IQJUN

GEOCHEMISTRY OF GROUND WATER IN THE 

1 SACRAMENTO VALLEY, CALIFORNIA

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1401-B

Geochemistry of 

Ground Water in the 

Sacramento Valley, California

By LAURENCE C. HULL

CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1401-B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1984
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Hull, Laurence C.

Geochemistry of ground water in the Sacramento Valley, California

(Geological Survey Professional Paper 1401-B)

(Central Valley of California RASA Project)

Bibliography: 36 p.

1. Water, Underground California Sacramento Valley. 2. Water chemistry.

I. Title. II. Series. III. Series: Central Valley of California RASA Project.

GB1025.C2H781984 551.4'9'097945 84-600016

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

Abstract ..................

Introduction ................

Geography ...............

Purpose and scope ..........

Data compilation ............

Well-numbering system ........

Acknowledgments ...........

Description of the study area ......

Geology and geomorphology .....

Geologic synopsis .........

Geomorphic units .........

Hydrogeology .............

Hydrochemical facies ...........

Criteria and procedure for division . 

Descriptions of hydrochemical facies

Tuscan volcanic rocks ......

Victor Plain ...........

Butte Basin ............

Sutter Basin ...........

North alluvial fans ........

South alluvial fans ........

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Temporal changes ...............

Methods of data analysis ........

Dissolved solids ..............

Hydrochemical facies .........

1 Nitrate ...................

Hydrochemical facies .........

Hypothesis testing ............

Processes controlling ground-water chemistry 

Methods of data analysis ........

Principal components analysis ....

Trend surface analysis ........

Processes and their spatial variation . . 

Mineral stability relations ...........

Mineralogy of sediments .........

Well 12N/01E-34Q ..........

Well 19N/01W-32G ..........

Saturation indices .............

Activity diagrams .............

Summary and conclusions ...........

References ...................

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ILLUSTRATIONS

PLATE 1. 

2.

Maps of the Sacramento Valley, California, showing geomorphology and water-table contours In pocket 

Maps of the Sacramento Valley, California, showing wells used to examine linear trends in chemistry with time In pocket

FIGURE

9.

10.

11.

12.

13.

Location map showing Sacramento Valley of California ..................................

Diagram showing well-numbering system ..........................................

Diagram showing major-ion composition of water from wells of various depths in section 32 of township 19 N., range

iw. .............................................................

Diagram showing major-ion composition of water from springs in the Clear Lake area and from wells in a high boron

area northwest of Woodland. ...............................................

Graph of chloride versus boron for springs in the Clear Lake area and for wells in a high boron area northwest of

Woodland. ..........................................................

Graphs of average yearly dissolved-solids concentrations versus time for the six hydrochemical facies in the Sacramento

Valley. ............................................................

Frequency histogram of dissolved nitrate-nitrogen concentrations in wells 1912-13. ..................

Graphs of average yearly nitrate-nitrogen concentrations versus time for the six hydrochemical facies in the Sacramento

Valley. ............................................................

Contour map of the trend surface of factor scores for component one. .........................

Contour map of the trend surface of factor scores for component two. .........................

Electron micrograph of sediments from the 407-foot depth of well 19N/01W-32G. ...................

Diagram showing phase relations among minerals and aqueous solution in terms of pH-VapAl"1" 3 and p[H4Si04] at 20°C. . 

Diagram showing phase relations among minerals and coexisting aqueous solution in terms of log [Mg+2]/[H+1 ]2 and

log [K+1 ]/[H+1] at 20°C. ..................................................

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in
IV CONTENTS

TABLES

Page

TABLE 1. Variation in chemical constituents explained by variations between hydrochemical facies. ............... B8

2. Geometric mean, minimum, and maximum of chemical concentrations for each of the hydrochemical facies, Sacramento

Valley. ............................................................ 9

3. Chemical composition of water from wells of various depths, section 32 of township 19 N., range 1 W. ...... 10

4. Data for wells and springs plotted in figures 4 and 5. .................................. 12

5. Analyses of variance and coefficients for the regression of average yearly dissolved solids with time for the six

hydrochemical facies in the Sacramento Valley. ..................................... 16

6. Wells showing significant trends in dissolved-solids concentration. ............................ 17

7. Analyses of variance and coefficients for the regression of average yearly nitrate-nitrogen concentrations with time

for the six hydrochemical facies in the Sacramento Valley. .............................. 19

8. Wells showing significant trends in nitrate-nitrogen concentration. ............................ 20

9. Chemical variables included in the principal components analysis of the Sacramento Valley. ............. 22

10. Results of principal components analysis on chemical data from the Sacramento Valley. ............... 23

11. Average chemical compositions of surface water entering the Sacramento Valley. ................... 25

12. Dissolved-oxygen concentrations in well water in the south alluvial fans facies. .................... 27

13. Mineralogic compositions of sidewall samples from U.S. Geological Survey test well 12N/01E-34Q. ......... 27

14. Minerals identified in sidewall samples from U.S. Geological Survey test well 12N/0.1E-34Q. ............ 28

15. Percent mineral composition of sidewall samples from U.S. Geological Survey test well 19N/01W-32G. ....... 28

16. Mean, minimum, and maximum saturation indices in Sacramento Valley ground water for selected mineral phases. . 30

CONVERSION FACTORS

Factors for converting inch-pound units to the International System (SI) of units are given below: 

Multiply By To obtain

acres 0.004047 km2 (square kilometers)

acre-ft (acre-feet) 0.001233 hm3 (cubic hectometers)

ft (feet) 0.3048 m (meters)

mi (miles) 1.609 km (kilometers)

mi2 (square miles) 2.590 km2 (square kilometers)

Degrees Fahrenheit are converted to degrees Celsius by using the formula: °C=(°F-32)/1.8 

Additional abbreviations:

(micrograms per liter) 

mm (millimeters) 

mg/L (milligrams per liter) 

(mg/L)/yr (milligrams per liter per year)

National Geodetic Vertical Datwm of 1929 (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 mean sea level. NGVD of 1929 is sometimes referred to as sea level in this report.
CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

GEOCHEMISTRY OF GROUND WATER IN THE 

SACRAMENTO VALLEY, CALIFORNIA

By LAURENCE C. HULL

ABSTRACT

A geochemical investigation was done to evaluate temporal changes 

in water quality and to determine the processes that control water 

chemistry in shallow ground water of the Sacramento Valley, California. A total of 671 chemical analyses from 651 wells were used to 

study the processes controlling water chemistry. Temporal variations 

in dissolved-solids and nitrate concentrations were studied in 140 and 

62 wells, respectively, having multiple observations with time.

The Sacramento Valley can be divided into six hydrochemical facies 

having distinctive chemical compositions. Waters from the Tuscan volcanic rocks and Victor Plain facies, along the eastern margin, have 

lower concentrations of dissolved solids and higher concentrations of 

silica than the waters from other areas of the Sacramento Valley. 

Ground waters in the basin hydrochemical facies (Butte Basin and 

Sutter Basin) along the valley axis are higher in iron, manganese, 

arsenic, and potassium. West side facies, the north alluvial fans and 

south alluvial fans, are low in silica. The south alluvial fans facies 

is higher in dissolved solids and has very high boron concentrations.

Significant increases have occurred in dissolved-solids and nitrate 

concentrations since the mid-1950's. Dissolved solids increased in five 

of the six hydrochemical facies, with rates ranging from 0.95 milligrams per liter per year in the Tuscan volcanic rocks facies to 4.75 

milligrams per liter per year in the south alluvial fans facies. Nitratenitrogen did not significantly increase in the Tuscan volcanic rocks, 

Butte Basin, or Sutter Basin hydrochemical facies. Rates of increase 

of 0.036, 0.036, and 0.099 milligrams per liter per year occurred in 

the Victor Plain, south alluvial fans, and north alluvial fans, respectively.

Two processes control over 50 percent of the spatial variation in 

ground-water chemistry. Thirty-six percent of the variation in the 

data is explained by the effects of recharge water chemistry on the 

chemistry of ground water. Low dissolved solids and high silica from 

east-side recharge produces fairly uniform conditions in east-side 

ground waters. Recharge from the northwest is intermediate in dissolved solids and silica. Southwest recharge is high in dissolved solids, 

boron, and fluoride. High boron concentrations in the ground water 

originate in thermal springs in the Coast Ranges. A second important 

process consists of reactions occurring in the fine-grained sediments 

of the flood basin in the central part of the Sacramento Valley. Reducing conditions here produce higher concentrations of manganese, iron, 

and arsenic. Nitrate concentrations are low, probably reflecting denitrification reactions.

Precipitation and dissolution of aluminosilicates appear to control 

the relation between silica and aluminum in parts of the valley. Silica 

and aluminum in ground waters of the Sutter Basin "hydrochemical 

facies show parallel behavior and have a ratio of 3 to 1. In the Tuscan 

volcanic rocks facies silica and aluminum are inversely related and 

are controlled by the solubility of halloysite. The clay minerals halloysite and illite were identified in the sediments. The clay mineral 

predicted to be the most stable in equilibrium with valley ground

water is montmorillonite. Halloysite represents an intermediate step 

in the weathering of primary minerals to more stable secondary 

phases.

INTRODUCTION

GEOGRAPHY

The Sacramento Valley comprises the northern third 

of the Central Valley of California in the north-central 

part of the State (fig. 1). It is a sedimentary basin 

bounded by mountains to the east (Sierra Nevada), 

northeast (Cascade Range), and west (Coast Ranges). 

To the south, the valley merges with the delta of the 

Sacramento and San Joaquin Rivers. The area lies between 38°30' and 40°15 ; N. latitude, and 121°15' and 

122°30' W. longitude. The valley covers about 4,400 

square miles, is 150 miles in length from Red Bluff to 

the delta, and ranges in width from about 8 miles at 

Red Bluff to about 50 miles near the southern end. The 

topography consists of generally flat plains, with low 

dissected hills near the margins. The elevation of the 

valley at the southern end is near sea level, increasing 

northward and reaching 300 feet above sea level near 

Red Bluff. The relatively flat topography of the valley 

is interrupted near the center, where a volcanic neck 

of late Tertiary age, Sutter Buttes, rises 2,100 feet 

above the valley floor.

The Sacramento River enters the valley from the 

north and flows along the entire length of the valley 

before entering the delta region to the south. The 

Feather River is the only other principal stream that 

flows through the valley. The Sacramento and Feather 

Rivers are fed by tributaries draining the surrounding 

mountain areas. From the Sierra Nevada come the 

American, Bear, Yuba, and Feather Rivers. The Cascade Range is drained primarily by Big Chico, Butte, 

Mill, and Deer Creeks in the area adjacent to the Sacramento Valley. The streams entering the valley from 

the west rise in the Coast Ranges and are much smaller 

than the streams from the east. Thomes, Stony, Cache, 

and Putah Creeks are the only sizable streams entering

Bi
B2 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

EXPLANATION

Central Valley Boundary

Sacramento Valley 

Sacramento-San Joaquin Delta

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FIGURE 1. Location map showing Sacramento Valley of California.

the west side of the valley. (See pi. IB for stream locations.)

Little precipitation falls in the valley during the summer growing season, and irrigation is necessary to support extensive agricultural development. Early irrigated farming began in the mid-1800's as the gold miners moving to California created a demand for 

foodstuffs. Development of irrigation was slow, however, until the drought of 1864 stimulated the construction of new irrigation projects. During both world wars, 

heavy demand for agricultural products spurred periods 

of rapid development. Ground water has always lagged 

behind surface water as a source of irrigation water, 

amounting to only about 10 percent of the total use 

in the late 1910's. Since the late 1950's, however, the 

use of ground water has been increasing, and in 1970,
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B3

it accounted for about 40 percent of all irrigation water 

used in the Sacramento Valley.

PURPOSE AND SCOPE

The U.S. Geological Survey began a series of hydrologic investigations the regional aquifer systems analysis (RASA) program to aid effective management of 

the Nation's ground-water resources by providing information on the hydrology and geochemistry of regional 

aquifer systems (Bennett, 1979). The Central Valley 

aquifer project, covering the Central Valley of California, is one of 29 such projects throughout the country 

and was started because of the long history of groundwater development and the importance of the area's 

agricultural production to the national economy (Bertoldi, 1979). Specific water-quality information needed 

for effective management in the future includes (1) the 

general inorganic chemical character of water from 

aquifers currently in use, (2) delineation of areas where 

degradation is occurring and where degradation may 

occur in the future, (3) the changes that have occurred 

in ground-water quality with time, and (4) the processes 

that control ground-water chemistry. This report 

evaluates temporal changes and geochemical processes 

as they influence the chemical quality of shallow ground 

water in the Sacramento Valley, the northern third of 

California's Central Valley. A companion report (Fogelman, 1983) deals with spatial variations in the chemical 

character of ground water and the delineation of areas 

of potential water-quality problems for this same study 

region.

DATA COMPILATION

Collection of additional water-quality samples was unnecessary because of the extensive data base of the 

U.S. Geological Survey and California Department of 

Water Resources. Chemical analyses from 653 wells 

throughout the Sacramento Valley sampled by the 

Geological Survey between 1974 and 1976 were already 

in the Survey's water-quality file (Fogelman, 1975, 

1976; Fogelman and Rockwell, 1977). From the unpublished files of the California Department of Water Resources, an additional 2,136 analyses from 281 wells 

were added to the water-quality file. During the summer of 1979, 63 additional samples were collected by 

the Geological Survey in Yolo County to extend coverage into an area where high boron concentrations 

suggested a need for water-quality evaluation. All wells 

from which chemical analyses were used in this study 

have driller's logs on file with the California Department of Water Resources. Thus, reasonably reliable information is available about depths and perforated intervals for these wells. Some of the samples represented multiple observations over time from single 

wells, and these were used for the temporal studies. 

Samples for the period 1974-79 were examined, and 

complete analyses with less than 5 percent error in the 

electrical balance between cations and anions were used 

to study the spatial variation in the valley. These latter 

data consisted of 671 analyses from 651 wells.

Manipulation of data files and the statistical analyses 

used in this report were accomplished using the Statistical Analysis System (SAS) 1 programs on the U.S. 

Geological Survey's Reston computer system.

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; the range number, east or 

west; and the section number. A section is further divided into sixteen 40-acre tracts lettered consecutively 

(except I and 0), beginning with A in the northeast 

corner of the section and progressing in a sinusoidal 

manner to R in the southeast corner. Within the 40-acre 

tract, wells are sequentially numbered in the order they 

are inventoried. The final letter refers to the base line 

and meridian relative to which the townships and 

ranges are numbered. For the Sacramento Valley, this 

is the Mount Diablo base line and meridian (M). Figure 

2 shows how the well number 11N/02E-23N01M is derived.

ACKNOWLEDGMENTS

The California Department of Water Resources has 

cooperated with the U.S. Geological Survey in past 

studies during which much of the data analyzed in this 

report were collected. The Department also provided 

additional unpublished chemical-quality data from their 

files. The author would like to thank J. R. Slack of 

the Geological Survey for his helpful suggestions on 

trend analysis. The cooperation of the residents of the 

Central Valley who have permitted the Geological Survey to collect water samples from their wells is also 

appreciated.

DESCRIPTION OF THE STUDY AREA

GEOLOGY AND GEOMORPHOLOGY

The Geology and geomorphology of the Sacramento 

Valley have been covered in detail by Olmsted and 

Davis (1961), the California Division of Mines and Geol-

'Use of trade names does not imply endorsement and is used for information only.
B4 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

SECTION 23

11N/2E-23N01MR. 2 E.

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FIGURE 2. Well-numbering system.

ogy (1966), and the California Department of Water Resources (1978a). Detailed maps and complete descriptions of the geology can be found in these publications. 

In this section, the geology and geomorphology are only 

briefly reviewed, with particular attention given to 

water-bearing characteristics and importance for water 

quality.

GEOLOGIC SYNOPSIS

The Sacramento Valley is an elongated structural 

trough trending to the northwest through central 

California. The trough is filled with a thick accumulation of sediments, perhaps as much as 50,000 feet in 

the deepest parts. Deposition has been occurring since 

early in the Cretaceous Period, with the bulk of the 

deposition occurring in a marine environment. A 

maximum of only 3,000 feet of continental deposits has 

been laid down as a veneer over the marine sediments. 

During early Tertiary time, the Cretaceous sediments

west of the valley were folded and uplifted, forming 

the Coast Ranges and isolating the Sacramento Valley 

from the ocean to the west.

Early- and middle-Tertiary sedimentation was 

primarily confined to the southern and eastern parts 

of the valley, with only those sediments adjacent to 

the Sierra Nevada being continental deposits. During 

late Tertiary time, continental deposits were laid down 

throughout the valley, and deposition continued into 

Quaternary time. These continental deposits include the 

Tehama, Tuscan, Laguna, and Mehrten Formations and 

form some of the most important aquifers in the valley. 

Also, near the end of the Tertiary Period, andesitic lava 

was extruded through the older marine sediments to 

form Sutter Buttes, a prominent volcanic neck in the 

middle of the Sacramento Valley.

During Pleistocene time, continental deposition continued throughout the valley. Alluvial fans and plains 

developed from sediments eroded from the Sierra 

Nevada to the east and the Coast Ranges to the west. 

Continued action of rivers flowing through the valley 

produced alluvial channel deposits and natural levees 

along the axis of the valley and produced flood basins 

adjacent to the rivers.

GEOMORPHIC UNITS

The Sacramento Valley is generally characterized by 

alluvial plains and fans bounded by rugged mountains. 

The geomorphic units described by Olmsted and Davis 

(1961) are (1) Sierra Nevada, (2) Coast Ranges, (3) Cascade Range, (4) low hills and dissected uplands, (5) alluvial plains and fans, (6) flood basins, (7) stream channels 

and natural levees, and (8) Sutter Buttes. In this section, the location and extent of the geomorphic units 

are described, and the geologic formations composing 

each unit discussed. Attention will be given to the 

lithology, mineralogy, and water-bearing characteristics 

of the geologic units where information is available. The 

location of the geomorphic units are illustrated on plate 

IA.

(1) Sierra Nevada. The Sacramento Valley is bounded 

on the east by the mountainous Sierra Nevada, a massive block of the Earth's crust that has been uplifted 

and tilted westward. The block extends under the sediments of the valley and underlies the eastern part at 

depth. The Sierra Nevada is composed of intrusive 

igneous rocks, metamorphosed volcanic rocks, and Paleozoic sedimentary rocks. The intrusive rocks are 

mainly granodiorite and quartz diorite with andesitic 

and rhyolitic extrusive rocks. In addition to the feldspars, quartz, and biotite of the igneous rocks, a wide 

variety of other minerals is present. These include cal-
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B5

cite, magnesite, barite, serpentine, metal sulfides, and 

others. The massive rocks of the Sierra Nevada contain 

little water, with the little that is available occurring 

in joints and fractures. The water quality of streams 

draining the Sierra Nevada is generally very good.

(2) Coast Ranges. The Coast Ranges, adjacent to the 

west of the valley, consist of Cretaceous marine sedimentary rocks folded and uplifted into ridges trending 

northwest-southeast, parallel to the valley. The Cretaceous rocks dip eastward and underlie much of the Sacramento Valley at depth. Also present in the Coast 

Ranges is the Franciscan Complex, which consists of 

highly metamorphosed marine sedimentary rocks of late 

Mesozoic age. The major rock type is this geologic unit 

is greywacke, interbedded with shales and minor limestone units. The primary minerals of the sedimentary 

rocks are plagioclase and quartz. Many lithic fragments 

present contain chert, chlorite, and mica. Ultramafic 

minerals have been metamorphosed to serpentine and 

magnesite. The marine sedimentary rocks are mostly 

consolidated and contain little water. Some units retain 

connate saline water, and streams draining the Coast 

Ranges may be contaminated by saline springs.

(3) Cascade Range. Between Oroville and Red Bluff, 

the northeastern part of the Sacramento Valley is 

bounded by the Cascade Range geomorphic unit. Adjacent to the valley, this unit is composed of the gently 

southwestward dipping Tuscan Formation of Pliocene 

age. On the surface, the Tuscan Formation is extensively eroded, and southwestward flowing streams have 

incised deep canyons. Beneath the valley, the Tuscan 

extends westward under younger valley sediments as 

far as the Sacramento River, and there the Tuscan interfingers with the Tehama Formation. The Tuscan is 

composed of volcanic breccia, coarse- to fine-grained 

tuffs, and volcanic sands, all of primarily andesitic composition. The most abundant mineral in the black sands 

found in wells drilled into this unit is plagioclase, and 

accessory quartz and pyroxenes. Volcanic glass is also 

abundant from the mudflow and tuff deposits. The Tuscan is an important aquifer in the northeastern part 

of the valley, and it contains fresh water at all depths.

(4a) Low hills and dissected uplands, east side. The 

dissected alluvial uplands leading up to the Sierra 

Nevada on the east side of the valley are composed 

of Tertiary and lower Quaternary sediments. This 

gently rolling terrain merges with the foothills of the 

Sierra Nevada to the east and the low alluvial plains 

to the west. The oldest rocks here are of the Eocene 

lone Formation, overlain by the Oligocene, Miocene, 

and Pliocene volcanic deposits of the Valley Springs and 

Mehrten Formations. These units extend westward to 

the trough of the valley, but they are generally too

deep to be tapped as aquifers except in the foothills 

area. Overlying the volcanic sediments are upper Tertiary and Quarternary continental deposits eroded from 

the Sierra Nevada. The older Tertiary volcanic sediments are composites of volcanic sands, ash and 

mudflows, and tuff layers of rhyolitic and andesitic composition. Particular minerals noted in these units include biotite, hornblende, orthoclase, plagioclase, and 

quartz. The lone Formation contains kaolinitic clay 

layers. The younger deposits, the Laguna Formation 

and Fair Oaks Formation of Shlemon (1967), are sand, 

gravel, and silt deposits intercalated by clay layers. The 

Quaternary deposits are poorly sorted, and wells 

finished in these units produce only moderate quantities 

of water.

(4b) Low hills and dissected uplands, northeast. The 

low hills on the northeast margin of the valley consist 

of reworked sediments eroded from the underlying Tuscan Formation. The hills show the characteristic form 

of alluvial fans but are deeply trenched by the streams 

draining the Cascade Range. These fanglomerate deposits, as they are called, are mainly cemented sands, 

gravels, and silts, and have a mineral composition similar to that of the Tuscan Formation from which they 

are derived. The fanglomerate is also an important 

source of water in this area.

(4c) Low hills and dissected uplands, west side. The 

low hills on the west side of the valley are composed 

of the Tehama Formation of Pliocene age. These deposits extend from Red Bluff on the north almost as 

far south as Fairfield. The Tehama is made up of poorly 

sorted fluviatile sediments, primarily beds of sandy silt 

and silty clay with some lenses of crossbedded sands 

and gravels. Many of the coarser layers are cemented 

by calcium carbonate that reduces their permeability. 

The Tehama extends eastward to about the middle of 

the valley and is generally within 150 to 200 feet of 

land surface. Tectonic activity since deposition has 

folded the Tehama into a number of structures such 

as the Dunnigan Hills, Corning Ridge, and Plainfield 

Ridge along the valley margin. Sediments originated 

in the Coast Ranges and are rich in plagioclase and 

magnesium silicates, such as chlorite and serpentine. 

The Tehama is an important potential aquifer, although 

currently most water is the area is being pumped from 

more permeable alluvial fan deposits nearer the surface.

(5a) Alluvial plains and fans, east side. The alluvial 

plain on the east side of the valley is referred to as 

the Victor Plain by Olmsted and Davis (1961). The Victor Formation2 of Olmsted and Davis (1961), which

2Recent work of Marchand and Allwardt (1981, p. 207) virtually eliminates the use of 

the Victor Formation for the northeastern part of the San Joaquin Valley. However, because 

no similar recent work has been done in the Sacramento Valley, the Victor Formation 

as defined and mapped by Olmsted and Davis (1961, p. 93) is used in this report.
B6 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

ranges in thickness from 50 to 100 feet, covers the surface throughout the area and overlies the Laguna Formation, a Pleistocene continental deposit. The plain extends from the northern San Joaquin Valley to north 

of Chico, where it pinches out between the fanglomerate of the Cascade Range and the Sacramento River 

flood plain. The Victor Formation is composed of alluvial sediments carried down from the Sierra Nevada 

and contains numerous channel deposits. The sediments 

become generally finer grained towards the west with 

bluish- and greenish-gray clays indicating reducing deposition environments near the center of the valley. Opposite the Sierra Nevada, the material contains extensively weathered feldspars, quartz, mica, biotite, and 

layers of arkosic sandstones. Opposite the Cascade 

Range, the Victor Formation is composed of reworked 

volcanic detritus from the Tuscan Formation. Water is 

generally available throughout the Victor Formation, 

though many high-capacity irrigation wells also tap the 

underlying Laguna Formation.

(5b) Alluvial plains and fans, west side. The low alluvial fans on the west side of the valley have been laid 

down by streams draining the Coast Ranges. The fans 

are aggrading and have poorly developed soil horizons. 

The northernmost fan is that of Stony Creek, which 

extends from the valley margin to the Sacramento 

River. South of this fan, a series of small fans built 

up by ephemeral streams extends only a short distance 

away from the Coast Ranges. Cache Creek and Putah 

Creek, south of the Dunnigan Hills, also have developed 

rather large fans which coalesce to form Putah Plain. 

The lithology of these fans is varied but generally is 

coarse-grained and poorly sorted. The mineralogy reflects the source areas in the Coast Ranges. The alluvial 

fans are generally good aquifers, especially where old 

stream channel deposits are penetrated.

(6) Flood basins. Low areas adjacent to the Sacramento River were subject to frequent flooding before 

extensive construction of levees in modern times. In 

these basins, fine-grained sediments have settled out 

of flood waters to form poorly permeable deposits. Deposits between 50 and 170 feet thick of yellow clay, 

indicating oxidizing conditions, overlie blue clays reflecting reducing conditions at depth. Few wells are 

drilled in these areas, and the only water generally 

available is in tongues of sand and gravel from old channel deposits.

In Sutter Basin, ground water contains high chloride 

concentrations and is unsuitable for irrigation purposes. 

The origin of this high chloride water is thought to be 

from Cretaceous marine sedimentary rocks brought 

closer to the surface by faulting associated with the 

formation of Sutter Buttes.

(7) Stream channels and natural levees. Along the 

major rivers and streams throughout the valley, Holocene alluvial deposits form flood plains and natural 

levees. The levees slope gently away from the channels 

and merge into the surrounding flood basin deposits. 

The deposits are generally coarse grained and poorly 

sorted. The varied mineralogy reflects the many 

sources from which these sediments are derived. Wells 

drilled in these deposits are hydraulically connected to 

the adjacent rivers and can produce large quantities 

of water.

(8) Sutter Buttes. Sutter Buttes, a volcanic neck of 

late Tertiary age, rises abruptly 2,100 feet above the 

surrounding plains. The core of the buttes is andesite, 

surrounded by alluvial fans created from the erosion 

of the igneous rocks. The deposits are poorly sorted 

and contain tuff, volcanic breccia, and sands. Some Cretaceous marine sedimentary rocks exposed at the buttes 

were forced upwards by the rising lava. The volcanic 

rocks yield little water, but the adjoining alluvium produces moderate amounts.

HYDROGEOLOGY

The direction of ground-water movement can be determined from the configuration of the water table 

where ground water occurs in saturated sediments and 

is not confined. Ground-water movement is influenced 

by gravity, and so is downhill as determined by the 

altitude of the water table. Plate IB is a map showing 

lines of equal altitude of the water table for the Sacramento Valley in the spring of 1975. Although extensive changes occurred in some areas during the drought 

of 1976 and 1977, the 1975 map reflects the general 

conditions throughout the valley as they existed when 

most of the water-quality samples were collected during 

the mid-1970's.

In the north part of the valley, ground waters flow 

to the south and away from the valley walls. The Sacramento River is a gaining stream north of Colusa and 

is a major ground-water discharge zone in the north 

part of the valley. Stony Creek alluvial fan is a particularly important area for recharge. South of Colusa, the 

Sacramento River becomes a losing stream, contributing water to the adjacent sediments. In the west-central part of the valley, between Colusa and Woodland, 

recharge from the west infiltrates the valley sediments 

and moves toward pumping depressions east of the 

Dunnigan Hills. In the east-central part of the valley, 

from Oroville to Marysville, the Feather, Yuba, and 

Bear Rivers recharge the ground water near the valley 

margin. Once on the valley floor, the Feather River 

becomes a gaining stream east of Sutter Buttes, but 

below Marysville it begins to lose water to pumping 

depressions to the east and west. A large pumping depression in the southeast draws water from the Ameri-
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B7

can and Sacramento Rivers in the vicinity of Sacramento. In the southwest, much ground-water recharge occurs in the Cache and Putah Creek alluvial 

fans. This water moves eastward toward pumping depressions near Woodland and Da vis. Some recharge 

from Cache Creek flows around the end of the Dunnigan Hills to pumping depressions north of Woodland.

The water table in the Sacramento Valley is generally 

close to the surface, and in only a few places is it more 

than 80 feet below the surface. Depth to water in the 

flood basins and alluvium along the Sacramento and 

Feather Rivers ranges from 10 to 20 feet. This very 

shallow water table extends eastward to include much 

of the western part of the Victor Plain. In the alluvial 

fans along the west side of the valley, the depth to 

water is between 10 and 40 feet. Lowering of water 

levels in pumping depressions north of Woodland and 

south of Davis, however, has increased the depth to 

water in those places. Along the southeast margin of 

the valley, the depth to water increases from less than 

10 feet, near the flood basins, to more than 80 feet 

in the foothills. In the fanglomerate of the Cascade 

Range around Chico, the depth to water is generally 

10 to 40 feet.

The valley boundary illustrated on plate IB is the 

approximate boundary of the principal alluvial aquifers 

currently used in the Sacramento Valley. There is no 

single definition of the Sacramento Valley, and different 

boundaries are drawn for different purposes. The 

boundary shown is approximately that used in a 

ground-water-flow model of the Sacramento Valley 

(California Department of Water Resources, 1978a, Appendix A). However, the definition of the Sacramento 

Valley ground-water basin by the State of California 

encompasses a much larger area. The boundary is a 

reference shape to show continuity among illustrations.

On the east side of the valley, the margin coincides 

with the contact between the alluvium of the low hills 

and dissected uplands geomorphic province, and the 

older rocks of the Sierra Nevada and Cascade Range 

provinces. On the west side, the division is at the contact between the younger alluvium of the alluvial fans 

and plains, and the older alluvium of the low hills and 

dissected uplands geomorphic province. This allows delineation of the Dunnigan Hills, and excludes a large, 

undeveloped and sparsely populated area to the northwest of the valley.

Some wells outside the principal alluvial aquifers 

were included in the data analysis, as the boundary of 

those aquifers is somewhat arbitrary. Because these 

wells lie in recharge zones, their characteristics are important to an understanding of ground-water chemistry 

in the rest of the valley, even though they are outside 

the area of the principal alluvial aquifers.

HYDROCHEMICAL FACIES

The concept of hydrochemical facies as described by 

Back (1966) is used to designate zones or areas in which 

ground waters share unique chemical characteristics. 

The differences between facies reflect different controls 

on water chemistry from the lithology and mineralogy 

of, flow patterns in, and sources of recharge to an 

aquifer. The prefix "hydro" is used as a modifier of 

"chemical" to emphasize the interdependence of chemistry and hydrology. The ground water in the Sacramento 

Valley shows a great diversity in chemical compositions. By dividing the valley into hydrochemical facies, 

the differences can be better related to differences in 

the sedimentary matrix holding the water and the 

sources of ground-water recharge.

CRITERIA AND PROCEDURE FOR DIVISION

While hydrochemical facies have commonly been defined in terms of water types (as determined by the 

predominance of ions in terms of equivalents per million; Back, 1966), the definition of hydrochemical facies 

can be based on any unique chemical signature. For 

the Sacramento Valley, this division was accomplished 

using differences in the concentrations of some of the 

dissolved chemical constituents.

An initial division of the valley was attempted by 

studying maps showing the geomorphic units, the water 

table, and the distribution of individual chemical constituents. Outlines of these areas were drawn up to the 

nearest quarter township, and a tentative hydrochemical facies was determined for each well. This initial division broke the valley into three areas extending northwest-southeast, parallel to the axes of the valley along 

the west side, the east side, and in the central basins. 

Each of these areas was divided into a northern and 

southern part resulting in six tentative divisions.

After this initial division, these six areas were tested 

to determine if they could be successfully differentiated 

on the basis of their chemistries. Linear discriminant 

function analysis (Davis, 1973; Drake and Harmon, 

1973) was used for this purpose. In linear discriminant 

function analysis, a linear combination of variables is 

used to distinguish between previously defined groups. 

After the function is defined, it can be used to classify 

new samples of unknown origin into one of the original 

groups.

To determine which chemical constituents would be 

the best to use in the discriminant analysis, the amount 

of variation in the major chemical species associated 

with the differences between hydrochemical facies was 

determined. The results of this analysis of variance are 

shown in table 1. The first five species in this table
B8 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

TABLE 1. Variation in chemical constituents 

explained by variations between hydrochemical 

fades

Constituent

Silica.......................

Sodium ....................

Magnesium ...............

Calcium....................

Sulfate.....................

Percent variation 

explained

......... 62.7

......... 48.5

......... 44.7

......... 39.4

......... 36.8

......... 30.1

.......... 27.2

.......... 15.1

.......... 9.6

were picked for the analysis. Because chloride is closely 

associated with sodium in the valley, it was passed over 

and magnesium added to give a total of six chemical 

constituents to use in the discriminant analysis.

Of the 671 chemical analyses available, only 340 had 

measurements for all the species and could therefore 

be used in the analysis. Most of the reduction in sample 

size was due to the much smaller number (358) of silica 

analyses. However, because silica showed by far the 

greatest amount of variation between the tentative hydrochemical facies, it was decided to use silica at the 

cost of reducing the number of samples that could be 

classified using the discriminant function. Samples, 

which lack measurements of all six of the chemical 

species used, were subsequently classified on the basis 

of nearby wells.

About one-fifth of the eligible samples were used to 

define the discriminant functions for the six hydrochemical facies. Using these functions, the remaining eligible 

samples were classified using the chemistry, and this 

classification was compared to the prior classification 

from the map data. Misclassified observations were of 

two types. Type 1 misclassifications were scattered 

throughout other facies and showed no pattern nor 

were particularly close to the boundary between two 

areas.Type 2 misclassifications were clustered in an adjacent hydrochemical facies near the boundary. In the 

latter case, the boundaries were redrawn and the chemical analyses reclassified.

The discriminant functions were recalculated several 

times with adjustments in boundaries between each calculation. The final analysis showed that 76 percent of 

the samples could be correctly classified by hydrochemical unit using the six chemical species. Thirteen percent 

of the samples were misclassified into the adjacent 

northern or southern portion in the same part of the 

valley, indicating an association between west side, east 

side, and basin pairs. On the basis of the classification 

of wells in a quarter township, each quarter township 

was assigned to its final hydrochemical facies. Other 

wells missing some of the chemical species used for the

classification were classified by their quarter township 

location.

The discriminant analysis showed that enough chemical variation existed among the six hydrochemical facies 

to consider them chemically distinct. The final boundaries of the hydrochemical facies are shown on plate 

IB. On the east side of the valley, one of the facies 

lies opposite the Cascade Range, and one opposite the 

Sierra Nevada. The flood basins along the axis of the 

valley are divided into two more facies, one in Butte 

Basin and including Sutter Buttes, the second in the 

Colusa, Sutter, and American Basins. The final two hydrochemical facies are opposite the Coast Ranges and 

are divided north and south near Colusa. In the next 

section, the chemical characteristics of the six hydrochemical facies are presented and compared, and differences in chemistry are discussed.

DESCRIPTIONS OF HYDROCHEMICAL FACIES

Table 2 shows the geometric mean, minimum, and 

maximum of chemical concentrations for each of the hydrochemical facies. To determine if the average concentrations of chemical species in the hydrochemical facies 

are significantly different, the differences between 

geometric means were tested using a t test. The results 

of these comparisons are illustrated quantitatively in 

the same table.

TUSCAN VOLCANIC ROCKS

The Tuscan volcanic rocks hydrochemical facies is 

along the northeastern margin of the Sacramento Valley (pi. IB) adjacent to the Cascade Range. The 

geologic formations in this area are the Tuscan Formation and more recent alluvial sediments derived from 

erosion of the Tuscan. Recharge is from the Cascade 

Range geomorphic province, and ground water is either 

discharged to the Sacramento River or moves into 

Butte Basin south of Chico. The Tuscan and its derivatives contain andesitic sands and extensive tuff deposits. Volcanic glass and plagioclase are abundant in 

these sediments.

The chemistry of ground water in the area reflects 

the low concentrations of dissolved solids carried by recharge from the Cascade Range, having low mean concentrations of magnesium, sodium, bicarbonate, sulfate, 

and chloride. Silica concentrations are high due to solution of volcanic glass, and the facies has the highest 

average nitrate-nitrogen concentration of the six. The 

maximum boron concentration is rather high, although 

the mean concentration is only moderate. Boron in excess of 1.0 mg/L occurs in only three wells, all to the 

east of the Sacramento River northeast of Red Bluff.
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B9

TABLE 2. Geometric mean, minimum, and maximum of chemical concentrations for each of the hydrochemical fades, Sacramento Valley

[Figure above line refers to geometric mean; figures below line refer to minimum-maximum range. H, high; L, low; VH, very high; VL, very low. NOTE. These comparisons are 

relative to the other hydrochemical facies and do not necessarily describe the absolute values. Labels were assigned only where the difference between two geometric means 

tested significant at the 95 percent level]

Chemical species

Dissolved solids 

(mg/L)

Calcium 

(mg/L)

Magnesium 

(mg/L)

Sodium 

(mg/L)

Potasium 

(mg/L)

Bicarbonate 

(mg/L)

Sulfate 

(mg/L)

Chloride 

(mg/L)

Fluoride 

(mg/L)

Nitrate - N 

(mg/L)

Phosphate - P 

(mg/L)

Silica 

(mg/L)

Iron 

(M«/L)

Manganese 

(n.g/L)

Arsenic 

(M-g/L)

Boron 

(mg/L)

Tuscan 

volcanic rocks

L 231

137-671 

26

14-75 

L 19

9.1-60 

L 14

3.9-68 

1.3

0.3-5.9 

L 170

98^400 

L 8.1

0.0-71 t<i

VL 6.5

0.9-97 

0.11

0.1-0.3 

H 2.7

0.2-27 

0.05

0.0-0.31

H 51 

35-67

4.8

0-170 

3.4

0-10

1,2 

0-4

0.066 

0-1.50

Victor 

Plain

L 220

137-1100 

L 21

5.7-170 

VL 12

3.6-120 

19

7.2-180 

1.3

0.4-4.5 

VL 130

68-504 

L 8.0

0.2-140 

15

2.0-380 

0.18

0.1-10.0

0.81 

0-61

0.11

0.01-0.39

H 55 

25-74

5.2

0-600 

5.5

0-280

2.2 

0-20

L 0.023 

0-0.60

Butte 

Basin

' 290

129-723 

32

5.5-98 

24

5.0-93 

20

6.3-93 

H 1.9

0.6-5.0 

230

44-522 

12

0.5-150 

L 11

1.0-210 

0.12

0.1-0.3 

1.0

0-20 

0.11

0.01-0.45

H 51 

31-75

8.0

0-1900 

9.1

0-1700

H 5.5 

0-50

L 0.034 

0-0.37

Sutler 

Basin

H 400

97-2040 

32

6.9-220 

24

2.2-160 

H 55

5.9-360 

H 1.7

0.3-5.4 

H 270

70-897 

17

0.3-590 

H 31

2.4-920 

0.11

0.0-0.4

L 0.12 

0-14

0.15

0.01-0.50

39 

23-73

H 11

0-520 

H 75

0-2300

H 5.8 

0-100

H 0.140 

0-1.10

North alluvial 

fans

280

108-1170 

H 39

8.2-99 

24

7.6-70 

22

7.5-240 

L 0.8

0.4-2.1 

240

83-560 

18

1.6-400 

15

2.7-170 

0.16

0.0-0.6 

1.7

0.01-11 

0.06

0.02-0.30

L 26

8.5-52

9.7

0-280 

8.0

0-70

1.7 

0-6

0.068 

0-0.58

South alluvial 

fans

H 400

175-1420

H 39 

13-140

H 33

3.5-140 

H 52

15-320 

1.2

0.2-7.0 

VH 310

130-1060 

21

1.8-180 

H 37

2.2-580 

H 0.29

0.1-1.0 

1.8

0-12 

0.07

0.01-0.42

L 28 

12-54

8.0

0-180 

2.8

0-2000

2.3 

0-31

VH 0.400 

0-8.10

The source of the high boron water is Tuscan Spring, 

located along Salt Creek in Tehama County in section 

32 of township 28 N., range 2 W. (Waring, 1965).

VICTOR PLAIN

The Victor Plain hydrochemical facies is along the 

east and southeast margin of the valley, and extends 

westward to near the Feather and Sacramento Rivers. 

Recharge is primarily from the Sierra Nevada to the 

east, although pumping depressions southeast of Marysville and north of Sacramento induce recharge from the

rivers. North of Marysville, water is discharged into 

the Feather River or passed into Butte Basin. South 

of Marysville, most of the ground-water discharge 

seems to be by pumping. Along the valley margin, the 

aquifers are volcanic sands with associated tuffaceous 

deposits. In the central and western parts of this facies, 

the volcanic units dip under the Victor Formation composed of alluvium from the Sierra Nevada.

The Victor Plain facies has a low average dissolvedsolids content reflecting recharge from the Sierra 

Nevada. Concentrations of calcium, magnesium, bicarbonate, boron, and sulfate are all low in this area. Silica
BIO CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

is high, reflecting the volcanic material present in the 

sediments.

BUTTE BASIN

The Butte Basin hydrochemical facies roughly coincides with the Butte flood basin east of the Sacramento 

River. The basin extends from Chico on the north to 

Yuba City on the south and lies between the Feather 

and Sacramento Rivers. Sutter Buttes is included in 

this facies as too few chemical analyses were available 

from this area to justify a separate facies. The basin 

receives recharge from the Tuscan volcanic rocks facies 

and from Victor Plain to the east. A small amount of 

recharge also occurs in the vicinity of Sutter Buttes. 

Ground water leaves the Butte Basin facies to enter 

the Sacramento River and to enter the Sutter Basin 

facies east of Sutter Buttes. In the flood basins the 

sediments are fine grained and reducing.

Ground water in the Butte Basin facies has a somewhat higher average dissolved-solids concentration than 

the two eastern margin facies possibly reflecting longer 

subsurface residence times or a change in sediment 

lithology. Chloride and boron are the only two species 

that have low average concentrations in this facies. The 

high silica concentrations may be a result of recharge 

from the Tuscan volcanic rocks facies and from volcanic 

material around Sutter Buttes. High arsenic concentrations imply deposition in a reducing environment (Garrels and Christ, 1965). This facies has the highest average potassium concentration, which may reflect the 

longer residence times or a change in mineralogy of 

the sediments.

Information on the variation in water chemistry with 

depth was obtained from a U.S. Geological Survey test 

well drilled in section 32 of township 19 N., range 1 

W. Three piezometer tubes were installed at different 

depths and isolated by cement packers. Data on the 

shallow water at the site are available from a previous 

study (Fogelman, 1976).

Analyses from the three piezometers and from the 

shallow water are shown in table 3. The percent composition of the major ionic species in terms of milliequivalents per liter are provided in figure 3. Between 80

and 595 feet, a decrease in magnesium is balanced by 

an increase in sodium, with calcium remaining about 

the same. There is a slight increase in the percent of 

chloride in the ground water over this same interval, 

but the net change in anion composition is very small. 

Below 595 feet, however, calcium and magnesium decrease, both in percent composition and concentration, 

and sodium increases. Chloride also increases, becoming 

the dominant anion by a depth of 968 feet. Even at 

1,333 feet, however, the dissolved-solids concentration 

of the water is only moderate, at 526 mg/L, and the 

boron concentration remains low.

SUTTER BASIN

The Sutter Basin hydrochemical facies is between the 

Sacramento and Feather Rivers south of Sutter Buttes. 

This facies extends west of the Sacramento River near 

Colusa to include part of Colusa Basin and includes 

some of the American Basin east of the Feather River. 

Recharge comes from Sutter Buttes, from the Butte 

Basin hydrochemical facies east of Sutter Buttes, and 

from both the Sacramento and Feather Rivers. Sediments in the flood basins are generally fine grained and 

reducing at depth.

The Sutter Basin facies is tied with the south alluvial 

fans facies for the highest average dissolved-solids concentration. This facies is also high in sodium, chloride, 

bicarbonate, potassium, and boron. The effects of reducing conditions are evident in the relatively higher 

concentrations of iron, manganese, and arsenic in this 

area, all of which are more soluble in reducing environments (Garrels and Christ, 1965). The very low average 

nitrate concentration in this facies is also indicative of 

reducing conditions, where nitrate can be removed by 

denitrification reactions.

The origin of the high dissolved-solids water is 

thought to be connate marine water moving upward 

along fault zones created when Sutter Buttes was 

emplaced (Curtin, 1971). Throughout much of the southern part of Sutter Basin, water of very high salinity 

(specific conductance greater than 3,000 micromhos) is 

located within 400 feet of the surface.

TABLE 3. Chemical composition of water from wells of various depths, section 32 of township 19 N., range 1 W.

Sample

No.

(fig. 3)

1

2

3

4

Site

identification

19N/01W-32A01M

19N/01W-32G03M

19N/01W-32G02M

19N/01W-32G01M

Depth of

well

(feet)

80

595

968

1,333

Calcium

(mg/L)

28

25

12

15

Magnesium

(mg/L)

24

4.4

2.6

2.8

Sodium

(mg/L)

34

62

110

160

Bicarbonate

(mg/L)

260

210

150

110

Sulfate

(mg/L)

12

12

3.6

3.4

Chloride

(mg/L)

12

22

110

230

Nitrate-N

(mg/L)

0.59

.03

.02

.12

Boron

(mg/L)

0.12

.15

.18

.24

Dissolved

solids

(mg/L)

276

276

340

526
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA Bll

Ca 

CATIONS

PERCENTAGE REACTING VALUES 

MILLIEQUIVALENTS PER LITER

c

ANIONS

FIGURE 3. Major-ion composition of water from wells of various depths in section 32 of township 19 N., range 1 W. Numbers refer

to sample numbers in table 3."

NORTH ALLUVIAL FANS

The north alluvial fans hydrochemical facies is along 

the northwestern margin of the valley. Ground water 

occurs primarily in alluvial fan deposits and is recharged by streams draining the Coast Ranges. The 

alluvial fan of Stony Creek is a major source of recharge 

in this area, with flow radiating away from the fan to 

the north, east, and south. Ground-water flow in this 

facies is generally east and south. Most of the water

is discharged to the Sacramento River along the eastern 

margin of the facies, although some moves into Colusa 

Basin along the southern boundary. Sediments in the 

alluvial fans are primarily derived from Mesozoic 

marine rocks of the Coast Ranges with sediments derived from Franciscan metamorphic rocks carried down 

by some of the larger streams.

The average dissolved-solids concentration in the 

north alluvial fans facies is moderate, and most of the 

chemical-constituent averages fall between those of the
B12 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

other facies. Calcium is the only species with a high 

average concentration in this facies; this is from the 

waters of Stony Creek, which are high in calcium. Silica 

is much lower in waters on the west side of the valley, 

and this is shown by the low average silica concentration.

SOUTH ALLUVIAL FANS

Located along the southwestern margin of the valley 

in alluvial fans adjacent to the Coast Ranges, the south 

alluvial fans hydrochemical facies extends from Putah 

Creek north to township 15 N. Recharge from the 

Coast Ranges is provided by numerous ephemeral 

streams north of township 10 N., and by Cache and 

Putah Creeks in townships south of 11 N. Recharge 

from Cache Creek moves around the end of the Dunnigan Hills to pumping depressions north of Woodland. 

Some recharge is also drawn from the Sacramento 

River to these same pumping depressions.

The south alluvial fans hydrochemical facies is distinguished by having high average concentrations for 

many of the chemical species measured. Dissolved solids, calcium, magnesium, sodium, chloride, and fluoride 

all have high average concentrations here. Boron and 

bicarbonate concentrations in this facies average very 

high. With little volcanic material to provide silica, its 

average concentration is low, as in the north alluvial 

fans.

Boron and chloride concentrations in the ground 

water of this facies average the highest in the valley, 

and boron concentrations as high as 8.1 mg/L have been 

measured in township 11 N., range 2 E. The source 

of the chloride and boron has been attributed to the 

streams draining the Coast Ranges (Bertoldi, 1976). Information obtained from a deep test well drilled by the 

U.S. Geological Survey in section 34 of township 12 

N., range IE., indicated another possible source. The 

test well 12N/01E-34Q was drilled to a total depth of 

2,400 feet and piezometers tubes installed at 942 feet, 

1,396 feet, and 2,120 feet. No sample was obtained from 

the 942-foot depth, but analyses from the two deeper 

piezometers are shown in table 4. At a depth of 2,120 

feet, water containing 12 mg/L boron and 660 mg/L 

chloride was encountered. Piezometric heads in the test 

well indicate upward movement of water, which may 

be contaminating the^ shallow water.

The source of boron and chloride in the streams 

draining the Coast Ranges is warm saline springs, a 

number of which occur in the Clear Lake area. Partial 

chemical analyses from these springs are also shown 

in table 4. Boron concentrations are as high as 310 mg/L 

in these springs. To determine the source of the high 

boron concentrations in the ground water in township 

UN., range 2 E., the percent composition in milliequiTABLE 4. Data for wells and springs plotted in figures 4 and 5

[Samples 1-2 from U.S. Geological Survey test well; 3-9, reported by Fogelman (1976); 

10, average for 1960-76; 11-15, reported by White and others (1973), with locations as 

indicated therein]

Sample Site

No. identification

(fig. 4)

Depth to first Dissolved Chloride Boron Temperature

opening solids (mg/L) (mg/L) (°C)

(feet) (mg/L)

1

2

3

4

5

6

7

8

9

10

11

12

13

14 

15

12N/01E-34Q01M

12N/01E^34Q02M

11N/02E-20K06M

11N/02E-18R01M

11N/02E-18C02M

11N/02E-23N02M

11N/02E-14B03M

11N/02E-30C01M

11N/02E-29A02M

Cache Creek at Capay

Wilbur Springs

Elgin Mine Spring

Abbott Mine Spring

Grizzly Spring 

Wilbur Oil Test

2,120

1,396

387

264

205

200

198

184

151

...

1,310

482

1,060

321

1,470

746

307

741

816

268

27,134

28,872

7,250

12,400 

19,080

660

95

210

39

210

66

37

96

140

36

9,700

11

1

3 

11

,000

,900

,940 

,400

12

1

310

240

56

178 

23

1.

3.

1.

8,

2,

1.

3.

4.

.5

,1

,7

.4

,1

,5

.1

,3

8

21.0

21.5

20.0

18.5

18.0

21.0

17.5

18.0

18.0

55.0

68.5 =30

17.5 

21.3

valents per liter of the wells in this township were plotted along with the deep well water and the warm spring 

water (fig. 4). The Wilbur Oil Test is a wildcat oil well 

that produces cold water also high in boron and 

chloride.

From figure 4 there is no clear indication of whether 

the water in the wells (sample Nos. 3-9) is derived from 

the thermal springs (sample Nos. 11-14) or from deep 

ground water in the area (sample No. 1). Two of the 

thermal springs, Wilbur Oil Test well and the deep 

well, are very high in percent sodium. Two of the thermal springs show an increase in percent magnesium, 

but the percent calcium remains very low.

The deep ground water and the Wilbur Oil Test well 

have a much higher percentage of chloride than the 

thermal springs, which are enriched in carbon dioxide 

from metamorphism at depth (Barnes and others, 1973). 

As no clear trends emerged in the plot of the percent 

compositions in milliequivalents per liter, a second plot 

of chloride versus boron concentrations was made (fig. 

5). The dashed diagonal line in figure 5 represents the 

dilution of Wilbur Springs water with water containing 

no boron or chloride. The thermal springs all plot very 

close to this line, having both high chloride and boron 

concentrations. The shallow wells in township 11 N., 

range 2 E., all plot very close to the the line representing dilution of Wilbur Springs water, as does the average composition of Cache Creek. Wilbur Oil Test well 

and the deep wells in the valley plot to the right of 

the dilution Line, which indicates more chloride relative 

to boron in these waters. From figure 5 the source of 

the high chloride and boron concentrations is confirmed 

as water from the thermal springs in the Coast Ranges 

carried into the valley by streams, such as Cache 

Creek. Distinct differences between the cold waters,
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B13

CATIONS

PERCENTAGE REACTING VALUES 

MILLEQUIVALENTS PER LITER

ANIONS

FIGURE 4. Major-ion composition of water from springs in the Clear Lake area and from wells in a high boron area northwest of Woodland.

Numbers refer to sample numbers in table 4.

such as Wilbur Oil Test and well 12N/01E-34Q01, and 

the thermal waters can be found in the boron-tochloride ratio and the percent of bicarbonate ion in the 

waters. The hot waters probably receive inputs of carbon dioxide and boron from a gas phase produced during metamorphism of the marine sediments at depth. 

Cold waters are not enriched in volatile components and 

therefore have lower boron-to-chloride ratios and lower 

percent bicarbonate contents. The shallow wells in

township UN., range 2 E., have the higher boron-tochloride ratios indicative of a thermal water source.

TEMPORAL CHANGES

Ground-water-quality degradation may be occurring 

in the Sacramento Valley because of increasing development accompanied by more intensive agriculture and 

denser habitation. Such effects have been noted in the
B14 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

1000

.11

100

/ K

14. / 

/

/

/

o

oc

00 10

.15

9 /

V

.3 .,

10 -.V

10

A

100 1000

CHLORIDE, IN MILLIGRAMS PER LITER

10,000

FIGURE 5. Plot of chloride versus boron for springs in the Clear Lake area and for wells in a high boron area northwest of Woodland. 

Numbers refer to sample numbers in table 4. Dashed line indicates dilution of Wilbur Springs water with water containing no 

boron or chloride.

San Joaquin Valley (Kern County Water Agency, 1979), 

in San Bernardino County (Klein and Bradford, 1979), 

and in the Colorado River Basin (El-Ashry, 1980). Much 

of the problem is related to infiltrating irrigation water 

that (1) has been concentrated by evapotranspiration, 

(2) has dissolved salts added as fertilizers and soil conditioners, and (3) has leached natural salts from the 

unsaturated zone. Additional degradation can occur 

from nonagricultural activities, such as the disposal of 

sewage and industrial wastes.

In this section, water-quality data from the Sacramento Valley are examined for linear trends with 

time. Data in each of the hydrochemical facies are combined to derive an average trend for each of the areas.

Individual wells are also examined for temporal changes 

to show more localized variations. The locations of wells 

used to study the temporal changes are plotted on a 

map of the valley, and those wells showing significant 

changes are labeled with symbols indicating the direction of change (pi. 2).

Two variables are used in this study, dissolved-solids 

and nitrate concentrations. Dissolved solids measures 

the total amount of dissolved chemicals in a water, and 

it gives a general indication of overall water quality. 

Nitrate is derived from a number of potential pollution 

sources, such as fertilizers and sewage. Increases in 

nitrate would indicate specific influences of man's activities. These two variables will give a fairly com-
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B15

prehensive evaluation of any temporal changes that 

may be occurring.

METHODS OF DATA ANALYSIS

To determine the average changes occurring in a hydrochemical facies, some technique was needed so that 

wells with different average dissolved-solids or nitrate 

concentrations could be compared. This was accomplished by standardizing the data for each well to 

have a mean of zero and a standard deviation of one. 

Data from all the wells for each year could then be 

averaged with equal weight being given to each well. 

This data manipulation procedure produced a data set 

with one observation per year for the period of record 

in each hydrochemical facies. The yearly observations 

represented the average for all the wells for which data 

were available in each hydrochemical facies.

Trends in the chemistry of the facies were then 

studied using a linear regression of the standardized 

and averaged dissolved-solids or nitrate concentrations 

versus time. One of the fundamental assumptions of regression analysis is that the observations are independently distributed. For sequences of data collected over 

time, this requirement is sometimes violated because 

the data are autocorrelated. Because of memory effects 

in natural processes, measurements taken at one time 

may be correlated to measurements taken at previous 

times.

To test the extent to which the dissolved solids and 

nitrate data suffered from autocorrelation problems, lag 

one serial correlations were calculated, and cumulative 

frequency histograms of these autocorrelations compiled. Matalas, Slack, and Wallis (1975) have generated 

a series of empirical cumulative frequency histograms 

for data sequences having a range of serial correlations. 

By comparing the cumulative frequency histograms for 

dissolved solids and nitrate in the Sacramento Valley 

to these empirical histograms, the probable population 

autocorrelations were determined to be about 0.3 for 

dissolved solids and between 0.3 and 0.5 for nitrate.

Serial correlations of this magnitude indicate a tendency for subsequent values to be positively correlated 

to previous values. As a result, the trends determined 

from the regression analyses may not be as strong as 

indicated. Positive autocorrelations of this magnitude, 

however, will not significantly affect the conclusions of 

the temporal study.

The equations describing the average temporal 

changes for the hydrochemical facies were transformed 

back to concentrations by destandardizing the regression equations. For each hydrochemical facies, the average of the means and standard deviations of the individual wells was calculated. By multiplying the regression equations by these average standard deviations 

and adding the average means, equations were obtained 

that give the average trend in a facies in terms of concentrations. This equation describes what might be observed in a typical well in each facies, and no individual 

well can be expected to show the same relation.

The equations produced by this analysis are not predictive equations, but merely reflect the average rate 

of change over the period of record, from about 1955 

to 1977. An example of the problems that can arise 

if they are used as predictive equations is demonstrated 

by considering what happens if some of the equations 

are used to estimate past concentrations. The equation 

describing the average changes in nitrate-nitrogen concentrations for the Victor Plain hydrochemical facies indicates negative concentrations in the ground water 

prior to 1939. This is a physical impossibility and arises 

because the equation was used for a purpose for which 

it was not intended. While predictions of future concentrations of dissolved solids and nitrate-nitrogen may not 

produce negative results, the values obtained could be 

just as meaningless.

In addition to testing for average trends throughout 

a hydrochemical facies, individual wells were examined 

for temporal variations. Where a significant trend was 

noted for an individual well, the equation was recorded 

and the well's location plotted on a map of the valley 

with a symbol indicating the direction of the trend (pi. 

2).

Few wells in the valley have multiple observations 

over time. To minimize the effects of random fluctuations, only wells with seven or more observations for 

dissolved solids and six or more observations for nitrate 

were used. There was no attempt to assure a random 

distribution of wells throughout the valley, and all wells 

in the valley or along the margins having a sufficient 

number of observations were used. Time is measured 

as the year minus 1900. For example, for the year 1975, 

time is equal to 197&-1900 or 75.

DISSOLVED SOLIDS

Dissolved solids is a measure of the total amount of 

dissolved chemicals in water. An increase in any of the 

individual constituents will be reflected by an increase 

in the dissolved solids. While not many historical dissolved-solids measurements are available, many specific-conductance measurements have been made. Specific 

conductance is proportional to the quantity and valence 

of ionized chemicals in a water. Because almost all the 

major dissolved chemical constituents in a water are 

ionized, dissolved solids and specific conductance are 

closely related.
B16 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

It was felt that dissolved solids would have more 

meaning for most users of this report; therefore, specific conductivities were transformed to dissolved-solids 

concentrations. The relation between dissolved solids 

and specific conductance will ^depend on the specific 

ionized species present and the amount of uncharged 

species, mainly silica, present in the water. For this 

reason, separate equations were derived for different 

parts of the valley. These areas do not exactly coincide 

with the hydrochemical facies as they were finally defined, but the division follows the same basic pattern. 

Thus, the effects of different water chemistries on the 

relation between dissolved solids and conductance were 

taken into account during the transformation.

Equations that describe the relation between dissolved solids and specific conductance were derived from 

a linear regression on data from about 650 wells having 

measurements of both dissolved solids and conductance. 

A linear equation with both slope and intercept terms 

was used rather than a ratio. The intercept terms of 

all the linear equations were significantly different from 

zero, which indicates that the linear equations better 

fit the data than ratios.

Because the calculation of dissolved solids from conductance is a simple linear transformation, the calculation in no way affects the presence of trends in the 

data. All dissolved-solids values for a well were estimated from conductance measurements even if some 

measured dissolved-solids concentrations were available. This prevented any changes that might be induced 

by using dissolved-solids data from different sources.

Locations of wells examined for trends in dissolved 

solids are shown on plate 2A. The changes for each 

hydrochemical facies are discussed in the next section 

and coefficients for the equations describing the average temporal changes are given in table 5. Coefficients 

for individual wells are summarized in table 6.

HYDROCHEMICAL FACIES

Dissolved solids in the Tuscan volcanic rocks hydrochemical facies show an average annual increase of 0.95 

mg/L. The trend line accounts for 31 percent of the 

variation in the standardized dissolved-solids data averaged over all wells by year. A plot of the trend line 

is shown in figure 6. Temporal variations are significant 

in five of the 19 wells for which historical data are available (table 6). Two of these wells show rising trends 

and three show decreasing trends.

TABLE 5. Analyses of variance and coefficients for the regression of average yearly dissolved solids with time for

the six hydrochemical facies in the Sacramento Valley

[t=(year-1900); for example, t=(1980-1900)=80. NS, linear trend not significant at the 90 percent confidence level]

Hydrochemical Number of

facies wells

Tuscan ° 19

volcanic rocks

Victor Plain 20

Butte Basin 12

Sutter Basin 10

Basin1

North alluvial 62

fans

South alluvial 18

fans

Source

Model

Error

Total

Model

Error

Total

Model

Error

Total

Model

Error

Total

Model

Error

Total

Model

Error

Total

Degrees

of

freedom

1

20

21

1

23

24

1

18

19

1

23

24

1

19

20

1

21

22

Sum of

squares

1.52

3.43

4.95

2.50

4.13

6.63

1.17

1.68

2.85

.03

15.21

15.24

1.05

1.41

2.46

7.41

2.39

9.80

Probability

F of exceeding

ratio F ratio r2 A

8.89 0.01 0.31 194

13.91 <0.01 .38 236

12.55 <0.01 .41 197

.05 .83 .00 

14.11 <0.01 .43 222

65.16 <0.01 .76 184

MODEL

Dissolved solids=A+B(

90 percent 90 percent

confidence confidence

limit B limit

±36 0.95 ±0.55

±48 1.61 ±0.74

±45 1.37 ±0.67

NS

±31 1.02 ±.46

±67 4.75 ±1.01
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA 

TABLE 6. Wells showing significant trends in dissolved-solids concentration

[For the model: Dissolved solids=A+Bt, where t is the year minus 1900]

BIT

Well A

Standard 

error

Student's 

t B

Standard 

error

Student's 

t

Probability 

of exceeding 

tforB

TUSCAN VOLCANIC ROCKS

23N/01W-09L01M 

25N/02W-16P01M 

26N/02W-09E01M 

27N/03W-15N01M 

27N/03W-22B01M

-24.6 

666.4 

909.5 

1055.6 

-122.4

73.2 

174.5 

257.0 

288.1 

223.1

0.34 

3.82 

3.54 

3.66 

.55

5.72 

-6.09 

-7.44 

-9.56 

6.30

1.12 

2.42 

3.56 

3.92 

3.06

5.12 

2.52 

2.09 

2.44 

2.06

<0.01

.01 

.04 

.02 

.04

VICTOR PLAIN

06N/07E-23A01M 

18N/04E-28M01M

892.6 

325.8

373.4 

209.4

2.39 

1.56

-10.32 

22.03

5.61 

3.02

1.84 

7.29

0.07 

.01

BUTTE BASIN

16N/04E-09D01M 

17N/01W-30K03M

-389.3 

1833.2

181.4 

515.4

2.15 

3.56

8.57 

-21.27

2.80 

7.04

3.06 

3.02

<0.01 

<0.01

SUTTER BASIN

15N/03E-15H04M 

16N/02W-25B02M

-910.0 

2428.1

485.4 

395.1

1.87 

6.15

' 20.92 

-24.98

6.82 

5.56

3.07 

4.49

<0.01 

<0.01

NORTH ALLUVIAL FANS

17N/03W-33R01M 

18N/02W-01E01M 

18N/04W-02F01M 

19N/02W-23N01M 

20N/02W-22E01M

21N/02W-15C01M 

21N/03W-02Q01M 

22N/01W-29C01M 

22N/02W-03A01M 

22N/02W-04C02M

22N/03W-06H01M 

22N/03W-22Q01M 

23N/03W-35B01M 

24N/03W-24P01M 

25N/03W-31R01M

383.2 

-716.6 

-817.1 

^64.2 

-618.4

-241.0 

-121.4 

40.2 

125.3 

^59.7

-922.3 

20.0 

-69.5 

-724.6 

89.9

76.6 

77.8 

112.5 

77.8 

442.7

84.5 

84.6 

87.8 

77.8 

376.8

553.3

77.8 

84.3 

364.0

77.8

5.01 

9.21 

7.26 

5.97 

1.40

2.85 

1.43 

.46 

1.61 

1.22

1.67 

.26 

.82 

1.99 

1.16

3.24 

16.37 

22.73 

14.03 

11.31

8.77 

7.41 

3.98 

3.37 

10.53

20.71 

3.88 

3.40 

16.36 

3.49

1.13 

1.16 

1.74 

1.16 

5.97

1.25 

1.25 

1.30 

1.16 

5.11

7.62 

1.16 

1.25 

4.95 

1.16

2.87 

14.16 

13.07 

12.14 

1.89

7.04 

5.95 

3.06 

2.91 

2.06

2.72 ' 

3.35 

2.73 

3.30 

3.02

<0.01 

<0.01 

<0.01 

<0.01 

.06

<0.01 

<0.01 

<0.01 

<0.01 

.04

<0.01 

<0.01 

<0.01 

<0.01 

<0.01

SOUTH ALLUVIAL FANS

06N/01E-19L02M 

06N/01E-19Q01M 

06N/01W-01B04M 

07N/02E-34C02M 

09N/04E-33L01M

13N/01W-07A01M 

13N/01W-36002M 

14N/03W-11H01M

-661.9 

101.0 

-166.4 

-152.3 

-740.0

335.8 

-32.3 

7.9

209.8 

207.1 

280.1 

185.0 

169.8

139.7 

117.0 

166.3

3.15 

.49 

.59 

.82 

4.36

2.40 

.28 

.05

18.11 

5.94 

7.48 

10.56 

28.96

7.43 

4.67 

4.45

3.15 

3.05 

4.04 

2.81 

2.75

2.04 

1.74 

2.46

5.76 

1.95 

1.85 3.75 

10.55

3.64 

2.68 

1.81

<0.01 

.05 

.07 

<0.01 

<0.01

<0.01 

<0.01 

.07
B18 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

TUSCAN VOLCANIC ROCKS SUTTER BASIN

290

280

270

260

250

240

230

1950

600

550

500

450

400

1955 1960 1965 1970 

VICTOR PLAIN

1975 1980

1950 1955 1960 1965 1970 

NORTH ALLUVIAL FANS

1975 1980

400

375

350

325

300

1950

330

320

310

300

290

280

270

260

1950 1955 1960 1965 1970 

SOUTH ALLUVIAL FANS

1975 1980

1955 1960 1965 1970 

BUTTE BASIN

1975 1980

310

300

290

280

270

260

1950

.

1955 1960 1965 1970 1975

575

550

525

500

475

450

425

400

1980 1950 

TIME, IN YEARS

1955 1960 1965 1970 1975 1980

FIGURE 6. Average yearly dissolved-solids concentrations versus time for the six hydrochemical facies in the Sacramento Valley. The 

coefficients and significance tests for the trend lines are given in table 5. If no line is shown, then the slope of the trend line is 

not significantly different from zero at the 90 percent confidence level. The technique by which the average yearly dissolved-solids 

values were derived is explained in the text.

The average yearly increase in dissolved solids for 

the Victor Plain facies is 1.61 mg/L. The trend line, 

which explains 38 percent of the variation in the standardized data, is shown in figure 6. Only two of the 

20 wells studied in this facies showed significant 

changes, one of which showed an increase, and the 

other a decrease with time.

For Butte Basin, a temporal trend having a slope 

of 1.37 (mg/L)/yr explains 41 percent of the variation 

in the averaged data. In this facies also, two wells 

showed significant trends. One well demonstrated an 

increase and the other a decrease in dissolved solids 

with time.

Sutter Basin is the only hydrochemical facies to show 

no significant temporal change in average dissolved-solids concentration. Two of the 10 wells did show significant trends, but in opposite directions.

An increase in the average dissolved-solids concentration of the north alluvial fans facies of 1.02 (mg/L)/yr 

explains 43 percent of the variation in the yearly average data. A plot of the trend line is shown in figure 

6. Of 62 wells that have seven or more observations 

in this area, 15 show significant upward trends, and 

none decrease.

Seventy-six percent of the variation in the average 

yearly dissolved-solids data are explained by a linear 

trend in the south alluvial fans facies. The slope is very 

steep 4.75 (mg/L)/yr and indicates a very rapid increase 

in dissolved solids (fig. 6). Of the 18 wells examined 

in this facies, 8 had significant increases in dissolved 

solids. There were no wells that showed decreases.
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B19

NITRATE

A certain amount of nitrate is present in precipitation, and chemical reactions in the soil zone may add 

to this amount. Thus, even under natural conditions, 

a finite amount of nitrate will exist in ground water. 

An estimate of natural nitrate concentrations in the 

Sacramento Valley can be made from data gathered 

during the period 1912-13 (Bryan, 1923). The median 

nitrate-nitrogen concentration in these 43 samples is 

0.67 mg/L. The highest nitrate-nitrogen concentration 

measured was 14 mg/L, which suggests some wells 

were polluted as long as 70 years ago. From the frequency histogram of the 1912-13 nitrate data (fig. 7), 

it is estimated that under natural conditions, no more 

than about 3 mg/L nitrate-nitrogen would be expected 

in the ground water.

By the period 1974-78, the median nitrate-nitrogen 

concentration had more than doubled to 1.6 mg/L. The 

maximum observed nitrate-nitrogen concentration also 

increased, with two wells containing 27 mg/L and one 

well 61 mg/L. These increases likely reflect the influences of man's activities, and the extent of temporal 

changes was, therefore, investigated to determine the 

magnitude of the problem.

20

15 -

o

10 

o

LLJ 

CC.

0 1 2 3 4 5 6 ' ' 13 14 

NITRATE-NITROGEN, IN MILLIGRAMS PER LITER

FIGURE 7. Frequency histogram of dissolved nitrate-nitrogen 

concentrations in wells, 1912-13. Data from Bryan (1923).

Locations of wells examined for trends in nitrate are 

shown on plate 2B. The changes for each hydrochemical 

facies are discussed further and coefficients for the equations describing the average temporal changes are 

given in table 7. Coefficients for individual wells are 

summarized in table 8.

TABLE 7. Analyses of variance and coefficients for the regression of average yearly nitrate-nitrogen concentrations 

with time for the six hydrochemical facies in the Sacramento Valley

[t=(year-1900); for example, t=(1980-1900)=80. NS, linear trend not significant at the 90 percent confidence level]

MODEL 

Nitrate nitrogen=A+Bt

Hydrochemical Number of

facies wells

Tuscan 5

volcanic rocks

Victor Plain 13

Butte Basin 3

Sutter Basin 2

North alluvial 26 

fans

South alluvial 13

fans

Source

Model

Error

Total

Model

Error

Total

Model

Error

Total

Model

Error

Total'

Model

Error

Total

Model

Error

Total

Degrees

of

freedom

1

15

16

1

17

18

1

14

15

1

10

11

1

18

19

1

18

19

Sum of

squares

0.88

10.57

11.45

6.05

3.51

9.56

.69

5.26

5.95

.65

7.77

8.42

5.02

1.82

6.84

2.03

4.73

6.76

Probability 90 percent

F of exceeding confidence

ratio F ratio r2 A limit

1.25 0.28 0.08 

29.31 <0.01 .63 -1.402 ±0.751

1.85 .20 .12 

.84 .38 .08 

49.67 <0.01 .73 ^3.786 ±1.625

7.72 .01 .30 -0.506 ±1.514

90 percent

confidence

B limit

NS

0.036 ±0.012

NS

NS

.099 ±0.024

.036 ±0.023
B20 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

TABLE 8. Wells showing significant trends in nitrate-nitrogen concentration

[For the model: Nitrate-nitrogen=A + Bt, where t is the year minus 1900]

Well A

Standard 

error

Student's 

t B

Standard 

error

Student's 

t

Probability 

of exceeding 

tforB

TUSCAN VOLCANIC ROCKS

23N/01W-09L01M 

25N/02W-07K01M

-25.33 

-7.96

3.82 

3.74

6.63 

2.13

0.53

.17

0.06 

.06

8.78

2.89

<0.01 

<0.01

VICTOR PLAIN

10N/05E-06M02M 

11N/06E-16M01M 

13N/05E-13D01M 

14N/05E-32R03M 

18N/04E-07A01M

-6.58 

-9.38 

-3.98 

-14.48 

-0.81

1.62 

1.35 

2.09 

2.19 

1.09

4.06 

6.96 

1.90 

6.60 

.75

0.12 

.18 

.08 

.23 

.03

0.03 

.02 

.03 

.03 

.02

4.85 

8.56 

2.31 

7.38 

1.87

<0.01 

<0.01 

.02 

<0.01 

.06

BUTTE BASIN

SUTTER BASIN

None ....................

NORTH ALLUVIAL FANS

18N/04W-02F01M 

20N/03W-02D01M 

21N/02W-15C01M 

21N/03W-02Q01M 

22N/01W-29C01M

22N/02W-03A01M 

25N/03W-31R01M

-73.57 

-11.39 

-15.47 

-33.53 

-8.35

-10.26 

-10.34

6.37 

7.49 

5.62 

7.80 

6.41

6.16 

6.88

11.54 

1.52 

2.75 

4.30 

1.30

1.67 

1.50

1.38 

.23 

.29

.58 

.17

.28 

.24

0.10 

.12 

.08 

.12 

.10

.10 

.10

13.96 

1.95 

3.43 

4.89 

1.70

2.92 

2.30

<0.01 

.05 

<0.01 

<0.01 

.09

<0.01 

.02

SOUTH ALLUVIAL FANS

09N/02E-10D01M 

13N/01W-07A01M 

13N/01W-08B01M 

14N/03W-11H01M 

14N/03W-14Q02M

26.38 

-10.19 

-11.80 

^.40 

-25.22

6.93 

2.66 

2.36 

2.75 

5.95

3.81 -0.38 

3.84 .21 

5.00 .22 

1.60 .10 

4.24 .38

0.11 

.04 

.04 

.04 

.08

3.44 

5.26 

6.13 

2.33 

4.55

<0.01 

'.02

HYDROCHEMICAL FACIES

No significant average trend was found for the Tuscan volcanic rocks hydrochemical facies. However, two 

of the five wells examined did show significant upward 

trends in nitrate concentrations.

A linear trend with a slope of 0.036 (mg/L)/yr explains 63 percent of the temporal variation in the average yearly nitrate-nitrogen concentrations in the Victor 

Plain facies. A plot of this trend line is shown in figure 

8. Of 13 wells in this area that had 6 or more observations, 5 showed significiant upward trends. None of the 

wells showed a significant decrease.

Neither Butte nor Sutter Basin had a significant average trend in nitrate. Also, there were no individual 

wells in either that showed any significant changes. 

However, with only three wells in Butte Basin and two 

in Sutter Basin having six or more observations, there 

are too few samples for the results of the test to be 

conclusive.

The nitrate-nitrogen concentrations in the north alluvial fans hydrochemical facies show a distinct upwards
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B21

TUSCAN VOLCANIC ROCKS SUTTER BASIN

4.5 

4.0 

3.5 

3.0 

2.5 

2.0 

1.5

1.0

1950

1.0 

0.8 

0.6 

0.4 

0.2

0.0

1950

T .. T t '

1955 1960 1965 1970 1975 1980

1955 1960 1965 1970 1975 1980

NORTH ALLUVIAL FANS

VICTOR PLAIN

4.5 

4.0 

3.5 

3.0 

2.5 

2.0

1.5

1950 1955 1960 1965 1970 

SOUTH ALLUVIAL FANS

1975 1980

1955 1960 1965 1970 1975 1980

BUTTE BASIN

7.0 

6.0 

5.0 

4.0 

3.0

2.0

1950

2.60

2.40

2.20

2.00

1.80

1.60

1.40

1955 1960 1965 1970 1975

1.20

1980 1950

TIME, IN YEARS

1955 1960 1965 1970 1975 1980

FIGURE 8. Average yearly nitrate-nitrogen concentrations versus time for the six hydrochemical facies in the Sacramento Valley. The 

coefficients and significance tests for the trend lines are given in table 7. If no line is shown, then the slope of the trend line is 

not significantly different from zero at the 90 percent confidence level. The technique by which the average yearly nitrate-nitrogen 

values were derived is explained in the text.

trend with a slope of 0.099 (mg/L)/yr. Seven of the 26 

wells in this facies showed significant upwards trends. 

The plot of the trend in figure 8 shows that the average 

concentration of nitrate-nitrogen in this facies has exceeded 3.0 mg/L, the maximum expected under natural 

conditions.

For the south alluvial fans hydrochemical facies, a 

linear trend having a slope of 0.036 (mg/L)/yr explains 

30 percent of the temporal variation in the average annual nitrate-nitrogen concentrations. Thirteen wells had 

six or more observations over time, and 5 of them show 

significant temporal changes. Four have upwards 

trends, and 1 a downwards trend.

HYPOTHESIS TESTING

Because trends in the data for the individual wells 

were tested at the 90 percent confidence level, there

is a 10 percent chance that no significant trend exists 

in the valley. The significant slopes that were observed 

could result from normally distributed random fluctuations about a true true slope of zero. If this were the 

case, 5 percent of the wells could show a significant 

increase, and 5 percent a significant decrease, although 

no real trend existed. If more than 5 percent of the 

dissolved-solids or nitrate wells show significant trends 

in the same direction, then it can be concluded that 

there has been a significant change for that species.

Of 140 wells in the Sacramento Valley tested for 

trends in dissolved solids, 28 (20 percent) showed an 

increase and 7 (5 percent) showed a decrease with time. 

Under the null hypothesis that no change in dissolved 

solids is occurring at the 90 percent confidence level, 

it would be expected that by random variation, 7 of 

the wells would show a significant decrease and 7 would
B22 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

show a significant increase. For dissolved solids, the 

number of wells showing increases exceeds the number 

expected by random variation, and the increase in dissolved solids is significant.

Of 62 wells tested for trends in nitrate, 18 (29 percent) showed increases, but only 1 (2 percent) showed 

a decrease with time. Under the null hypothesis of no 

change, 3 can be expected to increase and 3 to decrease 

by random variation at the 90 percent confidence level. 

Because 29 percent of the wells showed significant increases in nitrate, the null hypothesis can be rejected, 

which indicates significant increases in nitrate are occurring in the Sacramento Valley.

PROCESSES CONTROLLING 

GROUND-WATER CHEMISTRY

Differences in the chemical composition of ground 

water among the six hydrochemical facies in the Sacramento Valley indicate that either different processes 

control the water chemistry in different areas, or that 

there are spatial variations in the same process. In this 

section, the major controlling processes are delineated 

from the chemical data using principal components analysis. Factor scores are than calculated for each well 

that reflect the magnitude of each component in that 

well. By studying the spatial trends in the factor scores 

using trend surface analysis, the areal variations in processes can be determined. Contour maps of these surfaces show the spatial distributions of the more important processes controlling ground-water chemistry.

METHODS OF DATA ANALYSIS

PRINCIPAL COMPONENTS ANALYSIS

Principal components analysis (Rummel, 1970; Davis, 

1973) is used to separate the variance of a data matrix 

into common and unique components. Variables, in this 

case dissolved chemical species, that are controlled by 

the same process will share a common variance structure. Principal components analysis will group the variables on the basis of this shared variation and will calculate a correlation between variables and components. 

These components represent independent sources of 

variation in a data matrix and can be interpreted in 

terms of controlling processes using knowledge of 

geochemical principles.

From the correlations, or loadings as they are 

termed, between variables and components, factor 

scores can be calculated for each well that measure the 

intensity of a component in that well. These factor

scores are a composite of all the chemical species that 

load on a component and summarize similar behavior 

in a number of different species. Where several of the 

chemical species are controlled by the same process, 

this transformation to factor scores reduces the number 

of variables needed to account for the significant variation in the data. The factor scores can be further 

analyzed as any other numerical variable.

TREND SURFACE ANALYSIS

Trend surface analysis (Davis, 1973) is a technique 

for separating variation in geographically distributed 

data into regional and local parts. Either of these parts 

might be the goal of a particular study. For the distribution of processes in the Sacramento Valley, however, 

the regional variation is of interest. A trend surface 

is delineated by a polynomial equation that predicts the 

magnitude of some variable in terms of the geographic 

coordinates of a set of observations. The criteria for 

fitting the polynomial is to minimize the squared deviations about the surface. In this study, the trend surface 

represents the spatial variation in processes controlling 

ground-water chemistry. By contouring the surface, 

maps can be prepared that show the spatial variation 

in the magnitude of the process.

PROCESSES AND THEIR SPATIAL VARIATION

For the Sacramento Valley, 15 variables (table 9) 

were included in the principal components analysis. A 

major assumption of components analysis is that the 

data are normally distributed. This requirement was 

reasonably well met by using logarithmic transformations of the chemical concentrations. Four principal 

components explained 73 percent of the total variation 

in the data and accounted for all the significant common 

variation (that is, only these four components had 

eigenvalues greater than one). The four individual components explained 36, 17, 11, and 9 percent of the total 

variation. Because neither the third nor fourth component yielded clear interpretations that could be attributed to processes, only the first two components are 

presented. These two components, however, explain

TABLE 9. Chemical variables included in the principal components 

analysis of the Sacramento Valley

Dissolved solids

Calcium 

Magnesium 

Sodium

Potassium

Manganese 

Arsenic 

Boron

Silica

Bicarbonate 

Sulfate 

Chloride

Nitrate

Phosphate 

Fluoride
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B23

TABLE 10. Results of principal components analysis on chemical 

data from the Sacramento Valley

[All loadings with absolute values less than 0.30 have been omitted for clarity]

Components

Communalities

Dissolved solids ...............

Calcium...........................

Magnesium ......................

Sodium ...........................

Bicarbonate .....................

Chloride..........................

Sulfate.............. ..............

Boron .............................

Fluoride..........................

Silica..............................

Potassium........................

Nitrate ...........................

Phosphate .......................

Manganese ......................

Arsenic ...........................

Percent variation

explained......................

Cumulative percent

variation explained........

0.94

.76

.81

.83

.85

.76

.63

.65

.30

-0.54

36

36

-0.46

.42

.67

-0.71

.50

.63

.62

17

53

0.93

.60

.67

.76

.72

.62

.41

.42

.30

.46

.48

.52

.30

.41

.38

over 50 percent of the variation in the chemical data 

from the Sacramento Valley.

Table 10 shows the results of the principal components analysis. On component one, dissolved solids, calcium, magnesium, sodium, bicarbonate, chloride, sulfate, boron, and fluoride all have positive loadings. 

Silica is the only species to have a significant negative 

loading on this component. The inverse loadings of dissolved solids and silica indicate that low silica concentrations tend to be found in conjunction with high dissolved-solids concentrations, and vice versa. The positive loadings of all the major chemical species on this 

factor indicate a source high in calcium, magnesium, 

sodium, bicarbonate, chloride, and sulfate. This source 

is also high in fluoride and boron, and low in silica.

The trend surface map of this component (fig. 9) 

shows a distinct linear trend across the valley, which 

accounts for 36 percent of the spatial variation in component one. Low values of component one along the 

eastern margin and in the northern end of the valley 

indicate low dissolved-solids and high silica concentrations. In the southwestern corner, high concentrations 

of dissolved species and boron are indicated. This distribution in the ground water is very similar to the distribution of water chemistry in surface streams draining 

into the valley. Table 11 shows the average concentrations of dissolved species in streams entering the valley 

from certain areas. East side streams are low in dissolved solids, and contain twice as much silica as streams

from the southwestern margin. The silica in east side 

streams is likely from the weathering of volcanic detritus in the Sierra Nevada and Cascade Range. High 

dissolved-solids concentrations in the southwest originate in thermal springs in the Coast Ranges.

The first principal component represents the effects 

of the chemistry of recharge to the valley on the 

chemistry of the ground water. This process is active 

along both margins of the valley and produces fairly 

uniform chemical compositions along the eastern side. 

Along the western margin, changes in surface-water 

chemistry from north to south induce changes in 

ground-water chemistry. The effects of recharge 

chemistry are the single most important control on 

ground-water chemistry, accounting for over one-third 

of the total variation in the chemical data.

The second principal component (table 10) explains 

17 percent of the variation in the chemical data, and 

contains positive loadings of silica, potassium, phosphate, manganese, and arsenic. Nitrate and fluoride 

have negative loadings on this component. The trend 

surface for component two (fig. 10) explains 38 percent 

of the spatial variation in this component. The pattern 

indicates high factor scores in the center of the valley 

with low values along the margins. The occurrence of 

high factor scores in wells in the flood basins suggests 

that the fine-grained sediments and reducing conditions 

in the basins are an important control on water chemistry.

Measurements of dissolved oxygen in the ground 

water in the south alluvial fans facies (table 12) show 

an average of 3.8 mg/L of dissolved oxygen. Although 

no data are available for other parts of the valley, it 

is reasonable to assume that the coarse-grained sediments along the valley margins would contain some dissolved oxygen. In the flood basins, where sediments 

are water saturated and fine grained, reducing conditions probably exist. The increase in manganese and 

arsenic, and decrease in nitrate, support an oxidationreduction effect. The solubility of manganese and arsenic increases in reducing conditions, and nitrate could 

be removed by denitrification.

Longer residence times in the fine-grained sediments 

of the flood basins and weathering of volcanic detritus 

from Sutter Buttes could supply the higher silica and 

potassium concentrations indicated by component two. 

The inverse relation between phosphate and fluoride 

probably reflects spatial differences in their sources 

rather than equilibrium with a mineral phase, such as 

apatite. Phosphate, introduced along with nitrate, 

would not be removed by reduction reactions and would 

persist in the ground water. Fluoride is probably derived from thermal springs in the hills surrounding the 

valley and lacks a source in the central basins.
B24 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

122°30' 122°00' 121°30' 121°00'

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Principal components analysis shows that two components account for over 50 percent of the variation in 

the chemical data from the Sacramento Valley. While

other processes will affect water chemistry, they seem 

to be either more localized or much less important than 

the two discussed. Recharge from the valley margins
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B25

TABLE 11. Average chemical compositions of surface water entering 

the Sacramento Valley

Species 

(mg/L)

Magnesium ...................

Sodium ........................

Chloride.......................

Sulfate....... ..................

Fluoride................ ......

Silica...........................

East side

85

11

4.7

6.3

55

5.4

7.0

.10

.08

22

Northern 

west side

215

36

12

13

164

16

28

.07

.08

14

Southern 

west side

592

53

40

101

314

150

82

.51

.4

11

EXPLANATION OF LOCALITIES

East side: Antelope Creek, Mill Creek, Big Chico Creek, Butte Creek, Feather River,

Yuba River, Bear River, and American River.

Northern west side: Red Bank Creek, Elder Creek, Thomes Creek, and Stony Creek. 

Southern west side: Stone Corral Creek, Lurline Creek, Freshwater Creek, Salt Creek

(west of Williams), Cortina Creek, Sand Creek, Oat Creek, Cache Creek, and Putah

Creek.

is the most important control on ground-water chemistry, with reactions in the flood basin sediments also 

being important.

MINERAL STABILITY RELATIONS

Sediments deposited in the Sacramento Valley have 

been derived from erosion of the surrounding mountains. These mountains contain igneous and metamorphic rocks which were formed at higher temperatures 

and (or) pressures than are found at shallow depths 

in the valley sediments. Because the environment of 

deposition is radically different from that of formation, 

these minerals are generally not in equilibrium with 

their surroundings. As a result, the minerals react with 

the ground water, which dissolves the unstable primary 

minerals and forms secondary minerals that are more 

stable. Reaction rates may be very slow, so the process 

is gradual and can take tens of millions of years or 

longer. During this diagenesis, phases, such as pyroxenes, feldspars, micas, and volcanic glass, weather to 

more stable phases, such as clay minerals and iron hydroxides.

Water chemistry will be affected by water-rock interactions, and the three parts of the system (primary minerals, aqueous solution, and secondary minerals) are interdependent. Using thermodynamic calculations, the 

mineral phases that will be stable in equilibrium with 

solutions of various compositions can be predicted. It 

cannot be justifiably presumed, however, that the mineral assemblage and the ground-water chemistry are 

in themodynamic equilibrium, particularly where 

aluminosilicate minerals are involved. Water chemistry 

may reflect incomplete reactions or partial equilibrium 

with metastable phases.

In this section, the relations between water chemistry and aquifer mineralogy are examined. Mineralogy 

is presented from two U.S. Geological Survey test 

wells. Mineral saturation indices are examined to determine any likely phases that may be controlling water 

chemistry. Finally, stability and metastability fields of 

mineral phases are compared to measured water 

chemistry to determine what secondary minerals will 

be most stable in contact with Sacramento Valley 

ground water.

MINERALOGY OF SEDIMENTS

Sidewall samples were collected from various depths 

in two test wells drilled by the U.S. Geological Survey 

during 1979. Data from two wells do not provide a comprehensive evaluation of sediment mineralogy, but they 

do provide a partial guide to what minerals may be 

present. Not all the samples have been analyzed as of 

the time of this writing and additional mineralogic data 

will be published in the future as basic data reports.

WELL 12N/01E-34Q

The mineralogy of sidewall samples from well 12N/ 

01E-34Q was determined by optical microscope identification of the 0.5- to 0.125-mm size fraction. Table 13 

shows the percent compositions of minerals, in this size 

range, for five samples collected from 323 feet to 897 

feet. Data from an additional sample at 1,845 feet is 

also given. Feldspars compose an average of about twothirds of the mineral grains, with about one-fifth being 

quartz and one-tenth being rock fragments. In all the 

shallower samples, the percent plagioclase having an 

anorthite content greater than 34 percent is smaller in 

the fine fraction than in the medium. The sample from 

1,845 feet has an unusually small amount of anorthiterich plagioclase in the medium fraction. The albitic 

plagioclase component increases between medium and 

fine fractions in all but one sample. The smaller size 

fractions, which are chemically more reactive, are dominated by albitic plagioclase.

Lithic fragments from a gravel at 1,856 feet, having 

a distinct-greenish hue, were examined by X-ray diffraction. The sample near this depth was unusually rich 

in heavy lithic fragments (table 13). X-ray analysis of 

these fragments indicates that the composition is 

quartz, plagioclase, and chlorite (Ivan Barnes, U.S. 

Geological Survey, personal commun., 1979). The 

mineralogy of the sediments determined by optical 

methods may not reflect the complete mineralogy, especially where some minerals are contained in rock fragments which are not differentiated mineralogically. 

Mineral phases identified in samples from this well are 

shown in table 14.
B26 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

122°30' 122°00' 121°30' 121°00'

40° _ 00'

39° 

30'

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 x xxxxeeaaaaaaaaaaaaaaBBaaaaBaaaaaaaaaepeeeeau 

xxxxxeeeeaeaaaeaaaaaaaaaaaBBaaaaaaBaeeeaeeao

 xxxxxeeeaeeeaaaaaaaaBaaaaaaaaaaaaaaaeeaaeeo

 xxxxxaaeeaaeeaaaaaBaaaaaaaaaBaaaBeaaeeeeea

 *xxxxxxeeeaaaeaeaaaaaaaaaaBaaaaaaaeafleaeeea

 ***xxxxxaeea8eeeaaaaaaaaaaaaaaaBaaaeeaaaeex

FIGURE 10. Trend surface of factor scores for component two.

WELL 19N/01W-32G

The mineralogy of a number of fine-grained layers 

was determined by X-ray diffraction analysis of sidewall 

samples from well 19N/01W-32G. The results of the

analyses are shown in table 15. Below a depth of about 

562 feet, a large component of X-ray amorphous material is present in the sediments. Whether this is primary volcanic glass or an intermediate weathering
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B27

TABLE 12. Dissolved-oxygen concentrations 

in well water in the south alluvial fans 

fades

Well

08N/03E-05N04M

08N/03E-06M01M

08N/01E-06B01M

08N/01E-17C01M

09N/03E-31L01M

09N/02E-18N01M

09N/02E-33E01M

09N/01E-35H02M

09N/01W-25M01M

10N/02E-03E01M

11N/01W-01D01M

Mean...........

Date

08-01-79

08-01-79

07-31-79

07-26-79

08-01-79

07-31-79

07-31-79

07-31-79

07-31-79 -

07-25-79

07-25-79

Dissolved 

oxygen 

(mg/L)

1.9

2.0

5.9

5.0

2.0

4.4

7.8

3.8

3.2

2.8

3.5

3.8

product, such as an amorphous aluminosilicate (Paces, 

1978) was not investigated. The two clay minerals present in quantity are halloysite and illite. The presence 

of halloysite rather than kaolinite was determined using 

transmission electron microscopy. Figure 11 shows an 

electron micrograph of sediments from the 407-foot 

depth. The tubular structures at the center and top 

of the figure are halloysite, with poorly formed flakes 

of illite also visible. The dark, blocky particles are finegrained quartz and plagioclase.

Optical examination of sand-size grains shows that 

the mineralogy is consistent with an andesitic sand. The 

major mineral is plagioclase, which is zoned and shows 

etching. Some pyroxenes are present and are clear to

light green. Many of the grains are stained by ferric 

hydroxides.

SATURATION INDICES

According to the law of mass action, when an aqueous 

solution is in equilibrium with a mineral, the ion-activity 

product for that mineral will equal the thermodynamic 

equilibrium constant. The degree of saturation of a 

water can be represented by the saturation index (SI), 

which is the logarithm of the quotient of the ion-activity 

product (IAP) divided by the equilibrium constant (K):

K (1)

where

SI=saturation index 

IAP=ion-activity product, and

K=thermodynamic equilibrium constant. 

The saturation index has a value of zero when the 

solution is in equilibrium with a solid phase. A positive 

saturation index indicates supersaturation and a negative index undersaturation with respect to some mineral. Minerals that are present in an aquifer and have 

negative saturation indices can dissolve. Minerals that 

have positive saturation indices may precipitate from 

solution, although whether this actually occurs depends 

on additional factors. By examining the saturation indices of water samples collected from the Sacramento 

Valley, some idea can be obtained about which minerals 

may be influencing ground-water chemistry.

TABLE 13. Mineralogic compositions ofsidewall samples from U.S. Geological Survey test well 12N/01E-34Q

c coarse >0.5 mm.

m medium 0.5-0.25 mm.

f fine 0.25-0.125 mm.

vf very fine 0.125-0.0625 mm.

p pan <0.0625 mm.

VAB very abundant, from 25-50 percent of the heavy minerals. 

AB abundant, from 10-25 percent of the heavy minerals. 

SP sparse, from 1-4 percent of the heavy minerals.

Texture, in percent

Composition, in percent: 

Potassium feldspar ....................

Plagioclase (Anorthite>34) .........

Lithic fragments .......................

Heavy minerals1 .......................

Nonopaque heavy grains:

....... 7/i

..... 12

..... 25

..... 28

..... 20

..... 11

3

323

2/41/34/17

f

13 

44 

10 

23 

10 

1

VAB 

VAB 

VAB 

AB

467

2/2/8/43/45

m f

6 16 

40 33 

24 14 

20 28 

7 6 

3 3

VAB 

VAB 

VAB 

SP

685

3/5/19/38/36

m

27 

22 

25 

20 

4 

2

f

13 

32 

16 

26 

10 

2

VAB 

VAB 

VAB

SP

788

60/22/10/3/5

m

15 

21 

23 

21 

14 

5

f

14 

36 

12 

18 

13 

6

VAB 

VAB 

VAB 

AB

897

8/52/28/4/8

m

15 

28 

23 

7 

21 

6

f

12 

39 

14 

13 

12 

13

AB 

AB 

AB 

AB

1845

59/18/6/4/12

m

10 

19 

3 

31 

21 

15

f

12 

24 

13 

18 

11 

23

AB 

AB 

AB 

VAB

'Heavy minerals have specific gravities greater than 2.9.
B28 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

TABLE 14. Minerals identified in sidewall samples from U.S. 

Geological Survey test well 12N/01E-34Q, in alphabetical order

Actinolite-tremolite (series)1

Amphiboles

Analcite

Apatite

Augite-ferroaugite (series) 1

Basic glass

Biotite

Bronzite-ferrohypersthene

(series) 1 

Chlorite 

Chromite 

Chrysotile 

Clinozoisite 

Cordierite 

Cummingtonite

Diopside-hedenbergite (series)1 

Enstatite-bronzite (series)1 

Epidote 

Fluorite 

Garnet (red) 

Glauconite

Glaucophane

Graphite

Hematite (some specular)

Hercynite

Hornblende (green, brown, and

oxy-varieties) 

Hypersthene 

Lawsonite 

Marcasite 

Pigeonite

Plagioclase (series)1 

Potash feldspar

(undifferentiated) 

Pumpellyite

Richterite (part of series)1 

Riebeckite 

Serpentine (?) 

Sideromelane 

Sphene (titanite) 

Spinel 

Staurolite 

Zircon 

Zoisite

'"Series" means several varieties within the series were found.

Saturation indices were calculated for 271 water 

analyses selected from those published by Fogelman 

(1975, 1976) and Fogelman and Rockwell (1977). Of the 

271, 153 had aluminum analyses and 183 had iron

analyses. Temperature, pH, and alkalinity were determined in the field. Samples for cation determinations 

were passed through a 0.1-micrometer filter and 

acidified with concentrated nitric acid. Some controversy exists as to the validity of aluminum and iron 

data gathered in this manner; however, the possible 

error introduced is generally small (Kennedy and 

others, 1974; Barnes, 1975). Some of the supersaturation in the aluminosilicate and iron minerals may be 

due to particulate iron and aluminum passing through 

the filter. Saturation indices were calculated using 

WATEQF (Plummer, and others, 1978).

Table 16 shows the mean, minimum, and maximum 

saturation indices for selected mineral phases along 

with log equilibrium constants at 20°C, the average 

temperature of ground waters in the Sacramento Valley. Although many minerals, such as the feldspars and 

montmorillonite, show variations in chemical composition, saturation indices were calculated for ideal mineral 

phases. Therefore, stability calculations for these minerals only approximate actual conditions, but they do 

provide useftil indicators of the types of reactions that 

may be occurring.

TABLE 15. Percent mineral composition of sidewall samples from 

U.S. Geological Survey test well 19N/01W-32G

[Tr, trace; unid., unidentified; inter., intermediate; ND, not detected]

Depth 

(feet)

260

353

407

478

562

630

930

984

1,098

1,193

1,390

Quartz

35-45

40

35-40

35-40

10-20

20-25

15-20

5-15

5-10

5-15

5-10

Plagioclase Amorphous Halloysite

25-35

15-20

20-25

25-30

15-25

15-20

10-15

5-10

5-10

5-10

2-5

ND

ND

ND

ND

Major

Inter.

Inter.

Major

Major

Major

Major

15-20

10-15

10-15

10-15

5-15

5-10

10-15

10-15

5-10

5-10

2-5

Illite

5-10

ND

2-5

5-10

2-5

2-10

5-10

1-5

1-5

1-5

2-5

Other

Tr unid.

Tr minor.

Tr minor.

5-10 Chlorite.

ND.

Tr unid.

ND.

Tr unid.

ND.

Tr minor.

ND.

Most waters are close to saturation with respect to 

silicaglass, as might be expected from the volcanic detritus in the sediments. All samples were supersaturated with respect to amorphous ferric hydroxide, 

which may reflect the variability in solubility with particle size or particulate iron passing through the 0.1- 

micrometer filter during sampling. The waters tend to 

be at saturation with respect to gibbsite and slightly 

below saturation with respect to amorphous aluminum 

hydroxide.

Ground water is fairly close to saturation with respect to the carbonate minerals, calcite, dolomite, and 

magnesite. No carbonate minerals were recorded in 

sediments from either of the wells examined for 

mineralogy. Apparently, the formation of secondary 

carbonate minerals is of limited extent despite a 

number of samples indicating supersaturation with respect to calcite.

Potassium and sodium feldspars, which are abundant 

in the sediments, average close to saturation in 

Sacramento Valley ground water. Dissolution of these 

phases probably contributes to water chemistry where 

ground water is undersaturated with respect to these 

feldspars. All water samples are undersaturated with 

respect to anorthite, which is less abundant in the sediments. Halloysite is the only clay mineral with which 

the ground water is not supersaturated. Any of the 

other clay minerals could potentially form from most 

of the waters. When stability fields are considered, 

however, only one phase will be the most stable. These 

phase relations can be illustrated using activity diagrams.

ACTIVITY DIAGRAMS

Activity diagrams can be used to illustrate the phase 

relations among primary and secondary minerals and 

to predict the most stable secondary phases as a function of Water chemistry. Such diagrams have been used 

by Feth, Roberson, and Polzer (1964), Tardy (1971) and 

White (1979) to depict weathering reactions in ground
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B29

FIGURE 11. Electron micrograph of sediments from the 407-foot depth of well 19N/01W-32G. (Photograph by

Technology of Materials, Santa Barbara, Calif.)

waters. The technique for calculating such diagrams is 

given in Garrels and Christ (1965), and Helgeson, 

Brown, and Leeper (1969) give numerous examples of 

such diagrams. A slightly different type of diagram, 

though basically similar, was presented by Kittrick 

(1969), and it depicts equilibrium between solid phases 

and an aqueous solution.

As an example of the calculation of the equilibrium 

boundary between two phases, the calculation of the 

phase boundary between illite and montmorillonite is 

given below. Formulas for these minerals are taken 

from WATEQF (Plummer, and others, 1978). For the 

reaction illite ^ montmorillonite, the chemical equation 

is written
B30 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

TABLE 16. Mean, minimum, and maximum saturation indices in 

Sacramento Valley ground water for selected mineral phases

Saturation index

Phase 1 at 20 °C Mean Minimum Maximum

Silica glass..........

Fe(OH)3 amorph..

A1(OH)3 amorph...

Gibbsite ..............

Fluorite ..............

Hydroxyapatite....

Strengite.............

Calcite................

Dolomite .............

Magnesite... .........

Adularia..... .........

Albite. ................

Anorthite ............

Halloysite............

Kaolinite .............

Illite...................

Montmorillonites:

1. Calcium .......

2. Belle Fouche.

3. Aberdeen.....

Muscovite............

Chlorite ..............

271

183

153

153

269

270

183

271

271

271

153

153

153

153

153

153

153

95

95

153

153

-3.07

4.885

-31.77

-32.95

-11.02

-59.57

-26.37

^8.45

-16.92

^8.16

-20.96

-18.32

-19.55

-33.38

-37.52

^10.99

-45.73

3-34.97

3-29.69

^9.94

-91.29

-0.14

1.31

-1.13

.05

-2.38

-2.22

-0.61

-0.33

-0.54

-0.53

.22

-0.82

-3.27

-0.89

3.26

2.31

3.40

7.49

6.69

1.35

-1.83

-0.58

.62

-2.32

-1.15

-3.50

-7.57

-2.06

-1.80

-3.37

-1.96

-1.81

-2.93

^5.95

-3.76

.32

-1.33

-0.37

4.04

3.43

-3.12

-12.11

0.16

3.67

.03

1.22

-0.10

1.64

1.05

.50

1.24

.43

1.66

.47

-1.77

1.38

5.58

4.53

5.84

9.01

8.10

4.61

3.52

'Chemical formulas for these phases are as written in WATEQF (Plummer and others, 

1978.)

2Data from the latest version of WATEQF. 

^e log equilibrium constant at 25°C is used because no enthalpy is given in WATEQF.

(K0. 6Mg0. 25Al2.3Si3.50 1 o(OH)2)+2.48H4Si04

+0.52Fe+3+0.45Mg+2*+ 

3.13H20+0.6K+1 +1.86H+1

). (2)

At the phase boundary, the ion-activity product for this 

reaction will'equal the equilibrium constant (K):

[K + T6[H+1] 1 - 86

[H4Si04]2- 48[Fe+3]0- 52[Mg+2]()- 45 =K. (3)

Brackets denote free ion activities, which are concentrations corrected for the nonideality of aqueous solutions. Rewriting in terms of log activities gives the equation:

0.61 log [K+1]/[H+1 ]-0.52 log [Fe+3]/[H+1]3 

-0.45 log [Mg+2]/[H+1]2-2.48 log [H4Si04]=log K. (4)

To plot this line on a two-dimensional graph, the equation must be reduced to two variables. The assumptions that may be considered when plotting phase diagrams include (1) controlling the concentration of a component by equilibrium with a solid phase, (2) conserving 

a component in solid phases during phase transformations, and (3) assigning a specified concentration for a

species in solution, commonly the average for that 

species in the study area.

In all plots below, it is assumed that the logarithm 

of the activity quotient of free ferric iron to hydrogen 

cubed (log [Fe+3]/[H+1]3) is controlled by the solubility 

of amorphous ferric hydroxide at 4.885 (Plummer and 

others, 1978). The average for the log activity ratio of 

potassium to hydrogen (log [K+1]/[H+1]) in the valley 

is 2.75, and ratio of magnesium to hydrogen squared 

(log [Mg+2]/[H+1]2) is 11.25. The average log activity 

of silica in ground waters is -3.21. The use of these 

averages where a concentration must be assumed in 

the calculation of a phase transformation does not significantly affect the depicted relations between data 

points and phase boundaries in the following activity 

diagrams.

For the equilibrium between illite and montmorillonite, log [Fe 3]/[H+1]3 is assumed to be controlled by 

the solubility of ferric hydroxide, and log [H4Si04] is 

assigned a value of -3.21. Replacing these quantities 

in equation 4 gives the following equation for the phase 

boundary between illite and montmorillonite:

log [K+1]/[H+1]=0.122+0.743 log [Mg+2]/[H+1f. (5)

Two types of diagrams are presented here to illustrate 

phase relations in the system MgO-K20-Fe203-Al203- 

Si02-H20. The diagrams depart, to some extent, from 

the traditional presentation in that metastable phases 

are included. Recent work by Paces (1973, 1978) and 

Busenberg (1978) indicates that during the weathering 

of feldspars, the initial solids formed are amorphous to 

poorly crystalline aluminosilicates which may be reversible with respect to the aqueous solution. Subsequent 

crystallization results in the formation of a metastable 

aluminosilicate commonly called halloysite. Eventually, 

the thermodynamically most stable mineral will form. 

This pathway between primary and secondary minerals 

generally follows Ostwald's Rule (Darken and Gurry, 

1953), which states that when a system is proceeding 

from a less to a more stable state, the final most stable 

state is not necessarily reached directly but may be 

attained through a series of intermediate states. 

Neither halloysite nor illite are stable with respect to 

montmorillonite or kaolinite, yet they are present in 

the sediments. Activity, diagrams showing metastable 

states indicate the intermediate steps, as well as the 

final secondary minerals that are expected to form.

All minerals included in the following diagrams are 

from WATEQF and have formulas and log equilibrium 

constants from that program. Because no information 

is available on the chemistry of clay minerals from the 

Sacramento Valley, particularly illite and montmorillonite, no better substitute is available. This use of surro-
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B31

gate minerals will introduce some uncertainty in the 

exact locations of phase boundaries in the diagrams. 

Considering the uncertainty in the knowledge of thermodynamic data on montmorillonites and illites, it is 

doubtful if full knowledge of mineral chemistry would 

improve the situation. The montmorillonite used in all 

the diagrams is Aberdeen montmorillonite (Kittrick, 

1971) with modifications by Truesdell and Jones (1974).

Figure 12 shows the stability fields of minerals as 

a function of pH-VapAl* 3 and p[H4Si04], where p stands 

for -log as in pH=-log [H+1 ], and pAl+3=-log [Al+3]. 

This diagram depicts the formation of secondary minerals from an aqueous solution. The solution, in this case, 

is assumed to have the average ion activity ratios and 

silica activity observed in the Sacramento Valley. No 

solid phase will form from solutions with low aluminum 

to hydrogen ratios and low silica concentrations (bottom 

right corner of figure 12). At higher aluminum to hydrogen ratios, the stable solid phase changes from gibbsite 

to kaolinite to montmorillonite with increasing silica. 

Halloysite and amorphous aluminum hydroxide phase 

boundaries are dashed to emphasize that they are both 

metastable phases. A solution in equilibrium with halloysite will be supersaturated with respect to 

montmorillonite or kaolinite. All samples from the valley plot in the montmorillonite stability field. Only 10 

percent of the samples are supersaturated with respect 

to halloysite.

Regression analysis was used to determine if any significant linear relation exists between aluminum and 

silica. Each hydrochemical facies was treated separately 

to preserve as much detail as possible. Only two of the 

regression equations were significant, those for the 

Tuscan volcanic rocks and the Sutter Basin facies.

The equation for the Tuscan volcanic rocks facies is

pH-V3pAr3=2.10+0.39p [H4SiOJ (6)

with r2 equal to 0.26 and coefficients and their 90 percent confidence limits of

2.10 ±0.83 

and

0.39 ±0.27. 

Rewriting the equation in terms of log activities gives .

0.33 log [Ar3]/[H+1]3 =2.10-0.39 log [H4Si04]. (7)

The inverse relation between the aluminum to hydrogen 

ratio and silica is indicative of control by a mineral 

phase. As silica increases in solution, log [Al+3]/[H"f1 ]3 

decreases to compensate. 

The slope is close to and not significantly different

from 0.33. Assuming that it is 0.33 and rearranging 

gives

[H+ 1 ]3 ~ iU ' (8)

The solution of halloysite or kaolinite is given by the 

equation:

1/2Al2Si205(OH)4+3H+1±51/2H20+Ar3 +H4Si04 . (9) 

The ion activity product is

(10)

For halloysite log K equals 6.83 and for kaolinite, 4.76. 

The close agreement between the ion activity product 

of halloysite in ground water from the Tuscan volcanic 

rocks facies with the equilibrium constant is strong evidence that halloysite is controlling the aluminum and 

silica concentrations in ground water.

The Sutter Basin hydrochemical facies also showed 

a significant relation between pH-VspAl"4" 3 and 

p[H4Si04].

pH-y3pAl+3=6.13-0.92p[H4Si04] (11)

with r2 equal to 0.27 and coefficients and their 90 percent confidence limits of

and

6.13±2.91 

0.92±0.91. 

Rewriting the equation in terms of log activities gives

0.33 log [Al+3]/[H+1]3=6.13+0.92 log [H4Si04]. (12)

The positive slope is indicative of solution control of 

water chemistry, with both aluminum and silica increasing as some mineral phase is attacked by hydrogen ion. 

The slope is close to and not significantly different from 

one. This gives an aluminum to silica ratio in the dissolving phase of 1 to 3. The most likely minerals are, 

therefore, potassium and sodium feldspars, which have 

an aluminum to silica ratio of 1 to 3. This aluminum 

to silica ratio, along with the etched surfaces of feldspar 

grains observed under the microscope, may reflect 

stoichiometric dissolution of feldspars as found by Holdren and Berner (1979) and Berner and Holdren (1979). 

Figure 13 shows the phase stability fields for minerals in terms of the log activity ratios of magnesium to
B32 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

3.8

3.6

3.4

3.2

I 

a

3.0

2.8

2.6

2.4

Ox

00

/o w o o o

/°8° ° 0°°

0,0

00

O 00

o

CO

_Amorph.^j_(OH)i 

Solution

Gibbsite

Solution

2.5 3.0 3.5 4.0 

p[H4SiO4]

4.5 5.0 5.5

FIGURE 12. Phase relations among minerals and aqueous solution in terms of pH-V3pAl+3 and p[H4Si04] at 20°C. (NOTE. The 

aqueous solution is assumed to have the following ion activity ratios: log [Fe +3]/[H+1]3 =4.885, log [K+1 ]/[H+1]=2.75, and 

log [Mg+2]/[H +1]2 =11.25.)

hydrogen squared and potassium to hydrogen. Aluminum is assumed to be strictly conserved in solid phases 

during phase transformations. Assumptions about iron 

and silica activities are as described at the beginning 

of this section on activity diagrams.

Figure 13 is actually two diagrams superimposed, so 

that illite, a metastable phase, can be shown. Illite is 

unstable with respect to montmorillonite or adularia if 

the three phases are considered simultaneously. The 

solid lines represent phase relations among the minerals
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B33

16

14

12

10

en

IIChlorite

Montmorillonite

Montmorillonite

Kaolinite

10

Log [K+]/[H+]

FIGURE 13. Phase relations among minerals and coexisting aqueous solution in terms of log [Mg+2]/[H +1 ]2 

and log [K+1]/[H+1] at 20°C. (NOTE. Phase relations among chlorite, montmorillonite, illite, and kaolinite 

are shown by solid lines, and among illite, adularia, and muscovite by dashed lines. The aqueous solution 

is assumed to have the following activities of dissolved species: log [Fe +3]/[H+1]3 =4.885, log [H4Si04]=-3.21.)

chlorite, montmorillonite, illite, and kaolinite. Among 

these minerals, montmorillonite is the most stable 

phase in equilibrium with ground water in the valley. 

The dashed lines represent phase relations among

adularia, muscovite, and illite. Of these three minerals, 

illite is the most stable in contact with Sacramento Valley ground waters. 

Illite is highly resistant to weathering (Berner, 1971),
B34 CENTRAL VALLEY OF CALIFORNIA RASA PROJECT

and most of the illite found in samples from well 19N/ 

01W-32G is probably detrital. Some of the illite may 

have formed from the weathering of primary minerals. 

Illite has been reported to form from orthoclase (Stumm 

and Morgan, 1970) and from muscovite (Berner, 1971).

Log activities from ground water in the Sacramento 

Valley plot in the stability field of a montmorillonite 

having the formula and stability of Aberdeen 

montmorillonite. Differences in activity ratios of ions 

in solution will probably produce montmorillonites of 

somewhat different composition in the Sacramento Valley, but some form of montmorillonite will be the most 

stable phase. Metastable phases either form as intermediate steps during the diagenesis of primary minerals 

(halloysite) or persist due to resistance to weathering 

(illite).

SUMMARY AND CONCLUSIONS

Shallow sediments in the Sacramento Valley were 

laid down as alluvial deposits in a freshwater environment. These sediments, from which ground-water samples were collected, have always been in contact with 

freshwater. Sediments along the valley margins are 

coarse grained, poorly sorted, and oxidized. In the central part of the valley, flood-basin deposits are finer 

grained and reduced. Most of the streams entering the 

valley are losing streams, at least over part of their 

courses, and much of the ground-water recharge is from 

this source.

The chemistry of ground water in the valley is greatly influenced by the chemistry of this recharge from 

the valley margins. Approximately one-third of the variation in ground-water chemistry can be attributed to 

this effect. The reducing conditions and finer grained 

sediments in the midvalley flood basins also influence 

ground-water chemistry. These processes have distinct 

spatial patterns that produce spatial variations in the 

chemical character of ground water in different areas.

Six areas, or hydrochemical facies, were delineated 

on the basis of geology, hydrology, and distinctions in 

chemical composition of ground water. Two of the facies 

lie along the eastern margin and receive recharge from 

the Cascade Range and Sierra Nevada. These waters 

are low in dissolved solids and high in silica. Two facies 

are located in the midvalley flood basins, and reducing 

conditions there produce higher concentrations of iron, 

manganese, and arsenic in the ground water. While the 

relative concentrations of these species are higher in 

the flood basins than in the rest of the valley, the absolute concentrations remain fairly low. Upwelling of 

saline water from underlying marine sediments contributes to high dissolved-solids concentrations in ground 

water in the southern parts of the flood basins.

Water along the west side of the valley is low in 

silica, and the water grades from moderate dissolvedsolids levels in the north to high dissolved-solids levels 

in the south. Ground water in the southwest of the valley is also high in boron. The source of the high boron 

and dissolved-solids concentrations is thermal springs 

in the Coast Ranges. Two facies were delineated along 

the western margin to separate the waters having moderate and high dissolved solids. Because of the 

heterogeneity of ground-water chemistry in the valley, 

division into areas having more uniform chemical compositions provides additional detail in any investigation 

of water chemistry.

Dissolved-solids and nitrate concentrations were 

examined for linear trends with time for each of the 

hydrochemical facies. Significant increases in both were 

observed in parts of the valley. On the east side of 

the valley, the Tuscan volcanic rocks facies showed a 

significant increase in dissolved solids of 0.95 (mg/L)/yr 

but no significant change in nitrate concentrations. The 

Victor Plain facies showed significant increases in both 

dissolved solids [1.6 (mg/L)/yr] and nitrate-nitrogen 

[0.036 (mg/L)/yr]. Neither Butte Basin nor Sutter Basin 

hydrochemical facies, in the midvalley flood basins, 

showed a significant trend in nitrate with time. There 

were very few wells with multiple nitrate measurements, however, and the lack of significant changes 

may be due to the small sample size. Dissolved-solids 

concentrations in Butte Basin have been increasing at 

an average rate of 1.37 (mg/L)/yr, but are unchanged 

in Sutter Basin.

Both the north and south alluvial fans facies, adjacent 

to the Coast Ranges, show significant upward trends 

in dissolved solids and nitrate. The south alluvial fans 

facies has the greatest rate of increase for dissolved 

solids [4.75 (mg/L)/yr] of all the facies, and the north 

alluvial fans facies has the greatest rate of increase for 

nitrate-nitrogen [0.099 (mg/L)/yr]. The midvalley basins 

have fine-grained poorly permeable soils that protect 

the ground water from contamination. Infiltration rates 

are slow, and reducing conditions permit the removal 

of nitrate by denitrification reactions. Along the valley 

margins, however, significant increases in dissolved solids and nitrates have occurred since the mid-1950's.

Alkali feldspars and halloysite appear to be the most 

significant aluminosilicate minerals affecting water 

chemistry. Where waters are undersaturated, dissolution of feldspars takes place, bringing the water to near 

saturation with respect to adularia and albite. There 

is a general parallel increase in aluminum and silica reflecting the solution of aluminosilicates by ground 

water. In Sutter Basin, the relation between log [Al"1" 3]/ 

[H+1]3 and log [H4Si04] indicates an increase in both 

aluminum and silica at a ratio 1 to 3. In the Tuscan
GEOCHEMISTRY OF GROUND WATER IN THE SACRAMENTO VALLEY, CALIFORNIA B35

volcanic rocks facies, the relation between log [Ar 3]/ 

[H+1 ]3 and log [H4SiOJ is inverse. Rearranging the regression equation yields the same equation as for 

equilibrium between halloysite and an aqueous solution. 

The regression constant is very close to the equilibrium 

constant for halloysite, which supports the theory of 

solubility control by halloysite. Illite and halloysite 

were identified in sediments from the Sacramento Valley. Neither of these clay minerals is stable in the 

ground-water environment. The most stable clay minerals in equilibrium ground water of the valley is 

montmorillonite. Halloysite occurs as a mestastable intermediate step in the weathering of primary minerals 

to stable secondary minerals.

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