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PROGRAM

CALIFORNIA

Scientific Investigations Report 2009-5266

U.S. Geological Survey

U.S. Department of the Interior

Status and Understanding of Groundwater Quality in the Central-Eastside

San Joaquin Basin, 2006: California GAMA Priority Basin Project

In cooperation with the California State Water Resources Control Board

A product of the California Groundwater Ambient Monitoring and Assessment (GAMA) Program

C A L I F O R N I A

Water Boards
Sampling priority

Study area

Low-use basins

Priority basins

Areas that are outside DWR-defined

 groundwater basins

Front Cover Map: Groundwater basins categorized by sampling priority. Location of groundwater basin

 boundaries from California Department of Water Resources (DWR, 2003). 

Cover Photographs: 

Front cover: Almond orchard in Stanislaus County, California. (Photograph taken by Isabel Pimentel, U.S.

Geological Survey.) 

Back cover: Monitoring well next to irrigation canal in Stanislaus County, California. (Photograph taken by

Tyler Johnson, U.S. Geological Survey.)
Status and Understanding of Groundwater 

Quality in the Central–Eastside San 

Joaquin Basin, 2006: California GAMA 

Priority Basin Project

By Matthew K. Landon, Kenneth Belitz, Bryant C. Jurgens, Justin T. Kulongoski, 

and Tyler D. Johnson

Prepared in cooperation with the California State Water Resources Control Board

Scientific Investigations Report 2009–5266

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Landon, M.K., Belitz, Kenneth, Jurgens, B.C., Kulongoski, J.T., and Johnson, T.D., 2010, Status and understanding of 

groundwater quality in the Central–Eastside San Joaquin Basin, 2006: California GAMA Priority Basin project: U.S. 

Geological Survey Scientific Investigations Report 2009–5266, 97 p. 
iii

Contents

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

Introduction ....................................................................................................................................................3

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

Description of Study Unit..............................................................................................................................5

Methods ........................................................................................................................................................11

Design of Sampling Networks ..........................................................................................................13

Status Assessment Methods............................................................................................................16

Identification of CDPH Data for Status Assessment............................................................17

Historically and Currently High Constituents ...............................................................17

Selection of CDPH Grid Wells.........................................................................................17

Selection of CDPH–Other Wells ....................................................................................18

Relative-Concentrations ..........................................................................................................18

Estimation of Aquifer-Scale Proportions ...............................................................................20

Selection Criteria for Constituents of Interest ......................................................................21

Understanding Assessment Methods.............................................................................................21

Ancillary Data Sets ...................................................................................................................22

Groundwater-Age Classification.............................................................................................23

Statistical Analysis ....................................................................................................................24

Attributes of Wells .......................................................................................................................................24

Land Use...............................................................................................................................................24

Depth ..................................................................................................................................................25

Lateral Position....................................................................................................................................25

Groundwater Age................................................................................................................................25

Geochemical Conditions....................................................................................................................25

Correlations between Explanatory Factors....................................................................................31

Status and Understanding of Water Quality............................................................................................31

Inorganic Constituents.......................................................................................................................36

Trace Elements...........................................................................................................................36

Status Assessment for Trace Elements .......................................................................36

Understanding Assessment for Arsenic.......................................................................40

Understanding Assessment for Vanadium...................................................................48

Understanding Assessment for Lead ............................................................................50

Radioactive Constituents..........................................................................................................50

Status Assessment for Radioactive Constituents ......................................................50

Understanding Assessment for Uranium......................................................................50

Nutrients......................................................................................................................................52

Status Assessment for Nutrients ...................................................................................52

Understanding Assessment for Nitrate ........................................................................52
iv

Inorganic Constituents with Secondary Maximum Contaminant Levels .........................57

Status Assessment for Inorganic Constituents with Secondary Maximum 

Contaminant Levels ............................................................................................57

Understanding Assessment for Manganese ...............................................................57

Understanding Assessment for Iron..............................................................................57

Understanding Assessment for TDS .............................................................................57

Organic Constituents..........................................................................................................................59

Trihalomethanes.........................................................................................................................62

Status Assessment for Trihalomethanes ......................................................................62

Understanding Assessment for Trihalomethanes .......................................................62

Solvents.......................................................................................................................................66

Status Assessment for Solvents ...................................................................................66

Understanding Assessment for Solvents .....................................................................66

Other VOCs..................................................................................................................................66

Fumigants....................................................................................................................................67

Status Assessment for Fumigants .................................................................................67

Understanding Assessment for DBCP ..........................................................................68

Herbicides...................................................................................................................................68

Status Assessment for Herbicides ................................................................................68

Understanding Assessment for Herbicides ................................................................69

Insecticides.................................................................................................................................72

Special Interest Constituents ...........................................................................................................72

Status Assessment for Special Interest Constituents.........................................................72

Understanding Assessment for Perchlorate.........................................................................72

Constituents Sampled for in a Subset of Wells..............................................................................74

Summary .......................................................................................................................................................74

Acknowledgments.......................................................................................................................................77

References....................................................................................................................................................77

Appendix A. Attributes of USGS–GAMA and CDPH Grid Wells .......................................................84

Appendix B. Comparison of CDPH and USGS–GAMA Data..............................................................95
v

Figures

Figure 1. Map showing location of Central Eastside Groundwater Ambient Monitoring

and Assessment (GAMA) study unit and California hydrogeologic provinces ...... 4

Figure 2. Map showing geographic features of the Central Eastside, California,

Groundwater Ambient Monitoring and Assessment (GAMA) study unit ............ 6

Figure 3. Ternary diagram showing proportions of urban, agricultural, and natural land

uses for study unit, study areas, and wells, Central Eastside, California,

Groundwater Ambient Monitoring and Assessment (GAMA) study unit ............ 8

Figure 4. Map showing land use in the Central Eastside, California, Groundwater

Ambient Monitoring and Assessment (GAMA) study unit and locations of grid

and understanding wells .................................................................. 9

Figure 5. Geologic map of the Central Eastside, California, Groundwater Ambient 

Monitoring and Assessment (GAMA) study unit ....................................... 10

Figure 6. Maps showing conceptual diagram of regional lateral groundwater flow and,

vertical flow influenced by agricultural practices and natural discharge zones,

for the aquifer system of the Central Eastside, California, Groundwater Ambient

Monitoring and Assessment (GAMA) study unit ....................................... 12

Figure 7. Map showing locations of grid cells, grid and understanding wells sampled

during March–June 2006, and grid wells at which data for inorganic

constituents from the California Department of Public Health were used,

Central Eastside, California, Groundwater Ambient Monitoring and Assessment

(GAMA) study unit ........................................................................... 14

Figure 8. Boxplots showing construction characteristics for grid and understanding

wells, Central Eastside, California, Groundwater Ambient Monitoring and

Assessment (GAMA) study unit............................................................ 26

Figure 9. Map showing locations of wells and normalized lateral position from eastern 

margin of valley, Central Eastside, California, Groundwater Ambient Monitoring

and Assessment (GAMA) study unit ...................................................... 27

Figure 10. Graphs showing relation of classified groundwater age to depth to top of

perforations below land surface, depth to the bottom of the well below land

surface, and bar chart showing number of wells with modern, mixed, and

pre-modern groundwater age classifications in each of three depth categories,

Central Eastside, California, Groundwater Ambient Monitoring and Assessment

(GAMA) study unit ........................................................................ 28

Figure 11. Diagram showing relation of oxidation-reduction condition indicators (dissolved

oxygen, nitrate, manganese, and iron) to lateral position and depth of

perforated interval of wells, Central Eastside, California, Groundwater Ambient

Monitoring and Assessment (GAMA) study unit ....................................... 30

Figure 12. Plots showing values of pH, map of pH in USGS grid and USGS understanding

wells representative of the primary aquifer and the most recent analysis during

March 1, 2003–February 28, 2006 for CDPH wells, and graph showing pH and

well depth by well type and groundwater-age classification, Central Eastside,

California, Groundwater Ambient Monitoring and Assessment (GAMA) study

unit............................................................................................. 32

Figure 13. Graphs showing maximum relative-concentration in grid wells for constituents

detected, by type of constituent, in the Central Eastside, California,

Groundwater Ambient Monitoring and Assessment (GAMA) study unit ............ 34
vi

Figure 14. Dot plots showing relative-concentrations of selected trace elements,

radioactive constituents, nutrients, and inorganic constituents with secondary

maximum contaminant levels in grid wells, Central Eastside, California,

Groundwater Ambient Monitoring and Assessment (GAMA) study unit ............ 41

Figure 15. Maps showing concentrations of selected inorganic constituents in USGS grid

and USGS understanding wells representative of the primary aquifer and the

most recent analysis during March 1, 2003–February 28, 2006 for CDPH wells, 

Central Eastside, California, Groundwater Ambient Monitoring and Assessment

(GAMA) study unit ........................................................................ 42

Figure 16. Plots relating arsenic to explanatory factors and boxplots showing relation of

arsenic to classified groundwater age, and graphs showing relation of arsenic

to well depth, and pH, Central Eastside, California, Groundwater Ambient

Monitoring and Assessment (GAMA) study unit ....................................... 45

Figure 17. Plot showing relation of vanadium to pH, Central Eastside, California, 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit ............ 49

Figure 18. Plots showing relation of uranium concentrations to groundwater-age

classification, depth to top of perforations, and normalized lateral position,

Central Eastside, California, Groundwater Ambient Monitoring and Assessment

(GAMA) study unit ........................................................................... 51

Figure 19. Plots relating nitrate (as nitrogen) to explanatory factors, Central Eastside, 

California, Groundwater Ambient Monitoring and Assessment (GAMA) 

study unit .................................................................................... 53

Figure 20. Graphs showing relations of total dissolved solids to depth to the top of the

perforations, and normalized lateral position, Central Eastside, California,

Groundwater Ambient Monitoring and Assessment (GAMA) study unit ............ 58

Figure 21. Graph of detection frequency and maximum relative-concentration for organic

and special interest constituents detected in grid wells, Central Eastside,

California, Groundwater Ambient Monitoring and Assessment (GAMA) study

unit .......................................................................................... 60

Figure 22. Graphs of detection frequency and relative-concentrations in grid wells of

selected organic and special interest constituents in the Central Eastside,

California, Groundwater Ambient Monitoring and Assessment (GAMA) study

unit, March–June 2006 ..................................................................... 61

Figure 23. Maps showing detections and concentration ranges of selected organic and

special interest constituents in U.S. Geological Survey (USGS) grid and USGS

understanding wells representative of the primary aquifer and the most recent

analysis during March 1, 2003–February 28, 2006 for California Department of 

Public Health (CDPH) wells, sum of trihalomethanes, wells having one or more

solvents at low, moderate, or high relative-concentrations, DBCP, and sum of

herbicides, and perchlorate, Central Eastside, California, Groundwater Ambient

Monitoring and Assessment (GAMA) study unit ....................................... 63

Figure 24. Graphs showing relation of detection frequency of trihalomethanes to percent 

urban land use and depth to the top of perforations, Central Eastside, California,

 Groundwater Ambient Monitoring and Assessment (GAMA) study unit ............ 65
vii

Figure 25. Graph showing relation of detection frequency of solvents to percent urban 

land use and depth to the top of perforations, Central Eastside, California, 

Groundwater Ambient Monitoring and Assessment (GAMA) study .................. 67

Figure 26. Graph showing relation of detection frequency of DBCP to percent orchard 

and (or) vineyard land use and depth to top of perforations, Central Eastside, 

California, Groundwater Ambient Monitoring and Assessment (GAMA) 

study unit .................................................................................... 68

Figure 27. Graphs showing relations of sum of herbicides to groundwater-age 

classification, depth to top of perforations, and percent natural land use, 

and pesticide dectection frequency to percent urban land use and depth to 

the top of perforations, Central Eastside, California, Groundwater Ambient 

Monitoring and Assessment (GAMA) study unit ....................................... 70

Figure 28. Graphs showing relations of perchlorate concentrations to groundwater-age

classification, perchlorate detection frequency to percent orchard and (or) 

vineyard land use and depth to the top of perforations below land surface, and 

perchlorate detection frequency to percent urban land use and depth to the top 

of perforations below land surface, Central Eastside, California, Groundwater

Ambient Monitoring and Assessment (GAMA) study unit .............................. 73

Figure A1. Map showing identification numbers and locations of grid and understanding

wells sampled during March–June, 2006, and grid wells at which data for

inorganic constituents from the California Department of Public Health were

used, Central Eastside, California, Groundwater Ambient Monitoring and

Assessment (GAMA) study unit............................................................ 84

Figure B1. Graph of paired major-ion and nitrate concentrations from wells sampled by the

Groundwater Ambient Monitoring and Assessment (GAMA) program in

March–June 2006 and the most recent available analysis in the California

Department of Health Services, for the Central Eastside, California, 

study unit .................................................................................... 95

Figure B2. Piper diagram for U.S. Geological Survey (USGS) grid and understanding wells

and all wells in the California Department of Public Health (CDPH) database

that have a charge imbalance of less than 10 percent, Central Eastside,

California, study unit ........................................................................ 96

Tables

Table 1. Summary of analyte groups and number of wells sampled for different

analytical schedules, Central Eastside Groundwater Ambient Monitoring and

Assessment (GAMA) study unit, March–June 2006 .................................... 15

Table 2. Comparison of the number of compounds and laboratory reporting levels or

median method detection limits by analyte group or analyte for California

Department of Public Health data and data collected by the U.S. Geological

Survey for the Central Eastside Groundwater Ambient Monitoring and

Assessment (GAMA) study unit............................................................ 16
viii

Table 3. Inorganic constituents, and number of grid wells per constituent, Central

Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study

unit............................................................................................. 19

Table 4. Results of Wilcoxon rank-sum tests for differences in values of selected

water-quality constituents and for differences in potential explanatory factors,

between modern, mixed, and pre-modern groundwater age categories, Central

Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study

unit............................................................................................. 29

Table 5. Results of non-parametric (Spearman's method) analysis of correlations

between selected potential explanatory factors, Central Eastside Groundwater

Ambient Monitoring and Assessment (GAMA) study unit .............................. 33

Table 6. Number of constituents analyzed and detected by benchmark and constituent

class or analyte group, Central Eastside Groundwater Ambient Monitoring and

Assessment (GAMA) study unit, March to June 2006 ................................. 35

Table 7A. Aquifer-scale proportions using grid-based and spatially weighted methods for

those constituents that had concentrations above water-quality benchmarks

(high relative-concentrations) during the current period (March 1, 2003, to

February 28, 2006) from the California Department of Public Health data base

or in samples collected from grid wells, March–June 2006, Central Eastside

Groundwater Ambient Monitoring and Assessment study unit ..................... 37

Table 7B. Aquifer-scale proportions using grid-based and spatially weighted methods for

those constituents that had concentrations above water-quality benchmarks

(high relative-concentrations) during April 24, 1976, to February 28, 2003, 

(historically high) from the California Department of Public Health database

or had moderate relative-concentrations in samples collected from grid wells,

March–June 2006, Central Eastside Groundwater Ambient Monitoring and

Assessment study unit ..................................................................... 38

Table 8. Aquifer-scale proportions for constituent classes, Central Eastside

Groundwater Ambient Monitoring and Assessment (GAMA) Program study

unit............................................................................................. 40

Table 9. Results of non-parametric (Spearman's method) analysis of correlations

between selected water-quality constituents and potential explanatory factors,

Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA)

study unit .................................................................................... 47

Table A1. Identifiers and explanatory factor attributes of grid and understanding wells

sampled during March–June, 2006, and grid wells at which data for inorganic

constituents from the California Department of Public Health were used,

Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA)

study .......................................................................................... 85

Table A2. Summary of groundwater age data and classification into modern, mixed, and

pre-modern age categories for samples collected during March–June, 2006,

Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA)

study unit .................................................................................... 88

Table A3. Concentrations of constituents used to classify oxidation-reduction conditions

in groundwater, nitrogen and oxygen isotopic values of nitrate, and excess

nitrogen gas concentrations, Central Eastside Groundwater Ambient

Monitoring and Assessment (GAMA) study unit ....................................... 91
ix

Abbreviations and Acronyms

AB Assembly Bill (through the California State Assembly)

AL-US U.S. Environmental Protection Agency action level 

CE-QPC Central Eastside Upland Basin study area grid well

CE-QPCFP Central Eastside Upland Basin study area flow-path well (understanding well)

CE-QPCMW Central Eastside Upland Basin study area monitoring well (not necessarily on flow 

path)

E estimated or having a higher degree of uncertainty

GAMA Groundwater Ambient Monitoring and Assessment program

HAL-US U.S. Environmental Protection Agency lifetime health advisory level 

LRL laboratory reporting level

LSD land-surface datum

LT-MDL long-term method detection level

MCL-CA California Department of Public Health maximum contaminant level 

MCL-US U.S. Environmental Protection Agency maximum contaminant level 

MDL method detection limit

MER Merced study area grid well

MERFP Merced study area flow-path well (understanding well)

MERMW Merced study area monitoring well (not necessarily on flow path)

MOD Modesto study area grid well

MODFP Modesto study area flow-path well (understanding well)

MODMW Modesto study area monitoring well (not necessarily on flow path)

MRL minimum reporting level

NAVD 88 North American Vertical Datum 1988

NL-CA California Department of Public Health notification level 

NWIS National Water Information System (USGS)

PBP Priority Basin Project

PSW Public-Supply Wells

QC quality control

QPC Quaternary Pleistocene age semiconsolidated deposits (a feature of the Uplands 

study area)

RPD relative percent difference

RSD relative standard deviation

RSD5-US U.S. Environmental Protection Agency risk-specific dose at a risk factor of 10-5

SMCL-CA California Department of Public Health secondary maximum contaminant level 

SMCL-US U.S. Environmental Protection Agency secondary maximum contaminant level 

TDS total dissolved solids

TRLK Turlock study area grid well

TRLKFP Turlock study area flow-path well (understanding well)

TRLKMW Turlock study area monitoring well (not necessarily on flow path)

US United States
x

V Analyte detected in associated blanks; therefore, the value was excluded from the 

groundwater quality analyses

Organizations

CDPH California Department of Public Health (Department of Health Services prior to July 

1, 2007)

CDWR California Department of Water Resources

LLNL Lawrence Livermore National Laboratory

MWH Montgomery Watson Harza Laboratory

NAWQA National Water Quality Assessment (USGS)

NWQL National Water Quality Laboratory (USGS)

SWRCB State Water Resources Control Board (California)

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Selected chemical names

Ammonia-N ammonia as nitrogen

DBCP 1,2-dibromo-3-chloropropane

EDB 1,2-dibromoethane

NDMA N-nitrosodimethylamine

Nitrate-N nitrate as nitrogen

Nitrite-N nitrite as nitrogen

PCE tetrachloroethene

1,2,3-TCP 1,2,3-trichloropropane

TDS total dissolved solids

THM trihalomethane

VOC volatile organic compound

δ15N-nitrate nitrate delta nitrogen-15-to-nitrogen-14 isotopic ratio or delta nitrogen-15 of 

dissolved nitrate

δ18O-nitrate nitrate oxygen-18-to-oxygen-16 isotopic ratio or delta oxygen-18 of dissolved 

nitrate

Units of Measure

cm3

 STP g–1 cubic centimeters at standard temperature and pressure per gram 

δ delta notation; the ratio of a heavier isotope to the more common lighter isotope of 

an element, relative to a standard reference material, expressed as per mil

ft foot (feet)

in. inch

kg kilogram

L liter

mg/L milligrams per liter (parts per million)

mi mile

mL milliliter
xi

mm millimeter

μg/L micrograms per liter (parts per billion)

μS/cm microsiemens per centimeter

pCi/L picocuries per liter

per mil parts per thousand

pmc percent modern carbon

TU tritium unit

> greater than

< less than

≤ less than or equal to 

°C degrees Celsius

°F degrees Fahrenheit

 % percent

Notes

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

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

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

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

micrograms per liter is equivalent to parts per billion (ppb).
xii

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Abstract

Groundwater quality in the approximately 1,695-squaremile Central Eastside San Joaquin Basin (Central Eastside) 

study unit was investigated as part of the Priority Basin 

Project (PBP) of the Groundwater Ambient Monitoring 

and Assessment (GAMA) Program. The GAMA PBP 

was developed in response to the California Groundwater 

Quality Monitoring Act of 2001, and is being conducted 

by the California State Water Resources Control Board 

in collaboration with the U.S. Geological Survey and the 

Lawrence Livermore National Laboratory. The GAMA 

Central Eastside study unit was designed to provide a spatially 

unbiased assessment of untreated-groundwater quality, as 

well as a statistically consistent basis for comparing water 

quality throughout California. During March through June 

2006, samples were collected from 78 wells in Stanislaus 

and Merced Counties, 58 of which were selected using a 

spatially distributed, randomized grid-based method to provide 

statistical representation of the study unit (grid wells), and 20 

of which were sampled to evaluate changes in water chemistry 

along groundwater-flow paths (understanding wells). Waterquality data from the California Department of Public Health 

(CDPH) database also were used for the assessment.

An assessment of the current status of the groundwater 

quality included collecting samples from wells for analysis 

of anthropogenic constituents such as volatile organic 

compounds (VOC) and pesticides, as well as naturally 

occurring constituents such as major ions and trace elements. 

The assessment of status is intended to characterize the 

quality of untreated-groundwater resources within the primary 

aquifer system, not the treated drinking water delivered to 

consumers by water purveyors. The primary aquifer system 

(hereinafter, primary aquifer) is defined as that part of the 

aquifer corresponding to the perforation interval of wells 

listed in the CDPH database for the Central Eastside study 

unit. The quality of groundwater in shallower or deeper 

water-bearing zones may differ from that in the primary 

aquifer; shallower groundwater may be more vulnerable to 

surficial contamination. The primary aquifer is represented 

by the grid wells, of which 90 percent had depths to the tops 

of their perforations of about 80 to 330 feet and depths to 

bottom of about 100 to 670 feet. Relative-concentrations 

(sample concentration divided by benchmark concentration) 

were used as the primary metric for assessing the status of 

water quality for those constituents that have Federal and (or) 

California human health or aesthetic benchmarks. A relativeconcentration greater than (>) 1.0 indicates a concentration 

above a benchmark, and less than or equal to (≤) 1.0 indicates 

a concentration equal to or below a benchmark. For organic 

and special interest constituents, relative-concentrations were 

classified as high (>1.0), moderate (≤1.0 and >0.1), or low 

(≤0.1). For inorganic constituents, relative-concentrations 

were classified as high (>1.0), moderate (≤1.0 and >0.5), 

or low (≤0.5). The threshold between low and moderate 

classifications was lower for organic and special interest 

constituents than for inorganic constituents because organic 

constituents generally are less prevalent and have smaller 

relative-concentrations than inorganic constituents.

Grid-based and spatially-weighted approaches, the 

latter incorporating data from all CDPH wells, were used 

to evaluate the proportion of the primary aquifer (aquiferscale proportions) with high, moderate, or low relativeconcentrations. For individual constituents or classes 

of constituents, the aquifer-scale high proportion is the 

percentage of the area of the study unit having high relativeconcentrations within the depth-zones of the primary aquifer. 

Aquifer-scale moderate and low proportions are defined 

similarly. Spatially-weighted aquifer-scale high proportions 

nearly always fell within the 90-percent confidence interval of 

grid-based aquifer-scale high proportions, indicating that the 

grid-based approach yielded statistically equivalent results to 

the spatially-weighted approach incorporating CDPH data.

Status and Understanding of Groundwater Quality in the 

Central–Eastside San Joaquin Basin, 2006: California 

GAMA Priority Basin Project 

By Matthew K. Landon, Kenneth Belitz, Bryant C. Jurgens, Justin T. Kulongoski, and Tyler D. Johnson
2 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

The status assessment for inorganic constituents showed 

that inorganic constituents (one or more) were high, relative 

to human-health benchmarks, in 18.0 percent of the primary 

aquifer, moderate in 44.0 percent, and low in 38.0 percent. 

Of inorganic constituents with human-health benchmarks, 

arsenic, vanadium, and nitrate were detected at high relativeconcentrations in 15.6 percent, 3.6 percent, and 2.1 percent, 

respectively, of the primary aquifer. Of inorganic constituents 

with secondary maximum contaminant levels (SMCL), 

manganese, iron, and TDS were detected at high relativeconcentrations in 4.5 percent, 2.2 percent, and 1.7 percent, 

respectively, of the primary aquifer.

The status assessment for organic constituents showed 

that organic constituents (one or more) were high, relative 

to human-health benchmarks, in a smaller proportion of the 

primary aquifer (1.2 percent) than inorganic constituents 

(18.0 percent). Organic constituents had moderate relativeconcentrations in 14.3 percent, and had low relativeconcentrations or were not detected in 84.5 percent, of 

the primary aquifer. The proportion of the primary aquifer 

with high relative-concentrations of organic constituents 

reflected high proportions of the discontinued soil fumigant 

1,2-dibromo-3-chlororopane (DBCP; 1.0 percent) and 

the solvent tetrachloroethene (PCE; 0.2 percent). Most 

of the organic and special interest constituents detected 

in groundwater in the Central Eastside study unit have 

human-health benchmarks. Of the 205 organic and 

special interest constituents analyzed for, 36 constituents 

were detected. Of these constituents, 32 were detected 

only at low relative-concentrations. Four constituents, 

chloroform, carbon tetrachloride, DBCP, and perchlorate, 

were detected at moderate relative-concentrations in grid 

wells. Nine organic and special-interest constituents were 

detected frequently (detected in greater than 10 percent 

of samples): the trihalomethanes chloroform, bromoform, 

bromodichloromethane, and dibromochloromethane; 

the solvent PCE; the herbicides atrazine, simazine, and 

metolachlor, and special-interest constituent perchlorate.

An assessment of understanding of the groundwater 

quality included sampling of understanding wells, some of 

which were perforated in shallower or deeper portions of 

the aquifer system than the primary aquifer, and analysis of 

correlations of groundwater quality with land use, depth, age 

classification, and other potential explanatory factors. 

The understanding assessment indicated that the 

concentrations of many constituents were related to depth 

and groundwater age. However, concentrations of individual 

constituents or constituent classes also were sometimes related 

to geochemical conditions, lateral position in the flow system, 

or land use. 

High and moderate relative-concentrations of uranium, 

nitrate, and total dissolved solids (TDS) were detected in 

some wells where the tops of perforations are within the 

upper 200 feet of the aquifer system. In wells with the depth 

to the top of perforations below this depth, concentrations 

were low. A similar pattern occurred for the sum of herbicide 

concentrations. These vertical water-chemistry patterns are 

consistent with the hydrogeologic setting, in which return 

flows from agricultural and urban land use are the major 

source of recharge, and withdrawals for irrigation and 

urban supply are the major source of discharge, resulting in 

substantial vertical components of groundwater flow. 

The decrease in concentrations of many constituents with 

depth reflects in part that groundwater gets older with depth. 

Tritium, helium-isotopes, and carbon-14 data were used to 

classify the predominant age of groundwater samples into 

three categories: modern (water that has entered the aquifer in 

the last 50 years), pre-modern (water that entered the aquifer 

more than 50 years, up to tens of thousands of years, ago), 

and mixed (mixtures of waters with modern and pre-modern 

ages). Uranium, nitrate, and herbicide concentrations were 

significantly higher in groundwater having modern- and 

mixed-ages than pre-modern ages, indicating that these 

constituents may be affected by anthropogenic activities in the 

last 50 years. 

Other patterns in the distribution of nitrate, uranium, and 

TDS are evident. Isotopic and geochemical data are consistent 

with partial denitrification of nitrate in some reducing 

groundwaters in the western and deeper parts of the flow 

system. Uranium and TDS concentrations increase from east 

to west across the valley, along the direction of regional lateral 

groundwater flow. 

High and moderate relative-concentrations of arsenic 

can be attributed to reductive dissolution of manganese or 

iron oxides, or to desorption by high pH waters. Arsenic 

concentrations also increased with increasing depth and 

groundwater age. High to moderate relative-concentrations 

of vanadium primarily are related to high pH under oxic 

conditions.

The frequency of detections of DBCP was greater 

in areas with orchard-vineyard land use >40 percent and 

at depths <200 feet. THMs and solvents were correlated 

positively with percent urban land use. Herbicide 

concentrations were correlated negatively with percent 

natural land use. Perchlorate concentrations were significantly 

greater in waters having modern and mixed ages than waters 

having pre-modern ages and were significantly and positively 

correlated with two land uses—percent orchard/vineyard land 

use and percent urban land use.
Purpose and Scope 3

Introduction 

To assess the quality of ambient groundwater in aquifers 

used for drinking water supply and to establish a baseline 

groundwater quality monitoring program, the State Water 

Resources Control Board (SWRCB), in collaboration with 

the U.S. Geological Survey (USGS) and Lawrence Livermore 

National Laboratory (LLNL), implemented the Groundwater 

Ambient Monitoring and Assessment (GAMA) Program 

(http://www.swrcb.ca.gov/gama). The SWRCB initiated the 

GAMA Program in 2000 in response to a legislative mandate 

(Supplemental Report of the 1999 Budget Act 1999-00 Fiscal 

Year). The GAMA Priority Basins Project was initiated in 

response to the Groundwater Quality Monitoring Act of 

2001 {Sections 10780-10782.3 of the California Water Code, 

Assembly Bill 599}) to assess and monitor the quality of 

groundwater in California. The statewide GAMA program 

currently consists of three projects: GAMA Priority Basin 

Project, conducted by the USGS (http://ca.water.usgs.gov/

gama); GAMA Domestic Well Project, conducted by the 

SWRCB; and GAMA Special Studies, conducted by LLNL. 

On a statewide basis, the Priority Basin Project focused 

primarily on the deeper portion of the groundwater resource 

and the SWRCB Domestic Well Project generally focused on 

the shallower aquifer systems.

The GAMA Priority Basin Project is a comprehensive 

assessment of statewide groundwater quality designed to 

help better understand and identify risks to groundwater 

resources, and to increase the availability of information 

about groundwater quality to the public. For the Priority 

Basin Project, the USGS, in collaboration with the SWRCB, 

developed the monitoring plan to assess groundwater 

basins through direct and other statistically reliable sample 

approaches (Belitz and others, 2003; State Water Resources 

Control Board, 2003). Additional partners in the GAMA PBP 

include the California Department of Public Health (CDPH), 

California Department of Pesticide Regulation (CDPR), 

California Department of Water Resources (CDWR), and local 

water agencies and well owners. Local participation in the 

PBP is voluntary.

The range of hydrologic, geologic, and climatic 

conditions that exist in California must be considered in 

an assessment of groundwater quality. Belitz and others 

(2003) partitioned the state into ten hydrogeologic provinces, 

each with distinctive hydrologic, geologic, and climatic 

characteristics (fig. 1). Most of these hydrogeologic 

provinces include groundwater basins designated by the 

CDWR (California Department of Water Resources, 2003). 

Groundwater basins generally consist of relatively permeable, 

unconsolidated deposits of alluvial or volcanic origin. Eighty 

percent of California’s approximately 16,000 public-supply 

wells are located in designated groundwater basins. Some 

groundwater basins, such as the San Joaquin Valley basin, 

cover large areas and are further divided into groundwater 

subbasins by CDWR. Groundwater basins and subbasins were 

prioritized for sampling on the basis of the number of publicsupply wells, with secondary consideration given to municipal 

groundwater use, agricultural pumping, registered pesticide 

applications, and the number of historic leaking underground 

fuel tanks (Belitz and others, 2003). In addition, some 

groundwater basins or areas outside of designated basins, 

having relatively few public-supply wells, were assigned 

high-priority status so that all hydrogeologic provinces would 

be represented in the assessment. The 116 priority basins 

were grouped into 35 study units and include approximately 

85 percent of public-supply wells in California. Study 

units usually include one or more study areas. Study areas 

generally correspond to CDWR-defined groundwater basins or 

subbasins. 

Purpose and Scope

The GAMA PBP includes three types of water-quality 

assessments in each study unit: (1) Status: assessment of the 

current quality of the untreated-groundwater resource in the 

primary aquifer system (hereinafter, primary aquifer); (2) 

Understanding: identification of the natural and human factors 

affecting groundwater quality; and (3) Trends: detection of 

changes in groundwater quality (Kulongoski and Belitz, 

2004). PBP status assessments are designed to provide a 

statistically robust characterization of untreated-groundwater 

quality in the primary aquifers at the basin scale (Belitz and 

others, 2003). The statistically robust design also allows for 

comparison between basins and for synthesis of results at 

regional and statewide scales.

This report is one of a series of assessment reports 

presenting the status and understanding of current waterquality conditions in GAMA study units. Subsequent 

efforts will address the trends aspects of the water-quality 

assessments. This report describes groundwater-quality 

conditions in the Central Eastside San Joaquin Basin GAMA 

study unit, hereinafter referred to as the Central Eastside 

study unit (fig. 1). The purposes of this report are to (1) briefly 

describe the hydrogeologic setting, (2) assess the current status 

of untreated-groundwater quality in the primary aquifer, and 

(3) assess the relations between water quality and selected 

potential explanatory factors (understanding). 
4 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Figure 1. Location of Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study unit and California 

hydrogeologic provinces (modified from Belitz and others, 2003). 

Basin and Range

Central

Valley

Klamath

Mountains

Desert

Cascades and

Modoc Plateau

Transverse Ranges and

selected Peninsular Ranges

Southern Coast

Ranges

Sierra

Nevada

Northern

Coast

Ranges

CENTRALEASTSIDE

GAMA

STUDY UNIT

(figure 2)

San Diego

Drainages

Bakersfield

San

Francisco

Sacramento

OREGON

NEVADA

MEXICO

ARIZONA

Redding

Los Angeles

San Diego

PA C I F I C O C E A N

0 200 MILES

0 200 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

Provinces from Belitz and others, 2003
Description of Study Unit 5

The status assessment in this report includes analysis of 

water-quality data from 58 wells, mostly public-supply wells 

(PSWs), but including other wells with similar perforation 

depth intervals, selected for sampling by USGS within 

spatially distributed grid cells across the Central Eastside 

study unit; hereinafter, these wells are referred to as USGS 

grid wells. Samples were collected from wells for analysis of 

anthropogenic constituents such as volatile organic compounds 

(VOCs) and pesticides, as well as naturally occurring 

constituents such as major ions and trace elements. Waterquality data from the California Department of Public Health 

(CDPH) database also were used to supplement data collected 

by USGS for the PBP. The primary aquifer is defined as that 

part of the aquifer corresponding to the perforation interval 

of wells listed in the CDPH database for the Central Eastside 

study unit. The CDPH database lists wells used for municipal 

and community drinking water supplies, and includes wells 

from systems classified as non-transient (such as cities, towns, 

and mobile-home parks) and transient (such as schools, 

campgrounds, and restaurants). Shallow groundwater wells, 

such as private domestic and environmental monitoring wells, 

may be particularly at risk because of surficial contamination. 

As a result, concentrations of contaminants such as VOCs 

and nitrate can be higher in shallow wells than deeper wells 

(Burow and others, 2007). GAMA’s Voluntary Domestic Well 

Project, conducted by the SWRCB, is designed to assess water 

quality in the shallower parts of the aquifer system. 

For the purposes of providing context, the waterquality data discussed in this report were compared to State 

and Federal drinking-water regulatory and nonregulatory 

benchmarks for treated drinking water. The assessments in this 

report are intended to characterize the quality of untreatedgroundwater resources in the primary aquifer within the study 

unit, not the treated drinking water delivered to consumers 

by water purveyors; after withdrawal from the ground, water 

typically is treated, disinfected, and (or) blended with other 

waters to maintain acceptable water quality. Regulatory 

benchmarks apply to treated water that is served to the 

consumer, not to untreated groundwater.

The understanding assessment includes data from 

20 wells sampled by USGS for the purpose of understanding 

(hereinafter, USGS understanding wells), used with gridwell data to assess the relations between water quality and 

selected potential explanatory factors. Some wells selected 

for understanding purposes had perforations in shallower or 

deeper zones (above or below the primary aquifer); therefore, 

some of the USGS understanding wells do not represent the 

primary aquifer. In addition to anthropogenic and naturally 

occurring constituents, samples also were collected at the 

understanding wells and some USGS grid wells for analysis 

of constituents that can be used as hydrologic tracers or 

geochemical indicators. Potential explanatory factors 

examined included land use, depth, lateral position in the 

flow system, indicators of groundwater age, and geochemicalcondition indicators. A comprehensive analysis of all possible 

explanatory factors is beyond the scope of this report. 

In addition to describing the findings of the assessments 

for status and understanding, this report provides description 

of methods used in designing the sampling network, 

identification of CDPH data for use in the status assessment, 

estimation of aquifer-scale proportions of high, moderate, 

and low relative-concentrations of constituents, analysis of 

ancillary data sets, classification of groundwater age, and 

statistical and graphical approaches used in analyzing waterquality data. Water-quality data for samples collected by 

the USGS for the GAMA PBP (hereinafter, USGS–GAMA 

sampling) in the Central Eastside study unit and details of 

sample collection, analysis, and quality-assurance procedures 

for the Central Eastside study unit are presented by Landon 

and Belitz (2008). 

Description of Study Unit

The Central Eastside study unit lies within the Central 

Valley Hydrogeologic Province described by Belitz and 

others (2003) and includes three CDWR San Joaquin Valley 

groundwater subbasins: Modesto, Turlock, and Merced 

(California Department of Water Resources, 2003). The study 

unit is bounded by the San Joaquin River to the west, the 

Sierra Nevada Mountains to the east, the Stanislaus River to 

the north, and the Chowchilla groundwater subbasin to the 

south (fig. 2).

The Central Eastside study unit is divided into four 

separate study areas: the Modesto study area (MOD), the 

Turlock study area (TRLK), the Merced study area (MER), 

and the Uplands study area (CE-QPC) (fig. 2). The exterior 

boundaries of the Modesto, Turlock, and Merced study 

areas correspond to the CDWR groundwater subbasins of 

the same names. However, these study areas differ from the 

CDWR subbasins in that the Quaternary–Pleistocene-age 

semiconsolidated (QPC) deposits (Jennings, 1977) were 

designated as a separate study area, the Uplands study area. 

The QPC areas were designated as a separate study area 

because of their wide extent, unique geology and topography, 

and generally higher percentage of natural land use in 

comparison with the rest of the Modesto, Turlock, and Merced 

subbasins. The QPC areas also were designated as separate 

study areas in PBP study units to the north in the Central 

Valley (Bennett and others, 2006; Dawson and others, 2007), 
6 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Figure 2. Geographic features of the Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study 

unit. 

Study area boundary

Groundwater subbasin boundary

 (From California Department of 

 Water Resources, 2003) 

Chowchilla Groundwater Subbasin

Merced

Study area:

Modesto

Uplands

Turlock

S I E R R A N E V A D A

MERCED

COUNTY

STANISLAUS

COUNTY

MARIPOSA

COUNTY

MADERA

COUNTY

SAN JOAQUIN

COUNTY

TUOLUMNE

COUNTY

CALAVERAS

COUNTY

San Joaquin River

Merced River

Merced River

Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

20 MILES

20 KILOMETERS

EXPLANATION

0 10

0

121 00’ 120 30’

38 00’

37 30’

37 00’

Merced

Turlock

Modesto
Description of Study Unit 7

facilitating comparison of water quality in similar upland areas 

from north to south along the valley. 

The Central Eastside study unit has a Mediterranean 

climate, with hot and dry summers, and winters that are cool 

and moist. Average rainfall across the study unit ranges from 

11 inches (in.) in the southern and western portions of the 

study unit to 15 in. in the eastern to northeastern portions of 

the study unit (California Department of Water Resources, 

2004a,b, and 2006). Climatic data from six National Climatic 

Data Center stations (Denair, Knights Ferry, LeGrand, 

Merced, Modesto, Turlock; Western Regional Climate Center, 

2007) in the study unit having 18 to 74 years of record indicate 

that 88 to 90 percent of precipitation occurs during November 

through April. 

Several creeks and rivers drain the Central Eastside 

study unit. The Stanislaus, Tuolumne, and Merced Rivers, as 

well as their tributaries, are the primary streams in the study 

unit (fig. 2), but most of their flow is derived from the Sierra 

Nevada Mountains to the east. Each of these rivers ultimately 

drains into the San Joaquin River, which flows northwest 

and empties into the Sacramento–San Joaquin Delta, which 

discharges into the San Francisco Bay.

Land use in the study unit is 59 percent agricultural, 

34 percent natural, and 7 percent urban, on the basis 

of classification of USGS National Land Cover Data 

(Volgelmann and others, 2001; Price and others, 2003) (fig. 3). 

The natural land-use areas mostly are grassland. Natural land 

use is dominant in the eastern parts of the study unit (fig. 4). 

Areas of natural land use, primarily grasslands, also are in the 

southwestern part of the Merced study area, where a relatively 

shallow water table limits the suitability of the land for growth 

of agricultural crops. The primary crops are almonds, walnuts, 

peaches, grapes, grain, corn, pasture, and alfalfa (California 

Department of Water Resources, 2009a,b). The largest 

urban areas are the cities of Modesto, Turlock, and Merced. 

Additional areas of urban land use are located along the 

Stanislaus and Tuolumne Rivers and in the northern part of the 

Merced study area (fig. 4). Land use in the Merced, Modesto, 

and Turlock study areas is predominantly agricultural, whereas 

land use in the Uplands study area is predominantly natural 

(fig. 3). The Modesto study area was the most urbanized in the 

study unit with 59 percent agricultural, 22 percent natural, and 

19 percent urban land use. 

The main water-bearing units within the Modesto, 

Turlock, and Merced study areas include the unconsolidated 

alluvial-fan deposits of the Pleistocene-age Riverbank 

Formation, the deeper unconsolidated Pleistocene-age Turlock 

Lake and Pliocene-age Laguna Formations, and the semiconsolidated Miocene-Pliocene-age Mehrten Formation 

(Burow and others, 2004; California Department of Water 

Resources, 2004a,b, 2006). Holocene flood-basin and dune 

deposits (fig. 5) generally are not saturated except near 

major rivers (Burow and others, 2004). Older consolidated 

deposits outcrop in the eastern portion of the Modesto, 

Turlock, and Merced study areas (east of the Uplands study 

area) and include the Valley Springs Formation of OligoceneMiocene-age, and the Ione Formation of Eocene age (fig. 5). 

These older consolidated deposits generally yield only small 

quantities of water. The main water-bearing units within the 

Uplands study area include the unconsolidated alluvial-fan 

deposits of the Pleistocene-age Turlock Lake and Plioceneage Laguna Formations, and consolidated deposits of the 

Miocene–Pliocene-age Mehrten Formation (Burow and others, 

2004; California Department of Water Resources, 2004a).

Groundwater conditions are unconfined, semi-confined, 

and confined in different zones of the groundwater system 

in the Central Eastside study unit. The base of freshwater, 

where estimated, generally is more than 700 feet (ft) below 

land surface (Page, 1973), but may be as shallow as 300 ft in 

parts of the study unit (Burow and others, 2004). Unconfined 

conditions are present in unconsolidated deposits above 

and east of the Corcoran Clay Member of the Turlock Lake 

Formation (Marchand and Allwardt, 1981), which underlies 

the southwestern half of the study unit (fig. 5) at depths 

ranging from 50 to 250 ft (Davis and others, 1959; Page 

and Balding, 1973; Page, 1986; Burow and others, 2004; 

California Department of Water Resources, 2004a,b, 2006). 

Confined conditions are present below the Corcoran Clay. 

Semi-confined conditions are present at depth east of the 

Corcoran Clay, because of many discontinuous clay lenses. 

Primary sources of recharge are percolation of irrigation 

return, precipitation, seepage from reservoirs and rivers, 

and urban return (Burow and others, 2004; Phillips and 

others, 2007). The irrigation supply is provided primarily by 

surface water draining from the Sierra Nevada, and stored 

in reservoirs (Phillips and others, 2007). The surface-water 

supplies are managed by irrigation districts and delivered to 

agricultural users through hundreds of miles of lined canals. 

Primary sources of discharge are pumping withdrawals 

for irrigation and municipal water supply, evaporation 

from areas with a shallow depth to water, and discharge to 

streams. Agricultural irrigation supplied by surface water and 

groundwater accounts for about 95 percent of the total water 

use in the region (Burow and others, 2004). 

The Modesto study area covers approximately 277 mi2, 

and is located primarily in Stanislaus County. The study area 

is bounded on the north by the Stanislaus River, on the west 

by the San Joaquin River, on the south by the Tuolumne 

River, and on the east by the foothills of the Sierra Nevada 

Mountains (fig. 2). The City of Modesto is the largest city, 

with a population of 206,872 in 2003 (U.S. Census Bureau, 

2007). Outside of urban areas, land use in the Modesto study 

area is predominantly irrigated agricultural fields, except in 

the extreme eastern portion where grasslands predominate 

(fig. 4). An extensive distribution system diverts water from 

the Tuolumne River for irrigation of crops and for municipal 
8 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

supply for the City of Modesto. Water also is diverted for 

irrigation from the Stanislaus River. 

The Turlock study area covers approximately 446 mi2, 

and is located in Stanislaus and Merced Counties. The study 

area is bounded on the north by the Tuolumne River, on the 

west by the San Joaquin River, on the south by the Merced 

River, and on the east by the foothills of the Sierra Nevada 

Mountains (fig. 2). The city of Turlock is the largest city, with 

a population of 13,467 in 2003 (U.S. Census Bureau, 2007). 

Land use in the Turlock study area predominantly is irrigated 

agricultural fields except in the extreme eastern portion where 

grasslands predominate (fig. 4). An extensive distribution 

system diverts water from the Tuolumne and Merced Rivers 

for irrigation of crops. 

The Merced study area covers approximately 668 mi2

and is located primarily in Merced County. The study area 

is bounded on the north by the Merced River, on the west 

by the San Joaquin River, on the south by the Chowchilla 

subbasin, and on the east by the foothills of the Sierra Nevada 

Mountains (fig. 2). The city of Merced is the largest city, with 

a population of 19,512 in 2003 (U.S. Census Bureau, 2007). 

Land use in the Merced study area predominantly is irrigated 

agricultural fields except in the extreme eastern portion where 

grasslands predominate and in some portions of the southwest 

(fig. 4). An extensive distribution system diverts water from 

20

40

60

80

100

0

100 80 60 40 20 0

100

80

60

40

20

0

PERCENT URBAN

PERCENT AGRICULTURE

PERCENT NATURAL

20

40

60

80

100

0

100 80 60 40 20 0

100

80

60

40

20

0

PERCENT URBAN

PERCENT AGRICULTURE

PERCENT NATURAL

Understanding

Uplands grid

EXPLANATION EXPLANATION

Merced grid

Modesto grid

Turlock grid

Study unit Wells

Data point:

Land use in

entire area

Data point:

Average for

grid wells

Uplands

Merced

Modesto

Turlock

Figure 3. Ternary diagram showing proportions of urban, agricultural, and natural land uses for study unit, study areas, and wells, 

Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.
Description of Study Unit 9

San Joaquin River

Merced River

Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

(3)

(2) (2)

(2)

(2)

(2)

Merced

Uplands

Uplands

Uplands

Turlock

Modesto

38 00’

37 00’

121 00’ 120 30’

37 30’

S I E R R A N E V A D A

Urban

Land use

Agricultural

Natural

Study area boundary

USGS grid well

CDPH grid well

USGS understanding well

Number of wells sampled at locations (if greater than 1)

EXPLANATION

(2)

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

20 MILES

20 KILOMETERS

0 10

0

Figure 4. Land use in the Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit and 

locations of grid and understanding wells.
10 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Study area boundaries

Corcoran Clay Eocene marine consolidated

Eocence nonmarine consolidated

Miocene consolidated Jurassic marine 

Mesozoic volcanics

Quaternary alluvium

Plio-Pleistocene (QPC)

Quaternary sand dunes

Tertiary volcanic

Mesozoic granitics

MERCED

COUNTY

STANISLAUS

COUNTY

MARIPOSA

COUNTY

MADERA

COUNTY

SAN JOAQUIN

COUNTY

TUOLUMNE

COUNTY

CALAVERAS

COUNTY

San Joaquin River

Merced River

Merced River

Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Geology from Saucedo and others (2000)

Corcoran clay from Page (1986)

20 MILES

20 KILOMETERS

EXPLANATION

0 10

0

121 00’ 120 30’

38 00’

37 30’

37 00’

Merced

Turlock

Modesto

Uplands

Uplands

Uplands

S I E R R A N E V A D A

(3)

(2) (2)

(2)

(2)

(2)

(2) Number of wells sampled at

 locations (if greater than 1)

USGS grid well

CDPH grid well

USGS understanding well

Figure 5. Geologic map of the Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.
Methods 11

the Merced River for irrigation of crops in the northern and 

central parts of the Merced study area.

The Uplands study area covers approximately 304 mi2, 

and is located in Stanislaus and Merced Counties (fig. 2). 

There are three major areas of Uplands study area, one 

within the eastern portions of each of the Modesto, Turlock, 

and Merced study areas. Land use in the Uplands study area 

primarily is grassland and irrigated agricultural fields, which 

mostly occur in the western portions of the study area (fig. 4). 

Agricultural lands primarily are irrigated with groundwater 

except in the Uplands study area within the Modesto study 

area, where surface water diverted from the Stanislaus River is 

used for irrigation.

The conceptual model of the groundwater-flow system, 

based on previous investigations of Burow and others (2004) 

and Phillips and others (2007), is shown in figure 6. Regional 

lateral flow of groundwater from northeast to southwest across 

the study unit is driven by topography, the discharge of water 

to the San Joaquin River, and evaporation in the western part 

of the flow system. Because irrigation return flows are the 

major source of recharge and withdrawals for irrigation are 

the major source of discharge, there are substantial vertical 

components of flow (Burow and others, 2008b). These vertical 

flow components enhance vertical movement of water from 

recharge areas to the perforated intervals of withdrawal 

wells within shallow to intermediate depths in the system. 

These processes occur in both agricultural and urban areas. 

Groundwater age is vertically stratified, with water less 

than 50 years old in the upper parts of the system and water 

that may be tens of thousands of years old at depth (Burow 

and others, 2008a). In the western part of the study unit, 

the Corcoran Clay may restrict the interaction between the 

underlying confined and overlying unconfined groundwater. 

However, well-bores open to the aquifer above and below the 

Corcoran Clay permit water exchange across the Corcoran 

(Williamson and others, 1989). At the western end of the flow 

system, there is upward movement of groundwater towards the 

San Joaquin River. Wells perforated below the Corcoran Clay 

in the western third to half of the study unit are less abundant 

than in the rest of the study unit because poor-quality water in 

some areas [relatively high total dissolved solids and (or) iron 

and manganese] limit the use of the water for irrigation and 

domestic supply (Davis and others, 1959; Bertoldi and others, 

1991). 

Methods 

Methods used for the PBP were selected to achieve the 

following objectives: (1) design a sampling plan suitable for 

statistical analysis; (2) combine selected existing CDPH data 

with data collected by the USGS for the PBP to assess water 

quality; and (3) evaluate proportions of the primary aquifer 

having high, moderate, and low concentrations, relative 

to water-quality benchmarks, for constituent classes and 

individual constituents of interest, (4) compile and classify 

relevant ancillary data to identify relations of potential 

explanatory factors to water quality, and (5) investigate 

statistical relations between potential explanatory factors and 

water quality to provide understanding of the factors affecting 

the occurrence of constituents. Additional discussion of the 

methods used in the Central Eastside study unit can be found 

in Landon and Belitz (2008), including methods used to (1) 

collect samples in a consistent manner, (2) analyze samples 

using proven and reliable laboratory methods, (3) assure the 

quality of the groundwater data, and (4) maintain data securely 

and with relevant documentation.

The status assessment was designed to determine the 

quality of untreated groundwater resources used for public 

drinking-water supply. The study unit was divided into four 

study areas, and each study area was divided into grid cells. 

One randomly selected well per grid cell was sampled by the 

USGS for selected constituents (USGS grid wells). Data were 

selected from the CDPH database (hereinafter, CDPH grid 

wells) to supplement USGS grid well data for constituents 

not sampled by the USGS for each grid cell, and data for 

additional wells was selected from the CDPH database (CDPH 

other wells) for supplemental analysis. Using these data, gridbased and spatially-weighted approaches were used to assess 

proportions of high, moderate, and low concentrations of 

constituents and constituent classes in the primary aquifer. The 

grid-based approach uses one well per grid cell (hereinafter 

grid wells, which may include USGS grid wells and CDPH 

grid wells), in a spatially distributed randomized well network, 

to assess aquifer-scale proportions. The spatially-weighted 

approach includes many wells per grid cell, including grid 

and CDPH other wells. For an individual constituent or class 

of constituents, the aquifer-scale proportion is the percentage 

of the area of the study unit having concentrations above a 

specified threshold within the depth zones of the primary 

aquifer. 

The understanding assessment was designed to evaluate 

the natural and human factors that affect groundwater quality 

within groundwater basins, and at regional to statewide scales. 

Given the complexity of aquifer systems and the uncertainties 

associated with the sources of elevated concentrations, the 

understanding assessment relies upon the use of multiple 

lines of evidence to investigate the relations between 

observed water quality and potential explanatory factors. 

More detailed descriptions of the methods used in the status 

and understanding assessments are provided in the following 

sections. 
12 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Generalized direction of 

regional groundwater flow

Water table

SEMI-CONSOLIDATED DEPOSITS Regional flow

FLOOD-PLAIN

DEPOSITS

Corcoran Clay confining unit

AGRICULTURAL RECHARGE

Public-supply wells Irrigation wells

San Joaquin

River

San Joaquin

River

NOT TO SCALE

Aquifer

B

A

20 KILOMETERS

0 10 20 MILES

100

FLUVIALLY DEPOSITED

ALLUVIAL FAN DEPOSITS

121 15’ 121 00’ 120 45’ 120 30’

37 15’

37 30’

37 45’

Figure 6. Conceptual diagram of (A) regional lateral groundwater flow and, (B) vertical flow influenced by agricultural practices and 

natural discharge zones, for the aquifer system of the Central Eastside, California, Groundwater Ambient Monitoring and Assessment 

(GAMA) study unit (modified from Burow and others, 2004; Phillips and others, 2007).
Methods 13

Design of Sampling Networks

The wells selected for sampling by USGS in this study 

reflect the combination of two well-selection strategies. 

First, 58 wells (USGS grid wells) were selected to provide a 

statistically unbiased, spatially distributed assessment of the 

quality of groundwater resources used for public drinkingwater supply. For constituents not analyzed in samples from 

all USGS grid wells, data from USGS grid wells were 

augmented with data from CDPH grid wells to populate 

the grid with data. Second, 20 wells, including monitoring, 

irrigation, drainage, and domestic wells, were selected for 

sampling by USGS for the PBP to provide greater density 

of extensive water chemistry data in several areas to address 

specific groundwater-quality issues in the study unit and 

to provide additional information for the understanding 

assessment (USGS understanding wells). 

The spatially distributed wells (USGS grid wells) were 

selected using a randomized grid-based method (Scott, 

1990). Each of the study areas was subdivided into grid cells 

approximating 28 mi2 (fig. 7). This grid-cell size met PBP 

objectives for the Central Valley hydrogeologic province of 

a sampling density of one well per 19.3–38.6 mi2 (50–100 

square kilometers), while having at least 10 grid cells per 

study area. The variable shapes of the equal-area grid cells 

are drawn objectively using the method of Scott (1990) and 

are influenced by the irregular shapes of the boundaries of the 

study areas (fig. 7). Geographic features of the study areas 

may force the same grid cell to be divided into multiple pieces 

to obtain the designated coverage area for each cell. Locations 

of wells listed in statewide databases maintained by the CDPH 

and USGS were plotted, and one public-supply well per grid 

cell was selected that met basic sampling criteria (for example, 

sampling point prior to treatment, capability to pump for 

several hours, and available well-construction information) 

and for which permission to sample could be obtained 

(Landon and Belitz, 2008). If a grid cell did not contain 

accessible public-supply wells, then commercial, irrigation, or 

domestic wells were considered for sampling. The USGS grid 

wells were sampled by USGS for the Priority Basins Project 

but are owned by other organizations or individuals. USGS 

grid wells in the Central Eastside study unit were numbered in 

the order of sample collection with the prefix varying by study 

area: the Modesto study area (MOD), the Turlock study area 

(TRLK), the Merced study area (MER), and the Uplands study 

area (CE-QPC) (fig. A1, appendix A).

One USGS grid well was sampled in 58 of the 60 grid 

cells, including 10 of the 10 grid cells in the Modesto study 

area, 16 of the 16 grid cells in the Turlock study area, 23 of 

the 24 grid cells in the Merced study area, and 9 of the 10 grid 

cells in the Upland Basins study area (fig. 7). The two grid 

cells where samples were not collected had few wells, and 

permission to sample was not granted for wells in those cells.

The 58 USGS grid wells sampled included 43 publicsupply, 8 domestic, 3 commercial, 3 irrigation wells, and one 

monitoring well. One monitoring well, MER-21, was included 

with the USGS grid wells because no public-supply, irrigation, 

commercial, or domestic wells were available for sampling in 

its grid cell. The monitoring well had perforated intervals of 

comparable length and depths to other grid wells used in the 

study.

Twenty additional wells were selected for sampling for 

the purpose of understanding, with particular focus on factors 

affecting nitrate, uranium, and arsenic concentrations in 

groundwater. USGS understanding wells sampled in the four 

study areas were designated as either flow-path (CE-QPCFP, 

MERFP, MODFP and TRLKFP) or other monitoring wells 

(MODMW, TRLKMW, and MERMW). The understanding 

wells included 14 monitoring, 3 irrigation, 2 drainage wells 

and 1 domestic well. Thirteen of the monitoring wells sampled 

for understanding purposes were installed by USGS as part 

of previous investigations (Burow and others, 1998a; Burow 

and others, 2004). The remaining USGS understanding wells 

were sampled by USGS for the Priority Basins Project but 

are owned by other organizations. USGS understanding wells 

were selected: (1) along an approximate regional groundwater 

flow path across the Modesto study area (fig. 7), (2) to 

compare water quality between shallow-to-intermediate depths 

of <200 ft where most of the USGS understanding wells are 

perforated and depths >200 ft where public-supply wells are 

perforated, and (3) to characterize source waters that may mix 

within the aquifer system, including saline waters that underlie 

the freshwater aquifer, recharge of surface water through 

canals, and groundwater recharged from valley precipitation 

rather than from surface water from the Sierra Nevada. 

The wells in the Central Eastside study unit were 

sampled using a tiered analytical approach during March 

through June 2006 (Landon and Belitz, 2008). All wells were 

sampled for a standard set of constituents, including field 

parameters, organic constituents, and selected hydrologic 

tracers (table 1). The standard set of constituents was termed 

the “fast” schedule. Thirty-eight USGS grid wells and one 

USGS understanding well were sampled for the constituents 

on the fast schedule only. Wells on the “intermediate” schedule 

were sampled for all the constituents on the fast schedule, 

plus additional field parameters, inorganic constituents, and 

isotopic tracers of solute sources and processes (table 1). 

Five USGS grid wells and 19 USGS understanding wells were 

sampled for the constituents on the intermediate schedule. 

Wells on the “slow” schedule were sampled for all the 

constituents on the intermediate schedule, plus additional 

low-level organic constituents and radioactive and microbial 

constituents (table 1). Fifteen USGS grid wells were sampled 

for the constituents for the slow schedule.
14 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Figure 7. Locations of grid cells, grid and understanding wells sampled during March–June 2006, and grid wells at which data for 

inorganic constituents from the California Department of Public Health were used, Central Eastside, California, Groundwater Ambient 

Monitoring and Assessment (GAMA) study unit.

MERCED

COUNTY

STANISLAUS

COUNTY

MARIPOSA

COUNTY

MADERA

COUNTY

SAN JOAQUIN

COUNTY

TUOLUMNE

COUNTY

CALAVERAS

COUNTY

Study area

 boundary

Grid cell and

 number

Merced

Study areas:

Modesto

Uplands

Turlock

San Joaquin River

Merced River

Merced River

Don Pedro Reservoir

Tulloch Reservoir

Stanislaus River

Turlock Lake

Modesto Reservoir

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

20 MILES

20 KILOMETERS

EXPLANATION

0 10

0

121 00’ 120 30’

38 00’

37 30’

37 00’

Merced

Turlock

Modesto

CDPH well

(3)

(2) (2)

(2)

(2)

(2)

Number of wells sampled

 at USGS understanding-

 well locations (if greater

 than 1)

(2)

USGS grid well

CDPH grid well

USGS understanding well

12

12

13

13 13

17

14

14

14

16

16

14

23

22

21

20

20

20

20

24

15

15

15

15 10

10

9

8

7 7

7

8

8

8

7

7

7

7

6

6

6

6 6

5

5

5

5

5

5

4

4

4

4

4

3

3

3

3

2

2

2

2

2

2

1

1

1

1

1

9

9 9 9

9

11

11

10 10

10

10

10

10

10

19

18
Methods 15

Schedule

 Fast Intermediate Slow 

Total number of wells 39 24 15

Number of grid wells sampled 38 5 15

Number of understanding wells sampled 1 19 0

Analyte Groups Number of analytes

Dissolved oxygen, specific conductance, temperature 3 3 3

Volatile Organic Compounds (VOC) 1 85 85 85

Pesticides and degradates 61 61 61

Perchlorate 1 1 1

Noble gases & tritium 2 7 7 7

Stable isotopes of hydrogen and oxygen in water 2 2 2

Pharmaceuticals 3 16 16 16

Alkalinity, pH 2 2

Nutrients 5 5

Dissolved organic carbon 1 1

Nitrogen and oxygen isotopes of nitrate 2 2

Dissolved gases (nitrogen, argon, methane) 4 3 3

Major and minor ions, trace elements 5 36 36

Arsenic, chromium, and iron species 6 6

Uranium isotopes 3 3

Sulfur and oxygen isotopes of sulfate 2 2

Carbon isotopes 2 2

Boron and strontium isotopes 3 2 2

Tritium 6 1 1

Gasoline oxygenates 7 3

NDMA and low-level 123-TCP 8 2

Polar pesticides and degradates 9 54

Radon-222 1

Radium isotopes 2

Gross alpha and beta radioactivity 4

Microbial constituents 4

Low-level halogenated VOCs (chlorofluorocarbons) 3,10 25

Sum: 175 240 335

Table 1. Summary of analyte groups and number of wells sampled for different analytical schedules, Central Eastside Groundwater 

Ambient Monitoring and Assessment (GAMA) study unit, March–June 2006. 

[123-TCP, 1,2,3-Trichloropropane; NDMA, N-Nitrosodimethylamine; USGS, U.S. Geological Survey; LRL, laboratory reporting level]

1

 Includes 10 constituents classified as fumigants or fumigant synthesis byproducts. 2

 Analyzed at Lawrence Livermore National Laboratory, Livermore, California.

3

 Results are not discussed in this report or Landon and Belitz (2008).

4

 Includes one analyte, dissolved nitrogen gas, in common with noble gas analyses. However, the different nitrogen values are used for different purposes and 

are counted as separate analyses. 5

 Includes 1 constituent, uranium, classified as a radioactive constituent later in this report. 6

 Analyzed at USGS Stable Isotope and Tritium Laboratory, Menlo Park, California.

7

 Does not include 5 constituents in common with VOCs.

8

 Includes one analyte, 123-TCP, in common with VOC analyses. However, the LRL for the low-level analysis is 0.005 microgram per liter (μg/L) compared 

to 0.18 μg/L for the VOC analysis. Therefore, the low-level analysis is counted as a separate analysis. 9

 Does not include 4 constituents in common with pesticides and degradates.

10 Includes 22 analytes in common with VOC analyses (Plummer and others, 2008). However, the LRLs for the low-level analyses are two to three orders of 

magnitude lower than for the VOC analyses. Therefore, the low-level analyses are counted as separate analyses.
16 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Analytes for the fast schedule mostly are common 

between PBP study units (Wright and others, 2005; Bennett 

and others, 2006; Kulongoski and others, 2006; Kulongoski 

and Belitz, 2007; Dawson and others, 2007; Fram and Belitz, 

2007), and are collected at all grid wells. Analytes for the 

slow schedule also mostly are common between PBP study 

units and are collected at approximately 25 percent of USGS 

grid wells. Analytes for the intermediate schedule, beyond 

those included on the fast schedule, primarily were selected to 

better understand processes relevant to water quality in each 

study unit. In the Central Eastside study unit, analytes for the 

intermediate schedule were selected to better understand the 

occurrence of uranium, nitrate, and arsenic. 

California State Assembly Bill AB599 directs the 

GAMA program to utilize existing monitoring data along 

with collection of new data to fill gaps. The existing statewide 

source of data for public-supply wells, the CDPH database, 

contains data for constituents with water-quality benchmarks 

regulated as part of Title 22. Although water-quality data 

also are collected by other organizations for local studies 

and specific purposes, the CDPH data are the only statewide 

database of public-supply well data available. Procedures for 

selecting data from the CDPH database are described in the 

next section of this report.

Data for some constituents, including VOCs, pesticides, 

inorganic constituents, and radioactive constituents, are 

available from both USGS–GAMA and CDPH data (table 2). 

However, a larger number of VOCs and pesticides are 

analyzed in the USGS–GAMA data than are available in 

the CDPH data. In addition, LRLs (laboratory reporting 

levels) for USGS–GAMA data typically were one to two 

orders of magnitude less than those for analyses compiled by 

CDPH (table 2), indicating that lower concentrations can be 

detected in the USGS-GAMA data. The USGS–GAMA data 

were designed to complement the CDPH data by providing 

a larger number of analytes and lower laboratory reporting 

levels compared to the CDPH data, although the CDPH data 

are available for a larger number of wells. Both data sets are 

utilized in the status assessment.

The USGS–GAMA data includes hydrologic tracers and 

geochemical indicators that are not regulated water-quality 

constituents with benchmarks. These constituents are of 

importance for understanding groundwater quality and are 

discussed for that purpose in this report. 

Status Assessment Methods

The status assessment was done by using the following 

methods. First, selected CDPH data were identified and 

incorporated into the dataset to be analyzed. Second, waterquality data were normalized to their respective water-quality 

benchmarks by calculating their relative-concentrations. 

Aquifer-scale proportions of high, moderate, and low relativeconcentrations of individual constituents and constituent 

classes were then determined on the basis of two approaches: 

(1) grid-based, and (2) spatially-weighted. Results for these 

approaches were compared to detection frequency, and the 

aquifer-scale proportions from the grid-based or spatiallyweighted approaches were used to identify constituents of 

interest for further discussion.

Analyte groups or analytes

CDPH GAMA

Number of 

compounds

Median 

MDL

Number of 

compounds

Median 

LRL

Median 

units

Volatile organic compounds plus gasoline 

oxygenates (including fumigants)

83 0.5 88 0.06 μg/L

Pesticides plus degradates 74 1 115 0.019 μg/L

Nutrients, major and minor ions 12 0.4 19 0.06 mg/L

Trace elements 22 10 25 0.12 μg/L

Radioactivity constituents 4 1 11 0.045 1 pCi/L

Perchlorate 1 4 1 0.5 μg/L

N-Nitrosodimethylamine (NDMA) nc nc 1 0.002 μg/L

Pharmaceutical constituents nc nc 16 0.021 2 μg/L 1

 Value is a median SSMDC for 11 radioactive constituents collected and analyzed by GAMA.

2

 Value is a median MDL.

Table 2. Comparison of the number of compounds and laboratory reporting levels or median method detection limits by analyte group 

or analyte for California Department of Public Health data and data collected by the U.S. Geological Survey for the Central Eastside 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit.

[CDPH, California Department of Public Health; LRL, laboratory reporting level; MDL, method detection limit; mg/L, milligrams per liter; μg/L, micrograms 

per liter; SSMDC, sample specific minimum detectable concentration; nc, not collected]
Methods 17

Identification of CDPH Data for 

Status Assessment

The CDPH database is large, and targeted retrievals are 

required to manageably use the data to assess water-quality. 

For example, for the Central Eastside study unit, the historical 

CDPH database contains more than 600,000 records for more 

than 700 wells. 

CDPH data were used in three ways for the status 

assessment. First, the CDPH data were used, along with 

USGS grid data, to identify constituents that have ever 

had concentrations above water-quality benchmarks in the 

Central–Eastside study unit so that these constituents could 

be included in the status assessment. Second, the CDPH 

database was used as a supplemental source of inorganic data 

for grid wells where a complete suite of inorganic data were 

not collected during USGS–GAMA sampling. Third, data 

from other wells from the CDPH database were used for the 

purpose of assessing proportions of the primary aquifer having 

high, moderate, and low relative-concentrations, by using 

spatially-weighted and detection frequency approaches. 

Historically and Currently High Constituents

Constituents that had high concentrations at some time 

during the full period of record (April 24, 1976, through 

February 28, 2006) of CDPH data in the Central–Eastside 

study unit, but did not have high concentrations in the most 

recent 3-year period of CDPH data or in USGS grid data, 

were identified as historically high. These constituents do not 

reflect current conditions on which the status assessment is 

based. Constituents that had high concentrations during the 

most recent 3 years of data from CDPH available at the time 

of data analysis (March 1, 2003, through February 28, 2006, 

hereinafter, current period), or had high concentrations in the 

USGS grid data, were identified as currently high. For each 

constituent with either current or historic high concentrations, 

spatially-weighted aquifer-scale proportions and detection 

frequencies in the primary aquifer were computed using the 

most recent concentration available for the current period.

Selection of CDPH Grid Wells

VOC, pesticide, and perchlorate data were collected 

at all 58 USGS grid wells. Because the USGS–GAMA data 

included more constituents, and were analyzed at lower LRLs, 

than the CDPH data, these data were the primary data used in 

the status assessment for these constituent classes. 

Samples for analysis of inorganic constituents were 

collected from 20 of the 58 USGS grid wells. For grid cells 

without USGS–GAMA inorganic data, a decision tree was 

used to select CDPH inorganic data for a single well in each 

cell.

The first choice for the CDPH grid well in a cell was to 

select the same well as the USGS grid well sampled, provided 

this well had the needed inorganic data from CDPH that met 

quality-control criteria. Cation/anion balance (Hem, 1989) was 

used as the quality-control assessment metric. A cation/anion 

balance of greater than or equal to (≥) 10 percent indicates 

uncertainty in the quality of the data. The most recent CDPH 

data from the well were evaluated to determine whether the 

CDPH data had a cation/anion balance <10 percent. If so, 

the CDPH inorganic data from the well were selected for 

use as CDPH grid-well data for inorganic constituents. It 

was assumed that analyses with acceptable-quality majorion data also had acceptable-quality data for trace elements, 

nutrients, and radiochemical constituents. This step resulted 

in the selection of inorganic data from the CDPH database for 

12 wells that also were USGS grid wells. For identification 

purposes, data from the CDPH for these grid wells were 

assigned GAMA identifications numbers equivalent to the 

GAMA USGS grid well but with “DG” (for “CDPH/GAMA”) 

inserted between the study area prefix and sequence number 

(for example, CDPH grid well MER-DG-04 is the same well 

as USGS grid well MER-04, table A1, fig. A1).

If the first step did not yield CDPH inorganic data for a 

grid cell, the second step was to search the CDPH database to 

identify the randomly ranked well with the smallest rank that 

had a cation/anion balance <10 percent. This step resulted in 

the selection of eight grid wells where CDPH inorganic data 

were used in the absence of inorganic data at the USGS grid 

wells. These CDPH grid wells were not co-located with the 

USGS grid well for the same cell. 

If no CDPH wells in a grid cell had a cation/anion 

balance <10 percent or there was insufficient data to calculate 

the balance, the third choice for the CDPH grid well was to 

select the lowest randomly ranked CDPH well that had any 

of the needed inorganic data. This step resulted in selection 

of 10 grid wells where CDPH inorganic data were used. 

Five of these 10 wells also were USGS grid wells. At these 

wells, the CDPH database provided arsenic-only data at one 

well, nitrate-only data at two wells, and data for several other 

inorganic constituents at two wells. The other five wells 

selected in this step were CDPH grid wells that were not 

co-located with USGS grid wells. For identification purposes, 

the five CDPH grid wells that were not co-located with USGS 

grid wells were assigned GAMA identification numbers 

equivalent to the GAMA USGS grid well for the cell, but with 

“DPH” inserted between the study area prefix and sequence 

number (for example, CDPH grid well MER-DPH-14 is in the 

same grid cell as USGS grid well MER-14, table A1, fig. A1).

Five of the CDPH grid wells (of the 13 not co-located 

with USGS grid wells) were located just outside of the target 

grid cells, in adjacent cells within the portion of the cell 

closest to the target cell. CDPH grid wells from adjacent cells 

were selected to increase the coverage of inorganic data in 

parts of the study unit where these data were relatively sparse. 

The CDPH grid wells were selected from adjacent cells only 

when they were located within the half of the adjacent cell 

closest to the target cell and were located in a different half of 

the cell from the grid well for that adjacent cell. 
18 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

The result of this algorithm was selection of one grid 

well per cell (CDPH grid wells) having inorganic data from 

the CDPH database for those cells where inorganic data were 

not collected during USGS–GAMA sampling. Inorganic 

data from the CDPH database were used for 30 CDPH grid 

wells (table A1). Data were available for 29 CDPH grid wells 

for nitrate and for 23 to 25 wells for most other inorganic 

constituents (table 3). In combination with USGS grid well 

inorganic data (20 wells), inorganic data were available for 

grid wells located within 48 of the 60 grid cells. Analysis 

of the combined data sets to evaluate the occurrence of 

high or moderate relative-concentrations was not affected 

by differences in LRLs between USGS–GAMA and CDPH 

data because concentrations above one-half of water quality 

benchmarks generally were substantially higher than the 

highest CDPH reporting limits. The locations and USGS–

GAMA identification numbers of grid and understanding 

wells (fig. A1), and attributes of CDPH grid wells (table A1) 

are presented in Appendix A. Several types of comparisons 

between USGS–GAMA and CDPH data are described in 

Appendix B.

Selection of CDPH–Other Wells 

The most recent data during the current period (March 

1, 2003, through February 28, 2006) for all CDPH wells in 

the study unit were used in spatially-weighted approaches 

to assess proportions of the primary aquifer having high, 

moderate, and low relative-concentrations for selected 

constituents. This approach resulted in a single concentration 

value being assigned to each well for each constituent 

assessed. All CDPH wells coded as untreated groundwater, 

including active, abandoned, destroyed, and inactive wells, 

with water-quality data for the current period, were included in 

the dataset for CDPH other wells.

Relative-Concentrations 

Relative-concentrations were used as the primary basis 

for evaluating groundwater quality and were calculated from 

equation 1:

 Relative-concentration = sample concentration

benchmark concentration [1]

Toccalino and others (2004), Toccalino and Norman 

(2006), and Rowe and others (2007), previously used the 

ratios of measured concentration to a benchmark (either 

Maximum Contaminant Levels (MCLs) or Health-Based 

Screening Levels (HBSLs), and defined this ratio as the 

benchmark quotient. Relative-concentrations used in this 

report are equivalent to the benchmark quotient of Toccalino 

and others (2004) for constituents with MCLs. HBSLs were 

not used in this report, as they currently are not used as 

benchmarks by California drinking-water regulatory agencies. 

Relative-concentrations provide context for the concentrations 

at which constituents are detected, but can be computed only 

for compounds with water-quality benchmarks. Relativeconcentrations of less than or equal to one (≤1) indicate a 

sample concentration less than the benchmark, and values 

greater than one (>1) indicate a sample concentration greater 

than the benchmark.

Regulatory and nonregulatory benchmarks apply 

to water that is served to the consumer, not to untreated 

groundwater. However, to provide context for the waterquality results, concentrations of constituents measured in 

the untreated groundwater were compared with human-health 

benchmarks established by the U.S. Environmental Protection 

Agency (USEPA) and CDPH, and benchmarks established 

for aesthetic concerns (secondary maximum contaminant 

levels, SMCL-CA) by CDPH (U.S. Environmental Protection 

Agency, 2006; California Department of Public Health, 

2007). The human-health benchmarks used include MCLs—

maximum contaminant levels, NLs—notification levels, 

HALs—health advisory levels, AL—Action Levels, and 

RSD-5s—risk specific dose (1 in 100,000). When available, 

Federal benchmarks were used except when California 

benchmarks were lower. Additional information on the types 

and concentrations of benchmarks for all constituents analyzed 

is provided by Landon and Belitz (2008). 

For the purposes of simplicity and clarity, relativeconcentrations were classified into high, moderate, and 

low categories. Inorganic constituents tend to be more 

prevalent and have larger maximum relative-concentrations 

than do organic and special interest constituents. Inorganic 

constituents tend to be present naturally in groundwater 

whereas organic and special interest constituents generally 

are present as a result of anthropogenic effects. Therefore, a 

smaller threshold value was used to distinguish between low 

and moderate relative-concentrations of organic and special 

interest constituents, in comparison to inorganic constituents. 

Use of a single threshold value to distinguish between low 

and moderate relative-concentrations of all constituents 

would have resulted in a greater number of moderate relativeconcentrations of inorganic constituents (due to their greater 

prevalence and relative-concentrations) and fewer moderate 

relative-concentrations of organic and special interest 

constituents than would be warranted. 
Methods 19

Constituent type Constituent

Number of grid wells 

sampled by 

USGS GAMA

Number of grid 

wells selected 

from CDPH

Nutrient-HBB Ammonia-N 20 0

Nutrient-HBB Nitrite-N 20 25

Nutrient-HBB Nitrate-N 19 29

Trace element-HBB Aluminum 20 23

Trace element-HBB Antimony 20 23

Trace element-HBB Arsenic 20 25

Trace element-HBB Barium 20 23

Trace element-HBB Beryllium 20 23

Trace element-HBB Boron 20 4

Trace element-HBB Cadmium 20 23

Trace element-HBB Chromium 20 23

Trace element-HBB Copper 20 24

Trace element-HBB Lead 20 23

Trace element-HBB Mercury 20 23

Trace element-HBB Molybdenum 20 0

Trace element-HBB Nickel 20 23

Trace element-HBB Selenium 20 23

Trace element-HBB Strontium 20 0

Trace element-HBB Thallium 20 23

Trace element-HBB Vanadium 20 8

Minor ion-HBB Fluoride 20 25

Trace element-SMCL Iron 20 24

Trace element-SMCL Manganese 20 24

Trace element-SMCL Silver 20 23

Trace element-SMCL Zinc 20 24

Major ion-SMCL Chloride 20 25

Major ion-SMCL Sulfate 20 25

Major ion-SMCL Total dissolved solids 20 26

Radioactive-HBB Uranium 20 13

Radioactive-HBB Gross alpha radioactivity 15 21

Radioactive-HBB Gross beta radioactivity 15 0

Radioactive-HBB Radon-222 15 0

Radioactive-HBB Radium-226, -228 15 8

Table 3. Inorganic constituents, and number of grid wells per constituent, Central Eastside Groundwater Ambient Monitoring and 

Assessment (GAMA) study unit. 

[CDPH, California Department of Public Health; N, nitrogen; SMCL, Secondary maximum contaminant level; HBB, Health-based benchmark (including all 

benchmark types except SMCL); USGS, U.S. Geological Survey]

1

 USGS GAMA analyses are for nitrate plus nitrite. However, nitrite concentrations are negligible compared to nitrate. 
20 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Inorganic constituents were classified by their maximum 

relative-concentration into three groups: 

• maximum relative-concentration >1 (high constituents 

or relative-concentrations; a high constituent is defined 

as a constituent that has at least one high relativeconcentration), 

• maximum relative-concentration ≤1 and >0.5 

(moderate constituents or relative-concentrations; a 

moderate constituent is defined as a constituent that 

has at least one moderate relative-concentration but no 

high relative-concentrations), 

• maximum relative-concentration ≤0.5 (low constituents 

or relative-concentrations; a low constituent is 

defined as a constituent that has only low relativeconcentrations).

Organic constituents also were classified by their 

maximum relative-concentration into three groups: 

• maximum relative-concentration >1 (high constituents 

or relative-concentrations), 

• maximum relative-concentration ≤1 and >0.1 

(moderate constituents or relative-concentrations), and 

• maximum relative-concentration ≤0.1 (low constituents 

or relative-concentrations). 

The USEPA previously has used a concentration of 0.1 

of the regulatory benchmark as a threshold at or above which 

the USEPA wants to be informed of a pesticide’s presence in 

surface water or groundwater (U.S. Environmental Protection 

Agency, 1998). This EPA threshold is equivalent to a relativeconcentration of 0.1, used in this report as a threshold to 

identify noteworthy (moderate or high) concentrations of 

organic and special-interest constituents that occur in the 

primary aquifer. 

Estimation of Aquifer-Scale Proportions

The aquifer-scale proportion for a particular constituent 

is the percentage of the area of the study unit having 

relative-concentrations of that constituent within a specified 

category (high, moderate, or low) in the primary aquifer. Two 

primary approaches were taken to calculate the aquifer-scale 

proportions in the Central Eastside study unit. The approaches 

used were: 

1. Grid-based: One well per grid cell was used to represent 

the primary aquifer. Aquifer-scale proportions for the 

high, moderate, and low relative-concentration categories 

were calculated from the detection frequencies of high, 

moderate, and low relative-concentrations in grid wells. 

Confidence intervals for grid-based detection frequencies 

of concentrations in the high category were computed by 

using the Jeffrey’s interval for the binomial distribution 

(Brown and others, 2001). The grid-based estimate is 

spatially unbiased. However, the grid-based approach may 

not identify constituents that are present at high relativeconcentrations in small proportions of the primary aquifer. 

2. Spatially-weighted: Within the current period, all 

available data from the following sources were used 

to calculate the aquifer-scale proportions for the high, 

moderate, and low relative-concentration categories: 

USGS grid, CDPH data (one analysis per well), and 

USGS understanding wells having perforation depth 

intervals representative of the primary aquifer (irrigation 

wells CE-QPC-01, MODFP-04, and MERFP-01). 

Other USGS understanding wells that were drainage, 

monitoring, or domestic wells were excluded because 

these wells were partially or entirely perforated at 

shallower depths than the primary aquifer or because of 

incomplete perforation data. The aquifer-scale proportions 

are computed using cell-declustering (Isaaks and 

Srivastava, 1989). In cell-declustering, the proportions 

(high, moderate, or low) are computed by first computing 

the proportion of wells (high, moderate, or low) in each 

grid-cell, and then averaging the proportions computed 

for each cell. The resulting proportions are spatially 

unbiased (Isaaks and Srivastava, 1989).

The detection frequency of high relative-concentrations 

was also calculated using the same data as the spatiallyweighted approach. The detection frequency approach 

is not spatially unbiased because the CDPH and USGS 

understanding wells are not distributed uniformly. 

Consequently, the data from areas with a high density of wells 

could have a disproportionately high weight compared to 

data from areas with a sparser density of wells. Aquifer-scale 

proportions for the high category that were calculated using 

the detection-frequency approach are provided for comparison 

in this report. However, grid-based and spatially-weighted 

approaches were used to assess aquifer-scale proportions 

because they are spatially unbiased.

The assessment of status was based on aquifer-scale 

proportions estimated primarily on the grid-based approach, 

and secondarily on the spatially-weighted approach. Estimates 

of moderate and low aquifer-scale proportions were selected 

primarily from the grid-based approach because, for some 

constituents, the MDLs for CDPH data included in the 

spatially-weighted approach were too high to distinguish 

between moderate and low relative-concentrations. 

Aquifer-scale high proportions of constituents were 

based on grid-based estimates unless the high proportion was 

zero using the grid-based approach and nonzero using the 

spatially-weighted approach; then, the estimate was based on 

the spatially-weighted approach. In these cases, it was likely 

that the constituent was present in high proportions in such 

a small fraction of the aquifer that it was more reliable to 

quantify its aquifer-scale high proportion with the spatiallyweighted rather than the grid-based approach. Aquifer-scale 

high proportions discussed in this report are grid-based values, 

unless otherwise noted. 
Methods 21

For the assessment of status, aquifer-scale proportions of 

high relative-concentrations were determined for individual 

constituents and for classes of constituents. The classes of 

organic constituents having human-health benchmarks for 

which aquifer-scale proportions were calculated include 

trihalomethanes, solvents, other VOCs, fumigants, herbicides, 

and insecticides. The classes of inorganic constituents 

having human-health benchmarks for which aquiferscale proportions were calculated include trace elements, 

radioactive constituents, and nutrients. Among constituents 

having aesthetic benchmarks, aquifer-scale proportions were 

calculated for total dissolved solids, manganese, and iron. 

Most aquifer-scale proportions for constituent classes 

were grid-based estimates, but spatially-weighted estimates 

were reported in some cases. For constituent classes that 

include individual constituents with spatially-weighted high 

proportions, the high proportions for the class were based on 

the spatially-weighted high proportions for constituents within 

the class. For constituent classes with high proportions based 

on the spatially-weighted approach, moderate proportions for 

constituent classes were calculated by using the grid-based 

moderate proportions minus the spatially-weighted high 

proportions. In this case, aquifer-scale high proportions were 

likely to be too small to be detected by using the grid-based 

approach and were more likely to be included in moderate 

than low grid-based proportions. The consequence of this 

approach is that the moderate proportions for constituent 

classes discussed later in this report are minimum estimates 

and the low proportions are maximum estimates. 

The grid-based and spatially-weighted estimation of 

aquifer-scale proportions, based on an equal-area grid cell 

network, are intended to characterize the water quality of the 

depth-zones of the primary aquifer throughout the study unit. 

These approaches assign weights to wells on the basis of a 

single well per cell (grid-based) or the number of wells per 

cells (spatially-weighted). Another possible approach would 

have been to assign weights to wells on the basis of water use 

(withdrawal rate). However, water-use data for wells generally 

are not available. The weights assigned cells by using either 

the grid-based or spatially-weighted approaches do not 

include the effects of areal variations in the thickness of the 

primary aquifer. However, because the alluvial sediments in 

the San Joaquin Valley are thousands of feet thick (Faunt and 

others, 2009) and water quality mostly is stratified vertically, 

variations in the thickness of the primary aquifer that is or 

could be used for drinking-water supply across the Central 

Eastside are likely to be minor. 

In the Central Eastside study unit, 90 percent of grid 

wells had depths to the tops of the perforations of about 80 

to 330 ft and depths to the bottom of the well of about 100 to 

670 ft. The aquifer-scale proportions discussed in this report 

do not represent the shallower or deeper parts of the aquifer 

system than these intervals. Previous investigations in the 

study unit have shown that groundwater in shallow parts of 

the aquifer generally is of poorer quality than at greater depths 

in the aquifer (Burow and others, 1998a,b, 2007, 2008b; 

Jurgens and others, 2008). The State Water Board’s Voluntary 

Domestic Well Project is designed to evaluate water quality 

in the shallow parts of the aquifer. Similarly, water quality 

at greater depths than those typically used for public supply 

can be of different quality, particularly with respect to higher 

dissolved solids concentrations (Page, 1973).

Selection Criteria for Constituents of Interest

Given the large number of analytes (>300), an algorithm 

was used to select those constituents of greatest importance for 

discussion in the report. Constituents were first classified into 

those whose maximum relative-concentrations in grid wells 

were high, moderate, or low. For all constituents having high 

or moderate relative-concentrations, grid-based aquifer-scale 

proportions and relative-concentration graphs are presented in 

this report. Individual constituents having only low relativeconcentrations are not discussed in this report except when 

they are organic constituents having detection frequencies 

>10 percent or are useful as hydrologic tracers for the 

assessment for understanding. A detection was defined as any 

analytical result with a reported concentration above the MDL 

for that constituent. Detection frequencies were calculated on 

the basis of the number of wells with a detection, divided by 

the total number of wells sampled for that constituent. Organic 

constituents that were detected in more than 10 percent of 

grid wells, even if all relative-concentrations were low, are 

discussed in this report because of their prevalence. 

Understanding Assessment Methods

For the assessment for understanding, all constituents 

having grid-based or spatially-weighted aquifer-scale high 

proportions of >2 percent were analyzed for relations to 

potential explanatory factors. Constituents of interest with 

aquifer-scale high proportions of ≤2 percent generally were 

aggregated by compound class and then analyzed for relations 

to potential explanatory factors.

A finite set of potential explanatory factors, including 

land use, depth, lateral position in the flow system, 

groundwater-age classification, and geochemical-condition 

indicators, were analyzed in relation to constituents of 

interest for the understanding assessment. The purpose of this 

assessment was to place the observed water quality within the 

context of physical and chemical processes. A comprehensive 

analysis of all possible explanatory factors is beyond the scope 

of this report. 
22 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Statistical tests were used to identify significant 

correlations between the concentrations of constituents of 

interest and potential explanatory factors. Selected correlated 

data that were most valuable for improving understanding 

of factors influencing water quality are shown in the figures 

in this report. Some significantly correlated data were not 

graphed because they were affected by correlations between 

explanatory factors. The assessment for understanding 

generally discusses concentrations rather than relativeconcentrations, to more directly relate constituents of interest 

to potential explanatory factors. 

The wells used in the understanding assessment included 

USGS and CDPH grid wells and USGS understanding 

wells. CDPH other wells were not used in the understanding 

assessment because age tracer, dissolved oxygen, and 

sometimes well construction data were not available. In 

addition, considerable effort would have been needed to 

attribute the well-construction data for the large number 

of CDPH other wells. For different potential explanatory 

factors, correlations were tested using either the set of grid 

plus understanding wells or grid wells only. Because the 

USGS understanding wells primarily represented relatively 

shallow groundwater in agricultural areas that were not 

selected randomly on a spatially distributed grid, they were 

excluded from analysis of relations between water quality and 

areally distributed explanatory factors (land use and lateral 

position) to avoid bias due to clustering. However, USGS 

understanding wells were included in analysis of relations 

between water quality and vertically distributed explanatory 

factors (depth, classified groundwater age, and oxidationreduction characteristics). To identify relations to vertically 

distributed explanatory factors, it was necessary to include 

USGS understanding wells, generally representing shallow 

parts of the aquifer, to have data spanning a sufficient range 

of depths. For analysis of relations of constituents to orchard 

and (or) vineyard land use, a subcategory of agricultural 

land use, USGS understanding wells were included because 

they represented a sufficient range of agricultural land-use 

categories. 

Ancillary Data Sets 

Land use was classified by using an “enhanced” version 

of the satellite-derived (30-m pixel resolution), nationwide 

USGS National Land Cover Dataset (Volgelmann and others, 

2001; Price and others, 2003). This dataset has been used in 

previous national and regional studies relating land use to 

water quality (Gilliom and others, 2006; Zogorski and others, 

2006). The data represent land use approximately during the 

early 1990s. The imagery is classified into 25 land-cover 

classifications (Nakagaki and Wolock, 2005). These 25 landcover classifications were aggregated into three principal 

land-use categories: urban, agricultural, and natural. Landuse statistics for the study unit, study areas, and for circles 

with a radius of 500 meters (m) around each study well were 

calculated on the basis of these classified datasets using 

ArcGIS (version 9.2). 

Well-construction data primarily were determined 

from driller’s logs. More rarely, well-construction data were 

obtained from ancillary records of well owners or from the 

USGS National Water Information System database. Well 

identification verification procedures are described by Landon 

and Belitz (2008).

The lateral position of wells within the valley serves as a 

proxy for the horizontal position in the regional groundwaterflow system. Regionally, groundwater primarily flows from 

the eastern margin of the valley-fill deposits along the Sierra 

Nevada mountain front towards the southwest to the western 

margin of the flow system, represented by the San Joaquin 

River (fig. 6). The groundwater-flow system has vertical 

flow components as well as horizontal flow components 

that deviate from the regional northeast-to-southwest flow 

direction in response to withdrawals and recharge related 

to groundwater development for irrigation since the early 

to mid 1900s (Phillips and others, 2007; Burow and others, 

2008a,b; California Department of Water Resources, 2008). 

These vertical and nonparallel horizontal flow components 

are superimposed on the topographically driven regional flow 

system. The aquifer system also contains large quantities 

of groundwater that was recharged before the modern flow 

system developed; under predominantly natural conditions, 

groundwater primarily had moved from northeast to 

southwest. 

The normalized lateral position (hereinafter, lateral 

position) was calculated as part of a regional groundwaterflow modeling study for a set of 30 × 30-m-wide cells in 

the San Joaquin Valley (Faunt and others, 2009). Lateral 

positions were assigned to wells residing within those cells 

using ArcGIS (version 9.2). The lateral position of each well 

was calculated as the ratio of (a) the distance from the well 

to the San Joaquin River and (b) the total distance from the 

San Joaquin River to the east edge of the valley. The east 

edge of the valley was represented by the eastern boundary 

of the valley fill deposits and was assigned a value of 1. In 

the Central Eastside, the valley trough was represented by 

the position of the San Joaquin River and was assigned a 

value of 0. Both boundaries were represented as approximate 

line segments and lateral position was calculated along lines 

perpendicular to both bounding lines. Higher values of lateral 

position indicate locations in the upgradient or proximal 

portion of the flow system and lower values of lateral position 

indicate locations in the downgradient or distal portion of the 

flow system. Plotting of data, with respect to lateral position, 

also allows for aggregation of areally distributed data into a 

single, diagrammatic cross section across the study unit.
Methods 23

Groundwater-Age Classification

Groundwater-dating techniques provide a measure 

of the time since the groundwater was last in contact with 

the atmosphere. Groundwater age has been identified as a 

critical variable affecting groundwater quality (Cook and 

Böhlke, 2000; Böhlke, 2002) and was included in the GAMA 

design and assessment for understanding for this reason. 

Techniques aimed at estimating groundwater residence times 

or ‘age’ include those based on tritium (3H; Tolstikhin and 

Kamensskiy, 1969; Torgersen and others, 1979) and 3H in 

combination with its decay product helium-3 (3He) (Schlosser 

and others, 1988, 1989), carbon-14 (14C) activities (Vogel and 

Ehhalt, 1963; Plummer and others, 1993; and Kalin, 2000), 

and dissolved noble gases, particularly helium-4 accumulation 

(Davis and DeWiest, 1966; Andrews and Lee, 1979; Cey and 

others, 2008; Kulongoski and others, 2008). Collection and 

analysis methods and analytical results for 3H, 14C, and noble 

gases are described by Landon and Belitz (2008). 

Quantities of 3H much larger than natural production 

were introduced into the atmosphere from above-ground 

nuclear bomb testing between 1951 and 1980 (peak 

production in 1963) (Michel, 1989; Solomon and Cook, 

2000). Consequently, the presence of 3H, a radioactive 

isotope of hydrogen with a half-life of 12.32 years (Lucas and 

Unterweger, 2000), in groundwater may be used to identify 

water that has exchanged with the atmosphere in the past 

50 years. By determining the ratio of 3H to its decay product 

3He, the time that the water has resided in the aquifer can be 

calculated more precisely than using 3H alone (Solomon and 

others, 1992). 14C is a radioactive isotope of carbon having a 

half-life of 5,730 years, formed naturally in the atmosphere 

that can be used to estimate groundwater ages ranging from 

approximately 1,000 to 30,000 years before present (Clark 

and Fritz, 1997). 14C values also increased in the atmosphere 

following atmospheric nuclear weapons testing in the 1950s 

and 1960s (Kalin, 2000). Calculated 14C ages in this study 

are referred to as “uncorrected” because they have not been 

adjusted to consider exchanges with sedimentary sources 

of carbon (Fontes and Garnier, 1979; Kalin, 2000). The 

14C age is calculated on the basis of the decrease in 14C 

activity, expressed as percent modern carbon (pmc), owing 

to radioactive decay with time since groundwater recharge, 

relative to an assumed initial 14C concentration (Clark and 

Fritz, 1997). A mean initial 14C activity of 99 percent modern 

carbon (pmc) is assumed for this study, with estimated errors 

on calculated groundwater ages of up to ±20 percent.

Helium (He) is a naturally occurring inert gas produced 

by the radioactive decay of lithium, thorium, and uranium in 

the earth. Measured groundwater He concentrations represent 

the sum of air-equilibrated He, He originating from excess 

dissolved-air bubbles, terrigenic He, and tritiogenic 3He. The 

presence of terrigenic He in groundwater, from its production 

in aquifer material or deeper in the crust, is indicative of long 

groundwater residence times. The amount of terrigenic He is 

defined as the concentration of the total measured He, minus 

He from air-equilibration and excess dissolved-air bubbles, 

which is estimated from recharge temperatures and neon 

concentrations. Samples in which more than 5 percent of the 

total He is terrigenic He (percent terrigenic He) likely are to 

represent groundwater with a residence time of more than 

100 years. 

Recharge temperatures for 75 samples were determined 

from dissolved neon, argon, krypton, and xenon using methods 

described in Aeschbach-Hertig and others (1999). Recharge 

temperatures were modeled using procedures described by 

Aeschbach-Hertig and others (2000). 

3H /3He apparent ages were computed as described 

in Solomon and Cook (2000). Samples with terrigenic He 

>5 percent have greater uncertainty in computed 3H/3He 

apparent ages because of sensitivity to the 3He/4He ratio of the 

terrigenic He (Plummer and others, 2000). The 3He/4He ratios 

of samples were determined by linear regression of the percent 

of terrigenic He against the δ3He {δ3He = [(Rmeas/Ratm) –1] 

x 100} of samples with less than 1 tritium unit (Plummer and 

others, 2000).

In this study, the ages of samples are classified as 

pre-modern, modern, and mixed. Groundwater with 3H 

less than 1 tritium unit (TU), percent terrigenic He greater 

than 5 percent, and 14C less than 90 pmc is designated as 

pre-modern. Pre-modern groundwater is defined as having 

recharged prior to about 1950. The pre-modern category 

could include groundwater that recharged from about 56 to 

tens of thousands of years ago. Groundwater with 3H greater 

than 1 TU, percent terrigenic He less than 5 percent, and 

14C greater than 90 pmc is designated as modern. Modern 

groundwater is defined as having recharged after about 1950. 

Samples with both pre-modern and modern components are 

designated as mixed groundwater. In reality, pre-modern 

groundwater could contain small fractions of modern water 

and modern groundwater could contain small fractions 

of pre-modern water. Previous investigations have used a 

range of tritium values from 0.3 to 1.0 TU as thresholds for 

distinguishing pre-1950 from post-1950 water (Michel, 1989; 

Plummer and others, 1993, p. 260; Michel and Schroeder, 

1994; Clark and Fritz, 1997, p. 185; Manning and Solomon, 

2005). By using a tritium value of 1.0 TU, at the upper end 

of the range used in the literature, for the threshold in this 

study, the age classification allows a slightly larger fraction 

of modern water to be present in a classified pre-modern 

age distribution than if a lower threshold were used. A lower 

threshold for tritium would result in fewer wells classified 

as pre-modern rather than mixed water, when other tracers, 

such as 14C and terrigenic He, would have suggested that they 

primarily were pre-modern water. This higher threshold was 

considered more appropriate for this study since many of the 

wells are long-screened production wells and some mixing of 

at least some water of pre-modern and modern age likely is to 

have occurred. 
24 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Apparent age estimates, on the basis of the 3H/3He and 

14C techniques, are reported in table A2. These apparent 

ages generally assume piston flow of water from recharge 

areas to the well without mixing with waters of another 

age. Because of uncertainties in age, in particular caused by 

mixing of water of widely different ages in wells with long 

perforation intervals and high withdrawal rates, these apparent 

age estimates were not used for statistically quantifying the 

relation between age and water quality in this report. Whereas 

more sophisticated lumped parameter models for analyzing 

age distributions that incorporate mixing are available 

(Cook and Böhlke, 2000), use of these alternative models to 

characterize age mixtures was beyond the scope of this report. 

Rather, classification into modern, mixed, and pre-modern 

categories was considered sufficient to provide a simple 

characterization for the purposes of examining relations of 

groundwater quality to age. 

Statistical Analysis

Nonparametric statistical methods were used to test the 

significance of correlations between water-quality constituents 

and potential explanatory factors. Nonparametric statistics 

are robust techniques that generally are not affected by 

outliers and do not require that the data follow any particular 

distribution (Helsel and Hirsch, 2002). The significance level 

(p) used for hypothesis testing for this report was compared 

to a threshold value (α) of 5 percent (α = 0.05) to evaluate 

whether the relation was statistically significant (p < α). 

Correlations were investigated using Spearman’s method to 

calculate the rank-order correlation coefficient (ρ) between 

continuous variables. The values of ρ can range from +1.0 

(perfect positive correlation), through 0.0 (no correlation), to 

–1.0 (perfect negative correlation). For potential explanatory 

factors that were classified into categories (for example, 

groundwater age categories of modern, mixed, and premodern), the values of water-quality parameters between 

the categories were compared using the Wilcoxon rank-sum 

test. The Wilcoxon rank-sum test can be used to compare 

two independent populations (data groups or categories) to 

determine whether one population contains larger values 

than the other (Helsel and Hirsch, 2002). The null hypothesis 

for the Wilcoxon rank-sum test is that there is no significant 

difference between the observations of the two independent 

data groups being tested. The Wilcoxon rank-sum test 

was used for multiple comparisons of two independent 

groups rather than the multiple-stage Kruskal–Wallis test 

for identifying differences between three or more groups, 

although the probability of falsely identifying a significant 

difference between groups is higher for a set of Wilcoxon 

rank-sum tests than the Kruskal–Wallis test (Helsel and 

Hirsch, 2002). However, given the potentially large and 

variable number of differences to be evaluated, the Wilcoxon 

rank-sum test was selected as a consistent and practical direct 

test of differences instead of the Kruskal–Wallis test, which 

requires multiple stages and is more complex to describe and 

implement. All statistical analysis was done using S-PLUS for 

Windows, version 7.0, Professional Edition (TIBCO Software 

Inc.).

Attributes of Wells

The values assigned to wells for potential explanatory 

factors are described in this section. Relations among 

explanatory factors that could affect apparent relations 

between explanatory factors and water quality also are 

described. 

Land Use

On average, land use around grid wells tended to contain 

more urban land use, and less agricultural and natural land 

use, than the study unit or study areas as a whole (fig. 3). 

This was particularly the case for the Modesto study area, 

which had an average of 56 percent urban land use within 

500 m of grid wells, but had 19 percent urban land use for the 

study area as a whole (figs. 3 and 4). The higher percentage 

of urbanized land use surrounding the grid wells reflects that 

public-supply wells often are located in or near communities. 

For the Modesto study area, the larger discrepancy between 

average urban land use for grid wells and the study area 

reflects that most of the grid cells intersected the urban area, 

which is larger than in other study areas, permitting wells in 

urban areas to be selected as grid wells in most cells. The grid 

well data, particularly in the Modesto study area, may reflect 

greater urban influence than might be expected on the basis of 

the average land use of the study areas. 

Understanding wells primarily were located in 

agricultural areas (fig. 3), with 17 of the 20 wells having 

greater than 90 percent agricultural land use within 500 m. 

Three of the understanding wells were located in areas with 

>75 percent natural land use. All of the understanding wells 

had <7 percent urban land use within 500 m.

An additional subcategory of agricultural land use 

included in the analysis was percent orchard and vineyard 

land use. Orchard and vineyard land use previously has 

been shown to be related to concentrations of nitrate and 

selected pesticides in parts of the eastern San Joaquin Valley 

(Domagalski, 1997; Burow and others, 1998a). For grid 

wells, percent orchard and vineyard land use within 500 m 

ranged from 0 to 95 percent, with a median of 1.3 percent. For 

understanding wells, percent orchard and vineyard land use 

ranged from 0 to 98 percent, with a median of 48 percent. 
Attributes of Wells 25

Depth

Grid wells, primarily used for public supply, had depths 

ranging from 75 to 923 ft below land surface, with a median 

of 284 ft (fig. 8). Depths to the top of the perforations ranged 

from 72 to 457 ft, with a median of 162 ft. The perforation 

length was as much as 656 ft with a median of 104 ft. These 

sets of depths represent different sets of wells because the well 

depths were known for more wells than were depths to the 

tops of the perforations.

The understanding wells mostly were shallower and had 

shorter screens than the grid wells (fig. 8). The median well 

depth, depth to the top of perforations, and perforation length 

were 113, 102, and 10 ft, respectively, for understanding wells. 

The relatively short perforation lengths and shallow depths 

reflect that 14 of the 20 understanding wells sampled (19 with 

construction data) were monitoring wells. 

Lateral Position

Grid wells, selected using a spatially distributed 

randomized design, were distributed across the entire range of 

lateral positions (fig. 9). Wells having lateral positions of 0.20 

to 0.39, 0.40 to 0.59, and 0.60 to 0.79, made up 29 percent, 

21 percent, and 24 percent of the total grid wells, respectively. 

Wells having lateral positions of 0.0 to <0.20 (distal or 

western) and >0.80 (proximal or eastern) made up 13 percent 

and 14 percent of the total grid wells, respectively. 

Groundwater Age

Of the 78 groundwater samples obtained by the USGS 

for the PBP, 24 were classified as modern, 27 were mixed, and 

21 were pre-modern (table A2). Samples from six wells could 

not be classified because the age-tracer data were incomplete 

or did not meet all quality-assurance checks. Of these six 

wells, one well had no tritium or noble-gas data, two wells had 

tritium >1 TU but no noble-gas data and could be modern or 

mixed (labeled modern/mixed on table A2), and three wells 

had noble-gas data indicating >5 percent terrigenic He but 

no tritium data and could be mixed or pre-modern (labeled 

mixed/pre-modern on table A2). These wells with incomplete 

data were not included in the discussion below.

Classified groundwater ages generally became older with 

increasing depth to the tops of the perforations (fig. 10A). 

The depths to the tops of the perforations were significantly 

shallower for wells having modern and mixed ages as 

compared to those having pre-modern ages (table 4). The 

depths to the tops of the perforations were not significantly 

different between wells having modern and mixed ages. 

Relative to well depth, wells classified as modern were 

significantly shallower than wells classified as mixed or premodern (table 4, fig. 10B). However, depths of wells having 

mixed and pre-modern ages were not significantly different 

(table 4). 

Most wells perforated entirely within the upper 200 ft 

of the aquifer were modern (fig. 10c). Most wells perforated 

entirely at depths >200 ft were pre-modern. Wells with the top 

of the perforation <200 ft but the bottom of the well >200 ft 

mostly were mixed. 

In each of the three depth categories, there are some wells 

with each of the classified ages. These results indicate that 

there are local variations in the general age-depth relation. For 

example, the two wells with perforations >200 ft and modern 

ages are located in the eastern (upgradient) part of the study 

unit along the Stanislaus River (MOD-09) or adjacent to a 

storage reservoir of water diverted from the Tuolumne River 

(CE-QPC-03, table A2, fig. A1). Four wells with perforations 

<200 ft and pre-modern ages are located either near the eastern 

or the western boundaries of the study unit.

Geochemical Conditions

Geochemical conditions investigated as potential 

explanatory factors in this report include oxidation-reduction 

characteristics and pH. 

Oxidation-reduction (redox) conditions influence 

the transport of many organic and inorganic constituents 

(McMahon and Chapelle, 2008). Redox conditions along 

groundwater-flow paths commonly proceed along a welldocumented sequence of Terminal Electron Acceptor 

Processes (TEAPs), in which a single TEAP typically 

dominates at a particular time and aquifer location (Chapelle 

and others, 1995; Chapelle, 2001). The predominant 

TEAPs are oxygen-reduction (oxic), nitrate-reduction, 

manganese-reduction, iron-reduction, sulfate-reduction, and 

methanogenesis. The presence of redox-sensitive chemical 

species indicating more than one TEAP may indicate mixing 

of waters from different redox zones upgradient of the well, 

that the well is screened across more than one redox zone, or 

spatial heterogeneity in microbial activity in the aquifer. In 

addition, different redox couples may not indicate a consistent 

redox condition, reflecting electrochemical disequilibrium in 

groundwater (Lindburg and Runnels, 1984). Thus, assessments 

of redox conditions could be complicated by mixing and (or) 

disequilibrium. 

Classification of redox conditions on the basis of the 

framework of McMahon and Chapelle (2008) for the 78 wells 

sampled by USGS–GAMA is shown in table A3. Figure 11

shows wells classified by their dissolved oxygen (DO) 

concentrations plotted by normalized lateral position on the 

x-axis and depth on the y-axis. This figure represents all of the 

wells having well construction and DO data in the study unit 

presented on a single composite cross-section. 
26 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

30 Number of values

75th percentile

Median

25th percentile

25th percentile minus 1.5 times

the interquartile range

75th percentile plus 1.5 times

the interquartile range

Upper detached

Lower detached

61 54 54 19 16 16

Well

depth

Top of

perforations

Perforation

length

Well

depth

Top of

perforations

Perforation

length

Grid wells Understanding wells

1,000

800

600

400

200

0

DEPTH BELOW LAND SURFACE OR LENGTH, FEET

Figure 8. Boxplots showing construction characteristics for grid and understanding wells, Central Eastside, California, Groundwater 

Ambient Monitoring and Assessment (GAMA) study unit.
Attributes of Wells 27

Grid cell

MERCED

COUNTY

STANISLAUS

COUNTY

MARIPOSA

COUNTY

MADERA

COUNTY

SAN JOAQUIN

COUNTY

TUOLUMNE

COUNTY

CALAVERAS

COUNTY

San Joaquin River

Merced River

Merced River

Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

20

25

30

0

5

10

15

 

NORMALIZED LATERAL POSITION 

PERCENTAGE OF WELLS

<0.2 0.20-

0.39

0.40-

0.59

0.60-

0.79

>0.80

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

20 MILES

20 KILOMETERS

EXPLANATION

0 10

0

121 00’ 120 30’

38 00’

37 30’

37 00’

Merced

Turlock

Modesto

Valley trough

( = 0 )

0.2

0.4

0.6

0.8

East edge of

valley ( = 1 )

Normalized lateral position

Distance from well to valley trough

Distance from east edge to valley trough =

USGS grid well

CDPH grid well

USGS understanding well

Figure 9. Wells and normalized lateral position from eastern margin of valley, Central Eastside, California, Groundwater Ambient 

Monitoring and Assessment (GAMA) study unit.
28 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Modern

Mixed

Pre-modern

NUMBER OF WELLS

DEPTH OF

PERFORATIONS

BELOW LAND

SURFACE

0 5 10 15

Entire perforated

interval < 200 feet

Entire perforated

interval 200 feet

Top of perforated

interval < 200 feet;

Bottom of perforated interval

 200 feet

0 17

100

200

DEPTH TO TOP OF PERFORATIONS, IN FEET 300

400

23 16

Modern Mixed Pre-modern

GROUNDWATER AGE MIXTURE CLASSIFICATION

0 22

200

400

WELL DEPTH, IN FEET

600

800

1,000

500

25 19

Modern Mixed Pre-modern

GROUNDWATER AGE MIXTURE CLASSIFICATION

16 Number of values

EXPLANATION

EXPLANATION

75th percentile

Median

25th percentile

25th percentile minus 1.5 times

the interquartile range

75th percentile plus 1.5 times

the interquartile range

Upper detached

Lower detached

A

B

C

Figure 10. Relation of classified groundwater age to (A) depth to top of perforations below land surface, 

(B) depth to the bottom of the well below land surface, and (C) bar chart showing number of wells with modern, mixed, and pre-modern groundwater 

age classifications in each of three depth categories, Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study 

unit.
Attributes of Wells 29

Groundwater in the Central Eastside primarily was oxic 

(86 percent of USGS grid wells and 70 percent of USGS 

understanding wells) but becomes more reducing with depth 

and near the western (distal or downgradient) end of the study 

area. Most groundwater in the upper 500 ft of the aquifer has 

DO >2.0 mg/L, except for near the western end of the flow 

system (fig. 11). Intermediate DO concentrations of 0.5 to 

2.0 mg/L, or DO of <0.5 mg/L, was increasingly prevalent at 

depths of about 500 ft or more, and in the western quarter of 

the study unit. The lateral position and depth of wells having 

DO of 0.5 to 2.0 mg/L is consistent with general transitions 

from higher to lower DO concentrations with increasing depth 

and from east to west. 

A minority of wells (14 percent of USGS grid wells, 

30 percent of USGS understanding wells) had reducing 

conditions ranging from suboxic to methanogenesis (table A3). 

The reducing conditions primarily occurred in the western 

quarter of the study unit or in relatively deep groundwater 

(fig. 11). Wells with lateral positions less than about 0.25 

primarily were suboxic or more reduced (fig. 11). These 

results are consistent with those of previous investigations, 

including Davis and others (1959), Bertoldi and others (1991), 

Dubrovsky and others (1993), Chapelle and others (1995), and 

Burow and others (1998b), noting that groundwater typically 

becomes more reduced towards the central trough of the San 

Joaquin Valley. The range of reducing conditions from suboxic 

to methanogenesis may reflect natural spatial variability 

in geochemical conditions as well as mixing of water with 

variable redox characteristics in wells with long perforated 

intervals.

Modern compared 

with mixed

Mixed compared 

with pre-modern

Modern compared 

with pre-modern

Z Z Z

Selected water-quality constituents

Arsenic –2.282 ns –2.029

Vanadium ns ns ns

Lead ns ns –2.076

Uranium ns 2.128 2.607

Nitrate plus nitrite as nitrogen 2.718 2.637 3.756

Manganese ns ns ns

Iron ns ns ns

Total dissolved solids ns ns ns

Sum of trihalomethane concentrations 1 ns ns ns

Sum of solvent concentrations 1 ns ns ns

Sum of herbicide concentrations ns 3.920 2.823

Perchlorate ns 2.138 3.030

Potential explanatory factors

Percent urban land use ns 2.274 ns

Percent agricultural land use ns ns ns

Percent natural land use 2.520 ns ns

Depth to top of perforations ns –2.842 –3.369

Well depth –2.313 ns –3.726

Dissolved oxygen ns 2.759 3.112

pH ns –2.184 –2.690

1

 Grid wells only. Variables not footnoted include grid plus understanding wells.

Table 4. Results of Wilcoxon rank-sum tests for differences in values of selected water-quality constituents and for differences in 

potential explanatory factors, between modern, mixed, and pre-modern groundwater age categories, Central Eastside Groundwater 

Ambient Monitoring and Assessment (GAMA) study unit.

[Wilcoxon rank sum tests, based on large-sample approximation with continuity correction; Z, test statistic for Wilcoxon test; Z values are shown for Wilcoxon 

tests in which the two populations were determined to be significantly different (two-sided test) on the basis of p values (significance level of the Wilcoxon test, 

values not shown) less than threshold value (α) of 0.05; ns, test indicates no significant difference between groups; blue Z values, significantly positive Z value 

(first group has larger values than the second); red Z values, significantly negative Z value (first group has smaller values than the second)]
30 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

0

200

400

600

800

1,000

0 0.2 0.4 0.6 0.8 1.0

NORMALIZED LATERAL POSITION

Generalized direction of groundwater flow

DEPTH, IN FEET BELOW LAND SURFACE

Wells with DO < 0.5 milligrams per liter are labeled with oxidation-reduction (redox) classification (McMahon and Chapelle, 2008):

Sub, Suboxic; NO3, nitrate-reducing; NO3/Mn, mixed nitrate and manganese reducing; Fe, iron reducing; CH4, methanogenic; remaining

wells with DO < 0.5 milligrams per liter (unlabeled) are anoxic (no other redox data but DO).

Wells with DO > 2.0 milligrams per liter (blue) and 0.5 – 2.0 milligrams per liter (yellow) are classified as oxic. Wells with DO of

0.5 – 2.0 milligrams per liter are shown to illustrate distribution of waters that may have transitional or mixed redox conditions.

Top

Fe

Sub

Sub

Sub

Fe

CH4

NO3 NO3/

Mn

NO3 /Mn NO3

Bottom

Perforated interval > 2.0

Dissolved oxygen (DO) in milligrams per liter

EXPLANATION

0.5 - 2.0 < 0.5

Valley trough East edge

of valley

Figure 11. Diagram showing relation of oxidation-reduction condition indicators (dissolved oxygen, nitrate, manganese, and iron) 

to lateral position and depth of perforated interval of wells, Central Eastside, California, Groundwater Ambient Monitoring and 

Assessment (GAMA) study unit.
Status and Understanding of Water Quality 31

Although the redox classification described above is 

valuable for characterizing the range and spatial distribution 

of redox conditions, hereinafter, DO concentrations are used 

as the factor for evaluating relations of redox conditions with 

concentrations of water-quality constituents. DO was used 

as a redox indicator because (1) data were available for 77 of 

the 78 USGS–GAMA sampled wells, whereas other redox 

indicators were available for fewer than 50 USGS–GAMA 

wells; (2) DO is a continuous numerical variable, which 

permits more straightforward analysis of correlations with 

water-quality data, rather than a categorical variable like redox 

classification; and (3) most groundwater in the study unit was 

classified as oxic, so the numbers of wells within the various 

reducing classes generally were too small for meaningful 

statistical analysis. Some redox-sensitive constituents such as 

arsenic also were evaluated for relations with the occurrence 

of manganese- and iron-reducing conditions, but there were 

not enough wells having these characteristics for statistical 

correlations to be meaningful.

pH has been identified as being positively correlated 

with concentrations of arsenic in the San Joaquin Valley 

(Belitz and others, 2003; Welch and others, 2006; Izbicki and 

others, 2008). pH ranged from 6.3 to 8.3 in 66 USGS grid and 

understanding wells in the Central Eastside study unit. USGS–

GAMA and CDPH other data showed some pH values >8.0 in 

the central Turlock and Modesto study areas and the southern 

and northern Merced study area (fig. 12A). 

Correlations between Explanatory Factors

It is important to identify significant correlations between 

explanatory factors because apparent correlations between 

an explanatory variable and a water-quality constituent could 

be affected by correlations between two or more explanatory 

factors. Statistically significant correlations between 

explanatory factors are indicated in table 5 and noted below, 

with the exception of relations between groundwater age and 

depth, which were discussed earlier. 

Percent natural land use is correlated significantly 

(positively) with lateral position (table 5). This correlation 

reflects that land use primarily is natural in the eastern 

portion of the study unit, whereas agricultural and urban 

land uses become more prevalent in the central and western 

portions (fig. 4). Percent orchard and (or) vineyard land use 

is correlated significantly (negatively) with well depth (but 

not with depth to top of perforations) (table 5). This apparent 

correlation may reflect that several of the USGS understanding 

wells were shallow monitoring wells installed in areas 

with large percentages of orchard and vineyard land use. 

Correlations between different land uses were not calculated, 

as these data were not independent.

Concentrations of DO were correlated significantly 

(positively) with lateral position (table 5), consistent with the 

visual patterns evident in figure 11. Wilcoxon tests indicated 

significantly higher DO in wells classified as having modern 

and mixed ages compared to pre-modern ages (table 4). 

Wells having modern and mixed classified ages did not have 

significantly different DO. 

pH was correlated positively with well depth and depth 

to the top of the perforations (table 5, fig. 12B). Wilcoxon 

tests indicated significantly lower pH in wells classified as 

having modern and mixed ages compared to pre-modern ages 

(table 4). For wells having similar depths, those of pre-modern 

age often had higher pH than those of mixed and modern age 

(fig. 12B). Wells having modern and mixed classified ages did 

not have significantly different pH. Also, pH was correlated 

significantly (positively) with urban land use. This relation 

may reflect that grid wells with >50 percent urban land use 

and pH ≥ 7.8 generally had larger perforation intervals than 

wells with <50 percent urban land use. The longer perforation 

intervals may draw relatively high pH waters at depth to the 

wells. Another observation consistent with this interpretation 

is that wells with mixed ages had a significantly larger percent 

urban land use than pre-modern wells (table 4). The negative 

correlation between pH and orchard and (or) vineyard land 

use probably reflects that both of these factors are related to 

well depth (table 5). Implications of correlations between 

explanatory factors are discussed later in the report as part of 

analysis of factors affecting individual constituents.

Status and Understanding of Water 

Quality

Approximately 11,000 individual analytical results were 

included in the assessment of groundwater quality for the 

Central Eastside study unit. The results are summarized in 

figure 13. Detected constituents having data in <20 grid wells 

(hereinafter, constituents sampled for in a subset of wells) are 

identified with green dots on figure 13 to distinguish them 

from constituents having data in a larger number of grid 

wells (24 to 58 wells, yellow dots). All of the constituents 

shown have human-health benchmarks except for those in the 

inorganic-SMCL group, which have aesthetic benchmarks 

(Landon and Belitz, 2008). 

Inorganic constituents detected at high maximum 

relative-concentrations in USGS or CDPH grid wells were 

arsenic, vanadium, lead, nitrate, manganese, and TDS; 

moderate maximum relative-concentrations were detected 

for these constituents and boron, uranium, and gross alpha 

radioactivity. Organic and special-interest constituents were 

not detected at high relative-concentrations in USGS grid 

wells; moderate relative-concentrations were detected for 

carbon tetrachloride, chloroform, DBCP, and perchlorate. 

Constituents with moderate or high maximum relativeconcentrations are discussed individually later in this report.
32 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

0

6.8 7.0 7.47.2 7.6 7.8 8.0 8.48.2

200

pH

400

600

800

1,000

Other wells

Modern

Public-supply wells

Mixed

Pre-modern

Modern

Mixed

Pre-modern

No age data

WELL DEPTH BELOW LAND SURFACE, FEET

 = -0.457, p = 0.001

EXPLANATION

B

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

A S I E R R A N E V A D A

Urban

Land use

Agricultural

Natural

EXPLANATION

USGS GAMA pH

< 7.5

7.5 - 7.9

> 8.0

CDPH pH

< 7.5

7.5 - 7.9

> 8.0

Study area boundary

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

20 MILES

20 KILOMETERS

0 10

0

Figure 12. Plots showing values of pH, (A) map of pH in USGS grid and USGS understanding wells representative of the primary aquifer 

and the most recent analysis during March 1, 2003–February 28, 2006 for CDPH wells, and (B) graph showing pH and well depth by well 

type and groundwater-age classification, Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study 

unit. 
Status and Understanding of Water Quality 33

Wells included in analysis

Normalized 

lateral position 

from 

valley trough

Depth to top of 

perforations 

below land 

surface 

(feet)

Depth of well 

below land 

surface 

(feet)

Dissolved oxygen 

concentration 

(mg/L)

pH, 

pH units

Percent orchard 

and vineyard 

land use

ρ ρ ρ ρ ρ ρ

Grid wells Percent urban land use ns ns ns ns 0.325 NC

Grid wells Percent agricultural land use ns ns ns ns ns NCGrid wells Percent natural land use 0.325 ns ns ns ns NCGrid wells Normalized lateral position 

from valley trough

ns ns 0.368 ns ns

Grid and understanding wells Depth to top of perforations 

below land surface, in feet

0.765 ns 0.401 ns

Grid and understanding wells Depth of well below land 

surface, in feet

ns 0.457 –0.268

Grid and understanding wells Dissolved oxygen 

concentration, in mg/L

ns ns

Grid and understanding wells pH, pH units –0.334

Table 5. Results of non-parametric (Spearman's method) analysis of correlations between selected potential explanatory factors, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study unit.[ρ, Spearman's correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated on the basis of p values (significance level of the Spearman's test) less 

than threshold value (α) of 0.05; ns, Spearman's test indicates no significant correlation between groups; blue text, significant positive correlation; red text, significant negative correlation; NC, not calculated 

due to lack of independence of variables]
34 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Figure 13. Maximum relative-concentration in grid wells for constituents detected, by type of constituent, in the Central Eastside, 

California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.

Most of the organic and special-interest constituents 

detected (31 of the 36) have some type of human-health 

benchmark (table 6). Of the five detected constituents 

that did not have benchmarks, three (de-ethylatrazine, 

de-isopropylatrazine, 3,4-dichloroanaline) are pesticide 

degradates. Some of the parent compounds (atrazine, diuron) 

of these degradates have human-health benchmarks. The other 

two detected constituents that did not have benchmarks were 

a solvent, dibromomethane, and an herbicide, norflurazon 

(Landon and Belitz, 2008). 

In contrast to organic and special-interest constituents, 

inorganic constituents nearly always were detected (48 of 51, 

table 6). Approximately one-third of inorganic constituents (15 

of 51) had no human-health or aesthetic benchmarks. Most of 

these constituents without benchmarks are major or minor ions 

that are present naturally in nearly all groundwater. 

Perchlorate

DBCP

1,2-Dichloro- propane

1,1-Dichloroethene 1,2,3

-TCP

(HAL)

Atrazine

Simazine

Fumigant Pesticide

Moderate

Organic and special interest constituents Inorganic constituents

Low

Moderate

High

Moderate

Low

Volatile

Organic

Compound

0.01

0.1

1

10

0.001

Number of 

compounds

detected < 0.001 9 0 4 0 1 0 0 0

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

Inorganic Radioactive

constituents

having

SMCL’s

Trace

elements

Special Nutrient

interest

Chloroform

Toluene

Chloromethane

cis-1,2-Dichloroethene

Dichloromethane

Benzene

PCE

Bromodichloromethane

Carbon

tetrachloride

Arsenic

Uranium

Manganese

TDS

(upper)

Sulfate

(upper)

Iron

Chloride

(upper)

Gross alpha

Gross beta

Radon

Radium

226+228

Boron

Barium

Chromium

Strontium

Molybdenum

Nickel

Aluminum

Antimony

Fluoride

Vanadium

Lead

Selenium

Cadmium

Nitrite-N

Nitrate-N

Zinc

Zinc

Copper

Titanium

Constituents with analyses in >20 grid wells and wells are spatially representative-Name and center of

symbol is location of data unless indicated by following location line: 

EXPLANATION

Constituents with analyses in <20 grid wells and wells are not spatially representative-Name and center of

symbol is location of data unless indicated by following location line:

Dibromochloromethane

Bromoform

TCE

TCE, Trichloroethene; PCE, Tetrachloroethene; DBCP, 1,2-Dibromo-3-chloropropane; 1,2,3-TCP, 1,2,3-Trichloropropane;

HAL, U.S. Environmental Protection Agency Lifetime Health Advisory Level; N, Nitrogen; TDS, Total Dissolved Solids; 

>, greater than; <, less than; SMCL, secondary maximum contaminant level; upper water-quality benchmark was used for

calculating relative-concentrations when multiple benchmarks exist

Diuron

Diuron
Status and Understanding of Water Quality 35

Benchmark

type

Number of constituents

Analyzed Detected 

VOCs and Gasoline Oxygenates (excluding fumigants)

MCL 29 16

HAL 6 2

NL 14 3

RSD5 2 0

AL 0 0

SMCL 0 0

None 27 1

Total 78 22

Fumigants

MCL 4 2

HAL 2 1

NL 0 0

RSD5 2 0

AL 0 0

SMCL 0 0

None 2 0

Total 10 3

Pesticides and degradates (NWQL Schedule 2003)

MCL 3 3

HAL 14 2

NL 0 0

RSD5 3 0

AL 0 0

SMCL 0 0

None 41 2

Total 61 7

Polar pesticides and degradates (NWQL Schedule 2060)

MCL 9 0

HAL 9 1

NL 0 0

RSD5 0 0

Table 6. Number of constituents analyzed and detected by benchmark and constituent class or analyte group, Central Eastside 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, March to June 2006. 

[VOCs, Volatile Organic Compounds; NWQL, USGS National Water Quality Laboratory; USEPA, U.S. Environmental Protection Agency; CDPH, California 

Department of Public Health; MCL, USEPA or CDPH Maximum Contaminant Level; HAL, USEPA Health Advisory Level; NL, CDPH Notification Level; 

RSD5, USEPA Risk Specific Dose at 10–5; AL, USEPA Action Level; SMCL, USEPA or CDPH Secondary Maximum Contaminant Level]

Benchmark

type

Number of constituents

Analyzed Detected 

Polar pesticides and degradates (NWQL Schedule 2060)—Continued

AL 0 0

SMCL 0 0

None 36 2

Total 54 3

Special interest constituents

MCL 0 0

HAL 0 0

NL 2 1

RSD5 0 0

AL 0 0

SMCL 0 0

None 0 0

Total 2 1

Sum of Organic and Special Interest Constituents

MCL 45 21

HAL 31 6

NL 16 4

RSD5 7 0

AL 0 0

SMCL 0 0

None 106 5

Total 205 36

Sum of Inorganic constituents

MCL 22 20

HAL 4 4

NL 2 2

RSD5 0 0

AL 2 2

SMCL 6 5

None 15 15

Total 51 48
36 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Relations to potential explanatory factors were analyzed 

for three individual constituents or parameters that were 

the only constituents in their class with moderate relativeconcentrations, aquifer-scale-high proportions <2 percent, 

or detection frequencies >10 percent. The three constituents 

were: total dissolved solids (TDS) (in the constituent class 

of inorganic constituents with SMCLs); 1,2-dibromo-3-

chloropropane (DBCP) (in the constituent class of fumigants); 

and perchlorate (in the class of constituents of special interest 

to the PBP).

Inorganic Constituents

Nine inorganic constituents met the selection criteria 

of having maximum relative-concentrations >0.5 (moderate 

or high) in the grid-based assessment (fig. 13): the trace 

elements arsenic, boron, lead, and vanadium; the radioactive 

constituents uranium and gross alpha radioactivity; the nutrient 

nitrate; and the inorganic constituents with SMCLs manganese 

and TDS. In addition, iron met the selection criteria of having 

>2 percent high relative-concentrations based on the spatiallyweighted analysis (table 7A). Analysis of potential explanatory 

factors is included below for arsenic, vanadium, lead, uranium, 

nitrate, manganese, iron, and TDS. 

For inorganic constituents having human-health 

benchmarks (nutrients, trace elements, and radioactive 

constituents), 18.0 percent of the aquifer had high relativeconcentrations of at least one constituent, 44.0 percent had 

moderate relative-concentrations of at least one constituent, 

and 38.0 percent had low relative-concentrations of all 

constituents (table 8). The aquifer-scale high proportion of 

inorganic constituents reflected high relative-concentrations 

of trace elements in 17.4 percent of the aquifer, radioactive 

constituents in 3.6 percent of the aquifer, and nitrate in 2.1 

percent of the aquifer. 

Trace Elements

As a class, 17.4 percent of the aquifer had high relativeconcentrations of at least one trace element having humanhealth benchmarks, 32.6 percent had moderate relativeconcentrations of at least one trace element, and 50.0 percent 

had low relative-concentrations of all trace elements (table 8). 

The aquifer-scale high proportion of trace elements reflected 

high relative-concentrations of arsenic in 15.6 percent of the 

aquifer and vanadium in 3.6 percent of the aquifer (table 7A). 

A high relative-concentration of lead and arsenic co-occurred 

in one grid well (Landon and Belitz, 2008); consequently, 

the high value of lead did not change the aquifer-scale high 

proportion of trace elements as a constituent class. 

Status Assessment for Trace Elements 

Figure 14 shows relative-concentrations of inorganic 

constituents with high or moderate maximum relativeconcentrations in USGS or CDPH grid wells. Figure 15

shows data for six constituents (arsenic, vanadium, uranium, 

nitrate, manganese, and total dissolved solids) from USGS 

grid wells, USGS understanding wells that were considered to 

be representative of the primary aquifer, and the most recent 

analysis during March 1, 2003,–February 28, 2006, from all 

CDPH wells. Maps for these six constituents are shown in 

figure 15 to illustrate the spatial distribution of concentrations 

of inorganic constituents having high and moderate aquiferscale proportions greater than 10 percent. 

Arsenic had high relative-concentrations in 15.6 percent 

of the aquifer and moderate relative-concentrations in 

28.9 percent (table 7A, fig. 14). High relative-concentrations 

of arsenic occurred in the Merced, west-central Turlock, and 

the western Modesto study areas (fig. 15A). Moderate relativeconcentrations of arsenic were present in all four study areas. 

Boron was detected at moderate relative-concentrations 

in 4.2 percent of the aquifer (table 7B). The single grid well 

having a moderate relative-concentration of boron was located 

in the Turlock study area (fig. 14). 

Lead was detected at a high relative-concentration in 

2.4 percent of the aquifer (table 7A). The single grid well 

having a high relative-concentration of lead was located in 

the Turlock study area (fig. 14). Lead had a moderate relativeconcentration in 2.4 percent of the aquifer. The single grid 

well having a moderate relative-concentration of lead was 

located in the Merced study area (fig. 14). Although lead had a 

maximum relative-concentration that was higher than all other 

constituents except arsenic (fig. 13), it was detected at high 

relative-concentrations only in one grid well and in two other 

CDPH wells (table 7A).

Vanadium had high relative-concentrations in 3.6 percent 

of the aquifer and moderate relative-concentrations in 

21.4 percent (table 7A, fig. 14). High relative-concentrations 

of vanadium primarily occurred in the central part of 

the Turlock study area (fig. 15B). Moderate relativeconcentrations occurred throughout the central portion of the 

study unit, particularly within the Turlock study area, and one 

moderate value occurred in the upland study area (fig. 15B).

Three trace elements (antimony, copper, and selenium), 

had aquifer-scale high proportions from the spatially-weighted 

approach of 0.2 to 0.3 percent of the aquifer (table 7A) but 

were not detected at high relative-concentrations in grid wells. 

The trace elements, barium and boron, had high relativeconcentrations in at least one well in the CDPH database 

before 2003 but not during the current period analyzed 

(table 7B). The high relative-concentrations for barium and 

boron represented historic rather than current conditions.
Status and Understanding of Water Quality 37

Constituent Threshold type Threshold value Threshold units

Grid-based aquifer-scale proportion 1

90-percent 

confidence 

interval for gridbased 

high proportion 2

Detection-frequency-based 

aquifer-scale proportion 3

Spatially 

weighted 

aquiferscale 

proportion 3

Number 

of wells

High 

proportion 

(in percent)

Moderate 

proportion 

(in percent)

Low 

proportion 

(in percent)

Lower 

limit 

(in 

percent)

Upper 

limit 

(in 

percent)

Number 

of wells

Number of 

high wells

High 

proportion 

(in percent)

High 

proportion 

(in percent)

Trace Elements

Arsenic MCL-US 10 μg/L 45 15.6 28.9 55.6 8.3 25.9 348 31 8.9 12.7Vanadium NL-CA 50 μg/L 28 3.6 21.4 75.0 0.6 13.1 133 7 5.3 1.4Lead MCL-US 15 μg/L 42 2.4 2.4 95.2 0.4 8.9 325 2 0.6 0.4Copper MCL-US 1,300 μg/L 44 0.0 0.0 100.0 0.0 3.0 332 1 0.3 0.3Antimony MCL-US 6 μg/L 43 0.0 0.0 100.0 0.0 3.1 336 1 0.3 0.2

Selenium MCL-US 50 μg/L 43 0.0 0.0 100.0 0.0 3.1 337 1 0.3 0.2

Radioactive Constituents

Gross-alpha radioactivity, 

30 day count

MCL-US 15 pCi/L 37 0.0 10.8 89.2 0.0 3.6 282 23 8.2 5.9

Uranium MCL-US 30 μg/L 33 0.0 15.2 84.8 0.0 4.0 191 12 6.3 3.6

Nutrients

Nitrate, as nitrogen MCL-US 10 mg/L 48 2.1 14.6 83.3 0.4 7.9 480 24 5.0 3.4

Inorganic constituents with SMCLs

Manganese SMCL-CA 50 μg/L 44 4.5 9.1 86.4 1.3 12.0 332 15 4.5 10.2

Iron SMCL-CA 300 μg/L 44 0.0 0.0 100.0 0.0 3.0 334 8 2.4 2.2

Total dissolved solids 

(TDS)

SMCL-CA 1,000 mg/L 58 1.7 8.6 89.7 0.3 6.5 363 4 1.1 1.1

Solvents

Tetrachloroethene (PCE) MCL-US 5 μg/L 58 0.0 0.0 100.0 0.0 2.3 365 3 0.8 0.2

Fumigants

1,2-Dibromo-3-

chloropropane (DBCP)

MCL-US 0.2 μg/L 58 0.0 8.6 91.4 0.0 2.3 372 13 3.5 1.0

1 Grid-based aquifer-scale proportions for organic constituents are based on samples collected by the U.S. Geological Survey from 58 grid wells during March–June 2006. 2 Based on the Jeffrey's interval for the binomial distribution (Brown and others, 2001). 2 Based on most recent analysis for each CDPH well during March 1, 2003–February 28, 2006, combined with GAMA grid-based data.

Table 7A. Aquifer-scale proportions using grid-based and spatially weighted methods for those constituents that had concentrations above water-quality benchmarks (high relative-concentrations) during the current period (March 1, 2003, to February 28, 2006) from the California Department of Public Health database or in samples collected from grid wells, March–June 2006, Central Eastside Groundwater Ambient Monitoring and Assessment study unit. [High; concentrations greater than water-quality benchmark; moderate, concentrations between 11 and 100 percent of benchmark for organic constituents or 51 and 100 percent of benchmark for inorganic constituents; low, concentrations less than or equal to 10 percent of benchmark for organic constituents or 50 percent of benchmark for inorganic constituents; CDPH, California Department of Public Health; MCL-US; U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, CDPH maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; NA, not applicable]
38 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project Table 7B. Aquifer-scale proportions using grid-based and spatially weighted methods for those constituents that had concentrations above water-quality benchmarks (high relative-concentrations) during April 24, 1976, to February 28, 2003, (historically high) from the California Department of Public Health database or had moderate relative- concentrations in samples collected from grid wells, March–June 2006, Central Eastside Groundwater Ambient Monitoring and Assessment study unit. —Continued [High, concentrations greater than water-quality benchmark; moderate, concentrations between 11 and 100 percent of benchmark for organic constituents or 51 and 100 percent of benchmark for inorganic constituents; low, concentrations less than or equal to 10 percent of benchmark for organic constituents or 50 percent of benchmark for inorganic constituents; CDPH, California Department of Public Health; MCL-US; U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, CDPH maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; NA, not applicable] Constituent Threshold type Threshold value Threshold units Date of most recent high value Grid-based aquifer-scale proportion 1 90-percent confidence interval for grid-based high proportion 2 Detection-frequency-based aquifer-scale proportion 3 Spatially weighted aquifer- scale proportion 3 Number of wells High proportion (in percent) Moderate proportion (in percent) Low proportion (in percent) Lower limit (in percent) Upper limit (in percent) Number of wells Number of high wells High proportion (in percent) High proportion (in percent) Trace Elements Boron NL-CA 1,000 μg/L 7/16/2002 4 24 0.0 4.2 95.8 0.0 5.4 109 0 0.0 0.0 Barium MCL-CA 1,000 μg/L 7/21/1992 4 43 0.0 0.0 100.0 0.0 3.1 339 0 0.0 0.0 Nutrients Nitrite, as nitrogen MCL-US 1 mg/L 2/24/2004 5 45 0.0 0.0 100.0 0.0 2.9 394 0 0.0 0.0 Inorganic constitutents with SMCLs Chloride SMCL-CA 500 mg/L 2/8/2000 4 45 0.0 0.0 100.0 0.0 2.9 334 0 0.0 0.0 Trihalomethanes Chloroform MCL-US 80 6 μg/L none 58 0.0 3.4 96.6 0.0 2.3 363 0 0.0 0.0 Solvents Carbon tet- rachloride- (tetrachlo- romethane) MCL-CA 0.5 μg/L 6/15/2005 5 58 0.0 3.4 96.6 0.0 2.3 365 0 0.0 0.0 Trichloro- ethene (TCE) MCL-US 5 μg/L 10/25/2005 5 58 0.0 0.0 100.0 0.0 2.3 365 0 0.0 0.0 Dichloro- methane (methylene chloride) MCL-US 5 μg/L 3/1/1994 4 58 0.0 0.0 100.0 0.0 2.3 365 0 0.0 0.0 1,2-Dichloro- ethane MCL-CA 0.5 μg/L 12/3/1997 4 58 0.0 0.0 100.0 0.0 2.3 365 0 0.0 0.0 Table 7B. Aquifer-scale proportions using grid-based and spatially weighted methods for those constituents that had concentrations above water-quality benchmarks (high relative-concentrations) during April 24, 1976, to February 28, 2003, (historically high) from the California Department of Public Health database or had moderate relative concentrations in samples collected from grid wells, March–June 2006, Central Eastside Groundwater Ambient Monitoring and Assessment study unit. [High, concentrations greater than water-quality benchmark; moderate, concentrations between 11 and 100 percent of benchmark for organic constituents or 51 and 100 percent of benchmark for inorganic constituents; low, concentrations less than or equal to 10 percent of benchmark for organic constituents or 50 percent of benchmark for inorganic constituents; CDPH, California Department of Public Health; MCL-US; U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, CDPH maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; NA, not applicable]
Status and Understanding of Water Quality 39

Table 7B. Aquifer-scale proportions using grid-based and spatially weighted methods for those constituents that had concentrations above water-quality benchmarks (high relative-concentrations) during April 24, 1976, to February 28, 2003, (historically high) from the California Department of Public Health database or had moderate relativeconcentrations in samples collected from grid wells, March–June 2006, Central Eastside Groundwater Ambient Monitoring and Assessment study unit. —Continued[High, concentrations greater than water-quality benchmark; moderate, concentrations between 11 and 100 percent of benchmark for organic constituents or 51 and 100 percent of benchmark for inorganic constituents; low, concentrations less than or equal to 10 percent of benchmark for organic constituents or 50 percent of benchmark for inorganic constituents; CDPH, California Department of Public Health; MCL-US; U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, CDPH maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; NA, not applicable]

Constituent Threshold type Threshold value Threshold units Date of most recent high value Grid-based aquifer-scale proportion 1

90-percent confidence interval for grid-based 

high proportion 2

Detection-frequency-based 

aquifer-scale proportion 3

Spatially 

weighted 

aquiferscale 

proportion 3

Number 

of wells

High 

proportion 

(in percent)

Moderate 

proportion 

(in percent)

Low 

proportion 

(in percent)

Lower 

limit 

(in percent)

Upper 

limit 

(in percent)

Number 

of wells

Number of 

high wells

High 

proportion 

(in percent)

High 

proportion 

(in percent)

Other VOCs

Vinyl chloride MCL-CA 0.5 μg/L 2/14/1989 4 58 0.0 0.0 100.0 0.0 2.3 365 0 0.0 0.0

Naphthalene NL-CA 17 μg/L 10/27/1992 4 58 0.0 0.0 100.0 0.0 2.3 338 0 0.0 0.0

Fumigants

1,2-Dibromoethane 

(EDB)

MCL-US 0.05 μg/L 3/16/1999 4 58 0.0 0.0 100.0 0.0 2.3 362 0 0.0 0.0

Herbicides

Atrazine MCL-CA 1 μg/L 4/11/1989 4 58 0.0 0.0 100.0 0.0 2.3 261 0 0.0 0.0

Insecticide

Diazinon HAL-US 1 μg/L 3/10/1994 4 58 0.0 0.0 100.0 0.0 2.3 230 0 0.0 0.0

Constituents of special interest

Perchlorate MCL-CA 6 μg/L none 58 0.0 20.7 79.3 0.0 2.3 189 0 0.0 0.0 1 Grid-based aquifer-scale proportions for organic constituents are based on samples collected by the U.S. Geological Survey from 58 grid wells during March–June 2006. 

2 Based on the Jeffrey’s interval for the binomial distribution (Brown and others, 2001).3 Based on most recent analysis for each CDPH well during March 1, 2003–February 28, 2006, combined with GAMA grid-based data. 4 Date of most recent high value is not within the 3-year period of CDPH data analyzed and were not used in calculating high aquifer proportion for current conditions. 5 High value occurred in at least one well within the 3-year period of CDPH data analyzed but not in the most recent sample. 6 The MCL-US threshold for trihalomethanes is the sum of chloroform, bromoform, bromodichloromethane, and dibromochloromethane.
40 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Constituent class

Aquifer-scale proportion

High relativeconcentrations 

(percent)

Moderate relativeconcentrations

(percent)

Low relativeconcentrations

(percent)

Inorganics with health-based benchmarks

Trace elements 17.4 32.6 50.0

Radioactive 1 3.6 2 11.6 84.8

Nutrients 2.1 14.6 83.3

Any inorganic with health-based benchmarks 18.0 44.0 38.0

Inorganics with aesthetic benchmarks

Total dissolved solids (SMCL) 1.7 8.6 89.7

Manganese and(or) iron (SMCL) 4.5 9.1 86.4

Organics with health-based benchmarks

Trihalomethanes 0.0 3.4 96.6

Solvents 1 0.2 3.4 96.4

Other VOCs 0.0 0.0 100.0

Fumigants 1 1.0 2 7.6 91.4

Herbicides 0.0 0.0 100.0

Insecticides 0.0 0.0 100.0

Any organic with health-based benchmarks 3 1.2 2 14.3 84.5

Constituents of special interest

Perchlorate (constituent of special interest) 0.0 20.7 79.3

Table 8. Aquifer-scale proportions for constituent classes, Central Eastside Groundwater Ambient Monitoring and Assessment 

(GAMA) Program study unit.

[CDPH, California Department of Public Health; THMs, Trihalomethanes; PCE, tetrachloroethene; TCE, trichloroethene; VOCs, volatile organic compounds; 

DBCP, 1,2-Dibromo-3-chloropropane; SMCL, secondary maximum contaminant level; values are grid based unless otherwise noted]

1

 Spatially weighted value.

2

 Calculated from the grid-based moderate proportion for the class minus the spatially weighted high proportion. Because the CDPH data usually could not 

be used to distinguish low from moderate relative-concentrations for all constituents in a class, the grid-based data were used to estimate low aquifer-scale 

proportions. Grid-based moderate proportions were adjusted downward by the amount of the spatially weighted high proportions because the CDPH data could 

generally be used to distinguish between high and moderate relative-concentrations. 3

 Maximum high proportion is shown. The high proportion is between a minimum of 1.0 percent, based on the fumigant DBCP, and a maximum of 

1.2 percent, assuming no co-occurrence of high values of the solvent PCE and DBCP. Additional statistical estimates considering overlap of high constituents 

are beyond the scope of this report.

Understanding Assessment for Arsenic

Arsenic concentrations were significantly higher in older 

(table 4, fig. 16A) and deeper (table 9, fig. 16B) groundwater. 

Arsenic concentrations in samples having a groundwater age 

classified as modern were significantly lower than samples 

classified as having mixed or pre-modern ages (table 4, 

fig. 16A) but were not significantly different between samples 

having mixed and pre-modern ages. Izbicki and others (2008) 

also determined that relatively high concentrations of arsenic 

were associated with older groundwaters in a study area in the 

San Joaquin Valley located north of the Central Eastside study 

unit. Arsenic concentrations also were correlated significantly 

(negatively) with normalized lateral position (table 9); this 

correlation reflects that most high concentrations of arsenic 

were detected in the western portion of the study unit, whereas 

most samples from the eastern portion of the study unit had 

low concentrations of arsenic (fig. 15A). 

Previous investigations of arsenic in the San Joaquin 

Valley (Belitz and others, 2003; Welch and others, 2006; 

Izbicki and others, 2008) and literature reviews (Welch and 

others, 2000; Stollenwerk, 2003) primarily have attributed 

elevated arsenic in groundwater to two mechanisms. One is 

the release of arsenic from dissolution of iron or manganese 

oxyhydroxides under iron- or manganese-reducing conditions. 

The other is desorption from aquifer sediments or inhibition of 

sorption to aquifer sediments with increasing pH. 
Status and Understanding of Water Quality 41

0 2 3 4

RELATIVE-CONCENTRATION, DIMENSIONLESS

1

Trace elements

Radioactive constituents

Inorganic constituents having secondary maximum contaminant levels

Nutrient

Nitrate

nitrogen

Arsenic

Boron

Lead

Vanadium

Manganese

Total dissolved

solids

Uranium

Gross

alpha activity

Uplands study area (5 or fewer grid wells)

EXPLANATION

Merced study area (17 or fewer grid wells)

Modesto study area (10 or fewer grid wells)

Turlock study area (12 or fewer grid wells)

Figure 14. Dot plots showing relative-concentrations of selected trace elements, radioactive constituents, nutrients, and inorganic 

constituents with secondary maximum contaminant levels in grid wells, Central Eastside, California, Groundwater Ambient Monitoring 

and Assessment (GAMA) study unit.
42 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

20 MILES

20 KILOMETERS

0 10

0

20 MILES

20 KILOMETERS

0 10

0

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

A

B

Urban

Land use

Agricultural

Natural

Urban

Land use

Agricultural

Natural

EXPLANATION

USGS GAMA arsenic

Low (< 5 micrograms per liter)

Moderate (5 - 10 micrograms per liter)

High (> 10 micrograms per liter)

CDPH arsenic

Low (< 5 micrograms per liter)

Moderate (5 - 10 micrograms per liter)

High (> 10 micrograms per liter)

Study area boundary

EXPLANATION

USGS GAMA vanadium

Low (< 25 micrograms per liter)

Moderate (25 - 50 micrograms per liter)

High (> 50 micrograms per liter)

CDPH vanadium

Low (< 25 micrograms per liter)

Moderate (25 - 50 micrograms per liter)

High (> 50 micrograms per liter)

Study area boundary

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Figure 15. Concentrations of selected inorganic constituents in USGS grid and USGS understanding wells representative of the 

primary aquifer and the most recent analysis during March 1, 2003–February 28, 2006, for CDPH wells, Central Eastside, California, 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit (A) arsenic, (B) vanadium, (C) uranium, 

(D) nitrate nitrogen, (E) manganese, and (F) total dissolved solids.
Status and Understanding of Water Quality 43

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

0 10

0

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

C

D

Urban

Land use

Agricultural

Natural

Urban

Land use

Agricultural

Natural

EXPLANATION

USGS GAMA uranium

Low (< 15 micrograms per liter)

Moderate (15 - 30 micrograms per liter)

High (> 30 micrograms per liter)

CDPH uranium

Low (< 15 micrograms per liter)

Moderate (15 - 30 micrograms per liter)

High (> 30 micrograms per liter)

Study area boundary

EXPLANATION

USGS GAMA nitrate (as nitrogen)

Low (< 5 milligrams per liter)

Moderate (5 - 10 milligrams per liter)

High (> 10 milligrams per liter)

CDPH nitrate (as nitrogen)

Low (< 5 milligrams per liter)

Moderate (5 - 10 milligrams per liter)

High (> 10 milligrams per liter)

Study area boundary

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

20 MILES

20 KILOMETERS

0 10

0

20 MILES

20 KILOMETERS

0 10

0

Figure 15. Continued. 
44 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

E

F

Urban

Land use

Agricultural

Natural

Urban

Land use

Agricultural

Natural

EXPLANATION

USGS GAMA manganese

Low (< 25 micrograms per liter)

Moderate (25 - 50 micrograms

 per liter)

High (> 50 micrograms per liter)

CDPH manganese

Low (< 25 micrograms per liter)

Moderate (25 - 50 micrograms

per liter)

High (> 50 micrograms per liter)

Study area boundary

EXPLANATION

USGS GAMA total dissolved solids

Low (< 500 milligrams per liter)

Moderate (500 - 1,000 milligrams

 per liter)

High (> 1,000 milligrams per liter)

CDPH total dissolved solids

Low (< 500 milligrams per liter)

Moderate (500 - 1,000 milligrams

 per liter)

High (> 1,000 milligrams per liter)

Study area boundary

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

0 10

0

20 MILES

20 KILOMETERS

10

20 MILES

20 KILOMETERS

0 10

0

Figure 15. Continued. 
Status and Understanding of Water Quality 45

Figure 16. Plots relating arsenic to explanatory factors and (A). boxplots showing relation of arsenic to classified groundwater age, 

and graphs showing relation of arsenic to (B). well depth, and (C). pH, Central Eastside, California, Groundwater Ambient Monitoring and 

Assessment (GAMA) study unit. 

MCL

40

200

400

600

800

1,000

WELL DEPTH BELOW LAND SURFACE, IN FEET

10 20 30

ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER 

0.5 times MCL

EXPLANATION

 = 0.365, p = 0.005

Manganese 50 micrograms per liter

Iron 100 micrograms per liter

pH < 8.0

Manganese 50 micrograms per liter

Iron < 100 micrograms per liter

pH < 8.0

Manganese < 50 micrograms per liter

Iron 100 micrograms per liter

pH 8.0

Manganese < 50 micrograms per liter

Iron < 100 micrograms per liter

pH 8.0

Manganese < 50 micrograms per liter

Iron < 100 micrograms per liter

pH < 8.0

0

13

ARSENIC, IN MICROGRAMS PER LITER

15 11

Modern Mixed Pre-modern

GROUNDWATER AGE MIXTURE CLASSIFICATION

16 Number of values

EXPLANATION

75th percentile

Median

25th percentile

25th percentile minus 1.5 times

the interquartile range

75th percentile plus 1.5 times

the interquartile range

Upper detached

Lower detached

20

10

15

5

No detection

No

detection

Modern significantly different from 

pre-modern and mixed at = 0.05

A

B
46 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

In the Central Eastside study unit, evidence for the first 

mechanism includes association of high and moderate relativeconcentrations of arsenic with manganese- and iron-reducing 

conditions in the western part of the study unit and the 

occurrence of arsenite as the dominant arsenic species in some 

wells with iron-reducing conditions. Six of the eight wells 

having manganese >50 μg/L (manganese-reducing conditions) 

and (or) iron >100 μg/L (iron-reducing conditions) had high 

(>10 μg/L) or moderate (5-10 μg/L) relative-concentrations of 

arsenic (fig. 16B; not all wells are shown because well depth 

was unknown for two wells). These results imply that arsenic 

mobilization can be related to either manganese- or ironreducing conditions. 

The presence of arsenite as the dominant dissolved 

arsenic species in iron-reduced water in the Central Eastside is 

consistent with mobilization under iron-reducing conditions. 

The two wells having the highest iron concentrations also had 

arsenite (As+3) as the dominant arsenic species, in contrast 

to 37 wells in the study unit with lower iron concentrations, 

which had arsenate (As+5) as the dominant arsenic species 

(Landon and Belitz, 2009, tables 10 and 11). Analysis of core 

samples from the Modesto area for arsenic species indicated 

that arsenate was the dominant form present in the aquifer 

sediments (Jurgens and others, 2008). However, under 

iron-reducing conditions, laboratory reaction experiments 

by Islam and others (2004) on sediments from Bangladesh 

have shown that arsenite can be the dominant arsenic species 

resulting from reductive dissolution of iron oxides even when 

arsenate is the dominant arsenic species in the solid-phase. At 

conditions less than iron-reducing conditions, it is expected 

that dissolved arsenate would not be reduced to arsenite; 

this observation fits with the observation that arsenate is the 

dominant species in all but some iron-reduced waters from 

the Central Eastside. The significant correlation (negative) 

of arsenic with normalized lateral distance from the valley 

trough (table 9) likely reflects the increasingly reduced redox 

conditions that occur in the western part of the valley. 

ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER 

6.86.6 7.0 7.47.2 7.6 7.8 8.0 8.48.2

No detection

10

20

30

40

pH STANDARD UNITS

MCL

0.5 times MCL

EXPLANATION

 = 0.321, p = 0.028

Manganese 50 micrograms per liter

Iron 100 micrograms per liter

Dissolved oxygen < 0.5 micrograms per liter

Manganese 50 micrograms per liter

Iron < 100 micrograms per liter

Dissolved oxygen < 0.5 micrograms per liter

Manganese < 50 micrograms per liter

Iron 100 micrograms per liter

Dissolved oxygen 0.5 micrograms per liter

Manganese < 50 micrograms per liter

Iron < 100 micrograms per liter

Dissolved oxygen < 0.5 micrograms per liter

Manganese < 50 micrograms per liter

Iron < 100 micrograms per liter

Dissolved oxygen 0.5 micrograms per liter

Manganese < 50 micrograms per liter

Iron < 100 micrograms per liter

Dissolved oxygen unknown (CDPH)

C

Figure 16. Continued. 
Status and Understanding of Water Quality 47

Constituent 

group

Constituent Benchmark type

High 

aquifer 

proportion 

(percent)

Data analyzed: Grid plus understanding wells Data analyzed: Grid wells

Depth to 

top of 

perforations 

Well 

depth

Dissolved 

oxygen pH

Percent 

orchard/ 

vineyard 

land use 1

Normalized 

lateral 

position 

from valley 

trough

Percent 

urban land 

use 1

Percent 

agricultural 

land use 1

Percent 

natural land 

use 1

ρ ρ ρ ρ ρ ρ ρ ρ ρ

Inorganic constituents

Arsenic 2 MCL-US 15.6 ns 0.365 ns 0.321 ns –0.262 0.374 ns –0.308

Vanadium 2 NL-CA 3.6 ns ns 0.402 ns ns ns 0.493 –0.414 –0.388Lead 2 MCL-US 2.4 ns ns ns ns –0.274 ns –0.322 ns 0.424Uranium 2 MCL-US 3.6 –0.562 –0.555 ns –0.341 ns –0.550 ns ns nsNitrate 2 MCL-US 2.1 –0.622 –0.563 0.439 –0.288 0.288 ns 0.334 ns nsManganese 2 SMCL-CA 4.5 ns ns –0.458 ns ns ns –0.426 0.345 ns

Iron 2 SMCL-CA 2.2 ns ns –0.351 ns ns ns –0.258 ns ns

TDS 3 SMCL-CA 1.7 –0.378 –0.329 –0.383 –0.316 ns –0.532 ns ns ns

Organic constituents and constituent classes

THMS, total 4 MCL-US 0.0 ns ns 0.228 ns ns ns 0.669 –0.298 –0.341Solvents, sum of 

concentrations 4

variable 0.2 ns ns ns ns ns ns 0.572 –0.296 ns

DBCP 3 MCL-US 1.0 ns ns ns ns 0.260 ns ns ns nsHerbicides, sum of 

concentrations 4

variable 0.0 –0.361 –0.233 0.230 –0.539 ns ns 0.283 ns –0.410

Constituents of special interest

Perchlorate 3 MCL-US 0.0 ns ns 0.245 ns 0.228 ns 0.415 ns ns

Table 9. Results of non-parametric (Spearman's method) analysis of correlations between selected water-quality constituents and potential explanatory factors, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study unit. [ρ, Spearman's correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated on the basis of p values (significance level of the Spearman's test) less than threshold value (α) of 0.05; ns, Spearman's test indicates no significant correlation between variables; blue text, significant positive correlation; red text, significant negative correlation; DBCP, 

1,2-Dibromo-3-chloropropane; THMs, trihalomethanes; TDS, total dissolved solids; SC, specific conductance (an electrical measure of TDS)]

1 Land use percentages are within a circle with a radius of 500 meters centered around each well included in analysis. 2 Constituents with > 2 percent high aquifer proportion.3 Single constituents with high values < 2 percent, moderate values, or detection frequencies at any concentration > 10 percent, which represent classes of constituents.4 Classes of compounds that include constituents with high values < 2 percent, moderate values, or detection frequencies at any concentration > 10 percent.
48 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Evidence for the second mechanism includes association 

of high arsenic with high pH in some wells (figs. 16B,C). 

Eight of the 19 wells having pH ≥ 8 had high or moderate 

relative-concentrations of arsenic (fig. 16C). Arsenic and pH 

were correlated significantly (positively) (table 9, fig. 16C). 

The variability of arsenic concentrations in groundwater with 

oxic alkaline (pH > 7) conditions suggests that the relation 

of arsenic concentrations to pH is complex and perhaps 

less dominant than the relation of arsenic concentrations to 

iron- and manganese-reducing conditions. Five wells with 

arsenic >10 μg/L, pH ≥7.5, and oxic conditions (fig. 16C) 

were located in the central Turlock and southern Merced 

study areas. One well had pH of 8.3 and iron >100 μg/L 

(figs. 16B,C). This well has a long perforation interval from 

230 to 500 ft, and may reflect mixing of groundwater affected 

by iron-reducing and alkaline–arsenic mobilization processes. 

The correlations of arsenic with urban land use (positive) 

and percent natural land use (negative) likely reflect other 

factors. Because pH is correlated to well depth and percent 

urban land use (table 5), high arsenic concentrations in 

urban wells probably reflect higher pH and relatively old 

groundwater. Similar results for vanadium are discussed in 

more detail in the following section.

Understanding Assessment for Vanadium

Vanadium concentrations were correlated significantly 

(positively) with dissolved oxygen and percent urban land 

use (table 9). Moderate-to-high vanadium concentrations 

often were associated with high pH. The correlation of 

vanadium and percent urban land use may reflect other factors 

independent of land use. These analyses suggest that high-tomoderate vanadium concentrations primarily were associated 

with oxic conditions with increasing pH. 

The positive correlation of vanadium with DO primarily 

reflected that vanadium concentrations were low in most 

samples having DO <0.5; many of the wells having low 

vanadium and DO had pre-modern ages. In addition, 

manganese and vanadium concentrations were significantly 

negatively correlated (ρ = –0.437, p = 0.003). These 

correlations are both consistent with smaller concentrations of 

vanadium in reducing groundwater.

 Similar to arsenic, vanadium adsorbs to iron and 

manganese oxyhydroxides on sediment surfaces, and 

desorbs from sediment or is inhibited from sorbing as pH 

increases (Hem, 1989; Naeem and others, 2007). Because 

arsenic and vanadium can be affected by similar processes, 

it was expected that concentrations of both constituents 

would increase with pH. For oxic (DO >0.5 mg/L) samples, 

vanadium and arsenic concentrations generally both increased 

as pH increased (figs. 16C and 17). 

The study data indicate that high pH contributes to 

mobilization of vanadium in oxic groundwater, including in 

some samples with pre-modern ages. Because the dominant 

vanadium forms are anionic complexes with oxygen and 

hydrogen, the solubility of vanadium is highest in oxidizing 

alkaline environments (Hem, 1989). Although pH and 

vanadium were not correlated overall in the Central Eastside 

study unit (table 9), they were correlated in samples having 

DO >0.5 mg/L (ρ = 0.386, p = 0.041, fig. 17). 

Most moderate and high vanadium concentrations were 

associated with pH ≥7.5 under oxic conditions (fig. 17). 

Among 21 wells with oxic conditions and pH ≥7.5, high or 

moderate relative-concentrations of vanadium occurred in 

9 wells (43 percent). Among 23 wells with reducing conditions 

(manganese–reducing, iron–reducing, or DO < 0.5 mg/L), 

or pH < 7.5, moderate relative-concentrations of vanadium 

occurred in 3 wells (13 percent). Although high and moderate 

relative-concentrations of vanadium primarily occurred in 

groundwater with pH ≥7.5 and oxic conditions, a majority 

of samples representing these conditions (57 percent) had 

low relative-concentrations of vanadium. Thus, oxic alkaline 

conditions do not always result in moderate or high vanadium. 

Wells with oxic alkaline conditions and low vanadium 

occurred across a range of study areas and land-use settings.

The three USGS grid wells with the highest vanadium 

concentrations (43 to 50 μg/L) in the Central Eastside were 

oxic, had pH of 8.0 to 8.2 (fig. 17), and had depths to the 

tops of the perforations >200 ft and pre-modern ages. These 

three wells (TRLK-05, TRLK-01, and TRLK-11, fig. A1, 

tables A1,A2) were located in the Turlock study area. 

Although vanadium concentrations were not significantly 

related to groundwater-age classification overall, the 

association of the highest vanadium concentrations with premodern groundwater suggests that groundwater residence time 

in the aquifer under oxic alkaline conditions could result in 

increased vanadium concentrations. 

The significant, positive correlation between urban land 

use and vanadium may reflect relations between urban land 

use, well depth, and pH. Relations between anthropogenic 

sources and vanadium in groundwater have been noted in 

some studies. Mejia and others (2007) attributed elevated 

vanadium in soil and groundwater to particulate emissions 

from burning fuel with high vanadium content. Elevated 

vanadium in air and soil also can be related to oil refinery 

processing, domestic heating, and automobile emissions (Soldi 

and others, 1996). Several factors need to be considered in 

analyzing the relation of vanadium and urban land use in 

the Central Eastside study unit. First, the highest vanadium 

concentrations are associated with three wells in the central 

Turlock study area having high pH, DO, and pre-modern 

age (figs. 15B, 17). These wells happen to be located in 

areas of relatively high urban land use. The pre-modern ages 

imply that vanadium concentrations would be unaffected 

by modern urban land use. Although wells with pre-modern 

ages can contain minor amounts of water affected by 

modern land use, simple mixing calculations suggest that 

vanadium concentrations in modern water would have to be 

unrealistically high to represent the source of vanadium to 

these wells. However, after removing all pre-modern wells 

from the data set, vanadium and percent urban land use still 

were significantly and positively correlated in grid wells 
Status and Understanding of Water Quality 49

(ρ = 0.491, p = 0.008). The remaining grid wells with 

moderate vanadium concentrations are located primarily in 

urban areas in the Modesto study area and the northern part 

of the Merced study area and have mixed ages, relatively 

large well depths of 395 to 526 ft, and pH of 7.5–7.8 with DO 

>4.8 mg/L (conditions favorable for high to moderate relativeconcentrations of vanadium). These wells could reflect a 

mixture of pre-modern water with high pH, DO, and vanadium 

with modern water having smaller pH and vanadium. 

Although age tracer and DO data are not available for CDPH 

other wells, comparison of the most recent vanadium and pH 

data for CDPH other wells during March 1, 2003–February 

28, 2006, (46 wells, vanadium data shown on fig. 15B) 

indicates that 4 of the 6 CDPH wells with vanadium >50 μg/L 

also have pH of 7.8–8.1. Alternatively, given the strength of 

the correlation between vanadium and percent urban land use 

after removing pre-modern wells, it is possible that there are 

urban sources or processes that might enhance mobilization 

of natural vanadium that may be contributing to moderate 

vanadium concentrations in the Central Eastside study unit. 

Moderate vanadium concentrations also occurred in 

four USGS understanding wells (MODFP-03, MERMW05, TRLKFP-01, TRLKMW-04; fig. A1) that were shallow, 

entirely perforated within 100 ft of land surface, and located 

in areas of agricultural or natural land use (table A1). Two of 

these wells had pH of 7.5–7.7 and oxic conditions, indicating 

oxic alkaline conditions contributing to vanadium mobility 

occurred in these shallow aquifer settings. The other two wells 

had pH <7.5 and one of the wells had reducing conditions; 

processes resulting in moderate vanadium concentrations in 

these wells are unknown but the data suggest that shallow 

sources of vanadium also may occur. 

Figure 17. Plot showing relation of vanadium to pH, Central Eastside, California, Groundwater Ambient Monitoring and Assessment 

(GAMA) study unit.

6.5 7 7.5 8 8.5

VANADIUM, IN MICROGRAMS PER LITER

pH, STANDARD UNITS

No detection

10

20

30

40

50

60

Manganese- or Iron-reducing

 conditions

Dissolved oxygen < 0.5 milligrams

 per liter, manganese- or iron-reducing

 conditions not present or insufficent

 data to evaluate

Dissolved oxygen 0.5

 milligrams per liter

CA-NL

0.5 times CA-NL

EXPLANATION

 = 0.208, p = 0.159
50 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Understanding Assessment for Lead

Lead concentrations in samples having a groundwater 

age classified as modern were significantly lower than 

samples classified as having a pre-modern age, but were 

not significantly different between samples having mixed 

and modern ages (table 4). Correlations of lead with 

urban (negative) and natural (positive) land use (table 9) 

primarily reflected relations among samples having relativeconcentrations of lead <0.1; the explanation for these apparent 

relations is unknown but is of relatively little consequence for 

groundwater quality because the concentrations were so small. 

Radioactive Constituents

For radioactive constituents as a class, 3.6 percent of 

the aquifer had high relative-concentrations of at least one 

radioactive constituent having human-health benchmarks, 

11.6 percent had moderate relative-concentrations of at 

least one trace element, and 84.8 percent had low relativeconcentrations of all radioactive constituents (table 8). As a 

class, radioactive constituents had aquifer-scale proportions 

that were dominated by the occurrence of uranium (table 7A). 

Status Assessment for Radioactive Constituents 

Uranium had high relative-concentrations in 3.6 percent 

of the aquifer (spatially-weighted value, table 7A). High 

relative-concentrations of uranium primarily were located in

the Modesto and Turlock study areas (fig. 15C). Uranium had 

moderate aquifer proportions in 15.2 percent of the aquifer 

(table 7A). Moderate relative-concentrations of uranium in 

grid wells and CDPH wells were present in the Modesto, 

Turlock, and Merced study areas. 

Gross alpha radioactivity had a moderate aquifer-scale 

proportion of 10.8 percent (table 7A). Four grid wells having 

moderate relative-concentrations were located in the Modesto 

study area (fig. 14).

Uranium and gross alpha radioactivity were positively 

correlated significantly (ρ = 0.630, p = 0.001). Uranium was 

the most abundant radioactive constituent present in the 

Central Eastside study unit (Landon and Belitz, 2008). These 

results suggest that gross alpha radioactivity measurements 

primarily reflect uranium concentrations in the Central 

Eastside. For this reason, gross alpha radioactivity is not 

discussed further in this report but is considered to reflect 

uranium concentrations, for which more data are available for 

the understanding assessment.

Understanding Assessment for Uranium

Uranium concentrations were significantly larger 

in modern and mixed age groundwater than pre-modern 

age groundwater (table 4, fig. 18A), and were correlated 

significantly (negatively) with depth to the tops of the 

perforations (table 9, fig. 18B) and with normalized lateral 

distance from the valley trough (table 9, fig. 18C). The 

correlation with normalized lateral position was determined on 

the basis of grid wells only, in order to remove relations with 

respect to depth. 

In addition to the significant decrease in uranium 

concentrations with increasing depth (of the top of the 

perforations, fig. 18B), uranium concentrations also were 

correlated significantly (positively) with calcium (ρ = 0.638, 

p = <0.001) and alkalinity (ρ = 0.773, p = <0.001). The 

results for the Central Eastside mirror results of a local-scale 

investigation in the Modesto area (Jurgens and others, 2008) 

and a regional investigation in the eastern San Joaquin Valley 

(Jurgens and others, 2009). Elevated uranium in shallow 

groundwater was attributed by Jurgens and others (2008, 

2009) to enhanced desorption of uranium from sediments 

by irrigation and urban recharge having high bicarbonate 

(alkalinity) concentrations. Increases in uranium-activity ratios 

(UAR, ratios of uranium-234 to uranium-238 isotopes) with 

depth also are consistent with mobilization of uranium from 

shallow sediments (Jurgens and others, 2008). In this study, 

UAR was negatively correlated strongly with depth to the 

tops of the perforations (ρ = 0.127, p = <0.001), similar to the 

results of the local Modesto study. 

The association of higher uranium with modern and 

mixed ages is consistent with the mobilization of uranium by 

irrigation and urban recharge in the shallow part of the aquifer, 

as described above. The pattern of decreasing uranium with 

increasing groundwater age and depth is opposite the pattern 

for arsenic, which increased with depth and groundwater 

age. These divergent patterns with depth reflect different 

mobilization processes. 

Uranium concentrations increase from east to west across 

the study unit (table 9, fig. 18C). This correlation may be 

influenced by a land-use gradient from east to west across the 

study unit from primarily natural land use in the eastern part 

of the study unit to primarily agricultural and urban land use, 

and intensive irrigation in the western portion of the study 

unit (fig. 4). Percent natural land use is correlated significantly 

(positively) with normalized lateral position (table 5), but was 

not correlated directly with uranium concentrations (table 9). 

Additional linear regression analysis, not described in this 

report, indicated that uranium concentrations still were related 

to normalized lateral position after accounting for possible 

differences in depth to the top of perforations from west to 

east. The relation between uranium and normalized lateral 

position also may reflect other factors such as predevelopment 

groundwater conditions, geology, or historical land-use 

patterns; further investigation is beyond the scope of this 

report. 
Status and Understanding of Water Quality 51

MCL

100

0

100

200

300

400

0.01

500

DEPTH TO TOP OF PERFORATIONS, IN FEET

0.1 1 10

URANIUM CONCENTRATION, IN MICROGRAMS PER LITER 

0.5 times MCL

 = -0.562, p < 0.001

Other wells

Modern

Public-supply wells

Mixed

Pre-modern

Modern

Mixed

Pre-modern

No age data

EXPLANATION

Pre-modern significantly different from 

modern and mixed at = 0.05

13

URANIUM, IN MICROGRAMS PER LITER

15 11

Modern Mixed Pre-modern

GROUNDWATER AGE MIXTURE CLASSIFICATION

16 Number of values

EXPLANATION

75th percentile

Median

25th percentile

25th percentile minus 1.5 times

interquartile range

75th percentile plus 1.5 times

interquartile range

Upper detached

Lower detached

100

10

1

0.1

A

B

Figure 18. Relation of uranium concentrations to (A). groundwater-age classification, (B). depth to top of perforations, and (C). 

normalized lateral position, Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.
52 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Nutrients

For nutrients as a class, 2.1 percent of the aquifer 

had high relative-concentrations of at least one nutrient 

having human-health benchmarks, 14.6 percent had 

moderate relative-concentrations of at least one nutrient, 

and 83.3 percent had low relative-concentrations of all 

nutrients (table 8). As a class, nutrients had aquifer-scale 

proportions that were dominated by the occurrence of nitrate 

(table 7A). Nitrite was detected in one well at a high relativeconcentration in the CDPH database during the current period 

but not in the most recent sample from that well (table 7B).

Status Assessment for Nutrients

Nitrate had high relative-concentrations in 2.1 percent of 

the aquifer, moderate relative-concentrations in 14.6 percent 

of the aquifer, and low relative-concentrations in 83.3 percent 

(table 7A). High relative-concentrations of nitrate primarily 

occur in the west-central Turlock, central Modesto, and 

northern Merced study areas (fig. 15D). Moderate relativeconcentrations of nitrate are distributed widely across the 

study unit (figs. 14 and 15D). 

Understanding Assessment for Nitrate

Nitrate concentrations were significantly greater in wells 

having modern, compared to mixed and pre-modern, ages 

(table 4, fig. 19A). Nitrate also was correlated significantly 

(negatively) with depth to the tops of the perforations 

(fig. 19B) and correlated significantly (positively) with 

dissolved oxygen (fig. 19C) and percent orchard and vineyard 

land use (fig. 19D; table 9). Some of the explanatory factors 

related to nitrate are themselves related—dissolved oxygen, 

groundwater age, and depth to the top of perforations (tables 4

and 5). Nitrate in groundwater has been studied extensively in 

the eastern San Joaquin Valley (for example, Dubrovsky and 

others, 1998; Burow and others, 2008b). 

0.01

0.1

1

10

100

NORMALIZED LATERAL POSITION

0 0.2 0.4 0.6 0.8 1

URANIUM CONCENTRATION, IN MICROGRAMS PER LITER 

> 200 feet modern

> 200 feet mixed

> 200 feet pre-modern

< 200 feet modern

< 200 feet mixed

No depth or no age data

EXPLANATION

MCL

0.5 times MCL

Depth to top of perforations

below land surface and

age classification

 = -0.550, p = 0.002

C

Figure 18. Continued. 
Status and Understanding of Water Quality 53

MCL

60

0

100

200

300

400

500

DEPTH TO TOP OF PERFORATIONS, IN FEET

10 20 30 40 50

NITRATE-NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER 

0.5 times MCL

 = -0.622, p < 0.001

Other wells

Modern

Public-supply wells

Mixed

Pre-modern

Modern

Mixed

Pre-modern

No age data

EXPLANATION

All differences significant at

13

NITRATE-NITROGEN,

IN MILLIGRAMS PER LITER

15 10

Modern Mixed Pre-modern

GROUNDWATER AGE MIXTURE CLASSIFICATION

16 Number of values

EXPLANATION

75th percentile

Median

25th percentile

25th percentile minus 1.5 times

interquartile range

75th percentile plus 1.5 times

interquartile range

Upper detached

Lower detached

60

40

20

10

30

50

No detection

No

detection

 = 0.05

A

B

Figure 19. Plots relating nitrate (as nitrogen) to explanatory factors. (A) Boxplots showing relation to classified groundwater age, and graphs showing 

(B) relation to depth to top of perforations, (C relation to dissolved oxygen, (D) graph showing relation of orchard and vineyard land use and depth to the top of 

perforations to detection frequency of nitrate (as nitrogen) >5 mg/L, (E) graph showing nitrogen and oxygen isotopic values of nitrate, classified by dissolved 

oxygen concentration, with nitrate (as nitrogen) concentration and dissolved excess nitrogen gas concentration shown for those samples suspected of being 

affected by denitrification, Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit. 
54 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project DETECTION FREQUENCY FOR NITRATE-NITROGEN GREATER THAN 5 MILLIGRAMS PER LITER, IN PERCENT

n = 12 n = 5 n = 25 n = 11

100

80

60

40

20

0

< 200

Depth to the top of perforations, in feet below land surface

Orchard and vineyard land use, in percent

 > 200 < 200 > 200

 0% 0%

> 40 > 40 < 40 < 40

6 7 8 9

60

50

40

30

20

0

10

No detection NITRATE NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER

1 2 3 4 5

DISSOLVED OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER 

> 200 feet-Mixed

< 200 feet-Pre-modern

> 200 feet-Pre-modern

< 200 feet-Modern

> 200 feet-Modern

No depth or no age data

< 200 feet-Mixed

MCL

0.5 times MCL

EXPLANATION

Depth to top of perforations

below land surface and

age classification

 = 0.439, p = 0.002

C

D

Figure 19. Continued. 
Status and Understanding of Water Quality 55

For grid and understanding wells having construction 

data, nitrate concentrations in excess of the US–MCL of 

10 mg/L (as nitrogen) occurred in some wells having depths 

to the top of the perforations <200 ft; these wells were not 

used for public supply. All public-supply wells having well 

construction data had nitrate (as nitrogen) <10 mg/L. Wells 

having depths to the top of the perforations >200 ft had nitrate 

concentrations that were low (<5 mg/L as N) (fig. 19B). 

Among wells with depths to the top of the perforations 

<200 ft, lower nitrate concentrations in public-supply wells 

than other wells may reflect that public-supply wells have 

longer perforated intervals. Consequently, high nitrate water 

from shallow depths may be diluted with low nitrate water 

from deep depths in long-screened public-supply wells. 

Decreases in nitrate concentrations with depth primarily 

may reflect increasing groundwater age. Nitrate concentrations 

were significantly different between wells having modern, 

mixed, and pre-modern age distributions (fig. 19A; table 4). 

Under the generally widespread oxic conditions in 

most of the aquifer in the Central Eastside study unit, the 

distribution of nitrate in the aquifer is not expected to be 

controlled by denitrification. Dispersion and mixing of nitrate 

in modern recharge waters with older waters having low 

nitrate is expected to influence nitrate distribution, as has 

been shown to occur in simulation studies in the eastern San 

Joaquin Valley (Burow and others, 1999; 2008b; Weissman 

and others, 2002).

Nitrate may be affected by denitrification in reducing 

groundwaters in the western and deeper parts of the flow 

system. Nitrate-N concentrations also were correlated 

significantly (positively) with dissolved oxygen (fig. 19C). 

Denitrification of nitrate to dissolved nitrogen gas (N2) 

and intermediate products in reducing groundwater have 

been identified in numerous studies (review by Kendall, 

1998). Local studies within the Central Eastside study unit 

have confirmed that denitrification occurs in parts of the 

aquifer (Singleton and others, 2007; Green and others, 2008; 

McMahon and others, 2008). Isotopic and geochemical 

evidence for denitrification in the Central Eastside study unit 

are described in the following paragraphs.

Figure 19. Continued. 

Nitrate (NO3) in

 precipitation

Nitrate (NO3) in fertilizer

Ammonia

(NH4) in 

fertilizer

and

precipitation

Manure and

septic waste

Soil Nitrogen

E

E

Dissolved oxygen > 2.0 milligrams

 per liter

Dissolved oxygen 0.5 milligrams

 per liter

Dissolved oxygen 0.6 - 2.0

 milligrams per liter

Denitrification

 Slope = 0.5

Measured nitrate (as nitrogen)-In

 milligrams per liter

Excess nitrogen gas-In milligrams

 per liter of equivalent nitrate as

 nitrogen dentrified

Nitrogen source ranges from

Kendall (1998)

EXPLANATION

0.06

8.5

4.4

4.7

5.0

2.2

21

1.8

50

6.0

0.06

8.5

30

20

10

0

-10

-10 0 10 20 30 40

DELTA NITROGEN-15 OF NITRATE, IN PER MIL

DELTA OXYGEN-18 OF NITRATE, IN PER MIL
56 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Values of nitrogen isotopes of nitrate ( δ15N-nitrate) and 

oxygen isotopes of nitrate (δ18O-nitrate) for samples having 

reducing groundwaters (DO <0.5 mg/L), plot along lines that 

are consistent with denitrification of nitrate (fig. 19E). As 

denitrification occurs, the δ15N and δ18O values of the residual 

nitrate tend to increase along a line having a slope between 

about 0.5 and 1.0 on a plot of δ15N versus δ18O (Böttcher and 

others, 1990; Mengis and others, 1999). 

Excess N2 (table A3, fig. 19E) in reducing water was 

consistent with the occurrence of denitrification. Excess N2

was estimated by comparing measured concentrations of argon 

(Ar) and N2 with those expected in water in equilibrium with 

the atmosphere (Singleton and others, 2005; Singleton and 

others, 2007). Uncertainties include the recharge temperature 

and the amount of excess air (Vogel and others, 1981; Böhlke, 

2002). Therefore, excess N2 from denitrification cannot 

be uniquely determined from measuring Ar and N2 alone. 

Excess N2 was estimated by adjusting the values until the 

calculated recharge temperatures determined from Ar and 

N2 were close to those estimated on the basis of noble-gas 

concentrations (Michael J. Singleton, Lawrence Livermore 

National Laboratory, written commun., March 12, 2007). 

These analyses indicated that eight samples with reducing or 

mixed redox groundwater had 1.1 to 11.3 mg/L of excess N2

(table A3). Three of these samples (two samples not shown 

on fig. 19E because the nitrate-N concentrations were too low 

for isotopic values to be measured) had nitrate-N at or below 

detection level, suggesting that complete denitrification had 

occurred. Four of these samples (MODFP-02, TRLKFP-01, 

TRLKFP-02, and TRLKMW-01) had nitrate-N of 4.4 to 

50 mg/L (table A3); these samples all were located in the 

western quarter of the study unit (fig. A1, table A1) in an 

unconfined portion of the aquifer above the Corcoran clay. 

Data from these four samples are consistent with partial 

denitrification of nitrate or mixing of waters with differing 

amounts of denitrification in these wells screened in the 

unconfined aquifer overlying the Corcoran clay.

Nitrate concentrations were not related strongly to land 

use in this study. Previous investigations in the eastern San 

Joaquin Valley have found positive correlations between 

nitrate-N concentrations and percent agricultural land use 

(Burow and others, 1998a,b, 2007). These relations reflect 

that irrigated agricultural land use has been the dominant land 

use in the eastern San Joaquin Valley for several decades to 

as much as a century, and increasing quantities of nitrogen 

fertilizers have been applied on agricultural lands since the 

1950s (Burow and others, 2007, 2008b). However, nitrate 

and agricultural land use were not correlated significantly 

in this study on the basis of all grid wells (table 9), or grid 

wells having depths to the top of perforations of <200 ft. The 

absence of a relation between agricultural land use and nitrate 

in this study probably reflects that the wells included in the 

analysis were perforated at depths where modern recharge 

affected by agricultural land use often is mixed with deep 

pre-modern water unaffected by modern land use. Nitrate 

was correlated positively with percent urban land use in grid 

wells (table 9). This relation probably reflects other factors 

such as differences in well construction and groundwater 

ages between urban and nonurban areas. A plot of nitrate 

against urban land use showed no clear visual patterns; the 

correlation appears to reflect that several wells having nitrate 

of <1 mg/L (as nitrogen), depth to the top of perforations 

>200 ft, and pre-modern ages, were located in areas with little 

or no urban land use. The apparent relation of nitrate to urban 

land use may reflect a relation between the occurrence of 

mixed ages and urban land use; wells having water of mixed 

ages had a significantly larger percent urban land use and 

nitrate than wells having water of pre-modern age (table 4). In 

previous investigations of shallow young groundwater, nitrate 

concentrations were lower beneath parts of the Modesto urban 

area than beneath adjacent agricultural areas (Burow and 

others, 2008a; Jurgens and others, 2008).

Nitrate was correlated significantly (positively) to 

percent orchard and vineyard land use on the basis of gridplus-understanding wells (table 9). The correlation was 

strongest (ρ = 0.481, p = 0.004) among wells having depths 

to the tops of the perforations of <200 ft; there was not a 

significant correlation for wells having depths to the tops of 

the perforations of >200 ft. Thus, higher nitrate occurs in 

shallow groundwater beneath orchard and vineyard-land use 

areas (fig. 19D). 

Sources of nitrate in groundwater have been investigated 

in the literature by comparing nitrogen and oxygen isotopic 

values of groundwater samples to ranges of values expected 

from different sources (Kendall, 1998). Most groundwater 

samples from the Central Eastside study unit have isotopic 

ratios that plot within the overlap between ammonia fertilizer, 

soil nitrogen, and animal waste sources (fig. 19E). Therefore, 

information on nitrate sources to groundwater cannot be 

determined uniquely from the nitrogen and oxygen isotope 

values. Samples plotting in the manure and septic waste range 

primarily are those samples that show evidence of being 

affected by denitrification, which shifts isotopic values upward 

and to the right. A few samples with δ15N-nitrate values of 

about 10 to 11 per mil could be consistent with animal waste 

sources. Although animal waste sources sometimes can be 

distinguished from fertilizer, soil nitrogen, and precipitation 

sources using δ15N or δ18O of nitrate, different animal waste 

sources, such as human versus livestock, generally cannot be 

distinguished (Kendall, 1998).
Status and Understanding of Water Quality 57

Inorganic Constituents with Secondary 

Maximum Contaminant Levels

CDPH has established nonenforceable benchmarks 

(SMCL-CAs) that are based on aesthetic properties rather 

than on human-health concerns for selected constituents. For 

TDS and the major ions chloride and sulfate, CDPH defines 

a “recommended” and an “upper” SMCL-CA. The “upper” 

SMCL-CA benchmarks were used for computing relativeconcentrations in this report; a relative concentration of 0.5 

corresponds to concentrations equal to the recommended 

SMCL-CA for each of these constituents. The minor elements 

manganese and iron also have SMCL-CA benchmarks. 

TDS for USGS–GAMA wells used in the analysis 

was calculated from specific conductance (SC) using a 

linear regression equation so that all grid wells would have 

TDS values determined using the same method. The linear 

regression equation (TDS = 0.6164*SC +38.9) was developed 

from data for 39 USGS grid and understanding wells in the 

Central Eastside having both measured SC and TDS data. 

SC, an electrical measure of TDS, was available in all 

78 USGS grid and USGS understanding wells, whereas 

laboratory-measured TDS data (as residue on evaporation) 

only were available for 39 of these wells. The predicted TDS 

using the regression equation closely matched measured TDS 

(r2 = 0.999). Because there were similar numbers of CDPH 

wells available with both TDS and SC measurements, TDS 

values from CDPH were used directly and combined with 

USGS–GAMA-calculated TDS values. 

Status Assessment for Inorganic Constituents with 

Secondary Maximum Contaminant Levels 

Manganese had high relative-concentrations in 

4.5 percent of the aquifer (table 7A). Moderate relativeconcentrations of manganese occurred in 9.1 percent of the 

aquifer (table 7A). Wells having moderate-to-high relativeconcentrations of manganese primarily were located in the 

western and southern parts of the study unit (fig. 15E) or 

deeper parts of the aquifer, as indicated by the distribution of 

reduced groundwater on figure 11. Iron had a aquifer-scale 

high proportion of 2.2 percent (spatially-weighted), with high 

relative-concentrations in eight wells in the CDPH database 

(table 7A). For the constituent class manganese and iron, the 

aquifer-scale proportions (table 8) were determined on the 

basis of grid-based aquifer proportions for manganese. 

TDS had high relative-concentrations in 1.7 percent of 

the aquifer and moderate relative-concentrations in 8.6 percent 

(table 7A). Moderate relative-concentrations of TDS were 

present in the Merced, Modesto, and Turlock study areas, but 

not the Uplands study area (figs. 14 and 15F). 

Chloride had high relative-concentrations in at least 

one well in the CDPH database before March 1, 2003, but 

not during the current period analyzed (table 7B). These 

high relative-concentrations represented historic, rather than 

current, conditions.

Understanding Assessment for Manganese

Manganese was correlated significantly (negatively) 

with DO (table 9). DO <0.5 mg/L and manganese >50 mg/L 

generally are consistent with reducing conditions (table A3). 

Manganese also was correlated significantly (negatively) 

with percent urban land use (table 9). However, the correlation 

probably reflects that high manganese primarily occurs in the 

western and southern parts of the study unit (fig. 15E), which 

has little urban land use. Manganese concentrations in oxic 

waters in both urban and nonurban land settings were low 

(table A3).

Understanding Assessment for Iron

The distribution of iron concentrations were described 

earlier in the sections describing redox characteristics and 

arsenic. High concentrations of iron occur in the western 

portion of the study unit and at large depths in the aquifer 

(fig. 11), reflecting the natural distribution of iron-reducing 

conditions that result from reductive dissolution of iron oxides 

present in the aquifer sediments. 

Understanding Assessment for TDS

TDS was correlated significantly (negatively) with 

depth to the top of the perforations (fig. 20A) and well depth, 

although the former correlation was stronger (table 9). TDS 

also was negatively correlated with normalized lateral distance 

from the valley trough (grid wells only) (table 9, fig. 20B) and 

DO (table 9). The apparent correlation between TDS and DO 

likely is a result of the correlation of both factors with lateral 

position. 

TDS generally was higher in wells with depths to 

the tops of the perforations <200 ft (fig. 20A). Wells with 

depths to the tops of the perforations >200 ft generally had 

fairly uniform TDS of about 200 mg/L, with the exception 

of monitoring well MODFP-01, with a TDS of 5,810 mg/L 

(relative-concentration of 5.81), discussed below. The higher 

TDS in the upper 200 ft of the aquifer is similar to the patterns 

for nitrate (fig. 19B) and uranium (fig. 18B). The higher 

concentrations near the water table imply greater loading of 

dissolved constituents to groundwater in recent decades, which 

could reflect several factors, including increases in recharge, 

changes in soil chemistry due to historical changes in land 

use, greater chemical use at the land surface, and changes in 

consumptive water use by plants. 
58 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

100

200

300

400

500

100 1,000 10,000

0 0.2 0.4 0.6 0.8 1.0

DEPTH TO TOP OF PERFORATIONS

BELOW LAND SURFACE, IN FEET 

TOTAL DISSOLVED SOLIDS, IN MILLIGRAMS PER LITER

100

200

2,000

500

1,000

5,000

10,000

TOTAL DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

NORMALIZED LATERAL POSITION

0

Other wells

Modern

Public-supply wells

Mixed

Pre-modern

Modern

Mixed

Pre-modern

No age data

EXPLANATION

> 200 feet modern

> 200 feet mixed

> 200 feet pre-modern

< 200 feet modern

< 200 feet mixed

No depth or no age data

< 200 feet pre-modern

SMCL

0.5 times SMCL

SMCL

0.5 times SMCL

 = -0.378, p = 0.001

 = -0.532, p < 0.001

EXPLANATION

Depth to top of perforations

below land surface and

age classification

A

B

Figure 20. Relations of total dissolved solids to (A). depth to the top of the perforations, and, (B). normalized lateral position, Central 

Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.
Status and Understanding of Water Quality 59

TDS significantly increased from east to west across the 

study unit (fig. 20B). Moderate to high relative-concentrations 

primarily occurred in the downgradient part of the flow system 

having normalized lateral position <0.3. This pattern was 

evident for all well depths, as well as modern, mixed, and 

pre-modern ages. Increases in TDS from east to west across 

the eastern San Joaquin Valley have been noted previously 

(Bertoldi and others, 1991). The increase could reflect a 

variety of factors, both natural and anthropogenic, including: 

historical groundwater discharge and evapotranspiration 

patterns, irrigation return and irrigation recycling, addition of 

salts from anthropogenic activities at or near the land surface, 

rock/water interaction along regional groundwater-flow paths 

and upwelling of more saline groundwater influenced by 

interactions with deep marine or lacustrine sediments near the 

valley trough. Detailed analysis of the processes accounting 

for these patterns is beyond the scope of this report.

The high TDS in MODFP-01 most likely results from 

mixing with deeper saline waters underlying the freshwater 

aquifer. MODFP-01 is a monitoring well that is not used for 

public supply. This well is perforated from 269 to 274 ft below 

land surface, just below the Corcoran clay, and is located in 

the western part of the Modesto study area (fig. A1). This well 

has distinctive chemistry with the highest concentrations of 

several inorganic constituents measured in the Central Eastside 

study, including chloride (3,130 mg/L), sodium (1,010 mg/L), 

iron (1,870 μg/L), and manganese (3,940 μg/L), as well as the 

lowest values of sulfate (<0.18 mg/L) and δ13C (–44.5 per mil) 

(Landon and Belitz, 2008). The well is highly reducing, with 

methanogenic conditions (table A3). The chloride-to-iodide 

ratio for MODFP-01 is distinctive from other wells sampled 

in the Central Eastside and is in the range of groundwater 

affected by interactions with marine rocks (Izbicki and others, 

2006). Although the depth to the base of freshwater, classified 

as SC <3,000 μS/cm (Page, 1973), typically is deeper than 

300 ft (Burow and others, 2004), saline water could be present 

at shallower depths beneath the Corcoran clay, as a result of 

regional upward hydraulic gradients causing upward flow of 

deep groundwater.

Organic Constituents

The organic compounds are organized by constituent 

class, including three classes of VOCs (trihalomethanes, 

solvents, and other VOCs), fumigants, and two classes of 

pesticides (herbicides and insecticides). VOCs can be present 

in paints, solvents, fuels, fuel additives, refrigerants, and can 

be byproducts of drinking water and other water disinfection, 

and are characterized by their tendency to evaporate 

(Ivahnenko and Barbash, 2004; Zogorski and others, 2006). 

Fumigants used to control pests also are a type of VOC, but 

are discussed separately in this report because their uses and 

distribution differ from other VOCs. Pesticides are used to 

control weeds, insects, fungi, and other pests in agricultural, 

urban, and suburban settings. 

Maximum relative-concentration and detection 

frequency were used as selection criteria for organic and 

special-interest constituents and are shown in figure 21. Ten 

organic constituents analyzed for all USGS grid wells met the 

selection criteria of having maximum relative-concentrations 

>0.1 and (or) detection frequency ≥10 percent (figs. 21 and 

22). 

All concentrations of VOCs in USGS–GAMA-collected 

samples from the Central Eastside study unit were below 

human-health benchmarks. Of the 22 VOCs detected, two 

(carbon tetrachloride and chloroform) had moderate relativeconcentrations in one or more grid wells (fig. 22). The other 

20 VOCs were detected at low relative-concentrations, some 

with constituents having maximum relative-concentrations 

as low as 2.5 x 10–5 (fig. 21). Five VOCs were detected 

in more than 10 percent of the grid wells, including: 

the trihalomethanes (THMs) chloroform, bromoform, 

bromodichloromethane, and dibromochloromethane; and the 

solvent tetrachloroethene (PCE, fig. 22). The other 17 VOCs 

had detection frequencies of <10 percent. One or more VOCs 

were detected in 47 percent of the grid wells. 

Of the 115 pesticides and pesticide degradates analyzed, 

10 were detected (Landon and Belitz, 2008; table 6). 

The detected compounds were all herbicides or herbicide 

degradates; no insecticides were detected. Of the 10 herbicides 

and herbicide degradates detected, six were parent compounds 

with benchmarks (atrazine, simazine, diuron, metolachlor, 

prometon, hexazinone), one was a parent compound without 

a benchmark (norflurazon), and three were degradates not 

having a benchmark (de-ethylatrazine, de-isopropylatrazine, 

and 3,4-dichloroaniline). Results for the herbicide diuron, 

sampled for 15 of 58 grid wells, are discussed in the 

section “Constituents Sampled for in a Subset of Wells.” 

All concentrations of pesticides were below human-health 

benchmarks. The individual constituents that were not 

detected, and the wells sampled for different pesticides and 

pesticide degradates, in the Central Eastside study unit are 

listed by Landon and Belitz (2008).

For organic constituents having human-health 

benchmarks, 1.2 percent of the aquifer had high relativeconcentrations of at least one constituent, 14.3 percent had 

moderate relative-concentrations of at least one constituent, 

and 84.5 percent had low relative-concentrations of all 

constituents (table 8). The aquifer-scale high proportion of 

organic constituents reflected high relative-concentrations 

of the fumigant DBCP in 1.0 percent of the aquifer, and the 

solvent PCE in 0.2 percent of the aquifer. 
60 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

1,2-Dichloropropane

DBCP

Carbon tetrachloride

Dichloromethane

Perchlorate

Chloroform

Simazine

Metolachlor Carbon disulfide

Trichlorofluoromethane

m- and p-xylene

Hexazinone

n-Propylbenzene

Dichlorodifluoromethane

Dibromochloromethane

1,2,3-TCP (HAL) Toluene

1,1-Dichloroethene

cis-1,2-Dichloroethene

1,2,4-Trimethylbenzene

1,2,2-Trichlororifluoroethane

Bromochloromethane

Ethylbenzene

Prometon

PCE Benzene

TCE

Bromodichloromethane

Bromoform

Chloromethane

Atrazine

1 10 100

0.01

0.001

0.0001

0.1

1

10

1e-005

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

DETECTION FREQUENCY AT ANY CONCENTRATION, PERCENT

EXPLANATION

TCE, Trichloroethene; PCE, Tetrachloroethene; DBCP, 1,2-Dibromo-3-chloropropane; 1,2,3-TCP, 1,2,3-Trichloropropane;

HAL, U.S. Environmental Protection Agency Lifetime Health Advisory Level

Detected at relative concentration >1 in one or more wells in the Central Eastside in the California Department of

Public Health Data Base for March 1, 2003 to February 28, 2006

Diuron

Atrazine

Diuron

Moderate

Low

Moderate

High

*

*

*

Constituents with >20 grid wells and spatially representative wells sampled-Name and center of

symbol is location of data unless indicated by following location line: 

Constituents with <20 grid wells and wells sampled not spatially representative-Name and center of

symbol is location of data unless indicated by following location line:

Figure 21. Detection frequency and maximum relative-concentration for organic and special interest constituents detected in grid 

wells, Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.
Status and Understanding of Water Quality 61

Figure 22. Detection frequency and relative-concentrations in grid wells of selected organic and special interest constituents in the 

Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit, March–June 2006.

0 10 20 30 40 50 60 70 80 90 100 0.001 0.01 0.1 1 10

STUDY-UNIT DETECTION FREQUENCY,

IN PERCENT RELATIVE-CONCENTRATION, DIMENSIONLESS**

*

*

* Detected at relative-concentration > 1 in one or more wells

 in the Central Eastside in the California Department of Public Health

 database for March 1, 2003 - February 28, 2006. 

**Wells with relative-concentrations <0.001 are not plotted. However,

 these wells are included in the study-unit detection frequencies

 shown on the left. 10-percent detection frequency

threshold used as selection

criteria for constituents of interest

PCE

Carbon

tetrachloride

Metolachlor

Simazine

Atrazine

No detections > 0.001

DBCP

Perchlorate

Uplands (9 grid wells)

EXPLANATION

Merced (23 grid wells)

Modesto (10 grid wells)

Turlock (16 grid wells)

Solvents

Fumigant

Herbicides

Constituent of special interest

Trihalomethanes

Total THMs

Chloroform

Bromoform

Dibromochloromethane

Bromodichloromethane

Low Moderate High
62 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Trihalomethanes

In addition to individual THMs, the detection frequency 

and relative-concentrations of total THMs (sum of the four 

individual THMs) are shown on figure 22 because (1) there 

is an MCL for total THMs, and, (2) all four THM compounds 

had detection frequencies of 10 percent or higher in grid wells 

in the study unit. For the purpose of presentation, relativeconcentrations of the individual THMs were computed 

using the MCL-US of 80 μg/L, corresponding to total THM 

concentrations. 

Status Assessment for Trihalomethanes

One or more THMs were detected in 31 percent of 

the grid wells in the study unit (fig. 22); no high relativeconcentrations were detected. THMs (on the basis of 

chloroform) had moderate relative-concentrations in 3.4 

percent of the aquifer (tables 7B and 8). THMs had low 

relative-concentrations or were not detected in 96.6 percent of 

the aquifer. Comparison of the relative-concentrations for total 

THMs and chloroform illustrates that chloroform accounted 

for most of the total THMs in most samples (17 of 21; Landon 

and Belitz, 2008). Nationally, chloroform was the most 

detected frequently VOC in aquifers as indicated by the USGS 

National Water Quality Assessment (NAWQA) program 

(Zogorski and others, 2006). THMs were more prevalent in the 

Modesto study area (detection frequency of 80 percent) than 

other study areas (detection frequency <25 percent, Landon 

and Belitz, 2008). 

Understanding Assessment for Trihalomethanes

The sum of THM concentrations was correlated 

significantly (positively) with percent urban land use (table 9, 

figs. 23A and 24). The detection frequency of THMs in grid 

wells having >40 percent urban land use was about 82 percent, 

compared with about 10 percent for wells having <40 percent 

urban land use. Nationally, THM concentrations also have 

been correlated strongly with percent urban land use (Zogorski 

and others, 2006). Potential urban sources of THMs include 

recharge from landscape irrigation that uses disinfected 

water, leakage from distribution or sewer systems, as well as 

industrial and commercial sources (Ivahnenko and Barbash, 

2004). THMs also were significantly negatively correlated 

with percent agricultural land use and percent natural land use 

(table 9), further indicating the strong relation with urban land 

use.

Although THM concentrations were not directly 

correlated with depth (table 9), most of the wells with 

detections of THMs had depths to the tops of the perforations 

of <200 ft (fig. 24). Detection frequencies for four categories 

of wells classified by depths to the top of the perforations 

and urban land use were determined: shallow-urban (<200 ft, 

>40 percent urban, 11 wells), deep-urban (>200 ft, >40 percent 

urban, 6 wells), shallow-less-urban (<200 ft, <40 percent 

urban, 24 wells), and deep-less-urban (<200 ft, <40 percent 

urban, 11 wells) (fig. 24). Detection frequencies were higher in 

shallow-urban (91 percent), than in deep-urban (67 percent), 

and shallow-less-urban (13 percent), or deep-less-urban wells 

(9 percent). 

THM concentrations were correlated significantly 

(positively) with DO (table 9). THMs were not detected in 

wells having DO <3 mg/L. The relation between THMs and 

DO could reflect a combination of degradation processes 

of THMs in reducing groundwater, or sources of THMs 

co-located with localized areas of high recharge from urban 

return flows that also might promote relatively high DO. 

THMs usually are considered to be resistant to degradation, 

except in parts of the aquifer that have sulfate-reduction or 

more highly reducing conditions (Barbash, 2007); however, 

THM degradation under more oxic conditions has been 

noted and has been attributed to degradation in reduced 

microenvironments within aquifers that receive artificial 

recharge (Pavelic and others, 2006). Positive correlations of 

THMs and DO also have been noted in nationwide analysis 

(Squillace and others, 2004; Zogorski and others, 2006). 

THM concentrations were not significantly different 

between wells having different groundwater-age classifications 

(table 4), even for wells having DO >0.5 mg/L and urban land 

use >40 percent. However, most of the THM detections (11 of 

16) occurred in wells both with mixed ages and depths to the 

tops of the perforations <200 ft. Two wells had detections of 

THMs but had pre-modern ages and depths to the tops of the 

perforations >200 ft. Because the practice of disinfection of 

water supplies using chlorination predates atmospheric nuclear 

testing, THMs could occur in relatively deep wells without 

the apparent presence of modern recharge water (from the last 

50 years). However, the age classifications do not exclude the 

possibility that a very small amount of modern water could be 

mixed into a sample. The presence of THMs with the absence 

of modern-age water could reflect short-circuit mechanisms 

owing to well construction or well operation practices (Jurgens 

and others, 2008; Landon and others, 2009); investigating 

these mechanisms is beyond the scope of this report. 
Status and Understanding of Water Quality 63

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

A

B

Urban

Land use

Agricultural

Natural

Urban

Land use

Agricultural

Natural

EXPLANATION

Study area boundary

Study area boundary

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Not detected (< 0.01 micrograms

 per liter)

Low (0.01 - 8 micrograms per liter)

Moderate (> 8 - 80 micrograms

 per liter)

CDPH trihalomethanes

USGS GAMA trihalomethanes

Low or not detected (< 0.5 - 8

 micrograms per liter)

Moderate (8 - 80 micrograms

 per liter)

Not detected

Low (< 0.1)

Moderate (> 0.1 - 1.0)

CDPH solvents

USGS GAMA solvents

Low or not detected (< 0.1)

Moderate (0.1 - 1.0)

High (> 1.0 )

20 MILES

20 KILOMETERS

0 10

0

20 MILES

20 KILOMETERS

0 10

0

Figure 23. Detections and concentration ranges of selected organic and special interest constituents in U.S. Geological Survey (USGS) grid and 

USGS understanding wells representative of the primary aquifer and the most recent analysis during March 1, 2003–February 28, 2006 for California 

Department of Public Health (CDPH) wells, (A). sum of trihalomethanes, (B). wells having one or more solvents at low, moderate, or high relativeconcentrations, (C). DBCP, and (D). sum of herbicides, and (E). perchlorate, Central Eastside, California, Groundwater Ambient Monitoring and 

Assessment (GAMA) study unit 
64 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Figure 23. Continued. 

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

S I E R R A N E V A D A

C

D

Urban

Land use

Agricultural

Natural

Urban

Land use

Agricultural

Natural

EXPLANATION

USGS GAMA DBCP

Not detected (< 0.03 micrograms

 per liter)

Moderate (0.03 - 0.20 micrograms

 per liter)

CDPH DBCP

Study area boundary

EXPLANATION

USGS GAMA herbicides

CDPH herbicides

Study area boundary

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Low or not detected (< 0.01 micro-

 grams per liter)

Low (0.01 - 0.02 micrograms per

 liter)

Moderate (0.03 - 0.20 micro-

 grams per liter)

High (> 0.20 micrograms per liter)

Not detected

Low (< 0.01 - 0.10 micrograms

 per liter)

Not high (< 0.1 micrograms

 per liter)

20 MILES

20 KILOMETERS

0 10

0

20 MILES

20 KILOMETERS

0 10

0
Status and Understanding of Water Quality 65

Figure 23. Continued. 

San Joaquin River

Merced River Don Pedro Reservoir

Tulloch Reservoir

Tuolumne River

Stanislaus River

Turlock Lake

Modesto Reservoir

Merced River

Merced

Turlock

Modesto

37 30’

121 00’ 120 30’

E S I E R R A N E V A D A

Urban

Land use

Agricultural

Natural

EXPLANATION

Study area boundary

Not detected (< 0.5 micrograms

 per liter)

Low (0.5 - 0.6 micrograms per liter)

Moderate (0.6 - 1.5 micrograms

 per liter)

CDPH perchlorate

USGS GAMA perchlorate

Not detected (< 4.0 micrograms

 per liter)

20 MILES

20 KILOMETERS

0 10

0

DETECTION FREQUENCY OF ONE OR MORE

TRIHALOMETHANES, IN PERCENT

n = 11 n = 6 n = 24 n = 11

100

80

60

40

20

0

< 200

Depth to the top of perforations, in feet below land surface

Urban land use, in percent

 > 200 < 200 > 200

> 40 > 40 < 40 < 40

Figure 24. Relation of detection frequency of trihalomethanes to percent urban land use and depth to the top of perforations, Central 

Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.
66 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Solvents

Solvents are used for a variety of industrial, commercial, 

and domestic purposes (Zogorski and others, 2006). 

Solvents meeting the selection criteria were PCE and carbon 

tetrachloride (figs. 21 and 22). 

Status Assessment for Solvents 

As a class, 0.2 percent of the aquifer had high relativeconcentrations (spatially-weighted) of at least one solvent, and 

3.4 percent had moderate relative-concentrations of at least 

one solvent (table 8). Solvents had low relative-concentrations, 

or were not detected, in 96.4 percent of the aquifer.

Moderate relative-concentrations of solvents 

in 3.4 percent of the aquifer reflected the grid-based 

moderate aquifer-scale proportions for carbon tetrachloride 

(3.4 percent). 

The solvent PCE was detected in 19 percent of grid 

wells in the study unit, with all detections at low relativeconcentrations (fig. 22). PCE had an aquifer-scale high 

proportion of 0.2 percent (spatially-weighted value, table 7A). 

As with THMs, the detection frequency of PCE was highest 

in the Modesto study area, 40 percent, as compared with 

detection frequencies of <17 percent in other study areas 

(Landon and Belitz, 2008). PCE was the second most 

frequently detected VOC in aquifers on the basis of national 

assessments by the USGS NAWQA program (Zogorski and 

others, 2006) and analysis of CDPH data across California 

(Williams and others, 2002). 

Carbon tetrachloride was detected in 6.8 percent of grid 

wells in the study unit (Landon and Belitz, 2008). Carbon 

tetrachloride was not detected at high relative-concentrations, 

had moderate relative-concentrations in 3.4 percent of the 

aquifer, and had low relative-concentrations in 3.4 percent. 

The detection frequency of carbon tetrachloride in the 

Modesto study area was 30 percent, larger than detection 

frequencies of 0 to 6 percent for other study areas (Landon and 

Belitz, 2008). Carbon tetrachloride has been used as a solvent 

in a variety of manufacturing and commercial applications 

in addition to historical use as a fumigant at grain storage 

facilities (Zogorski and others, 2006). Nationally, carbon 

tetrachloride was detected in 1.3 percent of wells sampled by 

the USGS NAWQA program; concentrations always were less 

than the USEPA MCL (Zogorski and others, 2006). 

The solvent trichloroethene (TCE) had a high 

value in one well within the current period but no high 

relative-concentrations were detected in the most recent 

analyses (table 7B). The solvents dichloromethane and 

1,2-dichloroethane had high relative-concentrations in at 

least one well in the CDPH data before March 1, 2003, but 

not during the current period analyzed (table 7B). These high 

values represented historic rather than current conditions. 

Understanding Assessment for Solvents

Similar to THMs, the sum of solvent concentrations 

was correlated significantly (positively) with percent urban 

land use (table 9, fig. 25). The sum of solvents was calculated 

by summing the concentrations of all seven solvents 

detected: PCE, carbon tetrachloride, TCE, dichloromethane, 

dibromomethane, cis-1,2-dichlroethene, and n-propylbenzene. 

The detection frequency of solvents in wells having 

>40 percent urban land use was about 65 percent, significantly 

larger than the detection frequency of about 15 percent for 

wells having <40 percent urban land use. Solvents were 

detected in samples from wells primarily in the Modesto, 

Turlock, and Merced urban areas (fig. 23B). 

Similar to THMs, solvent concentrations were not 

correlated directly with depth (table 9), but most of the wells 

with detections of solvents had depths of <200 ft to the tops 

of the perforations and urban land use of >40 percent (fig. 25). 

Detection frequencies were higher in shallow-urban wells 

(82 percent), than in deep-urban wells (33 percent), shallow 

less-urban (13 percent), or deep less-urban (0 percent) wells. 

Solvent concentrations were not correlated significantly 

with any other factors, suggesting that the distribution of 

sources in urban land-use settings is the dominant explanatory 

factor affecting solvent distribution. Nationally, solvent 

concentrations also have been correlated strongly with percent 

urban land use (Zogorski and others, 2006; Moran and others, 

2007). A previous investigation in the Modesto area found 

urban land use to be the best predictor for the detection of 

VOCs (Wright and others, 2004).

Although solvent concentrations were not significantly 

different between different age classes (table 4), detection 

frequencies for solvents were less in pre-modern waters 

(5 percent, 1 of 20 wells) than in mixed wells (44 percent, 12 

of 27 wells) and modern wells (21 percent, 5 of 24). Because 

some solvents were used before 1950, solvents could be 

present in pre-modern water without invoking mixing of small 

amounts of modern water in the predominantly pre-modern 

age distribution. Similar to THMs, solvents in pre-modern 

water could reflect short-circuit mechanisms due to well 

construction or well operation processes or other nonadvective 

transport processes. 

Other VOCs

For the class “other VOCs”, there were no grid-based 

moderate or high aquifer-scale proportions (table 7B). There 

also were no high relative-concentrations of constituents in 

this class in the CDPH database during the current period 

analyzed. Other VOCs vinyl chloride and naphthalene had 

high concentrations in at least one well in the CDPH data 

before March 1, 2003, but represented historic values rather 

than current conditions.
Status and Understanding of Water Quality 67

Fumigants

Ten VOCs used primarily as fumigants to control pests 

in agriculture and in households, or synthesis byproducts 

included in fumigant mixtures, were grouped into the 

constituent class of fumigants. The classification of nine of 

these constituents as fumigants was determined by the USGS 

National Water Quality Assessment Program (Zogorski and 

others, 2006; listed by Landon and Belitz, 2008, table 3A). 

Although classified as having a primary use as a solvent and 

in the synthesis of some organic compounds (Zogorski and 

others, 2006), 1,2,3-trichloropropane (1,2,3-TCP) has been 

identified as synthesis byproduct in fumigant mixtures in use 

from the 1950s until the early 1980s (Oki and Giambelluca, 

1987; Zebarth and others, 1998), including use in the San 

Joaquin Valley (Domagalski and Dubrovsky, 1991), and has 

been detected in groundwater in areas where fumigants have 

been used (Zogorski and others, 2006). Consequently, 1,2,3-

TCP was included in the fumigants category in this report, but 

actually represents a fumigant synthesis byproduct. Results 

of low-level analyses of 1,2,3-TCP in a subset of wells are 

discussed in “Constituents Sampled for in a Subset of Wells.”

Status Assessment for Fumigants

One fumigant, DBCP, met the selection criteria (fig. 21). 

Moderate relative-concentrations of DBCP occurred in 

8.6 percent of the aquifer (table 7A). Relative-concentrations 

of DBCP ranged from 0.30 to 0.80 in grid wells (fig. 22). 

Detection frequencies of DBCP were similar in three of the 

four study areas: 6 percent in Turlock, 10 percent in Modesto, 

and 13 percent in Merced. DBCP was not detected in the 

Uplands study area. DBCP was used as a soil fumigant to 

control nematodes, primarily on orchards and vineyards but 

also some row crops, in California approximately between 

1955 and 1977 (Domagalski, 1997; Peoples and others, 1980). 

Use of DBCP was discontinued by the California Department 

of Food and Agriculture in 1977 because of its detection in 

groundwater and toxicity (California State Water Resources 

Control Board, 2002). DBCP was the most frequently detected 

fumigant or pesticide detected in groundwater samples 

collected from the San Joaquin Valley during 1971–88 

(Domagalski, 1997) and in California as a whole up to 1999 

(Troiano and others, 2001). Detection frequencies of DBCP 

in groundwater in the Central Valley have been higher than 

most other parts of the country because of its historical use 

on orchards and vineyards (Dubrovsky and others, 1998; 

Zogorski and others, 2006). 

DBCP had an aquifer-scale high proportion of 1.0 percent 

(spatially-weighted), the highest proportion of any organic 

constituent (table 7A). Although high relative-concentrations 

were not detected in grid wells, the spatially-weighted aquiferscale high proportion fell within the 90-percent confidence 

interval for the grid-based aquifer-scale high proportion 

(table 7A). 

High relative-concentrations of DBCP occurred in 

parts of the north-central Merced, north-central Turlock, and 

central Modesto study areas (fig. 23C). Moderate relativeconcentrations of DBCP occur in the north-central Merced, 

south-central and north-central Turlock, and central Modesto 

study areas. 

DETECTION FREQUENCY, IN PERCENT

n = 11 n = 6 n = 24 n = 11

100

80

60

40

20

0

< 200

Depth to the top of perforations, in feet below land surface

Urban land use, in percent

 > 200 < 200 > 200

0%

> 40 > 40 < 40 < 40

Figure 25. Relation of detection frequency of solvents to percent urban land use and depth to the top of perforations, Central Eastside, 

California, Groundwater Ambient Monitoring and Assessment (GAMA) study.
68 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

 Because DBCP was the only fumigant having moderate 

and high relative-concentrations, the aquifer-scale proportions 

for fumigants as a class were equivalent to those for DBCP 

(table 8). The moderate aquifer-scale proportion for fumigants 

(7.6 percent) was calculated from the grid-based moderate 

aquifer-scale proportion for DBCP (8.6 percent) minus 

the spatially-weighted aquifer-scale high proportion of 

1.0 percent.

The fumigant EDB had high relative-concentrations in 

at least one well in the CDPH data before March 1, 2003, 

but not during the current period analyzed (table 7B). These 

high values represented historic values rather than current 

conditions.

Understanding Assessment for DBCP

DBCP was correlated significantly (positively) with 

orchard and vineyard land use (table 9) despite being detected 

in only 10 percent of the grid plus understanding wells. 

Detection frequencies of DBCP were higher (25 percent) in 

shallower (depth to top of perforations <200 ft) wells having 

orchard and vineyard land use >40 percent than in deeper 

wells (depth to the top of the perforations >200 ft) or orchard 

and vineyard land <40 percent (fig. 26). The relation between 

orchard and vineyard land use and DBCP also may partially 

reflect a relation between explanatory factors—a significant 

negative correlation between orchard and vineyard land use 

and well depth (but not depth to top of perforations) (table 5). 

DBCP was not correlated significantly with depth to top of 

perforations or well depth (table 9), even for wells having 

>40 percent orchard and vineyard land use. Relatively low 

detection frequencies of DBCP may mask relations with depth. 

However, 6 of the 7 wells with known well construction where 

DBCP was detected had a depth to the top of the perforations 

<200 ft.

DBCP was not correlated significantly with other 

explanatory factors. DBCP was correlated positively with 

nitrate (as nitrogen) concentrations (ρ = 0.338, p=0.015), 

although nitrate was detected much more widely. 

Herbicides

All detections of herbicides in samples from the Central 

Eastside study unit were at low relative-concentrations 

(figs. 21 and 22). At least one herbicide was detected 

in 57 percent of the 58 grid wells sampled. The highest 

maximum relative-concentration of 0.014 was for atrazine. 

Status Assessment for Herbicides

Three herbicides, simazine, atrazine, and metolachlor, 

were detected in 10 percent or more of the grid wells (figs. 21

and 22). Simazine, atrazine, and metolachlor also were among 

the most commonly detected herbicides in groundwater in 

major aquifers across the United States (Gilliom and others, 

2006). Four additional herbicides or herbicide degradates, 

de-ethylatrazine (2-chloro-4-isopropylamino-6-amino-striazine), 3,4-dichloroaniline, prometon, and hexazinone, 

analyzed for all grid wells, were detected in <10 percent 

of the primary aquifer, all at low relative-concentrations. 

Concentrations of these four constituents were summed with 

simazine, atrazine, and metolachlor to calculate the sum of 

herbicide concentrations. 

DETECTION FREQUENCY, IN PERCENT

n = 12 n = 4

0%

n = 39 n = 15

< 200

Depth to the top of perforations, in feet below land surface

Orchard and vineyard land use, in percent

 > 200 < 200 > 200

> 40 > 40 < 40 < 40

100

80

60

40

20

0

Figure 26. Relation of detection frequency of DBCP to percent orchard and (or) vineyard land use and depth to top of perforations, 

Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.
Status and Understanding of Water Quality 69

Simazine was the most commonly detected herbicide 

(detected in 34 percent of the 58 grid wells), with detection 

frequencies varying by study area: 60 percent in Modesto, 

48 percent in Merced, 19 percent in Turlock, and 0 percent 

in the Uplands study area. Simazine historically has had 

the highest use on vineyards and orchards in the study unit, 

but also has been used on right-of-ways for weed control 

(Domagalski and Dubrovsky, 1991). Simazine was the 

most frequently detected triazine herbicide in groundwater 

in California (Troiano and others, 2001) and in a regional 

survey of 183 wells of the San Joaquin Valley in 1985–87 

(Domagalski and Dubrovsky, 1991). 

Atrazine was detected in 31 percent of the 58 grid wells. 

Detection frequencies of atrazine were highest in the Modesto 

study area (70 percent), with comparatively lower values of 

<30 percent in the other three study areas (Landon and Belitz, 

2008). Registered use of atrazine has been discontinued nearly 

entirely in the study unit since at least the late 1980s, but 

historically was used for weed control along roadsides and in 

row crops such as corn and has been detected widely in the 

San Joaquin Valley (Domagalski and Dubrovsky, 1991; Burow 

and others, 1998a; Troiano and others, 2001). Atrazine had 

high relative-concentrations in at least one well in the CDPH 

data before 2001, but not during the current period analyzed 

(table 7B). These high values represented historic rather than 

current conditions.

Metolachlor was detected in 10 percent of the 58 grid 

wells. Metolachlor was detected in 19 percent of grid wells in 

Turlock and 13 percent of grid wells in Merced, but was not 

detected in the Modesto or Uplands study areas. Metolachlor 

is an herbicide used primarily on corn in the study unit (Burow 

and others, 1998a). 

For the class herbicides, there were no grid-based 

moderate or high aquifer-scale proportions (table 7B). 

Similarly, there were no current high relative-concentrations of 

constituents in these classes in the CDPH database.

Understanding Assessment for Herbicides 

Concentrations of herbicides were significantly lower 

in pre-modern than modern- or mixed-age waters (fig. 27A, 

table 4). However, herbicides were detected in 6 of 20 

wells (30 percent) classified as having pre-modern ages. 

Concentrations of herbicides were not significantly different 

between modern and mixed-age waters.

Herbicides primarily were detected in wells with depths 

to the tops of the perforations <200 ft (fig. 27B). However, 

herbicides were detected in some wells with depths to 

the tops of perforations as large as 335 ft. The detection 

frequency of herbicides for wells with the depths to the top 

of the perforations <200 ft was 75 percent, whereas for wells 

with depths to the top of perforations >200 ft, the detection 

frequency was 32 percent. Herbicides also were correlated 

positively with DO (table 9), but this relation may result from 

the correlation of DO and depth (table 5).

Herbicide concentrations were not correlated significantly 

with percent agricultural land use, but were correlated 

significantly (negatively) with percent natural land use 

(figs. 23D and 27C, table 9) and were correlated (positively) 

with percent urban land use (figs. 27D, table 9). Atrazine 

(along with its degradation product de-ethylatrazine) and 

simazine are the herbicides with the largest detection 

frequencies and concentrations in the Central Eastside; 

both herbicides have been used in agricultural and urban 

settings (Gilliom and others, 2006). The detection frequency 

of herbicides was 69 percent in wells with natural land use 

≤40 percent, implying >60 percent agricultural and urban land 

use, and 31 percent in wells with natural land use >40 percent. 

The correlation of urban land use with herbicides was weaker 

than the relation with percent natural land use. However, 

detection frequencies of herbicides were higher for urban land 

use >40 percent than for urban land use ≤40 percent, although 

the relation of herbicide detection frequency to urban land use 

was weaker than the relation to depth of top of perforations 

(fig. 27D).

 The detection of low concentrations of herbicides in 

30 percent of wells classified as having a pre-modern age 

probably reflects the presence of modern water (containing 

herbicides) in quantities too small to detect with 3H and He 

tracers, given the mixing of waters of widely varying age 

in long-screened production wells. The age tracers indicate 

that these wells predominantly are pre-modern. The presence 

of pesticides can be used to constrain the age of the young 

fraction of groundwater mixtures (Plummer and others, 1993). 

Atrazine first was registered for use in the United States in 

1958 (U.S. Environmental Protection Agency, 2003), and 

simazine first was introduced in 1956 (Gunasekara, 2004). The 

presence of atrazine or de-ethylatrazine in two pre-modern 

samples and simazine in two pre-modern samples indicates 

that some fraction of the water sample was recharged after 

the late 1950s. Metolachlor was first registered in 1976 (U.S. 

Environmental Protection Agency, 1995). The presence of 

metolachlor in two wells with a pre-modern age implies a 

fraction of the water was recharged after 1976. The higher 

detection frequency of herbicides (30 percent), as compared 

to THMs (10 percent) and solvents (5 percent) in pre-modern 

water, may reflect a wider areal extent of herbicide use than 

disinfected water use or solvent use in the Central Eastside. 

It also is possible that the LRLs for herbicides, which are 

lower than for THMs or solvents, could result in a larger 

number of detections and thus, higher detection frequencies 

of herbicides than THMs and solvents. The THMs, solvents, 

and herbicides are related to urban sources, as indicated by 

the positive correlation with urban land use and results of 

previous investigations (Zogorski and others, 2006). However, 

herbicides also are used widely in agricultural areas (Gilliom 

and others, 2006). 
70 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Figure 27. Relations of sum of herbicides to (A) groundwater-age classification, (B) depth to top of perforations, and (C) percent 

natural land use, and (D) pesticide detection frequency to percent urban land use and depth to the top of perforations, Central Eastside, 

California, Groundwater Ambient Monitoring and Assessment (GAMA) study unit.

0

No

detection

0.05 0.1 0.15

100

200

300

400

500

DEPTH TO TOP OF PERFORATIONS, IN FEET

SUM OF HERBICIDE CONCENTRATION, IN MICROGRAMS PER LITER 

Other wells

Modern

Public-supply wells

Mixed

Pre-modern

Modern

Mixed

Pre-modern

No age data

EXPLANATION

Modern, mixed, pre-modern

Age classification:

 = -0.620, p 0.001 

24

SUM OF HERBICIDES,

IN MICROGRAMS PER LITER

27 20

Modern Mixed Pre-modern

GROUNDWATER AGE MIXTURE CLASSIFICATION

20 Number of values

EXPLANATION

75th percentile

Median

25th percentile

25th percentile minus 1.5 times

interquartile range

75th percentile plus 1.5 times

interquartile range

Upper detached

No detection Lower detached

0.01

0.1

Pre-modern significantly different from 

modern and mixed at = 0.05

B

A
Status and Understanding of Water Quality 71

Figure 27. Continued. 

DETECTION FREQUENCY, IN PERCENT

n = 11 n = 6 n = 24 n = 11 100

80

60

40

20

0

< 200

Depth to the top of perforations, in feet below land surface

 > 200 < 200 > 200

> 40 > 40 < 40 < 40

Urban land use, in percent

No detection

0.01

0.1

NATURAL LAND USE, IN PERCENT

0 20 40 60 80 100

SUM OF HERBICIDES, IN MICROGRAMS PER LITER 

 = -0.524, p < 0.001

> 200 feet modern

> 200 feet mixed

> 200 feet pre-modern

< 200 feet modern

< 200 feet mixed

No depth or no age data

EXPLANATION

< 200 feet pre-modern

Depth to top of perforations

below land surface and

age classification

C

D
72 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

The detections of herbicides in relatively deep wells 

having pre-modern ages potentially could be influenced by 

short-circuiting mechanisms that allow small quantities of 

modern water with dissolved herbicides to enter the wells, 

although most of the water extracted from the wells has 

pre-modern ages. For example, in a local study in Modesto, 

Jurgens and others (2008) found evidence of movement of 

waters from the shallow aquifer, with higher concentrations 

of many constituents, including herbicides, down the well 

bore of a public-supply well and into storage in the aquifer 

adjacent to the deepest part of the well under decreased 

pumping conditions during the winter. It is possible that 

these or other short-circuiting processes could account for the 

presence of trace amounts of herbicides in groundwater having 

an apparently pre-modern age. Also, increased pumping 

from a deeper aquifer will increase the downward hydraulic 

gradient between shallow and deeper aquifer zones, resulting 

in increased downward migration of water from the shallow 

aquifer.

Although organic constituents mostly had low or 

moderate relative-concentrations (except for some high 

relative-concentrations of DBCP and PCE), these constituents 

can serve as tracers of groundwater influenced by modern 

recharge and land-use activities and their relation to potential 

explanatory factors contributes to understanding factors 

affecting concentrations of constituents that have high 

concentrations. 

Insecticides

Insecticides were not detected at moderate or high 

relative-concentrations (table 7B). Also, there were no current 

high relative-concentrations of constituents in this class in the 

CDPH database.

Special Interest Constituents

Constituents of special interest analyzed for the Central 

Eastside study unit were NDMA, 1,2,3-TCP, and perchlorate. 

These constituents were selected because recently they have 

been found in, or are considered to have the potential to reach, 

drinking-water supplies (California Department of Public 

Health, 2008a,b,c). NDMA was not detected in the 15 grid 

wells sampled (Landon and Belitz, 2008). Results of lowlevel analyses of 1,2,3-TCP are discussed in the Constituents 

Sampled for in a Subset of Wells section of this report. 

Perchlorate met the selection criteria of having a 

maximum relative-concentration ≥0.1 and a detection 

frequency ≥10 percent (fig. 21). Perchlorate was included in 

explanatory factor analysis because it is the only constituent of 

special interest having moderate relative-concentrations.

Status Assessment for Special Interest 

Constituents

The maximum relative-concentration of perchlorate 

in grid wells from the Central Eastside study unit was 0.25 

(MCL-CA of 6 μg/L) (fig. 22). The MRL (minimum reporting 

level) for perchlorate of 0.5 μg/L corresponds with a relativeconcentration of 0.083, a value close to the distinction between 

moderate and low values. Moderate relative-concentrations 

of perchlorate occurred in 20.7 percent of the aquifer and low 

relative-concentrations or nondetects occurred in 79.3 percent. 

Perchlorate had the highest detection frequency of 70 percent 

in the Modesto study area (fig. 23E; Landon and Belitz, 

2008). Detection frequencies were lower in the other three 

study areas, ranging from 6 to 17 percent. Perchlorate was not 

detected historically in the Central Eastside CDPH database 

above the LRL of 4 μg/L, which is considerably higher than 

the LRL of 0.5 μg/L for the PBP.

Understanding Assessment for Perchlorate

Concentrations of perchlorate were significantly higher in 

modern and mixed-age waters than in pre-modern age waters 

(table 4, fig. 28A). Perchlorate was not significantly different 

between water having modern or mixed ages. One shallow 

monitoring well sampled for the purposes of understanding 

had a concentration of 8.8 μg/L; this well was not included in 

aquifer proportion calculations because it did not represent the 

primary aquifer. Perchlorate concentrations in all other wells 

were 0.5 to 1.6 μg/L. Perchlorate concentrations were not 

correlated with depth to the top of the perforations or bottom 

of the well but were correlated significantly (positively) with 

dissolved oxygen concentrations (table 9). While perchlorate 

biodegrades under anoxic conditions in some aquifers 

(Sturchio and others, 2007), the apparent relation between 

perchlorate and DO in the Central Eastside may result from 

relations of both DO and perchlorate with modern-age and 

mixed-age groundwater. 
Status and Understanding of Water Quality 73

DETECTION FREQUENCY, IN PERCENT

n = 12 n = 4 n = 39 n = 15

100

80

60

40

20

0

DETECTION FREQUENCY, IN PERCENT

n = 11 n = 6 n = 24 n = 11

100

80

60

40

20

0

24

PERCHLORATE, IN MICROGRAMS PER LITER

27 20

Modern Mixed Pre-modern

GROUNDWATER AGE MIXTURE CLASSIFICATION

20 Number of values

EXPLANATION

75th percentile

Median

25th percentile

25th percentile minus 1.5 times

interquartile range

75th percentile plus 1.5 times

interquartile range

Upper detached

Lower detached

Minimum reporting level

10

1

2

3

4

5

0.1

0.2

0.3

0.4

0.5

Pre-modern significantly different from 

modern and mixed at = 0.05

MRL

MRL

C

A

B

< 200

Depth to the top of perforations, in feet below land surface

Orchard and vineyard land use, in percent

 > 200 < 200 > 200

> 40 > 40 < 40 < 40

< 200

Depth to the top of perforations, in feet below land surface

Urban land use, in percent

 > 200 < 200 > 200

> 40 > 40 < 40 < 40

Figure 28. Relations of (A) perchlorate concentrations to groundwater-age classification, (B) perchlorate detection frequency to percent orchard 

and (or) vineyard land use and depth to the top of perforations below land surface, and (C) perchlorate detection frequency to percent urban land use 

and depth to the top of perforations below land surface, Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) study 

unit.
74 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Perchlorate was correlated significantly (positively) with 

orchard and vineyard land use, and urban land use (table 9). 

Perchlorate detection frequencies were higher in wells having 

high proportions of either urban land use or orchard and 

(or) vineyard land use than in wells having small fractions 

of either of these land uses (fig. 28B,C). The distribution of 

detections of perchlorate in modern- and mixed-age waters 

beneath disparate land-use areas suggests multiple potential 

sources. Possible anthropogenic sources of perchlorate could 

include nitrate fertilizers mined from the Atacama Desert of 

Chile that have been used historically on some orchard crops 

(Dasgupta and others, 2006) or industrial, manufacturing, or 

commercial uses such as explosives, road flares, automobile 

air-bag systems, and other products (Parker and others, 

2008). Perchlorate derived from natural atmospheric and 

soil processes has been detected in groundwater in some 

highly arid desert environments (Dasgupta and others, 2005; 

Plummer and others, 2006); the influence of natural processes 

on perchlorate concentrations in groundwater in the Central 

Eastside study unit is currently unknown and beyond the scope 

of this investigation. 

Constituents Sampled for in a Subset of Wells

For 62 constituents, including polar pesticides, selected 

trace elements, and selected radioactive constituents, data were 

available for only a subset of grid wells (<20 wells) (Landon 

and Belitz, 2008). Of these, two organic constituents, diuron 

and 1,2,3-TCP, met the selection criteria of having detection 

frequencies >10 percent in the samples collected. None of the 

constituents analyzed for a subset of wells met the selection 

criteria of having moderate or high relative-concentrations 

(fig. 13). 

Diuron was detected in 20 percent of the 15 grid wells 

sampled for polar pesticides in the study unit (fig. 21). It was 

detected in two wells in the Merced study area and one well 

in the Modesto study area. Diuron has been used extensively 

throughout the San Joaquin Valley on orchards, particularly 

oranges, but has also been used for nonagricultural purposes 

such as weed control on roadways (Domagalski, 1997). 

Nationally, the detection frequency of diuron was <1 percent 

(Gilliom and others, 2006), but in California it has been 

detected in about 6 percent of well samples (Troiano and 

others, 2001). 

For the fumigant synthesis byproduct 1,2,3-TCP, samples 

analyzed with a MRL of 0.005 μg/L (Montgomery Watson 

Harza Laboratories) indicated detections in 3 of the 15 grid 

wells (20 percent detection frequency). In comparison, in 

samples analyzed with an LRL of 0.18 μg/L (USGS NWQL), 

1,2,3-TCP was detected in 2 of 58 grid wells (3 percent 

detection frequency). In two wells, 1,2,3-TCP was detected 

using both methods. 1,2,3-TCP was detected in the Merced 

and Turlock study areas. The maximum relative-concentration 

of 1,2,3-TCP was 0.022. The HAL-US of 40 μg/L was used 

as the water-quality benchmark instead of the NL-CA of 

0.005 μg/L, resulting in relative-concentrations for 1,2,3-TCP 

that were on a similar scale to other organic constituents. 

Nationally, the detection frequency of 1,2,3-TCP above 

0.20 μg/L was <1 percent (Zogorski and others, 2006).

For radon-222, there are two benchmarks, a proposed 

MCL of 300 pCi/L and a proposed alternative MCL-US of 

4,000 pCi/L. The alternative MCL-US will apply if the State 

or local water agency has an approved multimedia mitigation 

program to address radon in indoor air (U.S. Environmental 

Protection Agency, 1999). This alternative MCL-US is 

anticipated to represent the most common benchmark and 

was used for computing relative-concentrations for this study. 

Compared to this benchmark, all relative-concentrations of 

radon-222 were low.

Summary 

The Groundwater Ambient Monitoring and Assessment 

(GAMA) Program was created by the California State Water 

Resources Control Board (State Water Board) to provide a 

comprehensive groundwater-quality baseline for the State 

of California. The program is a comprehensive assessment 

of statewide groundwater quality designed to improve 

ambient groundwater-quality monitoring, and to increase 

the availability of information about groundwater quality 

to the public. The GAMA program includes the Priority 

Basin Project, conducted by the USGS in collaboration with 

the State Water Board and Lawrence Livermore National 

Laboratory (LLNL). This report is one of a series of reports 

presenting the status and understanding of current waterquality conditions in study units of the GAMA Priority Basin 

Project (PBP).

The 1,695-square-mile Central Eastside GAMA study 

unit lies in the Central Valley Hydrogeologic province and 

contains three groundwater subbasins (Modesto, Turlock, and 

Merced). The Central Eastside study unit was divided into 

four study areas: Merced, Modesto, Turlock, and Uplands. 

The purposes of this report are to (1) briefly describe the 

hydrogeologic setting of the Central Eastside study unit, (2) 

assess the current status of untreated-groundwater quality in 

the primary aquifer in the Central Eastside study unit, and 

(3) assess the relations between water quality and selected 

potential explanatory factors for the purpose of understanding.
Summary 75

The GAMA PBP is designed to provide a statistically 

robust characterization of untreated-groundwater quality in the 

primary aquifer at the basin-scale. Fifty-eight grid wells were 

selected randomly within spatially distributed grid cells across 

the Central Eastside study unit. Samples were collected from 

these USGS grid wells for analysis of 175 to 335 constituents. 

Data from the most recent 3-year period available at the time 

of analysis (March 1, 2003,–February 28, 2006) meeting ionic 

charge balance criteria were selected from the CDPH database 

to supplement USGS grid wells for inorganic constituents not 

sampled for by the USGS in each grid cell (CDPH grid wells). 

In addition, the most recent analyses from all wells in the in 

the CDPH database (CDPH other wells) were incorporated 

in the analysis. Using these data, grid-based and spatiallyweighted approaches were used to assess proportions (aquiferscale proportions) of high, moderate, and low relativeconcentrations of constituents and constituent classes in the 

primary aquifer. 

The status assessment is intended to characterize 

the quality of untreated-groundwater resources in the 

primary aquifer within the study unit. To provide context, 

concentrations of constituents measured in the untreated 

groundwater were compared with regulatory and 

nonregulatory human-health and aesthetic benchmarks. 

Given the large number of analytes, an objective 

algorithm was used to select those constituents of greatest 

importance to water quality in the primary aquifer for 

discussion in the report. Relative-concentrations (sample 

concentration divided by benchmark concentration) were used 

as the primary metric for evaluating groundwater-quality. 

Constituents were classified into those whose maximum 

relative-concentrations were high, moderate, or low. Those 

constituents with maximum relative-concentrations >1 were 

classified as high. For inorganic constituents, maximum 

relative-concentrations ≤1 and >0.5 were classified as 

moderate, and maximum relative-concentrations ≤0.5 were 

classified as low. For organic and special-interest constituents, 

maximum relative-concentrations ≤1 and >0.1 were classified 

as moderate, and maximum relative-concentrations ≤0.1 were 

classified as low. 

Selected constituents and constituent classes having high 

or moderate relative-concentrations or organic and specialinterest constituents with detection frequencies >10 percent 

were selected for additional analysis. These constituents or 

classes were tested for relations to a finite set of potential 

explanatory factors that included land use, depth, lateral 

position in the flow system, groundwater-age classification, 

and geochemical-condition indicators. 

On the basis of 3H (tritium), noble gas, and 14C data, 

groundwater ages were classified into modern, mixed, and 

pre-modern categories. Of the wells for which sufficient 

data were available for classification, samples from 24 wells 

were modern, 27 were mixed, and 21 were pre-modern. 

Most samples from wells perforated entirely within the 

upper 200 ft of the aquifer were modern. Most samples from 

wells perforated entirely at depths ≥200 ft were pre-modern. 

Samples from wells with the top of the perforation <200 ft, but 

with the bottom of the well ≥200 ft mostly were mixed.

Groundwater in the Central Eastside primarily was oxic 

(81 percent of sampled wells), but became more reducing with 

depth and near the western (distal or downgradient) end of the 

study area. pH values increased with larger depths and older 

groundwater ages. 

The status assessment showed that one or more 

inorganic constituents were high, relative to human-health 

benchmarks, in 18.0 percent of the primary aquifer, moderate 

in 44.0 percent, and low in 38.0 percent. Aquifer-scale 

high proportions of inorganic constituents reflected high 

relative-concentrations of trace elements in 17.4 percent of 

the aquifer, radioactive constituents in 3.6 percent of the 

aquifer, and nutrients in 2.1 percent of the aquifer. Inorganic 

constituents with aquifer-scale high proportions were arsenic 

(15.6 percent), vanadium (3.6 percent), lead (2.4 percent), 

uranium (3.6 percent), gross alpha radioactivity (5.9 percent), 

and nitrate (2.1 percent). Inorganic constituents with moderate 

aquifer-scale proportions were arsenic (28.9 percent), boron 

(4.2 percent), lead (2.4 percent), vanadium (21.4 percent), 

uranium (15.2 percent), gross alpha radioactivity 

(10.8 percent), and nitrate (14.6 percent). Spatially-weighted 

aquifer-scale high proportions nearly always fell within the 

90-percent confidence interval of grid-based aquifer-scale 

high proportions, indicating that the grid-based approach 

yielded statistically equivalent results to the spatially-weighted 

approach incorporating CDPH data.

The status assessment for organic and special-interest 

constituents showed that one or more of these constituents 

were high, relative to human-health benchmarks, in a smaller 

proportion of the primary aquifer (1.2 percent) than inorganic 

constituents (18.0 percent). Organic constituents had moderate 

relative-concentrations in 14.3 percent of the primary 

aquifer and low relative-concentrations in 84.5 percent. 

The proportion of the primary aquifer with high relativeconcentrations of organic constituents reflected aquiferscale high proportions of the discontinued soil fumigant 

1,2-dibromo-3-chlororopane (DBCP, 1.0 percent) and the 

solvent tetrachloroethene (PCE, 0.2 percent). Four organic 

and special-interest constituents were detected at moderate 

relative-concentrations in grid wells, including: chloroform 

(3.4 percent), carbon tetrachloride (3.4 percent), DBCP 

(8.6 percent), and perchlorate (20.7 percent). 

The status assessment for organic constituents indicated 

that 22 of the 78 VOCs (not including fumigants) and 10 of 

the 115 pesticides and pesticide degradates analyzed in grid 

wells were detected. All detections of VOCs and pesticides 

in grid-well samples from the Central Eastside study unit 

were below human-health benchmarks. Of the 22 VOCs 

detected, 20 had low maximum relative-concentrations. 

Of the 22 VOCs detected, 17 had detection frequencies 

of <10 percent. Five VOCs were detected in ≥10 percent 

of the grid wells—the THMs chloroform, bromoform, 

bromodichloromethane, and dibromochloromethane, and the 

solvent PCE. For the six parent herbicides with benchmarks, 
76 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

all had low relative-concentrations. Of the five herbicide 

parent compounds detected, analyzed in all 58 grid wells, 

and having benchmarks, three had detection frequencies 

≥10 percent. Atrazine, simazine, and metolachlor were 

detected in 31 percent, 34 percent, and 10 percent of grid 

wells, respectively. Perchlorate was detected in 22 percent of 

grid wells.

The understanding assessment indicated that the 

concentrations of many constituents were related to depth 

and groundwater age. However, concentrations of individual 

constituents or constituent classes also were sometimes related 

to geochemical conditions, lateral position in the flow system, 

or land use.

Nitrate, uranium, and total dissolved solids concentrations 

had higher concentrations in the upper 200 ft than in the 

deeper parts of the aquifer system. There was a similar a 

contrast in concentrations for a number of constituents that did 

not have human-health benchmarks or that were detected only 

at low relative-concentrations (such as sum of herbicides). 

Constituents with these vertical profiles included most major 

ions (calcium, magnesium, sodium, bicarbonate, chloride, and 

sulfate), and several trace elements (strontium, barium, cobalt, 

and lithium). Isotopic values of boron, sulfate, strontium, 

carbon, and uranium also had contrasting values between the 

upper 200 ft and greater depths. A detailed discussion of the 

processes affecting these additional constituents and tracers is 

beyond the scope of this report. 

The concentration profiles for multiple constituents 

suggests that water chemistry generally differs between 

the upper 200 ft of the aquifer and depths >200 ft. The 

zonation of water chemistry with depth is consistent with the 

hydrogeologic setting, in which return flows from agricultural 

and urban land use are the major source of recharge, and 

withdrawals for irrigation are the major source of discharge, 

resulting in substantial vertical components of groundwater 

flow.

The decrease in concentrations of many constituents with 

depth reflects, in part, that groundwater generally gets older 

with depth in the Central Eastside study unit. Uranium, nitrate, 

herbicide, and perchlorate concentrations were significantly 

larger in groundwater having modern and mixed ages than 

pre-modern ages, indicating that these constituents may be 

affected by anthropogenic activities in the last 50 years.

Other patterns in the distribution of nitrate, uranium, and 

TDS were evident. Nitrate concentrations were correlated 

positively with dissolved oxygen. Nitrate concentrations 

and percent orchard and vineyard land use were correlated 

in groundwater sampled from <200 ft below land surface. 

Additional isotopic and geochemical data are consistent with 

partial denitrification of nitrate in some reducing groundwaters 

in the western and deeper parts of the flow system. 

Geochemical and isotopic patterns for this study are similar to 

those of a local-scale study in Modesto and a regional-scale 

study in the eastern San Joaquin Valley in which elevated 

uranium in shallow groundwater was attributed to desorption 

of uranium from sediments by irrigation and urban recharge. 

Uranium and TDS increase from east to west across the valley, 

along the direction of regional lateral groundwater flow.

Mobilization of the trace element arsenic by reductive 

dissolution of manganese or iron oxides and desorption 

by high pH waters explains high-to-moderate arsenic 

concentrations in the Central Eastside study unit. The primary 

variable influencing moderate-to-high concentrations of the 

trace element vanadium appears to be high pH, particularly in 

oxic waters. The pattern of increasing arsenic with increasing 

groundwater age and depth was opposite to the pattern for 

uranium, which decreased with depth and groundwater 

age. These divergent patterns with depth reflect different 

mobilization processes. High concentrations of another 

trace element, manganese (SMCL), occurred in reduced 

groundwater located in the western and deep parts of the 

aquifer in the study unit. 

For organic and special interest constituents, THMs 

and solvents are significantly and positively correlated with 

percent urban land use. THM concentrations also were 

significantly and positively correlated with DO. DBCP, 

although detected infrequently, primarily was detected in areas 

with a high proportion of orchard and vineyard land use and 

at depths <200 ft. Herbicide concentrations were negatively 

correlated with percent natural land use and positively 

correlated with percent urban land use. Perchlorate was 

significantly and positively correlated with two land uses—

percent orchard/vineyard land use and percent urban land use.

Generally, there were many similarities in groundwater 

quality areally across the study unit. However, some variations 

in groundwater quality between study areas were evident, 

most likely as a result of differences in land use, lateral 

position, redox, and pH. The Modesto study area had higher 

detection frequencies of THMs, solvents, and perchlorate than 

other study areas; these constituents were positively correlated 

with urban land use and the higher detection frequencies in 

Modesto are consistent with greater percentages of urban land 

use than in the other study areas. The grid wells, particularly 

in the Modesto study area, had more urbanized land use than 

the study areas as a whole; therefore, the grid well data may 

reflect greater urban influence than might be expected on the 

basis of the average land use of the study areas. Detection 

frequencies of herbicides generally were lower in the Uplands 

study area than in the Modesto, Turlock, and Merced study 

areas; herbicides were correlated negatively with natural 

land use and the lower detection frequencies in the uplands 

study area may reflect greater amounts of natural land use 

in the uplands study area compared to the other study areas. 

High concentrations of arsenic and manganese occurred in 

the western Modesto, Turlock, and Merced study areas but 

did not occur in the Uplands study area located in the eastern 

portion of the study unit; this pattern reflects that groundwater 

becomes more reducing in the western part of the study unit. 

Concentrations of uranium and TDS also increased from 

east to west across the study unit, probably reflecting several 

processes that vary with valley position, and concentrations of 

these constituents were lower in the Uplands study area than 

in the Modesto, Turlock, and Merced study areas.
References 77

At the low concentrations at which they generally were 

present, VOCs, pesticides, and perchlorate primarily are 

tracers of groundwater that has recharged in the decades 

since these compounds began to be used for industrial and 

commercial purposes. Low-level analyses provide an early 

awareness of constituents whose presence in groundwater 

at low concentrations may be important for prioritization of 

monitoring of water quality in the future. 

Acknowledgments

The authors thank the following cooperators for their 

support: the State Water Board, LLNL, California Department 

of Public Health, and California Department of Water 

Resources. We especially thank the cooperating well owners 

and water purveyors for their generosity in allowing the 

USGS to collect samples from their wells. Funding for this 

work was provided by State of California bonds authorized by 

Proposition 50 and administered by the State Water Board.

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84 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Appendix A. Attributes of USGS–GAMA and CDPH Grid Wells

MERCED

COUNTY

STANISLAUS

COUNTY

MARIPOSA

COUNTY

MADERA

COUNTY

SAN JOAQUIN

COUNTY

TUOLUMNE

COUNTY

CALAVERAS

COUNTY

Turlock

Modesto

MODFP-01, 02, 03

MODFP-04

MODMW-01

TRLKFP-01

TRLKFP-02

TRLKMW-01, 02 TRLKMW-03, 04

TRLKMW-05

CE-QPC-FP01

MERFP-01

MERFP-02

MERMW-01, 02

MERMW-03, 04

MERMW-05

MERMW-05

MER-DG-04

MER-DPH-23

MER-DG-13

MER-DPH-15

MER-DG-05 MER-DG-08

MER-DPH-16

MER-DPH-14

MER-DPH-01

MER-DPH-19

MER-DPH-07

MER-DG-06

TRLK-DPH-08

TRLK-DPH-16

TRLK-DG-14

TRLK-DG-15

TRLK-DPH-13

MOD-DPH-10

MOD-DG-04

CE-QPC-DG-02

CE-QPC-DPH-04

MOD-DG-08

MOD-DG-06

MOD-DG-07

TRLK-DPH-06

MOD-DG-03

TRLK-DG-04

MOD-DG-05

TRLK-DPH-07

CE-QPC-DG-06

MER-22

MER-06

MER-07 MER-11

MER-17

MER-18

MER-20

MER-21

MER-21

MER-12

MER-13

MER-23

MER-01 MER-04

MER-05 MER-08

MER-09

MER-19

TRLK-03

MER-15

MER-02

TRLK-10

MER-16

MOD-01

MOD-02

MOD-03

MOD-04

MOD-05

MOD-07

MOD-06

MOD-10

TRLK-04

TRLK-05

TRLK-07

TRLK-08 TRLK-09

TRLK-11

TRLK-12

TRLK-06

MER-14

MOD-08

MER-03

MER-10

TRLK-13

TRLK-01 TRLK-02

TRLK-14

TRLK-15

TRLK-16

CE-QPC-01

CE-QPC-09

MOD-09

CE-QPC-04

CE-QPC-06

CE-QPC-07

CE-QPC-08

CE-QPC-02 CE-QPC-05

CE-QPC-03

12

12

13

13 13

17

14

14

14

16

16

14

23

22

21

20

20

20

20

24

15

15

15

15 10

10

9

8

7 7

7

8

8

8

7

7

7

7

6

6

6

6 6

5

5

5

5

5

5

4

4

4

4

4

3

3

3

3

2

2

2

2

2

2

1

1

1

1

1

9

9 9 9

9

11

11

10

10

10

10

10

10

19

18

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

20 MILES

20 KILOMETERS

EXPLANATION

0 10

0

121 00’ 120 30’

38 00’

37 30’

37 00’

Merced

Identification numbers

 of wells

USGS grid well

CDPH grid well

USGS understanding

 well

Study area

 boundary

Merced

Study areas:

Modesto

Uplands

Turlock Grid cell and

 number

Figure A1. Identification numbers and locations of grid and understanding wells sampled during March–June, 2006, and grid wells at which data for inorganic 

constituents from the California Department of Public Health were used, Central Eastside, California, Groundwater Ambient Monitoring and Assessment (GAMA) 

study unit.
Appendix A. Attributes of USGS–GAMA and CDPH Grid Wells 85

Table A1.  Identifiers and explanatory factor attributes of grid and understanding wells sampled during March–June, 2006, and

grid wells at which data for inorganic constituents from the California Department of Public Health were used, Central Eastside

Groundwater Ambient Monitoring and Assessment (GAMA) study.—Continued

[CDPH, California Department of Public Health; CE-QPC, Uplands study area well; MER, Merced study area well; MOD, Modesto study area well; TRLK, 

Turlock study area well; FP, flow-path well; MW, monitoring well; ft, feet; m, meter; LSD, land surface datum; na, not available; nc, not calculated; USGS, 

U.S. Geological Survey; MER-##, naming convention for USGS-grid well; MER-DG-##, naming convention for CDPH data from a USGS-grid well; MERDPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public supply well; COM, commercial well; DOM, domestic well; IRR, irrigation 

well; MON, monitoring well; DRAIN, drainage well; USGS, U.S. Geological Survey; age-tracer data reported in table A2; oxidation-reduction data reported in 

table A3]

USGS 

GAMA well 

identification 

number

CDPH 

GAMA well 

identification 

number

Well 

type

Agricultural 

land use 

within 500-m 

of the well 

(percent)

Natural land 

use within 

500-m of 

the well 

(percent)

Urban land 

use within 

500-m of 

the well 

(percent)

Construction information

Normalized 

lateral 

position 

(dimensionless)

Well depth 

(ft below 

LSD)

Top of 

perforations 

(ft below LSD)

Bottom of 

perforations 

(ft below LSD)

Length from 

top of 

uppermost 

perforated 

interval to 

bottom 

of well 

(ft below LSD)

Grid wells

CE-QPC-01 none PSW 46.8 34.1 19.0 280 na na na 0.73

CE-QPC-02 CE-QPCDG-02PSW 40.9 0.6 58.5 300 124 na 176 0.65

CE-QPC-03 none PSW 0.2 99.7 0.1 285 228 na 57 0.76

CE-QPC-04 none COM 7.4 92.6 0.0 336 252 na 84 0.66

CE-QPC-05 none DOM 44.3 54.0 1.7 117 97 117 20 0.85

CE-QPC-06 CE-QPCDG-06PSW 59.6 37.7 2.7 358 183 na 175 0.67

CE-QPC-07 none COM 21.8 78.2 0.0 380 270 380 110 0.86

CE-QPC-08 none DOM 1.4 95.3 3.3 109 na na na 0.65

CE-QPC-09 none DOM 12.0 88.0 0.0 100 na na na 1.00

MER-01 none DOM 56.7 43.3 0.0 130 110 130 20 0.00

MER-02 none PSW 89.2 1.0 9.7 na na na na 0.32

MER-03 none PSW 4.0 11.3 84.7 526 305 515 221 0.43

MER-04 MER-DG-04 PSW 65.2 0.5 34.4 75 na na na 0.04

MER-05 MER-DG-05 PSW 52.5 47.5 0.0 268 248 268 20 0.18

MER-06 MER-DG-06 PSW 44.9 0.1 55.0 178 130 150 48 0.73

MER-07 none PSW 99.7 0.0 0.3 na na na na 0.67

MER-08 MER-DG-08 PSW 99.1 0.8 0.1 380 130 380 250 0.22

MER-09 none PSW 61.4 2.7 35.9 266 174 249 92 0.59

MER-10 none PSW 10.2 5.6 84.2 294 102 294 192 0.49

MER-11 none PSW 48.0 15.8 36.2 630 234 620 396 0.81

MER-12 none IRR 34.6 65.3 0.1 210 110 210 100 0.12

MER-13 MER-DG-13 PSW 45.2 54.8 0.0 355 110 350 245 0.24

MER-14 none PSW 17.3 13.2 69.5 734 261 730 473 0.47

MER-15 none PSW 95.6 4.2 0.1 185 na 185 na 0.57

MER-16 none PSW 64.1 14.3 21.5 167 152 167 15 0.41

MER-17 none DOM 16.3 81.1 2.6 na na na na 0.88

MER-18 none DOM 88.2 11.8 0.0 200 na na na 0.70

MER-19 none DOM 74.8 22.0 3.2 na na na na 0.41

MER-20 none IRR 66.9 33.1 0.0 923 335 909 588 0.93

MER-21 none MON 13.5 86.5 0.0 345 225 345 120 0.48

MER-22 none DOM 0.0 99.4 0.6 130 na na na 0.99

MER-23 none COM 100.0 0.0 0.0 485 350 475 135 0.14

MOD-01 none PSW 87.3 1.0 11.7 81 81 na 0 0.21

MOD-02 none PSW 0.0 0.7 99.3 395 91 366 304 0.35

MOD-03 MOD-DG-03 PSW 1.3 2.9 95.9 255 180 255 75 0.28

MOD-04 MOD-DG-04 PSW 34.0 29.3 36.7 304 164 276 140 0.27

MOD-05 MOD-DG-05 PSW 35.3 9.3 55.4 292 139 271 153 0.41

Table A1.  Identifiers and explanatory factor attributes of grid and understanding wells sampled during March–June, 2006, and grid

wells at which data for inorganic constituents from the California Department of Public Health were used, Central Eastside Groundwater

Ambient Monitoring and Assessment (GAMA) study.

[CDPH, California Department of Public Health; CE-QPC, Uplands study area well; MER, Merced study area well; MOD, Modesto study area well; TRLK, 

Turlock study area well; FP, flow-path well; MW, monitoring well; ft, feet; m, meter; LSD, land surface datum; na, not available; nc, not calculated; USGS, 

U.S. Geological Survey; MER-##, naming convention for USGS-grid well; MER-DG-##, naming convention for CDPH data from a USGS-grid well; MERDPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public supply well; COM, commercial well; DOM, domestic well; IRR, irrigation 

well; MON, monitoring well; DRAIN, drainage well; USGS, U.S. Geological Survey; age-tracer data reported in table A2; oxidation-reduction data reported in 

table A3]
86 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Table A1. Identifiers and explanatory factor attributes of grid and understanding wells sampled during March–June, 2006, and 

grid wells at which data for inorganic constituents from the California Department of Public Health were used, Central Eastside 

Groundwater Ambient Monitoring and Assessment (GAMA) study.—Continued

[CDPH, California Department of Public Health; CE-QPC, Uplands study area well; MER, Merced study area well; MOD, Modesto study area well; TRLK, 

Turlock study area well; FP, flow-path well; MW, monitoring well; ft, feet; m, meter; LSD, land surface datum; na, not available; nc, not calculated; USGS, 

U.S. Geological Survey; MER-##, naming convention for USGS-grid well; MER-DG-##, naming convention for CDPH data from a USGS-grid well; MERDPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public supply well; COM, commercial well; DOM, domestic well; IRR, irrigation 

well; MON, monitoring well; DRAIN, drainage well; USGS, U.S. Geological Survey; age-tracer data reported in table A2; oxidation-reduction data reported in 

table A3]

USGS 

GAMA well 

identification 

number

CDPH 

GAMA well 

identification 

number

Well 

type

Agricultural 

land use 

within 500-m 

of the well 

(percent)

Natural land 

use within 

500-m of 

the well 

(percent)

Urban land 

use within 

500-m of 

the well 

(percent)

Construction information

Normalized 

lateral 

position 

(dimensionless)

Well depth 

(ft below 

LSD)

Top of 

perforations 

(ft below LSD)

Bottom of 

perforations 

(ft below LSD)

Length from 

top of 

uppermost 

perforated 

interval to 

bottom 

of well 

(ft below LSD)

MOD-06 MOD-DG-06 PSW 0.0 9.7 90.3 216 128 192 88 0.32

MOD-07 MOD-DG-07 PSW 0.3 1.7 97.9 231 155 231 76 0.29

MOD-08 MOD-DG-08 PSW 65.3 1.6 33.1 296 200 292 96 0.61

MOD-09 none PSW 49.6 7.7 42.7 338 307 338 31 0.75

MOD-10 none IRR 99.8 0.2 0.0 815 159 815 656 0.84

TRLK-01 none PSW 0.5 4.4 95.2 348 280 348 68 0.31

TRLK-02 none PSW 48.2 0.5 51.3 272 180 252 92 0.33

TRLK-03 none PSW 75.4 4.5 20.2 497 230 460 267 0.44

TRLK-04 TRLK-DG-04 PSW 39.2 2.5 58.3 332 104 na 228 0.61

TRLK-05 none PSW 0.0 0.7 99.3 472 204 457 268 0.32

TRLK-06 none PSW 50.6 6.9 42.5 221 105 213 116 0.24

TRLK-07 none PSW 0.0 95.8 4.2 200 100 196 100 0.81

TRLK-08 none PSW 62.0 38.0 0.0 128 72 na 56 0.70

TRLK-09 none PSW 54.5 19.6 25.9 338 76 na 262 0.87

TRLK-10 none PSW 46.5 43.5 10.0 116 72 112 44 0.00

TRLK-11 none PSW 21.6 1.1 77.2 410 210 400 200 0.39

TRLK-12 none PSW 40.1 58.8 1.1 158 108 na 50 0.00

TRLK-13 none PSW 91.6 1.0 7.3 104 84 104 20 0.09

TRLK-14 TRLK-DG-14 PSW 100.0 0.0 0.0 360 160 355 200 0.22

TRLK-15 TRLK-DG-15 PSW 99.9 0.1 0.0 400 330 390 70 0.24

TRLK-16 none COM 73.0 27.0 0.0 na na na na 0.79

none CE-QPCDPH-04PSW 7.4 92.6 0.0 340 232 336 108 0.66

none MER-DPH-01 PSW 58.8 8.6 32.6 na na na na 0.27

none MER-DPH-07 PSW 58.1 10.9 31.0 672 457 672 215 0.54

none MER-DPH-14 PSW nc nc nc na na na na 0.45

none MER-DPH-15 PSW 99.9 0.1 0.0 na na na na 0.55

none MER-DPH-16 PSW 23.0 4.8 72.2 na na na na 0.44

none MER-DPH-19 PSW 12.7 9.6 77.7 198 110 176 88 0.48

none MER-DPH-23 PSW nc nc nc 315 240 300 na 0.07

none MOD-DPH-10 PSW 81.9 14.4 3.7 260 200 260 60 0.63

none TRLK-DPH-06 PSW nc nc nc 266 109 221 157 0.24

none TRLK-DPH-07 PSW nc nc nc na na na na 0.84

none TRLK-DPH-08 PSW 74.3 12.8 12.8 na na na na 0.30

none TRLK-DPH-13 PSW nc nc nc na na na na 0.15

none TRLK-DPH-16 PSW nc nc nc na na na na 0.82
Appendix A. Attributes of USGS–GAMA and CDPH Grid Wells 87

Table A1. Identifiers and explanatory factor attributes of grid and understanding wells sampled during March–June, 2006, and 

grid wells at which data for inorganic constituents from the California Department of Public Health were used, Central Eastside 

Groundwater Ambient Monitoring and Assessment (GAMA) study.—Continued

[CDPH, California Department of Public Health; CE-QPC, Uplands study area well; MER, Merced study area well; MOD, Modesto study area well; TRLK, 

Turlock study area well; FP, flow-path well; MW, monitoring well; ft, feet; m, meter; LSD, land surface datum; na, not available; nc, not calculated; USGS, 

U.S. Geological Survey; MER-##, naming convention for USGS-grid well; MER-DG-##, naming convention for CDPH data from a USGS-grid well; MERDPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public supply well; COM, commercial well; DOM, domestic well; IRR, irrigation 

well; MON, monitoring well; DRAIN, drainage well; USGS, U.S. Geological Survey; age-tracer data reported in table A2; oxidation-reduction data reported in 

table A3]

USGS 

GAMA well 

identification 

number

CDPH 

GAMA well 

identification 

number

Well 

type

Agricultural 

land use 

within 500-m 

of the well 

(percent)

Natural land 

use within 

500-m of 

the well 

(percent)

Urban land 

use within 

500-m of 

the well 

(percent)

Construction information

Normalized 

lateral 

position 

(dimensionless)

Well depth 

(ft below 

LSD)

Top of 

perforations 

(ft below LSD)

Bottom of 

perforations 

(ft below LSD)

Length from 

top of 

uppermost 

perforated 

interval to 

bottom 

of well 

(ft below LSD)

USGS-Understanding Wells

CE-QPC-FP01 none IRR 97.1 1.5 1.4 611 315 606 296 0.71

MERFP-01 none IRR 24.1 75.9 0.0 368 na na na 0.10

MERFP-02 none DOM 14.0 86.0 0.0 na na na na 0.91

MERMW-01 none MON 97.4 0.1 2.5 88 78 83 10 0.38

MERMW-02 none MON 97.4 0.1 2.5 148 138 143 10 0.38

MERMW-03 none MON 97.8 2.2 0.0 56 46 51 10 0.43

MERMW-04 none MON 97.8 2.2 0.0 168 158 163 10 0.43

MERMW-05 none MON 17.3 79.2 3.6 68 23 68 45 0.48

MODFP-01 none MON 100.0 0.0 0.0 280 269 274 11 0.15

MODFP-02 none MON 100.0 0.0 0.0 183 174 179 9 0.15

MODFP-03 none MON 100.0 0.0 0.0 35 25 30 10 0.15

MODFP-04 none IRR 98.9 1.1 0.0 275 116 275 159 0.50

MODMW-01 none MON 94.6 3.2 2.2 260 100 260 160 0.48

TRLKFP-01 none DRAIN 92.1 1.4 6.5 80 na na na 0.09

TRLKFP-02 none DRAIN 100.0 0.0 0.0 58 na na na 0.22

TRLKMW-01 none MON 99.9 0.1 0.0 113 103 108 10 0.11

TRLKMW-02 none MON 99.9 0.1 0.0 46 36 41 10 0.11

TRLKMW-03 none MON 98.1 1.7 0.2 171 161 166 10 0.44

TRLKMW-04 none MON 98.1 1.7 0.2 101 91 96 10 0.44

TRLKMW-05 none MON 100.0 0.0 0.0 97 87 92 10 0.39
88 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project Table A2. Summary of groundwater age data and classification into modern, mixed, and pre-modern age categories for samples collected during March–June, 2006, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study unit.—Continued [°C, degrees celsius; 3He/4He, Helium-3/Helium-4; 14C, carbon-14; pmc, percent modern carbon; <, less than; >, greater than; na, not applicable; blank field, no data; —, not detected; e, estimated] GAMA well identification number Recharge temperature (°C) Recharge temperature uncertainty, (°C) Tritium, (tritium units) Tritium uncertainty (tritium units) Terrigenic helium (percent of total helium)

3H/3Hetri

age 

(years)

3H/3Hetri

age 

uncertainty 

(years)

14C 

(pmc)

14C 

counting 

uncertainty

(pmc)

14C 

age 

(uncorrected) 

(years)

14C age 

uncertainty 

(uncorrected) 

(years)

Age 

classification

Grid Wells

CE-QPC-01 18.3 <0.1 10.85 0.60 0.0 32.7 1.0 110.0 0.6 <1,000 na Modern

CE-QPC-02 17.8 <0.1 2.79 0.13 82.8 >50 na Mixed

CE-QPC-03 15.3 <0.1 8.50 0.38 0.0 32.1 0.9 Modern

CE-QPC-04 19.6 <0.1 5.2 >50 na Mixed/Premodern

CE-QPC-05 19.6 <0.1 0.31 0.17 8.6 >50 na Pre-modern

CE-QPC-06 21.0 <0.1 0.85 0.11 23.6 >50 na Pre-modernCE-QPC-07 24.1 0.7

— 0.31 59.4 >50 na 48.7 0.2 5,900 1 Pre-modern

CE-QPC-08 18.0 0.2 4.81 0.23 0.3 29.9 0.9 Modern

CE-QPC-09 16.6 0.8 3.32 0.31 28.2 16.3 2.2 110.3 0.5 <1,000 na Mixed

MER-01 16.2 <0.1 0.75 0.09 98.7 >50 na Pre-modern

MER-02 21.7 0.1 5.20 0.41 79.0 41.9 1.6 82.1 0.4 1,500 87 Mixed

MER-03 17.5 <0.1 1.60 0.19 95.0 >50 na 34.4 0.3 8,700 97 MixedMER-04 22.6 0.6 3.65 0.19 0.0 10.9 0.5 Modern

MER-05 16.5 0.1 0.55 0.07 89.0 >50 na Pre-modern

MER-06 19.1 0.2 96.9 >50 na Mixed/

Pre-modern

MER-07 18.5e 2.7e 24.9 >50 na Mixed/

Pre-modern

MER-08 no dataMER-09 18.8 <0.1 5.20 0.41 28.6 32.1 1.6 105.2 0.4 <1,000 na Mixed

MER-10 16.8 0.3 4.70 0.41 39.6 36.6 1.7 96.6 0.4 <1,000 na MixedMER-11 15.7 <0.1 4.83 0.31 41.4 32.4 1.6 94.9 0.4 <1,000 na MixedMER-12 12.1 <0.1 1.60 0.31 44.9 16.5 6.7 100.8 0.4 <1,000 na MixedMER-13 18.6 <0.1 0.34 0.19 23.0 >50 na Pre-modern

MER-14 17.6 0.1 0.97 0.13 97.8 >50 na Pre-modernMER-15 18.9 <0.1 8.47 0.35 0.0 24.7 0.8 Modern

MER-16 20.6 0.1 4.66 0.21 90.5 >50 na Mixed

MER-17 20.6 0.2 5.02 0.27 0.0 2.3 0.1 Modern

MER-18 20.5 <0.1 0.51 0.09 16.6 >50 na Pre-modern

MER-19 18.8 <0.1 11.44 0.47 6.0 14.9 1.0 Mixed

Table A2. Summary of groundwater age data and classification into modern, mixed, and pre-modern age categories for samples collected during March–June, 2006, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study unit.[°C, degrees celsius; 3He/4He, Helium-3/Helium-4; 14C, carbon-14; pmc, percent modern carbon; <, less than; >, greater than; na, not applicable; blank field, no data; —, not detected; e, estimated]
Appendix A. Attributes of USGS–GAMA and CDPH Grid Wells 89

Table A2. Summary of groundwater age data and classification into modern, mixed, and pre-modern age categories for samples collected during March–June, 2006, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study unit.—Continued[°C, degrees celsius; 3He/4He, Helium-3/Helium-4; 14C, carbon-14; pmc, percent modern carbon; <, less than; >, greater than; na, not applicable; blank field, no data; —, not detected; e, estimated]

GAMA well 

identification 

number

Recharge 

temperature 

(°C)

Recharge 

temperature 

uncertainty, 

(°C)

Tritium, 

(tritium 

units)

Tritium 

uncertainty 

(tritium units)

Terrigenic 

helium 

(percent 

of total 

helium)

3H/3Hetri

age 

(years)

3H/3Hetri

age 

uncertainty 

(years)

14C 

(pmc)

14C 

counting 

uncertainty

(pmc)

14C 

age 

(uncorrected) 

(years)

14C age 

uncertainty 

(uncorrected) 

(years)

Age 

classification

MER-20 10.4 8.9 0.50 0.31 77.6 >50 na 27.0 0.2 10,800 15 Pre-modern

MER-21 20.7 <0.1

— 0.19 27.1 >50 na 51.2 0.3 5,400 87 Pre-modern

MER-22 23.1 1.0 0.56 0.10 97.6 >50 na Pre-modern

MER-23 15.6 <0.1 0.11 0.05 64.5 >50 na Pre-modernMOD-01 17.9 <0.1 5.61 0.41 77.1 30.5 1.5 91.4 0.4 <1,000 na Mixed

MOD-02 19.7 <0.1 4.01 0.41 58.2 39.9 1.9 97.0 0.4 <1,000 na MixedMOD-03 13.1 0.3 4.11 0.20 1.8 Not Datable Modern

MOD-04 15.6 <0.1 1.55 0.10 18.9 37.2 2.0 Mixed

MOD-05 15.9 <0.1 4.82 0.20 53.4 36.2 1.2 MixedMOD-06 18.2 <0.1 2.40 0.13 80.6 >50 na MixedMOD-07 16.7 <0.1 4.62 0.32 95.7 >50 na MixedMOD-08 17.5 <0.1 5.32 0.22 68.1 45.8 1.1 MixedMOD-09 15.4 1.8 1.00 0.31 0.0 36.8 4.9 93.4 0.5 <1,000 na Modern

MOD-10 20.6 0.2 1.52 0.13 29.7 >50 na Mixed

TRLK-01 16.8 0.1

— 0.19 95.5 >50 na 14.0 0.2 16,200 96 Pre-modern

TRLK-02 18.8 <0.1 1.69 0.19 77.1 46.9 2.3 55.4 0.3 4,800 40 MixedTRLK-03 17.3 <0.1 0.46 0.07 83.9 >50 na 19.0 0.2 13,700 56 Pre-modernTRLK-04 18.1 <0.1 8.02 0.33 66.0 33.0 1.1 Mixed

TRLK-05 17.8 <0.1 0.41 0.19 88.8 >50 na 26.7 0.2 10,800 118 Pre-modern

TRLK-06 20.7 <0.1 9.21 0.36 58.5 33.1 1.1 Mixed

TRLK-07 16.1 <0.1 5.42 0.39 0.0 19.5 1.0 Modern

TRLK-08 19.8 <0.1 4.52 0.32 0.0 17.6 0.9 ModernTRLK-09 18.4 <0.1 4.04 0.21 0.0 18.5 0.7 ModernTRLK-10 16.9 <0.1 2.51 0.31 44.2 25.0 2.6 90.8 0.5 <1,000 na Mixed

TRLK-11 17.9 0.1 0.50 0.19 69.6 >50 na 39.4 0.3 7,600 88 Pre-modern

TRLK-12 19.8 0.2 6.12 0.61 4.4 42.3 1.7 Modern

TRLK-13 12.73 0.56 Modern/

Mixed

TRLK-14 18.5e 2.7e 1.64 0.68 97.4 >50 na Mixed

TRLK-15 18.6 0.3 0.51 0.07 98.4 >50 na Pre-modern

TRLK-16 22.8 0.1 1.08 0.11 0.0 6.4 0.5 Modern
90 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project Table A2. Summary of groundwater age data and classification into modern, mixed, and pre-modern age categories for samples collected during March–June, 2006, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) study unit.—Continued [°C, degrees celsius; 3He/4He, Helium-3/Helium-4; 14C, carbon-14; pmc, percent modern carbon; <, less than; >, greater than; na, not applicable; blank field, no data; —, not detected; e, estimated] GAMA well identification number Recharge temperature (°C) Recharge temperature uncertainty, (°C) Tritium, (tritium units) Tritium uncertainty (tritium units) Terrigenic helium (percent of total helium)

3H/3Hetri

age 

(years)

3H/3Hetri

age 

uncertainty 

(years)

14C 

(pmc)

14C 

counting 

uncertainty

(pmc)

14C 

age 

(uncorrected) 

(years)

14C age 

uncertainty 

(uncorrected) 

(years)

Age 

classification

Understanding Wells

CE-QPCFP-01 1.74 0.13 Modern/

Mixed

MERFP-01 14.2 <0.1 0.91 0.31 57.5 >50 na 80.2 0.3 1,700 71 Pre-modern

MERFP-02 21.0 <0.1

— 0.31 35.6 >50 na 3.2 0.1 28,400 369 Pre-modern

MERMW-01 19.7 1.0 4.70 0.41 0.0 10.0 0.7 116.5 0.6 <1,000 na Modern

MERMW-02 19.4 <0.1 8.03 0.50 0.0 33.0 1.1 128.7 0.6 <1,000 na ModernMERMW-03 22.8 <0.1 9.72 0.60 0.0 37.7 1.1 111.4 0.5 <1,000 na ModernMERMW-04 20.4 0.3 4.33 0.41 18.0 32.7 1.8 92.0 0.5 <1,000 na MixedMERMW-05 24.5 <0.1 2.10 0.31 0.0 4.2 0.6 94.5 0.4 <1,000 na ModernMODFP-01 14.1 4.3

— 0.19 99.4 >50 na 0.9 0.1 39,000 1,042 Pre-modern

MODFP-02 16.3 0.1 2.92 0.31 95.6 44.3 3.3 72.7 0.3 2,600 7 Mixed

MODFP-03 21.7 0.2 3.20 0.31 5.9 >50 na 88.6 0.4 <1,000 na Mixed

MODFP-04 17.8 0.1 10.94 0.60 0.0 35.5 1.0 119.2 0.6 <1,000 na ModernMODMW-01 21.8 <0.1 9.34 0.60 0.0 15.2 0.8 122.1 0.6 <1,000 na ModernTRLKFP-01 18.8 <0.1 4.61 0.41 0.0 6.1 0.5 107.5 0.4 <1,000 na ModernTRLKFP-02 18.8 0.1 3.82 0.31 13.9 19.5 2.1 105.8 0.4 <1,000 na MixedTRLKMW-01 17.8 <0.1 0.91 0.19 86.2 >50 na 67.2 0.3 3,200 49 Pre-modern

TRLKMW-02 17.9 <0.1 9.44 0.60 0.0 29.7 1.1 114.5 0.5 <1,000 na Modern

TRLKMW-03 18.8 <0.1 6.11 0.41 0.0 33.4 1.2 103.0 0.5 <1,000 na ModernTRLKMW-04 18.8 <0.1 5.61 0.41 0.0 26.8 1.1 99.7 0.4 <1,000 na ModernTRLKMW-05 21.1 <0.1 7.62 0.50 0.0 12.5 0.7 113.7 0.6 <1,000 na Modern
Appendix A. Attributes of USGS–GAMA and CDPH Grid Wells 91

Table A3. Concentrations of constituents used to classify oxidation-reduction conditions in groundwater, nitrogen and oxygen isotopic 

values of nitrate, and excess nitrogen gas concentrations, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) 

study unit.—Continued

[Redox, oxidation-reduction; mg/L, milligram per liter; μg/L, microgram per liter; oxic, dissolved oxygen > 0.5; anoxic, dissolved oxygen < 0.5 but no other 

redox indicators available; suboxic, dissolved oxygen < 0.5 but redox status cannot be further discerned because multiple indicators are below thresholds; 

NO3-red, nitrate reducing; Mn-red, manganese reducing; Fe-red, iron reducing; >, greater than; <, less than; Mix – Y/Z, mixture of waters with different redox 

ranging from most oxidized (Y) to most reduced (Z); —, no detection (values reported as zero) in CDPH database, method detection limit variable; blank 

values, no data; CDPH, California Department of Public Health;USGS, U.S. Geological Survey]

USGS 

GAMA well 

identification 

number

CDPH 

GAMA well 

identification 

number

Redox constituents

Redox 

classification

δ15N 

of nitrate 

(per mil)

δ18O 

of nitrate 

(per mil)

Excess 

N2

(mg/L)

Dissolved

oxygen 1

Nitrate 

plus nitrite 

nitrogen 2

Manganese 2 Iron 2 Sulfate 2,3

Redox threshold value 4 >0.5 >0.5 >50 >100 4

Units mg/L mg/L μg/L μg/L mg/L

Grid wells

CE-QPC-01 none 7.2 4.4 0.3 8 7.6 oxic 6.6 3.4 <0.4

CE-QPC-02 CE-QPCDG-024.7 4.1 — — 8.0 oxic

CE-QPC-03 none no data

CE-QPC-04 none 6.0 oxic

CE-QPC-05 none 8.0 oxic

CE-QPC-06 CE-QPCDG-068.0 oxic

CE-QPC-07 none 3.0 3.3 <0.2 <6 16.0 oxic 4.6 3.7 1.4

CE-QPC-08 none 4.5 oxic

CE-QPC-09 none 7.2 2.1 0.2 E5 10.5 oxic 4.3 3.9 <0.4

MER-01 none 0.3 anoxic

MER-02 none 4.4 5.3 2.2 <6 21.7 oxic 5.0 2.1 2.7

MER-03 none 4.7 3.4 <0.2 <6 12.2 oxic 5.7 3.5 0.6

MER-04 MER-DG-04 1.0 oxic

MER-05 MER-DG-05 0.2 — — <100 10.7 suboxic

MER-06 MER-DG-06 2.6 2.8 — <100 19.7 oxic

MER-07 none 5.7 oxic

MER-08 MER-DG-08 <0.2 anoxic

MER-09 none 5.8 5.9 <0.2 <6 10.6 oxic 4.7 3.7 0.4

MER-10 none 3.3 2.7 <0.2 <6 11.6 oxic 6.6 4.2 <0.4

MER-11 none 1.5 0.8 26 17 19.0 oxic 5.8 6.2 0.5

MER-12 none 0.5 0.8 8 8 70.6 oxic 9.2 7.6 <0.4

MER-13 MER-DG-13 3.1 2.8 — < 100 17.7 oxic

MER-14 none 1.1 oxic

MER-15 none 7.7 oxic

MER-16 none 5.1 oxic

MER-17 none 3.2 oxic

MER-18 none 5.3 oxic

MER-19 none 2.6 oxic

MER-20 none <0.2 109 122 79.6 Fe-red 33.3 22.3 8.5

MER-21 none 0.2 0.4 46 20 12.9 suboxic 7.8 4.7 1.3

MER-22 none 1.8 oxic

MER-23 none <0.2 anoxic

Table A3. Concentrations of constituents used to classify oxidation-reduction conditions in groundwater, nitrogen and oxygen isotopic 

values of nitrate, and excess nitrogen gas concentrations, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) 

study unit.

[Redox, oxidation-reduction; mg/L, milligram per liter; μg/L, microgram per liter; oxic, dissolved oxygen > 0.5; anoxic, dissolved oxygen < 0.5 but no other 

redox indicators available; suboxic, dissolved oxygen < 0.5 but redox status cannot be further discerned because multiple indicators are below thresholds; NO3-

red, nitrate reducing; Mn-red, manganese reducing; Fe-red, iron reducing; >, greater than; <, less than; Mix – Y/Z, mixture of waters with different redox ranging 

from most oxidized (Y) to most reduced (Z); —, no detection (values reported as zero) in CDPH database, method detection limit variable; blank values, no data; 

CDPH, California Department of Public Health; USGS, U.S. Geological Survey]
92 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Table A3. Concentrations of constituents used to classify oxidation-reduction conditions in groundwater, nitrogen and oxygen isotopic 

values of nitrate, and excess nitrogen gas concentrations, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) 

study unit.—Continued

[Redox, oxidation-reduction; mg/L, milligram per liter; μg/L, microgram per liter; oxic, dissolved oxygen > 0.5; anoxic, dissolved oxygen < 0.5 but no other 

redox indicators available; suboxic, dissolved oxygen < 0.5 but redox status cannot be further discerned because multiple indicators are below thresholds; 

NO3-red, nitrate reducing; Mn-red, manganese reducing; Fe-red, iron reducing; >, greater than; <, less than; Mix – Y/Z, mixture of waters with different redox 

ranging from most oxidized (Y) to most reduced (Z); —, no detection (values reported as zero) in CDPH database, method detection limit variable; blank 

values, no data; CDPH, California Department of Public Health;USGS, U.S. Geological Survey]

USGS 

GAMA well 

identification 

number

CDPH 

GAMA well 

identification 

number

Redox constituents

Redox 

classification

δ15N 

of nitrate 

(per mil)

δ18O 

of nitrate 

(per mil)

Excess 

N2

(mg/L)

Dissolved

oxygen 1

Nitrate 

plus nitrite 

nitrogen 2

Manganese 2 Iron 2 Sulfate 2,3

Redox threshold value 4 >0.5 >0.5 >50 >100 4

Units mg/L mg/L μg/L μg/L mg/L

MOD-01 none 6.7 4.8 <0.2 <6 12.3 oxic 7.6 3.7 1.6

MOD-02 none 6.5 6.3 <0.2 <6 23.8 oxic 6.2 1.7 1.6

MOD-03 MOD-DG-03 8.5 oxic

MOD-04 MOD-DG-04 3.8 3.8 — — 8.0 oxic 7.2 4.6

MOD-05 MOD-DG-05 3.2 3.8 — — 21.0 oxic

MOD-06 MOD-DG-06 4.8 3.2 — — 14.0 oxic

MOD-07 MOD-DG-07 3.4 oxic

MOD-08 MOD-DG-08 5.6 4.3 — — 9.0 oxic

MOD-09 none 5.6 2.9 <0.2 <6 5.0 oxic

MOD-10 none 5.7 oxic

TRLK-01 none 3.5 1.4 <0.2 <6 3.7 oxic 5.3 1.5 2.9

TRLK-02 none 3.9 3.0 0.2 <6 6.1 oxic 6.8 3.7 1.5

TRLK-03 none 2.4 1.4 9.4 101 2.2 oxic 4.9 3.5

TRLK-04 TRLK-DG-04 3.1 oxic

TRLK-05 none 3.1 2.7 <0.2 <6 5.1 oxic 6.7 4.7 <0.4

TRLK-06 none 3.8 8.1 — — 27.0 oxic

TRLK-07 none 5.6 oxic

TRLK-08 none 5.2 oxic

TRLK-09 none 3.5 oxic

TRLK-10 none 3.8 5.6 <0.2 <6 170.0 oxic 4.4 1.1 1.3

TRLK-11 none 4.3 2.1 <0.2 <6 5.9 oxic 6.6 2.0 0.9

TRLK-12 none 0.8 oxic

TRLK-13 none 0.2 anoxic

TRLK-14 TRLK-DG-14 0.7 0.8 — 70 1.4 oxic

TRLK-15 TRLK-DG-15 0.3 anoxic

TRLK-16 none 2.0 oxic

none CE-QPCDPH-042.1 — <20 3.2 oxic to 

NO3-red

none MER-DPH-01 2.8 — <100 23.3 oxic to 

NO3-red

none MER-DPH-07 2.0 — — 15.0 oxic to 

NO3-red
Appendix A. Attributes of USGS–GAMA and CDPH Grid Wells 93

Table A3. Concentrations of constituents used to classify oxidation-reduction conditions in groundwater, nitrogen and oxygen isotopic 

values of nitrate, and excess nitrogen gas concentrations, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) 

study unit.—Continued

[Redox, oxidation-reduction; mg/L, milligram per liter; μg/L, microgram per liter; oxic, dissolved oxygen > 0.5; anoxic, dissolved oxygen < 0.5 but no other 

redox indicators available; suboxic, dissolved oxygen < 0.5 but redox status cannot be further discerned because multiple indicators are below thresholds; 

NO3-red, nitrate reducing; Mn-red, manganese reducing; Fe-red, iron reducing; >, greater than; <, less than; Mix – Y/Z, mixture of waters with different redox 

ranging from most oxidized (Y) to most reduced (Z); —, no detection (values reported as zero) in CDPH database, method detection limit variable; blank 

values, no data; CDPH, California Department of Public Health;USGS, U.S. Geological Survey]

USGS 

GAMA well 

identification 

number

CDPH 

GAMA well 

identification 

number

Redox constituents

Redox 

classification

δ15N 

of nitrate 

(per mil)

δ18O 

of nitrate 

(per mil)

Excess 

N2

(mg/L)

Dissolved

oxygen 1

Nitrate 

plus nitrite 

nitrogen 2

Manganese 2 Iron 2 Sulfate 2,3

Redox threshold value 4 >0.5 >0.5 >50 >100 4

Units mg/L mg/L μg/L μg/L mg/L

none MER-DPH-14 1.1 — — 9.0 oxic to 

NO3-red

none MER-DPH-15 0.6 — <100 2.6 oxic to 

NO3-red

none MER-DPH-16 4.1 40 — 12.0 oxic to 

NO3-red

none MER-DPH-19 2.7 — — 11.0 oxic to 

NO3-red

none MOD-DPH-03 7.9 — — 28.0 oxic to 

NO3-red

none MOD-DPH-07 3.6 — — 19.0 oxic to 

NO3-red

none MOD-DPH-10 3.9 — <100 5.8 oxic to 

NO3-red

none TRLK-DPH-04 — 30 <20 3.0 oxic to 

suboxic

none TRLK-DPH-08 — — <100 5.3 oxic to 

suboxic

none TRLK-DPH-13 15.5 91 <100 48.3 oxic to 

Mn-red

none TRLK-DPH-15 0.7 10 <20 3.5 oxic to 

NO3-red
94 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Table A3. Concentrations of constituents used to classify oxidation-reduction conditions in groundwater, nitrogen and oxygen isotopic 

values of nitrate, and excess nitrogen gas concentrations, Central Eastside Groundwater Ambient Monitoring and Assessment (GAMA) 

study unit.—Continued

[Redox, oxidation-reduction; mg/L, milligram per liter; μg/L, microgram per liter; oxic, dissolved oxygen > 0.5; anoxic, dissolved oxygen < 0.5 but no other 

redox indicators available; suboxic, dissolved oxygen < 0.5 but redox status cannot be further discerned because multiple indicators are below thresholds; 

NO3-red, nitrate reducing; Mn-red, manganese reducing; Fe-red, iron reducing; >, greater than; <, less than; Mix – Y/Z, mixture of waters with different redox 

ranging from most oxidized (Y) to most reduced (Z); —, no detection (values reported as zero) in CDPH database, method detection limit variable; blank 

values, no data; CDPH, California Department of Public Health;USGS, U.S. Geological Survey]

USGS 

GAMA well 

identification 

number

CDPH 

GAMA well 

identification 

number

Redox constituents

Redox 

classification

δ15N 

of nitrate 

(per mil)

δ18O 

of nitrate 

(per mil)

Excess 

N2

(mg/L)

Dissolved

oxygen 1

Nitrate 

plus nitrite 

nitrogen 2

Manganese 2 Iron 2 Sulfate 2,3

Redox threshold value 4 >0.5 >0.5 >50 >100 4

Units mg/L mg/L μg/L μg/L mg/L

USGS-understanding wells

CE-QPC-FP01 none 3.6 oxic

MERFP-01 none <0.2 <0.06 443 189 108.0 Fe-red 1.1

MERFP-02 none 0.5 <0.06 84 20 74.3 Mix oxic/

Mn-red

11.3

MERMW-01 none 7.4 19.0 1.2 <6 32.1 oxic 3.6 3.8 <0.4

MERMW-02 none 5.6 17.1 <0.2 <6 30.9 oxic 7.6 0.1 0.4

MERMW-03 none 1.8 5.4 0.6 <6 24.8 oxic 9.8 5.7 1.3

MERMW-04 none 2.6 2.4 <0.2 <6 9.6 oxic 6.7 3.0 1.0

MERMW-05 none 8.0 7.5 2.9 E4 19.2 oxic 3.2 5.3 1.7

MODFP-01 none <0.2 <0.06 3940 1870 <0.18 Methanogenic9.3

MODFP-02 none 0.3 4.4 216 13 46.1 Mix NO3-

red/Mn-red

9.3 11.2 4.7

MODFP-03 none 4.9 13.2 E0.1 E5 28.1 oxic 7.4 0.5 0.5

MODFP-04 none 5.6 4.9 0.5 E3 10.9 oxic

MODMW-01 none 7.2 12.4 E0.1 <6 53.0 oxic 9.1 2.3 1.4

TRLKFP-01 none 0.3 49.9 1130 <6 84.9 Mix NO3-

red/Mn-red

15.7 4.7 6.0

TRLKFP-02 none 0.3 21.2 22 <6 32.4 NO3-red 11.3 8.9 1.8

TRLKMW-01 none 0.2 5.0 9.4 <6 12.4 NO3-red 11.4 10.3 2.2

TRLKMW-02 none 1.4 13.6 10 <6 47.8 oxic 8.6 10.3 1.6

TRLKMW-03 none 5.7 14.8 E0.2 <6 35.7 oxic 5.7 2.9 1.0

TRLKMW-04 none 6.0 14.7 E0.1 <6 27.8 oxic 7.2 2.0 1.3

TRLKMW-05 none 7.2 39.2 E0.1 <6 161.0 oxic 3.6 2.7 0.4 1

 Dissolved oxygen values measured by USGS.

2

 Values for wells with CDPH GAMA identification are from CDPH database. Values for wells with no CDPH GAMA identification were measured in 

samples collected by USGS for GAMA. 3

 Hydrogen sulfide odor, an indicator of sulfate-reducing conditions, was not detected in any of wells sampled by USGS for GAMA.

4

 Redox classification and thresholds based on McMahon and Chapelle (2008). 
Appendix B. Comparison of CDPH and USGS–GAMA Data 95

Appendix B. Comparison of CDPH 

and USGS–GAMA Data

Comparisons of CDPH and GAMA data were done to 

assess the validity of using data from these different sources in 

combination. Because LRLs for most organic constituents and 

trace elements were substantially lower for USGS GAMAcollected data than the MDLs used for CDPH data (table 2), 

it generally was not possible to meaningfully compare 

concentrations of these constituent types in individual wells. 

However, concentrations of major ions and nitrate, which 

generally are prevalent at concentrations substantially above 

LRLs, were compared for each well having data from both 

sources. Thirteen wells had some major-ion and nitrate 

data in common between the data sets. Wilcoxon signed 

rank tests of paired analyses for eight different constituents 

(calcium, magnesium, sodium, alkalinity, chloride, sulfate, 

TDS, nitrate-N) having concentrations above the LRL in 

both databases indicated no significant differences between 

USGS–GAMA and CDPH data for these constituents. Because 

of concerns that the small number of wells prevented a 

statistically robust analysis for each individual constituent, the 

data for these eight constituents also were combined into one 

data set so that there was a large enough data set (97 pairs) for 

meaningful statistical comparison. A nonparametric signed 

rank test indicated no significant differences between the 

paired USGS–GAMA and CDPH data (z = 0.74, p = 0.410). 

While differences between the paired data sets occurred for 

a few wells, most sample pairs plotted close to a 1 to 1 line 

(fig. B1). The relative percent difference (RPD) was calculated 

for each data pair. The median RPD was 6.1 percent; 75 

percent of the RPD values were <20 percent. These direct 

comparisons indicated that the GAMA and CDPH inorganic 

data were not significantly different. 

Combined GAMA and CDPH major ion data for grid 

wells were plotted on piper diagrams (Piper, 1944) with 

all CDPH major ion data to determine whether the grid 

wells represented the range of groundwater types that have 

historically been observed in the study unit. Piper diagrams 

show the relative abundance of major cations and anions (on 

a charge equivalent basis) as a percentage of the total ion 

content of the water (fig. B2). Piper diagrams often are used to 

define groundwater type (Hem, 1985). All CDPH data having 

cation/anion data and a cation/anion balance of <10 percent 

were retrieved and plotted on these piper diagrams for 

comparison with grid well data.

0.1

1

10

100

1,000

0.1 1 10 100 1,000

CONCENTRATION, CDPH DATA, IN MILLIGRAMS PER LITER

CONCENTRATION, USGS-GAMA DATA, IN MILLIGRAMS PER LITER

Calcium

Magnesium

Sodium

Alkalinity

Chloride

Sulfate

TDS

Nitrate nitrogen

1:1 line

Figure B1. Paired major-ion and nitrate concentrations from wells sampled by the Groundwater Ambient Monitoring and Assessment 

(GAMA) program in March–June 2006 and the most recent available analysis in the California Department of Health Services, for the 

Central Eastside, California, study unit.
96 Status and Understanding of Groundwater Quality, Central–Eastside San Joaquin Basin, 2006: GAMA Priority Basin Project

Figure B2. Piper diagram for U.S. Geological Survey (USGS) grid and understanding wells and all wells in the California Department of 

Public Health (CDPH) database that have a charge imbalance of less than 10 percent, Central Eastside, California, study unit.

CDPH well (most recent analysis with charge imbalance 

 less than 10 percent)

USGS understanding well

USGS grid well

EXPLANATION

PERCENT

CALCIUM

100 

80 

60 

40 

20 

0 

0 

20 

40 

60 

80 

100 

MAGNESIUM

0 

20 

40 

60 

80 

100 

SODIUM PLUS POTASSIUM

CHLORIDE, FLUORIDE, NITRITE PLUS NITRATE

0 

20 

40 

60 

80 

100 

100 

80 

60 

40 

20 

0 

 CARBONATE PLUS BICARBONATE

100 

80 

60 

40 

20 

0 

SULFATE

0 

20 

40 

60 

80 

100 

SULFATE PLUS CHLORIDE

0 

20 

40 

60 

80 

100 

CALCIUM PLUS MAGNESIUM

100 

80 

60 

40 

20 

0 

100 

80 

60 

40 

20 

0 

PERCENT

PERCENT
Appendix B. Comparison of CDPH and USGS–GAMA Data 97

A similar range of water types were evident from grid 

wells and historical CDPH data (fig. B2). In most wells, no 

single cation accounted for more than 60 percent of the total 

cations, and bicarbonate accounted for more than 60 percent 

of the total anions; these samples are described as mixed 

cation-bicarbonate type waters. There also were many wells 

that were mixed cation-mixed anion type waters, indicating 

that no single cation and no single anion accounted for more 

than 60 percent of the total. A minority of wells are classified 

as sodium-chloride type waters, indicating that sodium and 

chloride accounted for more than 60 percent of the total 

cations and anions, respectively. 

The similarity of the range of relative abundance of major 

cations and anions in grid wells to the set of all CDPH wells 

indicates that the grid wells represent most of the diversity 

of water types present within the Central Eastside study unit. 

Two minor differences between grid data and CDPH data were 

evident, as described in the paragraphs below.

First, a minority of CDPH wells (14 of 222, 6.3 percent) 

had chloride as a dominant anion (lower right of anion 

diagram, fig. B2). One of 44 grid wells (2.3 percent) had 

this water type; the upper limit of the 90-percent confidence 

interval of the grid-based estimate was 7.3 percent. The 

CDPH wells having ≥60 percent chloride, nitrate, and 

fluoride (chloride is dominant) are located in parts of three 

cells located in the northern Turlock and southern Modesto 

study areas, along the Tuolumne River and within the 

Modesto urban area. The cumulative area of this cluster of 

chloride-type waters is less than that of one grid cell. The 

one grid well having ≥ 60 percent chloride was within this 

chloride-type cluster. However, the grid wells in the other two 

cells overlapping with the chloride-type cluster did not fall 

within the area with these waters. Considering the localized 

distribution of these chloride-type waters within parts of 

three cells, it is not surprising that only one grid well had 

chloride-type water. 

Second, there were two GAMA grid wells having higher 

sulfate percentages than any CDPH wells (fig. B2). These 

GAMA grid wells were located in the southeastern Merced 

and western Turlock study areas. There were no CDPH wells 

within the cell or the part of the cell where these wells resided. 

Thus, it is likely that these two relatively high sulfate grid well 

samples represent water types not represented in the CDPH 

database due to the nonuniform distribution of CDPH wells 

across the study unit.
Manuscript approved for publication, December 9, 2009

Prepared by the USGS Enterprise Publishing Network,

Publishing Service Center, Sacramento, California

For more information concerning the research in this report, contact the

California Water Science Center Director,

U.S. Geological Survey, 6000 J Street

Sacramento, California 95819

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

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Landon and others-Status and Understanding of Groundwater Quality in the Central-Eastside San Joaquin Basin,

2006: California GAMA Priority Basin Project-SIR 2009-5266

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