U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4096

Prepared in cooperation with

PAJARO VALLEY WATER MANAGEMENT AGENCY 

GEOHYDROLOGIC FRAMEWORK OF RECHARGE AND

SEAWATER INTRUSION IN THE PAJARO VALLEY,

SANTA CRUZ AND MONTEREY COUNTIES, CALIFORNIA

Pajaro Valley

Water Management

Agency

Pajaro Valley

Water Management
Base image for cover illustration provided

courtesy of the Monterey Bay Aquarium Research Institute.
Geohydrologic Framework of Recharge and 

Seawater Intrusion in the Pajaro Valley, Santa 

Cruz and Monterey Counties, California

By Randall T. Hanson

Prepared in cooperation with the

PAJARO VALLEY WATER MANAGEMENT AGENCY

Sacramento, California

2003

Water-Resources Investigations Report 03-4096

U.S. GEOLOGICAL SURVEY
For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall—Suite 2012

Sacramento CA 95819-6129

http://ca.water.usgs.gov/

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.

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director
Contents iii

CONTENTS

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

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

Purpose and Scope ...................................................................................................................................... 6

Approach to Investigation........................................................................................................................... 8

Description of Study Area........................................................................................................................... 8

Climate............................................................................................................................................... 8

Population .......................................................................................................................................... 8

Land and Water Use........................................................................................................................... 8

Acknowledgments.............................................................................................................................. 9

Geohydrology....................................................................................................................................................... 9

Hydrology............................................................................................................................................................. 14

Precipitation ................................................................................................................................................ 14

Streamflow .................................................................................................................................................. 14

Ground Water.............................................................................................................................................. 25

Coastal Monitoring-Well Sites........................................................................................................... 25

Long-Term Water Levels and Seawater Intrusion............................................................................. 35

Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry...................................................... 35

Characteristics of Water from Monitoring and Supply Wells .................................................................... 39

Source, Age, and Movement of Ground Water........................................................................................... 45

Seawater Intrusion and Saline Ground Water............................................................................................. 53

Summary and Conclusions................................................................................................................................... 58

Selected References.............................................................................................................................................. 59

Appendix 1. Summary Analysis of Geophysical, Lithologic, and Well-Construction 

Data Analysis for Selected Monitoring Wells in the Pajaro Valley................................................64

Appendix 2. Water-Chemistry Data for Selected Monitoring Wells and Surface Waters 

in the Pajaro Valley.........................................................................................................................77
iv Figures

FIGURES

Figure 1. Maps showing (A) Pajaro Valley study area and Salinas Valley locations for ground-water 

sampling sites, (B) outline of Pajaro Water Management Agency and selected features in the 

Pajaro Valley study area, California, (C) distribution of pumping wells, water agency 

boundaries, and sampling locations in the Pajaro Valley, California, and (D) distribution of 

pumping wells with depth to top of perforations in the Pajaro Valley, California............................ 3

Figure 2. Map showing geology, selected wells used to estimate aquifer thicknesses, geologic outcrops, 

and sites for geophysical logging in the Pajaro Valley, Santa Cruz and Monterey Counties, 

California ........................................................................................................................................... 10

Figure 3. Geologic section showing relation between geology and selected water-quality attributes 

along the coast in the Pajaro Valley, Santa Cruz, and Monterey Counties, California ..................... 11

Figure 4. Graph showing cumulative departure of precipitation at Watsonville, delineation of wet and 

dry periods, and estimated ground-water pumpage for Pajaro Valley, Santa Cruz, and Monterey 

Counties, California ........................................................................................................................... 15

Figure 5. Graphs showing relations of monthly precipitation to streamflow for Corralitos Creek near 

Freedom, Santa Cruz County, California, 1958–97........................................................................... 16

Figure 6. Graphs showing relation between streamflow and water chemistry for selected sites on 

the Pajaro River and for Corralitos Creek at Freedom, Santa Cruz County, California.................... 17

Figure 7. Graphs showing flow duration for the Pajaro River at Chittenden, 1949–98, for Corralitos 

Creek near Corralitos, 1958–73, and for Corralitos Creek at Freedom, California 1957–98, 

Santa Cruz County, California........................................................................................................... 24

Figure 8. Hydrographs and chemographs from the PV-1 multiple-well monitoring site in the San 

Andreas subarea of the Pajaro Valley, Santa Cruz County, California ............................................. 26

Figure 9. Hydrographs and chemographs from the PV-3 multiple-well monitoring site in the Pajaro 

River Mouth subarea of the Pajaro Valley, Santa Cruz County, California...................................... 28

Figure 10. Hydrographs and chemographs from the PV-4 multiple-well monitoring site in the Springfield 

subarea of the Pajaro Valley, Monterey County, California.............................................................. 29

Figure 11. Hydrographs and chemographs from the PV-6 multiple-well monitoring site in the Pajaro 

River Mouth subarea of the Pajaro Valley, Santa Cruz County, California...................................... 30

Figure 12. Hydrographs and chemographs from the PV-8 multiple-well monitoring site in the San 

Andreas/Harkins Slough subareas of the Pajaro Valley, Santa Cruz County, California.................. 31

Figure 13. Hydrographs and chemographs from the PV-11 multiple-well monitoring site in the San 

Andreas subarea of the Pajaro Valley, Santa Cruz County, California ............................................. 32

Figure 14. Hydrographs and chemographs from the SC-A3 multiple-well monitoring site in the 

La Selva Beach area of Soquel Creek Water District north of Pajaro Valley, Santa Cruz 

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

Figure 15. Hydrographs and chemographs from the SC-A4 multiple-well monitoring site in the 

La Selva Beach area of Soquel Creek Water District north of Pajaro Valley, Santa Cruz 

County, California ............................................................................................................................. 34

Figure 16. Hydrographs from multiple-well PV-3 and deep well 12S/1E-25B3 in the Pajaro River 

Mouth subarea and wells 12S/3E-8C and 12S/3E-7J2 in the Pajaro subarea of the Pajaro 

Valley, Santa Cruz County, California .............................................................................................. 36

Figure 17. Hydrographs from multiple-well PV-4 and well 12S/2E-30M2 in the Springfield subarea 

of the Pajaro Valley, Monterey County, California........................................................................... 38

Figure 18. Map showing distribution of boron, chloride, sulfate, and specific conductance for selected 

wells in the Pajaro Valley, Santa Cruz County, California ............................................................... 40
Figures v

Figure 19. Diagrams showing major-ion chemistry for selected ground-water and surface-water 

samples from the Pajaro Valley 1989, 1996, and 1998 ..................................................................... 43

Figure 20. Graph showing deuterium and oxygen-isotope values for selected ground-water and 

surface-water samples from the Pajaro Valley and adjacent Salinas Valley, California, 

and the seawater mixing line for Pajaro Valley ground-water samples............................................. 46

Figure 21. Graphs showing strontium-isotope ratios plotted against strontium for selected ground-water 

and surface-water samples from the Pajaro Valley and Salinas Valley, California .......................... 47

Figure 22. Graphs showing chloride values plotted against the ratio of chloride to boron, 

chloride to iodide, and chloride to bromide for ground-water and surface-water sites in the 

Pajaro Valley and adjacent Salinas Valley, California ...................................................................... 50

Figure 23. Graphs showing boron-isotope values plotted against the ratio of chloride to boron, and 

sulfur-34 isotope values plotted against sulfate concentration for selected ground-water and 

surface-water samples from the Pajaro Valley and adjacent Salinas Valley, California................... 55
vi Tables

TABLES

Table 1. Summary of streamflow attributes for selected streams in the Pajaro Valley, California................. 23
Conversion Factors, Vertical Datum, Water-Quality Information and Abbreviations vii

CONVERSION FACTORS, VERTICAL DATUM, WATER-QUALITY 

INFORMATION, AND ABBREVIATIONS

CONVERSION FACTORS 

Temperature is given in degrees Celsius (oC), which can be converted to degrees Fahrenheit (oF) by 

the following equation:

oF = 1.8(oC) + 32.

VERTICAL DATUM

Sea Level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD 

of 1929)—a geodetic datum derived from general adjustments of the first-order level nets of both the 

United States and Canada, formerly called Sea Level Datum of 1929.

WATER-QUALITY INFORMATION

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

micrograms per liter (μg/L). Milligrams per liter is equivalent to “parts per million” and micrograms 

per liter is equivalent to “parts per billion.” Selected constituents also are expressed in terms of 

millimoles, which is the concentration in milligrams per liter divided by the atomic weight of the 

element. Selected constituents also are expressed in terms of milliequivalents per liter, which is the 

concentration in milligrams per liter divided by the valence of the element. Specific conductance is 

given in microseimens per centimeter at 25 oC (μS/cm at 25 oC). Tritium activity is given in picocuries 

per liter (pCi/L). Carbon-14 data are expressed as percent modern carbon (pmc), and carbon-13 data 

are expressed in delta notation as per mil differences relative to the ratio of carbon-13 to carbon-12.

Multiply By To obtain

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer

square mile (mi2) 12.590 square kilometers

acre-foot (acre-ft) 0.001233 cubic hectometer

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

cubic foot per second (ft3/s) 0.02832 cubic meter per second

acre 0.4047 hectometer

pound 0.4536 kilogram

pound per cubic foot (lb/ft3) 0.01602

gram per cubic centimeter 

(g/cm3)
viii Conversion Factors, Vertical Datum, Water-Quality Information and Abbreviations

ABBREVIATIONS

PV Pajaro Valley

EM electromagnetic induction

ft bls feet below land surface

km/s kilometers per second

km-g/s-cm3 kilometers-grams per second-centimeter cubed

MCL (Primary) maximum contaminant level

mmho/m milli-mhos per meter

per mil parts per thousand

pmc percent modern carbon

SC Soquel Creek

SMCL secondary maximum contaminant level (EPA) 

ybp years before present

Organizations

EPA U.S. Environmental Protection Agency 

PVWMA Pajaro Valley Water Management Agency

MCWRA Monterey County Water Resources Agency

SCWD Soquel Creek Water District

USGS U.S. Geological Survey
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. The identification consists of the township number, north or south; the range number, east or west, 

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

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

the southwest corner. Within the 40-acre tracts, wells are sequentially numbered in the order they are inventoried. 

The final letter refers to the base line and meridian. In California, there are three base lines and meridians; 

Humboldt (H), Mount Diablo (M), and San Bernadino (S). All wells in the study area are referenced to the Mount 

Diablo base line and meridian (M). Well numbers consist of 15 characters and follow the format 

012S003E25B003M. In this report, well numbers (except in tables) are abbreviated and written 12S/3E-25B3. 

Wells in the same township and range are referred to by only their section designation, 2M1. 

12S/3E-25B3

D C B A

E F G H

M L K J

N P Q R

6 5 4 3 2 1

7 8 9 10 11 12

18 17 16 15 14 13

19 20 21 22 23 24

30 29 28 26 25

31 32 33 34 35 36

27

R3E R4E

T12S

T11S

T10S

R2E

RANGE

TOWNSHIP

R1W R1E

T13S

T14S

R3E

SECTION 25

T12S

MERIDIAN

25

Well-numbering diagram (Note: maps in this report use abbreviated well numbers such as "25B3")
Abstract 1

Geohydrologic Framework of Recharge and Seawater 

Intrusion in the Pajaro Valley, Santa Cruz and Monterey 

Counties, California

By Randall T. Hanson

ABSTRACT

Pajaro Valley is a coastal watershed of 160 

square miles located along Monterey Bay north of 

Elkhorn Slough and south of the city of Santa 

Cruz. The valley has been predominantly 

developed for agriculture since the late 1800s. In 

1984 the Pajaro Valley Water Management 

Agency (PVWMA) was formed and was delegated 

with the responsibility of the management of the 

water resources within the Pajaro Valley by the 

State of California. About 84 percent of the water 

is used for agriculture and 16 percent is used for 

industrial and municipal water supply; almost all 

of the demand is supplied by ground water. 

Ground-water pumpage varies with seasonal and 

climatic periods.

The alluvial aquifers are composed of 

Quaternary- and Tertiary-aged sediments that are 

layered marine and terrestrial coarse-grained 

deposits separated by extensive fine-grained 

deposits that potentially restrict vertical movement 

of ground water and seawater intrusion in the 

coastal subareas. The coarse-grained deposits, 

which persist over large areas, control pumpage 

and related seawater intrusion. The Aromas Sand 

crops out throughout the north and central parts of 

the PVWMA area and offshore on the continental 

shelf and in Monterey submarine canyon. Because 

many of the wells in the coastal and inland 

subregions are screened at depths of 200 to 400 

feet below land surface, a direct avenue is 

provided for seawater intrusion through the 

coarse-grained deposits of the shallower alluvium 

and Aromas Sand. Geophysical logs from 

monitoring wells indicate discrete zones of saline 

water that are related to pumpage and seawater 

intrusion in the aquifers of the shallower alluvium 

and upper Aromas Sand in the upper-aquifer 

system and to deeper saline waters in the lower 

Aromas Sand within the lower-aquifer system.

The precipitation data indicate that there 

were at least nine dry and nine wet periods that 

range from 2 to 19 years during the period of 

record, 1880–1997. The ground-water pumpage, 

runoff, streamflow and related water quality of 

streamflow also vary with seasonal and climatic 

periods.

Recharge occurs from deep percolation of 

precipitation and from infiltration of streamflow. 

Streamflow originates from local runoff and from 

outside the valley as inflow from the Pajaro River. 

Although partly regulated, streamflow in the 

Pajaro River at Chittenden is less than 200 cubic 

feet per second 88 percent of the time and is less 

than 12 cubic feet per second 50 percent of the 

time. Streamflow water-quality data suggest that 

there may be several sources of poor-quality water 

that contribute to elevated chloride, sulfate, and 

nitrate concentrations in streamflow. The poor 

water quality occurring during lower streamflows 

indicates that low flows may be an additional 

source of salinity for ground-water recharge as 

streamflow infiltration along the Pajaro River.
2 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

The geochemical data from this study 

indicate that the two main sources of recharge are 

deep percolation of local runoff and streamflow 

infiltration of Pajaro River water. The geophysical 

and geochemical data suggest that only the 

shallow alluvial aquifer and parts of the upper 

Aromas Sand that constitute the upper-aquifer 

system are being replenished by recent recharge in 

the coastal areas of Pajaro Valley and represent the 

renewable ground-water resources. These data 

also suggest that there is very little vertical flow 

through the layered aquifer systems in the coastal 

regions. The confining aquitards are laterally 

extensive but may be missing in places owing to 

fluvial erosion or offsetting by fault movement. 

Geochemical and geophysical data indicate that 

the ground water from some parts of the upper and 

lower Aromas Sand in the coastal regions was 

recharged thousands of years ago and may, in part, 

represent nonrenewable ground-water resources. 

The analysis of major-ion chemistry, in 

combination with isotope and traceelement/chloride ratios, indicates that the coastal 

ground-water and surface-water samples can be 

described in seven groups. These groups represent 

recently recharged shallow aquifers (Group 1), 

shallower aquifers containing older water (Group 

2), shallow aquifers containing recently intruded 

seawater (Group 3), deeper aquifers containing 

older seawater (Group 4), deeper aquifers 

containing older freshwater (Group 5), Pajaro 

River streamflow (Group 6), and local runoff from 

streams and sloughs (Group 7).

The ground-water levels have been near or 

below sea level at most coastal monitoring wells 

since their installation in the 1980s. Water-level 

changes include long-term decline, climate-period 

changes, and seasonal changes from agricultural 

pumping. The seasonal and long-term declines 

owing to dry years and increased pumpage from 

the lower Aromas Sand are similar to the waterlevel declines measured in other local deep wells 

since 1970. At the time of this study (1998–99), 

water levels in most coastal and some inland 

water-supply wells as far inland as Murphy 

Crossing in the Pajaro subarea and in the 

Springfield subarea were below the estimated 

water levels of 5 to 15 feet above sea level required 

to stop seawater intrusion.

The predominant source of the chloride in 

the coastal regions of Pajaro Valley may be recent 

seawater for aquifers in the older alluvium and the 

upper Aromas Sand and old seawater or connate 

water in the aquifers of the lower Aromas Sand. 

Geochemical and geophysical data indicate that 

saline water is vertically restricted for both the 

recent seawater intrusion in the older shallow 

alluvium and upper Aromas Sand and for the older 

seawater or connate water present in the lower 

Aromas Sand in Pajaro Valley. 

INTRODUCTION

The Pajaro Valley Water Management Agency 

(PVWMA) was established in 1984 by the State of 

California as the agency charged with ground-water 

management in Pajaro Valley (figs. 1A, B). The 

PVWMA must deal with issues of both water supply 

and water quality. Managing water resources for 

agricultural and municipal users requires maintaining a 

reliable water supply to meet growing demands while 

preventing ground-water overdraft and the related 

degradation of water quality. Important regional waterquality concerns include increased nitrate 

concentrations and seawater intrusion. To protect the 

quantity and quality of the ground-water supplies, the 

PVWMA has implemented a plan to prevent further 

seawater intrusion. In order to evaluate how these 

activities can be conducted most effectively, it is 

necessary to improve the understanding of the 

hydrogeology and geochemistry of the water resources 

in the Pajaro Valley.

A Basin Management Plan (BMP) developed for 

the Pajaro Valley in 1993 (Montgomery Watson, 1993) 

is continuing to be refined to help provide reliable 

water resources. In order to effectively implement the 

BMP, it is necessary to identify the parts of the aquifer 

systems that are not in connection with sources of 

recent recharge, the parts of the aquifer system that are 

being actively recharged in the coastal regions and 

represent the renewable resource, and the sources and 

movement of the natural and artificial recharge and 

seawater intrusion.
Introduction 3

Pacific Ocean

Monterey Bay

Pajaro

River

Watershed

Pajaro

Valley

Study Area

Salinas River

Watershed

Santa

Clara

Co

Santa

Cruz

Co

Merced

Co

Stanislaus

Co

Fresno

Co

Monterey

Co

San

Benito

Co

Soquel Santa Cruz

Pajaro

Salinas

River

River

Corralitos

Creek

ElkhornSlough

Corralitos

Freedom

Monterey

Marina Salinas

King

City

King

City

Watsonville

Santa Cruz Mts

S a linas

V alley

R3W R1W R1E R3E R5E R7E R9E R11E R13E

T

12

S

T

14

S

T

16

S

T

18

S

T

20

S

T

10

S

T

8

S

0 5 10 15 20 Miles

0 5 10 15 20 Kilometers

A

Pajaro Valley

Water Management

Agency boundry

122°00' 121°45' 121°30' 121°15' 121°00' 120°45'

37°

00'

36°

45'

36°

30'

36°

15'

37°

15'

P a c i f i c O c e a n

CALI FORNIA

N

PAJARO

VALLEY

STUDY

AREA

Santa

Maria

Salinas Valley well

Figure 1. (A) Pajaro Valley study area and Salinas Valley locations for ground-water sampling sites, (B) outline of Pajaro Water Management Agency and 

selected features in the Pajaro Valley study area, California, (C) distribution of pumping wells, water agency boundaries, and sampling locations in the 

Pajaro Valley, California, and (D) distribution of pumping wells with depth to top of perforations in the Pajaro Valley, California.
4 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

20

40

60

80

100

300

500

900 700

Pacific Ocean

Monterey Bay

Santa

Clara

Co

Santa

Clara

Co

Santa

Cruz

Co

Monterey

Co

San

Benito

Co

Soquel

Santa Cruz

Murphy

Crossing Pajaro

River

Corralitos

Aptos

Creek

Creek

Soquel

Creek

Elkhorn

Watsonville Harkins

Gallighan

Slough

Hanson

Slough

Struve

Slough

Slough

College Lake

Kelly Lake

Tynan Lake

Pinto Lake

Slough

Slough

La Selva

Beach

Sunset

Beach

Monterey Submarine CanyonMonterey Submarine Canyon

Buena

Vista

Palm

Beach

San Andreas Dunes

Giberson Dunes Giberson Dunes

Springfield

Terrace

Corralitos

Freedom

Pajaro

Salsipuedes

Aromas

Watsonville

Bolsa de

Cayetano

Mattos

Gulch

R2W R1W R1E R2E R3E R4E

T

12

S

T

13

S

T

11

S

T

10

S

T

9

S

EXPLANATION

Pajaro River watershed

Outside Pajaro River

watershed

Pajaro Valley Water

Management Agency

boundary (USBR, 1998)

City of Watsonville

service area

River or streams

Bathymetry contours—

Number is depth below sea

level in meters. Interval varies.

Data from MBARI, 2000

0 5 10 Miles

0 5 10 Kilometers

122°00' 121°52'30"

36°

52'

30"

37°

00'

37°

07'

30"

B 121°45' 121 121°37'30" °30'

Figure 1.—Continued.
Introduction 5

City of Santa Cruz

Service Area

Pajaro Valley Water

Management Agency

Alco Water

Company

Service Area

Pacific Ocean

Monterey

Bay

ZayanteVergeles

Fault

Zone

San Andreas

Fault Zone

Santa Cruz Basin

AG-1

AG-2

PV-14

PR-16.5

PR-14.4

PR-10.9

CO-4–4.2

W-019

W-011

W-08

PV-12

PV-8

PV-1

PV-3

PV-5

12S/3E-8C1

12S/3E-7J2

12S/1E-25B3

12S/1E-24L1

12S/1E-24L3

11159200

11159000

PR-2.2

WS-1.6

PV-4A

DLN-D

HDL

PV-11

PV-13

SC-A4

SC-A3 SC-A2

SC-A5

PV-6

12S/1E-25G1 12S/2E-30M2

11159150

City of Watsonville

Service Area

Central

Water

District

Soquel Creek

Water District

0 5 10 Miles

0 5 10 Kilometers

C

122°00' 121°52'30"

36°

52'

30"

37°

00'

37°

07'

30"

121°45' 121 121°37'30" °30'

R2W R1W R1E R2E R3E R4E

T

12

S

T

13

S

T

11

S

T

10

S

T

9

S

Figure 1.—Continued.
6 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

The PVWMA implemented a monitoring-well 

network along the coast in 1989 to assess seawater 

intrusion (fig. 1C). Water-level and water-quality data 

have been collected from water-supply wells and five 

multiple-well monitoring sites. Data collected from 

these wells provide a basis for refining the 

understanding the source, movement, and distribution 

of recharge and seawater intrusion in the coastal region 

of Pajaro Valley that is related to the pumpage of 

ground water from hundreds of wells (fig. 1D).

Purpose and Scope

The purpose and scope of this study was to 

collect and analyze hydrologic, water-chemistry, and 

geologic data that will help improve the understanding 

of the ground-water flow systems and the relation 

between recharge, pumpage, seawater intrusion and 

other water-quality issues for the aquifers within the 

Pajaro Valley. Because water-resource management 

issues such as overdraft, conjunctive-use, and water 

quality are linked to the collective management of the 

ground-water resources, the following issues were 

investigated and are discussed in this report:

(1) Sources of chloride, sulfate, nitrate, and other 

constituents that contribute to poor ground-water 

quality;

(2) Vertical distribution of seawater intrusion and 

how it relates to the current definition of aquifers;

(3) Movement of recharge water through the 

ground-water system and its affect on local groundwater quality; and

(4) The relation of recharge and pumpage to 

seawater intrusion.

EXPLANATION

Known

Inferred

Concealed

Water-supply wells—

Multiple-well monitoring sites—

Pajaro River watershed

Outside Pajaro River watershed

River or stream

Bathymetry contours—Interval varies. Data

from MBARI, 2000

Faults—

From USGS (Brabb and others, 1997),

CDMG (Wagner and others, 2000)

Soquel Creek Water District

Monterey County Water Resources Agency

USGS streamflow/PVWMA Surface-water

sampling sites

USGS Streamflow gage

Pajaro Valley Water Management Agency

City of Watsonville

Long-term monitoring wells

12

11

10

1

2

3

4

5

6

7

8

9

13

Highlands North

Highlands South

Beach Road

Corralitos

Harkins Slough

San Andreas

Watsonville

Salsipuedes

Pajaro

Granite Ridge

Pajaro River Mouth

Springfield Terrace

Monterey Bay

Surface-water sites—

Ground-water subareas—

(Tegavhi and Smith, 1999d)

Figure 1.—Continued.
Introduction 7

Monterey Bay

0 5 Miles

0 5 Kilometers

EXPLANATION

Pajaro River watershed

Outside Pajaro River watershed

Pajaro Valley Water

Management Agency

(USBR, 1998) City of Watsonville service area

See figure 1C for list of groundwater subareas

River or streams

Bathymetry contours

(MBARI, 2000)

36°

52'

30"

37°

00'

Depth to top of perforations—

in feet below land surface

Equal to or less than 105

106–200

201–300

301–400

401–730

D

Ground-water

subareas

121°52'30" 121 121°37'30" °45'

Figure 1.—Continued.
8 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

This report summarizes the source, age, and 

movement of recharge to ground water and the nature 

and extent of seawater intrusion in the coastal region of 

the Pajaro Valley. 

Approach to Investigation 

Existing geologic data were compiled from 

selected supply wells (Doug Coty, PVWMA, written 

commun., 1999) and integrated with other sources of 

geologic information. In addition, geophysical data 

were collected from selected monitoring wells in 

August 1999. The methods used and the data collected 

from geophysical logging and compiled from other 

sources are described in Appendix 1. Hydrologic data 

were compiled from precipitation gages, streamflow 

gages, and ground-water monitoring wells (Doug Coty, 

PVWMA, written commun., 1999). Additional waterchemistry data was provided by Soquel Creek Water 

District (SCWD) for selected monitoring wells and 

Soquel Creek (Jeff Gailey, SCWD, written commun., 

1999). Water-chemistry samples of selected monitoring 

wells and surface waters were collected and analyzed 

by the U.S. Geological Survey (USGS) during the 

period August 1998–May 1999 (table A2.1). Waterchemistry data-collection methods and analysis are 

summarized in Appendix 2. These data were used to 

improve the geohydrologic framework of recharge and 

seawater intrusion and they provide a baseline for the 

future comparison of performance of the water-supply 

options that will ultimately be employed to resolve the 

water-supply problems of the Pajaro Valley.

Description of Study Area

Pajaro Valley is a coastal watershed of 160 mi2 

in the southern part of Santa Cruz County and the 

northern part of Monterey County; the Pajaro River is 

the county boundary. The valley is the coastal part of 

the larger Pajaro River Basin (fig. 1A). The valley is 

drained by the Pajaro River and its tributaries, of which 

Corralitos Creek is the largest (figs. 1A, B). This 

coastal watershed is located along Monterey Bay south 

of the city of Santa Cruz and north of Elkhorn Slough 

and its offshore extension, Monterey submarine canyon 

(fig. 1B). The valley has been developed predominantly 

for agriculture since the late 1800s but also contains the 

city of Watsonville and other small towns and suburban 

areas (fig. 1B). The PVWMA service area encompasses 

about 70,000 acres, of which about 40 percent is used 

for agriculture.

Climate

The climate of the Monterey Bay region is 

temperate with mild summers and wet and cool 

winters. The mean rainfall for Pajaro Valley ranges 

from 16 in. near the coast to more than 40 in. in the 

foothills of the Santa Cruz Mountains. The average 

rainfall for the city of Watsonville is about 22 in. for a 

109-year period of record. 

Population 

The total population in the valley has grown 

from 60,000 in 1990 to about 80,000 by 1996 

(Association of Bay Area Governments, 1997). The 

city of Watsonville is a major urban area in the valley 

with a population of about 31,100 in 1990 (fig. 1C). 

There are extensive suburban areas and the additional 

smaller communities of Pajaro, Freedom, Corralitos, 

and Aromas. 

Land and Water Use 

 Sixty-seven percent of the valley is classified as 

native vegetation, and agriculture remains the primary 

human use of land and water in Pajaro Valley. About 84 

percent of the water is currently used for agriculture 

and about 16 percent for industrial and municipal 

supply (Teghavi and Smith, 1999b). Average 

agricultural water use was estimated to range from 

about 50,000 acre-ft/yr (1964–87) to 56,000 acre-ft/yr 

(1987–97) (Teghavi and Smith, 1999a, b). Agriculture 

increased from about 30,500 acres in 1966 to about 

34,700 acres in 1997 (about 24 percent of land use), a 

14-percent increase over 32 years (Teghavi and Smith, 

1999a); this is an increase of about 0.4 percent per year. 

The increased acreage is accompanied by an increase in 

truck crops and decrease in deciduous crops. Most of 

the increase in truck crop is strawberries, which 

increased from 7 percent to 23 percent of agricultural 

acreage between 1966 and 1997 (Teghavi and Smith, 

1999a).
Geohydrology 9

Urban water use was estimated to have increased 

over 34 years (1964–1997) at about 2.5 percent per 

year (Teghavi and Smith, 1999b). More recently, urban 

water use increased from 9,000 acre-ft/yr (1964–87) to 

10,000 acre-ft/yr (1987–97) (Teghavi and Smith, 

1999b). Almost all of the water demand is supplied 

from wells pumping ground water (fig. 1D).

Acknowledgments

This study could not have been accomplished 

without the assistance of personnel from the Pajaro 

Valley Water Management Agency (PVWMA), Dr. 

Andy Fisher of the University of California at Santa 

Cruz (UCSC), Ms. Laura Brown and Mr. Jeff Gailey of 

the Soquel Creek Water District (SCWD), Mr. Bruce 

Laquerge of the county of Santa Cruz, and Mr. Galin 

Swain of the city of Watsonville. Other local 

municipalities, water mutuals, and numerous other well 

owners also provided well data. This study represents a 

cooperative effort with PVWMA and is part of an 

ensemble of studies and projects that were 

implemented to help the agency better understand the 

water-resources issues and apply solutions to the watersupply problems of overdraft and seawater intrusion. 

For this reason the author acknowledges the 

cooperation of the principal consultants involved in the 

related projects and studies: Lyndel Melton, Ali 

Teghavi, Chris Smith, Matt Zadar, and Martin Feeney.

The author also acknowledges the contributions 

of Dr. Avner Vengosh and Dr. Jim Gill of UCSC for the 

geochemical data collected, in part with the USGS, in 

the Salinas Valley. The author also thanks Tom Bullen 

and John Fitzpatrick of the USGS for analyses of 

strontium and boron isotopes and the technical input of 

Tom Bullen, John Izbicki, and Michael Land of the 

USGS.

GEOHYDROLOGY

The geology of Pajaro Valley and adjacent parts 

of the Monterey Bay has been compiled in various 

studies (California State Water Resources Board, 1953; 

Green, 1970; Dupre, 1975; Luhdorff and Scalmanini, 

1987a, b, 1988; Brabb and others, 1997; Teghavi and 

Smith, 1999c; Wagner and others, 2000; Rosenberg, 

2001) related to the geohydrologic framework of 

Pajaro Valley. The following geohydrologic description 

is a brief summary of these previous studies.

The alluvial aquifers in the Pajaro Valley are 

underlain by granitic basement rocks of Cretaceous age 

that generally occur at depths of 2,000 to 4,000 ft along 

the coast (Johnson and others, 1988). Above the 

relatively impermeable basement rocks are a sequence 

of poorly permeable consolidated rocks composed of 

mostly marine shales, mudstones, clay, silt, fine sand, 

conglomerate, and minor deposits of volcanic rock of 

Eocene to Miocene age (Johnson and others, 1988). 

The important geologic units relative to ground-water 

flow above the consolidated marine rocks are a 

sequence of westward-dipping sedimentary rocks of 

late Tertiary to Quaternary age. These rocks include the 

marine deposits of the poorly consolidated Purisima 

Formation of Miocene to Pliocene age and range from 

the land surface along the northern and eastern 

boundaries of the valley to depths of 800 to 900 ft 

below land surface at the mouth of the Pajaro River 

(Johnson and others, 1988). The Purisima Formation 

outcrops to the north within the Soquel Creek Water 

District (SCWD) area (fig. 2). The Purisima Formation 

is overlain by the unconsolidated Aromas Sand and 

terrace deposits of Pleistocene age. The Aromas Sand 

crops out throughout the north and central parts of the 

PVWMA area and offshore on the continental shelf and 

in Monterey submarine canyon (fig. 2). These rocks are 

overlain in places by unconsolidated alluvium, dune 

deposits, and younger marine sediments of Holocene 

age (Johnson and others, 1988) (fig. 2).

Although there is no formal designation of 

aquifers in the Pajaro Valley, the ground-water flow 

system generally can be subdivided by the major 

geologic units–the Purisima Formation, the Aromas 

Sand, the older alluvium, and the younger alluvium. In 

addition, the Aromas can be subdivided into an upper 

and lower portion on the basis of lithology and 

geophysical characteristics (fig. 3). The flow of ground 

water through the alluvial aquifers generally has been 

characterized as flowing from the mountains to the 

coast with the exception of local cones of depression in 

the area of the city of Watsonville and along the mouth 

of the Pajaro River (Johnson and others, 1988, fig. 5). 

The aquifers additionally can be grouped with the 

alluvial deposits and upper Aromas Sand representing 

the upper-aquifer system and the lower Aromas Sand 

and Purisima Formation representing the lower-aquifer 

system.
10 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

Pacific Ocean

Monterey

Bay

ZayanteVergeles

Fault

Zone

U

D

? ?

U

D

U D

U

D

U

D

Pajaro River

Corralitos

Fault Zone

San Andreas

Fault Zone

Santa Cruz Basin

Watsonville

Santa Cruz Mts

R2W R1W R1E R2E R3E R4E

T

12

S

T

13

S

T

11

S

T

10

S

T

9

S

EXPLANATION

Pajaro River

watershed

Outside Pajaro

River watershed

River or stream

Geologic section—

See figure 3.

Bathymetry

contours—

(Interval varies.

Data from

MBARI, 2000)

Faults (USGS)—

(Brabb and

others, 1997),

Known

Concealed

Inferred

Fault zone (Modified from

Dupré, 1975.)

Aromas Sand

Purisima Formation

Selected geologic outcrops—

(Brabb and others, 1997;

Wagner and others, 2000)

Fault (U = upthrown side)

Probable Fault

Possible Fault

Anticline

Syncline

5–24

25–49

50–74

75–124

125–185

Upper aquitard

aggregate thickness—

in feet

0 5

0 5 10 Kilometers

10 Miles

5 10 Kilometers

122°00' 121°52'30"

36°

52'

30"

37°

00'

37°

07'

30"

121°45' 121 121°37'30" °30'

PV-1

PV-3

A

A'

A'

A

PV-8

PV-4A

PV-1

PV-3

PV-6

PV-4A

Figure 2. Geology, selected wells used to estimate aquifer thicknesses, geologic outcrops, and sites for geophysical logging in the Pajaro Valley, Santa 

Cruz and Monterey Counties, California.
Geohydrology 11

300200100

Sea

100200300400500600700800

level

0 1,000 2,000 3,000 4,000 5,000 6,000

DISTANCE, IN FEET

DEPTH, IN FEET

A'

A

Flood Plain

Pajaro

River

PV-1*

PV-3*

PV-4A

PV-8*

PV-6*

* indicates well projected to cross section

Geology modified from Luhdorff & Scalmanini (1987)

Older sea waterSelected fine-grained

sediments

Upper Aromas Sand

Lower Aromas Sand

Basal sand& Gravels

Upper Purisima Formation

"Blue" ClayEstuarine M

D

S616

31

3,290

M187

13,317

D

208 D

Recent sea water

10,183

M

246

M 9,663

S54

S25

D

4,742

SM

MD

25

S18

S 119

EXPLANATION

Electrical-resistivity log

Well screen—no tritium Well screen sample depth

Well screen with tritiumChloride concentration,

in mg/L for 1998 samples

4,742

ShallowShallow-MediumMediumMedium-DeepDeep

S

SM

M

MD

D

LOWER AROMAS PURISIMA FORMATION UPPER AROMAS

RECENT

SEDIMENTS

Upper-aquifer system

Lower-aquifer system

?

?

?

?

?

?

?

?

Shallow Alluvial Aquifers

Younger and older alluvium,undifferentiated

?

?

?

?

?

?

?

Figure 3. Relation between geology and selected water-quality attributes along the coast in the Pajaro Valley, Santa Cruz, and Monterey Counties, California. 
12 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

The geologic factors that, in part, control the 

distribution of recharge, pumpage, and related seawater 

intrusion include the layering of the sediments, the 

potential connectivity of the coarse-grained deposits 

that allows deep percolation of recharge to replenish 

pumpage, and the potential barriers to ground-water 

flow or infiltration of recharge (fig. 2). The 

hydrogeology and its relation to seawater intrusion was 

summarized by Johnson (1982):

“The upper confined or semiconfined Quaternary 

aquifer has seawater intrusion. This aquifer consists of 

the main water-bearing unit of the alluvium and the 

upper part of the Aromas Sand which is in direct 

geologic and hydrologic contact with the basal gravel 

unit of the alluvium. The aquifer’s depth near the coast 

is presently interpreted as corresponding to an intruded 

interval of 100–200 ft below sea level. The alluvial part 

of the aquifer extends up the valley floor east of the city 

of Watsonville into the upper part of the Pajaro Valley 

where its sediments are generally coarser grained than 

those between Watsonville and the coast. The upper 

part of the Aromas Sand outcrops north of the alluvial 

flood plain into a major ground-water recharge area, 

extends south under the Springfield coastal terrace, and 

outcrops to the southeast in north Monterey 

County....The connection between the 180-foot aquifer 

in the Salinas Valley and the 100 to 200 ft interval of 

the Quaternary aquifer in the Pajaro Valley is the upper 

Aromas Sand which comprises part of both principally 

alluvial aquifers.

“There is also a deeper aquifer layer entirely 

within the Aromas Sand, defined by wells pumping in 

the 300–600 ft interval below sea level, that shows 

some early indications of higher chlorides and possible 

intrusion along its coastal margin at the mouth of the 

Pajaro Valley. It might be considered as an area of 

potential intrusion, though not enough is known about 

this interval north of the Springfield coastal terrace.”

The distribution of wells, well screens, and 

related pumpage from major aquifers in Pajaro Valley 

are controlled, in part, by the fine-grained deposits 

within the geologic units (Appendix 1; figs. 1D, 2, 3). 

The coarse-grained deposits, which persist over large 

areas, control pumpage, and related seawater intrusion. 

For example, the shallowest sediments within the upper 

100 to 200 ft are a combination of eolian, terrace, and 

fluvial deposits (Dupre, 1975, 1998). The distribution 

of wells with perforations that start in these deposits is 

widespread and is exceptionally dense in the inland 

subareas, the Springfield Terrace subarea, and along 

the Pajaro River and Corralitos Creek and their 

tributaries (figs. 1C, D). The distribution of wells with 

screens that start at depths of 200 to 400 ft and pump 

from the Aromas Sand are concentrated in the coastal 

and inland subregions that bound the city of 

Watsonville (fig. 1D). Both the Aromas Sand and the 

Purisima Formation dip to the southeast from the 

Soquel–Aptos area (fig. 1B) where ground water is 

supplied to predominantly municipal users by SCWD. 

Wells with screens that start in these deeper formations 

are located in the northern parts of the Harkins Slough 

and Corralitos subareas, in the Pajaro River mouth 

subarea, and to the south in the Highlands subareas 

(figs. 1C, D). This distribution of depth to well screens 

and related pumpage controls the large inland pumping 

depression that spans the Springfield Terrace–Pajaro 

River Mouth subareas inland to the Watsonville, 

Salsipuedes, and Corralitos subareas (fig. 1C; Johnson 

and others, 1988, fig. 5).
Geohydrology 13

Fine-grained deposits persist over large areas and 

have been mapped as a continuous layer within the 

flood-plain areas of the Pajaro River and Corralitos 

Creek (Johnson and others, 1988) and as discontinuous 

layers throughout most of the inland subareas. The 

variable aggregate thickness of shallow fine-grained 

deposits determined from drillers’ logs, and multiple 

fine-grained layers indicated by the geophysical logs, 

suggests that these layers may not be homogeneous or 

laterally extensive (Appendix 1; figs. 2, 3). The thick 

lacusterine and marine fine-grained layers also reoccur 

vertically throughout the cyclic sedimentation as a 

result of 11 sea-level changes in recent geologic history 

that affected most of the Pajaro Valley (Dupre, 1975). 

The aggregate thickness of fine-grained layers in the 

Corralitos subarea is coincident with a region that has 

both perched and deep aquifers (Johnson and others, 

1988, fig. 4) (fig. 2). These fine-grained layers range in 

thickness from 5 to as much as 185 ft in the Corralitos 

and Harkins Slough subareas (fig. 2) and represent a 

potentially significant barrier to the vertical flow of 

ground water in the inland and coastal subareas as 

shown in the geologic section of the layered coastal 

aquifers (fig. 3). The coastal section through the 

monitoring wells also illustrates that these layers, 

referred to as “aquitards,” are laterally extensive 

parallel to the coast and separate the major aquifers and 

even the individual water-bearing zones within aquifers 

(figs. 2, 3). Because of the layering in coastal aquifer 

systems such as Pajaro Valley, the seawater intrusion is 

vertically restricted. Massive fine-grained deposits also 

are present to the south of the Springfield Terrace 

subarea (Fugro West, Inc., 1995) exemplified in the 

geophysical logs from test hole PV-5 and monitoring 

well PV-4A (Appendix 1, figs. A1.3, A1.4,). This 

massive clay layer may represent an ancestral 

submarine canyon in a manner similar to the massive 

clay deposits adjacent to Hueneme submarine canyon 

in the Ventura area (Hanson and others, 2002, fig. 8).

The San Andreas, Zayante-Vergeles, and 

Corralitos fault zones cross the Pajaro Valley (fig. 2) 

and may affect the flow of ground water. The San 

Andreas Fault zone trends northwest along the foothills 

of the Santa Cruz Mountains (fig. 2). This major fault 

zone may provide at least a partial barrier to groundwater flow (Teghavi and Smith, 1999c) along the 

northeastern side of the valley (fig. 2). The ZayanteVergeles Fault zone is southwest of and parallel to the 

San Andreas Fault zone. The Zayante-Vergeles Fault 

zone passes through the Corralitos subarea and has 

been mapped or inferred throughout Santa Cruz and 

northern Monterey Counties (Green, 1970; Dupre, 

1975; Brabb and others, 1997; Wagner and others, 

2000). This potentially active fault zone is coincident 

with ponds on the northeast side such as College, Pinto, 

Kelly, and Tynan Lakes (fig. 1B). Like the San Andreas 

Fault, it is predominantly a right-lateral strike-slip fault 

that is also delineated by offsets in modern stream 

channels (fig. 2); its relatively high seismicity suggests 

movement throughout the Quaternary and Holocene 

periods (Dupre, 1975). The surface vertical 

displacement of about 3 ft has been mapped near Pinto 

Lake in the Holocene-aged deposits (Dupre, 1975), and 

small springs historically discharged near the town of 

Corralitos near the fault trace. The persistence of 

drainage networks and the small displacements suggest 

that although this fault zone could be an important 

barrier to ground-water flow in the Purisima Formation 

or lower Aromas Sand, the flow of ground water 

through the younger fluvial sediments may not be 

disrupted along stream channels where erosion and 

deposition have persisted. The Corralitos Fault zone 

(fig. 2) represents a convergent compressional wrench 

fault zone between the San Andreas and ZayanteVergeles Fault zones that occurs in the Salsipuedes 

subarea. Although this is also a region of high 

seismicity, there are no known recent displacements 

(Dupre, 1975), and the nature of this fault zone as a 

barrier to ground-water flow remains uncertain. The 

area between the San Andreas and Zayante-Vergeles 

Fault zones is a deep graben (fig. 2) referred to as the 

“Santa Cruz basin” (Dupre, 1975). 

Other minor faults (fig. 2) have been inferred 

from lineations onshore, and a few minor faults have 

been mapped offshore in the Aromas Sand (Green, 

1970; Wagner and others, 2000). The northern side of 

the Monterey submarine canyon also is bounded by 

faults (fig. 2), which may affect potential seawater 

intrusion along these submarine outcrops (Green, 1970; 

Wagner and others, 2000). The role and importance of 

faults as hydraulic barriers or sources of poor-quality 

water remains uncertain. 
14 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

HYDROLOGY

Hydrologic data collected as part of this study 

were incorporated with existing hydrologic and 

geologic data to further define the geohydrologic 

framework of recharge and seawater intrusion. The 

hydrologic data collected prior to this study included 

precipitation, streamflow, ground-water levels, and 

water-chemistry data from streamflow and groundwater samples (Doug Coty, written commun., Pajaro 

Valley Water Management Agency, 1998). Previous 

studies have helped identify the extent and nature of the 

geohydrologic framework (California State Water 

Resources Board, 1953; Green, 1970; Muir, 1972, 

1974, 1977; Luhdorff & Scalmanini, 1987a, b, 1988; 

Johnson and others, 1988; Dupre, 1975, 1998) and 

have helped classify the potential recharge and related 

land use for the Pajaro Valley (Muir and Johnson, 

1979; Teghavi and Smith, 1999a).

Precipitation

The deep percolation of precipitation is a 

significant source of recharge in the Pajaro Valley. The 

rainfall varies from year to year from less than 40 

percent to more than 200 percent of the 109-year 

period-of-record mean, which as noted earlier, ranges 

from 16 in. near the coast to 40 in. in the foothills 

(California State Water Resources Board, 1953). The 

climatic cycles yield changes in precipitation and 

recharge (components of supply) that cause changes in 

pumpage (component of demand) and related changes 

in ground-water storage and seawater intrusion. In 

addition to multi-year cycles, precipitation is seasonal; 

about 90 percent of the rainfall in Pajaro Valley 

typically occurs between November and April. Wet and 

dry climate periods were delineated on the basis of the 

rising and falling limbs of a cumulative departure curve 

of precipitation at Watsonville (fig. 4). On the basis of 

this classification scheme, there have been nine dryyear and nine wet-year periods lasting from 2 to 19 

years for the period of record, 1880–1997 (fig. 4). 

Droughts within dry-year periods have been as short as 

the severe drought of 1976–77 and as long as the 

sustained 7-year drought of 1986–92. These multi-year 

cycles also can result in seasonal changes in the use of 

ground water such as the 20-percent increase in 

pumpage from 1996 to 1997 that was caused by the 

relatively dry spring in 1997 (Teghavi and Smith, 

1999b). Based on streamflow at Corralitos Creek and 

precipitation at Watsonville, the increased runoff 

threshold, precipitation required before increased 

runoff occurs, for different months range from 0 in. in 

summer months to more than 3 in. in January (fig. 5). 

Thus a series of climatic and seasonal changes in 

precipitation and runoff control the local ground-water 

recharge and tributary-runoff components of 

streamflow. 

Streamflow

Along with deep percolation of precipitation, 

streamflow infiltration is the other major source of 

natural recharge in Pajaro Valley. Streamflow occurs 

from runoff that originates outside the valley that enters 

as streamflow in the Pajaro River. Runoff from within 

the valley also flows through the tributaries to the 

Pajaro River. Streamflow is currently measured at two 

gages that record the flow into Pajaro Valley of the 

Pajaro River at Chittenden (11159000) and Corralitos 

Creek at Freedom (11159200) (figs. 1B, C). A third 

gaging station measured the downstream flow of 

Corralitos Creek at Corralitos (11159150) further 

upstream (fig. 1A) for the period 1958–1973. The 

Pajaro River flows all year at Chittenden, with a base 

flow of about 7 ft3/s (5,071 acre-ft/yr) for 1996–98 

streamflow (fig. 6A). Flow in the Pajaro River is partly 

regulated upstream from the Pajaro Valley by upstream 

releases and diversions. There are no downstream 

gaging stations on the Pajaro River to assess 

streamflow loss, flood-control dike-failure warning, or 

ground-water/surface-water flow-model performance. 

A better understanding of the distribution of 

streamflow losses and gains, as well as better estimates 

of streambed conductivity, are needed to improve 

estimates of the quantity, timing, and quality of 

streamflow infiltration.
Hydrology 15

188018841888189218961900190419081912191619201924192819321936194019441948195219561960196419681972197619801984198819921997

YEAR

0

10

20

30

40

50

60

70

80

90

100

Watsonville Precipitation

ANNUAL PUMPAGE, IN THOUSANDS OF ACRE FEET

CUMULATIVE DEPARTURE OF PRECIPITATION, IN INCHES

–60

–80

–40

–20

0

20

40

60

80

100

120

DRY WET DRY WET DRY W D WET DRY W DRY W D W D WET DRY W

Annual Estimated Pumpage

EXPLANATION

Figure 4. Cumulative departure of precipitation at Watsonville, delineation of wet and dry periods, and estimated ground-water pumpage for Pajaro 

Valley, Santa Cruz, and Monterey Counties, California. 
16 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10

0 1 2 3 4 5 6 7 8 9 10

PRECIPITATION, IN INCHES PRECIPITATION, IN INCHES PRECIPITATION, IN INCHES

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10

INCREASED RUNOFF THRESHOLD

1.3

2.2

2.5

3.3

3

2

0.6

0.3

0.02 0 0

0.3

0

0.5

1

1.5

2

2.5

3

3.5

OCTNOVDECJANFEBMARAPRMAYJUNJULAUGSEP

MONTH

PRECIPITATION THRESHOLD,

IN INCHES

January

April

July

October

February

May

August

November

March

June

September

December

AVERAGE DAILY STREAMFLOW,

IN CUBIC FEET PER SECOND

AVERAGE DAILY STREAMFLOW,

IN CUBIC FEET PER SECOND

AVERAGE DAILY STREAMFLOW,

IN CUBIC FEET PER SECOND

AVERAGE DAILY STREAMFLOW,

IN CUBIC FEET PER SECOND

0

50

100

150

200

250

300

0

50

100

150

200

250

300

0

50

100

150

200

250

0

20

40

60

80

100

120

140

160

0

10

20

30

40

0

2

4

6

8

10

0

1

2

3

4

5

6

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

0.2

0.4

0.6

0.8

1.0

1.2

0

50

100

150

200

0

10

20

30

40

0

1

2

3

4

5

Figure 5. Relations of monthly precipitation to streamflow for Corralitos Creek near Freedom, Santa Cruz County, California, 1958–97. 
Hydrology 17

EXPLANATION

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

YEAR

STREAMFLOW, IN CUBIC FEET PER SECOND

CHEMICAL CONCENTRATION, IN MILLIGRAMS PER LITER

Pajaro River streamflow

at Chittenden (P.R.-16.5)

Chloride Sufate Nitrate as

nitrate

1

10

100

1,000

0.1

1

10

100

1,000

10,000

100,000

A

Figure 6. Relation between streamflow and water chemistry for selected sites on the Pajaro River and for Corralitos Creek at Freedom, Santa Cruz 

County, California. 
18 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

1

10

100

1,000

0.1 1 10 100 1,000 10,000

STREAMFLOW, IN CUBIC FEET PER SECOND

CHLORIDE AND SULFATE, IN MILLIGRAMS PER LITER

0.1

1

10

100

NITRATE AS NITRATE, IN MILLIGRAMS PER LITER

Streamflow vs Chloride Streamflow vs Sulfate

EXPLANATION

Streamflow vs Nitrate as nitrate

B

Figure 6.—Continued.
Hydrology 19

1995 1996 1997 1998 1999 2000

1

10

100

1,000

10,000

0.1

1

10

100

1,000

10,000

100,000

YEAR

STREAMFLOW, IN CUBIC FEET PER SECOND

CHEMICAL CONCENTRATION, IN MILLIGRAMS PER LITER

Pajaro River streamflow

at Chittenden (PR-16.5) 11159000 USGS

P.R. at Aromas Bridge(Chloride) (PR-14.4)

P.R. at Murphy Crossing(Chloride) (PR-10.9)

P.R. at Thurwatcher(Nitrate as nitrate) (PR-2.2)

P.R. at Thurwatcher(Chloride) (PR-2.2)

EXPLANATION

C

Figure 6.—Continued.
20 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

1995 1996 1997 1998 1999 2000

YEAR

STREAMFLOW, IN CUBIC FEET PER SECOND

CHEMICAL CONCENTRATION, IN MILLIGRAMS PER LITER

0.1

1

10

0.1

1

10

100

1,000

10,000

100,000

EXPLANATION

Pajaro River streamflow

at Chittenden (P.R.-16.5)

Pajaro River

at Murphy Crossing (P.R.-10.9)

(Boron)

Pajaro River

at Thurwatcher (P.R.-2.2)

(Boron)

D

Figure 6.—Continued.
Hydrology 21

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

STREAMFLOW, IN CUBIC FEET PER SECOND

CHEMICAL CONCENTRATION, IN MILLIGRAMS PER LITER

0.1

1

10

100

0.1

1

10

100

1,000

10,000

EXPLANATION

CORRALITOS CREEK STREAMFLOW AT FREEDOM 11159200 USGS

CORRALITOS CREEK AT GREEN VALLEY (Chloride) (CO4-4.2)

CORRALITOS CREEK AT GREEN VALLEY (Nitrate as nitrate) (CO4-4.2)

E

Figure 6.—Continued.
22 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

The streamflow record from the Pajaro River at 

Chittenden shows changes in streamflow and base 

flow, as well as relation to concentrations of selected 

chemical constituents, for climatic and seasonal cycles 

(figs. 6A–E). The annual geometric-mean streamflow 

for the 58-year period of record is about 10,200 acreft/yr (table 1). The seasonal geometric-mean 

streamflows range from less than 400 acre-ft/season in 

summer dry-year periods to almost 36,000 acreft/season in winter wet-year periods. Based on 

streamflow-duration analysis, streamflows of less than 

200 ft.3/s occur about 88 percent of the time on the 

Pajaro River (fig. 7), with mean daily flows less than 12 

ft3/s 50 percent of the days. Streamflows that probably 

represent floodflows exceed or equal about 770 ft3/s 5 

percent of the time (about 18 days per year) (fig. 7). 

The geometric-mean annual streamflow for wet-year 

periods is about 4 times larger than for dry-year periods 

(table 1, fig. 7). Collectively, this suggests that the 

smaller streamflows are a substantial part of the 

streamflow available for ground-water recharge and 

that climatic cycles influence the time as well as the 

quantity and quality of streamflow available for 

additional water supply, fish habitat, or ground-water 

recharge. 

The streamflow from Corralitos Creek at 

Freedom shows similar climatic and seasonal changes 

in streamflow (fig. 6E). Streamflow on Corralitos 

Creek also is subject to diversions as great as 8 ft3/s 

(5,800 acre-ft/yr), but diversions generally are on the 

order of about 1,000 acre-ft/yr for water supply by the 

Watsonville Water Works upstream from Corralitos. 

The annual geometric-mean streamflow for the 42-year 

period of record for the gage at Freedom is about 1,400 

acre-ft/yr. (table 1). The seasonal geometric-mean 

streamflows range from about 40 acre-ft/season in 

summer dry-year periods to almost 3,700 acreft/season in winter wet-year periods. The streamflowduration graph indicates that mean daily flows exceed 

or equal 0.4 ft3/s 50 percent of the days and equal or 

exceed 79 ft3/s 5 percent of the time (about 18 days per 

year) (fig. 7). The geometric-mean annual streamflow 

for wet-year periods is about 2.4 times larger than for 

dry-year periods for Corralitos Creek at Freedom

(table 1; fig. 6E).

The water quality of streamflow on the Pajaro 

River changes with flow (fig. 6) and affects the water 

quality of recharge. Increased chloride and sulfate 

concentrations occur during low-flow periods, which 

are coincident with the summer/fall agricultural 

pumping season (figs. 6A, B; Teghavi and Smith, 

1999d). Nitrate appears to increase with increased 

streamflow during the 1980s (figs. 6A, B). The changes 

in chloride and sulfate appear to vary together for 

climatic periods but may not vary together for some 

seasons. Sulfate and the trace-constituent boron 

(figs. 6A, D) appear to increase with increasing flow for 

some periods, for example 1983, but the lack of 

synoptic data makes covariation with streamflow 

difficult to determine using existing data. These data 

may suggest that there are several sources of poorquality water that contribute runoff to streamflow. 

These sources need to be compared with pesticides, 

herbicide, and wastewater loadings to help delineate 

the multiple sources of poor-quality water that occur 

during some streamflow periods. The relations between 

water quality and streamflow is similar to that 

measured on the Santa Clara River, Ventura County, 

which contains poorer quality water at low flow that is 

partly from ground-water contributions that are 

rejected to the surface as base flow in the bedrock 

constrictions at subbasin boundaries (Densmore and 

others, 1992). The source and concentrations of these 

chemical constituents may affect water-resources 

management and the implementation of specific watersupply options. The downstream chloride 

concentrations for the period 1995–97 at Aromas 

Bridge, Murphy Crossing, and Thurwatcher Road show 

similar changes in concentration that are generally 

opposite to changes in streamflow. The chloride 

concentrations in streamflow at Murphy Crossing and 

Thurwatcher Road (fig. 1C, PR-2.2) appear to change 

similarly with changes upstream at Aromas Bridge 

(fig. 6C). Additional sources of chloride such as 

irrigation or urban runoff, discharge of treated 

wastewater, or additional ground-water or surfacewater sources could contribute to differences in 

chloride at downstream sites but remain uncertain 

owing to the lack of synoptic streamflow and water 

chemistry data. The streamflow and water quality 

relations of the data from the Chittenden streamflow 

gage indicate that low flows of the Pajaro River may be 

an additional source of salinity for ground-water 

recharge.
Hydrology 23

Table 1. Summary of streamflow attributes for selected streams in the Pajaro Valley, California 

[AMBAG, Association of Bay Area Governments; CSWRCB, California State Water Resources Control Board; MW, Montgomery Watson; USGS, U.S. 

Geological Survey; —, no data]

Time period

(source of summary data)

Pajaro River at Chittenden 

[11159000/PR-16.5]1

Average annual streamflow

(acre-feet per year, except as noted)

1First number is U.S. Geological Survey station number, and second number is Pajaro Valley Water Management Agency station number.

Corralitos Creek At Freedom

[11159200/CO-2.3]1

Average annual streamflow

(acre-feet per year, except as noted)

[1946–49]

(CSWRCB, 1953)

21,500 (1946–47)

 7,300 (1947–48)

24,100 (1948–49)

—

[1984]

(AMBAG, 1985)

105,000 11,350

[1964–92]

(MW, 1993)

114,500 2,300

[1940–1998]2

(USGS)

2(a) arithmetic mean for period of record; (b) geometric mean for period of record; (c) geometric mean of streamflow for wet- and dry-year climatic 

periods.

(a) 124,000

(b) 10,200

(c) 19,900–4,700

(a) 11,900

(b) 1,400

(c) 2,000-800

[Winter–spring]3

(USGS)

3(a) winter- and spring-season geometric means for period of record; (b)winter-season geometric mean for wet- and dry-year climatic periods; (c) spring 

geometric mean of streamflow for wet- and dry-year climatic periods of the period of record. All streamflows are reported in acre-feet per season.

(a)14,400–4,000

(b) 35,700–4,800

(c) 7,600–1,800

(a) 1,600–300

(b) 3,700–500

(c) 600–100

[Summer–fall]4

(USGS)

4(a) summer- and fall-season geometric means for period of record; (b) summer-season geometric mean for wet- and dry-year climatic periods; (c) fall 

geometric mean of streamflow for wet- and dry-year climatic periods of the period of record. All streamflows are reported in acre-feet per season.

(a) 700–1,100

(b) 1,200–400

(c) 1,900–600

(a) 50–200

(b) 50–40

(c) 220–170
24 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

CUMULATIVE FRACTION EXCEEDING OR EQUALING

0.010.020.050.100.200.500.800.900.950.980.99

1 10 100 1,000 10,000

Wet years

Dry years

Total period of record

MEAN-DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

1 10 100 1,000

Wet years

Dry years

Total period of record

1 10 100 1,000

Wet years

Dry years

Total period of record

A BC

Figure 7. Flow duration for the Pajaro River at Chittenden, 1949–98, for Corralitos Creek near Corralitos, 1958–73, and for Corralitos Creek at Freedom, California 1957–98, Santa Cruz 

County, California. 
Hydrology 25

Ground Water

The coastal monitoring wells provide 

information to assess water levels and seawater 

intrusion near the coast (fig. 1C). Longer term waterlevel records from coastal and inland water-supply 

wells reflect water levels and changes similar to those 

of the monitoring-well sites. In general, changes in 

ground-water levels from coastal wells show seasonal 

changes from agricultural pumpage in the Pajaro 

Valley. The lack of long-term decline in coastal wells 

may be due, in part, to water-level declines from 

pumpage with related storage depletion and subsequent 

water-level recoveries supplied, in part, by seawater 

intrusion. 

The water-level declines from sustained 

pumpage have resulted in seawater intrusion in Pajaro 

Valley. Mann (1988) encapsulated the nature of 

seawater intrusion when he wrote to the director of the 

Pajaro Valley Water Management Agency:

“It is generally agreed that there is seawater 

intrusion in the coastal portions of Pajaro Valley. It probably started as early as 1947....Geologic conditions favor seawater intrusion....It is 

generally recognized that below sea level 

pumping levels in wells close to the coast carry 

with them the danger of inland movement of 

sea water, especially in shallow aquifers. What 

is not recognized generally is that the critical 

fresh water level must be maintained some distance above sea level to avoid any inland 

movement of sea water....In the Quaternary 

alluvium beneath Pajaro Valley, there is a basal 

gravel, the bottom of which is about 200 feet 

below sea level. To stabilize or stop such 

movement, the fresh water head must be at +5 

or higher. Thus, if water levels in wells tapping 

the alluvial aquifer are above sea level, but 

below elevation +5, it must be presumed that 

sea water is wedging in toward the shoreline 

from the subsea outcrop. In the deeper Aromas 

aquifer (assuming a depth of 600 feet) the necessary fresh water head would be +15....Under 

present conditions in the PVWMA, there is a 

pumping trough well inland from the coast, 

and a widespread area of below-sea-level water 

levels. Under these conditions, the sizes and 

depths of cones of depression near the coast 

are irrelevant to the regional problems, for sea 

water intrusion would continue even if there 

were no coastal cones of depression.”

Coastal Monitoring-Well Sites

Water-level altitudes from multiple-well 

observation sites along the coast (figs. 1C, 8–17) from 

La Selva Beach (PV-1, fig. 8) south to Springfield 

Terrace (PV-4A, fig. 10) are generally below sea level 

with some wells showing declining winter water levels 

during the 7-year drought (1986–92). In the time after 

this drought, water levels in the monitoring wells have 

recovered a few feet but were still less than the height, 

5 to 15 ft above sea level, required to maintain the 

seaward flow of ground water and prevent seawater 

intrusion. Most water levels in 1998 ranged from 0 to 5 

ft above sea level. Summer water levels in the aquifers 

in the lower Aromas Sand near the mouth of the Pajaro 

River were still as much as 15 ft below sea level at 

well-site PV-3 (fig. 9) and 20 ft below sea level at wellsite PV-6 (fig. 11).

The water-level hydrographs from the multiplewell sites in areas where less of the pumpage is for 

agriculture, such as the San Andreas (PV-1, fig. 8) and 

La Selva Beach (SC-3A and SC-4A, figs. 14, 15) areas, 

do not contain large seasonal water-level changes. The 

hydrographs from monitoring-well sites show that the 

water levels and water-level declines are similar in the 

shallow and medium wells but differ from the 

hydrographs for many of the deeper wells. These 

water-level differences with depth can result in 

downward pressure gradients that can promote the 

potential for vertical leakage of ground water through 

natural leakage or through wellbore flow in wells 

screened across multiple aquifers. Water-level 

differences between upper and lower aquifers are the 

largest during the irrigation season. These patterns of 

water-level declines and water-level changes are 

similar to those measured in other coastal basins with 

agricultural and municipal use, such as the Ventura 

(Hanson and others, 2002) and Santa Barbara areas 

(Freckleton and others, 1998).
26 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

CHLORIDE, IN MILLIGRAMS PER LITER

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

PV-1 (Shallow 70–90 feet)

EXPLANATION

PV-1 (Medium 140–230 feet)

PV-1 (Deep 250–270 feet)

PV-1 (Shallow-Chloride)

PV-1 (Medium-Chloride)

PV-1 (Deep-Chloride)

A

Figure 8. The PV-1 multiple-well monitoring site in the San Andreas subarea of the Pajaro Valley, Santa Cruz County, California. 
Hydrology 27

PV-1 (Shallow 70–90 feet)

EXPLANATION

PV-1 (Medium 140–230 feet)

PV-1 (Deep 250–270 feet)

PV-1(Shallow-Chloride)

PV-1(Shallow-Nitrate as nitrate)

PV-1(Medium-Nitrate as nitrate)

PV-1(Deep-Nitrate as nitrate)

B

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

CHEMICAL CONCENTRATION, IN MILLIGRAMS PER LITER

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

0

100

200

300

400

500

Figure 8.—Continued.
28 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

PV-3 (Shallow 140–170 feet)

EXPLANATION

PV-3 (Medium 250–270 feet)

PV-3 (Deep 380–480 feet)

PV-3 (Shallow-Chloride)

PV-3 (Medium-Chloride)

PV-3 (Deep-Chloride)

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

CHLORIDE, IN MILLIGRAMS PER LITER

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

Figure 9. The PV-3 multiple-well monitoring site in the Pajaro River Mouth subarea of the Pajaro Valley, Santa Cruz County, California. 
Hydrology 29

PV-4 (Shallow 110 feet)

EXPLANATION

PV-4 (Medium 160 feet)

PV-4 (Shallow-Chloride)

PV-4 (Medium-Chloride)

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

CHLORIDE, IN MILLIGRAMS PER LITER

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

Figure 10. The PV-4 multiple-well monitoring site in the Springfield subarea of the Pajaro Valley, Monterey County, California. 
30 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

EXPLANATION

PV-6(Shallow-Chloride)

PV-6(Shallow/Medium-Chloride)

PV-6(Medium/Deep-Chloride)

Pajaro River at Thurwatcher (P.R.-2.2) (Chloride)

PV-6(Deep-Chloride)

PV-6 (Shallow 180 feet)

PV-6 (Shallow/Medium 280 feet)

PV-6 (Medium/Deep 660 feet)

PV-6 (Deep 750 feet)

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

CHLORIDE, IN MILLIGRAMS PER LITER

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

0

5

10

15

20

Figure 11. The PV-6 multiple-well monitoring site in the Pajaro River Mouth subarea of the Pajaro Valley, Santa Cruz County, California. 
Hydrology 31

PV-8 (Shallow 200 feet)

EXPLANATION

PV-8 (Medium 530 feet)

PV-8 (Deep 590 feet)

PV-8 (Shallow-Chloride)

PV-8 (Medium-Chloride)

PV-8 (Deep-Chloride)

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

CHLORIDE, IN MILLIGRAMS PER LITER

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

45

50

Figure 12. The PV-8 multiple-well monitoring site in the San Andreas/Harkins Slough subareas of the Pajaro Valley, Santa Cruz County, California. 
32 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

0

100

200

300

400

500

600

700

800

900

1,000

CHEMICAL CONCENTRATION, IN MILLIGRAMS PER LITER

PV-11 (55–115 feet)

EXPLANATION

PV-11 (Chloride)

PV-11 (Nitrate as nitrate)

Figure 13. The PV-11 multiple-well monitoring site in the San Andreas subarea of the Pajaro Valley, Santa Cruz County, California. 
Hydrology 33

SC-A3 (C) (Shallow 125–145 feet)

EXPLANATION

SC-A3 (B) (Medium 230–270 feet)

SC-A3 (A) (Deep 290–310 feet)

SC-A3 (C) (Shallow-Chloride)

SC-A3 (B) (Medium-Chloride)

SC-A3 (A) (Deep-Chloride)

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

CHLORIDE, IN MILLIGRAMS PER LITER

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

Figure 14. The SC-A3 multiple-well monitoring site in the La Selva Beach area of Soquel Creek Water District north of Pajaro Valley, Santa Cruz 

County, California.
34 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

CHLORIDE, IN MILLIGRAMS PER LITER

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

EXPLANATION

SC-A4 (D) (Shallow-Chloride)

SC-A4 (C) (Shallow/Medium-Chloride)

SC-A4 (B) (Medium/Deep-Chloride)

SC-A4 (A) (Deep-Chloride)

SC-A4 (D) (Shallow 180 feet)

SC-A4 (C) (Shallow/Medium 280 feet)

SC-A4 (B) (Medium/Deep 660 feet)

SC-A4 (A) (Deep 750 feet)

Figure 15. The SC-A4 multiple-well monitoring site in the La Selva Beach area of Soquel Creek Water District north of Pajaro Valley, Santa Cruz 

County, California.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 35

Long-Term Water Levels and Seawater Intrusion

The first comprehensive study of Santa Cruz and 

Monterey Counties recognized seawater intrusion as 

early as 1943 and noted that this intrusion may be 

driven by seasonal pumping troughs of 15 ft below sea 

level (California State Water Resources Board, 1953). 

The seasonal pumping troughs persist throughout the 

coastal areas where ground-water withdrawals are 

predominantly agricultural pumpage (Johnson, 1983; 

Luhdorff & Scalmanini, 1987a, b; Johnson and others, 

1988; Bond, 1986; Bond and Bredehoeft, 1987). 

Simpson (1946) also noted some evidence of seawater 

intrusion into the coastal parts of the Pajaro Valley 

aquifer system. In 1974, the most extensive seawater 

intrusion was in the aquifers from 100 to 200 ft depth 

below sea level; intrusion was present to a lesser extent 

in the lower aquifers in the Aromas Sand (300 to 600 ft 

below sea level) (Muir, 1974). 

Comparison of long-term ground-water levels 

from coastal water-supply wells and monitoring wells 

(PV-3) indicate water levels have been below sea level 

since 1970 in the Pajaro River Mouth subarea 

(fig. 16A). Comparisons with the coastal monitoring 

wells in the Springfield subarea shows that long-term 

declines of more than 20 ft occurred prior to the 1990s 

and increasing water-level differences of more than 10 

ft are occurring in the Aromas Sand (12S/2E-30M2 

relative to shallower aquifers (PV-4A) (fig. 17). Longterm water levels generally have been below sea level 

but vary periodically with the climatic cycles in the 

Springfield subarea since 1965 (fig. 17). Similarly, 

water levels from wells in the Murphy Crossing area 

show shallow inland water levels that are above coastal 

water levels except during dry periods (figs. 16A, B). 

The water levels near Murphy Crossing show 

substantial decline during the drought of the late 1980s 

and are below sea level in the early 1990s. Recent 

assessment of the perched and regional water levels in 

the Corralitos subarea indicate that water levels in the 

aquifers of the Aromas Sand in this region also are near 

sea level (Koenig, 1992, 1994). The water levels from 

shallow and deeper water-supply wells from the 

Murphy Crossing region converge on water levels 

similar to those of coastal monitoring wells during 

drought periods as shown for the dry period 1983–1992 

(figs. 16A, B). The resulting inland cones of depression 

have resulted in regional water-level declines and 

widespread water-level altitudes near or below sea level 

that have persisted for several decades and may become 

more widespread during droughts.

EVALUATION OF RECHARGE AND 

SEAWATER INTRUSION BASED ON 

WATER CHEMISTRY

Ground-water sampling for this study was used 

to assess the nature and distribution of recharge and 

saline ground water, as well as the source, age, and 

movement of ground water in the coastal areas of the 

Pajaro Valley aquifer systems. The sampling and 

analysis included physical attributes, major ions and 

nutrients, selected trace elements, and selected 

isotopes. The trace elements used for geochemical 

interpretations included boron, strontium, barium, 

bromide, and iodide. The deuterium, oxygen, and 

tritium isotopes were collected for all ground-water 

sites. In addition, isotopes of carbon, sulfur, boron, and 

strontium were analyzed for selected sites. The data 

collected in this study are summarized in Appendix 2. 

Additional geochemical data provided by PVWMA 

and SCWD also were used to help interpret the nature 

and distribution of recharge and seawater intrusion.
36 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

–30

–20

–10

0

10

20

30

40

50

60

70

DRY WET DRY WET DRY WET

12S/3E-8C (Shallow 80-180 feet)

EXPLANATION

12S/1E-25B3 (Deep 495-580 feet)

PV-3 (Shallow 140-170 feet)

PV-3 (Medium 250-270 feet)

PV-3 (Deep 380-480 feet)

A

Figure 16. Multiple-well PV-3 and deep well 12S/1E-25B3 in the Pajaro River Mouth subarea and wells 12S/3E-8C and 12S/3E-7J2 in the Pajaro 

subarea of the Pajaro Valley, Santa Cruz County, California.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 37

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

WET DRY WET DRY WET D W D WET DRY W

12S/1E-25B3 (Deep 495-580 feet)

EXPLANATION

12S/3E-7J2 (Shallow 105-197 feet)

B

Figure 16.—Continued.
38 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

DRY WET WET D W DRY WET DRY WET

1957 1959 1961 1963 1965 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999

YEAR

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

–30

–25

–20

–15

–10

–5

0

5

10

15

20

25

30

35

40

12S/2E-30M2 (Medium 255-275 feet)

EXPLANATION

PV-4A (Shallow 110 feet)

PV-4A (Medium 160 feet)

Figure 17. Multiple-well PV-4 and well 12S/2E-30M2 in the Springfield subarea of the Pajaro Valley, Monterey County, California.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 39

Characteristics of Water from Monitoring and 

Supply Wells

The general distribution of poor-quality water 

from inorganic sources includes ground water that is 

high in sulfate and chloride (greater than 250 mg/L) in 

the coastal region and ground water that is high in 

sulfate (greater than 250 mg/L) near Murphy Crossing 

in the Salsipuedes subarea (figs. 1B,C, 18). A larger 

region of poor-quality water that has relatively high 

specific conductance (greater than 1000. μS/cm) and 

boron (greater than 300. μg/L) also extends 

northwestward away from the Pajaro River in the Santa 

Cruz basin portion of the Salsipuedes subarea east of 

the Zayante-Vergeles Fault (fig. 18). Many of the 

changes in water quality are coincident with changes in 

water levels and appear to be part of the ongoing longer 

term changes related to ground-water development.

Samples from the monitoring wells [PV-1 (M, 

D), PV-4 (M), PV-6 (D), PV-8 (D)] (figs. 1C, 18), and 

the two agricultural-drain samples, (AG-1.0 and 

AG-2.0) exceed the secondary drinking water standard 

maximum contaminant level (SMCL) concentration of 

250 mg/L of chloride (U.S. Environmental Protection 

Agency, 1994) (App. 2, table A2.1). The samples from 

PV-3 (S, M) and PV-8 (M) are approaching this 

chloride concentration. 

The samples from PV-1 (S, M), PV-6 (D), 

PV-8 (D), AG-1.0, and AG-2.0 exceed the SMCL 

concentration of 250 mg/L of sulfate (U.S. 

Environmental Protection Agency, 1994). The highest 

sulfate concentrations are in saline waters sampled 

from PV-1 (D) at 1,726 mg/L. Samples collected 

during the relatively lower summer streamflow from 

the Pajaro River (7.6 ft3/s) and Watsonville Slough 

(0.47 ft3/s) sites are approaching this level and are 

fairly similar in magnitude. These are similar to 

previous low-flow data from the Pajaro River (J.H. 

Kleinfelder & Assoc., 1983) and similar to the 

chemistry of base flow at subbasin boundaries in other 

coastal basins such as the Santa Clara River in Ventura, 

California (Izbicki and others, 1995). All but some of 

the samples from PV-6 (S, SM, MD) exceed the SMCL 

of 500 mg/L dissolved solids (U.S. Environmental 

Protection Agency, 1994). Samples from PV-6 (MD) 

and PV-12 exceed the SMCL concentration of

0.3 mg/L in iron (U.S. Environmental Protection 

Agency, 1994). 

The monitoring wells were sampled for nitrate 

because the presence of nitrate in ground water is 

generally indicative of recharge from percolation of 

irrigation waters as well as an indicator of nonpoint 

pollution (H. Esmaili & Associates, Inc., 1978). The 

shallow ground water at monitoring wells PV-1, PV-6, 

PV-4, PV-8, and PV-11 have nitrate concentrations 

greater than 10 mg/L NO3 as nitrogen (USEPA MCL) 

or 45 mg/L NO3 as nitrate. The nitrate concentrations 

vary, chloride shows little change, and ground-water 

levels increase at well PV-11 (fig. 13) in the San 

Andreas Dunes area (fig. 1B). The relation between 

nitrates, chlorides, and water levels remains uncertain, 

on the basis of this short-term data set. The nitrate is 

increasing with chloride at the shallow well at PV-1 

(fig. 8B). This well may be downgradient from 

percolation of irrigation water applied in the San 

Andreas Dunes area (fig. 1B). Samples from the PV-1 

(S) in the San Andreas subarea contains relatively high 

concentrations of nitrate that vary with chloride

(figs. 8A, B). Similarly, PV-4 (S), PV-6 (S), and 

PV-8 (S) all contain more than 10 mg/L nitrate that 

suggest some deep percolation of applied irrigation 

water. The surface-water and agricultural return-flow 

samples are also relatively high in nitrate (App. 2,

table A2.1). On the basis of these samples, the elevated 

nitrate concentrations appear to be restricted to surface 

waters and shallow ground waters that are receiving 

areal recharge containing chloride and nitrate from 

deep percolation of irrigation waters. 

The quality of ground water has been saline in 

aquifers in the lower Aromas Sand in the coastal 

regions since the monitoring wells were installed and 

first sampled, as shown by the water-quality data from 

monitoring wells PV-3 (D), PV-6 (D), and PV-8 (D) 

(figs. 9, 11, 12). Other chemical changes in ground 

water, however, are recent and may indicate additional 

seawater intrusion, infiltration of irrigation-return flow 

and runoff, and changing surface-water inflows. The 

complete chemistry and isotopic analyses have enabled 

identification of potential sources of recharge in the 

coastal regions, but the lack of inland monitoring wells 

makes it difficult to delineate the changes in chemistry 

of ground water as it moves through the aquifers along 

flow paths between the inland areas of recharge, the 

areas of pumpage, and the submarine outcrops. 
40 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

Pacific Ocean

Monterey Bay

Santa

Clara

Co

Santa

Cruz

Co

Monterey

Co

San

Benito

Co

San Andreas

Fault Zone

High sulfate

and chloride

High sulfate

and chloride

High sulfate

Soquel

Santa Cruz

Murphy Pa

Crossing

jaro

River

Corralitos

Aptos

Creek

Creek

Soquel

Creek

Elkhorn

Watsonville Harkins

Gallighan

Slough

Hanson

Slough

Struve

Slough

Slough

College

Lake

Pinto Lake

Slough

Slough

La Selva

Beach

Sunset

Beach

Buena

Vista

Palm

Beach

San Andreas Dunes

Giberson Dunes Giberson Dunes

Springfield

Terrace

Corralitos

Freedom

Watsonville Watsonville

Bolsa de

Cayetano

Mattos

Gulch

Salsipuedes

R2W R1W R1E R2E R3E R4E

T

12

S

T

13

S

T

11

S

T

10

S

T

9

S

EXPLANATION

Pajaro River

watershed

Outside Pajaro

River

watershed

Pajaro Valley

Water Management

Agency boundary

(USBR, 1998)

River or streams

Sulfate (SO4), in

milligrams per liter

Chloride (Cl), in

milligrams per liter

Boron (B), in

micrograms

per liter

Specific Conductance

(SC), in microsiemens

per centimeter at 25°

Celsius

251–500

501–1,700 301–400

401–600

601–2,600

1,001–2,400

2,401–10,000

10,001–28,100 251–1,000

1,001–13,000

Bathymetry contours—

Interval varies. Data

from MBARI, 2000

0 5 10 Miles

0 5 10 Kilometers

122°00' 121°52'30"

36°

52'

30"

37°

00'

37°

07'

30"

121°45' 121 121°37'30" °30'

Known

Inferred

Concealed

Faults—From USGS

(Brabb and others,

1997), CDMG (Wagner

and others, 2000)

A

ZayanteVergeles

Fault Zone

Figure 18. Distribution of boron, chloride, sulfate, and specific conductance for selected wells in the Pajaro Valley, Santa Cruz County, California.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 41

Pacific Ocean

Monterey Bay

Santa

Clara

Co

Santa

Cruz

Co

Monterey

Co

San

Benito

Co

ZayanteVergeles

Fault Zone San Andreas

Fault Zone

Soquel

Santa Cruz

Murphy

Pa

Crossing jaro

River River

Corralitos

Aptos

Creek

Creek

Soquel

Creek

Elkhorn

Watsonville Harkins

Gallighan

Slough

Hanson

Slough

Struve

Slough

Slough

College

Lake

Pinto Lake

Slough

Slough

La Selva

Beach

Sunset

Beach

Buena

Vista

Palm

Beach

San Andreas Dunes

Giberson Dunes Giberson Dunes

Springfield

Terrace

Corralitos

Freedom

Bolsa de

Cayetano

Salsipuedes

High specific

conductance

and boron

R2W R1W R1E R2E R3E R4E

T

12

S

T

13

S

T

11

S

T

10

S

T

9

S

0 5 10 Miles

0 5 10 Kilometers

122°00' 121°52'30"

36°

52'

30"

37°

00'

37°

07'

30"

121°45' 121 121°37'30" °30'

B

High specific

conductance

Mattos

Gulch

Watsonville Watsonville

Figure 18.—Continued.
42 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

As ground water flows from recharge areas 

through the aquifers, the chemistry changes owing to 

chemical reactions and mixing with other water along 

flow paths. Changes in major-ion chemistry observed 

as water flows through coastal aquifers have been 

explained as being a result of calcite precipitation and 

cation exchange (fig. 19A) (Izbicki and others, 1993, 

Izbicki and Martin, 1997). Changes in major-ion 

chemistry observed as seawater or other high-chloride 

water invades freshwater aquifers have been explained 

as being a result of mixing and cation exchange 

(fig. 19A) (Izbicki, 1991). Sulfate reduction and 

chloride-bromide dissolution also are occurring along 

the long lateral flow paths in this coastal aquifer system 

(fig. 19A). Base exchange may also be reflected in the 

older seawater samples (fig. 19A). Cation exchange and 

calcite precipitation are not observed in this limited 

coastal data set (figs. 19A, B).

Major-ion diagrams (Piper, 1944) were used to 

identify groups of wells having ground water of similar 

major-ion chemistry and to interpret mixing and other 

chemical reactions that occur along flow paths through 

aquifers. These diagrams show the relative 

concentration of major cations and anions, in 

milliequivalents per liter, to the total ionic content of 

the water. Ground water from monitoring wells and 

surface-water samples collected in 1998 plot into seven 

distinct groups in the major-ion diagrams (fig. 19B), 

and show similar grouping for 1989 and 1996. The 

seven groups of ground-water and surface-water 

samples are:

(1) Recent ground water—Shallow alluvial 

aquifers in Holocene- and late Pleistocene-aged 

alluvium and upper Aromas Sand recently recharged 

by a combination of local runoff and Pajaro River 

streamflow [PV-1 (S), PV-3 (S), PV-4A (S), PV-8 (S, 

M), PV-11, PV-12, AG-1.0];

(2) Older ground water—Shallow alluvial 

aquifers in Holocene- and late Pleistocene-aged 

deposits and upper Aromas Sand in the Pajaro River 

and La Selva Beach coastal areas related to older 

recharge from local runoff [PV-6 (S, SM, MD), 

SC-A4 (B, C, D), SC-A3 (C)];

(3) Recent seawater intrusion—Shallow alluvial 

aquifers with a mixture of recent seawater and shallow 

ground water [PV-1 (M, D), PV-3 (M), PV-4A (M), SCA3 (A)],

(4) Older seawater—Older seawater or connate 

water in aquifers in the lower Aromas Sand [PV-6 (D), 

PV-8 (D), SC-A3 (B), SC-A4 (A, B)]; and

 (5) Very old ground water—Older waters in 

aquifers in the lower Aromas Sand from coastal areas 

[PV-3 (D) and AG-2.0].

 (6) Pajaro River water—River inflow from San 

Benito and Santa Clara County watersheds [Pajaro 

River at Chittenden, Murphy Crossing, and 

Thurwatcher Road];

 (7) Local runoff—Local tributary streamflow 

and runoff from local precipitation [Watsonville 

Slough, Soquel Creek];

Water from shallow wells near sources of 

recharge (group 1) plot near the middle and slightly to 

the left and above surface-water samples (groups 6 and 

7) for all three years shown in the three diamondshaped diagrams (fig. 19B). Water from deeper wells 

and wells completed at the downgradient ends of long 

flow paths through the shallower aquifers (group 2) 

plot in the middle left in the central diamond (fig. 19B). 

Water having high chloride concentrations plot toward 

the upper right edge of the diagram and were divided 

into samples from wells that are approaching the 

composition of seawater (group 3) and older saline 

water that represents older seawater or connate water 

(group 4) that has undergone base exchange to a 

calcium-magnesium chloride water. An additional 

group of older coastal water (group 5) that have 

chloride concentrations of less than 250 mg/L also 

occurs along the coast in Pajaro Valley in aquifers in 

the lower Aromas Sand. The irrigation-return-flow 

drain sample AG-1.0 may represent a mixture of 

seawater and water from the shallower aquifers (group 

1) and the AG-2.0 sample is similar in chemical 

composition to the waters from the deeper aquifers 

screened at well PV-3 (D), for example.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 43

6

7

Base exchange–mixing

Sea water intrusion–mixing without

base exchange

Seawater

Cation exchange–calcite precipitation–

sulfate reduction

Ground-water–surface-water evolution

TYPICAL CHEMICAL PROCESSES

EXPLANATION

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

SULFATE PLUS CHLORIDE

SODIUM PLUS POTASSIUM

CALCIUM PLUS MAGNESIUM

CARBONATE PLUS BICARBONATE

Chittenden

(P.R.-16.5)

Pajaro River

2

1

Soquel Creek Thurwatcher

(P.R.-2.2)

Watsonville Slough

A

Figure 19. Major-ion chemistry for selected ground-water and surfacewater samples from the Pajaro Valley 1989, 1996, and 1998.
44 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

1989 1996 1998 100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

SULFATE PLUS CHLORIDE

SODIUM PLUS POTASSIUM

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

CALCIUM PLUS MAGNESIUM

100

80

60

40

20

0

CARBONATE PLUS BICARBONATE

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

2

6

6 6

7 7

3

3

3

1

1

1

5 5

4 4

4

2 2

110–180 feet

260–280 feet

510–640 feet

730–750 feet

130–200 feet

420–530 feet

570–590 feet

56–116 feet

100–210 feet

Seawater

12S/1E-10R1, 2, 3 (PV-1)

12S/1E-23R1, 2, 3 (PV-3)

12S/1E-18N1, 2, 3, 4 (PV-6)

12S/1E-11R1, 2, 3 (PV-8)

12S/1E-13M1 (PV-11)

11S/2E-34D3 (PV-12)

125–145 feet

230–270 feet

290–310 feet

11S/1E-33E1-3 (SC-A3)

220–240 feet

230–410 feet

480–500 feet

520–540 feet

12S/1E-3C1-4 (SC-A4)

 77–100 feet

12S/1E-11N1 (PV-13)

Murphy Crossing

(P.R.-10.9)

Chittenden

(P.R.-16.5)

Pajaro River

2

1

Soquel Creek

80–110 feet

130–160 feet

12S/1E-31E1, 2 (PV-4A)

Thurwatcher

(P.R.-2.2)

Watsonville Slough

AG-1.0

AG-2.0

Agricultural

drain

1

2

3

4

5

6

7

Recent ground water

Older ground water

Recent seawater intrusion

Older seawater

Very old ground water

Pajaro River water

Local runoff

Water-type

Group Numbers

(S)

(M)

(D)

(S)

(M)

(D)

(C)

(B)

(A)

(D)

(C)

(B)

(A)

(S)

(M)

(S)

(M)

(D)

(S)

(SM)

(MD)

(D)

70–90 feet

140–230 feet

250–270 feet

140–170 feet

250–270 feet

380–480 feet

B

Figure 19.—Continued.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 45

Selected major and minor ions and trace 

elements in water from monitoring wells from groups 3 

and 4 were compared with the average concentrations 

in seawater (Hem, 1985). For wells that have more than 

250 mg/L of chloride, the chloride concentration was 

about 28 percent of the average concentration of 

seawater (19,000 mg/L) (Hem, 1985) for well PV-6 

(D); about 50 to 54 percent for wells PV-1 (M), PV-4A 

(M), and PV-8 (D); and about 70 percent for well PV-1 

(D). In the wells that have more saline water, iodide 

was about 7 percent of the average concentration of 

seawater (0.06 mg/L) for PV-1 (M), PV-6 (D), and PV8 (D); about 40 percent for well PV-1 (D); about 2 

percent for well PV-4A (D). Well PV-1 (D) had a 

concentration of boron of about 1.6 mg/L, which is 

about 36 percent of the concentration in seawater (4.5 

mg/L). In contrast, the Pajaro River and agricultural 

drains were just less than 1 mg/L of boron, which 

suggests this is not the source of higher boron 

concentrations in the coastal regions. The highest 

barium concentrations were sampled from PV-3 (S), 

PV-8 (D), and PV-12. Barium in the more saline wells 

ranged from about 300 percent of the concentration of 

seawater (20 μg/L) for well PV-1 (M) to about 1,180 

percent for well PV-8 (D). Strontium concentrations 

greater than 1.0 mg/L and bromide concentrations 

greater than 10 mg/L occurred in samples from PV-1 

(M, D), PV-3 (S), PV-6 (D), and PV-8 (D). Strontium 

ranged from about 56 percent for well PV-1 (M) to 

about 160 percent of the concentration of seawater 

(8 mg/L) for well PV-8 (D) for the more saline wells. 

Thus the wells that have recent seawater intrusion 

(group 3) are approaching the concentration found in 

seawater for chloride, are depleted in iodide and boron, 

and enriched in barium and strontium. The wells that 

have older seawater (group 4) also are depleted in 

iodide and boron but are more enriched in calcium, 

magnesium, barium, and strontium relative to seawater 

and recent seawater intrusion. This suggests that there 

may be additional sources for these constituents.

Source, Age, and Movement of Ground Water

The source, age, and movement of ground water 

in the aquifers can be delineated, in part, based on the 

isotopic and chemical characteristics of the 

monitoring-well samples from selected aquifers and 

from the potential “end-members” represented by 

waters from nearby surface-water sites and upperaquifer-system wells from the adjacent Salinas Valley 

that, in part, also draw water from the Aromas Sand 

(Vengosh and others, 2002) south of Pajaro Valley. The 

range in stable isotopes, tritium, carbon-14 ages, and 

trace-element composition of water from wells 

collectively indicates multiple sources of recharge to 

aquifers underlying the Pajaro Valley. The stable 

isotopes of water (deuterium and oxygen-18) indicate 

that the ground-water and surface-water samples 

generally fall along the meteoric water line, with the 

more saline samples generally trending toward 

seawater (fig. 20). These data also generally agree with 

the seven groups of water identified from the major-ion 

diagrams (fig. 19B).

The most negative (lightest) deuterium and 

oxygen isotope values are from the deep well at PV-3 

(D) (fig. 20) (group 5); these values are more similar to 

those of water from the Sierra Nevada that is much 

lighter (-74 per mil delta-deuterium and -10.2 per mil 

delta-oxygen-18) owing to the higher altitudes and 

cooler temperatures in the mountains (Muir and 

Coplen, 1981) than local ground-water recharged from 

recent local precipitation. Therefore, the ground water 

from PV-3 (D) may be water recharged when the 

climate was wetter and cooler (Izbicki, 1996b). This 

would tend to suggest that the ground water at the coast 

in aquifers in the lower Aromas Sand is older than 

ground waters that is present in the shallower alluvial 

aquifers or water-bearing zones in parts of the upper 

Aromas Sand. In addition to the lighter deuterium and 

oxygen isotopes, the water from PV-3 (D) also is 

similar to the Salinas Valley samples that have 

strontium-87/86 ratios (fig. 21) above the value for 

coastal California granitic rocks (Faure and Powell, 

1972). This indicates that the source of the sediments 

for the aquifer in the lower Aromas Sand may be 

similar to that for the sediments of the Salinas Valley. 

These sediments, which are, in part, granitic may have 

been derived from the granitic-bedrock mountains that 

bound the Salinas Valley.
46 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

δD=6 δO

δD=8 δO + 10

(Craig, 1961)

–9 –8 –7 –6 –5 –4 –3 –2 –1 0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Thurwatcher (P.R.-2.2)

Chittenden (P.R.-16.5)

70–90 feet

140–230 feet

140–230 feet (Resample)

250–270 feet

140–170 feet

250–270 feet

380–480 feet

110–180 feet

260–280 feet

510–640 feet

730–750 feet

130–200 feet

420–530 feet

570–590 feet

56–116 feet

100–210 feet

AG-1.0

AG-2.0

Watsonville Slough

Seawater

Precipitation—Santa Maria

80–110 feet

130–160 feet

Seawater mixing line

12S/1E-10R1, 2, 3 (PV-1)

Pajaro River

12S/1E-23R1, 2, 3 (PV-3)

12S/1E-31E1, 2 (PV-4A)

12S/1E-18N1, 2, 3, 4 (PV-6)

12S/1E-11R1, 2, 3 (PV-8)

12S/1E-13M1 (PV-11)

11S/2E-34D3 (PV-12)

Agricultural drain

PV-3 (D)

PV-12

Salinas Valley

wells with seawater

Meteoric water line

–60

–50

–40

–30

–20

–10

0

EXPLANATION

Group 3

Group 3

Group 6

Group 2

Group 1

Group 1

Group 7

Group 5

Group 4

Group 5

1

2

3

4

5

6

7

Recent ground water

Older ground water

Recent seawater intrusion

Older seawater

Very old ground water

Pajaro River water

Local runoff

Water-type group numbers

(S) and description—

(M)

(M)

(D)

(S)

(M)

(S)

(SM)

(MD)

(D)

(S)

(M)

(D)

(S)

(M)

(D)

Figure 20. Deuterium and oxygen-isotope values for selected ground-water and surface-water samples from the Pajaro Valley and adjacent Salinas Valley, 

California, and the seawater mixing line for Pajaro Valley ground-water samples.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 47

.7055

.7060

.7065

.7070

.7075

.7080

.7085

.7090

.7095

.7100

1 10 100 1,000 10,000 100,000 1,000,000

STRONTIUM, IN MICROGRAMS PER LITER

87STRONTIUM/86STRONTIUM RATIO

River and Slough Samples

Seawater

"Older" seawater

PV-3 (D)

Coastal California

granitic rocks

(Faure and Powell, 1972)

Seawater

Gypsum sample—Salinas

EXPLANATION

Salinas Valley

wells with seawater

Recent

seawater

intrusion

Thurwatcher (P.R.-2.2)

Chittenden (P.R.-16.5)

70–90 feet

140–230 feet

140–230 feet (Resample)

250–270 feet

140–170 feet

250–270 feet

380–480 feet

110–180 feet

260–280 feet

510–640 feet

730–750 feet

130–200 feet

420–530 feet

570–590 feet

56–116 feet

100–210 feet

AG-1.0

AG-2.0

Watsonville Slough

80–110 feet

130–160 feet

12S/1E-10R1, 2, 3 (PV-1) Pajaro River

12S/1E-23R1, 2, 3 (PV-3)

12S/1E-31E1, 2 (PV-4A)

12S/1E-18N1, 2, 3, 4 (PV-6)

12S/1E-11R1, 2, 3 (PV-8)

12S/1E-13M1 (PV-11)

11S/2E-34D3 (PV-12)

Agricultural drain

(S)

(M)

(M)

(D)

(S)

(M)

(D)

(S)

(M)

(S)

(SM)

(MD)

(D)

(S)

(M)

(D)

Figure 21. Strontium-isotope ratios plotted against strontium for selected ground-water and surface-water samples from the Pajaro Valley and Salinas 

Valley, California.
48 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

The surface-water samples are all isotopically 

heavier than the composition of precipitation from 

Santa Maria, California, about 100 mi south of Pajaro 

Valley (fig. 20). The samples from the Pajaro River at 

Chittenden and Thurwatcher (group 6) (figs. 1C, 19B, 

and 20) are shifted to the right of the meteoric water 

line and may represent some evaporation in the river 

samples. The agricultural drain (AG) samples also are 

shifted to the right of the meteoric water line but less 

than are the Pajaro River samples, indicating some 

evaporation and possibly additional concentration of 

salts in irrigation water. The Watsonville Slough water 

(group 7) plots next to the meteoric water line, along 

with nearby shallow ground-water samples [PV-6 (S), 

PV-8 (S), and PV-11], and suggest a locally derived 

source of water that is different from the Pajaro River. 

The strontium-isotope samples (fig. 21) for all the 

surface-water samples are below the value for coastal 

California granite and are similar to most of the 

ground-water samples.

The isotopic values from the wells completed in 

the shallower aquifers (groups 1 and 2), are similar to 

the composition of precipitation from Santa Maria, 

California. The lighter values reflect a combination of 

shallow and deep coastal monitoring wells at PV-3,

PV-6, and PV-8 sites (fig. 20). The shallower 

monitoring wells may receive some combination of 

local deep infiltration and streamflow infiltration from 

the Pajaro River. The isotopic samples from the 

different depths at all of the other monitoring-well sites 

are different from each other, which would suggest that 

there is little to no vertical leakage between these 

water-bearing units. 

The least negative (heaviest) isotope values are 

from seawater intrusion or from the deeper, saline 

waters from wells screened in the lower Aromas Sand 

(fig. 20). Although chloride concentrations from 

ground-water samples are necessary to assess the 

potential for seawater intrusion, they are not a complete 

identifier of seawater intrusion when other sources of 

chloride may be present (Izbicki, 1992, 1996a, b). In 

addition to chloride, the stable isotopes of deuterium 

and oxygen were used to group the samples from PV-1 

(M, D), PV-4A (D), and PV-8 (D) that fall along a 

mixing line with seawater (fig. 20). Assuming the 

average oxygen-isotope composition for the coastal 

nonsaline water from the recently recharged shallower 

aquifers (group 1, -5.64 per mil), most of the recent 

seawater-intrusion samples indicate percentages of 

seawater that are within 10 percent of the chloride 

estimate and are consistent with recent seawater 

intrusion. In contrast, the percentages of seawater for 

the more saline well samples from the lower Aromas 

Sand are 30 percent for PV-8 (D) and 1 percent for 

PV-6 (D). These lower percentages are considerably 

less than the chloride-based estimates of percentage of 

seawater and suggest another or additional source of 

chloride in these saline waters. The seawater intrusionsamples from shallow wells in Salinas Valley are 

distinctly different from Pajaro Valley samples and 

may fall on a separate mixing line with seawater 

(fig. 20).

Samples from the shallowest ground-water sites 

[PV-1 (S, M, D), PV-3 (S, M), PV-8 (S), PV-11] contain 

detectable amounts of tritium (App. 2, table A2.1). 

This, along with the distribution of nitrates, would 

indicate that infiltration of irrigation and surface waters 

is actively recharging the shallow ground-water 

aquifers. These shallower aquifers represent renewable 

ground-water resources. The lack of tritium in water 

samples from most wells completed in the aquifers in 

the upper Aromas Sand beneath the Pajaro River Valley 

and in the lower Aromas Sand in the coastal areas 

indicate that these aquifers do not contain recent 

recharge. These deeper aquifers represent the 

nonrenewable ground-water resources. Although these 

data enable identification of processes occurring in the 

coastal regions, the quantification of the recharge to the 

deeper aquifers in the inland regions will require 

additional isotopic sampling of water-supply wells as 

well as the installation and sampling of multiple-well 

monitoring sites.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 49

The carbon-14 samples indicate that the oldest 

waters from coastal monitoring-well samples occur in 

well PV-3 (D) (group 5) (App. 2). The sample from this 

well contained 4.89 percent modern carbon (pmc), 

which yields an uncorrected ground-water age estimate 

of about 24,900 years before present (ybp). This 

ground-water age is consistent with the light deuterium 

and oxygen isotope values (fig. 20) that represent water 

recharged thousands of years ago under cooler climatic 

conditions. Other samples from monitoring wells 

completed in the upper and lower Aromas Sand that do 

not contain tritium range from 24.1 pmc at PV-6 (D) 

(about 11,800 ybp) (group 4), to 37.61 pmc at PV-8 (D) 

(about 8,100 ybp) (group 4), and 51.21 pmc at PV-6 (S) 

(about 5,500 ybp) (group 2). These percentages of 

modern carbon were adjusted for initial waters (App. 2) 

and represent corrected ages of about 23,300 ybp at 

PV-3 (D), 10,700 ybp at PV-6 (D), 7,000 ybp at PV-8 

(D), and 4,300 ybp at PV-6 (S) (App. 2, table A2.1). 

These estimated ages are interpretive and are subject to 

considerable uncertainty. Davis and Bentley (1982) 

estimated that errors in carbon-14 ages may be as much 

as 100 percent. Even considering this uncertainty, the 

results indicate that much of this ground water was 

probably recharged thousands of years before present. 

The trace elements boron, iodide, and bromide 

also were used to help determine the origin of ground 

water in coastal areas where seawater, high-chloride 

water from partly consolidated marine deposits, and 

irrigation-return water from shallow unconfined 

aquifers may contribute to high chloride concentrations 

in water from wells (Piper and Garrett, and others, 

1953). Graphical techniques that normalize changes in 

trace-element concentrations to changes in 

concentrations of conservative (non-reactive) tracers 

are useful in the interpretation of these data. Plots of 

the ratio of chloride to boron, iodide, and bromide as a 

function of chloride can be used to determine the 

relation of high-chloride waters from various sources 

(Izbicki, 1991, 1996a). 

The 1998 samples from Pajaro Valley show a 

wide range in boron, iodide, and chloride 

concentrations; seawater is in the middle-right portion 

of the diagram and most native, low-chloride groundwater and surface-water samples are in the lower left of 

the diagram (figs. 22 A, B). Although shallow wells 

such as PV-1 (M, D) and PV-4A (M) plot near the ratio 

for seawater, other saline samples from deeper coastal 

monitoring wells have ratios higher than that of 

seawater (figs. 22 A, B). Most ground-water samples do 

not plot along a potential mixing line between native 

water [PV-6 (S)] and high-chloride waters [PV-6 (D)] 

for ratios of chloride to boron (fig. 22A). Some boron 

can be removed from ground water through adsorption 

(Rai and Zachara, 1984), but the higher chloride 

concentrations and lower boron concentrations relative 

to seawater from these deeper aquifers may represent 

older seawater or additional salinity derived from partly 

consolidated marine deposits (Izbicki, 1991). Unlike 

some deep-aquifer samples from other coastal aquifers, 

these samples are depleted in iodide and boron and 

may not represent deep marine brines. 

Ratios of chloride to iodide, boron, and bromide 

(figs. 22A, B, C) indicate that some of the deeper saline 

waters are depleted in iodide, boron, and bromide 

relative to seawater. These data suggest that the waters 

from the lower Aromas Sand were enriched in chloride 

relative to these trace elements and may indicate other 

sources of chloride such as dissolution of in situ salts 

from these marine sediments. The potential mixing 

lines between Watsonville Slough, Pajaro River, or 

PV-12 and seawater represent the mixtures that would 

result if lateral mixing occurred along a flow path. In 

addition, the ground-water end-members that have an 

initial water similar to that to PV-6 (S) and a final water 

similar to PV-6 (D) represent the mixtures that would 

result if vertical mixing occurred in the coastal region. 

Most samples do not fall on these potential mixing 

lines, which suggests reduced mixing from lateral and 

vertical flow. However, the degree of lateral mixing in 

these layered aquifer systems remains uncertain on the 

basis of a data set that is largely restricted to the coastal 

regions of the aquifer systems. These potential mixing 

lines suggest that, where consistent with the hydraulics 

of the ground-water flow system, some samples are a 

mixture of local ground water (groups 1 and 2) with 

seawater, whereas very few samples appear to be a 

mixture of local ground water (groups 1 and 2) with 

water from aquifers in the lower Aromas Sand 

(group 5). 
50 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

0.1 1 10 100 1,000 10,000

CHLORIDE, IN MILLIMOLES PER LITER

CHLORIDE-TO-BORON RATIO, IN MILLIMOLES

10

100

1,000

10,000

100,000

Seawater

Watsonville

Slough

PV-6S

PV-6D

Pajaro

River

56–116 feet

100–210 feet

Watsonville Slough

Seawater

12S/1E-13M1 (PV-11)

11S/2E-34D3 (PV-12)

EXPLANATION

Seawater–Watsonville

Slough ideal mixing line

Seawater–Pajaro River

ideal mixing line

PV-6 (D)–PV-6 (S) ideal

mixing line

Salinas Valley

wells with

seawater

Recent

seawater

intrusion

"Older"

seawater

Thurwatcher (P.R.-2.2)

Chittenden (P.R.-16.5)

140–230 feet

140–230 feet (Resample)

250–270 feet

140–170 feet

250–270 feet

380–480 feet

110–180 feet

260–280 feet

510–640 feet

730–750 feet

130–200 feet

420–530 feet

570–590 feet

AG-1.0

AG-2.0 130–160 feet

12S/1E-10R1, 2, 3 (PV-1)

Pajaro River 12S/1E-23R1, 2, 3 (PV-3)

12S/1E-31E1, 2 (PV-4A)

12S/1E-18N1, 2, 3, 4 (PV-6)

12S/1E-11R1, 2, 3 (PV-8)

Agricultural drain

A

(S)

(M)

(D)

(S)

(M)

(D)

(M)

(S)

(SM)

(MD)

(D)

(S)

(M)

(D)

Figure 22. Chloride values plotted against the ratio of chloride to boron, chloride to iodide, and chloride to bromide for ground-water and surfacewater sites in the Pajaro Valley and adjacent Salinas Valley, California.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 51

CHLORIDE, IN MILLIMOLES PER LITER

CHLORIDE-TO-IODIDE RATIO, IN MILLIMOLES

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

0.1 1 10 100 1,000

Watsonville

Slough

PV-12

PV-6 (S)

PV-6 (D)

PV-4 (M)

Seawater

"Older"

seawater

Salinas

River

wells

(180 feet)

(480 feet)

56–116 feet

100–210 feet

Watsonville Slough

Seawater

12S/1E-13M1 (PV-11)

11S/2E-34D3 (PV-12)

EXPLANATION

Seawater–Watsonville

Slough ideal mixing line

Seawater–Pajaro River

ideal mixing line

PV-6 (D)–PV-6 (S)

ideal mixing line

Seawater–PV-12

ideal mixing line

Salinas Valley

wells with seawater

Salinas Valley

wells with

fresh water

Recent

seawater

intrusion

140–230 feet

140–230 feet (Resample)

250–270 feet

140–170 feet

250–270 feet

380–480 feet

130–160 feet

12S/1E-10R1, 2, 3 (PV-1)

12S/1E-23R1, 2, 3 (PV-3)

12S/1E-31E1, 2 (PV-4A)

110–180 feet

260–280 feet

510–640 feet

730–750 feet

130–200 feet

420–530 feet

570–590 feet

12S/1E-18N1, 2, 3, 4 (PV-6)

12S/1E-11R1, 2, 3 (PV-8)

Thurwatcher (P.R.-2.2)

Chittenden (P.R.-16.5)

AG-1.0

AG-2.0

Pajaro River

Agricultural drain

B

(S)

(M)

(D)

(S)

(M)

(D)

(M)

(S)

(SM)

(MD)

(D)

(S)

(M)

(D)

Pajaro

River

PV-8 (D)

Salinas

high nitrate

perched well

Figure 22.—Continued.
52 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

Figure 22.—Continued.

0.1 1 10 100 1,000

CHLORIDE, IN MILLIMOLES PER LITER

CHLORIDE-TO-BROMIDE RATIO, IN MILLIMOLES

Seawater

10

100

1,000

10,000

100,000

PV-6 (S) PV-6 (D)

Watsonville

Slough

Seawater

EXPLANATION

Seawater–Watsonville

Slough ideal mixing line

Seawater–Pajaro River

ideal mixing line

PV-6 (D)–PV-6 (S)

ideal mixing line

C

56–116 feet

100–210 feet

12S/1E-13M1 (PV-11) Watsonville Slough

11S/2E-34D3 (PV-12)

140–230 feet

140–230 feet (Resample)

250–270 feet

140–170 feet

250–270 feet

380–480 feet

130–160 feet

12S/1E-10R1, 2, 3 (PV-1)

12S/1E-23R1, 2, 3 (PV-3)

12S/1E-31E1, 2 (PV-4A)

110–180 feet

260–280 feet

510–640 feet

730–750 feet

130–200 feet

420–530 feet

570–590 feet

12S/1E-18N1, 2, 3, 4 (PV-6)

12S/1E-11R1, 2, 3 (PV-8)

Thurwatcher (P.R.-2.2)

Chittenden (P.R.-16.5)

AG-1.0

AG-2.0

Pajaro River

Agricultural drain

(S)

(M)

(D)

(S)

(M)

(D)

(M)

(S)

(SM)

(MD)

(D)

(S)

(M)

(D)

Pajaro

River
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 53

Some of the wells that plot near the traceelement potential mixing lines may not represent 

mixtures with recent seawater. The ratios of chloride to 

boron and chloride to iodide indicate that most groundwater samples from increasing depth do not fall on a 

mixing line that has seawater or deeper ground water as 

an end-member (figs. 22A, B). In addition, the 

temperature log collected at well PV-8 (M) (App. 1,

fig. A1.6) indicates that there is little to no vertical flow 

across the massive clay layers that occurs between 200 

and 380 feet bls and separates the aquifers in the upper 

and lower Aromas Sand. Therefore, these samples may 

represent different degrees of mixture of waters along 

different flow paths with increasing depth and not 

mixture from vertical flow. Conversely, the ratios of 

chloride to bromide (fig. 22C) indicate two possible 

types of mixing between local runoff such as 

Watsonville Slough water and seawater or mixing 

between Pajaro River water and seawater. The 

differences in ratios may represent a progressive 

dissolution of salts in the sediments that cause a 

relative enrichment of chloride relative to boron and 

iodide (figs. 22A, B) along layered flow paths that are 

separated by the fine-grained layers in the coastal 

regions.

The trace-element ratios with chloride also 

indicate that surface-water samples from the Pajaro 

River and Watsonville Slough are significantly 

different from water from PV-11 and PV-12, 

respectively. These two wells represent relatively 

shallow ground water that would be expected to 

represent some component of local recharge, which 

appears to be a mixture of shallow and older ground 

water that may result from percolation of agricultural 

irrigation. The relatively higher concentrations of iron 

and manganese in water from PV-12 are due, in part, to 

reduced conditions, with almost no dissolved oxygen 

present in this well (App. 2, table A2.1). Reduced 

conditions and subsequent sulfate reduction also have 

resulted in increased barium concentrations (App. 2, 

table A2.1). In addition, the deuterium and oxygen-18 

samples from PV-12 suggest that this ground water 

may represent a mixture of meteoric water with 

previously heated water adjacent to the ZayanteVergeles Fault zone (figs. 1C, 20). The trace-element 

ratios also are different than the distributions indicated 

for the Salinas Valley groups of samples (figs. 22A, B) 

that represent seawater intrusion, fresh water, or highnitrate samples (Vengosh and others, 2002).

Seawater Intrusion and Saline Ground Water

Saline ground water occurs in parts of the 

aquifers of the Pajaro Valley. Some of this saline 

ground water is related to seawater intrusion resulting 

from sustained ground-water pumpage in the coastal 

regions. However, other saline waters may have 

different sources that are not related to seawater 

intrusion. While high chloride concentrations are 

commonly an indicator of seawater intrusion, 

additional geochemical indicators, such as trace 

element ratios and isotopes are needed to differentiate 

other sources of saline water of an origin different from 

seawater.

Since 1989, the chloride concentrations of waterquality samples from multiple-well observation sites 

along the coast from La Selva Beach to Springfield 

Terrace (figs. 1B, C) have increased for selected 

monitoring wells (figs. 8–12, 14, and 15). Groundwater samples from PV-1 (M), PV-1 (D), PV-3 (M), 

PV-4 (M), PV-6 (D), PV-8 (D), SC-A3 (A), and SC-A4 

(A) have chloride concentrations, ranging from 4,000 

to about 18,000 mg/L. (figs. 8–12, 14, and 15). 

Chloride concentrations are trending towards the 

average chloride concentration of seawater (19,000 

mg/L) for aquifers in the upper Aromas Sand in the 

Sunset Beach area [PV-1 (D), fig. 8], in the Palm Beach 

area [PV-3 (M), fig.9], and in the Springfield Terrace 

area [PV-4 (M), fig. 10]. Chloride concentrations have 

increased in the lower Aromas Sand in the La Selva 

Beach area [SC-A3 (B), fig. 14]. The chloride 

chemographs from the coastal monitoring sites show 

consistently high chloride concentrations in aquifers in 

the lower Aromas Sand in the north-central coastal 

regions [PV-6 (D), PV-8 (D), SC-A3 (A), and 

SC-A4 (A); figs. 6, 12, 14, and 15] and in aquifers in 

the upper Aromas Sand in the Pajaro River mouth and 

Springfield Terrace areas to the south [PV-3 (M) and 

PV-4 (D); figs. 9, 10]. The chloride concentrations in 

the middle aquifers in the upper Aromas Sand aquifers 

have been increasing at Sunset Beach [PV-1 (M), 

fig. 8A] and began increasing in the La Selva Beach 

area to the north since 1993 [SC-A3 (B), 

figs. 1B, C, and 14]. Chloride concentrations may vary 

that have seasonal and other step changes in water 

levels at many of the wells with relatively higher 

chloride concentrations. The major-ion diagram in 

figure 19B indicates that ground-water samples from 

some of these sites completed in aquifers in the upper 

Aromas Sand (group 3) are trending towards the 

average composition of seawater (Hem, 1985).
54 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

Stable isotopes of boron and sulfur as well as 

tritium and carbon-14 also were used to help constrain 

the possible sources of older saline water (group 4) in 

the Pajaro Valley. The graphs of boron-isotope values 

from Pajaro Valley samples, combined with the anion 

ratios with chloride concentrations, indicate that the 

samples from aquifers in the lower Aromas Sand 

(group 4) are depleted in boron and in delta-boron-11 

(fig. 23A). In addition, all of the saline samples from 

monitoring wells in the Salinas Valley are also depleted 

in boron 11 and have smaller chloride-to-boron ratios 

relative to seawater (fig. 23A). This also suggests a 

different origin of the salinity for the samples from the 

lower Aromas Sand relative to the recent seawater 

intrusion samples from the Salinas or the Pajaro 

Valleys. In contrast, the sulfur isotopes (fig. 23B) show 

little difference between older and recent saline waters, 

which may indicate that these waters are, in part, older 

seawater. These older calcium-magnesium-chloride 

saline waters lack tritium and these ages were 

estimated to be between 7,000 to 10,700 years before 

present on the basis of carbon-14 samples (App. 2,

table A2.1). In addition, these wells with older waters 

(group 4) also are depleted in iodide as well as boron 

and enriched in barium and strontium relative to 

seawater and do not represent the intrusion of recent 

seawater. 

Geologic and geophysical data indicate that the 

layering in coastal aquifer systems such as Pajaro 

Valley, results in seawater intrusion that is vertically 

restricted. This is exemplified by the geophysical logs 

collected at PV-1, PV-6, and PV-8 (App. 1, figs. A1.1, 

A1.5, and A1.6). This vertical restriction can contribute 

to a relatively sharp interface within the coarse-grained 

layers of the aquifers system. For example, the 

geophysical logs from PV-1 (fig. A1.1) indicate that 

saline water is present in discrete layers, which is 

typical of the geometry for the leading edge of seawater 

intrusion seen in other coastal alluvial basins such as 

the Ventura area (Hanson and others, 2002) and Los 

Angeles (Newhouse and Hanson, 2000). Thus, the 

initial seawater intrusion is not a wedge but is 

selectively intruding coarse-grained water-bearing 

units that range in thickness from 10 to 150 ft. The low 

resistivity in the lower part of the original resistivity log 

also suggests that saline waters were already present in 

1988 when the monitoring well was installed, and the 

subsequent logs from 1999 indicate that additional 

shallower water-bearing zones now contain saline 

water (App. 1, fig. A1.1). The geophysical logs.

(App. 1) indicate that these deeper saline waters in 

PVWMA monitoring wells occur in relatively thin 

intervals that are vertically adjacent to fresher ground 

water. This is especially evident from the multiple 

screens of the middle monitoring well at PV-8 (App. 1, 

fig. A1.6) (420–530 ft depth, App. 2, table A2.1) where 

the upper screen is opposite fresher water and the lower 

part of the deeper screen penetrates a thin zone of 

saline water. 

The geophysical data also may indicate the 

presence of zones of saline water elsewhere in the 

lower Aromas Sand. For example, zones containing 

saline water were also indicated by higher 

electromagnetic (EM) conductivity and lower gamma 

activity in geophysical logs, at the bottom of 

monitoring well PV-8 (M) (App. 1, fig. A1.6) where 

composite salinity also has increased up to 245 mg/L of 

chloride by 1998 (fig. 12) and at the Dolan Road 

monitoring well, where poor-quality water appears to 

be present at a depth of about 365 to 395 ft (App. 1, 

fig. A1.11) adjacent to fine-grained layers. Because 

most sediments in the lower Aromas Sand were 

deposited during periods of rising ocean levels (Dupre, 

1998), the seawater that was initially present in these 

sediments may not have been completely flushed from 

the sediments during subsequent periods of falling sea 

level. The deeper thin zones of high salinity seen in the 

geophysical logs from PV-6 and PV-8 (App. 1, 

figs. A1.5, A1.6) may be indicative of this older saline 

water. Because sea level changes are cyclic, there is 

also the possibility that increasing salinity that is not 

modern seawater may be remnant portions of paleointrusion as was simulated for the adjacent SoquelAptos watershed (Essaid, 1992). Regardless of the 

specific mechanism, these deeper saline waters have 

been saline at least since the completion of the 

monitoring wells in Pajaro Valley, have been resident 

long enough to have undergone base exchange, and 

represent recharge from thousands of years ago. 
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 55

Recent

seawater

intrusion

Salinas Valley

wells with seawater

(180 feet)

(480 feet)

Salinas Valley

wells with

high nitrate

Seawater

Surface

waters

"Older"

seawater

Azevedo's strawberry

field (soil sample)—

(Vangosh and others, 2002)

Seawater

EXPLANATION

10 100 1,000 10,000 100,000

CHLORIDE-TO-BORON RATIO, IN MILLIMOLES PER LITER

DELTA BORON-11, IN PER MIL

0.0

10.0

20.0

30.0

40.0

50.0

60.0

Salinas Valley

wells with

freshwater

56–116 feet

100–210 feet

12S/1E-13M1 (PV-11) Watsonville Slough

11S/2E-34D3 (PV-12)

70–90 feet

140–230 feet

250–270 feet

140–170 feet

250–270 feet

380–480 feet

130–160 feet

12S/1E-10R1, 2, 3 (PV-1)

12S/1E-23R1, 2, 3 (PV-3)

12S/1E-31E1, 2 (PV-4A)

110–180 feet

260–280 feet

510–640 feet

730–750 feet

130–200 feet

420–530 feet

570–590 feet

12S/1E-18N1, 2, 3, 4 (PV-6)

12S/1E-11R1, 2, 3 (PV-8)

Thurwatcher (P.R.-2.2)

Chittenden (P.R.-16.5)

AG-1.0

AG-2.0

Pajaro River

Agricultural drain

A

(S)

(M)

(D)

(S)

(M)

(D)

(M)

(S)

(SM)

(MD)

(D)

(S)

(M)

(D)

Figure 23. Boron-isotope values plotted against the ratio of chloride to boron, and sulfur-34 isotope values plotted against sulfate concentration for 

selected ground-water and surface-water samples from the Pajaro Valley and adjacent Salinas Valley, California.
56 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

1 10 100 1,000 10,000

SULFATE, IN MILLIGRAMS PER LITER

DELTA SULFUR-34, IN PER MIL

–5

0

5

10

15

20

25

30

35

Seawater

PV-6 (S)

"Older" seawater

Recent seawater intrusion

Surface water

70–90 feet

140–230 feet

250–270 feet

110–180 feet

260–280 feet

510–640 feet

730–750 feet

130–200 feet

420–530 feet

570–590 feet

56–116 feet

100–210 feet Watsonville Slough Seawater

130–160 feet

12S/1E-10R1, 2, 3 (PV-1)

12S/1E-31E1, 2 (PV-4A)

12S/1E-18N1, 2, 3, 4 (PV-6) 12S/1E-11R1, 2, 3 (PV-8)

12S/1E-13M1 (PV-11)

11S/2E-34D3 (PV-12)

EXPLANATION

Thurwatcher (P.R.-2.2)

Chittenden (P.R.-16.5)

AG-1.0

AG-2.0

Pajaro River

Agricultural drain

(S)

(M)

(D)

(M)

(S)

(SM)

(MD)

(D)

(S)

(M)

(D)

B

Figure 23.—Continued.
Evaluation of Recharge and Seawater Intrusion Based on Water Chemistry 57

As noted above, not all coastal wells completed 

in the deeper aquifers have high chloride 

concentrations that are relatively constant. For 

example, two monitoring wells [SC-A3 (B), SC-A4 

(A)] with this water type (group 4) (fig. 19B) have 

shown increasing salinity in the adjacent La Selva 

Beach area (figs. 14, 15) since their installation in 1987 

and 1993, respectively. On the basis of major-ion 

chemistry, the samples from the La Selva Beach 

monitoring wells completed in the upper Aromas Sand 

[SC-A3 (B)] and the lower Aromas Sand [SC-A4 (A)] 

are similar to this older seawater group (group 4) while 

the saline sample from the upper Aromas Sand [SC-A3 

(A)] is similar to recent seawater (group 3) (fig. 19B). 

However, isotope samples that would be needed to help 

delineate the origin of these saline waters were not 

collected from monitoring wells used to monitor 

seawater intrusion by SCWD as part of this study. 

Thus, the origin of the salinity in these coastal 

monitoring wells remains uncertain on the basis of 

major-ion chemistry data alone and would require 

additional geochemical and geophysical data to 

determine their origin with more certainty. 

Low streamflows on the Pajaro River are another 

source of saline water in the Pajaro Valley. Streamflow 

hydrographs and chemographs for the Pajaro River 

indicate that chloride and sulfate concentrations 

increase during low flows (figs. 6A, B). The chloride 

and sulfate concentrations may exceed the SMCL of 

250 mg/L when streamflows are less than about 10 ft3/s 

(figs. 6A, B). On the basis of the streamflow duration, 

these low flows occur about half of the time (fig. 7) for 

the period of record. Concentrations as high as 600 

mg/L of chloride and 300 mg/L of sulfate were present 

in the Pajaro River during the low base flows of severe 

droughts such as 1976–77. Concentrations of chloride 

remain less than 40 mg/L on Corralitos Creek and also 

increase during low flows (fig. 6E). Conversely, the 

concentration of nitrate in streamflow in the Pajaro 

River tends to increase with increased flow 

(figs. 6A, B) and to increase during lower flows on 

Corralitos Creek (fig. 6E). The concentrations of nitrate 

generally increase with increased base flow 

streamflows up to about 20 ft3/s and do not exceeded 

the MCL of 45 mg/L of nitrate as nitrate when base 

flow streamflows are less than about 10 ft3/s during the 

1980s (fig. 6A). The ground-water recharge from 

streamflow infiltration is coincident with 

concentrations of boron greater than 0.3 mg/L 

(fig. 6D). The increased boron that occurs as a boron 

plume in the shallower alluvial aquifers adjacent to the 

Pajaro River was identified as a potential indicator of 

recharge as streamflow infiltration (J.H. Kleinfelder 

and Assoc., 1983).

In summary, geochemical and geophysical data 

indicate several sources of saline water. The saline 

waters in the PV-1 (M, D) and PV-4A (M) wells reflect 

recent seawater intrusion into the recent and older 

alluvium. The deeper saline waters in the PV-8 (D) and 

the PV-6 (D) wells may be older seawater. These older 

seawaters probably have undergone base exchange in 

which the sodium from the seawater has been 

exchanged (taken into sediments) with calcium and 

magnesium released from the fine-grained material in 

these sediments into the ground water. Further, they 

may represent seawater that was hypersaline and may 

also include salinity from another source. This is 

indicated by elevated chloride-to-iodide and chlorideto-boron ratios above the values for average seawater 

composition and by depleted boron-11 concentrations. 

This source could be dissolution of chloride salts that 

occur within the marine sediments of the deeper lower 

Aromas Sand and Purisima Formation. The chlorideto-bromide ratios, however, indicate that many of these 

waters are mixtures of water from either Pajaro River 

or Watsonville Slough and seawater. Thus, the 

geochemical data, in combination with the geophysical 

data, indicate that only the upper layers of the aquifer 

systems in selected areas are receiving recent recharge 

and that saline waters are potentially restricted to 

selected coarse-grained layers that contain recent 

seawater intrusion in the shallower alluvial aquifers 

and older seawater at depth in the coastal regions 

(fig. 3).
58 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

SUMMARY AND CONCLUSIONS

Pajaro Valley is a coastal watershed of 160 mi2

located along Monterey Bay north of Elkhorn Slough 

and south of the city of Santa Cruz. The valley has been 

predominantly developed for agriculture since the late 

1800s. In 1984 the Pajaro Valley Water Management 

Agency (PVWMA) was formed and was delegated 

with the responsibility of managing of the water 

resources within the Pajaro Valley by the State of 

California. About 84 percent of the water is used for 

agriculture and 16 percent is used for industrial and 

municipal water supply; almost all of the demand is 

supplied by ground water. Ground-water pumpage 

varies with seasonal and climatic periods.

The alluvial aquifers are composed of 

Quaternary- and Tertiary-aged sediments that are welllayered marine and terrestrial coarse-grained deposits 

separated by extensive fine-grained deposits. The finegrained deposits potentially restrict vertical movement 

of ground water and constrain seawater intrusion in the 

coastal subareas. The coarse-grained deposits persist 

over large areas and control the pumpage and related 

seawater intrusion. The Aromas Sand crops out 

throughout the north and central parts of the PVWMA 

area and offshore on the continental shelf and in 

Monterey submarine canyon. Because many of the 

wells in the coastal and inland subregions are screened 

at depths of 200 to 400 ft below land surface, a direct 

avenue is provided for seawater intrusion through the 

coarse-grained deposits. Geophysical logs from 

monitoring wells indicate discrete zones of saline water 

that are related to pumpage and seawater intrusion in 

the aquifers of the shallower alluvium and upper 

Aromas Sand in the upper-aquifer system and to deeper 

saline waters in the lower Aromas Sand within the 

lower-aquifer system. Several fault zones traverse the 

Pajaro Valley, but the importance of faults as hydraulic 

barriers to ground-water flow remains uncertain.

The precipitation data indicate that there are at 

least nine dry and nine wet periods that range in 

duration from 2 to 19 years for the period of record, 

1880–1997. These multiyear cycles have resulted in 

increases in ground-water pumpage of as much as 20 

percent during dry-year periods, especially when there 

is a lack of precipitation in the spring. Droughts within 

dry-year periods have been as short as the severe 

drought of 1976–77 and as long as the sustained 7-year 

drought of 1986–92. Runoff and streamflow, and the 

related water quality of streamflow, also vary with 

seasonal and climatic periods.

Recharge occurs from deep percolation of 

precipitation and from infiltration of streamflow. 

Streamflow originates from local runoff as well as 

runoff that originates outside the valley and enters as 

streamflow in the Pajaro River. Although partly 

regulated, streamflow on the Pajaro River at Chittenden 

is less than 200 ft3/s 88 percent of the time and is less 

than 12 ft3/s 50 percent of the time. The geometricmean streamflow on the Pajaro River at Chittenden in 

wet-year periods is about 4 times greater than in dryyear periods. Streamflow water-quality data suggest 

that there may be several sources of poor-quality water 

that contribute to elevated chloride, sulfate, and nitrate 

concentrations in streamflow. The poor water quality 

occurring during lower streamflows indicates that low 

flows may be an additional source of salinity for 

ground-water recharge as streamflow infiltration along 

the Pajaro River.

The geochemical data from this study indicate 

that the two main sources of natural recharge are deep 

percolation of local runoff and streamflow infiltration 

of Pajaro River water. Some of the poor-quality ground 

water may be from other sources that may include 

irrigation runoff, deep infiltration of irrigation returnflow, slough and lagoon leakage, dissolution of salts 

from saline sediments, old seawater, and infiltration of 

saline streamflow during low-flow periods. The 

geophysical and geochemical data suggest that only the 

shallow-alluvium aquifer and parts of the upper 

Aromas Sand are being replenished by recent recharge 

in the coastal areas of Pajaro Valley and represent the 

renewable ground-water resources. These data also 

suggest that there is very little vertical flow through the 

layered aquifer systems in the coastal regions. The 

confining aquitards are laterally extensive but may be 

missing in places owing to fluvial erosion or offsetting 

by fault movement. On the basis of tritium and carbon14 ages, the ground water from some parts of the upper 

and lower Aromas Sand was recharged thousands of 

years ago and may, in part, represent nonrenewable 

ground-water resources. 
Selected References 59

The analysis of major-ion chemistry, in 

combination with isotope and trace-element/chloride 

ratios, indicates that the coastal ground-water and 

surface-water samples can be described in seven 

groups. These groups represent recently recharged 

shallow aquifers (group 1), shallower aquifers with 

older water (group 2), shallow aquifers with recently 

intruded seawater (group 3), deeper aquifers with older 

seawater (group 4), deeper aquifers with older 

freshwater (group 5), Pajaro River streamflow 

(group 6), and recent local runoff from streams and 

sloughs (group 7).

The ground-water levels have been near or below 

sea level at most coastal monitoring wells since their 

installation in the 1980s. Water-level changes include 

long-term decline, climate-period changes, and 

seasonal changes from agricultural pumping. The 

water-level declines in monitoring wells during 

seasonal and dry climatic periods are related to 

decreased recharge and increased pumpage. Increased 

pumpage from the lower Aromas Sand and resulting 

water-level changes in coastal monitoring wells are 

similar to the changes in water levels measured in other 

local deep water-supply wells since 1970. At the time 

of this study (1998–99), water levels in most coastal 

and some inland water-supply wells in the Springfield 

subarea and as far inland as Murphy Crossing in the 

Pajaro subarea were below the estimated water levels 

of 5 to 15 ft above sea level required to stop seawater 

intrusion. The water levels also indicate lower 

pressures in the deeper aquifers, especially during 

seasonal pumping. Thus, long-term declines of more 

than 20 ft owing to regional and coastal pumping, 

combined with seasonal water-level changes from 

coastal agricultural pumping, are collectively causing 

seawater intrusion. Water levels near sea level in the 

inland regions represent a regional area of long-term 

ground-water decline from increased ground-water 

development. Inland regional decline combined with 

low water levels in the coastal areas sustains long-term 

seawater intrusion.

 The predominant source of the chloride in 

ground water in the Pajaro Valley may be recent 

seawater for aquifers in the older alluvium and the 

upper Aromas Sand and old seawater or connate water 

in the aquifers of the lower Aromas Sand. Groundwater samples from selected coastal monitoring wells 

[PV-6 (D), SC-A4 (A), PV-3 (M), PV-8 (D), PV-4 (M), 

PV-1 (M, D), and SC-A3 (A)] have chloride 

concentrations that range from 4,000 to about 18,000 

mg/L. Some of these samples approach the 

composition of seawater, whereas other selected deeper 

water-bearing zones in the lower Aromas Sand contain 

calcium-magnesium chloride waters. Chloride 

chemographs, along with the geophysical logs, indicate 

that saline water is vertically restricted to aquifers in 

the older shallow alluvium and the upper and lower 

Aromas Sand in the northern area, to aquifers in the 

lower Aromas Sand farther inland from the coast in the 

central area, and to aquifers in the upper Aromas Sand 

in the central-southern coastal areas of the Pajaro 

Valley. On the basis of the geochemistry and 

geophysical logs, the seawater intrusion is similar to 

that occurring in other coastal alluvial aquifers. The 

seawater occurs in the aquifers that include eolian 

sands and basal layers in coarse-grained fluvial and 

terrace deposits. In the coastal regions, these layers are 

commonly bounded by thickly bedded fine-grained 

layers resulting in little vertical flow of ground water or 

local deep percolation of recharge and vertically 

discreet horizontal flow of intruding seawater with 

relatively sharp fronts.

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Appendixes 63

APPENDIXES
64 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

Geophysical logs that included natural gamma, 

electromagnetic conductivity, temperature, and fluid 

resistivity were completed by the USGS at selected 

monitoring-well sites (PV-1, 3, 6, 8, 11, 12, 13, 14). 

No logs were collected at PV-4 because of petroleum 

products in the well vault that could have entered the 

monitoring wells. In addition, geophysical logs were 

completed at the Dolan Road monitoring-well site 

[DLN-(D)]. Geophysical logging was also attempted at 

the Hudson Landing site [HD- (D, S), but the 

geophysical tools could penetrate only to a depth of 

about 80 ft below land surface. Geophysical logs that 

include electrical resistivity and spontaneous-potential 

logs from the original drilling of the monitoring wells

(PV-1, 3, 4, 6, 8) and the test hole PV-5 (Luhdorff and 

Scalmanini, 1987a) also were digitized for purposes of 

comparison and analysis. The plots of all geophysical 

logs with lithologic and well-construction data are 

shown in appendix figures A1.1–A1.11, below. The 

SCWD and PVWMA monitoring-well numbering 

convention adds a letter to the local well number that 

designates the relative position of the well screens in 

the multiple-well site configuration as S (shallow), SM 

(shallow medium), M (medium), MD (medium deep), 

and D (deep). Similarly, the SCWD monitoring-well 

numbering convention adds a letter to the local well 

number from A through D that designates the relative 

position of the well screens in the multiple-well site 

configuration as D or C as the shallowest and A the 

deepest well.

The geophysical logging yielded additional 

information about the stratigraphy of the aquifers, the 

nature of ground-water flow and seawater intrusion, 

and the nature of samples taken from these monitoring 

wells. Electromagnetic (EM) conductivity logs are 

used to determine conductive sediments that contain 

poor-quality water or clay. Gamma logs are useful 

indicators of silt-and-clay layers that may be difficult to 

delineate based on conventional electric logs when 

poor-quality water is present. The combination of EM 

and gamma logs can be used to determine waterbearing zones with poor-quality water that may be 

related to saline water such as seawater intrusion. 

Temperature logs are useful indicators of lateral and 

vertical movement of ground water that may disrupt the 

natural linear geothermal gradient of the sediments. 

Electrical resistivity logs are useful for d33etermining 

layering and poor-quality water. Except for the EM log 

from well PV-1, all the other EM logs were collected in 

2-inch metal casings, which dampens the induced field 

and causes additional alterations from collars at pipe 

joints.

PV-1. The original electrical resistivity logs from 

the deeper test hole indicate that there was poor-quality 

water already present when the well was drilled in 

November 1988 below 250 ft below land surface (bls) 

and may represent the initial distribution of seawater 

intrusion. The gamma and EM logs from 1998 indicate 

that additional poor-quality water occurs in well PV-1 

(M) from 100 to 110 ft bls and from 145 to 180 ft bls 

that may indicate new seawater intrusion (fig. A1.1). 

The middle well [PV-1 (M)] was installed with two 

well screens that may straddle a stratigraphic boundary 

between the shallow and older alluvium. The upper 

screened interval for PV-1 (M) appears to represent a 

fresher zone that is underlain by a thin siltstone layer 

that is, in turn, underlain by a zone of progressively 

more saline water (145–180 ft bls). This relation is 

consistent with the results from the pumped and airlifted water-chemistry samples taken from this 

monitoring well. The air-lifted sample yielded water 

with almost three times more specific conductivity and 

chloride concentration (App. 2, table A2.1) than the 

pumped sample that may have skimmed fresher ground 

water from the upper screened interval.

APPENDIX 1. SUMMARY ANALYSIS OF GEOPHYSICAL, LITHOLOGIC, AND WELLCONSTRUCTION DATA ANALYSIS FOR SELECTED MONITORING WELLS IN THE PAJARO 

VALLEY
Appendix 1 65

Seawater

Intrusion

1988

?

0

200

400

64 inch

600

800

0 100

GAMMA,

IN COUNTS

PER

SECOND

–50 0 50

SPONTANEOUS

POTENTIAL,

IN

MILLIVOLTS

0 100

16/64 INCH

RESISTIVITY,

IN OHMMETERS

PAJARO VALLEY PV-1

12S/1E-10R1–3

1,000 100 10

EM

CONDUCTIVITY,

IN MILLIMHOS

PER METER

WELL

CONSTRUCTION

DEPTH, IN FEET BELOW LAND SURFACE

16 inch

R1 [PV-1 (D)]R2 [PV-1 (M)]R3 [PV-1 (S)]

GROUT SEALED ABANDONED

Sand

Clayey sand

Clayey sand

Sand

Sand

Sand, fine

Sand,coarse

Sand

Sand

Sand, coarsening

in bottom 10 feet

Sand, coarsening

in bottom 20 feet

Siltstone

interbed

Gravelly sand

Gravelly sand

LITHOLOGY

Screened

Interval

Additional

Seawater

Intrusion

1988-98

1998

Year log

recorded

1988

Year log

recorded

1988

Year log

recorded

1998

Year log

recorded

Figure A1.1. Geophysical logs and well construction for selected monitoring wells and test holes in the Pajaro Valley, Santa Cruz County, 

California.
66 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

PV-3. The geophysical logs indicate that poorquality water was already present when the well was 

drilled in 1988, as evidenced by the low resistivity for 

the interval from 180 to 360 ft bls (fig. A1.2). In 

addition, there may be discrete zones of poor-quality 

water from about 100 to 175 ft bls, most of which are 

not screened. Note that the logs completed in the deep 

well [PV-3(D)] do not extend to the reported total depth 

as shown in the diagram (Luhdorff and Scalmanini, 

1988) because the total depth is different than reported 

or the well has filled with sediment above the lowest 

screened interval since completion. The gamma and 

lithologic logs suggest that the thick fine-grained layer 

from 330 to 362 ft bls separates the water-bearing units 

in the Aromas Sand from the intruded-water-bearing 

zones in the recent and older alluvium. Shifts in the 

gamma log and EM log collectively show a 

stratigraphic break at about 362 ft bls that may be the 

upper boundary of the Aromas Sand. The EM log, 

although damped by the metal casing, appears to 

coincide with increased conductivity in the same zones 

that contained poor-quality water when the well was 

drilled. 

PV-4A and PV-5. As at the other coastal sites, the 

geophysical and drillers logs from monitoring well PV4A (fig. A1.3) and the geophysical logs from the test 

hole PV-5 (fig. A1.4) (Luhdorff and Scalmanini, 1988) 

indicate poor-quality water and very thick silt-and-clay 

layers from 180 to 545 ft bls. These thicker clay layers 

may represent deposits from an older submarine 

canyon that now represent a partial barrier north of the 

Salinas Valley between the Pajaro River and the 

Elkhorn Slough coastal area that was previously 

identified as an extension of the existing Monterey 

submarine canyon (Johnson, 1983).

PV-6. The geophysical logs for PV-6 also show 

poor-quality water below 740 ft bls (fig. A1.5) when 

the site was drilled in 1988. The fine-grained units 

occur from 385 to 410 and 465 to 480 ft bls. Based on 

the chemical sampling and the gamma and EM logs 

from 1998, the poorest quality water occurs in the zone 

740 to 750 ft bls, which is below a fine-grained layer at 

730 to 740 ft bls at the base of the Aromas Sand, which 

may also coincide with the boundary between geologic 

formations.

PV-8. The geophysical logs indicate that the 

shallow well is completed in an isothermal zone 

between about 120 and 200 ft bls that would indicate 

downward or lateral flow of ground water (fig. A1.6). 

Although the water-level altitudes would indicate a 

potential for downward flow of water to the lower 

aquifers, the temperature log indicates that little to no 

downward flow of water is occurring through the 

relatively fine-grained material occurring between 200 

and 415 ft bls. The 415-ft depth also is the upper 

boundary of the Aromas Sand. The two screens 

completed in the middle well [PV-8 (M)] are separated 

by a fine-grained layer at 440–460 ft bls. Based on the 

fluid resistivity log, the well is probably filled with 

water from the upper screen, but the water from the 

lower screen is relatively poor quality. This log, in 

combination with previous purge logs of water quality, 

indicates that water-quality samples from PV-8 (M) 

may be a mixture of two different ground waters 

resulting in unreliable interpretations of sample data.

PV-11–PV-14. The EM and gamma logs 

(figs. A1.7–A1.10, below) from these monitoring wells 

generally indicate no saline water, and several clay 

layers are present. The logs from monitoring well PV11, located in the San Andreas Dunes area west of 

Sunset Beach (CH2M HILL, 1994), indicate several 

clay layers at depths from 20 to 30, 33 to 37, and 

interbedded clay layers from 75 to 95 ft bls (fig. A1.7). 

The logs from monitoring well PV-12 also indicate 

(CH2M HILL, 1995) thick clay layers from 17 to 34 

and 36 to 60 ft bls (fig. A1.8). The logs from 

monitoring well PV-13 (fig. A1.9), located in the San 

Andreas Dunes area west of Sunset Beach (CH2M 

HILL, 1994), indicate several clay layers at depths 

from 35 to 43, 48 to 53, and 84 to 89 ft bls. The logs 

from monitoring well (fig. A1.10) confirm the previous 

information (CH2M HILL, 1995), with a clay layer or 

arkosic sand from 17 to 22 ft bls. 

DLN-(D). The geophysical logs (fig. A1.11) 

from the Monterey County Water Resource Agency 

monitoring well on Dolan Road indicate the possibility 

of poor-quality water in the red sand from 360 to 370 

and 380 to 395 ft bls (Fugro West, Inc., 1995).
Appendix 1 67

200

0

400

600

800

64 inch

1,000 0 50 100 –50 0 50 0 100

PAJARO VALLEY PV-3

12S/1E-23R2–4

1,000 10 100

16 inch

DEPTH, IN FEET BELOW LAND SURFACE

R2 [PV-3 (D)]R3 [PV-3 (M)]R4 [PV-3 (S)] GROUT SEALED

ABANDONED

Sand

Sand

Sand

Sand

Sand

Sand

Sand

Sand

Sand

Sand

Sand

Silty clay

Silty clay

Claystones and siltstones

Sand with clay interbeds

Sand

and

silty

clay

interbeds

Silty clay

Silty clay

Silty clay

Silty clay with

fine sand beds

Silty clay

Silty clay

Silty clay

Sand

Clayey sand/

Clay and sand

Clayey sand

Clayey sand

Clayey sand

Gravelly sand

Sandy gravel

LITHOLOGY

?? ? ? ?

GAMMA,

IN COUNTS

PER

SECOND

16/64 INCH

RESISTIVITY,

IN OHMMETERS

EM

CONDUCTIVITY,

IN MILLIMHO

PER METER

WELL

CONSTRUCTION

SPONTANEOUS

POTENTIAL,

IN

MILLIVOLTS

Screened

Interval

Cyclic peaks

above 350 feet

depth are metal

couplings in

well casing

1998

Year log

recorded

1998

Year log

recorded

1988

Year log

recorded

1988

Year log

recorded

Figure A1.2. Geophysical logs and well construction for selected monitoring wells and test holes in the Pajaro Valley, Santa Cruz County, California.
68 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

–10 0 50 100 0 60

PAJARO VALLEY PV-4A

12S/2E-31E1, 2

200

0

400

600

800

1,000

DEPTH, IN FEET BELOW LAND SURFACE

E1 [PV-4A (M)]E2 [PV-4A (S)]

GROUT SEALED ABANDONED ABANDONED

Sand

Mudstones and silty

sandstones

interbedded

Clay and sand

interbedded

Silty clay

Silty clay

Silty clay

Clayey sand

Silty clay with sand

Sandy clay

Sand

Gravelly sand

Gravelly sand

Silty sand

Clayey sand

Gravelly sand

SPONTANEOUS LITHOLOGY

POTENTIAL,

IN

MILLIVOLTS

16-INCH

RESISTIVITY,

IN OHMMETERS

WELL

CONSTRUCTION

Screened

Interval

1988

Year log

recorded

1988

Year log

recorded

Figure A1.3. Geophysical logs and well construction for selected monitoring wells and test holes in the Pajaro Valley, 

Monterey County, California.
Appendix 1 69

–30 0 30 0 40

200

0

400

600

800

1,000

DEPTH, IN FEET BELOW LAND SURFACE

PAJARO VALLEY PV-5

13S/2E-7C1

SPONTANEOUS

POTENTIAL,

IN

MILLIVOLTS

16 INCH

RESISTIVITY,

IN OHMMETERS

1988

Year log

recorded

1988

Year log

recorded

Figure A1.4. Geophysical logs for selected monitoring wells 

and test holes in the Pajaro Valley, Monterey County, 

California.
70 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

0

200

400

600

800

1,000 0 100 –50 0 50 0 100

PAJARO VALLEY PV-6

12S/1E-18N1–4

1,000 100

DEPTH, IN FEET BELOW LAND SURFACE

64 inch

16 inch

N1 [PV-6 (D)]N2 [PV-6 (MD)]N3 [PV-6 (SM)]N4 [PV-6 (S)]

GROUT SEALED ABANDONED

Sandy and silty clay

Sandy silty clay

Silty clay

Silty clay

Silty clay and

sand interbeds

Silty clay with sand

Silty sandy clay

Sandy clay

Silty clay with

sand interbeds

Silty sand

Silt and sandstone

Silt and sandstone

Siltstones and

claystones with

interbedded silty

sandstones

Sand

Sand

Sand

Sand

Sand with silty

clay layer

Gravelly sand with

clay layers

Gravelly sand with

clay layers

Silty sand

Sand

Gravelly sand

Clayey sand

Silty sandstone

Silty sandstone

Silty sandstone

Silty sandstone

GAMMA,

IN COUNTS

PER

SECOND

SPONTANEOUS

POTENTIAL,

IN

MILLIVOLTS

16/64 INCH

RESISTIVITY,

IN OHMMETERS

EM

CONDUCTIVITY,

IN MILLIMHOS

PER METER

WELL

CONSTRUCTION

LITHOLOGY

Screened

Interval

1998

Year log

recorded

1998

Year log

recorded

1988

Year log

recorded

1988

Year log

recorded

Figure A1.5. Geophysical logs and well construction for selected monitoring wells and test holes in the Pajaro Valley, Santa Cruz County, California.
Appendix 1 71

15 25

TEMPERATURE,

IN DEGREES

CENTIGRADE

200

0

400

600

800

1,000

25 50 75

GAMMA,

IN COUNTS

PER SECOND

–50 0 50 0 100

16/64 INCH

RESISTIVITY,

IN OHMMETERS

PAJARO VALLEY PV-8

12S/1E-11R1–3

0 5 10 15 20 25

FLUID

RESISTIVITY,

IN MICROSIEMENS

PER CENTIMETER

DEPTH, IN FEET BELOW LAND SURFACE

R1 [PV-8 (D)]R2 [PV-8 (M)]R3 [PV-8 (S)]

GROUT SEALED ABANDONED

1,000 100

EM

CONDUCTIVITY,

IN MILLIMHOS

PER METER

WELL

CONSTRUCTION

LITHOLOGY

16 inch

64 inch

Sand

Silty clay

Silty clay

Silty

clay

Silty clay

Clayey sand

Sand

Sand

Sandy claystone

Sand

Sand

Siltstone

Siltstone

Siltstone

Clay interbeds

Claystone interbeds

Silty clayey sand

Gravelly sand

Gravelly sand

Clayey sand

Sand

SPONTANEOUS

POTENTIAL,

IN

MILLIVOLTS

Screened

Interval

1998

Year log

recorded

1998

Year log

recorded

1998

Year log

recorded

1998

Year log

recorded

1988

Year log

recorded

1988

Year log

recorded

Figure A1.6. Geophysical logs and well construction for selected monitoring wells and test holes in the Pajaro Valley, Santa Cruz County, California.
72 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

15 20 25 30 45 60 75 90

PAJARO VALLEY PV-11

12S/1E-13M1

0 5 10 15 20 25

FLUID

RESISTIVITY,

IN MICROSIEMENS

PER CENTIMETER

100 10 1

M1

DEPTH, IN FEET BELOW LAND SURFACE

GAMMA,

IN COUNTS

PER

SECOND

TEMPERATURE,

IN DEGREES

CENTIGRADE

EM

CONDUCTIVITY,

IN MILLIMHOS

PER METER

WELL

CONSTRUCTION

0

25

50

75

100

125

150

175

200

Screened

Interval

1998

Year log

recorded

1998

Year log

recorded

1998

Year log

recorded

1998

Year log

recorded

Figure A1.7. Geophysical logs and well construction for selected monitoring wells and test holes in the Pajaro Valley, Santa Cruz County, 

California.
Appendix 1 73

30 45 60 75 90

PAJARO VALLEY PV-12

11S/2E-34D3

1,000 100 10

Metal in hole

DEPTH, IN FEET BELOW LAND SURFACE

GAMMA,

IN COUNTS

PER

SECOND

EM

CONDUCTIVITY,

IN MILLIMHOS

PER METER

0

25

50

75

100

125

150

175

200

1998

Year log

recorded

1998

Year log

recorded

Figure A1.8. Geophysical logs and well construction for 

selected monitoring wells and test holes in the Pajaro Valley, 

Santa Cruz County, California.
74 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

30 45 60 75 90

PAJARO VALLEY PV-13

1,000 100 10

DEPTH, IN FEET BELOW LAND SURFACE

GAMMA,

IN COUNTS

PER

SECOND

EM

CONDUCTIVITY,

IN MILLIMHOS

PER METER

0

25

50

75

100

125

150

175

200

12S/1E-11L1

1998

Year log

recorded

1998

Year log

recorded

Figure A1.9. Geophysical logs and well construction for 

selected monitoring wells and test holes in the Pajaro Valley, 

Santa Cruz County, California.
Appendix 1 75

0

10

20

30

40

50

30 45 60 75 90

PAJARO VALLEY PV-14

12S/3E-5N1

100 10

DEPTH, IN FEET BELOW LAND SURFACE

GAMMA,

IN COUNTS

PER

SECOND

EM

CONDUCTIVITY,

IN MILLIMHOS

PER METER

1998

Year log

recorded

1998

Year log

recorded

Figure A1.10. Geophysical logs and well construction for 

selected monitoring wells and test holes in the Pajaro Valley, 

Santa Cruz County, California.
76 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

200

0

400

600

800

1,000 30 60 90

DOLAN ROAD [DLN] (MCWRA)

13S/2E-15R2, 3

1,000 10 100

DEPTH, IN FEET BELOW LAND SURFACE

R2 [DLN (D)]R3[DLN (S)]

Clay

Sand

Sand

Sand

Sand

Sand

Sand

Clay with

sand stringers

Clay with

sand stringers

Clay

Clay

Clay

LITHOLOGY GAMMA,

IN COUNTS

PER

SECOND

EM

CONDUCTIVITY,

IN MILLIMHO

PER METER

WELL

CONSTRUCTION

Screened

Interval

Saline

water

1998

Year log

recorded

1998

Year log

recorded

Figure A1.11. Geophysical logs and well construction for selected monitoring wells and test 

holes in the Pajaro Valley, Santa Cruz County, California.
Appendix 2 77

All coastal monitoring wells (except PV-13), 

inland monitoring well PV-12, and selected surfacewater sources that represent the potential sources of 

natural recharge were sampled in August and 

December 1998 (fig. 1C; table A2.1). The surfacewater samples were collected in the Pajaro River at 

Chittenden (PR-16.5) and at Thurwatcher Bridge (PR2.2), at Watsonville Slough (WS-1.6), and from two 

agricultural sumps along the western part of Beach 

Rd.(AG-1.0, AG-2.0) that represent the return flow 

from irrigation with ground water. The surface-water 

samples were analyzed only for stable isotopes of 

deuterium, oxygen, sulfur, and boron. The boron and 

strontium-isotope samples were collected by the USGS 

along with the other water-chemistry samples for the 

selected monitoring wells and surface-water sites and 

analyzed by Tom Bullen and John Fitzpatrick of the 

USGS, Water Resources Discipline. The sulfur isotopes 

were analyzed by Tyler Coplen of the USGS, Water 

Resources Discipline. All of the samples presented in 

table A2.1 were collected and analyzed by the USGS.

Other chemistry data used in this report, other 

than the data for samples listed in table A2.1, were 

provided by PVWMA and SCWD. These data were 

collected by agency personnel and analyzed at five 

commercial laboratories: Monterey Bay Analytical 

Services (California State Department of Health 

Services Certification No. I-2385); Soil Control Labs 

(California State Department of Health Services 

Certification No. 1494); Toxscan, Inc. (California State 

Department of Health Services Certification No. 1515); 

Branchiforte Labs (California State Lab No. 1180); and 

Dellavalle Laboratory, Inc. (California State 

Department of Health Services Certification No. 1595) 

(Jonathan Leer, Pajaro Valley Water Management 

Agency, oral commun, 2002; California Department of 

Health Services, 2002). 

The methods of data collection and analyses of 

other upper-aquifer system wells in the Salinas Valley 

are described by Vengosh and others (2002). The 

average composition of seawater (Hem, 1985) was used 

for hydrologic and water-chemistry comparisons.

The water-chemistry samples were collected 

under the USGS protocol established for waterchemistry sampling (U.S. Geological Survey, 1997). 

Water-chemistry samples were obtained from the 

monitoring wells with positive-displacement pumps 

after well development. The water-chemistry data are 

summarized by groups of constituents that are used for 

geochemical and hydrologic interpretations. These 

groups include major anions; dissolved-solids 

concentration and specific conductance; iron and 

manganese; other selected trace elements; pH, 

temperature, and dissolved oxygen; total dissolved 

carbon; tritium; stable isotopes of deuterium, oxygen, 

sulfur, boron, and strontium; and the carbon (C-14 and 

C-13/12) isotopes.

Some samples may not be representative of the 

actual water chemistry in the aquifers screened by the 

monitoring wells. For example, a piece of sampling 

hose is lodged in well PV-3 (M) and has hampered 

sampling since 1994. This may suggest that the sample 

collected in August 1998 is skimmed from the top part 

of the screened interval and that down-hole pumps may 

not yield representative samples from this well. Air-lift 

or down-hole “thief” sampling could be attempted to 

overcome this problem. The sample from PV-8 (M) 

may also be yielding samples that are a mixture of the 

saline water of the lower screen and the less saline 

water from the upper screened interval. Similarly, the 

pumped sample from PV-1 (M) was lower in salinity 

and had a different isotope signature than that of the 

subsequent air-lifted sample (table A2.1). Down-hole 

sampling may be required to get reliable samples from 

these two screened water-bearing zones. 

APPENDIX 2. WATER-CHEMISTRY DATA FOR SELECTED MONITORING WELLS AND 

SURFACE WATERS IN THE PAJARO VALLEY
78 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

See footnote at end of table.

Table A2-1. Summary of selected USGS water-chemistry data from the Pajaro Valley, Santa Cruz and Monterey Counties, California

[USGS, U.S. Geological Survey; number below the compound is the data parameter code, which is a 5-digit number used in the USGS computerized data 

system, National Water Information System (NWIS), to uniquely identify a specific constituent or property; PVWMA, Pajaro Valley Water Management 

Agency; Degrees C (oC), degrees Celsius; μS/cm, microseimen per centimeter; mg/L, milligram per liter; μg/L, microgram per liter; per mil, parts per 

thousand; pCi/L, picocuries per liter; — no, data; <, less than]

 State well No.

(USGS gaging station 

No.)

PVWMA

(location name)

Depth of 

well screen, feet

below land surface

Sample 

date

Sample 

time

Water 

temperature

(

oC)

(00010)

Air 

temperature

(

oC)

(00020)

12S/1E-10R3 PV-1 (S) 70 – 90 16-Dec-98 0940 18 —

12S/1E-10R2 PV-1 (M) 140 – 230 28-Aug-98 1145 18 17

12S/1E-10R2 PV-1 (M) (Resample)1 140 – 230 11-May-99 1125 18 17

12S/1E-10R1 PV-1 (D) 250 – 270 28-Aug-98 1445 19 19

12S/1E-23R4 PV-3 (S) 140 – 170 27-Aug-98 1110 18 18.8

12S/1E-23R3 PV-3 (M) 250 – 270 27-Aug-98 1500 17 20

12S/1E-23R2 PV-3 (D) 380 – 480 27-Aug-98 1140 19.5 20.5

12S/2E-31E2 PV-4A (S) 80 – 110 14-Dec-98 1600 16.1 13.5

12S/2E-31E1 PV-4A (M) 130 – 160 14-Dec-98 1530 17.6 15

12S/2E-18M4 PV-6 (S) 110 – 180 26-Aug-98 1345 19 19

12S/2E-18N3 PV-6 (SM) 260 – 280 26-Aug-98 1710 19 17

12S/2E-18N2 PV-6 (MD) 510 – 640 26-Aug-98 1430 23.5 18

12S/2E-18N1 PV-6 (D) 730 – 750 16-Dec-98 1430 22 20

12S/1E-11R3 PV-8 (S) 130 – 200 25-Aug-98 1515 18 20

12S/1E-11R2 PV-8 (M) 420 – 530 25-Aug-98 1900 23 18.5

12S/1E-11R1 PV-8 (D) 570 – 590 25-Aug-98 1945 16.5 18.5

12S/1E-13M1 PV 11 56 – 116 25-Aug-98 1300 16.5 17

11S/2E-34D3 PV 12 100 – 210 13-Dec-98 1615 17.3 13

(11159000) PR-16.5 — 24-Aug-98 1345 21.5 23

— PR-2.2 — 25-Aug-98 1030 18 19.5

— WS-1.6 — 26-Aug-98 1015 16.5 18.5

— AG-1.0 — 26-Aug-98 1015 — 19

— AG-2.0 — 26-Aug-98 1030 — 19
Appendix 2 79

See footnote at end of table.

Table A2-1. Summary of selected USGS water-chemistry data from the Pajaro Valley, Santa Cruz and Monterey Counties, California—Continued

PVWMA

(location name)

Specific 

conductance, 

field

(μS/cm at 25oC)

(00095)

Specific 

conductance, 

laboratory

(μS/cm)

(90095)

Oxygen, 

dissolved

(mg/L)

(00300)

pH,

field

(standard units)

(00400)

pH, 

laboratory

(standard units)

(00403)

PV-1 (S) 1,121 1,115 7.4 7.36 7.82

PV-1 (M) 8,880 9,650 6.2 7.19 7.48

PV-1 (M) (Resample)1 26,800 24,400 — — 7.54

PV-1 (D) 32,900 34,400 4.4 7.07 7.49

PV-3 (S) 2,150 2,330 .3 7.52 7.80

PV-3 (M) 1,070 1,120 7.48 7.55

PV-3 (D) 1,080 1,110 .3 8.06 8.19

PV-4A (S) 1,920 — 3.48 7.46 —

PV-4A (M) 28,100 27,200 1.2 6.83 6.99

PV-6 (S) 529 540 7.5 7.87

PV-6 (SM) 459 480 .3 7.82 8.14

PV-6 (MD) 502 520 .2 7.98 8.21

PV-6 (D) 10,100 13,200 .15 7.5 7.35

PV-8 (S) 986 996 8.2 6.99 7.84

PV-8 (M) 1,060 1,070 .4 7.52 7.80

PV-8 (D) 25,200 26,800 7.09 7.11

PV 11 880 892 7.6 6.71 7.48

PV 12 861 861 .05 6.72 7.07

PR-16.5 1,660 1,640 8.9 8.40 8.23

PR-2.2 1,690 1,700 — 8.30 8.18

WS-1.6 1,310 1,330 7.2 7.80 7.83

AG-1.0 3,140 3,340 7.2 7.91 7.68

AG-2.0 3,560 3,800 7.6 8.01 7.62
80 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

See footnote at end of table.

Table A2-1. Summary of selected USGS water-chemistry data from the Pajaro Valley, Santa Cruz and Monterey Counties, California—Continued

PVWMA

(location name)

Calcium, 

dissolved

(mg/L as Ca)

(00915)

Magnesium,

dissolved

(mg/L as Mg)

(00925)

Sodium,

dissolved

(mg/L as Na)

(00930)

Potassium,

dissolved

(mg/L as K)

(00935)

PV-1 (S) 81.1 52.5 52.9 3.13

PV-1 (M) 650 487 563 10.8

PV-1 (M) (Resample)1 — — — —

PV-1 (D) 1,120 1,180 6,910 17.7

PV-3 (S) 168 129.1 80.6 7.59

PV-3 (M) 73.5 59.7 51.5 5.09

PV-3 (D) 29.0 35.4 132 16.2

PV-4A(S) — — — —

PV-4A(M) 1,200 1,280 4,890 20.5

PV-6 (S) 44.4 27.8 32.3 2.29

PV-6 (SM) 37.7 22.9 28.2 2.68

PV-6 (MD) 16.0 25.8 48.6 11.9

PV-6 (D) 811 994 928 83.0

PV-8 (S) 53.2 68.1 45.6 1.38

PV-8 (M) 72.5 63.0 32.1 3.21

PV-8 (D) 2,540 1,920 2,220 29.5

PV 11 49.4 55.6 40.5 .84

PV 12 91.0 38.8 41.7 1.76

PR-16.5 85.7 78.7 155 3.62

PR-2.2 81.2 89.5 149 5.04

WS-1.6 93.9 73.0 82.6 2.21

AG-1.0 18.0 199 236 .55

AG-2.0 92.2 144 507 1.89
Appendix 2 81

See footnote at end of table.

Table A2-1. Summary of selected USGS water-chemistry data from the Pajaro Valley, Santa Cruz and Monterey Counties, California—Continued

PVWMA

(location name)

Chloride,

dissolved

(mg/L as Cl)

(00940)

Sulfate,

dissolved

(mg/L as SO4)

(00945)

Fluoride, 

dissolved

(mg/L as F)

(00950)

Silica,

dissolved

(mg/L as SiO2)

(00955)

PV-1 (S) 119 134 0.2 44.129

PV-1 (M) 3,290 387 .1 63.0

PV-1 (M) 

(Resample)1

9,500 1,300 — —

PV-1 (D) 13,300 1,730 .1 34.6

PV-3 (S) 616 53.8 .1 31.9

PV-3 (M) 187 1.11 .12 33.2

PV-3 (D) 209 61.0 .1 29.1

PV-4A(S) — — — —

PV-4A(M) 9,660 1,310 .1 46.0

PV-6 (S) 18.1 10.8 .12 36.2

PV-6 (SM) 24.6 31.3 .1 39.2

PV-6 (MD) 31.4 33.3 .1 33.9

PV-6 (D) 4,740 648.9 .1 32.0

PV-8 (S) 54.0 148 .1 45.7

PV-8 (M) 246 24.8 .1 52.9

PV-8 (D) 10,200 1,460 .1 26.9

PV 11 61.5 122 .1 42.8

PV 12 33.1 57.6 .5 32.3

PR-16.5 140 236 .4 20.5

PR-2.2 162 260 .4 15.5

WS-1.6 35.8 264 .7 14.4

AG-1.0 569 348 .8 18.7

AG-2.0 570 429 1.2 15.1
82 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

See footnote at end of table.

Table A2-1. Summary of selected USGS water-chemistry data from the Pajaro Valley, Santa Cruz and Monterey Counties, California—Continued

PVWMA

(location name)

Nitrogen, 

ammonia

(mg/L as N)

(00608)

Nitrogen,

nitrite

(mg/L as N)

(00613)

Nitrogen 

ammonia & 

organic

(mg/L as N)

(00623)

NO2 + NO3

(mg/L as N)

(00631)

Phosphorus,

dissolved

(mg/L as P)

(00666)

Phosphorus,

ortho

(mg/L as P)

(00671)

Carbon,

organic,

dissolved

(mg/L as C)

(00681)

PV-1 (S) 0.021 0.012 0.1 28.6 0.066 0.071 0.7

PV-1 (M) <.020 <.010 <.100 6.83 .033 .046 .1

PV-1 (M) (Resample)1 — — — — — — —

PV-1 (D) .106 <.010 <.100 <.050 .014 .018 .2

PV-3 (S) .585 <.010 .529 <.050 .171 .146 .2

PV-3 (M) 3.88 <.010 4.27 <.050 1.46 1.28 3.3

PV-3 (D) <.020 <.010 <.100 <.050 <.010 .024 .2

PV-4A(S) .122 .01 .105 40.5 .062 .117 .5

PV-4A(M) — — — — — — 1.2

PV-6 (S) <.020 .043 .495 10.1 .113 .104 1.5

PV-6 (SM) .062 <.010 <.100 <.050 .109 .109 .3

PV-6 (MD) .029 <.010 <.100 <.050 .054 .05 .2

PV-6 (D) .074 .01 .1 .05 .026 .018 .1

PV-8 (S) <.020 <.010 .112 42.6 .099 .103 .8

PV-8 (M) <.020 <.010 <.100 <.050 .034 .039 <.100

PV-8 (D) .041 .059 <.100 4.81 .045 .04 .4

PV 11 <.020 <.010 .158 40.0 .044 .048 1.2

PV 12 1.25 .01 1.55 .05 .842 .667 2.7

PR-16.5 1.8 <.010 2.02 .156 .875 .903 5.2

PR-2.2 .021 <.010 .798 57.7 .262 .261 4.1

WS-1.6 .053 .115 .766 11.7 .108 .094 5.6

AG-1.0 <.020 <.010 .434 81.7 .143 .144 3.9

AG-2.0 .026 .104 .466 8.81 .012 .019 7.8
Appendix 2 83

See footnote at end of table.

Table A2-1. Summary of selected USGS water-chemistry data from the Pajaro Valley, Santa Cruz and Monterey Counties, California—Continued

PVWMA

(location name)

Barium,

dissolved

(μg/L as Ba)

(01005)

Boron, 

dissolved

(μg/L as B)

(01020)

Iron, 

dissolved

(μg/L as Fe)

(01046)

Manganese, 

dissolved

(μg/L as Mn)

(01056)

Strontium, 

dissolved

(μg/L as Sr)

(01080)

Iodide,

dissolved

(mg/L as I)

(71865)

Bromide,

dissolved

(mg/L as Br)

(71870)

Silica, 

dissolved

(mg/L)

(00955)

PV-1 (S) 31.8 51.8 35.3 34.3 553 0.003 0.40 44.1

PV-1(M) 60.12 146 50 157 4,480 .004 11.7 63

PV-1 (M) (Resample)1 — 627 — — 12,410 .003 34.5 —

PV-1 (D) 150 1,600 200 60 9,820 .024 40.8 34.60

PV-3 (S) 218 84.6 30 513 1,590 .028 2.29 31.9

PV-3 (M) 195 208 10 997 817 .025 .587 33.2

PV-3 (D) 3.78 69 10 13.6 197 .053 .787 29.1

PV-4A (S) — 51.7 — — — — — —

PV-4A (M) 229 1,020 100 49.9 10,600 .001 34.62 46

PV-6 (S) 106 126 10 139.5 283 .019 .048 36.2

PV-6 (SM) 35.7 90.4 10 1.8 222 .009 .092 39.2

PV-6 (MD) 33.8 56.9 10 281 115 .004 .115 33.9

PV-6 (D) 141 75.6 2,290 388 6,750 .001 15.5 32.0

PV-8 (S) 82.9 19.6 10 3 386 .003 4.29 45.7

PV-8 (M) 34.4 66.1 10 46.3 411 .001 .953 52.9

PV-8 (D) 236 60.5 200 602 12,950 .004 35.30 26.9

PV 11 63.4 43 10 3 423 .009 3.02 42.8

PV 12 391 105 367 4,100 571 .086 .201 32.3

PR-16.5 118 947 <10.0 13.3 758 .061 .512 20.5

PR-2.2 91.7 675 <10.0 5.13 768 .042 .556 15.5

WS-1.6 110 286 16.4 37.8 756 .031 2.31 14.4

AG-1.0 87 600 <30.0 <12.0 1,520 .036 2.12 18.7

AG-2.0 56 973 <30.0 <12.0 1,140 .058 2.34 15.1
84 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

See footnote at end of table.

Table A2-1. Summary of selected USGS water-chemistry data from the Pajaro Valley, Santa Cruz and Monterey Counties, California—Continued

PVWMA\

(location name)

Alkalinity

(mg/L as 

CaCO3)

(39036)

Alkalinity, 

dissolved

(mg/L as 

CaCO3)

(39086)

Dissolved 

solids

(180C)

(mg/L)

(70300)

Alkalinity,

dissolved

(mg/L as 

CaCO3)

(90410)

Delta, 

deuterium

(per mil)

(82082)

Delta, 

oxygen-18

(per mil)

(82085)

Tritium

(pCi/L)

(07000)

Strontium87/86

(ratio per mil)

(75978)

PV-1 (S) — — 650 130 -36.4 -5.7 18.1 0.70626

PV-1 (M) 104 105 7,030 — -30.8 -5.2 6.3 .70660

PV-1 (M) 

(Resample)1

— — 20,300 — -18.7 -3.27 6.3 —

PV-1 (D) 103 103 24,900 — -12.2 -2.25 5.1 .70671

PV-3 (S) 180 180 1,490 — -38.9 -6.4 <.3 .70676

PV-3 (M) 266 268 583 — -37.4 -6.01 .4 .70691

PV-3 (D) 136 137 594 — -46.7 -7.38 .3 .70828

PV-4A(S) — — — — -35.2 -5.32 8.2 .70701

PV-4A(M) — — 18,200 136 -19.2 -2.96 9.34 .70707

PV-6 (S) 257 257 372 — -35.3 -5.9 <.3 .70705

PV-6 (SM) 172 172 291 — -35.8 -5.96 <.3 .70713

PV-6 (MD) 185 185 313 — -35.3 -6.01 <.3 .70777

PV-6 (D) — — 8,360 84.5 -34.5 -5.57 <.3 .70716

PV-8 (S) 129 129 687 — -33.5 -5.52 14.2 .70600

PV-8 (M) 133 133 575 — -36 -6.07 <.3 .70691

PV-8 (D) 112 112 20,600 — -22.5 -3.92 <.3 .70670

PV 11 80.4 80.4 601 — -35.9 -5.57 10.4 .70586

PV 12 356 — 534 363 -21.8 -2.67 3.4 .70726

PR-16.5 421 423 1,030 — -34.8 -4.71 — .70734

PR-2.2 372 372 1,080 382 -32.8 -4.43 — .70735

WS-1.6 372 371 908 387 -28.9 -4.74 — .70735

AG-1.0 307 307 2,180 316 -31.5 -4.87 — .70750

AG-2.0 525 523 2,460 542 -31 -5.0 — .70759
Appendix 2 85

1The PV-1(M) was resampled by PVWMA by airlifting and analyzed by the USGS.

Table A2-1. Summary of selected USGS water-chemistry data from the Pajaro Valley, Santa Cruz and Monterey Counties, California—Continued

PVWMA

(location name)

Carbon-14

(percent modern 

carbon)

(49933)

Carbon-13/12

(per mil)

(82081)

Uncorrected age

(years before 

present)

Corrected age

(years before 

present)

Delta

boron-11

(per mil)

(—)

Delta

sulfur-34

(per mil)

(82086)

PV-1 (S) 108.6 -18.11 <50 — 25.2 9

PV-1 (M) 76.83 -14.55 2,200 — 44.7 18.9

PV-1 (M) 

(Resample)1

— — — — — —

PV-1 (D) 87.8 -7.7 1,000 — 59.2 21

PV-3 (S) 42.73 -17.96 7,000 6,000 18.5 —

PV-3 (M) 48.74 -14.36 5,900 4,900 7.5 —

PV-3 (D) 4.89 -15.76 24,900 23,300 27.2 —

PV-4A (S) 87.48 -17.97 1,100 — 23.7 8.4

PV-4A (M) 75.49 -13.02 2,300 1,300 50.9 20.1

PV-6 (S) 51.21 -15.03 5,500 4,300 13.7 30.7

PV-6 (SM) 48.13 -16.24 6,100 5,000 22.7 4.6

PV-6 (MD) 35.34 -17.19 8,600 7,500 31.7 2.8

PV-6 (D) 24.1 -18.37 11,800 10,700 27.0 20.8

PV-8 (S) 114.0 -20.4 <50 — 27.2 9.1

PV-8 (M) 45.81 -16.32 6,500 5,400 34.2 .8

PV-8 (D) 37.61 -13.11 8,100 7,000 31.5 20.7

PV 11 — — — — 22.5 6.3

PV 12 87.88 -17.83 1,000 — 17.5 .8

PR-16.5 — — — — 18.2 -1.1

PR-2.2 — — — — 13.2 1.2

WS-1.6 — — — — 19.2 6

AG-1.0 — — — — 13.5 12.6

AG-2.0 — — — — 16.7 12
86 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

Major-Ion, Minor-Ion, and Trace-Element 

Chemistry—Major-ions, minor-ions, and trace 

elements are used to assess the chemical characteristics 

of the ground-water samples and help to determine the 

source and movement of the ground water.

Major Anions—Chloride concentrations from 

monitoring-well samples ranged from 18 mg/L at PV-6 

(S) to 13,317 mg/L at PV-1 (D). The samples from the 

Watsonville Slough, PV-6 (S, SM, MD), PV-8 (S),

PV-11, and PV-12 contain less than 65 mg/L chloride. 

Previous samples from PV-3 (M) indicated that 

chloride concentrations increased from 3,920 to 6,690 

mg/L between 1989 and 1994. Sulfate concentrations 

from monitoring-well samples ranged from 1.1 mg/L at 

PV-3 (M) to 1,726 mg/L at PV-1 (D). Sulfate 

concentrations from surface-water samples ranged 

from 236 mg/L for the Pajaro River at Chittenden to 

429 mg/L for drain sample AG-2.0 (table A2.1).

Dissolved-Solids Concentration and Specific 

Conductance—Dissolved-solids concentrations from 

monitoring-well samples ranged from 291 mg/L for 

well PV-6 (SM) to 24,900 mg/L for well PV-1 (D). The 

specific conductance ranged from 459 μS/cm for well 

PV-6 (SM) to 32,900 μS/cm for well PV-1 (D). The 

lowest conductance for surface-water samples was 

1,310 mS/cm for Watsonville Slough but the 

agricultural drain samples exceeded 3,000 mS/cm.

Iron and Manganese—The iron and manganese 

concentrations from monitoring-well samples ranged 

from 10 μg/L for numerous wells to 2,288 μg/L (iron) 

for well PV-6 (D) and from 60 μg/L for PV-1 (D) to 

4,100 μg/L for PV-12 (manganese) (table A2.1). Iron 

and manganese concentrations in surface-water 

samples ranged from less than 10 μg/L for the Pajaro 

River to 16 μg/L for the Watsonville Slough

(table A2.1).

Other Selected Trace Elements—Ground-water 

samples from the monitoring-well sites contained 

iodide concentrations ranging from 0.001 mg/L for 

PV-4A (M) and PV-6 (D) to 0.086 mg/L for PV-12. 

Barium concentrations ranged from 3.8 μg/L for well 

PV-3 (D) to 391 μg/L for well PV-12. Strontium 

concentrations ranged from 115 μg/L for well PV-6 

(MD) to 12,950 μg/L for well PV-8 (D). Bromide 

concentrations ranged from less than 0.05 mg/L for 

well PV-6 (S) to 40.8 mg/L for well PV-1 (D). Boron 

concentrations ranged from 19.6 μg/L for well PV-8 (S) 

to 1,600 μg/L for well PV-1 (D). The surface-water 

boron concentrations were higher than those of many 

of the well samples and ranged from 286 to 973 μg/L 

(table A2.1).

pH, Temperature, and Dissolved Oxygen—The 

pH values from monitoring-well samples were slightly 

basic, ranging from 6.7 to 8.1(table A2.1). The 

temperature of the water samples from the monitoring 

wells ranged from 17 to 23.5 oCelsius (oC). The water 

samples from surface waters and agricultural drains 

ranged from 7.2 to 8.9 mg/L in dissolved-oxygen 

concentration. Samples from PV-1 (S, M, D), PV-4A 

(S, M), PV-11, PV-8 (S) ranged from 1.2 to 8.2 mg/L 

dissolved oxygen. The remaining ground-water 

samples had less than 0.4 mg/L dissolved oxygen. 

Samples from wells PV-3 (M), PV-6 (S), and PV-8 (D) 

contained no detectable dissolved oxygen. The lack of 

oxygen and the trace smell of hydrogen sulfide 

collectively indicate that the waters from these wells 

are under reduced conditions. The mixing of these 

waters with shallower oxygenated waters may result in 

the precipitation of minerals on well screens, within 

gravel packs and aquifer pore spaces, or within 

agricultural soils. Wells that are perforated across 

aquifers with different chemistry could be susceptible 

to encrustation.

Dissolved Organic Carbon—The concentration 

of dissolved organic carbon for the monitoring wells 

ranged from less than 0.1 mg/L for well PV-8 (M) to 

3.3 mg/L for well PV-3 (M). Many of the ground-water 

samples ranged between 0.1 and 0.8 mg/L, with the 

least carbon at depth (table A2.1). Dissolved organic 

carbon concentration was highest in surface-water and 

agricultural drain samples, ranging from 3.9 to 7.8 

mg/L (table A2.1). 

Total Dissolved Nitrate—The concentration of 

nitrate in monitoring wells ranged from less than 0.05 

mg/L to 42.6 mg/L (table A2.1) as nitrogen. The 

concentration of nitrate in surface-water samples 

ranged from 0.2 mg/L to 81.7 mg/L (table A2.1) as 

nitrogen. Samples from the Pajaro River at 

Thurwatcher and the drain at AG-1.0 exceeded 45 

mg/L NO3 as nitrogen. 

Stable Isotopes—Stable isotopes of oxygen and 

hydrogen (deuterium) are used to help determine the 

source of water, isotopes of boron help to identify the 

source of dissolved ions that contribute to poor-quality 

water, and isotopes of strontium help to infer the source 

of the sediments through which the water flows.
Appendix 2 87

Oxygen and Deuterium Isotopes —Oxygen-18 

and deuterium are naturally occurring stable isotopes 

of oxygen and hydrogen. Oxygen-18 (18O) and 

deuterium (D) abundance are expressed as ratios in 

delta notation as per mil (parts per thousand) 

differences relative to the standard known as Vienna 

Standard Mean Ocean Water (VSMOW) (Gonfiantini, 

1978). Because the source of most of the world’s 

precipitation is the evaporation of seawater, the delta18O and delta-D composition of precipitation 

throughout the world is linearly correlated. This 

relation is known as the meteoric water line (Craig, 

1961). The delta-18O and delta-D composition of 

ground water relative to the meteoric water line and 

relative to the isotopic composition of water from other 

sources is an indicator of the source and movement of 

ground water. The delta-18O and delta-D composition 

of water from monitoring wells ranged from -7.38 to 

-2.25 and -46.7 to -12.2, respectively (fig. 20; 

table A2.1). 

Boron Isotopes—Boron-10 and boron-11 are 

naturally occurring stable isotopes of boron. Natural 

and anthropogenic processes fractionate the Boron-11 

content relative to Boron-10. Because boron is 

ubiquitous and is a soluble ion in water and because 

boron isotopes have fractionated through geologic 

time, boron isotopes provide a combined indicator of 

the potential for natural sources of water such as 

seawater intrusion as well as anthropogenic sources of 

boron (Bassett, 1990; Vengosh and others, 1994, 2002) 

(see “Stable Isotopes” ). The abundance of Boron-11 

(

11B) is expressed in delta notation as per mil (parts per 

thousand) differences relative to the standard reference 

boron isotopic ratio of Boron-11 to Boron-10 of 4.161 

for National Bureau of Standards boric acid sample No. 

951 (Tom Bullen, U.S. Geological Survey, written 

commun., 2001). Delta-boron-11 concentration in 

ground-water samples from the monitoring wells 

ranged from 7.5 to 59.2 per mil (fig. 23A; table A2.1). 

Delta-boron-11 concentration in surface water samples 

ranged from 13.2 to 19.2 per mil (fig. 23A; table A2.1).

Strontium Isotopes—Strontium-87/86 isotopes 

are naturally occurring stable isotopes of strontium and 

are expressed as a ratio (Faure and Powell, 1972). 

Strontium from ground-water samples undergoes 

cation exchange between calcium and strontium from 

the surrounding sediments. This exchange process is 

relatively rapid for most ground-water flow rates and 

results in a strontium isotopic composition of ground 

water that reflects the isotopic composition of the 

aquifer sediments. Therefore, strontium isotopes are a 

useful indicator of the source of the sediments that 

compose the aquifer (Izbicki and others, 1994). 

Strontium isotopes also can be affected by base-ion 

exchange during seawater intrusion, but this effect may 

be too localized to be discernible from sampling of 

ground water on a regional scale. The strontium isotope 

ratio for coastal California granitic rocks is 0.7082 

(Faure and Powell, 1972), and the ratio for average 

modern seawater is 0.7092 (Vengosh and others, 2002). 

The strontium-87/86 ratio of ground-water samples 

from monitoring wells ranged from 0.70586 to 0.70828 

(fig. 21; table A2.1). The strontium-87/86 ratio of 

surface-water samples ranged from 0.70734 to 0.70759 

(fig. 21; table A2.1).

Sulfur Isotopes—Sulfur-32/34 isotopes are 

naturally occurring stable isotopes of sulfur and are 

expressed in delta notation as per mil (parts per 

thousand) differences relative to the standard reference 

sulfur isotopic ratio of troilite from the Vienna Canon 

Diablo meteorite (VCDT, Coplen and others, 2002). 

Delta-sulfur-34 concentration in ground-water samples 

from the monitoring wells ranged from 0.8 to 30.7 per 

mil (fig. 23B; table A2.1). Delta-sulfur-34 

concentration in surface-water samples ranged from 

-1.1 to 12.6 per mil (fig. 23B; table A2.1).
88 Geohydrologic Framework of Recharge and Seawater Intrusion in the Pajaro Valley, Santa Cruz and Monterey Counties, California

Unstable Isotopes—Unstable isotopes of water 

and carbon are used to identify the potential age of 

ground-water samples. 

Tritium—Tritium (3H) is a naturally occurring 

radioactive isotope of hydrogen having a half-life of 

12.4 years. The activity of tritium is measured in 

picocuries per liter (pCi/L); and one pCi/L is equivalent 

to about 2.2 disintegrations of tritium per minute or 

about one tritium atom in 3.1×1017 atoms of hydrogen. 

Prior to 1952, the tritium concentration of precipitation 

in coastal California was about 6.5 pCi/L (Izbicki, 

1996b). Beginning in 1952, about 800 kg of tritium was 

released as a result of the atmospheric testing of 

nuclear weapons (Michel, 1976) and the tritium 

activity of precipitation at Santa Maria, California, 

increased to more than 2,200 pCi/L (International 

Atomic Energy Agency, 1981). This release stopped in 

1962 with the signing of treaties banning the 

atmospheric testing of nuclear weapons. Since that 

time, tritium activity in precipitation has decreased to 

pre-1952 levels. Because tritium can be part of the 

water molecule and tritium activities are not 

significantly affected by reactions other than 

radioactive decay, it is an excellent tracer of the 

movement of water and relative age of water on time 

scales ranging from 0 to 50 years before present 

(Izbicki and others, 1993). Detectable tritium 

concentrations ranged from 18.08 pCi/L for well PV-1 

(S) to 0.416 pCi/L for well PV-3 (M), which is just 

above the detection limit of 0.3 pCi/L (table A2.1). 

Most of the wells at PV-3, PV-8, and PV-3 did not 

contain detectable tritium (table A2.1). The remaining 

samples from monitoring wells were at the tritiumdetection limit and contain waters recharged before 

1952. 

Carbon Isotopes—Carbon-14 (14C) is a 

naturally occurring radioactive isotope of carbon that 

has a half-life of about 5,730 years. Carbon-14 data are 

expressed as percent modern carbon (pmc) by 

comparing carbon-14 activities with the specific 

activity of National Bureau of Standards oxalic acid: 

12.88 disintegrations per minute per gram of carbon in 

the year 1950 equals 100-percent modern carbon. In 

addition to the naturally occurring carbon-14, carbon14 also was produced during the atmospheric testing of 

nuclear weapons. As a result, carbon-14 activities can 

exceed 100 percent modern carbon. Carbon-14 is a 

tracer of the movement and relative age of water on 

time scales ranging from several hundred to more than 

30,000 years before present. Because carbon-14 is not 

part of the water molecule, carbon-14 activities are 

affected by chemical reactions between dissolved 

constituents and aquifer material. As a result, carbon14 data must be corrected using chemical, 

mineralogical, and carbon-13 data to evaluate chemical 

reactions that occur within an aquifer and to estimate 

the actual age of a water sample. Davis and Bentley 

(1982) estimated that errors in carbon-14 ages may be 

as much as 100 percent. The carbon-14 ages were 

adjusted in this study on the basis of the percentage of 

carbon-14 for initial waters that represent recent 

recharge. The carbon-14 content from well PV-12 

(11S/3E-24D3M), with a carbon-14 content of 88 

percent and tritium present, was used for adjustment to 

initial water. These samples represent a relatively 

recently recharged ground water. The percent of 

modern carbon-14 is increased by the resulting ratio of 

carbon-14 values to 88 percent for the initial water of 

recent aquifer recharge. For wells with no detectable 

tritium in water samples, the values for uncorrected 

percent modern carbon ranged from 4.9 pmc for PV-3 

(D) to 51.2 pmc for PV-6 (S) (table A2.1). For wells 

containing tritium in water samples, the values for 

uncorrected percent modern carbon ranged from 75.5 

pmc for PV-4A (M) to 114.0 pmc for PV-8 (S)

(table A2.1).

Carbon-13 is a stable isotope of carbon that is 

commonly used in conjunction with carbon-14. 

Carbon-13 data are expressed as ratios in delta notation 

as per mil differences relative to the ratio of carbon-13 

to the more common isotope carbon-12 in standard 

Peedee Belemnite (PDB) (Gonfiantini, 1978). The 

delta-carbon-13 values for the monitoring wells ranged 

from -7.7 per mil for well PV-1 (D) to –20.4 per mil for 

well PV-8 (S) (table A2.1). Along with the presence of 

hydrogen sulfide noted during sampling of the 

monitoring wells, these data may indicate that the 

deeper wells are in a zone where reducing conditions 

and methanogenesis is occurring.
WRIR 03-4096 SANTA CRUZ AND MONTEREY COUNTIES, CALIFORNIA HANSON – GEOHYDROLOGIC FRAMEWORK OF RECHARGE AND SEAWATER INTRUSION IN THE PAJARO VALLEY,