Mirabel 

Heights

38°29’39” 122°51’08”

— 251.0 Mark West Creek 10/01/05 10/05/10 Inactive Discharge Continuous Yes.

Stream gages adjacent to Santa Rosa Plain watershed

16 NAPH 11456000 Napa River near St. Helena 38°30’41” 122°27’17” 193.2 78.8 Napa River 10/01/29 09/30/10 Active Discharge Continuous No.

17 NAPN 11458000 Napa River near Napa 38°22’06’’ 122°18’08’’ 24.7 218 Napa River 10/01/29 09/30/10 Active Discharge Continuous No.

18 SCAC 11458500 Sonoma Creek at Agua Caliente 38°19’24” 122°29’36” 94.3 58.4 Sonoma Creek 02/01/55 09/30/10 Active Discharge Continuous No.
Surface-Water Hydrology 43

Table 3. Summary statistics for streamflow records within and neighboring Santa Rosa Plain watershed (SRPW), Sonoma County, 

California.

[Abbreviations: Avg., average; cfs, cubic feet per second; ID, identifier; mm/dd/yy, month/day/year; No., number; USGS, U.S. Geological Survey; —, no data]

SRPW

gage

ID

SRPW

gage

code

USGS

gage

ID

USGS gage name

No. of 

days 

with 

data

No. of 

water 

years in 

record

Peak 

discharge 

(cfs)

Date of 

peak 

discharge

(mm/dd/yy)

Max daily 

mean 

discharge 

(cfs)

Date of 

max daily 

mean 

discharge

(mm/dd/yy)

Min daily 

mean 

discharge 

(cfs)

Date of 

min daily 

mean 

discharge

(mm/dd/yy)

1 MWCW 11465500 Mark West Creek near Windsor 425 2 3,970 01/04/08 1,750 01/04/08 0.86 10/01/07

2 COPE 11465660 Copeland Creek at Rohnert Park 849 3 512 12/26/06 255 01/20/10 0 Multiple

3 LAGC 11465680 Laguna de Santa Rosa near 

Cotati

4,347 11 3,980 12/31/05 2,450 12/31/05 0 Multiple

4 COLU 11465690 Colgan Creek near Santa Rosa 846 3 426 01/25/08 135 01/04/08 0 Multiple

5 COLL 11465700 Colgan Creek near Sebastopol 4,346 11 934 12/31/05 647 01/02/02 0 Multiple

6 LAGS 11465750 Laguna de Santa Rosa near 

Sebastopol

4,333 11 9,690 12/31/05 5,930 01/26/08 –63* 02/22/09

7 SRCU 11465800 Santa Rosa Creek near Santa 

Rosa

4,092 11 3,200 02/08/60 1,450 02/08/60 0 Multiple

8 BRSH 11466065 Brush Creek at Santa Rosa 1,092 4 2,390 12/31/05 713 12/31/05 0 10/01/08

9 MATC 11466170 Matanzas Creek at Santa Rosa 1,335 5 3,700 12/31/05 2,040 12/31/05 0.01 10/01/09

10 SRCS 11466200 Santa Rosa Creek at Santa Rosa 2,609 8 9,080 02/27/40 4,830 02/27/40 0 Multiple

11 SRCW 11466320 Santa Rosa Creek at Willowside 

Rd.

4,314 11 6,410 12/31/05 5,200 12/31/05 0.58 10/02/09

12 MWCM 11466800 Mark West Creek near Mirabel 

Heights

1,708 5 11,300 12/31/05 7,180 01/03/06 0 Multiple

16 NAPH 11456000 Napa River near St. Helena 25,232 68 18,300 12/31/05 13,700 02/17/86 0 Multiple

17 NAPN 11458000 Napa River near Napa 19,724 51 37,100 02/18/86 26,200 02/17/86 0 Multiple

18 SCAC 11458500 Sonoma Creek at Agua Caliente 13,018 35 20,300 12/31/05 8,180 12/31/05 0 Multiple

Table 3. Summary statistics for streamflow records within and neighboring Santa Rosa Plain watershed (SRPW), Sonoma County, 

California.—Continued

[Abbreviations: Avg., average; cfs, cubic feet per second; ID, identifier; mm/dd/yy, month/day/year; No., number; USGS, U.S. Geological Survey; –, no data]

SRPW

gage

ID

SRPW

gage

code

USGS

gage

ID

USGS gage name

No. of 

days with 

zero flow

No. of 

days with 

reverse 

flow

Avg. 

discharge 

(cfs)

Avg. fall 

(Oct–Dec) 

discharge 

(cfs)

Avg. winter 

(Jan–Mar) 

discharge 

(cfs)

Avg. spring 

(Apr–Jun) 

discharge 

(cfs)

Avg. summer 

(Jul–Sep) 

discharge 

(cfs)

1 MWCW 11465500 Mark West Creek near Windsor 0 0 46.8 13.2 95.1 7.6 —

2 COPE 11465660 Copeland Creek at Rohnert Park 113 0 7.6 2.2 14.3 4.6 —

3 LAGC 11465680 Laguna de Santa Rosa near 

Cotati

362 0 32.7 36.3 80.8 13.4 0.48

4 COLU 11465690 Colgan Creek near Santa Rosa 206 0 4.7 2.7 7.6 2.1 —

5 COLL 11465700 Colgan Creek near Sebastopol 1,225 0 8.5 10.3 20.7 3.1 0.04

6 LAGS 11465750 Laguna de Santa Rosa near 

Sebastopol

357 2 80.3 73.7 212.8 32.1 0.61

7 SRCU 11465800 Santa Rosa Creek near Santa 

Rosa

204 0 18.8 15.0 52.2 9.5 0.35

8 BRSH 11466065 Brush Creek at Santa Rosa 21 0 20.6 14.3 32.5 11.4 —

9 MATC 11466170 Matanzas Creek at Santa Rosa 0 0 31.8 21.8 48.3 20.3 —

10 SRCS 11466200 Santa Rosa Creek at Santa Rosa 68 0 104.5 96.0 195.9 44.0 1.20

11 SRCW 11466320 Santa Rosa Creek at Willowside 

Rd.

0 0 93.2 102.6 219.9 43.0 4.23

12 MWCM 11466800 Mark West Creek near Mirabel 

Heights

15 1 288.0 238.3 739.8 175.5 3.60

16 NAPH 11456000 Napa River near St. Helena 838 0 94.9 74.7 262.2 41.5 1.49

17 NAPN 11458000 Napa River near Napa 1,671 0 202.2 130.3 618.6 91.2 3.30

18 SCAC 11458500 Sonoma Creek at Agua Caliente 525 0 71.3 58.2 193.9 30.4 1.15

*

Backwater condition.
44 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

10

1

100

1,000

1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Average discharge by water year,

in cubic feet per second 

Water year 

See figure 7 for location of gages

See figure 7 for location of gages

A

NAPH 

NAPN 

SCAC LAGC 

COLL 

SRCU 

SRCS 

MWCM 

EXPLANATION Note: Thin lines with circles indicate gages outside of Santa Rosa 

Plain watershed (SRPW) with continuous records; Thick lines 

with squares indicate gages inside SRPW with continuous 

records; Dotted lines with triangles indicate gages inside SRPW 

with seasonal (Oct-Apr) records only

1

10

100

1,000

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Average discharge by water year,

in cubic feet per second 

Water year 

B

IP019208_FIGURE B06ab

Note: Thick lines with squares indicate gages 

inside SRPW with continuous records; Dashed 

lines with triangles indicate gages inside SRPW 

with seasonal (Oct-Apr) records only

EXPLANATION

LAGC 

COLL 

LAGS 

MWCM SRCW 

BRSH MATC 

COPE COLU 

MWCW SRCS 

Figure 6. Average water-year discharge for gages within and neighboring the Santa Rosa Plain watershed, Sonoma County, California: A, 

Long-term (water-year 1930–2010) records; B, Recent (water-year 2000–10) records. (COPE, Copeland Creek at Rohnert Park; LAGC, Laguna 

de Santa Rosa near Cotati; COLU, Colgan Creek near Santa Rosa; COLL, Colgan Creek near Sebastopol; LAGS, Laguna de Santa Rosa near 

Sebastopol; SRCU, Santa Rosa Creek near Santa Rosa; BRSH, Brush Creek at Santa Rosa; MATC, Matanzas Creek at Santa Rosa; SRCS, Santa 

Rosa Creek at Santa Rosa; SRCW, Santa Rosa Creek at Willowside Road; MWCW, Mark West Creek near Windsor; MWCM, Mark West Creek 

near Mirabel Heights; NAPH, Napa River near St. Helena; Napa River near Napa; SCAC, Sonoma Creek at Agua Caliente)
Surface-Water Hydrology 45

Lynch Creek

Petaluma River

Moore Creek

Mark West Creek

Conn Creek

Pope Creek

Chileno Creek

Dry Creek

Sonoma Creek Napa River

Putah Creek

Russian River

Keys Creek

Americano Creek

Stemple Creek

Russian

River

Napa River

Sonoma

Creek

SRCW

SRCS MATC

BRSH

SRCU

LAGS

MWCM

COLL

COLU

LAGC

COPE

MWCW

LAGG SRCA

SRCM

SCAC

NAPH

NAPN

0

0 5 10 Kilometers

5 10 Miles

122°50’ 122°40’ 122°30’ 122°20’

38°

40’

38°

30’

38°

20’

EXPLANATION

IP019208_Figure B07

Surface-water subbasins

Laguna de Santa Rosa 

Mark West Creek

Santa Rosa Creek

USGS stream gage

 and identifier

USGS stream gage

 (stage only) and identifier

Major streams SRCA

Santa Rosa Plain SCAC

 watershed boundary

Figure 7. U.S. Geological Survey stream gages within and neighboring the Santa Rosa Plain watershed, Sonoma County, California. (COPE, 

Copeland Creek at Rohnert Park; LAGC, Laguna de Santa Rosa near Cotati; COLU, Colgan Creek near Santa Rosa; COLL, Colgan Creek near 

Sebastopol; LAGS, Laguna de Santa Rosa near Sebastopol; LAGG, Laguna de Santa Rosa near Graton; SRCU, Santa Rosa Creek near Santa 

Rosa; SRCM, Santa Rosa Creek at Mission Blvd; BRSH, Brush Creek at Santa Rosa; SRCA, Santa Rosa Creek at Alderbrook Drive; MATC, 

Matanzas Creek at Santa Rosa; SRCS, Santa Rosa Creek at Santa Rosa; SRCW, Santa Rosa Creek at Willowside Road; MWCW, Mark West 

Creek near Windsor; MWCM, Mark West Creek near Mirabel Heights; NAPH, Napa River near St. Helena; Napa River near Napa; SCAC, 

Sonoma Creek at Agua Caliente)
46 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 8. Correlation between drainage basin area and average discharge for streamflow records within and around the Santa Rosa Plain 

watershed, Sonoma County, California. (COPE, Copeland Creek at Rohnert Park; LAGC, Laguna de Santa Rosa near Cotati; COLU, Colgan Creek 

near Santa Rosa; COLL, Colgan Creek near Sebastopol; LAGS, Laguna de Santa Rosa near Sebastopol; SRCU, Santa Rosa Creek near Santa 

Rosa; BRSH, Brush Creek at Santa Rosa; MATC, Matanzas Creek at Santa Rosa; SRCS, Santa Rosa Creek at Santa Rosa; SRCW, Santa Rosa 

Creek at Willowside Road; MWCW, Mark West Creek near Windsor; MWCM, Mark West Creek near Mirabel Heights; NAPH, Napa River near 

St. Helena; Napa River near Napa; SCAC, Sonoma Creek at Agua Caliente)

1

10

100

1,000

1 10 100 1,000

Average discharge (Qave), in cubic feet per second

Drainage basin area (DA), in square miles

Gages outside the Santa Rosa

 Plain watershed (SRPW)

Gages within the SRPW

Outside regression curve

SRPW regression curve

See figure 7 for location of gages

IP019208_Figure B08

SRPW regression results

Qave= 1.68 × DA0.922

r=−0.962

Outside regression results

Qave= 0.408 × DA0.820

r=−0.998

MWCM 

NAPN 

SRCS 

SCAC 

SRCW NAPH 

LAGS 

LAGC 

MWCW 

MATC 

BRSH SRCU 

COLL 

COPE 

COLU 
Surface-Water Hydrology 47

Figure 9. Monthly mean discharge for four selected stream gages in the Santa Rosa Plain watershed, Sonoma County, California, 1999–2010. 

Discharge rates of less than 0.01 cubic feet per second represent zero-flow conditions. (COLL, Colgan Creek near Sebastopol; LAGS, Laguna de 

Santa Rosa near Sebastopol; SRCW, Santa Rosa Creek at Willowside Road; MWCM, Mark West Creek near Mirabel Heights)

0.01 

0.1 

1 

10 

100 

1,000 

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 

Water year 

Monthly mean discharge, in cubic feet per second

MWCM SRCW 

LAGS COLL 

IP019208_Figure B09

Santa Rosa Plain watershed gages;

 see figure 7 for location

 

EXPLANATION
48 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 10. Average monthly discharge for gages having continuous stream discharge records in the Santa Rosa Plain watershed, 

Sonoma County, California: A, Laguna de Santa Rosa subbasin; B, Santa Rosa Creek subbasin; and C, Mark West Creek subbasin. 

(LAGC, Laguna de Santa Rosa near Cotati; COLL, Colgan Creek near Sebastopol; LAGS, Laguna de Santa Rosa near Sebastopol; SRCU, 

Santa Rosa Creek near Santa Rosa; SRCS, Santa Rosa Creek at Santa Rosa; SRCW, Santa Rosa Creek at Willowside Road; MWCM, 

Mark West Creek near Mirabel Heights)

IP019208_Figure B10abc Average monthly discharge, in cubic feet per second 

Water year by calendar month 

NovOct Dec Jan Feb Mar Apr May June July Aug Sept

0 

50 

100 

150 

200 

250 

300 

0 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

1,000 

50 

100 

150 

200 

250 

300 

LAGC 

COLL 

LAGS 

SRCU 

SRCS

SRCW

MWCM

Laguna de Santa Rosa subbasin

Santa Rosa Creek subbasin

Mark West Creek subbasin

Note scale change

A

B

C
Surface-Water Hydrology 49

Figure 11. Average monthly discharge for gages having seasonal-only (October–April) records in the Santa Rosa Plain watershed, 

Sonoma County, California. (COPE, Copeland Creek at Rohnert Park; COLU, Colgan Creek near Santa Rosa; BRSH, Brush Creek at Santa 

Rosa; MATC, Matanzas Creek at Santa Rosa; SRCS, Santa Rosa Creek at Santa Rosa; MWCW, Mark West Creek near Windsor)

Month 

1 

10 

100 

300 

October November December January February March April

IP019208_Figure B11

Average monthly discharge, in cubic feet per second

BRSH 

MATC 

MWCW 

COPE 

COLU 

SRCS 

conditions, calibrating hydrologic models to the winter stormflows, and evaluating the hydrologic response of drainage

basins having different basin characteristics within the SRPW.

Laguna de Santa Rosa Drainage Basin

COPE Gage

COPE is a seasonal gage in the urban area of Rohnert 

Park (fig. 4), but the 5.5 mi2 drainage upstream of the gage 

consists mostly of undeveloped and agricultural land in the 

eastern headwaters of the Laguna de Santa Rosa drainage 

basin that drain the western slope of the Sonoma Mountains 

(fig. 4). According to the NHD, the upper section of Copeland 

Creek is designated as perennial, whereas the lower section is 

designated as intermittent (Simley and Carswell, 2009). The 

gaging station includes a water-stage recorder and a creststage gage. The period of record for this gage was October 1, 

2006, to April 30, 2010 (fig. 6, table 2). The average October 

through April discharge measured at this gage was 7.6 ft3/s 

(table 3). The estimated maximum daily mean discharge of 

255 ft3/s was measured on January 20, 2010, and the maximum peak discharge of 512 ft3

/s was measured on December 

26, 2006. The stream was often dry during October at the location of the gage.

LAGC Gage

LAGC is a continuous gage on the main channel of the 

Laguna de Santa Rosa upstream of the Colgan Creek tributary in the Laguna de Santa Rosa drainage basin (fig. 4). The 

gage was active at the time of this study, with a continuous 

discharge record from November 6, 1998, to present (fig. 6, 

table 2). The gage is a water-stage recorder, and the upstream 

drainage area of 40.8 mi2 includes a combination of urban, 

agricultural, and undeveloped land uses. There is no regulation or diversion upstream of the gage, and discharge is not 

effected by backwater conditions in the Laguna de Santa Rosa. 

The mean discharge measured at this gage was 33 ft3/s, with a 

peak discharge of 3,980 ft3/s on December 31, 2005 (fig. 12, 

table 3). The maximum daily mean discharge of 2,450 ft3/s 

was also measured on December 31, 2005. The mean summer 

discharge was 0.48 ft3/s; however, a minimum daily mean discharge of zero was often measured during the summer months 

(fig. 12; table 3).

COLU Gage

COLU is a seasonal gage in the upper section of Colgan 

Creek in the north central part of the Laguna de Santa Rosa 

drainage basin (fig. 4) and, compared to the other gages, has 

the smallest drainage area with only 3.4 mi2. The gaging station includes a water-stage recorder and a crest-stage gage. 

Measured streamflow for this small drainage is important for

evaluating the hydrologic response of smaller catchments 

within the developed and urbanized areas of the SRPW. The 

period of record for the site was from October 2006 through 

September 30, 2010 (fig. 6), and the site was still active at the 

time of this study. The mean October through April discharge 

was 4.7 ft3

/s (table 3). The maximum daily mean discharge of 

135 ft3/s was measured on January 4, 2008, while the maximum peak flow of 564 ft3

/s was measured on January 20, 

2010. The mean discharge during October was 1.6 ft3/s, and 

the record includes many days with zero discharge.
50 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

COLL Gage

COLL is a continuous gage close to the mouth of Colgan 

Creek in the Laguna de Santa Rosa drainage basin and has 

a drainage area of 6.8 mi2 (fig. 4). Colgan Creek is a small 

tributary on the eastern side of the Laguna de Santa Rosa with 

a drainage consisting of developed agricultural and urbanized 

land in the south-central part of the Laguna de Santa Rosa 

drainage basin (fig. 4). The gaging station uses a water-stage 

recorder and a Doppler-velocity system. There is no regulation or diversion upstream of the gage; however, high flows

are effected by backwater conditions in the Laguna de Santa 

Rosa. The period of record for the gage was November 1998 

to September 30, 2010 (fig. 6, table 2). The mean discharge 

measured at this gage was 8.5 ft3

/s, with a peak discharge of 

934 ft3/s on December 31, 2005. The maximum daily mean 

discharge of 647 ft3/s was measured on January 2, 2002 

(fig. 13, table 3). The mean summer discharge was 0.04 ft3/s; 

however, a minimum daily mean discharge of zero was often 

measured during the summer months (fig. 13, table 3).

LAGS Gage

LAGS is a continuous gage in the lower end of the 

Laguna de Santa Rosa drainage basin, approximately 

1.5 mi upstream of the confluence of Santa Rosa Creek and

the Laguna de Santa Rosa (fig. 4). With an upstream catchment area of 79.6 mi2

, the streamflow measured at this gage is

representative of the surface-water outflow from the Laguna

de Santa Rosa drainage basin. The LAGS gage includes a 

water-stage recorder and a Doppler-velocity system that measures flow direction. There is no diversion or flow regulation

upstream of the gage; however, high flows can be strongly

affected by backwater conditions. The mean discharge 

measured at this gage was 80 ft3/s, with a peak discharge of 

9,690 ft3/s on December 31, 2005 (fig. 14, table 3). A maximum daily mean discharge of 5,930 ft3

/s was recorded on 

January 26, 2008. The mean summer discharge at this gage 

was 0.61 ft3/s; however, a minimum daily mean discharge of 

zero was often measured during the summer months (fig. 14, 

table 3). A minimum daily mean discharge of –63 ft3/s was 

measured on February 22, 2009, during a period of significant

flow reversal caused by backwater conditions in the Laguna de

Santa Rosa.

Santa Rosa Creek Drainage Basin

SRCU Gage

SRCU is a discontinued continuous stream gage in 

the upper reaches of Santa Rosa Creek, upstream of the 

city of Santa Rosa, with a drainage area of 12.5 mi2 (fig. 4). 

The period of record for the gage is August 1, 1959, through 

October 13, 1970. The daily mean discharge record consists of 

11 complete water years and is representative of the rainfallrunoff characteristics of the more rugged, undeveloped upper 

drainages in the Mayacmas Mountains along the eastern part 

of the Santa Rosa Creek drainage basin. The mean discharge 

measured at this gage was 19 ft3

/s, with a peak discharge of 

3,200 ft3/s measured on February 8, 1960. The maximum daily 

mean discharge of 1,450 ft3/s was measured on February 8, 

1960 (fig. 15, table 3). The mean summer discharge at this 

gage was 0.35 ft3/s; however, a minimum daily mean discharge of zero was often measured during the summer months 

(fig. 15, table 3).

Figure 12. Daily mean discharge measured at gage 11465680, Laguna de Santa Rosa near Cotati (LAGC), Sonoma County, California, 

1999–2010. Discharge rates of less than 0.01 cubic feet per second represent zero-flow conditions.

IP019208_Figure B12

0.01

0.1

1

10

100

1,000

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Daily mean discharge, in cubic feet per second

Water year

LAGC
Surface-Water Hydrology 51

BRSH Gage

BRSH is a seasonal gage close to the mouth of Brush 

Creek in the central part of the Santa Rosa Creek drainage 

basin (fig. 4) that has a drainage area of 10.1 mi2. The gage 

records include a combination of stage-only data from November 2002 to May 2005 and both discharge and stage data from 

October 2005 to April 2010 (fig. 6). Measured streamflow

at this gage is useful for characterizing the semi-rural area 

of Rincon Valley and first-order drainages in the Mayacmas

Mountains. The mean discharge for the gage was 21 ft3/s 

(table 3). A maximum daily mean discharge of 713 ft3/s and 

the maximum peak discharge of 2,390 ft3/s were both measured on December 31, 2005 (table 3).

MATC Gage

MATC is a seasonal gage on Matanzas Creek, downstream of the Spring Creek tributary in the south central part 

of the Santa Rosa Creek drainage basin (fig. 4), that has a 

drainage area of 21.0 mi2. The drainage area consists mostly 

of the less developed, more rugged and rural terrain of Bennett Valley and the surrounding Sonoma Mountains. Records 

for this gage include stage-only data from November 2002 

to April 2004 and both stream discharge and stage data from 

October 2004 to September 30, 2010 (the gage was active at 

the time of this study; fig. 6B). Measured streamflow at MATC

is useful for characterizing recharge and runoff in the upstream 

drainages and headwater areas of the Sonoma Mountains. The 

mean discharge for the gage was 32 ft3

/s (table 3). The maximum daily mean discharge of 2,040 ft3

/s and the maximum 

peak flow of 3,700 ft3/s were both recorded on December 31, 

2005. Minimum flows of 0.01 ft3/s were measured during 

October 2009. No periods of zero discharge were recorded at 

this gage.

SRCS Gage

SRCS is a seasonal gage on the main channel of Santa 

Rosa Creek in the south central part of the Santa Rosa Creek 

drainage basin (fig. 4) that has an upstream drainage area of 

57.0 mi2. Beginning in December 1939, the site has the earliest 

streamflow record for the SRPW; however, the earlier record

includes data only through September 1941 (fig. 6). The gage 

was inactive from October 1941 to September 2001. The gage 

was reactivated in October 2001 and operated as a seasonal 

gage, with some water years including records for September 

and May. Only stage data is available for water-year 2005. 

The gage was still active at the time of this study.

Discharge at gage SRCS can be affected by diversions 

for flood control (diversion of Santa Rosa Creek is diverted

into Santa Rosa Creek reservoir) and irrigation (these diversions are not recorded). The maximum daily mean discharge 

was 4,830 ft3/s (measured on February 27, 1940), and the 

minimum daily mean discharge was 0 ft3/s (measured multiple 

times; table 3). A maximum peak instantaneous discharge of 

9,080 ft3/s was recorded on February 27, 1940, and a minimum discharge of zero was measured from October 1, 1939, 

to December 31, 1939 (table 3). The measured average daily 

mean discharge at this site for water-years 2002 through 2009 

(October through April only) was 105 ft3

/s (table 3).

Figure 13. Daily mean discharge measured at gage 11465700, Colgan Creek near Sebastopol (COLL), Sonoma County, California 

1999–2010. Discharge rates of less than 0.01 cubic feet per second represent zero-flow conditions.

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

IP019208_Figure B13

0.01

0.1

1

10

100

1,000

10,000

Daily mean discharge, in cubic feet per second

Water year

COLL
52 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

SRCW Gage

SRCW is a continuous gage on the main channel of Santa 

Rosa Creek in the downstream end of the Santa Rosa Creek 

drainage basin, about 1.6 mi upstream of the confluence with

the Laguna de Santa Rosa, that has an upstream drainage 

of 77.6 mi2 (fig. 4). At the time of this study, the gage was 

still active, and approximately 12 years of nearly continuous stream-discharge data were available (from December 9, 

1998, to September 30, 2010; fig. 6, table 2). The daily mean 

discharge records are generally good for this gage; however, some diversion of streamflow for irrigation is possible.

Discharge rates of less than 0.01 ft3/s represent zero-flow conditions. During high flows, backwater effects from flooding in

the Laguna de Santa Rosa can affect discharge at the SRCW 

gage, and there can be diversions for flood control in the upper

parts of the drainage during the largest storms. Subjective 

analysis of the hydrograph indicated that these diversions were 

insignificant when compared to the natural flows, as indicated

by the general shape of the hydrograph (for example, the rising, peaks, and receding limbs).

The mean discharge measured at this gage was 93 ft3/s, 

with a peak discharge of 6,410 ft3/s measured on December 

31, 2005. The maximum daily mean discharge of 5,200 ft3/s 

was measured on December 31, 2005 (fig. 16). The mean 

summer (July through September) discharge at this gage was 

4.2 ft3/s; in contrast, the mean winter (January through March) 

discharge was 220 ft3/s (table 3).

Mark West Creek Drainage Basin

MWCW Gage

MWCW is a seasonal gage on the main branch of Mark 

West Creek in south central part of the Mark West Creek 

drainage basin (fig. 4), approximately 4 mi upstream of gage 

MWCM, that has a drainage area of 43 mi2. The MWCW gage 

has a relatively short record limited to water-years 2007 and 

2008 (from October 1, 2006, to January 7, 2009). The drainage 

basin upstream of the MWCW gage includes 49.9 percent of 

the Mark West Creek basin (as defined by CIWM) and represents mostly natural, undeveloped conditions. The channel is 

mostly perennial and includes the largest number of springs 

of all drainage basins within the SRPW (Simley and Carswell, 

2009).

Although the record for this gage is sparse (seasonal data 

spanning only 2 water years), the measured streamflow at this

site is useful for evaluating the rainfall-runoff response and the 

potential recharge magnitude for the hilly, rugged terrain of 

the Mayacmas Mountains in the northeastern part of the study 

area. The average October through April discharge for this site 

was 47 ft3/s. A maximum peak discharge of 3,970 ft3/s was 

recorded on January 4, 2008. Although summer flows were

not measured at this gage, the streamflow hydrographs showed

early October flows of 1 to 2 ft3/s, indicating that flows during

much of summer were probably less than 2 ft3/s.

Figure 14. Daily mean discharge measured at gage 11466200, Laguna de Santa Rosa near Sebastopol (LAGS), Sonoma County, 

California, 1999–2011. Discharge rates of less than 0.01 cubic feet per second represent zero-flow conditions with the exception of 

February 22, 2009, where the measured discharge equaled –63 cubic feet per second.

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 

IP019208_Figure B14

0.01

0.1

1

10

100

1,000

10,000

Daily mean discharge, in cubic feet per second

Water year

LAGS
Surface-Water Hydrology 53

MWCM Gage

MWCM is a continuous gage on the lowermost section 

of the main branch of Mark West Creek, close to the confluence with the Russian River (fig. 4). Of all available streamflow records for the SRPW, the record at the MWCM gage

is the most important in terms of developing the study-area 

water budget and for calibrating hydrologic models because 

the 251 mi2 upstream drainage area includes 96 percent of the 

SRPW study area. Streamflow measured at the MWCM gage

represents the integrated hydrologic response of the study area 

as a whole to climate, basin characteristics, and anthropogenic 

factors. The MWCM gage has a nearly continuous record for 

water-years 2006 through 2010 (fig. 17). With the exception 

of the flood-control facilities on Santa Rosa Creek, no flow

regulation significantly affects streamflow at MWCM. Some

flow diversions for irrigation of up to 6,000 acres are possible (these diversions are primarily on the Santa Rosa Creek 

tributary), but such diversions are not directly detectable in 

the streamflow records. During very high flow conditions, discharge records for MWCM can be poor as a result of backwater effects from the Russian River.

The mean discharge for the period of continuous data 

at the MWCM gage (water-years 2006 through 2008) was 

288.3 ft3

/s, with a peak daily mean discharge of 7,180 ft3/s 

on January 3, 2006, and a minimum discharge of zero for 

the period September 4, 2008, through September 18, 2008 

(fig. 17). As mentioned previously, backwater effects from the 

Russian River can impede and occasionally reverse streamflow at the MWCM gage. For example, December 31, 2005,

was the date of both the maximum recorded peak discharge 

of 7,180 ft3

/s and also the extreme minimum discharge of 

−830 ft3/s, indicating reverse (upstream) flow on Mark West

Creek in response to Russian River overflow. A maximum

gage height of 69.8 ft was recorded on January 1, 2006, corresponding to peak flood conditions on the Russian River. The

maximum daily mean discharge of 7,180 ft3/s was measured 

for Mark West Creek when flooding in the Russian River

subsided, 2 days after the maximum gage height was recorded. 

Therefore, the maximum daily mean discharge reflected a

rapid draining of the large volume of water held in the Laguna 

de Santa Rosa floodplain, rather than discharge caused by the

generation of runoff in response to precipitation within the 

Mark West Creek drainage basin.

Extension of the MWCM Streamflow Record

The streamflow records from the SRPW gages are too

short to show longer-term streamflow characteristics and

trends (fig. 6, table 2). A comparison of the time series for 

annual (water year) average discharge for the three gages 

neighboring the SRPW with long discharge records (NAPH, 

NAPN, and SCAC) to gages within the SRPW with the 

longest discharge records of l1 to 12 years (COLL, LAGC, 

SRCS, SRCU, and MWCM) indicated the limitations of the 

shorter-term records in adequately representing the longerterm streamflow characteristics of the SRPW (fig. 6A). For 

example, the extremely dry period during water-year 1977, 

and the wettest periods of 1983–1984 and 1995–1998, are 

missing from the streamflow records within the SRPW.

0.01

0.1

1

10

100

1,000

10,000

1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970

Daily mean discharge, in cubic feet per second

Water year

IP019208_Figure B15

SCRU

Figure 15. Daily mean discharge measured at gage 11466200, Santa Rosa Creek near Santa Rosa (SRCU), Sonoma County, California, 

1959–1970.
54 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

To estimate longer-term (greater than 30 year) streamflow variability, the daily mean discharge record for MWCM

was extended by using streamflow records for gage NAPN

(11458000, Napa River near Napa) on the Napa River (fig. 7). 

The NAPN gage recorded data from water-years 1930 through 

2010, and the MWCM gage recorded data from water years 

2006 through 2010 (table 2). Comparison of the streamflow

records at NAPN and MWCM indicated a positive correlation 

for water-years 2006 through 2010 (fig. 18). The daily mean 

discharge record for MWCM was extended to water years 

1960 through 2005 by using the maintenance of varianceextension, type 1 (MOVE.1) method (Hirsch, 1982) and the 

NAPN gage record (fig. 19). The MOVE.1 technique produces 

discharge estimates at the short-term gage with a statistical distribution similar to that expected if the discharge had 

actually been measured (Helsel and Hirsch, 1992, p. 277). It 

estimates the probability of extreme high or low discharge. 

To develop the MOVE.1 estimates, the linear correlations 

between daily mean discharge at the NAPN and MWCM 

gages were analyzed on a seasonal basis (fig. 18). Linear 

regression between NAPN and MWCM indicated significant

coefficients of determination (r2) ranging from a maximum of 

0.91 for spring to a minimum of 0.59 for fall (fig. 18).

For the purposes of this report, the flow at the MWCM

gage at which there is flooding is assumed to range from 6,000

to 8,000 ft3/s, which can result in local flooding of roadways

(Michael Webster, U.S. Geological Survey, written commun., 

2013). Note that the flowrate at which there is flooding is not

only dependent on the flow at the MWCM gage, but also on

the flow of the Russian River, as a consequence of backwater effects (see the “Laguna de Santa Rosa Drainage Basin”

section of this chapter). The combined daily mean discharge 

record (extended and measured) at the MWCM gage included 

62 days between October 1, 1959, and September 30, 2010, 

that had a daily mean discharge of 6,000 ft3/s or greater 

(fig. 19A). The combined record included 25 days that had 

a daily mean discharge of 8,000 ft3/s or greater (fig. 19A). 

In contrast to the high flows, the estimated maximum daily

mean discharge during the exceedingly dry water-year of 1977 

was only 71.4 ft3/s, which was measured on January 3, 1977. 

Seven water years, including water-year 1977, had relatively 

low maximum daily mean discharges of less than 2,000 ft3/s.

The extended daily mean discharge record for MWCM 

was combined with the measured record to develop a time 

series of annual discharge for water-years 1960 through 2010 

(fig. 19B). The long-term estimated mean discharge for the 

51-year time series was 265 ft3/s, compared to the measured 

mean discharge of 250 ft3/s for water-years 2006 through 

2010 and an estimated mean discharge of 266 ft3/s for the 

46-year extended record. In the combined record, a maximum 

estimated annual mean discharge was 663 ft3/s for wateryear 1983. In the measured record, a maximum annual mean 

discharge of 533 ft3/s was measured for water-year 2006. In 

addition to water-year 1983, two other years were estimated to 

have a mean discharge greater than 500 ft3

/s: 1982 and 1995. 

Seven water years were estimated to have a mean discharge 

less than 100 ft3/s: 1961, 1972, 1976, 1977, 1987, 1990, and 

1994. Water-years 1976 and 1977 were the two driest years 

in terms of discharge during the 51-year period, with a mean 

discharge of 40 ft3/s estimated for 1976 and only about 8 ft3/s 

estimated for 1977.

Figure 16. Daily mean discharge measured at gage 11466320, Santa Rosa Creek at Willowside Road (SRCW), Sonoma County, 

California, 1998–2010.

0.1

1

10

100

1,000

10,000

1998 2000 2002 2004 2006 2008

Water year

IP019208_Figure B16

Daily mean discharge, in cubic feet per second

SCRW
Surface-Water Hydrology 55

By using the combined record, a cumulative departure 

from the mean curve was also calculated to help evaluate wet 

and dry periods in terms of streamflow (fig. 19C). Dry periods 

are characterized by downward trends (for example, wateryears 1960–66 in figure 19C), and wet periods are characterized by upward trends (for example, water-years 1966–75 in 

figure 19C). Using these definitions of wet and dry periods, the

wet periods are water-years 1966–74, 1981–86, 1994–99, and 

2002–06. The dry periods are water-years 1960–66, 1976–81, 

1986-94, 1999–2002, and 2006–09.

Flow-Duration Analysis

A comparison of flow-duration curves for all continuous daily mean discharge records for the MWCM gage and 

its estimated flows was used to characterize the distribution

of streamflow for the SRPW (fig. 20). Flow duration for the 

MWCM gage indicated that a daily mean discharge of about 

600 ft3/s was exceeded about 10 percent of the time (fig. 20). 

The 1-percent daily mean discharge at the MWCM gage (the 

discharge that is equaled or exceeded 1 percent of the time) 

was equal to about 3,000 ft3/s for both the measured and 

extended record.

The flow-duration curves for the measured and extended

record at the MWCM gage were generally well matched for 

flows that were equaled or exceeded about 92 percent of the

time (fig. 20). For the highest flows that occur less than 1

percent of the time, daily mean discharge was higher for the 

estimated long-term record. The relatively short record for 

the MWCM gage probably is inadequate for representing the 

highest discharges at this site. For the driest 8 percent of flows

at MWCM, measured flows were usually greater than the

extended record (fig. 20). The measured flows were probably

higher than the extended record because baseflow and flows

associated with urbanization in recent decades tended to augment streamflow, especially during late summer and fall.

Summary of Streamflow Characteristics

Streamflow within the SRPW is strongly seasonal with

high winter flows and mostly intermittent summer flows. Most

of the streamflow is runoff generated in response to rainfall,

with about 90 percent of the total annual discharge volume 

from October through May. Streamflow is highly variable, not

only on a seasonal basis but also from year to year. Following exceptionally dry winters, streamflow data indicated that

streams designated by the NHD as perennial can become dry 

during the summer.

Winter streamflow is characterized by a relatively rapid

response time of overland flow reaching first-order streams

in the upper drainages and then continuing on to the main 

channels. The response time refers to the time of concentration 

of overland runoff into the main stream channels. The rapid 

response times are caused by a combination of factors, including both storm characteristics and basin characteristics. An 

important storm characteristic is that almost all precipitation 

within the SRPW is rain. Winter streamflow generally is well

correlated to rainfall intensity and magnitude. Antecedent 

Figure 17. Daily mean discharge measured at gage 11466800, Mark West Creek near Mirabel Heights (MWCM), Sonoma County, 

California, 2006–2010. Discharge rates of less than 0.01 cubic feet per second represent zero-flow conditions.

0.01

0.1

1

10

100

1,000

10,000

2006 2007 2008 2009 2010

Water year

IP019208_Figure B17

Daily mean discharge, in cubic feet per second

MWCM
56 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 18. Correlation between seasonal daily streamflow at NAPN and MWCM gages, along with MOVE.1-estimated streamflow 

for the Santa Rosa Plain watershed, Sonoma County, California: A, winter (January through March); B, spring (April through June); C, 

summer (July through September); and D, fall (October through December). (MWCM, Mark West Creek near Mirabel Heights; NAPN, 

Napa River near Napa)

0.01

1

100

10,000

10,000

0.01

1 100

Concurrent measured daily mean discharge

Maintainance of Variance-Extension, Type 1(MOVE.1) line

0.01 0.1 1 10 100

0.01

0.1

1

10

100

1 10 100 1,000 10,000

1

10

100

1,000

10,000

1 10 100 1,000 10,000

1

10

100

1,000

10,000

MWCM measured daily mean discharge, in cubic feet per second MWCM measured daily mean discharge, in cubic feet per second

NAPN measured daily mean discharge, in cubic feet per second

D Fall streamflow

A Winter streamflow B Spring streamflow

C Summer streamflow

Qpred = 33.1 × Q0.541

NAPN

Qpred = 1.08 × Q1.012

NAPN

r2 = 0.86 r2 = 0.91

r2 = 0.70 r2 = 0.59

Qpred = 4.02 × Q0.830

NAPN

Qpred = 3.35 × Q0.568

NAPN

IP019208_Figure B18
Surface-Water Hydrology 57

Figure 19. Extended streamflow record estimated at the lower Mark West Creek near Mirabel Heights (MWCM) gage. A, Estimated 

daily mean discharge for water-years 1960–2005 and measured daily mean discharge for water-years 2006–2010. B, Estimated annual 

(water year) discharge for water-years 1960–2005 and measured annual discharge for water-years 2006–2010. C, Cumulative departure 

from the mean curve for water-years 1960–2005, Santa Rosa Plain watershed, Sonoma County, California.

0

100

200

300

400

500

600

700

0

2,000

4,000

6,000

8,000

10,000

12,000

18,000

16,000

14,000

20,000

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Mean water-year discharge, in cubic feet per second

Water year

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Water year

BMean daily discharge, in cubic feet per second

A

IP019208_Figure B19ab

Measured record 

Extended record 

Long-term mean 

Extended record 

Measured record 
58 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

IP019208_Figure B19c

0

200

–200

–400

–600

400

600

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Water year

CAnnual cumulative departure from mean 

annual discharge, in cubic-feet per second 

 MWCM combined record 

Figure 19. Continued.

conditions also are important in determining runoff; an 

increase in storm frequency causes wetter soil and ground 

conditions, resulting in increased runoff in response to rainfall. 

Steep hillsides, thin soil and vegetation cover, and impervious 

surfaces in urban areas are basin characteristics that cause 

more rapid runoff.

There is typically some localized flooding in low-lying

areas each winter in response to the largest storms. The rapid 

response times for most drainages within the SRPW increases 

the potential for flooding in low lying areas of the basin,

especially within the 1-percent AEP floodplain of the Laguna

de Santa Rosa (fig. 5). Developed areas, primarily within 

Windsor, Cotati, and Rohnert Park, have been inundated in 

response to the larger storms associated with higher rainfall 

intensities. The potential for urban-area flooding tends to be

high in the low-lying, low-gradient sections of drainage basins 

downstream of areas with a high percentage of impervious 

land cover (rooftops, parking lots, and roadways).

As stated earlier, streamflow in the Laguna de Santa

Rosa, the lower Mark West Creek, and the lower Santa Rosa 

Creek can be slowed and even reversed because of backwater effects in the Laguna de Santa Rosa floodplain generated

by high flows on the Russian River. The Laguna de Santa

Rosa floodplain acts as a natural flood retention basin for the

Russian River by temporarily capturing and storing up to 

79,000 acre-ft of flood water (Sloop and others, 2007).

During summer, low-flow conditions are found throughout the SRPW, with most of the streamflow consisting of

baseflow and, in some cases, runoff from irrigation. Streamflow in the upper reaches of Santa Rosa Creek and Mark West

Creek drainages are perennial in many places, especially along 

sections of Mark West Creek, Matanzas Creek, Spring Creek, 

and Santa Rosa Creek. By late summer and early autumn, the 

natural flows in the upper channels diminish to less than 1 ft3/s 

for most locations. In contrast to winter streamflow, summer

streamflow in the lower reaches of the principal drainages is

generally less than 5 ft3/s and is mostly limited to the main 

channels, whereas the smaller tributaries remain dry throughout the summer. Summer streamflow in the lower reaches of

Santa Rosa Creek is often a combination of natural baseflow

from the upper drainages, urban runoff from the city of Santa 

Rosa, irrigation return flows, and outflows from reservoirs

(Lake Ralphine and Spring Lake). A large percentage of the 

summer streamflow does not discharge to the Russian River,

but rather flows into wetlands within the Laguna de Santa

Rosa, where it is lost to the atmosphere through evapotranspiration (ET) or infiltrates through the streambed, which can

ultimately become groundwater recharge.

Flood Control, Flow Diversions, and Drainage 

Modifications

The surface-water hydrology of the SRPW has been 

affected by a long history of anthropogenic influences on

stream channel and watershed characteristics related to landuse change and increasing population (Sloop and others, 

2007). From the initial settlements through modern-day urbanization, modifications have been made to stream channels

for flood control and to promote drainage across agricultural
Surface-Water Hydrology 59

land and developed areas. At the same time, changes in land 

use have generally resulted in an increase in the potential for 

flooding (Sloop and others, 2007). An increase in the imperviousness of the land surface caused by an increase in the 

percentage of area occupied by rooftops, parking lots, and 

roadways has resulted in more rapid runoff generation and 

an increase in total runoff volume downstream of urban areas 

(Sloop and others, 2007). In addition to flood-control modifications, the natural-drainage system has been modified by flow

diversions for irrigation and discharge of wastewater effluent.

Conversion of land to agriculture, housing developments, and 

commercial land uses have often been accompanied by stream 

channel and drainage modifications, such as channel straightening and deepening, to improve the drainage of low-lying 

areas and to dewater swamp and marsh land. In some cases, 

canals were constructed to improve the connectivity of the 

natural drainage system, especially across alluvial fans where 

natural channels are often not well defined (Sloop and others,

2007; Dawson and Sloop, 2010). Groundwater-resource development can lower water tables enough to decrease summer 

baseflow in some channel sections, whereas return flows from

irrigation and urban-area discharges can increase summer 

flows in other channel sections.

Flood Control

The SRPW includes five retention basins, all impounded

by earthen dams, that are used to mitigate flooding of Santa

Rosa Creek as it passes through the city of Santa Rosa. Four 

of these retention basins were constructed by the Natural 

Resources Conservation Service (NRCS) and the Sonoma 

County Flood Control and Water Conservation District during 

the early 1960s (U.S. Army Corps of Engineers, 2002) and are 

owned and operated by SCWA: Spring Lake, Matanzas Creek 

Reservoir, Piner Creek Reservoir, and Middle Fork Brush 

Creek Reservoir. A fifth retention basin, Lake Ilsanjo, was

constructed in 1956 and is owned and operated by the California Department of Parks and Recreation as part of Annadel 

State Park.

Spring Lake is located within the city of Santa Rosa 

(fig. 5) and is the largest flood-control facility in the SRPW

with a maximum storage capacity of 3,550 acre-ft and a 

surface area of 0.24 mi2

 (154 acres). Having a natural drainage 

area of only 0.55 mi2 (355 acres), Spring Lake also receives 

water through two channel diversions that connect the reservoir to the Santa Rosa Creek and Spring Creek drainages. 

The main flood-control diversion connects to the channel

of Santa Rosa Creek about 0.5 mi upstream of the reservoir 

and uses a concrete control sill along with an 8-foot diameter 

culvert and control orifice to regulate inflows into the reservoir. The diversion allows the first 840 ft3

/s of natural flow to

remain in the Santa Rosa Creek channel. As the flow in Santa

Rosa Creek exceeds 840 ft3/s, a portion of the flow is diverted

to the reservoir. There is a maximum diversion of 5,220 ft3/s 

for natural flows of 8,250 ft3/s and greater in the main channel 

of Santa Rosa Creek. A second diversion connects with the 

channel of Spring Creek about 1.0 mi south of the reservoir. 

Flow of about 10 ft3/s remains in the natural channel of Spring 

Creek; flows greater than 10 ft3/s and up to 1,000 ft3/s are 

diverted to Spring Lake.

IP019208_figure B20ab

Percentage of time the indicated discharge was equaled or exceeded 

0.01 

0.1 

1 

10 

100 

1,000 

10,000 

100,000 Daily mean discharge, in cubic feet per second

 Gage12_11466800_MWCM 

 Gage12_MOVE1_extended 

5 2 1 0.5 0.2 10 20 30 40 60 70 80 50 90 95 98 99 99.8 

Figure 20. Flow duration of daily mean discharge and modeled daily mean discharge for the lower Mark West Creek near Mirabel 

Heights (MWCM) gage, Santa Rosa Plain watershed, Sonoma County, California.
60 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Matanzas Creek Reservoir was built in 1963 and is 

located on Matanzas Creek in the upper section of the drainage 

(fig. 5). The reservoir has a maximum surface area of 62 acres, 

a maximum storage capacity of 1,500 acre-ft, and a catchment 

area of 11 mi2 (7,040 acres). Piner Creek Reservoir was built 

in 1962 on Paulin Creek (fig. 5) and has a maximum surface 

area of 19 acres, a maximum storage capacity of 172 acre-ft, 

and a catchment area of 2.05 mi2 (1,312 acres). Middle Fork 

Brush Creek Reservoir was built in 1961 and is located on the 

middle fork of Brush Creek (fig. 5). It has a maximum surface 

area of 20 acres, a maximum storage capacity of 138 acre-ft, 

and a catchment area of 2.24 mi2 (1,434 acres). Lake Ilsanjo 

is on Spring Creek (fig. 5) and has a maximum surface area 

of 67 acres, a maximum storage capacity of 395 acre-ft, and a 

drainage area of 1.71 mi2 (1,094 acres).

Several studies have been carried out within the SRPW to 

evaluate flood risk, specifically for Santa Rosa Creek and the

potential effects on the city of Santa Rosa (U.S. Army Corps 

of Engineers, 2002). By using precipitation-runoff models, a 

discharge with an AEP of 1 percent being equaled or exceeded 

was estimated to be 19,600 ft3/s at the mouth of Santa Rosa 

Creek, and 8,250 ft3/s was estimated at the upper Santa Rosa 

Creek channel just above the Spring Lake diversion. Additional estimates of discharges with an AEP of 1 percent for 

Santa Rosa Creek included 8,410 ft3/s above the Matanzas 

Creek confluence and 14,200 ft3/s above the Piner Creek 

confluence. Estimated discharges with an AEP of 1 percent

for other tributary streams included 1,200 ft3/s at the mouth 

of Spring Creek, 5,270 ft3/s at the mouth of Brush Creek, 

5,500 ft3/s at the mouth of Matanzas Creek, and 4,420 ft3/s 

at the mouth of Piner Creek (U.S. Army Corps of Engineers, 

2002).

Surface-Water Diversions

Surface-water diversions affecting the SRPW include 

internal diversions within the SRPW and diversions across 

the SRPW boundary. Internal diversions include flood control

(discussed previously) and minor diversions of flow from

Mark West Creek and Santa Rosa Creek to irrigate up to 

6,000 acres within the SRPW (U.S. Geological Survey, 2011). 

In the headwater areas, numerous small diversions of runoff 

from small, unnamed channels are likely on a localized scale 

to supply water for ponds and irrigation. The total magnitude 

of these diversions was assumed to be relatively small. The 

source of irrigation water within the SRPW was assumed to 

be either groundwater, recycled water, or some combination of 

the two.

A significant amount of Russian River water is diverted

into the SRPW, primarily for use as the municipal water 

supply for the town of Windsor and the cities of Santa Rosa, 

Rohnert Park, and Cotati. A portion of the imported municipal water is used for residential landscape irrigation and 

other uses, which could result in some increases in runoff 

and recharge in the SRPW. A minor amount of Russian River 

water (less than about 1,000 acre-ft/year) is used directly 

for irrigation within the SRPW (Donald Seymour, Sonoma 

County Water Agency, written commun., 2010), and this could 

also cause a small, localized increase in runoff and groundwater recharge. In this study, Russian River water used directly 

for irrigation was not considered a significant component of

the SRPW water balance.

Most of the imported Russian River water is ultimately 

processed at two wastewater-treatment facilities within the 

SRPW (fig. 21). Treated wastewater is discharged in a variety 

of ways: direct discharge to stream channels, land applications (irrigation and wetlands), and deliveries to the Geysers 

geothermal are outside of the SRPW (for power generation). 

As of January 2008, more than 14,000 acre-ft/yr of treated 

wastewater was delivered to the Geysers (City of Santa Rosa, 

2012); this is the largest use of treated wastewater. Most of the 

remaining water is applied on designated land parcels (fig. 21), 

as described later, to irrigate hay fields, grapevines, golf

courses, and urban parks during the spring and summer (Sloop 

and others, 2007).

Monthly records on the application of treated wastewater from the town of Windsor and the city of Santa Rosa 

used for irrigation, also referred to as reclaimed water, were 

available for water-years 1990 through 2009 (fig. 22). For the 

most part, land irrigated with treated wastewater was within 

the Laguna de Santa Rosa 1-percent AEP floodplain (fig. 21). 

Total monthly treated wastewater used for irrigation varied 

from zero during winter months to a maximum of about 

3,000 acre-ft during the summer months of water-years 1994 

and 1995 (fig. 22A). The annual volume of treated wastewater used for irrigation averaged about 10,200 acre-ft, with 

a maximum of 14,100 acre-ft used during water-year 2001, 

and a minimum of 7,400 acre-ft used during water-year 2009 

(fig. 22B). At the time of this study, the volume of treated 

wastewater used for irrigation in water year 2010 was not 

available.

Drainage Modifications

With the onset of more intensive agriculture from the 

early 1800s, many stream channels were modified to promote more rapid drainage of wetlands and vernal pools that 

developed on the alluvial fans during the wet winter season 

(Dawson and Sloop, 2010). Channels that were formerly 

disconnected on the alluvial fans became straightened 

and more connected by a network of roadside ditches and 

canals. In their natural state, stream channels shifted periodically across the alluvial fans during the wet season, with 

Copeland Creek occasionally switching watersheds between 

the Russian River and the Petaluma River drainage systems 

(Dawson and Sloop, 2010). With the conversion of land to 

ranching and agricultural uses, the stream network changed. 

Streams draining the mountains on the eastern side of the 

valley that fed seasonal wetlands and did not join with the 

Laguna de Santa Rosa, such as Copeland and Crane Creeks, 

were instead redirected by straight canals and drainage ditches 

into the main channel of the Laguna de Santa Rosa as early 
Surface-Water Hydrology 61

Mark West Creek

Matanzas Creek

Colgan Creek

Spring Creek

Pool Creek

Windsor Creek

Five Creek

Crane Creek

Pruit Creek

Paulin Creek

Copeland Creek

Irwin Creek

Santa Rosa Creek

Badger Creek

Roseland Creek

Mill Creek

Blucher Creek

Duer Creek

Laguna de Santa Rosa

Humbug Creek

Golf Creek

Hinebaugh Creek

Fulton Creek

Piner Creek

Porter Creek

Santa Rosa Creek

Copeland Creek

Crane Creek

Surface-water subbasins

Mark West Creek

Santa Rosa Creek

Laguna de Santa Rosa

EXPLANATION

Santa Rosa reclaimed-

 water parcels

Windsor reclaimed-

 water parcels Major streams

Santa Rosa Plain

 watershed

 boundary

IP019208_Figure B21

122°50’ 122°45’ 122°40’ 122°35’

38°

35’

38°

30’

38°

25’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Santa Rosa

Laguna Treatment

Plant

Windsor

Treatment

Plant

Figure 21. Location of land parcels where reclaimed water is applied for irrigation of crops and urban areas, Santa Rosa Plain 

watershed, Sonoma County, California.
62 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

0

500

1,000

1,500

2,000

2,500

3,000

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Applied reclaimed water, in acre-feet per month Applied reclaimed water, in acre-feet per year

Water year 

Water year 

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

B

A

IP019208_Figure 22ab

Figure 22. Reclaimed water applied as irrigation in Santa Rosa Plain watershed, Sonoma County, California, for water-years 1990–

2009: A, monthly totals; B, annual (water-year) totals.
Groundwater Hydrology 63

as the 1870s (Dawson and Sloop, 2010). The trend toward 

increasing connectivity of the drainage network is ongoing, 

with storm drains installed in housing developments and drainage tile placed under vineyards. The consequences of these 

historical changes probably include habitat loss, decreased 

groundwater recharge, decreased water quality, and increased 

erosion and flooding downstream (Dawson and Sloop, 2010).

More recently, channel modifications have included

channel restoration efforts to mitigate invasive vegetation, 

channel erosion, and sedimentation, and to improve riparian 

habitat and water quality. The ongoing channel restoration has 

included the stabilization of eroding channel banks by using 

riprap and native vegetation cover and the conversion of riparian areas to recreational uses, which includes the removal of 

underbrush.

Groundwater Hydrology

Groundwater systems in the study area include the saturated sedimentary rocks and sediments underlying the floor of

SRP and surrounding lowlands, as well as the volcanic rocks 

underlying the mountains in the eastern SRPW, where such 

rocks are sufficiently permeable to yield water. Beneath the

floor of SRP, the principal groundwater system is lithologically heterogeneous, but consists of one continuous body of 

saturated material. Groundwater in the principal aquifer is 

contained in the pore spaces of the Quaternary alluvial materials and Tertiary sedimentary rocks, including the Glen Ellen 

Formation, the Wilson Grove Formation, and the Petaluma 

Formation. Groundwater is also contained in locally permeable intervals within the Sonoma Volcanics and, to a much 

lesser extent, in fractured bedrock (fig. 23). Data compiled 

largely from previous investigations (Cardwell, 1958; Ford, 

1975; Herbst and others, 1982; Sweetkind and others, 2010) 

are used in this report to describe the aquifer system, aquifer 

properties, recharge and discharge, and groundwater flow in

the SRPW.

Groundwater Subbasins and Storage Units

The SRPW has been divided into groundwater subbasins 

by the California Department of Water Resources (CDWR; 

Swartz and Hauge, 2003). Groundwater storage units were 

defined for this study to help describe the groundwater hydrology of the SRPW.

Figure 23. Conceptual diagram of the groundwater system, Santa Rosa Plain watershed, Sonoma County, California.

ALLUVIUM & WILSON GROVE PETALUMA SONOMA GLEN ELLEN BEDROCK FORMATION FORMATION VOLCANICS FORMATION

GENERAL DIRECTION OF

GROUNDWATER MOVEMENT PRECIPITATION RUNOFF INFILTRATION

FAULT

ZONE

WATER

TABLE

Sebastopol fault

Rodgers Creek fault

Mayacmas Mts

Agriculture Rural

City
64 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Groundwater Subbasins

A groundwater basin is defined as an aquifer or aquifer

system bounded laterally, and at some depth, by rocks or 

geologic structures, which, by virtue of a distinct permeability 

contrast, hydraulically separate groundwater within the basin 

from that beneath neighboring land (California Department of 

Water Resources, 2003). Groundwater subbasins are subdivisions of groundwater basins delineated on the basis of geologic, hydrologic, or institutional boundaries. The study area 

includes the SRP and Rincon Valley groundwater subbasins, 

the eastern portion of the Wilson Grove Formation Highlands 

groundwater basin, the western half of the Kenwood Valley 

groundwater basin, the southern part of the Healdsburg area 

groundwater subbasin, and the southern part of the Alexander Valley groundwater subbasin (Swartz and Hauge, 2003; 

fig. 24).

The SRP groundwater subbasin is the largest groundwater subbasin in the study area and includes about 125 mi2

 that 

covers most of SRP (fig. 24). As mapped by the California 

Department of Water Resources (2003), the SRP groundwater subbasin includes water-bearing strata only within the 

Cenozoic sedimentary formations, and not within the Tertiary 

volcanic rocks, which are primarily the Sonoma Volcanics; 

however, the volcanic rocks provide substantial quantities 

of water to wells on the valley floor and in the mountains

along the eastern side of the valley. The groundwater-storage 

capacity of the SRP groundwater subbasin within the depth 

range of 10 to 200 ft was estimated to be about 950,000 acre-ft 

(Cardwell, 1958). The storage capacity over an average thickness of 400 ft was estimated to be about 4,300,000 acre-ft 

(Herbst and others, 1982). Neither estimate includes the 

amount of groundwater contained in the Sonoma Volcanics. 

The Sonoma Volcanics, however, contain discontinuous 

aquifers that in some of the more porous and permeable lithologic packages can yield several hundred to several thousand 

gallons of water per minute to wells (Cardwell, 1958). Note 

that the sustainable yield of the basin is less than the storage 

capacity because of potential negative effects of withdrawing 

all groundwater in storage, such as, land subsidence, depletion of environmental flows, and pumping of poor quality

groundwater.

The Rincon Valley groundwater subbasin includes about 

5,760 acres of relatively flat land in a structural depression that

lies about 1 to 2 mi east of the SRP (fig. 24). The small subbasin is bounded mostly by outcrops of the Sonoma Volcanics, 

but is hydrologically connected with the SRP groundwater 

subbasin through a topographically narrow gap underlain by 

alluvial deposits. Santa Rosa Creek flows westward through

the gap after receiving tributary water from Brush Creek, 

which flows intermittently and drains most of Rincon Valley.

The groundwater-storage capacity of the Rincon Valley subbasin within a depth range of 10 to 200 ft has been estimated 

to be about 21,000 acre-ft (Cardwell, 1958). Cardwell’s (1958) 

estimate of the average specific yield was 5.5 percent.

The Wilson Grove Formation Highlands groundwater 

basin covers about 81,280 acres of hilly terrain that extends 

westward from the western edge of SRP to the Pacific Ocean

(fig. 24). Only a small part of this basin along its eastern side 

is within the study area. The basin is underlain by sandstone of 

the Wilson Grove Formation, which rests unconformably on 

Mesozoic basement rocks. No published estimates of storage 

are available for the portion of the Wilson Grove Highlands 

groundwater basin within the SRPW.

Groundwater Storage Units

For the purposes of this study, five groundwater storage

units were defined on the basis of the work by Cardwell

(1958), hydrogeology, and fault locations (fig. 24). The 

Windsor Basin (WB) storage unit is located north of the 

Trenton Ridge fault, west of the Mayacmas Mountain foothills, and east of the Sebastopol fault (fig. 24). The Cotati 

Basin (CB) storage unit is located south of the Trenton 

Ridge fault, west of the Sonoma Mountain foothills, and east 

of the Sebastopol fault (fig. 24). The Wilson Grove (WG) 

storage unit is located between the Mendocino Range and 

the Sebastopol fault (fig. 24). The Valley (VAL) storage 

unit includes the alluvial fill of the Rincon Valley, Bennett

Valley, and the northern half of Kenwood Valley (fig. 24). The 

Uplands (UPL) storage unit includes the Sonoma Volcanics 

in the Mayacmas and Sonoma mountains east of the Rodgers 

Creek fault zone, but excludes the VAL storage unit (fig. 24).

Definition of Aquifer System

The Glen Ellen, Wilson Grove, and Petaluma Formations, 

and the Sonoma Volcanics have distinct aquifer properties and 

constitute the four principal water-bearing aquifer units in the 

study area (fig. 1). In the following discussion, the Glen Ellen 

is defined as a combination of the Quaternary alluvial deposits and Glen Ellen Formation (Sweetkind and others, 2010). 

The distribution, subsurface extent, and interfingering relations among the four principal formations reflect the history

of uplift and basin development in the SRPW, tectonic activity 

including offset along major basin-bounding faults, and the 

interaction between continental and marine sedimentation. 

Previous groundwater-resource investigations of the SRPW 

(Cardwell, 1958; Ford, 1975; Herbst and others, 1982) defined

the geology of the groundwater-flow system. Subsequent

geologic mapping and geophysical studies, and interpretation 

of borehole data, have refined the understanding of the basin

geometry and the identity and location of major basin-filling

units (Blake and others, 2002; Wagner and others, 2010; 

Graymer and others, 2007; McLaughlin and others, 2008; 

Langenheim and others, 2010; Sweetkind and others, 2010). 

Three-dimensional lithologic and stratigraphic models of the 

SRPW constructed from borehole data, geologic map, and 

geophysical data delineated the thickness, extent, and the distribution of subsurface geologic units (Sweetkind and others, 

2010). These 3D lithologic and statigraphic models were used 

in this study to define the SRPW aquifer system.
Groundwater Hydrology 65

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Mt. St. John thrust fault 

Gates Canyon thrust fault

Petrified Forest thrust zone

WindsorFulton

Santa

Rosa

Laguna

Cotati Plain

Rincon

Valley

Bennett

Valley Kenwood Valley

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Santa Rosa Plain

Wilson Grove Formation Highlands

Rincon Valley

Other subbasins

Windsor Basin

Wilson Grove

Valley

Uplands

Cotati Basin

EXPLANATION

P l a i n

S a n t a

R o s a

Groundwater subbasin Groundwater storage unit

Santa Rosa Plain

 watershed boundary

Inferred

 fault

Groundwater storage unit 

[after Cardwell (1958). Dashed lines,

approximate boundaries not explicitly

defined by Cardwell (1958).]

IP019208_Figure B24

Windsor

Basin

Valley

Cotati

Basin

Uplands

Uplands

Wilson

Grove

Rodgers Creek

 fault zone

Figure 24. Groundwater basins, subbasins, and storage units in the Santa Rosa Plain watershed, Sonoma County, California.
66 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Aquifer Extent and Distribution of Material 

Properties

Data from 2,683 selected boreholes gathered from multiple sources were used to define the subsurface stratigraphy and

lithologic heterogeneity of the four principal aquifer units of 

the SRPW (Sweetkind and others, 2010). The boreholes were 

selected from a much more numerous set of water-well data by 

selecting about 10 representative boreholes from each of the 

36 sections within a township and range, or about 10 boreholes 

within each square mile of the study area. In parts of the study 

area where the population density was low, drill-hole data 

were correspondingly sparse and fewer than 10 boreholes per 

square mile were available for analysis. The boreholes that 

were used represented those that contained the greatest amount 

of detail in the description of each interval, had a large number 

of downhole intervals described (as opposed to a single, long 

interval of “sand and gravel” or “alluvium”), were representative of downhole lithology of nearby holes, and represented a 

distribution of holes that were not clustered but were approximately equally distributed over the study area.

Lithologic descriptions from the 2,683 boreholes were 

simplified to 19 internally consistent lithologic classes (Sweetkind and others, 2010). The source of the lithologic data is 

mostly from drillers’ descriptions of material recovered at land 

surface as well drilling proceeded (drill cuttings). The lack of 

any uniform protocol in sampling and describing the drill cuttings, such as color charts, grain-size analysis, mineralogical 

examination, as well as the use of local terms for some materials, required interpretation to translate the original descriptions 

into geologically consistent terminology.

The resulting lithologic data from the selected boreholes 

were used to construct a 3D model of lithologic variations 

within the basin by interpolating and extrapolating data from 

boreholes by using a nearest-neighbor 3D-gridding process 

(Sweetkind and others, 2010). The 3D-gridding process is a 

cell-based solid-modeling approach where cell nodes with 

dimensions of about 820 ft in the horizontal dimensions and 

about 33 ft in the vertical dimension are sequentially assigned 

properties by looking outward horizontally from each node 

in search circles of ever-increasing diameter. The aspect ratio 

specified for the gridding process emphasized the horizontal

dimension over the vertical, thereby rendering interpolated 

drill-hole data as horizontal stratigraphic units (fig. 25). 

The approach is a simple spatial interpolation method that 

does not consider spatial structure of the data. Within the 

shallow basin fill, where data are abundant, local lithologic

variability is incorporated, and the 3D lithologic variability 

of the basin fill can be reasonably estimated. Where borehole

data are less abundant, such as at greater depths or in uplands, 

the lithologic model is more dependent on a smaller number of 

boreholes, and there is greater uncertainty in the understanding of lithologic variability. The resultant 3D lithologic model 

delineated the variability of lithologic character of each of 

the four principal water-bearing aquifer units (Sweetkind and 

others, 2010).

Vertical sections cut through the 3D lithologic model 

display a west-to-east transition from dominantly fine-grained

marine sands of the Wilson Grove Formation, on the west, to 

heterogeneous continental sediments of the Glen Ellen and 

Petaluma Formations, beneath the SRP, to Sonoma Volcanics interbedded with the Petaluma Formation in the uplands 

east of the Rodgers Creek fault zone (figs. 2 and 25). Thin 

(100 to 150 ft thick) and irregularly distributed coarse-grained 

deposits (sand and gravel), interpreted to be mainly Glen 

Ellen Formation (Sweetkind and other, 2010), overlie clayrich deposits, interpreted to be mainly Petaluma Formation 

(Sweetkind and others, 2010), throughout the SRP (figs. 2 and 

25). The modeled widespread subsurface extent of the Petaluma Formation contrasts with the relatively limited outcrop 

expression of the Petaluma Formation around the margins of 

the SRPW (fig. 2).

In order to tie the basin-fill lithology to a stratigraphic

context and to mapped surface exposures, a second 3D model 

was constructed that represents the configuration and relative

elevation of the top of each hydrogeologic unit (Sweetkind 

and others, 2010). This digital 3D hydrogeologic framework 

model (3D HFM) of the SRPW was constructed by using multiple geologic data sets including geologic maps, surface traces 

of faults, interpreted subsurface stratigraphic contacts from 

drill-hole data, and the results of geophysical models (Sweetkind and others, 2010). Subsurface stratigraphy was defined

from the borehole lithologic data through the identification of

distinctive lithologic packages tied, where possible, to highquality well control and to surface exposures (Sweetkind and 

others, 2010). Assignment of stratigraphic tops was fundamentally lithology-based, rather than fossil or age-based, and, as 

such, was “rock-stratigraphic,” rather than time stratigraphic.

Mappable lithologic sequences were identified in well data by

analyzing numerous serial cross sections across the SRPW and 

making stratigraphic correlations on the basis of rock type, 

bedding and sorting characteristics, stratigraphic succession, 

and an understanding of the relationship between the mapped 

geologic units and their lithologic characteristics.

The 3D HFM of the SRPW was constructed by standard 

subsurface mapping methods of creating isopach and structure-contour maps for each of the four principal aquifer units. 

The elevation of the tops of stratigraphic units and thickness 

for each of the four major aquifer units were contoured from 

map and well data by using simplified fault traces to bound

contoured regions (Sweetkind and others, 2010). The 3D 

HFM was constructed from gridded surfaces that defined

the top and base of each stratigraphic horizon, which were 

then stacked in stratigraphic sequence to form a 3D digital 

solid. The 3D stacking was guided by rules that controlled 

stratigraphic onlap, truncation of units, and minimum thickness. For computational simplicity, the 3D HFM generalizes 

complex stratigraphic interfingering and repeated units. As a

result, vertical sections derived from the 3D HFM differ in 

detail from published geologic cross sections (McLaughlin 

and others, 2008).
Groundwater Hydrology 67 IP019208_FigureB25

GravelSand and gravelSandstone and gravelSand SandstoneSand and claySandstone and clay

Clay, sand, and gravelClay, sand, and trace gravelClay and gravelClay and trace gravelClay and sandClay and sandstone

Clay

ConglomerateVolcanic conglomerateBasaltAsh and (or) tuffUndifferentiated basementNo data

B B'

A

Vertical exaggeration = 4X

A'

 Vertical exaggeration = 4X

SEA

LEVEL

FEET

–500

500

1,000

–1,000–1,500

SEA

LEVEL

FEET

–500

500

1,000

–1,000

–1,500

SEA

LEVEL

FEET

–500

–1,000–1,500

SEA

LEVEL

FEET

–500

500

–1,000

–1,500

Rodgers Creek

fault zone

Rodgers Creek

fault zone

Maacama

fault zone

Petrified Forest

thrust zone

Trenton Ridge

fault 

Sebastopol

fault 

Sebastopol

fault 

Trenton Ridge

 fault 

Bennett Valley

fault zone

Windsor Basin

Cotati Basin

Lithologic model from Sweetkind and others (2010)

base of lithology model basement rocks basement rocks

base of lithology model basement rocks

Faults were not used in construction of lithologymodel and are shown for illustrative purposes only. 

EXPLANATION 

Fault, sense of offset not shown

Figure 25. Vertical sections from three-dimensional lithologic model of the Santa Rosa Plain watershed, Sonoma County, California.
68 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Interpretive geologic sections based on the 3D HFM indicated that the Glen Ellen Formation is relatively thin (less than 

200 ft) throughout the SRP (section A–A’ on fig. 2), in contrast 

to previous estimates by Cardwell (1958) of 3,000 ft. Previous 

studies have generally interpreted Wilson Grove Formation 

and Neogene volcanic rocks to underlie the SRP (Cardwell, 

1958; Ford, 1975; Herbst and others, 1982). In contrast, the 

3D lithologic model and the 3D HFM of the SRP emphasize 

the lateral extent of clay-rich lithologies, interpreted to be 

mainly Petaluma Formation (Sweetkind and others, 2010), 

throughout the deeper parts of the basins that underlie the SRP 

(fig. 2). 

The groundwater-flow model developed during the next

phase of this project required spatially distributed estimates of 

aquifer properties. Stratigraphic and textural data from the 3D 

models were used to assess geologic factors that could affect 

hydraulic conductivity and storage properties. Lateral and 

vertical variations of sediment texture, including grain size, 

sorting, and bedding, can affect the direction and magnitude 

of groundwater flow. The results of the 3D lithology model

were classed by sediment texture to help characterize grainsize variations of the aquifer system (Sweetkind and others, 

2010; fig. 26). Textural classes were based on the percentage 

of coarse-grained lithologic classes and on degree of sorting in 

each cell; relative proportion of clay matrix was considered an 

important variable. Resultant texture classes included coarsegrained, intermediate, and fine-grained (fig. 26); volcanic 

rocks did not fit into this scheme and were retained as two

additional classes: tuff and basalt.

Hydrogeologic Characteristics of Aquifers

The water-bearing properties of the geologic units in the 

SRPW vary considerably because of geologic heterogeneity; 

estimation of these properties allows quantitative prediction of 

the hydraulic response of the aquifer to changes in recharge, 

pumping, and other stresses. Previous studies of the SRPW 

(Cardwell, 1958; Herbst and others, 1982) reported estimates 

of specific yield of various lithology types and geologic

formations by using a variety of methods (table 4). Specific

yield values in the SRPW have been estimated to range from 

0 to 25 percent (table 4). Coarse-grained, well-sorted sedimentary materials have high values of specific yield because

of the large amount of connected pore volume in the material; 

cemented deposits and clay-rich deposits have smaller total 

pore volumes and lower specific yields (table 4).

Although the thickness of valley-fill deposits in the northern and southern parts of the SRPW are known to be several 

thousands of feet thick on the basis of gravity measurements 

and sparse deep petroleum exploration holes, little is known 

about the physical characteristics or hydraulic properties of 

the sedimentary rocks below depths commonly drilled for 

water wells. Most of the water wells drilled on the valley 

floor through 2010 are 1,500 ft or less in depth. Of the wells

compiled for the geologic framework and textural models of 

the SRP, only 14 were drilled to depths greater than 1,500 ft. 

As such, well data provide sufficient lithologic information

to enable a useful semi-quantitative description of the aquifer 

system in the study area to a depth of about 1,500 ft.

5,127 drillers’ reports for water wells drilled in the study 

area provided data on well tests, including discharge rate, 

water-level drawdown, and the length of test. These data 

allow the calculation of specific capacity (fig. 27A), which 

is a measure of well productivity and is given in terms of 

gallons per minute per foot of drawdown (gpm/ft). These 

wells were widely distributed across the valley floor and

included a more sparse distribution of wells in the mountainous areas. The range of specific capacities is from 0.01 to

1,990 gpm/ft; however, only 8 values exceeded 100 gpm/ft, 

and only 161 exceeded 10 gpm/ft (fig. 27B). The median 

value of specific capacity for the group of 5,127 wells was

0.67 gpm/ft (fig. 27A), which equates to a transmissivity of 

about 130 square feet per day (ft2/d; Driscoll, 1986). The 

predominantly low specific capacity for wells in the study

area can be, in part, explained by the prevalence of finegrained materials present as thick beds and as matrix within 

coarser-grained materials (fig. 25). In addition, values for 

discharge rate and water-level drawdown were obtained from 

poorly-controlled and short-duration tests of wells shortly after 

drilling; such tests typically result in lower specific-capacity

values than those obtained from more rigorous aquifers tests. 

For most of the wells, the material penetrated in the subsurface 

Table 4. Reported specific-yield values by lithology and 

geologic formation, Santa Rosa Plain watershed, Sonoma County, 

California.

[Abbreviations: Sy, specific yield; <, less than; —, no data]

Lithology Sy 

(percentage)1

Geologic 

formation

Sy 

(percentage)2

Adobe, clay shale 3 Alluvial fan 

deposits

8 to 17

Cemented gravel, 

cemented sand, clay 

and gravel

5 Wilson Grove 

Formation

10 to 20

Cemented gravel, 

cemented sand, clay 

and gravel

5 Sonoma 

Volcanics

0 to 3

Silt, clay, sand and 

gravel, fine sand,

quicksand, sand and 

clay

10 Petaluma 

Formation

3 to 7

Silt, clay, sand and 

gravel, fine sand,

quicksand, sand and 

clay

10 Franciscan 

Complex

<3

Coarse sand, loose sand, 

medium sand

20 Glen Ellen 3 to 7

Gravel, sand and gravel 25 — —

1

Specific yield values from Caldwell, 1958.

2

Specific yield values from Herbst and others, 1982.
Groundwater Hydrology 69

IP019208_FigureB26

25,000-foot grid based on Calfornia State Plane system;

Lambert Conformal Conic projection, North American

datum 1983. 

Portion of numerical model grid for Santa Rosa Plain.

Model grid, shown as faint horizontal and vertical lines,

consists of cells 660 feet on a side.

Modified from Sweetkind and others (2010).

6,337,500

6,350,000 6,362,500

1,925,000

1,900,000

2000

2000

2000 2000

1000 1000 1000

1000 1000

3

2

2 3

2 1 1

2 2

2003

2002

2002 2003

1002 1001 1001

1002 1002

Sebastopol fault

Trenton Ridge fault

Formation Code Texture Class Strat_Text Code

2000, Wilson Grove Formation

1000, Glen Ellen Formation 1, coarse grained; 2, intermediate

3, fine grained

see Sweetkind and others (2010) for explanation

and discussion of all formation codes, texture

classes, and strat_text codes.

Glen Ellen Formation

Wilson Grove Formation

Sonoma Volcanics

Petaluma Formation

Fault

Drill hole from which data 

 were used in construction 

 of lithologic model

+ =

Figure 26. Assignment of sediment texture attributes, Santa Rosa Plain watershed, Sonoma County, California.
70 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

could not be confidently assigned to particular geologic formations. For a large percentage of the wells, it is likely, based 

on the well depth, that the wells were completed in more than 

one formation, resulting in a total transmissivity value that is 

controlled by coarse-grained deposits.

The most reliable well-based estimation of hydraulic 

properties in the study area comes from the limited number of aquifer tests that followed established procedures, 

most of which are large-capacity public-supply wells used 

for municipal water systems. The results of the pumping 

tests for 14 wells drilled in the Quaternary alluvial deposits, Wilson Grove Formation, and Glen Ellen formation are 

given in table 5, and the locations are shown in figure 28. The 

pumping-test results are included with the discussion of the 

hydraulic properties of the main water-bearing aquifer units of 

the SRPW.

Quaternary Alluvial Deposits

The generally coarse Quaternary alluvial deposits, and 

their close proximity to modern streams, allow for rapid 

recharge of precipitation and runoff to the groundwater system and exchanges between groundwater and surface water. 

Groundwater is unconfined in most places within the alluvial

deposits, but semi-confined conditions exist beneath thick layers of clay or silt in some areas.

The alluvial deposits compose minor aquifers of limited 

areal extent along major streams and beneath the alluvial fans 

on the eastern side of the SRP. The alluvial deposits that blanket the valley floor are generally poorly sorted and have large

fractions of clay, and are not considered to be a major aquifer. 

Within the study area, yields from wells that are completed 

only in alluvial deposits ranged from 1 to 650 gpm (Cardwell, 

1958). The highest yields were from wells on alluvial fans in 

the northern part of the study area near Mark West Creek.

Reported specific yield values of the alluvial deposits ranged from 8 to 17 percent (Herbst and others, 1982). 

This range is higher than the range of specific-yield values that

Cardwell (1958) estimated—between 6.8 and 11 percent—for

the upper 200 ft of material on the basis of drillers’ logs for 

over 900 wells in the SRPW. The two ranges of specific-yield

values are not directly comparable because the 200-ft depth 

range used by Cardwell could, in places, include unspecified

thicknesses of underlying formations that have lower specific

yield values than the Quaternary alluvial deposits.

The hydraulic conductivity values from the two available 

aquifer tests range from 2 to about 51 feet per day (ft/d), and 

storativity values range from about 0.0013 to 0.19 (table 5). 

The large range of hydraulic properties is consistent with the 

lithologic heterogeneity and varying degree of confinement of

alluvial fan deposits.

Glen Ellen Formation

Previous reports describing the Glen Ellen Formation 

(for example, Cardwell, 1958, and Ford, 1975) reported that it 

is as thick as 3,000 ft; however, Sweetkind and others (2010) 

reported that formation is a few hundred feet thick. Therefore, 

well yields, specific-capacity data, and hydraulic parameters

reported by previous authors are to be regarded with caution. 

The thickness of the Glen Ellen Formation reported by Sweetkind and others (2010) is used in this report.

Ford (1975) reported that wells perforated in the Glen 

Ellen Formation typically yield 15 to 30 gpm. Most wells 

in which the Glen Ellen Formation is the principal waterbearing unit have specific capacities of 10 gpm/ft or less;

the highest specific capacity is 30 gpm/ft in a well drilled in

the northern part of the study area on the fan between Windsor Creek and Mark West Creek. On the basis of the specific

capacity of selected wells, and the saturated thickness of Glen 

Ellen Formation they penetrated, Cardwell (1958) estimated 

the hydraulic conductivity of the upper part of the Glen 

Ellen Formation to be in the range of 13–23 ft/d. Kadir and 

McGuire (1987) reported the results of a pumping tests on a 

well (8N/9W-12Q2) perforated in the Glen Ellen Formation in 

which the hydraulic conductivity was estimated as 5 ft/d. The 

upper part of the Glen Ellen Formation, although finer grained

than the deeper part, is less compacted or cemented and, therefore, has higher hydraulic-conductivity values. The upper part 

of the Glen Ellen Formation is within the depth range common for wells located on the alluvial fans along the east side 

of the SRP. In the eastern part of the SRP, in Rincon Valley 

and the lower end of Bennett Valley, the Glen Ellen Formation 

has been deformed by folding and faulting (Cardwell, 1958; 

McLaughlin and others, 2008). In these areas, most wells 

that pump water from the Glen Ellen Formation do so from 

stratigraphically lower units of coarser, but more compacted 

and cemented, materials; thus, well yields are correspondingly 

lower.

The large amount of clay in the Glen Ellen Formation is 

in thin to thick beds of nearly pure clay-sized material; clay is 

also common in the matrix of coarser-grained units. The ubiquitous clay-sized material, degree of compaction, and cementation all limit the permeability of the Glen Ellen Formation. 

The specific-yield range for the Glen Ellen Formation given

by Herbst and others (1982) is between 3 and 7 percent. The 

large amount of clay-sized material in some areas is sufficient

to cause locally confined conditions. In the lowland area east

of Santa Rosa, the water in some wells drilled into the Glen 

Ellen Formation stood above the water table or even flowed to

land surface in the 1950s (Cardwell, 1958).

Wilson Grove Formation

The thick, sand-dominated Wilson Grove Formation is 

exposed in the low hills west of SRP and also is continuous to 

the east for an uncertain distance beneath SRP, where it is concealed by variable thicknesses of alluvial materials and Glen 

Ellen Formation and interfingers with the Petaluma Formation

(section B-B’ on fig. 2). Numerous wells obtain water from the 

Wilson Grove Formation in the western part of the study area. 

Cardwell (1958) cites a range of specific capacities between
Groundwater Hydrology 71

Figure 27. Specific-capacity data for wells in the Santa Rosa Plain watershed, Sonoma County, California: A, histogram; B, cumulative 

frequency.

0

1,000

2,000

3,000

4,000

5,000

6,000

0 50 100 150 200 250

Specific capacity, in gallons per minute per foot

Number of wells 

B

0

500

1,000

1,500

2,000

2,500

IP019208_Figure B27ab

0.1 0.5 1 1.5 2 5 10 50

Specific capacity, in gallons per minute per foot

Number of wells 

A

Total number of wells 5,127

5,093 wells with specific capacity

 less than 50 gpm/ft

4,990 wells with specific capacity

 less than 10 gpm/ft

Total number of wells 5,127

Median specific capacity:

 0.67 gpm/ft

* category includes 34 wells

 that have specific capacity

 between 50.1 and 1,990 gpm/ft

*

One well with

specific capacity

of 1,990 gpm/ft

not shown
72 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Table 5. Hydraulic properties from aquifer tests, Santa Rosa Plain watershed, Sonoma County, California.

[Abbreviations: ft, feet; gpd; gallon per day; gpm, gallon per minute; N/A, not applicable; —, no data]

State well 

number

Hole 

depth 

(ft)

Cased 

depth 

(ft)

Top 

perf 

(ft)

Bot 

perf

(ft)

Pumping 

rate 

(gpm)

Aquifer 

thickness 

(ft)

Specific 

capacity 

(gpm/ft)

Transmissivity 

(gpd/ft)

Hydraulic 

conductivity 

(ft/d)

Storativity Notes

Well pumping tests in Quarternary alluvial material

7N/8W-24A41 1,200 912 295 912 1,000 230 — 72,580–88,000 42.2–51.2 0.0013–0.0017 Analyzed using 

Jacob method 

and (or) Hantush 

and Jacob 

method.

8N/8W-33K12 400 400 100 400 106 300 7.9 4,825 2 0.19 Analyzed using 

Cooper-Jacob 

method.

Well pumping tests in Wilson Grove Formation

7N/9W-15Q21 642 405 122 394 1,200 1

500 14.8 26,800 7 0.0011 Analyzed using 

Jacob method.

7N/9W-25M13 790 770 310 750 2,070 440 17.4 26,100 — 0.00095 N/A.

7N/9W-25E11 1,521 830 410 805 1,200 1

560 40 24,800 6 N/A Analyzed using 

Jacob method.

6N/8W-7A21 815 — 650 800 1,500 1

160 N/A 10,100 8 0.0015 Analyzed using 

Hantush and 

Jacob method.

6N/8W-7A21 815 — 650 800 1,500 1

160 N/A 15,200 13 0.001 Analyzed using 

Jacob method.

7N/9W-36K13 1,506 1,065 425 1,045 1,248 — 6.3 9,450 — — N/A.

7N/9W-36K13 1,506 1,065 425 1,045 1,650 — 27.5 41,250 — — N/A.

7N/9W-36K13 1,506 1,065 425 1,045 1,709 — 13.9 20,850 — — N/A.

7N/9W-36K23 1,074 1,040 410 1,020 2,746 3

900 18.4 27,600 4 0.001–0.08 N/A.

6N/9W-2C14 600 600 332 600 444 268 8.9–25 13,350–37,500 7–19 — N/A.

6N/9W-2H14 776 530 237 468 925–1,050 231 23.6–75 35,400–112,500 21–65 — N/A.

6N/9W-2B14 647 528 138 528 850 390 18.5–37.2 27,750–55,800 10–19 — N/A.

6N/9W-1M34 1,015 572 172 552 1,210–2,017 380 23.2–69.9 34,800–104,850 12–37 — N/A.

6N/9W-1N_4 760 690 270 670 842 400 7–7.8 10,500–11,700 4 — N/A.

6N/9W-1N_4 760 690 270 670 1,220 400 6.3 9,450 3 — N/A.

Well pumping tests in Glen Ellen Formation

8N/9W-12Q21 500 500 270 495 200 1

150 N/A 5,870 5 N/A Analyzed using 

Jacob method.

1

Kadir and McGuire (1987).

2

Written communication, ENGEO, Inc. (2006).

3

Written communication, Sonoma County Water Agency (2009).

4

Written communication, city of Sebastapol (2009).
Groundwater Hydrology 73

Figure 28. Locations of wells with pumping tests, Santa Rosa Plain watershed, Sonoma County, California.

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Mt. St. John thrust fault 

Gates Canyon thrust fault

Petrified Forest thrust zone

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

Santa Rosa Plain

Wilson Grove

 Formation

 Highlands

Rincon Valley

Other subbasins

Groundwater subbasin

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B28

EXPLANATION

Well

Location of well

 and identifier

Cotati Basin Valley

Windsor Basin Upland

Wilson Grove

Groundwater storage units

Inferred fault

6N/9W-2C1 Rodgers Creek

 fault zone

8N/8W-33K1

8N/9W-12Q2

6N/8W-7A2

6N/9W-2C1

6N/9W-2B1

7N/9W-36K1,2

7N/9W-25E1

7N/8W-24A4

7N/9W-15Q2

7N/9W-25M1

6N/9W-2H1

6N/9W-1M3

6N/9W-1N_
74 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

2 and 29 gpm/ft for the Wilson Grove that were based mostly 

on short-duration pumping tests. Cardwell’s range of values, 

however, included wells to the west of the study area screened 

within relatively deep stratigraphic horizons of the Wilson 

Grove that are finer-grained because they were deposited

in deeper water. Within the study area, most wells screened 

partially or totally in the Wilson Grove Formation are within 

the upper stratigraphic horizons, which are coarser grained 

and more permeable than deep deposits to the west. Cardwell 

(1958) notes that the specific capacity of wells perforated in

the lower part of the Wilson Grove is generally 1 gpm/ft or 

less, whereas wells perforated in the upper part have specific

capacities of 5 to 6 gpm/ft. On the basis of short-term pumping tests and the saturated thickness open to wells, Cardwell 

(1958) estimated the hydraulic conductivity of the Wilson 

Grove Formation to range from 2 to 33 ft/d, with a mean of 

about 13 ft/d.

The predominance of relatively clean sand and the low 

degree of cementation in the Wilson Grove Formation result in 

moderate to high storativity. Herbst and others (1982) reported 

the specific yield to be in the range of 10 to 20 percent, higher

than any of the other rocks or sediments in the study area.

The hydraulic properties of Wilson Grove Formation 

have been determined from pumping tests in 11 wells located 

in the western part of the SRPW, in or near Sebastopol, since 

the late 1980s (table 5). The range of hydraulic-conductivity 

values from the 11 wells was from about 3 to 65 ft/d and storativity values ranged from 0.00095 to 0.08. The reason for the 

difference between the storage values reported by Herbst and 

others (1982) and the pumping tests in table 5 could be that 

the values reported Herbst and others (1982) reflected unconfined conditions, while the pumping tests reflected confined

conditions.

Petaluma Formation

The Petaluma Formation is dominated by fine-grained

materials, either in thick beds or as interstitial material in 

poorly sorted silty and clayey sands or gravels. Also, the Petaluma Formation is at least 3,000 ft thick in places within the 

study area, including a large part of the southern SRPW. Even 

though the formation is dominated by clay, thin, moderately 

to poorly sorted beds of sands and gravels can be encountered in sufficient quantity by deeper wells that yields greater

than 100 gpm are possible. Detailed stratigraphic analyses 

by Allen (2003) and Holland and others (2009) have identified three distinct members (lower, middle, and upper) in the

Petaluma Formation based, in part, on the dominant grainsize and sorting. This stratigraphic subdivision has utility 

for understanding the variations in hydraulic properties over 

the full depth range of the aquifer. The lower member is up 

to 750 ft thick and is predominantly dense beds of mudstone 

that have the lowest hydraulic conductivity within the formation. The formation coarsens in the 3,500-ft thick middle and 

upper parts, in which beds of poorly sorted sands and gravels 

result in increased hydraulic conductivity. In general, the beds 

of coarser materials are thin and not of great lateral extent. 

The productivity of wells drilled in the Petaluma Formation 

depends mostly on the total thickness of the thin, poorly sorted 

beds of sand and gravel perforated by the well. In general, the 

upper member of the Petaluma Formation is the most productive. Domestic wells drilled into the Petaluma Formation yield 

an average of about 20 gpm and range from 10 to 50 gpm 

(California Department of Water Resources, 1979).

In the study area, the Petaluma Formation can be thought 

of as an aquifer of last resort—widely distributed, but with a

relatively low productivity that is tapped for water when no 

better option is available at a particular location. Because of 

the complicated interfingering stratigraphic relations of the

Petaluma Formation with the Wilson Grove Formation and 

Sonoma Volcanics, some wells can pass from one formation 

into another more than once. The interfingering of the three

formations can also place relatively impermeable lavas or clay 

beds above more permeable sand or gravel beds, producing 

confined groundwater conditions. Wells spanning unconfined

and confined layers, however, can provide pathways for

groundwater to flow between layers that could affect both the

hydraulics and water quality of these areas. 

Because of the prevalence of silt- and clay-sized particles, specific yields are low, ranging from 3 to 7 percent, and

well yields are generally low across the study area (Cardwell, 

1958; Swartz and Hague, 2003). At the time of Cardwell’s 

study the Petaluma Formation was thought to be an important 

aquifer only in the northern part of Petaluma Valley. Estimates 

of transmissivity based on specific capacities of Rohnert Park

municipal wells range from 130 to 1,600 ft2/d (City of Rohnert 

Park, 2007); however, some of the wells also tap interbeds 

of Wilson Grove Formation, Sonoma Volcanics, or both 

(California Department of Water Resources, 1979). 

Sonoma Volcanics

The Sonoma Volcanics represent an important aquifer 

in parts of the SRPW, Sonoma Valley, and Napa Valley. The 

Sonoma Volcanics comprise a heterogeneous assemblage of 

lithologic types that have a broad range of hydraulic properties. Lithologies within the Sonoma Volcanics with the lowest 

specific yields and hydraulic conductivities include unfractured zones in welded tuffs and lavas, thick diatomaceous 

deposits, and some clay-rich lahar deposits. Hydrothermally 

altered volcanic rocks are generally rich in clay; although 

these alteration zones have limited areal extent, they also 

have low capacity to yield water. Lithologies with the greatest permeability include rubble zones between lava flows,

beds of scoria and coarse tephra, air-fall tuffs, and some 

coarse-grained facies of volcaniclastic units. The rubble zones 

between lava flows and air-fall tuffs could have wide areal

distributions and, therefore, could constitute significant aquifers in some areas. Fractured welded tuffs and lavas have low 

porosity and, therefore, store little water, but, in some cases, 

these units have relatively high transmissivity values where 

the fracture network is extensive.
Groundwater Hydrology 75

Water production from wells drilled into thick air-fall 

pumice units can exceed a few hundred gpm, but wells drilled 

into unfractured lavas or welded tuffs can produce less than 

10 gpm, and dry holes are sometimes encountered. For wells 

penetrating the Sonoma Volcanics, Ford (1975) gives a range 

of well yields between 10 and 50 gpm; however, some of 

the wells penetrate more than one formation, and the relative 

contributions are unknown.

Ford (1975) also reported specific-capacity values for

the Sonoma Volcanics ranging from 0.004 to 26.2 gpm/ft, 

which equates to a transmissivity range of 0.8 to 5,300 ft2/d. 

Herbst and others (1982) reported the specific yield of the

Sonoma Volcanics to be in the range of 0 to 15 percent (table 

4). Results from a 72-hour aquifer test performed in 1975 on 

a 739-ft deep well located in Bennett Valley provided an estimated transmissivity of 500 ft2

/d (Ford, 1975), or an average 

hydraulic conductivity of 0.68 ft/d.

Basement Rocks

Wells drilled into the basement rocks, which include 

the Great Valley Group, Franciscan Complex, and Coast 

Range ophiolite, generally produce small amounts of water 

(Cardwell, 1958; Kunkel and Upson, 1960; Page, 1986). 

Previous studies that characterized the basement rocks as 

non-water-bearing (Cardwell, 1958; Ford, 1975; Herbst and 

others, 1982; Kadir and McGuire, 1987) did so on the basis of 

a comparison with the overlying Tertiary and Quaternary formations that generally store, transmit, and yield much greater 

quantities of water to wells. Most of the permeability in the 

basement rocks is afforded by fracture networks that have 

developed in response to folding and faulting. In areas underlain by no aquifers more favorable than the basement, the 

most productive targets for drilling are highly fractured zones 

in well-indurated sedimentary rocks within the Great Valley 

Group (Kl on fig. 1) or the Franciscan Complex (Kjf on fig. 1). 

Many successful domestic wells of low capacity (5 gpm or 

less) have been completed in fractured basement rocks in the 

hills and mountains within the study area.

No water wells have been completed in basement rocks 

beneath the valley floor that could provide knowledge of

the hydraulic properties of these rocks where deeply buried. 

However, the deeply buried basement rocks are probably more 

compacted and cemented than those exposed at the surface 

or residing at shallow depth and, therefore, are less likely to 

produce significant quantities of water to wells.

Faults and Groundwater Flow

The most important hydrologic aspect of faults in the 

SRPW is the role they played in the development of the inland 

valleys and the great depth of some of the now sediment-filled

basins within them, especially the SRP and Sonoma Valley. 

Several faults cross the SRPW and serve as the main boundaries for the sedimentary basins beneath the SRPW. On a local 

scale, the faults are hydrologically important because they are 

planar features or zones across or within which the movement 

of groundwater can be inhibited or preferentially increased 

(Heyenkamp and others, 1999). Faulting breaks indurated 

rocks, producing zones of fractures that increase permeability 

and can provide preferential paths for groundwater flow. After

some length of time, however, the movement of groundwater 

through fractures can cause chemical weathering and cementation that reduce the permeability and convert the fault plane 

or zone into a groundwater barrier (Kharaka and others, 1999; 

Nelson and others, 1999). Faulting in unconsolidated sediments or indurated rocks can produce zones of fine-grained

fault gouge with low permeability that act as a groundwater 

barrier or zone of restricted groundwater flow (Wong and Zhu,

1999). Faults also can displace rocks or sediments in such 

ways that formations with very different hydraulic properties 

are made adjacent.

Some faults can allow fluids to move vertically and allow

deep waters to move to the surface or into shallow formations 

(Tamanyu, 1999). For example, hydrothermal systems that are 

active in Napa, Sonoma, and the SRP tend to have alignments 

of thermal springs and wells along and near valley-bounding 

faults (Youngs and others, 1983).

In general, faults are interpreted to form barriers to 

groundwater movement; however, to date, this has not been 

shown conclusively for faults in the SRPW. For example, 

Kadir and McGuire (1987) reported the results of a pumping test using well 6N/8W-7A2 and neighboring piezometers 

6N/8W-7A4-6. The authors stated that it could not be determined if the Sebastopol fault was a barrier to flow; however, it

was treated as a barrier in a groundwater-flow model developed by the authors. Herbst and others (1982) reported that 

the Sebastopol fault does act as a barrier to flow. Along any

particular fault, the hydraulic characteristics of the materials 

in the fault zone and the width of the zone can vary considerably, so that a fault can be a barrier along part of its length, but 

elsewhere allow or even enhance groundwater flow across it.

Groundwater Recharge and Discharge

Sources of groundwater recharge in the SRPW are infiltration of precipitation, streamflow, septic-tank effluent, and

irrigation. Groundwater discharges as baseflow in streams, discharge from springs, ET from phreatophytes, and groundwater 

pumpage. Groundwater recharge to and discharge from the 

SRPW also can be underflow in the saturated zone across the

SRPW boundary, with flows either into or out of neighboring

groundwater basins.

Groundwater Recharge

The principal sources of recharge to the groundwater 

system are direct infiltration of precipitation and infiltration

from streams within the SRPW. The 1951 groundwater-level 

contour map indicates that immediately west of the Rodgers 

Creek fault zone, in the WB and CB storage units, Santa Rosa 

Creek could be losing water to (recharging) the groundwater 

system (fig. 29A).
76 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Herbst and others (1982) reported the average annual 

recharge flux for the SRPW between 1960 and 1975 was about

29,300 acre-ft. Assuming this estimate is correct, the average annual recharge for the SRPW would be greater than this 

value because the SRPW includes areas not included in the 

1982 estimate: Kenwood Valley, the area west of the Sebastopol Fault, and the mountains that border the Santa Rosa Plain.

Other potential sources of groundwater recharge include 

underflow from the neighboring Petaluma area, Healdsburg

area, and Wilson Grove Formation Highlands groundwater 

basins and subbasin (fig. 28). Woolfenden and others (2011) 

estimated groundwater underflow into the SRPW by using a

preliminary integrated hydrologic model of the study area. 

Total estimated average annual groundwater underflow into

the SRPW ranged from about 1,100 to 1,300 acre-ft (Woolfenden and others, 2011).

Infiltration from septic tanks, leaking water-supply pipes,

irrigation water in excess of crop requirements, and crop frostprotection applications are assumed to be minor sources of 

recharge. Although recharge from excess irrigation sometimes 

can be a significant part of total recharge within some basins,

within this study area, it is assumed to be minor.

Groundwater Discharge

Groundwater discharges as baseflow in streams in parts

of the SRPW. The 1951 groundwater-level contour map 

indicates that east of the Rodgers Creek fault zone in the UPL 

and VAL storage units, the Santa Rosa, Spring, and Matanzas Creeks are receiving water from the groundwater system 

(gaining streams). About 5 mi west of the Rodgers Creek 

fault zone, water-level contour lines again indicate that Santa 

Rosa Creek is a gaining stream in the western part of the SRP 

(fig. 29A).

According to USGS 7.5 minute topographic maps and 

CDWR records, there are 28 mapped springs in the study 

area, only 3 of which are named (fig. 30). The mapped springs 

include both current and historical springs; some of the 

historical springs possibly are not currently (2012) flowing

or no longer exist as a result of anthropogenic land-surface 

modifications. In addition to the mapped springs, Cardwell

(1958) noted that groundwater discharged from the Wilson 

Grove Formation on the west side of the SRPW through 

springs and seeps. Most of the springs in the study area are 

gravity springs found on the steeper slopes or in gullies, where 

the water table intersects the land surface; however, some are 

contact springs found along the outcrop of the contact between 

a permeable and a low-permeability bed. Contact springs are 

relatively common in the Sonoma Volcanics and Glen Ellen 

Formation (Cardwell, 1958). Groundwater discharged from 

springs is a source of baseflow for streams or is lost to ET.

Springs are sensitive to changes in groundwater levels caused 

by natural variations in climate or by the development of 

groundwater resources, and their flows often diminish or stop

during dry periods or when nearby wells are pumped. Detailed 

inventories of springs in the study area are not available to 

substantiate any long-term discharge decreases in response 

to pumping from the large number of wells that have been 

installed during the past 60 years.

Cardwell (1958) estimated that the ET discharge from the 

Laguna de Santa Rosa ranged from 4,000 to 6,000 acre-ft/yr. 

Cardwell (1958) did not estimate the baseflow contribution

to the Laguna de Santa Rosa. Groundwater discharge to the 

Laguna de Santa Rosa in excess of that used by plants or lost 

to the atmosphere by evaporation flows to the lower reach of

Mark West Creek, which flows out of the study area.

There could be groundwater underflow out of the SRPW

to the Healdsburg area groundwater subbasin, the Wilson 

Grove Formation Highlands groundwater basin, and Petaluma 

Valley groundwater basin. Woolfenden and others (2011) 

estimated groundwater underflow out of the SRPW by using

a preliminary integrated hydrologic model of the study area. 

Total estimated average annual groundwater underflow out of

the SRPW ranged from about 75 to 200 acre-ft (Woolfenden 

and others, 2011).

The first drilled wells were completed in the study area

around 1875 (Cardwell, 1958). Cardwell estimated that, by 

1951, there were about 8,500 wells in the SRP. In the 1950s, 

the water for municipal supply in the cities of Sebastopol and 

Cotati was obtained solely from groundwater. During this 

period, the city of Santa Rosa derived its supply from both 

groundwater and surface water. Cardwell (1958) estimated 

that in 1949 about 2,800 acre-ft of groundwater were used for 

public supply in the Santa Rosa Valley. Except for relatively 

minor amounts of surface water used on fields near Mark

West Creek, Santa Rosa Creek, and the Laguna de Santa Rosa, 

almost all irrigation utilized groundwater (Cardwell, 1958). 

Cardwell (1958) estimated that in 1949 about 3,400 acre-ft of 

groundwater were used for irrigation in the Santa Rosa Valley, 

Bennett Valley, Rincon Valley, and Kenwood area.

As of 2010, groundwater was being used primarily for 

domestic, public-supply, and agricultural water supply in the 

study area. Pumping from private domestic and agricultural 

wells was not reported. Public-supply wells are concentrated 

near Rohnert Park and Sebastopol (fig. 31). The total annual 

pumping from public-supply wells for water years 1975–2010 

ranged from about 3,900 acre-ft in water year 1975 to about 

10,100 acre-ft in water year 2001 (fig. 32). Hevesi and others (2011) estimated agricultural pumpage for water years 

1974–2009 by using a calibrated watershed model of the 

study area with land-use data and monthly crop coefficients.

Daily-irrigation demand was estimated and used to estimate 

the spatial and temporal distribution of average monthly 

agricultural pumping for 1,072 agricultural wells. Total estimated agricultural water demand ranged from 9,000 acre-ft in 

water year 1974 to 46,600 acre-ft in water year 2008 (Hevesi 

and others, 2011). Domestic pumpage was estimated for 

1974–2010 by using population density and census tracts in 

rural areas, and an assumed per capita consumptive use factor 

of 0.19 acre-ft per person (California Department of Water 

Resources, 1994). Annual domestic pumpage estimates for 

water years 1975–2010 ranged from 12,100 in water year 1975 
Groundwater Hydrology 77

Figure 29. Groundwater-level contours for spring or fall, Santa Rosa Plain watershed, Sonoma County, California: A, spring 1951; B, 

spring 1974; C, spring 1980; D, fall 1980; E, spring 1990;. F, fall 1990; G, fall 2001;. H, spring 2007; and I, fall 2007.

Mt. St. John thrust fault 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29a

Line of equal water-level altitude, spring 1951.

 Contour interval is 20 feet. Hatchures

 indicate depressions. Modified from

 Cardwell, 1958

A

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust fault

Petrified Forest thrust zone

l l l l l l l l l

l l

80

80

80

120

120

120

160

160

200

200

400

300

300

160

80
78 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Mt. St. John thrust fault 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29b

Line of equal water-level altitude, spring 1974.

 Contour interval is 20 feet. Hatchures

 indicate depressions

B

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust fault

Petrified Forest thrust zone

l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l

200

160

160

120

120

120

160

120

80

80

80

260

Figure 29. Continued.
Groundwater Hydrology 79

Figure 29. Continued.

Mt. St. John thrust fault 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29c

Line of equal water-level altitude, spring 1980.

 Contour interval is 20 feet. Hatchures

 indicate depressions

C

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust fault

Petrified Forest thrust zone

l l l l l l

l

l

l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l

l l

l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

80

80

40

80

120

120

160

200

200

300
80 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 29. Continued.

Mt. St. John thrust fault 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29d

Line of equal water-level altitude, fall 1980.

 Contour interval is 20 feet. Hatchures

 indicate depressions

D

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust fault

Petrified Forest thrust zone

l l l l l

l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l

80

80

40

40

80

80

60

120

120

160

160

200

200

300

400
Groundwater Hydrology 81

Figure 29. Continued.

Mt. St. John thrust fault 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29e

Line of equal water-level altitude, spring 1990.

 Contour interval is 20 feet. Hatchures

 indicate depressions

E

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust fault

Petrified Forest thrust zone

l l l

l

l l l

l l

l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l

l l l l l l l l l l l

l l l l l l l l l

l l l l l l

l l l l l l l l l l l l l

80

80

80

40

120

120

120

160

200

300

400

200
82 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 29. Continued.

Mt. St. John

thrust 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29f

Line of equal water-level altitude, fall 1990.

 Contour interval is 20 feet. Hatchures

 indicate depressions

F

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust

Petrified Forest thrust zone

l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l

l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

80

80

40

80

80

0

120

160

160

120

120

200

200

300

400
Groundwater Hydrology 83

Figure 29. Continued.

Mt. St. John thrust fault

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg 

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29g

Line of equal water-level altitude, fall 2001.

Contour interval is 20 feet. Hatchures

 indicate depressions

H

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust fault

Petrified Forest thrust zone

l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

80

80

80

40

120

160 200

300

400

200
84 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 29. Continued.

Mt. St. John thrust fault 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29h

Line of equal water-level altitude, spring 2007.

 Contour interval is 20 feet. Hatchures

 indicate depressions

I

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust fault

Petrified Forest thrust zone

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l

l l l l l l l l l l

80

80

80

160

160

120

120

200

300

400

200
Groundwater Hydrology 85

Figure 29. Continued.

Mt. St. John

thrust 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B29i

Line of equal water-level altitude, fall 2007.

 Contour interval is 20 feet. Hatchures

 indicate depressions

J

Inferred

 fault

Rodgers Creek

 fault zone

EXPLANATION

Gates Canyon thrust

Petrified Forest thrust zone

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l

l l l l

l

80

60

40

80

80

160

160

120

200

200

300

400
86 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 30. Current and historical springs in the Santa Rosa Plain watershed, Sonoma County, California.

Mt. St. John thrust fault 

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Rodgers Creek

 fault zone

Spring [or seep]

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Big Spring

Hunter

Kawana Spring

Springs

IP019208_Figure B30 springs

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

Inferred

 fault

Gates Canyon thrust fault

Petrified Forest thrust zone

EXPLANATION
Groundwater Hydrology 87

to 23,400 acre-ft in water year 2002. The average total annual 

pumpage from all sources for water years 1975–2009 was 

about 47,400 acre-ft. The maximum total annual pumpage was 

about 78,500 acre-ft in water year 2008.

Groundwater Levels and Movement

Knowledge of groundwater levels and how they vary 

spatially and temporally is fundamental to understanding and 

managing water resources. The shape of the water-table and 

potentiometric surfaces, and combinations thereof, can reveal 

important hydrogeologic characteristics of the watershed, such 

as the locations of key areas of recharge and discharge, and 

geologic effects on groundwater flow. Declining long-term

hydrographs can indicate that an aquifer is being pumped in 

excess of recharge.

Basin-wide Groundwater Levels

Groundwater-level data collected by the USGS, CDWR, 

SCWA, and the municipalities were used to construct groundwater-level maps of the study area for selected seasons and 

years to show changes in levels caused by changing patterns 

of water use and weather conditions for 1951, 1974, 1980, 

1990, 2001, and 2007 (fig. 29A–I). In general, the water-level 

contours indicate that Santa Rosa Creek and tributaries gained 

water east of the Rodgers Creek fault zone, and Mark West 

and Santa Rosa creeks gained water in the western part of the 

SRP. The water-level contours are consistent with Copeland 

Creek gaining water. However, the water levels are generally 

50–230 ft bls; therefore, groundwater could not have been 

entering the stream.

1951 (Predevelopment) Conditions 

The earliest published groundwater-level data that have 

sufficient geographical distribution to allow an objective

representation of groundwater-level altitudes in the study area 

date from the late 1940s to early 1950s (Cardwell, 1958). 

Cardwell (1958) depicted groundwater-level contours across 

the SRPW and adjacent areas for spring of 1951 (reproduced 

in modified form as figure 29A). The 1951 groundwater-level 

map was based on data collected before water-resources utilization in the study area had reached an amount that significantly altered water balance for the basin (Cardwell, 1958) 

and, thus, is an approximation of pre-development conditions. 

The 1951 groundwater-level map was based on data from 

about 450 wells that ranged in depth between 24 and 1,048 ft 

(Cardwell, 1958). Most of these wells were probably perforated through more than one hydrogeologic unit; therefore, 

the measured water level was a composite of the water levels 

from each perforated unit. For the spring 1951 data, the depth 

to water beneath the valley floor in most places was between

5 and 20 ft bls (Cardwell, 1958). As stated earlier, the spring 

1951 groundwater-level contour map shows that groundwater 

moved toward, and discharged into, stream channels in most 

of the SRP, likely sustaining baseflow (fig. 29A). In addition, 

the contour map shows that, on a larger scale, groundwater flowed from the Mayacmas and Sonoma Mountains in

the UPL storage unit westward toward the Laguna de Santa 

Rosa on the western edge of the SRP and eastward from the 

highlands in the WG storage unit toward the Laguna de Santa 

Rosa.

Cardwell (1958) identified flowing wells in two parts

of the study area: (1) a 10-mi by 2-mi wide area along the 

Laguna de Santa Rosa and (2) in the VAL storage unit in 

Bennett Valley (fig. 29A). The flowing wells along and west

of the Laguna de Santa Rosa were indicative of groundwater 

upflow in this area and were consistent with the Laguna

de Santa Rosa being a focal point of pre-development 

groundwater discharge. Most of the flowing wells identified in the eastern part of the study area were located close

to mapped fault strands of the Rodgers Creek fault zone. The 

flowing wells also were within regions identified by Youngs

and others (1983) as “known warm water zones.” This observation is consistent with deep circulation of groundwater in 

some parts of the fault zones along the eastern side of SRPW 

(Cardwell, 1958).

Cardwell (1958) identified two groundwater-flow

divides. The first was located between Santa Rosa Creek and

Mark West Creek in the northern part of the basin (fig. 29A). 

The second divide was located about 1 mi southeast of Cotati, 

where the SRP narrows and terminates to the south (fig. 29A). 

Groundwater moved southwest and northwest, away from both 

divides (fig. 29A). The component of groundwater that moved 

southwest from the southern divide flowed out of the SRPW

into Petaluma Valley.

Groundwater-level altitudes beneath the Laguna de Santa 

Rosa in 1951 were the lowest of anywhere in the study area 

(fig. 29A). The Laguna de Santa Rosa is a broad area of natural 

groundwater discharge; this discharge supplied, and still 

supplies, water for high rates of ET from marshy areas and 

baseflow to the lower reaches of the Laguna de Santa Rosa

and Mark West Creek.

Groundwater levels in the Coast Ranges of California 

respond to precipitation. In many areas, groundwater levels 

can be positively correlated with the cumulative precipitation 

over several years (Faye, 1973; Farrar and Metzger, 2003). 

Precipitation records collected at Santa Rosa from 1906 to 

2010 were obtained from the California Data Exchange Center 

(http://cdec.water.ca.gov/, accessed June 2011) to evaluate the 

representativeness of the 1951 water levels (fig. 33). The total 

precipitation for Santa Rosa for water year 1951 (October 

1950 to September 1951) was 32.1 in., compared to a mean 

of 29.8 in. for the period 1906–2010 (fig. 33). Therefore, the 

amount of precipitation received in the wet season (November 

to March), prior to the time water levels were measured in 

1951, was about 8 percent above the long-term mean. However, precipitation was less than the mean for all the years 

from 1944 to 1950. The cumulative precipitation deficit for
88 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Mt. St. John thrust fault

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Rodgers Creek

 fault zone

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B31

Inferred

 fault

Public-supply well

Agricultural well

Domestic well

EXPLANATION

Gates Canyon thrust fault

Petrified Forest thrust zone

Figure 31. Location of public-supply wells in the Santa Rosa Plain watershed, Sonoma County, California.
Groundwater Hydrology 89

that seven-year period was over 41 in., which is equivalent to 

about 1.4 years of mean precipitation. The 7-year dry period 

that preceded the 1951 water year probably caused groundwater levels measured in April 1951 to be lower than average, 

but the overall effect on groundwater levels in the study area is 

uncertain.

1974 Conditions

The spring 1974 groundwater-level contour map is 

broadly similar to the 1951 map (figs. 29A and B). The contours show the dominant direction of groundwater flow was

from the east side of the plain toward the Laguna de Santa 

Rosa. However, the well-defined trough in contours near the

Laguna de Santa Rosa, evident in the 1951 map, was not well 

defined for 1974 because of the fewer number of data points

available (fig. 29B). Instead, there was a water-level depression in the southern part of the SRP near Rohnert Park-Cotati. 

Within this area, the data indicated that groundwater levels 

declined about 60 ft between 1951 and 1974 as a result of 

pumping. There was another water-level depression located 

between Mark West Creek and Santa Rosa Creek. This depression could have been caused by agricultural pumping as there 

are no municipal wells in the area. There were no depressions 

resulting from municipal pumping in the vicinities of Santa 

Rosa or Sebastopol. The spring 1974 water-level contours also 

indicated that Santa Rosa Creek could have lost water in the 

valleys and SRP (fig. 29B). The cause of Santa Rosa Creek 

becoming a losing stream is unclear, but it could have been 

caused by groundwater pumping.

1980 Conditions

The spring 1980 groundwater-level contour map indicates 

pumping depressions in the Rohnert Park-Cotati area and 

north of Sebastopol (fig. 29C). The same general distribution 

of heads as in 1974 is apparent, but the Laguna de Santa Rosa 

and Mark West Creek groundwater discharge area is better 

defined by the 60-ft contours along the west side of the map.

The pumping depression in the Rohnert Park-Cotati area, 

evident in the 1974 map, is defined by the spring 1980 data

as a more complex area of drawdown that is 20 ft deeper in 

places than in 1974 (fig. 29B and C). The spring 1980 waterlevel contours indicate that Santa Rosa Creek gained water 

east of the Rodgers Creek fault zone and that Mark West and 

Santa Rosa creeks gained water in the western end of the SRP 

(fig. 29C).

The groundwater-level contour map for fall 1980 is 

in similar shape and distribution to the spring 1980 map 

(fig. 29D); however, the fall data were measured at the end 

of a pumping and irrigation season that left groundwater 

levels generally 5 to 20 ft lower than in spring in most of 

the SRPW. In the discharge area along the Laguna de Santa 

Rosa, the fall levels were generally less than 10 ft lower than 

in spring. The pumping depression in the southern part of the 

area enlarged and deepened between spring and fall. Comparing the groundwater levels of spring 1980 to those of spring 

1951, declines of over 40 ft were measured in the Rohnert 

Park-Cotati area, and the area of decline covered about one 

quarter of the SRPW. The area of declining groundwater 

Figure 32. Total annual pumping from public-supply wells in the Santa Rosa Plain watershed, Sonoma County, California, 1975–2009.

0

1

2

3

4

5

6

7

8

9

10

11

12

200920072005200320011999199719951993

Year

IP019208_Figure B32

199119891987198519831981197919771975

Reported municipal pumpage, in thousands of acre-feet per year
90 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

levels expanded, despite the slightly above average amounts 

of precipitation received in 1978 and the 124 percent of the 

mean received in 1980 (fig. 33). A smaller area of decline can 

also be defined in the northern region near Windsor. The fall

1980 water-level contours also indicate that Santa Rosa Creek 

gained water east of the Rodgers Creek fault zone (fig. 29D).

1990 Conditions

The groundwater-level contour map for spring 1990 is 

very similar in shape and position to the spring 1980 map; 

however, the pumping depression in the southern part of the 

area is more restricted areally and appeared to have shifted 

slightly southward (fig. 29E). In addition, groundwater levels 

beneath the Laguna de Santa Rosa area were higher than in 

1980. The changes between spring 1980 and spring 1990 

indicate there was some reduction in pumping in the Rohnert 

Park-Cotati area or that recharge during the 1989–90 rainfall 

season was greater than average. An increase in recharge 

seems unlikely because only 21.4 in. of precipitation was 

received in water–year 1990, about 70 percent of the mean, 

and the prior 3 years were all well below average, with a total 

precipitation deficit of about 23.7 in., or nearly 80 percent of

the annual mean. The pumping depression expanded between 

spring and fall 1990 and deepened to include a large area 

where groundwater levels were below sea level (figs. 29E–F). 

The 1990 water-level contours indicate that Santa Rosa 

Creek gained water east of the Rodgers Creek fault zone 

(figs. 29E–F).

2001 Conditions

The spring 2001 groundwater-level contour map was 

based on measurements collected in mid-May, well past the 

rainy season and, presumably, past the peak groundwater 

levels; therefore, it is not presented. The fall 2001 groundwater-level contour map indicated that groundwater levels were 

generally lower than in fall 1990, as shown by the position 

of the 80-ft contour in the central part of the area (fig. 29G). 

The fall 2001 groundwater-level contours indicated that Santa 

Rosa Creek could be losing water east of the Rodgers Creek 

fault zone (fig. 29G). Water-year 2001 had relatively low 

precipitation (about 20 in.), which could explain Santa Rosa 

Creek losing water; however, the magnitude and duration were 

not equal to the 1976–77 or 1986–92 droughts (fig. 19C).

2007 Conditions

A comparison of the contours between spring and fall 

2007 revealed a similarity in the general shape and distribution of contours. Groundwater-level contours for spring 2007 

indicated that water levels were higher than the spring 2001 

levels and the southern pumping depression had disappeared 

(fig. 29H), indicating that long-term reductions in pumping in 

the Rohnert Park-Cotati area allowed recovery of groundwater 

levels to altitudes typical of the early 1970s. The 2007 waterlevel contours indicate that Santa Rosa Creek was gaining 

water east of the Rodgers Creek fault zone (fig. 29H–I).

Figure 33. Total annual precipitation for water-years 1906–2010, Santa Rosa Plain watershed, Sonoma County, California.

0 

10 

20 

30 

40 

50 

Long-term mean (29.8 inches)

60 

1906 1908 1910 1912 

1914 

1916 1918 1920 1922 

1924 

1926 1928 1930 1932 

1934 

1936 1938 1940 1942 

1944 

1946 1948 1950 1952 

1954 

1956 1958 1960 1962 

1964 

1966 1968 1970 1972 

1974 

1976 1978 1980 1982 

1984 

1986 1988 1990 1992 

1994 

1996 1998 2000 2002 

2004 

2006 2008 2010 

Water year

Total water year precipitation, in inches

IP019208 Figure B33

Data missing
Groundwater Hydrology 91

Water-level Changes in Monitoring Wells

Water-level data collected by the USGS, CDWR, SCWA, 

and others were used to construct water-level hydrographs for 

selected wells in the SRPW (fig. 34). Water-level hydrographs 

display the temporal variation in water levels at a particular 

location and can be used to refine the conceptualization of the

hydrologic system and to calibrate a groundwater-flow model.

Information on well construction and geology of the representative aquifer for the hydrograph wells is presented in table 6.

Windsor Basin Storage Unit

Well 7N/8W-24L1 is east of Santa Rosa in the Rodgers 

Creek fault zone in the southeastern part of the WB storage 

unit (fig. 34). Well 24L1 is perforated between 160 and 

200 ft bls in the Petaluma Formation. Water levels in well 

24L1 were fairly steady, with only 10–20 ft seasonal change in 

water levels (fig. 35A).

Wells 8N/8W-20Q1 and 29B1 are southeast of Windsor 

in the east-central part of the WB storage unit (fig. 34). Well 

20Q1 is 312 ft deep and is perforated in the Glen Ellen and 

Petaluma Formations (table 6). Well 29B1 is 64 ft deep and 

is perforated in the Glen Ellen Formation (table 6). The water 

levels in well 20Q1 were variable and showed declines in the 

late 1970s and in the early 1990s (fig. 35B). These declines 

could be responses to the 1976–77 and 1987–92 droughts 

(fig. 19C). The water levels in well 29B1 were fairly steady 

but showed seasonal variability (fig. 35C).

Well 8N/9W-13A2 is just east of Windsor in the northeastern part of the WB storage unit (fig. 34). Well 13A2 is 

109 ft deep and is perforated in the Glen Ellen Formation. 

The water levels varied over time; however, they generally 

increased about 30 ft between the late 1980s and the late 

1990s (fig. 35D). The variability in the water levels could be 

a response to climate in addition to seasonal pumping fluctuations. For example, the water-level low in 1977 corresponds 

to the drought in 1977 (fig. 19C). However, effects of the 

1987–92 drought is not evident in the water-level record.

Well 8N/9W-22R1 is southwest of Windsor in the 

western part of the WB storage unit (fig. 34) and is perforated 

between 122 and 142 ft bls in the Wilson Grove Formation. 

The water levels were fairly steady, but increased about a net 

10 ft between the early 1980s and 2010 (fig. 35E).

Well 8N/9W-36P1 is south of Windsor in the southwestern part of the WB storage unit (fig. 34) and is perforated 

between 711 and 1,010 ft bls in the Petaluma Formation. 

Overall, water levels were steady; however, the data between 

1978 and 1990 are sparse (fig. 35F). The water-level decline 

between 1976 and 1978 could be a response to the 1976–77 

drought. 

Table 6. Construction information of hydrograph wells, Santa 

Rosa Plain watershed, Sonoma County, California.

[Abbreviations: ft, feet; LSD, land surface datum; P, Petaluma Formation; 

QGE, Quaternary deposits and Glen Ellen Formation, undivided; SV, Sonoma 

Volcanics; WG, Wilson Grove Formation]

State well

identifier

LSD

(ft)

Well 

depth

(ft)

Top 

perforation

(ft)

Bottom 

perforation

(ft)

Formation

Hydrograph wells located in the Windsor Basin storage unit

7N/8W-24L01 185 200 160 200 P

8N/8W-20Q01 141 312 56 310 QGE and P

8N/8W-29B01 143 64 52 64 QGE

8N/9W-13A02 124 109 87 109 QGE

8N/9W-22R01 72 142 122 142 WG

8N/9W-36P01 94 1,010 711 1,010 P

Hydrograph wells located in the Cotati Basin storage unit

6N/8W-15J03 93 166 65 90 QGE

6N/8W-23H01 100 1,500 300 1,500 P

6N/8W-25C01 105 1

507 144 1490 P

6N/8W-26A01 101 462 288 462 P

6N/8W-26L01 99 94 54 94 QGE

6N/8W-27H01 95 82 62 82 QGE

7N/8W-21J01 124 360 148 360 P

7N/8W-35K01 128 206 185 205 P

Hydrograph wells located in the Valley storage unit

7N/7W-06H02 303 100 60 80 QGE

7N/7W-09P01 381 296 286 296 SV

7N/7W-19B01 204 85 45 85 QGE

Hydrograph wells located in the Wilson Grove storage unit

6N/9W-02C01 161 600 332 600 WG

7N/9W-15K01 71 70 59 69 WG

7N/9W-35D02 120 167 55 167 WG

Hydrograph wells located in the Uplands storage unit

6N/7W-03D01 535 254 234 253 P

6N/7W-03M01 484 61 41 61 P

Muliple-level monitoring site

6N/8W-07A04 78 80 60 80 QGE

6N/8W-07A05 78 257 237 257 P

6N/8W-07A06 78 570 550 570 WG

1

Bottom 131 ft cemented in some time between 1985 and 2002. New depth 

is 376 ft.
92 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Healdsburg fault

Rodgers Creek fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Mt. St. John thrust fault 

Gates Canyon thrust fault

Petrified Forest thrust zone

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

Santa Rosa Plain

Wilson Grove

 Formation

 Highlands

Rincon Valley

Other subbasins

Groundwater subbasin

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure B34

EXPLANATION

Well

Location of well and

 identifier—* (asterisk)

 indicates hydrograph

 shown on figure B41

Cotati Basin Valley

Upland

Wilson Grove

Windsor Basin

Groundwater storage units

Inferred fault

Rodgers Creek

 fault zone

7N/9W-35D2

6N/9W-2C1

7N/8W-35K1

N

7N/8W-24L1 7N/8W-21J1

6N/8W-27H1

6N/8W-27H1

6N/8W-26A1

6N/8W-26L1

6N/8W-25C1

6N/8W-23H1

6N/8W-15J3

6N/8W-7A4–6*

6N/8W-7A2*

6N/7W-3D1

8N/8W-29B1

8N/8W-20Q1

8N/9W-36P1

7N/9W-15K1

7N/7W-6H2

7N/7W-9P1

6N/7W-3M1

8N/9W-22R1

8N/9W-13A2

7N/7W-19B1

Figure 34. Locations of selected hydrograph wells, Santa Rosa Plain watershed, Sonoma County, California.
Groundwater Hydrology 93

200

150

50

100

Groundwater-level altitude, in feet above NGVD 29

Year

A

IP019208_Figure B35a

Total depth: 200 feet

Perforations: 160 to 200 feet

7N/8W-24L1

Land surface datum = 179 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Figure 35. Water-level data for selected wells in the Windsor Basin storage unit, 1974–2010, Santa Rosa Plain watershed, Sonoma 

County, California: A, 7N/8W-24L1; B, 8N/8W-20Q1; C, 8N/8W-29B1; D, 8N/9W-13A2; E, 8N/9W-22R1; and F, 8N/9W-36P1.

100

150

50

0

Groundwater-level altitude, in feet above NGVD 29

B

Total depth: 312 feet

Perforations: 56 to 310 feet

8N/8W-20Q1

Land surface datum = 141 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year
94 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 35. Continued.

IP019208_Figure B35c

C

50

0

100

150

Groundwater-level altitude, in feet above NGVD 29

Total depth: 64 feet

Perforations: 52 to 64 feet

8N/8W-29B1 

Land surface datum = 138 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

D

Year

50

0

100

150

Groundwater-level altitude, in feet above NGVD 29

Total depth: 109 feet

Perforations: 87 to 109 feet

8N/9W-13A2 

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Land surface datum = 124 feet
Groundwater Hydrology 95

100

150

50

0

Groundwater-level altitude, in feet above NGVD 29

E

IP019208_Figure B35e

Total depth: 145 feet

Perforations: 122 to 142 feet

8N/9W-22R1

Land surface datum = 72 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

100

–50

50

0

Groundwater-level altitude, in feet above NGVD 29

F

Total depth: 1,010 feet

Perforations: 711 to 1,010 feet

8N/9W-36P1

Land surface datum = 90 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

Figure 35. Continued.
96 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Cotati Basin Storage Unit

Wells 6N/8W-23H1, 25C1, and 26A1 are deep production wells located within 1 mi of each other in the Rohnert 

Park and Cotati area of the CB storage unit (fig. 34). Well 

23H1 is perforated from 300 to 1,500 ft bls, well 25C1 is 

perforated from 144 to 376 ft bls, and well 26A1 is perforated 

from 288 to 462 ft bls (table 6). All of three of these wells are 

perforated in the Petaluma Formation (table 6). Water levels in 

well 23H1 showed a net increase of about 40 ft between 1977 

and 2006; however, there was a water-level decline of at least 

100–150 ft through the mid-1990s that inversely corresponded 

to the pumping pattern of Rohnert Park production wells, such 

that the water levels recovered as pumping declined (figs. 36A

and 37). Water levels in well 25C1 showed a net increase of 

40–60 ft between 1977 and 2006; however, the water levels 

declined through the 1990s in response to pumping and 

recovered once pumping decreased (figs. 36B and 37). There 

was no net change in measured water levels in well 26A1 

between 1974 and 2005; however, the water levels declined 

through the 1990s in response to pumping and recovered once 

pumping decreased (figs. 36C and 37). The two deeper wells 

(23H1 and 26A1) showed similar water-level declines of about 

100–150 ft, while the water-level decline in well 25C1 was 

about 30–40 ft. These patterns could indicate that the deeper 

wells are in confined zones with lower storativity than the

shallower well and that the hydraulic connection between the 

deeper confined and shallow unconfined zones is impeded by

low permeability deposits.

Wells 6N/8W-15J3, 26L1, and 27H1 are shallow wells 

located in the Rohnert Park/Cotati area (fig. 34), and all are 

perforated in the Glen Ellen Formation. Well 15J3 is perforated between 65 and 90 ft bls, well 26L1 is perforated 

between 54 and 94 ft bls, and well 27H1 is perforated between 

62 and 82 ft bls (table 6). The water levels in well 15J3 

showed a net decline of about 5–10 ft between 1970 and 2005; 

however, water levels declined about 35 ft between 1970 and 

the mid 1990s and recovered about 25 ft by 2005 (fig. 36D). 

Water levels in well 26L1 showed no net change between 

1972 and 2004, although there was seasonal variability (fig.

36E). The water levels in well 27H1 showed a net increase of 

about 30 ft between 1989 and 2010 (fig. 36F). The water-level 

decline in well 15J3 and recoveries in wells 15J3 and 27H1 

could be in response to Rohnert Park pumping (fig. 37).

Wells 7N/8W-21J1 and 35K1 are in the Santa Rosa area 

south of the Trenton Ridge fault in the northeastern part of 

the CB storage unit, and both are perforated in the Petaluma 

Formation (fig. 34, table 6). Well 21J1 is perforated between 

148 and 360 ft bls; well 35K1 is perforated between 185 

and 205 ft bls. The water levels in these wells were steady 

or increased by a small amount. The water levels in well 

21J1 showed no net change between 1990 and 2011, but did 

show seasonal variability (fig. 36G). Water levels in well 

35K1 showed a net increase of 10 ft between 1989 and 2011 

(fig. 36H).

Valley Storage Unit

Wells 7N/7W-19B1 and 7N/7W-06H2 and 09P1 are in 

the VAL storage unit (fig. 34). Well 7N/7W-19B1 is in the 

Bennett Valley area (fig. 34) and is perforated between 45 and 

85 ft bls in the Glen Ellen Formation (table 6). The water levels in well 19B1 were fairly steady, but showed a net decline 

of about 10 ft between 1980 and 2010 (fig. 38A)

Well 7N/7W-06H2 is in the Rincon Valley (fig. 34) and is 

perforated between 60 and 80 ft bls in the Glen Ellen Formation (table 6). The water levels showed seasonal variability, 

declined around 2001, and slightly recovered around 2006 

(fig. 38B). Overall, there was a net decline in water level of 

about 10 ft between 1989 and 2011.

Well 7N/7W-09P1 is in the northern part of Kenwood 

Valley (fig. 34) and is perforated between 286 and 296 ft bls 

in the Sonoma Volcanics (table 6). Overall, the water levels 

showed a net increase of about 40 ft between 1989 and 2011 

(fig. 38C).

Wilson Grove Storage Unit

Wells 6N/9W-02C1, 7N/9W-15K1, and 35D2 are in the 

WG storage unit (fig. 34) and all are perforated in the Wilson 

Grove Formation. Well 2C1 is a production well that is 

perforated between 332 and 600 ft bls, well 15K1 is perforated between 59 and 69 ft bls, and well 35D2 is perforated 

between 55 and 167 ft bls (table 6). The water levels in well 

2C1 showed 20–40 ft of seasonal variability (80 ft maximum), 

which could be in response to pumping, and no long-term 

trend (fig. 39A). The water levels in well 15K1 showed a net 

increase of about 5 ft between 1980 and 2011 (fig. 39B). The 

water levels in well 35D2 showed no net change between 1970 

and 2005 (fig. 39C). 

Uplands Storage Unit

Wells 6N/7W-03D1 and 3M1 are in UPL storage unit 

in the Sonoma Mountains, and both are perforated in the 

Petaluma Formation (fig. 34, table 6). Well 3D1 is perforated between 234 and 253 ft bls, and well 3M1 is perforated 

between 41 and 61 ft bls (table 6). The water levels in well 

3D1 showed a long-term decline of about 30 ft (fig. 40A). 

The water levels in well 3M1 were steady (fig. 40B). The wells 

are about 0.5 mi apart; however, the water levels in well 3D1 

were more variable, and generally higher, than in well 3M1. 

Well 3M1 is next to Matanzas Creek, which could explain the 

steady nature of its water levels. In addition, the variability in 

the well 3D1 water levels could be due to pumping or climate 

variability.

Multiple-Level Monitoring Site

In 1977, SCWA constructed a multi-level monitoring 

site (6N/8W-07A4-6) on the west side of the basin in the CB 

storage unit (fig. 34). Well 7A4 is perforated between 60 and 

80 ft bls in the Glen Ellen Formation, well 7A5 is perforated 

between 237 and 257 ft bls in the Petaluma Formation, and 
Groundwater Hydrology 97

Figure 36. Water-level data for selected wells in the Cotati Basin storage unit, 1974–2010, Santa Rosa Plain watershed, Sonoma 

County, California: A, 6N/8W-23H1, B, 6N/8W-25C1, C, 6N/8W-26A1, D, 6N/8W-15J3, E, 6N/8W-26L1, F, 6N/8W-27H1, G, 7N/8W-21J1, and 

H, 7N/8W-35K1. Groundwater-level altitude, in feet above NGVD 29

A

–300

–250

–200

–150

–100

–50

0

50

6N/8W-23H1 

Total depth: 1,500 feet

Perforations: 300 to 1,500 feet

Land surface datum = 100 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

IP019208_Figure B36a
98 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 36. Continued. Groundwater-level altitude, in feet above NGVD 29

B

–100

–50

0

50

100

Total depth: 376 feet

Perforations: 144 to 376 feet

Land surface datum = 105 feet

6N/8W-25C1 

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

IP019208_Figure B36b

Groundwater-level altitude, in feet above NGVD 29

C

–200

–150

–100

–50

0

50

100

Total depth: 462 feet

Perforations: 288 to 462 feet

Land surface datum = 101 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

6N/8W-26A1 
Groundwater Hydrology 99

Figure 36. Continued. Groundwater-level altitude, in feet above NGVD 29

D

6N/8W-15J3 

IP019208_Figure B36d

0

50

100

150

1970 1974 1978 1982 1986 1990 

Year 

1994 1998 2002 2006 2010 

Total depth: 166 feet

Perforations: 65 to 90 feet

Land surface datum = 95 feet

0

50

100

150

Groundwater-level altitude, in feet above NGVD 29

6N/8W-26L1

Total depth: 94 feet

Perforations: 54 to 94 feet

Land surface datum = 100 feet

E

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year
100 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 36. Continued.

IP019208_Figure B36f

100

150

0

50

Groundwater-level altitude, in feet above NGVD 29

F

Total depth: 82 feet

Perforations: 62 to 82 feet

6N/8W-27H1 

Land surface datum = 97 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

100

150

0

50

Groundwater-level altitude, in feet above NGVD 29

G

Total depth: 360 feet

Perforations: 148 to 360 feet

7N/8W-21J1

Land surface datum = 122 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year
Groundwater Hydrology 101

100

150

0

50

Groundwater-level altitude, in feet above NGVD 29

H

IP019208_Figure B36h

Total depth: 206 feet

Perforations: 185 to 205 feet

7N/8W-35K1

Land surface datum = 127 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

Figure 36. Continued.
102 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 37. Total annual pumping from city of Rohnert Park production wells, Santa Rosa Plain watershed, Sonoma County, California, 

1973–2009.

0

1,000

2,000

3,000

4,000

5,000

6,000

1970 1975 1980 1985 1990

Year

1995 2000 2005 2010

Total annual pumpage from Rohnert Park wells, in acre-feet per year

IP019208_Figure B37
Groundwater Hydrology 103

Figure 38. Water-level data for selected wells in the Valley storage unit, 1974–2010, Santa Rosa Plain watershed, Sonoma County, 

California: A, 7N/7W-19B1, B, 7N/7W-06H2, and C, 7N/7W-09P1.

Year

200

250

100

150

Groundwater-level altitude, in feet above NGVD 29

A

IP019208_Figure B38a

Total depth: 85 feet

Perforations: 45 to 85 feet

7N/7W-19B1

Land surface datum = 204 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

300

350

200

250

Groundwater-level altitude, in feet above NGVD 29

B

Total depth: 100 feet

Perforations: 60 to 80 feet

7N/7W-6H2 

Land surface datum = 295 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year
104 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Figure 38. Continued.

400

300

350

250

Groundwater-level altitude, in feet above NGVD 29

C

IP019208_Figure B38c

Total depth: 296 feet

Perforations: 286 to 296 feet

7N/7W-9P1

Land surface datum = 382 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

100

150

50

0

Groundwater-level altitude, in feet above NGVD 29

A

Total depth: 600 feet

Perforations: 332 to 600 feet

6N/9W-2C1

Land surface datum = 161 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

Figure 39. Water-level data for selected wells in the Wilson Grove storage unit, 1974–2010, Santa Rosa Plain watershed, Sonoma 

County, California: A, 6N/9W-02C1, B, 7N/9W-15K1, and C, 7N/9W-35D2.
Groundwater Hydrology 105

0

50

100

150

Groundwater-level altitude, in feet above NGVD 29

IP019208_Figure B39b

B

Total depth: 90 feet

Perforations: 59 to 69 feet

7N/9W-15K1

Land surface datum = 71 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

Year

100

150

0

50

Groundwater-level altitude, in feet above NGVD 29

C

Total depth: 167 feet

Perforations: 55 to 167 feet

7N/9W-35D2

1970 1974 1978 1982 1986 1990 1994 1999 2002 2006 2010

Land surface datum = 135 feet

Figure 39. Continued.
106 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California Groundwater-level altitude, in feet above NGVD 29

A

IP019208_Figure B40a

400

450

500

550

Total depth: 254 feet

Perforations: 234 to 253 feet

6N/7W-3D1 

Land surface datum = 525 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

Groundwater-level altitude, in feet above NGVD 29

B

400

450

500

550

Total depth: 61 feet

Perforations: 51 to 61 feet

6N/7W-3M1 

Land surface datum = 480 feet

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 

Year

Figure 40. Water-level data for selected wells in the Uplands storage unit, 1974–2010, Santa Rosa Plain watershed, Sonoma County, 

California: A, 6N/7W-03D1 and B, 6N/7W-03M1.
Summary 107

well 7A6 is perforated between 550 and 570 ft bls in the Wilson Grove Formation (table 6). Well 6N/8W-07A2, a SCWA 

production well installed in late 1999, is about 360 ft to the 

northwest of the multi-level monitoring site (fig. 34). The production well is perforated from 650 to 800 ft bls in the Wilson 

Grove Formation. Comparison of the water-level hydrographs 

of the piezometers to the annual pumpage from the production 

well showed that pumping from well 7A2 affects water-levels 

in the piezometers differently (fig. 41). The data indicated 

that the water levels in the shallowest piezometer respond 

the least to pumping from well 7A2 (fig. 41). This could be 

the result of low permeability deposits in the Glen Ellen and 

Petaluma Formations, the unconfined storage properties of the

shallow aquifer, or both. The two deeper piezometers, however, particularly the 570-ft piezometer, have strong hydraulic 

communication with the pumping well, indicating that the 

570-ft piezometer and the pumping well are perforated in the 

same hydrogeologic unit, which is under confined conditions.

The data also indicated a downward vertical gradient between 

well 7A4 and the deeper wells; prior to pumping, the vertical 

gradients between 7A4 and 7A5 and between 7A4 and 7A6 

were about 0.11 and 0.09, respectively. After pumping started, 

the vertical gradients increased to about 0.28 and 0.31 between 

the same respective wells.

Summary

The principal water-bearing strata in the study area 

include the Tertiary and Quaternary sediments and sedimentary rocks underlying the SRP floor to depths of at least

1,500 ft, as well as permeable zones in Tertiary volcanic 

rocks in contact with the sedimentary section along the valley 

margin and interfingered with it beneath the valley floor. The

Tertiary material includes the Glen Ellen, Wilson Grove, and 

Petaluma Formations, and the Sonoma Volcanics. Geophysical investigations have identified two deep basins beneath

SRP: the Windsor Basin to the north is about 1.2 mi deep, 

and the Cotati Basin to the south is about 2 mi deep. Geologic 

materials within the depth range perforated by wells are most 

commonly fine-grained or poorly-sorted sediments that have

low hydraulic conductivities and specific yields that result in

low to modest well yields and large drawdowns. In most parts 

of the study area, shallow groundwater flow is unconfined, and

flow is confined at depth. The presence of fine-grained material—either as interbeds within coarser-grained materials or as

thick, but not laterally extensive, beds—imparts anisotropic

hydraulic characteristics typical of layered systems.

Figure 41. Water-level data for 6N/8W-07A4-6, 1977–2007, and daily pumpage data for 6N/8W-07A2, 2000–2007, Santa Rosa Plain, 

Sonoma County, California.

0

100

200

300

400

500

600

700

800

900

1,000

-100

-80

-60

-40

-20

0

20

40

60

80

100

1977 1979 1981 1983 1985 1987 1989 1991

Year

1993 1995 1997 1999 2001 2003 2005 2007

Annual pumpage, in millions of gallons

Groundwater-level altitude, in feet

IP019208_Figure B41

80 feet

257 feet

570 feet

Pumpage in millions

 of gallons

Well 6N/8W-7A4-6

 Piezometers at

Well 6N/8W-7A2
108 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

The available surface-water records indicate that a high 

percentage of streamflow is generated as overland flow, with

a relatively fast response time to the larger storm events. 

The streamflow is highly seasonal, with peak flows limited to

winter. Summer flows are low (less than 5 ft3/s). In general, 

December, January, and February are the months with the 

highest streamflow, while August and September are the driest

months. Year to year variability in streamflow is also high, in

terms of total annual flow, peak annual flow, and average summer flows. The primary source of the summer flow is likely

baseflow from groundwater discharge, although irrigation and

urban-area runoff also can contribute. Many of the streams are 

intermittent, with no flow during the summer; however, during

drier than normal periods, normally perennial streams can 

become dry by late summer and early fall.

The 1951 (predevelopment) water-level contour map 

shows that groundwater flowed from the Mayacmas and

Sonoma Mountains in the UPL storage unit westward toward 

the Laguna de Santa Rosa on the western edge of the SRP and 

eastward from the highlands in the WG storage unit toward the 

Laguna de Santa Rosa. The groundwater-level contour map 

also shows that groundwater moved toward, and discharged 

into, the stream channels, thereby, likely sustaining baseflow.

In the WB storage unit, water levels were fairly steady 

with some seasonal variability, and some hydrographs showed 

the effects of the 1976–77 and 1986–92 droughts. In the CB 

storage unit, water levels from wells in the area near Rohnert 

Park recovered to 1974 conditions by 2007 as a result of a 

large reduction in municipal pumping that began in 2000. In 

the VAL storage unit, water levels were fairly steady with 

some seasonal variability in the Bennett and Rincon valleys; 

however, water levels increased in the Kenwood Valley. In 

the WG storage unit, water levels showed seasonal variability 

but, generally, had no long-term trends. In the UPL storage 

unit, the water levels in the deeper well showed a long-term 

decline, while the water levels in the shallow well were steady. 

Sparse vertical profile data indicated that shallower wells have

limited hydraulic communication with deeper aquifer material; 

this could be caused by the extensive clays in the Petaluma 

Formation and indicate confined aquifer conditions at depth.

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112 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California
Chapter C. Groundwater Quality and Source and Age of 

Groundwater in the Santa Rosa Plain Watershed, Sonoma 

County, California

By Peter Martin, Loren F. Metzger, Jill N. Densmore, and Roy A. Schroeder

Introduction

Groundwater-quality and source and age of groundwater 

were characterized for the Santa Rosa Plain (SRP) and the surrounding area within the Santa Rosa Plain watershed (SRPW) 

by using analyses for selected physical properties, inorganic 

constituents, and stable and radioactive isotopes. This chapter 

describes (1) the areal, depth-dependent, and temporal variations in groundwater quality; (2) groundwater-quality constituents of particular concern for drinking water and irrigation; 

and (3) the recharge source and age of groundwater. Data used 

in this characterization were compiled from previous investigations; from databases maintained by the California Department of Public Health (CDPH), the California Department of 

Water Resources (CDWR), and various public-supply purveyors; and from analyses from surface-water sites and groundwater wells sampled either as part of this study or concurrently 

by private consultants. These data represent untreated water 

samples, which are not equivalent to analyses used for compliance to drinking-water quality standards. To place these results 

within a health-based context, however, these untreated water 

analyses were compared with federal (U.S. Environmental 

Protection Agency, or USEPA) and state (CDPH) drinkingwater regulatory standards. As with chapters A and B, groundwater conditions in the SRP portion of the study area are the 

primary focus of this chapter. For the purposes of this chapter, 

specific surface-water sites, springs, and groundwater wells

are identified by the abbreviations “SW,” “SPR,” and “W,”

respectively.

Previous Investigations 

One of the earliest published geohydrologic investigations of the SRPW to include water quality was by Cardwell 

(1958). His study included a limited assessment of water quality based on analyses from 200 wells, of which 80 analyses 

were relatively complete for major ions. Cardwell (1958) characterized the quality of groundwater in the SRPW as generally satisfactory for most uses on the basis of dissolved solids 

and hardness concentrations in the range of 250 to 350 mg/L 

and 60 to 160 mg/L, respectively. However, Cardwell (1958) 

noted several areas where concentrations of boron, iron, and 

manganese were high enough to classify water as being of 

unsatisfactory quality without treatment. Boron was cited as 

a constituent of particular concern on the western side of the 

Windsor Basin in samples from several irrigation wells perforated in the Glen Ellen Formation. Cardwell (1958) hypothesized that boron concentrations of up to 2,200 micrograms per 

liter (μg/L) in well samples from the area of township, range, 

and section (T/R/S) 8N/9W-23 could be attributable to deeply 

circulating groundwater rising along a fault zone. Iron and 

manganese were noted to be high (greater than the 1946 drinking water standard of 300 μg/L for both elements) in localized 

areas of the Wilson Grove Formation (formerly known as the 

Merced Formation; Fox, 1983). High concentrations of iron 

and manganese were especially prevalent in samples from 

wells perforated in thick sections of blue (unoxidized) sandstone and where the Wilson Grove Formation crops out on the 

western side of the SRP (Cardwell, 1958).

Water-quality conditions in the SRPW were described in 

detail in several reports by the CDWR. Ford (1975) described 

that groundwater in localized areas was of poor quality for 

agricultural and domestic use because of elevated concentrations of boron, sodium, iron, manganese, and dissolved solids. 

Herbst and others (1982), characterized groundwater quality 

by type based on major-ion composition and grouped water 

samples by geologic formations. Hardness was described by 

Herbst and others (1982) as one of the most widespread waterquality problems in the SRP as a result of an area of soft water 

(0 to 60 mg/L as calcium carbonate) northwest of the city 

of Santa Rosa and hard water (121 to 180 mg/L as calcium 

carbonate) to the west and southwest of the city of Santa Rosa. 

Areas of high-dissolved solids concentrations were identified in water from wells in the Rohnert Park and Cotati area

(6N/7W-18 and 6N/8W-16 and -26), north of Santa Rosa 

(7N/8W-13C), and east of Sebastopol (7N/8W-29 and -30). As 

with previous reports, boron was cited as a constituent of particular concern. High concentrations of boron were associated 

with sodium-bicarbonate type water, particularly along the 

Rodgers Creek fault and along an area that Herd and Helley 

(1976) described as a possible branch of the Rodgers Creek 
114 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

fault in the southeast portion of 8N/9W. This hypothesized 

branch of the Rodgers Creek fault coincides with the western 

end of the shallow west-northwest-striking Trenton Ridge 

fault, which was described earlier in this report (chapter B). 

Boron also was reported as being widespread in the Windsor 

area in samples from relatively shallow depths of about 60 

to 150 feet below land surface (ft bls), but at concentrations 

generally less than 500 μg/L (Herbst and others, 1982).

In 2000, the California State Water Resources Control 

Board (SWRCB), the U.S. Geological Survey (USGS), and the 

Lawrence Livermore National Laboratory (LLNL) initiated 

the Groundwater Ambient Monitoring Assessment (GAMA) 

Program to assess the quality of ambient groundwater in aquifers used for drinking-water supply and to establish a baseline groundwater-quality monitoring program. The GAMA 

Program partitioned California into 10 hydrogeologic provinces and 35 study units within these provinces (Belitz and 

others, 2003). Within each study unit, statistical and graphical methods to explain relations between water quality and 

various causative factors, such as well depth, groundwater 

age, oxidation-reduction (redox) status of the subsurface, 

and position along a conceptual flowpath (Belitz and others,

2003). One such study unit was the North San Francisco Bay 

(NSF) Study Unit, which includes the SRPW (Kulongoski 

and others, 2006; 2010). In 2004, samples of untreated water 

were collected from 89 wells in the NSF Study Unit, including 

28 wells from the SRPW. As with other GAMA study units, 

analysis of samples collected from wells in the SRPW focused 

primarily on organic constituents, including volatile organic 

compounds (VOCs), pesticides and pesticide degradates, 

and organic wastewater indicators (Kulongoski and others, 

2006). Fewer wells (five) within the SRPW were sampled for

inorganic constituents (major and minor elements and nutrients). Stable and radioactive isotopes were used to characterize the sources or ages of groundwater, including samples 

collected from 25 wells for oxygen (18O) and hydrogen (2H), 

27 wells for tritium (3H), and 5 wells for carbon-14 (14C) and 

carbon-13 (13C) (Kulongoski and others, 2006). Results for 

the NSF Study Unit as a whole, showed that no anthropogenic constituents were detected at concentrations higher than 

regulatory thresholds, and few naturally occurring constituents 

were detected at concentrations above regulatory thresholds 

(Kulongoski and others, 2006 and 2010).

Historical Monitoring

Groundwater quality in the SRPW has been monitored 

since at least 1947 to ensure minimum quality standards are 

met for a variety of water demands, including domestic, agricultural, and industrial use. The monitoring of groundwater for 

these various purposes has primarily focused on physical properties and inorganic constituents, such as specific conductance,

pH, temperature, dissolved solids, hardness, major ions, nutrients, and trace (minor) elements. In the last several decades, 

however, attention has shifted to include organic constituents 

such as VOCs, pesticides, organic wastewater indicators, 

and other constituents of potential concern associated with 

anthropogenic activities. For this study, analyses completed 

on 2 springs and 162 wells by various agencies were compiled 

to help describe the groundwater quality of the SRPW (fig. 1, 

table 1, and appendix A).

Water-quality sampling of springs in the SRPW consists of one analysis from each of two springs located in the 

Mayacmas Mountains (fig. 1A and appendix A). These two 

analyses from the mid-1970s include physical properties and 

inorganic constituents (major ions, selected trace elements, 

and nutrients).

Groundwater-quality data from wells in the SRPW 

includes one-time sampling for short-term studies and multiple 

samples collected over time from selected wells. The longest 

sustained water-quality monitoring effort in the SRPW has 

been done by the CDWR. Since the late 1940s, the CDWR has 

sampled and analyzed selected wells for major ions (calcium, 

magnesium, potassium, sodium, chloride, and sulfate), boron, 

nitrate, total dissolved solids, alkalinity, specific conductance,

pH, and water temperature.

Methods of Sample Collection and 

Analysis

The methods for the collection and analysis of surfacewater and groundwater samples for this study (collected 

between 2006 and 2010, referred to as recent samples) and for 

previous studies (primarily collected prior to 2006, referred to 

as historical samples) by the CDWR, the CDPH, and publicsupply purveyors have varied in accordance with different 

sampling objectives and in response to improvements in 

sampling protocols and analytical techniques. The sampling 

protocols and analytical techniques for the recent and historical samples are briefly described in the following sections.

The sampling protocols and analytical techniques for samples 

collected and analyzed as part of the NSF GAMA study are 

described in Kulongoski and others (2006). All non-USGS 

laboratories used as sources of data in this report have met the 

requirements of California Code of Regulations Title 22, Division 4, Chapter 19, on the certification of environmental laboratories, including successful participation in the California 

Environmental Laboratory Accreditation Program (ELAP).

Recent Sample Collection and Analysis

Fifteen surface-water samples (fig. 1A) were collected 

for the analysis of the stable isotopes of oxygen (18O) and 

hydrogen (2H) in the SRPW and from neighboring areas to the 

west of the study area (SW14 and SW18) to help determine 

the source of groundwater to the SRPW. Specific conductance

and water temperature were measured at the time of sample 

collection. For 18O and 2

H, unfiltered water samples were
Introduction 115

i

122°50’ 122°40’

38°

30’

38°

20’

12

116

101

R9W 

T

9

N 

R8W R7W R6W 

T

8

N 

T

7

N 

T

6

N 

T

5

N 

Russian

 River

Mark West Creek

Brush Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe

 Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

Mendocino Range

Sonoma Mountains

Mayacmas Mountains

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

0

0 5 10 Kilometers

5 10 Miles

IP019208_Figure C01a

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, Albers Equal Area Conic Projection EXPLANATION

Spring

Surface-water site

Cotati Basin Valley

Upland

Windsor Basin Wilson Grove

Groundwater storage units

SW4

SPR2

A

Rodgers Creek

 fault zone

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Mt. St. John thrust fault

Rodgers Creek fault

Inferred fault

Gates Canyon thrust fault

Petrified Forest thrust zone

SW17

SW15

SW3

SW5

SW6

SW8

SW9

SW13 SW7

SW14

SW2 SW1

SW10

SW21

SW18

SPR1 SPR2

Uplands

Uplands

Valley

Windsor

Basin

Wilson Grove

Cotati

Basin

Figure 1. Locations of A, surface-water sites and springs and B, wells sampled in the Santa Rosa Plain watershed, Sonoma County, 

California, 1947–2010.
116 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

i

122°50’ 122°40’

38°

30’

38°

20’

12

116

101

R9W 

T

9

N 

R8W R7W R6W 

T

8

N 

T

7

N 

T

6

N 

T

5

N 

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe

 Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

Mendocino Range

Sonoma Mountains

Mayacmas Mountains

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

0

0 5 10 Kilometers

5 10 Miles

IP019208_Figure C01b

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, Albers Equal Area Conic Projection EXPLANATION

Well used for

 section B-B’

Cotati Basin

Line of section

Valley

Upland

Windsor Basin Wilson Grove

Groundwater storage units

B B’

Shallow (0’ to 150’ below

 land surface datum)

Mixed

Deep (greater than 150’)

Construction unknown

Well perforated interval

B

Rodgers Creek

 fault zone

Inferred fault

B'

B

Upland

Upland

Valley

Windsor

Wilson Grove

Cotati

W8 W9

W7

W6

W5

W4

W3

W2

W1

W98

W97

W95

W94 W96

W93 W92 W91

W90

W89

W85

W84

W88

W87

W83

W101

W100

W82

W82

W81

W80

W79

W75

W74

W73

W71-72

W70

W68

W69

W67

W66

W63

W59

W49 W51 W55

W48

W47 W46

W45

W31

W29

W26

W27

W21

W20

W10

W162

W161

W160

W159

W156

W155

W152

W144 W148

W137

W134

W133

W131

W132

W130

W124

W121-122

W120

W117

W115

W107

W106

W123

W125

W105

W104

W102

W50

W42

W22

W62

W64

W65

W58

W52

W54

W41

W36 W44

W37

W38

W39-40

W57

W56

W43

W53

W61

W19

W17

W18

W15

W16

W12 W13 W11

W14

W32

W30

W33-35

W78

W76-77

W126-127

W128-9

W114

W113

W111

W110

W108

W112

W109

W116

W119 W28

W23

W24-25

W103

W99

W146

W147

W145

W139

W138

W140

W141–143

W136

W135

W157

W153

W151

W154

W158

W150

W149

W60

Figure 1. Continued.
Introduction 117

Table 1. Construction information for selected wells used to obtain water-quality data in the Santa Rosa Plain watershed, Sonoma County, California.—Continued[See figure 1 for locations (map numbers) of wells, springs, and surface-water sites. U.S. Geological Survey (USGS) site identification number is the unique number for each site in USGS National WaterInformation System (NWIS) database. Depths in feet below land surface (fbls). Depth Category based on average of perforated interval. Land surface altitude, where available, in feet above sea level which refers to the North American Vertical Datum of 1988 (NAVD of 1988). Abbreviations: CDPH, California Department of Public Health; CDWR, California Department of Water Resources; GAMA, Groundwater Ambient Monitoring and Assessment; owner, data provided by well owner; Unk, unknown; USGS-GAMA, Groundwater Ambient Monitoring and Assessment Program (USGS and California State Water Resources Control Board); <, actual value is less than value shown; —, no data]

Map

 number

Groundwater

storage

unit

State well

number

USGS site

identification

number

GAMA

identification

number

Depth

drilled

(fbls)

Depth

cased

(fbls)

Depth

 of top

perforation

(fbls)

Depth

 of bottom

perforation

(fbls)

Depth

category

Land

surface

altitude1

Period

of data

included

in this

report

Water quality

data source

W1 Wilson Grove 5N/8W-03G1 381827122434601 NSFVP-04 397 397 — — Unk 211 2004 USGS-GAMAW2 Uplands 6N/7W-05A1 — — 120 2120 — — Shallow — 1960 CDWR

W3 Uplands — — — 306 306 127 306 Mixed — 1996–2002 CDPH

W4 Cotati 6N/7W-17E1 — — 650 2650 — — Unk — 1957–1963 CDWR

W5 Cotati — — — 600 600 323 580 Deep — 1987 CDPH

W6 Cotati — — — 602 560 297 540 Deep — 1988 CDPH

W7 Cotati 6N/7W-18R1 382138122393901 — 250 250 18 240 Mixed 158 1957–2007 CDWR

W8 Cotati — — — 370 364 242 344 Deep — 1980–2007 CDPH, CDWR

W9 Uplands 6N/7W-23H1 382038122354001 NSFVOL-10 140 136 76 136 Shallow 1,704 2004–2006 CDPH, USGS-GAMA

W10 Cotati 6N/7W-28L1 382009122382601 NSFVP-01 921 864 441 862 Deep 331 1984–2009 CDPH, CDWR, USGS-GAMA

W11 Cotati — — — 325 325 100 — Unk — 1974 CDPH, CDWR

W12 Cotati — — — 430 415 225 395 Deep — 2005 CDPH

W13 Cotati — — — 417 400 100 390 Mixed — 2005–2008 CDPH

W14 Cotati 6N/7W-30D1 — — — — — — Unk — 1957–1961 CDWR

W15 Cotati — — — 470 420 120 380 Mixed 109 1982–2009 CDPH, CDWR, owner

W16 Cotati — — — 526 401 238 398 Deep — 2000–2009 CDPH

W17 Cotati — — — 689 498 220 480 Deep — 2000–2006 CDPH

W18 Cotati — — — 726 387 130 380 Mixed — 1989–2002 CDPH

W19 Cotati — — — 715 705 210 695 Deep — 1991 CDPH

W20 Cotati 6N/7W-30R1 381955122395901 — 150 150 120 150 Shallow 178 1976–2002 CDWR

W21 Cotati 6N/7W-31C1 381934122403601 NSFVP-06 716 700 170 680 Deep 144 1998–2007 CDPH, USGS-GAMAW22 Cotati — — — 700 603 160 590 Deep — 1998–2007 CDPH

W23 Cotati 6N/8W-02E1 382349122424201 — 172 172 167 172 Deep 114 1974–1984 CDWRW24 Cotati — — — — — — — Unk — 1994 CDPH

W25 Cotati — — — — — — — Unk — 1994 CDPH

W26 Cotati 6N/8W-02J2 382337122422902 NSFVP-23 530 530 369 530 Deep 115 2004 USGS-GAMAW27 Cotati — — — 900 900 274 880 Deep 108 2006 CDPH

W28 Cotati 6N/8W-03B1 382402122434401 — 60 60 50 60 Shallow 109 1957–2007 CDWR

W29 Cotati — — — — — — — Unk — 1996 CDPH

W30 Cotati 6N/8W-04R1 382315122443001 NSFVP-03 120 120 55 85 Shallow 95 2004 USGS-GAMA

W31 Cotati 6N/8W-05E1 — — 638 638 20 638 Mixed 3 90 1951 CDWR

W32 Cotati 6N/8W-07A2 382307122463801 NSFVP-12 815 808 650 800 Deep 82 1982–2009 CDPH, CDWR, USGS-GAMA, 

owner

Table 1. Construction information for selected wells used to obtain water-quality data in the Santa Rosa Plain watershed, Sonoma County, California.
118 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California Table 1. Construction information for selected wells used to obtain water-quality data in the Santa Rosa Plain watershed, Sonoma County, California.—Continued [See figure 1 for locations (map numbers) of wells, springs, and surface-water sites. U.S. Geological Survey (USGS) site identification number is the unique number for each site in USGS National

Water

Information System (NWIS) database. Depths in feet below land surface (fbls). Depth Category based on average of perforated interval. Land surface altitude, where available, in feet above sea level which refers to the North American Vertical Datum of 1988 (NAVD of 1988). Abbreviations: CDPH, California Department of Public Health; CDWR, California Department of Water Resources; GAMA, Groundwater Ambient Monitoring and Assessment; owner, data provided by well owner; Unk, unknown; USGS-GAMA, Groundwater Ambient Monitoring and Assessment Program (USGS and California State

Water Resources Control Board); <, actual value is less than value shown; —, no data]

Map

 number

Groundwater

storage

unit

State well

number

USGS site

identification

number

GAMA

identification

number

Depth

drilled

(fbls)

Depth

cased

(fbls)

Depth

 of top

perforation

(fbls)

Depth

 of bottom

perforation

(fbls)

Depth

category

Land

surface

altitude1

Period

of data

included

in this

report

Water quality

data source

W33 Cotati 6N/8W-07A4 382308122463903

— 900 80 60 80 Shallow 81 2007 USGS

W34 Cotati 6N/8W-07A5 382308122463902

— 900 257 237 257 Deep 81 2007 USGS

W35 Cotati 6N/8W-07A6 382308122463901

— 900 570 550 570 Deep 81 2007 USGS

W36 Cotati

—

—

— 615 502 302 462 Deep 103 1988–1989 CDPH, CDWR

W37 Cotati

—

—

— 520 510 120 490 Mixed

— 1998–2007 CDPH, CDWR

W38 Cotati

—

—

— 600 510 110 510 Mixed

— 1988–2009 CDPH

W39 Cotati

—

—

— 411 411 38 411 Mixed

— 1996 CDPH

W40 Cotati

—

—

— 606 445 300 440 Deep

— 2002–2008 CDPH

W41 Cotati

—

—

— 580 580 224 490 Deep 107 1976–2009 CDPH, CDWR, owner

W42 Cotati

—

—

— 624 562 298 522 Deep

— 1980–2007 CDPH, CDWR

W43 Cotati

—

—

— 1,328 1,290 275 1,275 Deep 113 1987–2005 CDPH, CDWR

W44 Cotati 6N/8W-13R3 382141122410901 NSFVP-05 600 502 130 450 Mixed 115 1988–2009 CDPH, USGS-GAMA

W45 Cotati

—

—

— 1,512 1,501 351 1,491 Deep 100 1981–2009 CDPH, CDWR

W46 Cotati

—

—

— 240 240 120 240 Mixed

— 1991 CDPH

W47 Cotati 6N/8W-16G14 —

—

—

—

—

— Unk

— 1959 CDWR

W48 Cotati 6N/8W-16R1

—

— 1,204 1,204

— 1,204 Unk

— 1958–1961 CDWR

W49 Wilson Grove

—

—

— 310 300 204 300 Deep

— 1996 CDPH

W50 Cotati

—

—

— 721 715 175 675 Deep

— 1999–2008 CDPH

W51 Cotati

—

—

— 610 590 190 580 Deep

— 1988–1997 CDPH

W52 Cotati

—

—

— 1,504 1,500 300 1,500 Deep 103 1987–2008 CDPH, CDWR

W53 Cotati

—

—

— 614 614 260 594 Deep

— 1998–2007 CDPH

W54 Cotati

—

—

— 496 496 224 494 Deep 106 1981–2008 CDPH, CDWR

W55 Cotati

—

—

— 725 424 161 421 Deep

— 2000–2009 CDPH

W56 Cotati

—

—

— 510 510 200 450 Deep 113 2002–2008 CDPH

W57 Cotati

—

—

— 460 405 190

— Deep

— 1980–2007 CDPH, CDWR, owner

W58 Cotati

—

—

— 630 458 265 458 Deep 104 1982–2009 CDPH, CDWR, owner

W59 Cotati

—

—

— 503 503

—

— Unk 108 1987–2008 CDPH, CDWR

W60 Cotati

—

—

— 500 500 118 478 Mixed 106 1982–2009 CDPH, CDWR, owner

W61 Cotati

—

—

— 710 688 156 666 Deep

— 2007 CDPH

W62 Cotati

—

—

— 610 500 60 425 Mixed 109 1988–2000 CDPH, CDWR

W63 Cotati 6N/8W-264 —

—

—

—

—

— Unk

— 1956 CDWR

W64 Cotati

—

—

— 462 462 288 462 Deep 100 1982–2009 CDPH, CDWR, owner

W65 Cotati

—

—

— 473 473 160 463 Deep 103 1982–2009 CDPH, CDWR, owner
Introduction 119

Table 1. Construction information for selected wells used to obtain water-quality data in the Santa Rosa Plain watershed, Sonoma County, California.—Continued[See figure 1 for locations (map numbers) of wells, springs, and surface-water sites. U.S. Geological Survey (USGS) site identification number is the unique number for each site in USGS National WaterInformation System (NWIS) database. Depths in feet below land surface (fbls). Depth Category based on average of perforated interval. Land surface altitude, where available, in feet above sea level which refers to the North American Vertical Datum of 1988 (NAVD of 1988). Abbreviations: CDPH, California Department of Public Health; CDWR, California Department of Water Resources; GAMA, Groundwater Ambient Monitoring and Assessment; owner, data provided by well owner; Unk, unknown; USGS-GAMA, Groundwater Ambient Monitoring and Assessment Program (USGS and California State Water Resources Control Board); <, actual value is less than value shown; —, no data]

Map

 number

Groundwater

storage

unit

State well

number

USGS site

identification

number

GAMA

identification

number

Depth

drilled

(fbls)

Depth

cased

(fbls)

Depth

 of top

perforation

(fbls)

Depth

 of bottom

perforation

(fbls)

Depth

category

Land

surface

altitude1

Period

of data

included

in this

report

Water quality

data source

W66 Cotati — — — 603 475 128 460 Mixed 102 1998–2007 CDPH, CDWR

W67 Cotati 6N/8W-26C2 382021122425701 NSFVP-07 795 685 295 670 Deep 95 1988–2009 CDPH / USGS

W68 Cotati — — — 350 337 135 337 Mixed — 1996 CDPH

W69 Wilson Grove 6N/8W-33M1 381915122452701 NSFWilson 

Grove-12

261 261 100 128 Shallow 203 2004 CDPH, USGS-GAMA

W70 Cotati — — — 660 552 370 400 Deep 100 1957–1987 CDPH, CDWR

W71 Cotati — — — 510 500 125 490 Mixed 120 1999–2008 CDPH, CDWR

W72 Cotati — — — 642 642 520 — Deep — 1996–2005 CDPH

W73 Cotati — — — 515 500 220 485 Deep — 1976–2008 CDPH, CDWR, owner

W74 Wilson Grove — — — 1,015 572 172 552 Deep — 1974–2009 CDPH, CDWR

W75 Wilson Grove 6N/9W-02B1 382400122491201 NSFWilson 

GroveFP-01

647 528 138 528 Mixed 83 2004 CDPH, CDWR, USGS-GAMA

W76 Wilson Grove 6N/9W-02C1 382352122493301 NSFWilson 

Grove-08

600 600 332 600 Deep 115 1960–2004 CDPH, CDWR, USGS-GAMA

W77A Wilson Grove 6N/9W-02C2 382352122493801 — 660 650 100 6120 Shallow 116 2006 USGS

W77B Wilson Grove 6N/9W-02C2 382352122493801 — 660 650 6340 6360 Deep 116 2006 USGSW77C Wilson Grove 6N/9W-02C2 382352122493801 — 660 650 6640 650 Deep 116 2006 USGSW78 Wilson Grove 6N/9W-02G1 — — 552 2552 — — Unk — 1957–1964 CDWR

W79 Wilson Grove — — — 776 530 237 468 Deep 83 1975–2008 CDPH, CDWR

W80 Wilson Grove 6N/9W-03R84 — — 280 2280 — — Unk — 1973 CDWR

W81 Wilson Grove 6N/9W-13F14 — — 220 2220 — — Unk — 1952 CDWR

W82 Wilson Grove 6N/9W-13J2 382153122480301 NSFWilson 

Grove-04

452 432 432 452 Deep 197 1997 CDPH, USGS-GAMA

W83 Valley 7N/7W-07K1 382754122402501 NSFVP-25 265 265 160 265 Deep 236 2004 USGS-GAMA

W84 Valley 7N/7W-14P1 — — 828 812 111 812 Mixed — 1961–1963 CDWR

W85 Valley 7N/7W-15C1 382737122372501 — — — — — Unk 378 1957–2007 CDWR

W86 Valley 7N/7W-17A2 382730122390201 — 1,018 1,008 84 1,008 Mixed 300 2007 USGS

W87 Valley — — — 160 160 — — Unk 211 1982–2005 CDPH, CDWR

W88 Valley 7N/7W-18R2 382647122400702 NSFVP-16 206 206 50 100 Shallow 205 1983–2005 CDPH, USGS-GAMA

W89A Valley 7N/7W-19G2 382623122403301 — 1,074 870 250 6290 Deep 215 2010 owner, USGS

W89B Valley 7N/7W-19G2 382623122403301 — 1,074 870 6500 6620 Deep 215 2010 owner, USGS

W89C Valley 7N/7W-19G2 382623122403301 — 1,074 870 6750 860 Deep 215 2010 owner, USGS

W90 Valley 7N/7W-29D1 382548122395501 — 588 588 40 — Mixed 243 1957–1984 CDWR
120 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California Table 1. Construction information for selected wells used to obtain water-quality data in the Santa Rosa Plain watershed, Sonoma County, California.—Continued [See figure 1 for locations (map numbers) of wells, springs, and surface-water sites. U.S. Geological Survey (USGS) site identification number is the unique number for each site in USGS National

Water

Information System (NWIS) database. Depths in feet below land surface (fbls). Depth Category based on average of perforated interval. Land surface altitude, where available, in feet above sea level which refers to the North American Vertical Datum of 1988 (NAVD of 1988). Abbreviations: CDPH, California Department of Public Health; CDWR, California Department of Water Resources; GAMA, Groundwater Ambient Monitoring and Assessment; owner, data provided by well owner; Unk, unknown; USGS-GAMA, Groundwater Ambient Monitoring and Assessment Program (USGS and California State

Water Resources Control Board); <, actual value is less than value shown; —, no data]

Map

 number

Groundwater

storage

unit

State well

number

USGS site

identification

number

GAMA

identification

number

Depth

drilled

(fbls)

Depth

cased

(fbls)

Depth

 of top

perforation

(fbls)

Depth

 of bottom

perforation

(fbls)

Depth

category

Land

surface

altitude1

Period

of data

included

in this

report

Water quality

data source

W91 Uplands 7N/8W-01M1 382849122420501 NSFVOL-01 323

—

—

— Unk 329 1983–2005 CDPH, CDWR, USGS-GAMA

W92 Windsor 7N/8W-03L1 382852122440301

—

— 135 17 95 Shallow 145 1957–1984 CDWR

W93 Windsor 7N/8W-05G1 382855122460501

—

—

—

—

— Unk 142 1957–1984 CDWR

W94 Windsor 7N/8W-08L1 382759122462001 NSFVP-02 350 350 65 341 Mixed 138 2004 USGS-GAMAW95 Windsor 7N/8W-09N1 382740122452601

— 63 63 35 63 Shallow 125 1975–2003 CDWR

W96 Windsor 7N/8W-10A1 382825122434101 NSFVP-24 85 85 65 85 Shallow 149 2004 USGS-GAMA

W97 Valley 7N/8W-134 —

—

—

—

—

— Unk

— 1957 CDWR

W98 Cotati 7N/8W-18Q1

—

— 811 2811 50

— Mixed

— 1947–1969 CDWR

W99 Valley

—

—

— 1,200 915 2120 910 Mixed 198 1958–2009 CDPH, CDWR

W100 Valley

—

—

— 291 290 50 288 Mixed 198 1983–2005 CDPH, CDWR

W101 Valley

—

—

— 1,000 940 280

— Deep

— 1950–2009 CDPH, CDWR

W102A Windsor 7N/8W-24F1 382622122414901

— 1,020 200 130 180 Mixed 196 2009 owner, USGS

W102B Windsor 7N/8W-24F1 382622122414901

— 1,020

— 6786 6806 Deep 196 2009 owner, USGS

W103 Cotati

—

—

— 315 296 165 292 Deep 3174 1996 CDPH

W104 Cotati

—

—

—

—

—

—

— Unk

— 1997 CDPH

W105 Cotati 7N/8W-27N2 382509122441701

— 65 65 45 65 Shallow 119 1974–1992 CDWR

W106 Cotati

—

—

— 256 256 116 256 Mixed

— 2001 CDPH

W107 Cotati 7N/8W-29K1 382513122455101

— 67 67 47 67 Shallow 99 1976–2007 CDWR

W108 Cotati 7N/8W-29M2 382518122463001

—

— 98 78 98 Shallow 95 1975–1992 CDWR

W109 Cotati 7N/8W-19P1 382510122462701 NSFVPFP-02 250 231 61 231 Mixed 95 2004 USGS-GAMA

W110 Cotati 7N/8W-30K1 

—

— 290 290 105 290 Mixed 395 1976–2005 CDWR

W111 Cotati 7N/8W-30P1 382459122472101

—

— 196

—

— Unk 86 1963–1979 CDWR

W112 Cotati 7N/8W-30P6

—

— 158 158 65 156 Mixed

— 1981–1996 CDWR

W113 Cotati 7N/8W-31C1

—

— 320 2320 

—

— Unk

— 1960 CDWR

W114 Cotati 7N/8W-31F1

—

— 780 2780 

—

— Unk

— 1957 CDWR

W115 Cotati 7N/8W-31L1

—

— 780 2780 

—

— Unk

— 1961 CDWR

W116 Cotati 7N/8W-33F1

—

— 110 2110 

—

— Shallow

— 1961 CDWR

W117 Cotati 7N/8W-33M1

—

— 452 452 140 452 Mixed

— 1957–1963 CDWR

W118 Cotati

—

—

— 503 503 143 503 Mixed

— 2005–2008 CDPH

W119 Cotati

—

—

— 220 220 75 220 Mixed

— 2005 CDPH

W120 Wilson Grove 7N/9W-094 —

—

—

—

—

— Unk

— 1951 CDWR

W121 Wilson Grove 7N/9W-09F1 382804122514701

—

— 186

—

— Unk 283 1957–1991 CDWR

W122 Wilson Grove 7N/9W-09F3 382807122515401

— 280 280 240 280 Deep 275 1978–2007 CDWR
Introduction 121

Table 1. Construction information for selected wells used to obtain water-quality data in the Santa Rosa Plain watershed, Sonoma County, California.—Continued[See figure 1 for locations (map numbers) of wells, springs, and surface-water sites. U.S. Geological Survey (USGS) site identification number is the unique number for each site in USGS National WaterInformation System (NWIS) database. Depths in feet below land surface (fbls). Depth Category based on average of perforated interval. Land surface altitude, where available, in feet above sea level which refers to the North American Vertical Datum of 1988 (NAVD of 1988). Abbreviations: CDPH, California Department of Public Health; CDWR, California Department of Water Resources; GAMA, Groundwater Ambient Monitoring and Assessment; owner, data provided by well owner; Unk, unknown; USGS-GAMA, Groundwater Ambient Monitoring and Assessment Program (USGS and California State Water Resources Control Board); <, actual value is less than value shown; —, no data]

Map

 number

Groundwater

storage

unit

State well

number

USGS site

identification

number

GAMA

identification

number

Depth

drilled

(fbls)

Depth

cased

(fbls)

Depth

 of top

perforation

(fbls)

Depth

 of bottom

perforation

(fbls)

Depth

category

Land

surface

altitude1

Period

of data

included

in this

report

Water quality

data source

W123 Cotati 7N/9W-12K1 382751122482501 NSFVP-15 135 60 40 60 Shallow 105 2004 USGS-GAMA

W124 Cotati 7N/9W-24G1 382619122482101 NSFVP-14 550 550 507 547 Deep 95 2004 USGS-GAMA

W125 Cotati — — — 250 20 — — Shallow — 1997 CDPH

W126 Cotati — — — 1,521 830 410 805 Deep 94 1999 CDPH, CDWR

W127 Cotati — — — 790 770 310 750 Deep — 2003 CDPH

W128 Cotati — — — 1,506 1,065 425 1,045 Deep 76 1982–2004 CDPH, CDWR

W129 Cotati 7N/9W-36K2 382429122480501 NSFVPFP-01 1,074 1,040 410 1,020 Deep 79 2002–2005 CDPH, USGS-GAMA

W130 Cotati 7N/9W-36M1 382419122484301 — — 288 — — Shallow 73 1957–1972 CDWR

W131 Uplands 8N/7W-07R1 383257122400501 NSFVOL-02 380 380 — — Unk 790 2004 USGS-GAMA

W132 Uplands 8N/7W-30E1 383053122410801 NSFVOL-09 — — — — Unk 1,155 2004 USGS-GAMAW133 Uplands 8N/8W-11G1 383319122424301 NSFVP-13 — — — — Unk 525 2004 USGS-GAMA

W134 Windsor 8N/8W-17F1 383231122462201 — 355 355 155 355 Deep 221 1975–1991 CDWR

W135 Windsor — — — 440 432 240 432 Deep — 1990–1993 CDPH

W136 Windsor 8N/8W-20Q1 383111122455701 — 312 312 56 310 Mixed 142 1950–2007 CDWR

W137 Uplands 8N/8W-21N1 383115122452701 — 406 394 294 394 Deep 223 1976–1991 CDWR

W138 Windsor — — — 512 352 180 342 Deep 153 1981–2009 CDPH, CDWR

W139 Windsor — — — 570 565 210 520 Deep — 2009 CDPH, owner

W140 Windsor 8N/8W-32K1 382937122460601 NSFVP-20 123 110 70 110 Shallow 141 2004 USGS-GAMA

W141 Windsor — — — 380 380 90 283 Mixed 153 1984 CDPH, CDWR

W142 Windsor — — — 295 282 165 270 Deep — 2003–2004 CDPH, owner

W143 Windsor — — — 690 545 158 541 Deep — 2002–09 CDPH, owner

W144 Windsor — — — 220 220 80 220 Mixed — 1994–2001 CDPH

W145 Windsor 8N/8W-33K1 382938122445401 — 400 400 — — Unk 158 1975–1984 CDWR

W146 Uplands — — — 558 558 364 558 Deep — 1980 CDPH

W147 Uplands — — — 470 464 323 464 Deep — 1996 CDPH

W148 Uplands 8N/8W-36R14 — — — — — — Unk — 1952 CDWR

W149 Windsor 8N/9W-10R1 — — 400 2400 — — Unk — 1952 CDWR

W150 Windsor — — — 637 558 105 192 Mixed — 1984 CDPH, CDWR

W151 Windsor 8N/9W-12P1 383254122483301 — 187 187 67 187 Mixed 113 1975–1984 CDWR

W152 Windsor 8N/9W-13A2 383243122480501 — 109 109 87 109 Shallow 123 1974–1993 CDWR

W153 Windsor 8N/9W-13C1 — — 202 202 96 200 Mixed — 1952 CDWR

W154 Windsor 8N/9W-13D2 — — 100 2100 — — Shallow — 1951 CDWR

W155 Windsor 8N/9W-13E1 — — 130 2130 — — Shallow — 1951 CDWR
122 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California Table 1. Construction information for selected wells used to obtain water-quality data in the Santa Rosa Plain watershed, Sonoma County, California.—Continued [See figure 1 for locations (map numbers) of wells, springs, and surface-water sites. U.S. Geological Survey (USGS) site identification number is the unique number for each site in USGS National

Water

Information System (NWIS) database. Depths in feet below land surface (fbls). Depth Category based on average of perforated interval. Land surface altitude, where available, in feet above sea level which refers to the North American Vertical Datum of 1988 (NAVD of 1988). Abbreviations: CDPH, California Department of Public Health; CDWR, California Department of Water Resources; GAMA, Groundwater Ambient Monitoring and Assessment; owner, data provided by well owner; Unk, unknown; USGS-GAMA, Groundwater Ambient Monitoring and Assessment Program (USGS and California State

Water Resources Control Board); <, actual value is less than value shown; —, no data]

Map

 number

Groundwater

storage

unit

State well

number

USGS site

identification

number

GAMA

identification

number

Depth

drilled

(fbls)

Depth

cased

(fbls)

Depth

 of top

perforation

(fbls)

Depth

 of bottom

perforation

(fbls)

Depth

category

Land

surface

altitude1

Period

of data

included

in this

report

Water quality

data source

W156 Windsor 8N/9W-13J2 383220122480501 NSFVP-09 400 400 260 400 Deep 118 1973–2008 CDPH, CDWR, USGS-GAMAW157 Windsor 8N/9W-13N1

—

— 208 188 14 184 Mixed

— 1951 CDWR

W158 Windsor 8N/9W-14A1

—

— 185 2185

—

— Unk

— 1951 CDWR

W159 Windsor 8N/9W-22R1

—

— 145 142 122 142 Shallow

— 2001–2005 CDWR

W160 Windsor 8N/9W-23D1

—

— 370 2370 35 165 Mixed

— 1959–1964 CDWR

W161 Windsor 8N/9W-26L1 383038122494301

— 265 265 246 265 Deep 118 1977–1984 CDWR

W162 Windsor 8N/9W-36P1

—

— 1,048 21,048

—

— Unk

— 1958–1984 CDWR

1Land-surface altitude listed where available.2Information uncertain.3Well used on section; land-surface altitude estimated from TOPO.4Unofficial or incomplete State well number based on approximate location.5Represents sample depth using packers at this site.6Missing footnote?
Methods of Sample Collection and Analysis 123

collected in 60-milliliter (mL) glass bottles by immersing 

the bottle beneath the water surface until filled. Samples and

measurements were collected from the center of flow with the

exception of Spring Lake (SW5), where they were collected 

from the end of a pier, and the Laguna de Santa Rosa (SW2 

and SW13) and the Russian River (SW18), where deep water 

necessitated collection from stream banks.

Groundwater samples were collected from six wells 

(W33–35, W77, W89, and W102; fig. 1B) by the USGS, 

private consulting firms (PES Environmental, Inc., or PES, for

the city of Sebastopol; and The Environment, Community, and 

Opportunity Network, or ECON, for the city of Santa Rosa), 

or a combination of both (table 2). All samples from W33–35 

were collected by USGS personnel. Samples from W77 were 

collected concurrently by the USGS and PES from three depth 

intervals (100–120, 340–360, and 640–650 ft bls) and by only 

PES from seven additional depth intervals. All samples from 

W89 and W102 were collected by ECON.

The samples from these wells were collected by using 

submersible pumps that were temporarily installed in monitor 

wells (W33-35, W89A, and W102A) and were put between 

inflatable packers in long-screened wells (W77, W89B, and

W89C) and in an open borehole (W102B; table 2). Inflatable

packers were used to sample 10 depth intervals in W77 (at 

10 to 20 ft intervals between 100–650 ft bls); 2 depth intervals 

in W89 (500–620 and 750–860 ft bls), and 1 depth interval in 

the open borehole of W102 (W102B; 786–806 ft bls) prior to 

the well casing installation (table 2). Prior to sample collection, each well or open borehole was pumped continuously to 

purge at least three casing volumes of water. Samples were 

collected after specific conductance, pH, and temperature

stabilized (three successive measurements within 5-percent of 

one another) to ensure representative samples.

Samples from wells W33–35, W77, W89, and W102 

were collected for the analysis of selected inorganic constituents, including major and minor (trace) elements, nutrients, 

chemical and physical properties (turbidity, dissolved oxygen, 

pH, specific conductance, temperature, and dissolved solids),

and stable and radioactive isotopes, including 18O, 2H, tritium 

(3

H), carbon-14 (14C), and carbon-13 (13C). These samples 

were collected, treated, and preserved following or by using 

procedures similar to those outlined by the U.S. Geological 

Survey (2010). Analyses were done using a combination of 

USGS and consultant-designated laboratories. All samples 

collected by the USGS from W33–35 and 3 of the 10 depth 

intervals for W77 were analyzed by the USGS or USGS-contracted laboratories. Samples collected from seven additional 

depth intervals for W77 by PES were analyzed for arsenic by 

Analytical Sciences of Petaluma, which is an ELAP-certified

laboratory (Carl Michelsen, PES, written commun., 2006). 

Samples collected from W89 and W102 by ECON were analyzed for major ions, trace elements, and nutrients by the city 

of Santa Rosa Utilities Department, Laguna Environment Lab, 

an ELAP-certified laboratory (Andy Rogers, ECON, written

commun., 2009 and 2010), and were analyzed for 18O, 2H, 3H, 14C, and 13C by the USGS Reston Stable Isotope Laboratory 

(RSIL) or USGS-contracted laboratories. Procedures for onsite measurements and laboratory analysis of samples by the 

non-USGS laboratories followed standard methods listed in 

table 3. Procedures for the collection and analysis of samples 

by the USGS or USGS-contracted laboratories are described 

in greater detail in the following paragraphs.

Five laboratories managed or contracted by the USGS 

performed chemical analyses for the SRPW study. The USGS 

National Water-Quality Laboratory (NWQL), in Denver, 

Colorado, performed analyses for inorganic analytes (major 

ions, trace elements, and nutrients) in samples collected from 

W33–35 and the three previously described depth intervals 

of W77. The following analytical methods were employed by 

the NWQL: major ions were analyzed by inductively coupled 

plasma with atomic-emission spectrometry (ICP-AES; Fishman and Friedman, 1989; Fishman, 1993; the American Public 

Health Association, 1998); trace elements were analyzed by 

ICP-AES, inductively coupled plasma with mass spectrometry (ICP-MS), auto-segmented-flow/ion-selective electrode,

colorimetry, and automated batch analyzer (Fishman and 

Friedman, 1989; Fishman, 1993; Struzeski and others, 1996; 

Garbarino, 1999; Garbarino and others, 2006); and nutrients 

were analyzed by colorimetry (Fishman, 1993; Patton and 

Truitt, 2000). The USGS Isotope Fractionation Project in 

Reston, Virginia (currently [2013] the Reston Stable Isotope 

Laboratory) analyzed surface and groundwater samples for 

18O and 2

H by using a hydrogen-water-equilibration technique 

(Coplen and others, 1991; K. Revesz and T. Coplen, U.S. 

Geological Survey internal standard operating procedure, written commun., 2004) and an automated version of the carbon 

dioxide equilibration technique of Epstein and Mayeda (1953; 

Revesz and Coplen, U.S. Geological Survey, internal standard 

operating procedure, written commun., 2004). The University 

of Miami Tritium Laboratory performed analysis on samples 

collected for tritium by using an electrolytic enrichment with 

gas proportional counting technique, as described by Ostlund 

and Dorsey (1975) and the University of Miami Tritium 

Laboratory (2010).

Samples for 14C and 13C were analyzed by two different USGS-contracted laboratories during the SRPW study: 

the University Waterloo Environmental Isotope Laboratory, 

Waterloo, Ontario, Canada, analyzed samples collected in 

2006 and 2007, and the Woods Hole Oceanographic Institute’s 

National Ocean Sciences Accelerator Mass Spectrometry 

Facility (NOSAMS), Woods Hole, Massachusetts, analyzed 

samples collected in 2009 and 2010. Both facilities used accelerator mass spectrometry techniques similar to the methodology described by Fifield (1999).

Historical Sample Collection and Analysis

Water-quality samples presented in this report from the 

CDWR (appendix A) were collected and analyzed following 

referenced methods of the American Public Health Association 

(2005) and the U.S. Environmental Protection Agency (1993, 
124 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Table 2. Summary of discrete-depth samples from selected wells, Santa Rosa Plain Watershed, Sonoma County, California, 2006–2010.

[See figure 1 for locations (map numbers) of wells. U.S. Geological Survey (USGS) identification number is the unique number for each site in USGS National

Water Information System (NWIS) database. Sample depth in feet below land surface. The five-digit number below the constituent name is the USGS parameter

code used to uniquely identify a specific constituent or property; not listed where no USGS values are shown. Alkalinity values collected and analyzed

by USGS represent field measurements with parameter code 39086; all others represent laboratory analyses. Abbreviations: CaCO3, calcium carbonate; 

E, estimated value; ECON, The Environment, Community, and Opportunity Network; ft, foot; in., inch; mg/L, milligram per liter; ND, non detect; PES, PES 

Environmental, Inc.; °C, degree Celsius; μS/cm, microsiemen per centimeter at 25°C; μg/L, microgram per liter; (L), measured in laboratory; <, actual value is

less than value shown; —, no data]

Map 

number

Groundwater 

storage 

unit

State well 

number

USGS 

identification 

number

Perforated 

or sampled 

depth

(fbls)

Sample 

date

(mm/dd/yyyy)

Collecting 

and 

analyzing 

agency

Method of sample collection

W33 Cotati 6N/8W-07A4 382308122463903 60–80 08/22/2007 USGS Dedicated 2 in. well in multiple-well 

monitoring site.

W34 Cotati 6N/8W-07A5 382308122463902 237–257 08/22/2007 USGS Dedicated 2 in. well in multiple-well 

monitoring site.

W35 Cotati 6N/8W-07A6 382308122463901 550–570 08/23/2007 USGS Dedicated 2 in. well in multiple-well 

monitoring site.

W77 Wilson Grove 6N/9W-02C2 382352122493801 100–120 10/16/2006 USGS Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 200–210 10/16/2006 PES Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 240–250 10/16/2006 PES Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 280–300 10/13/2006 PES Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 340–360 10/16/2006 USGS Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 420–440 10/13/2006 PES Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 490–510 10/13/2006 PES Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 540–560 10/13/2006 PES Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 590–610 10/12/2006 PES Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W77 Wilson Grove — — 640–650 10/12/2006 USGS Inflatable bladders (packers) used to isolate

sampled interval within open borehole.

W89A Valley 7N/7W-19G2 382623122403301 250–290 03/12/2010 ECON Dedicated 2 in. well in borehole.

W89B Valley — — 500–620 03/17/2010 ECON Inflatable bladders (packers) used to isolate

sampled interval within well screen.

W89C Valley — — 750–860 03/11/2010 ECON Inflatable bladders (packers) used to isolate

sampled interval within well screen.

W102A Windsor 7N/8W-24F1 382622122414901 130–180 08/24/2009 ECON Permanent well.

W102B Windsor — — 786–806 07/23/2009 ECON Inflatable bladders (packers) used to isolate

sampled interval within open borehole.
Methods of Sample Collection and Analysis 125

Table 3. Minimum reporting levels and analytical method references for water-quality analyses performed by non-U.S. Geological 

Survey laboratories, Santa Rosa Plain Watershed, Sonoma County, California, 2006–2010.

[All standard methods (SM) are from “Standard Methods for the Examination of Water and Wastewater” (American Public Health Association, 2005).

Abbreviations: CaCO3, calcium carbonate; ECON, The Environment, Community, and Opportunity Network; EPA, U.S. Environmental Protection Agency; 

EPA 200.7: “Trace Metals by ICP” (U.S. Environmental Protection Agency, 1994); EPA 200.8: “Trace Metals by ICP” (U.S. Environmental Protection Agency,

1994); EPA 200.9: “Methods for the Determination of Metals in Environmental Samples, Supplement 1” (U.S. Environmental Protection Agency, 1994);

EPA 245.1: “Methods for the Determination of Metals in Environmental Samples, Supplement 1” (U.S. Environmental Protection Agency, 1994); EPA 300.0:

“Methods for the Determination of Inorganic Anions by Ion Chromatography” (U.S. Environmental Protection Agency, 1993); mg/L, milligram per liter; NTU,

nephelometric turbidity unit; PES, PES Environmental, Inc.; μS/cm, microsiemen per centimeter at 25°C; °C, degree Celsius; μg/L, microgram per liter]

Property or constituent 

(unit of measurement)

Minimum 

reporting 

level

Reporting 

entity Analytical method reference

Physical properties, major ions, and nutrients

Dissolved oxygen (mg/L) 0.1 ECON, PES SM 4500 O G: Electrometric Method.

pH (standard units) 0.1 ECON, PES SM 4500 H+

 B: Electrometric Method.

Specific conductance (μS/cm) 10 ECON, PES SM 2510 A-B: Conductivity.

Temperature (°C) 0.1 ECON, PES SM 2550 B: Temperature.

Hardness, total (mg/L, as CaCO3

) 1.0 ECON SM 2340 B: Total Hardness by Calculation.

Calcium, dissolved (mg/L) 0.1 ECON EPA 200.7: Inductively Coupled Plasma—Atomic Emission Spectrometry.

Magnesium, dissolved (mg/L) 0.02 ECON EPA 200.7: Inductively Coupled Plasma—Atomic Emission Spectrometry.

Potassium, dissolved (mg/L) 0.5 ECON EPA 200.7: Inductively Coupled Plasma—Atomic Emission Spectrometry.

Sodium, dissolved (mg/L) 0.5 ECON EPA 200.7: Inductively Coupled Plasma—Atomic Emission Spectrometry.

Chloride, dissolved (mg/L) 1.0 ECON EPA 300.0: Ion Chromatography.

Fluoride, dissolved (mg/L) 0.1 ECON EPA 300.0: Ion Chromatography.

Sulfate, dissolved (mg/L) 0.5 ECON EPA 300.0: Ion Chromatography.

Nitrate as NO3

 (mg/L) 2.0 ECON EPA 300.0: Ion Chromatography.

Nitrite, dissolved as N (mg/L) 0.4 ECON SM 4500 NO2 B: Colorimetry.

Turbidity (NTU) 1 ECON, PES SM 2130 B: Temperature.

Trace elements

Aluminum, dissolved (μg/L) 10 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Antimony, dissolved (μg/L) 2.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Arsenic, dissolved (μg/L) 1.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Arsenic, dissolved (μg/L) 1.0 PES EPA 200.9: Stablized Temperature Graphite Furnance Atomic Absorption.

Barium, dissolved (μg/L) 1.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Beryllium, dissolved (μg/L) 0.5 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Boron, dissolved (μg/L) 50 ECON EPA 200.7: Inductively Coupled Plasma—Atomic Emission Spectrometry.

Cadmium, dissolved (μg/L) 1.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Chromium, dissolved (μg/L) 2.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Copper, dissolved (μg/L) 2.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Iron, dissolved (μg/L) 40 ECON EPA 200.7: Inductively Coupled Plasma—Atomic Emission Spectrometry.

Lead, dissolved (μg/L) 1.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Manganese, dissolved (μg/L) 1.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Mercury, dissolved (μg/L) 0.2 ECON EPA 245.1: Manual Cold-Vapor Atomic-Absorption Spectrometry.

Molybdenum, dissolved (μg/L) 2.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Nickel, dissolved (μg/L) 2.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Selenium, dissolved (μg/L) 2.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Silver, dissolved (μg/L) 10 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Thallium, dissolved (μg/L) 1.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Vanadium, dissolved (μg/L) 2.0 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.

Zinc, dissolved (μg/L) 20 ECON EPA 200.8: Inductively Coupled Plasma—Mass Spectrometry.
126 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

1994). All CDWR samples were analyzed at the California 

Department of Water Resources Bryte Analytical Laboratory in West Sacramento, California (Bruce Agee, California 

Department of Water Resources, written commun., 2005).

Water-quality samples presented in this report from public-supply purveyors and the CDPH (appendix A) were generally collected, analyzed, or both by consultants or laboratories 

contracted by individual public-supply purveyors. Sampling 

and analysis were done in accordance with requirements of 

California Code of Regulations Title 22 (California Department of Public Health, 2011), by using methods referenced by 

the American Public Health Association (2005) and the U.S. 

Environmental Protection Agency (1993, 1994).

Well Construction of Sampled Wells

Groundwater wells in this study area span a wide range of 

completed depths and perforated (screened) intervals (table 1). 

Most are production (municipal, irrigation, and domestic) 

wells and, therefore, are typically perforated over long intervals (greater than or equal to 100 ft). Some wells, especially 

the older ones, could have been constructed in uncased open 

holes, but this information is unavailable. Perforated intervals 

for many of the wells having water-quality data are composed 

either of a single, continuous interval or multiple, individual 

intervals covering 50 percent or more over the completed hole 

depth. For the purposes of this report, the wells were categorized into four main groups on the basis of well construction: 

(1) shallow wells—entire perforated or open interval above

150 ft bls, (2) deep wells—entire perforated or open interval

below 150 ft bls, (3) mixed depth wells—perforated or open

interval extends above and below 150 ft bls, and (4) unknown 

depth wells—the depth of the perforated or open interval is

unknown (table 1). A total of 167 wells were sampled: 24 were 

shallow, 69 were deep, 40 were mixed, and 34 were unknowndepth. Given the spatial distribution of sampled wells (fig. 1B) 

and the relative uniform distribution of sample depths, it is 

expected that the data are representative of the geochemistry 

of the SRPW. In the SRP, the shallow wells primarily sample 

the Glen Ellen Formation and the deep wells primarily sample 

the Petaluma Formation.

Chemical Character of Groundwater

Water-quality data compiled for this report (appendix A) 

were used to characterize the areal, vertical, and temporal 

variations in groundwater quality and to identify water-quality 

constituents of potential concern. These data include physical properties and inorganic constituents (major ions, trace 

elements, and nutrients) collected during 1947–2010, with 

an average sample date of about 1990. Wells with multiple 

chemical analyses are represented in selected summary tables 

and figures by a single value corresponding to the most recent

analysis for that particular well. 

Major-Ion Composition

The major-ion composition of groundwater is controlled 

by the natural chemistry of the recharge water, geochemical 

reactions in the subsurface, primarily dissolution and precipitation of minerals, and anthropogenic factors, such as the disposal of wastewater and irrigation return flows. The major-ion

composition of groundwater was characterized for this report 

by using trilinear and Stiff diagrams. Dissolved-solids concentrations of the samples were also used to help characterize the 

water quality of the SRPW.

A trilinear diagram shows the relative contribution of 

major cations and anions, on a charge-equivalent basis, to the 

ionic content of the water (Piper, 1944). Percentage scales 

along the sides of the diagram indicate the percentage of total 

cations or anions, in milliequivalents per liter (meq/L), of each 

major ion. Cations are shown in the left triangle, anions are 

shown in the right triangle, and the central diamond integrates 

the data (fig. 2). Trilinear diagrams are useful in depicting 

the range in chemical composition, in grouping water types, 

and in determining if there is simple mixing between chemically different water (Hem, 1992). In this report, the dominant 

cation and anion species are used to describe the water type of 

a water sample when a single cation or anion composes more 

than 60 percent of the total cations or anions, respectively. 

Where no one cation or anion exceeds 60 percent, the sample 

is described as mixed and the first and second most abundant

cations or anions are given for description purposes.

Stiff diagrams depict the concentrations of major ions in 

meq/L and indicate relative proportions of major ions (Stiff, 

1951). Analyses with similarly shaped diagrams represent 

groundwater of similar chemical characteristics with respect to 

major ions. Changes in the width of the diagrams indicate differences in the concentration of dissolved constituents. Water 

that contains higher concentrations of major ions has a larger 

polygon than does the diagram for water with lower concentrations. All Stiff diagrams in this report are shown at the same 

scale in units of meq/L (fig. 3). The left side of the diagram 

shows the major cations: sodium plus potassium at the top, 

calcium in the middle, and magnesium at the bottom. The right 

side of the diagram shows major anions: chloride plus fluoride

at the top, carbonate plus bicarbonate in the middle, and sulfate on the bottom.

The dissolved-solids concentration in water ordinarily 

is determined from the weight of the dry residue remaining 

after evaporation of an aliquot of the water sample and is 

referred to as residue on evaporation (ROE) (appendix A). The 

dissolved-solids concentration in water can also be computed 

if the concentrations of major ions are known. For this report, 

the computed dissolved-solids concentration represents the 

total concentration of all the major ions reported, but does not 

include an adjustment of bicarbonate to carbonate ions that 

commonly is made to make the computation comparable to the 

ROE value for the same sample. Because many of the samples 

were not analyzed for ROE, the dissolved-solids concentrations discussed in this report are the computed values presented in appendix A.
Chemical Character of Groundwater 127

Calcium

 100 

 80 

 60 

 40 

 20  0 

 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Fluoride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride

 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

IP019208_Figure C02a Uplands

C A T I O N S A N I O N S

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

A

Shallow (0’ to 150’ blsd)

Mixed

Deep (Greater than 150’)

Construction unknown

Well perforated interval

EXPLANATION

Uplands storage unit

Spring

Well number

W10

W10

Figure 2. Trilinear diagrams of the most recent, complete sample from selected surface-water sites, springs, and wells in storage units 

in the Santa Rosa Plain watershed, Sonoma County, California, 1947–2010: A, Uplands; B, Valley; C, Windsor; D, Cotati; and E, Wilson 

Grove.
128 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Calcium

 100 

 80 

 60 

 40 

 20  0 

 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Fluoride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride

 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

C A T I O N S A N I O N S

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

B

W97

W84

W84 Shallow (0’ to 150’ blsd)

Mixed

Deep (Greater than 150’)

Construction unknown

Well perforated interval

Well number

EXPLANATION

Valley storage unit

IP019208_Figure C02b Valley

Figure 2. Continued.
Chemical Character of Groundwater 129

Calcium

 100 

 80 

 60 

 40 

 20  0 

 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Fluoride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride

 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

IP019208_Figure C02c Windsor basin

C A T I O N S A N I O N S

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

C

Well number Shallow (0’ to 150’ blsd)

Mixed

Deep (Greater than 150’)

Construction unknown

Well perforated interval

EXPLANATION

Windsor Basin storage unit

W158

W155

W155

W154

W102B

W102A

Figure 2. Continued.
130 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Calcium

 100 

 80 

 60 

 40 

 20  0 

 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Fluoride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride

 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

IP019208_Figure C02d Cotati basin

C A T I O N S A N I O N S

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

D

Shallow (0’ to 150’ blsd)

Mixed

Deep (Greater than 150’)

Construction unknown

Well perforated interval

Well number

EXPLANATION

Cotati Basin storage unit

W47

W63

W63

Figure 2. Continued.
Chemical Character of Groundwater 131

Calcium

 100 

 80 

 60 

 40 

 20  0 

 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Fluoride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride

 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

IP019208_Figure C02e Wilson Grove

C A T I O N S A N I O N S

PERCENTAGE OF TOTAL MILLIEQUIVALENTS PER LITER

E

Shallow (0’ to 150’ blsd)

Mixed

Deep (Greater than 150’)

Construction unknown

Well perforated interval Well number

EXPLANATION

Wilson Grove storage unit

W81

W81

Figure 2. Continued.
132 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

i

122°50’ 122°40’

38°

30’

38°

20’

12

116

101

R9W 

T

9

N 

R8W R7W R6W 

T

8

N 

T

7

N 

T

6

N 

T

5

N 

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe

 Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

Santa Rosa Plain

Mendocino Range

Sonoma Mountains

Mayacmas Mountains

Upland

Upland

Valley

Windsor Basin

Wilson Grove

Cotati

B'

B

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

0

0 5 10 Kilometers

5 10 Miles

IP019208_Figure C03a

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

A

Local well number

Dissolved-solids

 concentration

Date of sample

 (mm/dd/yy)

Shallow well

Spring

 10 8 6 4 2 2 4 6 8 10 

Na Cl

Ca HCO3

Mg SO4

CATIONS ANIONS

STIFF DIAGRAM SCALE

Cotati Basin Valley

Upland

Windsor Basin Wilson Grove

Groundwater storage units

W33

TDS=314

8/22/07

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Rodgers Creek

 fault zone

B B’ Line of section

Inferred fault

Mt. St. John thrust fault

Gates Canyon thrust fault

Petrified Forest thrust zone

Milliequivalent per liter

W20

TDS=389

9/11/02

W2

TDS=383

4/19/60

W9

TDS=167

2/7/06

W28

TDS=400

9/17/07

W116

TDS=375

9/17/07

W105

TDS=481

7/30/92

SPR1

TDS=87

8/6/74

SPR2

TDS=102

8/6/74

W33

TDS=462

8/22/07

W107

TDS=638

9/18/07

W77A

TDS=340

10/16/06

W108

TDS=378

7/30/92

W92

TDS=400

5/16/84 W95

TDS=396

9/23/03

W152

TDS=267

W155 8/30/93

TDS=327

9/28/51

W159

TDS=319

8/23/05

W130

TDS=294

8/10/72

Figure 3. Stiff diagrams showing water-type for selected in the Santa Rosa Plain watershed, Sonoma County, California: A, shallow 

wells; B, deep wells; and C, wells along section B–B’.
Chemical Character of Groundwater 133

i

122°50’ 122°40’

38°

30’

38°

20’

12

116

101

R9W 

T

9

N 

R8W R7W R6W 

T

8

N 

T

7

N 

T

6

N 

T

5

N 

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe

 Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

Santa Rosa Plain

Mendocino Range

Sonoma Mountains

Mayacmas Mountains

Upland

Upland

Valley

Windsor Basin

Wilson Grove

Cotati

B'

B

W73

TDS=210

3/24/87

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

0

0 5 10 Kilometers

5 10 Miles

IP019208_Figure C03b

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection EXPLANATION

B

Local well number

Dissolved-solids

 concentration

Date of sample

 (mm/dd/yy)

Deep wells

 10 8 6 4 2 2 4 6 8 10

Na Cl

Ca HCO3

Mg SO4

CATIONS ANIONS

STIFF DIAGRAM SCALE

Cotati Basin Valley

Upland

Windsor Basin Wilson Grove

Groundwater storage units Rodgers Creek

 fault zone

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

B B’ Line of section

Inferred fault

Mt. St. John thrust fault

Gates Canyon thrust fault

Petrified Forest thrust zone

Milliequivalent per liter

W6

TDS=554

7/20/88

W16

TDS=241

7/21/09

W10

TDS=513

6/7/84

W19

TDS=435

6/18/91

W22

TDS=292

9/25/07

W23

TDS=244

5/16/84

W32

TDS=196

8/25/03

W102B

TDS=1,804

7/23/09

W34

TDS=460

8/22/07

W35

TDS=246

8/23/07

W40

TDS=389

7/16/08

W89A

TDS=503

3/12/10

W89B

TDS=405

3/11/10

W89C

TDS=398

3/17/10

W27

TDS=422

7/28/06

W41

TDS=462

7/21/09

W50

TDS=263

7/16/08 W56

TDS=368

7/16/08

W58

TDS=343

9/12/06

W49

TDS=242

2/14/96

W82

TDS=163

2/14/96

W74

TDS=318

2/7/77

W76

TDS=352

10/4/04

W77B

TDS=212

10/16/06

W77C

TDS=268

10/12/06

W65

TDS=427

7/21/09

W73

TDS=264

3/24/87

W103

TDS=344

2/26/96

W146

TDS=334

7/16/80

W147

TDS=330

4/10/96

W134

TDS=243

8/28/91 W137

TDS=422

8/28/91

W138

TDS=378

8/25/09

W139

TDS=393

8/25/09

W135

TDS=321

3/26/93

W127

TDS=212

8/25/03

W161

TDS=161

5/17/84

W143

TDS=337

8/25/89

W142

TDS=401

7/20/04

W156

TDS=248

5/17/84

W129

TDS=191

9/1/05

Figure 3. Continued.
134 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

sac12-0452_Figure C03c

B

C

B'

FEET

SEA

LEVEL

FEET

–1,000–2,000

SEA

LEVEL

1,000

0 6,000 12,000 18,000 24,000 30,000 36,000 42,000 48,000 54,000 60,000 66,000

–1,000–2,000

Rodgers Creek

fault zone

Sebastopol

fault 

Trenton

Ridge

 fault 

Bennett Valley

fault zone

Cotati Basin Wilson Grove Windsor

Basin

Southwest Northeast

Valley Uplands

Mayacmas Mountains

Distance along section, in feet 12.85x vertical exaggeration Geology modified from McLaughlin and others (2008) and Sweetkind and others (2010).

?

?

?

? ?

?

163

340 462

460

246

191

196

420

344

335

638

400 265

422

498

344

534

386 398

398

405

503 207

1,804

212

226

W82*

W77*

W33-35 W110* W107* W28*

W23* W103* W102 W89* W99* 

W85* W31*

W32*

W129*

W105*

W27*

W119*

W82*

Glen Ellen FormationWilson Grove Formation

Sonoma Volcanics

Petaluma Formation

Undifferentiated basement

EXPLANATION 

Well perforation (white box)

Reported Total dissolved solids (TDS) value

Well (black = production well, 

red = observation or discretedepth sampled test well)

Well map number (asterisk indicates well projected to section)

Fault, sense of offset not shown

 2 2 4 4 6 6 8 8 10 (meq/L) 10

Cl NaHCO3 Ca

SO4 Mg

CATIONS ANIONS

STIFF DIAGRAM SCALE

Laguna de Santa Rosa

Milliequivalent per liter

Figure 3. Continued.
Chemical Character of Groundwater 135

Areal and Depth-Related Patterns

As described in chapter B, the study area is subdivided 

into five groundwater storage units, (1) the mountainous

upland areas east of the city of Santa Rosa (UPL); (2) valleys 

located to the east of the SRP Valley (VAL), including Rincon 

Valley, Bennett Valley and the northern half of Kenwood 

Valley; (3) Windsor Basin (WB); (4) Cotati Basin (CB); and 

(4) the Wilson Grove area (WG; fig. 1). The WB and CB 

storage units compose the SRP. The major-ion composition 

and dissolved-solids concentration of the most recent sample 

compiled for each well (appendix A) were used to describe the 

chemical character in each groundwater storage unit.

Uplands (UPL) Storage Unit

Samples from springs SPR1 and SPR2 in the Mayacmas 

Mountains are a mixed cation-bicarbonate type water with 

dissolved-solids concentrations of about 100 mg/L or less—

the lowest dissolved-solids concentrations in the SRPW 

(fig. 3A; appendix A). These samples represent recharge that 

has undergone minor water-rock interactions, has migrated 

through geologic materials that are resistant to weathering, or 

both. As groundwater migrates through the UPL and downgradient storage units, the dissolved-solids concentration of the 

groundwater increases as a result of water-rock interactions 

and anthropogenic inputs, such as irrigation return flows and

septic-tank discharge. The median dissolved-solids concentration of samples from wells in the UPL storage unit was 

330 mg/L (table 4).

All well samples in the UPL storage unit were a mixed 

cation-bicarbonate or calcium/magnesium-bicarbonate type 

water, except for the sample from well W10, which was a 

sodium-bicarbonate type water (fig. 2A). The sample from 

W10 had a dissolved-solids concentration of 513 mg/L—

the highest dissolved-solids concentration in the storage 

unit (fig. 3B and appendix A). The sample also had a boron 

concentration of 1,800 μg/L, which was significantly higher

than the median concentration of 100 μg/L for the storage 

unit (table 4). Cardwell (1958) hypothesized that high boron 

concentrations in the SRPW could be attributable to deeply 

circulating groundwater rising along a fault zone. Well W10 

is within the Rodgers Creek fault zone (fig. 1); therefore, 

Cardwell’s hypothesis could explain the high boron concentration in the sample from well W10.

Valley (VAL) Storage Unit

All well samples from the VAL storage unit were a mixed 

cation-bicarbonate type water that had a higher percentage 

of sodium than samples from the UPL storage unit, with the 

exception of the samples from W84 and W97 (figs. 2A and 

2B). The sample from well W84 was a sodium/calciumbicarbonate/sulfate type water, and the sample from well 

W97 was a mixed cation-chloride/sulfate type water (fig. 2B). 

None of the samples were from wells perforated solely in the 

shallow aquifer, which could account for the higher percentage of sodium in the well samples. In an aquifer system, it is 

common for the dominant cation to shift from calcium to 

sodium as depth increases as a result of cation exchange, in 

which calcium in solution is exchanged with sodium on clay 

surfaces in the aquifer sediments (Hem, 1992). The median 

dissolved-solids concentration of samples from wells in the 

VAL storage unit was 392 mg/L, which was about 20 percent 

higher than the median of samples from wells in the UPL 

storage unit (table 4). This higher dissolved-solids in the VAL 

storage unit compared to the UPL storage unit could result 

from a greater number of sediment and water interactions in 

the alluvial and volcanic deposits of the VAL compared to the 

weathering-resistant hard rocks of the UPL.

Windsor Basin (WB) Storage Unit

Most of the well samples from the WB storage unit were 

mixed cation-bicarbonate and sodium-bicarbonate type waters 

(fig. 2C). A comparison of major-ion composition and depth 

revealed a subtle shift in major cations, from greater proportions of calcium and magnesium in shallow groundwater to 

greater proportions of sodium in deeper groundwater (fig. 2C). 

This finding is consistent with ion exchange between clays

and groundwater as distance and depth from recharge sources 

increases. The median dissolved-solids concentration of samples from wells in the WB storage unit was 321 mg/L, which 

was slightly lower than the median of samples from wells in 

the UPL and VAL storage units (table 4).

Several samples near Windsor (W154, W155, and W158) 

were a mixed cation-chloride type water (fig. 2C). The sample 

from W158 had a nitrate concentration, reported as nitrogen 

(NO3-N), of about 10 mg/L (appendix A). The greater proportions of chloride and high nitrate concentration were consistent 

with the effects of recharge of septic-tank effluent or agricultural return flow.

Sample W102B was a sodium-mixed anion type water, 

with chloride as the most abundant anion (fig. 2C). This 

sample was collected from 786–806 ft bls in the open borehole of well W102 when it was drilled (table 3). The sample 

contained the highest dissolved-solids (1,804 mg/L) and boron 

(2,100 μg/L) concentrations of all the samples in this report 

(appendix A). Well W102 is within the Rodgers Creek fault 

zone (fig. 3B). The high dissolved-solids and boron concentrations are probably attributable to deeply circulating groundwater rising along the fault zone.

Cotati Basin (CB) Storage Unit

Most well samples in the CB storage unit were a mixed 

cation-bicarbonate water type or a sodium-bicarbonate water 

type (fig. 2D). As in the WB storage unit, there was a subtle 

shift in major cations from greater proportions of calcium and 

magnesium in shallow groundwater to greater proportions of 

sodium plus potassium in deeper groundwater. The median 

dissolved-solids concentration of samples from wells in the 

CB storage unit was 362 mg/L, which was close to the median 

dissolved-solids concentration of samples from wells in the 

UPL and VAL storage units (table 4).
136 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Table 4. Summary of groundwater quality for most recent complete sample from selected wells, classified by storage unit and depth 

category, Santa Rosa Plain Watershed, Sonoma County, California, 1947–2010.—Continued

[See figure 1 for locations of wells. Abbreviations: MCL, maximum contaminant level; mg/L, milligram per liter; NL, notification level; SMCL, secondary

maximum contaminant level; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; —, no data]

Number 

of wells 

in shallow 

aquifer

Number 

of wells 

in deep 

aquifer

Total 

number 

of wells

Parameter

units

MCL, SMCL, or NL

Constituent

Specific 

conductance

(μS/cm)

1

900

Chloride

(mg/L)

1

250

Total 

dissolved 

solids 

computed

(mg/L)

1

500

Nitrate 

as N

(mg/L)

2

10

Arsenic

(μg/L)

3

10

Boron

(μg/L)

4

1,000

Iron

(μg/L)

5

300

Manganese

 (μg/L)

5

50

Uplands groundwater storage unit

2 — — Number of samples 2 2 2 1 0 1 1 1

2 — — Maximum 421 29.0 383 0.1 — 100 5,000 480

2 — — Minimum 230 4.0 167 0.1 — 100 5,000 480

2 — — Median 326 16.5 275 0.1 — 100 5,000 480

— 4 — Number of samples 4 4 4 2 2 2 3 2

— 4 — Maximum 616 42.0 513 0.4 10.0 1,800 195 110

— 4 — Minimum 388 3.0 330 0.3 3.4 400 40 30

— 4 — Median 471 10.2 378 0.3 6.7 1,100 80 70

— — 9 Number of samples 9 9 9 5 3 5 6 5

— — 9 Maximum 616 42.0 513 0.4 10.0 1,800 5,000 480

— — 9 Minimum 211 3.0 167 0.1 1.4 28 40 5

— — 9 Median 403 6.0 330 0.2 3.4 100 137 110.0

— — 9 Wells above regulatory level 0 0 1 0 0 1 2 3

Valley groundwater storage unit

0 — — Number of samples 0 0 0 0 0 0 0 0

0 — — Maximum — — — — — — — —

0 — — Minimum — — — — — — — —

0 — — Median — — — — — — — —

— 3 — Number of samples 3 3 3 3 1 3 3 3

— 3 — Maximum 516 30.6 503 <0.5 2.3 440 540 170

— 3 — Minimum 417 17.8 398 <0.5 2.3 420 110 52

— 3 — Median 475 17.9 405 <0.5 2.3 430 120 82

— — 8 Number of samples 8 8 8 3 2 7 6 5

— — 8 Maximum 516 30.6 503 7.6 2.3 600 4,600 170

— — 8 Minimum 117 7.1 75 0.2 1.1 50 110 52

— — 8 Median 446 17.9 392 0.6 1.7 430 215 82

— — 8 Wells above regulatory level 0 0 3 0 0 0 2 5

Windsor groundwater storage unit

7 — — Number of samples 7 7 7 5 1 6 2 2

7 — — Maximum 590 98.0 534 4.8 1.9 620 420 1,000

7 — — Minimum 339 8.0 267 0.2 1.9 <10 42 150

7 — — Median 477 39.0 396 0.9 1.9 100 231 575

— 9 — Number of samples 6 9 9 2 5 8 5 7

— 9 — Maximum 2,700 480.0 1,804 0.3 20.0 2,100 1,200 1,545

— 9 — Minimum 206 10.5 161 0.1 6.0 100 50 230

— 9 — Median 331 13.0 337 0.2 9.0 184 280 726

— — 29 Number of samples 26 29 29 15 7 24 15 16

— — 29 Maximum 2,700 480.0 1,804 17.6 20.0 2,100 1,200 1,545

— — 29 Minimum 206 7.0 161 0.02 1.9 40 10 20

— — 29 Median 377 26.0 321 0.5 6.0 106 320 675

— — 29 Wells above regulatory level 1 1 3 1 2 2 8 15

Table 4. Summary of groundwater quality for most recent complete sample from selected wells, classified by storage unit and depth 

category, Santa Rosa Plain Watershed, Sonoma County, California, 1947–2010.
Chemical Character of Groundwater 137

Table 4. Summary of groundwater quality for most recent complete sample from selected wells, classified by storage unit and depth 

category, Santa Rosa Plain Watershed, Sonoma County, California, 1947–2010.—Continued

[See figure 1 for locations of wells. Abbreviations: MCL, maximum contaminant level; mg/L, milligram per liter; NL, notification level; SMCL, secondary

maximum contaminant level; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; —, no data]

Number 

of wells 

in shallow 

aquifer

Number 

of wells 

in deep 

aquifer

Total 

number 

of wells

Parameter

units

MCL, SMCL, or NL

Constituent

Specific 

conductance

(μS/cm)

1

900

Chloride

(mg/L)

1

250

Total 

dissolved 

solids 

computed

(mg/L)

1

500

Nitrate 

as N

(mg/L)

2

10

Arsenic

(μg/L)

3

10

Boron

(μg/L)

4

1,000

Iron

(μg/L)

5

300

Manganese

 (μg/L)

5

50

Cotati groundwater storage unit

10 — — Number of samples 10 10 10 9 1 4 2 1

10 — — Maximum 852 89.0 638 16.4 1.8 146 186 306

10 — — Minimum 390 22.0 294 0.1 1.8 80 40 306

10 — — Median 567 35.0 388 4.4 1.8 100 113 306

— 40 — Number of samples 40 40 40 21 24 2 11 16

— 40 — Maximum 700 81.0 554 7.1 15.0 26 1,900 460

— 40 — Minimum 191 7.5 160 0.3 0.7 10 40 23

— 40 — Median 365 17.0 273 1.0 4.3 18 310 69

— — 87 Number of samples 86 87 87 57 40 18 21 27

— — 87 Maximum 4,420 1,240.0 2,755 16.4 17.0 1,500 7,200 1,400

— — 87 Minimum 191 7.5 160 0.1 0.7 10 40 23

— — 87 Median 456 26.0 362 2.0 4.0 100 300 86

— — 87 Wells above regulatory level 2 2 12 1 3 1 10 19

Wilson Grove groundwater storage unit

1 — — Number of samples 1 1 1 1 1 1 1 1

1 — — Maximum 385 16.1 340 0.5 4.5 15 25 9

1 — — Minimum 385 16.1 340 0.5 4.5 15 25 9

1 — — Median 385 16.1 340 0.5 4.5 15 25 9

— 8 — Number of samples 8 8 8 4 4 3 3 5

— 8 — Maximum 372 22.1 352 1.3 13.0 16 3,354 120

— 8 — Minimum 175 7.2 129 0.2 1.0 15 39 1

— 8 — Median 352 12.6 238 0.9 9.3 16 280 13

— — 15 Number of samples 15 15 15 11 6 7 6 7

— — 15 Maximum 385 22.1 352 1.3 13.0 100 3,354 120

— — 15 Minimum 91 7.2 105 0.1 1.0 10 10 1

— — 15 Median 336 13.2 233 0.5 6.0 16 34 13

— — 15 Wells above regulatory level 0 0 0 0 2 0 1 2

Total number of wells

20 64 148 Total wells above regulatory level 3 3 19 2 7 4 23 44

20 64 148 Total number of samples 144 148 148 91 58 61 54 60

1

State of California recommended SMCL.

2

State MCL.

3

Federal MCL.

4

State of California notification level.

5

State SMCL.
138 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Samples from well W47 and W63 in the CB storage 

unit were a mixed cation-chloride type water (fig. 2D). These 

samples had the highest percentage of chloride (greater than 

70 percent) in the CB storage unit, and contained dissolvedsolids concentrations in excess of 1,550 mg/L (appendix A). 

These wells fall along the trace of an unnamed fault, east of 

the Sebastopol fault (figs. 1B and 3C). Unfortunately, boron 

concentrations were not analyzed for these samples to help 

determine if deeply circulating groundwater rising along the 

fault was a possible source of water to these samples.

Stiff diagrams of deep wells showed variability in water 

type and dissolved-solids concentrations in well samples from 

closely spaced wells in the southern part of the CB storage 

unit (fig. 3B). Multiple faults are mapped or inferred in the 

southern part of the CB storage unit (Langenheim and others, 

2010), which could help explain the observed variability in 

water quality. Depth-dependent hydrologic, chemical, and 

isotopic data are needed to better understand this variability.

Wilson Grove (WG) Storage Unit

Most well samples in the WG storage unit were calciumbicarbonate type or mixed cation-bicarbonate type waters with 

dissolved-solids concentrations less than 300 mg/L (fig. 2E, 

table 4). Unlike the WB and CB storage units, there was no 

shift in major cations from greater proportions of calcium and 

magnesium in shallow groundwater to greater proportions of 

sodium plus potassium in deeper groundwater. The sample 

from well W81 was a sodium-bicarbonate/chloride type water, 

which was significantly different than other samples in this

storage unit (fig. 2E). Wells in the WG storage unit are perforated in the Wilson Grove Formation, which is composed of 

fine to medium-grained, moderately to well-sorted, tan to gray,

uncemented to weakly cemented marine sandstone (Sweetkind and others, 2010). Unlike the Glen Ellen and Petaluma 

Formations, the Wilson Grove Formation has only minor clay 

deposits, which provide limited opportunities for the loss of 

calcium from solution by means of ion exchange with sodium 

attached to clays.

Changes in Chemical Character along Geologic 

Section B–B’

Stiff diagrams and dissolved-solids concentrations of 

well samples along geologic section B–B’, modified from

Sweetkind and others (2010, see fig. A1-4) and chapter B (see 

figs. 2 and 25), were used to illustrate changes in water quality 

as groundwater flows through the aquifer system. A total of

21 wells, including depth-dependent samples from wells W77, 

W89, and W102, were projected onto geologic section B–B’ to 

compare water quality to geologic units and structure (fig. 3C).

Geologic Units along Section B–B’

Geologic section B-B’ extends from the Mayacmas 

Mountains in the UPL storage unit on the east to the highlands 

in the WG storage unit on the west (fig. 3C). The UPL storage 

unit consists primarily of undifferentiated basement rocks and 

Sonoma Volcanics. In general, the basement rocks have low 

permeability and are not considered a major water-bearing 

unit, except in fractures and weathered zones. The Sonoma 

Volcanics are a heterogeneous assemblage of continental 

volcanic and volcaniclastic rocks, including basalt, andesite, 

and rhyolite lavas interbedded with air-fall and ash-flow tuffs,

debris-flow deposits, and lacustrine deposits (Sweetkind and

others, 2010).

The VAL storage unit consists primarily of the Glen Ellen 

Formation and the Sonoma Volcanics. For purpose of waterquality characterization in this chapter, all late Pliocene and 

younger nonmarine deposits are considered part of the Glen 

Ellen Formation, including the surficial Quaternary deposits.

The Glen Ellen Formation consists of heterogeneous mixtures 

of tuffaceous clay, mud, bouldery to pebbly gravel, and sand 

and silt deposits with interbedded conglomerates (Sweetkind 

and others, 2010). Recent investigations indicated its thickness 

is about 100 to 150 ft throughout most of the SRPW (Sweetkind and others, 2010; chapter B).

The WB and CB storage units consist of 100–150 ft thick 

alluvial deposits of the Glen Ellen Formation underlain, for the 

most part, by the Petaluma Formation and Sonoma Volcanics 

and by the Wilson Grove Formation along their western edge 

(fig. 3C). The Sonoma Volcanics, however, were not identified

along the geologic section in the upper 2,000 ft of the aquifer 

system, and water-quality data from wells were only available from about the upper 1,200 ft. The Pliocene and Miocene 

Petaluma Formation is dominated by deposits of moderately to 

weakly consolidated silty to clayey mudstone with local beds 

and lenses of poorly sorted sandstone (Sweetkind and others, 

2010). The Petaluma Formation generally is finer grained then

the overlying Glen Ellen Formation. These heterogeneous 

clay-rich deposits interfinger with the Sonoma Volcanics to the

east and the Wilson Grove Formation to the west.

The WG storage unit consists almost entirely of the 

Wilson Grove Formation. The Pliocene and Late Miocene 

Wilson Grove Formation consists of consolidated, to weakly 

consolidated deposits of massive or thick-bedded, gray to 

buff, fine-grained to very fine grained, fossiliferous sand or

sandstone (Sweetkind and others, 2010). Compared to the 

Petaluma Formation, the Wilson Grove Formation is coarsergrained and more permeable.

Geologic Structure along Section B–B’

Geologic section B-B’ crosses multiple faults, including, from east to west, the Bennett Valley fault zone, Rodgers 

Creek fault zone, Trenton Ridge fault, and the Sebastopol 

fault. The Rodgers Creek and Bennett Valley fault zones 

are right-lateral faults and are branches of the San Andreas 

transform system in northern California (Langenheim and 

others, 2010). The Rodgers Creek fault zone forms the eastern 

boundary of the WB and CB storage units where they meet the 

UPL and VAL storage units (fig. 1). The Trenton Ridge fault 

is a northwest-southeast trending thrust fault that dips to the 

northeast and forms the boundary between the WB and CB 
Chemical Character of Groundwater 139

storage units (fig. 1). The Sebastopol fault forms the boundary 

between the WG and CB storage units and the western boundary of the WB storage unit (fig. 1). An unnamed fault, parallel 

and east of the Sebastopol fault, appears to truncate the eastern 

extent of the Wilson Grove Formation along geologic section 

B–B’ (fig. 3C).

Groundwater Flow along Section B–B’

Groundwater-level contour maps presented in chapter B

(see fig. 29A) indicate that prior to significant groundwater

development in the SRP, groundwater along the cross section flowed from the Mayacmas and Sonoma Mountains, in

the UPL storage unit, westward toward the Laguna de Santa 

Rosa, on the western edge of the SRP, and eastward from the 

highlands in the WG storage unit toward the Laguna de Santa 

Rosa. In the UPL and VAL storage units, water-level contour 

lines bend in the upstream direction where they cross the confluence of the Santa Rosa, Spring, and Matanzas creeks east of

the Rodgers Creek fault, indicating that streams are receiving 

water from the groundwater system (gaining streams). In contrast, immediately west of the Rodgers Creek fault zone in the 

WB and CB storage units, the water-level contour lines bend 

in the downstream direction where they cross the Santa Rosa 

Creek, indicating the stream is losing water to (recharging) the 

groundwater system (see chapter B, fig. 29A). About 5 mi west 

of the Rodgers Creek fault, water-level contour lines again 

bend in the downstream direction where they cross the Santa 

Rosa Creek, indicating that the stream is a gaining stream in 

the western part of the SRP (see chapter B, fig. 29A). Limited 

depth-dependent water-level data were available to determine 

the vertical flow gradient between geologic units along the

cross section.

General Chemical Composition

The variation in major-ion composition of groundwater 

along geologic section B-B’ is represented by Stiff diagrams 

and dissolved-solids concentrations for representative samples 

(fig. 3C). The groundwater was a mixed cation-bicarbonate 

type water, and most samples had relatively low dissolvedsolids concentrations (about 230 mg/L or less) on both ends of 

the geologic section. As groundwater flows from the Mayacmas Mountains to the west, its dissolved-solids concentration 

increased with increasing distance from the mountains. Depthdependent samples from the Sonoma Volcanics in well W89 

had dissolved-solids concentrations of about 400 to 500 mg/L, 

which was about twice that in the sample from well W85 near 

the Mayacmas Mountains.

West of the VAL storage unit, in the Rodgers Creek 

fault zone, groundwater sampled from 786 to 806 ft bls in the 

open borehole of well W102 (sample W102B) when it was 

being drilled had a dissolved-solids concentration of about 

1,800 mg/L and was a sodium-chloride type water. As stated 

previously, the high dissolved-solids concentration in well 

W102B was attributed to deeply circulating groundwater rising along the fault zone. The completed well W102, perforated from 130 to 180 ft bls in the Glen Ellen and Petaluma 

Formations, yielded a sodium-bicarbonate type water with a 

dissolved-solids concentration of 534 mg/L. The relatively 

high concentrations of sulfate, boron, and dissolved solids in 

shallow water from well W102 (sample W102A) compared 

to the sample from well W85 at the east end of the section 

indicated possible upward flow along the fault zone through

the deep aquifer and subsequent mixing with fresher groundwater in the shallow aquifer (appendix A). Additional depthdependent samples are needed upgradient and downgradient of 

the Rodgers Creek fault zone to confirm this observation.

Comparison of data from the Rodgers Creek fault zone 

to available data from downgradient (west) of the fault zone 

indicated that the high dissolved-solids concentration of water 

in the fault zone did not significantly affect the water quality

of wells in the WB and CB storage units. With the exception 

of the sample from well W107, all samples west of the Rodgers Creek fault zone had lower dissolved-solids concentrations 

than both the shallow and deep samples from well W102. 

The sample from well W103, the nearest downgradient well to 

well W102, was a mixed cation-bicarbonate type water with 

a dissolved-solids concentration of 344 mg/L. The difference in water type and its significantly lower dissolved-solids

concentration indicated that groundwater underflow across the

Rodgers Creek fault zone is minimal along geologic section 

B–B’. The Rodgers Creek fault zone likely restricts the lateral 

movement of water from the UPL and VAL storage units to the 

WB and CB storage units, forcing groundwater to discharge 

toward the stream channels east of the fault zone, as indicated 

by the groundwater-level contour maps presented in chapter B

(see fig. 29A). The mixed cation-bicarbonate type water with 

low dissolved-solids concentration sampled from well W103, 

downgradient of the Rodgers Creek fault zone, is probably the 

result of recharge from Santa Rosa Creek west (downgradient) 

of the fault zone.

Stiff diagrams for samples from wells perforated within 

the Wilson Grove Formation, along the western end of section 

B-B’, display a discernible difference in major-ion composition compared to samples from wells perforated in the Glen 

Ellen and Petaluma Formations to the east (fig. 3C). Calcium 

was the predominant cation of mixed cation-bicarbonate type 

water in samples from wells perforated in the Wilson Grove 

Formation. This water type is likely a reflection of recharge

from comparatively low-elevation hills to the west through the 

marine sandstone of the Wilson Grove Formation. Calcium 

carbonate is commonly present as cementing material for 

fossil-rich sedimentary deposits and is a likely source of calcium in solution. Moreover, the relative lack of clay minerals 

in the Wilson Grove Formation provides little opportunity for 

the replacement of calcium in solution with sodium by means 

of ion exchange with clays.

The dissolved-solids concentrations in samples from 

wells perforated almost exclusively in the Wilson Grove 

Formation along the cross section (wells W32, W35, W76, 

W77, W82, and W129) were less than about 350 mg/l, and 

had a mean of about 200 mg/L (fig. 3C). The samples from 

wells with perforations in the Glen Ellen and Petaluma 

Formations immediately east of the WG storage unit 
140 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

(W31, W33, and W34) were sodium-bicarbonate type water, 

and had dissolved-solids concentrations in excess of 420 mg/L 

(fig. 3C). The difference in water type and dissolved-solids 

concentrations indicated limited groundwater interaction 

between the Wilson Grove Formation and the neighboring 

Glen Ellen and Petaluma Formations. In addition, the low 

permeability clay layers in the Petaluma Formation limit the 

vertical groundwater movement between the Wilson Grove 

Formation and overlying Petaluma Formation.

Samples from shallow wells perforated or completed in 

the Glen Ellen Formation in the CB storage unit were primarily a mixed cation-bicarbonate type water in which magnesium was the most abundant cation, with the exception of the 

sample from well W33, which was a sodium-bicarbonate type 

water. Samples from wells perforated or completed in the 

Petaluma Formation, or in both the Glen Ellen and Petaluma 

Formations in the CB storage unit, were a mixed cation-bicarbonate type water in which sodium was the most abundant 

cation or were a sodium-bicarbonate water. In general, the 

dissolved-solids concentrations of samples from the shallow wells were higher than dissolved-solids concentrations 

of samples from the deep wells, which could be the result of 

anthropogenic factors, such as irrigation return flows. The

samples from wells perforated in the Petaluma Formation, or 

in both the Glen Ellen and Petaluma Formations in the CB 

storage unit, had higher dissolved-solids concentrations and a 

greater proportion of sodium than samples from wells perforated in the Wilson Grove Formation on the western end of the 

CB storage unit (fig. 3C). The difference in dissolved-solids 

concentration and water type indicated that the unnamed 

fault, east of the Sebastopol fault, is at least a partial barrier to 

groundwater flow.

Temporal Trends

Temporal trends in water quality in the study area were 

evaluated by using analyses of specific conductance and chloride concentrations from 33 wells presented in appendix A that 

had a minimum of 8 records spanning 20 or more years and 

included at least one analysis between 2000 and 2010. Specific

conductance is a general indicator of the water quality of a 

sample because it is directly related to the dissolved-solids 

concentration of the sample (Hem, 1992). Chloride was used 

in this study as a single-constituent indicator of general water 

quality because it is nonreactive geochemically and highly 

soluble. As a result of these properties, variations in chloride 

concentration are generally attributed to mixing with water 

from different sources (Hem, 1992). Because chloride is both 

a conservative ion and a major component of specific conductance, the two are usually strongly correlated.

The specific conductance of water from almost 75 percent

of the 33 wells (appendix A) assessed for this study increased 

over time, with about half increasing by more than 10 percent 

of the initial value during the period of record (24–60 years for 

individual wells). Chloride behaved similarly, with concentrations increasing in about 67 percent of the wells evaluated, 

and just over half increasing by more than 10 percent. Specific

conductance and chloride concentration decreased in approximately 15 and 30 percent of evaluated wells, respectively, by 

more than 10 percent.

The largest increases in specific conductance, chloride

concentration, or both were in wells W28, W41, W60, W65, 

and W73, located in the vicinity of the cities of Rohnert Park 

and Cotati (fig. 4; appendix A). The specific conductance of

water from these wells increased more than 50 percent: from 

310–380 μS/cm, in the early 1980s, to 520–610 μS/cm by 

2009 (fig. 4). Chloride concentrations in water from these 

same wells also increased by more than 50 percent, with the 

exception of well W73, where the increase was less than 

10 percent (fig. 4). Possible sources of increased specific

conductance in the Rohnert Park/Cotati area include, groundwater underflow of high dissolved-solids concentration water

present along the Rodgers Creek fault zone (for example, 

W102B), irrigation return flow, and septic-tank effluent or

leaking sewer pipes. Groundwater pumping and fluctuating

water levels could contribute to this possible change in the 

source of water to wells; municipal pumpage in Rohnert Park 

alone, more than doubled from about 2,500 acre-ft in 1976 to 

about 5,500 acre-ft in 1995, before being reduced to nearly 

zero by 2006 (see chapter B, fig. 37). As a consequence, 

water levels generally decreased between the mid-1970s and 

mid-1990s and then remained unchanged for a few years 

before increasing in the early 2000s (see chapter B, fig. 36). 

However, comparison of specific conductance and chloride

versus groundwater withdrawals and water levels indicated 

no better than a weak relationship. Because of the presence 

of numerous wells in this part of the study area, it is unclear 

whether observed water-quality changes were in response to 

local conditions at these particular wells or were in response to 

more regional effects. Depth-dependent hydrologic, chemical, 

and isotopic data are needed to better understand the source of 

the increased specific conductance and chloride values.

The greatest decreases in specific conductance and chloride concentration were in samples from well W107 (fig. 4). 

Well W107 historically yielded some of the highest values 

in the SRP for specific conductance and chloride concentration, with maximum values in excess of 1,200 μS/cm and 

150 mg/L, respectively (fig. 4; appendix A). The well is 

relatively shallow (perforated from 47 to 67 ft bls) and is in an 

area that has been classified as residential since 1974. Anthropogenic inputs could account for the relatively high specific

conductance and chloride concentration values; however, 

because land use has not changed substantially in the vicinity 

of this well, it is unclear why water quality improved recently.

Water-Quality Constituents of Potential Concern

Specific conductance, chloride, dissolved solids, nitrate,

arsenic, boron, iron, and manganese are water-quality constituents of potential concern in the SRPW because concentrations 

of these constituents in samples from some wells exceed state 
Chemical Character of Groundwater 141

i

122°40’

38°

30’

38°

20’

12

116

101

Upland

Upland

Valley

Windsor

Wilson

Grove

Cotati

R9W 

T

9

N 

R8W R7W R6W 

T

8

N 

T

7

N 

T

6

N 

T

5

N 

i 0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Healdsburg Fault

Carneros Fault

Sebastopol Fault

Trenton Ridge Fault

Bennett Valley Fault Zone

Maacama Fault Zone

Petrified Forest Thrust Zone

Rodgers Creek Fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure C04-1

Trend line

Chloride in milligrams per liter

Specific conductance in

 microsiemens per centimeter

EXPLANATION

Shallow (0’ to 150’ below

 land surface datum)

Mixed

Deep (Greater than 150’)

Construction unknown

Well perforated interval

Rodgers Creek

 Fault Zone Cotati Basin Valley

Upland

Windsor Basin Wilson Grove

Groundwater storage units

122°50’

Inferred fault

W85

W79

W74

W156

W122

W107

W32

W28

W101

W138

W136

W110

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W156

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W136

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

Year

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W138

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W85

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W101

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W107

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W28

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W32

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W74

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W79

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150 1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W110

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W122

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

Figure 4. Time-series plots of specific conductance and chloride for selected wells in the Santa Rosa Plain watershed, Sonoma 

County, California, 1950–2009.
142 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

i

122°40’

38°

30’

12

116

101

Upland

Upland

Valley

Windsor

Wilson

Grove

Cotati

R9W 

T

9

N 

R8W R7W R6W 

T

8

N 

T

7

N 

T

6

N 

T

5

N 

i

122°50’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Petaluma

Valley

Healdsburg Fault

Carneros Fault

Sebastopol Fault

Trenton Ridge Fault

Bennett Valley Fault Zone

Maacama Fault Zone

Petrified Forest Thrust Zone

Rodgers Creek Fault

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure C04-2 continued

Trend line

Chloride in milligrams per liter

Specific conductance in

 microsiemens per centimeter

EXPLANATION

Mixed

Deep (greater than 150’

 below land surface datum)

Well perforated interval

Rincon

Valley

Kenwood

Valley

Mendocino Range

Sonoma Mountains

Mayacmas Mountains

Rodgers Creek Cotati Basin Valley Fault Zone

Upland

Windsor Basin Wilson Grove

Groundwater storage units

Inferred fault

W73

W8

W60

W64 W15

W65

W58

W57

W7 W42 W41

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W57

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W7

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W8

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W15

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W73

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W60

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W64

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W65

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W58

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W42

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

1950

YEAR

0

300

600

900

1,500

1,200

1960 1970 1980 1990 2000 2010

W41

Chloride, in milligrams per liter

Specific conductance,

in microsiemens per centimeter

0

100

200

50

150

Figure 4. Continued.
Chemical Character of Groundwater 143

or federal recommended or mandatory regulatory standards 

for drinking water (table 4). This section describes the sources, 

properties, and concentrations of these constituents in the 

SRPW. The range, median, and number of wells above regulatory standards for each constituent were summarized by storage unit and depth (table 4). Only the most recent complete 

analysis from each well (appendix A) was used to compile this 

summary.

Specific Conductance

The presence of charged ionic species in solution makes 

the solution conductive. As ion concentrations increase, 

conductance increases; thus, specific conductance at 25 used

in this study provides an indication of total-ion or dissolvedsolids concentration. Specific conductance has a recommended secondary maximum contaminant level for California 

(SMCL-CA) of 900 μS/cm (U.S. Environmental Protection 

Agency, 2009; California Department Public Health, 2011).

In the SRPW study area, only 3 of the 144 samples 

from wells with specific conductance measurements

(about 2 percent) had values greater than or equal to the 

SMCL of 900 μS/cm (table 4). The highest specific conductance reported in table 4 is 4,420 μS/cm, which was 

measured in a sample from well W47 in the CB storage unit 

(appendix A).

Chloride 

Chloride occurs naturally in groundwater from the 

weathering and leaching of sedimentary rocks and soils, and 

the dissolution of salt deposits, such as the evaporate deposit 

halite or sodium chloride (NaCl; Hem, 1992; Mullaney 

and others, 2009). An additional source is saline “connate”

groundwater. The term connate implies that the solute source 

is fossil seawater trapped in the geologic formations when 

they were deposited (Hem, 1992). Anthropogenic sources 

of chloride include manufacturing, power generation, leachate from landfills, water softener backwash, and wastewater

(Hem, 1992). Chloride has a recommended SMCL-CA of 

250 mg/L (U.S. Environmental Protection Agency, 2009; 

California Department Public Health, 2011). The SMCL for 

chloride is based on aesthetic properties, including a salty taste 

and a common association with sodium, which is of concern 

to individuals susceptible to hypertension. Although not toxic 

to humans, chloride presents a common toxicity problem for 

plants (Ayers and Westcot, 1994). Because chloride is not 

absorbed or held back by soils, it is very mobile in groundwater and soil water; therefore, it is taken up by the crop, moves 

through the phloem, and accumulates in the leaves. Chloride 

tolerance varies broadly across different crops. Some of the 

more sensitive crops include avocado, strawberries, berries, 

and certain types of grapes and citrus (Ayers and Westcot, 

1994).

Only 3 of the 148 samples from wells in the SRPW study 

area with chloride analyses (about 2 percent) had concentrations greater than or equal to the SMCL of 250 mg/L (table 

4). The highest chloride concentration reported in table 4 is 

1,240 mg/L, which was measured in a sample from well W47 

in the CB storage unit (appendix A).

Dissolved Solids

Total dissolved solids (or dissolved solids) are a measure 

of all dissolved chemicals in water (Hem, 1992; California 

State Water Resources Control Board, 2010). Dissolved solids 

can be measured or calculated in several ways; most commonly, a summation of dissolved ions is used, if a complete 

analysis is available, or the residue that remains after evaporation at a specific temperature is weighed (residue-on-evaporation, or ROE, at 180 in this study). The computed dissolved 

solids can differ from the ROE value by 10–20 mg/L in either 

the positive or negative direction when the solids concentration is 100–500 mg/L (Hem, 1992); thus, a direct comparison of dissolved-solids concentrations from different labs is 

difficult, unless the method used by the analyzing laboratory

is known. For this report, the total dissolved-solids concentration was calculated as the summation of the reported major 

cations (calcium, magnesium, sodium, and potassium) and 

anions (chloride, sulfate, bicarbonate, fluoride, silica, and

nitrate, if available). Total dissolved solids has a recommended 

SMCL-CA of 500 mg/L (U.S. Environmental Protection 

Agency, 2009; California Department Public Health, 2011).

Of the 148 samples from wells in the SRPW study 

area with dissolved-solids analyses, 19 (about 13 percent) 

had concentrations greater than or equal to the SMCL of 

500 mg/L (table 4). The highest dissolved-solids concentration reported in table 4 is 2,755 mg/L, which was measured in 

a sample from well W47 in the CB storage unit (appendix A). 

In the WB and CB storage units, the median dissolved-solids 

concentration was higher in samples from shallow wells than 

in samples from deep wells (table 4), which could reflect

anthropogenic sources of dissolved-solids concentrations, such 

as irrigation-return flows, to the shallow aquifer.

Nitrate

Nitrate (NO3) is one of the most frequently identified contaminants in groundwater and is attributable to both anthropogenic and natural sources (Freeze and Cherry, 1979). Natural 

sources of nitrate include the atmosphere and decomposition 

(oxidation, or mineralization) of organic material (Hem, 

1992). Anthropogenic sources include fertilizers, septic-tank 

effluent, leaking sewers, and atmospheric deposition of nitrogen emissions (Hem, 1992). Nitrate concentrations in drinking 

water in excess of the maximum contaminant level (MCL) of 

10 mg/L as nitrogen (N), equivalent to 45 mg/L as NO3, are 

considered hazardous and can result in methemoglobinema 

(blue-baby syndrome) in small children (Hem, 1992).
144 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Only 2 of the 91 samples from wells in the SRPW 

study area with nitrate analyses (about 2 percent) had NO3-N 

concentrations greater than or equal to the MCL of 10 mg/L 

(table 4). NO3-N concentrations in the UPL storage unit, where 

anthropogenic contributions are expected to be low, were all 

less than or equal to 0.4 mg/L (table 4). On the basis of the 

NO3-N concentrations in the UPL storage unit, concentrations 

greater than 1 mg/L in the SRPW likely reflect anthropogenic

contributions. Based on this assumption, anthropogenic nitrate 

was present in some shallow wells of the WB and CB storage units that had median concentrations of 0.9 and 4.4 mg/L, 

respectively (table 4). As expected, NO3-N concentrations 

were lower in the deep wells of these storage units (median 

concentrations of 0.2 and 1.0, respectively; table 4). Concentrations are typically lower at depth because of greater 

distance from the anthropogenic source at land surface and 

increased opportunity over time and distance for nitrate to 

be diluted by mixing with low-nitrate groundwater and to be 

denitrified under anoxic conditions in the aquifer system.

Arsenic

Arsenic is a tasteless, odorless, semi-metallic element 

which can be present in surface and groundwater from natural 

and anthropogenic sources. Arsenic in the natural environment 

is most commonly associated with sulfide and with ferromanganese-oxide minerals, particularly in areas characterized by 

geothermal water or high evaporation rates (Welch and others, 

2000). Anthropogenic sources of arsenic include wood preservatives, pesticides, and used as a semi-conductor in the manufacture of microelectronics (Agency for Toxic Substances 

and Disease Registry, 2009). Industrial and agricultural uses 

of arsenic have been reduced in the United States over the 

last several decades because of the increasing recognition of 

its threat to human health (Agency for Toxic Substances and 

Disease Registry, 2009). The MCL for arsenic was lowered 

from 50 μg/L to 10 μg/L in 2006. Long-term exposure to 

arsenic in water can cause a variety of dermal, cardiovascular, 

respiratory, and neurological ailments, as well as an increased 

risk of some cancers (U.S. Environmental Protection Agency, 

2009; California Department of Public Health, 2011).

Of the 58 samples from wells in the SRPW study area 

with arsenic analyses, 7 (about 12 percent) had arsenic 

concentrations greater than or equal to the MCL of 10 μg/L 

(table 4). The highest arsenic concentration reported in table 4

is 20 μg/L, which was measured in a sample from well W156 

in the WB storage unit (appendix A). About 30 percent of the 

wells sampled for arsenic in the WB and WG storage units had 

arsenic concentrations in excess of the MCL.

Boron

Boron is a naturally occurring metalloid in many minerals. Natural sources of boron include the mineral tourmaline, 

which is present in igneous rocks, and evaporite minerals, 

such as borax, colemanite, and kernite (Hem, 1992; Reimann 

and Caritat, 1998). The most prevalent sources of boron in 

water are from the leaching of rocks and soils that contain 

borate or borosilicate minerals, wastewater with cleaning agents containing boron, and fertilizers and pesticides 

(California State Water Resources Control Board, 2010). 

High boron concentrations (in excess of 48,000 mg/L) have 

been associated with thermal springs (Hem, 1992). Although 

boron is a trace micronutrient necessary for metabolism of 

important substances such as calcium and magnesium, high 

concentrations of boric acid, the most common form in drinking water, can be toxic to humans. Symptoms of boric acid 

ingestion include gastrointestinal tract distress, vomiting, 

abdominal pain, diarrhea, and nausea (State Water Resources 

Control Board, 2010). Once boron compounds dissolve, 

they behave like a salt (dissolved anion) and are difficult to

remove by treatment. Boron has a health-based notification

level (NL-CA) of 1,000 μg/L. NL-CAs are established by 

the CDPH for some constituents in drinking water that lack 

MCLs. If a constituent is detected at concentrations greater 

than its NL-CA, California State law requires timely notification of local governing bodies and recommends consumer 

notification. Boron is essential to plant nutrition; however,

concentrations in a small excess of the required amount are 

toxic to some plants (Hem, 1992). For example, the optimal 

boron concentration range is 300–500 μg/L for citrus and 

grapes, but concentrations slightly higher are detrimental to 

these crops (Kabay and others, 2006).

Of the 61 samples from wells in the SRPW study area 

with boron analyses, 4 (about 7 percent) had boron concentrations greater than or equal to the NL-CA of 1,000 μg/L 

(table 4). The highest boron concentration reported in table 4

is 2,100 μg/L, which was measured in the deep sample 

from well W102 in the WB storage unit (sample W102B; 

appendix A). The median boron concentration was about 

100 μg/L in samples from wells with boron analyses in the 

WB and CB storage units, areas where most of the irrigated 

agriculture exists.

Iron

Iron is derived from natural weathering of many rocks 

and minerals whose iron content is relatively high, including 

pyroxenes, amphiboles, biotite, magnetite, and olivine (Hem, 

1992). When exposed to anoxic or suboxic conditions and 

low to near neutral pH, iron is readily dissolved in the form 

of ferrous (Fe2+) iron. Under oxidizing conditions (dissolved 

oxygen greater than 1–2 mg/L), and at all but a very low pH, 

iron exists as ferric (Fe3+) iron, which is much less soluble 

(Hem, 1992). The SMCL of iron is 300 μg/L (U.S. Environmental Protection Agency, 2009; California Department of 

Public Health, 2011). Excessive amounts of iron can cause 

aesthetic effects, such as an objectionable metallic taste and 

turning water a reddish or orange color that can stain plumbing fixtures and other surfaces, but normally, it does not pose

a health risk. Excessive iron can also have economic implications, including corrosion and encrustation of steel well casing, perforations, and related infrastructure.
Source and Age of Groundwater 145

Of the 54 samples from wells in the SRPW study area 

with iron analyses, 23 (about 43 percent) had iron concentrations greater than or equal to the SMCL of 300 μg/L 

(table 4). The highest iron concentration reported in table 4 is 

7,200 μg/L, which was measured in a sample from well W119 

in the CB storage unit (appendix A). About 50 percent of the 

samples from wells with iron analyses in the WB and CB storage units were at or above the SMCL.

Manganese

Manganese is derived from natural weathering of many 

rocks and minerals, including basalt, many olivines, pyroxene, 

and amphibole (Hem, 1992). Similar to iron, manganese is 

readily dissolved under reducing conditions (dissolved oxygen 

is absent) and low to near-neutral pH. In the presence of 

oxygen and carbonate or silicate at high pH, manganese will 

precipitate out of solution and form black-colored deposits 

that, in drinking water, are unpleasant in appearance and taste 

(Hem, 1992). At concentrations above the SMCL of 50 μg/L 

(U.S. Environmental Protection Agency, 2009; California 

Department of Public Health, 2011), manganese can have 

a metallic taste and cause staining of plumbing fixtures and

other surfaces, but normally, it does not pose a health risk.

Of the 60 samples from wells in the SRPW study area 

with manganese analyses, 44 (about 73 percent) had manganese concentrations greater than or equal to the SMCL 

of 50 μg/L (table 4). The highest manganese concentration 

reported in table 4 is 1,545 μg/L, which was measured in a 

sample from well W142 in the WB storage unit (appendix A). 

Of the samples from wells with manganese analyses in the 

WB and CB storage units, about 94 and 70 percent, respectively, were at or above the SMCL.

Source and Age of Groundwater

Stable and radioactive isotopes, including oxygen-18 

(18O), deuterium (2

H), tritium (3H), carbon-14 (14C), and 

carbon-13 (13C), were used to determine the sources and ages 

of water in the SRPW. Analyses from samples collected for 

this study, and selected data from the NSF GAMA study 

(Kulongoski and others, 2010, appendix B9, tables 6 and 7), 

were used to gain insight into recharge processes and the evolution of water quality in the study area.

Oxygen-18 and Deuterium

Two sets of samples were collected from 15 surfacewater sites during the fall of 2008 and spring of 2009 for analysis of oxygen-18 and deuterium (fig. 1A, table 5). Additional 

samples were collected from one surface-water site (SW21) 

during 2003–07 independent of this study (fig. 1A, table 5). 

Groundwater samples were collected from 32 wells, including discrete-depth zones in 3 wells (W77, W89, and W102) 

during 2004–10 (fig. 5A, table 5). Twenty-five of the groundwater samples were collected as part of the NSF GAMA study 

(Kulongoski and others, 2006).

Background

Oxygen-18 (18O) and deuterium (2H or D) are stable 

(nonradioactive), naturally occurring isotopes of oxygen and 

hydrogen. The abundance of heavier oxygen-18 and deuterium 

relative to isotopically lighter oxygen-16 (16O) and hydrogen-1 

or protium (1H) can be used to infer the source and the evaporative history of water. Oxygen-18 and deuterium data are 

expressed in delta notation (δ) as per mil (parts per thousand, 

‰) differences in the ratios of 18O/16O and 2H/1H in samples 

relative to a standard known as Vienna Standard Mean Ocean 

Water (VSMOW; Gat and Gonfiantini, 1981). By convention, the value of VSMOW is 0 per mil. Oxygen-18 (δ18O) 

and deuterium (δ2

H or δD) ratios are useful in a wide variety 

of hydrologic studies (Gat and Gonfiantini, 1981). Analytical

precision is generally within about 0.2 and 2 per mil for δ18O 

and δD, respectively (Coplen, 1994).

Because the source of much of the world’s precipitation 

is derived from the evaporation of seawater, the δ18O and δD 

compositions of precipitation near coasts throughout the world 

cluster along a line known as the global meteoric water line 

(GMWL; Craig, 1961), such that the following is found:

18   D O = + 8 10 (1)

Plotting the isotopic composition of precipitation shows a 

general trend from heavier (having more oxygen-18 and deuterium and, therefore, less negative values) to lighter (having 

less oxygen-18 and deuterium and, therefore, more negative 

values) from the equator to the poles (Gat and Gonfiantini,

1981). Storms that originate over cold waters in the Gulf of 

Alaska have a lighter isotopic composition than storms that 

originate over warm tropical waters in the vicinity of Hawaii 

or the Gulf of Mexico. Local meteoric water lines (LMWL) 

can be slightly offset from the GMWL by a different intercept term in equation 1 (deuterium excess), but are parallel 

to the GMWL. In the case of this study, there was no wellconstrained LMWL, so the GMWL was used as a surrogate for 

comparison with local samples.

Differences also result from fractionation (partitioning of 

molecules of different isotopic composition of the same element) as moist air masses move inland. As storms move inland 

from coastal areas, the concentration of heavier isotopes relative to lighter isotopes decreases because heavier isotopes are 

preferentially concentrated in the liquid phase, and lighter isotopes are preferentially concentrated in the vapor phase during 

repeated cycles of evaporation and condensation. In addition, 

precipitation that condenses at higher altitudes and at cooler 

temperatures tends to be isotopically lighter than precipitation 

that condenses at lower altitudes and warmer temperatures 

(Muir and Coplen, 1981).
146 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

i

122°50’ 122°40’

38°

30’

38°

20’

12

116

101

Upland

Upland

Valley

Windsor

Wilson Grove

Cotati

R9W 

T

9

N 

R8W R7W R6W 

T

8

N 

T

7

N 

T

6

N 

T

5

N 

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe

 Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

Santa Rosa Plain

Mendocino Range

Sonoma Mountains

Mayacmas Mountains

B'

B

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

0

0 5 10 Kilometers

5 10 Miles

IP019208_Figure C05a

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection EXPLANATION

Shallow (0’ to 150’ below

 land surface datum)

Mixed

Deep (greater than 150’)

Construction unknown

Well perforated interval Well number

Deuterium, in per mil

W76

−37.7

B B’ Line of section

Rodgers Creek

 fault zone

Inferred fault

W88

−43.0

W89A

−42.6

W89B

−42.1

W89C

−42.1

W77A

−37.7

W77B

−36.7

W77C

−37.8

W33

−44.8

W34

−49.3

W35

−36.2

W102A

−43.4

W102B

−47.1

W82

NM

W156

−40.7

W140

−38.0

W94

W123 −36.0

−36.1

W124

−42.7

W109

−42.7 W129

−38.9 W75

−37.7

W76

−37.7

W32

−35.9 W30

−37.7

W26

−40.9

W44

−41.3

W67

−42.2

W69

−38.3 W1

−38.6

W21

−39.9

W10

−41.0

W9

−42.3

W133

−39.2

W132

−41.2

W91

−41.0

W96

−39.1 W83

−41.1

W131

NM

Figure 5. Delta deuterium and oxygen-18 data from selected wells in the Santa Rosa Plain watershed, Sonoma County, California, 

2003–10: A, areally and B, along section B-B’.
Chemical Character of Groundwater 147 sac12-0452_Figure C05b_7

B Southwest Northeast

B B'

FEET

SEA

LEVEL

FEET

–1,000–2,000

SEA

LEVEL

1,000

0 6,000 12,000 18,000 24,000 30,000 36,000 42,000 48,000 54,000 60,000 66,000

–1,000–2,000

Rodgers Creek

fault zone

Sebastopol

fault 

Trenton

Ridge

 fault 

Bennett Valley

fault zone

Cotati Basin Wilson Grove Windsor

Basin

Valley Uplands

Mayacmas Mountains

Distance along section, in feet 12.85x vertical exaggeration Geology modified from McLaughlin and others (2008) and Sweetkind and others (2010).

?

?

?

? ?

?

−36.7

−36.2

−43.4

−47.1

−42.1

−42.9

−42.1

−49.3

−37.8

−37.8

−44.8

−37.7

−37.7

−38.9

−35.9

−42.7

−37.7

−38.7

−40.9

−36.0

−41.1

W77*

W33-35

W129*

W32* W102 W89* 

W83*

W75*

W76*

W124* W94* W30* W109* W26*

W124* Glen Ellen Formation

Wilson Grove Formation

Sonoma Volcanics

Petaluma Formation

Undifferentiated basement

EXPLANATION 

Well (black = production well, 

Well perforation (white box)

red = observation or discretedepth sampled test well)

Well map number (asterisk indicates well projected to section) Fault, sense of offset

 not shown

Deuterium,in per mil

−38.9

Laguna de Santa Rosa

Figure 5. Continued.
148 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California Table 5. Summary of isotopic data in samples from selected surface-water sites and wells, Santa Rosa Plain watershed and neighboring area, Sonoma County, California, 2003–10.—Continued [See figure 1 for locations of surface-water sites and figure 7 for locations of selected wells. Sample depths listed for multi-zone sampled wells and piezometers. δiE is delta notation, the ratio of the heavier isotope (i) to the more common lighter isotope of an element (E), relative to a standard reference material, expressed as per mil (parts per thousand). Abbreviations: Ave, avenue; Cr, circle; Crk, creek; Dr., drive; mm/dd/yyyy, month/day/year; Mtn., mountain; N/A, not applicable; no., number; nr, near; pCi/L, picocuries per liter; Rd., road; Sprs, springs; St., street; TU, tritium units; Unk, unknown; USGS, U.S. Geological Survey; –, no data; @, at; <, less than] Map no. Groundwater storage unit State well number or surface-water site name USGS identification number Depth category Sampled interval (feet) Sample date (mm/dd/yyyy) Delta deuterium (per mil) Delta oxygen-18 (per mil) Tritium (pCi/L) Tritium2 (TU) Carbon-14 (percent modern) Uncorrected carbon-14 age (years before present) Delta carbon-13 (per mil) Surface-water sites SW1 Uplands Crane Creek nr Rohnert Park 382051122383801 – – 10/24/2008 –35.5 –5.33 – – – – – SW1 Uplands Crane Creek nr Rohnert Park 382051122383801 – – 04/02/2009 –39.2 –6.10 – – – – – SW2 Cotati Laguna de Santa Rosa @ Stony Point 11465680 – – 10/23/2008 –25.1 –3.30 – – – – – SW2 Cotati Laguna de Santa Rosa @ Stony Point 11465680 – – 04/01/2009 –33.1 –4.92 – – – – – SW3 Uplands Santa Rosa Crk @ Wildwood Mtn. Rd. 382801122371801 – – 10/24/2008 –36.8 –5.83 – – – – – SW3 Uplands Santa Rosa Crk @ Wildwood Mtn. Rd. 382801122371801 – – 04/02/2009 –41.8 –6.76 – – – – – SW5 Uplands Spring Lake @ pier 382710122391401 – – 10/24/2008 –13.1 0.41 – – – – – SW5 Uplands Spring Lake @ pier 382710122391401 – – 04/02/2009 –36.2 –4.54 – – – – – SW6 Valley Brush Cr @ Hwy 12 11466065 – – 10/24/2008 –23.7 –3.44 – – – – – SW6 Valley Brush Cr @ Hwy 12 11466065 – – 04/02/2009 –37.4 –5.80 – – – – – SW7 Valley Matanzas Crk @ Bethards Dr. 382543122394601 – – 10/24/2008 –38.2 –5.89 – – – – – SW7 Valley Matanzas Crk @ Bethards Dr. 382543122394601 – – 04/02/2009 –40.0 –6.06 – – – – – SW8 Windsor Santa Rosa Crk @ Pierson St. 11466200 – – 10/24/2008 –37.9 –5.45 – – – – – SW8 Windsor Santa Rosa Crk @ Pierson St. 11466200 – – 04/02/2009 –40.1 –6.37 – – – – – SW9 Windsor Matanzas Cr @ Santa Rosa 11466170 – – 10/24/2008 –38.0 –5.54 – – – – – SW9 Windsor Matanzas Cr @ Santa Rosa 11466170 – – 04/02/2009 –39.0 –5.98 – – – – – SW10 Cotati Santa Rosa Crk @ Willowside Ave 11466320 – – 10/23/2008 –31.2 –4.53 – – – – – SW10 Cotati Santa Rosa Crk @ Willowside Ave 11466320 – – 04/01/2009 –38.9 –6.09 – – – – – SW13 Wilson Grove Laguna de Santa Rosa nr Sebastopol 11465750 – – 10/23/2008 –23.8 –3.07 – – – – – SW13 Wilson Grove Laguna de Santa Rosa nr Sebastopol 11465750 – – 04/01/2009 –33.8 –5.32 – – – – – SW14 N/A Jonive Cr nr Sebastopol 382417122512901 – – 10/23/2008 –31.4 –5.05 – – – – – SW14 N/A Jonive Cr nr Sebastopol 382417122512901 – – 04/01/2009 –34.6 –5.60 – – – – – SW15 Uplands Mark West Cr @ Mark West Sprs 383258122431201 – – 10/24/2008 –37.3 –5.79 – – – – –

SW15 Uplands Mark West Cr @ Mark West Sprs 383258122431201 – – 04/01/2009 –40.6 –6.56 – – – – –

Table 5. Summary of isotopic data in samples from selected surface-water sites and wells, Santa Rosa Plain watershed and neighboring area, Sonoma County, California, 2003–2010.
Chemical Character of Groundwater 149

Table 5. Summary of isotopic data in samples from selected surface-water sites and wells, Santa Rosa Plain watershed and neighboring area, Sonoma County, California, 2003–10.—Continued[See figure 1 for locations of surface-water sites and figure 7 for locations of selected wells. Sample depths listed for multi-zone sampled wells and piezometers. δiE is delta notation, the ratio of the heavier 

isotope (i) to the more common lighter isotope of an element (E), relative to a standard reference material, expressed as per mil (parts per thousand). Abbreviations: Ave, avenue; Cr, circle; Crk, creek; Dr., drive; mm/dd/yyyy, month/day/year; Mtn., mountain; N/A, not applicable; no., number; nr, near; pCi/L, picocuries per liter; Rd., road; Sprs, springs; St., street; TU, tritium units; Unk, unknown; USGS, U.S. 

Geological Survey; –, no data; @, at; <, less than]

Map 

no.

Groundwater 

storage unit

State well number or 

surface-water site name

USGS 

identification 

number

Depth 

category

Sampled 

interval

(feet)

Sample 

date

(mm/dd/yyyy)

Delta 

deuterium

(per mil)

Delta 

oxygen-18

(per mil)

Tritium

(pCi/L)

Tritium2

(TU)

Carbon-14

(percent 

modern)

Uncorrected 

carbon-14 

age

(years before 

present)

Delta 

carbon-13

(per mil)

Surface-water sites—Continued

SW17 Windsor Mark West Cr nr Windsor 11465500 – – 10/24/2008 –29.4 –4.19 – – – – –

SW17 Windsor Mark West Cr nr Windsor 11465500 – – 04/01/2009 –40.6 –6.48 – – – – –

SW18 N/A Russian River @ River Front Park 383101122514901 – – 10/23/2008 –43.7 –6.40 – – – – –

SW18 N/A Russian River @ River Front Park 383101122514901 – – 04/01/2009 –44.5 –6.68 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 09/07/2003 –34.5 –5.07 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 06/02/2004 –31.7 –4.86 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 07/19/2004 –31.3 –4.45 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 09/15/2004 –34.0 –4.54 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 09/14/2005 –34.1 –4.80 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 06/14/2006 –32.4 –4.91 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 07/20/2006 –30.3 –4.14 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 08/23/2006 –33.2 –4.65 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 09/22/2006 –33.6 –4.80 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 06/06/2007 –32.7 –4.71 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 08/22/2007 –31.2 –4.12 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 10/23/2008 –23.1 –3.68 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 04/01/2009 –37.3 –5.68 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 06/16/2010 –36.3 –5.49 – – – – –

SW21 Wilson Grove Mark West Cr nr Mirabel Heights 11466800 – – 08/25/2010 –35.1 –4.82 – – – – –

Wells

W1 Wilson Grove 5N/8W-03G1 381827122434601 Unk – 09/01/2004 –38.6 –6.03 <1 <0.3 – – –

W9 Uplands 6N/7W-23H1 382038122354001 Shallow 76–136 10/05/2004 –42.3 –6.79 7.7 2.4 – – –

W10 Cotati 6N/7W-28L1 382009122382601 Deep 441–862 08/31/2004 –41.0 –6.23 1.6 0.5 – – –

W21 Cotati 6N/7W-31C1 381934122403601 Deep 170–680 09/01/2004 –39.9 –6.01 1.6 0.5 – – –

W26 Cotati 6N/8W-02J2 382337122422902 Deep 369–530 09/20/2004 –40.9 –6.28 1.9 0.6 – – –

W30 Cotati 6N/8W-04R1 382315122443001 Shallow 55–85 08/31/2004 –37.7 –5.89 1.9 0.6 – – –

W32 Cotati 6N/8W-07A2 382307122463801 Deep 650–800 09/13/2004 –35.9 –5.68 ND – – – –

W33 Cotati 6N/8W-07A4 382308122463903 Shallow 60–80 08/22/2007 –44.8 –6.61 2<0.3 <0.1 16.7 14,000 –20.23

W34 Cotati 6N/8W-07A5 382308122463902 Deep 237–257 08/22/2007 –49.3 –7.33 2<0.3 <0.1 1.3 34,000 –21.10W35 Cotati 6N/8W-07A6 382308122463901 Deep 550–570 08/23/2007 –36.2 –5.67 2<0.3 <0.1 58.5 4,000 –22.86W44 Cotati 6N/8W-13R3 382141122410901 Mixed 130–450 09/01/2004 –41.3 –6.28 <1 <0.3 – – –W67 Cotati 6N/8W-26C2 382021122425701 Deep 295–670 09/02/2004 –42.2 –6.29 1 0.3 – – –
150 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California Table 5. Summary of isotopic data in samples from selected surface-water sites and wells, Santa Rosa Plain watershed and neighboring area, Sonoma County, California, 2003–10.—Continued [See figure 1 for locations of surface-water sites and figure 7 for locations of selected wells. Sample depths listed for multi-zone sampled wells and piezometers. δiE is delta notation, the ratio of the heavier isotope (i) to the more common lighter isotope of an element (E), relative to a standard reference material, expressed as per mil (parts per thousand). Abbreviations: Ave, avenue; Cr, circle; Crk, creek; Dr., drive; mm/dd/yyyy, month/day/year; Mtn., mountain; N/A, not applicable; no., number; nr, near; pCi/L, picocuries per liter; Rd., road; Sprs, springs; St., street; TU, tritium units; Unk, unknown; USGS, U.S. Geological Survey; –, no data; @, at; <, less than] Map no. Groundwater storage unit State well number or surface-water site name USGS identification number Depth category Sampled interval (feet) Sample date (mm/dd/yyyy) Delta deuterium (per mil) Delta oxygen-18 (per mil) Tritium (pCi/L) Tritium2 (TU) Carbon-14 (percent modern) Uncorrected carbon-14 age (years before present) Delta carbon-13 (per mil) Wells—Continued W69 Wilson Grove 6N/8W-33M1 381915122452701 Shallow 229–295 10/27/2004 –38.3 –5.92 ND – – – – W75 Wilson Grove 6N/9W-02B1 382400122491201 Mixed 138–528 10/05/2004 –37.7 –6.14 2.2 0.7 45.2 6,000 –15.48 W76 Wilson Grove 6N/9W-02C1 382352122493301 Deep 332–600 10/04/2004 –37.7 –6.12 2.6 0.8 57.5 4,000 –17.99 W77A Wilson Grove 6N/9W-02C2 382352122493801 Shallow 100–120 10/16/2006 –37.8 –6.06 2.1 0.6 57.8 4,000 –17.12 W77B Wilson Grove 6N/9W-02C2 382352122493801 Deep 340–360 10/16/2006 –36.7 –6.04 2<0.3 <0.1 38.5 7,000 –18.82 W77C Wilson Grove 6N/9W-02C2 382352122493801 Deep 640–650 10/12/2006 –37.8 –6.07 1.7 0.5 59.2 4,000 –16.91 W82 Wilson Grove 6N/9W-13J2 382153122480301 Deep 432–452 09/22/2004 – – <1 <0.3 – – – W83 Valley 7N/7W-07K1 382754122402501 Deep 160–265 09/23/2004 –41.1 –6.49 <1 <0.3 – – – W88 Valley 7N/7W-18R2 382647122400702 Shallow 50–100 09/15/2004 –43.0 –6.57 ND – – – – W89A Valley 7N/7W-19G2 382623122403301 Deep 250–290 03/12/2010 –42.6 –6.49 3.0 0.9 34.4 8,000 –14.77 W89B Valley 7N/7W-19G2 382623122403301 Deep 500–620 03/17/2010 –42.1 –6.46 0.7 0.2 27.1 10,000 –15.98 W89C Valley 7N/7W-19G2 382623122403301 Deep 750–860 03/11/2010 –42.1 –6.54 2<0.3 <0.1 26.9 10,000 –16.33 W91 Uplands 7N/8W-01M1 382849122420501 Unk – 09/15/2004 –41.0 –6.46 <1 <0.3 68.7 3,000 –19.88 W94 Windsor 7N/8W-08L1 382759122462001 Mixed 65–341 08/31/2004 –36.0 –5.65 2.2 0.7 – – – W96 Windsor 7N/8W-10A1 382825122434101 Shallow 65–85 09/22/2004 –39.1 –6.16 <1 <0.3 – – – W102A Windsor 7N/8W-24F1 382622122414901 Mixed 130–180 08/24/2009 –43.4 –6.57 2<0.3 <0.1 26.2 10,000 –17.13 W102B Windsor 7N/8W-24F1 382622122414901 Deep 786–806 07/23/2009 –47.1 –7.03 2<0.3 <0.1 3.4 27,000 –22.28 W109 Cotati 7N/8W-19P1 382510122462701 Mixed 61–231 09/30/2004 –38.7 –5.93 1.6 0.5 84.7 1,000 –18.12 W123 Cotati 7N/9W-12K1 382751122482501 Shallow 40–60 09/14/2004 –36.1 –5.82 5.1 1.6 – – – W124 Cotati 7N/9W-24G1 382619122482101 Deep 507–547 09/14/2004 –42.7 –6.53 <1 <0.3 – – – W129 Cotati 7N/9W-36K2 382429122480501 Deep 410–1,020 09/14/2004 –38.9 –6.25 <1 <0.3 24.0 11,000 –19.53 W131 Uplands 8N/7W-07R1 383257122400501 Unk – 09/15/2004 – – 1.6 0.5 – – – W132 Uplands 8N/7W-30E1 383053122410801 Unk – 09/29/2004 –41.2 –6.59 <1 <0.3 – – – W133 Uplands 8N/8W-11G1 383319122424301 Unk – 09/14/2004 –39.2 –6.30 <1 <0.3 – – – W140 Windsor 8N/8W-32K1 382937122460601 Shallow 70–110 09/16/2004 –38.0 –6.01 6.7 2.1 – – – W156 Windsor 8N/9W-13J2 383220122480501 Deep 260–400 09/02/2004 –40.7 –6.37 <1 <0.3 – – –

1 Calculated from tritium concentration in pCi/L using conversion factor of 3.2 pCi/L.2 Below long term minimum detection level (LT-MDL) of 0.3 pCi/L.
Source and Age of Groundwater 151

Isotopic Composition of Samples

The δ18O and δD values ranged from +0.41 to −7.33 and

−13.1 to −49.3 per mil, respectively, for surface- and groundwater samples in the SRPW (fig. 6A, table 5) and plotted on 

either side and along the GMWL (fig. 6A). Water that has 

undergone evaporation plots substantially to the right of the 

GMWL because as water evaporates, the isotopic values of 

the residual water become heavier, which forms an evaporative trend line with a slope of 3 to 6, in contrast to the slope 

of 8 for the GMWL (fig. 6A). The effects of evaporation are 

evident in samples from surface-water sites (fig. 6A). The 

surface-water samples that plot furthest to the right of the 

GMWL were collected during the warmer, drier months—

note, particularly, the samples collected in fall 2008, when 

antecedent effects of seasonal evaporation would be the greatest, from Spring Lake (SW5) and the Laguna de Santa Rosa 

(SW2 and SW13) (fig. 6A). Surface-water samples collected 

during cooler and wetter months, including samples collected 

in spring 2009, were generally isotopically lighter (more 

negative) and clustered closer to the GMWL, indicating little 

evaporation (fig. 6B). The isotopically lightest samples from 

the SRPW were collected in spring 2009 from SW3 on Santa 

Rosa Creek and SW15 on Mark West Creek (fig. 6B, table 5). 

Both sites receive runoff from the Mayacmas Mountains, 

where maximum elevations in the SRWP exceed 2,000 ft 

above sea level. As stated previously, precipitation that condenses at higher altitudes and cooler temperatures tends to be 

isotopically lighter than precipitation that condenses at lower 

altitudes and warmer temperatures. Therefore, precipitation 

and runoff from the Mayacmas Mountains would be expected 

to be isotopically lighter than precipitation that falls directly 

on the lower elevations of the SRP and the lower Mendocino 

Range to the west. Fall and spring δ18O and δD compositions 

of water from the Russian River, as represented by SW18 

(fig. 6B), however, were comparable, which is a reflection of

the river’s comparatively constant flows from a large drainage area, of which the SRPW is only a portion, coupled with 

continual dry-season releases from a large upstream reservoir.

Values of δ18O and δD ranged from −5.65 to −7.33 per

mil and from −35.9 to −49.3 per mil, respectively, for groundwater samples collected in the study area (fig. 6C, table 5). In 

general, the isotopic values of the well samples were similar 

to the range in isotopic values of the spring surface-water 

samples. With the exception of well samples from the UPL 

and WG storage units, the isotopic values generally plotted 

slightly below the GMWL (fig. 6D). This could indicate that 

these samples have experienced evaporation or have mixed 

with evaporated surface waters, or it could reflect a LMWL

that plots parallel to, but slightly to the right of, the GMWL. 

The position of LMWLs relative to the GWML in California 

can differ slightly as a function of elevation, latitude, and other 

factors (Kendall and Coplen, 2001; Bowen and Revenaugh, 

2003). Groundwater samples, however, generally plotted 

closer to the GMWL compared to fall surface-water samples, 

indicating that the groundwater undergoes less evaporation 

seasonally than surface-water (fgs. 6B and 6D).

Isotopic values exhibited no direct relationship to the 

depth of the well’s perforated interval. Samples from deep 

wells had both the heaviest and lightest measured values 

(fig. 6C).

In general, the isotopic values for water samples from 

the UPL and VAL storage units grouped together and, with 

the exceptions of samples from well W34 and W102B, fell 

within the lighter range of isotopic values for all well samples 

in the SRPW study area. The isotopic values of well samples 

in the UPL and VAL storage units were similar to the spring 

2009 isotopic values from surface-water sites SW3 and SW15 

(figs. 6B and 6D, table 5), indicating that precipitation and 

runoff from the Mayacmas Mountains is the probable source 

of recharge to these storage units. The isotopic values for 

samples from the WG storage unit also grouped together, 

but fell within the heavier range of isotopic values for wells 

in the study area (fig. 6D). The difference in isotopic values 

between the WG storage unit and the UPL and VAL storage 

units indicates that these storage units have different sources 

of recharge. Infiltration of precipitation and runoff from the

Wilson Grove highlands, which are at a lower altitude than the 

Mayacmas Mountains and, therefore, likely have precipitation 

with heavier isotopic values, is the probable source of recharge 

to wells in the WG storage unit.

The isotopic values for well samples from the WB and 

CB storage units extended over the entire range of isotopic 

values for all wells in the study area, indicating a mixture of 

water sources (fig. 6D). Most of the samples from wells in the 

WB and CB storage units were heavier (less negative) than 

well samples in the UPL and VAL storage units, indicating 

that groundwater underflow from the latter cannot be the sole

source of recharge to the downgradient WB and CB storage 

units. The heavier isotopic values, which showed only slight 

deviation from the GMWL, indicate that at least some of the 

recharge to the WB and CB storage units originates as precipitation falling directly on the lower elevations of the SRP.

The isotopic values in well samples W34 and W102B 

were significantly lighter (more negative) than the other

surface-water or well samples, indicating that recharge to both 

wells originated at higher altitudes or under cooler conditions than modern precipitation (fig. 6A, table 5). Uncorrected carbon-14 dates for these well samples, presented in 

the “Groundwater Age” section of this chapter, indicate these

wells were recharged near the end of the last North American 

glaciation, when it likely was colder, wetter, or both, which 

would cause isotope ratios to be lighter (more negative) than 

modern precipitation.

Changes in Isotopic Composition along Geologic 

Section B–B’

δD values of well samples were plotted along geologic 

section B–B’ (modified from Sweetkind and others, 2010,

fig. A1–4, and chapter B, figs. 2 and 25) to illustrate changes 

in the isotopic composition as groundwater flows through the

aquifer system (figs. 5A and 5B). A total of 16 wells, including 
152 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Global meteoric water line (Craig, 1961)

SW5

Shallow (0’ to 150’ below

 land surface datum)

Mixed

Deep (greater than 150’)

Construction unknown

Well perforated interval

-50

-60

-40

-30

-20

-10

0

10

-8 -7 -6 -5 -4 -3 -2 -1 0 1

IP019208_Figure C06a

Evaporative trend line

Surface-water site, fall 2008

Surface-water site, spring 2009

Surface-water site, other dates

 2003–10 for SW21

EXPLANATION

delta Oxygen-18, in per mil

delta Deuterium, in per mil

A

SW2 Surface-water site

SW2

SW13

Figure 6. Relation between delta deuterium and delta oxygen-18 for water samples from in the Santa Rosa Plain watershed, Sonoma 

County, California, 2003–10: A, all water samples; B, selected surface-water samples; C, selected groundwater samples by perforated 

interval; and D, selected groundwater samples by groundwater storage unit.
Source and Age of Groundwater 153

IP019208_Figure C06b

Surface-water site, fall 2008

Surface-water site, spring 2009

Surface-water site, other dates 2003–10 for SW21

EXPLANATION

-30

-35

-40

-45

-50

-55

-8 -7 -6 -5

delta Oxygen-18, in per mil

delta Deuterium, in per mil

Evaporative

trend line

Global meteoric water line (Craig, 1961)

B

SW18

SW3

SW14

SW13

SW15

SW17

SW8

SW9

SW7

SW10

SW1

SW21

SW6

SW14

SW1

SW18

SW7

SW8

SW9

SW15

Figure 6. Continued.
154 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

-30

-35

-40

-45

-50

-55

-8 -7 -6 -5

delta Oxygen-18, in per mil

delta Deuterium, in per mil

Shallow (0’ to 150’ below

 land surface datum)

Mixed

Deep (greater than 150’)

Construction unknown

Well perforated interval

EXPLANATION

Global meteoric water line (Craig, 1961)

C

IP019208_Figure C06c

W34 Well number

W34

W102B

Figure 6. Continued.
Source and Age of Groundwater 155

-30

-35

-40

-45

-50

-55

-8 -7 -6 -5

delta Oxygen-18, in per mil

delta Deuterium, in per mil

EXPLANATION

Global meteoric water line (Craig, 1961)

D

IP019208_Figure C06d

Cotati Basin Well number

Valley

Upland

Wilson Grove

Windsor Basin

Well by storage unit

W102B

W34

W34

Figure 6. Continued.
156 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

depth-dependent samples from wells W77, W89, and W102, 

were projected onto geologic section B–B’ for the comparison 

of δD values with geologic units and structure (fig. 5B).

Isotopic values from wells on both the east and west ends 

of the geologic section were relatively similar at all depths 

within each respective area (fig. 5B): δD values east of the 

Rodgers Creek fault ranged from −41.1 to −42.9 per mil, with

a mean composition of −42.0 per mil, whereas δD values 

west of the Sebastopol fault ranged from −36.7 to −37.8 per

mil, with a mean composition of −37.5 per mil. The relatively

uniform isotopic composition of samples within each area 

indicates water recharged under similar conditions with substantial mixing in the subsurface and contrasts with the greater 

variability in areas between the faults. The mean δD value of 

well samples east of the Rodgers Creek fault zone was about 

4.5 per mil lighter (more negative) than for well samples west 

of the Sebastopol fault. This difference can be explained by 

the higher altitude of the Maycamas and Sonoma Mountains 

on the east side of the SRPW, relative to the Mendocino Range 

on the west side. The altitude effect, which was inferred from 

measurements taken on the western flank of the Sierra Nevada

Mountains, is –0.56 per mil/100 ft for δD (Ingraham and Taylor, 1991; Rose and others, 1996). The Mayacmas and Sonoma 

Mountains are at least 1,000 ft higher than the Mendocino 

Range, which would equate to a −5.6 per mil difference in δD.

Greater variation in isotopic composition was found 

along the central part of geologic section B-B’, between the 

Rodgers Creek and Sebastopol faults, than between the mountains and the faults (fig. 5B, table 5). δD values from of the 

central part of the geologic section span the range of isotopic 

composition for all wells sampled in the study area, from 

−35.9 to −49.3 per mil. The deep sample from well W102

(W102B) had a δD of −47.1 per mil, the second lightest sample analyzed for this study. Well W102 lies within the Rodgers Creek fault zone (fig. 5A). As described in the “Chemical Character of Groundwater” section of this chapter, the

sample from well W102B had high dissolved-solids and boron 

concentrations, which were attributed to deeply circulating 

groundwater rising along the fault zone. The shallow sample 

from well W102 (W102A) had a δD value similar to samples 

from wells upgradient (east) of the Rodgers Creek fault.

δD values from well samples along the geologic section 

between the Trenton Ridge fault and the unnamed fault east 

of the Sebastopol fault ranged from −36.0 to −42.7 per mil,

with a mean composition of −39.2 per mil. These values were

heavier than the δD values in both the shallow and deep samples from well W102 and the mean isotopic composition for 

well samples east of the Rodgers Creek fault (−42.0 per mil).

Samples from wells perforated solely in the Glen Ellen or in 

the Glen Ellen and Petaluma formations between the faults 

have δD values slightly heavier (less negative) than samples 

perforated solely in the Petaluma Formation. This shift toward 

more negative δD values in the Petaluma Formation could 

result from recharge originating at a higher altitude, during a 

cooler climate, or both.

Samples from wells perforated in the Wilson Grove 

Formation, east of the Sebastopol fault (W32, W35, and 

W129) and west of the unnamed fault, had similar δD values 

as samples from wells in the WG storage unit west of the fault 

(fig. 5B). This similarity in isotope values indicates that these 

wells are receiving water from the Wilson Grove storage unit 

to the west, not groundwater underflow from the Petaluma

Formation to the east. These data indicate that the Sebastopol 

fault is not a complete barrier to groundwater underflow in

the Wilson Grove Formation. Samples from wells W33 in the 

Glen Ellen Formation and W34 in the Petaluma Formation, 

however, which also are located between these two faults, 

have significantly lighter δD values (−44.8 and −49.4 per mil,

respectively) than the well samples from the Wilson Grove 

Formation (−35.9 to −38.9 per mil) (fig. 5B). The isotopically 

light water in the samples from wells W33 and W34 indicates 

that this water probably was derived from precipitation that 

fell at higher altitudes in the eastern part of the SRPW, was 

recharged during a cooler and wetter climate, or both. As 

stated in the “Major-Ion Composition” section of this chapter, the water type of samples from wells W33 and W34 was 

similar to the water type of samples from deep wells in the CB 

east of the unnamed fault (fig. 3C). The similarity of isotopic 

signature and water type for wells W33 and W34 and the deep 

wells in the CB east of the unnamed fault is indicative of a 

similar source for these samples.

Tritium and Carbon-14

Tritium (3H) was analyzed in samples collected from 30 

wells, including discrete-depth zones in 3 of the wells (W77, 

W89, and W102), for a total of 35 samples (fig. 7A, table 5). 

Carbon-14 (14C) was analyzed in samples from 11 wells, 

including discrete-depth zones in the same 3 wells (W77, 

W89, and W102), for a total of 16 samples (fig. 7A, table 5). 

Most of the samples were collected and analyzed for the NSF 

GAMA study. Both 3H and 14C were used in this study to 

qualitatively and semi-quantitatively estimate the amount of 

time since the water was last in contact with the atmosphere 

(time since recharge).

Background

Tritium

Tritium is both a naturally occurring and an anthropogenically generated, short-lived, radioactive isotope of 

hydrogen, whose presence can be used to identify relatively 

young (post-1952) water (Clark and Fritz, 1997). It has a halflife of 12.32 years (Lucas and Unterweger, 2000). Because 

of its short half-life, it is useful for distinguishing between 

water that has been in the hydrologic cycle more or less than 

about 60 years. Tritium activity is measured in disintegrations per unit of time and, in this study, is reported in both 

tritium units (TU) and picocuries per liter (pCi/L); a tritium 

unit is one 3

H atom in 1018 atoms of hydrogen and equals 
Source and Age of Groundwater 157

3.2 pCi/L. The interaction of cosmic radiation with the upper 

atmosphere results in the creation of a global steady-state 

inventory of 3.5–4.5 kgs of 3H in the atmosphere, from which 

small amounts reach the Earth’s surface in precipitation (Lal 

and Peters, 1967; O’Brien and others, 1991). Approximately 

800 kg of 3H were released as a result of atmospheric testing 

of thermonuclear weapons during 1952–62 (Michel, 1976). 

This produced a spike in 3H concentrations in precipitation 

and in groundwater recharged during that time that was much 

higher than the natural level. Much smaller amounts of thermonuclear 3

H were released until 1980, after which all atmospheric testing ceased. By 1990, this anthropogenic source 

of 3H had been largely washed out of the atmosphere, and 

concentrations in precipitation had decreased close to natural, 

pre-bomb levels (Clark and Fritz, 1997). Minor amounts of 

anthropogenic 3H continue to be released to the atmosphere 

from nuclear power plants and related facilities that process 

nuclear material.

Because 3H is part of the water molecule, and its concentration is unaffected by reactions other than radioactive 

decay, it is an excellent tracer of modern water. Tritium data 

can be used in conjunction with data for its daughter product 

helium-3 (3He) to determine an absolute age. However, this 

method might not yield reliable tritium-based ages where crystalline rock is present because water from this source typically 

yields concentrations of terrigenic helium that greatly exceed 

concentrations derived from thermonuclear 3H (Solomon 

and Cook, 2000). In the absence of 3He concentrations, 3H 

data alone were used in this study to interpret qualitative and 

“mixed” ages of water.

Carbon-14

Carbon-14 is the naturally occurring, long-lived (halflife of 5,730 years), radioactive isotope of carbon that can 

sometimes be used to determine the age of groundwater far 

beyond the range for 3H. Carbon-14 data are expressed as 

percent modern carbon (pmc) by comparing 14C activities to 

the specific activity of National Bureau of Standards oxalic

acid; 12.88 disintegrations per minute per gram of carbon in 

the year 1950 equals 100 percent modern carbon (Stuiver and 

Polach, 1977). Carbon-14 is created in the upper atmosphere 

by the bombardment of nitrogen atoms by cosmic radiation. 

Thermonuclear testing and nuclear power plants have also 

contributed 14C to the atmosphere, and the burning of fossil 

fuels during the industrial age has “diluted” the level of 14C 

(Clark and Fritz, 1997). Atmospheric 14C is present as carbon 

dioxide (CO2), which can then be incorporated into various 

hydrospheric (oceans, lakes, and groundwater) and biospheric 

(plants and animals) reservoirs. Whether through infiltration

of water or the decay and release of biomass into the soil 

zone, once these intermediate sources of carbon are isolated 

from the atmosphere, the 14C content in the dissolved carbon 

steadily decreases.

In reality, 14C that has been isolated from the atmosphere 

is seldom only affected by just radioactive decay. Chemical 

reactions along a groundwater flowpath can dilute 14C by 

either the addition of dissolved inorganic carbon (DIC, which 

is carbonate plus bicarbonate) that lacks 14C, or by the removal 

of DIC that contains 14C (Clark and Fritz, 1997). 14C concentrations can be decreased, for example, when carbon is added 

to groundwater by the dissolution of calcite or dolomite. 

Because these minerals were formed long ago, they are devoid 

of 14C and are often said to contain “dead” carbon (Freeze and

Cherry, 1979). The addition of DIC from these sources dilutes 

the original 14C content to give the appearance of older water, 

as does the production of DIC from oxidation of organic matter that is devoid of 14C.

Ratios of the stable isotopes carbon-13 to the far more 

abundant carbon-12 (12C) were used in this study as indications of biogeochemical and carbon-exchange processes that 

can affect estimates of 14C ages. Because carbonate minerals and DIC exchange carbon isotopes (equilibration), albeit 

slowly, groundwater can acquire a less negative delta carbon-13 (δ13C) value as it moves along a flowpath from infiltration to discharge. Stable carbon isotopes can also be affected 

by decomposition (oxidation or mineralization) of organic 

matter buried in the aquifer because organic material has a 

more negative δ13C composition than does inorganic carbon 

so that carbon isotopes would become lighter. Stable carbon 

isotopes were used to make qualitative inferences about the 

extent to which these processes have caused the “calculated” 14C age to overestimate the actual time elapsed since recharge 

in this study.

To obtain a more accurate estimate of “true” age for

groundwater (the time elapsed between recharge and discharge) based on 14C, carbonate dissolution and exchange and 

organic mineralization need to be corrected for through the 

use of coupled groundwater-flow and geochemical-reaction

models that incorporate d13C and related chemical data. As 

geochemical modeling was beyond the scope of this study, 

14C ages in this report are “uncorrected” ages.

Groundwater Age in the Santa Rosa Plain 

Watershed

Tritium in water from wells in the study area ranged 

from less than the detection limit of 0.1 (for samples collected 

during 2006–10) to 2.4 TU (table 5). For this study, groundwater that had 3

H values greater than 0.3 TU (detection limit 

of samples collected during 2004) was interpreted to be water 

recharged after 1952, or modern recharge. A total of 15 of the 

35 well samples (about 43 percent) had 3H concentrations in 

excess of 0.3 TU (table 5). As expected, modern recharge was 

more prevalent in the shallow well samples. Of the 7 samples 

from shallow wells, 5 (about 70 percent) had 3H concentrations in excess of 0.3 TU, and 3 of the samples had 3

H 

concentrations of 1.6 TU or more; in contrast, only 7 of the 19 

samples from deep wells (about 37 percent) had 3H concentrations in excess of 0.3 TU, and all samples had 3

H concentrations of 0.9 TU or less (table 5). The vertical migration of 
158 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

i

122°50’ 122°40’

38°

30’

38°

20’

12

116

101

Uplands

Uplands

Valley

Windsor

Basin

Wilson Grove

Cotati

Basin

R9W 

T

9

N 

R8W R7W R6W 

T

8

N 

T

7

N 

T

6

N 

T

5

N 

Russian

 River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe

 Creek

Copeland Creek

Rincon

Valley

Kenwood

 Valley

Santa Rosa Plain

Mendocino Range

Sonoma Mountains

Mayacmas Mountains

B'

B

Windsor

Santa

Rosa

Sebastopol

Rohnert Park

Cotati

0

0 5 10 Kilometers

5 10 Miles

IP019208_Figure C07a

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection EXPLANATION

Shallow (0’ to 150’ below

 land surface datum)

Mixed

Deep (greater than 150’)

Construction unknown

Well perforated interval Well number

Tritium, in Tritium units

Uncorrected carbon-14 age,

 in years before present

W76

 0.8 

4,000

NM, not measured

<, less than

A

Cotati Basin Valley

Upland

Windsor Basin Wilson Grove

Groundwater storage units Rodgers Creek

 fault zone

B B’ Line of section

Inferred fault

W9

 2.4 

NM

W21

 0.5 

NM

W67

 0.3 

NM

W10

 0.5 

NM

W89A

 0.9 

8,000

W89B

 0.2 

10,000

W89C

 <0.1 

10,000

W77A

 0.6 

4,000

W77B

 <0.1 

7,000

W77C

 0.5 

4,000

W33

 <0.1 

14,000

W34

 <0.1 

34,000

W35

 <0.1 

4,000

W102A

 <0.1 

10,000

W102B

 <0.1 

27,000

W1

 <0.3 

NM

W82

 <0.3 

NM

W76

 0.8 

4,000

W75

 0.7 

6,000

W26

 0.6 

NM

W44

 <0.3 

NM

W30

 0.6 

NM

W109

 0.5 

1,000

W129

 <0.3 

10,000

W131

 0.5 

NM

W133

 <0.3 

NM

W132

 <0.3 

NM

W83

 <0.3 

NM

W91

 <0.3 

3,000

W124

 <0.3 

NM

W140

 2.1 

NM

W94

 0.7 

NM

W96

 <0.3 

NM

W123

 1.6 

NM

W156

 <0.3 

NM

Figure 7. Tritium and carbon-14 values from selected surface-water sites and wells in the Santa Rosa Plain watershed, Sonoma 

County, California, 2004–10: A, areally and B, along section B–B’.
Source and Age of Groundwater 159 sac12-0452_Figure C07b

B Southwest Northeast

B B'

FEET

SEA

LEVEL

FEET

–1,000–2,000

SEA

LEVEL

1,000

0 6,000 12,000 18,000 24,000 30,000 36,000 42,000 48,000 54,000 60,000 66,000

–1,000–2,000

Rodgers Creek

fault zone

Sebastopol

fault 

Trenton

Ridge

 fault 

Bennett Valley

fault zone

Cotati Basin Wilson Grove Windsor

Basin

Valley Uplands

Mayacmas Mountains

Distance along section, in feet 12.85x vertical exaggeration Geology modified from McLaughlin and others (2008) and Sweetkind and others (2010).

?

?

?

? ?

?

W76*

W77*

W33-35

W102 W89* W129*

W75*

W109*

W96* W83*

W94* W30* W26*

W82*

W82 Glen Ellen Formation

Wilson Grove Formation

Sonoma Volcanics

Petaluma Formation

Undifferentiated basement

EXPLANATION 

Well (black = production well, 

Well perforation (white box)

red = observation or discretedepth sampled test well)

Well map number (asterisk  indicates well projected to section) Fault, sense of offset

 not shown

Tritium, in tritium units (TU)/ uncorrected C-14 age, in years before present

Laguna de Santa Rosa

NM, not measured

<, less than

 0.6 

4,000

 <0.1 

14,000 <0.1 

10,000

 <0.1 

10,000

 0.2 

10,000

 0.9 

8,000

 <0.1 

27,000

 <0.1 

34,000

 <0.1 

4,000 <0.3 7,000

 0.5 

4,000

 <0.3 

NM

 0.6 

NM

 0.6 

NM

 0.7 

NM

 <0.3 

NM

 <0.3 

NM

 <0.3 

3,000

 0.5 

1,000

 <0.3 

11,000

 <0.3 

11,000

 0.7 

6,000

 0.8 

4,000

Figure 7. Continued.
160 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

recharge in the SRP probably is retarded by the presence of 

low permeability clay deposits in the Glen Ellen and Petaluma Formations. Only 3 of the 9 (about 33 percent) samples 

from the UPL and VAL storage units had 3H concentrations 

in excess of 0.3 TU (fig. 7A, table 5). The absence of modern 

water in many of these samples probably reflects the low vertical permeability of the basement rocks and Sonoma Volcanics 

that compose a large part of the UPL and VAL storage units. 

Most of the precipitation on these storage units probably 

becomes runoff that contributes to streamflow and potential

recharge in the downstream storage units.

Measured 14C activities for the 16 water samples ranged 

from 1.3 to 84.7 pmc (table 5). These 14C activities represent uncorrected ages of 1,000 to 34,000 years before present (fig. 7A, table 5). Of the 16 samples, 11 were from deep 

wells (entire perforated or open interval below 150 ft bls), of 

which all had uncorrected 14C ages of 4,000 years or older; 5 

of the deep well samples were 10,000 years or older (table 5). 

The relatively old age of the deep well samples supports 

the inference from the 3H data that the vertical migration of 

modern recharge probably is retarded by low permeability clay 

deposits in the Glen Ellen and Petaluma Formations.

The two oldest uncorrected 14C ages were from samples 

collected from the borehole at well W102 (786–806 ft bls) 

when the well was being constructed, (sample W102B; 

27,000 years) and from well W34 (34,000 years). Well W102 

is located along the eastern edge of the SRP within the Rodgers Creek fault zone (fig. 7A). As described in the “Chemical 

Character of Groundwater” section of this chapter, sample

W102B had high dissolved-solids and boron concentrations, which were hypothesized to be from deeply circulating 

groundwater rising along the fault zone. The old age of the 

water supports this hypothesis. Well W34 is in the western 

part of the CB storage unit near the Laguna de Santa Rosa, 

which is an area of natural groundwater discharge. The well 

is perforated in the Petaluma Formation from 237 to 257 ft 

bls. Groundwater-level contour maps, presented in chapter B

(see fig. 29A), indicate that groundwater moved from the UPL 

storage unit westward toward the Laguna de Santa Rosa and 

eastward from the highlands in the WG storage unit toward 

the Laguna de Santa Rosa, prior to significant groundwater development in the SRP. The sample from W34 more 

closely resembles that of groundwater in the eastern part of 

the SRP than groundwater in the WG storage unit (fig. 3C). 

The similarity of water type to well samples to the east and 

its old age indicate that well W34 receives water at the end 

of a long groundwater-flow path through the CB storage unit.

Because radiocarbon dates presented in this report are “uncorrected” for dissolution and exchange with calcium carbonate

and for oxidation of organic carbon in the sediment, they 

are to be considered as only a semi-quantitative indication 

of the groundwater age. Nevertheless, the implication that 

samples from W102 and W34 represent groundwater that 

was recharged during the Pleistocene epoch (more than about 

10,000 years before present) is supported by isotopically light 

oxygen-18 and deuterium ratios (table 5). The oxygen-18 and 

deuterium ratios are consistent with recharge during a cooler 

and wetter time than present.

Groundwater Age Along Geologic Section B–B’

Tritium and uncorrected 14C ages were plotted along 

geologic section B-B’ to illustrate changes in groundwater age 

as groundwater flows through the aquifer system (figs. 7A and 

7B). Unfortunately, none of the samples were collected from 

wells perforated solely in the Glen Ellen Formation in the VAL 

storage unit, on the eastern side of the geologic section, where 

modern recharge would be expected. The samples from the 

underlying Sonoma Volcanics had 3H concentrations less than 

0.3 TU, except for the shallow sample at well W89 (W89A), 

which had a 3H concentration of 0.9 TU. The uncorrected 14C 

age for this sample was 8,000 years before present, indicating 

that the sample from this well is a mixture of modern and old 

water. The deeper samples from this well (W89B and W89C) 

had uncorrected 14C ages of 10,000 years before present.

The shallow and deep samples from well W102, in 

the Rodgers Creek fault zone, had 3H concentrations less 

than 0.1 TU, indicating these samples contained water that 

was recharged before 1952. The uncorrected 14C ages of the 

samples ranged from 10,000 years in the shallow sample to 

27,000 years in the deep sample (fig. 7B). As indicated previously, high dissolved-solids and boron concentrations in the 

deeper sample were attributed to deeply circulating groundwater rising along the fault zone. The uncorrected 14C age in the 

shallow sample was similar to the uncorrected 14C age in the 

deeper samples from well W89 to the east. These data indicate 

upward movement of groundwater upgradient (east) of the 

Rodgers Creek fault zone, which is supported by groundwaterlevel contour maps presented in chapter B (see fig. 29A).

Available samples from all wells west of the Rodgers 

Creek fault zone and east of the unnamed fault east of the 

Sebastopol fault had 3H concentrations in excess of 0.3 TU, 

indicating that these samples contained at least some water 

that was recharged after 1952. Most of the wells with 3

H data 

in this portion of the cross-section, however, are relatively 

shallow. Carbon-14 was analyzed only for the sample from 

well W109 in this group of wells, which yielded an uncorrected 14C age of 1,000 years before present. Only one of these 

wells (well W26) was perforated solely in the Petaluma Formation, but, unfortunately, 14C was not analyzed on the sample 

from this well. The 3H data from these wells indicated modern 

recharge in the SRP downgradient of the Rodgers Creek fault 

zone.

Samples from wells perforated in the Wilson Grove 

Formation (W35, W75, W76, W77A–C, W82, and W129), 

on the western side of the geologic section, had 3H concentrations ranging from less than 0.1 to 0.8 TU and uncorrected 
Summary and Conclusions 161

14C ages ranging from 4,000 to 11,000 years before present (fig. 7B, table 5). The shallow (100–120 ft bls) and deep 

(640–650 ft bls) samples from well W77 and the samples 

from long-screened wells W75 (138–528 ft bls) and W76 

(332–600 ft bls) were the only groundwater samples from 

wells in the Wilson Grove Formation that had 3H concentrations in excess of 0.3 TU (fig. 7B, table 5). The samples from 

these wells were a mixture of modern and old water. One 

possible explanation for this mixture is movement of modern 

recharge from the shallow aquifer system to the deeper aquifer 

system through the long-screened wells when the wells are not 

being pumped. Groundwater samples from wells in the Wilson 

Grove Formation probably were younger than the reported 

uncorrected 14C age because of groundwater-aquifer interactions. As stated previously, the Wilson Grove Formation consists of fossiliferous sand or sandstone (Sweetkind and others, 

2010). Carbon-14 present in the groundwater in the Wilson 

Grove Formation can interact with “dead” carbon (devoid of 14C) present in the fossils and calcium carbonate cement in the 

sandstone, which would reduce the measured 14C activity in 

the groundwater and result in an apparent older 14C age.

Wells W35 and W129 are located between the Sebastopol 

fault and the unnamed fault to the east (fig. 7B). Both wells 

are totally (well W35) or partially (well W129) perforated in 

the Wilson Grove Formation (fig. 7B). Major ion data (fig. 3C) 

indicate that these wells predominately receive groundwater from the WG storage unit to the west. The sample from 

well W35 had a similar uncorrected 14C age as samples from 

the WG storage unit (4,000 years before present); however, 

the uncorrected 14C age for the sample from well W129 was 

significantly older (11,000 years before present) (fig. 7B). 

The older age of the sample from well W129 could be the 

result of its greater depth and the fact that it is partially perforated in the Petaluma Formation (fig. 7B).

Wells W33 and W34 are also located between the 

Sebastopol fault and the unnamed fault to the east, but are not 

perforated in the Wilson Grove Formation (fig. 7B). Well W33 

is perforated in the Glen Ellen Formation from 60 to 80 ft bls, 

and well W34 is perforated in the Petaluma Formation from 

237 to 257 ft bls. In contrast to the samples from wells W35 

and W129, the major-ion data for samples from wells W33 

and W34 indicated that these wells predominately received 

groundwater from the CB storage unit to the east (fig 3C). The 

uncorrected 14C ages for wells W33 and W34 were 14,000 and 

34,000 years before present, respectively (fig. 7B, table 5). 

The similarity of water type to samples from wells to the east 

and the very old age indicate that wells W33 and W34 receive 

water from the end of long groundwater flowpaths that traverse the CB storage unit. The age of water could be younger 

in well W33 than in well W34 because it passes through a 

shorter groundwater flowpath.

Summary and Conclusions

Groundwater quality was characterized for the SRPW 

by using analyses from previous investigations for selected 

physical properties and inorganic constituents compiled from 

databases maintained by the CDPH, the CDWR, and publicsupply purveyors from 1947 to 2010. These data were used to 

characterize areal, vertical, and temporal variations in groundwater quality and to identify constituents of potential concern. 

Stable and radioactive isotopes measured in groundwater 

samples collected in 2004 as part of the GAMA Program or 

measured in surface- and groundwater samples collected from 

2006 to 2010, as part of this study or concurrently by private 

consultants, were used to help identify the recharge source and 

age of groundwater in the study area.

The major ion composition and dissolved-solids concentration of well samples were used to characterize the water 

quality of the Uplands (UPL), Valley (VAL), Windsor Basin 

(WB), Cotati Basin (CB), and Wilson Grove (WG) storage 

units of the SRPW. Samples from springs in the Mayacmas 

Mountains were a mixed cation-bicarbonate type water with 

dissolved-solids concentrations less than about 100 mg/L—

the lowest dissolved-solids concentrations in the SRPW. As 

groundwater migrates through the UPL and downgradient storage units, the dissolved-solids concentration of the groundwater increased as a result of water-rock interactions and anthropogenic inputs, such as irrigation return flows and septic-tank

discharge. The median dissolved-solids concentrations of 

well samples in the UPL and VAL storage units were 330 and 

392 mg/L, respectively.

Most of the well samples from the WB and CB storage 

units are mixed cation-bicarbonate and sodium-bicarbonate 

type waters. A comparison of major-ion composition and depth 

shows a subtle shift in major cations from greater proportions 

of calcium and magnesium in shallow groundwater, to greater 

proportions of sodium plus potassium in deeper groundwater. 

This finding is consistent with increasing mineralization and

ion exchange between clays and groundwater, with increasing 

distance and depth from recharge sources.

Most well samples in the WG storage unit were calciumbicarbonate type or mixed cation-bicarbonate type waters with 

dissolved-solids concentrations less than 300 mg/L. Unlike 

the WB and CB storage units, there is no shift in major cations 

from greater proportions of calcium and magnesium in shallow groundwater, to greater proportions of sodium plus potassium in deeper groundwater. Wells in the WG storage unit are 

perforated in the Wilson Grove Formation, which is composed 

of marine sandstone. Unlike the Glen Ellen and Petaluma 

Formations, the Wilson Grove Formation has only minor clay 

deposits, so there is less potential for the loss of calcium from 

solution by means of ion exchange with sodium in association 

with clay minerals.
162 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Water from almost 75 percent of the 33 wells with 

records spanning 20 or more years had increased specific

conductance over time, and about half had increases of more 

than 10 percent during the period of record. Chloride behaved 

similarly; concentrations increased in about 67 percent of the 

wells evaluated, and just over half increased by more than 

10 percent. In approximately 15 and 30 percent of evaluated 

wells, specific conductance and chloride concentration, respectively, decreased by more than 10 percent in approximately. 

The greatest increases in specific conductance, chloride concentration, or both were in wells located in the vicinity of the 

cities of Rohnert Park and Cotati. Possible causes of increased 

specific conductance in the Rohnert Park/Cotati area include

groundwater underflow of high dissolved-solids concentration

water present along the Rodgers Creek fault zone, irrigation 

return flow, and septic-tank effluent or leaking sewer pipes.

Depth-dependent hydrologic, chemical, and isotopic data are 

needed to better understand the cause of the increased specific

conductance and chloride values.

Specific conductance, chloride, dissolved solids, nitrate,

arsenic, boron, iron, and manganese are water-quality constituents of potential concern in the SRPW because concentrations of these constituents in samples from some wells 

exceed state or federal recommended or mandatory regulatory 

standards for drinking water. About 43 percent of the samples 

analyzed for iron had concentrations greater than or equal to 

the SMCL of 300 μg/L, and about 73 percent of the samples

analyzed for manganese, had concentrations greater than 

or equal to the SMCL of 50 μg/L. About 12 percent of the

samples analyzed for arsenic had concentrations greater than 

or equal to the MCL of 10 μg/L, with about 30 percent of the

wells sampled in the WB and WG storage units exceeding the 

MCL. About 12 percent of the samples analyzed for dissolved 

solids had concentrations greater than or equal to the SMCL of 

500 mg/L. Boron concentrations were greater than or equal to 

regulatory standards in 7 percent of the samples with analyses. 

Specific conductance, chloride, and nitrate values were greater

or equal to regulatory standards in only about 2 percent of the 

samples with analyses.

Samples were collected from 15 surface-water sites during the fall of 2008 and spring of 2009, and from 32 wells during 2004–10, for the analysis of the stable isotopes of oxygen 

and hydrogen (δ18O and δD). In general, the isotopic values of 

the well samples were similar to the range of isotopic values 

measured in the spring surface-water samples. With the exception of well samples from the UPL and WG storage units, 

the isotopic values of groundwater samples generally plotted 

slightly below the GMWL, indicating that the samples could 

have been subject to some evaporation, been mixed with evaporated surface water, or been derived from recharge source 

areas with somewhat different meteoric water lines because of 

differing altitudes. The isotopic values displayed no relationship to the depth of a well’s perforated interval—samples from

deep wells had both the heaviest and lightest isotopic values 

analyzed. In general, the isotopic values of samples from the 

UPL and VAL storage units grouped together and were in the 

lighter range of all measured isotopic values. The isotopic values for well samples from the WG storage unit also grouped 

together, but fell within the heavier range of all isotopic values 

from wells in the SRPW. This difference can be explained by 

the higher altitude of the Maycamas and Sonoma Mountains 

on the east side of the SRPW relative to the Mendocino Range 

on the west side. Precipitation and subsequent recharge at 

the higher altitudes in the Maycamas and Sonoma Mountains 

would be isotopically lighter than precipitation and subsequent 

recharge at the lower altitudes in the Mendocino Range.

The isotopic values from wells in the WB and CB storage 

units spanned the entire range of isotopic values in the SRWP, 

indicating a mixture of water sources. Most of the isotopic 

values for samples from wells in the WB and CB storage units 

were heavier (less negative) than for well samples in the UPL 

and VAL storage units, indicating that groundwater underflow

from the UPL and VAL storage units cannot be the sole source 

of recharge to the WB and CB storage units. The heavier 

isotopic values, which only deviated slightly from the GMWL, 

indicated that at least some of the recharge to the WB and CB 

storage units originates as precipitation directly falling on the 

lower elevations of the SRP. The isotopic values of samples 

collected from a borehole in the Petaluma Formation within 

the Rodgers Creek fault zone, on the east side of the SRP, and 

from a well perforated in the Petaluma Formation near the 

Laguna de Santa Rosa, on the west side of the SRP, were significantly lighter (more negative) than the other surface-water

or well samples. The more negative values are consistent with 

recharge at higher altitudes or recharge under cooler conditions than have prevailed in the post-glacial period.

Tritium (3H) concentrations were analyzed in 35 samples 

collected from 30 wells and ranged from less than 0.3 (detection limit of samples collected in 2004) to 2.4 TU. About 

43 percent of the samples had detectable 3H concentrations, 

indicating that these samples contained some modern water 

(water recharged since 1952). As expected, modern recharge 

was more prevalent in the shallow well samples. Five of the 

seven samples from shallow wells (about 70 percent) had 3H 

concentrations greater than 0.3 TU, and three of the samples 

had 3

H concentrations of 1.6 TU or more. In contrast, only 7 

of the 19 samples from deep wells (about 37 percent) had 3H 

concentrations greater than 0.3 TU, and all samples had 3H 

concentrations of 0.9 TU or less. The vertical migration of 

recharge in the SRP is probably retarded by the presence of 

low permeability clay deposits in the Glen Ellen and Petaluma 

Formations. Only three of the nine (about 33 percent) samples 

from the UPL and VAL storage units had detectable 3H concentrations. The absence of modern water in many of these 

samples probably reflects the low vertical permeability of the

basement rocks and Sonoma Volcanics that compose the UPL 

and VAL storage units. Most of the precipitation on these storage units probably becomes runoff that contributes to streamflow and potential recharge in the downstream storage units.

Measured 14C activities for the 16 well samples analyzed 

ranged from 1.3–84.7 pmc. These 14C activities represent 

uncorrected ages of 1,000–34,000 years before present. The 
References Cited 163

deep well samples all had uncorrected 14C ages of 4,000 years 

or older, and five of the deep well samples were 10,000 years

or older. The relatively old age of the deep well samples 

supports the inference made from the 3H data that the vertical migration of modern recharge probably is retarded by low 

permeability clay deposits in the Glen Ellen and Petaluma 

formations.

The two oldest uncorrected carbon-14 ages (27,000 and 

34,000 years before present) were from samples collected 

from a borehole in the Petaluma Formation within the Rodgers Creek fault zone on the east side of the SRP (well W102) 

and from a well perforated in the Petaluma Formation near the 

Laguna de Santa Rosa on the west side of the SRP (well W34). 

The sample from well W102 in the Rodgers Creek fault zone 

had high dissolved-solids and boron concentrations, which 

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this report were “uncorrected” for dissolution and exchange

with calcium carbonate and for oxidation of organic carbon in 

the sediment, they are to be considered as only a semi-quantitative indication of the groundwater age. Nevertheless, the 

deduction that these samples represent groundwater recharged 

during the Pleistocene epoch (more than about 10,000 years 

before present) is supported by isotopically light oxygen-18 

and deuterium ratios, which indicate recharge during a cooler 

and wetter period than the present, such as the Pleistocene.

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589–604.
166 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California
Chapter D. Conceptual Model of Santa Rosa Plain 

Watershed Hydrologic System

By Tracy Nishikawa, Joseph A. Hevesi, Donald S. Sweetkind, and Peter Martin

Introduction—Purpose of 

Conceptualization

A conceptual model of the hydrologic system for the 

Santa Rosa Plain watershed (SRPW) will be used (1) to better 

understand the movement and storage of water in the SRPW; 

(2) to aid in the interpretation of stream-gage records, well 

hydrographs, and water-quality data; and (3) to provide a 

framework for the development of a numerical-flow model

needed to evaluate the response of the SRPW to current and 

historic hydrologic conditions and water uses and to predict 

the response of the hydrologic system to potential future 

conditions. The conceptual model of the SRPW is based on 

known and estimated physical and hydrologic characteristics 

of the surface-water and groundwater systems and how these 

characteristics influence the flow and storage of water in the

SRPW. Following Markstrom and others (2008), the hydrologic system of the SRPW is conceptualized as having three 

regions: (1) region 1, which consists of the plant canopy, the 

land surface, and the soil zone; (2) region 2, which consists 

of streams, lakes, and wetlands; and (3) region 3, which is 

the subsurface zone that consists of an unsaturated zone and 

an underlying saturated zone (fig. 1). Water is stored in each 

region, and the regions are linked by flow processes. The flow

of water into and out of each region, as well as the flow of

water within each region, is a function of mechanisms specific

to each region.

Region 1—Plant canopy,

snowpack, surfacedepression storage,

and soil zone

Region 3—Subsurface

(unsaturated and

saturated zones)

beneath soil zone

Region 2—Streams

and lakes

Surface runoff

Interflow

Groundwater discharge Groundwater discharge

Gravity drainage Leakage

Soil-moisture

dependent flow

Soil-moisture or

head-dependent flow Head-dependent flow

Figure 1. Schematic diagram of the general conceptual model of the Santa Rosa Plain watershed hydrologic system (modified from 

Markstrom and others, 2008).
168 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Boundaries and Boundary Conditions

The SRPW covers about 260 square miles (mi2) and 

includes a combination of previously defined surface-water

drainage divides and groundwater subbasin boundaries (fig. 2). 

Most of the SRPW area is contained within the surface-water 

drainage basin defined by the Mark West Creek drainage

(MWCD), the largest tributary, by area, to the Russian River. 

The SRPW also includes several smaller, peripheral surfacewater drainages to the northwest and south of the MWCD, 

where the study area was extended to better match the boundary of the Santa Rosa Plain groundwater subbasin. The surface 

watershed area overlies all of the Santa Rosa Plain and Rincon 

Valley groundwater subbasins, as well as the northern half 

of the Kenwood Valley basin, the eastern part of the Wilson 

Grove Formation Highlands groundwater basin, the southern 

part of the Healdsburg area groundwater subbasin, and the 

southern part of the Alexander Valley groundwater subbasin.

The SRPW boundary includes mostly naturally-defined

topographic drainage divides, such that surface-water inflows

from outside the boundary are not expected. Surface-water 

flows in the SRPW can exit as runoff, evapotranspiration

(ET), or consumptive use. Most runoff from the SRPW flows

to the Russian River drainage, and the majority of the runoff 

discharges from Mark West Creek (see chapter B, fig. 7); 

however, a few small areas along the southern boundary drain 

southward into San Pablo Bay.

As described in chapter B of this report, the groundwater 

basin underlying the SRPW is subdivided into five groundwater storage units: (1) the mountainous uplands areas (UPL) 

north, east, and southeast of the city of Santa Rosa; (2) valleys 

located to the east of the SRP Valley (VAL), including Rincon 

Valley, Bennett Valley and northern half of Kenwood Valley; (3) Windsor Basin (WB); (4) Cotati Basin (CB); and 

(4) the Wilson Grove area (WG; fig. 2). About 75 percent of 

the length of the defined SRPW boundary is interpreted to be

a no-flow boundary, where low-permeability basement rock,

a groundwater divide, or both prevent hydraulic communication between the SRPW groundwater basin and neighboring groundwater subbasins. The remaining SRPW boundary 

includes four segments where there is groundwater communication with adjacent groundwater subbasins or basins. 

These four boundary segments include (1) the VAL storage 

unit and southern half of Kenwood Valley basin in the east, 

(2) the CB storage unit and Petaluma Valley basin in the south, 

(3) the WG storage unit and Wilson Grove Formation Highlands basin in the southwest, and (4) the WB storage unit and 

the Healdsburg area subbasin in the northwest (fig. 2).

The lower boundary of the SRPW groundwater system 

is the contact with the low-permeability basement rock. It is 

assumed that the basement rock contributes negligible water 

to the groundwater system and does not store appreciable 

volumes of water. The theoretical upper boundary of the 

groundwater system is land surface, but varies spatially and 

temporally with water level. Inflows from the upper boundary

come from groundwater recharge. Outflows across the upper

boundary are ET and groundwater discharge to surface-water 

bodies.

Region 1: Plant Canopy, Land Surface, 

and Soil Zone

Region 1 of the SRPW—the plant canopy, the land

surface, and the soil zone—provides the primary link between

climatic factors and the SRPW hydrologic system. The plant 

canopy includes natural vegetation, crops (for example, row 

crops, pastures, vineyards, and orchards), and landscaped 

urbanized areas (for example, residential areas, parks, and 

golf courses). The three types of land surfaces included in 

the SRPW conceptual model are (1) areas covered by soil; 

(2) naturally exposed bedrock areas free of soil cover; and 

(3) areas covered by anthropogenic features such as buildings, roads, and parking lots. The soil zone represents the 

upper unsaturated zone and is conceptually defined as the

layer extending from the ground surface to the base of the root 

zone (Markstrom and others, 2008). The soil zone stores and 

transmits water between the atmosphere and the underlying 

unsaturated and saturated zones.

Inflows to Region 1

In the SRPW, the primary sources of water to region 1 

include (1) precipitation (primarily in the form of rainfall), 

(2) irrigation, (3) groundwater discharge, and (4) surface 

water. Inflows from precipitation and irrigation to the soil zone

only occur in pervious areas and are limited by the infiltration

capacity of the soil (or for some locations exposed bedrock). 

There is no natural surface-water inflow (including both runoff

and interflow) into the SRPW from neighboring areas; however, on a local scale within the SRPW, surface-water inflows

can be lateral redistribution of overland flow or interflow.

Precipitation

Precipitation is primarily rainfall in the SRPW (Cardwell, 

1958) and is the main source of water to region 1. According 

to the 30-year (1971–2000) normal precipitation estimated 

by PRISM (Daly and others, 2004), the mean annual precipitation for the SRPW is approximately 40 inches (in.), or 

560,000 acre-feet per year (acre-ft/yr), distributed over the 

watershed. Precipitation can be snow on rare occasions, especially at the higher altitudes (Cardwell, 1958); however, snow 

accumulations are negligible in the SRPW.
Boundaries and Boundary Conditions 169

Healdsburg fault

Sebastopol fault

Trenton Ridge fault

Bennett Valley fault zone

Maacama fault zone

Rodgers Creek fault

Gates Canyon thrust fault

Petrified Forest thrust zone

12

116

101

R9W 

T

9

N

R8W R7W R6W 

T

8

N

T

7

N

T

6

N

T

5

N

i

122°50’ 122°40’

38°

30’

38°

20’

0

0 5 10 Kilometers

5 10 Miles

Windsor

Santa Rosa

Sebastopol

Rohnert Park

Cotati

Russian River

Mark West Creek

Rincon Creek

Santa Rosa Creek

Santa Rosa Creek

Spring Creek

Matanzas Creek

Laguna de Santa Rosa

Washoe Creek

Copeland Creek

Rincon

Valley

Petaluma

Valley

Kenwood

 Valley

S a n t a R o s a P l a i n

M e n d o c i n o R a n g e

S o n o m a M o u n t a i n s

M a y a c m a s M o u n t a i n s

Base from U.S. Geological Survey digital data, 1:1,000,000,

downloaded 2003 State Plane Projection, Fipzone 402

Shaded relief base from 1:250,000 scale Digital Elevation Model:

sun illumination from northwest at 30 degrees above horizon

IP019208_Figure D02

EXPLANATION

Boundaries with neighboring

 groundwater subbasins

Inferred fault

Windsor Basin

Wilson Grove

Valley

Uplands

Cotati Basin

Groundwater storage unit

Windsor

Basin

Valley

Cotati

Basin

Uplands

Uplands

Wilson

Grove

Rodgers Creek

 fault zone

Mt. St. John thrust fault

Figure 2. Locations where groundwater in the Santa Rosa Plain watershed could be in hydraulic communication with neighboring 

groundwater subbasins, Santa Rosa Plain watershed, California.
170 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Irrigation Return Flows

On a local scale, irrigation return flows are a significant

inflow to region 1. The source for most of the irrigation is

groundwater pumped from within the SRPW; this represents 

a transfer of water from the saturated zone of region 3 to 

the soil zone. Agricultural pumpage is unreported; however, 

Hevesi and others (2011) estimated agricultural pumpage in 

the SRPW to range from 8,900 acre-foot (acre-ft) in 1974 to 

46,600 acre-ft in 2008 by using a calibrated watershed model 

of the study area.

Other sources of irrigation water, including reclaimed 

municipal wastewater and imported water from the Russian 

River, are primary sources of irrigation water for some locations. The use of imported Russian River water for irrigation 

is less than about 1,000 acre-ft/yr and does not represent a 

significant inflow to the SRPW (Donald Seymour, Sonoma

County Water Agency, written commun., 2010).

Inflow from irrigation is minimal to nonexistent during the wet, winter months for most years (except during 

droughts) and is highest during the late spring growing season 

and the warm, dry summer months. In the SRPW conceptual 

model, it is assumed that about 80 percent of water applied 

as irrigation is used by plants to satisfy crop demand, about 

15 percent becomes irrigation-return flow to regions 2 and 3,

and about 5 percent becomes surface-water runoff.

Groundwater Discharge

Groundwater in the SRPW often is discharged directly to 

a stream channel, lake, or spring in region 2. However, if the 

water table rises at an intra-channel location, groundwater can 

flow directly into the soil zone in region 1. Groundwater flow

to the soil zone increases the water content of the soil, which 

in turn provides more water available for ET. Cardwell (1958) 

estimated that groundwater discharge lost to ET in the Laguna 

de Santa Rosa ranged from 4,000 to 6,000 acre-ft/yr; this value 

is probably lower than ET for the entire SRPW. If the soil 

zone becomes fully saturated, groundwater can discharge to 

surface-water bodies and contribute to runoff, or can simply 

discharge to land surface. The runoff also can re-infiltrate the

soil zone downslope of the discharge zone.

Temporally, groundwater discharge can be an important inflow component to region 1 during wetter than normal

periods. Spatially, groundwater discharge can be important in 

the low-lying areas of the SRPW, such as the Laguna de Santa 

Rosa, and in the mountains where springs can discharge.

Outflows from Region 1

The primary outflows from region 1 in the SRPW conceptual model are ET, surface-water runoff, and infiltration to

the unsaturated zone.

Evapotranspiration

ET is the combined water loss to the atmosphere from 

evaporation of water and soil water and plant transpiration. 

ET is a function of potential evapotranspiration (PET), water 

availability, soil texture, vegetation type, vegetation density, 

and root depth. PET is the rate of ET that is possible given 

an unlimited water supply (California Irrigation Management 

Information System, 2004) under specific climate conditions

(Maidment, 1993). If water supply is limited, ET will be less 

than PET.

The average PET rate is 46–48 inches per year (in./yr), 

or about 660,000 acre-ft/year, distributed over the SRPW 

(California Irrigation Management Information System, 2004). 

Woolfenden and others (2011) estimated the total actual ET 

from the soil, unsaturated, and saturated zones (regions 1 and 

3) to be about 265,700 acre-ft/yr by using a preliminary watershed model. ET also occurs along many other stream channels 

in the SRPW where the water table is shallow enough for 

transpiration by phreatophytes.

Surface-Water Runoff

Precipitation can fall on pervious or impervious surfaces 

in the SRPW. Precipitation on pervious soils either infiltrates,

ponds on the soil surface (eventually infiltrating or running

off), runs off directly, or evaporates. Precipitation on impervious surfaces either ponds, runs off, or evaporates. For the 

purposes of the SRPW conceptual model, surface-water 

runoff includes overland flow and shallow subsurface flow

(interflow).

The magnitude of surface-water runoff relative to rainfall 

is a primary driver for flow processes within region 1 and

between region 1 and regions 2 and 3. Where runoff is high, 

less water is available for infiltration, ET, and, ultimately,

recharge. The spatial distribution of runoff in the SRPW 

depends on soil thickness and type, the permeability of the 

soil and underlying bedrock, land use and topography. For 

example, steep hillsides, thin soil and vegetation cover, and 

impervious surfaces in urban areas are characteristics that 

cause more rapid runoff. In the upper drainages in rugged 

areas of the mountains surrounding the SRPW, the soils are 

thinner, the bedrock is relatively impermeable, and the slopes 

are steep, making these areas more conducive to runoff. As 

defined by the U.S. Department of Agriculture (2007), runoff

potential is low along the western boundary, high to moderately high in the southern part of the Santa Rosa Plain and 

throughout the upland areas of the Sonoma and Mayacmas 

Mountains, and moderately low and moderate to high in the 

vicinity of Santa Rosa and throughout the northern part of the 

SRPW (see chapter A, fig. 10).
Region 2: Surface-Water Zone 171

Infiltration and Recharge

For this study, infiltration is defined as the rate of water

entering the soil zone, and recharge is defined as the rate of

water entering the saturated zone. The average annual rate of 

infiltration is unknown; however, Cardwell (1958) reported

a field experiment in the SRPW where about 27 percent of

precipitation infiltrated. Assuming the average annual precipitation is 40 in., the potential average annual infiltration

rate is about 153,000 acre-ft/yr over the entire SRPW. Note 

that Cardwell (1958) assumed that the infiltration equaled the

recharge rate and did not account for any losses such as ET.

Storage in Region 1

Water in region 1 is stored on and above the land surface 

on a seasonal basis (months) as retention storage and below 

the land surface as soil moisture in the soil zone. Retention 

storage includes three components: (1) interception storage, 

(2) pervious-area depression storage, and (3) impervious-area 

retention storage (Linsley and others, 1982). Interception storage is the water held by the surfaces of vegetation; this could 

be significant in the forested areas of the SRPW mountains.

Pervious-area depression storage is water ponded within 

depressions on the pervious land surface; this could be significant over short periods in flat-lying areas of the SRP. Impervious-area retention storage includes precipitation ponded on 

very low permeability surfaces, such as rooftops, parking lots, 

and exposed bedrock.

Soil-zone storage includes two components: field-capacity storage and fully saturated storage. Field-capacity storage 

is the volume of water retained by a soil after gravity drainage 

(Hillel, 1982); that is, air, water, and soil co-exist in a given 

volume of soil zone. Fully saturated storage is the volume of 

infiltrated water stored in all available pore spaces; that is,

only water and soil co-exist in a given volume of soil zone. 

Field-capacity and fully saturated storage are dependent on 

soil texture and thickness (Hillel, 1982); therefore, they are 

expected to be greater in areas with higher clay content and 

thicker soils—for example, the southern part of the SRPW

(see chapter A, figs. 7 and 8).

Region 2: Surface-Water Zone

In the SRPW conceptual model, inflows to region 2, the

surface-water zone, include (1) overland flow and interflow

from region 1, (2) groundwater discharge, (3) precipitation, 

and (4) reclaimed municipal wastewater. Outflows include

(1) surface-water discharge, (2) streambed losses (seepage), 

(3) evaporation, and (4) diversions (for example, irrigation).

In the SRPW conceptual model, the location of the 

hydrologic features of region 2 is defined by a combination

of the 1/24,000 scale high-resolution National Hydrographic 

Dataset (NHD; Simley and Carswell, 2009) and an updated 

hydrography map provided by Sonoma County Water Agency 

(Christopher Delaney, Sonoma County Water Agency, written 

commun., 2009). The NHD dataset includes mapped stream 

channels and water bodies (lakes and ponds). The refined

hydrography map is very similar to the NHD data, but provides an updated version of the hydrography (for example, 

updated engineered channels). As described in chapter B of 

this report, the majority of water bodies in the SRPW are small 

(less than 10 acres), and therefore, it is assumed that water 

bodies are not significant water sources, sinks, or storage

components.

Inflows to Region 2

The primary inflow to region 2 of the SRPW conceptual

model is surface-water runoff from region 1. The surfacewater runoff includes overland flow and shallow subsurface interflow. A secondary source of inflow to region 2 is

groundwater discharge; this source could be the only inflow

to region 2 during the dry season. Groundwater discharge to 

streams between July and September, as represented by average streamflow during these months, can range from 30 to

3,100 acre-ft/yr (see chapter B, table 3). Note that these values 

do not address the possibility of urban runoff or other sources 

adding to streamflow.

The groundwater-level contours indicate that groundwater could be discharging to the streams in the lower reaches 

of Mark West and Santa Rosa Creeks and the upper reaches 

of Santa Rosa Creek near the Rodgers Creek fault zone (see 

chapter B, fig. 29). Precipitation and reclaimed municipal 

wastewater also are inflows to region 2; however, these

sources are small compared to the other sources in the SRPW 

conceptual model.

Outflows from Region 2

The primary outflow from region 2 of the SRPW conceptual model is stream discharge from the SRPW (either to the 

Russian River watershed to the north or the Petaluma River 

watershed to the south). Secondary water outflows from region

2 are seepage into the underlying unsaturated zone, evaporation, and local diversions of streamflow for irrigation.

Streamflow data from a 5-year record on lower Mark

West Creek, about 2 mi upstream of the confluence with

the Russian River, provide some indication of the surfacewater outflows from region 2. During the 5 year period of

record (water-years 2006–2010), the average flow rate was

288 cubic feet per second (ft3/s), or about 210,000 acre-ft/yr. 

The area upstream of this gage includes about 90 percent of 

the total area of the SRPW and, therefore, provides a good 

estimate of the total surface-water outflow from region 2.
172 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Region 3: Subsurface Zone

Region 3, the subsurface zone of the SRPW conceptual 

model, includes the unsaturated and saturated zones. The 

areally extensive unsaturated zone extends from the bottom 

of the soil zone in region 1, or the bottom of the streambed or 

lakebed in region 2, to the top of the saturated zone (the water 

table) and is composed of unconsolidated materials (gravel, 

sand, silt, and clay), consolidated rock, water, and air. The 

saturated zone extends from the water table to the top of the 

low-permeability basement and is composed of unconsolidated materials, consolidated rock, and water. The thicknesses 

of the unsaturated zone and saturated zone vary with of the 

altitude of the water table. During relatively wet periods at 

some locations, the unsaturated zone can be absent if the water 

table intersects regions 1 or 2, thus creating a groundwaterdischarge zone.

Unsaturated Zone

Percolation through the bottom of the soil zone in 

region 1 and seepage through streambeds and lakebeds in 

region 2 become infiltration to the top of the unsaturated zone

and are a function of the physical characteristics of the unsaturated zone (vertical hydraulic conductivity, thickness, effective 

porosity, and vertical-saturation profile).

In the SRPW conceptual model, water flow in the

unsaturated zone is assumed to be vertical; however, in 

reality, lateral flow can result from vertical heterogeneity.

The long-term average rates of deep percolation, in response 

to infiltration from regions 1 and 2, generally are equal to the

long-term recharge rate; however, the annual, monthly, and 

daily recharge rates can be much different than rates of deep 

percolation as a result of the time delay and dampening effect 

of the unsaturated zone on transient, unsaturated flow.

Inflows to the Unsaturated Zone

Inflow to the unsaturated zone is infiltration from the

overlying soil zone of region 1, from overlying streambeds 

from region 2, and from septic-tank effluent. Areally, water

infiltrates the soil zone in locations where precipitation is high,

soils are more permeable, and ET is minimal. Locally, water 

infiltrates from runoff that collects in stream channels and

percolates through permeable streambeds in losing reaches of 

streams. The infiltration rate, or vertical flux, from regions 1

and 2 is limited by the vertical hydraulic conductivity of the 

unsaturated zone. Under conditions when infiltration from the

overlying zone is limited by a low hydraulic conductivity of 

the unsaturated zone, a fraction of the infiltration is rejected,

and this water remains in region 1. At the water table, capillary 

rise into the unsaturated zone contributes minor inflows; this is

assumed to be insignificant on the scale of the SRPW.

In the SRPW, infiltration from the soil zone is the greatest

in the WG, WB, CB, and VAL storage units in areas where the 

Wilson Grove and Glen Ellen Formations are present at land 

surface (see chapter B, fig. 1). In addition, infiltration is higher

where precipitation is higher (for example, the Wilson Grove 

highlands shown in chapter A, fig. 6). Although precipitation 

is high in the UPL storage unit (see chapter A, fig. 6), the 

infiltration is probably low because the Sonoma Volcanics and

Franciscan Formation are at land surface throughout most of 

the storage unit (see chapter B, fig. 1). These geologic units 

generally have low hydraulic conductivities, and the land 

surface is relatively steep, which precludes high infiltration

rates. Infiltration from stream channels can be greatest in the

transition from the UPL storage unit to the SRP, where streamflow is concentrated in the channel, the subsurface tends to be

more permeable (see chapter A, fig. 9), and the unsaturated 

zone is thick. As described in chapter B of this report, domestic pumpage is large in comparison to municipal pumpage; 

therefore, the potential recharge from septic-tank effluent also

could be large. However, basin-wide nitrate concentrations 

in groundwater are relatively low, as reported in chapter C of 

this report, which indicates that septic-tank effluent is not a

primary component of unsaturated-zone inflow.

Outflows from the Unsaturated Zone

Outflows from the unsaturated zone include recharge to

the saturated zone and transpiration by plants. Recharge to 

the saturated zone is the primary outflow from the unsaturated

zone and is dependent on the vertical permeability of the 

unsaturated and saturated interface. That is, if the interface 

is permeable, there is high recharge potential; however, if 

the interface is not permeable (permeable unsaturated zone 

overlying a low-permeability saturated zone), there is low 

recharge potential. Under conditions where the water table is 

rising, there is a decrease in the thickness of the unsaturated 

zone, and water stored in the unsaturated zone is added to the 

saturated zone, which contributes to recharge (this component 

of recharge is in addition to vertical percolation through the 

unsaturated zone).

In the SRPW, outflows from the unsaturated zone are

highest in the WG storage unit where the permeable Wilson 

Grove Formation is present and WB and CB storage units, 

immediately downgradient (west) of the Rodgers Creek fault 

zone, where coarse-grained deposits are present along the 

channels (see chapter B, fig. 1). In contrast, outflows from the

unsaturated zone are lower in the western parts of the WB and 

CB storage units because the unsaturated zone is dominated 

by fine-grained deposits of the Glen Ellen Formation (see

chapter B, fig. 1).
Region 3: Subsurface Zone 173

Saturated Zone

Important hydrologic characteristics of the saturated 

zone of the SRPW groundwater-flow system include those

that reflect the ability of the groundwater system to transmit, store, and release water; that allow for vertical passage 

of water between hydrogeologic units; and that control the 

flow of water across geologic or hydrologic boundaries. The

movement of water through the saturated groundwater system 

is controlled by topography, aquifer and aquitard properties 

(including thickness and orientation of layers, structures, and 

fault planes), and the magnitude and distribution of recharge 

and discharge, including pumping. Aquifer and aquitard properties depend on the type of sediments and rocks composing 

the hydrogeologic system. Geologic structures, such as fault 

planes or zones, either can be flow barriers or flow conduits,

depending on age, orientation, mineralization along the fault, 

and the juxtaposition of aquifers and aquitards across the fault.

Inflows to the saturated zone of region 3 include groundwater recharge from the unsaturated zone and underflow from

adjacent groundwater basins. Outflows from the saturated zone

include ET, discharge to streams and lakes, discharge to the 

soil zone, pumping, and underflow to adjacent basins.

Inflows to the Saturated Zone

Inflows to the saturated zone include groundwater

recharge from the unsaturated zone, direct recharge from 

streams, and underflow from adjacent basins. Herbst and

others (1982) estimated the average annual recharge to the 

Santa Rosa Plain groundwater basin between 1960 and 1975 

was about 29,300 acre-ft. Assuming this estimate is correct, 

the average annual recharge for the SRPW would be greater 

than this value because the SRPW includes areas not included 

in the 1982 estimate: the northern half of Kenwood Valley, 

the area west of the Sebastopol Fault, and the mountains that 

border the Santa Rosa Plain. A preliminary coupled watershed-groundwater-flow model of the SRPW for water-years

1976–2009 was used to estimate an average groundwater 

underflow to the SRPW of about 70 acre-ft/yr (Woolfenden

and others, 2011).

Outflows from the Saturated Zone

Discharge from the groundwater system can be 

(1) underflow to adjacent groundwater subbasins, (2) ET,

(3) discharge to springs and streams in region 2, (4) pumping, 

or (5) discharge to the soil zone in region 1. ET was discussed 

in the section entitled “Outflows from Region 1.” The average annual discharge from groundwater underflow estimated

by using a preliminary coupled watershed-groundwater-flow

model of the SRPW was about 1,200 acre-ft/yr during wateryears 1976–2009 (Woolfenden and others, 2011).

Groundwater discharges directly to streams where the 

water table is at or above the bottom of the streambed and 

contributes to the baseflow component of total streamflow in

region 2. Water-level contour maps indicate that Santa Rosa 

Creek consistently gains streamflow in the VAL storage unit.

Groundwater discharged from springs is a source of baseflow

for streams or is lost to ET. Springs are sensitive to changes 

in groundwater levels caused by natural variations in climate, 

by the development of groundwater resources, or by land-use 

changes (increases in impervious areas), and their flows often

diminish or stop during dry periods or when nearby wells are 

pumped.

In the SRPW, pumping for public-supply, agricultural, 

and domestic uses is the largest component of groundwater 

discharge. The total annual pumpage from 70 public-supply 

wells from 1975–2009 ranged from about 2,000 acre-ft to 

about 8,300 acre-ft. Agricultural pumpage is unreported; however, Hevesi and others (2011) estimated agricultural pumpage 

for 1,072 agricultural wells by using a calibrated watershed 

model of the study area. Total estimated agricultural water 

demand ranged from 8,900 acre-ft in 1974 to 46,600 acre-ft 

in 2008. Domestic pumpage is unreported also; however, it 

was estimated for 1974–2010 by using population density 

and census tracts in rural areas with an assumed per capita 

consumptive-use factor of 0.19 acre-ft per person (California 

Department of Water Resources, 1994). Annual domestic 

pumpage estimates ranged from 11,077 to 23,202 acre-ft during 1974–2010, and pumpage increased as the rural population grew. The average total pumpage from all sources for 

1975–2010 was about 48,000 acre-ft/yr. The maximum was 

about 53,400 acre-ft in 2008.

In the SRPW conceptual model, about 15 percent of 

the groundwater used directly for agricultural irrigation is 

assumed to be returned to the saturated zone as recharge, and 

about 5 percent is assumed to become runoff. In addition, 

an unknown, but likely small, percentage of domestic and 

municipal-well water used for landscape irrigation becomes 

recharge. A fraction of domestic-well water can contribute 

to recharge as leakage through underground septic systems. 

Municipal-well water can also indirectly contribute to recharge 

as reclaimed water used to supplement agricultural and landscape irrigation.

Pumping removes water from the saturated zone and can 

lower the water table, which in turn decreases baseflow in

streams. The magnitude and spatial distribution of baseflow

in the stream channels can decrease seasonally as pumping 

increases during the dry summer months to provide water for 

irrigation. In addition, baseflow can decrease with sustained

or increased pumping near streams. It has not been determined 

whether groundwater discharge to stream channels and springs 

has decreased in the SRPW in response to pumping from the 

large number of wells that were drilled between the 1950s and 

2010.
174 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Summary of Conceptual Model

The geologic, hydrologic, and water-quality data presented in chapters A–D were used to develop the conceptual 

model for the SRPW (fig. 1).

Precipitation

Precipitation, primarily as rainfall, is the major source 

of inflow to the SRPW. The mean annual precipitation for the

SRPW is approximately 40 in., or about 560,000 acre-ft/yr 

distributed over the 167,400 acre watershed. Precipitation is 

greatest (about 42–57 in./yr) in the Mayacmas and Sonoma 

Mountains in the UPL storage unit (see chapter A, fig. 6); 

however, because of the low permeability of the basement 

rocks and Sonoma Volcanics that compose these mountains, 

and the steep slope of the storage unit, most of the precipitation on this storage unit probably becomes runoff that contributes to streamflow and potential recharge in the downstream

storage units. In addition, there is direct infiltration of precipitation on the SRP.

Streamflow

Mark West Creek, Santa Rosa Creek, and Laguna de 

Santa Rosa are the major streams that drain the SRPW. The 

main channel of Mark West Creek originates in the Mayacmas 

Mountains and is perennial throughout much of its length 

in the UPL storage unit, with summer flows maintained by

numerous springs near the headwaters. Santa Rosa Creek 

also originates in the Mayacmas Mountains, in steep terrain, 

with mostly-natural vegetation. The upper part of Santa Rosa 

Creek and Matanzas Creek, one of its tributaries, are perennial 

streams in the UPL storage unit. As the streams flow through

the VAL storage unit, they gain flow until just east of the Rodgers Creek fault zone, where groundwater-level contours indicate groundwater from the VAL storage unit discharges to the 

stream channel. Immediately west of the Rodgers Creek fault 

zone, groundwater-level contours indicate that the Santa Rosa 

Creek loses water to (recharges) the SRP groundwater basin. 

As the Santa Rosa Creek reaches the western end of the SRP, 

the stream begins to gain flow and is perennial. The Laguna

de Santa Rosa, which originates in the southern part of the 

SRPW, is perennial in most sections.

Streamflow leaves the SRPW from Mark West Creek

into the Russian River. There is a stream gage located near 

the outlet (MWCM); however, the record is short (water-years 

2006–2010). Therefore, the MWCM record was extended 

(water years 1960–2010) by using the MOVE.1 technique 

and data from a neighboring stream gage on the Napa 

River (NAPN). The long-term estimated mean discharge 

for the extended 51-year time series was 265 ft3

/s, or about 

192,000 acre-ft/yr.

Aquifer System

The Glen Ellen, Wilson Grove, and Petaluma Formations, 

and the Sonoma Volcanics have distinct aquifer properties 

and constitute the four principal water-bearing aquifer units 

in the study area. In general, the aquifer units transition from 

Sonoma Volcanics interbedded with the Petaluma Formation 

in the UPL storage unit east of the Rodgers Creek fault zone, 

to the Glen Ellen Formation overlying the Sonoma Volcanics 

in the VAL storage unit, to heterogeneous continental sediments of the Glen Ellen and Petaluma Formations beneath the 

SRP, to dominantly fine-grained marine sands of the Wilson

Grove Formation on the west in the WG storage unit (fig. 3).

Earlier work reported the Glen Ellen Formation was 

as thick as 3,000 ft (Cardwell, 1958); however, recent work 

indicated that the formation is highly variable, but generally is a few hundred feet thick or less. In general, the fluvial

sediments of the Glen Ellen Formation have low to moderate 

permeability and tend to be more permeable in the VAL storage unit and on the eastern end of the SRP than the remainder 

of the SRPW. Wells perforated in the Glen Ellen Formation 

that include some beds of moderately- to well-sorted, coarsegrained materials yield large amounts of water to wells.

The 2,700-ft-thick, sand-dominated Wilson Grove 

Formation is exposed in the low hills of the WG storage unit, 

west of the SRP, and it continues to the east for an uncertain 

distance beneath the SRP, where it is concealed by variable 

thicknesses of alluvial materials and the Glen Ellen Formation 

and interfingers with the Petaluma Formation (fig. 3). The predominance of relatively well-sorted marine sand and the low 

degree of cementation in the Wilson Grove Formation result in 

moderate permeability and moderate to high storativity.

The Petaluma Formation is dominated by fine-grained

materials, either in thick beds or as interstitial material in 

poorly sorted silty and clayey sands or gravels. The formation 

is at least 3,000 ft thick in places within the study area, and 

has been divided into three distinct members (lower, middle, 

and upper) on the basis, in part, of the dominant grain-size 

and sorting. The lower member, which is up to 750 ft thick, is 

largely made up of dense beds of mudstone and has the lowest 

hydraulic conductivity. The formation coarsens in the middle 

and upper members, in which beds of poorly-sorted sands and 

gravels increase the hydraulic conductivity. The upper member 

is about 500 ft thick and unconformably overlies the middle 

member in the CB storage unit.

The Sonoma Volcanics include a thick accumulation of 

andesitic and basaltic tuffs containing interbedded lavas and 

volcaniclastic rocks and are an important aquifer in the UPL 

and VAL storage units (fig. 3). Lithologies with the greatest 

permeability include rubble zones between lava flows, beds

of scoria and coarse tephra, air-fall tuffs, and some coarsegrained facies of volcaniclastic units.
Summary of Conceptual Model 175

ALLUVIUM & WILSON GROVE PETALUMA SONOMA GLEN ELLEN BEDROCK FORMATION FORMATION VOLCANICS FORMATION

GENERAL DIRECTION OF GROUNDWATER

MOVEMENT WITH RELATIVE AGE PRECIPITATION RUNOFF INFILTRATION

FAULT

ZONE

WATER

TABLE

MILLENIA

MILLENIA CENTURIES

DECADES

YEARS

Sebastopol fault

Rodgers Creek fault

Mayacmas Mts

Agriculture Rural

City

DAYS

sac11-0378_Figure D03

CENTURIES

DECADES

Groundwater Flow

In general, groundwater flows from the mountains in the

UPL storage unit through the VAL storage unit and into the 

WB and CB storage units to the west, and from the Wilson 

Grove highlands in the WG storage unit on the west toward 

the Laguna de Santa Rosa on the western edge of the CB storage unit (fig. 3).

Samples from springs in the Mayacmas Mountains are 

a mixed cation-bicarbonate type water that have dissolvedsolids concentrations less than about 100 mg/L—the lowest

dissolved-solids concentrations in the SRPW. As groundwater migrates through the UPL and VAL storage units, the 

dissolved-solids concentration of the groundwater increases as 

a result of water-rock interactions and anthropogenic inputs, 

such as irrigation return flows and septic-tank discharge. The

median dissolved-solids concentrations of well samples in the 

UPL and VAL storage units were 330 and 392 mg/L, respectively. The isotopic values of well samples in the UPL and 

VAL storage units were similar to the spring 2009 isotopic 

values from surface-water sites in the Mayacmas Mountains 

(see chapter C, figs. 6B and 6D), indicating that precipitation 

and runoff from the Mayacmas Mountains is the probable 

source of recharge to the wells sampled in these storage units. 

Only three of the nine (about 33 percent) samples from the 

UPL and VAL storage units had detectable tritium concentrations, indicative of modern water (see chapter C, fig. 7A). 

The absence of modern water in many of these samples probably reflects the low vertical permeability of the basement

rocks and Sonoma Volcanics that compose the UPL and VAL 

storage units. Samples from wells perforated in the Sonoma 

Volcanics, in the VAL storage unit, had uncorrected 14C ages 

of 10,000 years before present, indicating that groundwater 

movement through the Sonoma Volcanics is relatively slow.

Groundwater-level and water-quality data indicate that 

the Rodgers Creek fault zone is a barrier to groundwater flow

between the VAL storage unit and the WB and CB storage 

units, which compose the SRP. Groundwater-level contours 

east of the Rodgers Creek fault zone indicated that groundwater is discharging to the stream channel, which probably is 

the result of the low permeability of the fault zone limiting the 

lateral movement of groundwater across the fault zone, so that 

it is forced into the stream channel (fig. 3).

A deep well sample in the Rogers Creek fault zone, 

near Santa Rosa, had a dissolved-solids concentration of 

1,510 mg/L, which is more than three times greater than the 

Figure 3. Conceptual model of the groundwater-flow system, Santa Rosa Plain watershed, Sonoma County, California.
176 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

dissolved-solids concentrations analyzed in almost all other 

wells. The sample was a sodium-chloride type water with 

concentrations of boron in excess of 2,000 mg/L, which is 

hypothesized to be from deeply circulating groundwater rising 

along the fault zone. The uncorrected 14C age of the sample 

was 27,000 years before present. The very old age of the water 

supported the hypothesized source of water to this well.

Comparison of data from the Rodgers Creek fault zone 

to available data from downgradient (west) of the fault zone 

indicated that the high dissolved-solids concentration of water 

in the fault zone did not significantly affect the water quality of wells in the WB and CB storage units (see chapter C, 

fig. 3C). Almost all well samples west of the Rodgers Creek 

fault zone had lower dissolved-solids concentrations than both 

the shallow and deep samples from the Rodgers Creek fault 

zone. The lower dissolved solids concentrations in samples 

from wells west of the Rodgers Creek fault zone indicated 

that groundwater underflow across the fault zone is minimal.

The Rodgers Creek fault zone probably restricts the lateral 

movement of water from the UPL and VAL storage units to the 

WB and CB storage units, forcing groundwater to discharge 

toward the stream channels east of the fault zone, as indicated 

by the groundwater-level contours (see chapter B, fig. 29A). 

The lower dissolved-solids concentration sampled from wells 

downgradient of the Rodgers Creek fault zone probably is the 

result of recharge west (downgradient) of the fault zone.

In the SRP, samples from most shallow wells perforated or completed in the Glen Ellen Formation are a mixed 

cation-bicarbonate type water in which magnesium is the most 

abundant cation. Samples from wells perforated or completed 

in the Petaluma Formation, or in both the Glen Ellen and Petaluma Formations in the SRP, are a mixed cation-bicarbonate 

type water in which sodium is the most abundant cation or are 

a sodium-bicarbonate water. In general, the dissolved-solids 

concentrations of samples from the shallow wells were higher 

than dissolved-solids concentrations of samples from the deep 

wells, which could be the result of anthropogenic factors such 

as irrigation-return flows. The samples from wells perforated

in the Petaluma Formation, or in both the Glen Ellen and Petaluma Formations in the CB storage unit, had higher dissolvedsolids concentrations and a higher proportion of sodium than 

samples from wells perforated in the Wilson Grove Formation 

on the western end of the CB storage unit (see chapter C, 

fig. 3C). The difference in dissolved-solids concentration and 

water type indicated that the unnamed fault, east of the Sebastopol fault, is at least a partial barrier to groundwater flow.

The stable isotopes of water (oxygen-18 and deuterium) 

indicated a mixture of water sources in the SRP (see chapter 

C, fig. 6D). Most of the samples from wells in the SRP were 

isotopically heavier (less negative) than streamflow and well

samples from the UPL and VAL storage units, indicating that 

streamflow and groundwater underflow from these storage

units cannot be the sole source of recharge to the SRP. The 

heavier isotopic values indicated that at least some of the 

recharge to the SRP originates as precipitation falling directly 

on the lower elevations of the SRP.

The dissolved-solids concentrations in samples from 

wells perforated almost exclusively in the Wilson Grove 

Formation were lower than in samples from wells perforated 

in the Glen Ellen and Petaluma Formations (see chapter C, 

fig. 3C). The samples from wells with perforations in the Glen 

Ellen and Petaluma Formations immediately east of the WG 

storage unit are sodium-bicarbonate type water with dissolvedsolids concentrations in excess of 420 mg/L (see chapter C, 

fig. 3C). The difference in water type and dissolved-solids 

concentrations indicated that there is little groundwater interaction between the Wilson Grove Formation and the neighboring Glenn Ellen and Petaluma Formations. Apparently, the 

Sebastopol fault limits the lateral groundwater movement from 

the WG storage unit to the CB storage unit, and low permeability clay layers in the Petaluma Formation limit the vertical 

groundwater movement between the Wilson Grove Formation 

and overlying Petaluma Formation.

Groundwater Age

Tritium (3H) concentrations were analyzed in 35 samples 

collected from 30 wells in the SRPW and ranged from less 

than 0.3 (detection limit of samples collected in 2004) to 

2.4 tritium units (TU). About 43 percent of the samples had 

detectable tritium concentrations, indicating that these samples 

contained some modern water (water recharged since 1952). 

As expected, modern recharge was more prevalent in the shallow well samples. The vertical migration of recharge in the 

SRP probably is retarded by the presence of low permeability 

clay deposits in the Glen Ellen and Petaluma Formations.

Measured 14C activities for the 16 well samples analyzed 

ranged from 1.3 to 84.7 percent modern carbon. These 14C 

activities represent uncorrected ages of 1,000–34,000 years 

before present. The deep-well samples all had uncorrected 14C 

ages of 4,000 years or older, and five of the deep well samples

were 10,000 years or older. The relatively old age of the deepwell samples supported the observation made from the tritium 

data that the vertical migration of modern recharge probably is 

restricted by low-permeability clay deposits in the Glen Ellen 

and Petaluma Formations. The low-permeability clay deposits 

also confine the deeper aquifer systems, which explains the

rapid response of the deeper aquifer systems to pumping and 

cessation of pumping. The oldest water sampled was from a 

well near the Laguna de Santa Rosa, which is near the end of 

a long groundwater-flow-line through the SRP groundwater

basin (fig. 3).

Data Gaps

Analyses of hydrologic, hydrogeologic, and geochemical 

data collected for this study indicated gaps in the data available for the SRPW. These gaps included a lack of depthdependent data, additional water-quality data, and agricultural 

and domestic pumpage data.
References Cited 177

There is only one nested piezometer site (wells W33–35) 

in the SRPW, and most wells are perforated over multiple 

aquifers. This lack of depth-dependent water-level and waterquality data makes it difficult to calibrate a groundwater-flow

model by aquifer or layer.

The variability observed in the water-quality data (for 

example, chloride and specific-conductance data) cannot

be explained by using the available dataset. Additional data 

collection (for example, isotopes, boron, and age dates) is 

necessary.

Groundwater pumping is a primary sink from the saturated zone, and the maximum total annual pumpage could 

be as much as 53,400 acre-ft. Of this total, only municipal 

pumping, which could make up only 15 percent of the total, is 

reported. The balance is composed of agricultural and domestic pumping, which are unreported. In order to develop and 

calibrate a numerical model of the SRPW, improved estimates 

of the unreported pumpage must be made, and the locations of 

the wells identified.

References Cited

California Department of Water Resource, 1994, California 

water plan update: Bulletin 160-93, October 1994, v. 2, 315 

p.

California Irrigation Management Information (CIMIS), 2005, 

Department of Water Resources and University of California, Davis: data accessed on March 31, 2010, at URL http://

www.cimis.water.ca.gov/.

Cardwell, G.T., 1958, Geology and ground water in the Santa 

Rosa and Petaluma areas, Sonoma County, California: U.S. 

Geological Survey Water-Supply Paper 1427, 273 p. and 5 

plates.

Daly, Christopher, Gibson, W.P., Doggett, Matthew, Smith, 

Joseph, and Taylor, George, 2004, Up-to-date monthly climate maps for the conterminous United States: Proceedings 

of the 14th American Meteorological Society Conference on 

Applied Climatology, 84th AMS Annual Meeting Combined 

Preprints, American Meteorological Society, Seattle, Washington, January 13–16, 2004, Paper P5.1, CD-ROM.

Herbst, C.M., Jacinto, D.M., and McGuire, R.A., 1982, Evaluation of ground water resources, Sonoma County, volume 

2: Santa Rosa Plain: California Department of Water 

Resources Bulletin 118-4, 107 p., 1 plate.

Hevesi, J.A., Woolfenden, L.R., Niswonger, R.G., Regan, 

R.S., and Nishikawa, Tracy, 2011, Decoupled application 

of the integrated hydrologic model, GSFLOW, to estimate 

agricultural irrigation in the Santa Rosa Plain, California, 

in Maxwell, R.M., Poeter, E.P., Hill, M.C., and Zheng,

Chunmiao, eds., MODFLOW and More 2011: Integrated 

hydrologic modeling–Conference proceedings, June 5–8, 

2011: Golden, Colorado, International Groundwater Modeling Center, p. 115–119.

Hillel, Daniel, 1982, Introduction to soil physics: San Diego, 

CA, Academic Press, Inc., 364 p.

Linsley, R.K., Kohler, M.A., and Paulus, J.L.H., 1982, Hydrology for engineers (3d. ed.): New York, NY, McGraw-Hill, 

Inc., 508 p.

Maidment, D.R., 1993, Handbook of hydrology: New York, 

NY, McGraw-Hill, Inc., 750 p.

Markstrom, S.L., Niswonger, R.G., Regan, R.S., Prudic, D.E., 

and Barlow, P.M., 2008, GSFLOW—Coupled groundwater

and surface-water flow model based on the integration of

the Precipitation-Runoff Modeling System (PRMS) and the 

Modular Ground-Water Flow Model (MODFLOW-2005): 

U.S. Geological Survey Techniques and Methods 6-D1, 240 

p.

Simley, J.D. and Carswell Jr., W.J., 2009, The National Map 

– Hydrography: U.S. Geological Survey Fact Sheet 2009-

3054, 4 p.

U.S. Department of Agriculture, Natural Resources Conservation Service, 2007, National Engineering Handbook, Part 

630 Hydrology, Chapter 7 Hydrologic Soil Groups: Washington, DC, 16 p.

Woolfenden, L.R., Hevesi, J.A., Niswonger, R.G., Regan, 

R.S., and Nishikawa, Tracy, 2011, Modeling a complex 

hydrologic system with an integrated hydrologic model 

in Maxwell, R.M., Poeter, E.P., Hill, M.C., and Zheng,

Chunmiao, eds., MODFLOW and More 2011: Integrated 

hydrologic modeling–Conference proceedings, June 5–8, 

2011: Golden, Colorado, International Groundwater Modeling Center, p. 134–138.
178 Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California

Appendix A. Selected chemical and physical properties and inorganic

constituents in samples from selected springs, and wells, Santa Rosa Plain 

Watershed, Sonoma County, California, 1947–2010.

Appendix A provided separately as a Microsoft Excel® file.
Prepared by the Sacramento Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

California Water Science Center

6000 J Street, Placer Hall

Sacramento, CA 95819

dc_ca@usgs.gov

or visit our Web site at:

http://ca.water.usgs.gov
Nishikawa—Hydrologic and Geochemical Characterization of the Santa Rosa Plain Watershed, Sonoma County, California—

Scientific Investigations Report 2013–5118

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