U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4229

Prepared in cooperation with the Napa County Department of Public Works

Ground-Water Resources in the Lower Milliken–Sarco– 

Tulucay Creeks Area, Southeastern Napa County, 

California, 2000–2002

Ground-Water Resources in the Lower 

Milliken–Sarco–Tulucay Creeks Area,

Southeastern Napa County, California, 2000–2002

By Christopher D. Farrar and Loren F. Metzger

Sacramento, California

2003

Water-Resources Investigations Report 03-4229

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with the

NAPA COUNTY DEPARTMENT OF PUBLIC WORKS 2008-19
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall, Suite 2012

6000 J Street

Sacramento, CA 95819-6129
Contents iii

CONTENTS

Abstract ................................................................................................................................................................ 1

Introduction .......................................................................................................................................................... 2

Purpose and Scope ...................................................................................................................................... 2

Description of Study Area........................................................................................................................... 4

Previous Investigations ............................................................................................................................... 6

Acknowledgments....................................................................................................................................... 10

Geology ................................................................................................................................................................ 10

Stratigraphy................................................................................................................................................. 10

Sonoma Volcanics.............................................................................................................................. 14

Quaternary Alluvial Deposits............................................................................................................. 14

Ground-Water Hydrology .................................................................................................................................... 15

Recharge...................................................................................................................................................... 16

Streamflow Gains and Losses............................................................................................................ 19

Methods of Data Collection and Analysis................................................................................ 19

Streamflow Measurements and Estimated Gains and Losses................................................... 21

Discharge..................................................................................................................................................... 23

Discharge from Pumpage................................................................................................................... 23

Ground-Water Underflow .................................................................................................................. 26

Ground-Water Levels and Movement......................................................................................................... 26

Monitoring Network .......................................................................................................................... 26

Ground-Water Levels......................................................................................................................... 28

Ground-Water Movement.................................................................................................................. 28

Annual and Seasonal Water-Level Fluctuations................................................................................ 31

Long-Term Changes in Ground-Water Levels .................................................................................. 37

Ground-Water Level Changes, 1975-2001 ........................................................................................ 42

Surface-Water and Ground-Water Quality .......................................................................................................... 44

Methods of Water Sampling and Analysis.................................................................................................. 44

Surface-Water and Ground-Water Chemistry............................................................................................. 46

Oxygen-18 and Deuterium.......................................................................................................................... 49

Background ........................................................................................................................................ 49

Stable Isotope Results ........................................................................................................................ 50

Ground-Water Age Dating.......................................................................................................................... 53

Chlorofluorocarbons .......................................................................................................................... 53

Tritium ............................................................................................................................................... 56

Water Temperature...................................................................................................................................... 57

Summary and Conclusions................................................................................................................................... 59

References Cited .................................................................................................................................................. 62
vi Figures

FIGURES

Figure 1. Map showing location of study area, southeastern Napa County, California....................................... 3

Figure 2. Map showing land use in 2000 in the lower Milliken–Sarco–Tulucay Creeks area, southeastern 

Napa County, California ....................................................................................................................... 5

Figure 3. Graphs showing annual precipitation for water years 1918–2002, and distribution of cumulative

precipitation for water years (WY) 2000–2002 at Napa State Hospital, southeastern Napa 

County, California ................................................................................................................................ 7

Figure 4. Map showing mean annual precipitation in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California, 1961-90................................................................................... 8

Figure 5. Map showing City of Napa Municipal Water Department distribution system, in the lower 

Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California .................................... 9

Figure 6. Map showing geology of, and cross section locations in, the lower Milliken–Sarco–Tulucay 

Creeks area, southeastern Napa County, California ............................................................................. 11

Figure 7. Generalized geologic cross sections in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California .................................................................................................. 12

Figure 8. Map showing generalized thickness of unconsolidated deposits in the lower Milliken–Sarco–

Tulucay Creeks area, southeastern Napa County, California ............................................................... 17

Figure 9. Conceptual model of the ground-water flow system in the lower Milliken–Sarco–Tulucay 

Creeks area, southeastern Napa County, California ............................................................................. 18

Figure 10. Map showing locations of streamflow-measurement stations and streambed infiltration zones in 

the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California .................... 20

Figure 11. Graphs showing streamflow measurements for the three seepage runs for selected parts of the 

Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California..... 22

Figure 12. Map showing geographic distribution of known active and potentially active wells per quarterquarter section in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County,

California .............................................................................................................................................. 24

Figure 13. Graph showing historical well development and departure from mean annual precipitation 

(at Napa State Hospital) in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa 

County, California, water years 1918-2002.......................................................................................... 25

Figure 14. Map showing locations of wells in the water-level monitoring network in the lower Milliken–

Sarco–Tulucay Creek area, southeastern Napa County, California...................................................... 27

Figure 15. Map showing generalized hydraulic head in the lower Milliken–Sarco–Tulucay Creeks area,

southeastern Napa County, California, October 2001 .......................................................................... 29

Figure 16. Map showing generalized hydraulic head in the lower Milliken–Sarco–Tulucay Creeks area,

southeastern Napa County, California, April 2002............................................................................... 30

Figure 17. Graphs showing change in water levels compared with well depth between October 2000-

October 2001 and October 2001-October 2002 in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California .................................................................................................. 32

Figure 18. Graphs showing change in water levels compared with well depth between April 2000-April 2001 

and April 2001-April 2002 in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa

County, California ................................................................................................................................ 33

Figure 19. Graphs showing change in water levels compared with well depth between October 2000-

April 2001, and October 2001-April 2002 in the lower Milliken–Sarco–Tulucay Creeks area,

southeastern Napa County, California .................................................................................................. 34

Figure 20. Map and graph showing continuous ground-water levels recorded in selected wells in the lower 

Milliken–Sarco-Tulucay Creeks area, southeastern Napa County, California..................................... 35

Figure 21. Graphs showing ground-water levels in selected wells and cumulative precipitation in the lower

Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California .................................... 36
Figures vii

Figure 22. Map showing locations of selected wells at which periodic water levels were made in the lower

Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, early 1960s through 

2002 ...................................................................................................................................................... 38

Figure 23. Graphs showing periodic water levels in selected wells in the lower Milliken–Sarco–Tulucay 

Creeks area, southeastern Napa County, California, early 1960s through 2002 .................................. 39

Figure 24. Map showing change in water levels in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California, autumn 1975 to autumn 2001................................................. 43

Figure 25. Map showing locations of ground-water, surface-water, and miscellaneous water-chemistry 

sampling sites in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, 

California .............................................................................................................................................. 45

Figure 26. Trilinear diagram showing chemical composition of water from selected ground-water and 

surface-water sampling sites in the lower Milliken–Sarco–Tulucay Creeks area, southeastern 

Napa County, California, September-November 2001......................................................................... 47

Figure 27. Graphs showing relation between delta deuterium and delta oxygen-18 in water samples for the 

lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California .......................... 50

Figure 28. Graph showing concentrations of chlorofluorocarbons CFC-11, CFC-12, and CFC-113 in North

American air ......................................................................................................................................... 54

Figure 29. Graph showing comparison of argon and nitrogen concentrations in ground-water samples from 

wells and calculated water temperatures at the time of recharge, lower Milliken–Sarco–Tulucay 

Creeks area, southeastern Napa County, California ............................................................................. 55

Figure 30. Graph showing tritium concentrations in samples from six wells and tritium concentrations 

expected in ground water in southeastern Napa County, California, that originated as precipitation

between 1953 and 2000, southeastern Napa County, California.......................................................... 57

Figure 31. Map and graphs showing temperature logs for selected wells in the lower Milliken–Sarco–

Tulucay Creeks area, southeastern Napa County, California ............................................................... 58
vi Tables

TABLES

Table 1. Summary of land-use change in the lower Milliken–Sarco–Tulucay Creeks area, southeastern 

Napa County, California, 1958–2000 ................................................................................................ 66

Table 2. Streamflow measurements, gain or loss of streamflow between stations, flow distance between 

stations, and rate of gain or loss of streamflow between stations at sites along Milliken, Sarco, 

and Tulucay Creeks and their tributaries, southeastern Napa County, California, 

April 17–26, 2000 .............................................................................................................................. 67

Table 3. Streamflow measurements, gain or loss of streamflow between stations, flow distance between 

stations, and rate of gain or loss of streamflow between stations at sites along Milliken, Sarco, 

and Tulucay Creeks and their tributaries, southeastern Napa County, California, March 13–16, 

2001 ................................................................................................................................................... 71

Table 4. Streamflow measurements, gain or loss of streamflow between stations, flow distance between 

stations, and rate of gain or loss of streamflow between stations at sites along Milliken, Sarco, 

and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 10–12, 

2001 ................................................................................................................................................... 75

Table 5. Construction data and water-level data for April and October of 2000 through 2002 for selected 

wells in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.... 79

Table 6. Summary of estimated ground-water pumping for domestic and irrigation uses in the lower 

Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, 2000–2002 ............. 85

Table 7. Summary of median water-level change in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California, 2000–2002 ........................................................................... 86

Table 8. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in 

southeastern Napa County, California, 2001 ..................................................................................... 87

Table 9. Field measurements and oxygen-18 and deuterium ratios in samples from streamflowmeasurement stations, lakes and ponds, springs, and wells, southeastern Napa County, 

California, 2001–02 ........................................................................................................................... 91

Table 10. Dissolved gas, chlorofluorocarbon (CFC), and tritium concentrations in samples from wells, and 

calculations of CFC concentrations in air and corresponding CFC-model age, southeastern Napa 

County, California, 2001 ................................................................................................................... 93

Table 11. Helium, hydrogen, and neon gas concentrations in water samples from wells, southeastern Napa 

County, California, 2001 ................................................................................................................... 94
Conversion Factors, Vertical Datum, Abbreviations, and Well-Numbering System vii

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS, and WELLNUMBERING SYSTEM

CONVERSION FACTORS

Temperature in degrees Celsius (oC) may be converted to degrees Fahrenheit (oF) as follows:

oF=1.8 oC+32.

Specific conductance is given microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25oC).

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

micrograms per liter (μg/L). 

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

foot per foot (ft/ft) 1 meter per meter

mile (mi) 1.609 kilometer

Area

acre 0.4047 hectare

square foot (ft2) 0.09290 square meter

square mile (mi2) 259.0 hectare

Volume

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

acre-foot per acre (acre-ft/acre) 0.0000003 cubic hectometer per square meter

gallons (gal) 3.785 liter

Flow rate

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

acre foot per day per mile [(acre-ft/d)/mi] 0.000766 cubic hectometer per second

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

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

cubic foot per day (ft3/d) 0.02832 cubic meter per day

cubic foot per second (ft3/s) 0.3048 cubic meter per second

gallon per minute (gal/min) 0.06309 liter per minute

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day
viii Conversion Factors, Vertical Datum, Abbreviations, and Well-Numbering System

Abbreviations

cm3/L cubic centimeters per liter 

hr/d hour per day

kg kilogram

L liter

mg/L milligrams per liter

mL milliter

pg/kg picogram per kilogram 

μg/L micrograms per liter

μS/cm microsiemens per centimeter

AL action level 

bls below land surface

DWR Department of Water Resources

GIS geographic information system

GPS global positioning system

GULP Groundwater Under Local Protection 

MCL maximum contaminant level

NWIS National Water Information System

pptv part per trillion by volume

STP standard temperature and pressure

USGS U.S. Geological Survey

V-SMOW Vienna Standard Mean Ocean Water

Ar argon

CaCO3 calcium carbonate

CFC chloroflurocarbons

CH4 methane

CO2 carbon dioxide

H2 hydrogen gas

1H hydrogen isotope

2H deuterium

3H tritium

He helium

3He helium-3

4He helium-4

H2S hydrogen sulfide

N2 nitrogen

Ne neon

O2 oxygen

16O oxygen-16

18O oxygen-18

TU tritium units

δ delta notation

‰ parts per thousand
Well-Numbering System ix

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 boustrophedonic manner to "R" in 

the southeast corner. Within the 40-acre tract, wells are numbered sequentially 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; Humbolt 

(H), Mount Diablo (M), and San Bernardino (S). All wells in the study area are referred to the Mount Diablo base 

line and meridian (M). Well numbers consist of 15 characters and follow the format 005N004W14J003M. In this 

report, well numbers are abbreviated and written 5N/4W-14J3. The following diagram shows how the number for 

well 5N/4W-14J3 is derived.
x 
Abstract 1

Ground-Water Resources in the Lower Milliken–Sarco–

Tulucay Creeks Area, Southeastern Napa County, 

California, 2000–2002

By Christopher D. Farrar and Loren F. Metzger

ABSTRACT

Ground water obtained from individual 

private wells is the sole source of water for about 

4,800 residents living in the lower Milliken–

Sarco–Tulucay Creeks area of southeastern Napa 

County. Increases in population and in irrigated 

vineyards during the past few decades have 

increased water demand. Estimated ground-water 

pumpage in 2000 was 5,350 acre-feet per year, an 

increase of about 80 percent since 1975. Water for 

agricultural irrigation is the dominant use, 

accounting for about 45 percent of the total. This 

increase in ground-water extraction has resulted in 

the general decline of ground-water levels. The 

purpose of this report is to present selected 

hydrologic data collected from 1975 to 2002 and 

to quantify changes in the ground-water system 

during the past 25 years. 

The study area lies in one of several 

prominent northwest-trending structural valleys in 

the North Coast Ranges. The area is underlain by 

alluvial deposits and volcanic rocks that exceed 

1,000 feet in thickness in some places. Alluvial 

deposits and tuff beds in the volcanic sequence are 

the principal source of water to wells.

The ground-water system is recharged by 

precipitation that infiltrates, in minor amounts, 

directly on the valley floor but mostly by 

infiltration in the Howell Mountains. Ground 

water moves laterally from the Howell Mountains 

into the study area. Although the area receives 

abundant winter precipitation in most years, nearly 

half of the precipitation is lost as surface runoff to 

the Napa River. Evapotranspiration also is high, 

accounting for nearly one-half of the total 

precipitation received. Because of the 

uncertainties in the estimates of precipitation, 

runoff, and evapotranspiration, a precise estimate 

of potential ground-water recharge cannot be 

made.

Large changes in ground-water levels 

occurred between 1975 and 2001. In much of the 

western part of the area, water levels increased; but 

in the central and eastern parts, water levels 

declined by 25 to 125 feet. Ground-water 

extraction produced three large pumping 

depressions in the northern and east-central parts 

of the area. The general decline in ground-water 

levels is a result of increases in ground-water 

pumpage and possibly changes in infiltration 

capacity caused by changes in land use. 

Ground-water-level declines during 

1960–2002 are evident in the records for 9 of 10 

key monitoring wells. In five of these wells, water 

levels dropped by greater than 20 feet since the 

1980s. The largest water-level declines have 

occurred since the mid 1970s, corresponding with 

a period of accelerated well construction and 

ground-water extraction.

Analysis of samples from 15 wells indicates 

that the chemical quality of ground water in the 

study generally is acceptable. However, arsenic 

concentrations in samples from five wells exceed 

the U.S. Environmental Protection Agency 

primary drinking-water standard of 10 micrograms 

per liter, and iron concentrations in samples from 

five wells exceed the U.S. Environmental 

Protection Agency and the California Department 

of Health Services secondary drinking-water 
2 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

standard of 300 micrograms per liter. Water from 

12 of 15 wells sampled contained concentrations 

of manganese that exceed the U.S. Environmental 

Protection Agency and the California Department 

of Health Services secondary drinking-water 

standard of 50 micrograms per liter. Two wells 

produced water that had boron in excess of the 

California Department of Health Services action 

level of 1 milligram per liter.

Stable isotope, chlorofluorocarbon, and 

tritium data indicate that ground water in the area 

is a mixture of waters that recharged the aquifer 

system at different times. The presence of 

chlorofluorocarbons and tritium in water from the 

study area is evidence that modern recharge (post 

1950) does take place. Water-temperature logs 

indicate that ground-water temperatures 

throughout the study area exceed 30°C at depths in 

excess of 600 feet. Further, water at depths greater 

than 600 feet in parts of the study area may contain 

objectionable concentrations of some constituents 

that may limit the use of the ground water.

INTRODUCTION

The Milliken–Sarco–Tulucay Creeks area, in 

southeastern Napa County, California, lies adjacent to 

the city of Napa and extends eastward into the Howell 

Mountains. This part of Napa County is approximately 

40 mi northeast of San Francisco. The Milliken, Sarco, 

and Tulucay Creeks are the main streams that drain the 

Howell Mountains between 38° 25 ́ and 38° 17 ́ north 

latitude; the creeks have a combined drainage area of 

42 mi2. Land-surface altitudes range from about 10 ft 

above sea level at the Napa River, in the southwest part 

of the area, to 1,877 ft above sea level on the summit of 

Mt. George in the Howell Mountains. The lower parts 

of the three drainage basins, which cover about 15 mi2

of rolling hills that extend westward from the mountain 

front to the Napa River, were the focus of this study 

(fig. 1). The study area has been extensively developed 

into agricultural land and rural home sites. Most home 

sites are on parcels larger than 1 acre.

Ground water is the only source of water in much 

of the lower Milliken–Sarco–Tulucay Creeks area. The 

city of Napa supplies part of this area with surface 

water delivered through a pipeline distribution system. 

For most of the area, however, each developed land 

parcel has an individual water system supplied by one 

or more wells. Single-family dwelling units, irrigated 

agriculture, and golf courses are the main users of the 

local ground water. An increase in ground-water 

extraction since the 1950s has resulted in the general 

decline of ground-water levels throughout the area. 

Declining ground-water levels are evidence of a 

ground-water system under stress. 

Purpose and Scope 

The U.S. Geological Survey (USGS) in 

cooperation with the Napa County Department of 

Public Works undertook this study to evaluate possible 

strategies for reducing water-level declines in the 

Milliken–Sarco–Tulucay Creeks area. A previous 

hydrologic assessment of the area was completed more 

than 25 years ago (Johnson, 1977). Many wells have 

been completed and significant hydrologic data have 

been collected since Johnson's study. These more 

recent data were used in this study to increase the level 

of knowledge of the local geohydrology of the area and 

to describe changes in land use, water use, and groundwater levels since 1975.

The purpose of this report is to present selected 

hydrologic data collected from 1975 to 2002 and to 

quantify changes in the ground-water system that 

occurred during the past 25 years. This information is 

essential for the future management of the groundwater system, which is the primary source of water 

supply for a rapidly developing area of the county. 

Specific objectives of the study were to refine the 

conceptual model of the geohydrologic framework, 

quantify changes in water use, describe present day 

ground-water conditions, identify the locations with the 

largest changes in ground-water storage, provide a 

more complete description of ground-water quality, 

and update the hydrologic budget of the lower 

Milliken–Sarco–Tulucay Creeks area. The emphasis of 

this study was on documenting changes in groundwater levels within the study area and identifying the 

principal causes of the ground-water level declines. An 

additional objective was to determine the source and 

movement of ground water in the different drainage 

basins.
Introduction 3

Mt. George x

GREEN

VALLEY

FAULT

ZONE

SODA CREEK FAULT

38

25'

00"

38

20'

00"

T7N

T6N

T6N

T5N

122 22' 30" R5W R4W 122 17' 30" R4W R3W 122 12' 30" R3W 122 07' 30

0

0

1

1

2 Miles

2 Kilometers

Napa Valley

H O WE L L

Napa M O UN

T

AINS

Base prepared by U.S. Geological Survey,

from digital elevation data, 2000

Cup and

Saucer area

Napa

River

Creek

Creek

Milliken

Creek

Sarco

Reservoir

Milliken

Tulucay

Storage

Unit 2

Storage

Unit 1

Storage

Unit 3

Storage

Unit 4

?

?

Tulucay Creek drainage basin

Milliken Creek

drainage basin

Sarco Creek

drainage basin

C a lifo rnia

 11458000

 11458000

Elev. 1,877 feet

San Francisco

Los

Angeles

Map

area

o

o o o o

o

?

EXPLANATION

Ground-water storage unit boundary (Johnson, 1977)

Drainage basin boundary

Study area boundary

Fault—Dashed where approximately located;

dotted where concealed (Johnson, 1977);

queried where uncertain

Napa city limit boundary

Stream-gaging station and identifier

Figure 1. Location of study area, southeastern Napa County, California.
4 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

To meet the objectives of this study, four 

principal tasks were identified: (1) evaluation of 

existing geohydrologic and geochemical data, (2) 

collection and analysis of new geohydrologic data 

including subsurface lithologic data, ground-water 

levels, and streamflow gains and losses, (3) collection 

and analysis of new water chemistry and isotopic data, 

and (4) updating estimates of water use and changes in 

ground-water storage and movement.

New hydrologic data presented in this report 

were collected between April 2000 and November 

2002. These data include ground-water levels, surfacewater discharge measurements, water chemistry 

including isotopic composition, and temperature logs 

in wells.

Description of Study Area

The study area, which is a topographic 

depression underlain by lava flows, tuff beds, and 

volcanic debris, is enclosed on the north, east, and 

south by the Howell Mountains and is bounded on the 

west by the Napa River (fig. 1). Johnson (1977) divided 

most of the study area into four storage units on the 

basis of geologic structure and surface-water drainage 

basins. However, parts of the study area near the 

western boundary are not included in these storage 

units. Storage unit 1 consists of 3,873 acres in the 

southern part of the study area and includes most of the 

lower reach of Tulucay Creek and several tributary 

streams. Storage unit 2 consists of about 1,638 acres of 

hilly land along the western side of the central part of 

the study area and is drained only by a few minor 

unnamed ephemeral streams. This area is known as 

The Cup and Saucer area in reference to the somewhat 

cup-shaped topography. Storage unit 3 consists of 

about 3,584 acres in the northern and eastern parts of 

the study area and is drained by Milliken and Sarco 

Creeks. Storage unit 4 comprises about 815 acres in a 

narrow area west of the Soda Creek Fault. 

Ground-water level and geochemical data 

collected during this study do not support the notion 

that these storage units are hydraulically separate. The 

storage unit designations, nevertheless, are useful 

because they are used by local governmental agencies 

and residents as names for partitions of the study area. 

For this reason the storage units are referred to in the 

text and the boundaries are shown in several figures in 

this report.

The study area consists of predominantly 

unincorporated land within the county of Napa, but it 

also includes a part of the city of Napa on the western 

side of the area. Land use is a combination of urban, 

agriculture, and unimproved open space (fig. 2). Landuse mapping by the California Department of 

Conservation in 2000 (Sherron Muma, California 

Department of Conservation, written commun., 2002) 

showed that unimproved open space with mixed 

residential use predominates, amounting to about 40 

percent of the total area, followed by urban use of 31 

percent, and irrigated agriculture mixed with 

residential use of 21 percent (table 1, at back of report). 

Major changes in land use have taken place in the study 

area over the past 40 years. On the basis of land-use 

analysis completed as a part of this study using the 

earliest available aerial photographs, land use for 

unimproved open space with mixed residential use 

decreased by 35 percent, urban use increased by 58 

percent, and irrigated agriculture with residential use 

increased by 304 percent since 1958. The dramatic 

increase in irrigated agriculture is attributable to the 

emergence of drip-irrigated vineyards for the 

production of wine grapes. Other agricultural uses, 

such as orchards, row crops, and non-irrigated grass 

and pasture lands, have been almost completely 

replaced by vineyards in the study area.

Census data for the year 2000 indicate that about 

16,500 people live in the study area (Association of 

Bay Area Governments, 2002), an increase of about 21 

percent since Johnson's study in the early 1970s . The 

population in the unincorporated county part of the 

study area is about 8,000. However, a direct 

comparison of the present day population in the county 

area with Johnson's data cannot be made because some 

previously unincorporated areas are now part of the 

city.

The climate of the study area is mediterranean, 

with distinct wet and dry seasons. About 90 percent of 

the area's yearly precipitation occurs from November 

through April. Mean annual precipitation at Napa State 

Hospital averaged about 24.5 inches from water years 

1918 through 2002 (fig. 3A) (National Oceanic and 

Atmospheric Administration, 2002). Figure 3A shows 

that annual precipitation in any given year can deviate 

as much as 200 percent from the 85-year average. 

During the study period, water years 2000 through 

2002, rainfall at Napa State Hospital ranged from about 

6 inches below average in 2001 to about 1.5 inches 

above average in 2000 and 2002 (fig. 3B).
Introduction 5

Figure 2. Land use in 2000 in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.

(Modified from Important Farmland Data, Napa County, California, 2000, 1:24,000 enlargement, Sherron Muma, California Department of Conservation, 

Farmland Mapping and Monitoring Program, written commun., 2000)
6 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

The areal distribution of mean annual 

precipitation for 1961–90 in the study area is shown in 

figure 4. Isohyetal contours show that precipitation 

generally increases from south to north and with 

increasing altitude. Average annual precipitation is as 

much as 40 inches in the highest altitudes of the Howell 

Mountains. This is about 65 percent more precipitation 

than the amount received at the lower altitudes (Daly 

and Taylor, 1998).

The average total amount of precipitation 

received in the Milliken, Sarco, and Tulucay Creeks 

drainage basins is about 69,000 acre-ft/yr based on the 

isohyetal map (fig. 4). Of this amount, about 29,000 

acre-ft/yr leaves the watershed as runoff in local 

streams to the Napa River. This estimate is based on 

streamflow records for stations on the Napa River and 

Tulucay Creek and is consistent with estimated unitrunoff for this area given in Rantz (1968). Johnson 

(1977) estimated that evapotranspiration in the basins 

consumes about 30,500 acre-ft/yr. An estimate of about 

34,000 acre-ft/yr is obtained when Johnson's estimate 

is adjusted for the slightly larger area mapped for this 

study. Using these estimates, it is clear that most of the 

water entering the basins leaves as runoff or 

evapotranspiration. Potential ground-water recharge 

can be calculated as the residual of total precipitation 

minus runoff and evapotranspiration, assuming no 

other inflows or outflows. Using this method, a residual 

of 6,000 acre-ft/yr is calculated based on the estimates 

made in this study. However, because of the uncertainty 

in the estimates of precipitation, runoff, and 

evapotranspiration, this value is not a precise estimate 

of potential ground-water recharge and should not be 

construed as the safe yield for the study area.

Residential water supplies in the study area are a 

combination of municipal and private sources. The City 

of Napa Municipal Water Department supplies almost 

79,000 people within the city limits of Napa and in 

several unincorporated parts of Napa County (Don 

Ridenhour, City of Napa Municipal Water Department, 

written commun., 2002). The water supplied by the city 

of Napa primarily is obtained from Lake Hennessey 

and the Milliken Reservoir (fig. 1) and imported 

surface water obtained from the North Bay Aqueduct 

through a contract with the State Water Project. County 

areas served by public water supply include residences 

in an area bounded by Monticello Road, Sarco Creek, 

and Vichy Avenue, and the Silverado Country Club 

east of Atlas Peak Road and north of Monticello Road 

(fig. 5). An estimated 3,200 people are served by about 

1,000 metered public-supply connections. The 

remaining estimated 4,800 residents of the county part 

of the study area rely on private water systems. Ground 

water is the predominant source of water for both 

domestic and irrigation use. 

Previous Investigations

Weaver (1949) carried out one of the earliest 

published geologic investigations that included the 

study area. His work defined the basic geology of the 

area in terms of stratigraphy and structure. Studies by 

Fox and others (1973) and Sims and others (1973) 

provide the most detailed geologic maps for the study 

area to date.

Hydrologic data have been collected in the study 

area since the late 1940s but some earlier sources of 

information also are available. Water levels have been 

measured in the study area since about the late 1910s as 

part of local or special studies and through cooperative 

efforts by various government entities [California 

Department of Water Resources (DWR), 1995]. 

Intermittent water-level measurements through the 

1940s were made primarily by the USGS. Regular 

long-term regional water-level monitoring by the 

USGS, DWR, and the Napa County Agricultural 

Advisor began in the late 1950s. Since 1973, Napa 

County in cooperation with DWR has collected waterlevel data semiannually for a network of about 10 

wells. These data were reviewed for this study and 

provided a basis for documenting long-term trends in 

ground-water levels throughout the study area. The 

study area also was included in a comprehensive 

hydrogeologic investigation of Napa and Sonoma 

Counties by the U.S. Geological Survey (Kunkel and 

Upson, 1960). Their data and interpretation provided 

the foundation for later hydrologic studies. In the mid1970s, the USGS carried out a more detailed study 

focused solely on the Milliken–Sarco–Tulucay Creek 

area (Johnson, 1977). 

The Regional Water Quality Control Board has 

developed a geographic information system (GIS) for 

Napa Valley (Jeff Kapellas, California Regional Water 

Quality Control Board, written commun., 2000) that 

includes surficial geology, soils, ground-water basins, 

well yield, depth to ground water, recharge areas, well 

data, Napa County septic systems, hazardous material 

storage sites, landfills, and land use.
Introduction 7

0

10

20

30

40

50

60

1918192119241927193019331936193919421945

1948195119541957196019631966196919721975197819811984198719901993199619992002

WATER YEAR

PRECIPITATION, IN INCHES

Mean annual precipitation 24.47 inches

0

2

6

4

10

12

16

20

24

28

8

14

18

22

26

30

DATE

CUMULATIVE PRECIPITATION, IN INCHES

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

WY 2001

WY 2002

WY 2000

A

B

Figure 3. (A) Annual precipitation for water years 1918–2002, and (B) distribution of cumulative precipitation for water years (WY) 2000–2002 at Napa 

State Hospital, southeastern Napa County, California.

(See figure 4 for areal distribution.)
8 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 4. Mean annual precipitation in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, 1961-90.

(Modified from Daly and Taylor, 1998)
Introduction 9

Figure 5. City of Napa Municipal Water Department distribution system, in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, 

California.
10 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Acknowledgments

The authors acknowledge the assistance of the 

staffs of the Napa County Departments of Public 

Works and Environmental Management, Napa County 

GIS Services, the City of Napa Municipal Water 

Department, the Central District of the Department of 

Water Resources, the California Department of 

Conservation, Farmland Mapping and Monitoring 

Program, and the resident volunteers of the 

Groundwater Under Local Protection (GULP) 

committee. Special thanks are extended to the more 

than 100 private property owners for allowing access to 

wells.

GEOLOGY

The study area lies in the southeastern part of 

Napa Valley, one of several prominent northwesttrending structural valleys in the North Coast Ranges. 

Napa Valley was formed between 1 and 2 million years 

ago by faulting and downwarping in the crust, 

processes that are related to plate tectonics and the 

transformation of a subduction zone into the strike-slip 

movement on the San Andreas and related faults 

(Howell and Swinchatt, 2000). Most of the study area 

is bounded on the west by the Soda Creek Fault and on 

the east by the steep west-facing slope of the Howell 

Mountains. The Green Valley Fault strikes northnorthwest through the Howell Mountains east of the 

study area (fig. 1). 

Stratigraphy

Geologic formations exposed at the surface 

include the Sonoma Volcanics and younger, thin, 

unconsolidated Quaternary alluvial deposits (fig. 6). 

Drillers' logs of a few wells drilled greater than 1,000 ft 

deep indicate that the thickness of the Sonoma 

Volcanics exceeds 1,000 ft in some parts of the study 

area. The Sonoma Volcanics unconformably overlie 

much older rocks—either of the Lower Cretaceous age 

Great Valley sequence, which mostly consist of highly 

indurated marine siltstones, sandstones, and 

conglomerates, or possibly of the Jurassic-Cretaceous 

age Franciscan Complex (Wagner and Bortugno, 

1982). Rocks in the Great Valley sequence and 

Franciscan Complex are highly lithified and generally 

of low permeability. In other areas of northern 

California, these geologic assemblages generally 

provide only small quantities of water to wells. The 

Great Valley sequence or Franciscan Complex form the 

bottom boundary of the ground-water basin in the 

study area because the rocks in these assemblages are 

relatively impermeable compared with the rocks of the 

Sonoma Volcanics and sediments in the Quaternary 

alluvial deposits. 

Figure 6. Explanation.
Geology 11

Figure 6. Geology of, and cross section locations in, the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.

(Modified from Johnson, 1977)
12 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Vertical exaggeration X2

600

400

200

200

Sea

level

1,000

1,000

2,000

Sea

level

Feet

Feet

Feet

METERS

A A' Napa River Milliken Creek Soda Creek Fault D–D'

Tsr

Tss

Tss

Tst

Tst

Tst

Qoal

Qyal Qoal

Tssd

0

0

2 Miles

2 Kilometers

Tsa

Tsa

Tsa

Vertical exaggeration X2

D–D'

D–D'

600

400

400

200

200

Sea

level

2,000

1,000

1,000

Sea

level

METERS

B B'

Tst

Tst

Tst

Qoal

Qoal

Tssd Qyal

Qyal

?

? ? Flows

Not mapped

Not mapped

Qoal Qf Qf Qyal Tsr

Cup and Saucer area

Napa River

Napa River

Soda Creek Fault Soda Creek Fault

Tsr

Tsa

Tsa Tsa

land surface

land surface

land surface

Datum is sea level

Vertical exaggeration X2

Datum is sea level

Datum is sea level

WEST

WEST EAST

EAST

EAST

600

600

400

400

200

200

Sea

level

2,000

2,000

1,000

1,000

Sea

level

METERS

Qf

Tst

Tsa

Tsa

Tsa

Qoal

Qoal Tssd

Not mapped

WEST

C C'

0

0

2 Miles

2 Kilometers

0

0

2 Miles

2 Kilometers

Tst

Tst

Tsa

Figure 7. Generalized geologic cross sections in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.

(Modified from Johnson, 1977)
Geology 13

Figure 7.—Continued.
14 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Sonoma Volcanics

The Sonoma Volcanics range in age from about 6 

to 3 million years old (A. Sarna-Wojcick, U.S. 

Geological Survey, oral commun., 2002) and are 

distributed over hundreds of square miles in Napa and 

Sonoma Counties. Within the study area, they are 

exposed in road cuts along Monticello Road through 

the Howell Mountain block, where a stratigraphic 

section 1,290 ft thick can be observed. First described 

by Weaver (1949), the Sonoma Volcanics consist of a 

complex variety of lithologies and compositions 

including basalt, andesite, and rhyolite lavas; tuffs; 

debris flows; diatomaceous lacustrine sediments; and 

sedimentary volcanic rocks. A detailed description of 

the Sonoma Volcanics is given in Kunkel and Upson 

(1960). For regional geologic mapping, the Sonoma 

Volcanics have been subdivided into five members 

(Sims and others, 1973; Fox and others, 1973). Johnson 

(1977) followed this stratigraphic nomenclature and 

described the Sonoma Volcanics as consisting of three 

volcanic members: the lower andesitic member (Tsa), 

the middle tuffaceous member (Tst), and the upper 

rhyolitic member (Tsr), separated by two subaqueous 

deposits: diatomaceous deposits (Tssd) and 

sedimentary deposits (Tss), interbedded between the 

volcanic units (fig. 7). This "layer-cake" model of the 

five members is a useful simplification for some 

purposes, but it ignores the true complexity in the 

distribution of the various lithologies found in the 

Sonoma Volcanics. Many lithologic units in the 

Sonoma Volcanics lack wide areal continuity and some 

units have a lenticular geometry or have interfingering 

contacts with adjacent lithologic units.

Erosion during the past 3 million years has 

modified the landscape and obscured the locations of 

volcanic centers and the relations between the various 

lithologic units. On the basis of studies of active 

volcanic systems elsewhere and the sequence of rocks 

derived from them, it is clear that the distribution of the 

different rock types in the study area is equally 

complex. It is likely that several volcanic vents were 

sources for the Sonoma Volcanics in the study area. 

Volcanic materials ejected from any one vent changed 

as the eruptions progressed from vent clearing to tephra 

(pumice and ash) eruptions to lava flows. Such 

sequences of volcanic activity from individual vents 

probably occurred repeatedly over periods of thousands 

of years. The volcanic activity produced a variety of 

rock types and chemical compositions, emplaced as 

lava flows, dikes, plugs, breccias, pumice beds, and 

avalanche deposits. The volcanic vents in proximity to 

each other may have been in different phases of 

eruptive evolution so that lava was being extruded in 

one location while tephra was being explosively ejected 

from a nearby vent. This type of activity results in a 

very heterogeneous sequence of rock types and 

compositions with depth and over distances of miles or 

less. 

Quaternary Alluvial Deposits

Thin unconsolidated, uncemented alluvial 

deposits blanket about 6 mi2 of the study area and 

directly overlie the Sonoma Volcanics. During previous 

studies (Kunkel and Upson, 1960; Sims and others, 

1973; Johnson, 1977), these alluvial deposits were 

subdivided into alluvial fan deposits, older alluvium, 

and younger alluvium on the basis of age and 

depositional environment. The alluvial fan deposits 

crop out mostly at the foot of the Howell Mountain 

block in the eastern part of the study area (fig. 6). The 

alluvial fan deposits are wedge-shaped bodies that are 

thickest near the mountain front; they are formed by 

colluvium, landslide debris, and alluvial material 

carried by ephemeral streams that discharged from 

steep canyons eroded into the mountain front. The 

deposits are composed of boulders, cobbles, gravel, 

sand, and finer-grained material. Generally the 

coarsest, more angular, and poorly sorted material 

occurs near the mountain front and the finer, more 

rounded, and better-sorted material occurs toward the 

central part of the basin.

The largest outcrops of older alluvium are in the 

southwestern and northwestern parts of the study area. 

The younger alluvium primarily crops out along 

present-day stream channels and associated flood 

plains. Both the older and younger alluvium consist of 

boulders, cobbles, gravel, sand, and fine-grained 

material that were deposited by streams over the past 

few thousand years. These deposits are better sorted 

and include more rounded clasts than the fan deposits 

and thus potentially are more permeable. 
Geology 15

Figure 8. Generalized thickness of unconsolidated deposits in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.
16 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 9. Conceptual model of the ground-water flow system in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.
Ground-Water Hydrology 17

GROUND-WATER HYDROLOGY

The principal water-bearing units in the study 

area are alluvial deposits west of the Soda Creek Fault 

and the tuffaceous member of the Sonoma Volcanics 

east of the fault. Alluvial deposits overlie the Sonoma 

Volcanics throughout much of the study area. The 

alluvial deposits are not highly productive aquifers in 

parts of the study area to the east of the Soda Creek 

Fault (storage units 1, 2, and 3) because they are fairly 

thin and generally are above the water table. West of 

the Soda Creek Fault (storage unit 4), the alluvial 

deposits are an important source of water because the 

deposits are considerably thicker and largely saturated. 

Throughout much of Napa Valley west of the study 

area, alluvial deposits constitute the major aquifer 

(Faye, 1973). Several tens to a few hundred gallons per 

minute of water can be pumped from wells completed 

in clean sand and gravel lenses in the younger alluvial 

deposits (Kunkel and Upson, 1960). Older alluvial 

deposits contain large fractions of clay and silt or are 

poorly sorted coarse- and fine-grained materials and in 

some beds are highly compacted or slightly cemented. 

The presence of large percentages of fine-grained 

materials, poor sorting, compaction, or cementation 

greatly reduces the permeability in many areas 

underlain by older alluvial deposits. Wells tapping such 

deposits generally produce less than 50 gal/min.

Alluvial fan deposits crop out over an extensive 

area at the base of the Howell Mountains. Although 

these deposits contain large percentages of boulders, 

cobbles, and gravel, the sorting generally is poor and 

some beds or lenses are cemented, greatly limiting the 

permeability. The topographic occurrence of the fan 

deposits places them above the zone of saturation in 

most of the area; therefore, these deposits are not an 

important source of water to wells. 

East of the Soda Creek Fault, ground water is 

pumped almost exclusively from the Sonoma 

Volcanics. The andesitic member is the basal member 

of the Sonoma Volcanics and underlies the entire study 

area (fig. 7). This member consists of andesitic and 

basaltic lavas that have little primary permeability 

except in the interflow zones. Fracture zones produced 

by faulting and folding provide some secondary 

permeability to this member and yield small amounts 

of water to wells. In storage unit 2, the andesitic 

member lies within 100 ft of the surface and is 

essentially the only source of water to wells.

The tuffaceous member overlies the lower 

andesitic member throughout most of the area and is 

the principal water-bearing unit within the study area. 

This unit is about 500 ft thick in storage units 1, 3, and 

4. Thick sections of uncemented and non-welded tuff 

or pumice beds can provide moderate to large 

quantities of water to wells. Well yields of 500 gal/min 

or more have been reported for wells open to the 

non-welded tuff. 

The upper rhyolite member overlies the 

tuffaceous member in the eastern parts of the study area 

and consists of low-permeability, banded rhyolitic lava 

with intercalated rhyolitic tuff. Many of the tuff beds in 

the rhyolite member are slightly to densely welded, 

which reduces intergranular permeability. In some 

areas, lava beds or welded tuffs in the rhyolite member 

confine ground water in the underlying tuffaceous 

member. 

The low permeability of the diatomaceous and 

sedimentary members of the Sonoma Volcanics 

restricts the downward movement of recharge water 

throughout much of the study area. The diatomaceous 

deposits are mostly in the eastern part of storage units 1 

and 3 and consist of diatomaceous clay and silt 

deposited in a lacustrine or paludal environment 

(Kunkel and Upson, 1960). The sedimentary deposits 

mostly are in storage unit 3 where they reach a 

maximum thickness of 250 ft. The diatomaceous and 

sedimentary members have low permeability and 

confine ground water in underlying geologic units. 

Within the study area, the largest amount of 

ground water per unit volume of material is contained 

in the uncemented, poorly consolidated, coarse clastic 

materials (either volcanic or sedimentary in origin), 

referred to in this report as unconsolidated material. 

The greatest potential for ground-water production is in 

the coarse clastic materials (sand, gravel, cobbles, 

pumice, and volcanic tephra). The least amount of 

ground water is contained in the well-lithified volcanic 

rocks (andesite, basalt, rhyolite, and welded tuff). 
18 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 10. Locations of streamflow-measurement stations and streambed infiltration zones in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California.
Ground-Water Hydrology 19

The thickness of the unconsolidated deposits and 

the percentage of coarse clastic materials within these 

deposits were determined by inspection of drillers' 

logs. Drillers' logs for the deepest well in each quarter 

section of land (160 acres) were examined: this 

included 69 wells that range in depth from 150 to 1,554 

ft. The term rock generally is used in the drillers' logs to 

describe well-lithified volcanic rocks. For this report, 

the thickness of the unconsolidated deposits included 

all material above the depth where materials were 

described predominantly as rock.

The thicknesses of the unconsolidated deposits 

within the study area are shown by 100-foot depth 

intervals in figure 8. Because of the complexity in the 

distribution of the lithologies in the study area, this 

generalized summary cannot be used to accurately 

determine the thickness of unconsolidated deposits at 

any particular well site. In general, the thickness of 

unconsolidated deposits is less than 100 ft at the 

northern, eastern, and southern perimeters of the study 

area and is as much as 500 ft or greater along the 

western perimeter. The thickness of the unconsolidated 

deposits generally is less than 200 ft in storage units 1 

and 2 and greater than 200 ft in storage unit 3. The 

thickness of the deposits is greater than 300 ft over 

about one-third of the area of storage unit 3. In storage 

unit 4, the thicknesses of the unconsolidated deposits 

range from less than 100 to more than 500 ft. The 

greater thicknesses of the unconsolidated deposits in 

storage unit 4 probably are related to the downward 

displacement of Sonoma Volcanics along the west side 

of the Soda Creek Fault and the contemporaneous 

deposition of sediments.

The thickness of unconsolidated deposits (fig. 8) 

is consistent with the results of a regional gravity 

survey (Youngs and others, 1989), which shows that 

the lowest gravity values occur along the western edge 

of the study area, corresponding with the areas 

underlain by thick sections of unconsolidated deposits. 

A gravity high occurs in a large part of the southern 

half of the study area where the unconsolidated 

deposits generally are less than 100 ft thick.

Drillers' logs also were examined to determine 

the percentage of uncemented coarse-grained materials 

contained in the upper 300 ft of unconsolidated 

deposits. Uncemented coarse-grained materials include 

boulders, cobbles, gravel, sand, pumice, or any 

combination of these not specifically described as 

cemented or imbedded in clay. The most productive 

wells generally are located in areas underlain by large 

amounts of uncemented coarse grained-material in the 

zone of saturation. Nearly one-third of the study area is 

underlain by deposits made up of greater than 10 

percent uncemented coarse-grained material. This 

includes a large part of storage unit 3 and the southern 

half of storage unit 4. More than half the study area is 

underlain by deposits having 5 percent or less 

uncemented coarse-grained material. This includes the 

eastern part of the study area overlying the syncline 

mapped by Fox and others (1973) and most of the 

southwest perimeter of the study area. Less than 20 

percent of storage unit 2 is underlain by deposits 

containing greater than 5 percent uncemented coarsegrained material. 

Recharge

The principal source of recharge to the groundwater system in the Milliken–Sarco–Tulucay Creeks 

drainage basin is precipitation within the basin; this 

recharge occurs as seepage from creeks, lakes, and 

man-made ponds, and areally as direct infiltration. 

Other significant sources of recharge are ground-water 

inflow from the Howell Mountains and, in the northern 

part of the area, ground-water inflow from the west. 

Minor sources of recharge include infiltration from 

septic tanks, leaking water-supply pipes, irrigation 

water in excess of crop requirements, and crop frostprotection applications. Although recharge from 

excess irrigation sometimes can be a significant part of 

total recharge within some basins, within this study 

area it is considered minor because the predominant 

crop is wine grapes and local growers use highly 

efficient drip systems. 

A schematic of the conceptual model of the 

ground-water system is shown in figure 9. The block 

diagram shows the geologic framework of the study 

area and the main components of the hydrologic 

budget, including direct recharge from precipitation, 

ground-water underflow into and out of the study area, 

streambed infiltration, surface-water outflow, and 

evapotranspiration. 
20 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 11. Streamflow measurements for the three seepage runs for selected parts of the (A) Milliken, (B) Sarco, and (C) Tulucay Creeks and their tributaries, 

southeastern Napa County, California.
Ground-Water Hydrology 21

Johnson (1977) estimated that the average annual 

recharge in the area of this study in 1975 was 5,400 

acre-ft/yr: 3,050 acre-ft/yr from streamflow infiltration; 

2,100 acre-ft/yr from subsurface inflow from the 

Howell Mountain block; and about 250 acre-ft/yr from 

direct infiltration of precipitation. According to 

Johnson (1977), most of the streamflow infiltration was 

restricted to 22 areas, defined as streambed-infiltration 

zones, along the eastern margin of the study area. 

Johnson's conclusions were based on measured 

streamflow losses in Milliken Creek, Sarco Creek, and 

a few unnamed streams along reaches that cross 

tuffaceous materials near the base of the Howell 

Mountain block. Streamflow measurements were 

collected along Milliken, Sarco and Tulucay Creeks 

and their tributaries as part of this study to help identify 

streambed infiltration.

Streamflow Gains and Losses

Streamflow gains and losses along Milliken, 

Sarco, and Tulucay Creeks and their tributaries were 

determined from streamflow measurements made 

during three seepage runs between April 2000 and 

April 2001. A seepage run consists of a series of 

streamflow measurements made at several sites along a 

stream to quantify streamflow gains and losses (Riggs, 

1972). A gaining reach is defined as one in which 

streamflow increases in the downstream direction due 

to ground-water inflow, tributary inflow, or 

precipitation (Blodgett and others, 1992). If groundwater inflow is the only source of streamflow gain, it 

may be referred to as a seepage gain. In contrast, a 

losing reach is defined as one in which streamflow 

decreases by infiltration to the subsurface or by 

evapotranspiration. A seepage loss is a decrease in 

streamflow attributable to infiltration only.

Methods of Data Collection and Analysis

Streamflow was measured or observed at 16 

stations in the Milliken Creek drainage basin, at 23 

stations in the Sarco Creek drainage basin, and at 33 

stations in the Tulucay Creek drainage basin (fig. 10). 

These stations were assigned numbers sequentially at 

the time of the initial measurement. 

No-flow conditions were observed at some of the 

stations for all three seepage runs during 2000 and 

2001 (April 17–26, 2000; March 13–16, 2001; and 

April 10–12, 2001). Flow was estimated when it was 

too low for direct measurement. The seepage runs were 

scheduled to avoid peak-flow conditions and periods of 

significant changes in stage, such as receding storm 

flows.

Most of the streamflow measurements were 

made using velocity-area methods [for a description of 

these methods see Rantz and others (1982)]. During 

low-flow conditions at stations where velocities were 

less than 0.2 ft/s and stream depths were less than 0.3 

ft, streamflow was measured using a modified 3-inch 

Parshall flume (Rantz and others, 1982). At stations 

where flows were too low and channel configurations 

unsuitable for conventional measurement techniques, 

miscellaneous methods including floats, volumetric, or 

visual estimates were used to estimate streamflow 

(Rantz and others, 1982).

The accuracy of streamflow measurements is 

largely dependent on flow conditions and measurement 

technique (Rantz and others, 1982). For this study, the 

accuracy of streamflow measurements was determined 

using a computer program (Sauer and Meyer, 1992) 

that determines the uncertainty or error of individual 

streamflow measurements. This program assigns a 

corresponding qualitative rating (excellent, good, fair, 

or poor) for each streamflow measurement. The 

streamflow-measurement rating ranged from good (2 to 

less than 5 percent error) to poor (greater than 8 percent 

error) for the individual pygmy meter measurements. 

In contrast, the measurements made using the modified 

3-inch Parshall flume generally were rated good 

(within 2 to 3 percent error). The miscellaneous 

discharge measurement methods (floats, volumetric, 

and visual estimates) generally had an accuracy of plus 

or minus 10 percent or more and were assigned a 

measurement rating of poor.

Streamflow Measurements and Estimated Gains and Losses

Streamflow gains and losses were calculated for 

each reach using streamflow measurements from 

successive stations. Duplicate measurements made 

during the same seepage run for individual sites were 

averaged prior to calculating gains and losses. 

Streamflow measurements, gains or losses between 

stations, flow distances, and rates of gain or loss of 

flow are shown in tables 2, 3, and 4 (at back of report) 

for the three seepage runs. The measurements indicate 

that few reaches have significant streamflow losses, 

and those that do are not consistently losing reaches. 

These data indicate that most winter runoff leaves the 

study area as streamflow to the Napa River and that 

only a small amount infiltrates beneath the streambeds. 
22 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 12. Geographic distribution of known active and potentially active wells per quarter-quarter section in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California.
Ground-Water Hydrology 23

Seepage measurements were attempted at the 22 

reaches, referred to as streambed infiltration zones by 

Johnson (1977); but because of limited access and poor 

channel conditions, measurements were made in only 

three of the zones: Atlas Peak Road tributary to 

Milliken Creek (station 30 to station 29), main stem of 

Milliken Creek (station 31 to station 34), and main 

stem of Sarco Creek (station 2 to station 1). On the 

Altas Peak Road tributary to Milliken Creek reach, 

minor losses of 0.1 acre-ft/d were calculated for April 

2000 and April 2001 (tables 2 and 4, respectively, at 

end of report), but a gain of about 0.3 acre-ft/d was 

calculated for March 2001 (table 3, at end of report). 

On the main stem of the Milliken Creek reach, 

streamflow losses of 4.4 and 0.1 acre-ft/d were 

calculated for the March and April 2001 seepage runs, 

respectively, but a streamflow gain of about 0.8 acreft/d was calculated for the April 2000 seepage run. On 

the main stem of the Sarco Creek reach, a streamflow 

loss of 0.1 acre-ft/d was calculated for the April 2001 

seepage run. The results for the three seepage runs do 

not show significant streamflow losses along the 

streambed-infiltration zones that were delineated by 

Johnson (1977). 

Measurements of streamflow as a function of 

stream distance for selected reaches of the main stem 

of Milliken Creek (fig. 11A), tributaries of Sarco Creek 

(fig. 11B), and tributaries of Tulucay Creek (fig. 11C) 

indicate a combination of gaining and losing reaches. 

Streamflow gains between some stations during the 

three seepage runs can be attributed to inflows from 

tributary streams, surface runoff, and water released 

from bank storage. Ground water also may be a source 

of streamflow gains in some of the lower reaches of 

0

10

20

30

40

50

60

191819201922192419261928193019321934193619381940194219441946194819501952195419561958

1960196219641966196819701972197419761978198019821984198619881990199219941996199820002002

-20

-15

-10

- 5

0

5

10

15

20

25

30

NUMBER OF NEW WELLS

WATER YEAR

DEPARTURE FROM MEAN ANNUAL PRECIPITATION,

IN INCHES

Figure 13. Historical well development (bars) and departure from mean annual precipitation (line) (at Napa State Hospital) in the lower Milliken– Sarco–Tulucay Creeks area, southeastern Napa County, California, water years 1918-2002. 

Well data incomplete for water year 2002.
24 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Milliken, Sarco, and Tulucay Creeks near the Napa 

River. For example, spring ground-water levels in well 

69 were about 8 ft below land surface (table 5, at end of 

report), indicating that the regional water table may 

coincide with the channel bottom along the lower part 

of Sarco Creek near station 11, particularly during the 

winter and early spring when water levels usually reach 

their maximum. 

Streamflow measurements from the three 

seepage runs indicate that significant infiltration occurs 

at various times in reaches other than the infiltration 

zones defined by Johnson (1977). On the main stem of 

Milliken Creek a loss of 16.5 acre-ft/d was calculated 

between stations 33 (Westgate Drive) and 38 (Atlas 

Peak Road) for the March 2001 seepage run (table 3, at 

end of report). This reach of Milliken Creek may be 

favorable for significant streambed infiltration because 

the underlying unconsolidated alluvial deposits are 

highly permeable (fig. 7, section A-A ́ ). Smaller losses 

were measured on parts of Sarco and Tulucay Creeks 

and their tributaries. For example, on the Hagen Road 

tributary to Sarco Creek, between the confluence of the 

Hagen Road and Third Avenue tributaries (26A/26B) 

and station 9 (Grange Hall), streamflow losses ranged 

from negligible during the April 2000 seepage run to 

1.1 acre-ft/d during the March 2001 seepage run. In the 

lower Tulucay Creek drainage basin, streamflow losses 

in the reach between station 16 (Mustang Road) and 

site 50 (Coombsville Road) averaged about 0.1 acreft/d for the three seepage runs. 

In summary, the seepage run data collected for 

this study indicate that the total streamflow loss along 

the 22 streambed infiltration zones along the eastern 

margin of the study may be less than the 3,050 acreft/yr estimated by Johnson (1977). However, the data 

for this current study also indicate that infiltration takes 

place in reaches downstream from Johnson's 

infiltration zones. Although additional seepage runs 

could more accurately quantify gains and losses, such 

an effort would be difficult because of poor channel 

conditions, multiple sources of surface inflow, and 

surface-water diversions. Other approaches to quantify 

gains and losses can be tried, such as dye-dilution, 

which is not adversely affected by boulder covered 

channels or shallow water depths (Kilpatrick and Cobb, 

1985). 

Discharge 

Ground-water discharge from the study area is 

predominantly pumpage from wells and underflow 

across the western boundary. A small amount of 

ground water discharges to streams. Johnson (1977) 

estimated that the total ground-water discharge in 1975 

was 5,650 acre-ft/yr: 3,000 acre-ft/yr of pumpage and 

2,650 acre-ft/yr of underflow from the study area 

toward the Napa River. For this study, ground-water 

pumpage and underflow across the western boundary 

were estimated for hydrologic conditions during the 

period 2000–2002. 

Discharge from Pumpage

A well inventory and (or) drill dates for January 

1950 or later indicate that approximately 800 wells 

were known or were assumed to be active in the study 

area. The number of wells per quarter-quarter section 

(40 acres) is shown in figure 12. The greatest number 

of wells is near Hagen Road, in the east-central part of 

the study area, and centered around Third Avenue 

between Coombsville Road and North Avenue in the 

southeastern part of the study area. 

The actual number of active wells in the study 

area is probably much larger than the approximately 

800 wells for which specific information is available. 

Records from the County Tax Assessor and the City of 

Napa Municipal Water Department identify about 

1,450 parcels as residential and about 132 parcels as 

either agricultural or agriculture mixed with residential 

use. Most of these parcels probably have individual 

water systems. Drillers' reports and parcel records 

indicate that there is an average of about 1.1 wells per 

residential parcel. The well inventory for this study 

confirmed that some residential properties have more 

than one well per parcel. Assuming an average of 1.1 

wells per parcel, there are an estimated 1,595 domestic 

wells on the 1,450 residential parcels (table 6). A 

similar examination of drillers' reports for irrigation 

wells on agricultural and mixed agricultural and 

residential properties indicates an average of about 1.4 

wells per agricultural or agricultural mixed with 

residential parcel. This translates to about 185 

irrigation wells in the study area.
Ground-Water Hydrology 25

Figure 14. Locations of wells in the water-level monitoring network in the lower Milliken–Sarco–Tulucay Creek area, southeastern Napa County, California.

Wells equipped with transducers for continuous monitoring identified by local well number (in red).
26 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Approximately 570 of the estimated 1,595 wells 

existing in 2002 were constructed between 1975 and 

2002, an increase of 56 percent from that estimated by 

Johnson (1977). Figure 13 shows the number of wells 

drilled annually between 1918 and 2002 compared to 

the departure from mean annual precipitation. The 

years with the greatest annual increases in the drilling 

of new wells generally coincide with periods when 

rainfall was at least 5 inches (20 percent) less than the 

annual mean. During the drought years of 1976–77 and 

1987–91, well drilling increased markedly with respect 

to non-drought years. However, fairly large numbers of 

new wells were drilled between 1993 and 2002, a 

period when rainfall was normal or above normal. This 

recent surge in new well construction may have been 

due to, at least in part, a ground-water protection 

ordinance introduced by Napa County in 1996 to 

regulate the conditions under which a new well can be 

constructed (Christine Secheli, Napa County 

Department of Environmental Management, oral 

commun., 2002). The ordinance was adopted in 1999. 

Drillers' reports and other well records suggest that an 

average of only three wells per year were drilled in the 

study area prior to 1975, whereas an estimated average 

of 22 wells per year were drilled between 1975 and 

2002. Although improved reporting over the years may 

account for part of the increased rate of well drilling, 

increases in both population and in irrigated agriculture 

probably account for most of the increase. 

The amount of ground water pumped from the 

study area only can be estimated because domestic 

wells in the area are not metered and electrical power 

consumption records for irrigation wells are not readily 

available. Ground-water pumpage for residential 

domestic use and for irrigation for agricultural and 

improved open space (golf courses, cemeteries, and 

public institutions) were estimated using several 

different approaches (table 6). 

Domestic pumpage was estimated using a wellbased method and a population-based method. The 

well-based method involved multiplying the estimated 

number of residential wells (1,595) by an estimated 

average pumping rate (20 gal/min) and an assumed 

average daily use value of 1 hr/d. The population-based 

method involved multiplying the estimated selfsupplied population in year 2000 (4,800) by an 

estimated per capita use value derived from water-use 

data for the Napa Municipal Water Department for 

1990-2000 (148 gallons per person per day). The wellbased method yielded a pumpage estimate of about 800 

acre-ft and the population-based method yielded a 

pumpage estimate of about 2,100 acre-ft (table 6, at 

end of text). 

Irrigation pumpage for agriculture was estimated 

using a well-based method and a land-use method. The 

well-based method involved multiplying the number of 

wells (185) by an estimated average pumping rate (75 

gal/min) and an assumed average daily use (about 2.9 

hr/d). The land-use method involved multiplying 

estimates of irrigated acreage (2,369 and 2,869 acres) 

by two different unit applied water coefficients for 

grapes (0.5 and 1.2 acre-ft/acre). The well-based 

method yielded a pumpage estimate of 2,690 acre-ft 

and the land-use method yielded pumpage estimates 

ranging from 1,180 to 3,440 acre-ft (table 6, at end of 

text).

Pumpage for irrigation of improved open space 

(golf courses, cemeteries, and public institutions) was 

estimated by multiplying acreage (391 acres) by a 

water coefficient for pasture (4.0 acre-ft/acre). The 

estimated annual pumpage for improved open space is 

about 1,560 acre-ft (table 6, at end of text).

In summary, estimates of the 2000-2002 annual 

ground-water pumpage in the study area range from 

3,600 to 7,100 acre-ft, and average 5,350 acre-ft. 

Assuming that the average of the estimate (5,350 acreft) represents the annual ground-water pumpage, 

annual ground-water pumpage has increased by 2,350 

acre-ft compared with the pumpage for 1975 of 3,000 

acre-ft (Johnson, 1977). The estimated increase in the 

quantity of annual ground-water pumpage between 

1975 and the 2000-2002 period is consistent with the 

marked increase in the number of new wells drilled in 

the study area and the large increase in irrigated 

agriculture during this period (fig. 13; table 1, at end of 

report).
Ground-Water Hydrology 27

12

121

Napa

Ground-water storage unit boundary

(Johnson, 1977)

122 17' 30" 122 12' 30"

38

20'

00"

38

17'

30"

T6N

T5N

R4W R3W

0

0

1

1

2 Miles

2 Kilometers

Cr

Marie

Sarco

Milliken

Tulucay

Napa

Cr

Cr

River

Cr

Soda

Cr

Kreuse Cr

Cr

Murphy

Spencer

29

29

Ground-water flow direction

Study area boundary; blue where coincides with

Napa River

o

o o

o

121

Maxwell

Bridge

RANCH RD

SILVERADO

TRAIL

SOSCOL

IMOLA

ST HAGEN RD

SILVERADO

TRAIL

MONTICELLO

RD

AVE

THIRD AVE

ST HELENA

HIGHWAY

VALLEY RD

ST

FIRST

AVE

ST

JEFFERSON

ST

FIRST

ATLAS PEAK RD

NORTH AVE

BROWNS

AVE LINCOLN

TRANCOS

OLD SONOMA RD

FOSTER

RD

AVE

FOURTH

AVE

THIRD

VICHY

AVE

BIG

MCKINLEY

RD

HARDMAN

AVE

AVE EAST THIRD

RD

GREEN VALLEY

SALVADOR

AVE

AVE OAK KNOLL

HIGHWAY NAPA-VALLEJO

121

EXPLANATION

Hydraulic-head contour, in feet above sea level—

Contour interval is variable. Dashed where uncertain

Well and local well number

19

20

24

34

114

16

110

105

141

9

8

7

6

4

2

99

98

97

95

93

92

90 91

89

87 86

85

84

83 82

81 79

78

76

75

74

72

70

69 68 67 66

65

63

61

58

57

56

55

54

53

52

51

50

49

48

47

45

44

43

41 42

40

38

37

36

35

33

32

31

29

26

25

23

22

21

19 18

17

15

14

13

10

145

144

143

142

124

122

120

119

118

117

116

113

104

102

100

-27L2

-27N1

140

121

0

-20 -10

200

-40

0

0

20

40

-40

0 -20

-60

-80

0

300

100

200

100

80 40

40

30

30

-30

0

100 190

110

10

10

Base prepared by U.S. Geological Survey,

from digital elevation data, 2000

0

-10

SODA CREEK

FAULT

Fault—Long dashed where approximately located; dotted where concealed (Johnson, 1977)

400

70

Figure 15. Generalized hydraulic head in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, October 2001.
28 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Napa

Ground-water storage unit boundary

(Johnson, 1977)

122 17' 30" 122 12' 30"

38

20'

00"

38

17'

30"

T6N

T5N

R4W R3W

0

0

1

1

2 Miles

2 Kilometers

Cr

Marie

Sarco

Milliken

Tulucay

Napa

Cr

Cr

River

Cr

Soda

Cr

Kreuse

Cr

Cr

Cr

Murphy

Spencer

o

o o

o

RANCH RD

SILVERADO

TRAIL

SOSCOL

IMOLA

ST HAGEN RD

SILVERADO

TRAIL

MONTICELLO

RD

AVE

THIRD AVE

ST HELENA

HIGHWAY

VALLEY RD

ST

FIRST

AVE

ST

JEFFERSON

ST

FIRST

ATLAS PEAK RD

NORTH AVE

BROWNS

AVE

LINCOLN

TRANCOS

OLD SONOMA RD

FOSTER

RD

AVE

FOURTH

AVE

THIRD

VICHY

AVE

BIG

MCKINLEY

RD

HARDMAN

AVE

AVE EAST THIRD

RD

GREEN

VALLEY

SALVADOR

AVE

AVE OAK KNOLL

HIGHWAY NAPA-VALLEJO

31

30

29

EXPLANATION

Hydraulic-head contour, in feet above sea level—

Contour interval is variable. Dashed where uncertain

Study area boundary; blue where coincides with

Napa River

20

24

34

114

16

110

105

141

9

8

7

6

4

2

99

98

97

95

93

92

91

90

89

87 86

85

84

83 82

79 81

78

76

75

74

72

70

69

68 67 66

65

63 61

60

58

57

56

55

54

53

52

51

50

49

48

47

45

44

43

42

41

40

38

37

36

35

33

32

26

25

23

22 21

19 18

17

15

14

13

10

145

144

143

142

124

122

120

119

118

117

116

113

104

102

100

-27L2

-27N1

140

121

140

200

200

-40

0

20

0

0

-60

60

400

100

100

150

40

70

-20

20

40

40

100

40

50

20

200

20

Base prepared by U.S. Geological Survey,

from digital elevation data, 2000

0

20

20

Ground-water flow direction

Well and local well number

19

Fault—Long dashed where approximately located; dotted where concealed (Johnson, 1977)

120

Maxwell

Bridge

50

0

40

SODA CREEK FAULT

29

121

29

121

12

20

121

120

300

Figure 16. Generalized hydraulic head in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, April 2002.
Ground-Water Hydrology 29

Ground-Water Underflow

Ground-water underflow out of the study area 

occurs along the western boundary of storage unit 1, 

and ground-water underflow into the study area occurs 

across the western boundary of storage unit 4. 

Underflow out of storage unit 1 and into storage unit 4 

was calculated on the basis of ground-water level data 

for 2000–2001. Insufficient ground-water level data 

were available for calculating underflow from storage 

unit 2, however, the quantity of underflow is probably 

small because of the low permeability of the deposits in 

this unit. The total quantity of underflow estimated for 

this study was compared with estimates made by 

Johnson (1977) for conditions during 1975. 

For this study, underflow was estimated using the 

following form of Darcy's Law: 

where Q is ground-water underflow (L3/T), K is the 

average hydraulic conductivity of the saturated, 

unconsolidated deposits (L/T), I is the hydraulic 

gradient (L/L), and A is the cross-sectional area (L2). 

The average hydraulic conductivity in storage unit 4, 

based on data from Johnson (1977), is 2 ft/d; the 

hydraulic gradient based on ground-water levels in 

2000-2001 was 7×10-3 ft/ft toward the study area, and 

the cross-sectional area was 4.5×106 ft2. Ground-water 

inflow along the western boundary of storage unit 4 

from outside the study area was calculated to be about 

530 acre-ft/yr during 2000-2001. The average 

hydraulic conductivity in storage unit 1, based on 

Johnson (1977), was 2 ft/d; the hydraulic gradient 

based on ground-water levels in 2000-2001 was 

1.5×10-2 ft/ft away from the study area, and the crosssectional area was 4.5×106 ft2. Ground-water 

underflow for storage unit 1 was calculated to be about 

1,130 acre-ft/yr. Therefore, the net ground-water 

underflow across the western boundaries of storage 

units 1 and 4 was about 600 acre-ft/yr leaving the study 

area. This value is about 2,050 acre-ft/yr less than the 

amount Johnson (1977) estimated for 1975. The 2,050 

ace-ft/yr decrease closely matches the estimated 

increase in ground-water pumpage between 1975 and 

2000 for the study area. Underflow across the western 

boundary has changed since 1975 because groundwater pumping has caused ground-water gradients in 

the study area to change.

Ground-Water Levels and Movement

Water-level monitoring can detect ground-water 

level declines related to excessive pumping and (or) 

deficient recharge. Water levels can be expressed in 

two ways: as depth-to-water below land surface or as 

hydraulic head. Hydraulic head is particularly useful 

because it expresses water level as an altitude relative 

to an arbitrary datum plane, such as sea level. Because 

ground water moves in the direction of decreasing 

hydraulic head, contour maps of hydraulic head can be 

used to determine the general direction of ground-water 

flow in an aquifer.

Monitoring Network

Water levels were measured semiannually by 

personnel from the Napa County Department of Public 

Works; as many as 120 wells were measured between 

the spring of 2000 and the spring of 2002 (fig. 14). The 

period of record for most wells is 1 year or more, but 

some were measured for less than 1 year. During the 

study, measurements at some wells were discontinued 

because of difficult access or well bore obstructions. As 

the study progressed, other wells were added to replace 

wells removed from the network or to improve areal 

coverage of the initial network. Measurements 

generally were made in April and October at the 

beginning and ending of the dry season, respectively.

Water levels in all the wells were measured using 

a 300-foot calibrated electric tape with graduations of 

0.01 ft, but measurements were recorded and reported 

to the nearest 0.1 ft (table 5, at end of report). Care was 

taken to ensure that measurements were not made 

while wells were pumping or recovering from recent 

pumping. Despite these efforts, some water-level 

measurements may have been made during non-static 

conditions. Measurements suspected of having been 

affected by recent or nearby pumping were excluded 

from the analysis of water-level conditions in this 

report (table 5, at end of report).

Q KIA = ,
30 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 17. Change in water levels compared with well depth between (A) October 2000-October 2001 and (B) October 2001-October 2002 in the lower 

Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.
Ground-Water Hydrology 31

Figure 18. Change in water levels compared with well depth between (A) April 2000-April 2001 and (B) April 2001-April 2002 in the lower Milliken–Sarco– Tulucay Creeks area, southeastern Napa County, California.
32 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 19. Change in water levels compared with well depth between (A) October 2000-April 2001, and (B) October 2001-April 2002 in the lower Milliken– Sarco–Tulucay Creeks area, southeastern Napa County, California.
Ground-Water Hydrology 33

Figure 20. Continuous ground-water levels recorded in selected wells in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, 

California.
34 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

65

70

75

80

85

90

95

100

105

110

115 0

5

10

15

20

25

30

35

40

45

50

3/10/01

3/5/01

11/11/01

11/29/01

1/5/02

1/23/02

10/27/01 (end of nearby pumping)

50

51

52

53

54

55

56

57

58

59

60 0

5

10

15

20

25

30

35

40

45

50

DEPTH TO WATER, IN FEET BELOW LAND SURFACE

CUMULATIVE PRECIPITATION, IN INCHES

DATE

A S O N D

2000

J F M A M J J A S O N D

2001

J F M A M J J A S O

2002

2/25/01

2/25/01

11/11/01

11/11/01

5/19/02

5/24-26/02

5/19/02

5/21/02

5N/4W-14J3 (local well no. 49)

6N/4W-23J1 (local well no. 2)

EXPLANATION

Precipitation

Depth to water

Figure 21. Ground-water levels in selected wells and cumulative precipitation in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa 

County, California.
Ground-Water Hydrology 35

In addition to the periodic measurements made 

using the calibrated electric tape, continuous 

measurements were made in six wells on which 

submersible pressure transducers and data loggers were 

installed (fig. 14). Staff from the Napa County 

Department of Public Works downloaded the data to a 

laptop computer every 3 months, replaced the 

datalogger batteries, and measured the water levels to 

evaluate transducer performance (Lee Driggers, Napa 

County Department of Public Works, written commun., 

2001). The continuous water-level data were converted 

from readings representing the submergence depth of 

the transducer below the water surface to values 

representing the depth to water below land surface. 

Transducer performance was evaluated by comparing 

water levels measured with a calibrated electric tape 

with water levels measured by the transducers; 

differences in water levels were attributed to transducer 

drift or to extraneous factors such as cable slippage. 

Corrections were applied to the data to compensate for 

these factors.

Land-surface altitudes at the wells were 

determined by Napa County Department of Public 

Works staff using a combination of differential Global 

Positioning System (GPS) surveying and third-order 

differential leveling surveying (Lee Driggers, Napa 

County Department of Public Works, written commun., 

2002). The land-surface altitudes at the wells in the 

monitoring network at the beginning of the study in 

2000 were derived from differential GPS surveying and 

have an accuracy of 0.5 ft. The land-surface altitudes at 

the wells added to the monitoring network in 2001 

were derived from third-order differential leveling and 

have an accuracy of less than 0.1 ft. Land-surface 

altitudes at some wells that were not included in either 

survey were determined by interpolating between the 

20-foot contour intervals on USGS 7-1/2-minute 

topographic maps for the Napa and Mt. George 

quadrangles. The contours have an accuracy of plus or 

minus 10 ft. 

Ground-Water Levels

Graphs and a map showing long-term water-level 

changes were made using both recent measurements 

collected as part of this study and historical water-level 

measurements obtained from the Napa County 

Department of Public Works (internal files) and the 

California Department of Water Resources (1995). 

These measurements do not necessarily represent the 

water table because hydraulic head can vary with depth 

in an aquifer. Therefore, the water levels in wells open 

to large depth intervals represent composite heads for 

the respective depth intervals. The correct 

interpretation of ground-water level data is, in part, 

dependent upon complete well-construction 

information, including total depth, perforation 

intervals, seals, and gravel-pack depth. Complete 

construction information, however, was not available 

for several of the wells in the 2000–2002 network, 

which limited analysis and interpretation of the data.

Ground-Water Movement

Maps of hydraulic head were made using waterlevel data from October 2001 and April 2002 (figs. 15

and 16) to determine approximate ground-water flow 

directions; the direction of ground-water movement is 

from areas of higher hydraulic head toward areas of 

lower hydraulic head. Hydraulic-head values were 

calculated by subtracting the measured depth to water 

from the land-surface altitudes at the wells. Under 

present-day conditions, the general direction of 

ground-water movement is from recharge areas in the 

mountains around the perimeter of the study area 

toward pumping depressions in storage units 1, 3, and 4 

(figs. 15 and 16). The locations of the pumping 

depressions in storage units 1 and 3 correspond with 

areas having the highest concentrations of active and 

potentially active wells (fig. 12). The water-level 

contours in figures 15 and 16 differ from the waterlevel contours in the maps by Johnson (1977) for April 

and September 1975. One obvious difference is the 

deep depression of hydraulic heads in the west-central 

part of storage unit 1 (figs. 15 and 16) that is not 

evident in the 1975 data. Other significant differences 

between 1975 and 2001–2002 are the deepening and 

broadening of the pumping depression in the southcentral part of storage unit 3 and the reduction in the 

depth and lateral extent of the pumping depression in 

the northwest part of storage unit 3. Changes in 

hydraulic head and directions of ground-water 

movement are more complicated in storage unit 4. A 

comparison of hydraulic heads for April 2002 with 

hydraulic heads for 1975 indicate a pumping 

depression in the southern part of storage unit 4 that 

was not evident in 1975. In September 1975, a large 

part of storage unit 4 was underlain by a pumping 

depression, but in October 2002 the depression was not 

as deep as it was in 1975 and it had shifted farther to 

the east.
36 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 22. Locations of selected wells at which periodic water levels were made in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa 

County, California, early 1960s through 2002.
Ground-Water Hydrology 37

0

20

40

60

80

100

1960 1970 1980 1990 2000

YEAR

DEPTH TO WATER, IN FEET BELOW LAND SURFACE

Well 11M1—77 feet

(unknown)

Well 14C1—220 feet

(unknown) Well 137—364 feet

(unknown)

Well 49—418 feet

(unknown)

0

20

40

60

80

100

1980 1985 1990 1995 2000

YEAR

Well 11M1—77 feet

(unknown)

Well 14C1—220 feet

(unknown)

Well 137—364 feet

(unknown)

Well 49—418 feet

(unknown)

A

B

Figure 23. Periodic water levels in selected wells in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, early 1960s 

through 2002. A, south group, all data. B, south group, highest water levels measured in spring. C, north group, all data. D, north group, highest water levels 

measured in spring. E, east-central group, all data. F, east-central group,highest water levels measured in spring.
38 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

0

25

50

75

100

125

150

1960 1970 1980 1990 2000

Well 2—700 feet (unknown)

Well 43—310 feet (unknown)

Well 56—210 feet (30–210)

Well 75—205 feet (45–205)

Well 35G3—260 feet (Unknown)

0

25

50

75

125

100

150

1980 1985 1990 1995 2000

YEAR

YEAR

DEPTH TO WATER, IN FEET BELOW LAND SURFACE

Well 35G3—260 feet

(unknown)

Well 43—310 feet

(unknown)

Well 2—700 feet

(unknown)

Well 75—205 feet

(45–205)

Well 56—210 feet

(30–210)

C

D

Figure 23.—Continued.
Ground-Water Hydrology 39

0

50

100

150

200

250

300

1960 1970 1980 1990 2000

DEPTH TO WATER, IN FEET BELOW LAND SURFACE

Well 20—208 feet

(130–207)

Well 10—320 feet

(unknown)

Well 41—610 feet

(unknown)

Well 95—195 feet

(155–185)

Well 91—415 feet

(315–415)

0

50

100

150

200

250

3001980 1985 1990 1995 2000

YEAR

YEAR

Well 20—208 feet

(130–207)

Well 41—610 feet

(unknown)

Well 91—415 feet

(315–415)

Well 95—195 feet

(155–185)

Well 10—320 feet

(unknown)

E

F

Figure 23.—Continued.
40 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Insufficient data were collected during this 

current study to determine precisely the direction of 

ground-water flow in storage unit 2. However, given 

that large pumping depressions were mapped for 

October 2001 and April 2002 near the northeast and 

southeast periphery of storage unit 2, it is likely that 

ground water moves from storage unit 2 toward these 

depressions. West of the surface-water divide, running 

north-south through the central part of storage unit 2, 

ground water probably generally moves westward 

toward the Napa River.

The Soda Creek Fault, on the west side of the 

study area, is a partial barrier to ground-water 

movement between storage units 3 and 4. This fault is 

described as a normal fault having more than 700 ft of 

vertical displacement (Weaver, 1949) that juxtaposes 

different geologic units on either side of the fault. In 

October 2001 water-level altitudes were about 10 ft 

higher on the east side of the fault than on the west side 

of the fault (figs. 15 and 16).

Annual and Seasonal Water-Level Fluctuations

Measured water levels and calculated hydraulic 

heads, as altitudes above sea level, in wells in the 

monitoring network are given in table 5 (at end of 

report) for October and April 2000 to 2002. Changes in 

water levels for October and April 2000 through 2002 

as related to well depth are shown in figures 17, 18, and 

19 and table 7, at end of report. Water levels generally 

declined between October 2000 and October 2001 

(median water-level change was −8.6 ft) and between 

October 2001 and October 2002 (median water-level 

change was −5.5 ft) (fig. 17A, B). Water levels 

generally were unchanged between April 2000 and 

April 2001 (median water-level change was +0.7 ft), 

but declined slightly between April 2001 and April 

2002 (median water-level change was -2.6 ft) (fig. 18A, 

B). Comparisons of annual water-level changes as 

related to well depth for autumn (fig. 17A, B) and for 

spring (fig. 18A, B) showed no clear correlation 

between water-level change and total depth of well. 

Water levels rose in almost every well between October 

2000 and April 2001 (fig. 19A) and between October 

2001 and April 2002 (fig. 19B); the median water-level 

change was +14.1 ft and +19.9 ft, respectively. Groundwater levels generally rise between October and April 

owing to a reduction of ground-water pumping during 

the winter rainy season, ground-water inflow from 

outside the study area, and possibly minor direct 

recharge. The slightly higher water-level rise during the 

winter of 2002 can be attributed to greater rainfall in 

2002 than in 2001 (fig. 3).

Water levels were measured continuously in six 

wells in the study area from 2000 to 2002 (fig. 20). 

Two of the wells (2 and 49) were not pumped; the other 

four wells (22, 67, 74, and 92) were pumped 

frequently. Water levels fluctuated 20 to 50 ft 

seasonally in wells 2, 67, 74, and 92 but fluctuated only 

5 to 15 ft seasonally in wells 22 and 49. The larger 

water-level fluctuations were in wells located near 

pumping depressions in storage units 1 and 3 (figs. 15

and 16). The continuous hydrographs show seasonal 

ground-water level changes more precisely than 

semiannual measurements. The hydrographs for the six 

continuously measured wells show that maximum 

annual ground-water levels can occur in individual 

wells any time between January and April and that the 

minimum annual ground-water level can occur any 

time between August and October. These data suggest 

continuous water-level monitoring is the most reliable 

means of determining the seasonal maximum and 

minimum ground-water levels. If other considerations 

necessitate a semiannual measurement schedule, then 

measuring during the periods March to April and 

September would be best for recording the maximum 

and minimum water levels. 
Ground-Water Hydrology 41

Ground-water storage unit

boundary (Johnson, 1977)

EXPLANATION

Water-level change, in feet

50 to 25

25 to 0

0 to -25

-25 to -50

-50 to -75

-75 to -100

-100 to -125

LINCOLN

Maxwell

Bridge

SILVERADO

TRAIL

SOSCOL ST

RD HAGEN

SILVERADO TRAIL

MONTICELLO

RD

AVE

THIRD AVE

ST

FIRST

AVE

COOMBSVILLE

ST

FIRST

ATLAS PEAK RD

RD

NORTH AVE

AVE

TRANCOS

AVE

FOURTH

AVE

THIRD

VICHY AVE

MCKINLEY

RD

HARDMAN AVE

AVE

EAST THIRD

RD

GREEN

VALLEY

AVE

OAK KNOLL

HIGHWAY NAPA-VALLEJO

121

121

RANCH RD

BIG

IMOLA

T6N

T5N

T6N

T5N

R4W R3W

0

0

1

1

2 Miles

2 Kilometers

Storage

Unit 2

Storage

Unit 1

Storage

Unit 3

Storage

Unit 4

Figure 24. Change in water levels in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, autumn 1975 to autumn 2001.
42 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Water levels in well 2 (unused) fluctuated about 

35 ft between the highest and the lowest levels. This is 

a response to recharge from nearby Milliken Creek and 

recovery after cessation of pumping in nearby wells 

used for golf course and landscape irrigation. 

Examination of the expanded hydrograph for well 2 

clearly illustrates the overall decline and daily 

fluctuations in water levels between March and 

November owing to nearby pumping (fig. 21). The 

transition from a cessation of nearby pumping to rapid 

water-level recovery in late October 2001 is marked by 

the beginning of a smooth and more steeply rising 

hydrograph. In late November 2001, the rate of waterlevel recovery increased in response to the onset of 

heavy winter rains several weeks earlier. A 2- to 3-

week lag time between precipitation and water-level 

response in January 2002 also is clearly evident in 

figure 21. The heavy precipitation tapered off in early 

January, and by the third week of January, the rate of 

water-level recovery also was leveling off.

The hydrograph for unused well 49 shows little 

water-level response to seasonal stresses due to the lack 

of pumping at nearby wells and its location away from 

major natural sources of recharge. Without complete 

well-construction information, an unambiguous 

interpretation of water-level fluctuations was not 

possible; however, water level seems to respond rapidly 

to precipitation (fig. 21). If any part of the well is 

screened in a confined aquifer, part of the water-level 

rise may be attributable to diminished pumping in 

distant wells.

Long-Term Changes in Ground-Water Levels

In the early 1900s, water flowed to land surface 

from many of the wells drilled in the area; by the 

1950s, most of the wells had ceased flowing (Kunkel 

and Upson, 1960). Although water-level records for 

this period were insufficient for creating hydrographs, 

long-term hydrographs for the early 1960s through 

2002 were created for 14 wells in the study area using 

periodic water-level measurements of 10 or more years. 

The locations of these wells are shown in figure 22, and 

the hydrographs for them are shown in figure 23. 

The hydrographs are divided into three groups on 

the basis of geographic location and similarities in 

water-level fluctuations. The three groups are 

delineated in figure 22 and are identified as south, 

north, and east-central. The depth range of perforated 

intervals is known for only five of the wells. The lack 

of information on the perforated intervals of nine of the 

wells limits understanding similarities and differences 

in ground-water level fluctuations and trends between 

wells and geographic locations. The ground-water level 

network can be improved by using only wells that have 

complete construction information. However, the longterm trends of ground-water levels in these 14 wells 

still are instructive because they show areas where 

ground-water level declines have been greatest and 

areas where ground-water levels have not changed 

significantly over periods of several years.

Water-level data for each group of wells are 

shown in two hydrographs—one shows all available 

data for each well from 1960 to 2002; the second shows 

the highest water level measured each spring from 

1980 to 2002. The data used for the spring water levels 

includes only values measured for March or April. The 

annual high water level generally occurs in the spring 

and is a measure of ground-water level recovery in the 

aquifer near the well. The amount of recovery depends 

upon recharge derived from precipitation during the 

previous season and the amount of reduction in groundwater pumping during October to February. 

Long-term water-level data were available from 

four wells in the southern part of the study area. . 

Water-level measurements were discontinued in the 

mid-1990s for wells 11M1 and 14C1, however, prior to 

that time, there was no significant long-term trend in 

increasing or decreasing water levels. The other two 

wells (49 and 137) in the south group show small 

water-level declines between the 1980s and 2002 of 

about 1 and 12 ft, respectively (fig. 23B). Annual 

water-level fluctuations in the wells in this area were 

generally much less than 10 ft.

Water-level declines have been greater in the 

northern part of the study area than in the southern part 

but have been less than those in the east-central part. 

Long-term water-level data were available for five 

wells in the northern part of the area (fig. 23C). Water 

levels in well 2 declined by about 70 ft between 1960 

and 1975 and by less than 10 ft between 1975 and 

2002. Water levels in well 6N/4W-35G3 declined about 

40 ft between 1962 and 1975, recovered to near 1962 

levels by the mid 1980s, and then declined by about 30 

ft by 1988, when measurements were discontinued. 

Wells 43, 56, and 75 have water-level data from about 

1980 to 2002, and the water levels in these wells 

declined by about 10 to 25 ft during this period (fig. 

23D). Annual water-level fluctuation in wells from this 

group were more than 25 ft during many years 

(fig. 23C).
Ground-Water Hydrology 43

Milliken

Frey

Lake

Madigan

Lake

Lakes

Leona

100

1,000

100

200

400

500

700

1,000

1,500

200

200

300

300

400

700

700

1,500

1,000

1,500

200

300

200

1,000

Sarco Milliken

Tulucay

Napa

Cr

Cr

River

Cr

Soda

Cr

Kreuse

Cr

Cr

Murphy

Cr

Spencer

Lake Camille

Cr

Marie

Maxwell

Bridge

RANCH RD

SILVERADO

TRAIL

SOSCOL

IMOLA

ST HAGEN RD

SILVERADO TRAIL MONTICELLO RD

AVE

THIRD AVE

ST HELENA

HIGHWAY

VALLEY RD

ST

FIRST AVE COOMBSVILLE ST

JEFFERSON

ST

FIRST

ATLAS PEAK RD

RD

12

NORTH AVE

BROWNS

AVE LINCOLN

TRANCOS

OLD SONOMA RD FOSTER

RD

AVE

FOURTH

AVE THIRD

VICHY

AVE

BIG

121 MCKINLEY

RD

HARDMAN AVE

AVE EAST THIRD

RD

GREEN

VALLEY

SALVADOR

AVE

AVE OAK KNOLL

HIGHWAY

NAPA-VALLEJO

Napa

12

29

29

29

121

WOODEN RD VALLEY

121

121

122 17' 30" 122 15' 00" 122 12' 30" 122 10' 00"

38

15'

00"

R4W R3W

0

0

1

1

2 Miles

2 Kilometers

o

o

o o o o

o

o

Reservoir

100

EXPLANATION

Ground-water storage unit boundary (Johnson, 1977)

Drainage basin boundary

38

20'

00"

38

22'

30"

38

17'

30"

T6N

T5N

Base prepared by U.S. Geological Survey, from digital elevation data, 2000

Fault—Long dashes where approximately located;

dotted where concealed (Johnson, 1977)

Well and identifier 36

Creek, lake, or pond sampling site and identifier

MC

9

6A

2

1

PS

MP

LM

LL

LF

DC

60

56

51

45

41

38

37/MCH

35

33/MCW 31

21

20A

16 14

13

11

VKP

OCS

LLC

LMO

49B

46B

46B

42B

28B

22B

22A

12

SCCP

17

7B

26B

26A

20B

7A

95

92

67

61

53

36

29

22

19

14

142

100

SODA CREEK FAULT

OCS

Spring and identifier

500

100

24

58

50

1,500

121

Topographic contour—Shows altitude of land surface. Contour interval variable. Datum is sea level

76

45

43

62

Figure 25. Locations of ground-water, surface-water, and miscellaneous water-chemistry sampling sites in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California.
44 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Long-term water-level data were available from 

five wells in the east-central part of the study area, and 

show that water-level declines were largest in this part 

of the study area (fig. 23E). Water levels in well 10, 

which has the longest period of record for the eastcentral group, declined by more than 100 ft between 

1962 and 2002. Annual high water levels measured in 

wells 20 and 41 (fig. 23F) declined between about 25 

and 70 ft between the 1980s and the 2000-2002 period. 

Annual water-level fluctuations in wells in this group 

range from about 10 to more than 50 ft (fig. 23E).

In summary, ground-water levels have declined 

since the 1960s over large parts of the study area 

especially in the central part. These water-level 

declines have occurred despite near average to above 

average precipitation for the past four decades. A 

decade by decade comparison of precipitation recorded 

at the Napa State Hospital for the 1960s, 70s, 80s, and 

90s shows that precipitation was 95, 99, 117, and 115 

percent of the average. Water levels declined in 9 of 10 

key monitoring wells (figs. 23B, 23D, and 23F) during 

the period 1980–2002. The water levels in five of these 

wells declined by greater than 20 ft since the 1980s. 

The general decline in ground-water levels in the study 

area may be attributed to an increase in ground-water 

pumping as more wells were constructed in the area 

especially since the mid 1970s.

Ground-Water Level Changes, 1975-2001

A water-level change map for the study area 

(fig. 24) was developed by computing the differences 

between the hydraulic-head map for autumn 2001 

(fig. 15) presented in this report and the hydraulichead map for autumn 1975 presented in Johnson 

(1977). Because most of the wells used to measure 

water levels during 2000-2002 were not the same wells 

measured by Johnson during 1974-75, the changes in 

figure 24 cannot be related to specific wells. In 

addition, it is important to recognize that water-level 

data used for the autumn 2001 and autumn 1975 

hydraulic-head maps represent composite hydraulic 

heads because the water levels were measured in wells 

that mostly were constructed with long gravel packs, 

long screens, or multiple-depth zones open to geologic 

formations.

The water-level changes shown in figure 24 are 

consistent with the water-level changes evident in the 

long-term hydrographs (fig. 23A–F). In general, water 

levels declined in the northeastern part of ground-water 

storage unit 1, in the central part of storage unit 2, and 

in the southeastern part of storage unit 3. Water levels 

during 1975–2001 declined 100 to 125 ft in an area east 

of Third Avenue, straddling storage units 1 and 3, and 

along and north of Hagen Road in an area that is 

underlain by a pumping depression (figs. 15 and 16). In 

the central part of storage unit 2 along First Avenue, 

ground-water levels during 1975–2001 declined as 

much as 75 to 100 ft. Water-level declines were 

moderate (25 to 75 ft) in large areas of storage units 1, 

2, and 3, including the western and the northern parts 

of storage unit 3 along Atlas Peak Road (fig. 24). 

Water-levels declines were small (0 to 25 ft) in storage 

unit 1 along Fourth Avenue between Imola Avenue and 

Coombsville Road and throughout a large part of the 

western half of storage unit 3. Water levels rose 0 to 50 

ft throughout much of the western part of storage unit 1 

between Imola Avenue, near Napa State Hospital, and 

at First Avenue south of North Avenue. Areas of 0- to 

25-foot water-level rises extended through the central 

and southern parts of storage unit 4, the southwest part 

of storage unit 3, and a small area in the northeast part 

of storage unit 3, in the vicinity of the Silverado 

Country Club. Although water-level monitoring in and 

west of storage unit 4 was less extensive during 2000–

2002 than during 1974–75 (Johnson, 1977), the limited 

data collected for this study indicate that water levels in 

most of this area rose less than 25 ft during 1975–2001.
Ground-Water Hydrology 45

Figure 26. Chemical composition of water from selected ground-water and surface-water sampling sites in the lower Milliken–Sarco–Tulucay Creeks area, 

southeastern Napa County, California, September-November 2001.
46 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

SURFACE-WATER AND GROUND-WATER 

QUALITY

Surface water and ground water were sampled 

for analyses of major ions, selected trace elements, 

silica, nutrients, the stable isotopes of oxygen and 

hydrogen, selected dissolved atmospheric gases 

[nitrogen (N2), argon (Ar), oxygen (O2), carbon 

dioxide (CO2), methane (CH4), helium (He), hydrogen 

(H2), and neon (Ne)], chloroflurocarbons (CFCs), and 

tritium. These data, summarized in tables 8 and 9 (at 

back of report) and tables10 and 11 (at back of report), 

were used to help characterize the areal variations in 

ground-water and surface-water chemistry and to help 

identify the ages and sources of ground waters. 

Surface-water samples were collected from sites along 

Milliken, Sarco, and Tulucay Creeks and their 

tributaries and at several lakes and ponds. Groundwater samples were collected from 15 wells in groundwater storage units 1 through 4 and from 2 springs in 

the Howell Mountains (fig. 25). Selection of wells was 

based on accessibility, type of well construction, and 

proximity to other wells selected for sampling in order 

to maximize the geographic coverage. 

In general, water chemistry and (or) isotopic 

composition can vary with depth in ground-water 

systems. Most wells in the study area have shallow 

(less than 50 ft) seals and are perforated over long 

intervals, from shallow depths (less than 100 ft) to near 

the bottom of the well. This type of well construction is 

used to maximize well yield, but it greatly restricts the 

number of wells that tap only the deeper parts of the 

ground-water system and limits the utility of chemical 

data for characterizing the ground-water flow system. 

Of the 15 wells sampled for this study, only 2 are 

known to have perforated intervals that begin at depths 

greater than 100 ft (wells 92 and 142). For this reason, 

no statistically meaningful comparison of water 

chemistry or isotopes between deep and shallow 

ground water can be made for the 15 samples collected.

Methods of Water Sampling and Analysis

Surface-water samples were collected using a 

DH-81 sampler according to methods given in Wilde 

and others (1999); for streams too shallow for this 

method, water was collected in a polyethylene beaker 

from near the centroid of flow. Ground-water samples 

from wells were collected from faucets either at or near 

the well head to minimize potential chemical alteration 

of the water between the well and the sampling point. 

Prior to the collection of the ground-water samples, the 

wells were purged a minimum of three casing volumes 

of water. Sequential measurements of specific 

conductance, pH, and temperature were made at 

5-minute intervals until readings had stabilized. All 

samples collected for the analysis of major ions, trace 

elements, silica, and nutrients were collected, treated, 

and preserved following procedures outlined by U.S. 

Geological Survey (1997 to present). These samples 

were analyzed at the USGS National Water Quality 

Laboratory in Denver, Colorado, using standard 

analytical methods described by Fishman and 

Friedman (1989), Fishman (1993), and Struzeski and 

others (1996).

Water samples for determinations of stable 

isotopes of hydrogen and oxygen were collected in 

unrinsed 60-mL glass bottles. Surface-water samples 

were collected directly from creeks, lakes, ponds, and 

springs by immersing the bottle until filled. Groundwater samples were bottom filled using a tygon tubing 

connected to the sampling point and allowed to 

overflow several sample volumes of water prior to 

being capped. Bottles were capped with conical-seal 

caps. Ratios of stable isotopes of oxygen and hydrogen 

were determined by the USGS Isotope Fractionation 

Project in Reston, Virginia, using a hydrogen waterequilibration technique (Coplen and others, 1991).

Samples for dissolved N2, Ar, O2, CO2, and CH4

analysis were collected by submerging 150-mL glass 

bottles in a plastic bucket that was continuously filled 

to overflowing with well water. A copper tube attached 

to the sample point and used to fill the plastic bucket 

was placed in the bottom of the glass sample bottle and 

allowed to displace several bottle volumes of water 

from the sample bottle prior to sealing the bottle. While 

the bottle was submerged in water, a rubber stopper 

pierced through by a syringe needle was firmly inserted 

in the bottle. The syringe needle allowed water, 

displaced by insertion of the stopper, and bubbles to 

escape. The syringe needle was removed with the bottle 

still submerged in water. A replicate sample was 

collected for each ground-water sampling site for 

quality assurance and control. Samples were stored and 

shipped on ice to the USGS Dissolved Gas Laboratory 

in Reston, Virginia, for analysis using gaschromatographic methods as described by the U.S. 

Geological Survey (2001).
Surface-Water and Ground-Water Quality 47

Water sampled for CFCs was pumped through 

copper tubing that was attached to the sampling point 

with hose fittings. The copper tubing was used to avoid 

contamination by chlorofluorocarbons that might 

diffuse through the tygon tubing used for chemical 

samples. Samples for CFCs were collected in five 

62-mL ampoules that had been pre-flushed with 

ultrapure nitrogen gas for 1 minute to exclude 

atmospheric gases. The ampoules were then flushed 

with well water for several minutes before being filled 

and sealed with an oxygen-MAPP gas torch. Samples 

for CFCs (CFC-11, CFC-12, and CFC-113) were 

analyzed by the USGS CFC Laboratory in Reston, 

Virginia, using purge-and-trap gas chromatography 

methods described by Busenberg and Plummer (1992).

Samples for dissolved helium (He), hydrogen 

(H2), and neon (Ne) were collected using the same 

bottles and procedures as described for the collection 

of dissolved gases. These samples were also collected 

in duplicate for quality assurance and control. Samples 

were analyzed for He by thermal conductivity detection 

at the USGS Dissolved Gas Laboratory in Reston, 

Virginia, using methods similar to those described by 

Sugisaki and others (1982). 

Samples for tritium analyses were collected in 

unrinsed 1-L polyethylene bottles. The bottles were 

bottom filled using a tygon tubing connected to the 

sampling point and allowed to overflow with several 

sample volumes of water prior to being capped. Bottles 

were sealed with conical-seal caps to minimize 

exchange with the atmosphere. These samples were 

analyzed at the USGS Tritium Laboratory in Menlo 

Park, California, by electrolytic enrichment and gas 

counting as described by Ostlund and Dorsey (1975).

Surface-Water and Ground-Water Chemistry

Selected samples were measured on site for 

dissolved oxygen, pH, specific conductance, water 

temperature, and alkalinity following procedures 

outlined by U.S. Geological Survey (1997 to present). 

Dissolved-oxygen concentrations ranged from less than 

0.1 to 8.9 milligrams per liter (mg/L); the highest 

concentrations were in the creek samples. The 

dissolved-oxygen concentrations exceeded 1 mg/L in 

water from six wells; the maximum concentration (6.6 

mg/L) was in a sample from well 53. 

The pH of all the samples ranged between 6.3 

and 8.6 (table 8, at end of report). The lowest and 

highest values, 6.3 and 8.6, respectively, do not meet 

the secondary drinking-water standard range of 6.5 to 

8.5 established for the protection of taste, odor, or 

appearance of drinking water (California Department 

of Health Services, 2003).

Specific conductance, a measurement of the 

ability of water to conduct an electrical current and an 

indicator of ionic concentration, varied widely 

depending on the type of the sample, the location, and 

the time of year. The specific conductance measured in 

stream samples ranged from 58 to 610 microsiemens 

per centimeter (μS/cm) (tables 8 and 9, at end of 

report). The lowest specific conductance values were 

measured in samples collected from creeks, lakes, and 

ponds on the Howell Mountain block. The highest 

specific conductance values were measured in samples 

collected from small culverts or tributaries to Milliken, 

Sarco, and Tulucay Creeks that drain areas of 

concentrated residential development (sites 20A, 21, 

and 9) or vineyards (sites 56, and 22B). Specific 

conductance of creek water was highest when 

streamflow was lowest. This suggests that recently 

discharged ground water makes up a greater part of 

streamflow during low-flow conditions than during 

times of high runoff. Specific conductance values in 

samples from lakes and ponds in the lower Milliken 

and Tulucay drainage basins in September 2001 ranged 

from 166 to 317 μS/cm. Samples from wells in the 

lower Milliken, Sarco, and Tulucay drainage basins in 

September and November 2001 ranged from 124 to 

1,220 μS/cm. Water from one well (61) exceeded the 

State secondary drinking-water standard of 900 μS/cm 

(California Department of Health Services, 2003).

Major-ion concentrations in surface- and groundwater samples are plotted in a trilinear diagram 

(fig. 26). A trilinear diagram shows the proportions of 

common cations and anions for comparison and 

classification of water samples independent of total 

analyte concentrations (Hem, 1985). Trilinear diagrams 

can be used to identify groups of samples that have 

similar relative ionic concentrations (Freeze and 

Cherry, 1979). Water samples from the lower Milliken–

Sarco–Tulucay Creeks area were separated into three 

groups, each group having a different relative chemical 

composition (fig. 26).
48 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Group 1 includes samples from six wells (19, 45, 

67, 76, 92, and 142), four of which exceed 350 ft in 

total depth, and all four surface-water samples 

[Murphy Creek (MC), a tributary of Tulucay Creek; 

Dairy Creek (DC), a tributary of Sarco Creek; and at 

two sites (site 33/MCW and site 37/MCH) along 

Milliken Creek]. Wells 45, 76, 92, and 142 are located 

close to creeks. Group 1 samples can be characterized 

as a mixed cation-bicarbonate type water. Sodium was 

the predominant cation in all group 1 water samples. 

Samples in this group had relatively low ionic 

concentrations compared with samples in groups 2 and 

3. Dissolved-solids concentrations (solids, sum of 

constituents) for the creek samples ranged from 128 to 

164 mg/L and for ground-water samples ranged from 

144 to 282 mg/L (compared with the median of 282 

mg/L for all 15 well samples).

Group 2 includes samples from three wells (22, 

36, and 100, all having total depths of less than 250 ft) 

in the southeastern part of the study area (fig. 25). The 

chemical composition of this group is characterized as 

a calcium-magnesium bicarbonate (two samples) or a 

calcium-magnesium mixed anion type water (one 

sample). The dissolved-solids concentrations for this 

group, which were higher than those for group 1, 

ranged from 217 to 435 mg/L.

Figure 27. Relation between delta deuterium and delta oxygen-18 in water samples for the lower Milliken–Sarco–Tulucay Creeks area, southeastern 

Napa County, California. A, all water samples. B, enlargement of part of figure 27A. C, creek samples grouped by month. D, creek samples for March and 

April 2001 grouped by range. E, ground-water samples and selected surface-water samples.
Surface-Water and Ground-Water Quality 49

Group 3 includes samples from six wells (14, 29, 

43, 53, 61, and 95). Five of the wells are located in or 

within 1 mi of the hilly region in ground-water storage 

unit 2 referred to as the Cup and Saucer (Johnson, 

1977). Group 3 samples are sodium-bicarbonate type 

water (fig. 26). Most samples in this group had higher 

ionic concentrations than the samples in groups 1 and 

2. Dissolved-solids concentrations ranged from 285 to 

732 mg/L. Concentrations of sodium ranged from 51 to 

247 mg/L and were higher compared with those in the 

samples from groups 1 and 2. Samples from wells in 

group 3 had the highest concentrations of chloride (175 

mg/L in the sample from well 61), fluoride (1.5 mg/L 

in the sample from well 61), and sulfate (80 mg/L in 

the sample from well 95).

Boron concentrations in samples from wells 14 

and 61 were 1.4 mg/L (1,440 micrograms per liter 

[μg/L]) and 11 mg/L (11,000 μg/L), respectively. 

These concentrations exceeded the California 

Department of Health Services action level (AL) of 1 

mg/L (California Department of Health Services, 

2002). The high boron concentration in well 61, as well 

as a strong hydrogen sulfide odor noted at the time of 

sampling and the overall chemical composition of the 

sample, indicates that the chemical composition of 

water in this well is similar to the composition of the 

samples collected in 1950 from two wells in the same 

area (Kunkel and Upson, 1960). Water from wells 

identified as State well numbers 5N/3W-6N2 and -6P1 

had boron concentrations in 1950 of 18 and 8 mg/L, 

respectively (Kunkel and Upson, 1960). Wells 5N/3W6N2 and -6P1 were of comparable depth to well 61 

(completed depth 260 ft) at 205 and 285 ft, 

respectively, suggesting that aquifers underlying the 

diatomaceous deposits of the Sonoma Volcanics in this 

particular area yield poor quality water.

Figure 27.—Continued.
50 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

The concentration of arsenic in samples from 

wells 29, 36, 43, 45 and 76 ranged from 11 to 67 μg/L. 

These concentrations exceeded the primary maximum 

contaminant level (MCL) of 10 μg/L (U.S. 

Environmental Protection Agency, 2002). Wells 43, 45, 

and 76 are located in the northwestern part of the study 

area near the Soda Creek Fault, where deep up-flow of 

ground water along the fault may account for the 

relatively high concentration of arsenic in these well 

samples compared with that in other well samples from 

the study area. 

Samples from several wells contained dissolved 

iron and manganese in concentrations exceeding the 

secondary Federal and State MCL of 300 μg/L for iron 

and 50 μg/L for manganese (U.S. Environmental 

Protection Agency, 2002; California Department of 

Health Services, 2003).

Variations in ground-water composition within 

the study area can be summarized as follows: 

1. Ground water from the perimeter of the study area, 

nearest recharge areas in the Howell Mountains, 

generally was less mineralized than ground water 

closer to the central part of the study area. Surfacewater and ground-water samples from the 

perimeter of the area can be characterized as 

calcium-magnesium bicarbonate composition, 

low dissolved solids, and relatively high silica 

concentrations compared with samples from other 

parts of the study area. 

2. Ground water in the southeastern part of the lower 

Tulucay drainage basin contains higher 

percentages of calcium and magnesium (greater 

hardness), slightly lower percentages of 

bicarbonate, and intermediate dissolved solids, 

compared with ground water in other parts of the 

study area.

3. Ground water in the central part of the study area, 

centered near the Cup and Saucer area, is 

characterized by higher dissolved-solids 

concentrations, a higher percentage of sodium, a 

higher percentage of chloride or sulfate, and 

higher concentrations of boron than ground water 

from areas closer to the Howell Mountains. 

Ground water from three wells (14, 61, 100), 

ranging in total depth from 228 to 260 ft, have the 

highest dissolved solids (all greater than 400 

mg/L) and highest chloride concentrations (54 to 

175 mg/L). These three wells are all located within 

about 0.5 mi of Coombsville Road and Fourth 

Avenue. 

4. Ground water having high arsenic concentrations 

may be related to deep upward circulation of 

ground water along the Soda Creek Fault. Water 

samples with the highest arsenic concentrations 

(16 to 67 μg/L) came from three wells located 

within 0.5 mi of the Soda Creek Fault in the 

northwest part of the area.

5. Water in contact with volcanic rocks often has high 

concentrations of dissolved boron, iron, and 

manganese (Hem, 1985). The volcanic rocks that 

underlie the study area are probably the source of 

the observed high concentration of the 

constituents.

Oxygen-18 and Deuterium

Water samples were collected from selected sites 

in the study area for analysis of oxygen-18 (18O) and 

deuterium (2H). Oxygen-18 and deuterium data can 

provide information on the source and movement of 

ground water.

Background

Oxygen-18 and deuterium are naturally 

occurring stable isotopes of oxygen and hydrogen. The 

abundance of oxygen-18 and deuterium relative to 

lighter oxygen-16 (16O) and hydrogen (1H) atoms can 

be used to help infer the source and the evaporative 

history of water. Oxygen-18 and deuterium abundances 

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):

and

δ

18O

O 18 O 16 ( ) ⁄ sample

O 18 O 16 ( ) ⁄ VSMOW

= --------------------------------------------------- – 1 × 1 000 ,

δD

H

2 H1 ( ) ⁄ sample

H

2 H1 ( ) ⁄ VSMOW

= --------------------------------------------- – 1 × 1 000 ,
Surface-Water and Ground-Water Quality 51

Because the source of much of the world's 

precipitation is derived from the evaporation of 

seawater, the δ18O and δD composition of precipitation 

throughout the world cluster along a line known as the 

global meteoric water line (Craig, 1961)

Differences in the isotopic composition of 

precipitation occur along this line if water vapor 

originated from evaporation of cooler or warmer 

seawater (Gat and Gonfiantini, 1981). Storms that 

originate over the 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. Differences also occur as the result of moist air 

masses moving over land; as storms move inland from 

coastal areas, the concentration of heavier isotopes 

relative to lighter isotopes decreases as water 

molecules repeatedly undergo evaporation and 

condensation. In addition, precipitation that condenses 

at high altitudes and at cool temperatures tends to be 

isotopically lighter than precipitation that forms at low 

altitudes and warm temperatures (Muir and Coplen, 

1981). Water that has not undergone evaporation will 

plot near the global meteoric water line.

Stable Isotope Results

The δ18O and δD values for the entire suite of 

water samples ranged from +4.04 to −7.95 per mil and 

−3.04 to −53.23 per mil, respectively (table 9, at end of 

report). These values plot on either side of and along 

the global meteoric water line (fig. 27A). Waters 

affected by evaporation plot to the right of the meteoric 

water line; the sample from a small artificial 

impoundment, designated as Van Koten pond for the 

purposes of this report, has the isotopically heaviest 

(least negative) water (δ18O and δD values, +4.04 and 

−3.04 per mil, respectively) and exemplifies the effect 

of evaporation. The sample from well 61, located in 

ground-water storage unit 1, was the isotopically 

lightest (most negative) water sampled (δ18O and δD 

values of −7.95 and −53.23 per mil, respectively). 

Figure 27B is an enlargement of part of the plot shown 

in figure 27A and shows in more detail the distribution 

of data for all samples except the three lake or pond 

samples that are most strongly affected by evaporation.

The δ18O and δD composition of surface-water 

samples from creeks in the lower Milliken, Sarco, and 

Tulucay drainage basins ranged from −5.11 to −7.62 

per mil for δ18O and from −37.51 to −50.54 per mil for 

δD (fig. 27C, table 9, at end of report). In contrast, the 

average δ18O and δD composition for the Napa River 

near Napa (Station number 1145800; fig. 1) for 1985-

87 was −5.64 and −37.32, respectively (Coplen and 

Kendall, 2000). The average isotopic composition of 

the Napa River is representative of the average for 

streams in the part of Napa Valley (about 218 mi2) 

upstream of station 1145800 and is heavier than the 

composition of nearly all the creek samples collected in 

the study area (fig. 27B). The creek water samples, 

excluding those from Milliken Creek, are composed of 

recently discharged ground water and seepage of 

shallow soil water (they were collected long after any 

significant rainfall). Milliken Creek is fed by controlled 

releases from Milliken Reservoir, therefore the water 

samples from this stream can contain large fractions of 

water derived from precipitation and surface-water 

runoff.

Seasonal variations in the isotopic composition 

of creek water between March 2001 and September 

2001 are illustrated in figure 27C. The isotopic 

composition of creek samples from March, April, and 

September 2001 became progressively heavier due to 

higher air and water temperatures that result in greater 

losses of lighter isotopes through evaporation. The 

isotopic composition of creek water was progressively 

lighter (more negative) from west (range 4W) to east 

(range 3W) (fig. 27D), probably because the higher 

altitude in the eastern part of the study area results in 

fractionation of the isotopes in rainfall. Evaporation 

from creeks, however, can cause isotopic 

concentrations to become heavier as water flows from 

the headwaters to locations in the western part of the 

study area.

δD δ

18 = O + 10.
52 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

The δ18O and δD composition of the samples 

from the lakes, ponds, and springs showed a wider 

variation of isotopic composition than the samples 

from the creeks, ranging from +4.04 to −7.10 per mil 

for δ18O and −3.04 to −46.90 per mil for δD (table 9, at 

end of report). Three of four lake and pond samples 

collected in September 2001 had the heaviest isotopic 

composition of any sample collected and analyzed. 

Samples from lower Lake Camille (LLC), located in 

the lower Tulucay Creek drainage basin, and from two 

private artificial ponds, Maher pond (MP), located in 

the lower Tulucay Creek drainage basin, and Van Koten 

pond (VKP), located in the Sarco Creek drainage basin, 

define an evaporative trend line (fig. 27A). The isotopic 

composition of the fourth sample collected in 

September 2001 from a pond at the Silverado Country 

Club (SCCP) plots slightly to the right of the global 

meteoric water line but within the same area of fig. 27B 

where most of the creek and ground-water samples 

plot. This sample most likely represents ground water 

pumped to fill and maintain the Silverado Country 

Club pond.

Four samples collected in January 2002 from 

three lakes, located east of the surface-water divide in 

the Howell Mountains, plot to right of the global 

meteoric water line (fig. 27E). The three samples from 

Lake Madigan and Lake Frey plot along the 

evaporative trend line and have an isotopic composition 

similar to the samples collected from Milliken Creek in 

September 2001. The fourth sample collected from 

Lake Leona, had an isotopic composition similar to 

most of the creek and ground-water samples, indicating 

that the water in this lake was relatively unaffected by 

evaporation compared with water from other sites, 

perhaps owing to recent runoff or ground-water inflow. 

Samples collected from Oak Canyon spring and 

Palmaz spring had a similar composition and plot close 

to the global meteoric water line (fig. 27B). The 

samples from Leona Lakes, Oak Canyon spring, and 

Palmaz spring probably are the most representative of 

the isotopic composition of ground-water recharge to 

the study area that originates in the Howell Mountains.

0

100

200

300

400

500

600

1940

1950 1960 1970 1980 1990 2000

YEAR

CHLOROFLUOROCARBON CONCENTRATIONS,

IN PARTS PER TRILLION BY VOLUME

CFC-12

CFC-11

CFC-113

Figure 28. Concentrations of chlorofluorocarbons CFC-11, CFC-12, and CFC-113 in North American air. 

(Modified from Plummer and Busenberg, U.S. Geological Survey, written commun., 2002)
Surface-Water and Ground-Water Quality 53

The δ18O and δD compositions of ground-water 

samples collected from wells in the study area 

generally were similar to the isotopic composition of 

the samples collected from creeks in spring 2001, 

ranging from −6.12 to −7.95 per mil for δ18O and 

−40.6 to −53.23 per mil for δD (fig. 27B; table 9, at end 

of report). The isotope samples from wells can be 

divided into two groups based on where they plot in 

figure 27E. Group I includes water samples from seven 

wells having δ18O and δD greater than −7 and −45 per 

mil, respectively. The seven wells are distributed 

throughout the study area: three in ground-water 

storage unit 1 (19, 36, and 100), one in ground-water 

storage unit 2 (29), two in ground-water storage unit 3 

(45 and 142), and one in ground-water storage unit 4 

(76). The water from well 36 had the heaviest isotopic 

composition of all ground-water samples possibly 

because of its proximity to Lake Camille, which may 

be a recharge source. The group I samples generally 

plot between the samples that are most representative 

of precipitation that recharges ground water in the 

Howell Mountains (samples from Leona Lakes, Palmaz 

spring, Dairy Creek, and Oak Canyon spring) and 

samples that were affected by evaporation (samples 

from Murphy and Milliken Creeks and from Lakes 

Madigan and Lake Frey) (fig. 27E): the samples 

probably represent mixing of infiltrated precipitation 

and partly evaporated water in lakes in the Howell 

Mountains.

Group II consists of water samples from eight 

wells having δ18O and δD less than −7 and −47 per mil, 

respectively; samples are lighter than the group I 

samples. The eight wells in group II are distributed 

among three of the four ground-water storage units—

three in storage unit 1 (14, 22, and 61), one in storage 

unit 2 (53), and four in storage unit 3 (43, 67, 92, and 

95). Samples from five of the six wells (14, 43, 53, 61, 

and 95) having the group-II isotopic composition had a 

group-3 water-chemistry characterized by sodiumchloride type water and high dissolved solids. Water 

from well 61 had the lightest (most negative) isotopic 

concentration and the greatest dissolved-solids 

concentration of any sample. The relatively light 

isotopic composition of the Group II samples, 

compared with that of the Group I samples, may be due 

to a mixture of water in creeks during the winter and 

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

10 15 20 25 30 35 40 45 50

30

0

5

25

10

15

0

5

10

15

20

20

NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER

ARGON CONCENTRATION, IN MILLIGRAMS PER LITER

Excess air, in cubic centimeters

Recharge temperature, in degrees Celsius

53

61

19

29

142 76

95 100

22

92

43

14

45 36

67

Well and number

Figure 29. Comparison of argon and nitrogen concentrations in ground-water samples from wells and calculated water temperatures at the time of 

recharge, lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.
54 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

water recharged in the Howell Mountains or from a 

mixture of recent and older recharge due to 

precipitation during a cooler and wetter climatic 

period. Over time, the older recharge water could have 

become mineralized due to water-rock interaction or 

this older mineralized water may have originated from 

deeper ground water, possibly including geothermal 

sources. 

Ground-Water Age Dating

Water samples were collected from 15 wells and 

analyzed for dissolved gases and chlorofluorocarbons 

(CFCs). Six of those wells also were sampled for 

tritium (3H), the radioactive isotope of hydrogen. 

Concentrations of CFCs and tritium can provide 

information on the ages of the water samples, which 

can be used to infer rates of ground-water flow. 

Ground-water age, when based on measurements of the 

concentrations of a chemical or isotope, refers to the 

time elapsed since the water containing the chemical or 

isotope was recharged and isolated from the 

atmosphere. Age dating ground water requires 

matching the concentrations of a chemical or isotope in 

the water to the historical atmospheric concentrations 

of the substances. Historical atmospheric 

concentrations of both CFCs and tritium are well 

documented (Plummer and Busenberg, 2000). 

Concentrations of dissolved gases were used to 

interpret the results of CFC analyses by providing data 

to estimate recharge temperatures, quantities of excess 

air, and redox conditions.

Chlorofluorocarbons

Chlorofluorocarbons CFC-11 (CFCl3), CFC-12 

(CF2Cl2), and CFC-113 (C2F3Cl3) are stable synthetic 

organic compounds that were first produced in the 

1930s for use as refrigerants, aerosol propellants, 

cleaning agents, solvents, and blowing agents for foam 

rubber and plastics (Busenberg and Plummer, 1992). 

CFCs provide excellent tracers and dating tools for 

modern water (0- to 50-year time scale). The analytical 

detection limit of CFC-12, CFC-11, and CFC-113 in 

water is about 0.3 picogram per kilogram (pg/kg) of 

water, corresponding to water recharged in 

approximately 1941, 1947, and 1955, respectively. 

Atmospheric CFC concentrations have increased 

steadily over time with CFC-11 and CFC-113 peaking 

in North America during the early 1990s (Niwot Ridge, 

Colorado) at values of about 275 and 85 parts per 

trillion by volume (pptv), respectively (fig. 28) 

(Plummer and Busenberg, 2000). CFC-12 may have 

reached its peak concentration in North America within 

the last several years (prior to 2002) at about 545 pptv. 

Once in the atmosphere, CFCs undergo equilibrium 

partitioning with water vapor and become incorporated 

into the hydrologic cycle through precipitation.

The accuracy of CFC-determined ground-water 

ages can be limited by chemical and physical processes 

including CFC exchange between recharge water and 

the atmosphere, degradation and sorption during transit 

along ground-water flow paths, and ground-water 

mixing (Plummer and Busenberg, 2000). Local pointsource CFC contamination, for example from septic 

tanks or leaking sewer lines, also may affect the 

accuracy of CFC-determined ages. Environmental 

contamination usually is indicated by large differences 

between CFC-11, CFC-12, and CFC-113 

concentrations or CFC concentrations that are greater 

than that possible for atmospheric concentrations. If 

CFC contamination occurs, CFC-determined ages will 

be interpreted as younger than the actual ages. Youngage bias of CFC-determined ages also can result if the 

recharge temperature and (or) recharge elevation are 

overestimated and if excess air (air entrapped in 

recharge water and transported into the saturated zone) 

is not accounted for in the age interpretation (Plummer 

and Busenberg, 2000). In contrast, CFC-determined 

ages will be interpreted as older in water from geologic 

formations that have thick unsaturated zones (greater 

than about 100 ft) because air in deep unsaturated 

zones tends to be older and therefore has lower CFC 

concentrations than the present-day atmosphere. Oldage bias of CFC-determined ages also can result from 

underestimation of the recharge temperature and (or) 

the recharge elevation, as well as from microbial 

degradation, particularly in anaerobic (oxygen-poor), 

sulfate-reducing, or methane rich environments 

(Plummer and Busenberg, 2000). Because of the 

difficulty in accounting for all of the physical and 

chemical processes that affect the concentrations of 

CFCs in ground water, age is usually referred to as 

"model" or "apparent" age.
Surface-Water and Ground-Water Quality 55

Calculation of CFC-model ages requires 

estimating water temperature at the time of recharge 

and determining the amount of excess air in samples. 

The water temperature at the time of recharge is needed 

because the solubility of gases varies as a function of 

temperature and atmospheric partial pressures 

(Busenberg and Plummer, 1992). Excess air is 

important because high concentrations, common in 

fractured rock aquifers, may result in calculated CFC 

ages younger than actual ages if not taken into account 

(Busenberg and Plummer, 1992). Water temperature at 

the time of recharge can be estimated from ratios of 

nitrogen (N2) and argon (Ar) corrected for excess air. 

Calculated recharge temperatures, based on nitrogen 

and argon-gas analyses of samples from 15 wells in the 

study area, range from 10.2 to 90.7 degrees Celsius 

(°C) (fig. 29; table 10, at end of report). The high 

recharge temperature (90.7°C), combined with a high 

excess air value of almost 25 cubic centimeters per liter 

(cm3/L) of solution in the sample from well 61 is 

attributable to degassing, specifically the loss of 

nitrogen and argon, from ground water in the presence 

of high methane (14.7 mg/L) and low oxygen (less than 

0.1 mg/L) concentrations. Excluding the sample from 

well 61, the mean recharge temperature for the 

remaining 14 well samples is 15.7°C. This value agrees 

well with the 30-year (1961–90) average air 

temperature for the Napa area of 14.7°C (National 

Oceanic and Atmospheric Administration, 2002). 

Figure 30. Tritium concentrations in samples from six wells and tritium concentrations expected in ground water in southeastern Napa County, California, 

that originated as precipitation between 1953 and 2000, southeastern Napa County, California.

0

10

20

30

40

50

60

70

80

90

1950 1960 1970 1980 1990 2000

YEAR PRECIPITATION ENTERED GROUND WATER AS RECHARGE

TRITIUM, IN TRITIUM UNITS

2010

100

Data range for six wells in study area

Tritium in samples from

six wells in study area

Expected tritium concentration
56 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Figure 31. Temperature logs for selected wells in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California.

(bls, below land surface)
Surface-Water and Ground-Water Quality 57

Table 10 (at end of report) contains ranges of 

recharge dates and CFC-model ages. The ages are the 

median ground-water ages that represent the 

differences between the midpoints of the recharge 

ranges and 2001, the year of sample collection. The 

ages correspond to recharge in the 1950s through the 

1980s for all well samples. The samples representing 

the youngest waters, based on CFC-model ages, are 

from wells 29 and 53 located in ground-water storage 

unit 2; water from these wells have median ages of 21 

and 15 years, respectively. Three of four water samples 

representing median ages between 25 and 35 years old 

are from wells located near creeks in ground-water 

storage units 3 and 4 (wells 45, 76, and 92). The 

younger age of these samples may be due to infiltration 

of modern surface water. Most of the samples (nine) 

have median ages between 40 and 50 years, including 

the samples from five of six wells in ground-water 

storage unit 1 (14, 19, 36, 61, and 100). The CFCmodel ages for the samples from the shallow and the 

deep wells are similar, supporting the conclusion that 

most recharge originates in the hills on the east side of 

the study area and flows laterally into shallow and 

deeper permeable layers (fig. 9). Strong vertical mixing 

of ground water or small contributions of water from 

wells from depths below 350 ft may explain the lack of 

clear age differences between the deep and the shallow 

wells. The young age dates for the samples from wells 

29 and 53, both of which are in storage unit 2, suggest 

that the hills underlain by tuff in the Cup and Saucer 

area may be a catchment area for some local recharge.

Age dates based on CFC concentrations should 

be used with caution due to the possibility of local 

contamination of ground water by CFCs or of 

modification of CFC concentrations in the aquifer by 

geochemical, biological, or hydrologic processes 

(Busenberg and Plummer, 1992). Samples from only 

seven wells (14, 36, 43, 61, 67, 95, and 100) had fairly 

consistent age dates (varying by 10 years or less) for all 

three CFC compounds and for the three samples from 

each well. The median age dates of the samples from 

these seven wells are between 1951 and 1961, but 

comments from the USGS CFC Laboratory caution 

that the samples from wells 14, 36, 61, and 100, as 

wells as the samples from wells 45, 76, and 142, may 

have degraded, degassed, or been contaminated by 

local sources of CFCs. For example, the age date of the 

sample from well 61 may be older than its actual age 

because of CFC losses caused by degassing and (or) 

chemical degradation under reducing conditions. 

Another example is the age date of the samples from 

well 76, which contained CFC-12 in concentrations 

greatly exceeding the historical atmospheric 

concentrations of this compound; this suggests possible 

contamination from a local source. 

Tritium

Tritium (3H) is the radioactive isotope of 

hydrogen; it has a half-life of 12.43 years. Tritium, 

because of its short half-life, is useful for determining 

the age of water that generally is less than 50 years old. 

Tritium activity is measured in disintegrations per unit 

of time and is commonly reported in tritium units (TU); 

each tritium unit equals one 3H atom in 1018 atoms of 

hydrogen. Approximately 800 kg of 3H were released 

as a result of atmospheric testing of nuclear weapons 

during 1952–62 (Michel, 1976). As a result, 3H 

concentrations in precipitation and in ground water 

recharged during that time increased. Because 3H is 

part of the water molecule and its concentration is not 

affected significantly by reactions other than 

radioactive decay, 3H is an excellent tracer of the 

movement of water on time scales ranging from 0 to 50 

years before present (2002).

Tritium can be used alone to determine the age of 

water, but in some cases this yields a non-unique value. 

A specific age can be determined if the amounts of 3H 

and its radiogenic daughter, helium (3He), are 

quantified (Clark and Fritz, 1997). Samples were 

collected from six wells with the intention of using the 

tritium-helium method to determine recent groundwater ages. However, results from analysis of dissolved 

helium (table 11, at end of report) indicated that this 

method may not yield reliable tritium-based ages 

because of the presence of helium [derived from the 

Earth's crust and mantle (terrigenic helium) ] that 

greatly exceeded concentrations derived from 

thermonuclear tritium. Elevated He concentrations and 

3He/4He ratios are common in some areas underlain by 

volcanic rocks or near fault zones; therefore, six 

samples were analyzed for tritium alone to provide 

qualitative estimates of recent ground-water ages.

Tritium concentrations expected in ground water 

in the Napa area, the source of which was precipitation 

between 1953 and 2000, are shown in figure 30. These 

expected concentrations were calculated by 

determining the radioactive decay of tritium between 

the time that the precipitation fell and the time that the 

ground water was sampled. Historical tritium 
58 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

concentrations were based on measured concentrations 

at several stations in North America and correlations 

with tritium concentrations in precipitation at Ottawa, 

Canada, and Vienna, Austria (Robert Michel, U.S. 

Geological Survey, written commun., 2002). Given a 

half-life of 12.43 years, ground-water samples 

collected in 2001 should have tritium levels of 0.6 TU 

if they were recharged entirely in 1952. A more recent 

recharge date (after 1952) would yield higher tritium 

levels; whereas, an older recharge date would yield 

lower levels (less than 0.6 TU). For this study, ground 

water that had 3H concentrations less than the detection 

limit of 0.3 TU was interpreted as water recharged 

prior to 1952; ground water that had detectable levels 

3H was interpreted as water recharged after 1952.

Concentrations of tritium ranged from 0.25 to 

2.12 TU in six of the ground-water samples (table 10). 

The concentrations in these samples are consistent with 

that for water recharged prior to 1956 (fig. 30), 

assuming the sampled waters are not mixtures of 

waters of two or more ages. Tritium concentrations 

ranged from 0.25 to 0.50 TU in samples collected from 

three shallow wells (22, 36, and 95), all less than 200 ft 

deep. Tritium concentrations ranged from 1.12 to 2.12 

TU for three deeper wells (29, 76, and 142), all greater 

than or equal to 350 ft in depth. The three shallow wells 

are perforated in fractured volcanic rocks and volcanic 

tuff. The low concentrations of tritium in water from 

these three wells may indicate that the water is 

relatively old and unmixed compared with water from 

the three deep wells, possibly owing to longer travel 

paths or slow movement through geologic units having 

low hydraulic conductivity. The higher concentrations 

of tritium in the three deep wells may be the result of 

the mixing of younger ground water with older ground 

water. The deep wells have long perforated sections in 

volcanic sediments or alluvium. Wells 76 and 142 are 

located less than 0.25 mi from creeks, and well 29 is 

located in an area that may receive local runoff from 

surrounding hills, which may be the sources of the 

recent recharge to these wells. Tritium ages for water 

from wells 36, 95, and 142 are in fair to good 

agreement with the CFC-model ages of the early 1960s 

or earlier.

Water Temperature 

Ground-water temperatures were measured at the 

land surface in water samples pumped from 15 wells; 

water-temperature logs were made from the water 

surface to the bottom of 11 other wells. The sample 

temperature measurements were made after several 

minutes of pumping, when temperatures were stable 

and measurements were within 0.5°C. The temperature 

logs were made using a precision thermistor suspended 

on a calibrated 4-conductor cable. The number and 

spacing of measurements were chosen on the basis of 

the total depth of the water-filled section of the well 

and the rate of temperature change with depth. 

Resistance at discrete depths was recorded after the 

thermistor output stabilized. The recorded resistance 

was converted to temperature using a polynomial 

calibration curve. This system provides temperature 

measurements accurate to 0.1°C.

The temperatures ranged from 17.5 to 27.0°C in 

the samples pumped from wells that ranged from 93 to 

500 ft in depth. The maximum temperatures in the 11 

wells that were logged ranged from 18.5 to 30.5°C. The 

temperature logs are for depths ranging from 68 to 592 

ft; in all cases, the maximum temperature was recorded 

at the bottom depths. Temperature logs for seven wells 

are shown in figure 31. Temperature logs are not shown 

for the remaining four logged wells because they had 

short water-filled sections that had less than 3°C of 

temperature variation.

The occurrence of warm ground water in some 

areas of Napa County has been known since the 1800s. 

Kunkel and Upson (1960) cite ground-water 

temperatures of as much as 60°C in wells completed in 

the Sonoma Volcanics northwest of the study area. The 

mean annual air temperature is about 14.7°C. Shallow 

ground water typically has a temperature close to the 

mean annual air temperature. Ground-water 

temperatures rise with depth owing to the prevailing 

geothermal gradient. Todd (1980) cites an average 

geothermal gradient of 1°C per 100 ft. On the basis of a 

linear best-fit correlation of temperature as related to 

depth, the geothermal gradient in the study area is 

about 2°C per 100 ft from a base temperature of about 

16°C. The local geothermal gradient is approximately 

twice the average for continental temperate zones 

worldwide.

All the wells having depths greater than 400 ft 

had water temperatures higher than 22°C. For the 22 

sites for which well depths and water temperatures 
Summary and Conclusions 59

were available, the estimated temperatures for depths 

of 500 ft below land surface, based on the bottom-hole 

temperature and a gradient of 0.02°C per foot, ranged 

from 22 to 29°C. Nineteen of the 22 sites have 

estimated temperatures greater than or equal to 25°C. 

The temperatures for the three sites that had lower 

temperatures ranged from 22 to 24°C. The 

temperatures at two of these three sites, which are 

located in the northern part of study area, probably are 

cooled by recharge from nearby perennial streams. 

Although the estimated temperatures at depths of 500 ft 

below land surface are fairly uniform throughout the 

study area, the highest temperatures (27–29°C) were 

from wells in the southern half of the study area, 

primarily in the northern and eastern parts of storage 

unit 1. The area that had the highest temperatures has 

diatomaceous sediments at the surface. The 

diatomaceous sediments have low permeability and 

restrict vertical flow. The lack of recharge may be a 

contributing factor to the high ground-water 

temperatures.

In many areas, ground water having elevated 

temperatures correlates with poor water quality. This is 

because the solubility of most common minerals 

increases with temperature. Thermal waters often 

contain trace elements such as arsenic, fluoride, and 

boron in concentrations that exceed drinking-water 

standards (Hem, 1985). No strong correlation between 

water chemistry and temperature was evident for the 15 

wells sampled in the study area.

The deepest well sampled for water quality is 

500 ft deep and is screened in shallow formations. 

Water in deep wells that were not sampled may be 

30°C or higher and the chemical quality may be 

diminished. The uniformly high water temperatures at 

depth in the study area could place a depth limitation 

on high quality ground water and limit the total amount 

of useable ground water in storage.

Temperature logs for wells 5N/3W-7G3, 5N/4W14J3, 6N/4W-22H3, and 6N/4W-23J1 show isothermal 

or near isothermal sections over lengths of 100 to 200 

ft. These isothermal sections are likely caused by 

lateral ground-water flow that masks the conductive 

temperature gradient. The isothermal sections in wells 

6N/4W-22H3 and 23J1, which are in the northern part 

of the study area, are from 150 to 350 ft and 300 to 500 

ft, respectively. The isothermal sections in wells 

5N/3W-7G3 and 5N/4W-14J3, which are in the 

southern part of the study area, are shallower (between 

depths of 50 to 200 ft). The differences in the depths of 

the isothermal sections may be due to differences in the 

depths of the geologic units that have the highest 

hydraulic conductivity; thicker and deeper in the 

northern part and thinner and shallower in the southern 

part. All four logs are consistent with a conceptual 

model of ground-water flow depicting recharge in the 

Howell Mountains, downward percolation of recharge 

water to permeable zones, and lateral flow away from 

the mountain front toward the Napa River (fig. 9).

SUMMARY AND CONCLUSIONS

The entire study area is underlain by the Sonoma 

Volcanics, a geologic formation consisting of a 

complex sequence of volcanic rocks and volcanic 

sedimentary units. The volcanic rocks are covered in 

places by thin clastic sedimentary formations. In 

general, the geologic units have low hydraulic 

conductivity, which inhibits recharge and limits well 

yields. Lithologic variations in the study area affect 

well productivity and water quality. Areas underlain by 

hard rock formations at shallow depths are more likely 

to have low well yields and to be more adversely 

affected by declining water levels (common in storage 

units 1, 2, and 3) than areas underlain by thick 

unconsolidated materials (typical in storage unit 4).

The predominant source of local recharge to the 

ground-water system is precipitation falling within the 

Milliken, Sarco, and Tulucay Creeks drainage basins; 

however, a small amount of the recharge may be from 

infiltration of water piped into the area from sources 

external to the drainage basins and from local sources 

including septic tank leakage. The principal source of 

ground-water replenishment to the study area is lateral 

flow of ground water that is recharged in the Howell 

Mountains to the east of the study area. Additional 

ground water enters the study area by inflow through 

the permeable alluvium along parts of the northwestern 

boundary of the study area. The amount of inflow 

varies depending on the local distribution of hydraulic 

heads. In 2000, ground-water outflow across the 

western boundary exceeded inflow by an estimated 600 

acre-ft/yr. Precipitation over the entire Milliken, Sarco, 

and Tulucay Creeks drainage basins totals an estimated 

69,000 acre-ft/yr. Nearly half of this amount (29,000 

acre-ft/yr) flows out of the study area as surface-water 

discharge to the Napa River. Evapotranspiration from 

the drainage basins is estimated to be about 34,000 
60 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

acre-ft/yr. This leaves 6,000 acre-ft/yr residual, but 

because of the uncertainity in estimates of 

precipitation, runoff, and evapotranspiration, this value 

is not a precise estimate of potential ground-water 

recharge and should not be construed as the safe yield 

for the study area.

Population and agricultural irrigation have 

increased substantially in the study area since 1975. An 

estimated 16,500 people lived in the area at the time of 

the 2000 census, an increase of 21 percent since 1975. 

About 4,800 people living in the area rely solely on 

ground water from individual private wells. Vineyards 

are the predominant use of agricultural land; in 2000, 

about 2,400 acres were cultivated. Increases in 

population and in irrigation for grape production in the 

past few decades have increased water demand. 

Ground-water pumpage was estimated to be about 

3,000 acre-ft/yr in 1975; by 2000, it was about 5,350 

acre-ft/yr, an increase of about 80 percent but could be 

as much as 7,100 acre-ft/yr. Ground-water pumpage for 

domestic use, improved open-space irrigation, and 

agriculture was estimated to be about 27, 29, and 43 

percent of the total pumpage, respectively.

Ground-water pumping has produced dramatic 

changes in hydraulic heads within the study area. 

Under pre-pumping conditions, hydraulic heads 

probably were highest around the perimeter of the area, 

and ground water generally flowed westward toward 

the Napa River from recharge areas in the mountains 

on the north, east, and south sides. Under present-day 

conditions, ground-water pumping has reduced 

hydraulic heads sufficiently to form three large groundwater level depressions within the study area. Under 

current (2001–2002) conditions, ground water flows 

from the perimeter predominantly toward the hydraulic 

depressions. A comparison of maps of hydraulic heads 

in 1975 and in 2001 showed that water levels increased 

in much of the western part of the area but declined by 

50 to 125 ft in pumping depressions in the central and 

eastern parts of the study area These two pumping 

depressions coincide with the areas having the highest 

density of active or potentially active wells in study 

area. Water levels declined by 25 to 50 ft in a third 

pumping depression, which is located in the 

northwestern part of the study area. Long-term 

hydrographs for wells in the study area indicate that the 

greatest rate of decline occurred after the early 1970s 

and coincides with an increase in the number of wells 

drilled in the study area. Declining ground-water levels 

evident over a large part of the Milliken, Sarco, and 

Tulucay Creeks area is an indication that current 

(2000–2002) ground-water use exceeds average 

ground-water replenishment.

The chemical quality of ground water in the 

study, which was based on the water samples collected 

from 15 wells, generally is acceptable. The water from 

some wells, however, contains one or more constituents 

in excess of the recommended standards for drinking 

water. Total dissolved solids ranged from 144 to 732 

mg/L; water in one sample exceeded the recommended 

standard of less than 500 mg/L. Arsenic concentrations 

ranged from less than 2 to 67 μg/L; concentrations in 

samples from five of the wells exceeded the standard of 

less than 10 μg/L. Samples from three wells located in 

the northwestern part of the study area, close to the 

Soda Creek Fault, had arsenic concentrations greater 

than 15 μg/L. The source of the arsenic may be ground 

water that circulates deeply along the fault zone. 

Samples from two wells located in the southern part of 

the study area had boron in excess of 1 mg/L. Water in 

samples from five wells had concentrations of iron that 

exceed the secondary drinking-water standard of 300 

μg/L. Water samples from 12 of the 15 wells contained 

concentrations of manganese that exceed the secondary 

drinking-water standard of 50 μg/L. The arsenic, boron, 

iron, and manganese probably are derived from 

minerals in the volcanic rocks or from the deeper and 

older rocks of the Franciscan Complex or Great Valley 

Sequence.

The observed variations in the chemical 

composition of ground water are consistent with that 

shown by the conceptual model of the study area which 

shows recharge in the Howell Mountains and lateral 

inflow around the northern, eastern, and southern parts 

of the study area. As ground water moves into and 

through the study area it reacts with and dissolves 

minerals in the rocks and sediments along the flow 

path, increasing the concentrations of some elements. 

The samples with the highest concentrations of 

dissolved solids were from wells in the central part of 

the study area in and around the Cup and Saucer area. 

Major ion compositions in samples from storage units 

1 and 3 are variable, but the ranges in variation overlap 

to a considerable degree. This suggests that recharge 

water to both areas is chemically similar and that 

similar reactions take place along ground-water flow 
Summary and Conclusions 61

paths in both areas. The lack of chemical distinctions 

between water from storage units 1 and 3 is consistent 

with the hydraulic connections between the two storage 

units, which behave as one contiguous ground-water 

reservoir.

The maximum ground-water temperatures 

measured in 15 samples pumped from wells and from 

11 temperature logs ranged from 17.5 to 30.5°C in 

wells that ranged from 68 to 592 ft deep. The 

temperature gradient for the study area was calculated 

to be about 0.02°C per foot, approximately double the 

average for continental United States. The deepest 

temperature logs suggest that temperatures at depths 

greater than 600 ft may exceed 30°C in parts of the 

area. No correlation between temperature and chemical 

composition is apparent for the 15 ground-water 

samples; but as a general principal, the solubilities of 

inorganic compounds increase with increasing 

temperatures, except for bicarbonate. On the basis of 

this general principal, ground water at depths greater 

than 600 ft in parts of the study area may contain 

objectionable concentrations of some constituents. 

Abnormally high water temperatures may limit the use 

of ground water in some parts of the study area. 

Ratios of the stable isotopes of hydrogen and 

oxygen showed a greater range in the surface-water 

samples than in the ground-water samples; this is 

because the surface-water samples were affected by 

evaporation, which concentrates the heavier isotopes. 

Samples from lakes, ponds, and Milliken Creek 

(downstream from Milliken Reservoir) collected 

during the warmest and driest part of the year had the 

heaviest isotopic ratios because of evaporation. Most of 

the other surface-water samples, including those from 

unregulated creeks and springs, plot along or close to 

the global meteoric water line. The isotopic ratios of 

the ground-water samples plot in two distinct groups (I 

and II) along the global meteoric water line. Samples in 

group I plot between the slightly lighter samples from 

springs and the slightly heavier samples from Milliken 

Creek. The waters in group I samples probably are 

mixtures of precipitation that infiltrated in the Howell 

Mountains and infiltrated surface water affected by 

evaporation. The group II samples are lighter than the 

group I samples and probably were mixtures of 

precipitation that infiltrated in the Howell Mountains 

and winter streamflow or water that was recharged 

during an earlier time when climatic conditions were 

cooler than at present. Most of the group II samples 

came from wells that also produced water with higher 

dissolved-solids concentrations than the wells in group 

I. The higher dissolved-solids concentrations are an 

indication that these waters have been in the groundwater system longer than waters with lower dissolvedsolids concentrations.

Ground-water ages based on chloroflurocarbon 

and tritium analyses ranged from 15 to about 50 years 

before present (2002). These ages, especially the CFCdetermined ages, should be used with caution because 

the actual ages may have been overestimated or 

underestimated because of difficulties in accounting for 

all the geochemical, biological, and hydrologic 

processes. Samples for age dating probably were 

mixtures of waters of different ages because most of 

the wells available for sampling for this study were 

constructed with gravel packs and long screens open to 

the geologic formation. The presence of tritium and 

CFCs in water from the study area is evidence that 

modern recharge (post 1950) does take place.

The results of this study indicate that ground 

water is being depleted under current pumping and 

recharge conditions. To achieve a hydrologic balance 

that stops ground-water level declines, it would be 

necessary to supply additional water to the area or to 

decrease the amount of ground-water withdrawn. If 

ground-water levels continue to decline, especially near 

the western boundary of the area, additional ground 

water may flow into the area from the main part of 

Napa Valley. However, increasing ground-water inflow 

from the west probably would reduce future water-level 

declines in the study area but would not increase heads 

in the main pumping depressions along Hagen Road 

and in the Coombsville area.

The area of this current (2002) study was 

subdivided by Johnson (1977) into four ground-water 

storage units on the basis of topography and geology. 

Results of this current study found evidence that 

storage units 1 and 3 are not hydraulically separate. 

The low hydraulic conductivity of most of the geologic 

units in the study area greatly restrict the feasibility of 

artificial recharge through wells or from surface 

retention facilities. Encouraging reductions in groundwater pumping by supplying imported water or 

reclaimed water to users in and near the pumping 

depressions might hold the greatest promise of 

reducing or reversing ground-water level declines. 

Small improvements to the hydrologic balance may be 

made by increasing recharge along streams by building 

a number of retention dams, directing water from 

agricultural subdrains to infiltration ponds rather than 
62 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

to storm drains or streams, or increasing the use of 

best-management practices for irrigation of agricultural 

lands.

Hydrologic monitoring cannot improve the water 

balance of an area, but data from monitoring programs 

can be used to better understand how a particular 

hydrologic system functions and to determine changes 

that take place in the system in response to waterresource use. The long-term records of ground-water 

levels in the study area were critical for analysis of 

present-day conditions and changes that have happened 

over the past few decades. Likewise, land-use, surfacewater discharge, and climatological data are critical for 

understanding the water balance.

Most wells in the study area were constructed 

with gravel packs and long screens open to the geologic 

formation to maximize well yield. This type of well 

construction is not optimal for ground-water 

monitoring because depth-dependent differences in 

hydraulic head, water chemistry, and ground-water age 

cannot be distinguished. Future monitoring of the 

ground-water system can be improved by using wells 

perforated at short depth intervals, including the 

shallow wells (100 to 200 ft); intermediate depth wells 

(greater than 200 to 400 ft); and deep wells (greater 

than 400 ft). Because locating existing supply wells 

with these characteristics may not be possible, it would 

be necessary to drill new wells specifically for 

monitoring. The new monitoring wells can be drilled 

and constructed to tap and isolate different depth zones 

to provide information on vertical variations in 

hydraulic head and water quality.

A continuation of the semi-annual water-level 

measurements in the current (2002) network of about 

10 wells may provide enough data to track changes in 

the ground-water system over periods of a few years. 

The timing of the annual maximum and minimum 

water levels in any given year varies depending on the 

timing and quantity of precipitation and pumping 

during the antecedent period. Data from the use of 

continuous water-level monitoring at some wells may 

help to accurately identify seasonal fluctuations in 

water levels. To prepare maps of hydraulic heads would 

require a larger number of monitoring wells. Groundwater level measurements made on a 5-year cycle in a 

network similar to that used for this and earlier studies 

(consisting of more than 100 wells) would provide 

sufficient data for constructing hydraulic-head maps 

and water-level change maps. These maps could be 

used to determine the effectiveness of waterconservation programs or of the use of imported water 

for reducing local ground-water use in parts of the 

study area.

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U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 9, chap. A4, at 

http://pubs.water.usgs.gov/twri9A4

Youngs, L.G., Chapman, R.H., and Chase, G.W., 1989, 

Complete Bouguer gravity map of the Napa Valley area, 

Napa and Sonoma Counties, California: California 

Division of Mines and Geology Open-File Report 89-3, 

scale 1:48,000.
Tables 65

TABLES
66 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Land-use category

Total acreage inventoried

Percent

change 1958 Percent of 

total 2000 Percent of total

Urban (residential, commercial, and industrial) 2,252 20 3,567 31 +58

Improved open space (golf course, cemetery) 120 1 391 4 +226

Irrigated agriculture with residential (vineyard) 586 5 2,369 21 +304

Non-irrigated agriculture with residential (grassland, pasture) 1,340 12 388 3 –71

Unimproved open space with residential 7,023 62 4,562 40 –35

Surface water (ponds, lakes) 30 <1 74 <1 +147

Total 11,351 100 11,351 100

Table 1. Summary of land-use change in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, 1958–2000

[See figure 2 for distribution of land use in 2000. <, actual value is less than value shown]
Tables 67

See footnotes at end of table.

Table 2. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 17–26, 2000—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station 

identifier Location (ft3/s) (acre-ft/d)

Milliken Creek Drainage Basin—Silverado Trail tributary

36 2200 block of Silverado Trail, 0.7 mi 

north of West Trancas Road (near old 

gage site 11458120) < 0.5 < 0.1 estimated poor na na na

Milliken Creek Drainage Basin—Atlas Peak Road tributary

30 Atlas Peak Road, 0.6 mi north of Westgate 

Drive 0.11 0.2 float poor na na na

29 Atlas Peak Road 0.04 0.1 volumetric poor –0.1 0.6 –0.2

Milliken Creek Drainage Basin—Silverado Country Club tributary

5 East of Hillcrest Drive near Westgate 

Drive 0.02 < 0.1 float poor na na na

6A West of Hillcrest Drive near Westgate 

Drive 1 0.11 1 0.2 flume/float good na < 0.1 na

Milliken Creek Drainage Basin—Hillcrest Drive tributary

3 2100 block Monticello Road 0.00 0.00 observed na na na na

4 Silverado Country Club, east of Hillcrest 

Drive, near St. Andrews Drive 0.04 0.1 flume good +0.1 0.4 +0.2

Milliken Creek Drainage Basin—Milliken Creek main stem

31 0.5 mi northeast of Westgate Drive 6.67 13.2 pygmy fair na na na

32 0.4 mi northeast of Westgate Drive 6.99 13.8 pygmy fair +0.6 0.1 +6.0

34 0.2 mi northeast of Westgate Drive 7.07 14.0 pygmy good +0.2 0.2 +1.0

33 Westgate Drive bridge 6.76 13.4 pygmy good –0.6 0.2 –3.0

38 Altas Peak Road 8.53 16.9 pygmy fair +3.5 1.2 +2.9

37 Hedgeside Avenue (old gage site 

11458100) 8.19 16.2 pygmy good –0.7 0.4 –1.8

35 West Trancas Road bridge 9.15 18.1 pygmy good +1.9 1.0 +1.9

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch, south fork

24 0.2 mi east of 2200 block Third Avenue 0.05 0.1 estimated poor na na na

52 2200 Third Avenue < 0.01 < 0.1 estimated poor 2 <–0.1 0.2 2 <–0.5

25 Unnamed tributary, west of Third Avenue 

above confluence with Third Avenue 

branch, south fork 0.05 0.1 estimated poor na na na

Below confluence 3 4 < 0.06 4 0.1 na na 2 + 0.1 0.2 2 +0.5

22A 2400 block Third Avenue 0.01 < 0.1 estimated poor 2 <–0.1 0.1 2 < –1.0

Table 2. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 17–26, 2000

[See figure 10 for station locations. Measurement rating: excellent (less than 2 percent error), good (2 percent to less than 5 percent error), fair (5 to 8 percent 

error), poor (greater than 8 percent error). ft3/s, cubic foot per second; acre-ft/d, acre-foot per day; mi, mile; (acre-ft/d)/mi, acre-foot per day per mile. <, actual 

value is less than value shown; na, not applicable]
68 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch, middle fork

23 NW 1/4, Section 5, T5N, R3W 0.00 0.0 observed na na na na

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch main stem

51 SE 1/4, Section 31, T6N, R3W 0.02 < 0.1 flume fair na na na

22B 2400 block Third Avenue < 0.05 < 0.1 estimated poor 2< +0.1 0.3 2< +0.3

22A/22B Below confluence 5 6 < 0.06 6 0.1 na na 2 +0.1 na na

26A Third Avenue and Hagen Road 7 0.12 7 0.2 flume good 2 +0.1 0.5 2 +0.2

Sarco Creek Drainage Basin—Hagen Road tributary, Napa Valley Country Club tributary

55 NW1/4, Section 6, T5N, R3W 0.00 0.0 observed na na na na

27 Above confluence with Hagen Road 

tributary main stem 0.05 0.1 flume fair +0.1 0.8 +0.1

Sarco Creek Drainage Basin—Hagen Road tributary main stem

28A NE 1/4 Section 31, T6N, R3W 0.06 0.1 flume fair na na na

26B East branch Hagen Road tributary 8 0.20 8 0.4 pygmy/flume 8 poor/fair +0.3 0.6 +0.5

26A/26B Below confluence 9 10 0.32 10 0.6 na na 2 +0.2 na na

9 Grange Hall 0.31 0.6 pgymy poor 2 0.0 0.8 2 0.0

Sarco Creek Drainage Basin—La Grande Avenue/Mt. George Avenue tributary, north branch

8A SW 1/4 Section 30, T6N, R3W 0.01 < 0.1 estimated poor na na na

Sarco Creek Drainage Basin—La Grande Avenue/Mt. George Avenue tributary main stem

7B Vichy Avenue bridge 0.18 0.4 flume poor na na na

Sarco Creek Drainage Basin - Sarco Creek main stem

2 SW 1/4 Section 19, T6N, R3W 2.14 4.2 pgymy good na na na

1 Langley Park 8.00 15.8 estimated poor 2 +11.6 0.8 2 +14.5

7A Vichy Avenue bridge, east side 2.03 4.0 pgymy good 2 –11.8 0.6 2 –19.7

10 Vichy Avenue bridge, west side 0.90 1.8 pgymy good –2.2 na na

11 Silverado Trail bridge 1.14 2.2 pygmy poor +0.4 1.4 +0.3

Tulucay Creek Drainage Basin–Murphy Creek tributary, Wild Horse Valley Road branch

14 Wild Horse Valley Road 0.04 0.1 flume good na na na

15 Mustang Road 0.06 0.1 flume good < +0.1 0.6 < +0.1

Table 2. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 17–26, 2000—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station 

identifier Location (ft3/s) (acre-ft/d)

See footnotes at end of table.
Tables 69

Tulucay Creek Drainage Basin—Murphy Creek tributary main stem

16 Mustang Road 0.38 0.8 pygmy poor na na na

50 Coombsville Road 0.30 0.6 pygmy fair –0.2 0.9 –0.2

Tulucay Creek Drainage Basin—Spencer Creek tributary main stem

12 SW 1/4, Section 16, T5N, R3W 0.10 0.2 estimated poor na na na

13 Green Valley Road 0.29 0.6 pygmy fair 2 +0.4 1.4 2 +0.3

Tulucay Creek Drainage Basin—Third Avenue tributary, west culvert

20A Above confluence with Third Avenue 

tributary main stem 0.04 0.1 flume good na na na

Tulucay Creek Drainage Basin—Third Avenue tributary main stem

18 Kirkland Road 0.05 0.1 estimated poor na na na

19 East Third Avenue 0.04 0.1 flume good 2 0.0 0.1 2 0.0

20B Above confluence with Third Avenue 

tributary west culvert 0.04 0.1 flume good 0.0 0.7 0.0

20A/20B Below confluence 11 12 0.08 12 0.2 na na 2 +0.1 na na

Tulucay Creek Drainage Basin—Kreuse Creek tributary main stem

49A Fourth Avenue 0.01 < 0.1 estimated poor na na na

48 Los Robles Drive 0.00 0.0 observed na 2 <–0.1 0.9 2 <–0.1

Tulucay Creek Drainage Basin—Marie Creek tributary, Penny Lane branch

40 Penny Lane 0.02 < 0.1 flume fair na na na

44A Shurtleff Park above confluence with 

Marie Creek tributary 0.10 0.2 pygmy poor +0.1 0.5 +0.2

Tulucay Creek Drainage Basin—Marie Creek tributary main stem

46A SW 1/4, Section 18, T5N, R3W 0.30 0.6 pygmy poor na na na

42A Lower Lake Camille, west spillway 0.15 0.3 pygmy poor 13 na na 13 na

43A Shurtleff Park above confluence with 

Penny Lane branch 0.01 < 0.1 estimated poor 2 –0.2 0.8 2 –0.2

44A/43A Below confluence 14 15 0.11 15 0.2 na na 2 +0.1 na na

39 Terrace Drive 0.00 0.0 observed na 2 –0.2 0.4 2 < –0.5

Table 2. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 17–26, 2000—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station 

identifier Location (ft3/s) (acre-ft/d)

See footnotes at end of table.
70 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Tulucay Creek Drainage Basin—Tulucay Creek main stem

17 Fourth Avenue 0.66 1.3 pgymy good na na na

45 Shurtleff Avenue. 1.04 2.1 pygmy fair +0.8 1.4 +0.6

41 Tulucay Creek at Napa (discontinued 

stream gage 11458350) 2.08 4.1 pygmy good +2.0 0.7 +2.9

Table 2. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 17–26, 2000—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station 

identifier Location (ft3/s) (acre-ft/d)

1 Summation of two small culverts; (A) main east–west channel from site 5, and (B) secondary north–south culvert parallel to Hillcrest Drive. 2 Estimated.

3 Below confluence of unnamed tributary west of Third Avenue and south fork of Third Avenue branch of Hagen Road tributary to Sarco Creek. 4 Summation of sites 52 and 25 streamflow estimates.

5 Below confluence of upper and middle forks of Third Avenue branch of Hagen Road tributary to Sarco Creek. 6 Summation of sites 22A and 22B streamflow measurements/estimates.

7 Average of two streamflow measurements on two separate days (4/21/00 and 4/26/00). 8 Average of two streamflow measurements on two separate days (4/21/00 and 4/26/00) and two different methods, pygmy and flume, respectively. 9 Below confluence of main stem of Third Avenue branch of Hagen Road tributary and Hagen Road tributary main stem. 10 Summation of sites 26A and 26B streamflow measurements/estimates. 11 Below confluence of Third Avenue tributary main stem and Third Avenue tributary west culvert. 12 Summation of sites 20A and 20B streamflow measurements. 13Not comparable with previous site.

14 Below confluence of Marie Creek tributary, Penny Lane branch, and Marie Creek tributary main stem. 15 Summation of sites 44A and 43A streamflow measurements.
Tables 71

Table 3. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, March 13–16, 2001—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station

identifier Location (ft3/s) (acre-ft/d)

Milliken Creek Drainage Basin—Silverado Trail tributary

36 2200 block of Silverado Trail, 0.7 mi 

north of West Trancas Road (near old 

gage site 11458120)

0.72 1.4 pygmy fair na na na

Milliken Creek Drainage Basin—Atlas Peak Road tributary

30 Atlas Peak Road, 0.6 mi north of Westgate 

Drive 0.13 0.3 pygmy poor na na na

29 Atlas Peak Road 0.30 0.6 pygmy fair +0.3 0.6 +0.5

Milliken Creek Drainage Basin—Silverado Country Club tributary

5 East of Hillcrest Drive near Westgate 

Drive

0.02 < 0.1 flume good na na na

6A West of Hillcrest Drive near Westgate 

Drive, main east–west channel

0.03 0.1 flume good na < 0.1 na

6B West of Hillcrest Drive near Westgate 

Drive, secondary north–south culvert

0.01 < 0.1 flume good na na na

6A/6B Below confluence 1 2 0.04 2 0.1 na na na na na

Milliken Creek Drainage Basin—Hillcrest Drive tributary

3 2100 block Monticello Road 0.04 0.1 flume good na na na

4 Silverado Country Club, east of Hillcrest 

Drive, near St. Andrews Drive 0.02 < 0.1 flume good <–0.1 0.4 < –0.2

Milliken Creek Drainage Basin—Milliken Creek main stem

31 0.5 mi northeast of Westgate Drive 46.2 91.5 AA meter good na na na

32 0.4 mi northeast of Westgate Drive 53.8 106.5 AA meter good +15.0 0.1 +150.0

34 0.2 mi northeast of Westgate Drive 44.0 87.1 AA meter good –19.4 0.2 –97.0

33 Westgate Drive bridge 51.7 102.4 AA meter good +15.3 0.2 +76.5

38 Altas Peak Road 43.4 85.9 AA meter good –16.5 1.2 –13.8

64 1100 block Monticello Road 44.6 88.3 AA meter good +2.4 0.7 +3.4

35 West Trancas Road bridge 42.5 84.2 AA meter good –4.1 0.6 –6.8

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch, south fork

24 0.2 mi east of 2200 block Third Avenue 0.02 < 0.1 flume good na na na

52 2200 Third Avenue 0.03 0.1 flume good < +0.1 0.2 < +0.5

22A 2400 block Third Avenue 3 0.08 3 0.2 flume good +0.1 0.3 +0.3

Table 3. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, March 13–16, 2001

[See figure 10 for station locations. Measurement rating: excellent (less than 2 percent error), good (2 percent to less than 5 percent error), fair (5 to 8 percent 

error), poor (greater than 8 percent error). ft3/s, cubic foot per second; acre-ft/d, acre-foot per day; mi, mile; (acre-ft/d)/mi, acre-foot per day per mile. <, actual 

value is less than value shown; na, not applicable]

See footnotes at end of table.
72 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch, middle fork

23 NW 1/4, Section 5, T5N, R3W 0.03 0.1 flume good na na na

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch main stem (Dairy Creek) 

51 SE 1/4, Section 31, T6N, R3W 0.07 0.1 flume good na na na

22B 2400 block Third Avenue 0.15 0.3 flume good +0.2 0.3 +0.7

22A/22B Below confluence 4 5 0.22 5 0.4 na na 6 +0.1 na na

26A Third Avenue and Hagen Road 0.48 1.0 pygmy poor +0.6 0.5 +1.2

Sarco Creek Drainage Basin—Hagen Road tributary, Napa Valley Country Club tributary

55 NW1/4, Section 6, T5N, R3W < 0.01 < 0.1 estimated poor na na na

27 Above confluence with Hagen Road 

tributary main stem 0.11 0.2 pygmy poor +0.1 0.8 +0.1

Sarco Creek Drainage Basin—Hagen Road tributary main stem

28B NW 1/4 Section 31, T6N, R3W 0.46 0.9 pygmy fair na na na

26B East branch Hagen Road tributary 0.50 1.0 pygmy fair +0.1 0.1 +1.0

26A/26B Below confluence 7 8 0.98 8 1.9 na na 6 +0.9 na na

9 Grange Hall 0.43 0.8 pygmy poor 6 –1.1 0.8 6 –1.4

Sarco Creek Drainage Basin—La Grande Avenue/Mt. George Avenue tributary, north branch

8A SW 1/4 Section 30, T6N, R3W 0.02 < 0.1 flume good na na na

Sarco Creek Drainage Basin—La Grande Avenue/Mt. George Avenue tributary main stem

65 Mt. George Avenue 0.00 0.0 observed na na na na

Above confluence 9 10 0.10 10 0.2 na na 6 +0.2 0.5 6 +0.4

8B SE 1/4 Section 25, T6N, R4W 0.12 0.2 flume good na na na

7B Vichy Avenue bridge 0.62 1.2 pygmy fair +1.0 0.5 +2.0

Sarco Creek Drainage Basin—Sarco Creek main stem

2 SW 1/4 Section 19, T6N, R3W 0.83 1.6 pygmy fair na na na

7A Vichy Avenue bridge, east side 1.05 2.1 pygmy fair +0.5 1.4 +0.4

10 Vichy Avenue bridge, west side 1.49 3.0 pygmy fair +0.9 na na

11 Silverado Trail bridge 2.34 4.6 pygmy fair +1.6 1.4 +1.1

Tulucay Creek Drainage Basin—Murphy Creek tributary, Wild Horse Valley Road branch

59 NW 1/4 Section 9, T5N, R3W 0.01 < 0.1 flume fair na na na

14 Wild Horse Valley Road 0.06 0.1 flume good +0.1 0.4 +0.2

15 Mustang Road 0.12 0.2 flume fair +0.1 0.6 +0.2

Table 3. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, March 13–16, 2001—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station

identifier Location (ft3/s) (acre-ft/d)

See footnotes at end of table.
Tables 73

Tulucay Creek Drainage Basin—Murphy Creek tributary main stem

16 Mustang Road 0.71 1.4 pygmy fair na na na

50 Coombsville Road 0.72 1.4 pygmy fair 0.0 0.9 0.0

Tulucay Creek Drainage Basin—Spencer Creek tributary main stem

12 SW 1/4, Section 16, T5N, R3W 0.64 1.3 pygmy fair na na na

13 Green Valley Road 0.87 1.7 pygmy fair +0.4 1.4 +0.3

Tulucay Creek Drainage Basin—Third Avenue tributary, west culvert

56 North Avenue 0.12 0.2 flume good na na na

20A Above confluence with Third Avenue 

tributary main stem 0.27 0.5 pygmy fair +0.3 0.7 +0.4

Tulucay Creek Drainage Basin—Third Avenue tributary, middle branch

57 Country Lane < 0.01 < 0.1 estimated poor na na na

Tulucay Creek Drainage Basin—Third Avenue tributary main stem

58 4300 block East Third Avenue 0.09 0.2 pygmy poor na na na

18 Kirkland Road 0.49 1.0 float fair +0.8 0.5 +1.6

19 East Third Avenue 0.30 0.6 pygmy fair –0.4 0.1 –4.0

20B Above confluence with Third Avenue 

tributary west culvert 0.32 0.6 pygmy good 0.0 0.7 0.0

20A/20B Below confluence 11 12 0.59 12 1.2 na na 6+0.6 na na

Tulucay Creek Drainage Basin—Kreuse Creek tributary, south branch

61 End of Madrone Drive 0.01 < 0.1 flume good na na na

Tulucay Creek Drainage Basin—Kreuse Creek tributary main stem

60 Kreuzer Lane 0.30 0.6 pygmy poor na na na

49B Near end of Penny Lane 0.35 0.7 pygmy poor +0.1 1.3 +0.1

48 Los Robles Drive 0.33 0.6 pygmy poor –0.1 0.4 –0.2

Tulucay Creek Drainage Basin—Marie Creek tributary, Penny Lane branch

40 Penny Lane 0.04 0.1 flume good na na na

44A Above confluence13 14 0.88 14 1.7 estimated na 6 +1.6 0.5 6 +3.2

Tulucay Creek Drainage Basin—Marie Creek tributary main stem

46B West of 46A in SW 1/4, Section 18, T5N, 

R3W 1.10 2.2 pygmy fair na na na

63 SE 1/4 Section 13, T5N, R4W 1.20 2.4 pygmy fair +0.2 0.2 +1.0

Table 3. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, March 13–16, 2001—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station

identifier Location (ft3/s) (acre-ft/d)

See footnotes at end of table.
74 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Tulucay Creek Drainage Basin—Marie Creek tributary main stem—Continued

62 Minor culvert, northwest side of Upper 

Lake Camille

0.19 0.4 pygmy poor 15 na na 15 na

42B Lower Lake Camille, north spillway 0.65 1.3 pygmy fair 15 na na 15 na

43B Shurtleff Park at Shetler Avenue 0.00 0.0 observed na 15 na na 15 na

44B Shurtleff Park below confluence with 

Marie Creek tributary, Penny Lane 

branch 0.88 1.7 pygmy fair +1.7 0.5 15 na

39 Terrace Drive 0.56 1.1 pygmy poor –0.6 0.4 –1.5

Tulucay Creek Drainage Basin—Tulucay Creek main stem

17 Fourth Avenue 1.93 3.8 pygmy good na na na

45 Shurtleff Avenue. 4.38 8.7 pygmy fair +4.9 1.4 +3.5

41 Tulucay Creek at Napa (discontinued 

stream gage 11458350) 5.62 11.1 pygmy fair +2.4 0.7 +3.4

Table 3. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, March 13–16, 2001—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station

identifier Location (ft3/s) (acre-ft/d)

1 Below confluence of Silverado Country Club tributary main east–west channel and secondary north–south culvert. 2 Summation of sites 6A/6B streamflow measurements.

3 Average of two streamflow measurements at slightly different locations on same day (3/14/01). 4 Below confluence of upper and middle forks of Third Avenue branch of Hagen Road tributary to Sarco Creek. 5 Summation of sites 22A and 22B streamflow measurements.

6 Estimated.

7 Below confluence of main stem of Third Avenue branch of Hagen Road tributary and Hagen Road tributary main stem. 8 Summation of sites 26A and 26B streamflow measurements. 9 Above confluence of La Grande Avenue/Mt. George Avenue tributary, north branch and La Grande Avenue/Mt. George Avenue tributary main stem. 10 Difference of sites 8B and 8A streamflow measurements. 11 Below confluence of Third Avenue tributary main stem and Third Avenue tributary west culvert. 12Summation of sites 20A and 20B streamflow measurements.

13 Above confluence of Marie Creek tributary, Penny Lane branch, and Marie Creek tributary main stem. 14 Difference of sites 44B and 43B streamflow measurements. 15 Not comparable with previous site.
Tables 75

Table 4. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 10–12, 2001—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station

identifier Location (ft3/s) (acre-ft/d)

Milliken Creek Drainage Basin—Silverado Trail tributary

36 2200 block of Silverado Trail, 0.7 mi 

north of West Trancas Road (near old 

gage site 11458120) 0.03 0.1 flume good na na na

Milliken Creek Drainage Basin—Atlas Peak Road tributary

30 Atlas Peak Road, 0.6 mi north of Westgate 

Drive 0.06 0.1 pygmy poor na na na

29 Atlas Peak Road 0.00 0.0 observed na –0.1 0.6 –0.2

Milliken Creek Drainage Basin—Silverado Country Club tributary

5 East of Hillcrest Drive near Westgate 

Drive 0.00 0.0 observed na na na na

6A West of Hillcrest Drive near Westgate 

Drive, main east–west channel < 0.01 < 0.1 volumetric good na < 0.1 na

6B West of Hillcrest Drive near Westgate 

Drive, secondary north–south culvert < 0.01 < 0.1 flume good na na na

6A/6B Below confluence 1 2 < 0.01 2 < 0.1 na na na na na

Milliken Creek Drainage Basin—Hillcrest Drive tributary

3 2100 block Monticello Road < 0.01 < 0.1 estimated poor na na na

4 Silverado Country Club, east of Hillcrest 

Drive, near St. Andrews Drive 0.00 0.0 observed na na 0.4 na

Milliken Creek Drainage Basin—Milliken Creek main stem

31 0.5 mi northeast of Westgate Drive 3.40 6.7 pygmy good na na na

32 0.4 mi northeast of Westgate Drive 2.23 4.4 pygmy good –2.3 0.1 –23.0

34 0.2 mi northeast of Westgate Drive 3.36 6.6 pygmy good +2.2 0.2 +11.0

33 Westgate Drive bridge 2.40 4.8 pygmy good –1.8 0.2 –9.0

38 Altas Peak Road 2.53 5.0 pygmy fair +0.2 1.2 +0.2

37 Hedgeside Avenue (old gage site 

11458100) 3.13 6.2 pygmy good +1.2 0.4 +3.0

64 1100 block Monticello Road 3.12 6.2 pygmy fair 0.0 0.4 0.0

35 West Trancas Road bridge 3.25 6.4 pygmy good +0.2 0.6 +0.3

Table 4. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of 

streamflow between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 10–12, 

2001

[See figure 10 for station location. Measurement rating: excellent (less than 2 percent error), good (2 percent to less than 5 percent error), fair (5 to 8 percent 

error), poor (greater than 8 percent error). ft3/s, cubic foot per second; acre-ft/d, acre-foot per day; mi, mile; (acre-ft/d)/mi, acre-foot per day per mile. <, 

See footnotes at end of table.
76 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch, south fork

24 0.2 mi east of 2200 block Third Avenue 0.00 0.0 observed na na na na

52 2200 Third Avenue < 0.01 < 0.1 estimated poor 3 < +0.1 0.2 3 < +0.5

25 Unnamed tributary, west of Third Avenue 

above confluence with Third Avenue 

branch, south fork < 0.01 < 0.1 estimated poor na na na

Below confluence 4 5 < 0.01 5 < 0.1 na na 3 0.0 0.2 3 0.0

22A 2400 block Third Avenue 6 < 0.01 < 0.1 flume good 0.0 0.1 0.0

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch, middle fork

23 NW 1/4, Section 5, T5N, R3W 0.02 < 0.1 flume good na na na

Sarco Creek Drainage Basin—Hagen Road tributary, Third Avenue branch main stem (Dairy Creek)

51 SE 1/4., Section 31, T6N, R3W 0.02 < 0.1 flume good na na na

22B 2400 block Third Avenue 6 0.02 6 < 0.1 flume fair 0.0 0.3 0.0

22A/22B Below confluence 7 8 < 0.03 8 < 0.1 na na 3 < +0.1 na na

26A Third Avenue and Hagen Road 0.08 0.2 flume good +0.1 0.5 +0.2

Sarco Creek Drainage Basin—Hagen Road tributary, Napa Valley Country Club tributary

55 NW1/4, Section 6, T5N, R3W 0.00 0.0 observed na na na na

27 Above confluence with Hagen Road 

tributary main stem 0.01 < 0.1 flume fair +0.1 0.8 +0.1

Sarco Creek Drainage Basin—Hagen Road tributary main stem

28B NW 1/4 Section 31, T6N, R3W 0.16 0.3 pygmy fair na na na

26B East branch Hagen Road tributary 0.15 0.3 flume good 0.0 0.1 0.0

26A/26B Below confluence 9 10 0.23 10 0.5 na na 3 +0.2 na na

9 Grange Hall 0.09 0.2 pgymy poor 3 –0.3 0.8 3–0.3

Sarco Creek Drainage Basin—La Grande Avenue/Mt. George Avenue tributary, north branch

8A SW 1/4 Section 30, T6N, R3W < 0.01 < 0.1 flume fair na na na

Sarco Creek Drainage Basin—La Grande Avenue/Mt. George Avenue tributary main stem

65 Mt. George Avenue 0.00 0.0 observed na na na na

Above confluence 11 12 0.00 12 0.0 na na 3 0.0 0.5 3 0.0

8B SE 1/4 Section 25, T6N, R4W < 0.01 < 0.1 flume poor na na na

7B Vichy Avenue bridge 13 0.08 13 0.2 estimated poor 3 +0.1 0.5 3 +0.2

Table 4. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 10–12, 2001—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station

identifier Location (ft3/s) (acre-ft/d)

See footnotes at end of table.
Tables 77

Sarco Creek Drainage Basin—Sarco Creek main stem

2 SW 1/4 Section 19, T6N, R3W 0.25 0.5 pgymy fair na na na

1 Langley Park 0.18 0.4 pgymy poor –0.1 0.8 –0.1

7A Vichy Avenue bridge, east side 0.08 0.2 pgymy poor –0.2 0.6 –0.3

10 Vichy Avenue bridge, west side 0.16 0.3 pgymy poor +0.1 na na

11 Silverado Trail bridge 0.13 0.2 pygmy poor –0.1 1.4 –0.1

Tulucay Creek Drainage Basin—Murphy Creek tributary, Wild Horse Valley Road branch

59 NW 1/4 Section 9, T5N, R3W < 0.01 < 0.1 estimated poor na na na

14 Wild Horse Valley Road 0.01 < 0.1 flume fair 3 < +0.1 0.4 3 < +0.2

15 Mustang Road 0.02 < 0.1 flume fair < +0.1 0.6 < +0.2

Tulucay Creek Drainage Basin—Murphy Creek tributary main stem

16 Mustang Road 0.19 0.4 flume poor na na na

50 Coombsville Road 0.13 0.3 flume good –0.1 0.9 –0.1

Tulucay Creek Drainage Basin—Spencer Creek tributary main stem

12 SW 1/4, Section 16, T5N, R3W 0.00 0.0 observed na na na na

13 Green Valley Road 0.17 0.3 pygmy fair +0.3 1.4 +0.2

Tulucay Creek Drainage Basin—Third Avenue tributary, west culvert

56 North Avenue < 0.01 < 0.1 flume good na na na

20A Above confluence with Third Avenue 

tributary main stem 0.02 < 0.1 flume good < +0.1 0.7 < +0.1

Tulucay Creek Drainage Basin—Third Avenue tributary, middle branch

57 Country Lane 0.00 0.0 observed na na na na

Tulucay Creek Drainage Basin—Third Avenue tributary main stem

58 4300 block East Third Avenue 0.01 < 0.1 flume fair na na na

18 Kirkland Road 0.01 < 0.1 volumetric fair 0.0 0.5 0.0

19 East Third Avenue 0.00 0.0 observed na <–0.1 0.1 <–1.0

20B Above confluence with Third Avenue 

tributary west culvert 0.06 0.1 flume good +0.1 0.7 +0.1

20A/20B Below confluence 14 15 0.08 15 0.2 na na +0.1 na na

Tulucay Creek Drainage Basin—Kreuse Creek tributary, south branch

61 End of Madrone Drive 0.00 0.0 observed na na na na

Table 4. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 10–12, 2001—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station

identifier Location (ft3/s) (acre-ft/d)

See footnotes at end of table.
78 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Tulucay Creek Drainage Basin—Kreuse Creek tributary main stem

60 Kreuzer Lane 0.03 0.1 flume good na na na

49B Near end of Penny Lane 0.00 0.0 observed na –0.1 1.3 –0.1

48 Los Robles Drive 0.00 0.0 observed na 0.0 0.4 0.0

Tulucay Creek Drainage Basin—Marie Creek tributary, Penny Lane branch

40 Penny Lane < 0.01 < 0.1 flume good na na na

44A Above confluence 16 17 0.05 17 0.1 na na 3 +0.1 0.5 3 +0.2

Tulucay Creek Drainage Basin - Marie Creek tributary main stem

46B West of 46A in SW 1/4, Section 18, T5N, 

R3W 0.23 0.5 pygmy poor na na na

63 SE 1/4 Section 13, T5N, R4W 0.21 0.4 pygmy poor –0.1 0.2 –0.5

62 Minor culvert, northwest side of Upper 

Lake Camille 0.01 < 0.1 estimated poor 18 na na 18 na

42B Lower Lake Camille, north spillway 0.02 < 0.1 estimated poor 18 na na 18 na

43B Shurtleff Park at Shetler Avenue < 0.01 < 0.1 estimated poor 18 na na 18 na

44B Shurtleff Park below confluence with 

Marie Creek tributary, Penny Lane 

branch 0.05 0.1 flume good 3<+0.1 0.5 18 na

39 Terrace Drive 0.00 0.0 observed na –0.1 0.4 –0.2

Tulucay Creek Drainage Basin—Tulucay Creek main stem

17 Fourth Avenue 0.33 0.6 pgymy fair na na na

45 Shurtleff Avenue 0.78 1.5 pygmy fair +0.9 1.4 +0.6

41 Tulucay Creek at Napa (discontinued 

stream gage 11458350) 2.81 5.6 pygmy fair +4.1 0.7 +5.8

Table 4. Streamflow measurements, gain or loss of streamflow between stations, flow distance between stations, and rate of gain or loss of streamflow 

between stations at sites along Milliken, Sarco, and Tulucay Creeks and their tributaries, southeastern Napa County, California, April 10–12, 2001—

Continued

Station Streamflow Measurement 

method

Measurement 

rating

Gain (+) or 

loss (–) of 

streamflow 

between 

stations

(acre-ft/d)

Flow

distance 

between 

stations

(mi)

Rate of gain (+) 

or loss (–) of 

streamflow 

between 

stations

[(acre-ft/d)/mi]

Station

identifier Location (ft3

/s) (acre-ft/d)

1Below confluence of Silverado Country Club tributary main east–west channel and secondary north–south culvert. 2 Summation of sites 6A/6B streamflow measurements.

3 Estimated.

4 Below confluence of unnamed tributary west of Third Avenue and south fork of Third Avenue branch of Hagen Road tributary to Sarco Creek. 5 Summation of sites 52 and 25 streamflow estimates. 6 Measured on 4/18/01.

7 Below confluence of upper and middle forks of Third Avenue branch of Hagen Road tributary to Sarco Creek. 8 Summation of sites 22A and 22B streamflow measurements.

9 Below confluence of main stem of Third Avenue branch of Hagen Road tributary and Hagen Road tributary main stem. 10 Summation of sites 26A and 26B streamflow measurements. 11 Above confluence of La Grande Avenue/Mt. George Avenue tributary, north branch and La Grande Avenue/Mt. George Avenue tributary main stem. 12 Difference of sites 8B and 8A streamflow measurements. 13 Difference of sites 10 and 7A streamflow measurements. 14 Below confluence of Third Avenue tributary main stem and Third Avenue tributary west culvert. 15 Summation of sites 20A and 20B streamflow measurements. 16 Above confluence of Marie Creek tributary, Penny Lane branch and Marie Creek tributary main stem. 17 Difference of sites 44B and 43B streamflow measurements. 18 Not comparable with previous site.
Tables 79

Table 5. Construction data and water-level data for April and October of 2000 through 2002 for selected wells in the lower Milliken–Sarco–Tulucay Creeks area, south

California—Continued

State well

No.

USGS site

identification

number

Local

well

number

Well

depth

Depth of 

perforated 

interval

Elevation 

of land 

surface

April 2000

water level

October 2000

water level

April 2001

water level

October 2001

water level

A

w

Depth Altitude Depth Altitude Depth Altitude Depth Altitude Dep

5N/3W-5M1 381818122133201 10 320 — 259.6 — — 170.9 88.7 154.4 105.2 186.6 73.0 165N/3W-6A11 381858122132601 92 368 148–368 357.8 — — 148.5 209.3 147.9 209.9 158.9 198.9 145N/3W-6B2 381831122140501 91 415 315–415 314.1 — — — — 211.0 103.1 240.8 73.3 225N/3W-6E1 381841122142801 103 — — 318.8 — — 210.7 108.1 206.0 112.8 — — — 5N/3W-6E2 381842122142901 104 730 170–730 315.7 190.5 125.2 203.4 112.3 211.1 104.6 210.8 104.9 205N/3W-6J2 381821122134001 110 530 170–530 241.8 — — — — — — 170.2 71.6 145N/3W-6J3 381819122134001 9 215 — 242.3 — — 126.5 115.8 102.1 140.2 126.7 115.6 115N/3W-6J42 381820122135101 — 3 455 260–455 4227 — — — — — — — — 125N/3W-6K2 381819122135301 6 500 140–500 222.2 117.2 105.0 131.3 90.9 118.3 103.9 149.6 72.6 125N/3W-6L1 381830122141201 90 380 260–380 299.2 209.9 89.2 215.0 84.2 212.5 86.6 243.0 56.2 225N/3W-6L2 381824122140801 94 285 165–285 228.6 107.5 121.1 121.2 107.4 105.2 123.4 — — 105N/3W-6M3 381820122144001 74 300 100–300 129.4 73.1 56.3 84.8 44.6 71.0 58.4 91.2 38.2 75N/3W-6N4 381814122142901 63 400 — 130.8 43.4 87.4 90.9 39.9 71.6 59.2 98.3 32.5 95N/3W-6N5 381807122143401 73 460 200–460 123.5 — — — — 164.5 –41.0 191.9 5–68.4 1655N/3W-6N61 381814122143101 61 260 80–260 124.4 69.4 55.0 81.7 42.7 67.0 57.4 89.9 34.5 65N/3W-6P3 381809122141401 87 185 — 147.6 47.4 100.2 55.7 91.9 38.7 108.9 74.6 73.0 45N/3W-6P5 381814122141601 140 298 100–298 174.6 — — — — — — 107.6 67.0 75N/3W-6Q1 381812122141201 89 6228 24–88 156.8 62.5 94.3 70.0 86.8 51.3 105.5 86.0 70.8 55N/3W-6Q3 381815122135201 7 350 70–350 279.6 169.3 110.3 169.0 110.6 168.6 111.0 168.8 110.8 — 5N/3W-6Q4 381815122135101 105 175 — 277.8 157.7 120.1 157.7 120.1 156.7 121.1 160.1 117.7 155N/3W-6Q5 381815122135901 93 397 177–397 195.0 7 93.5 7101.4 100.1 94.8 86.3 108.6 118.5 76.4 95N/3W-6R1 381817122134901 8 180 — 250.7 128.9 121.8 133.7 117.0 130.5 120.2 141.1 109.6 135N/3W-6R2 381813122133701 17 400 220–400 255.0 — — 155.7 99.3 146.5 108.5 173.8 81.2 165N/3W-6R3 381813122134101 106 207 — 255.3 — — 111.6 143.6 — — 129.8 5125.5 — 5N/3W-7B1 381801122140601 117 6215 — 143.0 — — — — 748.4 794.6 69.3 73.7 4

See footnotes at end of table.

Table 5. Construction data and water-level data for April and October of 2000 through 2002 for selected wells in the lower Milliken–Sarco–Tulucay Creeks area, soCalifornia[State well No.: See well-numbering diagram in text. See figures 14, 25, and 31 for well locations. USGS (U.S. Geological Survey) identification number consists of latit

number. Depths in feet below land surface; well depth, completed well depth unless otherwise noted. Elevation of land surface in feet above sea level. Altitude, altitude o

above sea level. —, no data]
80 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002 5N/3W-7B2 381759122140501 118 240 120–240 149.2

—

—

—

— 750.1 799.0 71.2 78.0 

5N/3W-7C3 381744122141901 20 208 130–207 131.6

—

— 62.4 69.2 37.8 93.8 78.3 53.3

5N/3W-7C4 381802122142001 85 240

— 137.3 60.1 77.2 71.0 66.3 54.5 82.8 83.0 54.3 

5N/3W-7C5 381804122141501 86 275

— 138.2 61.8 76.4 68.4 69.8 53.6 84.6 87.4 50.8 

5N/3W-7D3 381801122144201 72 245 120–245 133.2 102.3 30.9 110.9 22.3 99.2 34.0 107.3 25.9 5N/3W-7D4 381753122143901 71 186

— 122.4

—

— 104.4 18.0

—

—

—

— 

5N/3W-7E4 381744122142801 78 160

— 109.4

—

— 66.8 42.6 51.2 58.2 84.3 25.1 

5N/3W-7E5 381748122143601 70 175 75–175 119.8 8 83.8 836.0 100.6 19.2 77.4 42.4 8112.2 87.6 5N/3W-7E6 381744122143201 77 110

— 105.3 50.0 55.3 63.6 41.7 46.0 59.3

—

— 

5N/3W-7E7 381746122143301 79 154 30–100 115.4 43.8 71.6 94.5 20.9 69.8 45.6 107.1 8.3 5N/3W-7E8 381744122142701 16 135

— 114.8 34.2 80.6 69.1 45.7 32.9 81.9 83.5 31.3 

5N/3W-7F1 381749122141801 24

—

— 122.6 38.0 84.6 47.7 74.8 33.0 89.6 61.9 60.6 

5N/3W-7F2 381751122142001 84

—

— 129.1 50.9 78.2 55.8 73.3 42.4 86.7 67.6 61.5 

5N/3W-7F3 381748122142401 81 290 50–290 114.9 41.2 73.7 44.4 70.5 28.2 86.7 68.4 46.5 5N/3W-7F4 381748122142402 82 145

— 115.6 39.9 75.6 42.7 72.8 27.9 87.6 60.1 55.4 

5N/3W-7F5 381749122142501 83 162

— 115.0 41.6 73.4 45.7 69.3 29.7 85.3 63.2 51.8 

5N/3W-7G21 381740122140001 100 228

— 154.6 42.6 112.0 51.2 103.4 34.1 120.5 62.5 92.1 

5N/3W-7G32 381745122135901 145 250 50–250 168.3

—

—

—

—

—

— 77.9 90.4 

5N/3W-7H4 381744122134301 21 355 175–355 203.2

—

— 75.2 128.0 59.4 143.8 90.6 112.6 

5N/3W-7H5 381744122135301 115

—

— 179.9

—

—

—

— 740.5 7139.4

—

— 

5N/3W-7L3 381738122142401 114

—

— 120.4

—

—

—

—

—

— 67.0 53.4 

5N/3W-7M4 381732122142801 32

—

— 98.3 12.9 85.4 25.0 73.3 13.8 84.5 28.6 69.7 

5N/3W-7N2 381717122143501 34 150

— 147.5 45.4 102.1 53.7 93.8 46.4 101.1 61.1 86.4 

5N/3W-7N3 381720122143601 33 110 50–110 127.3 29.5 97.8 40.0 87.3 31.5 95.8 45.0 82.3 5N/3W-7PL6 381719122140801 50 170 70–100 189.2 64.1 125.1 66.2 123.0 65.1 124.1 69.0 120.2 

Table 5. Construction data and water-level data for April and October of 2000 through 2002 for selected wells in the lower Milliken–Sarco–Tulucay Creeks area

California—Continued

State well

No.

USGS site

identification

number

Local

well

number

Well

depth

Depth of 

perforated 

interval

Elevation 

of land 

surface

April 2000

water level

October 2000

water level

April 2001

water level

October 2001

water level

Depth Altitude Depth Altitude Depth Altitude Depth Altitude 

See footnotes at end of table.
Tables 81

5N/3W-8E11 381746122133101 22 135 — 254.2 85.6 168.6 93.6 160.6 87.4 166.8 94.0 160.2 85N/3W-8L2 381735122131401 124 310 — 360.4 — — — — 765.0 7295.4 69.1 291.3 65N/3W-17C2 381700122131201 116 — — 684.7 — — — — 7205.2 7479.5 195.7 489.0 8205N/3W-18D1 381712122144101 35 92 — 135.9 29.5 106.4 40.4 95.5 44.3 91.6 50.6 85.3 35N/4W-1C2 381856122152101 30 200 — 105.0 38.5 66.5 30.6 74.4 29.7 75.3 — — 25N/4W-1F2 381831122153001 27 420 260–420 165.4 121.6 43.8 121.2 44.2 118.9 46.5 8139.3 5, 8 26.1 — 5N/4W-1F31 381841122152401 29 350 50–350 131.4 103.5 27.9 113.4 18.0 113.1 18.3 127.2 4.2 115N/4W-1J2 381824122145301 12 460 300–460 272.1 — — — — 180.5 91.6 — — — 5N/4W-1J3 381827122144701 141 535 115–535 156.8 — — — — — — 120.9 35.9 8125N/4W-1K2 381820122151001 59 280 — 218.5 — — 210.0 8.5 195.2 23.3 — — — 5N/4W-1L1 381830122152001 28 400 140–400 149.7 121.9 27.8 134.0 15.8 — — — — 125N/4W-1L2 381831122152401 119 150 50–150 135.8 — — — — 796.2 739.6 102.8 33.0 95N/4W-1R2 381809122145101 60 117 — 141.7 108.9 32.8 113.5 28.2 108.2 33.5 dry dry dr5N/4W-1R3 381808122145201 144 260 140–260 138.0 — — — — — — 117.3 20.7 105N/4W-1R42 381812122145201 — 9595 275–595 4150 — — — — — — — — 115N/4W-2Q1 381814122161701 113 — — 83.4 — — — — 26.6 56.8 39.9 43.6 35N/4W-12B4 381753122151001 48 6100 — 101.0 29.7 71.3 35.9 65.1 31.9 69.1 37.7 63.3 35N/4W-12B5 381755122151001 25 — — 99.5 27.5 72.0 34.0 65.6 29.6 69.9 35.6 63.9 35N/4W-12B6 381756122150401 26 155 — 113.3 113.2 0.1 119.7 –6.4 112.9 0.4 115.3 –2.0 105N/4W-12G11 381740122150201 14 240 60–240 82.6 23.8 58.8 35.4 47.2 25.9 56.7 42.0 40.6 2

Table 5. Construction data and water-level data for April and October of 2000 through 2002 for selected wells in the lower Milliken–Sarco–Tulucay Creeks area, south

California—Continued

State well

No.

USGS site

identification

number

Local

well

number

Well

depth

Depth of 

perforated 

interval

Elevation 

of land 

surface

April 2000

water level

October 2000

water level

April 2001

water level

October 2001

water level

A

w

Depth Altitude Depth Altitude Depth Altitude Depth Altitude Dep

See footnotes at end of table.
82 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002 5N/4W-12H2 381744122145001 15 10 

—

— 104.3 8102.2 8 2.1 114.0 –9.7 92.2 12.1 8120.9 8–16.6 810

5N/4W-12J2 381738122145701 13 285

— 82.9 43.0 39.9 57.7 25.2 48.5 34.4 68.1 14.8 4

5N/4W-13C1 381709122152101 143 11 

—

— 85.6

—

—

—

—

—

— 48.8 36.8 4

5N/4W-13G4 381648122151501 18 210 105–126 120.6 16.1 104.5 19.0 101.6 17.0 103.6 22.8 97.8 15N/4W-13H1 381700122145001 137 364

— 4132

—

— 20.8 111.2 19.0 113.0

—

— 1

5N/4W-13H31 381649122144901 36 130 70–130 156.6 28.9 127.7 36.7 119.9 33.3 123.3 41.5 115.1 35N/4W-13J11 381646122145601 19 360 45–360 144.9 21.2 123.7 29.0 115.9 34.8 110.1 41.8 103.1 35N/4W-14J32 381644122154601 49 6,12399

— 84.7

—

— 55.6 29.1 54.8 29.9 57.9 26.8 5

6N/3W-30P1 381958122141601 58

—

— 168.0

—

— 186.2 –18.2 103.0 65.0 190.0 –22.0 11

6N/3W-31D1 381941122143201 65 585 65–565 145.9 193.5 –47.6 205.4 –59.5 178.3 –32.4 217.0 –71.1 186N/3W-31D2 381939122142501 66 555

— 156.3 193.3 –37.0 214.2 –57.9 186.2 –29.9 225.6 –69.3 19

6N/3W-31F2 381933122141301 68 510

— 161.7 198.3 –36.6 219.1 –57.4 190.9 –29.2 229.9 –68.2 19

6N/3W-31F3 381925122142101 41 610

— 147.7 190.2 –42.5 205.2 –57.5 177.6 –29.9 216.2 –68.5 19

6N/3W-31F41 381935122141601 67 486

— 161.8 201.5 –39.6

—

— 189.2 –27.4 232.3 –70.4 19

6N/3W-31G2 381926122140201 42 382 160–382 165.4 202.3 –36.9

—

—

—

— 234.5 –69.1 20

6N/4W-13E1 382204122154501 5 510 80–510 515.0 285.8 229.2 265.2 249.8 284.8 230.2 273.9 5241.1

—

6N/4W-14Q1 382143122160301 4 385 55–315 138.4 99.4 39.0 116.2 22.2 94.9 43.5 126.6 11.8 106N/4W-15R51 382135122165901 76 395 60–395 94.0 47.2 46.8 66.9 27.1 45.2 48.8 74.4 19.6 46N/4W-22H32 382114122165801

—

13500 120–500 465

—

—

—

—

—

—

—

— 5

6N/4W-22R1 382047122170501 75 205 45–205 37.5

—

— 51.5 –14.0 27.3 10.2 51.1 –13.6 3

Table 5. Construction data and water-level data for April and October of 2000 through 2002 for selected wells in the lower Milliken–Sarco–Tulucay Creeks area, south

California—Continued

State well

No.

USGS site

identification

number

Local

well

number

Well

depth

Depth of 

perforated 

interval

Elevation 

of land 

surface

April 2000

water level

October 2000

water level

April 2001

water level

October 2001

water level

A

w

Depth Altitude Depth Altitude Depth Altitude Depth Altitude Dep

See footnotes at end of table.
Tables 83

6N/4W-23B1 382128122161001 3 612 352–612 144.8 46.6 98.2 143.5 1.3 100.7 44.1 155.7 5–10.9 106N/4W-23J12 382053122154701 2 700 — 89.6 — — 99.0 –9.4 80.3 9.3 106.2 –16.6 76N/4W-23K3 382103122161301 44 300 60–300 126.4 104.7 21.7 112.9 13.5 107.2 19.2 119.8 6.6 116N/4W-23Q31 382050122160901 43 310 150–310 100.1 — — 102.6 –2.5 79.6 20.5 97.1 3.0 86N/4W-25E3 382019122153201 99 154 134–154 94.3 47.7 46.6 63.5 30.8 53.0 41.3 70.8 23.5 56N/4W-25G1 382016122145801 51 175 — 119.7 131.7 –12.0 135.8 –16.1 128.7 –9.0 142.6 –22.9 136N/4W-25G21 382017122145801 142 440 160–440 119.9 — — — — — — 142.7 –22.8 136N/4W-25J1 382003122145001 52 360 238–360 129.0 143.9 –14.9 152.7 –23.7 146.4 –17.4 161.9 –32.9 146N/4W-26B2 382035122160601 57 132 — 71.5 58.7 12.8 65.6 5.9 53.9 17.6 73.0 –1.5 56N/4W-26B32 382039122161901 23 205 — 71.7 767.0 74.7 73.4 –1.7 62.5 9.2 96.0 –24.3 66N/4W-26F2 382022122162601 55 153 — 66.8 63.7 3.1 74.4 –7.6 65.5 1.3 90.0 –23.2 76N/4W-26G1 382035122161101 56 210 30–210 60.1 — — 66.6 –6.5 860.8 8–.7 75.2 –15.1 56N/4W-26G2 382021122161401 47 156 — 53.1 — — 48.0 5.1 34.7 18.4 57.0 –3.9 36N/4W-26G31 382018122161301 45 93 — 48.4 29.1 19.3 43.2 5.2 29.1 19.3 53.4 –5.0 36N/4W-26L5 382008122162801 122 210 60–150 55.8 — — — — 767.8 7–12.0 82.9 –27.2 76N/4W-26R3 381956122155101 102 147 — 77.5 — — 64.7 12.8 45.4 32.1 62.3 15.2 56N/4W-27L214 — — 120 60–120 50 1526.0 1524.0 1537.0 1513.0 1526.3 1523.7 1541.9 158.1 1526N/4W-27N1 381953122175401 — 141 39–141 50 — — 39.5 10.5 23.3 26.7 41.7 8.3 26N/4W-35G5 381929122160701 69 — — 39.0 8.1 30.9 14.4 24.6 8.2 30.8 14.3 24.7 6N/4W-35H1 381927122155001 120 297 157–297 68.4 — — — — 47.8 20.6 62.6 5.8 4

Table 5. Construction data and water-level data for April and October of 2000 through 2002 for selected wells in the lower Milliken–Sarco–Tulucay Creeks area, south

California—Continued

State well

No.

USGS site

identification

number

Local

well

number

Well

depth

Depth of 

perforated 

interval

Elevation 

of land 

surface

April 2000

water level

October 2000

water level

April 2001

water level

October 2001

water level

A

w

Depth Altitude Depth Altitude Depth Altitude Depth Altitude Dep
84 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002 6N/4W-36A1 381947122145401 98 375 240–375 122.2 132.5 –10.3 153.6 –31.4 129.4 –7.2 158.2 –36.0 13

6N/4W-36B4 381937122150901 96 87

— 105.9 49.5 56.4 50.7 55.2 51.2 54.7 52.2 553.7 5

6N/4W-36E1 381927122154001 37 100

— 78.1 38.2 39.9 50.5 27.6 42.8 35.3 55.5 22.6 4

6N/4W-36G1 381939122150401 95 195 155–185 112.6

—

— 137.8 –25.2 122.5 –9.9 139.2 –26.6 12

6N/4W-36H4 381926122144201 40 6, 16 485

— 105.0

—

— 191.4 –86.4 152.6 –47.6 189.6 –84.6 15

6N/4W-36H6 381935122145501 97 600 100–600 138.2 180.5 –42.2 195.7 –57.4 176.0 –37.8 185.7 –47.4 176N/4W-36J3 381923122145601 38

—

— 87.3 126.8 –39.5 167.6 –80.3 125.9 –38.6 181.4 –94.1 14

6N/4W-36K2 381910122150101 54

—

— 103.4

—

— 183.1 –79.7 160.6 –57.2 185.1 –81.7 17

6N/4W-36P1 381907122152301 31 382

— 128.4 155.5 –27.1 210.0 –81.6 163.7 –35.3 224.3 –95.9 16

6N/4W-36R11 381905122145601 53 530 30–530 126.2 106.1 20.1

—

— 119.2 7.0 125.3 0.9 10

6N/4W-36R2 381907122145001 121 17≥178

— 154.0

—

—

—

—

7 139.1 7 14.9 155.8 –1.8 13

Table 5. Construction data and water-level data for April and October of 2000 through 2002 for selected wells in the lower Milliken–Sarco–Tulucay Creeks area, south

California—Continued

State well

No.

USGS site

identification

number

Local

well

number

Well

depth

Depth of 

perforated 

interval

Elevation 

of land 

surface

April 2000

water level

October 2000

water level

April 2001

water level

October 2001

water level

A

w

Depth Altitude Depth Altitude Depth Altitude Depth Altitude Dep

1 Well used for water-chemistry sampling.2 Well used for temperature logging.3 Well depth remeasured by USGS in April 2002 is approximately 450 feet.4 Determined from topographic map with an accuracy of plus or minus 10 feet.5 Water-level measurement not included in the contouring (figs. 15 and 16).6 Reported drill or hole depth.7 Water-level measurement made in mid-May.8 Water-level measurement may have been affected by recent or nearby pumping, but if measured in October 2001 or April 2002 was included in the contouring on 9 Well depth remeasured by USGS in April 2002 is approximately 590 feet.10Well depth 120 to 130 feet according to owner.11Well depth 250 to 300 feet according to owner.12Well depth remeasured by USGS in April 2002 is approximately 418 feet.13Well depth remeasured by USGS in April 2002 is approximately 520 feet.14Well monitored by the California Department of Water Resources (California Department of Water Resources, accessed August 1, 2002).15Water-level data from California Department of Water Resources (California Department of Water Resources, accessed August 1, 2002).16 Well depth approximately 475 feet according to owner.17Well depth at least 178 feet according to owner.
Tables 85

Basis of estimate

Well to

 parcel

ratio

Number

of

parcels

Estimated 

number

of

wells

Estimated 

average 

pumping 

rate

(gal/min)

Assumed 

average 

daily use 

(hours)

Area

 (acres)

Unit

applied 

water

(acre-ft/acre)

Approxim

populati

(2000 

census

Domestic Use (residential use excluding residential mixed with agricultural properties)

Well and parcel data 1 1.1 1,450 1,595 2 20 3 0.98 na na na Population and water use data na na na na na na na 4 4,800 

Average 

Irrigation Use—Agriculture (includes agriculture mixed with residential use)

Well and parcel data 6 1.4 132 185 7 75 8 2.88 na na na FMMP land use and DWR water use coefficient na na na na na 9 2,369 10 1.2 na FMMP land use and local water use coefficient na na na na na 9 2,369 11 0.5 na Napa County land use and DWR water use coefficient na na na na na 12 2,869 10 1.2 na Napa County land use and local water use coefficient na na na na na 12 2,869 11 0.5 na 

MinimumMaximumAverage

Irrigation Use—Improved open space (includes golf courses, cemeteries, and public institutions)

FMMP land use and DWR water use coefficient na na na na na 9 391 13 4.0 na 

1 Estimated from 643 wells identified on 577 residential properties.2 Approximately 0.45 of average reported well yield (48 gal/min) for 589 wells identified on driller’s logs as domestic use.3 Average daily pumping time for residential wells, from Metzger and Fio (1997).4 Estimated population of county portion of study area not served by public supply water.5 Average daily per capita water use for the City of Napa Municipal Water Department, 1990–2001 (Alan Aguilar, Department of Water Resources, written commu

Napa, written commun., 2002). In comparison, per capita rate for part of study area outside of city limits estimated to be 156 gallons per person per day based on 2002 wat

Municipal Water Department (Gil Harrington, City of Napa, written commun., 2002).

6 Estimated from the identification of 104 wells distributed among 73 agricultural and agriculture mixed with residential properties.7 Approximately 0.45 of average reported well yield (168 gal/min) for 93 wells identified on driller’s logs as irrigation use.8 Average daily pumping time for institutional wells used primarily for landscape irrigation, from Metzger and Fio (1997).9 Average determined from FMMP year 2000 land use. See figure 2.10Average amount of water applied per acre for grapes in Napa County in 1980 (California Department of Water Resources, 1986).11Average amount of water applied per acre for grapes in Napa County in 2001 according to local residents (John Stewart, Napa County Department of Public Wor12Acreage determined from Napa County Assessor parcel database (2002).13Average amount of water applied per acre for pasture in Napa County in 1980 (California Department of Water Resources, 1986).

Table 6. Summary of estimated ground-water pumping for domestic and irrigation uses in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa Count[Well to parcel ratio based on distribution of identifiable wells amongst identifiable properties. Estimated average yield is approximately 0.45 of reported average yield of 

of 0.45 based on the proportion of measured to reported yields for 11 wells included in this study. Assumed average daily use values from Metzger and Fio (1997). Unit ap

of average countywide irrigation water applied during 1980. DWR, California Department of Water Resources. FMMP, California Department of Conservation Farmland 

Program. gal/min, gallon per minute; acre-ft/acre, acre-foot per acre. na, not applicable] 
86 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Table 7. Summary of median water-level change in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, southeastern Napa County, 

California, 2000–2002—Continued

Time period

Storage unit 1 Storage unit 2 Storage unit 3 Storage unit 4 All storage units

Number of 

wells

measured

Median

Number of 

wells

measured

Median

Number of 

wells 

measured

Median

Number of 

well

measured

s

Median

Number of 

wells

measured

Median

October 2000–October 2001 45 –10.4 4 –14.0 31 –7.4 5 –4.9 85 –8.6

October 2001–October 2002 51 –5.9 4 –6.5 27 –5.0 5 –7.2 87 –5.5

Average annual change: 48 –8.2 4 –10.3 29 –6.2 5 –6.1 86 –7.1

April 2000– April 2001 38 0.5 6 –8.9 20 2.0 3 –0.3 67 0.7

April 2001-April 2002 47 –2.3 6 –1.0 30 –4.2 5 2.2 88 –2.6

Average annual change: 43 –0.9 6 –5.0 25 –1.1 4 1.0 78 –1.0

October 2000–April 2001 50 13.6 6 1.6 32 19.0 5 16.2 93 14.1

October 2001–April 2002 60 20.8 7 9.1 34 20.1 6 18.8 107 19.9

Average annual change: 55 17.2 7 5.4 33 19.6 6 17.5 100 17.0

Table 7. Summary of median water-level change in the lower Milliken–Sarco–Tulucay Creeks area, southeastern Napa County, California, 2000–2002

[Median water-level change in feet]
Tables 87

Table 9. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in southeastern Nape County, California, 2001—

Continued

Stream site identifier

or State well No.

(abbreviated or

local identifier)

USGS

identification No.

Sample

date

Solids, 

residue on 

evaporation 

at 180°C,

dissolved

(mg/L)

[70300]

Solids, 

sum of 

constituents, 

dissolved

(mg/L)

[70301]

Oxygen,

dissolved

(mg/L)

[00300]

pH,

field

(standard

units)

[00400]

Specific

conductance,

field

(μS/am)

[00095]

Temperature,

water

(°C)

[00010]

Streamflow-measurement stations

Murphy Creek (MC) 381739122140001 9-19-01 174 164 8.9 7.3 130 15.0

Dairy Creek (DC) 381909122132701 9-18-01 150 146 8.9 7.4 97 14.5

Site 33 (MCW) 382130122153501 9-20-01 130 128 — 7.1 172 15.5

Site 37 (MCH) 382017122161101 9-20-01 136 133 — 7.2 184 18.0

Wells

5N/3W-6A1 (92) 381858122132601 9-18-01 194 196 3.5 7.2 124 26.0

5N/3W-6N6 (61) 381814122143101 11-06-01 640 732 <.1 7.8 1 1,220 23.0

5N/3W-7G2 (100) 381740122140001 9-19-01 451 435 <.1 6.7 638 19.5

5N/3W-8E1 (22) 381746122133101 9-19-01 282 282 1.3 6.9 356 21.5

5N/4W-1F3 (29) 381841122152401 11-07-01 278 286 3.2 7.4 386 20.5

5N/4W-12G1 (14) 381740122150201 11-06-01 (

2) 417 <.1 8.0 564 20.0

5N/4W-13H3 (36) 381649122144901 11-07-01 230 217 2.4 1 6.3 255 19.0

5N/4W-13J1 (19) 381646122145601 11-08-01 216 217 .6 7.1 248 23.5

6N/3W-31F4 (67) 381935122141601 9-21-01 218 144 <.1 7.4 265 27.0

6N/4W-15R3 (76) 382135122165901 11-08-01 220 227 1.6 6.7 269 20.0

6N/4W-23Q3 (43) 382050122160901 9-20-01 288 296 .1 7.3 402 21.5

6N/4W-25G2 (142) 382017122145801 9-17-01 256 255 <.1 7.3 260 24.0

6N/4W-26G3 (45) 382018122161301 9-20-01 279 282 .1 7.0 404 17.5

6N/4W-36G1 (95) 381939122150401 9-18-01 324 334 <.1 6.6 310 20.0

6N/4W-36R1 (53) 381905122145601 11-05-01 280 285 6.6 1 8.6 428 26.0

Table 8. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in southeastern Napa County, California, 2001

[State well No.: See well-numbering diagram on page ix. See figure 25 for location of streamflow-measurement stations, lakes and ponds, springs, and wells. 

USGS (U.S. Geological Survey) identification No. consists of latitude, longitude, and sequence number. Parameter code, in brackets, is a five-digit number in 

the USGS computerized data system, National Water Information System (NWIS, used to uniquely identify a specific constituent or property. CaCO3, calcium 

carbonate; mg/L, milligram per liter; μS/cm, microsiemen per centimeter at 25°C; °C, degree Celsius; μg/L, microgram per liter. e, value estimated by the 

USGS National Water Quality Laboratory, Denver, Colorado; <, actual value is less than value shown; —, no data]

See footnotes at end of table.
88 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Table 9. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in southeastern Napa County, California, 2001—

Continued

Stream site identifier

or

State well No.

(abbreviated or

local identifier)

Sample 

date

Hardness,

total

(mg/L as

CaCO3)

[00900]

Calcium,

dissolved

(mg/L)

[00915]

Magnesium, 

dissolved 

(mg/L)

[00925]

Potassium, 

dissolved 

(mg/L)

[00935]

Sodium, 

dissolved 

(mg/L)

[00930]

Alkalinity, 

field

(mg/L as

CaCO3)

[39086]

Bicarbonate, 

field

(mg/L)

[00453]

Streamflow-measurement stations

Murphy Creek (MC) 9-19-01 52 11.2 5.75 2.68 15.2 59 72

Dairy Creek (DC) 9-18-01 27 6.02 3.02 3.90 11.7 45 54

Site 33 (MCW) 9-20-01 46 9.47 5.43 2.75 16.7 59 71

Site 37 (MCH) 9-20-01 46 9.53 5.38 3.59 18.5 66 79

Wells

5N/3W-6A1 (92) 9-18-01 71 16.1 7.36 4.17 24.3 114 138

5N/3W-6N6 (61) 11-06-01 65 19.2 4.1 1.51 247 328 395

5N/3W-7G2 (100) 9-19-01 220 49.5 24.4 4.74 39.3 157 189

5N/3W-8E1 (22) 9-19-01 120 24.8 14.7 3.19 24.2 124 148

5N/4W-1F3 (29) 11-07-01 40 14.1 1.11 .69 72.1 98 118

5N/4W-12G1 (14) 11-06-01 37 11.6 2.06 5.09 109 223 268

5N/4W-13H3 (36) 11-07-01 100 18.4 13.1 1.52 10.6 90 108

5N/4W-13J1 (19) 11-08-01 79 16.7 9.14 1.82 21.9 108 132

6N/3W-31F4 (67) 9-21-01 74 17.7 7.32 4.29 26.9 113 139

6N/4W-15R3 (76) 11-08-01 73 12.9 9.83 6.67 23.3 98 118

6N/4W-23Q3 (43) 9-20-01 70 14.0 8.51 20.4 50.9 192 232

6N/4W-25G2 (142) 9-17-01 76 16.4 8.47 5.63 35.2 115 138

6N/4W-26G3 (45) 9-20-01 110 23.9 12.6 11.8 34.0 163 197

6N/4W-36G1 (95) 9-18-01 52 9.48 6.84 9.25 62.7 110 133

6N/4W-36R1 (53) 11-05-01 10 2.52 .793 1.15 97.7 176 183

Table 8. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in southeastern Napa County, California, 2001—

Continued
Tables 89

Table 9. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in southeastern Napa County, California, 2001—

Continued

Stream site identifier

or

State well No.

(abbreviated or

local identifier)

Sample 

date

Chloride,

dissolved

(mg/L)

[00940]

Fluoride,

dissolved

(mg/L)

[00950]

Silica,

dissolved

(mg/L)

[00955]

Sulfate,

dissolved

(mg/L)

[00945]

Nitrogen,

ammonia,

dissolved

(mg/L)

[00608]

Nitrite plus 

nitrate as N, 

dissolved

(mg/L)

[00631]

Nitrogen,

nitrite, 

dissolved 

(mg/L)

[00613]

Streamflow-measurement stations

Murphy Creek (MC) 9-19-01 10.9 <0.2 74.4 7.7 <0.04 e 0.03 <0.006

Dairy Creek (DC) 9-18-01 7.45 .2 85.4 1.0 <.04 .08 <.006

Site 33 (MCW) 9-20-01 12.9 <.2 42.7 2.1 0.04 .09 <.006

Site 37 (MCH) 9-20-01 11.5 e .1 41.7 3.3 <.04 e .03 <.006

Wells

5N/3W-6A1 (92) 9-18-01 6.18 0.3 65.2 4.3 <0.04 <0.05 e 0.003

5N/3W-6N6 (61) 11-06-01 175 1.5 46.0 30.2 1.47 <.05 <.008

5N/3W-7G2 (100) 9-19-01 67.1 .3 81.7 71.7 .15 <.05 <.006

5N/3W-8E1 (22) 9-19-01 10.2 .3 88.8 41.9 e .04 <.05 e .004

5N/4W-1F3 (29) 11-07-01 16.2 .6 48.6 61.6 <.04 2.90 <.008

5N/4W-12G1 (14) 11-06-01 54.0 .6 56.5 43.9 .53 <.05 <.008

5N/4W-13H3 (36) 11-07-01 10.7 .2 84.5 14.8 <.04 2.09 <.008

5N/4W-13J1 (19) 11-08-01 11.6 .2 86.5 3.4 .05 e .04 <.008

6N/3W-31F4 (67) 9-21-01 11.1 .3 — 7.7 .06 e .02 <.006

6N/4W-15R3 (76) 11-08-01 12.9 .2 81.9 19.7 e .03 <.05 <.008

6N/4W-23Q3 (43) 9-20-01 9.98 e .1 75.4 .6 .24 <.05 <.006

6N/4W-25G2 (142) 9-17-01 16.2 .4 81.4 22.0 .04 <.05 <.006

6N/4W-26G3 (45) 9-20-01 24.1 <.2 71.3 5.5 .24 <.05 <.006

6N/4W-36G1 (95) 9-18-01 15.4 e .1 76.1 80.3 2.57 <.05 <.006

6N/4W-36R1 (53) 9-19-01 15.5 .9 40.8 20.3 .06 <.05 <.008

Table 8. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in southeastern Napa County, California, 2001—

Continued
90 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Table 9. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in southeastern Napa County, California, 2001—

Continued

Stream site identifier

or

State well No.

(abbreviated or 

local identifier)

Sample 

date

Phosphorus,

orthophosphate,

dissolved

(mg/L)

[00671]

Arsenic,

dissolved

(μg/L)

[01000]

Boron,

dissolved

(μg/L)

[01020]

Iron,

dissolved

(μg/L)

[01046]

Lithium,

dissolved

(μg/L)

[01130]

Manganese,

dissolved

(μg/L)

[01056]

Streamflow-measurement stations

Murphy Creek (MC) 9-19-01 0.15 e 1 20 62 7.4 e 2.9

Dairy Creek (DC) 9-18-01 .17 e 2 20 32 10.5 e 1.9

Site 33 (MCW) 9-20-01 <.02 <2 200 55 13.8 e 2.3

Site 37 (MCH) 9-20-01 e .01 <2 160 59 12.6 8.1

Wells

5N/3W-6A1 (92) 9-18-01 0.04 2 60 28 43.0 1 136

5N/3W-6N6 (61) 11-06-01 .02 <2 3 11,000 <10 90.7 1 93.0

5N/3W-7G2 (100) 9-19-01 <.02 <2 830 1,710 58.3 1 314

5N/3W-8E1 (22) 9-19-01 .04 4 110 1 991 43.4 1 261

5N/4W-1F3 (29) 11-07-01 .06 1 11 230 <10 19.1 <2.0

5N/4W-12G1 (14) 11-06-01 .03 6 3 1,440 11 34.6 1 54.5

5N/4W-13H3 (36) 11-07-01 .12 1 11 e 10 e 7 7.7 5.6

5N/4W-13J1 (19) 11-08-01 .07 3 140 102 97.5 1 81.4

6N/3W-31F4 (67) 9-21-01 e .02 e 1 200 18 50.3 1 117

6N/4W-15R3 (76) 11-08-01 .17 1 17 70 31 51.4 1 309

6N/4W-23Q3 (43) 9-20-01 .33 1 67 160 12 82.6 1 491

6N/4W-25G2 (142) 9-17-01 .04 6 380 1 310 51.4 1 131

6N/4W-26G3 (45) 9-20-01 .14 1 16 200 1 462 62.7 1 831

6N/4W-36G1 (95) 9-18-01 .62 3 490 1 2,290 187 1 656

6N/4W-36R1 (53) 9-19-01 .02 4 900 18 36.8 19.9

Table 8. Field data and laboratory analyses of samples from streamflow-measurement stations and wells in southeastern Napa County, California, 2001—

Continued

1Value exceeds the maximum contaminant level (MCL) or is outside of the acceptable range for primary or secondary Federal and State drinking-water 

standards (California Department of Water Resources, 1997; U.S. Environmental Protection Agency, 2002). 2Insufficient sample for analysis.

3Value exceeds State active level (California Department of Health Services, 2002).
Tables 91

Table 9. Field measurements and oxygen-18 and deuterium ratios in samples from streamflow- measurement stations, lakes and ponds, springs, and 

wells, southeastern Napa County, California, 2001–02—Continued

Station identifier

or State well No.

(abbreviated or

local identifier)

USGS

Identification No. Subarea Sampledate

Instaneous

streamflow

(ft3/s)

Delta

oxygen-18

(per mil)

Delta

deuterium

(per mil)

Field measurements

Specific 

conductance

(μS/cm)

Water 

temperature

(°C)

Streamflow-measurement stations

Site 12 381644122123001 Tulucay 3-14-01 0.64 –7.61 –50.52 58 16.0

Site 13 381728122133801 Tulucay 3-13-01 .87 –7.39 –48.9 114 13.0

Site 14 381744122123601 Tulucay 3-13-01 .06 –7.58 –49.9 81 12.0

Site 16 381739122131101 Tulucay 3-13-01 .71 –7.29 –46.59 110 12.5

Site 17 381730122143101 Tulucay 3-14-01 1.93 –7.29 –49.55 155 11.0

Site 20A 381745122143401 Tulucay 3-14-01 .27 –6.93 –48.12 470 16.0

Site 20B 381745122143402 Tulucay 3-14-01 .32 –7.07 –48.03 242 13.0

Site 41 381709122163301 Tulucay 3-13-01

4-10-01

5.62

2.81

–6.87

–6.25

–46.98

–43.36

264

306

11.5

11.5

Site 42B 381639122151001 Tulucay 3-13-01 .65 –6.61 –45.17 120 15.0

Site 45 381723122155501 Tulucay 3-13-01

4-10-01

4.38

.78

–7.00

–6.30

–47.63

–43.7

270

337

12.0

11.0

Site 46B 381634122142901 Tulucay 3-13-01 1.10 –7.00 –45.77 99 11.5

Site 49B 381707122151801 Tulucay 3-14-01 .35 –7.28 –48.66 122 11.0

Site 50 381738122140601 Tulucay 3-13-01 .72 –7.20 –48.06 — 12.5

Site 56 381813122140601 Tulucay 3-15-01 .12 –6.88 –48.18 610 10.0

Site 58 381758122131601 Tulucay 3-15-01 .09 –7.55 –50.54 72 10.0

Site 60 381653122140201 Tulucay 3-14-01 .30 –7.46 –49.51 96 11.5

Site 62 381638122150501 Tulucay 3-13-01 .19 –6.96 –47.82 109 12.5

Murphy Creek (MC) 381739122140001 Tulucay 9-19-01 .10 –5.71 –39.34 130 15.0

Site 21 381743122161001 (

1) 3-14-01 .01 –6.78 –47.41 498 13.0

Site 1 382024122145901 Sarco 4-12-01 .18 –6.89 –45.77 110 9.5

Site 2 382046122142501 Sarco 3-16-01 .83 –7.43 –47.16 90 9.5

Site 7A 381956122151101 Sarco 3-15-01 1.05 –7.41 –47.38 113 11.0

Site 7B 381956122151102 Sarco 3-15-01 .62 –6.63 –45.18 246 14.0

Site 9 381923122145701 Sarco 3-14-01 .43 –6.52 –44.21 462 13.5

Site 11 381922122162201 Sarco 3-15-01

4-11-01

2.34

.13

–7.01

–6.57

–46.38

–44.55

210

270

11.5

12.5

Site 22A 381901122135201 Sarco 3-14-01 .08 –6.88 –47.92 488 13.0

Site 22B 381900122135101 Sarco 3-14-01 .15 –7.27 –48.81 213 12.0

Site 24 381849122132901 Sarco 3-14-01 .02 –7.29 –46.5 109 12.0

Site 26A 381922122141201 Sarco 3-14-01 .48 –6.79 –45.58 328 14.5

Site 26B 381922122141202 Sarco 3-14-01 .50 –7.50 –49.01 143 13.0

Site 28B 381925122140801 Sarco 3-14-01 .46 –7.58 –48.12 138 12.0

Site 51 381906122133501 Sarco 3-14-01 .07 –7.28 –48.24 128 12.0

Dairy Creek (DC) 381909122132701 Sarco 9-18-01 .04 –6.98 –45.36 97 14.5

Site 6A 382104122151401 Milliken 3-15-01 .03 –7.05 –44.99 119 11.5

Site 31 382139122150801 Milliken 3-15-01

4-11-01

46.2

3.40

–7.59

–7.12

–48.59

–45.38

61

104

10.0

12.0

Site 33 (MCW) 382130122153501 Milliken 3-15-01

4-11-01

9-20-01

51.7

2.40

.43

–7.62

–7.15

–5.32

–47.61

–44.57

–37.72

61

107

172

10.0

12.0

15.5

Table 9. Field measurements and oxygen-18 and deuterium ratios in samples from streamflow-measurement stations, lakes and ponds, springs, and 

wells, southeastern Napa County, California, 2001–02

[See figure 25 for location of streamflow-measurement stations, lakes and ponds, springs, and wells. USGS (U.S. Geological Survey) identification No. 

consists of latitude, longitude, and sequence number. State well No.: see well-numbering diagram on page ix. Subarea “outside” refers to sites located outside 

of the Milliken, Sarco, and Tulucay drainage basins. ft3/s, cubic foot per second; per mil, parts per thousand; μS/cm, microsiemen per centimeter at 25°C; °C, 

degree Celsius; μg/L, microgram per liter. —, no data]

See footnote at end of table.
92 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Streamflow-measurement stations—Continued

Site 35 381931122162901 Milliken 3-15-01

4-12-01

42.5

3.25

–7.54

–6.91

–47.55

–43.88

78

153

11.0

11.0

Site 37 (MCH) 382017122161101 Milliken 4-11-01

9-20-01

3.13

.46

–6.96

–5.11

–43.32

–37.51

156

184

13.0

18.0

Site 38 382031122155301 Milliken 3-15-01

4-11-01

43.4

2.53

–7.54

–6.93

–48.08

–44.74

71

149

10.0

13.0

Lakes and ponds

lower Lake Camille (LLC) 381638122151601 Tulucay 9-21-01 –1.92 –25.68 166 20.0

Maher pond (MP) 381752122132801 Tulucay 9-19-01 –.36 –19.42 220 —

Van Koten pond (VKP) 381934122150701 Sarco 9-18-01 4.04 –3.04 — —

Silverado Country Club pond 

(SCCP)

382030122154201 Milliken 9-21-01 –6.75 –44.67 317 22.0

Lake Frey (LF) 381749122112101 (

2) 1-15-02 –4.96 –37.66 64 9.5

Lake Leona (LL) 382041122122801 (

2) 1-15-02 –7.00 –46.90 88 9.0

Lake Madigan (LM) 381829122113501 (

2) 1-15-02 –4.95 –38.48 70 10.0

Lake Madigan outflow (LMO) 381829122113401 (

2) 1-15-02 –5.00 –37.88 69 9.5

Springs

Oak Canyon spring (OCS) 381848122131801 Sarco 1-16-02 –6.79 –44.25 98 11.5

Palmaz spring (PS) 381956122122701 Sarco 2-06-02 –7.10 –45.91 73 14.0

Wells

5N/3W-6A1 (92) 381858122132601 Storage Unit 3 9-18-01 –7.59 –49.06 124 26.0

5N/3W-6N6 (61) 381814122143101 Storage Unit 1 11-06-01 –7.95 –53.23 1,220 23.0

5N/3W-7G2 (100) 381740122140001 Storage Unit 1 9-19-01 –6.83 –44.8 638 19.5

5N/3W-8E1 (22) 381746122133101 Storage Unit 1 9-19-01 –7.22 –47.68 356 21.5

5N/4W-1F3 (29) 381841122152401 Storage Unit 2 11-07-01 –6.27 –41.54 386 20.5

5N/4W-12G1 (14) 381740122150201 Storage Unit 1 11-06-01 –7.13 –47.35 564 20.0

5N/4W-13H3 (36) 381649122144901 Storage Unit 1 11-07-01 –6.12 –40.6 255 19.0

5N/4W-13J1 (19) 381646122145601 Storage Unit 1 11-08-01 –6.69 –42.46 248 23.5

6N/3W-31F4 (67) 381935122141601 Storage Unit 3 9-21-01 –7.29 –47.72 265 27.0

6N/4W-15R3 (76) 382135122165901 Storage Unit 4 11-08-01 –6.44 –42.28 269 20.0

6N/4W-23Q3 (43) 382050122160901 Storage Unit 3 9-20-01 –7.17 –47.58 402 21.5

6N/4W-25G2 (142) 382017122145801 Storage Unit 3 9-17-01 –6.93 –43.45 260 24.0

6N/4W-26G3 (45) 382018122161301 Storage Unit 3 9-20-01 –6.43 –42.52 404 17.5

6N/4W-36G1 (95) 381939122150401 Storage Unit 3 9-18-01 –7.26 –48.99 310 20.0

6N/4W-36R1 (53) 381905122145601 Storage Unit 2 11-05-01 –7.47 –50.58 428 26.0

Table 9. Field measurements and oxygen-18 and deuterium ratios in samples from streamflow- measurement stations, lakes and ponds, springs, and 

wells, southeastern Napa County, California, 2001–02—Continued

Station identifier

or State well No.

(abbreviated or

local identifier)

USGS

Identification No. Subarea Sample

date

Instaneous

streamflow

(ft3/s)

Delta

oxygen-18

(per mil)

Delta

deuterium

(per mil)

Field measurements

Specific 

conductance

(μS/cm)

Water 

temperature

(°C)

1Outside of the Tulucay Creek subarea.

2Outside subarea in the Howell Mountains.
Tables 93

Table 11. Dissolved gas, CFC, and tritium concentrations in samples from wells, and calculations of CFC concentrations in air and corresponding CFC-model age, south

2001

State well No. 

(local well number 

in parenthesis)

Laboratory measurements Calculated CFC 

concentrations in air 

Dissolved gas

N2/Ar 

temp. 1

(oC)

Excess

air 1

(cm3/L)

CFC

Tritium 

concentration

(TU)

N2 1

(mg/L)

Ar 1

(mg/L)

O2 1

(mg/L)

CO2 1

(mg/L)

CH4 1

(mg/L)

CFC-11(pg/kg; 

mean)

CFC-12(pg/kg; 

mean)

CFC-113

(pg/kg; 

mean)

CFC-11

(pptv)

CFC-12

(pptv)

CFC-113

(pptv)

Time labo

5N/3W-6A1 (92) 32.0 0.9 2.9 9.0 0.0 13.5 16.0 62.7 71.0 23.2 — 27.4 133.0 24.5 Late 195N/3W-6N6 (61) 27.0 .5 .0 4.8 14.7 2 90.7 24.9 1.6 42.0 .0 — 2 .8 2 83.1 2 .0 1950s o

probdega

H2S

5N/3W-7G2 (100) 22.8 .8 .1 32.6 .0 10.2 5.5 1.1 19.6 .0 — .4 31.6 .0 Mid to 

coul

5N/3W-8E1 (22) 36.9 .9 .1 16.5 .0 15.3 21.7 21.1 35.1 6.1 .32 10.2 71.5 7.2 Mid 195N/4W-1F3 (29) 18.6 .7 3.2 4.1 .0 12.3 1.9 119.0 169.3 15.9 1.50 48.8 299.9 15.7 Late 195N/4W-12G1 (14) 23.6 .7 .0 1.7 1.0 23.3 10.2 1.7 9.8 .0 — 1.2 27.9 .0 Mid to 

coul

5N/4W-13H3 (36) 21.7 .7 .7 41.0 .0 16.1 6.3 12.0 6.8 .0 .50 6.0 14.3 .0 Mid 19

CFC

5N/4W-13J1 (19) 17.7 .6 .1 9.6 .0 17.8 2.7 3 19.0 313.2 3 1.9 — 3 10.2 3 29.8 3 2.5 Late 196N/3W-31F4 (67) 20.9 .7 .1 4.4 .1 13.6 4.7 3.1 5.4 .0 — 1.4 10.1 .0 Early 16N/4W-15R3 (76) 21.4 .7 .1 15.7 .0 13.4 5.1 26.7 1,450.8 6.5 2.12 11.7 2,720.1 6.8 Early 1

12 

6N/4W-23Q3 (43) 22.6 .6 .1 10.7 .0 24.2 9.4 4.9 9.0 .0 — 3.6 26.4 .0 Mid to 6N/4W-25G2 (142) 21.1 .7 .1 6.9 .3 13.5 4.9 4.9 15.7 1.7 1.12 2.2 29.8 1.8 Mid 19

degr

6N/4W-26G3 (45) 24.4 .7 .1 20.9 .9 20.8 10.5 3.8 76.7 2.3 — 2.4 197.6 3.5 Mid 19

degr

6N/4W-36G1 (95) 21.5 .7 .1 30.5 .0 11.4 4.6 4.0 24.1 .0 .25 1.5 40.9 .0 Early 16N/4W-36R1 (53) 16.5 .6 3.6 .7 .0 21.8 2.6 3 122.4 3171.2 3 35.3 — 379.8 3 459.2 3 58.0 Mid to 

Table 10. Dissolved gas, chlorofluorocarbon (CFC), and tritium concentrations in samples from wells, and calculations of CFC concentrations in air and corresponding Napa County, California, 2001[State well No.: See well-numbering diagram on page ix. See figure 25 for locations of wells. Mean of CFC-laboratory measurements from three of five ampoules, unless site; the remaining two ampoules used for quality control. N2, nitrogen; Ar, argon; O2, oxygen; CO2, carbon dioxide; CH4, methane; CFC, chlorofluorocarbon; H2S, hydro

liter; °C, degree Celsius; cm3/L, centimeter cubed per liter; pg/kg, picogram per kilogram; TU, tritium units; pptv, parts per trillion by volume. —, no data]

1Mean of measurements for two samples collected at each site.2The 30-year (1961–90) average air temperature at Napa, California (National Oceanic Atmospheric Administration, 2002); 14.7°C was used for this calculation.3Mean of measurements from two ampoules; the third ampoule was “contaminated” (concentration greater than 2001 air) (Plummer and Busenberg, U.S. Geologic

2002).
94 Ground-Water Resources in the Lower Milliken–Sarco–Tulucay Creeks Area, Southeastern Napa County, California, 2000–2002

Table 12. Helium, hydrogen, and neon gas concentrations in water 

samples from wells, southeastern Napa County, California, 2001

State well No. 

(local well number 

in parenthesis)

Dissolved gas concentrations1

(10-8 cc/g at STP)

He H2 Ne

5N/3W-6N6 (61) 534.3 3.8 —

5N/3W-7G2 (100) 29.8 74.5 22.6

5N/3W-8E1 (22) 27.7 74.6 28.0

5N/4W-12G1 (14) 96.7 6.6 —

6N/4W-26G3 (45) 255.3 26.4 —

6N/4W-36G1 (95) 20.6 1.9 31.0

Table 11. Helium, hydrogen, and neon gas concentrations in water 

samples from wells, southeastern Napa County, California, 2001

[See figure 25 for location of wells. cc/g, cubic centimeter per gram; STP, 

standard temperature and pressure; He, helium; H2, hydrogen; Ne, neon. 

—, no data]

1Mean for two samples collected at each site.