GROUND WATER IN THE

CENTRAL VALLEY, CALIFORNIAA SUMMARY REPORT

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1401-A
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Ground Water in the 

Central Valley, California 

A Summary Report

By GILBERT L. BERTOLDI, RICHARD H. JOHNSTON, and K.D. EVENSON 

REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL VALLEY, CALIFORNIA

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1401-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1991
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this

publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Bertoldi, G.L.

Ground water in the Central Valley, California : a summary report / by Gilbert L. Bertoldi, Richard H. Johnston, 

and K.D. Evenson.

p. cm. (Regional aquifer-system analysis) (U.S. Geological Survey professional paper ; 1401-A) 

Includes bibliographical references. 

Supt. of Docs, no.: I 19.16:1401-A

1. Water, Underground California Central Valley (Valley) I. Johnston, Richard H. II. Evenson, Kristin D. 

III. Title. IV. Series. V. Series: U.S. Geological Survey professional paper ; 1401-A.

GB1025.C2B47 1991 555.7'9'097945-dc 20 89-600312

CIP

For sale by the Books and Open-File Reports Section, U.S. Geological Survey 

Federal Center, Box 25425, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program was started in 

1978 following a congressional mandate to develop quantitative appraisals of 

the major ground-water systems of the United States. The RASA Program 

represents a systematic effort to study a number of the Nation's most 

important aquifer systems, which in aggregate underlie much of the country 

and which represent an important component of the Nation's total water 

supply. In general, the boundaries of these studies are identified by the 

hydrologic extent of each system and accordingly transcend the political 

subdivisions to which investigations have often arbitrarily been limited in the 

past. The broad objective for each study is to assemble geologic, hydrologic, 

and geochemical information, to analyze and develop an understanding of the 

system, and to develop predictive capabilities that will contribute to the 

effective management of the system. The use of computer simulation is an 

important element of the RASA studies, both to develop an understanding of 

the natural, undisturbed hydrologic system and the changes brought about in 

it by human activities, and to provide a means of predicting the regional 

effects of future pumping or other stresses.

The final interpretive results of the RASA Program are presented in a series 

of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number, 

and where the volume of interpretive material warrants, separate topical 

chapters that consider the principal elements of the investigation may be 

published. The series of RASA interpretive reports begins with Professional 

Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available. /^i

Dallas L. Peck 

Director

CONTENTS

Page

Foreword III

Abstract Al

Introduction 1

Purpose and scope 2

Basin environment 2

Previous investigations 7

Well-numbering system 7

Hydrogeology 7

Geologic setting 8

Water-bearing characteristics of the aquifer system

(post-Eocene continental deposits) 12

Storage coefficient 12

Hydraulic conductivity 14

Ground-water flow system 15

Regional flow 15

Ground water in storage 21

Ground-water development 22

History of development 22

Depth and yield of wells 22

Effects of ground-water withdrawal on the Central Valley

aquifer system 23

Page

Ground-water development Continued

Effects of ground-water withdrawal on the Central Valley

aquifer system Continued

Changes to the ground-water flow system 23

Changes in aquifer storage 26

Land subsidence 27

Mechanics of land subsidence 30

Areal extent and effects 32

Quality of ground water 34

Base of freshwater 34

Dissolved solids 36

Hydrochemical facies 36

Sacramento Valley 37

San Joaquin Valley 37

Problem compounds 39

Chloride 39

Boron 39

Nitrate 40

Effects of human activities 40

Selected references 41

ILLUSTRATIONS

FIGURE 1. Maps showing location of study area 

2. Photograph of Sutter Buttes 

3. Graphs showing precipitation in the Sierra Nevada and runoff in the Central Valley 

4. Graphs showing mean annual precipitation and cumulative departure of precipitation from mean value, 1860 to 1980, at four 

Central Valley stations 

5. Diagram showing well-numbering system 

6. Generalized oblique view of part of Central Valley structural trough 

7. Map showing approximate areas of marine deposition during the Tertiary 

8. Map showing thickness of Central Valley aquifer system 

9. Geologic and hydrologic units and equivalent layers in digital flow model 

10. Diagrams showing concepts of Central Valley aquifer system

A3 

5

5

6 

 7

9

10

11

13

16

11. Map showing estimated predevelopment water table 18

12. Representative hydrographs showing long-term changes in ground-water levels in the Central Valley 19

13. Map showing water table in 1976 20

14. Generalized sections showing predevelopment and current ground-water flow conditions in the San Joaquin Valley 21

15. Maps showing ground-water pumpage in the Central Valley 24

16. Diagrams showing change in regional ground-water flow system due to development 26

17. Diagrams showing change in effective vertical conductance of Central Valley aquifer system due to development 26

18. Map showing estimated change in hydraulic head in lower pumped zone from 1860 to 1961 28

19. Map showing areal extent of land subsidence in the Central Valley 29

20. Graph showing relation of ground-water storage to hydraulic head in a compacting aquifer system 30

21. Diagrams showing influence of hydrogeology and well construction on potential for land subsidence 31

22. Graphs showing changes in hydraulic head and compaction at two wells in subsiding areas of San Joaquin Valley, 1960-80 32

23. Photograph showing magnitude of subsidence at a site 10 miles southwest of Mendota in the San Joaquin Valley 33

24. Map showing distribution of dissolved-solids concentrations in ground water of the Central Valley 35

25. Map showing chemical types of ground water in the Central Valley 38
VI CONTENTS

TABLES

TABLE 1. Laboratory values of selected hydraulic and physical properties of unconsolidated sediment in the Central Valley2. Specific-storage values (Ss) for aquifers in the Central Valley A14 

15

CONVERSION FACTORS

For readers who wish to convert measurements from the inch-pound system of units to the metric 

system of units, the conversion factors are listed below:

Multiply By To obtain

acre 0.4047

acre-ft (acre-foot) 0.001233

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

ft (foot) 0.3048

ft/d (foot per day) 0.3048

gal/min (gallons per minute) 0.06308

(gal/min)/ft (gallons per minute per foot) 0.2070

in. (inch) 25.4

in./yr (inch per year) 25.4

mi (mile) 1.609

mi2 (square mile) 2.590

square hectometer

cubic hectometer

cubic hectometer per year

meter

meter per day

liter per second

cubic meter per second per meter

millimeter

millimeters per year

kilometer

square kilometer

In this report, "sea level" refers to the National Geodetic Vertical Datum (NGVD) of 1929 a 

geodetic datum derived from a general adjustment of the first-order level nets of both the United 

States and Canada, formerly called the Sea Level Datum of 1929.
REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIAA SUMMARY REPORT

By GILBERT L. BERTOLDI, RICHARD H. JOHNSTON, and K.D. EVENSON

ABSTRACT

The agricultural productivity of the Central Valley depends on 

irrigation. Half of the 22 million acre-feet of irrigation water applied 

annually is ground water. The valley is a long, narrow structural trough 

filled with about 32,000 feet of sediment in the south and as much as 

50,000 feet in the north. Nearly all the fresh ground water is contained 

in the continental rocks and deposits younger than Eocene age. 

Streamflow, an important factor in recharging the aquifer system, is 

influenced by precipitation in the mountains surrounding the valley. 

The majority of recharge from infiltration of streamflow occurs on the 

east side of the valley.

Ground-water pumpage, which greatly exceeds the natural recharge rate, has dramatically altered the ground-water flow in the 

Central Valley. During the 1960's and 1970's, the recharge rate was 

more than five times that of the predevelopment period and was largely 

derived from percolation of imported surface water or recirculated 

pumped ground water rather than precipitation and recharge from 

streams. Prior to development, most ground water was discharged as 

evapotranspiration; however, in recent years, most discharge has been 

well pumpage. Computer simulation of the Central Valley aquifer 

system suggests that the total flow through the system has increased 

from about 2 million acre-feet per year to nearly 12 million acre-feet per 

year. The vertical movement of ground water has been artificially 

enhanced by many of the 100,000 irrigation wells that contain long 

intervals of perforated casing. When unpumped, these wells permit 

vertical flow between permeable layers within the aquifer system.

The total fresh ground water presently (1986) in storage in the 

upper 1,000 feet of the aquifer system is about 800 million acre-feet. 

During the 1960's and 1970's, ground water in storage was depleted at 

an average rate of 800,000 acre-feet annually.

In the San Joaquin Valley from the 1940's to the late 1960's, 

substantial withdrawals of ground water were accompanied by hundreds of feet of head decline. This head decline caused inelastic 

compaction of fine-grained beds, resulting in land subsidence that is 

unequaled anywhere else in the world. More than one-half of the San 

Joaquin Valley (or about 5,200 square miles) underwent subsidence of 

more than 1 foot. In one location, subsidence exceeded 29 feet. Within 

the areas of heavy withdrawals, subsidence is greatest where the 

aquifer system contains thick sections of montmorillonite clay. Land 

subsidence created engineering and economic problems, including 

damage to canals and drainage systems, and loss of irrigation wells 

caused by casing failure.

More recently (since the drought of 1976-77), surface-water 

imports have increased, ground-water pumpage has decreased, and in

Manuscript approved for publication May 22, 1987.

places, ground-water levels have recovered. Land subsidence has 

virtually ceased; however, it could resume with increased pumpage, if 

water levels decline below previous lows.

Ground-water quality in the Central Valley is generally influenced 

by the water from streams that are a major source of recharge. In 

general, water on the east side of the valley and from east-side streams 

contains low concentrations of dissolved solids compared to water on the 

west side and from west-side streams. Concentrations of dissolved 

solids in ground water generally are lower in the northern part of the 

valley than in the southern part. There are, however, localized 

exceptions in many places through the valley. Local concentrations of 

boron, chloride, and nitrate in the ground water of the Central Valley 

are large enough to be a problem either to crops or humans.

Human activities have some influence on the concentration and 

location of water-quality problems in the valley. Significant increases in 

concentrations of dissolved solids and, specifically, dissolved nitrate 

indicate that ground-water quality is degrading as a result of increasing 

application of fertilizer in agricultural areas and the growth of urban 

population. Pesticides such as dibromochloropropane (DBCP) as well as 

selenium and other trace elements in agricultural drainage water cause 

ecological and health problems in the San Joaquin Valley.

INTRODUCTION

In 1978 the U.S. Geological Survey began a series of 

ground-water investigations, the Regional Aquifer-System Analysis Program (RASA), as described in the 

"Foreword." The aquifer system in the Central Valley of 

California is one of 28 major aquifer systems in the 

country. It was selected for study because of the valley's 

long history of ground-water development and the importance of the area's agricultural production to the 

national economy (Bertoldi, 1979). Information needed 

for effective management of this aquifer system in the 

future includes (1) hydrogeologic framework of the valley, (2) hydraulic characteristics of the porous media 

(alluvium) through which ground water flows, (3) understanding of the ground-water-flow system including sources and rates of recharge and discharge, (4) chemical 

character of the ground water, (5) processes that control 

ground-water chemistry, and (6) effects of past and 

current human activities on the aquifer system.

The 5-year Central Valley aquifer study included the 

collection, analysis, and evaluation of data and preparaAl
A2 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

tion of preliminary reports on water quality by Fogelman 

(1982a, 1982b, 1983); water use by Williamson (1981) and 

Diamond and Williamson (1983); hydrogeology by Page 

(1981, 1983), French, Page, and Bertoldi (1982, 1983), 

Page and Bertoldi (1983), French, Page, Bertoldi, and 

Fogelman (1983), and Berkstresser and others (1985); 

and ground-water hydraulics, regional flow, and aquifer 

mechanics by Williamson and Prudic (1986) and Prudic 

and Williamson (1986). In addition, Nady and Larragueta 

(1983a, 1983b) and Mullen and Nady (1985) described 

streamflow and irrigation development.

PURPOSE AND SCOPE

The purpose of Professional Paper 1401 is to describe 

major aspects of the geology, hydrology, and geochemistry of the Central Valley aquifer system. These descriptions are derived largely from the study results and 

preliminary reports of the 5-year study; however, they 

also utilize the extensive hydrologic literature on the 

California Central Valley (see references cited in chapters A-D).

Professional Paper 1401 consists of the following chapters:

Chapter A (this report) summarizes the important 

aspects of the geologic framework, regional groundwater flow, effects of development, and ground-water 

quality in the Central Valley.

Chapter B (Hull, 1984) describes the geochemistry of 

ground water in the Sacramento Valley.

Chapter C (Page, 1986) describes the geologic framework of the Central Valley, with emphasis on textural 

changes in the alluvial deposits that constitute the 

aquifer system.

Chapter D (Williamson and others, 1989) discusses 

ground-water hydraulics, with emphasis on an analysis of 

regional ground-water flow prior to and after extensive 

ground-water withdrawals. This regional analysis is 

based on computer simulation and presents a new, 

somewhat different concept of the aquifer system.

BASIN ENVIRONMENT

The Central Valley of California, viewed from the air 

or on a shaded relief map (fig. 1A), stands out as a notable 

topographic basin. It is about 400 mi long and averages 

about 50 mi in width. Surrounded on all sides by 

mountain ranges, the valley has only one natural outlet 

through which surface water drains. That outlet, the 

Carquinez Strait, cuts through the central Coast Ranges 

(fig. 1A) on the west boundary of the valley. In this 

study, the boundary of the Central Valley represents the 

areal extent of the valley's aquifer system rather than a 

physiographic boundary. The aquifer system's boundary

is defined as coincident with the topographically highest 

occurrence of alluvial fan or alluvial plain deposits (alluvial boundary) of Pleistocene or Holocene age. In the 

northern part of the valley, discordant with the uniform 

flatness of the landscape, is the only notable topographic 

feature, Sutter Buttes (figs. 1A and 2). There, north and 

south Buttes, remnants of an ancient volcanic plug, rise 

to altitudes of 1,860 and 2,130 ft above sea level, 

respectively.

The Central Valley is composed of parts of four 

hydrographic subregions or drainage basins named for 

the major natural surface-water feature in each subregion (fig. I A). Sacramento Valley, the northernmost 

third of the Central Valley, has an area of about 4,400 mi2 

and is drained by its namesake, the Sacramento River. Of 

the four hydrographic subregions, the Sacramento Valley 

is the least intensively developed. San Joaquin Valley, 

the southern two-thirds of the Central Valley, is made up 

of parts of two subregions: the San Joaquin Basin and, at 

the southern end, a basin of interior drainage called the 

Tulare Basin after a Pleistocene lake that occupied most 

of the area. The fourth hydrographic subregion is the 

Delta, a low-lying area that drains directly to the 

Sacramento-San Joaquin Delta rather than to either river 

(fig. 1A). The lower part of the Delta subregion consists 

of wetlands interspersed with hundreds of miles of 

channels and numerous islands.

Climate in the Central Valley is the Mediterranean 

type (Blair and Fite, 1957, p. 323). Average annual 

precipitation ranges from 13 to 26 in. in the Sacramento 

Valley and from 5 to 16 in. in the San Joaquin Valley. 

About 85 percent of the annual precipitation occurs from 

November to April. Summers are hot; winters are mild, 

allowing a long growing season. In contrast to the low 

precipitation in the valley, mean annual precipitation in 

the adjacent Sierra Nevada increases with altitude and 

ranges from 40 to more than 90 in. (Rantz, 1969). Much 

of the precipitation in the mountains is snow, especially in 

the higher southern Sierra Nevada. Variations in the 

volume of snowpack and delays in its melting produce 

differences in the timing of runoff in the two valleys. 

Peak runoff into the Sacramento Valley generally lags 

peak precipitation in the surrounding mountains by 1 to 

2 months whereas peak runoff in the San Joaquin Valley 

generally lags peak precipitation by 5 to 6 months (fig. 3).

Streamflow, a very important factor in the water 

supply of the Central Valley, is almost entirely dependent on precipitation in the Sierra Nevada and part of the 

Klamath Mountains in the north (fig. 1A). No perennial 

streams of any significant size enter the valley from the 

west, except for Stony Creek, Cache Creek, and Putah 

Creek at the northwest end of the valley (fig. IB). Mean 

annual streamflow entering the Central Valley around its 

perimeter is 31.7 million acre-ft.
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A3

Agriculture is the major commercial activity in the 

valley, providing jobs for about 30 percent of the population and cash receipts to farmers of about $13 billion in 

1983. The Central Valley contains 5 of the top 10 

agricultural counties (in value of crops sold) in the United

States, including Fresno (number 1), Kern (number 2), 

and Kings (number 3). To support this level of agricultural activity in an area that is deficient in precipitation 

requires a substantial amount of irrigation water. During 

the 1960's and 1970's, an average of 22 million acre-ft of

120

EXPLANATION

HYDROGRAPHIC SUBREGIONS CONSTITUTING 

THE CENTRAL VALLEY

S Sacramento

D Delta

SJ San Joaquin

T Tulare

 AREAL EXTENT OF CENTRAL VALLEY 

AQUIFER SYSTEM

HYDROGRAPHIC SUBREGION BOUNDARY

^"VSJjNSn1-- ^«'V '

^ *M? f * - % - 1 -

.A^ '«Jv\ /--A .T-*:' ;

50 100 KILOMETERS

EX'CO

FIGURE 1. Location of study area. A, Shaded relief map of California showing Central Valley drainage basins. B, Counties

and cities of the Central Valley.
A4 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

irrigation water was required annually about one-half 

from ground water and one-half from surface water. 

During drought years, the amount supplied by ground

water increases; during wet years, the amount supplied 

by surface water increases. During the early to middle 

1980's, the overall usage of irrigation water increased

123° 122C 121' 120° 119° 118°

40'

39'

38'

37'

36'

35° -

EXPLANATION

___ __ COUNTY BOUNDARY (some counties bounded 

entirely by rivers)

( Siockton /\ 

CONTRA-^ I SANJOAQUIN/ 

COSTA W /

Modestn

Alluvial 

boundary

I . 1

Hanford \

Kettleman ( 

i Hills 

I KINGS

100 KILOMETERS

FIGURE 1. Continued
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A5

slightly and a higher proportion came from surface-water 

sources.

If precipitation and runoff were distributed uniformly 

in space and time, then average values could be relied 

upon by water users and managers. However, both 

precipitation and runoff in the valley vary widely during each year and from year to year (figs. 3, 4). The 

cumulative departure graphs (fig. 4) show wetter than 

normal periods (cumulative departure increases) and 

drier than normal periods (cumulative departure decreases) since the middle 1800's. Because annual precipitation at Fresno and Bakersfield is much less than 

at Red Bluff and Sacramento, the magnitude of departure is also much less.

A fairly stable measure of the variability of runoff in 

the Central Valley has been the sum of the annual flow of 

the 15 largest streams because one end of the Central 

Valley may have less-than-normal precipitation while 

the other end may have above-normal precipitation. Only 

twice between 1961 and 1977 was the total annual flow 

of the 15 largest streams within 10 percent of the mean 

annual flow of these streams, and only in 7 years of the 

44-year-period record was the total annual flow within 10 

percent of the mean annual flow. These records indicate

I I I I I I i ill I

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

B

n -I L-I n. [1.

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

EXPLANATION

[ | PRECIPITATION ^ RUNOFF

FIGURE 3. Precipitation in the Sierra Nevada and runoff in the 

Central Valley (modified from Williamson and others, 1989). A, 

Sacramento Valley. B, San Joaquin Valley.

FIGURE 2. Sutter Buttes. View northeastward from southwest edge of buttes.
FIGURE

4. Meanannual precipitatio 

(from William

n and cumulative depaison and others, 1989).

c

8 

8

08 8 

3 8

8 1i 8 1

D Mean ann

i 1 . !II 1!!...

ual, 5.9inches

i I ,.|hi

&

-

-

c

3 

8 

8o

8

o

8 3 

8

8 I

ETTTT: r

PRECIPITATION, IN F 

3 8 8 \

C Mean ann

! ,1 I iu. ,

jal, 9.7 inches

i, , III 1

ERCENT 

1

3 

8

0

8 

3 

$

AGE OF MEAN/

S < 2

L

ANNUAL* -i ro r 3 en o i

===== to _

8§5 - jal, 18.0inches

o5 _s

o

8

0

8 3 

S

8 Ii 8 8 i

A Mean ann

. ,li, |,||

ual, 23.2 inches

Ih ,!

H

Q i iO 5! 

> tr1H »

^ 

cc 

H 

H

55 w w O) cp fo _4O o o o o o 

o

CUMULATIVE DEPARTURE, IN INCHES
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A7

that runoff in the valley is highly variable. Because of 

such high variability of both precipitation and runoff, 

early settlers soon learned that ground water was a more 

dependable source than surface water.

PREVIOUS INVESTIGATIONS

Because of the long history of ground-water development and its severe impacts, many hydrologic investigations have been done in the Central Valley by the 

California Department of Water Resources, the U.S. 

Geological Survey, and various local agencies. The earliest systematic study of California's water resources was 

by Hall (1886, 1889). Two important early studies were a 

summary of ground-water resources in the San Joaquin 

Valley by Mendenhall and others (1916) and a description 

of the geology and ground-water resources in the Sacramento Valley by Bryan (1923). These two studies were 

particularly useful in the investigations of the regional 

flow system described in chapter D of this series 

(Williamson and others, 1989) because they documented 

hydrologic conditions prior to large-scale irrigation development.

In the early 1950's, the State of California and the U.S. 

Geological Survey cooperated in a series of ground-water 

reconnaissance studies that revealed nearly continuous 

annual declines of ground-water levels for large areas of 

the San Joaquin Valley and for some interstream areas of 

the Sacramento Valley. Two reports prepared during the 

1950's provided summary descriptions of the groundwater hydrology of the San Joaquin Valley (Davis and 

others, 1959) and the Sacramento Valley (Olmsted and 

Davis, 1961).

Comprehensive investigations of land subsidence in the 

San Joaquin Valley have been carried out since the 1950's 

by the U.S. Geological Survey under the direction of 

Joseph F. Poland. These landmark studies clearly show 

the relation between ground-water-level decline, compaction of fine-grained sediments, and land subsidence. 

Land subsidence in the Central Valley was described in 

several reports by Bull (1972), Lofgren (1975), Bull and 

Miller (1975), Bull (1975), Bull and Poland (1975), Poland 

and others (1975), and Ireland and others (1984). The 

mechanics of compacting sediments were described by 

Meade (1964,1967, 1968), Riley (1970), Miller and others 

(1971), and Poland and Ireland (1988).

A bibliography of nearly 600 reports that describe 

ground water in the Central Valley was compiled by 

Bertoldi (1979).

WELL-NUMBERING SYSTEM

Wells are identified according to their location in the 

rectangular system used for the subdivision of public

lands. The identification consists of the township number, north or south of a base line; the range number, east 

or west of a meridian; and the section number. A section 

is divided into sixteen 40-acre tracts lettered consecutively (excluding I and 0), beginning with A in the 

northeast corner of the section and progressing to R in 

the southeast corner (fig. 5). Within the 40-acre tract, 

wells are sequentially numbered in the order they are 

inventoried. The final letter refers to the base line and 

meridian relative to which the townships and ranges are 

numbered. For the Central Valley, this is the Mount 

Diablo base line and meridian (M).

HYDROGEOLOGY

The Central Valley is a long, narrow, northwesttrending, asymmetric structural trough that has been 

filled with about 32,000 ft of sediment in the southern 

part and as much as 50,000 ft in the northern part (Page,

RANGE

1W 1E 2E 3E

11N

SECTION 23

FIGURE 5. Well-numbering system.
A8 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

1986). Sedimentary deposits include both marine and 

continental deposits and range in age from Jurassic to 

Holocene. The valley is bordered on the east by the 

Sierra Nevada and on the west by the Coast Ranges (fig. 

6). The Sierra Nevada is the source of most of the 

sediment that underlies the valley.

GEOLOGIC SETTING

Although there are differing viewpoints among geologists as to the details of the origin of structural features 

we can view today in the Central Valley, there is general 

agreement on the larger aspects of the emergence of 

California as a landmass and the subsequent formation of 

the valley (King, 1977, p. 177). The formation of the 

Sierra Nevada and the Coast Ranges is important to 

understanding the deposition of aquifer material in the 

Central Valley as well as to understanding the distribution and movement of ground water.

To the east of the Central Valley is the Sierra Nevada. 

It is the largest single mountain range in the conterminous United States and is about 350 mi long and 55 to 80 

mi wide about as long and wide as the Central Valley. 

The Sierra Nevada is composed primarily of granite and 

related plutonic rocks but includes metasedimentary and 

metavolcanic rocks that range in age from Late Jurassic 

to Ordovician or perhaps older. The Sierra Nevada 

batholith was emplaced after the Late Jurassic but prior 

to the Late Cretaceous (Shelton, 1966, p. 385).

The Sierra Nevada, like the Central Valley, is asymmetric; the east side is considerably steeper than the 

west side (fig. 6), suggesting that the block was tilted 

upward toward the east. At the base of the east slope is 

evidence of recent faulting at several locales, while steep 

canyons have been cut into the gently sloping west side. 

Based on the steep canyons, Shelton (1966, p. 388) 

deduced that the huge granitic batholith was at a low 

elevation long enough to acquire a fairly flat erosional 

surface before it was tilted in successive stages. Wells 

drilled in the San Joaquin and Sacramento Valleys 

penetrated granitic rocks at increasing depths toward the 

west, indicating that the granite exposed in the Sierra 

Nevada is only a small part of the whole mass.

Uplift of the granitic rocks resulted in increased 

precipitation in the Sierra Nevada, particularly near its 

crest, which in places exceeds 14,000 ft altitude. Warm, 

moist airmasses from the Pacific Ocean are forced aloft by 

the mountain range, causing the airmasses to cool and the 

moisture to condense, resulting in heavy precipitation 

that exceeds 80 in./yr in places (Rantz, 1969). Because 

the crest of the range is near the east side, much of the 

runoff is westward to the Central Valley and is a major 

source of water to the valley. The streams that debouch

from the Sierra Nevada also have supplied a major part 

of the recent and older Cenozoic sediments that have 

been deposited in the valley.

Bailey and others (1970) described the significance and 

processes of the overland thrusting of the Mesozoic 

marine sediments to the formation of the Coast Ranges 

and to the deformation of the Great Valley sequence. Two 

important hydrologic points can be made from Bailey's 

geologic discussion. First, the emergence of the Coast 

Range thrust and subsequent development of the Coast 

Ranges established an orographic barrier for moistureladen onshore oceanic winds; as a result, the Central 

Valley effectively was put into a rain shadow since the 

formation of its west boundary an important factor in 

considering the source and distribution of ground-water 

recharge to the valley. A second factor that needs to be 

emphasized is the extensive deformation of the marine 

beds of the Great Valley sequence on the west side of the 

valley. In addition to establishing the asymmetric nature 

of the valley trough, these highly contorted beds may 

form a fault zone under the west side of the valley 

(Oakeshott, 1971, p. 289). The fault zone and the beds are 

barriers to ground-water flow. Variations in the chemical 

quality of ground water and in hydraulic heads are 

observed in closely spaced wells of similar construction 

drilled near the east boundary of the Coast Ranges 

(Hotchkiss and Balding, 1971).

Although the ancestral Sacramento and San Joaquin 

Valleys were created by the emergence of many areas of 

the Coast Ranges by middle Cretaceous time, parts of the 

Central Valley remained inundated by the Pacific Ocean 

until late Pliocene time (about 2 to 3 million years before 

present). These inundated areas were continuously 

changing in size and shape as the Coast Ranges emerged. 

As a result, both marine and continental sediments were 

deposited. Marine deposition was dominant in the Central Valley from the Paleocene to the beginning of the 

Oligocene (fig. 7). During the early Oligocene, marine 

deposition was restricted to the southern part of the San 

Joaquin Valley; during the Miocene, marine deposits 

were laid down along the west flank of the San Joaquin 

Valley and throughout most of the southern San Joaquin 

Valley (fig. 7). The seas had retreated by the Pleistocene, 

and only continental sediments were deposited during 

the Pleistocene and Holocene.

Marine deposits of Tertiary age, therefore, underlie 

large parts of the Central Valley; they crop out around 

Sutter Buttes, along the southwest flank of the Sacramento Valley, and along the west, southwest, south, and 

southeast flanks of the San Joaquin Valley.

Because of many changes in the depositional environment, the marine deposits differ greatly in sediment 

type, sorting, and thickness, and have been given nu-
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A9

merous names by petroleum geologists (Park and Weddle, 1959, pi. 3; Sacramento Petroleum Association, 1962, 

figs. 6, 7,10, 20, and 27, and table 2). In places the marine 

deposits provided the source material for the overlying 

continental deposits that form the freshwater aquifers of 

the valley. Generally, the marine deposits contain saline 

water, some of which has migrated into adjacent and 

overlying freshwater aquifers.

In a few places in the San Joaquin Valley, the marine 

rocks and deposits have been flushed of saline water and 

contain freshwater, which they yield to wells. In the 

Sacramento Valley, no marine deposits were reported as 

yielding freshwater to wells, although Olmsted and Davis 

(1961, p. 134) reported that marine rocks were flushed of 

connate water locally. The marine rocks, then, provide 

very little freshwater in the Central Valley.

Continental deposits of post-Eocene age overlie the 

marine deposits and contain most of the freshwater 

aquifers in the Central Valley. An important contribution 

to the quantification of storage capacity in the aquifer 

system was made when Page (1983) successfully mapped

texture change in the continental deposits overlying 

predominantly marine rocks and deposits in about 1,000 

mi2 of the San Joaquin Valley.

In this report and in chapter D (Williamson and others, 

1989), the base of the aquifer system is taken as coincident with the base of the post-Eocene continental deposits. This is not strictly true in the southeastern San 

Joaquin Valley where sandy marine beds underlying the 

continental deposits contain freshwater and are hydrologically part of the aquifer system. The thickness of the 

aquifer system, based largely on a thickness map of 

post-Eocene continental deposits prepared by R. W. Page 

(U.S. Geological Survey, written commun., 1981; Page, 

1974), is shown in figure 8. The thickness of the aquifer 

system averages about 2,400 ft and increases from north 

to south, with a maximum thickness of more than 9,000 ft 

near Bakersfield. However, the contact between continental and the underlying marine deposits is not always 

certain because the two types of deposits interfinger in 

some places, particularly near the south end of the valley. 

DeLaveaga (1952, p. 102) suggested that the continental

IT SHASTA

" "~~~"~~ IT?__- ^T=^= ~ J PtOFILE WITHOUT VERTICAL EXAGGERATION \

6. Generalized oblique view (northward) of part of Central Valley structural trough. Coast Ranges lie to the east; Sierra Nevada to the 

west. (From "Geology Illustrated" by John S. Shelton. Copyright 1966, W.H. Freeman and Company. Used with permission.)
A10 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

deposits may be as much as 15,000 ft thick, but 9,000 ft 

is shown as the maximum aquifer system thickness (fig. 

8). The omission of all continental deposits beneath the 

9,000-foot level does not affect the analysis of groundwater flow (described later) because (1) the total probable volume affected is less than 1 percent of the total 

volume of the aquifer system and (2) the deeper part of 

the continental deposits is so far below practical pumping

youngest

EXPLANATION

AREAS OF MARINE DEPOSITION 

DURING THE TERTIARY

BOUNDARY OF CENTRAL VALLEY

Paleocene

FIGURE 7. Approximate areas of marine deposition during the Tertiary.
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA All

123 C 122 C 121' 120C 119 C 118 C

40'

39'

38'

37'

36'

35° ~

EXPLANATION

LINE OF EQUAL THICKNESS OF CONTINENTAL DEPOSITS 

Interval 1,000 feet

FIGURE 8. Thickness of Central Valley aquifer system (largely post-Eocene continental deposits). From Page (1974) and R.W. Page (U.S.

Geological Survey, written commun., 1981).
A12 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

limits that an error in estimating their thickness has no 

effect on the flow-system analysis.

WATER-BEARING CHARACTERISTICS OF

THE AQUIFER SYSTEM 

(POST-EOCENE CONTINENTAL DEPOSITS)

The post-Eocene continental deposits that constitute 

the Central Valley aquifer system contain mostly fluvial 

deposits and interbedded lacustrine deposits but include 

some volcanic material. The continental deposits consist 

predominantly of lenses of gravel, sand, silt, and clay. 

The numerous lenses of fine-grained (silt, sandy silt, 

sandy clay, and clay) sediments are distributed throughout the valley and in most places constitute over 50 

percent of the total thickness penetrated by wells, as 

determined from electric logs (Page, 1986).

Most of these fine-grained lenses are not areally 

extensive; however, several major ones were mapped, 

principally near the axis of the San Joaquin Valley. The 

most notable is the Corcoran Clay Member (Pleistocene) 

of the Tulare Formation (Pliocene and Pleistocene), 

which is part of the modified E-clay of Page (1986) and 

underlies most of the west side of the San Joaquin Valley. 

This diatomaceous clay unit underlies an area of approximately 5,000 mi2 (Page, 1986) and ranges in thickness 

from near zero to at least 160 ft beneath the present bed 

of Tulare Lake (Davis and others, 1959; Page, 1986). The 

northern extent of the Corcoran Clay Member is not 

known because of the lack of well data north of Stockton, 

particularly in the Delta area. A diatomaceous clay 

similar in composition to that of the Corcoran Clay 

Member was present in a test hole drilled northwest of 

Sacramento, and drillers have filed reports showing a 

diatomaceous clay in several deeper wells north of 

Stockton (Page and Bertoldi, 1983). Laboratory tests of 

the clay indicate that it is highly susceptible to compaction, like the Corcoran Clay Member; however, the clay 

was not present in six other test holes northwest of 

Sacramento, so the full extent of it is not known.

The Corcoran Clay Member is important to the hydraulics of the aquifer system in that prior to development it acted as an effective confining unit. However, the 

drilling of large-diameter wells through the Corcoran and 

the practice of perforating wells both above and below it 

have made the present effectiveness of the Corcoran as a 

confining unit questionable.

In the basis of drillers' logs, electric logs from gas wells 

and the water wells, plus information from seven U.S. 

Geological Survey test holes drilled as part of this study, 

Page (1986) concluded that no extensive fine-grained 

lenses underlie the Sacramento Valley. However, there 

are two areas of mostly fine-grained sediments interbedded with coarse-grained sediments along the northeast 

flank of the Sacramento Valley adjacent to and south of

Chico (Page, 1986, fig. 8), at depths from 600 to 900 ft, 

and along the southwest flank of the Sacramento Valley 

north of Cache Creek in T. 3 N., at depths from about 600 

to 2,700 ft (Page, 1986, figs. 8-14).

Relating aquifers within the post-Eocene continental 

deposits to specific formations in the subsurface is 

difficult. In the valley, investigators use mainly physiography, weathering characteristics, and soils to map upper 

Cenozoic formations; however, in the subsurface, especially under saturated conditions, equivalents of surface 

units cannot be mapped with any certainty because 

differences in lithology are not apparent. In the Central 

Valley, then, physical properties of the aquifer materials 

and the distribution of these properties are more important than the delineation of formation boundaries to 

understanding regional and local flow patterns and to 

quantifying water in storage. The general relations in the 

Sacramento and San Joaquin Valleys among geologic 

units, hydrologic units, and layers used in the computer 

simulation of ground-water flow are shown in figure 9.

STORAGE COEFFICIENT

Storage coefficient is the amount of water that can be 

released from or added to the ground-water reservoir. It 

is usually defined as the volume of water an aquifer 

system releases from or takes into storage per unit 

surface area of aquifer per unit change in head (Lohman 

and others, 1972, p. 8). In the zone of water-table 

fluctuations, the storage coefficient is virtually equal to 

the amount of water released from storage by gravity 

drainage, referred to as specific yield. Below the zone of 

water-table fluctuations, the storage coefficient is the 

amount of water released by compression of the sediments and expansion of the water. This amount is usually 

much less than the amount released by gravity drainage.

Laboratory values of specific yield and porosity are 

shown in table 1. For the purposes of uniformity, only 

reported values obtained by the "sample saturation and 

drainage" method described by Johnson (1967, p. D5) 

were used in table 1. In general, sand yields more water 

from gravity drainage than fine-grained deposits like silt 

and clay, even though the porosities are nearly the same. 

The fine-grained deposits usually have much smaller 

specific-yield values because the tiny pores do not drain 

readily.

Williamson and others (1989, table 7) used specificyield values for aquifer materials similar to those shown 

in table 1 and then estimated an aggregated specific yield 

for the first few hundred feet of saturated sediment on 

the basis of lithologic descriptions from about 17,000 well 

logs. They estimated an average specific yield of 7 

percent for the Sacramento Valley, 8 percent for the 

Delta area, and 10 percent for the San Joaquin Valley and 

Tulare Basin.
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A13

The Central Valley aquifer system contains numerous 

fine-grained (clay and silt) randomly distributed lenses 

that, in general, constitute over 50 percent of the total

thickness of the system. Because of the large percentage 

of fine-grained sediments, Williamson and others (1989, 

table 7) concluded that the part of the aquifer system

Generalized section of geologic units. Reported maximum thickness, 

in feet, is in parentheses (adapted from Page, 1986, table 1)

Hydrologic unit used in

many reports such as

Bloyd (1978)

Layers in digital flow

model (Williamson

and others, 1989)

Flood basin deposits (160) Primarily clay, silt, and some sand; include 

muck, peat, and other organic soils in Delta area. Restrict yield to wells 

and impede vertical movement of water.

River deposits (115) Primarily gravel, sand, and silt; include minor 

amounts of clay. Among the more permeable deposits in valley.

Continental rocks and deposits (3,000 ± ) Heterogeneous mix of poorly 

sorted clay, silt, sand, and gravel; include some beds of claystone, siltstone, 

and sandstone. Younger deposits (Pliocene to Holocene) form major aquifer 

system in valley. Older deposits (Eocene to Pliocene) include breccia, conglomerate, and some volcanic rocks and deposits. Older deposits close to 

land surface on east side are generally small yield aquifers.

Volcanic rocks and deposits (1,000) Younger (Pliocene and Pleistocene) 

rocks and deposits include tuff and tuff breccia associated with Sutter 

Buttes; locally important source of water. Older (Miocene and Pliocene) 

volcanic rocks and deposits include andesite, obsidian, pumice, tuff, volcanic 

breccia, gravel, sand, volcanic mud flows, and some basalt. The rocks and 

deposits are important aquifers in the northeast part of valley where close 

to land surface.

Unconfined to locally 

confined aquifer

Layer 4

Most wells tap this

layer; unconfined

storage

Layer 3

Some wells tap this

layer; elastic and

inelastic confined storage

Layer 2

No wells; elastic and 

inelastic confined storage

Layer 1

No wells; elastic 

confined storage

Generalized section of geologic units. Reported maximum thickness, 

in feet, is in parentheses (adapted from Page, 1986, table 2)

Hydrologic unit used in

many reports such as

Poland and Lofgren (1984)

Layers in digital flow

model (Wilfiamson

and others, 1989)

ernary

*>

3

9

£3

1

3

or

Tertiary anc

f

H

Flood basin deposits (100) Primarily clay, silt, and some sand; include 

muck, peat, and other organic soils in Delta area. Restrict yield to wells and 

impede vertical movement of water.

amounts of clay. Among the more permeable deposits in valley.

Lacustrine and marsh deposits (3,600+) Primarily clay and silt; include 

some sand. Thickest beneath Tulare Lake bed. Include three widespread 

clay units - A, C, and modified E clay. Modified E clay includes the 

Corcoran Clay Member of the Tulare Formation. Impede vertical movement

of water.

Continental rocks and deposits (15,000) Heterogeneous mix of poorly 

sorted clay, silt, sand, and gravel; include some beds of mudstone, claystone, 

shale, siltstone, sandstone, and conglomerate. Form major aquifer system 

in valley.

Marine rocks and deposits Primarily sand, clay, silt, sandstone,

shale, mudstone, and siltstone. Locally yield fresh water to wells, mainly on 

the southeast side of the valley but also on the west side near Kettleman

Hills.

Upper water-bearing 

zone 1 ; unconfined 

to semiconfined

Principal \ AU *

(modified E clay)/

/

Lower water-bearing zone 1 ; 

semiconfined to confined. 

Extends to base of freshwater which is variable "X,

^\

Below the depth of water 

wells. In many areas,

post-Eocene deposits 

contain saline water

Layer 4

Manv wells tap this

layer; unconfined

storage

Layer 3

Many wells tap this 

layer; elastic and 

inelastic confined storage

Layer 2

Some wells tap this 

layer; elastic and 

inelastic confined storage

Layer 1

confined storage

upper and lower water-bearing zones are undifferentiated where the modified E clay (includes Corcoran Clay Member of the Tulare Formation) 

is absent.

B

FIGURE 9. Geologic and hydrologic units and equivalent layers in digital flow model. A, Sacramento Valley. B, San Joaquin Valley.
A14 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

TABLE 1. Laboratory values of selected hydraulic and physical properties of unconsolidated

sediment in the Central Valley

[ft/d, feet per day; <, less than; , no data]

Sediment size

Sand ........................

Clayey sand ..............

Sand-silt-clay ............

PllflVPV <5llt

Silty sand .................

Sandy silt .................

Silt ..........................

Silty clay ..................

Clay ........................

Number of samples _ ... . , ,, Specific yield1 used to determine 

.-.,,, .. (percent) specific yield and porosity

126 

28 

95 

107 

137 

49 

79

86 

0

19-35

27 

10-28

16 

2-20

12

<l-7

3.5 

<1-15

7.5 

1-12

7.5

1-7

3

<l-8

4 

No pure clays

Porosity1 

(percent)

31-65

40 

28-52

37 

31-56

37 

32-61

42 

25-41

34 

34-37

36 

34-56

43 

35-52

43 

analyzed

Average hydraulic 

conductivity2 

(ft/d)

 ,. , Hori- Vertical , zontal

11.5 14

.02 .02 

.0001 

.21 .16 

.02 .13 

.0002 

.0001 .002

1Range of values above line; mean value below line. Specific yield and porosity values were compiled from Stearns and others (1930), 

Piper and others (1939), Johnson (1967), and Johnson and others (1968).

laboratory determinations of hydraulic conductivity values were obtained from the U.S. Bureau of Reclamation, Sacramento, 

California, and Johnson and others (1968).

below the upper few hundred feet should be considered 

confined in the sense that the vertical permeabilities of 

sediments are much lower than the horizontal permeabilities. In the confined aquifers, water released by compression of fine-grained lenses, rather than that released 

from dewatering of pore space, may be the major source 

of water release from storage (Jacob, 1940). Therefore, it 

is necessary to define and measure another storage 

parameter, specific storage (Ss). This parameter is the 

volume of water released from or taken into storage per 

unit volume of aquifer material per unit change in head 

(Lohman and others, 1972, p. 13). Below the zone of 

water-table fluctuation, only Ss and the thickness of the 

aquifer are used to calculate water in storage. However, 

when effective (grain-to-grain) stress is increased, some 

of the fine-grained lenses undergo reorientation and 

deformation. Therefore, Ss has two significant values. If 

water released from storage is due to the expansion of 

water and compressibility of the aquifer in response to a 

decrease in hydraulic head, the specific storage is elastic. 

Conversely, if a decrease in hydraulic head causes 

deformation and reorientation of sediments in finegrained lenses, the specific storage is inelastic. The 

coefficients of elastic and inelastic specific storage derived from field tests and computer simulation are shown 

in table 2. Note that values of inelastic specific storage 

are much larger than those of elastic specific storage. For 

their simulation of regional ground-water flow, Williamson and others (1989) used the average specific-storage 

values in table 2.

HYDRAULIC CONDUCTIVITY

The term "hydraulic conductivity" (K) allows relative 

comparison of the transmission properties of different 

aquifers or parts thereof. The hydraulic conductivity of a 

saturated porous medium (aquifer material) is the volume 

of water that the material will transmit in a unit of time 

through a cross section of unit area, under a hydraulic 

gradient of unit change in head through a unit length of 

flow (Lohman and others, 1972, p. 6).

Average horizontal hydraulic conductivity (Kh) in the 

valley ranged from 14 ft/d for sand to 0.002 ft/d for silty 

clay (table 1) as determined from laboratory tests of core
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA 

TABLE 2. Specific-storage values (SJ for aquifers in the Central Valley

Specific-storage 

coefficient 

(per foot)

A15

Source of data Remarks

Fine 

grained

Coarse 

grained

Elastic

Poland 

(1961, p. B53)

1.4x10-

Riley and 

McClelland 

(1972, p. 77)

Helm 

(1978, p. 193))

0.7 to lx 10-

4.5x10-

Assumed 1,000 ft of 

aquifer thickness 

of which 700 ft is 

coarse and 300 ft 

is fine. Estimate is 

for coarse material 

only.

Detailed leaky-aquifer analysis of 

pumping tests near 

Fresno, San Joaquin Valley.

Average of several 

model runs.

Inelastic

Poland 

(1961, p. B53)

Helm 

(1978, p. 193)

2x10-

3x10-

For 300 ft of clay 

with inelastic storage coefficient of 5xlO-2.

Average for seven 

sites; range, 

1.4xl(T*to 

6.7 xKT4 .

samples. However, the average Kh of the entire Central 

Valley aquifer system is estimated to be 6 ft/d based on 

calibration of a regional ground-water flow model 

(Williamson and others, 1989). This value is somewhat 

less than the average value for sand but probably reflects 

the lateral discontinuity of sand beds and more accurately 

represents the conductivity that controls ground-water 

flow on a regional scale. The average hydraulic conductivity of the Sacramento Valley is about one-half the 

average for the San Joaquin Valley, probably because of 

more fine-grained volcanic-derived sediments in the 

Sacramento Valley (Williamson and others, 1989).

GROUND-WATER FLOW SYSTEM

REGIONAL FLOW

Regional ground-water flow in the Central Valley is 

strongly influenced by the numerous clay and silt lenses

that are present in the aquifer system. Different viewpoints on the role of the fine-grained lenses have resulted 

in two concepts of the aquifer system, as follows:

1. Until recently, most investigators considered the 

Sacramento Valley as containing one unconfined 

aquifer (Bloyd, 1978) and the San Joaquin Valley as 

containing two aquifers separated by a regional 

confining unit. The San Joaquin sequence was described in descending order by Poland and Lofgren 

(1984) as a semiconfined aquifer (upper water-bearing zone), a regional confining unit (Corcoran Clay 

Member of the Tulare Formation), and a confined 

aquifer (lower water-bearing zone).

2. More recently, Williamson and others (1989) proposed the concept of a single heterogeneous aquifer 

system for the Central Valley. This concept is that 

"the entire thickness of continental deposits is one 

aquifer system that has varying vertical leakance and 

confinement depending upon the properties of fine-
A16 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

grained sediments." Below the upper few hundred 

feet everywhere, they consider the aquifer to be 

virtually confined. These two concepts of the aquifer 

system in the Central Valley are shown in figure 10.

Lithologic studies described in chapter C (Page, 1986) 

show that the aquifer system contains many isolated 

lenses of sand, silt, and clay. The fine-grained lenses, 

although limited in lateral extent, constitute more than 

50 percent of the system and have an aggregate thickness 

of as much as several thousand feet. In contrast, the 

Corcoran Clay Member, a confining unit, ranges in 

thickness from zero to 160 ft and has an average thickness 

of 55 ft (Williamson and others, 1989). Vertical head 

differences are present nearly everywhere in the Central 

Valley. Head differences, as much as 400 ft, were 

observed in wells of different depths in some areas on the 

west side of the San Joaquin Valley. These large head

differences result from very heavy pumpage in the lower 

zone combined with the resistance to vertical flow 

provided by the fine-grained lenses in the aquifer system. 

Although some head difference in wells is observed 

across the Corcoran Clay Member, an even greater head 

difference occurs in wells that tap the intervals above and 

below the Corcoran. In addition, numerous wells that 

contain perforated sections both above and below the 

Corcoran Clay Member show little vertical head difference. Thus, the Corcoran is much less important than the 

combined effect of the many fine-grained lenses in 

controlling vertical flow. In summary, the concept of a 

single heterogeneous aquifer system is supported by the 

presence of numerous fine-grained lenses and the hydraulic response of the system to pumping.

The concept of a single, mostly confined, heterogeneous 

aquifer system was used as the basis for the computer 

simulation of regional ground-water flow described in

WEST EAST WEST EAST

Unconfined to semiconfined aquifer '. '., water-bearing zone) .-? .' 

SACRAMENTO VALLEY

Principal confining unit

^orcoran Clay Member ^iSSeTrTlimit of confinedXaquifer 

of the Tulare Formation) M

SAN JOAQUIN VALLEY

WEST EAST

Numerous overlapping, 

discontinuous clay beds

B

FIGURE 10. Concepts of Central Valley aquifer system. A, Concept of aquifers used in many hydrologic reports of the Central Valley; 

Sacramento Valley (adapted from Bloyd, 1978); and San Joaquin Valley (adapted from Poland and Lofgren, 1984). B, Concept of single 

heterogeneous aquifer with varying vertical leakance and confinement (adapted from Williamson and others, 1989).
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A17

chapter D (Williamson and others, 1989). The simulation 

utilized the U.S. Geological Survey's three-dimensional 

finite-difference model (Trescott, 1975; Trescott and 

Larsen, 1976). The model was modified to include a 

procedure first described by Meyer and Carr (1979) that 

simulates the effects of land subsidence due to inelastic 

compaction of clays (Prudic and Williamson, 1986). The 

resulting model considers the valley deposits as one 

aquifer system characterized by variations in vertical 

leakage properties. The leakage depends not only on the 

vertical permeability of the sediments, but also on the 

density of wells constructed with long perforated sections 

or multiple screens, because such wells provide vertical 

hydraulic connection within the aquifer system.

Four aquifer layers were specified within the model: an 

upper layer representing the shallow water-table zone, 

two middle layers representing the lower pumped zone, 

and a basal layer representing the continental deposits 

below the deepest wells in the valley (fig. 9). The model 

simulated recharge from precipitation, streams, and 

irrigation returns and simulated discharge to streams, 

evapotranspiration, and wells. Emphasis was placed on 

simulation of the period from 1961 to 1977 because of 

availability of data and because this period was representative of long-term climatic conditions including wet 

years and dry years. The discussion of regional groundwater flow presented here draws heavily on the results of 

simulation by Williamson and others (1989).

The natural pattern of ground-water movement and 

the rates of recharge and discharge have been significantly altered by water development. Prior to development, ground water generally moved from recharge 

areas in the higher ground surrounding the Central 

Valley toward topographically low areas in the center of 

the valley. The general pattern of lateral flow in the 

valley before development is shown by the water-table 

contour map in figure 11. Note that ground water flowed 

largely toward the Sacramento or San Joaquin Rivers 

except in the southern San Joaquin Valley, where flow 

was toward Tulare Lake.

Recharge was supplied primarily by streams entering 

the valley from the Sierra Nevada and Klamath Mountains and, to a lesser extent, directly from precipitation. 

The streamflow (mostly snow meltwater) was largest 

from January to June. Recharge via stream channels took 

place mostly in their upper reaches shortly after entering 

the valley. Downward hydraulic gradients undoubtedly 

were present in these recharge areas, but they are not 

well documented because of the scarcity of data from 

older deep wells.

Prior to irrigation development, most ground water 

was discharged as evapotranspiration in the central 

trough of the valley, and to a lesser extent, it was 

discharged to streams. Potential evapotranspiration in

the valley's center is about 49 in/yr, exceeding precipitation rates. The occurrence of upward direction of 

hydraulic gradients in the central part of the valley is 

shown by the large area of flowing wells that were 

documented prior to 1900 (Hall, 1889; Mendenhall and 

others, 1916). In the southern San Joaquin Valley, 

ground water was discharged to Tulare Lake and as 

evapotranspiration in the area surrounding it (shown by 

the closed depression in fig. 11). Water discharging to 

stream channels flowed into the Sacramento and San 

Joaquin Rivers, then into the Delta and westward into 

San Francisco Bay.

The regional hydraulic gradients in the aquifer system 

were steeper in the Sacramento Valley than in the San 

Joaquin Valley for the following reasons: (1) The outlet at 

the confluence of the Sacramento and San Joaquin Rivers 

is closer to the northern end of the Central Valley, (2) 

recharge rates were higher in the Sacramento Valley, 

and (3) average permeabilities are lower in the Sacramento Valley.

The ground-water flow system has been greatly altered by large-scale ground-water development and very 

large diversions and redistribution of surface water 

through the Central Valley. Heavy pumpage from wells, 

averaging 11.5 million acre-ft annually during the 1960's 

and 1970's, combined with increased recharge due to 

irrigation returns from redistributed surface water, 

caused changes in ground-water levels throughout most 

of the Central Valley. Examples of long-term groundwater-level changes in some wells caused by the water 

development are shown in figure 12.

The configuration of the water table in 1976 (fig. 13) 

shows the effects of heavy pumpage from wells. Ground 

water now flows primarily toward cones of depression at 

pumping centers rather than toward the preexisting 

natural discharge areas along the Sacramento and San 

Joaquin Rivers and around Tulare Lake, but there is still 

a large component of flow toward the Delta area. The 

largest ground-water-level declines have occurred in the 

western and southern parts of the San Joaquin Valley. 

Declines are much less in the Sacramento Valley, but a 

major pumping depression has formed just north of the 

Delta. Recharge from irrigation returns has caused 

ground-water levels to rise above their predevelopment 

levels in parts of northwestern San Joaquin Valley and 

parts of central Sacramento Valley. (See further discussion of water-level declines in the section "Effects of 

Ground-Water Withdrawal on the Central Valley Aquifer 

System.")

The combination of increased recharge to the water 

table and increased pumping from the lower zone has 

caused a reversal in the direction of the hydraulic 

gradient (from upward to downward) in the center of the 

Central Valley (Williamson and others, 1989). Large
A18

123 C

REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA 

122° 121° 120° 119° 118 C

40°

39°

38°

37 C

36 C

35 C

EXPLANATION

 120 WATER-TABLE CONTOUR - Shows altitude, in feet. 

Contour interval, 40 and 100 feet. Datum is 

sea level

FIGURE 11. Estimated predevelopment water table (modified from Williamson and others, 1989).
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A19

250

200

150

100

50

uj Sea 

> level 

UJ

CO

LLJ

0

50

300

10N/1E-33A01M

Amtude, 120 feet 

\

10N/1E-28K01M

Altitude, 109 feet

10N/1E-23Q02M

Altitude, 87 feet

g

LLJ

CD 200

OC

O

UJ

> 100

O

CD

<

fc Sea

level

100

200

^ 300

UJ

-J 350

cc

UJ

I II I I 

NOTE: Vertical scale is 2 times that of other hydrographs

20S/18E-36D01M

Altitude, 260 feet 

Depth, 1,400 feet

20S/18E-11Q03M

Altitude, 268 feet 

Perforations, 1,885 to 1,930 feet

20S/18E-11Q01M

Altitude, 268 feet 

Depth, 710 feet

D 20S/18E-06D01M 

** Altitude, 317 feet

Depth, 1,017 feet

_|_____I_____I_____I_____I_____I_____I

20S/19E-09A01M

Altitude, 224 feet

300

250

200

150

100

50

31S/29E-9A01M

Altitude, 468 feet

I I I 

31S/29E-29A01M

-Altitude, 400 feet 

Depth, 530 feet

31S/29E-20M02M

Altitude, 392 feet 

Perforations, 170 to 270 feet

31S/29E-34A01M

Altitude, 413 feet 

Depth, 800 feet

31S/29E-12M01lti

Altitude, 513 feet 

Perforations, 380 to 982 feet

1925 30 35 40 45 50 55 

YEAR

60 65 70 75 1980

FIGURE 12. Representative hydrographs showing long-term changes in ground-water levels in the Central Valley 

(modified from Williamson and others, 1989). See figure 5 for well-numbering system. A, Wells in Central 

Valley near Woodland. B, Wells on west side of San Joaquin Valley north of Kettleman City. C, Wells in 

southern San Joaquin Valley near Bakersfield.
A20

123C

REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA 

122° 121° 120° 119° 118C

40 c

39 C

38C

37 C

36 C

35° -

400

500

600

EXPLANATION

WATER-TABLE CONTOUR - Shows altitude of water table 

in feet; variable contour interval. Dashed where 

approximately located. Hachures indicate depression. 

Datum is sea level

FIGURE 13. Water table in 1976 (modified from Williamson and others, 1989).
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A21

withdrawals of ground water from the west side of the 

San Joaquin Valley have profoundly affected hydraulic 

gradients and flow patterns there. The heads in the lower 

zone (originally above land surface) are now below sea 

level and the direction of lateral ground-water flow has 

been reversed (Bull and Miller, 1975). Water in the lower 

zone previously flowed toward the center of the valley 

and discharged in the slough near the San Joaquin River 

(fig. 14). However, by the 1960's, flow was mostly toward 

the pumping center on the west side of the valley.

The total flow through the aquifer system has increased from about 2 million acre-ft/yr prior to development to nearly 12 million acre-ft/yr after development 

(Williamson and others, 1989). The increased pumpage is 

supplied largely by increased recharge, mostly from 

imported surface water or recirculated pumped water. 

Increased pumpage of ground water has not only changed

the amount of regional flow, it has also decreased the 

amount of water in storage and caused the land surface to 

subside over a large area. These effects are discussed in 

the section "Effects of Ground-Water Withdrawal on the 

Central Valley Aquifer System."

GROUND WATER IN STORAGE

The quantity of water in storage in the aquifers of the 

Central Valley has been estimated by several investigators. All such estimates are based on use of average 

values of specific yield for different lithologies and an 

arbitrary thickness of the aquifer system. Earlier investigators restricted their estimates to the shallow part of 

the aquifer system. Olmsted and Davis (1961) estimated 

that there were 28 million acre-ft of water in storage in 

the upper 200 ft of sediments in the Sacramento Valley.

SOUTHWEST NORTHEAST

Potentiometric surface 

lower zone, 1906

Corcoran Clay Member 

of the Tulare Formation

VERTICAL EXAGGERATION X 20

2,000

SOUTHWEST

FEET

2,000 "I

1,000 -

NORTHEAST

Corcoran Clay Member 

of the Tulare Formation

1,000 -

2,000

10 KILOMETERS

FIGURE 14. Predevelopment and current ground-water flow conditions (shown by arrows) in the San Joaquin Valley (from Bull and Miller,

1975, fig. 20). A, About 1900. B, 1966.
A22 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

This estimate was based on dividing the sediments into 

four lithologic groups (channel deposits, alluvial plain and 

fan deposits, dissected alluvial deposits, and valley deposits) and assigning storage characteristics to each 

group. Davis and others (1959) estimated that there were 

121 million acre-ft in the upper 200 ft of the aquifer 

system in the San Joaquin Valley.

As part of this RASA study, Williamson and others 

(1989) estimated that there were about 830 million acre-ft 

of freshwater in the upper 1,000 ft of the continental 

deposits in the Central Valley (as of 1961). This estimate 

was derived from a study of several thousand well logs, 

in which values of specific yield were assigned to depth 

intervals according to texture as mapped by Page (1986). 

Average values of specific yield for designated aquifer 

layers and subareas were then computed and are given in 

table 7 of Williamson and others (1989). The thickness of 

the aquifer system was taken as the difference between 

the 1961 water table and the lesser of either the depth to 

base of freshwater, or depth to the base of continental 

deposits, or 1,000-foot depth. The product of the specific 

yields and thicknesses, so derived, provided values of 

ground water in storage as follows:

Area

Sacramento Valley . 

Delta ...................

San Joaquin Valley 

Tulare Basin..........

Average

specific yield

0.07

0.08

0.10

0.10

Volume of

ground water

in storage

(million acre-ft)

170

130

160

370

Central Valley 0.09 830

As discussed in the section "Effects of Ground Water 

Withdrawal on the Central Valley Aquifer System," 

ground water in storage was depleted at an average rate 

of 800,000 acre-ft annually during the 1960's and 1970's. 

Thus, the total ground water in storage as of 1986 was 

probably about 810 million acre-ft.

GROUND-WATER DEVELOPMENT

HISTORY OF DEVELOPMENT

Ground-water development began in the Central Valley about 1880. However, development of surface water, 

primarily for irrigation, had been underway for the 

previous 100 years. By 1900, an extensive system of 

canals had been built to supply surface water to the 

southern San Joaquin Valley, and ground water was 

providing only a very small part of the irrigation water.

After 1900, the construction of wells and rate of 

ground-water withdrawal increased slowly. By 1913,

total well pumpage for the Central Valley was estimated 

to be 360,000 acre-ft annually. During the 1940's and 

1950's, the pumpage of ground water for irrigation 

increased sharply. During the 1960's and 1970's, groundwater pumpage averaged about 11.5 million acre-ft/yr 

and was providing about 50 percent of the water used for 

irrigation. This withdrawal rate represented about 20 

percent of the total yearly ground-water pumpage in the 

United States during that time. Pumpage for domestic 

and industrial use rose slightly during the 1960's and 

1970's, but by 1977, it constituted only 5 percent of the 

total ground-water withdrawal. A summary of groundwater pumpage in the Central Valley from 1961 to 1977 

was provided by Diamond and Williamson (1983).

In the late 1960's, increased importation of surface 

water in some areas caused ground-water pumpage to 

decline and many wells to be unused. However, a drought 

in 1976 and 1977 decreased the availability of imported 

surface water, and ground-water pumpage increased 

sharply, reaching a maximum of 15 million acre-ft in 1977. 

Thus, in recent years, annual ground-water pumpage has 

fluctuated depending upon the availability of imported 

surface water. The areal distribution of the relatively 

light ground-water pumpage during the wet year of 1975 

is compared with the heavy pumpage during the drought 

year of 1977 in figure 15.

During the early 1980's, ground-water pumpage decreased slightly from the 11.5 million acre-ft annual rate 

of the 1960's and 1970's, and the 1980's pumpage is about 

equal to the estimated recharge. Pumping in the Central 

Valley is seasonal, and most of the water is withdrawn 

during the spring-summer growing season. The autumnwinter period is usually a period of water-level recovery. 

Historically, the highest withdrawal rates have been in 

the drier areas the south-central part of the San Joaquin Valley.

DEPTH AND YIELD OF WELLS

Most of the (approximately) 100,000 high-capacity 

wells in the Central Valley are used for either irrigation 

or public water supply. Yields in excess of 1,000 gal/min 

are generally required and can be obtained nearly everywhere. The depth at which such yields can be obtained, 

however, varies depending on the local geology. Poorquality water at shallow depths in some areas requires 

deep wells.

Well depths in the Sacramento Valley are generally 

less than those in the San Joaquin Valley, and they range 

from an average depth of 120 ft in the highly permeable 

areas to nearly 500 ft in the less permeable areas. An 

analysis of performance tests on 2,783 wells reported by 

Olmsted and Davis (1961) indicated that most of those 

wells yielded 250 to 1,700 gal/min. For these wells,
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A23

specific capacities ranged from about 20 to 100 (gal/ 

min)/ft of drawdown, and the saturated thickness tapped 

ranged from about 100 to 400 ft.

Well depths in the San Joaquin Valley range from 

about 100 to 3,500 ft. The deepest wells are in the 

west-central and south-central parts of the valley, where 

the primary source of water is the lower zone. Elsewhere 

in the San Joaquin Valley, most wells tap the upper zone. 

For example, in the eastern part of the Los BanosKettleman City area, Bull and Miller (1975) noted that 

wells tapping the highly permeable upper-zone sands 

may be only 150 to 200 ft deep and yield 1,500 gal/min. In 

the western part of that area, however, where the upper 

zone has low permeability, wells must be 2,500 to 3,500 ft 

deep to obtain adequate yields (900 to 1,200 gal/min).

Davis and others (1964) summarized data from 15,000 

well-performance tests in the San Joaquin Valley. They 

noted that most wells yielded 500 to 1,500 gal/min with 

specific capacities ranging from 10 to 100 (gal/min)/ft of 

drawdown.

EFFECTS OF GROUND-WATER WITHDRAWAL ON 

THE CENTRAL VALLEY AQUIFER SYSTEM

The effects of ground-water withdrawal on the Central 

Valley aquifer system were investigated by computer 

simulations of ground-water flow prior to and following 

development as described in chapter D (Williamson and 

others, 1989). As noted earlier, the valley deposits (clay, 

silt, sand, and gravel) were simulated as one aquifer 

system characterized by variations in vertical leakance 

properties. The leakance depends not only on the vertical 

permeability of the sediments but also on the density of 

wells and their construction. Many of the wells are 

constructed with long intervals of perforated casing that 

connect several aquifer layers and thus greatly increase 

the vertical hydraulic connection through the aquifer 

system.

CHANGES TO THE GROUND-WATER FLOW SYSTEM

Before ground-water development, the flow system of 

the Central Valley was in a state of dynamic equilibrium natural recharge was balanced by natural discharge (fig. 16). As described earlier, ground water 

flowed toward the axial part of the valley and discharged 

primarily as evapotranspiration from marshes that existed prior to development. Some discharge also occurred 

along stream channels where aquifer heads were higher 

than stream stages. The total flow through the aquifer 

system was small (about 2 million acre-ft/yr) compared to 

the surface-water inflow (about 32 million acre-ft/yr). 

Total precipitation in the valley was estimated to be 12.4 

million acre-ft/yr, and total evapotranspiration directly

from precipitation was estimated to be 10.9 million 

acre-ft/yr, leaving 1.5 million acre-ft/yr of water to 

recharge the aquifer system.

The construction of about 100,000 irrigation wells and 

annual ground-water withdrawals of about 11 million 

acre-ft during the 1960's and 1970's, together with 

greatly increased recharge from irrigation returns (derived from imported surface water and recirculated 

pumped water), have significantly altered the groundwater flow pattern of the Central Valley aquifer system. 

Because ground-water pumpage and recharge from irrigation water since the 1960's has greatly exceeded the 

estimated predevelopment recharge rate (fig. 16), flow is 

largely from areas recharged by imported irrigation 

water toward areas of irrigation pumpage. Flow through 

the aquifer system increased nearly sixfold from about 

2 to nearly 12 million acre-ft/yr. The values shown in 

figure 16 do not include water that recharged the aquifer 

system only to be discharged a short distance away. 

Thus, total ground-water flow during the 1960's and 

1970's, which represents both regional and local flow 

systems, was greater than that presented in figure 16. 

Simulation suggests that downward flow from the shallow deposits and from the upper part of the lower 

pumped zone has increased by an order of magnitude 

(Williamson and others, 1989).

Water during the 1960's and 1970's was supplied 

principally by irrigation returns and, to a lesser extent, 

by natural recharge and by continuing depletion of 

aquifer storage. However, during the early 1980's, 

ground-water pumpage decreased slightly and was about 

equal to the combined rate of natural recharge and 

irrigation returns. Direct evapotranspiration from the 

ground-water reservoir was almost completely eliminated owing to lowering of the water table.

The aquifer system's ability to transmit water vertically has changed in direct response to the construction of 

about 100,000 irrigation wells (fig. 17). Most of the wells 

in the Central Valley contain perforated casing throughout their lower two-thirds (Diamond and Williamson, 

1983). Where the Corcoran Clay Member is present, the 

perforated sections of many wells fall above and below 

this confining unit to provide direct hydraulic connection 

vertically through the perforated zone. Vertical flow is 

substantial inside many unpumped wells. On the basis of 

current-meter traverses in 16 wells, Davis and others 

(1964) concluded that vertical flow through about 3,000 

wells that pierced confining beds was about 100,000 

acre-ft/yr in the western part of the San Joaquin Valley. 

Probably an even greater amount of flow occurs through 

wells in the rest of the Central Valley (Williamson and 

others, 1989).

Conversely, decreased vertical flow through the confining beds probably resulted from the inelastic compac-
A24

123°

REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

122° 121° 120° 119° 118°

40'

39'

38'

37'

36'

35° ~

Red Bluff

EXPLANATION

LINE OF EQUAL ANNUAL PUMPAGE 

Interval 1 foot

Alluvial 

boundary

50 100 MILES

50 100 KILOMETERS

FIGURE 15. Ground-water pumpage in the Central Valley for 1975 and 1977 (modified from Williamson and others, 1989). A, 1975 (wet year).

B, 1977 (drought year).
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A25

123 C 122° 121' 120° 119 C 118 C

40°

39°

38C

37 C

36 C

35° -

EXPLANATION

LINE OF EQUAL ANNUAL PUMPAGE 

Interval 1 foot

B

FIGURE 15. Continued
A26 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

tion of fine-grained materials within the aquifer system. 

Thus, the vertical permeability of the Corcoran Clay 

Member and the many clay beds in the section may have 

been reduced by 1.5 to 6 times (Williamson and others, 

1989).

Changes in the aquifer system's vertical flow were 

investigated with the finite-difference flow model as 

described in chapter D (Williamson and others, 1989). 

Simulations suggest that the average vertical leakance 

increased by about an order of magnitude from the 

predevelopment era to the 1970's. This increase is due to 

the large number of wells that contain long intervals of 

perforated casing (fig. 17). In some areas of the valley, 

the simulated increase in vertical leakage was more than 

three orders of magnitude. Such localized, very large 

increases are most logically explained by vertical movement of water through many wells. Simulations intended 

to determine possible decreases in vertical leakance due 

to inelastic compaction of clays were inconclusive because 

of the dominant effect of vertical flow through unpumped 

wells.

Calculations presented in chapter D indicate that if 

large-diameter wells perforated over a long interval are 

evenly distributed, the vertical leakance of one well is 

about the same as that of the fine-grained beds in about 

7 mi2 of the aquifer system. Therefore, in areas with 

many wells, the vertical flow is significantly altered by 

well density.

CHANGES IN AQUIFER STORAGE

Ground-water levels have been significantly altered by 

development in the Central Valley (fig. 12). For the most 

part, long-term declines of the water table are less than

100 ft except locally in the southern part of the San 

Joaquin Valley. In a few areas, increased recharge from 

irrigation returns has caused the water table to rise as

Recharge from precipitation 

(1.5)

\

Evapotranspiration 

(1.7)

.. CENTRAL VALLEY:.. 

'AQUIFER SYSTEMc 

Stream gain v 

(0-3) ^

Stream loss 

(0-5)

Surface-water bodies

Recharge from precipitation 

(1.5)

Irrigation return includes ground-water

Pumpage 

(11.5)

and surface-water sources 

(9.0)

Stream gain 

. CENTRAL VALLEY a (0-3)

AQUIFER SYSTEM'

Surface-water bodies

Stream loss 

(0.5)

B

Decrease in ground-water storage 

(0.8)

FIGURE 16. Change in regional ground-water flow system due to 

development (all values in millions of acre-feet per year). A, Predevelopment. B, Average rates during 1960's and 1970's.

Sand

Sand

' '-.'. .Sand..

' '.' -. : ::.':-'.

-.'-.' 

 -.- .-- '-'

US Compacted clayjgggl

B

Sand ': " "-

Well with long perforated 

interval - not pumping

FIGURE 17. Change in effective vertical conductance of Central Valley aquifer system due to development; size of arrows represents relative 

magnitude of flow. A, Predevelopment effective vertical conductance. B, Current effective vertical conductance.
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A27

much as 40 ft. Head declines in the lower pumped zone in 

the Sacramento and northern San Joaquin Valleys generally are small less than 80 ft (fig. 18). However, in the 

heavily pumped western and southern parts of the San 

Joaquin Valley, heads have declined from 100 to 400 ft 

since development began. Since the late 1960's, the 

increased availability of imported surface water in these 

areas and the accompanying decrease in ground-water 

pumpage has stopped the long-term decline and allowed 

some recovery of ground-water levels. Year-to-year 

changes in ground-water levels have reflected the availability of surface water. During wet or average years, 

more imported surface water is available for irrigation; as 

a result, well pumpage decreases and ground-water 

levels rise. During drought years, such as 1976 and 1977, 

less surface water is available, wells are more heavily 

pumped, and ground-water levels decline.

When heads have declined sufficiently in the lower 

pumped zone for inelastic compaction of clay beds to 

occur, the rate of water-level decline slows. This slower 

rate results because the effective storage coefficient is 

significantly increased (Williamson and others, 1989).

The result of the decline in gruund-water levels from 

the start of development until 1977 has been the loss of an 

estimated 60 million acre-ft of aquifer storage. This 

depletion of storage is made up of three components.

1. L^ng-term lowering of the water table that results 

from dewatering of the shallow sediments 40 million acre-ft.

2. Inelastic compaction (permanent reduction of pore 

space) 17 million acre-ft.

3. Elastic storage (compression of sediments and expansion of water) 3 million acre-ft.

The changes in storage were calculated with the 

computer model as described in chapter D (Williamson 

and others, 1989). The decrease in storage due to 

dewatering is the product of specific yield, water-table 

decline, and area of the shallow aquifer dewatered. 

Similarly, the change in elastic storage is the product of 

elastic specific storage, thickness of confined aquifer, 

head decline, and area of aquifer affected.

The loss of storage from inelastic compaction of clay 

beds causes a permanent loss of pore space that in turn is 

balanced by an equivalent volume of land subsidence (see 

next section). Extensive leveling by the National Geodetic Survey over many years has established the extent 

of land subsidence in the Central Valley (Poland and 

others, 1975; Ireland and others, 1984). The 17 million 

acre-ft of storage loss attributed to inelastic compaction 

of fine-grained sediments is simply the volume of land 

subsidence derived from these surveys.

During the 1960's and 1970's, the annual decrease in 

ground-water storage was about 800,000 acre-ft, representing about 7 percent of annual pumpage (fig. 16). The 

long-term decrease in aquifer storage of 60 million 

acre-ft, although very large, represents only a small part 

of the more than 800 million acre-ft of freshwater stored 

in the upper 1,000 ft of sediments in the Central Valley. 

Nevertheless, the lowering of water levels in the upper 

and lower zones caused a significant increase in pumping 

lifts and thus a significant increase in the cost of pumping 

ground water. During the early 1980's, ground-water 

pumpage decreased, ground-water levels rose in many 

areas, and there was virtually no further decrease in 

ground-water storage.

LAND SUBSIDENCE

The largest volume of land subsidence in the world 

caused by human activities is in the Central Valley. The 

area affected by subsidence includes much of the southern part of the San Joaquin Valley and smaller areas in 

the Sacramento Valley and the Delta (fig. 19). By far, the 

largest volume of land subsidence is caused by groundwater pumpage and the resulting compaction of clay in 

the San Joaquin Valley. However, other processes have 

contributed to land subsidence locally as described by 

Poland and Evenson (1966) and Poland and Lofgren 

(1984). Briefly, the five processes that cause subsidence 

are:

1. Compaction of fine-grained sediments in the aquifer 

system resulting from head declines due to heavy 

ground-water pumpage.

2. Compaction of sediments in petroleum reservoir 

rocks caused by oil and gas extraction.

3. Hydrocompaction compaction of moisture-deficient 

sediments following the first application of water.

4. Compaction of peat soils following land drainage.

5. Tectonic subsidence.

Compaction of peat soils and subsequent land subsidence has occurred in an area of about 450 mi2 in the Delta 

area formed at the confluence of the San Joaquin and 

Sacramento Rivers (Poland and Evenson, 1966). Islands 

in the Delta area that were originally at or slightly above 

sea level are now 10 to 20 ft below sea level. This type of 

subsidence results from oxidation and compaction of peat 

soils following drainage of marshlands for agriculture. 

This area was drained in the middle and late 1800's, which 

resulted in subsidence that is continuing today. Increased 

pumping is required to maintain a lowered water table for 

cultivation.

Hydrocompaction refers to the compaction of moisturedeficient deposits above the water table following the
A28

123 C

REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA 

122° 121° 120° 119° 118 C

40'

39'

38'

37'

36'

35'

EXPLANATION

CHANGE IN HYDRAULIC HEAD, IN FEET 

Rise of 0 to 10

Decline from 0 to 40 

Decline from 40 to 80 

Decline from 80 to 120 

Decline from 120 to 200 

Decline from 200 to 300 

Decline from 300 to 400

Decline of more than 400 

R

FIGURE 18. Estimated change in hydraulic head in lower pumped zone from 1860 to 1961 (modified from Williamson and others, 1989).
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A29

123 C 122 C 12V 120 C 119 C 118 C

40 C

39 c

38 C

37 C

36 C

35° -

EXPLANATION

AREA OF SUBSIDENCE DUE TO 

HYDROCOMPACTION

AREA WHERE SUBSIDENCE DUE TO WATERLEVEL DECLINE IS MORE THAN 1 FOOT

AREA OF SUBSIDENCE DUE TO COMPACTION OF 

PEAT AND BASED ON FIELD SURVEYS IN 1952

AREA WHERE SUBSIDENCE DUE TO 

COMPACTION OF PEAT IS MORE THAN 

10 FEET AS OF 1981

Davis-Zamora Area

Los Banos-Kettleman City Area

50 100 MILES

50 100 KILOMETERS

Tulare-Wasco Area

Arvin-Maricopa Area

FIGURE 19. Areal extent of land subsidence in the Central Valley (modified from Williamson and others, 1989).
A30 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

first application of water. Dry areas along the west and 

south margins of the San Joaquin Valley have subsided in 

such a manner (fig. 19). Within these areas, subsidence of 

5 to 10 ft is common (Poland and Evenson, 1966).

Compaction of sediments due to the withdrawal of oil 

and gas has caused land subsidence locally; however, the 

magnitude is uncertain. Subsidence of less than 1 ft has 

been attributed to this process in the oil fields near 

Bakersfield by Lofgren (1975).

Subsidence due to tectonic movement has been negligible compared to the other four processes during the last 

100 years, according to Williamson and others (1989).

Land subsidence in California due to ground-water 

withdrawal has been extensively studied by the U.S. 

Geological Survey since the mid-1950's. The pioneer work 

by Geological Survey hydrologist Joseph F. Poland and 

his colleagues established many of the principles of the 

mechanics of land subsidence as well as field measurement techniques. Their studies were reported largely in 

U.S. Geological Survey Professional Papers. Areal investigations of land subsidence are described in Professional Paper 437, chapters A-I. Studies of the geology, 

physical properties, and compaction mechanisms of sediments in subsiding areas are described in Professional 

Paper 497, chapters A-G.

The principal field methods used to determine the 

magnitude of land subsidence in California have been 

extensometer wells and precise leveling. A network of 

bench marks was established and precise leveling was 

done by the National Geodetic Survey as well as State 

and municipal government agencies. Extensometer wells 

were used to measure the change in thickness of the 

compacting sediments. Such wells consist of a heavy 

weight anchored into the formation below the bottom of 

the well casing and a cable attached to the weight on one 

end and a counterweight at the other end. A recorder 

provided continuous measurement of the movement of 

the land surface with respect to the anchor weight. For 

a summary of field methods to measure land subsidence, 

the interested reader is referred to a UNESCO guidebook on land subsidence (Poland, 1984).

MECHANICS OF LAND SUBSIDENCE

Land subsidence due to withdrawal of ground water is 

caused by compaction of clay within an aquifer system. 

When pumpage causes the hydraulic head to decline 

below the preconsolidation stress level, the effective 

stress (grain-to-grain load) increases and the clay is 

compacted, releasing water to the aquifer system. A 

brief summary of the mechanics of land subsidence is 

given here. This discussion is based largely on detailed 

analysis of the stresses involved in land subsidence as 

presented by Lofgren (1968) and Poland (1984, p. 37-54).

The classic equation for effective stress (originally developed by Karl Terzaghi and described in Terzaghi and 

Peck, 1967) is as follows:

where 

P

P

tO'iJ?

P' = P -

is effective stress (effective overburden pressure or grain-to-grain load), 

is total stress (geostatic pressure), and 

is pore pressure (fluid pressure).

As the hydraulic head is reduced in a confined aquifer 

(sand and (or) gravel), the geostatic pressure is not 

significantly changed. Thus, the decreased pore pressure 

causes increased grain-to-grain load. The compaction of 

the aquifer is small, immediate, and largely recoverable. 

However, for confining beds (clay and silt) with much 

lower permeability but higher specific storage, the response is quite different. The adjustment of pore pressure in the confining beds to head decline in the aquifer 

proceeds slowly (after months or years). Compaction is 

substantial and largely unrecoverable. If pumping 

ceases, heads recover, and compaction of the confining 

beds eventually ceases (though it may continue for some 

time). If pumping resumes, the confining beds will not be 

compacted until the head declines below the head (critical 

head, fig. 20) of the previous pumping period (providing 

the compaction was completed during the previous pumping period). The loss of inelastic storage from the 

compacting clay is not recoverable. The recovery of 

heads to prepumping levels is not accompanied by a 

recovery of storage lost to compaction.

Inelastic ., (compaction)-"^ 

storage

 \ " I ii IBIUSUI; i

\ (compaction)' 

istorage [

TIME

FIGURE 20. Relation of ground-water storage to hydraulic head in a 

compacting aquifer system (modified from Prudic and Williamson, 

1986).
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A31

The relation of storage and hydraulic head in a compacting aquifer system is shown in figure 20. Note that 

head declines are rapid when pumped water is derived 

from elastic storage but are slow when it is derived from 

inelastic storage.

Water derived from compaction has varied from a few 

percent to more than 60 percent of the pumped water in 

the subsiding areas. These differences were attributed to 

variations in geology and well construction, as discussed 

in chapter C (Page, 1986), and are summarized here. The 

percentage of fine-grained deposits within the stressed 

interval and the mineralogy of these deposits are important factors. In the Los Banos-Kettleman City area, an 

area of maximum land subsidence, the highest percentage of fine-grained deposits anywhere in the San Joaquin 

Valley lies within the upper 2,000 ft of the aquifer 

system. Bull (1975, p. F49) suggested that within this 

area, interlayering of thin-bedded, compressible finegrained sediments with permeable coarse-grained sediments resulted in rapid and substantial compaction in 

response to water-level declines. Compaction is less with 

the same water-level declines in areas of coarse-grained 

beds.

The type of clay mineral present influences subsidence; 

montmorillonite is highly susceptible to compaction and is

the predominant clay mineral in the major subsiding 

areas of the San Joaquin Valley. Differences in hydraulic 

head throughout the pumped interval also affect compaction; the rate of compaction is less when the water table 

and artesian head are lowered simultaneously, as in a 

well that is perforated in both water-table and confined 

zones.

The factors influencing inelastic compaction and land 

subsidence are summarized in figure 21. Where wells are 

perforated in confined zones of an aquifer system that 

contain numerous thin lenses of compressible montmorillonite clay, inelastic compaction will be at a maximum. 

However, where wells tap both water-table and confined 

zones and much of the perforated section falls within 

coarse-grained deposits, compaction will be minimal.

The measured compaction in relation to head decline at 

two wells in subsiding areas from 1960 to 1980 is shown 

in figure 22. At each site, the 1960's were marked by 

steady head decline and a high rate of compaction. The 

decrease in ground-water pumpage in the early to middle 

1970's was accompanied by a steady recovery of water 

levels and greatly reduced compaction. The resumption 

of large ground-water withdrawals during the 1976-77 

drought was marked by a sharp decline in water levels 

and a short period of renewed compaction. Ireland and

Large 

head difference -i^

Fine-grained layers;

 ^=^=^^1^-=E^--

 ' - . .'.'.-

 Sand " ." .-.' ". '.

"i Cone of depression . . .

mostly montmorillonite clay ~^=^-

 _^ 

.'.-.-'. -.

Artesian head 

(lower sand)

Artesian head 

(lower sand)

Sand

"~~ ~- .' .. Numerous thin

0 " './ : fine-grained lenses Sand . . .-.-.-. Of mostly

j ;. ' ' ' ' montmorillonite clay

I I

Sand

: . -. -. ' j ]...- .' .'. .-.'.-.'. .

Fine-grained layers;

little or no 

montmorillonite clay

I I

.1 i: i i

i i

Sand

B

FIGURE 21. Influence of hydrogeology and well construction on potential for land subsidence. Dashed lines denote perforated 

sections of wells. A, High potential for subsidence. B, Low potential for subsidence.
A32 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

others (1984) reported that hydraulic heads generally 

declined 10 to 20 times as fast during the drought as 

during the period of long-term drawdown and compaction 

that ended in the late 1960's. In 1975, hydraulic heads 

were much higher than the lowest heads reached during

350

400

450

500

111 111 550

w

Q

O

_i 

LU 

CD

DC 

111

$ 

O

600 0.5

O

300

350

400

450

500

550

1.0 UJ ai 

u.

0.5

1960 1965 1970 1975 

YEAR

1980 1985

FIGURE 22. Changes in hydraulic head and compaction at two 

wells in subsiding areas of San Joaquin Valley, 1960 to 1980 

(modified from Ireland and others, 1984). A, Arvin-Maricopa 

area (well depth 1,480 ft). B, Los Banos-Kettleman City area 

(well depth 1,358 ft).

the 1960's; consequently, when heavy pumpage resumed 

during the 1976-77 drought, very little compaction occurred. Pumpage was supplied by elastic storage (a small 

quantity) rather than by inelastic storage, so heads 

declined rapidly. Following the drought, recovery to 

predrought water levels was rapid and compaction virtually ceased.

AREAL EXTENT AND EFFECTS

In the Central Valley, land has subsided largely in the 

San Joaquin Valley south of the Merced River. This 

subsidence was extensively documented by Poland and 

others (1975) and by Ireland and others (1984), and it is 

briefly summarized here. These two reports present 

detailed contour maps and profiles showing the areal 

extent and magnitude of subsidence and hydrographs 

relating water levels and compaction.

More than one-half of the San Joaquin Valley, or about 

5,200 mi2, has undergone land subsidence of more than 1 

ft (fig. 19). Three major areas of subsidence within the 

valley are the Los Banos-Kettleman City area (western 

Fresno County), the Tulare-Wasco area (Tulare County), 

and the Arvin-Maricopa area (Kern County) (Poland and 

Lofgren, 1984). Of these, the Los Banos-Kettleman area 

underwent by far the largest volume of subsidence, 

amounting to two-thirds of the subsidence observed in 

the Central Valley up to 1980. This long trough-like area, 

extending for about 80 mi along the west margin of the 

valley, contains three depressions all characterized by 

more than 20 ft of subsidence at their centers. The 

maximum subsidence recorded in the United States (29.6 

ft) is within one of these depressions in western Fresno 

County near the town of Mendota (fig. 23) (Ireland and 

others, 1984).

Subsidence began in the San Joaquin Valley in the 

1920's and increased slowly until World War II. Very 

large increases in ground-water pumpage during the 

1940's and 1950's caused the volume of subsidence to 

increase dramatically. Pumpage increased further 

through the mid-1960's at an average withdrawal rate of 

nearly 12 million acre-ft/yr and subsidence increased 

accordingly. As of 1970, the total volume of subsidence 

was 15.6 million acre-ft/yr (Poland and others, 1975).

FIGURE 23. Magnitude of subsidence at a site 10 mi southwest of 

Mendota in the San Joaquin Valley. Joseph F. Poland, principal 

subsidence researcher of the U.S. Geological Survey, alongside a 

power pole that shows approximate position of land surface in 1925, 

1955, and 1977. Land surface was lowered 29.6 ft from 1925 to 1977.
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A33
A34 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

Ground-water pumpage declined sharply when surface 

water from the Sacramento Valley became available to 

the western San Joaquin Valley via the California Aqueduct in the late 1960's. Since 1970, there has been very 

little subsidence except during the 1976-77 drought, 

when pumpage from wells sharply increased and compaction briefly resumed (fig. 22). As of 1983, land 

subsidence in the San Joaquin Valley had either slowed 

considerably or stopped (Ireland, 1986). Locally in 

Fresno and Kings Counties, between 1977 and 1982, 

rebound of the land surface was significant (about 0.5 ft), 

indicating that subsidence during the 1976-77 drought 

was elastic. In the future, subsidence will resume only if 

renewed pumpage is sufficiently heavy to cause groundwater levels to drop below their previous lows.

In the Sacramento Valley, subsidence of more than 1 ft 

is limited to the Davis-Zamora area in the southern part 

of the valley northwest of Sacramento (fig. 19), where 

land subsidence of about 2 ft was reported by Lofgren 

and Ireland (1973). However, some additional leveling 

suggests that subsidence increased between 1973 and 

1979 (Williamson and others, 1989).

Subsidence in the Central Valley has created engineering and economic problems, although many are not 

noticeable by casual observation because land subsided 

over such large areas at the same time. Subsidence of 

canals and irrigation and drainage systems has resulted 

in cracking and the loss of carrying capacity. In areas 

susceptible to hydrocompaction, it has been necessary to 

precompact sediments by prolonged wetting before construction of canals a costly procedure. Failure of well 

casings due to compressional stress resulted in the loss of 

thousands of irrigation wells during the 1950's and 1960's. 

Frequent surveying to determine elevations of bench 

marks has been required for construction purposes and 

revision of topographic maps.

QUALITY OF GROUND WATER

Historically, ground-water quality of the Central Valley has been studied as three units: the Sacramento 

Valley, the San Joaquin Valley, and the Delta area (fig. 

1). Olmsted and Davis (1961) described ground-water 

quality of the Sacramento Valley, Hull (1984) described 

ground-water quality of the Sacramento Valley exclusive 

of the southern part near the Delta, and Davis and others 

(1959) described ground-water quality of the San Joaquin 

Valley south of the Delta. Numerous authors of the U.S. 

Geological Survey described ground-water quality conditions in smaller parts of the valley, as indicated in the 

references.

Central Valley ground-water chemistry is influenced 

by water from streams that enter the valley from the 

surrounding mountains and provide most of the natural 

recharge (Davis and others, 1959; Hull, 1984). The 

quality of water in streams that enter the valley from the 

east is influenced by the granitic Sierra Nevada and is 

notably different from the quality of water in streams 

from the west, which is influenced by the marine sediments of the Coast Ranges. In general, the east side, the 

axial part, and the west side of the Central Valley are 

characterized by distinctive ground-water chemistry. 

The water chemistry is further influenced by an increase 

in reducing conditions and cation exchange processes as 

the water moves through the sediments.

Davis and others (1959) divided the San Joaquin Valley 

into three areas of ground-water-quality characteristics: 

the east side, the axial part, and the west side. In 

general, ground water on the east side is bicarbonate 

type and has low to moderate dissolved-solids concentrations, ground water of the axial part differs greatly in 

chemical types and generally contains higher concentrations of dissolved solids than does water on the east side, 

and ground water on the west side is typically a sulfate or 

bicarbonate type and contains higher concentrations of 

dissolved solids than does water on the east side (Davis 

and others, 1959).

Davis and others (1959) further divided ground water 

in the San Joaquin Valley into three vertical zones: 

unconfined, semiconfined, and confined. Confined waters 

generally have lower concentrations of dissolved solids 

and higher percent sodium. The confined waters also 

differ in chemical types from the unconfined waters 

owing to cation-exchange reactions, which occur on the 

clay particles (Davis and others, 1959).

Hull (1984), in a detailed study of the Sacramento 

Valley, delineated six hydrochemical facies: two on the 

east side, two in the center of the valley, and two on the 

west side. In general, ground water on the east side is 

low in dissolved solids and high in silica, reflecting the 

quality of recharge water from the granitic rock of the 

Sierra Nevada. Reducing conditions produce high concentrations of dissolved iron, manganese, and arsenic in 

the central part of the valley. Ground water on the west 

side is lower in silica and higher in dissolved-solids 

concentrations than ground water on the east side. Also, 

dissolved-solids concentrations tend to increase from 

north to south along the axis of the Sacramento Valley 

(fig. 24).

BASE OF FRESHWATER

The base of freshwater less than 2,000 mg/L (milligrams per liter) of dissolved solids in the Sacramento
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A35

123C 122° 121' 120C 119° 118C

40C

39C

38 C

37 C

36 C

35 c

EXPLANATION

DISSOLVED-SOLIDS CONCENTRATION 

(in milligrams per liter)

Less than 200

200 to 500 

501 to 1,500 

More than 1,500 

No data

FIGURE 24. Distribution of dissolved-solids concentrations in ground water of the Central Valley.
A36 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

Valley was delineated by Berkstresser (1973). With the 

exception of several localized pods of shallow saline 

water, ground water in the continental and volcanic 

deposits of the Sacramento Valley is fresh. Several 

localized shallow saline zones were described by Berkstresser. Of these, four are now known to be underlain by 

freshwater. These shallow saline zones, underlain by 

freshwater, are located around the northern part of the 

Delta, along the Sacramento River, adjacent to the 

Sutter and Yolo bypasses, and around the base of Sutter 

Buttes. The zone around Sutter Buttes reflects the 

configuration of the underlying marine deposits (Berkstresser, 1973).

The base of freshwater as delineated by Page (1973b) 

for the San Joaquin Valley is more complex. In the San 

Joaquin Valley, the base of freshwater lies within the 

unconsolidated continental deposits of Pliocene to Holocene age, the more consolidated marine and sedimentary 

deposits of Tertiary age, and the igneous and metamorphic rocks of pre-Tertiary age. Unlike the Sacramento 

Valley, the base of freshwater in the San Joaquin Valley 

is underlain by a saline water body. The depths to the 

base of freshwater in the San Joaquin Valley range from 

less than 100 to more than 3,500 ft below land surface.

DISSOLVED SOLIDS

The areal distribution of dissolved-solids concentrations in ground water of the Central Valley is shown in 

figure 24. The map for the Sacramento Valley was 

prepared using a combination of data for dissolved solids 

measured by the residue-on-evaporation method (72 

percent of the analyses) (Brown and others, 1970, p. 145) 

and data derived from specific conductance measurements (Fogelman, 1982a).

The map for the San Joaquin Valley was produced 

primarily from dissolved-solids measurements made using the residue-on-evaporation method. Where data were 

sparse, the map was modified by examining additional 

dissolved-solids data calculated as the sum of the dissolved constituents. Because 2,000 mg/L of dissolved 

solids is considered to be the maximum concentration in 

freshwater (Olmsted and Davis, 1961, p. 134), only those 

wells that yielded water with a dissolved-solids concentration of less than 2,000 mg/L were used to prepare the 

map. An exception to this was made along the southwest 

margin of the San Joaquin Valley, where shallow ground 

water has high dissolved-solids concentrations. The water samples were collected between 1974 and 1982 in the 

Sacramento Valley and between 1934 and 1985 in the San 

Joaquin Valley.

Because figure 24 is a two-dimensional representation 

of data compiled from existing wells with a wide variety 

of depths and screen lengths, the map cannot show

vertical variations in water quality. It is therefore a 

generalization of the dissolved-solids concentrations that 

are likely to be found in a particular area, and it is most 

representative of the ground-water zones commonly 

used. About 11 percent of the wells in the Sacramento 

Valley data set yielded water with dissolved-solids concentrations that were higher or lower than the mapped 

interval. This proportion of concentrations not conforming to mapped intervals is probably similar for the San 

Joaquin Valley (Fogelman, 1982b).

The U.S. Environmental Protection Agency (1979) 

secondary drinking-water standards recommend a limit 

of 500 mg/L for dissolved solids. The California Domestic 

Water Quality Regulations allow a maximum of 1,000 

mg/L if water of better quality is not available. However, 

because dissolved-solids concentrations only indicate the 

total amount of dissolved constituents in water, the 

usability of water that exceeds 500 mg/L needs to be 

evaluated according to the concentration of each chemical 

constituent.

Dissolved-solids concentrations are lower in the northern part and along the east side of the Central Valley. 

Dissolved solids are higher in the south-central part of 

the Sacramento Valley and in the western part of the San 

Joaquin Valley. This distribution reflects the low concentrations of dissolved solids in recharge water that originates in the Cascade Range and the Sierra Nevada, and 

the predominant regional ground-water flow pattern.

In the Sacramento Valley, dissolved-solids concentrations generally do not exceed 500 mg/L. Two large areas 

of shallow ground water in which concentrations of 

dissolved solids range from 500-1,500 mg/L are present 

in the southern part of the Sacramento Valley (fig. 24). 

One area is south of the Sutter Buttes in the Sutter basin, 

and the other is west of the Sacramento River extending 

from West Sacramento on the north to the confluence of 

the Sacramento and San Joaquin Rivers on the south (fig. 

24).

In the San Joaquin Valley, dissolved-solids concentrations are lower on the east side and higher on the west 

side of the valley. In the center and on the east side of the 

valley, dissolved-solids concentrations generally do not 

exceed 500 mg/L; on the west side, most of the ground 

water contains concentrations of dissolved solids in 

excess of 500 mg/L. Concentrations of dissolved solids 

generally increase to the west, and concentrations in 

excess of 2,000 mg/L are not uncommon along the west 

margin of the valley.

HYDROCHEMICAL FACIES

Where a few ions dominate the dissolved-solids content 

of ground water, the term "hydrochemical fades" (Back, 

1961) is used to describe the dominant ion patterns. The
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A37

classification of ground water into hydrochemical facies 

or chemical water types is based on the relative concentration, in chemical equivalents, of cations and anions in 

the water. The cation and anion that represent at least 50 

percent of the total ions are used to designate the 

chemical water type, such as magnesium bicarbonate. If 

no one cation or anion amounts to 50 percent, the water 

is designated by the two ions that make up the largest 

percentages, such as calcium magnesium bicarbonate.

The distribution of chemical water types in the Central 

Valley aquifer system is shown in figure 25. This map 

shows only the general distribution pattern of water 

types. Because the map does not show vertical variations, these variations are discussed herein, where appropriate. Areas where data are insufficient to define the 

water type were left blank.

Throughout the Sacramento Valley, with the exception 

of part of the Sutter basin, bicarbonate is the predominant anion in the ground water. Ground water in the 

northern and eastern parts of the Sacramento Valley has 

fairly homogeneous chemical character, with calcium and 

magnesium being the predominant cations. Two areas, 

one along Stony Creek and one along the Feather River, 

stand out as being exclusively calcium or magnesium 

bicarbonate, respectively, reflecting the recharge waters 

from the streams that drain into the valley at these 

locations (Fogelman, 1983).

South of the Sutter Buttes, water types are more 

complex and sodium is the predominant cation. Sodium 

bicarbonate type water is predominant along Salt Creek 

and downstream from the confluence of Salt Creek and 

the Sacramento River. The sodium ion in this area is 

probably derived from saline water in the Cretaceous 

formations that are drained by Salt Creek (Fogelman, 

1983). Downgradient on the west side of the valley, and 

extending in a belt across the valley, is an area where the 

chemical type of ground water is predominantly magnesium sodium or sodium magnesium bicarbonate. The 

most notable exceptions to these water types are the 

areas adjacent to Cache and Putah Creeks and a small 

area around Dunnigan. The chemical water types in these 

areas are magnesium, calcium magnesium, or magnesium 

calcium bicarbonate. The water chemistry in the Dunnigan area is influenced largely by the Pliocene Tehama 

Formation, of which the Dunnigan Hills are largely 

composed (Fogelman, 1983). Recharge from Cache and 

Putah Creeks is the likely source of water in these areas. 

South of Putah Creek, sodium is again the predominant 

cation, although small pockets of calcium sodium and 

magnesium sodium ground water are also present.

Magnesium and calcium are the predominant cations in 

most of the southeastern part of the Sacramento Valley, 

and in particular, that area underlying the drainages of 

the American and Cosumnes Rivers. Smaller areas of 

sodium, sodium calcium, and sodium magnesium bicarbonate water types are also present in the southeastern 

Sacramento Valley.

The most notable exception to the predominance of 

bicarbonate as the major anion is the area in the southern 

part of Sutter basin just south of the Sutter Buttes. In 

this area, calcium, magnesium, and sodium, as well as 

chloride, bicarbonate, and sulfate may be found in any 

combination. Sutter Buttes may be the source of the high 

sodium and chloride concentrations (Fogelman, 1983), or 

the source may be a shallow layer of saline water 

surrounding the base of the Sutter Buttes (Berkstresser, 

1973).

SAN JOAQUIN VALLEY

The distribution of water types or hydrochemical facies 

in the San Joaquin Valley is more complex than in the 

Sacramento Valley. The most important difference is the 

presence of chloride and sulfate as well as bicarbonate as 

the dominant anions. Generally, chloride predominates in 

the northwest, sulfate predominates in the southwest, 

and bicarbonate predominates in the east. The major 

exceptions to this are the Hanford-Visalia area, where 

chloride and bicarbonate predominate, and the extreme 

southeastern part of the valley, where bicarbonate, 

sulfate, and chloride are all present in varying concentrations.

On the basis of water types, three areas of the San 

Joaquin Valley can be delineated: the east side, predominantly bicarbonate; the axial trough, variable anion 

composition; and the west side, predominantly sulfate 

and chloride.

Water types of the east San Joaquin Valley are fairly 

uniform, most commonly resembling the chemical types 

of the local surface water that recharges the ground 

water (Dale and others, 1966; Croft and Gordon, 1968; 

Page and LeBlanc, 1969; Mitten and others, 1970; Sorenson, 1981). Calcium bicarbonate, calcium sodium bicarbonate, sodium calcium bicarbonate, and calcium 

magnesium bicarbonate are the predominant water types 

of ground water of the east side.

Because the axial trough has been the discharge area in 

the past for the San Joaquin Valley, ground water in this 

area is a combination of water from the east side and the 

west side (Croft and Gordon, 1968; Bertoldi, 1971). Local 

recharge from streams and surface water imported via 

canals that infiltrates from irrigated fields to the water 

table (Hotchkiss and Balding, 1971) also affects the 

ground-water chemistry of the axial trough.
A38 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

123 C 122 C 12V 120C 119 C 118C

40°

39'

I I

EXPLANATION

WATER TYPES 

Magnesium bicarbonate

Calcium bicarbonate

Sodium bicarbonate

Magnesium sodium bicarbonate or 

sodium magnesium bicarbonate

Calcium magnesium bicarbonate or 

magnesium calcium bicarbonate

Sodium calcium bicarbonate or 

calcium sodium bicarbonate

Calcium, magnesium, sodium, chloride, 

sulfate, and bicarbonate may occur 

in any combination

Sodium or calcium, chloride or sulfate are 

most common in various combinations

No data

38'

37'

36 C

35° -

c

50 100 MILES

50 100 KILOMETERS

FIGURE 25. Chemical types of ground water in the Central Valley. 

[Chemical types shown are approximated from maps and descriptions in the following reports: Davis and Poland, 1957; Wood and 

Davis, 1959; Wood and Dale, 1964; Dale and others, 1966; Croft and

Gordon, 1968; Page and LeBlanc, 1969; Bertoldi, 1971; Hotchkiss 

and Balding, 1971; Page and Balding, 1973; Sorenson, 1981; 

Fogelman, 1983; Evenson, 1985; Johnson, 1985.]
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A39

Two depth zones are recognizable in the axial trough. 

In the lower zone, water types are generally sodium 

bicarbonate, sodium chloride, and calcium sodium bicarbonate. Ground water in the upper zone is more variable 

than in the lower zone.

Generally, sodium and calcium are the dominant cations of the axial trough ground water. In the northern 

part of the valley, chloride and bicarbonate are the 

dominant anions, and in the southern part of the valley, 

sulfate and bicarbonate are the dominant anions.

There is also a large amount of areal and vertical 

variability of water types on the west side of the San 

Joaquin Valley; here, in places, the Corcoran Clay 

Member of the Tulare Formation separates different 

types of ground water. Water types in the zone above the 

Corcoran Clay Member are less variable than those of the 

axial trough. Sodium is the predominant cation in this 

area with few exceptions. Sodium and sodium calcium 

predominate in the south, and sodium and calcium sodium 

predominate in the north. Sulfate is the predominant 

anion in the zone above the Corcoran Clay Member in the 

south (Davis and Poland, 1957; Wood and Davis, 1959; 

Hotchkiss and Balding, 1971), whereas chloride and 

bicarbonate predominate in the north. The presence of 

bicarbonate in the north is attributed to recharge from 

intermittent streams (Hotchkiss and Balding, 1971).

Chemical analyses for wells perforated below the 

Corcoran Clay Member north of the Fresno-Merced 

County line are limited, but the water type is probably 

similar to that above the clay. South of this area to the 

Tulare Lake bed, the water type below the Corcoran 

Clay Member is generally sodium sulfate (Bertoldi, 

1971). The few wells that are perforated below the 

Corcoran Clay Member south of the Tulare Lake bed tap 

ground water that is generally sodium chloride (Dale and 

others, 1966).

PROBLEM COMPOUNDS

Local concentrations of boron, chloride, and nitrate in 

the Central Valley are high enough to be a problem either 

to crops or to humans. Other constituents, such as 

pesticides and trace metals, have been investigated only 

on a random basis and, with the exception of selenium in 

the western San Joaquin Valley, are not known to be a 

problem.

CHLORIDE

High chloride concentrations are generally not considered a health hazard. On the basis of taste preference, the 

U.S. Environmental Protection Agency (1979) recommends a limit of 250 mg/L for chloride in drinking water. 

High chloride concentrations can be toxic to plants, but

salinity usually impairs growth before chloride alone 

reaches toxic levels. Water with chloride concentrations 

up to 700 mg/L can be used on most crops, depending on 

soils and irrigation practices, without impairing growth 

(National Academy of Sciences and National Academy of 

Engineering, 1973).

Chloride concentrations of Central Valley ground water generally are less than 250 mg/L; however, several 

areas are notable for having higher chloride concentrations.

Two bands of high chloride concentrations are located 

in the Sacramento Valley. One band is adjacent to Salt 

Creek near Williams; the other is adjacent to Petroleum 

and Salt Creeks near Arbuckle. In these areas, high 

chloride concentrations are attributed to the recharge 

from local streams (Bertoldi, 1976). A third area of high 

chloride concentrations is south of Sutter Buttes, in the 

southwestern part of the Sutter basin. This area coincides with a shallow saline water body previously described by Berkstresser (1973), which is probably the 

source of the high chloride concentrations.

The most notable locations of high chloride concentrations in the San Joaquin Valley are in the northwestern 

and north-central part of the valley along the course of 

the San Joaquin River and adjacent lowlands. Within this 

area, depth to the base of freshwater is shallower (500 ft 

or less) than elsewhere in the valley. Ground-water flow 

was upward prior to development and, currently, flow 

remains upward locally in this area (Williamson and 

others, 1989). Therefore, the most probable source of 

high chloride in the shallow ground water is upward flow 

of saline ground water. Sorenson (1981) mapped high 

chloride concentrations adjacent to and west of the San 

Joaquin River in San Joaquin and Contra Costa Counties. 

Others mapped high chloride concentrations all along the 

San Joaquin River (Page and LeBlanc, 1969; Mitten and 

others, 1970; Hotchkiss and Balding, 1971; Page and 

Balding, 1973).

BORON

Boron is a critical element in irrigation water supplies. 

In small quantities, boron is an essential micronutrient; 

however, boron becomes toxic to sensitive plants at 

concentrations as low as 0.75 mg/L and is toxic to most 

crops at concentrations exceeding 4.0 mg/L. Within this 

range, crops have been classified into three categories of 

boron tolerance: sensitive (less than 1 mg/L), semitolerant (1-2 mg/L), and tolerant (more than 2-4 mg/L) 

(National Academy of Sciences and National Academy of 

Engineering, 1973).

Boron is found in concentrations greater than 0.75 

mg/L in several areas of the Central Valley. Small areas 

of high boron concentrations have been observed in the
A40 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA

extreme northern and extreme southern parts of the 

valley. Fogelman (1983) delineated an area east of Red 

Bluff where ground water had concentrations of boron 

exceeding 0.75 mg/L. These high boron concentrations 

are attributed to the nearby Pliocene Tuscan Formation 

(Fogelman, 1983).

Wood and Dale (1964) reported concentrations of boron 

generally greater than 3 mg/L in the area southwest of 

Bakersfield and ranging from 1 to 4 mg/L in an area 

southeast of Bakersfield. Dale and others (1966) noted 

boron concentrations as high as 4.2 mg/L near Buttonwillow Ridge and Buena Vista Slough. The nearby 

marine sedimentary deposits were the probable source of 

the high boron concentrations of the west side, whereas 

the east side concentrations were probably derived from 

continental sedimentary deposits (Wood and Dale, 1964).

A large area of high boron concentrations in the 

southwestern part of the Sacramento Valley extends 

from Arbuckle on the north to Rio Vista on the south. 

There is one band of low boron water extending through 

the center of this area from Vacaville to West Sacramento. High boron concentrations in this area were 

attributed to marine deposits of the Upper Cretaceous 

Chico Formation and Lower Cretaceous rocks of the 

Coast Ranges, from which the recharge water is derived 

(Fogelman, 1983).

Another large area of high boron concentrations, in the 

northwestern part of the San Joaquin Valley, extends 

from the northernmost edge of the valley west of the San 

Joaquin River to the Kings-Fresno County line. Bertoldi 

(1971) reported high boron concentrations in the lower 

zone in the southern part of this area. Bertoldi also found 

high boron concentrations near the Diablo Range, indicating that these marine sediments of the range are the 

likely source of the boron. Sorenson (1981) also attributed 

high boron concentrations in the northern part of the 

valley to the marine sediments of the Coast Ranges.

NITRATE

Nitrate toxicity usually does not affect adults, but it 

can cause a blood disorder known as methemoglobinemia, 

which is sometimes fatal in infants and young children. 

The recommended maximum concentration in drinking 

water for nitrate (as nitrogen) is 10 mg/L (National 

Academy of Sciences and National Academy of Engineering, 1973).

Nitrate in irrigation water is usually considered an 

asset because of its value as a fertilizer. However, some 

crops such as sugar beets, apricots, grapes, citrus, and 

avocados are sensitive and may be adversely affected by 

high nitrate concentrations. Problems can result from 

concentrations as low as 5 mg/L, and severe problems 

result from concentrations above 30 mg/L (Ayers, 1977).

Concentrations of nitrate as nitrogen exceeding 30 

mg/L are very rare and extremely localized in the Central 

Valley. However, there are several areas where concentrations exceed 10 mg/L.

Several potential problem areas were delineated in the 

Sacramento Valley with respect to nitrates in drinking 

water (Fogelman, 1983). Hull (1984) estimated the maximum concentration of nitrate under natural conditions of 

the Sacramento Valley to be 3 mg/L and considered that 

areas having 5.5 mg/L or more are those in which nitrate 

concentrations may be increasing. Forty wells in the 

Chico-Corning area contained water in which nitratenitrogen concentrations exceeded 5.5 mg/L, and 25 

percent of these wells contained water with concentrations exceeding 10 mg/L. In the Gridley-Marysville area, 

21 wells contained water in which concentrations of 

nitrate-nitrogen exceeded 5.5 mg/L, and 33 percent of 

these contained water with concentrations exceeding 10 

mg/L. In both areas, the wells containing high nitrate 

concentrations are shallow, and surface contamination 

from leaching of applied nitrate fertilizers, urban wastetreatment facilities, or septic systems was suggested as 

the probable cause (Fogelman, 1983).

Sorenson (1981) reported nitrate concentrations 

greater than 5 mg/L over a large part of southern San 

Joaquin County between Lodi and Stockton. These high 

concentrations were attributed to agricultural practices.

In several other small areas of the San Joaquin Valley, 

ground water contains concentrations of nitrate exceeding 10 mg/L. Such an area south of Bakersfield was 

identified by Wood and Dale (1964) and 2 years later 

another such area slightly north of Bakersfield was 

identified by Dale and others (1966). High concentrations 

also have been reported around the Fresno metropolitan 

area. In this area, nitrate concentrations decrease with 

depth (Page and LeBlanc, 1969), indicating surface 

contamination. Sporadic high concentrations of nitrate 

were also found near the foothills of the Sierra Nevada in 

the Hanford-Visalia area (Croft and Gordon, 1968). Other 

occurrences of nitrate-nitrogen exceeding 10 mg/L are 

extremely localized and usually are attributed to localized 

pollution sources such as septic tanks, dairies, or feed lots 

(Sorenson, 1981; Bertoldi, 1971).

EFFECTS OF HUMAN ACTIVITIES

Temporal changes in dissolved-solids and nitrate concentrations in the Sacramento Valley were studied by 

Hull (1984). Dissolved-solids concentrations were used as 

an indicator of changes in the overall water quality, and 

nitrate concentrations were used as an indicator of 

human sources such as applied fertilizers or human 

waste. Significant increases in concentrations of both 

dissolved solids and nitrates were observed, indicating
GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A41

that ground-water quality is degrading as a result of 

increasing application of agricultural chemicals and 

growth of urban population.

The concentrations of dissolved solids increased significantly since the 1950's throughout the Sacramento 

Valley except for an area south of Sutter Buttes between 

the Sacramento and Feather Rivers (Hull, 1984). However, increases in nitrate concentrations were found only 

on the west side and in the southeastern part of the valley 

(Hull, 1984). Hull (U.S. Geological Survey, oral commun., 1984) indicated that the rate of nitrate build-up has 

been increasing since 1912. He found that in 1912 to 1913, 

2.2 percent of the wells had nitrate concentrations 

greater than 5.5 mg/L; between 1960 and 1969, 4.9 

percent had concentrations exceeding 5.5 mg/L, and from 

1974 to 1978, 10.5 percent had concentrations exceeding 

5.5 mg/L.

Studies to determine human impact on ground-water 

quality in the San Joaquin Valley were not done, largely 

because different time periods of sample collection and 

the use of different analytical techniques make the 

comparability of the existing data questionable. Because 

agricultural practices in the San Joaquin Valley are 

similar to those in the Sacramento Valley, it is likely that 

ground-water quality in the San Joaquin Valley is also 

degrading as a result of human activities. However, 

differences in geology, soils, and irrigation techniques 

could all affect the impact of human activities. Areas of 

possible contamination of ground water due to human 

activities in the San Joaquin Valley were mapped by 

Templin (1984, pi. 8). Sources listed were industrial, 

municipal and domestic solid-waste disposal sites, and 

agricultural chemical build-up. This map suggests that 

human-induced contamination exists throughout much of 

the valley.

The use of pesticides is another possible impact on 

ground water that has not been studied in detail. 

Pesticides have been used intensively in the Central 

Valley for many years and because dissolved-solids and 

nitrate concentrations are increasing, it is likely that 

pesticide concentrations in ground water also are increasing.

The presence of dibromochloropropane (DBCP) in 

ground water in the San Joaquin Valley was mapped by 

Templin (1984, pi. 7D). The presence of this pesticide at 

levels above 0.0005 mg/L near Bakersfield, Fresno, 

Modesto, and north of Merced and Stockton coincides 

with land-use patterns. More specifically, these areas are 

occupied by orchards and vineyards, where DBCP is 

commonly used. The presence of DBCP in the San 

Joaquin Valley suggests that other pesticides have probably built up in ground water of the Central Valley.

Selenium, which is toxic to humans and animals at very 

low concentrations, occurs naturally in the soils and

ground water on the west side of the San Joaquin Valley 

(Deverel and others, 1984). Ecological and health effects 

of selenium and other trace elements in agricultural 

drainage water on the west side of the San Joaquin Valley 

have become subjects of extensive study since high 

incidences of mortality and birth defects were observed 

in waterfowl nesting in the area where drainage water is 

discharged (U.S. Bureau of Reclamation, 1984). In a 

preliminary study (Deverel and others, 1984), the areal 

distribution of selenium and other inorganic constituents 

was examined along the west side of the valley west of 

Fresno. Selenium concentrations (median concentrations 

of 10-11 mg/L) were highest in the central and southern 

parts (south of Los Banos and south of Mendota) of the 

area studied. Extensive studies by U.S. Department of 

Interior agencies (Bureau of Reclamation, Fish and 

Wildlife Service, and the Geological Survey) and also 

many studies by universities, State water resources 

agencies, and concerned local agencies are still in 

progress (1989).

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Riley, F.S., 1970, Land-surface tilting near Wheeler Ridge, southern 

San Joaquin Valley, California: U.S. Geological Survey Professional Paper 497-G, 29 p.

Riley, F.S., and McClelland, E.J., 1972, Application of the modified 

theory of leaky aquifers to a compressible multiple-aquifer system: 

U.S. Geological Survey open-file report, 86 p.

Sacramento Petroleum Association, 1962, Typical gas fields of the 

Sacramento Valley, California, in Geologic guide to the gas and oil 

fields in northern California: California Division of Mines and 

Geology Bulletin 181, p. 99-148.

Shelton, J.S., 1966, Geology illustrated: San Francisco, W.H. Freeman, 

434 p.

Sorenson, S.K., 1981, Chemical quality of ground water in San Joaquin 

and part of Contra Costa Counties, California: U.S. Geological 

Survey Water-Resources Investigations Report 81-26, 32 p.

Stearns, H.T., Robinson, T.W., and Taylor, G.H., 1930, Geology and 

water resources of the Mokelumne area, California: U.S. Geological Survey Water-Supply Paper 619, 402 p.

Templin, W.E., 1984, Ground-water quality monitoring network design 

for the San Joaquin Valley ground-water basin, California: U.S. 

Geological Survey Water-Resources Investigations Report 

83-4080, 133 p.

Terzaghi, K.C., and Peck, R.B., 1967, Soil mechanics in engineering 

practice [2d ed.]: New York, John Wiley, 729 p.

Thomas, H.E., and Phoenix, D.A., 1976, Summary appraisal of the 

Nation's ground-water resources California region: U.S. Geological Survey Professional Paper 813-E, p. E1-E51.

Trescott, P.C., 1975, Documentation of finite-difference model for 

simulation of three-dimensional ground-water flow: U.S. Geological Survey Open-File Report 75-438, 101 p.

Trescott, P.C., and Larson, S.P., 1976, Supplement to Open-File 

Report 75-438, Documentation of finite difference model for 

simulation of three-dimensional ground-water flow: U.S. Geological Survey Open-File Report 76-591, 21 p.

U.S. Bureau of Reclamation, 1984, Kesterson Reservoir and waterfowl: 

Information Bulletin 2, 11 p.

U.S. Environmental Protection Agency, 1979, National secondary 

drinking water regulations: Federal Register, v. 44, no. 140, July 

19, 1979, p. 42195-42202.

Williamson, A.K., 1981, Evapotranspiration of applied water, Central 

Valley, California, 1957-78: U.S. Geological Survey Water-Resources Investigations Report 81-45, 56 p.

Williamson, A.K., and Prudic, D.E., 1986, Simulation of flow and 

compaction in the regional aquifer system of the Central Valley of 

California, U.S.A., in Johnson, A.I., Carbogm'n, Laura, and 

Ubertini, L., eds., Land subsidence: 3d International Symposium 

on Land Subsidence, Venice, Italy, March 18-26, 1984, Proceedings, International Association of Hydrological Sciences Publication 151, p. 271-280.

Williamson, A.K., Prudic, D.E., and Swain, L.A., 1989, Ground-water 

flow in the Central Valley, California: U.S. Geological Survey 

Professional Paper 1401-D, 127 p.

Wood, P.R., and Dale, R.H., 1964, Geology and ground-water features 

of the Edison-Maricopa area, Kern County, California: U.S. 

Geological Survey Water-Supply Paper 1656, 103 p.

Wood, P.R., and Da vis, G.H., 1959, Ground-water conditions in the 

Avenal-McKittrick area, Kings and Kern Counties, California: 

U.S. Geological Survey Water-Supply Paper 1457, 141 p.

-A- U S. GOVERNMENT PRINTING OFFICE: 1991 587-824


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists 

and engineers. Included are reports on the results of resource studies 

and of topographic, hydrologic, and geologic investigations. They also 

include collections of related papers addressing different aspects of a 

single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations, as 

well as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present 

significant interpretive results of hydrologic investigations of wide 

interest to professional geologists, hydrologists, and engineers. The 

series covers investigations in all phases of hydrology, including 

hydrogeology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for 

distribution at no cost to the public. Information is usually of short-term 

interest.

Water-Resources Investigations Reports are papers of an 

interpretive nature made available to the public outside the formal 

USGS publications series. Copies are reproduced on request unlike 

formal USGS publications, and they are also available for public 

inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, 

maps, and other material that are made available for public consultation 

at depositories. They are a nonpermanent form of publication that may 

be cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 7.5- or 15-minute quadrangle formats (scales 

mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include 

structure and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales; they show results of surveys using 

geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which reflect subsurface structures that are of economic or 

geologic significance. Many maps include correlations with the geologyMiscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also 

includes 7.5-minute quadrangle photogeologic maps on planimetric 

bases that show geology as interpreted from aerial photographs, Series 

also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial 

geology, stratigraphy, and structural relations in certain coal-resource 

areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum 

potential.

Miscellaneous Field Studies Maps are multicolor or blackand-white maps on topographic or planimetric bases on quadrangle or 

irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or blackand-white maps on topographic or planimetric bases presenting a wide 

range of geohydrologic data of both regular and irregular areas; 

principal scale is 1:24,000, and regional studies are at 1:250,000 scale 

or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, 

Books and Open-File Reports Section, Federal Center, Box 25425, 

Denver, CO 80225. (See latest Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be 

purchased by mail and over the counter in paperback book form and as 

a set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be 

purchased by mail and over the counter in paperback book form and as 

a set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may 

be purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by 

mail and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by 

mail and over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback 

booklet form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" are available free of charge by mail or may 

be obtained over the counter in paperback booklet form only. Those 

wishing a free subscription to the monthly catalog "New Publications of 

the U.S. Geological Survey" should write to the U.S. Geological 

Survey, 582 National Center, Reston, VA 22092.

Note. Prices of Government publications listed in older catalogs, announcements, and publications may be incorrect. Therefore, 

the prices charged may differ from the prices in catalogs, announcements, and publications.