Ground-Water Quality in the Southeastern 


Sacramento Valley Aquifer, California, 1996


By Barbara J. Milby Dawson 

U.S. GEOLOGICAL SURVEY


Water-Resources Investigations Report 01-4125


NATIONAL WATER-QUALITY ASSESSMENT PROGRAM


6211-13


Sacramento, California 

2001 
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey. 

For additional information write to:	 Copies of this report can be purchased

from: 

U.S. Geological Survey 

Water Resources Division 

Placer Hall, Suite 2012 

6000 J Street 

Sacramento, California 95819-6129 

U.S. Geological Survey


Branch of Information Services


Box 25286


Denver, CO 80225-0286

FOREWORD 

The U.S. Geological Survey (USGS) is committed to 

serve the Nation with accurate and timely scientific information that helps enhance and protect the overall quality of 

life, and facilitates effective management of water, biological, energy, and mineral resources. (http://www.usgs.gov/). 

Information on the quality of the Nation’s water resources is 

of critical interest to the USGS because it is so integrally 

linked to the long-term availability of water that is clean and 

safe for drinking and recreation and that is suitable for 

industry, irrigation, and habitat for fish and wildlife. Escalating population growth and increasing demands for the 

multiple water uses make water availability, now measured 

in terms of quantity and quality, even more critical to the 

long-term sustainability of our communities and ecosystems. 

The USGS implemented the National Water-Quality 

Assessment (NAWQA) Program to support national, 

regional, and local information needs and decisions related 

to water-quality management and policy. (http:// 

water.usgs.gov/nawqa). Shaped by and coordinated with 

ongoing efforts of other Federal, State, and local agencies, 

the NAWQA Program is designed to answer: What is the 

condition of our Nation’s streams and ground water? How 

are the conditions changing over time? How do natural features and human activities affect the quality of streams and 

ground water, and where are those effects most pronounced? By combining information on water chemistry, 

physical characteristics, stream habitat, and aquatic life, the 

NAWQA Program aims to provide science-based insights 

for current and emerging water issues and priorities. 

NAWQA results can contribute to informed decisions that 

result in practical and effective water-resource management 

and strategies that protect and restore water quality. 

Since 1991, the NAWQA Program has implemented 

interdisciplinary assessments in more than 50 of the 

Nation’s most important river basins and aquifers, referred 

to as Study Units. (http://water.usgs.gov/nawqa/ 

nawqamap.html). Collectively, these Study Units account 

for more than 60 percent of the overall water use and population served by public water supply, and are representative 

of the Nation’s major hydrologic landscapes, priority ecological resources, and agricultural, urban, and natural 

sources of contamination. 

Each assessment is guided by a nationally consistent 

study design and methods of sampling and analysis. The 

assessments thereby build local knowledge about 

quality issues and trends in a particular stream or 

while providing an understanding of how and wh

quality varies regionally and nationally. The cons

multi-scale approach helps to determine if certain 

water-quality issues are isolated or pervasive, and 

direct comparisons of how human activities and n

cesses affect water quality and ecological health 

Nation’s diverse geographic and environmental s

Comprehensive assessments on pesticides, nutrie

tile organic compounds, trace metals, and aquatic 

are developed at the national scale through comp

analysis of the Study-Unit findings. (http://water.

nawqa/natsyn.html). 

The USGS places high value on the commun

and dissemination of credible, timely, and relevan

so that the most recent and available knowledge a

resources can be applied in management and poli

sions. We hope this NAWQA publication will pr

the needed insights and information to meet your 

thereby foster increased awareness and involvem

protection and restoration of our Nation’s waters

The NAWQA Program recognizes that a nati

assessment by a single program cannot address a

resource issues of interest. External coordination 

els is critical for a fully integrated understanding 

sheds and for cost-effective management, regulat

conservation of our Nation’s water resources. The 

therefore, depends extensively on the advice, coo

and information from other Federal, State, interst

and local agencies, non-government organization

academia, and other stakeholder groups. The assi

suggestions of all are greatly appreciated. 

Robert M. Hirsch

Associate Director for Water
CONTENTS 

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

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1


Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Study Area Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Climate and Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Water Use and Land Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Study Design and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Study Area and Well Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Water Sample Collection and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Data Reporting and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7


Well Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7


Quality-Control Data Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8


Ground-Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9


General Water Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9


Inorganic Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10


Nutrient Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10


Dissolved Organic Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


Stable Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


Tritium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


Radon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13


Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13


Volatile Organic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15


Factors Affecting Ground-Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Ground-Water Redox Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Depth to Water Table and Well Opening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Land Use, Pesticide Use, and Fertilizer Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

FIGURES 

1.	 Location of Sacramento Valley aquifer, Sacramento subunit study area, Sacramento River Basin, and 

physiographic provinces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

2. Locations of wells sampled in the southeastern Sacramento Valley aquifer, California . . . . . . . . . . . . . . . . . . . . . 4 

3. Well locations and surficial geology in the southeastern Sacramento Valley aquifer, California . . . . . . . . . . . . . . 6 

4.	 Major-ion composition of ground-water samples from the southeastern Sacramento Valley aquifer, 

California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

5.	 Relation between delta (δ) deuterium and δ oxygen-18 in ground water, surface water, and rain water 

in parts of the Sacramento Valley, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

6.	 Number of pesticides detected in wells sampled in the southeastern Sacramento Valley aquifer, 

California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Contents V 
7.	 Number of volatile organic compounds detected in wells sampled in the southeastern Sacramento Valley 

aquifer, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

8.	 Ground-water redox condition in wells sampled in the southeastern Sacramento Valley aquifer, 

California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

TABLES 

1. Laboratory analyses methods for measured water-quality constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

2. Well construction information for wells sampled in rice areas in the Sacramento Valley, 

California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

3. Summary of mean relative standard deviations for replicate samples of inorganic constituents, nutrient 

constituents, and dissolved organic carbon for ground-water studies in the Sacramento River Basin, 

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

4. Summary of general water chemistry in ground water in the southeastern Sacramento Valley aquifer, 

California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

5. Comparison of selected ground-water-quality constituents measured in the southeastern Sacramento 

Valley aquifer, California, 1996, with those measured in other National Water-Quality Assessment Program 

studies in drinking water aquifers, 1992–1995 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

6. Summary of inorganic constituents detected in ground water in the southeastern Sacramento Valley 

aquifer, California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

7. Summary of nutrient constituents and dissolved organic carbon detected in ground water in the 

southeastern Sacramento Valley aquifer, California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

8. Summary of isotopes detected in ground water in the southeastern Sacramento Valley aquifer, 

California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

9. Summary of pesticides in ground water in the southeastern Sacramento Valley aquifer, 

California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

10. Pesticides and degradation products not detected in ground water in the southeastern Sacramento 

Valley aquifer, California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

11. Summary of volatile organic compounds detected in ground water in the southeastern Sacramento 

Valley aquifer, California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

12. Volatile organic compounds not detected in ground water in the southeastern Sacramento Valley 

aquifer, California, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

13. Amount of nitrogen applied as fertilizer in the Sacramento Subunit study area . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

VI Contents 
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS


Multiply By To obtain 

centimeter (cm) 0.3937 inch 

centimeter per year (cm/y) .3937 inch per year 

cubic meter per day (m3/d) .000811 cubic foot per day 

meter (m) 3.281 foot 

meter per day (m/d) 3.281 foot per day 

square kilometer (km2) .3861 square mile 

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

°F = (1.8)°C + 32 

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

Abbreviations and Acronyms 

μg/L microgram per liter


μS/cm microsiemen per centimeter


mg/L milligram per liter


pCi/L picocurie per liter


CADHS California Department of Health Services


DOC dissolved organic carbon


DWR California Department of Water Resources


DWS drinking-water standard


EPA U.S. Environmental Protection Agency


HAL lifetime health advisory level


MCL maximum contaminant level


NAWQA National Water-Quality Assessment (Program)


NWQL National Water Quality Laboratory


QC quality control


SMCL secondary maximum contaminant level


USGS U.S. Geological Survey


VOC volatile organic compound


Contents VII 
Ground-Water Quality in the Southeastern Sacramento 

Valley Aquifer, California, 1996 

By Barbara J. Milby Dawson 

ABSTRACT 

In 1996, the U.S. Geological Survey sampled 

29 domestic wells and 2 monitoring wells in the 

southeastern Sacramento Valley as part of the U.S. 

Geological Survey’s National Water-Quality 

Assessment (NAWQA) Program. This area, designated 

as the NAWQA Sacramento subunit study area, was 

chosen because it had the largest amount of groundwater use in the Sacramento River Basin. The 

Sacramento subunit study area is about 4,400 square 

kilometers and includes intense agricultural and urban 

development. The wells sampled ranged from 14.9 to 

79.2 meters deep. Ground-water samples from 31 wells 

were analyzed for 6 field measurements, 14 inorganic 

constituents, 6 nutrient constituents, organic carbon, 

86 pesticides, 87 volatile organic compounds, tritium 

(hydrogen-3), radon-222, deuterium (hydrogen-2), and 

oxygen-18. 

Nitrate levels were lower than the 2000 drinkingwater standards in all but one well, but many detections 

were in the range that indicated an effect by human 

activities on ground-water quality. Radon was detected 

in all wells, and was measured at levels above the 

proposed Federal 2000 maximum contaminant level in 

90 percent of the wells. Five pesticides and one pesticide degradation product were detected in groundwater samples and concentrations were below 2000 

drinking-water standards. All pesticides detected 

during this study have been used in the Sacramento 

Valley. Thirteen volatile organic compounds were 

detected in ground water. One detection of trichloroethene was above Federal 2000 drinking-water 

standards, and another, tetrachloromethane, was above 

California 1997 drinking-water standards; both 

occurred in a well that had eight volatile organic compound detections and is near a known source of groundwater contamination. Pesticides and volatile organic 

compounds were detected in agricultural and urban 

areas; both pesticides and volatile organic compounds 

were detected at a higher frequency in urban wells. 

Ground-water chemistry indicates that natural 

processes and human activities are affecting groundwater quality in the upper part of the southeastern 

Sacramento Valley aquifer. The factors identified as 

having an influence on ground-water quality were 

redox condition in the aquifer, depth within the aquifer, 

and land use overlying the aquifer. Nitrate concentrations showed a statistical correlation with each of these 

factors. Detections of pesticides and volatile organic 

compounds were too few to compare concentrations 

with the various factors, but the types of synthetic 

compounds detected were consistent with the surrounding land use. Sixty-one percent of the wells sampled in this study showed the effect of human activities 

on ground-water quality in the form of a nitrate concentration over 3 milligrams per liter or a detection of a 

pesticide or volatile organic compound. In general, the 

water quality in the southeastern Sacramento Valley 

aquifer was found suitable for most uses. 

INTRODUCTION 

In 1996, the U.S. Geological Survey (USGS) 

conducted a ground-water-quality study on the southeastern side of the Sacramento Valley in northern 

California (fig. 1). This study is one component of the 

USGS National Water-Quality Assessment (NAWQA) 

Program, which began in 1991. The NAWQA Program 

is described in Hirsch and others (1988). NAWQA 

studies began in the Sacramento River Basin in 1994; 

data from these studies will be compared to similar 

studies throughout the United States to assess the 

quality of the Nation’s water resources, to assess longterm changes in the Nation’s water quality, and to 

identify natural and human factors that govern the 

Nation’s water quality. 

Introduction 1 
Colusa 

Sutter 

Buttes 

Sacramento 

Arcade 

Creek 

Sacramento R iver 

Feather River 

Putah Creek 

Cache Creek 

0 

0 20 

20 40 MILES 

40 KILOMETERS 

Sacramento River 

Basin Study Area 

Central 

Valley 

Sacramento 

Valley 

Physiographic provinces 

EXPLANATION 

Great Basin 

Sierra 

Nevada 

Coast 

Ranges 

Middle 

Cascade 

Mountains 

Klamath 

Mountains 

Sacramento 

Valley 

Rivers and streams 

Reservoirs 

American

River 

Marysville 

Southeastern Sacramento Valley 

aquifer (Sacramento subunit 

study area) 

Sacramento Valley 

aquifer 

Sacramento River Basin 

CALIFORNIA 

121° 

122° 

41° 

123° 

40° 

39° 

123° 

122° 

121° 

39° 

40° 

41° 

Figure 1. Location of Sacramento Valley aquifer, Sacramento subunit study area, Sacramento River Basin, and physiographic provinces. 

2 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 
This study is one of three ground-water studies 

conducted by the Sacramento River Basin NAWQA 

Program during 1996–1998, and is part of the NAWQA 

ground-water study component called a study-unit 

survey. The objective of a study-unit survey is to assess 

the overall water quality in the aquifers that supply the 

highest amount of drinking water within the study 

basin. In the Sacramento River Basin, the study-unit 

survey will consist of at least three subunit surveys 

within the Sacramento Valley aquifer, which is the 

largest aquifer within the basin and supplies the most 

drinking water from ground water. This report 

describes the first subunit survey to be conducted, 

designated the Sacramento subunit, which assessed the 

general water chemistry and quality of the ground 

water in the upper part of the southeastern Sacramento 

Valley aquifer, including the occurrence and 

distribution of arsenic, nitrate, radon, pesticides, and 

volatile organic compounds (VOC). 

Purpose and Scope 

This report describes the ground-water quality in 

the upper part of the southeastern Sacramento Valley 

aquifer, California, and factors that may be affecting 

that water quality, such as human activities (land use, 

pesticide use, and fertilizer use) and natural conditions 

(depth to water level in relation to top of well opening 

and redox condition). Ground-water samples from 31 

wells (fig. 2) were analyzed for 6 field measurements, 

14 inorganic constituents, 6 nutrient constituents, 

organic carbon, 86 pesticides, 87 VOCs, tritium 

(hydrogen-3), radon-222, deuterium (hydrogen-2) and 

oxygen-18. Measured concentrations were compared 

to data on ground-water redox conditions, depth to 

water table and well opening, land use, and pesticide 

and fertilizer use. 

Study Area Description 

Climate and Population 

The Sacramento subunit is in the southeastern 

part of the Sacramento Valley (fig. 1). The Sacramento 

Valley occupies the northern one-third of California’s 

Central Valley. The Sacramento subunit is about 

4,400 km2 (square kilometer) and includes intense 

agricultural and urban development. Land-surface 

elevations range from sea level to about 150 m (meter) 

above sea level along the eastern edge of the valley. The 

southeastern Sacramento Valley has a Mediterranean 

climate (Blair and Fite, 1957, p. 323) with hot, dry 

summers and wet, mild winters. Rainfall in the study 

area ranges from about 43 to 53 cm/y (centimeter per 

year) (Daly and Taylor, 1998), almost all of which 

occurs between late autumn and early spring. The total 

population of the study area in 1990 was about 

1,276,000, which was mostly in the Sacramento 

metropolitan area; the population density ranges from 

less than 100 per square kilometer in agricultural areas 

to greater than 4,800 per square kilometer in parts of the 

Sacramento metropolitan area (Hitt, 1994). 

Water Use and Land Use 

Ground water and surface water in the study area 

are used for drinking water and irrigation. Drinkingwater supply in the Sacramento metropolitan area is a 

mix of surface water and ground water; drinking water 

in rural areas and small towns is supplied mostly from 

ground water. About one-half of the drinking water in 

this study area is supplied from ground water. Total 

ground-water use in 1995 was about 2 million cubic 

meters per day. About one-third of the study area is 

irrigated by a combination of ground water and surface 

water. [All water-use data are from Perlman (1999).] 

The principle land uses in the study area are agriculture 

and urban; the main agricultural land uses are rice crops 

and fruit and nut orchards. Other land uses include 

grain, field crops, tomatoes, and pasture (Domagalski 

and others, 1998). 

Hydrogeology 

The wells sampled for the Sacramento subunit 

study tap the upper 80 m of the Sacramento Valley 

aquifer, a single heterogeneous aquifer with no 

continuous confining layers or other distinct internal 

boundaries that occupies the northern one-third of 

California’s Central Valley (fig. 1). The aquifer occupies a structural trough between the Sierra Nevada and 

Coast Ranges (fig. 1) and consists of thousands of 

meters of sediment eroded from the surrounding 

mountains (Page, 1986). In most areas, fine-grained 

sediments make up more than 50 percent of the aquifer 

system (Page, 1986). Intensive development of surfaceand ground-water resources for water supply and flood 

control during the last 100 years has greatly altered the 

hydrologic system in the Sacramento Valley; discharge 

and recharge have increased from natural conditions 

due to ground-water pumping and irrigation return 

(Bertoldi and others, 1991). Regional ground-water 

flow extends from the sides of the valley toward the 

center and southward. Ground-water conditions grade 

downward from unconfined to semiconfined to confined below the upper hundred meters. Pumping 

depressions have developed in some areas from 

pumping for irrigation and drinking-water supply 

(Williamson and others, 1989). 

The alluvial deposits that compose the upper part 

of the southeastern Sacramento Valley aquifer range in 

Introduction 3 
121° 

122° 30' 

39° 

30' 

EXPLANATION 

SOLANO 

COUNTY 

SACRAMENTO 

COUNTY 

YOLO 

COUNTY 

SUTTER 

COUNTY 

YUBA 

COUNTY 

BUTTE 

COUNTY 

PLACER 

COUNTY 

COLUSA 

COUNTY 

GLENN 

COUNTY 

Marysville 

Colusa 

31 

Sacramento 

0 

0 0 

10 

20 KILOMETERS 

20 MILES 

30 

29 28 

27 

24 

22 

20 

18 

17 

15 13 

14 16 

19 

21 

23 

26 25 

11 12 

10 

9 8 

6 7

4 3

2 

1 

5 

Sutter 

Buttes 

1

Well location and number 

27 Domestic well 

4 Monitoring well 

Southeastern Sacramento 

Valley aquifer (Sacramento 

subunit study area) 

Reservoirs 

Rivers and streams T15N 

County boundaries 

39° 

38° 

30' 

Figure 2. Locations of wells sampled in the southeastern Sacramento Valley aquifer, California. 

4 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 
age from Eocene to Holocene (fig. 3). These deposits, 

eroded from the Sierra Nevada, Sutter Buttes, and other 

areas of the basin, consist of clay, silt, sand, and gravels 

that have been transported and deposited by the 

Sacramento River and its east-side tributaries (Helley 

and Harwood, 1985). The sediment source areas 

consist mainly of metamorphic, granitic, and volcanic 

rock types. Recharge is thought to occur mainly along 

the river channels, in irrigated areas, and, to some 

extent, from precipitation. Estimated soil permeabilities throughout most of the study area generally are 

less than 3 m/d (meters per day) and can range up to 

6 m/d (Bertoldi, 1974). 

Study Design and Methods 

Study Area and Well Selection 

For this study, the Sacramento Valley Aquifer 

was divided into subunits that could be characterized 

by sampling 30 wells; divisions were based on the 

source of sediment that composes the aquifer. The 

Sacramento subunit study area, which mostly consists 

of sediment derived from the Sierra Nevada to the east, 

has the highest ground-water use in the Sacramento 

River Basin and was selected by the Sacramento River 

Basin NAWQA Program to be sampled in 1996. 

The criteria used for well selection in this study 

follow those published in Lapham and others (1997). 

After selecting the study boundaries, a computer geographic information system was used to divide the 

Sacramento subunit into 30 equal-area cells. A computer program (Scott, 1990) was used to randomly 

select and prioritize the wells from the USGS well 

database that met the well-selection criteria and were 

located in one of the 30 cells. The same program also 

was used to randomly prioritize grid points in each cell; 

township/range/section of the selected grid points was 

determined, and records of wells in these township/ 

range/sections were obtained from the California 

Department of Water Resources (DWR). The selected 

wells then were field inventoried and a final evaluation 

of each well was made and permission to sample the 

well was obtained from the property owner. If a well 

did not meet the selection criteria or the owner would 

not give permission to sample, the search was expanded 

to the other wells within that cell, and, in some cases, a 

suitable well in an adjacent cell was selected. Thirtyone wells were selected for sampling, 29 domestic 

wells and a pair (shallow and deep) of monitoring 

wells. 

Water Sample Collection and Analysis 

The wells selected for this study were sampled 

between May and August 1996. Sampling equipment 

consisted of Teflon tubing with stainless steel fittings 

attached to a faucet on the well head or discharge line at 

a point before pressure tanks or treatment. Samples 

were collected and processed following the protocols in 

Koterba and others (1995) with the exception of 

dissolved organic carbon (DOC) samples, which were 

filtered using a Teflon filter holder and an electric, 

oil-free vacuum pump. 

Before pumping, the water level in the well was 

measured and used to calculate the volume of standing 

water in the well casing. The well then was purged by 

pumping at least three full casing-volumes of water. 

While the well was being purged, water temperature, 

pH, specific conductance, and dissolved oxygen were 

monitored. Ground-water samples were collected after 

three full casing-volumes were pumped or when the 

field measurements were stable. After all the samples 

were collected at each site, the sampling equipment was 

cleaned and stored in plastic bags. Periodically, qualitycontrol (QC) samples were collected to document 

sample bias and variability. 

All ground-water samples were shipped to the 

USGS National Water Quality Laboratory (NWQL) in 

Arvada, Colorado, for analysis, except tritium samples, 

which were analyzed at the USGS Tritium Laboratory 

in Menlo Park, California, and stable isotope samples, 

which were analyzed at the USGS Isotope Laboratory 

in Reston, Virginia. Analytical methods used in this 

study are listed in table 1. Semiquantitative 

(presence/absence) measurements for ferrous iron and 

hydrogen sulfide also were made in the field using 

colorimetric methods. 

Data Reporting and Analysis 

Water temperature is reported in degrees Celsius. 

Specific conductance values are reported in microsiemens per centimeter. Alkalinity and hardness are 

reported in milligrams per liter as calcium carbonate. 

Dissolved concentrations of oxygen, solids (residueon-evaporation at 180ο

C), organic carbon and inorganic 

and nutrient compounds are reported in milligrams 

per liter (mg/L). Nitrate, nitrite, and ammonia are 

reported in milligrams per liter as nitrogen. Nitrate 

values in this report are equivalent to nitrate plus nitrite 

as no detections of nitrite were greater than 

0.01 mg/L. Phosphorus and orthophosphorus are 

reported in milligrams per liter as phosphorus. Pesticide 

and VOC concentrations are reported in micrograms 

per liter. Radon and tritium concentrations are reported 

in picocuries per liter (pCi/L). Deuterium and oxygen18 data are reported as ratios per mill or parts per 

thousand of deuterium to hydrogen and oxygen-18 to 

oxygen-16, respectively. All data used in this report are 

available on the Web at 

<http://infotrek.er.usgs.gov/wdbctx/nawqa/nawqa.home>. 

Introduction 5 
121° 

122° 30' 

39° 

0 

0 10 

10 

20 KILOMETERS 

20 MILES 

31 

30 

29 

28 

27 

24 

22 

20 

18 

17 

15 13 14 

16 

19 

21 

23 

26 25 

11 

12 

10 

9 

8 

7 

6 

4 3 

2 

1 

5 

EXPLANATION 

Rocks and deposits of the Southeastern 

Sacramento Valley aquifer 

30' 

39° 

38° 

30' 

Qb Flood-basin deposits, Holocene 

Qr River deposits, Holocene

QTc 

Tcpm 

t Dredge tailings, Holocene 

Continental rocks and deposits, 

Pliocene to Holocene 

Continental rocks and deposits, 

Miocene and Pliocene 

Tcmo Continental rocks and deposits, 

Oligocene and Miocene 

Continental rocks and deposits, 

Eocene Tce 

Other rocks and deposits shown on this map 

QTvs }

Volcanic rocks and deposits at 

Sutter Buttes, Pliocene and 

QTv Pleistocene

Tvu 

Tm 

pTs 

pTg 

pTm 

Volcanic rocks, uncertain age 

Marine rocks and deposits, 

Paleocene and Eocene 

Marine rocks, pre-Tertiary 

Granitic rocks, pre-Tertiary 

Metamorphic rocks, pre-Tertiary 

Reservoirs 

3 Location and number of well sampled 

for this study 

Modified from Page, 1986. 

Figure 3. Well locations and surficial geology in the southeastern Sacramento Valley aquifer, California. 

6 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 
Table 1. Laboratory analyses methods for measured water-quality constituents 

Constituent group Analysis method Reference 

Deuterium/hydrogen Hydrogen equilibrium and mass spectrometry Coplen and others (1991) 

Dissolved organic carbon Ultraviolet-promoted persulfate oxidation and infrared spectrometry Brenton and Arnett (1993) 

Inorganic constituents Atomic absorption spectrometry, colorimetry, ion-exchange Fishman and Friedman (1989) 

chromatography, or inductively coupled plasma atomic emission Fishman (1993) 

spectrometry or mass spectrometry Struzeski and others (1996) 

Nutrient constituents Colorimetry Patton and Truitt (1992) 

Fishman (1993) 

Oxygen-18/oxygen-16 Carbon dioxide equilibrium Epstein and Mayeda (1953) 

Pesticides Solid-phase extraction on a carbon-18 cartridge and gas Zaugg and others (1995) 

chromatography/mass spectrometry Lindley and others (1996)

Pesticide Solid-phase extraction on a Carbopak-B cartridge and high 

performance liquid chromatography with ultraviolet detection 

Werner and others (1996) 

Radon-222 Liquid scintillation counting American Society for Testing and 

Materials (1992) 

Tritium Electrolytic enrichment with gas counting Ostlund and Dorsey (1975) 

Volatile organic compounds Purge and trap capillary gas chromatography/mass spectrometry Rose and Schroeder (1995) 

In this report, water-quality constituents are 

compared to the year 2000 drinking-water standards 

(DWS) set by the U.S. Environmental Protection 

Agency (EPA) and to 1997 DWS set by the California 

Department of Health Services (CADHS); CADHS 

standards are used for constituents not currently 

regulated by the EPA or for those constituents with a 

CADHS standard lower than the EPA standard. 

Drinking-water standards used for comparison are the 

maximum contaminant level (MCL), which is the 

maximum concentration of a contaminant permissible 

in a public water system; the secondary maximum 

contaminant level (SMCL), which is a guideline for 

taste, odor, and appearance of drinking water (EPA 

SMCLs are unenforceable, while CADHS SMCLs are 

enforceable) and the lifetime health advisory level 

(HAL), which is the concentration of a chemical in 

drinking water that is not expected to cause any adverse 

noncarcinogenic effects during a lifetime of exposure 

(California Department of Water Resources, 1997; U.S. 

Environmental Protection Agency, 2000). 

Summary statistics include median values 

because most ground-water chemistry data have nonnormal distributions and many outliers. The Spearman 

rank correlation test was used to determine the degree 

of correlation between two different variables. The data 

for each variable were ranked, and the ranks of each 

pair of data were compared to see if the higher- and 

lower-ranking data of one variable corresponded to the 

higher- and lower-ranking data of the other variable. 

The linear correlation coefficient, rho, (Spearman’s 

rho) is computed on the ranks of the data; rho varies 

between –1 and 1, with values closer to –1 indicating an 

inverse correlation, values closer to 1 indicating a 

positive correlation, and values closer to 0 indicating no 

correlation (Helsel and Hirsch, 1995). A significance 

level of 95 percent confidence was used for all statistical tests in this report. The words “significant” and 

“correlation” always are used in the statistical sense. 

Acknowledgments 

The author thanks the homeowners and businesses in the Sacramento Valley and the Geology 

Department at California State University, Sacramento, 

for allowing the USGS to sample their wells during this 

study. Their cooperation has added much to the understanding of ground-water quality in the Sacramento 

Valley. 

WELL DESCRIPTIONS 

Well construction data for the wells sampled 

during this study are listed in table 2. These wells 

included twenty-nine 10- to 25-cm (centimeter)­

diameter wells used for domestic or irrigation supply 

and two 5- and 10-cm-diameter wells used for monitoring. Well depths ranged from 14.9 to 79.2 m below 

land surface, with the depth to the top of the opening in 

the well casings ranging from 8.5 to 65.5 m below land 

surface. Well casings are steel or polyvinyl chloride. 

Land-use information was obtained from the DWR. 

Land use around the wells sampled in 1996 was verified 

in the field by the USGS. Water levels were measured 

during field inventory and again during sampling. Water 

levels ranged from 0.7 to 48.8 m below land surface, 

with a median value of 7.5 m below land surface. 

Well Descriptions 7 
5

10

15

20

25

30

Table 2. Well construction information for wells sampled in rice areas in the Sacramento Valley, California, 1996 

[USGS, U.S. Geological Survey; masl, meter above sea level; mbls, meter below land surface; ND, no data] 

Well 

number USGS Site ID State well number 

Land 

surface Well depth 

elevation (mbls) 

(masl) 

Open 

interval 

(mbls) 

Depth to 

ground 

water 

(mbls) 

Water 

level 

elevation 

(masl) 

Date waterlevel 

measured 

1 381923121255001 006N005E33Q001M 5.5 48.2 42.1–48.2 17.4 –11.9 5/23/96 

2 382855121221601 007N005E01R001M 14.0 79.2 59.4–79.2 28.5 –14.5 6/26/96 

3 383304121192501 008N006E16B002M 22.9 45.7 30.5–45.7 25.2 –2.4 5/2/96 

4 383350121254301 008N005E09H001M 9.9 14.9 8.2–14.3 7.3 2.6 5/13/96 

383352121254002 008N005E09H003M 9.7 63.4 58.8–63.4 11.2 –1.4 5/13/96 

6 383801121333801 009N004E17J002M 8.5 45.7 42.4–45.7 4.3 4.2 5/20/96 

7 383914121124901 009N007E09B001M 82.6 76.2 38.1–76.2 17.6 65.0 5/30/96 

8 384301121195101 010N006E16P001M 42.4 70.1 64.0–70.1 48.8 –6.4 4/16/96 

9 384330121265601 010N005E17H001M 17.4 73.2 64.6–73.2 29.9 –12.5 8/6/96 

384455121292101 010N004E01K001M 12.2 49.4 42.7–48.8 16.9 –4.7 7/12/96 

11 384736121411501 011N003E18N001M 8.8 68.0 60.7–65.5 5.8 3.0 1

4/2/85 

12 384949121233501 011N005E02M001M 25.9 54.9 36.6–54.9 27.7 –1.8 5/30/96 

13 385432121213001 012N005E12A001M 33.8 33.5 21.3–30.5 15.6 18.2 6/3/96 

14 385432121451401 012N002E09A001M 6.7 47.2 42.7–47.2 0.9 5.8 6/6/96 

385546121312801 013N004E33J001M 14.6 47.2 30.5–47.2 5.1 9.5 6/4/96 

16 385550121352201 013N003E36L001M 10.7 16.8 13.4–16.8 2.6 8.0 8/8/96 

17 385718121290401 013N004E24N001M 19.2 64.9 24.4–64.9 3.4 15.8 3/21/96 

18 385914121215801 013N005E12Q002M 38.1 32.6 29.3–32.6 14.5 23.6 5/21/96 

19 390301121391001 014N003E20H003M 13.4 38.1 20.7–38.1 4.4 9.0 5/22/96 

390333121250701 014N005E16Q001M 30.5 71.6 62.2–71.6 28.4 2.1 6/3/96 

21 390342121415501 014N002E13L002M 11.3 27.4 18.0–27.4 0.7 10.6 5/22/96 

22 390743121273601 015N005E30C001M 26.2 61.0 48.8–61.0 18.3 7.9 7/3/96 

23 390756121411901 015N002E24J001M 14.6 25.9 11.0–25.9 2.3 12.3 6/26/96 

24 390945121354601 015N003E12M001M 18.3 21.3 12.2–21.3 5.8 12.5 8/5/96 

390954121394302 015N003E08F002M 17.4 35.1 21.3–35.1 6.8 10.6 6/13/96 

26 391016121411701 015N002E01R001M 17.4 16.2 8.5–16.2 2.2 15.2 6/12/96 

27 391806121484501 017N001E25D001M 23.2 27.4 18.3–27.4 10.0 13.2 6/12/96 

28 392121121393401 017N003E05L001M 28.7 29.0 18.3–29.0 2.2 26.4 6/10/96 

29 392209121320301 018N004E33L001M 33.8 42.7 ND 5.3 28.5 6/10/96 

392636121324501 018N004E05M001M 54.9 34.7 28.7–34.7 16.0 38.8 6/11/96 

31 392945121350001 019N003E13P001M 45.7 52.4 48.5–52.4 7.5 38.2 6/11/96 

1

Most recent water level available for this well. 

QUALITY-CONTROL DATA ANALYSES 

Quality-control samples, in addition to the 

ground-water samples, were collected to evaluate the 

bias and variability of the ground-water chemistry data 

measured in this study. QC sample types included fieldblank samples, replicate ground-water samples, and 

field-spiked ground-water samples. Blank and replicate 

sample data from all three ground-water studies in the 

Sacramento River Basin from 1996 to 1998 were used 

for this analysis. 

Blank samples were collected to evaluate bias in 

ground-water chemical data from contamination introduced during the sample collection and analysis of 

inorganic constituents, nutrient constituents, DOC, 

pesticides, and VOCs. On the basis of contamination in 

blank samples, environmental data were affected for 

ammonia and DOC. The concentrations of these two 

compounds measured in the ground water were all 

within three standard deviations of the mean of the 

concentrations measured in the field blanks. This 

indicates that measured concentrations of these 

8 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 
constituents may be partly or entirely due to contamination that was introduced during sampling or analysis; 

these data were not used for data analyses in this report. 

DOC contamination in field blanks may have come 

from the cleaning methods used to prevent contamination of other organic constituents during sampling; this 

contamination may not be present in the ground-water 

samples. Sources of ammonia contamination are not 

known. 

Replicate samples were collected to determine 

variability of the ground-water chemical data for inorganic constituents and for DOC. A total of 11 replicate 

sample pairs were used for this evaluation. The mean 

relative standard deviations for all constituents were 

less than 15 percent and for most constituents were less 

than 5 percent (table 3). 

Surrogates (compounds that behave similarly to 

pesticide or VOC analytes) were added to all pesticide 

and VOC samples to evaluate the ability of the sampling and analysis methods to detect the target analytes 

in each ground-water sample. The mean recoveries of 

the surrogates in all ground-water samples for pesticides and VOCs were within the control limits determined by the NWQL for 1996 (R.W. Brenton, U.S. 

Geological Survey National Water Quality Laboratory, 

unpub. data, 1997; M.P. Schroeder, U.S. Geological 

Survey National Water Quality Laboratory, unpub. 

data, 1997; B. Connor, U.S. Geological Survey 

National Water Quality Laboratory, unpub. data, 1998). 

Spike solutions containing known amounts of 

some target analytes were added to some replicate 

ground-water samples to evaluate bias of those pesticide and VOC analyses. Seventy-nine of 86 pesticides 

were evaluated with field-spiked samples; 77 of these 

had mean recoveries within NWQL’s 1996 control 

limits (R.W. Brenton, U.S. Geological Survey National 

Water Quality Laboratory, unpub. data, 1997; M.P. 

Schroeder, U.S. Geological Survey National Water 

Quality Laboratory, unpub data, 1997). Thirteen VOCs 

were evaluated with field-spiked samples; 12 had mean 

recoveries within NWQL’s 1996–1998 control limits 

(B. Connor, U.S. Geological Survey National Water 

Quality Laboratory, unpub. data, 1998). The mean 

recoveries of two compounds, atrazine (a pesticide) and 

tetrachloromethane (a VOC), detected in ground-water 

samples were slightly higher (1 percent and 4 percent, 

respectively) than the upper control limits, indicating 

that detected values of those compounds may be biased 

slightly high in some ground-water samples. The mean 

recovery of the pesticide chloramben was lower than 

the lower control limit, indicating that chloramben may 

not have been detected if present in low concentrations 

in some ground-water samples. Replicates of VOC 

field-spiked samples also were collected; the relative 

standard deviation of all VOC-spike compounds was 

less than 20 percent. 

GROUND-WATER QUALITY 

General Water Chemistry 

Summary statistics for general water chemistry 

measurements are listed in table 4. The pH in one well 

was below the secondary DWS (measured at 6.4; 

SMCL is 6.5–8.5; U.S. Environmental Protection 

Agency, 2000). Secondary DWSs for specific 

conductance, measured at 1,402 and 2,270 μS/cm 

(microsiemen per centimeter) (SMCL=900 μS/cm; 

California Department of Water Resources, 1997), and 

Table 3. Summary of mean relative standard deviations for replicate samples of inorganic constituents, nutrient constituents, and dissolved 

organic carbon for ground-water studies in the Sacramento River Basin, California 

Mean relative standard deviation Mean relative standard deviation Mean relative standard deviation 

of less than 5 percent of less than 10 percent of less than 15 percent 

Arsenic 

Calcium 

Chloride1 

Iron 

Hardness as calcium 

carbonate (CaCO3) 

Magnesium


Nitrite


Potassium


Silica


Sodium


Sulfate


Total dissolved solids


Ammonia1 Boron1


Dissolved organic carbon1 Bromide1


Fluoride1 Orthophosphorus1


Nitrate1


Phosphorus1


1

One to two individual replicate pairs had a relative standard deviation of greater than 20 percent. 

Ground-Water Quality 9 
Table 4. Summary of general water chemistry in ground water in the southeastern Sacramento Valley aquifer, California, 1996 

[°C, degrees Celsius; CaCO3, calcium carbonate; SMCL, secondary maximum contaminant level; μS/cm, microsiemen per centimeter; 

mg/L, milligram per liter]; —, no value available] 

Number of 

Drinking- Drinking- wells 

Minimum Median Maximum water exceeding General water chemistry measurement measurement measurement water standard a drinking- standard type water 

standard 

Alkalinity, mg/L as CaCO3 53 136 345 — — — 

Dissolved solids, mg/L, residue on 134 258 1,750 500 SMCL1 3 

evaporation at 180°C 

Dissolved oxygen, mg/L 0 4.2 7.6 — — — 

Hardness, total, mg/L as CaCO3 48 (soft) 135 (hard) 940 (very hard) — — — 

pH, standard units 6.4 7.4 8 6.5–8.5 SMCL1 1 

Specific conductance, μS/cm at 25°C 159 390 2,270 900 SMCL2 2 

Water temperature, °C 16.8 19.6 21 — — — 

1

U.S. Environmental Protection Agency, 2000. 2

California Department of Water Resources, 1997. 

dissolved solids, measured at 912 and 1,750 mg/L 

(SMCL=500 mg/L; U.S. Environmental Protection 

Agency, 2000) were exceeded in two wells. The SMCL 

for dissolved solids also was exceeded in a third well 

(measured at 569 mg/L). The median concentration of 

dissolved solids measured in this study (258 mg/L) was 

slightly lower than the national median for drinkingwater aquifers (298 mg/L) measured in other NAWQA 

studies from 1992–1995 (table 5) (Gilliom and others, 

1998, p. 18). 

Inorganic Constituents 

The major inorganic constituents (those with 

median concentrations greater than 1 mg/L) detected in 

ground water in this study were bicarbonate, silica, 

calcium, sodium, chloride, magnesium, sulfate, and 

potassium; summary statistics for these inorganic 

constituents are listed in table 6. The major water types 

found in this study were magnesium-calciumbicarbonate waters (fig. 4), with some wells having 

sodium-bicarbonate water types and others having a 

more mixed water type (magnesium-calcium-sodiumbicarbonate-chloride). The higher concentrations of 

sodium and chloride in some wells may be due to 

natural or man-made causes. The minor inorganic 

constituents (those with median concentrations less 

than 1 mg/L) that were detected in ground water in this 

study were boron, fluoride, manganese, bromide, iron, 

and arsenic (table 6). 

Drinking-water standards were exceeded for five 

inorganic constituents. The SMCL for chloride (250 

mg/L; U.S. Environmental Protection Agency, 2000) 

was exceeded in 1 of 31 wells (3 percent). The HAL for 

boron (0.6 mg/L; U.S. Environmental Protection 

Agency, 2000) was exceeded in 2 of 31 wells 

(6 percent); these wells also exceeded the water-quality 

criterion for boron in irrigation water (0.75 mg/L; 

National Academy of Sciences and National Academy 

of Engineering, 1972). The SMCL for iron (300 μg/L; 

U.S. Environmental Protection Agency, 2000) was 

exceeded in 3 of 31 wells (10 percent). The SMCL for 

manganese (50 μg/L; U.S. Environmental Protection 

Agency, 2000) was exceeded in 5 of 31 wells (16 

percent) sampled during this study. Arsenic was detected 

in 28 of 31 wells (90 percent); all concentrations were 

below the current 1996 EPA MCL (50 μg/L; U.S. 

Environmental Protection Agency, 1996). Lower MCLs 

for arsenic have been proposed [0.005 mg/L (U.S. 

Environmental Protection Agency, 2000); 0.010 mg/L 

(U.S. Environmental Protection Agency (2001a)]; 

however, those MCLs are being withdrawn and a new 

MCL is under review (U.S. Environmental Protection 

Agency, 2001b). The EPA also has an HAL for arsenic 

of 0.002 mg/L (U.S. Environmental Protection Agency, 

2000); 20 out of 31 wells (64 percent) have an arsenic 

concentration over the HAL. 

Nutrient Constituents 

The nutrient inorganic constituents detected in 

ground water during this study were nitrate, nitrite, 

ammonia, phosphorus, and orthophosphorus (table 7). 

However, detections of ammonia in field blanks indicate 

that the concentrations of ammonia in ground water may 

be partly or entirely due to contamination introduced 

during sample collection or analysis. The MCL for 

nitrate (10 mg/L as nitrogen; U.S. Environmental 

Protection Agency, 2000) was exceeded in 1 of 31 wells 

(3 percent). Nitrate was detected in 29 of 31 wells 

sampled (94 percent). Eight of the wells (26 percent) had 

nitrate values greater than 3 mg/L. Ground water 

containing 3 mg/L or more nitrate is considered to be 

affected by human activity (Madison and Brunett, 1984). 

10 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 
Table 5. Comparison of selected ground-water-quality constituents 

measured in the southeastern Sacramento Valley aquifer, 

California, 1996, with those measured in other National WaterQuality Assessment Program studies in drinking water aquifers, 

1992–1995 

[mg/L, milligram per liter; pCi/L, picocurie per liter. —, no value 

available] 

National Median median concentration concentration in southeastern Compound Sacramento in drinking 

water Valley aquifer, aquifers, 1996 1992–1995 

Dissolved solids (mg/L) 258 298 

Nitrate + nitrite (mg/L) 1.4 1.0 

Radon (pCi/L) 495 450 

Detection National 

frequency in detection 

southeastern frequency in 

Compound Sacramento drinking water 

Valley aquifers, 

aquifer, 1996 1992–1995 

(percent) (percent) 

Pesticides, total 35.5 39.6 

Atrazine 6.4 19.1 

Bentazon 16.7 1.3 

Bromacil 3.2 0.6 

Desethyl atrazine 12.9 17.5 

Simazine 9.7 6.3 

Tebuthiuron 3.2 0.9 

Volatile organic 38.7 10 

compounds, total 

1,1-Dichloroethane 6.4 — 

1,2,4-Trimethylbenzene 9.7 0.0 

1,2-Dichloroethane 3.2 0.0 

Bromodichloromethane 3.2 1.2 

cis-1,2-Dichloroethene 3.2 0.2 

(1,2-DCE) 

Dichlorodifluoromethane 9.7 1.2 

(Freon 12) 

Methyl tert-butyl ether 3.2 2.8 

(MTBE) 

Styrene 3.2 0.0 

Tetrachloroethene (PCE) 6.4 2.0 

Tetrachloromethane 3.2 0.2 

Trichloroethene (TCE) 12.9 0.9 

Trichlorofluoromethane 3.2 1.4 

(Freon 11) 

Trichloromethane 6.4 6.8 

(Chloroform) 

Nitrite was detected in 3 of 31 wells (10 percent) at 

0.01 mg/L. The median concentration for nitrate + 

nitrite in this study was 1.4 mg/L, which is higher than 

the national NAWQA median concentration of 

1.0 mg/L in drinking-water aquifers (table 5) (Gilliom 

and others, 1998, p. 17). 

Dissolved Organic Carbon 

Dissolved organic carbon was detected in 27 of 

31 wells (87 percent) (table 7). The median concentration of DOC detected in ground water in this study was 

0.3 mg/L, which is lower than the median DOC concentration detected in wells throughout the Nation 

(0.7 mg/L, Leenheer and others, 1974). Detections of 

DOC in field blanks indicate that all concentrations 

measured in ground water in this study may be partly or 

entirely due to contamination introduced during sample 

collection or analysis. 

Stable Isotopes 

The naturally occurring stable isotopes 

deuterium (hydrogen-2) and oxygen-18 were measured 

in 22 wells sampled for this study (table 8). Figure 5 

shows a plot of these data along with values measured 

in surface water and rainfall for this and previous 

studies (Davisson and others, 1993). The large rivers of 

the Sacramento Valley that originate to the north and 

east carry large amounts of precipitation and snowmelt 

from higher elevations that is isotopically lighter (green 

triangles in fig. 5). Smaller creeks originating in the 

west and in the Sacramento Valley carry valley rainfall 

and irrigation drainage water that is isotopically heavier 

(blue triangles in fig. 5). Both groups of data plot along 

the line calculated for North American Meteoric Waters 

(Coplen, 1993, p. 235). Ground-water stable isotope 

data from the upper part of the southeastern Sacramento 

Valley aquifer (red circles in fig. 5) overlaps and spans 

the gap between the two groups of surface-water data 

along the North American Meteoric Water Line. This 

indicates that ground water in the study area is a combination of water from large rivers (high elevation 

precipitation) and from valley precipitation and surface 

water. No distinct areal or vertical pattern within the 

aquifer was found between wells with lighter or heavier 

stable isotopes. 

Tritium 

Tritium (hydrogen-3) was detected in 18 of 22 

wells (82 percent) (table 8). Tritium concentrations 

were below the 1997 CADHS drinking-water standard 

(20,000 pCi/L; California Department of Water 

Resources, 1997) in all wells. Tritium, a naturally 

occurring and man-made radioactive isotope of hydrogen with a short half-life of 12.43 years, can be used to 

determine whether ground water has been recharged 

since the earlier 1950s [the beginning of nuclear bomb 

testing that produced tritium levels up to three orders of 

magnitude higher than natural concentrations (Michel, 

1989, p. 2)]. Ground water that originated as 

Ground-Water Quality 11 
Table 6. Summary of inorganic constituents detected in ground water in the southeastern Sacramento Valley aquifer, California, 1996 

[mg/L, milligram per liter; μg/L, microgram per liter; SMCL, secondary maximum contaminant level; MCL, maximum contaminant level; 

HAL, health advisory level; —, no value available] 

Compound 

Number of 

detections/ 

samples 

Minimum 

detection 

Median 

detection 

Maximum 

detection 

Drinking water 

standard 

Drinking 

water 

standard 

type 

Number of wells 

exceeding a 

drinking-water 

standard 

Arsenic, μg/L as As 28/31 1 5 46 50 MCL1 0 

Bicarbonate, mg/L as HCO3 31/31 67 160 413 — — — 

Boron, μg/L as B 31/31 12 42 1100 600 HAL1 2 

Bromide, mg/L as Br 30/31 0.02 0.06 1.2 — — — 

Calcium, mg/L as Ca 31/31 10 24 210 — — — 

Chloride, mg/L as Cl 31/31 2.0 17 620 250 SMCL1 1 

Fluoride, μg/L as F 22/31 0.1 0.2 0.3 4 MCL1 0 

Iron, μg/L as Fe 20/31 3 7 1600 300 SMCL1 3 

Magnesium, mg/L as Mg 31/31 5.0 17 100 — — — 

Manganese, μg/L as Mn 9/31 1 80 870 50 SMCL1 5 

Potassium, mg/L as K 31/31 0.40 1.4 4.1 — — — 

Silica, mg/L as SiO2 31/31 24 58 86 — — — 

Sodium, mg/L as Na 31/31 5.7 20 120 — — — 

Sulfate, mg/L as SO4 31/31 1.0 11 130 250 SMCL1 0 

1

U.S. Environmental Protection Agency, 2000. 

20 

40 

60 

80 

20 

40 

60 

80 

80 

60 

40 

20 

80 

60 

40 

20 

20 

40 

60 

80 

80 

60 

40 

20 

80 

60 

40 

20 

20 

40 

60 

80 

80 60 40 20 20 40 60 80

Magnesium (Mg) 

Sulfate (SO

Sodium (Na) + Potassium (K) 

Carbonate (CO

Calcium (Ca) + Magnesium (Mg)

Sulfate (SO

Ca 

4 

4 

4 

Mg SO

Cl 

3 

3 

Na+K 

) 

) 

) + Bicarbonate (HCO

) + Chloride (Cl) 

HCO3+CO3 Calcium (Ca) Chloride (Cl) 

C A T I O N S Percent milliequivalents per liter 

Figure 4. Major-ion composition of ground-water samples from the southeastern Sacramento Valley aquifer, California. 

A N I O N S 

12 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 
Table 7. Summary of nutrient constituents and dissolved organic carbon detected in ground water in the southeastern Sacramento Valley 

aquifer, California, 1996 

[All units shown in milligrams per liter; HAL, health advisory level; MCL, maximum contaminant level; C, carbon; N, nitrogen; P, 

phosporous; —, no value available] 

Number 

Drinking- of wells 

Number of detec- Minimum Median detec- Maximum Drinking- water exceeding Compound water tions/samples detection tion detection standard standard a drinkingtype water 

standard 

Ammonia, as N 24/31 0.02 0.02 0.11 30 HAL1 0 

Nitrate, as N 29/31 .06 1.4 12 10 MCL1 1 

Nitrite, as N 3/31 .01 .01 .01 1 MCL1 0 

Orthophosphorus, as P 31/31 .03 .1 .4 — — — 

Phosphorus, as P 30/31 .03 .1 .45 — — — 

Dissolved organic carbon, as C 27/31 .2 .3 .7 — — — 

1

U.S. Environmental Protection Agency, 2000. 

precipitation and recharged before the 1950s would 

now have a tritium concentration of about 1 pCi/L 

(Plummer and others, 1993). Current tritium 

concentrations in rainfall are about 44 pCi/L 

[1989–1993 average in rainfall at Menlo Park, 

California (International Atomic Energy Agency/World 

Meteorological Organization, 1998)]. The concentrations of tritium measured in ground water in the upper 

part of the southeastern Sacramento Valley aquifer 

ranged from 4 to 67 pCi/L; four wells contained no 

measurable tritium. This indicates that most of the 

ground water sampled was at least partly recharged in 

the last 45 years. It is possible that some wells yield a 

mixture of old and young ground water. 

Radon 

Radon-222 was detected in all the wells sampled, 

with a median concentration of 495 pCi/L in the study 

area (table 8). The proposed MCL for radon-222 (300 

pCi/L; U.S. Environmental Protection Agency, 2000) 

was exceeded in 28 of 31 wells (90 percent). The 

median concentration of radon from this study slightly 

exceeded the NAWQA median concentration of 

450 pCi/L in drinking-water aquifers (table 5) (Gilliom 

and others, 1998, p. 19). 

Radon is a naturally produced radioactive isotope 

(radon-222) that occurs as a gas; it is a by-product of the 

natural decay of uranium that is present in small quantities in certain rock types, such as the granitic rocks 

found in the Sierra Nevada and sediment derived from 

those rocks in the Sacramento Valley. Radon gas is 

soluble in water and is transported in ground water. 

When water containing radon gas is exposed to air (as 

when pumped out of an aquifer and used as tap or 

shower water), the radon diffuses into the air where it 

can be inhaled. 

Pesticides 

Of the 86 pesticides and pesticide degradation 

products that were analyzed for in ground-water samples for this study, only 5 pesticides were detected in 

study wells (listed in order of decreasing frequency of 

detection): bentazon, simazine, atrazine, bromacil, and 

tebuthiuron (table 9). One degradation or breakdown 

product of atrazine, desethyl atrazine, also was 

detected. The pesticides and pesticide degradation 

Table 8. Summary of isotopes detected in ground water in the southeastern Sacramento Valley aquifer, California, 1996 

—, no value available; pCi/L, picocurie per liter; MCL, maximum contaminant level] 

Number 

Isotope Units 

Number of 

detections/ 

samples 

Minimum 

detection 

Median 

detection 

Maximum 

Detection 

Drinkingwater 

standard 

Drinkingwater 

standard 

type 

of wells 

exceeding 

a drinkingwater 

standard 

Deuterium Ratio per mill 22/22 –78.8 –59.3 –50.5 — — — 

Oxygen-18 Ratio per mill 22/22 –10.9 –8.33 –6.82 — — — 

Radon-222 pCi/L 30/30 230 495 940 300 MCL1 28 

Tritium pCi/L 18/22 4 19 67 20,000 MCL2 0 

1

Under review by the U.S. Environmental Protection Agency (2000). 2

California Department of Water Resources, 1997. 

Ground-Water Quality 13 
-30 

-40 

-50 

-60 

-70 

-80 

-90-12 -11 -10 -9 -8 -7 -6 -5 -4 

δ OXYGEN-18, IN PER MILL 

Figure 5. Relation between delta (δ) deuterium and δ oxygen-18 in ground water, surface water, and rain water in parts of the Sacramento 

δ DEUTERIUM, IN PER MILL 

Groundwater from southeastern Sacramento 

Valley aquifer, 1996 

North American meteoric water line 

(Coplen, 1993) 

Average rainfall for the southern Sacramento 

Valley (Davisson and others, 1993) 

Surface water originating in the Coast Ranges 

and Sacramento Valley (data from this study 

and Davisson and others, 1993) 

Surface water originating in the Klamath Mountains 

and Sierra Nevada 

study and Davisson and others, 1993) 

(data from this 

Valley, California. 

products not detected in ground-water samples in the 

southeastern Sacramento Valley aquifer are listed in 

table 10. A total of one, two, or three pesticides 

(table 9) were found in 9 of 31 wells (29 percent) that 

were sampled for this study. This frequency is less than 

the NAWQA median frequency of about 40 percent for 

pesticide detections in a drinking-water aquifer 

(table 5) (Gilliom and others, 1998, p. 17). The 

locations of wells in which one or more pesticides were 

detected are shown in figure 6. Forty-six percent of the 

detections were estimated concentrations (the 

compound was present in the water sample, but the 

concentration was below the lowest calibration 

standard). All pesticides detected in this study were 

below the 2000 drinking-water standards (California 

Department of Water Resources, 1997; U.S. 

Environmental Protection Agency, 2000) (table 9). 

Atrazine and its breakdown product, desethyl atrazine, 

Table 9. Summary of pesticides in ground water in the southeastern Sacramento Valley aquifer, California, 1996 

[All units shown in micrograms per liter; E, estimated value, compound was detected at a concentration below the lowest calibration standard; MCL, maximum contaminant level; HAL, health advisory level; —, no value available] 

Number 

Pesticide 

Number of 

detections/ 

samples 

Minimum 

detection 

Maximum 

detection 

Drinkingwater 

standard 

Drinkingwater 

standard 

type 

of wells 

exceeding 

a drinkingwater 

standard 

Atrazine 2/31 0.001 0.001 3 MCL1 0 

Bentazon 4/24 E .02 E 1.3 18 MCL2 0 

Bromacil 1/31 .34 — 90 HAL1 0 

Desethyl atrazine 4/31 E .004 E .044 — — — 

Simazine 3/31 .006 .077 4 MCL1 0 

Tebuthiuron 1/31 .032 — 500 HAL1 0 

1

U.S. Environmental Protection Agency, 2000. 2

California Department of Water Resources, 1997. 

14 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 
Table 10. Pesticides and degradation products not detected in 

ground water in the southeastern Sacramento Valley aquifer, 

California, 1996 

Pesticides not detected 

2,4-D Fonofos 

2,4-DB gamma-HCH 

2,4,5-T Linuron 

2,4,5-TP Malathion 

Acetochlor MCPA 

Acifluorfen MCPB 

alpha-HCH Methiocarb 

Alachlor Methomyl 

Aldicarb Methyl azinphos 

Benfluralin Methyl parathion 

Bromoxynil Metolachlor 

Butylate Metribuzin 

Carbaryl Molinate 

Carbofuran Napropamide 

Chloramben Neburon 

Chlorothalonil Norflurazon 

Chlorpyrifos Oryzalin 

cis-Permethrin Oxamyl 

Clopyralid Parathion 

Cyanazine Pebulate 

Dacthal, mono-acid Pendimethaline 

DCPA Phorate 

Diazinon Picloram 

Dicamba Prometon 

Dichlobenil Pronamide 

Dichlorprop Propachlor 

Dieldrin Propanil 

Dinoseb Propargite 

Disulfoton Propham 

Diuron Propoxur 

DNOC Terbacil 

EPTC Terbufos 

Esfenvalerate Thiobencarb 

Ethalfluralin Triallate 

Ethoprop Triclopyr 

Fenuron Trifluralin 

Fluometuron 

Pesticide degradation products not detected 

Aldicarb sulfone Carbofuran, -3-hydroxy 

Aldicarb sulfoxide 

were detected less frequently than in other NAWQA 

drinking-water aquifer studies, whereas bentazon, 

simazine, bromacil, and tebuthiuron were detected 

more frequently (table 5) (U.S. Geological Survey 

Pesticide National Synthesis Project, 1998). 

The five pesticides detected in ground-water 

samples collected during this study have been used in 

the study area, according to data from the California 

Department of Pesticide Regulation (1997). The most 

frequently detected pesticide, bentazon, was used on 

rice crops, but was banned in 1989 because of 

detections in ground water (Miller-Maes and others, 

1993) Bentazon continues to be detected in ground 

water in many parts of the Sacramento Valley, including 

this study area (Bartkowiak and others, 1998). The 

major uses for atrazine, bromacil, and tebuthiuron are 

for weed control in right-of-way areas and for landscape maintenance. Simazine is used for weed control 

in right-of-way areas and for landscape maintenance, 

and also on many crops grown in the study area, 

including nut and fruit orchards (California Department 

of Pesticide Regulation, 1997). 

Volatile Organic Compounds 

One, two, three, or eight VOCs were detected in 

12 of 31 wells (39 percent) sampled for in this study 

(table 11, fig. 7). This is higher than the NAWQA 

median frequency of 10 percent for a VOC detection in 

drinking-water aquifers (table 5) (Gilliom and others, 

1998, p. 18). Sixty-seven percent of these detections 

were estimated concentrations (the compound was 

present in the water sample, but the concentration was 

below the lowest calibration standard). Of the 87 VOCs 

analyzed for in this study, 13 VOCs were detected in 

ground-water samples (listed in order of decreasing 

frequency of detection) (table 11): trichloroethene, 

dichlorodifluoromethane, 1,2,4-trimethylbenzene, 

trichloromethane, tetrachloroethene, 1,1-

dichloroethane, tetrachloromethane, 1,2-

dichloroethane, trichlorofluoromethane, cis-1,2-

dichloroethene, styrene, methyl tert-butyl ether, and 

bromodichloromethane. The VOCs not detected in 

ground-water samples from the southeastern 

Sacramento Valley aquifer are listed in table 12. In one 

of the wells sampled, eight VOCs were detected, two of 

which exceeded the MCLs—tetrachloromethane, 

measured at 1.2 μg/L [MCL=0.5 μg/L; California 

Department of Water Resources (1997)], and trichloroethene, measured at 5.5 μg/L [MCL=5 μg/L; U.S. 

Environmental Protection Agency (2000)] (table 11). 

This well is near a site of known contamination, and the 

compounds detected are consistent with contamination 

associated with that site. Of the VOCs detected, 12 were 

detected more frequently in this study than in NAWQA 

drinking-water aquifer studies nationwide between 

1992–1995; only trichloromethane was detected less 

frequently (table 5) (U.S. Geological Survey Volatile 

Organic Compound National Synthesis Project, 1998). 

The 13 VOCs detected in ground-water samples in this 

study have many different uses, including gasoline 

components, pesticides, solvents, degreasers, and 

refrigerants, and can be by-products of drinking-water 

chlorination (Vershueren, 1983; U.S. Environmental 

Protection Agency, 1998). No information is available 

on the amount or uses for these VOCs in the 

Sacramento Valley. 

Ground-Water Quality 15 
16 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 

Marysville 

Colusa 

Sacramento 

0 

0 0 

10 

20 KILOMETERS 

20 MILES 

Study area 

Reservoirs 

Rivers and streams 

0 Pesticides 

1 Pesticide 

2 Pesticides 

3 Pesticides 

Cities 

Well locations, well numbers, and 

number of pesticides detected 

31 

30 

29 28 

27 

24 

22 

20 

18 

17 

15 13 

14 16 

19 

21 

23 

26 25 

11 

12 

10 

9 8 

6 7

3 

4 

2 

1 

5 

2 

1 

3 

4 

121° 

122° 30'

39° 

30' 

39° 

38° 

30' 

Figure 6. Number of pesticides detected in wells sampled in the southeastern Sacramento Valley aquifer, California. 

1
Table 11. Summary of volatile organic compounds detected in ground water in the southeastern Sacramento Valley aquifer, California, 1996 

[All units shown in micrograms per liter; E, estimated value, compound was detected at a concentration below the lowest calibration 

standard; —, no value available; MCL, maximum contaminant level; HAL, health advisory level] 

Number 

Number of Minimum Maximum Drinking- Drinking- of wells 

Volatile organic compound detections/ detection detection water water exceeding 

samples standard standard a drinkingtype water 

standard 

1,1-Dichloroethane 

1,2,4-Trimethylbenzene 

1,2-Dichloroethane 

Bromodichloromethane 

cis-1,2-Dichloroethene 

Dichlorodifluoromethane 

Methyl tert-butyl ether 

Styrene 

Tetrachloroethene 

Tetrachloromethane 

Trichloroethene 

Trichlorofluoromethane 

Trichloromethane 

2/31 E 0.02 E 0.04 — — — 

3/31 E .01 E .02 — — — 

1/31 .19 — 0.5 MCL1 0 

1/31 E .03 — 100 MCL1 0 

1/31 .43 — 6 MCL2 0 

3/31 E .04 E .29 1,000 HAL1 0 

1/31 E .06 — 20 HAL1 0 

1/31 E .06 — 100 MCL1 0 

2/31 .58 .97 5 MCL1 0 

1/31 1.2 — .5 MCL2 1 

4/31 E .01 5.5 5 MCL1 1 

1/31 E . 04 — 150 MCL2 0 

3/31 E .03 1.1 100 MCL1 0 

1

U.S. Environmental Protection Agency, 2000. 2

California Department of Water Resources, 1997. 

FACTORS AFFECTING GROUND-WATER QUALITY 

Ground-Water Redox Condition 

The oxidation-reduction (redox) state of water 

can affect which compounds are present in that water. 

Water with chemistry indicating oxidizing chemical 

reactions is referred to as oxic; water with chemistry 

indicating reducing chemical reactions is referred to as 

anoxic. As it is easy to introduce oxygen into ground 

water during the sampling process, the classification of 

ground water as oxic or anoxic is based on the concentrations of several compounds that are sensitive to 

dissolved oxygen concentration (such as nitrate, iron, 

manganese, and sulfate), rather than dissolved oxygen 

concentrations (Saad and Thorstenson, 1998). 

Hull (1984) proposed that reducing conditions in 

the fine-grained sediments in flood-basin areas are a 

major factor influencing the ground-water chemistry of 

the Sacramento Valley. Concentrations of dissolved 

oxygen, nitrate, nitrite, iron, manganese, sulfate, and 

sulfide in this study indicate that anoxic conditions 

exist in parts of the southeastern Sacramento Valley 

aquifer. Wells having dissolved oxygen concentrations 

of at least 1 mg/L, nitrogen present as principally 

nitrate, sulfur present as sulfate (no sulfides), and very 

low concentrations of manganese and iron were 

classified as oxic. Wells having low concentrations of 

dissolved oxygen, nitrate, and sulfate, and relatively 

high concentrations of dissolved iron and manganese, 

and the presence of sulfide were classified as anoxic. 

Ten of 31 wells (32 percent) in this study have 

chemistry indicating anoxic conditions; these wells are 

near the center of the Sacramento Valley, and correspond to the areas with finer-grained flood basin or 

alluvial deposits (fig. 8). 

A significant correlation was found between 

nitrate concentrations and dissolved oxygen concentrations (p=0.006, rho=0.49; Spearman’s rank correlation), indicating that reducing chemical reactions that 

decrease the dissolved oxygen concentrations may be 

decreasing the nitrate concentrations and preventing 

them from exceeding the MCL in some areas. A 

significant inverse correlation also was found between 

arsenic and dissolved oxygen concentrations (p=0.001, 

rho=–0.56; Spearman’s rank correlation), suggesting 

that the presence and concentration of arsenic is related 

to the redox condition of the ground water. 

No significant correlations were found between 

the number of pesticides or VOCs detected in a well and 

the dissolved oxygen concentration (p=0.77, rho=0.05 

for pesticides and p=0.54, rho=0.12 for VOCs; 

Spearman’s rank correlation). The number of pesticides 

or VOCs detected in this study were too few to compare 

their concentrations with dissolved oxygen 

concentrations, but an inverse correlation has been 

found in other studies (Barbash and Resek, 1996). 

Depth to Water Table and Well Opening 

Depth from land surface to the water table or to 

the top of the well opening can influence the presence 

and amount of a compound in ground water. In this 

study, pesticides were detected in wells as deep as 49 m 

Factors Affecting Ground-Water Quality 17 
18 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 

Marysville 

Colusa 

Sacramento 

0 

0 0 

10 

20 KILOMETERS 

20 MILES 

Study area 

Reservoirs 

Rivers and streams 

Well locations, well number, 

and number of volatile organic 

compounds (VOCs) detected 

0 VOCs 

1 VOC 

2 VOCs 

3 VOCs 

8 VOCs 

31 

30 

29 28 

27 

24 

22 

20 

18 

17 

15 13 

14 16 

19 

21 

23 

26 25 

11 

12 

10 

9 8 

6 7

4 

3 

2 

1 

5 

19 

14 

15 

6 

3 

121° 

122° 30'

39° 

30' 

39° 

38° 

30' 

Cities 

Figure 7. Number of volatile organic compounds detected in wells sampled in the southeastern Sacramento Valley aquifer, California. 

1
Table 12. Volatile organic compounds not detected in ground water in the southeastern Sacramento Valley aquifer, California, 1996 

Volatile organic compounds not detected 

1,1,1,2-Tetrachloroethane


1,1,1-Trichloroethane


1,1,2,2-Tetrachloroethane


1,1,2-Trichloro-1,2,2-trifluoroethane (Freon 113)


1,1,2-Trichloroethane


1,1-Dichloroethene


1,1-Dichloropropene


1,2,3,4-Tetramethylbenzene


1,2,3,5-Tetramethylbenzene


1,2,3-Trichlorobenzene


1,2,3-Trichloropropane


1,2,3-Trimethylbenzene


1,2,4-Trichlorobenzene


1,2-Dibromo-3-chloropropane (DBCP)


1,2-Dibromoethane (EDB)


1,2-Dichlorobenzene


1,2-Dichloropropane


1,2-Dimethylbenzene


1,3,5-Trimethylbenzene


1,3-Dichlorobenzene


1,3-Dichloropropene


1,4-Dichlorobenzene


1,4-Dimethylbenzene


2,2-Dichloropropane


2-Butanone


2-Chlorotoluene


2-Ethyltoluene


2-Hexanone


2-Propenal


2-Propenenitrile


3-Chloro-1-propene


4-Chlorotoluene


4-Methyl-2-pentanone


Acetone


Benzene


Bromobenzene


Bromochloromethane


Bromoethene


Bromomethane


Carbon Disulfide


Chlorobenzene


Chlorodibromomethane


Chloroethane


Chloroethene (Vinyl chloride)


Chloromethane


cis-1,3-Dichloropropene


Dibromomethane


Dichloromethane


Diethyl ether


Diisopropyl ether


Ethyl methacrylate


Ethyl tert-butyl ether (ETBE)


Ethylbenzene


Hexachlorobutadiene


Hexachloroethane


Iodomethane


Isopropylbenzene


Methyl acrylate


Methyl acrylonitrile


Methyl methacrylate


Methylbenzene (Toluene)


n-Butylbenzene


n-Propylbenzene


Napthalene


p-Isopropyltoluene


sec-Butylbenzene


tert-Amyl methyl ether (TAME)


tert-Butylbenzene


Tetrahydrofuran


trans-1,2-Dichloroethene


trans-1,3-Dichloropropene


trans-1,4-Dichloro-2-butene


Tribromomethane


Vinyl acetate


below land surface, and VOCs were detected in wells as 

deep as 62 m below land surface (depth to top of well 

opening). Three different depths (depth to water, depth 

from land surface to top of well opening, and depth 

from water level to well opening) have been found to 

influence the occurrence and concentrations of nitrate 

and pesticides (Barbash and Resek, 1996; Rupert, 

1997). 

An inverse correlation was found in this study 

between depth to well opening and nitrate concentration in oxygenated wells (p=0.02, rho=–0.52; Spearman’s rank correlation). No correlation was found 

between nitrate concentration and depth to water 

(p=0.64, rho=0.11; Spearman’s rank correlation); 

however, an inverse correlation was found between 

depth of well opening below water level and nitrate 

concentration (p=0.01, rho=–0.52; Spearman’s rank 

correlation). These two correlations suggest that well 

construction (depth) may be a factor in the occurrence 

and concentration of nitrate detected in the wells 

sampled during this study. 

No correlation was found between arsenic 

concentration and depth of water table or well opening 

(p=0.10, rho=–0.32, and p=0.94, rho=–0.01, respectively; Spearman’s rank correlation). No correlation 

was found between the number of pesticides detected in 

a well and the depth of the well opening below the water 

table (p=0.09, rho=–0.38; Spearman’s rank correlation). No correlation was found between the number of 

pesticides detected in a well and the depth of the well 

opening below land surface or the depth of the water 

level below land surface (respectively: p=0.16, 

rho=–0.26 and p=0.85, rho=–0.04; Spearman’s rank 

correlation). No correlation was found between depth 

to water table or well opening and the number of VOCs 

detected in a well. 

Factors Affecting Ground-Water Quality 19 
20 Ground-Water Quality in the Southeastern Sacramento Valley Aquifer, California, 1996 

Marysville 

Colusa 

Sacramento 

0 

0 0 

10 

20 KILOMETERS 

20 MILES 

Study area 

Reservoirs 

Rivers and streams 

Well locations, well numbers, and 

redox conditions in wells 

Oxic 

Anoxic 

31 

30 

29 28 

27 

24 

22 

20 

18 

17 

15 13 

14 16 

19 

21 

23 

26 25 

11 12 

10 

9 8 

6 7

4 3

2 

1 

5 

2 

9 

121° 

122° 30'

39° 

30' 

39° 

38° 

30' 

Cities 

Figure 8. Ground-water redox condition in wells sampled in the southeastern Sacramento Valley aquifer, California. 

1
Land Use, Pesticide Use, and Fertilizer Use 

The amount and types of compounds that are 

used by humans can influence which compounds will 

be present in the ground water in that area. Tritium 

concentrations indicate that ground water in the study 

area has been exposed to surface conditions in the 

Sacramento Valley since the 1950s, and therefore, 

pesticides and nitrate in fertilizers at land surface could 

enter the ground water. 

Pesticides and VOCs were detected in urban and 

agricultural areas; higher percentages of both chemicals were detected in wells surrounded by urban land 

use. No data were available on the amounts or types of 

VOCs used in the study area. The types of compounds 

detected in this study are consistent with the land use 

surrounding each well. The compounds detected in 

urban wells were consistent with urban activities 

(nonagricultural pesticide use, such as landscape 

maintenance, right-of-way weed control, pest control, 

and VOCs associated with gasoline, industrial uses, or 

chlorinated drinking water). Those detected in agricultural areas are consistent with agricultural activities 

(pesticides used on crops or rights-of-way and VOCs 

that may be present in pesticides or gasoline). 

Data on fertilizer sales in 1987 are available for 

each county in the study area (Battaglin and Goolsby, 

1995). For this study, the amount of nitrogen calculated 

from these data is used to estimate the amount of 

nitrogen from fertilizer applied to the land surface in 

each county (table 13). A correlation was found 

between the estimated amount of nitrogen applied in 

each county and the nitrate concentrations measured in 

oxygenated wells in that county (p=0.02, rho=0.50; 

Spearman’s rank correlation). Also, the nitrate concentration that exceeded the EPA MCL was in the county 

with the highest estimated amount of applied nitrogen. 

This correlation suggests that, in addition to depth from 

land surface to well opening and the redox condition, 

nitrate concentrations in ground water may be related to 

the amount of nitrogen applied to the land surface. 

Table 13. Amount of nitrogen applied as fertilizer in the Sacramento 

subunit study area 

[Calculated from fertilizer sales by county in 1987 (Battaglin and 

Goolsby, 1995)] 

County Nitrogen (pounds per acre) 

Butte 20 

Placer 2 

Sacramento 20 

Sutter 60 

Yuba 20 

SUMMARY AND CONCLUSIONS 

In 1996, the U.S. Geological Survey sampled 

29 domestic wells and 2 monitoring wells in the southeastern Sacramento Valley Aquifer as part the National 

Water-Quality Assessment Program. Measurements of 

inorganic constituents, nitrate, tritium, and stable 

isotopes indicate that ground water in the study area 

originated as precipitation that fell over the Coast 

Ranges, the Sacramento Valley, and the Sierra Nevada, 

was at least partly recharged since the 1950s and shows 

the effect of human activities. In general, the water 

quality in the southeastern Sacramento Valley aquifer 

was found suitable for most uses. Four of 31 wells 

(13 percent) exceeded a maximum contaminant level 

(MCL) or health advisory level drinking-water standard 

(DWS) for any one compound. Nitrate was detected in 

29 of 31 wells (94 percent) at levels below 2000 in all 

but one well. Radon-222 was detected in all wells sampled, and it was measured at levels above the proposed 

Federal MCL in 90 percent of the wells. Pesticides were 

detected in 29 percent of the wells sampled at levels 

below 2000 DWS. Volatile organic compounds (VOC) 

were detected in 39 percent of the wells sampled at 

levels below 2000 DWS in all but one well. That well is 

near a source of ground-water contamination, and water 

from that well contained eight VOCs, two of which 

(tetrachloromethane and trichlorethene) were above 

either California or Federal MCL DWS. 

Some of the water-quality components identified 

in this study can be linked to natural processes. Evidence of the effects of natural activities on groundwater quality found in this study included a positive 

correlation between nitrate and dissolved oxygen 

concentrations, suggesting that nitrate concentrations 

are being lowered by reducing chemical reactions in the 

ground water; inverse correlation between arsenic and 

dissolved oxygen concentrations but no correlation 

between arsenic concentrations and depth to well 

opening or water level; and detections of radon-222, 

which are naturally occurring in all wells. 

Evidence of the effect of human activities on 

ground-water quality was found in 61 percent of the 

wells sampled in this study in the form of a nitrate 

concentration of more than 3 milligrams per liter, or 

detections of pesticides or VOCs. Pesticides and VOCs 

were detected in urban and agricultural areas; both 

types of synthetic compounds were detected in a higher 

percentage of urban wells, although no statistical 

correlation was found between land-use type or population density and number of synthetic compounds 

detected in a well. The types of synthetic compounds 

detected were consistent with the land use in the area 

Summary and Conclusions 21 
(VOCs and pesticides related to urban activities were 

detected in urban wells; VOCs and pesticides related to 

agricultural activities were detected in wells located in 

agricultural areas). 

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