U.S. Department of the Interior

U.S. Geological Survey

Data Series 706

Prepared in cooperation with the California State Water Resources Control Board

Groundwater-Quality Data in the Western San Joaquin Valley 

Study Unit, 2010: Results from the California GAMA Program

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

FRESNO COUNTY

MERCED COUNTY

MADERA COUNTY

MONTEREY COUNTY

SAN BENITO

COUNTY

SANTA CLARA

COUNTY

MARIPOSA COUNTY

KINGS

COUNTY

STANISLAUS COUNTY

Fresno

Modesto

Mendota

Gustine

Patterson

Los Banos

Kettleman City

5

5

99

101

101

41

33

152

180

198

C o a s t R a n g e s

Tuolumne

Merced

Joaquin

River

River

River

Salinas

River

San 

Kings

River

Westside

subbasin

Delta-Mendota

subbasin
Cover photographs:

Top right: Irrigation well, Gustine, California. (Photograph taken by Gregory Brewster, U.S. Geological Survey).

Bottom right: Alfalfa field, Five Points, California. (Photograph taken by Tyler Johnson, U.S. Geological Survey).
Groundwater-Quality Data in the Western 

San Joaquin Valley Study Unit, 2010: Results 

from the California GAMA Program

By Timothy M. Mathany, Matthew K. Landon, Jennifer L. Shelton, and Kenneth Belitz

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

Prepared in cooperation with the California State Water Resources Control Board 

Data Series 706

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: 2013

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Mathany, T.M., Landon, M.K., Shelton, J.L., and Belitz, Kenneth, 2013, Groundwater-quality data in the Western 

San Joaquin Valley study unit, 2010—Results from the California GAMA Program: U.S. Geological Survey Data 

Series 706, 102 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................5

Hydrogeologic Setting ..................................................................................................................................5

Western San Joaquin Valley Study Unit—Delta-Mendota Subbasin Study Area.....................7

Western San Joaquin Valley Study Unit—Westside Subbasin Study Area...............................7

Methods ..........................................................................................................................................................9

Study Design..........................................................................................................................................9

Sample Collection and Analysis.......................................................................................................12

Data Reporting.....................................................................................................................................12

Quality-Assurance Methods.............................................................................................................12

Quality-Control Results ......................................................................................................................12

Comparison Benchmarks ..................................................................................................................13

Water-Quality Results ................................................................................................................................14

Groundwater-Quality Data ................................................................................................................14

Water-Quality Indicators .........................................................................................................14

Organic Constituents.................................................................................................................15

Constituents of Special Interest..............................................................................................15

Inorganic Constituents..............................................................................................................15

Isotopic Tracers and Dissolved Gases...................................................................................17

Future Work ..................................................................................................................................................17

Summary .......................................................................................................................................................17

Acknowledgments.......................................................................................................................................19

References Cited..........................................................................................................................................19

Appendix .......................................................................................................................................................78
iv

Tables

Table 1. Identification, sampling, and construction information for wells sampled 

for the Western San Joaquin Valley Groundwater Ambient Monitoring and 

Assessment study unit, California, March to July 2010 ................................. 25

Table 2. Classes of chemical constituents and water-quality indicators collected for the 

slow and the intermediate well-sampling schedules in the Western San Joaquin 

Valley Groundwater Ambient Monitoring and Assessment study unit, California, 

March to July 2010 ........................................................................ 27

Table 3A. Volatile organic compounds, primary uses or sources, comparative 

benchmarks, and reporting information for the U.S. Geological Survey 

National Water Quality Laboratory Schedule 2020 .................................... 28

Table 3B. Low-level fumigants, primary uses or sources, comparative benchmarks, and 

reporting information for the U.S. Geological Survey National Water Quality 

Laboratory Schedule 1306 .................................................................. 32

Table 3C. Pesticides and pesticide degradates, primary uses or sources, comparative 

benchmarks, and reporting information for the U.S. Geological Survey 

National Water Quality Laboratory Schedule 2033 .................................... 33

Table 3D. Constituents of special interest, primary uses or sources, comparative 

benchmarks, and reporting information for Weck Laboratories, Inc., City of 

Industry, California, analyses ............................................................ 36

Table 3E. Trace elements, comparative benchmarks, and reporting information for the 

U.S. Geological Survey National Water Quality Laboratory Schedule 1948 ......... 37

Table 3F. Nutrients and dissolved organic carbon, comparative benchmarks, and 

reporting information for the U.S. Geological Survey National Water Quality 

Laboratory Schedule 2755 and Laboratory Code 2613 ................................. 38

Table 3G. Major and minor ions, silica, total dissolved solids, and alkalinity, comparative 

benchmarks, and reporting information for the U.S. Geological Survey 

National Water Quality Laboratory Schedule 1948 .................................... 39

Figures

Figure 1. Map showing hydrogeologic provinces of California and the location of 

the Western San Joaquin Valley Groundwater Ambient Monitoring and 

Assessment study unit ..................................................................... 4

Figure 2. Map showing boundaries of the Delta-Mendota subbasin and the Westside 

subbasin study areas, geologic formations, major cities, major roads, and 

hydrologic features ........................................................................ 6

Figure 3. Map showing the Delta-Mendota subbasin and the Westside subbasin study 

areas, and the boundaries of the California Department of Water Resources 

defined groundwater subbasins ......................................................... 8

Figure 4. Map showing the Delta-Mendota subbasin and the Westside subbasin study 

areas, the distribution of the grid cells, and the location of California Department 

of Public Health wells ..................................................................... 10

Figure 5. Map showing the Delta-Mendota subbasin and the Westside subbasin study 

areas, the distribution of the grid cells, and the location of sampled grid and 

understanding wells ........................................................................ 11
v

Table 3H. Total arsenic and iron (unfiltered) and arsenic, chromium, and iron species 

(filtered), comparative benchmarks, and reporting information for the 

U.S. Geological Survey National Research Laboratory Trace Metal Laboratory, 

Boulder, Colorado, analyses ............................................................... 39

Table 3I. Isotopic tracers, comparison benchmarks, and reporting information for 

laboratory analyses ........................................................................ 40

Table 4. Water-quality indicators in samples collected for the Western San Joaquin 

Valley Groundwater Ambient Monitoring and Assessment study unit, 

California, March to June 2010 ............................................................ 41

Table 5. Volatile organic compounds and low-level fumigants detected in samples 

collected for the Western San Joaquin Valley Groundwater Ambient 

Monitoring and Assessment study unit, California, March to June 2010 ............ 46

Table 6. Pesticides and pesticide degradates detected in samples collected for 

the Western San Joaquin Valley Groundwater Ambient Monitoring and 

Assessment study unit, California, March to July 2010 ................................. 51

Table 7. Constituents of special interest detected in the samples collected for 

the Western San Joaquin Valley Groundwater Ambient Monitoring and 

Assessment study unit, California, March to July 2010 ................................. 55

Table 8. Trace elements collected for the Western San Joaquin Valley Groundwater 

Ambient Monitoring and Assessment study unit, California, 

March to July 2010 ........................................................................ 56

Table 9. Nutrients and dissolved organic carbon detected in samples collected for 

the Western San Joaquin Valley Groundwater Ambient Monitoring and 

Assessment study unit, California, March to July 2010 ................................. 63

Table 10. Major and minor ions, silica, and total dissolved solids detected in samples 

collected for the Western San Joaquin Valley Groundwater Ambient 

Monitoring and Assessment study unit, California, March to July 2010 ............ 66

Table 11. Total arsenic and iron (unfiltered) and arsenic, chromium, and iron species 

(filtered) detected in samples collected for the Western San Joaquin Valley 

Groundwater Ambient Monitoring and Assessment study unit, California, 

March to June 2010 ........................................................................ 69

Table 12. Isotopic tracers detected in samples collected for the Western San Joaquin 

Valley Groundwater Ambient Monitoring and Assessment study unit, California, 

March to June 2010 ........................................................................ 72

Table 13. Results for analyses of dissolved standard gases detected in samples collected 

for the Western San Joaquin Valley Groundwater Ambient Monitoring and 

Assessment study unit, California, March to July 2010 ................................. 76

Tables—Continued
vi

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Volume

quart (qt) 0.9464 liter (L)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg)

SI to Inch/Pound

Multiply By To obtain

Length

micrometer (μm) 3.3 x 10–6 foot (ft)

centimeter (cm) 0.3937 inch (in.)

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2) 0.3861 square mile (mi2)

Volume

liter (L) 1.057 quart (qt)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound, avoirdupois (lb)

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).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Land-surface altitude, as used in this report, refers to distance above the vertical datum and is 

reported as feet above mean sea level (ft above msl).

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). 

Activities of radioactive constituents in water (except uranium) are given in picocuries per liter 

(pCi/L). 
vii

Selected Terms and Symbols

α confidence level

cm3 STP/g cubic centimeters of gas at standard temperature and pressure (0 degrees 

Celsius and 1 atmosphere of pressure) per gram of water

δ iE delta notation, the ratio of a heavier isotope of an element (iE) to the more 

common lighter isotope of that element, relative to a standard reference 

material, expressed as per mil

= equal to

> greater than

≥ greater than or equal to

< less than

≤ less than or equal to

N Normal (1-gram-equivalent per liter of solution)

— not detected

p significant level

pH a measure of the acidity or basicity of a solution

pK1,2 negative logarithms of the acid dissociation constants for carbonic acid

pKw negative logarithm of the acid dissociation constant of water

R result

z test statistic

± plus or minus

* value is at a concentration greater than benchmark level

** value is at a concentration greater than upper benchmark level

Abbreviations and Acronyms

AL action level

AL-US action level (USEPA)

APE Alternate Place Entry program designed for USGS sampling

CAS Chemical Abstract Service (American Chemical Society)

CASRN Chemical Abstract Service (American Chemical Society) Registry Number®

CSU combined standard uncertainty

DM Delta-Mendota subbasin study area

E estimated or having a higher degree of uncertainty

GAMA Groundwater Ambient Monitoring and Assessment Program

GPS Global Positioning System

HAL lifetime health advisory level

HAL-US lifetime health advisory level (USEPA)

HPLC high-performance liquid chromatography

IBSP Inorganic Blind Sample Project (USGS)

LRL laboratory reporting level

LSD land surface datum

LT-MDL long-term method detection level

MCL maximum contaminant level

MCL-CA maximum contaminant level (CDPH)
viii

MCL-US maximum contaminant level (USEPA)

MDL method detection limit

MRL minimum reporting level

MU method uncertainty

na not available

nc not collected

NELAP National Environmental Laboratory Accreditation Program

NFM National Field Manual (USGS)

NFQA National Field Quality Assurance Program (USGS)

NL notification level

NL-CA notification level (CDPH)

np no preference

nv no measured value or no value in category

NWIS National Water Information System (USGS)

PBP Priority Basin Project

PCFF Personal Computer Field Form program designed for USGS sampling

QA quality assurance

QC quality control

RL reporting level

RPD relative percent difference

RSD relative standard deviation

RSD5-US risk-specific dose at 10–5 (USEPA)

SD standard deviation

SMCL secondary maximum contaminant level

SMCL-CA secondary maximum contaminant level (CDPH)

SMCL-US secondary maximum contaminant level (USEPA)

SRL study reporting level (concentration cutoff for applying the ≤ symbol)

ssLC sample-specific critical level

U.S. United States

UV ultraviolet

VCDT Vienna Canon Diablo Troilite (the international reference standard for sulfur 

isotopes)

VPDB Vienna Pee Dee Belemnite (the international reference standard for carbon 

isotopes)

VSMOW Vienna Standard Mean Ocean Water (an isotopic water standard defined in 1968 

by the International Atomic Energy Agency)

WS Westside subbasin study area

WSJV Western San Joaquin Valley study unit

Organizations

BQS Branch of Quality Systems (USGS)

CDPH California Department of Public Health

CDPR California Department of Pesticide Regulation

CDWR California Department of Water Resources

LLNL Lawrence Livermore National Laboratory

NAWQA National Water-Quality Assessment Program (USGS)

Abbreviations and Acronyms—Continued
ix

NIST National Institute of Standards and Technology

NRP National Research Program (USGS)

NWQL National Water Quality Laboratory (USGS)

SITL Stable Isotope and Tritium Laboratory, Menlo Park, California (USGS)

SWRCB California State Water Resources Control Board

TML Trace Metal Laboratory, Boulder, Colorado (USGS)

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Weck Weck Laboratories Inc., City of Industry, California

Selected Chemical Names

CaCO3 calcium carbonate

CO3

2– carbonate

CFC chlorofluorocarbon

DBCP 1,2-dibromo-3-chloropropane

DOC dissolved organic carbon

EDB 1,2-dibromoethane

EPTC S-ethyl-dipropylthiocarbamate

H2O water

HCl hydrochloric acid

HCO3

– bicarbonate

MTBE methyl tert-butyl ether

Na2S2O3 sodium thiosulfate

NDMA N-nitrosodimethylamine

NO2

–+NO3– nitrite plus nitrate (as nitrogen)

PCE perchloroethene (tetrachloroethene)

SiO2 silicon dioxide

TCE trichloroethene

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

TDS total dissolved solids

THM trihalomethane

VOC volatile organic compound

Abbreviations and Acronyms—Continued
x
Abstract 

Groundwater quality in the approximately 2,170-squaremile Western San Joaquin Valley (WSJV) study unit was 

investigated by the U.S. Geological Survey (USGS) from 

March to July 2010, as part of the California State Water 

Resources Control Board (SWRCB) Groundwater Ambient 

Monitoring and Assessment (GAMA) Program’s Priority 

Basin Project (PBP). The GAMA-PBP was developed in 

response to the California Groundwater Quality Monitoring 

Act of 2001 and is being conducted in collaboration with 

the SWRCB and Lawrence Livermore National Laboratory 

(LLNL). The WSJV study unit was the twenty-ninth study 

unit to be sampled as part of the GAMA-PBP.

The GAMA Western San Joaquin Valley study was 

designed to provide a spatially unbiased assessment of 

untreated-groundwater quality in the primary aquifer 

system, and to facilitate statistically consistent comparisons 

of untreated groundwater quality throughout California. 

The primary aquifer system is defined as parts of aquifers 

corresponding to the perforation intervals of wells listed in 

the California Department of Public Health (CDPH) database 

for the WSJV study unit. Groundwater quality in the primary 

aquifer system may differ from the quality in the shallower 

or deeper water-bearing zones; shallow groundwater may be 

more vulnerable to surficial contamination.

In the WSJV study unit, groundwater samples were 

collected from 58 wells in 2 study areas (Delta-Mendota 

subbasin and Westside subbasin) in Stanislaus, Merced, 

Madera, Fresno, and Kings Counties. Thirty-nine of the wells 

were selected by using a spatially distributed, randomized 

grid-based method to provide statistical representation of the 

study unit (grid wells), and 19 wells were selected to aid in the 

understanding of aquifer-system flow and related groundwaterquality issues (understanding wells).

The groundwater samples were analyzed for 

organic constituents (volatile organic compounds 

[VOCs], low-level fumigants, and pesticides and 

pesticide degradates), constituents of special interest 

(perchlorate, N-nitrosodimethylamine [NDMA], and 

1,2,3-trichloropropane [1,2,3-TCP]), and naturally occurring 

inorganic constituents (trace elements, nutrients, dissolved 

organic carbon [DOC], major and minor ions, silica, total 

dissolved solids [TDS], alkalinity, total arsenic and iron 

[unfiltered] and arsenic, chromium, and iron species [filtered]). 

Isotopic tracers (stable isotopes of hydrogen, oxygen, and 

boron in water, stable isotopes of nitrogen and oxygen in 

dissolved nitrate, stable isotopes of sulfur in dissolved sulfate, 

isotopic ratios of strontium in water, stable isotopes of carbon 

in dissolved inorganic carbon, activities of tritium, and 

carbon-14 abundance), dissolved standard gases (methane, 

carbon dioxide, nitrogen, oxygen, and argon), and dissolved 

noble gases (argon, helium-4, krypton, neon, and xenon) 

were measured to help identify sources and ages of sampled 

groundwater. In total, 245 constituents and 8 water-quality 

indicators were measured.

Quality-control samples (blanks, replicates, or matrix 

spikes) were collected at 16 percent of the wells in the 

WSJV study unit, and the results for these samples were 

used to evaluate the quality of the data from the groundwater 

samples. Blanks rarely contained detectable concentrations of 

any constituent, suggesting that contamination from sample 

collection procedures was not a significant source of bias 

in the data for the groundwater samples. Replicate samples 

all were within acceptable limits of variability. Matrixspike recoveries were within the acceptable range (70 to 

130 percent) for approximately 87 percent of the compounds.

This study did not evaluate the quality of water delivered 

to consumers. After withdrawal, groundwater typically is 

treated, disinfected, and (or) blended with other waters to 

maintain water quality. Regulatory benchmarks apply to 

water that is delivered to the consumer, not to untreated 

groundwater. However, to provide some context for the 

results, concentrations of constituents measured in the 

untreated groundwater were compared with regulatory and 

non-regulatory health-based benchmarks established by the 

U.S. Environmental Protection Agency (USEPA) and CDPH, 

and to non-regulatory benchmarks established for aesthetic 

concerns by CDPH. Comparisons between data collected 

for this study and benchmarks for drinking water are for 

illustrative purposes only and are not indicative of compliance 

or non-compliance with those benchmarks.

Groundwater-Quality Data in the Western San Joaquin 

Valley Study Unit, 2010: Results from the California GAMA 

Program

By Timothy M. Mathany, Matthew K. Landon, Jennifer L. Shelton, and Kenneth Belitz
2 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Most inorganic constituents detected in groundwater 

samples from the 39 grid wells were detected at concentrations 

less than health-based benchmarks. Detections of organic and 

special-interest constituents from grid wells sampled in the 

WSJV study unit also were less than health-based benchmarks.

In total, VOCs were detected in 12 of the 39 grid wells 

sampled (approximately 31 percent), pesticides and pesticide 

degradates were detected in 9 grid wells (approximately 

23 percent), and perchlorate was detected in 15 grid wells 

(approximately 38 percent). 

Trace elements, major and minor ions, and nutrients 

were sampled for at 39 grid wells; most concentrations were 

less than health-based benchmarks. Exceptions include two 

detections of arsenic greater than the USEPA maximum 

contaminant level (MCL-US) of 10 micrograms per liter 

(μg/L), 20 detections of boron greater than the CDPH 

notification level (NL-CA) of 1,000 μg/L, 2 detections of 

molybdenum greater than the USEPA lifetime health advisory 

level (HAL-US) of 40 μg/L, 1 detection of selenium greater 

than the MCL-US of 50 μg/L, 2 detections of strontium 

greater than the HAL-US of 4,000 μg/L, and 3 detections of 

nitrate greater than the MCL-US of 10 μg/L. 

Results for inorganic constituents with non-healthbased benchmarks (iron, manganese, chloride, sulfate, and 

TDS) showed that iron concentrations greater than the 

CDPH secondary maximum contaminant level (SMCL-CA) 

of 300 μg/L were detected in five grid wells. Manganese 

concentrations greater than the SMCL-CA of 50 μg/L 

were detected in 16 grid wells. Chloride concentrations 

greater than the recommended SMCL-CA benchmark of 

250 milligrams per liter (mg/L) were detected in 14 grid 

wells, and concentrations in 5 of these wells also were greater 

than the upper SMCL-CA benchmark of 500 mg/L. Sulfate 

concentrations greater than the recommended SMCL-CA 

benchmark of 250 mg/L were measured in 21 grid wells, 

and concentrations in 13 of these wells also were greater 

than the SMCL-CA upper benchmark of 500 mg/L. TDS 

concentrations greater than the SMCL-CA recommended 

benchmark of 500 mg/L were measured in 36 grid wells, and 

concentrations in 20 of these wells also were greater than the 

SMCL-CA upper benchmark of 1,000 mg/L.

Introduction 

About one-half of the water used for public and 

domestic drinking-water supply in California is groundwater 

(Kenny and others, 2009). To assess the quality of ambient 

groundwater in aquifers used for public drinking-water supply 

and to establish a baseline groundwater-quality monitoring 

program, the California State Water Resources Control Board 

(SWRCB), in cooperation with the U.S. Geological Survey 

(USGS) and Lawrence Livermore National Laboratory 

(LLNL), implemented the Groundwater Ambient Monitoring 

and Assessment (GAMA) Program in 2000 (California 

Environmental Protection Agency, 2011, Web site at http://

www.waterboards.ca.gov/water_issues/programs/gama/). 

The main goals of the GAMA Program are to improve 

groundwater monitoring and to increase the availability of 

groundwater-quality data to the public. The GAMA Program 

currently consists of four projects: (1) the GAMA Priority 

Basin Project (PBP) conducted by the USGS (U.S. Geological 

Survey, 2011a, California Water Science Center Web site at 

http://ca.water.usgs.gov/gama/); (2) the GAMA Domestic 

Well Project conducted by the SWRCB; (3) GAMA Special 

Studies conducted by LLNL; and (4) GeoTracker GAMA, 

conducted by the SWRCB. The GAMA-PBP primarily 

focuses on the deep part of the groundwater resource, which 

is typically used for public drinking-water supply. The GAMA 

Domestic Well Project generally focuses on the shallow 

aquifer systems, which may be particularly at risk as a result 

of surficial contamination. The GAMA Special Studies Project 

focuses on using research methods to help explain the source, 

fate, transport, and occurrence of chemicals that can affect 

groundwater quality. GeoTracker GAMA is an online interface 

serving data from the GAMA Program and other efforts to the 

public (http://geotracker.waterboards.ca.gov/).

The GAMA Program was initiated by the SWRCB 

in 2000 and later expanded by the Groundwater Quality 

Monitoring Act of 2001 (State of California, 2001a, b; 

Sections 10780–10782.3 of the California Water Code, 

Assembly Bill 599). The GAMA-PBP assesses groundwater 

quality in key groundwater basins that account for more 

than 90 percent of all groundwater used for public supply in 

the State. For the GAMA-PBP, 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; 

California State Water Resources Control Board, 2003). 

Additional partners in the GAMA-PBP include LLNL, 

California Department of Public Health (CDPH), California 

Department of Water Resources (CDWR), California 

Department of Pesticide Regulation (CDPR), local water 

agencies, and well owners (Kulongoski and Belitz, 2004). 

Participation in the GAMA-PBP is entirely voluntary.

The GAMA-PBP is unique in California because it 

includes many chemical analyses that are not otherwise 

available in the statewide water-quality monitoring datasets. 

Groundwater samples collected for the GAMA-PBP are 

analyzed for approximately 300 chemical constituents using 

methods with lower detection limits than required by the 

CDPH for regulatory monitoring of water from drinking-water 

wells. These analyses will be especially useful for providing 

an early indication of changes in groundwater quality. In 

addition, the GAMA-PBP analyzes samples for a suite of 

constituents more extensive than required by CDPH and for 

a suite of chemical and isotopic tracers for understanding 

hydrologic and geochemical processes. This understanding of 

groundwater composition is useful for identifying the natural 

and human factors affecting water quality. Understanding 
Introduction 3

the occurrence and distribution of chemical constituents of 

significance to water quality is important for the long-term 

management and protection of groundwater resources.

The range of hydrologic, geologic, and climatic 

conditions in California must be considered in an assessment 

of groundwater quality. Belitz and others (2003) partitioned 

the State into 10 hydrogeologic provinces, each with 

distinctive hydrologic, geologic, and climatic characteristics: 

Cascades and Modoc Plateau, Klamath Mountains, Northern 

Coast Ranges, Central Valley, Sierra Nevada, Basin 

and Range, Southern Coast Ranges, Transverse Ranges 

and selected Peninsular Ranges, Desert, and San Diego 

Drainages (fig. 1). These 10 hydrogeologic provinces include 

groundwater basins and subbasins designated by the CDWR 

(California Department of Water Resources, 2003). The 

lithology of the groundwater basins and subbasins generally 

consist of relatively permeable, unconsolidated deposits of 

alluvial or volcanic origin. Eighty percent of California’s 

approximately 16,000 active and standby drinking-water 

wells listed in the statewide database maintained by the 

CDPH (hereinafter referred to as CDPH wells) are located in 

groundwater basins and subbasins within the 10 hydrogeologic 

provinces. Groundwater basins and subbasins were prioritized 

for sampling on the basis of the number of CDPH wells in 

the basin, with secondary consideration given to municipal 

groundwater use, agricultural pumping, the number of 

formerly leaking underground fuel tanks, and the number of 

registered pesticide applications (Belitz and others, 2003). 

Of the 472 basins and subbasins designated by the CDWR, 

116 priority basins were selected and grouped into 35 GAMA 

study units, representing approximately 95 percent of the 

CDPH wells in California. In addition, some areas outside of 

the defined groundwater basins were included in the adjacent 

study unit to achieve representation of the 20 percent of the 

CDPH wells not located in the groundwater basins.

The data collected in each study unit are used for 

three types of water-quality assessments: (1) Status—

assessment of the current quality of the groundwater 

resource; (2) Understanding—identification of the natural 

and human factors affecting groundwater quality; and 

(3) Trends—detection of changes in groundwater quality 

over time (Kulongoski and Belitz, 2004). The assessments 

are intended to characterize the quality of groundwater in the 

primary aquifer systems of the study units, not the treated 

drinking water delivered to consumers by water purveyors. 

The primary aquifer systems are defined as parts of aquifers 

corresponding to the perforation intervals of wells listed in 

the CDPH databases for the study units. The CDPH database 

lists wells used for municipal and community drinking-water 

supplies, and includes wells from systems classified as nontransient (such as those in cities, towns, and mobile-home 

parks) and transient (such as those in schools, campgrounds, 

and restaurants). Collectively, the CDPH refers to these wells 

as “public-supply” wells. Groundwater quality in the primary 

aquifers may differ from the quality in the shallow or deep 

parts of the aquifer system. In particular, shallow groundwater 

may be more vulnerable to surface contamination. As a result, 

samples from shallow wells (such as many private domestic 

wells and groundwater monitoring wells) can have greater 

concentrations of constituents (such as volatile organic 

compounds [VOCs] and nitrate) from anthropogenic sources 

than samples from wells screened in the underlying primary 

aquifer systems (Landon and others, 2010).

All published analytical data and quality-assurance/

quality-control (QA/QC) data collected for the GAMA 

Program are stored in the web-based Geotracker Database 

(California State Water Resources Control Board, 2009, 

website at https://geotracker.waterboards.ca.gov/gama/). The 

Geotracker Database also stores groundwater-quality data and 

related reports collected by other State agencies, such as the 

CDPH, CDWR, CDPR, and data collected by the SWRCB 

and Regional Boards from groundwater monitoring wells at 

contaminated or remediated sites.

The Western San Joaquin Valley study unit (hereinafter 

referred to as the WSJV study unit) lies within the Central 

Valley hydrogeologic province described by Belitz and others 

(2003) (fig. 1). Groundwater is a major source of public 

drinking-water supply for many of the communities located 

in the WSJV study unit (Water Education Foundation, 2006); 

therefore, the WSJV study unit was considered high priority 

for sampling to provide adequate representation of the Central 

Valley hydrogeologic province. 

This USGS Data Series Report is one of a series of 

reports presenting the water-quality data collected by the 

USGS in the WSJV study unit for the GAMA-PBP. Data 

Series Reports for each of the GAMA-PBP study units 

are available from the U.S. Geological Survey (2011b), at 

http://ca.water.usgs.gov/projects/gama/includes/GAMA_

publications.html.
4 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Figure 1. Hydrogeologic provinces of California and the location of the Western San Joaquin Valley (WSJV) Groundwater Ambient 

Monitoring and Assessment (GAMA) study unit.

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

San Diego

Drainages

Western

San Joaquin Valley

Study Unit

(figure 3)

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.

IP-027484_Figure 01

Westside

subbasin

Delta-Mendota

subbasin
Hydrogeologic Setting 5

Purpose and Scope

The purposes of this report are to describe (1) the 

hydrogeologic setting of the WSJV study unit, the study 

design, and the study methods; (2) the analytical results for 

groundwater samples collected in the WSJV study unit, and 

(3) the results of QC analysis. Groundwater samples were 

analyzed for water-quality indicators; organic, special-interest, 

and inorganic constituents; isotopic tracers; and dissolved 

gases. The chemical data presented in this report were 

evaluated by comparison to State and Federal drinking-water 

standards. The health-based and non-health based benchmarks 

considered for this report are those established by the 

U.S. Environmental Protection Agency (USEPA) and (or) the 

CDPH. Discussion of the factors that influence the distribution 

and occurrence of the constituents detected in groundwater 

samples will be the subject of subsequent publications.

Hydrogeologic Setting 

The WSJV study unit is located in the western part of the 

Central Valley and includes two CDWR-defined San Joaquin 

Valley groundwater subbasins: Delta-Mendota and Westside 

(California Department of Water Resources, 2003). Combined, 

these subbasins define the extent of the WSJV study unit and 

cover an area of approximately 2,170 square miles (mi2) in 

Stanislaus, Merced, Madera, Fresno, and Kings Counties, 

California (fig. 2). The study unit is bounded by the Southern 

Coast Ranges, the Stanislaus/San Joaquin county line, and 

numerous CDWR-defined subbasins (California Department 

of Water Resources, 2006a, b) (fig. 2).

The climate in the study unit is classified as 

Mediterranean, arid to semiarid, with hot, dry summers and 

cool, moist winters (U.S. Department of Commerce, National 

Climatic Data Center, 2010). Average rainfall across the study 

unit ranges from 6 inches (in.) in the southern parts of the 

Westside subbasin study area to 12 in. in the northern parts 

of the Delta-Mendota subbasin study area (PRISM Climate 

Group, Oregon State University, 2010). Approximately 85 to 

90 percent of annual precipitation occurs as rainfall during 

November through April (Western Regional Climate Center, 

2011).

The primary aquifer system in the study unit occurs in 

unconsolidated alluvial and continental deposits of the Tulare 

Formation (Hotchkiss and Balding, 1971; Belitz and Heimes, 

1990; California Department of Water Resources, 2006a, b). 

The Tulare Formation is composed of mostly unconsolidated 

clay, silt, sand, and gravel of Pliocene and Pleistocene age. 

The Corcoran Clay Member of the Tulare Formation acts as 

the major restrictive structure to vertical groundwater and 

is found at depths of about 100 to 850 feet (ft) throughout 

the study unit (California Department of Water Resources, 

2006a, b). The Corcoran Clay occurs in the subsurface 

throughout the study unit except along the western and 

eastern margins where the Quaternary alluvial and continental 

sediments pinch out (fig. 2).

The upper part of the Tulare Formation above the 

Corcoran Clay can be unconfined, semi-confined, or confined, 

as a result of the presence of many discontinuous lenses of 

clay and silt that occur throughout the alluvial and continental 

deposits (Hotchkiss and Balding, 1971). In the upper zone, 

groundwater flow is typically from the San Joaquin Valley 

margins towards the valley center and from south to north, 

following the elevation trend of the valley (Hotchkiss and 

Balding, 1971; Faunt, 2009). 

The lower part of the Tulare Formation below the 

Corcoran Clay is confined (Belitz and Heimes, 1990). This 

lower zone is underlain by saline water that may be influenced 

by interactions with marine sediments at greater depths. In the 

lower zone, groundwater flow in the system is predominantly 

towards the western margin of the San Joaquin Valley, but 

is strongly influenced by local variations in pumping stress 

(Belitz and Heimes, 1990; Faunt, 2009). 

On the basis of differences in textures, hydrologic 

properties, and oxidation states, the unconsolidated deposits in 

the semi-confined to unconfined zone of the Tulare Formation 

can be divided into two major hydrogeologic units: Coast 

Ranges alluvium and Sierran sands (Belitz and Heimes, 1990). 

The Coast Ranges alluvium (mapped as Quaternary 

alluvium on fig. 2), derived from the Coast Ranges to the 

west, is generally oxidized, and ranges in thickness from 

850 ft along the Coast Ranges to 0 ft in the eastern edge of 

the study unit (Belitz and Heimes, 1990). The Coast Ranges 

alluvium was deposited in a series of coalescing alluvial fans 

from streams draining the Coast Ranges. Sediments deposited 

near the fan head, where the stream enters the valley, are 

typically the coarsest, and sediment texture generally becomes 

finer with increasing distance from the fan head and stream 

channels (Belitz and Heimes, 1990). 

The Sierran sands (mapped as Quaternary other 

sediments on fig. 2) predominantly consist of well-sorted 

micaeous sand that is derived from the Sierra Nevada to 

the east. These deposits are up to 400–500 ft thick in the 

San Joaquin Valley trough and thin to the west, where they 

interfinger with the Coast Ranges alluvium. The Sierran sands 

are highly permeable and predominantly chemically reduced 

in the valley trough (Belitz and Heimes, 1990). The reduced 

sediments are gray-green, whereas oxidized sediments are 

yellow, orange, or brown as a result of oxidation of iron 

minerals in the sediments (Davis and others, 1959; Hotchkiss 

and Balding, 1971). 

In addition to the Coast Ranges alluvium and Sierran 

sands, continental deposits and non-marine and marine 

sediments (mapped as Tertiary deposits and sediments on 

fig. 2) occur along the western margin of the study unit. These 

deposits are generally located above modern streambeds or 

the water table, but have coarse texture that may contribute to 

groundwater recharge along the western margin of the study 

unit (Hotchkiss and Balding, 1971).
6 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Figure 2. Boundaries of the Delta-Mendota (DM) subbasin and the Westside (WS) subbasin study areas, geologic 

formations, major cities, major roads, and hydrologic features.

IP027484_Figure 02_geology

FRESNO COUNTY

MERCED COUNTY

MADERA COUNTY

MONTEREY COUNTY

SAN BENITO

COUNTY

MARIPOSA COUNTY

STANISLAUS COUNTY

KINGS

COUNTY

Newman

Fresno

Gustine

Modesto

Mendota

Patterson

Los Banos

Kettleman City

580

5

5

99

99

101

101

49

41

198 41

198

33

33

152

180 C o a s t R a n g e s

EXPLANATION

Extent of the Pleistocene-age

 Corcoran Clay member of

 the Tulare Formation

Quaternary alluvium

Quaternary other sediments

Delta-Mendota subbasin

 study area

Westside subbasin

 study area

0 10 20 MILES 5

0 10 20 KILOMETERS 5

15

15

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

121° 120°

37°

36°

Tertiary deposits and sediments

Mesozoic and Paleozoic

 igneous and metamorphic 

 rocks

Geology from Saucedo and others, 2000
Hydrogeologic Setting 7

Western San Joaquin Valley Study Unit—

Delta-Mendota Subbasin Study Area

The Delta-Mendota subbasin study area (hereinafter 

referred to as the DM study area) is 1,170 mi2 in area and lies 

in the northwestern part of the San Joaquin Valley (fig. 3). 

The DM study area mostly lies between the San Joaquin 

River near the San Joaquin Valley trough on the east and the 

Coast Ranges on the west. The study area is approximately 

90 miles (mi) long from southeast to northwest parallel to 

the San Joaquin Valley axis and is 6 to 28 mi wide from the 

southwest valley margin to the San Joaquin River in the 

northeast. The city of Los Banos is the only community in 

the study area with a population greater than 10,000 people 

and had an estimated 2006 population of 34,717 (U.S. Census 

Bureau, 2011).

Land-surface altitudes in the study area range from 

approximately 30 feet above mean sea level (ft above msl) 

near the northeastern corner, to about 1,500 ft above msl 

near the southwestern corner; most of the study area has a 

land surface less than 150 ft above msl. The San Joaquin 

River at the eastern edge of the study area is the largest 

surface drainage feature in the study area and interacts with 

groundwater (forming wetlands) in the eastern part of the 

study area. Numerous small streams that flow intermittently 

after rainstorms drain from the Coast Ranges to the east into 

the study area. However, only Orestimba and Los Banos 

Creeks maintain channels to the San Joaquin River. When 

flowing, these tributary streams lose most of their water to 

seepage and evaporation before reaching the San Joaquin 

River (Hotchkiss and Balding, 1971). Water is imported into 

the study area from the Sacramento-San Joaquin Rivers Delta 

to the north through the Delta-Mendota canal, which runs 

the length of the study area from northwest to southeast, and 

through the San Luis Canal, which runs along the western 

edge of the San Joaquin Valley. Water is also imported from 

the San Joaquin River (Mendota Pool) at the southeast corner 

of the study area by way of the Outside, Main, and Chowchilla 

canals (fig. 3). 

Groundwater recharge in the study area primarily occurs 

from the percolation of agricultural return (from applied 

surface water and groundwater used for irrigation). To a lesser 

extent, ambient recharge also occurs from seepage losses 

along the San Joaquin River and infiltration of runoff from the 

Coast Ranges into tributary streams. Groundwater discharge 

from the study area is primarily due to groundwater pumping 

and subsurface outflow to other parts of the San Joaquin 

Valley (California Department of Water Resources, 2006a, b; 

Faunt, 2009). 

Western San Joaquin Valley Study Unit—

Westside Subbasin Study Area

The Westside subbasin study area (hereinafter referred 

to as the WS study area) is 1,000 mi2 in area and lies in the 

southwestern part of the San Joaquin Valley (fig. 3). The 

WS study area and groundwater subbasin correspond to 

the boundaries of the Westlands Water District (California 

Department of Water Resources, 2006b). The study area is 

approximately 60 mi long from southeast to northwest parallel 

to the San Joaquin Valley axis and is 13 to 24 mi wide from 

the southwest valley margin to the northeastern boundary. 

There are no communities in the study area with a population 

greater than 10,000 people (U.S. Census Bureau, 2011).

Land-surface altitudes in the study area range from 

approximately 160 ft above msl near the northeastern 

corner of the study area to about 1,400 ft above msl near the 

western margin of the study area; most of the study area has 

land-surface altitudes less than 500 ft above msl. The major 

surface-water features in the study area are the California 

Aqueduct and numerous streams that intermittently flow 

after rainstorms from the Coast Ranges into the study area. 

The largest of these intermittent streams are the Panoche, 

Los Gatos, and Zapato Chino Creeks. When flowing, these 

streams usually lose their water to seepage and evaporation 

before reaching the San Joaquin River (Belitz and Heimes, 

1990). Since about 1967, water has been imported into the 

study area from the Sacramento-San Joaquin Rivers Delta to 

the north through the California Aqueduct (fig. 3). 

Under current conditions, groundwater recharge in 

the study area is primarily from irrigation return flow, with 

lesser amounts of infiltration of runoff from the Coast Ranges 

along tributary streams. Subsurface groundwater inflow from 

adjacent groundwater subbasins also occurs (Faunt, 2009). 

Groundwater discharge from the study area is primarily due 

to groundwater pumping (California Department of Water 

Resources, 2006a, b; Faunt, 2009). Under predevelopment 

conditions, groundwater recharge in the study area was 

primarily from infiltration of stream water draining the 

Coast Ranges, and groundwater discharge was primarily by 

evapotranspiration and streamflow along the San Joaquin 

Valley trough (Belitz and Heimes, 1990). 
8 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Figure 3. The Delta-Mendota (DM) subbasin and the Westside (WS) subbasin study areas, and the boundaries 

of the California Department of Water Resources (CDWR) defined groundwater subbasins.

IP027484_Figure 03

Zapato Chino

Creek

FRESNO COUNTY

MERCED COUNTY

MADERA COUNTY

MONTEREY COUNTY

SAN BENITO

COUNTY

MARIPOSA COUNTY

KINGS

COUNTY

STANISLAUS COUNTY

SAN JOAQUIN

COUNTY

Newman

Fresno

Gustine

Modesto

Mendota

Patterson

Los Banos

Kettleman City

580

5

5

99

99

101

101

49

41

198 41

198

33

33

152

180

Tuolumne

Merced

Orestimba

Creek

San Luis

Reservoir

Delta-Mendota

Canal

San Luis

Canal

Creek

Los Banos

Los Gatos Creek

C o a s t R a n g e s

Joaquin San 

San Joaquin

Outside

Canal

Mendota Pool

Kings Subbasin

Tulare Lake

Subbasin

River

River

River

Fresno

River

River

Kings

River

Bear

Creek

Owens

Creek

Main Canal

Panoche

Creek

Chowchilla

Canal

EXPLANATION

Delta-Mendota subbasin Boundary of Western

San Joaquin Valley

Study Unit

Waterbodies

Westside subbasin County boundary

Pleasant Valley Subbasin

121° 120°

37°

36°

0 10 20 MILES

0 10 20 KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

California Aqueduct

STUDY AREA
Methods 9

Methods 

Methods used for the GAMA-PBP were selected to 

achieve the following objectives: (1) design a sampling plan 

for suitable statistical representation; (2) collect samples in 

a consistent manner; (3) analyze samples by using proven 

and reliable laboratory methods; (4) assure the quality of the 

groundwater data; and (5) maintain data securely and with 

relevant documentation. The appendix contains detailed 

descriptions of the sample-collection protocols and analytical 

methods, the QA plan, and the results of analyses of QC 

samples.

Study Design

The wells selected for sampling in this study reflect 

the combination of two well-selection strategies. For the 

first strategy, 39 randomized wells were selected to provide 

a statistically unbiased, spatially distributed assessment 

of the quality of groundwater resources used for public 

drinking-water supply. Wells sampled as part of the spatially 

distributed, randomized grid-cell network, hereinafter, are 

referred to as “grid wells.” For the second strategy, 19 nonrandomized wells (hereinafter referred to as “understanding 

wells”) were sampled in the DM and WS study areas to 

aid in the understanding of aquifer-system flow and related 

groundwater-quality issues.

The spatially distributed wells were selected using a 

randomized grid-based method (Scott, 1990). The randomized 

grid-based method divides the study unit into equal-area grid 

cells; however, geographic features may force a grid cell to be 

divided into multiple pieces to obtain the designated coverage 

area for each cell. For instance, a part of a grid cell may be 

separated by a mountain range, but the sum of the cell parts is 

still considered one grid cell.

In the WSJV study unit, the approach was to sample one 

available CDPH well located within each grid cell. If a grid 

cell contained more than one CDPH well, each well randomly 

was assigned a rank. The highest ranking well that met basic 

sampling criteria (for example, sampling point located prior 

to treatment or capability to pump for several hours) was 

sampled. If a grid cell contained no accessible CDPH wells, 

then other types of wells, such as irrigation, domestic, and 

(or) commercial wells were considered for sampling. These 

non-CDPH wells were identified from USGS databases or 

by door-to-door canvassing. Wells with depths and screened 

intervals similar to CDPH wells in the area were selected. 

In this fashion, one well was selected for possible sampling 

in each cell to provide a spatially distributed, randomized 

monitoring network.

The DM study area was divided into 30 equal-area grid 

cells of approximately 38.9 mi2

 (approximately 100 square 

kilometers [km2]) in area each; wells in 29 of the 30 grid cells 

were sampled (figs. 4, 5). The WS study area was divided 

into 10 equal-area grid cells of approximately 38.9 mi2

(approximately 100 km2) in area each; wells in all 10 grid cells 

were sampled.

The design of the grid cells differed between the DM and 

WS study areas because of differences in the distributions and 

prevalence of CDPH wells. In the DM study area, the entire 

study area is divided into 30 equal-area cells. Most of the cells 

within the study area contain CDPH wells; 149 CDPH wells 

are in the study area (fig. 4). Consistent with GAMA-PBP 

design in other study units with many CDPH wells, the grid 

cells cover the entire study area.

In contrast, the WS study area had only 16 CDPH wells 

(fig. 4). Therefore, our approach involved using circular buffer 

areas around each CDPH well to delineate the grid cells was 

used. This allowed assessment of the part of the primary 

aquifer that is currently used for water supply. This strategy 

has been used in other GAMA-PBP study units with few and 

(or) unevenly distributed CDPH wells (Bennett and others, 

2009; Schmitt and others, 2009). Buffer areas in the WS study 

area (within the WSJV study unit) have a radius of about 4 mi 

(fig. 5). 

The 39 grid wells sampled in the WSJV study unit were 

numbered in the order in which samples were collected in each 

study area with the prefix varying by study area: “DM” for the 

Delta-Mendota subbasin and “WS” for the Westside subbasin 

(fig. 5). A grid well could not be located in one WS grid cell, 

so an available CDPH well located slightly outside of the 

study area boundary (WS-09, fig. 5) was sampled. Although 

the CDPH well was situated 1.3 mi away from the boundary 

of the WS study area, it was close to the eastern boundary and 

was considered to be representative of groundwater quality in 

the eastern part of the WS study area.

The 19 understanding wells sampled as part of the 

study were not included in the statistical characterization 

of water quality in the WSJV study unit because inclusion 

of these wells would lead to the overrepresentation of some 

cells. These additional wells were numbered in the order 

of sample collection, with the prefix modified from those 

used by the grid cells: “DM-U” or “WS-U” (“U” indicates 

“understanding”) (fig. 5). Of the understanding wells, 15 were 

short-screened monitoring wells open at different depths in 

the aquifer system at 6 geographic locations. The monitoring 

wells were sampled for the purpose of understanding vertical 

differences in groundwater quality and age in these six 

different parts of the WSJV study unit.

The GAMA alphanumeric identification number for each 

well, along with the date sampled, sampling schedule, landsurface altitude, well type, and available well-construction 

information is shown in table 1. Groundwater samples were 

collected during the period from March to July 2010. Grid 

wells included 18 CDPH wells, 10 irrigation wells, 8 domestic 

wells, 1 institutional well, and 2 unused wells. Understanding 

wells included 15 monitoring wells, 3 CDPH wells, and 

1 domestic well. 
10 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Figure 4. The Delta-Mendota (DM) subbasin and the Westside (WS) subbasin study areas, the distribution of the 

grid cells, and the location of California Department of Public Health (CDPH) wells.

IP027484_Figure 04

FRESNO COUNTY

MERCED COUNTY

MADERA COUNTY

MONTEREY COUNTY

SAN BENITO COUNTY

MARIPOSA COUNTY

KINGS

COUNTY

SAN JOAQUIN

COUNTY

STANISLAUS COUNTY

Newman

Fresno

Gustine

Modesto

Mendota

Patterson

Los Banos

Kettleman City

580

5

5

99

99

101

101

49

41

198 41

198

33

33

152

180

EXPLANATION

Delta-Mendota subbasin and grid cells

Westside subbasin

Westside subbasin grid cells California Department of Public Health

 (CDPH) well

Waterbodies

121° 120°

37°

36°

County boundary

0 10 20 MILES

0 10 20 KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection
Methods 11

IP027484_Figure 05

FRESNO COUNTY

MERCED COUNTY

MADERA COUNTY

MONTEREY COUNTY

SAN BENITO COUNTY

MARIPOSA COUNTY

KINGS

COUNTY

STANISLAUS COUNTY

SAN JOAQUIN

COUNTY

Newman

Fresno

Gustine

Modesto

Mendota

Patterson

Los Banos

Kettleman City

580

5

5

99

99

101

49

41

198 41

198

33

33

152

180

DM-U-13

DM-U-01

WS-10

WS-09

WS-08

WS-07

WS-06

WS-05

WS-04

WS-03

WS-02

WS-01

DM-29

DM-28

DM-27

DM-26

DM-25

DM-24

DM-23

DM-22

DM-21

DM-20

DM-19 DM-18

DM-17

DM-16

DM-15

DM-14

DM-13

DM-12

DM-11

DM-10

DM-09

DM-08

DM-07

DM-06

DM-05

DM-03 DM-04

DM-02

DM-01

WS-U-03

WS-U-02

WS-U-01

DM-U-14-16

DM-U-10-12

DM-U-07-09

DM-U-04-06

DM-U-02,03

EXPLANATION

WS-U-01

DM-06 Grid well and identifier

Understanding well and identifier

Sampled Wells

Delta-Mendota subbasin and grid cells

Westside subbasin

Westside subbasin grid cells

Waterbodies

121° 120°

37°

36°

0 10 20 MILES

0 10 20 KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

County boundary

Figure 5. The Delta-Mendota (DM) subbasin and the Westside (WS) subbasin study areas, the distribution 

of the grid cells, and the location of sampled grid and understanding wells.
12 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Well locations were verified by using a global positioning 

system (GPS), 1:24,000-scale USGS topographic maps, 

comparison with existing well information in USGS 

and CDPH databases, and information provided by well 

owners, drillers’ logs, and (or) other sources of construction 

information. Well location and information were recorded 

in the field on field sheets, and electronically on field laptop 

computers using the Alternate Place Entry (APE) program 

designed by the USGS. All information was verified and then 

uploaded into the USGS National Water Information System 

(NWIS) database. Well location, well owner, and well-use 

information is confidential.

The wells in the WSJV study unit were sampled by 

using a tiered analytical approach. All wells were sampled 

for a standard set of constituents: VOCs, low-level 

fumigants, pesticides and pesticide degradates, perchlorate, 

1,2,3-trichloropropane (1,2,3-TCP), trace elements, 

nutrients, major and minor ions, silica, total dissolved solids 

(TDS), total arsenic and iron (unfiltered), arsenic and iron 

species (filtered), stable isotopes of hydrogen and oxygen 

in water, stable isotopes of carbon in dissolved inorganic 

carbon and carbon-14 abundance, activities of tritium, and 

dissolved noble gases. The standard set of constituents was 

termed the “intermediate” sampling schedule (table 2). 

Wells on the “slow” sampling schedule included all of the 

constituents on the intermediate sampling schedule, plus 

N-nitrosodimethylamine (NDMA), dissolved organic carbon 

(DOC), field alkalinity, chromium species (filtered), stable 

isotopes of nitrogen and oxygen in dissolved nitrate, stable 

isotopes of sulfur in dissolved sulfate, stable isotopes of boron 

in water, isotopic ratios of strontium in water, and dissolved 

standard gases. 

Intermediate and slow refer to the relative time required 

to sample the well for all constituents on the schedule. Three 

intermediate or two slow wells generally could be sampled 

in 1 day. In the WSJV study unit, 25 of the wells were 

sampled on the intermediate sampling schedule (24 grid and 

1 understanding), and 33 wells were sampled on the slow

sampling schedule (15 grid and 18 understanding) (table 1).

Sample Collection and Analysis

Samples were collected by using USGS National Field 

Manual (NFM) (U.S. Geological Survey, variously dated) 

and modified USGS National Water-Quality Assessment 

(NAWQA) Program (Koterba and others, 1995) sampling 

protocols. These sampling protocols were followed so 

that samples representative of groundwater in the aquifer 

were collected at each site, and so that the samples were 

collected and handled in ways that minimized the potential 

for contamination; this will also allow comparison with data 

collected by GAMA-PBP throughout California and with other 

USGS projects in California and the Nation.

Groundwater samples were analyzed for 85 VOCs 

(table 3A); 2 low-level fumigants (table 3B); 83 pesticides 

and pesticide degradates (table 3C); 3 constituents of special 

interest (table 3D); 24 trace elements (table 3E); 5 nutrients 

and DOC (table 3F); 9 major and minor ions, silica, TDS, 

and alkalinity (table 3G); total arsenic and iron (unfiltered) 

and arsenic, chromium, and iron species (filtered) (table 3H), 

8 stable isotopes, carbon-14 abundance, activities of tritium, 

11 dissolved standard and noble gases, and helium stable 

isotopic ratios (table 3I). The methods used for sample 

collection and analysis are described in the appendix section 

titled “Sample Collection and Analysis.”

Data Reporting

Many of the constituents presented in this report have 

different laboratory reporting conventions; some of the 

constituents were analyzed by seperate analytical laboratories 

or methods. The appendix section titled “Data Reporting” 

gives details for the laboratory reporting conventions for all 

constituents and the method preferences for those constituents 

that were analyzed by multiple methods or by multiple 

laboratories in the WSJV study unit.

Quality-Assurance Methods

The QA/QC procedures used for this study followed the 

protocols described in the NFM (U.S. Geological Survey, 

variously dated) and used by the NAWQA Program (Koterba 

and others, 1995). The QA plan followed by the USGS 

National Water Quality Laboratory (NWQL), the primary 

laboratory used to analyze samples for this study, is described 

in Pirkey and Glodt (1998) and Maloney (2005). QC samples 

collected in the WSJV study unit are blanks, replicates, and 

matrix and surrogate spikes. QC samples were collected 

to evaluate potential contamination, as well as bias and 

variability of the data that may have resulted from sample 

collection, processing, storage, transportation, and laboratory 

analysis. QA/QC procedures and results are described in the 

appendix section titled “Quality-Assurance Methods.”

Quality-Control Results

Results of QC analyses (blanks, replicates, and matrix 

and surrogate spikes) were used to evaluate the quality of the 

data for the groundwater samples. On the basis of detections 

in laboratory and field blanks collected for this and previous 

GAMA-PBP study units, the laboratory reporting levels 

(LRLs) for 4 VOCs and 11 inorganic constituents were raised 

in this report (Olsen and others, 2010; Fram and others, 2012). 

The GAMA Program refers to these raised reporting levels as 

“study reporting levels” (SRLs). Detections of the four VOCs 

with concentrations less than the raised reporting levels were 

reclassified as non-detections and not presented in this report. 

Detections of the 11 inorganic constituents with concentrations 

less than the SRL are flagged with a less than or equal to 
Methods 13

symbol (≤) in this report (see tables 5, 8, 9, A3 and additional 

discussion in the appendix section titled “Blanks”) because the 

concentrations of these constituents could be less than or equal 

to the reported value. 

Variability for 100 percent of the replicate pairs for 

constituents detected in samples was within the acceptable 

limits. Results from the replicates confirm that the procedures 

used to collect and analyze the samples did not introduce 

unacceptable levels of contamination or variability (see 

tables A4A-B and additional discussion in the appendix 

section titled “Replicates”). Median matrix-spike recoveries 

for 21 of the 172 organic and special-interest constituents 

analyzed were outside the acceptable range of between 70 and 

130 percent. Constituents for which low recoveries occurred 

might not have been detected in some samples if they were 

present at concentrations near the LRLs, and constituents with 

high recoveries may indicate that reported values could be 

greater than what is in the sample (see tables 3B, 3D, A5B–C

and additional discussion in the appendix section titled 

“Matrix Spikes”). 

Comparison Benchmarks

Concentrations of constituents detected in groundwater 

samples were compared with USEPA and CDPH regulatory 

and non-regulatory drinking-water health-based benchmarks 

and benchmarks established for aesthetic purposes (California 

Department of Public Health, 2008a, b; U.S. Environmental 

Protection Agency, 2008a, b). The chemical data presented 

in this report are meant to characterize the quality of the 

untreated groundwater within the primary aquifer system 

of the WSJV study unit and are not intended to represent 

the treated drinking water delivered to consumers by water 

purveyors. The chemical composition of treated drinking 

water may differ from untreated groundwater because treated 

drinking water may be subjected to disinfection, filtration, 

mixing with other waters, and (or) exposure to the atmosphere 

prior to its delivery to consumers. Comparisons of untreated 

groundwater to benchmarks are for illustrative purposes only 

and are not indicative of compliance or non-compliance with 

drinking-water regulations. The following benchmarks were 

used for comparisons:

• MCL—Maximum Contaminant Level. Legally 

enforceable standards that apply to public-water 

systems and are designed to protect public health by 

limiting the levels of contaminants in drinking water. 

MCLs established by the USEPA are the minimum 

standards with which States are required to comply, 

and individual States may choose to set more stringent 

standards. CDPH has established MCLs for additional 

constituents not regulated by the USEPA, as well as 

lowered the benchmark concentrations for a number 

of constituents with MCLs established by the USEPA. 

In this report, a benchmark set by the USEPA and 

adopted by CDPH is labeled “MCL-US,” and one set 

by CDPH that is more stringent than the MCL-US is 

labeled “MCL-CA.” Well owners are notified when 

constituents are detected at concentrations greater than 

an MCL-US or an MCL-CA benchmark in samples 

collected for the GAMA-PBP, but these detections do 

not constitute violations of CDPH regulations.

• AL—Action Level. Legally enforceable standards 

that apply to public-water systems and are designed to 

protect public health by limiting the levels of copper 

and lead in drinking water. Detections of copper or 

lead greater than the action-level benchmarks trigger 

requirements for mandatory water treatment to reduce 

the corrosiveness of water to water pipes. The action 

levels established by the USEPA and CDPH are the 

same; thus, these benchmarks are labeled “AL-US” in 

this report.

• SMCL—Secondary Maximum Contaminant Level. 

Non-enforceable standards applied to constituents that 

affect the aesthetic qualities of drinking water, such 

as taste, odor, and color, or the technical qualities of 

drinking water, such as scaling and staining. Both the 

USEPA and CDPH define SMCLs, but unlike MCLs, 

SMCLs established by CDPH are not required to be 

at least as stringent as those established by USEPA. 

SMCLs established by CDPH are used in this report 

(SMCL-CA) for all constituents that have SMCL-CA 

values. The SMCL-US is used for pH because no 

SMCL-CA has been defined.

• NL—Notification Level. Health-based notification 

levels established by CDPH (NL-CA) for some of 

the constituents in drinking water that lack MCLs. 

If a constituent is detected in drinking water at 

concentrations greater than its NL-CA, California State 

law requires timely notification of local governing 

bodies and recommends consumer notification.

• HAL—Lifetime Health Advisory Level. The 

maximum concentration of a constituent at which its 

presence in drinking water is not expected to cause any 

adverse carcinogenic effects for a lifetime of exposure. 

HALs are established by the USEPA (HAL-US) and 

are calculated assuming consumption of 2 liters (L) 

(2.1 quarts) of water per day over a 70-year lifetime by 

a 70-kilogram (154-pound) adult and that 20 percent of 

a person’s exposure comes from drinking water.

• RSD5—Risk-Specific Dose. The concentration 

of a constituent in drinking water corresponding 

to an excess estimated lifetime cancer risk of 1 

in 100,000. RSD5 is an acronym for risk-specific 

dose at the 10−5 risk level (10−5 equals 1/100,000). 

RSD5s are calculated by dividing the 10–4 cancer 

risk concentration established by the USEPA by 10 

(RSD5-US).
14 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

For constituents with MCLs, detections in groundwater 

samples were compared to the MCL-US or MCL-CA. 

Constituents with SMCLs were compared with the 

SMCL-CA. For chloride, sulfate, specific conductance, and 

total dissolved solids, CDPH defines a “recommended” 

and an “upper” SMCL-CA; detections of these constituents 

in groundwater samples were compared with both levels. 

The SMCL-US levels for these constituents correspond to 

their recommended SMCL-CAs. Detected concentrations 

of constituents without an MCL or SMCL were compared 

to the NL-CA. For constituents without an MCL, SMCL, 

or NL-CA, detected concentrations were compared with the 

HAL-US. For constituents without an MCL, SMCL, NL-CA, 

or HAL-US, detected concentrations were compared with 

the RSD5-US. For constituents without an MCL, SMCL, 

NL-CA, HAL-US, or RSD5-US, detected concentrations were 

compared with the AL-US. Note that using this hierarchy 

to select the comparison benchmark for a constituent with 

more than one type of established benchmark will not 

necessarily result in selection of the benchmark with the 

lowest concentration. For example, for zinc the SMCL-CA 

is 5,000 micrograms per liter (μg/L) and the HAL-US is 

2,000 μg/L, but the comparison benchmark selected by this 

hierarchy is the SMCL-CA. The comparison benchmarks used 

in this report are listed in table 3A–I for all constituents and in 

tables 4–13 for constituents detected in groundwater samples 

from the WSJV study unit. Not all constituents analyzed for 

this study have established benchmarks available. Detections 

of constituents at concentrations greater than the selected 

comparison benchmarks are marked with asterisk (*) in 

tables 4, 5, and 8–11. 

Water-Quality Results 

Groundwater-Quality Data

Results from analyses of groundwater samples from the 

WSJV study unit are presented in tables 4–13. Groundwater 

samples collected in the WSJV study unit were analyzed for 

245 constituents; 138 of those constituents were not detected 

in any of the samples, and 100 constituents were detected 

(tables 3A–I, 4). 

For organic and special-interest constituent classes, the 

results tables (tables 5−7) include only constituents that were 

detected and the following summary statistics: the number of 

wells in which each constituent was detected, the frequency 

at which it was detected (in the study unit and in each study 

area), and the total number of constituents detected at each 

well. For the inorganic and isotopic constituent classes, 

the results tables (tables 8−13) include all of the wells, 

constituents, and samples analyzed. 

Water-quality indicators measured in the field and at the 

NWQL are included in table 4. The results of groundwater 

analyses organized by compound classes are presented in 

tables 5–13:

• Organic constituents

• Volatile organic compounds and low-level fumigants 

(table 5)

• Pesticides and pesticide degradates (table 6)

• Constituents of special interest (table 7)

• Inorganic constituents

• Trace elements (table 8)

• Nutrients and dissolved organic carbon (table 9)

• Major and minor ions, silica, and total dissolved 

solids (table 10)

• Total arsenic and iron (unfiltered) and arsenic, 

chromium, and iron species (filtered) (table 11)

• Isotopic tracers (table 12)

• Dissolved standard gases (table 13)

Water-Quality Indicators 

Measurements of turbidity, dissolved oxygen, pH, 

specific conductance, alkalinity, and associated parameters 

(water temperature and bicarbonate and carbonate 

concentrations) are presented in table 4. Turbidity, 

dissolved oxygen, alkalinity, and bicarbonate and carbonate 

concentrations are used as indicators of natural processes that 

affect water chemistry. The pH value indicates the acidity of 

the water. Specific conductance is the measure of electrical 

conductivity of the water and is proportional to the amount of 

dissolved solids in the water.

Field pH values for 2 of the 39 grid wells sampled 

(1 in the DM study area and 1 in the WS study area) in the 

WSJV study unit exceeded the SMCL-US range for pH 

(<6.5 or >8.5) (table 4). Low pH in water may contribute 

to corrosion of pipes, and high pH in water may contribute 

to scaling. Laboratory pH values may be dissimilar to field 

pH values because the pH of groundwater may change upon 

removal from the ambient environment and exposure to the 

atmosphere; therefore, the field measurements are preferred 

over the laboratory results.

Field specific-conductance values for 33 of the 39 grid 

wells sampled were greater than the recommended SMCL-CA 

of 900 microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25 °C) (approximately 85 percent) (table 4). Field 
Water-Quality Results 15

specific-conductance values for 25 of 29 grid wells sampled 

in the DM study area were greater than the recommended 

SMCL-CA, and 16 of these samples had field specificconductance values greater than the upper benchmark of 

1,600 μS/cm at 25 °C. Field specific-conductance values for 

8 of 10 grid wells sampled in the WS study area were greater 

than the recommended SMCL-CA, and 4 of these samples had 

values that were greater than the upper SMCL-CA. 

Field specific-conductance values for 15 of the 

19 understanding wells sampled (12 DM-U and 3 WS-U) 

were greater than the recommended SMCL-CA. Four DM-U 

well samples and 1 WS-U well sample had field specificconductance values greater than the upper SMCL-CA.

Organic Constituents

VOCs are present in paints, solvents, fuels, fuel additives, 

refrigerants, fumigants, and disinfected water and are 

characterized by their tendency to evaporate. VOCs generally 

persist longer in groundwater than in surface water because 

groundwater is isolated from the atmosphere. 

Of the 85 VOCs analyzed in WSJV study unit, 22 were 

detected in groundwater samples; all concentrations in the 

grid wells sampled were less than health-based benchmarks. 

Benzene was detected at concentrations greater than the 

MCL-CA of 1 μg/L in samples from the three WS-U wells 

(table 5). One or more VOCs were detected in 13 of the 

39 grid wells sampled (about 33 percent detection frequency; 

table 5). Chloroform (trichloromethane) was the only VOC 

detected in more than 10 percent of the grid wells in the 

WSJV study unit. Chloroform is among the most commonly 

detected VOCs in groundwater nationally (Zogorski and 

others, 2006).

VOCs were detected in 10 of the 29 grid wells sampled 

in the DM study area (approximately 34 percent). Chloroform 

was detected in more than 10 percent of the grid wells in the 

DM study area. VOCs were detected in 3 of the 10 grid wells 

sampled in the WS study area (30 percent). Chloroform and 

carbon disulfide were each detected in at least 10 percent of 

the grid wells in the WS study area. One or more VOCs were 

detected in nine understanding well samples (6 DM-U and 

3 WS-U).

The low-level fumigants DBCP and EDB were sampled 

for at all 58 wells in the WSJV study unit. DBCP was detected 

in two grid wells sampled in the DM study area, whereas, 

EDB was not detected in any of the wells in the WSJV study 

unit (table 5).

Pesticides are chemicals used to control weeds, insects, 

fungi, and other pests in agricultural, urban, and suburban 

settings, and include herbicides, insecticides, and fungicides. 

Pesticide degradates are the product of the environmental 

transformations of the parent pesticide, and they can have 

similar properties to the parent pesticide. Of the 83 pesticides 

and pesticide degradates analyzed in the WSJV study unit, 

16 were detected in groundwater samples; all concentrations 

were less than health-based benchmarks (table 6). One or 

more pesticide compounds were detected in 9 of the 39 grid 

wells sampled (about 23 percent detection frequency; table 6). 

Simazine and deethylatrazine (2-chloro-4-isopropylamino-6-

amino-s-triazine) were detected in more than 10 percent of the 

grid wells in the WSJV study unit. 

Pesticides and pesticide degradates were detected in 9 of 

the 29 grid wells sampled in the DM study area (31 percent). 

Deethylatrazine, atrazine, hexazinone, and S-ethyldipropylthiocarbamate (EPTC) were detected in more than 

10 percent of the grid wells in the DM study area. Simazine, 

deethylatrazine, and atrazine are among the nation’s most 

commonly detected pesticide compounds in groundwater 

(Gilliom and others, 2006). 

No pesticide and (or) pesticide degradate was detected 

in any of the 10 grid wells sampled in the WS study area. 

One or more pesticide compounds were detected in 11 of the 

understanding wells sampled (10 DM-U and 1 WS-U).

Constituents of Special Interest

Perchlorate, NDMA, and 1,2,3-TCP are constituents 

of special interest in California because they may adversely 

affect water quality and recently have been detected in water 

supplies (California Department of Public Health, 2008b). 

Perchlorate and 1,2,3-TCP were analyzed for at all 58 wells, 

and NDMA was sampled for at the 33 slow wells in the 

WSJV study unit. NDMA was not detected in any of the grid 

or understanding wells; as a result, NDMA is not included in 

table 7. 

In total, perchlorate was detected in 15 of 39 grid wells 

sampled (about 38 percent detection frequency; table 7). 

Perchlorate was detected in 13 of the 29 grid wells sampled 

in the DM study area (approximately 45 percent). Perchlorate 

was detected in 2 of the 10 grid wells sampled in the 

WS study area (20 percent). Perchlorate was not detected at 

concentrations greater than the MCL-CA of 6 μg/L in any of 

the grid or understanding wells sampled in the WSJV study 

unit. 1,2,3-TCP was detected in one DM-U well sample 

(table 7).

Inorganic Constituents

Unlike the organic constituents and the constituents 

of special interest, most of the inorganic constituents are 

naturally present in groundwater, although their concentrations 

may be influenced by human activities. Inorganic constituents 

were sampled for at all 58 wells in the WSJV study unit. 

Health-based or non-health-based benchmarks have 

been established for 21 of 24 trace elements analyzed in 

this study (table 3E). Of the 17 trace elements with healthbased benchmarks, most detected concentrations in the 

wells sampled in the WSJV were less than established 

benchmarks. The exceptions were detections of arsenic, boron, 

molybdenum, selenium, strontium, and uranium (table 8). 
16 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Arsenic concentrations greater than the MCL-US of 

10 μg/L were detected in two grid wells sampled in the 

DM study area (table 8). Additionally, arsenic was measured 

at concentrations greater than the MCL-US in five of the 

understanding wells sampled (three DM-U and two WS-U). 

Boron concentrations greater than the NL-CA of 

1,000 μg/L were detected in 20 grid wells sampled (16 in the 

DM study area and 4 in the WS study area; table 8). Boron 

concentrations greater than the NL-CA were also detected in 

seven of the understanding wells sampled (six DM-U and one 

WS-U).

Molybdenum concentrations greater than the HAL-US 

of 40 μg/L were detected in two grid wells sampled in the 

DM study area (table 8). Additionally, molybdenum was 

measured at a concentration greater than the HAL-US in one 

DM-U well sample. 

A selenium concentration greater than the MCL-US 

of 50 μg/L was detected in one grid well sampled in the 

DM study area (table 8). There were no detections of selenium 

greater than the MCL-US in the WS study area or in the 

WSJV understanding wells.

Strontium concentrations greater than the HAL-US of 

4,000 μg/L were detected in two grid wells sampled in the 

DM study area (table 8). Additionally, strontium was measured 

at a concentration greater than the HAL-US in one DM-U well 

sample. 

Uranium concentrations greater than the MCL-US of 

30 μg/L were detected in two DM-U well samples (table 8). 

There were no detections of uranium greater than the 

MCL-US in the WSJV grid wells.

Nutrients (nitrogen and phosphorus) and DOC present 

in groundwater can affect biological activity in aquifers and 

in surface-water bodies that receive groundwater discharge. 

Inorganic nitrogen may be present in the form of ammonia, 

nitrite, or nitrate, depending on the oxidation-reduction state 

of the groundwater. 

All concentrations of ammonia and nitrite measured in 

the WSJV study unit were less than health-based benchmarks. 

Concentrations of nitrate greater than the MCL-US of 

10 milligrams per liter (mg/L) were detected in three grid 

wells sampled in the DM study area and in six DM-U well 

samples (table 9). There were no detections of nitrate greater 

than the MCL-US in grid wells sampled in the WS study area. 

The CDPH has established non-health-based secondary 

benchmarks (SMCL-CAs) that are based on aesthetic 

properties (such as taste, color, and odor) and technical 

properties (such as scaling and staining) of water, rather than 

on human-health concerns for iron, manganese, silver, zinc, 

chloride, sulfate, and TDS. 

Iron and manganese are trace elements whose 

concentrations are affected by the oxidation-reduction state 

of the groundwater. Iron concentrations greater than the 

SMCL-CA of 300 μg/L were detected in five grid wells 

sampled in the DM study area (table 8).

Manganese concentrations greater than the SMCL-CA of 

50 μg/L were detected in 16 grid wells (13 in the DM study 

area and 3 in the WS study area; table 8). Additionally, 

manganese was measured at concentrations greater than the 

SMCL-CA in eight of the understanding wells sampled (six 

DM-U and two WS-U). 

Chloride concentrations greater than the recommended 

SMCL-CA benchmark of 250 mg/L were detected in 

14 grid wells sampled in the DM study area, and 5 of these 

detections were greater than the upper SMCL-CA benchmark 

of 500 mg/L (table 10). Chloride was not measured at 

concentrations greater than the recommended SMCL-CA 

benchmark in any of the grid wells sampled in the WS study 

area. Chloride was measured at concentrations greater than 

the recommended SMCL-CA in six of the understanding wells 

sampled (five DM-U and one WS-U).

Sulfate concentrations greater than the recommended 

SMCL-CA benchmark of 250 mg/L were detected in 

21 WSJV grid wells (13 in the DM study area and 8 in the 

WS study area; table 10), and 13 of these detections (9 in 

the DM study area and 4 in the WS study area) were greater 

than the upper SMCL-CA benchmark of 500 mg/L. Sulfate 

concentrations greater than the recommended SMCL-CA of 

250 mg/L were detected in nine understanding wells sampled 

(eight DM-U and one WS-U), and four of these detections 

(three DM-U and one WS-U) were greater than the upper 

SMCL-CA. 

TDS concentrations greater than the recommended 

SMCL-CA benchmark of 500 mg/L were detected in 

36 WSJV grid wells sampled (27 in the DM study area and 

9 in the WS study area; table 10), and 20 of these detections 

(16 in the DM study area and 4 in the WS study area) were 

greater than the upper SMCL-CA benchmark of 1,000 mg/L. 

TDS concentrations greater than the recommended 

SMCL-CA benchmark of 500 mg/L were detected in 15 of the 

understanding wells sampled (12 DM-U and 3 WS-U), and 

7 of these detections (6 DM-U and 1 WS-U) were greater than 

the upper SMCL-CA. 

Arsenic, chromium, and iron occur as different species, 

depending on the oxidation-reduction state of the groundwater. 

The oxidized and reduced species have different solubilities 

in groundwater, and the relative proportions of the oxidized 

and reduced species of each element can be used in the 

interpretation of the oxidation–reduction conditions of the 

aquifer. 

Concentrations of total arsenic and total iron (unfiltered) 

and arsenic, chromium, and iron (filtered), and the dissolved 

concentration of either the reduced or the oxidized species 

of the element, are reported in table 11. Concentrations of 

total arsenic, total chromium, and total iron reported by 

the NWQL in table 8 are considered to be more accurate 

than the concentrations reported by the USGS National 

Research Program (NRP) Trace Metal Laboratory (TML), 

Boulder, Colorado, in table 11 (see the appendix section 

titled “Constituents Determined by Multiple Methods or 

Laboratories”).
Summary 17

Isotopic Tracers and Dissolved Gases

The isotopic ratios, activities, and concentrations of 

many elements are used as tracers of hydrologic processes 

(Clark and Fritz, 1997). Stable isotope ratios of hydrogen and 

oxygen in water (δ2H and δ18O) aid in the interpretation of the 

sources of groundwater recharge. Stable isotopic ratios reflect 

the altitude, latitude, and temperature of precipitation and also 

the extent of evaporation of the water in surface-water bodies 

or soils prior to infiltration into the aquifer or directly from 

groundwater close to land surface once in the aquifer system. 

In the WSJV study unit, the stable isotope ratios of hydrogen 

in water ranged from –88.0 to –43.3 per mil, and oxygen in 

water ranged from –11.6 to –5.15 per mil (table 12).

Stable isotope ratios of nitrogen and oxygen in dissolved 

nitrate (δ15N and δ18O) can be used to aid in interpretation of 

sources and processes affecting nitrate in aquifers. The stable 

isotopic ratios of nitrogen in dissolved nitrate ranged from 

2.20 to 20.90 per mil, and oxygen in dissolved nitrate ranged 

from –0.41 to 15.97 per mil (table 12).

Stable isotope ratios of sulfur in dissolved sulfate (δ34S) 

can be used to aid in the characterization of the groundwater 

system by determining the source of dissolved sulfur species 

and (or) aid in the understanding of chemical reactions 

occurring in groundwater systems, such as the microbial 

reduction of sulfate or the oxidation of sulfide. The stable 

isotope ratios of sulfur in dissolved sulfate ranged from 

–11.22 to 7.05 per mil (table 12).

Stable isotope ratios of boron in water (δ11B) may be 

useful in distinguishing the sources of boron in groundwater. 

Natural sources include igneous rocks, evaporate minerals, 

seawater, and geothermal waters. Boron isotope variations 

have been shown to reflect mixing in the regional groundwater 

and the boron isotope fractionalization caused by boron 

removal by adsorption onto clays. As a result, boron isotopes 

are a useful tracer in groundwater systems in which the role 

of clay and minerals can be clearly identified (as a tracer for 

anthropogenic boron or as a tracer for seawater contamination, 

for example) (U.S. Geological Survey, 2004). The stable 

isotope ratios of boron in water ranged from 4.7 to 58.1 per 

mil (table 12).

Isotopic ratios of strontium in water (87Sr/86Sr) reflect the 

amounts of strontium (and other inorganic constituents) the 

aquifer materials are contributing to the groundwater system. 

In regions that have diverse geology, strontium isotope ratios 

may aid in identifying groundwater-flow paths. The isotopic 

ratios of strontium ranged from 0.7047 to 0.7079 (table 12).

Tritium activities and helium isotope ratios provide 

information about the age (time since recharge) of 

groundwater. Tritium is a short-lived radioactive isotope of 

hydrogen that is incorporated into the water molecule. Low 

levels of tritium are produced continuously by interaction 

of cosmic radiation with the Earth’s atmosphere, and a large 

amount of tritium was produced as a result of atmospheric 

testing of nuclear weapons between 1952 and 1963. Thus, 

concentrations of tritium greater than background levels 

generally indicate the presence of water recharged since the 

early 1950s. Helium isotope ratios can be used in conjunction 

with tritium concentrations to estimate ages for young 

groundwater. Of the isotopic tracer constituents analyzed 

for this study, tritium is the only one with a health-based 

benchmark. All measured tritium activities in samples from 

the wells in the WSJV study unit were more than three orders 

of magnitude less than the MCL-CA benchmark (table 12).

Carbon-14 is a radioactive isotope of carbon and an 

age-dating tracer. Low levels of carbon-14 are produced 

continuously by interaction of cosmic radiation with the 

Earth’s atmosphere and are incorporated into atmospheric 

carbon dioxide. Carbon dioxide dissolves in precipitation, 

surface water, and groundwater exposed to the atmosphere, 

thereby entering the hydrologic cycle. Because carbon-14 

decays with a half-life of approximately 5,700 years, low 

activities of carbon-14, relative to modern values, generally 

indicate a presence of groundwater that is several thousand 

years old or more. Carbon-14 activities ranged from 0.340 

to 106.1 percent modern carbon, and the stable isotopes of 

carbon in dissolved inorganic carbon (δ13C) ranged from 

–37.47 to –6.03 per mil (table 12).

Gases dissolve in water that is in contact with the 

atmosphere, and the solubilities of the different gas species 

vary with temperature. Concentrations of dissolved standard 

and noble gases are used to estimate the conditions of 

groundwater recharge, particularly the temperature of the 

water at the time of recharge. Results for analyses of dissolved 

standard gases (methane, carbon dioxide, nitrogen, oxygen, 

and argon) are presented in table 13.

Future Work

Subsequent reports for the WSJV study unit will be 

focused on assessment of the data presented in this report by 

using statistical, qualitative, and quantitative approaches to 

evaluate the natural and human factors affecting groundwater 

quality in the WSJV study unit. Water-quality data contained 

in the CDPH databases will be used in combination with the 

data that are presented in this report. These subsequent reports 

will include the LLNL results (dissolved noble gas, tritium, 

and helium isotope ratios) for the WSJV study unit.

Summary 

Groundwater quality in the approximately 2,170-squaremile Western San Joaquin Valley (WSJV) study unit was 

investigated by the U.S. Geological Survey (USGS) from 

March to July 2010, as part of the Priority Basin Project 

(PBP) of the California State Water Resources Control 

Board (SWRCB) Groundwater Ambient Monitoring and 

Assessment (GAMA) Program. The GAMA Program was 

created to provide a comprehensive baseline of groundwater 
18 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

quality in the State. The GAMA-PBP was created as a 

result of 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. The GAMA-PBP is being conducted by the 

USGS in cooperation with the SWRCB and Lawrence 

Livermore National Laboratory (LLNL).

The GAMA WSJV study was designed to provide a 

spatially unbiased assessment of untreated-groundwater 

quality in the primary aquifer system and to facilitate 

statistically consistent comparisons of untreated-groundwater 

quality throughout California. The primary aquifer system is 

defined as parts of aquifers corresponding to the perforation 

intervals of wells listed in the California Department of 

Public Health (CDPH) database for the WSJV study unit. 

Groundwater quality in the primary aquifer system may differ 

from the quality in the shallower or deeper water-bearing 

zones; shallow groundwater may be more vulnerable to 

surficial contamination. 

This study did not attempt to evaluate the quality of water 

delivered to consumers; after withdrawal from the ground, 

water typically is treated, disinfected, and blended with other 

waters to maintain acceptable water quality. The benchmarks 

used in this report apply to treated water that is served to the 

consumer, not to untreated groundwater. However, to provide 

some context for the results, concentrations of constituents 

measured in these groundwater samples were compared with 

benchmarks established by the U.S. Environmental Protection 

Agency (USEPA) and CDPH.

The WSJV study unit is located within the Central 

Valley hydrologic province and includes two groundwater 

subbasins defined by the California Department of Water 

Resources: Delta-Mendota and Westside. The WSJV study 

included assessment of the groundwater quality from 58 wells 

in Stanislaus, Merced, Madera, Fresno, and Kings Counties, 

California. Thirty-nine wells were selected using a randomized 

grid approach to achieve statistically unbiased representation 

of groundwater used for public drinking-water supplies (grid 

wells). Nineteen wells were selected to provide additional 

sampling density to aid in the understanding of aquifer-system 

flow and related groundwater-quality issues (understanding 

wells). 

Groundwater samples were analyzed for water-quality 

indicators, organic constituents, constituents of special 

interest, and inorganic constituents. Isotopic tracers also were 

measured to provide a dataset that will be used to interpret the 

sources and ages of groundwater. In total, 245 constituents 

and 8 water-quality indicators were measured. This report 

describes the sampling, analytical, and quality-assurance 

methods used in the study and presents the results of the 

chemical analyses of the groundwater samples.

Quality-control samples (blanks, replicates, or matrix 

spikes) were collected at 16 percent of the wells in the 

WSJV study unit, and the results for these samples were 

used to evaluate the quality of the data for the groundwater 

samples. Blanks rarely contained detectable concentrations of 

any constituent. All replicate samples were within acceptable 

limits of variability, and matrix-spike recoveries were 

generally within the acceptable range.

Organic and inorganic constituents were sampled at 

all 58 wells in the WSJV study unit. Most detections were 

measured at concentrations less than health-based benchmarks. 

In the Delta-Mendota subbasin study area grid wells, 

2 detections of arsenic were greater than the USEPA 

maximum contaminant level (MCL-US), 16 detections of 

boron were greater than the CDPH notification level (NLCA), 2 detections of molybdenum were greater than the 

USEPA lifetime health advisory level (HAL-US), 1 detection 

of selenium was greater than the MCL-US, 2 detections of 

strontium were greater than the HAL-US, and 3 detections 

of nitrate were greater than the MCL-US. In the Westside 

subbasin study area grid wells, four detections of boron were 

at concentrations greater than the NL-CA.

In the WSJV study unit understanding wells, three 

detections of benzene were greater than the CDPH maximum 

contaminant level (MCL-CA), five detections arsenic were 

greater than the MCL-US, seven detections of boron were 

greater than the NL-CA, one detection of molybdenum was 

greater than the HAL-US, one detection of strontium was 

greater than the HAL-US, two detections of uranium were 

greater than the MCL-US, and six detections of nitrate were 

greater than the MCL-US.

In the Delta-Mendota subbasin study area grid wells, 

5 detections of iron were greater than the CDPH secondary 

maximum contaminant level (SMCL-CA), 13 detections of 

manganese were greater than the SMCL-CA, 14 detections 

of chloride were greater than the recommended SMCL-CA 

(5 of the detections were greater than the upper SMCL-CA), 

13 detections of sulfate were greater than the SMCL-CA (9 of 

the detections were greater than the upper SMCL-CA), and 

27 detections of TDS were greater than the recommended 

SMCL-CA (16 of the detections were greater than the upper 

SMCL-CA).

In the Westside subbasin study area grid wells, three 

detections of manganese were greater than the SMCL-CA, 

eight detections of sulfate were greater than the SMCL-CA 

(four of the detections were greater than the upper 

SMCL-CA), and nine detections of TDS were greater than the 

recommended SMCL-CA (four of the detections were greater 

than the upper SMCL-CA).

In the WSJV study unit understanding wells, 8 detections 

of manganese were greater than the SMCL-CA, 6 detections 

of chloride were greater than the recommended SMCL-CA, 

9 detections of sulfate were greater than the SMCL-CA (4 of 

the detections were greater than the upper SMCL-CA), and 

15 detections of TDS were greater than the recommended 

SMCL-CA (7 of the detections were greater than the upper 

SMCL-CA).
References Cited 19

Acknowledgments

The authors thank the following cooperators for their 

support: the California State Water Resources Control 

Board, California Department of Public Health, California 

Department of Water Resources, and Lawrence Livermore 

National Laboratory. We especially thank the well owners and 

water purveyors for allowing the U.S. Geological Survey to 

collect samples from their wells.

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Table 1 25

Table 1. Identification, sampling, and construction information for wells sampled for the Western San Joaquin Valley (WSJV) 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.

[Sampling schedule is described in table 2. Land-surface datum (LSD) is a datum plane that is approximately at land surface at each well. The altitude of the 

LSD is described in feet above the North American Vertical Datum of 1988 (NAVD 88). GAMA well identification number: DM, Delta-Mendota subbasin 

study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin 

study area understanding well. Other abbreviations: ft, feet; na, not available]

Sampling information

Well type

Construction information

GAMA well 

identification 

number

Date sampled 

(m/dd/ yyyy)

Sampling 

schedule

Altitude of LSD 

(ft above 

NAVD 88)

Well depth 

(ft below LSD)

Depth to top 

perforation 

(ft below LSD)

Depth to bottom 

perforation 

(ft below LSD)

WSJV grid wells (39 wells sampled)

Delta-Mendota subbasin study area (29 grid wells sampled)

DM-01 3/1/2010 Intermediate 165 Production 913 663 903

DM-02 3/1/2010 Slow 162 Production 250 140 250

DM-03 3/2/2010 Slow 123 Production 310 164 308

DM-04 3/2/2010 Intermediate 110 Production 180 115 172

DM-05 3/3/2010 Slow 147 Production 230 115 220

DM-06 3/4/2010 Slow 87 Production 597 342 587

DM-07 3/8/2010 Intermediate 101 Production 300 na na

DM-08 3/8/2010 Intermediate 146 Production 260 185 na

DM-09 3/9/2010 Slow 90 Production 630 na na

DM-10 3/9/2010 Slow 97 Production 450 370 444

DM-11 3/10/2010 Intermediate 52 Production 168 146 168

DM-12 3/11/2010 Intermediate 131 Production 350 na na

DM-13 3/17/2010 Intermediate 154 Production 500 400 500

DM-14 3/18/2010 Intermediate 63 Production 104 na na

DM-15 4/12/2010 Intermediate 145 Production 218 198 218

DM-16 4/12/2010 Intermediate 114 Production 150 130 150

DM-17 4/13/2010 Intermediate 1,154 Production 280 na na

DM-18 4/13/2010 Intermediate 348 Unused 555 na na

DM-19 4/14/2010 Slow 228 Production 350 250 350

DM-20 4/15/2010 Intermediate 140 Production 210 190 210

DM-21 4/15/2010 Intermediate 225 Production 500 na na

DM-22 6/15/2010 Intermediate 150 Production 355 280 345

DM-23 6/15/2010 Intermediate 111 Production 200 140 200

DM-24 6/16/2010 Intermediate 122 Production 150 130 150

DM-25 6/16/2010 Slow 101 Production 150 130 150

DM-26 6/17/2010 Slow 143 Production 280 160 280

DM-27 6/24/2010 Slow 97 Production 575 350 575

DM-28 6/29/2010 Slow 196 Production 178 na na

DM-29 6/30/2010 Intermediate 94 Production 230 na na

Table 1
26 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Sampling information

Well type

Construction information

GAMA well 

identification 

number

Date sampled 

(m/dd/ yyyy)

Sampling 

schedule

Altitude of LSD 

(ft above 

NAVD 88)

Well depth 

(ft below LSD)

Depth to top 

perforation 

(ft below LSD)

Depth to bottom 

perforation 

(ft below LSD)

WSJV grid wells (39 wells sampled)—Continued

Westside subbasin study area (10 grid wells sampled)

WS-01 3/15/2010 Intermediate 482 Production 1,000 400 1,000

WS-02 6/7/2010 Intermediate 218 Production 678 348 678

WS-03 6/7/2010 Slow 310 Production 2,109 na na

WS-04 6/8/2010 Intermediate 237 Production 620 300 620

WS-05 6/8/2010 Slow 274 Production 1,550 650 na

WS-06 6/9/2010 Intermediate 231 Production na na na

WS-07 6/10/2010 Intermediate 239 Production 1,020 650 na

WS-08 6/22/2010 Intermediate 361 Unused 1,500 592 na

WS-09 6/23/2010 Slow 185 Production 610 580 600

WS-10 7/8/2010 Slow 232 Production 600 320 600

WSJV understanding wells (19 wells sampled)

DM-U-01 3/16/2010 Intermediate 97 Production na na na

WS-U-01 6/9/2010 Slow 247 Production 630 400 630

WS-U-02 6/10/2010 Slow 278 Production 570 210 545

WS-U-03 6/23/2010 Slow 611 Production 850 na na

Monitoring wells (15 wells sampled)

DM-U-02 3/29/2010 Slow 69 Monitoring 113 105 110

DM-U-03 3/29/2010 Slow 69 Monitoring 20 12 17

DM-U-04 3/30/2010 Slow 175 Monitoring 110 90 110

DM-U-05 3/30/2010 Slow 175 Monitoring 355 335 355

DM-U-06 3/30/2010 Slow 175 Monitoring 160 150 160

DM-U-07 3/31/2010 Slow 168 Monitoring 550 530 550

DM-U-08 3/31/2010 Slow 168 Monitoring 395 375 395

DM-U-09 3/31/2010 Slow 168 Monitoring 215 195 215

DM-U-10 4/6/2010 Slow 166 Monitoring 550 530 550

DM-U-11 4/6/2010 Slow 166 Monitoring 78 58 78

DM-U-12 4/7/2010 Slow 166 Monitoring 350 330 350

DM-U-13 5/18/2010 Slow 190 Monitoring 380 240 260

DM-U-14 5/19/2010 Slow 115 Monitoring 430 390 410

DM-U-15 5/19/2010 Slow 115 Monitoring 115 95 115

DM-U-16 5/20/2010 Slow 115 Monitoring 255 235 255

Table 1. Identification, sampling, and construction information for wells sampled for the Western San Joaquin Valley (WSJV) 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued

[Sampling schedule is described in table 2. Land-surface datum (LSD) is a datum plane that is approximately at land surface at each well. The altitude of the 

LSD is described in feet above the North American Vertical Datum of 1988 (NAVD 88). GAMA well identification number: DM, Delta-Mendota subbasin 

study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin 

study area understanding well. Other abbreviations: ft, feet; na, not available]
Table 2 27

Table 2. Classes of chemical constituents and water-quality indicators collected for the slow and the intermediate well-sampling 

schedules in the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, 

March to July 2010. 

Constituent classes Slow 

schedule

Intermediate 

schedule

Constituent 

list table

Results 

table

Water-quality indicators 

Dissolved oxygen, temperature, pH, and specific conductance X X 4

Turbidity X 4

Field alkalinity, bicarbonate, and carbonate X 4

Organic constituents

Volatile organic compounds (VOCs) X X 3A 5

Low-level fumigants X X 3B 5

Pesticides and pesticide degradates X X 3C 6

Constituents of special interest

Perchlorate X X 3D 7

1,2,3-Trichloropropane (1,2,3-TCP) X X 3D 7

N-Nitrosodimethylamine (NDMA) X 3D none 1

Inorganic constituents

Trace elements X X 3E 8

Nutrients X X 3F 9

Dissolved organic carbon (DOC) X 3F 9

Major and minor ions, silica, and total dissolved solids (TDS) X X 3G 10

Laboratory alkalinity, bicarbonate, and carbonate X X 3G 4

Total arsenic and iron (unfiltered) and arsenic and iron species (filtered) X X 3H 11

Chromium species (filtered) X 3H 11

Stable isotopes

Stable isotopes of hydrogen and oxygen in water X X 3I 12

Stable isotopes of nitrogen and oxygen in nitrate X 3I 12

Stable isotopes of sulfur in dissolved sulfate X 3I 12

Stable isotopes of boron in water X 3I 12

Isotopic ratios of strontium in water X 3I 12

Stable isotopes of carbon in dissolved inorganic carbon and carbon-14 abundance X X 3I 12

Radioactivity and dissolved gases

Tritium X X 3I 12

Dissolved gases X 3I 13

Dissolved noble gases, tritium, and helium isotope ratios X X 3I none 2

 1 Constituent not detected in groundwater samples.

2

 Results for dissolved noble gases, tritium, and helium isotope ratios analyzed by Lawrence Livermore National Laboratory (LLNL), Livermore, California, 

were not completed in time for inclusion in this report; results will be presented in a subsequent publication.

Table 

2
28 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 3A. Volatile organic compounds (VOCs), primary uses or sources, comparative benchmarks, and reporting information for the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 2020. [The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; RSD5-US, USEPA risk specific dose at a risk factor of 10−5. Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; SRL, study reporting level; THM, trihalomethane; D, detected in groundwater samples (table 5); na, not available; μg/L, micrograms per liter; —, not detected; WSJV, Western San Joaquin Valley study unit; ≤, less than or equal to] Constituent (synonym or abbreviation) Primary use or source USGS parameter code CAS number 1 LRL or SRL (μg/L) Benchmark type Benchmark level (μg/L) Detection Acetone Solvent 81552 67-64-1 3.4 na na

—

Acrylonitrile Organic synthesis 34215 107-13-1 0.8 RSD5-US 0.6

—

tert-Amyl methyl ether (TAME) Gasoline oxygenate 50005 994-05-8 0.06 na na

—

Benzene Gasoline hydrocarbon 34030 71-43-2 0.026 MCL-CA 1 D

Bromobenzene Solvent 81555 108-86-1 0.022 na na

—

Bromochloromethane Fire retardant 77297 74-97-5 0.06 HAL-US 90

—

Bromodichloromethane Disinfection byproduct (THM) 32101 75-27-4 0.034 MCL-US 3 80

—

Bromoform (Tribromomethane) Disinfection byproduct (THM) 32104 75-25-2 0.1 MCL-US 3 80

D

Bromomethane (Methyl bromide) Fumigant 34413 74-83-9 0.2 HAL-US 10

—

n-Butylbenzene Gasoline hydrocarbon 77342 104-51-8 0.08 NL-CA 260

—

sec-Butylbenzene Gasoline hydrocarbon 77350 135-98-8 0.034 NL-CA 260

—

tert-Butylbenzene Gasoline hydrocarbon 77353 98-06-6 0.06 NL-CA 260

—

Carbon disulfide Organic synthesis 77041 75-15-0 4 0.06 NL-CA 160

D

Carbon tetrachloride (Tetrachloromethane) Solvent 32102 56-23-5 0.052 MCL-CA 0.5

—

Chlorobenzene Solvent 34301 108-90-7 0.016 MCL-CA 70

—

Chloroethane Solvent 34311 75-00-3 0.06 na na

—

Chloroform (Trichloromethane) Disinfection byproduct (THM) 32106 67-66-3 5 0.02 MCL-US 3 80

D

Chloromethane Solvent 34418 74-87-3 0.14 HAL-US 30

D

3-Chloropropene Organic synthesis 78109 107-05-1 0.08 na na

—

2-Chlorotoluene Solvent 77275 95-49-8 0.028 NL-CA 140

—

4-Chlorotoluene Solvent 77277 106-43-4 0.042 NL-CA 140

—

Dibromochloromethane Disinfection byproduct (THM) 32105 124-48-1 0.12 MCL-US 3 80

D

1,2-Dibromo-3-chloropropane (DBCP) 6 Fumigant 82625 96-12-8 0.34 MCL-US 0.2

D

1,2-Dibromoethane (EDB) 6 Fumigant 77651 106-93-4 0.05 MCL-US 0.05

—

Dibromomethane Solvent 30217 74-95-3 0.05 na na

—

1,2-Dichlorobenzene Solvent 34536 95-50-1 0.028 MCL-CA 600

—

1,3-Dichlorobenzene Solvent 34566 541-73-1 0.024 HAL-US 600

—

1,4-Dichlorobenzene Fumigant 34571 106-46-7 0.026 MCL-CA 5 —

trans-1,4-Dichloro-2-butene Organic synthesis 73547 110-57-6 0.36 na na

—

Table 3A
Table 3A 29

Constituent 

(synonym or abbreviation)

Primary use 

or source

USGS 

parameter code

CAS 

number 1

LRL or SRL 

(μg/L)

Benchmark 

type

Benchmark 

level 

(μg/L)

Detection

Dichlorodifluoromethane (CFC-12) Refrigerant 34668 75-71-8 0.1 NL-CA 1,000 —

1,1-Dichloroethane (1,1-DCA) Solvent 34496 75-34-3 0.044 MCL-CA 5 D

1,2-Dichloroethane (1,2-DCA) Solvent 32103 107-06-2 0.08 MCL-CA 0.5 —

1,1-Dichloroethene (1,1-DCE) Organic synthesis 34501 75-35-4 0.022 MCL-CA 6 —

cis-1,2-Dichloroethene (cis-1,2-DCE) Solvent 77093 156-59-2 0.022 MCL-CA 6 —

trans-1,2-Dichloroethene (trans-1,2-DCE) Solvent 34546 156-60-5 0.018 MCL-CA 10 —

1,2-Dichloropropane Fumigant 34541 78-87-5 0.026 MCL-US 5 D

1,3-Dichloropropane Fumigant 77173 142-28-9 0.06 na na —

2,2-Dichloropropane Fumigant 77170 594-20-7 0.06 na na —

1,1-Dichloropropene Organic synthesis 77168 563-58-6 0.03 na na —

cis-1,3-Dichloropropene Fumigant 34704 10061-01-5 0.1 RSD5-US 7 4 —

trans-1,3-Dichloropropene Fumigant 34699 10061-02-6 0.14 RSD5-US 7 4 —Diethyl ether Solvent 81576 60-29-7 0.08 na na —

Diisopropyl ether (DIPE) Gasoline oxygenate 81577 108-20-3 0.06 na na —

Ethylbenzene Gasoline hydrocarbon 34371 100-41-4 2 0.1 MCL-CA 300 D 2

Ethyl tert-butyl ether (ETBE) Gasoline oxygenate 50004 637-92-3 0.032 na na —

Ethyl methacrylate Organic synthesis 73570 97-63-2 0.14 na na —

o-Ethyl toluene (1-Ethyl-2-methyl benzene) Gasoline hydrocarbon 77220 611-14-3 0.032 na na D

Hexachlorobutadiene Organic synthesis 39702 87-68-3 0.06 RSD5-US 9 —Hexachloroethane Solvent 34396 67-72-1 0.14 HAL-US 1 —2-Hexanone (n-Butyl methyl ketone) Solvent 77103 591-78-6 0.46 na na —

Iodomethane (Methyl iodide) Organic synthesis 77424 74-88-4 0.26 na na —

Isopropylbenzene Gasoline hydrocarbon 77223 98-82-8 0.042 NL-CA 770 D

1,3-Dichlorobenzene Solvent 34566 541-73-1 0.024 HAL-US 600 —1,4-Dichlorobenzene Fumigant 34571 106-46-7 0.026 MCL-CA 5 —

trans-1,4-Dichloro-2-butene Organic synthesis 73547 110-57-6 0.36 na na —

Dichlorodifluoromethane (CFC-12) Refrigerant 34668 75-71-8 0.1 NL-CA 1,000 —

1,1-Dichloroethane (1,1-DCA) Solvent 34496 75-34-3 0.044 MCL-CA 5 D

Table 3A. Volatile organic compounds (VOCs), primary uses or sources, comparative benchmarks, and reporting information for the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 2020.—Continued[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; RSD5-US, USEPA risk specific dose at a risk factor of 10−5. Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; SRL, study reporting level; THM, 

trihalomethane; D, detected in groundwater samples (table 5); na, not available; μg/L, micrograms per liter; —, not detected; WSJV, Western San Joaquin Valley study unit; ≤, less than or equal to]
30 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Constituent (synonym or abbreviation) Primary use or source USGS parameter code CAS number 1 LRL or SRL (μg/L) Benchmark type Benchmark level (μg/L) Detection 1,2-Dichloroethane (1,2-DCA) Solvent 32103 107-06-2 0.08 MCL-CA 0.5

—

1,1-Dichloroethene (1,1-DCE) Organic synthesis 34501 75-35-4 0.022 MCL-CA 6 —

cis-1,2-Dichloroethene (cis-1,2-DCE) Solvent 77093 156-59-2 0.022 MCL-CA 6 —

trans-1,2-Dichloroethene (trans-1,2-DCE) Solvent 34546 156-60-5 0.018 MCL-CA 10

—

1,2-Dichloropropane Fumigant 34541 78-87-5 0.026 MCL-US 5 D

1,3-Dichloropropane Fumigant 77173 142-28-9 0.06 na na

—

2,2-Dichloropropane Fumigant 77170 594-20-7 0.06 na na

—

1,1-Dichloropropene Organic synthesis 77168 563-58-6 0.03 na na

—

cis-1,3-Dichloropropene Fumigant 34704 10061-01-5 0.1 RSD5-US 7 4

—

trans-1,3-Dichloropropene Fumigant 34699 10061-02-6 0.14 RSD5-US 7 4

—

Diethyl ether Solvent 81576 60-29-7 0.08 na na

—

Diisopropyl ether (DIPE) Gasoline oxygenate 81577 108-20-3 0.06 na na

—

Ethylbenzene Gasoline hydrocarbon 34371 100-41-4 2 0.1 MCL-CA 300 D 2

Ethyl tert-butyl ether (ETBE) Gasoline oxygenate 50004 637-92-3 0.032 na na

—

Ethyl methacrylate Organic synthesis 73570 97-63-2 0.14 na na

—

o-Ethyl toluene (1-Ethyl-2-methyl benzene) Gasoline hydrocarbon 77220 611-14-3 0.032 na na

D

Hexachlorobutadiene Organic synthesis 39702 87-68-3 0.06 RSD5-US 9 —Hexachloroethane Solvent 34396 67-72-1 0.14 HAL-US 1 —2-Hexanone (n-Butyl methyl ketone) Solvent 77103 591-78-6 0.46 na na

—

Iodomethane (Methyl iodide) Organic synthesis 77424 74-88-4 0.26 na na

—

Isopropylbenzene Gasoline hydrocarbon 77223 98-82-8 0.042 NL-CA 770

D

1,2,3-Trimethylbenzene Gasoline hydrocarbon 77221 526-73-8 0.06 na na

D

1,2,4-Trimethylbenzene Gasoline hydrocarbon 77222 95-63-6 2 0.56 NL-CA 330

—

1,3,5-Trimethylbenzene Organic synthesis 77226 108-67-8 0.032 NL-CA 330

—

Vinyl bromide (Bromoethene) Fire retardant 50002 593-60-2 0.12 na na

—

Vinyl chloride (Chloroethene) Organic synthesis 39175 75-01-4 0.06 MCL-CA 0.5

—

Table 3A. Volatile organic compounds (VOCs), primary uses or sources, comparative benchmarks, and reporting information for the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 2020.—Continued[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; RSD5-US, USEPA risk specific dose at a risk factor of 10−5. Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; SRL, study reporting level; THM, 

trihalomethane; D, detected in groundwater samples (table 5); na, not available; μg/L, micrograms per liter; —, not detected; WSJV, Western San Joaquin Valley study unit; ≤, less than or equal to]
Table 3A 31

Constituent 

(synonym or abbreviation)

Primary use 

or source

USGS 

parameter code

CAS 

number 1

LRL or SRL 

(μg/L)

Benchmark 

type

Benchmark 

level 

(μg/L)

Detection

m- plus p-Xylene Gasoline hydrocarbon 85795 108-38-3 / 106-42-3 2 0.4 MCL-CA 8 1,750 D 2

o-Xylene Gasoline hydrocarbon 77135 95-47-6 2 0.1 MCL-CA 8 1,750 D 2

1 This report contains CAS Registry Numbers® (CASRNs), which is a Registered Trademark of the American Chemical Society. CAS recommends the verification of the CASRNs through CAS Client 

ServicesSM.

2 The SRLs were defined by Fram and others (2012). Values measured at concentrations less than or equal to the SRL have been flagged with a ≤ symbol, reclassified as non-detections, and not included in the 

calculations of detection frequencies (with the exception of wells WS-U-01 and WS-U-02 [see the appendix section titled “Detections in Blanks and Application of SRLs” for details]).

3 The MCL-US benchmark for trihalomethanes is the sum of chloroform, bromoform, bromodichloromethane, and dibromochloromethane.4 The SRL was defined based on the highest concentration detected in the WSJV blanks. Values measured at concentrations less than or equal to the SRL have been flagged with a ≤ symbol, reclassified as 

non-detections, and not included in the calculations of detection frequencies.

5 The SRL was defined based on a detection in the WSJV equipment blank and only can potentially affect monitoring well samples. Values measured at concentrations less than or equal to the SRL have been 

flagged with a ≤ symbol and reclassified as non-detections.

6 Analyzed on NWQL Schedules 2020 and 1306.7 The RSD5 benchmark for 1,3-dichloropropene is the sum of its isomers (cis and trans).8 The MCL-CA benchmarks for m- plus p-xylene and o-xylene is the sum of all three xylene compounds.

Table 3A. Volatile organic compounds (VOCs), primary uses or sources, comparative benchmarks, and reporting information for the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 2020.—Continued[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; RSD5-US, USEPA risk specific dose at a risk factor of 10−5. Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; SRL, study reporting level; THM, 

trihalomethane; D, detected in groundwater samples (table 5); na, not available; μg/L, micrograms per liter; —, not detected; WSJV, Western San Joaquin Valley study unit; ≤, less than or equal to]
32 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 3B. Low-level fumigants, primary uses or sources, comparative benchmarks, and reporting information for the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 1306. [The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency maximum contaminant level. Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; D, detected in groundwater samples (table 5); μg/L, micrograms per liter; —, not detected] Constituent (synonym or abbreviation) Primary use or source USGS parameter code CAS number LRL (μg/L) Benchmark type Benchmark level (μg/L) Detection 1,2-Dibromo-3-chloropropane (DBCP) 1 Fumigant 82625 96-12-8 0.03 MCL-US 0.2

D

1,2-Dibromoethane (EDB) 1 Fumigant 77651 106-93-4 0.018 MCL-US 0.05

—

1 Analyzed on NWQL Schedules 1306 and 2020.

Table 3B
Table 3C 33

Table 3C. Pesticides and pesticide degradates, primary uses or sources, comparative benchmarks, and reporting information for the 

U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 2033.

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of 

March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as 

MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health 

advisory level; MCL-CA, California Department of Public Health maximum contaminant level; MCL-US, USEPA maximum contaminant level; RSD5-US, 

USEPA risk specific dose at a risk factor of 10−5; Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; D, detected in 

groundwater samples (table 6); na, not available; μg/L, micrograms per liter; —, not detected]

Constituent 

(synonym or abbreviation)

Primary use 

or source

USGS 

parameter 

code

CAS 

number

LRL 

(μg/L)

Benchmark 

type

Benchmark 

level 

(μg/L)

Detection

Acetochlor Herbicide 49260 34256-82-1 0.010 na na —

Alachlor Herbicide 46342 15972-60-8 0.008 MCL-US 2 —

Atrazine Herbicide 39632 1912-24-9 0.007 MCL-CA 1 D

Azinphos-methyl Insecticide 82686 86-50-0 0.12 na na —

Azinphos-methyl oxon Insecticide degradate 61635 961-22-8 0.042 na na — 1

Benfluralin Herbicide 82673 1861-40-1 0.014 na na — 1

Carbaryl Insecticide 82680 63-25-2 0.06 RSD5-US 400 —

Carbofuran Insecticide 82674 1563-66-2 0.060 MCL-CA 18 —

2-Chloro-2,6-diethylacetanilide Herbicide degradate 61618 6967-29-9 0.010 na na —

4-Chloro-2-methylphenol Herbicide degradate 61633 1570-64-5 0.0032 na na —

Chlorpyrifos Insecticide 38933 2921-88-2 0.010 HAL-US 2 —

Chlorpyrifos oxon Insecticide degradate 61636 5598-15-2 0.05 na na — 1

Cyanazine Herbicide 04041 21725-46-2 0.022 HAL-US 1 —

Cyfluthrin Insecticide 61585 68359-37-5 0.016 na na — 1

λ-Cyhalothrin Insecticide 61595 91465-08-6 0.010 na na — 1

Cypermethrin Insecticide 61586 52315-07-8 0.020 na na — 1

DCPA (Dacthal) Herbicide 82682 1861-32-1 0.0076 HAL-US 70 D

Deethylatrazine (2-Chloro-4-

isopropylamino-6-amino-striazine)

Herbicide degradate 04040 6190-65-4 0.014 na na D

Desulfinylfipronil Insecticide degradate 62170 na 0.012 na na —

Desulfinylfipronil amide Insecticide degradate 62169 na 0.029 na na —

Diazinon Insecticide 39572 333-41-5 0.005 HAL-US 1 —

Diazinon oxon Insecticide degradate 61638 962-58-3 0.0060 na na —

3,4-Dichloroaniline Herbicide degradate 61625 95-76-1 0.0042 na na D

3,5-Dichloroaniline Herbicide degradate 61627 626-43-7 0.0030 na na D

Dichlorvos Insecticide 38775 62-73-7 0.02 na na — 1

Dicrotophos Insecticide 38454 141-66-2 0.08 na na — 1

Dieldrin Insecticide 39381 60-57-1 0.009 RSD5-US 0.02 —

2,6-Diethylaniline Herbicide degradate 82660 579-66-8 0.006 na na D

Dimethoate Insecticide 82662 60-51-5 0.006 na na — 1

Disulfoton Insecticide 82677 298-04-4 0.04 HAL-US 0.7 —

Disulfoton sulfone Insecticide degradate 61640 2497-06-5 0.0136 na na —

α-Endosulfan Insecticide 34362 959-98-8 0.006 na na —

Endosulfan sulfate Insecticide degradate 61590 1031-07-8 0.014 na na —

Ethion Insecticide 82346 563-12-2 0.008 na na — 1

Ethion monoxon Insecticide degradate 61644 17356-42-2 0.021 na na —

Ethoprophos Herbicide 82672 13194-48-4 0.016 na na —

S-Ethyl-dipropylthiocarbamate 

(EPTC)

Herbicide 82668 759-94-4 0.0020 na na D

Table 3C
34 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Constituent 

(synonym or abbreviation)

Primary use 

or source

USGS 

parameter 

code

CAS 

number

LRL 

(μg/L)

Benchmark 

type

Benchmark 

level 

(μg/L)

Detection

2-Ethyl-6-methylaniline Herbicide degradate 61620 24549-06-2 0.0098 na na D

Fenamiphos Insecticide 61591 22224-92-6 0.030 HAL-US 0.7 —

Fenamiphos sulfone Insecticide degradate 61645 31972-44-8 0.053 na na —

Fenamiphos sulfoxide Insecticide degradate 61646 31972-43-7 0.08 na na — 1

Fipronil Insecticide 62166 120068-37-3 0.018 na na —

Fipronil sulfide Insecticide degradate 62167 120067-83-6 0.013 na na —

Fipronil sulfone Insecticide degradate 62168 120068-36-2 0.024 na na —

Fonofos Insecticide 04095 944-22-9 0.0044 HAL-US 10 —

Hexazinone Herbicide 04025 51235-04-2 0.008 HAL-US 400 D 1

Iprodione Fungicide 61593 36734-19-7 0.014 na na —

Isofenphos Insecticide 61594 25311-71-1 0.006 na na —

Malaoxon Insecticide degradate 61652 1634-78-2 0.08 na na —

Malathion Insecticide 39532 121-75-5 0.016 HAL-US 100 —

Metalaxyl Fungicide 61596 57837-19-1 0.007 na na D

Methidathion Insecticide 61598 950-37-8 0.006 na na —

Metolachlor Herbicide 39415 51218-45-2 0.014 HAL-US 700 D

Metribuzin Herbicide 82630 21087-64-9 0.012 HAL-US 70 —

Molinate Herbicide 82671 2212-67-1 0.002 MCL-CA 20 D

Myclobutanil Fungicide 61599 88671-89-0 0.010 na na —

1-Naphthol Insecticide degradate 49295 90-15-3 0.036 na na — 1

Oxyfluorfen Herbicide 61600 42874-03-3 0.010 na na — 1

Paraoxon-methyl Insecticide degradate 61664 950-35-6 0.010 na na — 1

Parathion-methyl Insecticide 82667 298-00-0 0.008 HAL-US 1 —

Pendimethalin Herbicide 82683 40487-42-1 0.012 na na —

cis-Permethrin Insecticide 82687 54774-45-7 0.014 na na — 1

Phorate Insecticide 82664 298-02-2 0.02 na na —

Phorate oxon Insecticide degradate 61666 2600-69-3 0.027 na na —

Phosmet Insecticide 61601 732-11-6 0.034 na na — 1

Phosmet oxon Insecticide degradate 61668 3735-33-9 0.0511 na na — 1

Prometon Herbicide 04037 1610-18-0 0.012 HAL-US 100 D

Prometryn Herbicide 04036 7287-19-6 0.006 na na D

Pronamide Herbicide 82676 23950-58-5 0.0036 RSD5-US 20 —

Propanil Herbicide 82679 709-98-8 0.010 na na —

Propargite Insecticide 82685 2312-35-8 0.020 na na — 1

cis-Propiconazole Fungicide 79846 60207-90-1 0.006 na na — 2

trans-Propiconazole Fungicide 79847 60207-90-1 0.02 na na —

Simazine Herbicide 04035 122-34-9 0.006 MCL-US 4 D

Tebuconazole Fungicide 62852 107534-96-3 0.020 na na —

Tebuthiuron Herbicide 82670 34014-18-1 0.028 HAL-US 500 D 2

Table 3C. Pesticides and pesticide degradates, primary uses or sources, comparative benchmarks, and reporting information for the 

U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 2033.—Continued

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of 

March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as 

MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health 

advisory level; MCL-CA, California Department of Public Health maximum contaminant level; MCL-US, USEPA maximum contaminant level; RSD5-US, 

USEPA risk specific dose at a risk factor of 10−5; Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; D, detected in 

groundwater samples (table 6); na, not available; μg/L, micrograms per liter; —, not detected]

Table 3C
Table 3C 35

Constituent 

(synonym or abbreviation)

Primary use 

or source

USGS 

parameter 

code

CAS 

number

LRL 

(μg/L)

Benchmark 

type

Benchmark 

level 

(μg/L)

Detection

Tefluthrin Insecticide 61606 79538-32-2 0.010 na na — 1

Terbufos Insecticide 82675 13071-79-9 0.018 HAL-US 0.4 —

Terbufos oxon sulfone Insecticide degradate 61674 56070-15-6 0.045 na na — 1

Terbuthylazine Herbicide 04022 5915-41-3 0.006 na na —

Thiobencarb Herbicide 82681 28249-77-6 0.016 MCL-CA 70 —

Tribufos Defoliant 61610 78-48-8 0.018 na na — 1

Trifluralin Herbicide 82661 1582-09-8 0.018 HAL-US 10 —

1

 The median matrix-spike recovery was less than 70 percent. Low recoveries may indicate that the compound might not have been detected in some samples 

if it was present at very low concentrations.

2

 The median matrix-spike recovery was greater than 130 percent. High recoveries may indicate that reported values could be greater than what is in the 

sample.

Table 3C. Pesticides and pesticide degradates, primary uses or sources, comparative benchmarks, and reporting information for the 

U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 2033.—Continued

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of 

March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as 

MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health 

advisory level; MCL-CA, California Department of Public Health maximum contaminant level; MCL-US, USEPA maximum contaminant level; RSD5-US, 

USEPA risk specific dose at a risk factor of 10−5; Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; D, detected in 

groundwater samples (table 6); na, not available; μg/L, micrograms per liter; —, not detected]
36 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 3D. Constituents of special interest, primary uses or sources, comparative benchmarks, and reporting information for Weck Laboratories, Inc., City of Industry, California, analyses. [The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. MRL, benchmark type, and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency lifetime health advisory; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; NL-CA, CDPH notification level. Other abbreviations: CAS, Chemical Abstract Service; MRL, minimum reporting level; D, detected in groundwater samples (table 7); μg/L, micrograms per liter; —, not detected] Constituent (synonym or abbreviation) Primary use or source USGS parameter code CAS number MRL (μg/L) Benchmark type Benchmark level (μg/L) Detection Perchlorate Rocket fuel, fireworks, flares 63790 14797-73-0 0.10 MCL-CA 6 D 1,2,3-Trichloropropane (1,2,3-TCP) Fumigant, solvent 77443 96-18-4 0.005 HAL-US 40

D

N-Nitrosodimethylamine (NDMA) Disinfection byproduct 34438 62-75-9 0.002 NL-CA 0.010

—

Table 3D
Table 3E 37

Table 3E. Trace elements, comparative benchmarks, and reporting information for the U.S. Geological Survey (USGS) National Water 

Quality Laboratory (NWQL) Schedule 1948.

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of 

March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as 

MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, 

USEPA lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum 

contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: CAS, Chemical 

Abstract Service; LRL, laboratory reporting level; SRL, study reporting level; D, detected in groundwater samples (table 8); na, not available; μg/L, micrograms 

per liter; —, not detected; WSJV, Western San Joaquin Valley study unit]

Constituent USGS 

parameter code

CAS 

number

LRL or SRL 

(μg/L)

Benchmark 

type

Benchmark level 

(μg/L) Detection

Aluminum 01106 7429-90-5 1

 1.6 MCL-CA 1,000 D

Antimony 01095 7440-36-0 0.054 MCL-US 6 D

Arsenic 01000 7440-38-2 0.044 MCL-US 10 D

Barium 01005 7440-39-3 1

 0.36 MCL-CA 1,000 D

Beryllium 01010 7440-41-7 0.012 MCL-US 4 D

Boron 01020 7440-42-8 2.8 NL-CA 1,000 D

Cadmium 01025 7440-43-9 0.02 MCL-US 5 D

Chromium 01030 7440-47-3 1

 0.42 MCL-CA 50 D

Cobalt 01035 7440-48-4 2

 0.33 na na D

Copper 01040 7440-50-8 1

 1.7 AL-US 1,300 D

Iron 01046 7439-89-6 1

 6 SMCL-CA 300 D

Lead 01049 7439-92-1 1

 0.65 AL-US 15 D

Lithium 01130 7439-93-2 0.44 na na D

Manganese 01056 7439-96-5 2

 0.52 SMCL-CA 50 D

Molybdenum 01060 7439-98-7 0.028 HAL-US 40 D

Nickel 01065 7440-02-0 1

 0.36 MCL-CA 100 D

Selenium 01145 7782-49-2 0.040 MCL-US 50 D

Silver 01075 7440-22-4 2

 0.01 SMCL-CA 100 D

Strontium 01080 7440-24-6 0.40 HAL-US 4,000 D

Thallium 01057 7440-28-0 0.020 MCL-US 2 —

Tungsten 01155 7440-33-7 1

 0.11 na na D

Uranium 22703 7440-61-1 0.008 MCL-US 30 D

Vanadium 01085 7440-62-2 0.16 NL-CA 50 D

Zinc 01090 7440-66-6 1

 4.8 SMCL-CA 3 5,000 D

1

 The SRL was defined by Olsen and others (2010).

2

 The SRL was defined based on the highest concentration detected in the WSJV blanks.

3

 The secondary maximum contaminant level for zinc is listed as SMCL-CA because SMCLs established by CDPH are used in this report for all constituents 

that have SMCL-CA values.

Table 3E
38 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 3F. Nutrients and dissolved organic carbon (DOC), comparative benchmarks, and reporting information for the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 2755 and Laboratory Code 2613. [The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health advisory level; MCL-US, USEPA maximum contaminant level. Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory reporting level; SRL, study reporting level; D, detected in groundwater samples (table 9); na, not available; mg/L, milligrams per liter; WSJV, Western San Joaquin Valley study unit; ≤, less than or equal to] Constituent USGS parameter code CAS number LRL or SRL (mg/L) Benchmark type Benchmark level (mg/L) Detection Ammonia (as nitrogen) 00608 7664-41-7 2 0.016 HAL-US 3 24.7

D

Nitrate plus nitrite (as nitrogen) 1 00631 na 0.04 MCL-US 10

D

Nitrite (as nitrogen) 00613 14797-65-0 2 0.001 MCL-US 1 D

Total nitrogen (ammonia + nitrite + nitrate + organic nitrogen) 62854 17778-88-0 0.10 na na

D

Phosphate, orthophosphate (as phosphorus) 00671 14265-44-2 0.008 na na

D

Dissolved organic carbon (DOC) 00681 na 0.66 na na

D

1 Nitrite plus nitrate (as nitrogen) is referred to as nitrate in the text for clarity.2 The SRL was defined based on a detection in a WSJV blank. Values measured at concentrations less than or equal to the SRL are reclassified as ≤ the value reported by the laboratory.3 The HAL-US is 30 mg/L “as ammonia.” To facilitate comparison to the analytical results, we have converted and reported this HAL-US as 24.7 mg/L “as nitrogen.”

Table 3F
Table 3H 39

Table 3G. Major and minor ions, silica, total dissolved solids (TDS), and alkalinity, comparative benchmarks, and reporting information 

for the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) Schedule 1948.

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. LRL, benchmark type, and benchmark level as of 

March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and 

as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health (CDPH) maximum 

contaminant level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: CAS, Chemical Abstract Service; LRL, laboratory 

reporting level; D, detected in groundwater samples (table 10); na, not available; mg/L, milligrams per liter; SiO2, silicon dioxide; CaCO3, calcium carbonate]

Constituent 

(synonym or abbreviation)

USGS 

parameter code

CAS 

number

 LRL 

(mg/L)

Benchmark 

type

Benchmark level 

(mg/L) Detection

Bromide 71870 24959-67-9 0.02 na na D

Calcium 00915 7440-70-2 0.044 na na D

Chloride 00940 16887-00-6 0.12 SMCL-CA 1

 250 (500) D

Fluoride 00950 16984-48-8 0.08 MCL-CA 2 D

Iodide 71865 7553-56-2 0.002 na na D

Magnesium 00925 7439-95-4 0.016 na na D

Potassium 00935 7440-09-7 0.064 na na D

Sodium 00930 7440-23-5 0.10 na na D

Sulfate 00945 14808-79-8 0.18 SMCL-CA 1

 250 (500) D

Silica (as SiO2

) 00955 7631-86-9 0.058 na na D

Total dissolved solids (TDS) 70300 na 10 SMCL-CA 1

 500 (1,000) D

Laboratory alkalinity (as CaCO3

) 2 29801 na 1 na na D

1

 The SMCL-CA benchmarks for chloride, sulfate, and TDS have recommended and upper benchmark levels. The upper benchmark level is shown in 

parentheses.

2

 Laboratory alkalinity results are presented in table 4.

Table 3H. Total arsenic and iron (unfiltered) and arsenic, chromium, and iron species (filtered), comparative benchmarks, and 

reporting information for the U.S. Geological Survey (USGS) National Research Laboratory (NRP) Trace Metal Laboratory (TML), 

Boulder, Colorado, analyses.

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. MDL, benchmark type, and benchmark level as of 

March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and 

as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health (CDPH) maximum 

contaminant level; MCL-US, U.S. Environmental Protection Agency maximum contaminant level; SMCL-CA, CDPH secondary maximum contaminant level. 

Other abbreviations: CAS, Chemical Abstract Service; MDL, method detection limit; na, not available; μg/L, micrograms per liter; D, detected in groundwater 

samples (table 11)]

Constituent USGS 

parameter code

CAS 

number

MDL 

(μg/L)

Benchmark 

type

Benchmark level 

(μg/L) Detection

Arsenic (total, unfiltered) na na 0.5 MCL-US 10 D

Arsenic-III (filtered) 99034 22569-72-8 1 na na D

Arsenic (total, filtered) 99033 7440-38-2 0.5 MCL-US 10 D

Chromium (total, filtered) 01030 7440-47-3 1 MCL-CA 50 D

Chromium-VI (filtered) 01032 18540-29-9 1 na na D

Iron (total, unfiltered) na na 2 SMCL-CA 300 D

Iron-II (filtered) 01047 7439-89-6 2 na na D

Iron (total, filtered) 01046 7439-89-6 2 SMCL-CA 300 D

Table 

3G

Table 

3H
40 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Table 3I. Isotopic tracers, comparison benchmarks, and reporting information for laboratory analyses.

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. Laboratory entity codes in the USGS National Water 

Information System (NWIS) for laboratories other than the USGS National Water Quality Laboratory (NWQL) are given in parentheses after the laboratory 

names. Benchmark type and benchmark value as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the 

MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department 

of Public Health maximum contaminant level. Elements: H, hydrogen; O, oxygen; N, nitrogen; S, sulfur; B, boron; Sr, strontium; C, carbon. Reporting 

units: μg/L, micrograms per liter; mg/L, milligrams per liter; pCi/L, picocuries per liter; cm3 STP/g, cubic centimeters of gas at standard temperature and 

pressure per gram of water. Other abbreviations: CAS, Chemical Abstract Service; na, not available; D, detected in groundwater samples (tables 12 and 13); 

%, percent; pmc, percent modern carbon; NRP, National Research Program]

Constituent

USGS 

parameter 

code

CAS 

number

Method 

uncertainty

Reporting 

units

Benchmark 

type

Benchmark 

level 

(pCi/L)

Detection

Dissolved standard gases 1

Methane 76994 74-82-8 na μg/L na na D

Carbon dioxide 00405 124-39-9 na mg/L na na D

Nitrogen 00597 7727-37-9 na mg/L na na D

Oxygen 62971 7782-44-7 na mg/L na na D

Argon 82043 7440-37-1 na mg/L na na D

Dissolved noble gases 2

Argon 85563 7440-37-1 2% cm3 STP/g na na na

Helium-4 85561 7440-59-7 2% cm3

 STP/g na na na

Krypton 85565 7439-90-9 2% cm3

 STP/g na na na

Neon 61046 7440-01-09 2% cm3

 STP/g na na na

Xenon 85567 7440-63-3 2% cm3

 STP/g na na na

Isotope ratios

Helium-3 / Helium-4 2 61040 na / 7440-59-7 0.75% atom ratio na na na

d2H in water 3 82082 na 2 per mil na na D

d18O in water 3 82085 na 0.20 per mil na na D

d15N in dissolved nitrate 3 82690 na 0.50 per mil na na D

d18O in dissolved nitrate 3 63041 na 1.00 per mil na na D

d34S in dissolved sulfate 3 49932 na 0.4 per mil na na D

d11B in water 4 62648 na na per mil na na D

Strontium isotope ratio (87Sr/86Sr) 4 75978 na 0.00005 atom ratio na na D

d13C in dissolved inorganic carbon 5 82081 na 0.05 per mil na na D

Radioactive constituents

C-14 5 49933 14762-75-5 0.0015 pmc na na D

Tritium 2, 6 07000 10028-17-8 1 pCi/L MCL-CA 20,000 D

1

 USGS Chlorofluorocarbon Laboratory, Reston, Virginia (USGSCFCVA).

2

 Lawrence Livermore National Laboratory, Livermore, California (CA-LLNL).

3

 USGS Stable Isotope Laboratory, Reston, Virginia (USGSSIVA).

4

 USGS NRP Metals Isotope Research Laboratory, Menlo Park, California (USGSMICA).

5

 Woods Hole Oceanographic Institution, National Ocean Sciences Accelerator Mass Spectrometry Facility [NOMAS], Woods Hole, Massachusetts 

(MA-WHAMS).

6

 USGS Stable Isotope and Tritium Laboratory [SITL], Menlo Park, California (USGSH3CA). SITL tritium data are reported using the conventions used by 

the USGS for radioactive consituents (see table 12).

Table 3I
Table 4 41

Table 4. Water-quality indicators in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, 

California, March to June 2010.

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and RL as of March 1, 2010. Benchmark type: SMCL-US, U.S. Environmental Protection Agency secondary maximum contaminant level; SMCL-CA, California Department of Public Health secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; °C, degree Celsius; mg/L, milligrams per liter; nc, not collected; na, not available; NTRU, nephelometric turbidity unit; RL, reporting level or range; μS/cm, microsiemens per centimeter; <, less than; >, greater than; CaCO3, calcium carbonate; *, value is at a concentration greater than the benchmark level; **, value is at a 

concentration greater than the upper benchmark level; —, not detected]

GAMA well 

identification 

number

Turbidity, 

field 

(NTRU) 

(63676)

Dissolved 

oxygen, 

field 

(mg/L) 

(00300) 

Water 

temperature, 

field 

(°C) 

(00010)

pH, 

laboratory 

(standard 

units) 

(00403)

pH, 

field 

(standard 

units) 

(00400)

Specific 

conductance, 

laboratory 

(μS/cm 

at 25°C) 

(90095)

Specific 

conductance, 

field 

(μS/cm 

at 25°C) 

(00095)

Alkalinity, 

laboratory 

(mg/L as 

CaCO3) 

(29801)

Alkalinity, 

field 

(mg/L as 

CaCO3) 

(29802)

Bicarbonate, 

field 

(mg/L) 

(63786)

Bicarbonate 3

(mg/L)

Carbonate, 

field 

(mg/L) 

(63788)

Carbonate 3

(mg/L)

Benchmark type

SMCL-CA na na SMCL-US SMCL-US SMCL-CA SMCL-CA na na na na na na

Benchmark level

1 5 na na <6.5 or >8.5 <6.5 or >8.5 2 900 (1,600) 2 900 (1,600) na na na na na na

[RL] [0.1] [0.2] [0.0-38.5] [0-14] [0-14] [5] [5] [1] [1] [1] [1] [1] [1]

WSJV grid wells (39 wells sampled)Delta-Mendota subbasin study area (29 grid wells sampled)DM-01 nc <0.2 24.0 7.9 8.0 *1,140 *1,050 118 nc nc 143 nc 0.5

DM-02 nc <0.2 19.0 7.8 7.8 **2,410 **2,170 230 216 262 279 1.1 0.8

DM-03 nc 3.1 20.5 7.5 7.4 847 846 214 202 245 260 0.4 0.4

DM-04 nc <0.2 18.5 7.8 7.6 661 673 182 nc nc 221 nc 0.6

DM-05 nc <0.2 19.5 7.2 7.2 619 626 97.0 92.4 112 118 0.1 0.1

DM-06 nc 1.7 21.0 7.6 7.5 *1,590 *1,570 194 187 227 236 0.5 0.4

DM-07 nc 5.6 21.0 7.7 7.5 *1,160 *1,190 226 nc nc 274 nc 0.6

DM-08 nc 5.0 22.5 7.6 7.5 *1,540 *1,560 280 nc nc 340 nc 0.6

DM-09 nc 0.9 20.5 7.6 7.6 *1,510 *1,500 227 216 263 276 0.6 0.5

DM-10 nc 1.0 22.0 7.8 7.9 **1,830 **1,800 198 190 229 240 0.9 0.7

DM-11 nc 6.0 19.5 7.7 7.7 **1,840 **1,830 233 nc nc 283 nc 0.7

DM-12 nc 5.0 21.0 7.5 7.6 *1,120 *1,110 199 nc nc 242 nc 0.4

DM-13 nc <0.2 25.5 7.9 7.9 **1,740 **1,700 157 nc nc 190 nc 0.7

DM-14 nc 0.5 20.5 7.6 7.5 **3,030 **3,030 327 nc nc 397 nc 0.7

DM-15 nc <0.2 23.0 7.6 7.5 **2,350 **2,350 72.0 nc nc 87.5 nc 0.2

Table 4
42 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 GAMA well identification number Turbidity, field (NTRU) (63676) Dissolved oxygen, field (mg/L) (00300) Water temperature, field (°C) (00010) pH, laboratory (standard units) (00403) pH, field (standard units) (00400) Specific conductance, laboratory (μS/cm at 25°C) (90095) Specific conductance, field (μS/cm at 25°C) (00095) Alkalinity, laboratory (mg/L as CaCO3) (29801) Alkalinity, field (mg/L as CaCO3) (29802) Bicarbonate, field (mg/L) (63786) Bicarbonate 3 (mg/L) Carbonate, field (mg/L) (63788) Carbonate 3 (mg/L) Benchmark type SMCL-CA na na SMCL-US SMCL-US SMCL-CA SMCL-CA na na na na na na Benchmark level

1 5 na na <6.5 or >8.5 <6.5 or >8.5 2 900 (1,600) 2 900 (1,600) na na na na na na

[RL] [0.1] [0.2] [0.0-38.5] [0-14] [0-14] [5] [5] [1] [1] [1] [1] [1] [1]

WSJV grid wells (39 wells sampled)—ContinuedDelta-Mendota subbasin study area (29 grid wells sampled)—ContinuedDM-16 nc <0.2 19.0 7.7 7.5 **1,990 **2,000 82.0 nc nc 99.5 nc 0.2

DM-17 nc <0.2 43.0 *9.0 *8.7 **4,110 **4,150 35.0 nc nc 38.5 nc 1.8

DM-18 nc <0.2 25.5 7.5 7.5 **3,470 **3,520 72.0 nc nc 87.6 nc 0.1

DM-19 nc 0.2 23.0 7.7 7.7 **1,730 **1,700 69.0 103 125 83.8 0.4 0.2

DM-20 nc 7.1 20.0 7.6 7.5 **1,800 **1,830 78.0 nc nc 94.8 nc 0.2

DM-21 nc 6.7 20.0 7.7 7.6 *1,220 *1,200 223 nc nc 271 nc 0.6

DM-22 nc <0.2 21.5 7.3 7.1 **4,850 **4,850 207 nc nc 252 nc 0.2

DM-23 nc 1.3 21.0 7.8 7.6 803 805 185 nc nc 224 nc 0.7

DM-24 nc <0.2 19.0 7.1 7.0 **2,080 **1,970 265 nc nc 323 nc 0.2

DM-25 nc <0.2 19.5 7.0 7.3 *932 *920 166 4 161 4 196 202 4 0.3 0.1

DM-26 nc 4.6 19.5 7.3 7.1 *1,250 *1,240 296 4 269 4 327 360 4 0.3 0.3DM-27 nc <0.2 26.0 8.1 7.9 **2,820 **2,920 119 112 136 143

— 0.8

DM-28 nc 6.3 20.0 7.4 7.1 **3,850 **3,740 214 199 242 260 0.4 0.3

DM-29 nc <0.2 18.0 6.9 6.8 **1,610 **1,650 201 nc nc 245 nc 0.1

Table 4. Water-quality indicators in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and RL as of March 1, 2010. Benchmark type: SMCL-US, U.S. Environmental Protection Agency secondary maximum contaminant level; SMCL-CA, California Department of Public Health secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; °C, degree Celsius; mg/L, milligrams per liter; nc, not collected; na, not available; NTRU, nephelometric turbidity unit; RL, reporting level or range; μS/cm, microsiemens per centimeter; <, less than; >, greater than; CaCO3, calcium carbonate; *, value is at a concentration greater than the benchmark level; **, value is at a 

concentration greater than the upper benchmark level; —, not detected]
Table 4 43

GAMA well 

identification 

number

Turbidity, 

field 

(NTRU) 

(63676)

Dissolved 

oxygen, 

field 

(mg/L) 

(00300) 

Water 

temperature, 

field 

(°C) 

(00010)

pH, 

laboratory 

(standard 

units) 

(00403)

pH, 

field 

(standard 

units) 

(00400)

Specific 

conductance, 

laboratory 

(μS/cm 

at 25°C) 

(90095)

Specific 

conductance, 

field 

(μS/cm 

at 25°C) 

(00095)

Alkalinity, 

laboratory 

(mg/L as 

CaCO3) 

(29801)

Alkalinity, 

field 

(mg/L as 

CaCO3) 

(29802)

Bicarbonate, 

field 

(mg/L) 

(63786)

Bicarbonate 3

(mg/L)

Carbonate, 

field 

(mg/L) 

(63788)

Carbonate 3

(mg/L)

Benchmark type

SMCL-CA na na SMCL-US SMCL-US SMCL-CA SMCL-CA na na na na na na

Benchmark level

1 5 na na <6.5 or >8.5 <6.5 or >8.5 2 900 (1,600) 2 900 (1,600) na na na na na na

[RL] [0.1] [0.2] [0.0-38.5] [0-14] [0-14] [5] [5] [1] [1] [1] [1] [1] [1]

WSJV grid wells (39 wells sampled)—ContinuedWestside subbasin study area (10 grid wells sampled)WS-01 nc 10.1 24.0 7.6 7.7 *1,510 *1,470 144 nc nc 175 nc 0.3

WS-02 nc <0.2 23.0 8.4 8.3 763 763 330 nc nc 393 nc 4.5

WS-03 nc <0.2 26.0 7.9 7.8 **1,820 **1,830 76.0 65.5 78.8 92.0 0.5 0.3

WS-04 nc <0.2 24.0 8.1 8.1 *1,380 *1,380 54.0 nc nc 65.0 nc 0.4

WS-05 nc <0.2 32.0 *8.6 *8.6 **1,640 **1,680 62.0 54.8 66.8 72.7 — 1.3

WS-06 nc <0.2 22.0 8.5 8.5 **2,950 **2,970 11.0 nc nc 12.8 nc 0.2

WS-07 nc 0.2 28.0 8.2 8.3 *1,280 *1,320 78.0 nc nc 93.7 nc 0.7

WS-08 nc 0.9 28.0 8.3 8.5 858 889 58.0 nc nc 69.3 nc 0.6

WS-09 nc <0.2 23.0 8.0 7.6 *1,010 *1,060 200 189 231 242 — 1.1

WS-10 0.3 <0.2 24.0 8.0 7.9 **1,980 **1,970 89.0 82.1 99.0 108 0.5 0.5

WSJV understanding wells (19 wells sampled)DM-U-01 nc <0.2 21.0 7.5 7.7 *1,390 *1,360 206 nc nc 251 nc 0.4

WS-U-01 nc <0.2 26.0 8.3 8.3 *1,480 *1,520 135 131 160 162 — 1.5

WS-U-02 nc <0.2 26.5 8.3 8.3 *1,160 *1,180 79.0 79.9 97.4 94.5 — 0.9

WS-U-03 nc <0.2 29.5 8.1 8.0 **1,700 **1,740 62.0 57.0 69.5 74.7 — 0.4

Table 4. Water-quality indicators in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and RL as of March 1, 2010. Benchmark type: SMCL-US, U.S. Environmental Protection Agency secondary maximum contaminant level; SMCL-CA, California Department of Public Health secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; °C, degree Celsius; mg/L, milligrams per liter; nc, not collected; na, not available; NTRU, nephelometric turbidity unit; RL, reporting level or range; μS/cm, microsiemens per centimeter; <, less than; >, greater than; CaCO3, calcium carbonate; *, value is at a concentration greater than the benchmark level; **, value is at a 

concentration greater than the upper benchmark level; —, not detected]
44 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 GAMA well identification number Turbidity, field (NTRU) (63676) Dissolved oxygen, field (mg/L) (00300) Water temperature, field (°C) (00010) pH, laboratory (standard units) (00403) pH, field (standard units) (00400) Specific conductance, laboratory (μS/cm at 25°C) (90095) Specific conductance, field (μS/cm at 25°C) (00095) Alkalinity, laboratory (mg/L as CaCO3) (29801) Alkalinity, field (mg/L as CaCO3) (29802) Bicarbonate, field (mg/L) (63786) Bicarbonate 3 (mg/L) Carbonate, field (mg/L) (63788) Carbonate 3 (mg/L) Benchmark type SMCL-CA na na SMCL-US SMCL-US SMCL-CA SMCL-CA na na na na na na Benchmark level

1 5 na na <6.5 or >8.5 <6.5 or >8.5 2 900 (1,600) 2 900 (1,600) na na na na na na

[RL] [0.1] [0.2] [0.0-38.5] [0-14] [0-14] [5] [5] [1] [1] [1] [1] [1] [1]

WSJV understanding wells (19 wells sampled)—ContinuedMonitoring wells (15 wells sampled)DM-U-02 nc 0.5 21.0 7.3 7.3 **3,060 **3,040 408 4 392 4 476 497 4 1.0 0.5

DM-U-03 nc 6.6 22.0 7.1 7.0 **3,270 **3,270 346 4 332 4 403 422 4 0.4 0.2DM-U-04 1.2 3.0 20.5 7.6 7.4 686 691 176 156 190 214

— 0.4

DM-U-05 0.4 5.6 22.0 7.6 7.7 728 738 182 160 196 221

— 0.4

DM-U-06 4.4 2.0 20.5 7.8 7.7 644 642 185 164 200 224

— 0.7

DM-U-07 0.8 0.4 25.5 8.3 8.3 *1,570 *1,580 298 266 324 357

— 3.3

DM-U-08 6.0 0.3 24.5 7.9 7.8 *1,410 *1,400 161 145 176 195

— 0.7

DM-U-09 1.8 0.2 22.0 7.5 7.4 **2,690 **2,660 109 93.0 113 133

— 0.2

DM-U-10 0.3 0.3 22.0 8.1 7.9 **2,570 **2,610 133 4 128 4 154 160 4 1.0 0.9

DM-U-11 3.0 0.3 19.0 7.2 7.3 *948 *961 600 4 246 4 298 731 4 0.5 0.5

Table 4. Water-quality indicators in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and RL as of March 1, 2010. Benchmark type: SMCL-US, U.S. Environmental Protection Agency secondary maximum contaminant level; SMCL-CA, California Department of Public Health secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; °C, degree Celsius; mg/L, milligrams per liter; nc, not collected; na, not available; NTRU, nephelometric turbidity unit; RL, reporting level or range; μS/cm, microsiemens per centimeter; <, less than; >, greater than; CaCO3, calcium carbonate; *, value is at a concentration greater than the benchmark level; **, value is at a 

concentration greater than the upper benchmark level; —, not detected]
Table 4 45

GAMA well 

identification 

number

Turbidity, 

field 

(NTRU) 

(63676)

Dissolved 

oxygen, 

field 

(mg/L) 

(00300) 

Water 

temperature, 

field 

(°C) 

(00010)

pH, 

laboratory 

(standard 

units) 

(00403)

pH, 

field 

(standard 

units) 

(00400)

Specific 

conductance, 

laboratory 

(μS/cm 

at 25°C) 

(90095)

Specific 

conductance, 

field 

(μS/cm 

at 25°C) 

(00095)

Alkalinity, 

laboratory 

(mg/L as 

CaCO3) 

(29801)

Alkalinity, 

field 

(mg/L as 

CaCO3) 

(29802)

Bicarbonate, 

field 

(mg/L) 

(63786)

Bicarbonate 3

(mg/L)

Carbonate, 

field 

(mg/L) 

(63788)

Carbonate 3

(mg/L)

Benchmark type

SMCL-CA na na SMCL-US SMCL-US SMCL-CA SMCL-CA na na na na na na

Benchmark level

1 5 na na <6.5 or >8.5 <6.5 or >8.5 2 900 (1,600) 2 900 (1,600) na na na na na na

[RL] [0.1] [0.2] [0.0-38.5] [0-14] [0-14] [5] [5] [1] [1] [1] [1] [1] [1]

WSJV understanding wells (19 wells sampled)—Continued

Monitoring wells (15 wells sampled)—ContinuedDM-U-12 0.6 0.6 21.0 7.8 7.9 *1,360 *1,380 177 4 170 4 204 215 4 1.4 0.6

DM-U-13 0.5 7.9 23.0 7.4 7.2 415 430 106 101 124 129 — 0.1

DM-U-14 0.4 3.5 22.5 7.8 7.6 *1,320 *1,340 192 185 224 233 0.7 0.7

DM-U-15 nc 4.8 20.0 7.7 7.4 898 *939 221 216 262 268 0.7 0.6

DM-U-16 0.9 9.3 23.0 7.7 7.5 *1,530 *1,550 155 148 179 188 0.5 0.4

1 The SMCL-CA benchmark for turbidity pertains only to surface-water samples.2 The SMCL-CA benchmark for specific conductance has recommended and upper benchmark levels. The upper benchmark level is shown in parentheses.3 Bicarbonate and carbonate concentrations were calculated from the laboratory alkalinity and pH values using the advanced speciation method (http://or.water.usgs.gov/alk/methods.html) with pK1 = 6.35, 

pK2 = 10.33, and pKW = 14.

4 Alkalinity, bicarbonate, and carbonate values generated by the incremental titration method (USGS parameter codes: 39086, 00453, and 00452; respectively).

Table 4. Water-quality indicators in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and RL as of March 1, 2010. Benchmark type: SMCL-US, U.S. Environmental Protection Agency secondary maximum contaminant level; SMCL-CA, California Department of Public Health secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; °C, degree Celsius; mg/L, milligrams per liter; nc, not collected; na, not available; NTRU, nephelometric turbidity unit; RL, reporting level or range; μS/cm, microsiemens per centimeter; <, less than; >, greater than; CaCO3, calcium carbonate; *, value is at a concentration greater than the benchmark level; **, value is at a 

concentration greater than the upper benchmark level; —, not detected]
46 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 5. Volatile organic compounds (VOCs) and low-level fumigants detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010. [The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3A. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin subbasin study area understanding well; WS, Westside study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; NL-CA, CDPH notification level; HAL-US, USEPA lifetime health advisory level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; ≤, less than or equal to; LRL, laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; *, value is at a concentration greater than the benchmark level; NWQL, USGS National Water Quality Laboratory] Primary use or source Trihalomethane Natural Fumigant Gasoline oxygenate Solvent GAMA well identification number Chloroform (Trichloromethane) (μg/L) (32106) Dibromo- chloromethane (μg/L) (32105) Bromoform (Tribromomethane) (μg/L) (32104) Carbon disulfide (μg/L) (77041) 1,2-Dibromo-3- chloropropane (DBCP)1 (μg/L) (82625) 1,2-Dichloro- propane (μg/L) (34541) Methyl tert- butyl ether (MTBE) (μg/L) (78032) 1,1-Dichloro- ethane (1,1-DCA) (μg/L) (34496) Trichloro- ethene (TCE) (μg/L) (39180) Chloro- methane (μg/L) (34418) Benchmark type MCL-US MCL-US MCL-US NL-CA MCL-US MCL-US MCL-CA MCL-CA MCL-US HAL-US Benchmark level 3 80 3 80 3 80 160 0.2 5 13 5 5 30 [LRL or SRL] [0.03, 5 0.02] [0.12] [0.1] [0.03] 6 [0.03] [0.026] [0.1] [0.044] [0.022] [0.14] WSJV grid wells (39 wells sampled) Number of wells with detections

4 1 1 3 2 1 2 1 1 0

Detection frequency 

(percent)

10.1 2.6 2.6 7.7 5.1 2.6 5.1 2.6 2.6 0

Total detectionsDelta-Mendota subbasin study area (29 grid wells sampled)DM-03 0.06

—

—

—

—

—

—

— E0.03

—

DM-05

—

—

—

—

—

— E0.05

—

—

—

DM-06

— E0.10 0.73

—

—

—

—

—

—

—

DM-08 0.31

—

—

—

—

—

—

—

—

—

DM-11

—

—

—

— E0.04

—

—

—

—

—

DM-17

—

—

— E1.06

—

—

—

—

—

—

DM-20

—

—

—

— 0.07

—

—

—

—

—

DM-21 0.06

—

—

—

—

—

—

—

—

—

DM-24

—

—

— ≤0.03

—

— E0.06 E0.04

—

—

DM-29

—

—

—

—

— 0.04

—

—

—

—

Number of wells with 

detections

3 1 1 1 2 1 2 1 1 0

Detection frequency 

(percent)

10.3 3.4 3.4 3.4 6.9 3.4 6.9 3.4 3.4 0

Total detections

Table 5
Table 5 47

Primary use or source Trihalomethane Natural Fumigant Gasoline oxygenate Solvent

GAMA well 

identification number

Chloroform 

(Trichloromethane) 

(μg/L) 

(32106)

Dibromochloromethane 

(μg/L) 

(32105)

Bromoform 

(Tribromomethane) 

(μg/L) 

(32104)

Carbon 

disulfide 

(μg/L) 

(77041)

1,2-Dibromo-3-

chloropropane 

(DBCP)1

(μg/L) 

(82625)

1,2-Dichloropropane 

(μg/L) 

(34541)

Methyl tertbutyl ether 

(MTBE) 

(μg/L) 

(78032)

1,1-Dichloroethane 

(1,1-DCA) 

(μg/L) 

(34496)

Trichloroethene 

(TCE) 

(μg/L) 

(39180)

Chloromethane 

(μg/L) 

(34418)

Benchmark type MCL-US MCL-US MCL-US NL-CA MCL-US MCL-US MCL-CA MCL-CA MCL-US HAL-US

Benchmark level 3 80 3 80 3 80 160 0.2 5 13 5 5 30

[LRL or SRL] [0.03, 5 0.02] [0.12] [0.1] [0.03] 6 [0.03] [0.026] [0.1] [0.044] [0.022] [0.14]

WSJV grid wells (39 wells sampled)—ContinuedWestside subbasin study area (10 grid wells sampled)WS-02 — — — E0.38 — — — — — —

WS-06 — — — 0.05 — — — — — —

WS-09 0.03 — — — — — — — — —

Number of wells with 

detections

1 0 0 2 0 0 0 0 0 0

Detection frequency 

(percent)

10.0 0 0 20.0 0 0 0 0 0 0

Total detectionsWSJV understanding wells (19 wells sampled) 7

WS-U-01 — — — 0.10 — — — — — —

WS-U-02 — — — — — — — — — —

WS-U-03 — — — — — — — — — —

Montioring wells (15 wells sampled)DM-U-04 — — — — — — — — — E0.01

DM-U-05 — — — — — — — — — E0.02

DM-U-07 — — — — — — — — — E0.02

DM-U-11 — — — — — — — — — —

DM-U-14 — — — ≤0.03 — — — — — E0.01

DM-U-16 ≤0.02 — — — — — — — — —

Table 5. Volatile organic compounds (VOCs) and low-level fumigants detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3A. GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin subbasin study area understanding well; WS, Westside study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CAare identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; NL-CA, CDPH notification level; HAL-US, USEPA lifetime health advisory level. Other abbreviations: USGS, U.S. Geological Survey; 

E, estimated or having a higher degree of uncertainty; ≤, less than or equal to; LRL, laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; 

*, value is at a concentration greater than the benchmark level; NWQL, USGS National Water Quality Laboratory]
48 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Primary use or source Hydrocarbons Detections per well VOC detection summary GAMA well identification number Benzene (μg/L) (34030) 1,2,3-Tri- methyl- benzene (μg/L) (77221) 1,2,3,4-Tetra- methyl- benzene (μg/L) (49999) 1,2,3,5-Tetra- methyl- benzene (μg/L) (50000) o-Ethyl toluene (1-Ethyl- 2-methyl benzene) (μg/L) (77220) 4-Isopropyl- 1-methyl benzene (μg/L) (77356) Isopropyl- benzene (μg/L) (77223) n-Propyl- benzene (μg/L) (77224) Ethyl- benzene 2 (μg/L) (34371) Toluene 2 (μg/L) (34010) m- plus p-Xylene 2 (μg/L) (85795) o-Xylene 2 (μg/L) (77135) Benchmark type MCL-CA na na na na na NL-CA NL-CA MCL-CA MCL-CA MCL-CA MCL-CA Benchmark level 1 na na na na na 770 260 300 150 4 1,750 4 1,750 [LRL or SRL] [0.026] [0.06] [0.08] [0.08] [0.032] [0.06] [0.042] [0.036] [0.036] [0.69] [0.08] [0.032] WSJV grid wells (39 wells sampled) Number of wells with detections 1 0 0 0 0 0 0 0 0 0 0 0 13 Detection frequency (percent) 2.6 0 0 0 0 0 0 0 0 0 0 0 33 Total detections 17

Delta-Mendota subbasin study area (29 grid wells sampled)DM-03 — — — — — — — — — — — — 2

DM-05 — — — — — — — — — — — — 1

DM-06 — — — — — — — — — — — — 2

DM-08 — — — — — — — — — — — — 1

DM-11 — — — — — — — — — — — — 1

DM-17 E0.02 — — — — — — — — — — — 2

DM-20 — — — — — — — — — — — — 1

DM-21 — — — — — — — — — — — — 1

DM-24 — — — — — — — — — — — — 2

DM-29 — — — — — — — — — — — — 1

Table 5. Volatile organic compounds (VOCs) and low-level fumigants detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3A. GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin subbasin study area understanding well; WS, Westside study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CAare identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; NL-CA, CDPH notification level; HAL-US, USEPA lifetime health advisory level. Other abbreviations: USGS, U.S. Geological Survey; 

E, estimated or having a higher degree of uncertainty; ≤, less than or equal to; LRL, laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; 

*, value is at a concentration greater than the benchmark level; NWQL, USGS National Water Quality Laboratory]
Table 5 49

Primary use or 

source Hydrocarbons

GAMA well 

identification 

number

Benzene 

(μg/L) 

(34030)

1,2,3-Trimethylbenzene 

(μg/L) 

(77221)

1,2,3,4-Tetramethylbenzene 

(μg/L) 

(49999)

1,2,3,5-Tetramethylbenzene 

(μg/L) 

(50000)

o-Ethyl 

toluene 

(1-Ethyl2-methyl 

benzene) 

(μg/L) 

(77220)

4-Isopropyl1-methyl 

benzene 

(μg/L) 

(77356)

Isopropylbenzene 

(μg/L) 

(77223)

n-Propylbenzene 

(μg/L) 

(77224)

Ethylbenzene 2

(μg/L) 

(34371)

Toluene 2

(μg/L) 

(34010)

m- plus 

p-Xylene 2

(μg/L) 

(85795)

o-Xylene 2

(μg/L) 

(77135)

Detections 

per well

VOC 

detection 

summary

Benchmark type MCL-CA na na na na na NL-CA NL-CA MCL-CA MCL-CA MCL-CA MCL-CA

Benchmark level 1 na na na na na 770 260 300 150 4 1,750 4 1,750

[LRL or SRL] [0.026] [0.06] [0.08] [0.08] [0.032] [0.06] [0.042] [0.036] [0.036] [0.69] [0.08] [0.032]

WSJV grid wells (39 wells sampled)—ContinuedDelta-Mendota subbasin study area (29 grid wells sampled)—ContinuedNumber of wells with 

detections

1 0 0 0 0 0 0 0 0 0 0 0 10

Detection frequency 

(percent)

3.4 0 0 0 0 0 0 0 0 0 0 0 34

Total detections 14Westside subbasin study area (10 grid wells sampled)WS-02 — — — — — — — — — — — — 1

WS-06 — — — — — — — — — — — — 1

WS-09 — — — — — — — — — — — — 1

Number of wells with 

detections

0 0 0 0 0 0 0 0 0 0 0 0 3

Detection frequency 

(percent)

0 0 0 0 0 0 0 0 0 0 0 0 30

Total detections 3WSJV understanding wells (19 wells sampled) 7

WS-U-01 *6.98 — — — — — — — 0.04 — — 0.05 3

WS-U-02 *138 E0.04 — — 0.05 — 0.07 E0.01 0.78 0.03 0.10 0.11 9

WS-U-03 *2.85 — — — — — — — — — — — 1

Table 5. Volatile organic compounds (VOCs) and low-level fumigants detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3A. GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin subbasin study area understanding well; WS, Westside study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CAare identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; NL-CA, CDPH notification level; HAL-US, USEPA lifetime health advisory level. Other abbreviations: USGS, U.S. Geological Survey; 

E, estimated or having a higher degree of uncertainty; ≤, less than or equal to; LRL, laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; 

*, value is at a concentration greater than the benchmark level; NWQL, USGS National Water Quality Laboratory]
50 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Primary use or source Hydrocarbons GAMA well identification number Benzene (μg/L) (34030) 1,2,3-Tri- methyl- benzene (μg/L) (77221) 1,2,3,4-Tetra- methyl- benzene (μg/L) (49999) 1,2,3,5-Tetra- methyl- benzene (μg/L) (50000)

o-Ethyl 

toluene 

(1-Ethyl2-methyl 

benzene) 

(μg/L) 

(77220)

4-Isopropyl1-methyl 

benzene 

(μg/L) 

(77356)

Isopropylbenzene 

(μg/L) 

(77223)

n-Propylbenzene 

(μg/L) 

(77224)

Ethylbenzene 2

(μg/L) 

(34371)

Toluene 2

(μg/L) 

(34010)

m- plus 

p-Xylene 2

(μg/L) 

(85795)

o-Xylene 2

(μg/L) 

(77135)

Detections 

per well

VOC 

detection 

summary

Benchmark type MCL-CA na na na na na NL-CA NL-CA MCL-CA MCL-CA MCL-CA MCL-CA

Benchmark level 1 na na na na na 770 260 300 150 4 1,750 4 1,750

[LRL or SRL] [0.026] [0.06] [0.08] [0.08] [0.032] [0.06] [0.042] [0.036] [0.036] [0.69] [0.08] [0.032]

WSJV understanding wells (19 wells sampled) 7 —Continued

Monitoring wells (15 wells sampled)DM-U-04

—

—

—

—

—

—

—

—

—

—

—

— 1

DM-U-05

—

—

—

—

—

—

—

—

—

—

—

— 1

DM-U-07

—

—

—

—

—

—

—

—

—

—

—

— 1

DM-U-11 E0.03 E0.02 0.20 0.60

— E0.03

—

—

— ≤0.02

—

— 5

DM-U-14

—

—

—

—

—

—

—

—

—

—

—

— 1

DM-U-16

—

—

—

—

—

—

—

—

—

—

—

— 1

1 Anaylzed on NWQL Schedules 2020 and 1306.2 The SRLs were defined by Fram and others (2012). Values measured at concentrations less than or equal to the SRL have been flagged with a ≤ symbol, reclassified as non-detections, and not included in the 

calculations of detection frequencies (with the exception of wells WS-U-01 and WS-U-02 [see the appendix section titled “Detections in Blanks and Application of SRLs” for details]).

3 The MCL-US benchmark for trihalomethanes is the sum of chloroform, bromoform, bromodichloromethane, and dibromochloromethane.4 The MCL-CA benchmarks for 

m- plus 

p-xylene and o-xylene is the sum all three xylene compounds.

5 The SRL was defined based on a detection in the WSJV equipment blank and only can potentially affect monitoring well samples. Values measured at concentrations less than or equal to the SRL have been 

flagged with a ≤ symbol and reclassified as non-detections.

6 The SRL was defined based on the highest concentration detected in the WSJV blanks. Values measured at concentrations less than or equal to the SRL have been flagged with a ≤ symbol, reclassified as 

non-detections, and not included in the calculations of detection frequencies.

7 Understanding wells were not included in statistical calculations.

Table 5. Volatile organic compounds (VOCs) and low-level fumigants detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3A. GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin subbasin study area understanding well; WS, Westside study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CAare identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; NL-CA, CDPH notification level; HAL-US, USEPA lifetime health advisory level. Other abbreviations: USGS, U.S. Geological Survey; 

E, estimated or having a higher degree of uncertainty; ≤, less than or equal to; LRL, laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; 

*, value is at a concentration greater than the benchmark level; NWQL, USGS National Water Quality Laboratory]
Table 6 51

Table 6 Table 6. Pesticides and pesticide degradates detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010. [The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and then listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3B. GAMA well 

identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta Mendota subbasin study area understanding well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency (USEPA) maximum contaminant level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, laboratory reporting level; μg/L, micrograms per liter; na, not available; —, not detected]

Primary use or source Herbicide

GAMA well identification number

Simazine 

(μg/L) 

(04035)

Atrazine 

(μg/L) 

(39632)

Hexazinone 

(μg/L) 

(04025)

S-Ethyldipropylthiocarbamate 

(EPTC) 

(μg/L) 

(82668)

Molinate 

(μg/L) 

(82671)

Prometryn 

(μg/L) 

(04036)

Metolachlor 

(μg/L) 

(39415)

Prometon 

(μg/L) 

(04037)

DCPA 

(Dacthal) 

(μg/L) 

(82682)

Tebuthiuron 

(μg/L) 

(82670)

Benchmark type MCL-US MCL-CA HAL-US na MCL-CA na HAL-US HAL-US HAL-US HAL-US

Benchmark level 4 1 400 na 20 na 700 400 70 500

[LRL] [0.006] [0.007] [0.008] [0.002] [0.0026] [0.006] [0.014] [0.012] [0.0076] [0.028]

WSJV grid wells (39 wells sampled)Number of wells with detections 6 3 3 3 1 1 0 0 0 0

Detection frequency (percent) 15.4 7.7 7.7 7.7 2.6 2.6 0 0 0 0

Total DetectionsDelta-Mendota subbasin study area (29 grid wells sampled)DM-02 — — — 0.008 E0.007 — — — — —

DM-03 E0.006 E0.006 — — — — — — — —DM-05 — — 0.029 E0.007 — — — — — —

DM-08 0.010 — — — — — — — — —

DM-20 E0.006 E0.006 — — — — — — — —DM-21 0.008 — — — — — — — — —

DM-26 0.011 — — — — — — — — —

DM-28 E0.007 0.019 0.009 — — — — — — —DM-29 — — 0.045 0.074 — 0.006 — — — —

Number of wells with detections 6 3 3 3 1 1 0 0 0 0

Detection frequency (percent) 20.7 10.3 10.3 10.3 3.4 3.4 0 0 0 0

Total Detections
Primary use or source Herbicide

GAMA well identification number

Simazine 

(μg/L) 

(04035)

Atrazine 

(μg/L) 

(39632)

Hexazinone 

(μg/L) 

(04025)

S-Ethyldipropylthiocarbamate 

(EPTC) 

(μg/L) 

(82668)

Molinate 

(μg/L) 

(82671)

Prometryn 

(μg/L) 

(04036)

Metolachlor 

(μg/L) 

(39415)

Prometon 

(μg/L) 

(04037)

DCPA 

(Dacthal) 

(μg/L) 

(82682)

Tebuthiuron 

(μg/L) 

(82670)

Benchmark type MCL-US MCL-CA HAL-US na MCL-CA na HAL-US HAL-US HAL-US HAL-US

Benchmark level 4 1 400 na 20 na 700 400 70 500

[LRL] [0.006] [0.007] [0.008] [0.002] [0.0026] [0.006] [0.014] [0.012] [0.0076] [0.028]

WSJV understanding wells (19 wells sampled) 1

DM-U-01

—

— 0.012

—

— E0.001

—

—

— E0.01

WS-U-01

— 0.013

—

—

—

— 0.015

—

—

—

Montioring wells (15 wells sampled)DM-U-02 E0.007

—

—

—

—

— E0.014

—

—

—

DM-U-03

—

—

—

—

—

—

— 0.02

—

—

DM-U-05

—

—

—

—

—

—

—

—

—

—

DM-U-06 E0.008

—

—

—

—

—

—

—

—

—

DM-U-10

—

—

—

—

—

—

—

—

—

—

DM-U-11

—

— 0.027 0.014 E0.007

—

—

— E0.004

—

DM-U-14 0.01 E0.007 0.024

—

—

— 0.023

—

—

—

DM-U-15 0.033 E0.007 0.076

—

—

— 0.045 0.02

—

—

DM-U-16 0.019 0.015

—

—

—

— E0.013

—

—

—

52 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 6. Pesticides and pesticide degradates detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued [The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and then listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3B. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta Mendota subbasin study area understanding well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency (USEPA) maximum contaminant level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, laboratory reporting level; μg/L, micrograms per liter; na, not available; —, not detected]
Primary use or source Herbicide degradate Fungicide

Pesticide 

detections 

per well

Pesticide detection 

summary

GAMA well 

identification number

Deethylatrazine 

(2-Chloro-4-isopropylamino-6-

amino-s-triazine) 

(μg/L) 

(04040)

3,4-Dichloroaniline 

(μg/L) 

(61625)

2,6-Diethylaniline 

(μg/L) 

(82660)

2-Ethyl-6-

methylaniline 

(μg/L) 

(61620)

3,5-Dichloroaniline 

(μg/L) 

(61627)

Metalaxyl 

(μg/L) 

(61596)

Benchmark type na na na na na na

Benchmark level na na na na na na

[LRL] [0.014] [0.0042] [0.006] [0.0098] [0.003] [0.007]WSJV grid wells (39 wells sampled)Number of wells with detections 6 2 1 1 1 0 9

Detection frequency (percent) 15.4 5.1 2.6 2.6 2.6 0 23

Total detections 28Delta-Mendota subbasin study area (29 grid wells sampled)DM-02 — — — — — — 2

DM-03 E0.012 — — — — — 3DM-05 — — — — — — 2

DM-08 E0.013 — — — — — 2DM-20 E0.009 — — — — — 3DM-21 E0.007 — — — — — 2DM-26 E0.008 — — — — — 2

DM-28 E0.028 E0.004 — — 0.004 — 6

DM-29 — E0.005 E0.005 E0.002 — — 6

Number of wells with detections 6 2 1 1 1 0 9

Detection frequency (percent) 20.7 6.9 3.4 3.4 3.4 0 31

Total detections 28

Table 6 53

Table 6. Pesticides and pesticide degradates detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and then listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3B. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta Mendota subbasin study area understanding well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency (USEPA) maximum contaminant level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, laboratory reporting level; μg/L, micrograms per liter; na, not available; —, not detected]
Primary use or source Herbicide degradate Fungicide

Pesticide 

detections 

per well

Pesticide detection 

summary

GAMA well 

identification number

Deethylatrazine 

(2-Chloro-4-isopropylamino-6-

amino-s-triazine) 

(μg/L) 

(04040)

3,4-Dichloroaniline 

(μg/L) 

(61625)

2,6-Diethylaniline 

(μg/L) 

(82660)

2-Ethyl-6-

methylaniline 

(μg/L) 

(61620)

3,5-Dichloroaniline 

(μg/L) 

(61627)

Metalaxyl 

(μg/L) 

(61596)

Benchmark type na na na na na na

Benchmark level na na na na na na

[LRL] [0.014] [0.0042] [0.006] [0.0098] [0.003] [0.007]WSJV understanding wells (19 wells sampled) 1

DM-U-01

—

—

—

—

—

— 3

WS-U-01

—

—

—

—

—

— 2

Montioring wells (15 wells sampled)DM-U-02

—

—

—

—

—

— 2

DM-U-03

—

—

—

—

—

— 1

DM-U-05 E0.009

—

—

—

—

— 1

DM-U-06

—

—

—

—

—

— 1

DM-U-10

— E0.008

—

—

—

— 1

DM-U-11

— E0.215

—

—

—

— 5

DM-U-14 E0.011

—

—

—

— 0.015 6

DM-U-15 E0.009 E0.006

—

—

— 0.054 8

DM-U-16 E0.016

—

—

—

—

— 4

1 Understanding wells were not included in statistical calculations.

54 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 6. Pesticides and pesticide degradates detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued [The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed, but only samples with detections are listed. Detected constituents are grouped by primary use or source and then listed in order of decreasing detection frequency in the 39 grid wells. All constituents are listed in table 3B. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta Mendota subbasin study area understanding well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-US, U.S. Environmental Protection Agency (USEPA) maximum contaminant level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, laboratory reporting level; μg/L, micrograms per liter; na, not available; —, not detected]
Table 7 55

Table 7. Constituents of special interest detected in the samples collected for the Western San Joaquin Valley (WSJV) Groundwater 

Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Information about the constituent 

given in table 3D. Samples from all 58 wells were analyzed for perchlorate and 1,2,3-TCP. Samples from the 33 slow wells were analyzed for NDMA. Only well 

samples with detections are listed. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin 

study area understanding well; WS, Westside subbasin study area grid well. Benchmark type, benchmark level, and MRL as of March 1, 2010. Benchmark 

type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is 

lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health maximum contaminant level; HAL-US, U.S. Environmental 

Protection Agency lifetime health advisory level. Other abbreviations: USGS, U.S. Geological Survey; 1,2,3-TCP, 1,2,3-Trichloropropane; NDMA, 

N-Nitrosodimethylamine; MRL, method reporting level; μg/L, micrograms per liter; —, not detected]

GAMA well identification number

Perchlorate 

(μg/L) 

(63790)

1,2,3-TCP 

(μg/L) 

(77443)

Benchmark type MCL-CA HAL-US

Benchmark level 6 40

[MRL] [0.10] [0.005]

WSJV grid wells (39 wells sampled)

Number of wells with detections 15 0

Detection frequency (percent) 38 0

Delta-Mendota subbasin study area (29 grid wells sampled)

DM-03 0.33 —

DM-06 1.20 —

DM-07 0.44 —

DM-08 0.29 —

DM-09 0.33 —

DM-10 0.30 —

DM-11 1.29 —

DM-12 1.32 —

DM-14 0.30 —

DM-20 4.53 —

DM-26 0.46 —

DM-28 1.32 —

Number of wells with detections 13 0

Detection frequency (percent) 45 0

Westside subbasin study area (10 grid wells sampled)

WS-01 1.15 —

WS-03 0.12 —

Number of wells with detections 2 0

Detection frequency (percent) 20 0

WSJV understanding wells (19 wells sampled) 1

Monitoring wells (15 wells sampled)

DM-U-02 0.29 —

DM-U-03 0.62 —

DM-U-04 0.16 —

DM-U-05 0.47 —

DM-U-06 0.12 —

DM-U-15 0.48 —

DM-U-16 2.51 0.016

1

 Understanding wells were not included in statistical calculations.

Table 7
56 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 8. Trace elements collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010. [The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed. Information about the constituents given in table 3E. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; ≤, less than or equal to; *, value is at a concentration greater than the benchmark level] GAMA well identification number Aluminum (μg/L) (01106) Antimony (μg/L) (01095) Arsenic (μg/L) (01000) Barium (μg/L) (01005) Beryllium (μg/L) (01010) Boron (μg/L) (01020) Cadmium (μg/L) (01025) Chromium (μg/L) (01030) Cobalt (μg/L) (01035) Copper (μg/L) (01040) Iron (μg/L) (01046) Lead (μg/L) (01049) Benchmark type MCL-CA MCL-US MCL-US MCL-CA MCL-US NL-CA MCL-US MCL-CA na AL-US SMCL-CA AL-US Benchmark level 1,000 6 10 1,000 4 1,000 5 50 na 1,300 300 15 [LRL or SRL] [1.6] 1 [0.054] [0.044] [0.36] 1 [0.012] [2.8] [0.02] [0.42] 1 [0.33] 2 [1.7] 1 [6] 1 [0.65] 1 WSJV grid wells (39 wells sampled) Delta-Mendota subbasin study area (29 grid wells sampled) DM-01 5.7 0.10 *15.7 14.0 E0.01 *1,130 0.18

— ≤0.01

— 40 ≤0.02

DM-02

—

— 0.19 55.0 0.01 *1,360 0.06

— ≤0.08

— *524 ≤0.11

DM-03

— 0.74 7.1 66.0

— 673

— 29.5 ≤0.07

—

— ≤0.03

DM-04

— E0.03 9.6 294

— 162 E0.02

— ≤0.06 3.6 40 2.47

DM-05 E1.8

— 3.5 132

— 249 0.03

— ≤0.02

— *400

—

DM-06

—

— 1.2 25.0

— 564

— 26.2 ≤0.05

— 10 ≤0.20

DM-07

—

— 1.8 69.0

— *1,630

— 11.7 ≤0.05 12.0 10 1.00

DM-08

—

— 2.0 82.0

— *1,620

— 2.4 ≤0.05

— ≤4 ≤0.15

DM-09

— E0.10 1.0 43.0

— 666 E0.03 4.2 ≤0.10

— ≤4 1.20

DM-10

—

— 2.3 48.0

— *1,540 E0.08 1.6 ≤0.05

—

— ≤0.11

DM-11

— E0.09 1.7 50.0

— 627

— 21.1 ≤0.13

— ≤5 ≤0.06

DM-12

—

— 0.5 21.0

— *1,320

— 11.6 ≤0.07

— 6 1.42

DM-13

—

— 1.7 46.0

— *2,440

—

— ≤0.16

— 61

—

DM-14

— 0.16 1.2 40.0

— *1,260

— 24.4 ≤0.16 E2.1

— ≤0.27

DM-15

— E0.30 2.1 13.0

— *3,070 0.22

— 0.34

—

— 1.04

DM-16

— E0.08 5.0 346

— 740 E0.06

— ≤0.15

— 82 1.06

DM-17

— E0.83 E1.2 12.0

— *13,700

—

— ≤0.27

—

—

—

DM-18

— E0.33 1.5 25.0

— *2,710

—

— ≤0.24

— *3,260

—

DM-19

—

— 0.85 11.0

— *3,220 0.34 4.8 ≤0.14

—

— ≤0.27

Table 8
Table 8 57

GAMA well 

identification number

Aluminum 

(μg/L) 

(01106)

Antimony 

(μg/L) 

(01095)

Arsenic 

(μg/L) 

(01000)

Barium 

(μg/L) 

(01005) 

Beryllium 

(μg/L) 

(01010)

Boron 

(μg/L) 

(01020)

Cadmium 

(μg/L) 

(01025)

Chromium 

(μg/L) 

(01030)

Cobalt 

(μg/L) 

(01035)

Copper 

(μg/L) 

(01040)

Iron 

(μg/L) 

(01046)

Lead 

(μg/L) 

(01049)

Benchmark type MCL-CA MCL-US MCL-US MCL-CA MCL-US NL-CA MCL-US MCL-CA na AL-US SMCL-CA AL-US

Benchmark level 1,000 6 10 1,000 4 1,000 5 50 na 1,300 300 15[LRL or SRL] [1.6] 1 [0.054] [0.044] [0.36] 1 [0.012] [2.8] [0.02] [0.42] 1 [0.33] 2 [1.7] 1 [6] 1 [0.65] 1

WSJV grid wells (39 wells sampled)—ContinuedDelta-Mendota subbasin study area (29 grid wells sampled)—ContinuedDM-20 — — 0.84 28.0 — 394 — 16.6 0.40 — ≤4 0.82DM-21 — E0.22 0.79 27.0 — *2,820 — 1.1 ≤0.12 — 87 ≤0.09

DM-22 — 0.17 9.3 31.0 0.03 *1,350 0.08 ≤0.32 0.69 — 187 —

DM-23 E2.0 0.52 9.4 86.0 0.02 932 0.04 31.0 ≤0.09 — — ≤0.05

DM-24 — E0.05 3.7 127 E0.01 471 0.02 ≤0.12 ≤0.21 — *1,160 ≤0.06

DM-25 — — 7.6 57.0 E0.01 379 0.06 ≤0.07 ≤0.09 — 270 ≤0.18

DM-26 — E0.05 1.4 54.0 0.01 757 E0.02 13.6 ≤0.19 2.1 7 1.56

DM-27 E3.4 E0.07 7.9 19.0 0.05 *1,970 E0.10 ≤0.24 ≤0.11 — 63 —DM-28 12.3 E0.07 1.2 34.0 0.03 *1,970 — 23.2 0.51 ≤1.4 17 1.04

DM-29 — — *15.6 223 E0.01 224 0.02 — ≤0.23 — *11,200 ≤0.03

Westside subbasin study area (10 grid wells sampled)WS-01 — — 0.98 22.0 — 613 — 4.4 ≤0.17 ≤1.5 25 ≤0.12

WS-02 7.8 — 0.42 37.0 0.03 *1,120 0.07 ≤0.19 ≤0.10 ≤0.98 14 ≤0.54

WS-03 12.9 0.06 1.6 17.0 E0.01 549 0.10 ≤0.22 0.59 — ≤3 1.20

WS-04 11.0 0.06 2.3 19.0 0.02 772 0.11 ≤0.11 0.35 ≤1.6 67 ≤0.29

WS-05 24.7 0.07 5.4 34.0 0.03 *1,350 0.20 ≤0.23 ≤0.23 — 8 —

WS-06 E6.7 E0.09 2.8 11.0 E0.02 *1,030 0.27 — 2.4 — E7 ≤0.49WS-07 4.7 — 2.7 20.0 0.02 *1,100 0.11 — ≤0.12 — 40 —

WS-08 E2.8 0.11 7.8 10.0 E0.01 500 0.06 ≤0.07 ≤0.11 — 41 0.68

WS-09 E2.7 — 1.1 90.0 0.02 940 0.06 ≤0.23 ≤0.10 — 88 ≤0.49

WS-10 E2.5 — 3.7 27.0 0.02 933 0.07 — 0.65 — 24 ≤0.05

Table 8. Trace elements collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed. Information about the constituents given in table 3E. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, 

laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; ≤, less than or equal to; *, value is at a concentration greater than the benchmark level]
58 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 GAMA well identification number Aluminum (μg/L) (01106) Antimony (μg/L) (01095) Arsenic (μg/L) (01000) Barium (μg/L) (01005) Beryllium (μg/L) (01010) Boron (μg/L) (01020) Cadmium (μg/L) (01025) Chromium (μg/L) (01030) Cobalt (μg/L) (01035) Copper (μg/L) (01040) Iron (μg/L) (01046) Lead (μg/L) (01049) Benchmark type MCL-CA MCL-US MCL-US MCL-CA MCL-US NL-CA MCL-US MCL-CA na AL-US SMCL-CA AL-US Benchmark level 1,000 6 10 1,000 4 1,000 5 50 na 1,300 300 15 [LRL or SRL] [1.6] 1 [0.054] [0.044] [0.36] 1 [0.012] [2.8] [0.02] [0.42] 1 [0.33] 2 [1.7] 1 [6] 1 [0.65] 1 WSJV understanding wells (19 wells sampled) DM-U-01 — E0.04 *13.4 66.0 — 850 E0.02 — ≤0.15 — 122 ≤0.02 WS-U-01 E2.3 0.12 *17.1 47.0 E0.01 718 0.06 ≤0.20 ≤0.08 — 21 — WS-U-02 E2.6 0.10 *11.9 35.0 E0.01 590 0.06 ≤0.24 ≤0.07 — 7 ≤0.02 WS-U-03 9.2 E0.03 3.8 11.0 0.03 *1,240 E0.05 ≤0.10 ≤0.15 — 43 ≤0.10 Monitoring wells (15 wells sampled) DM-U-02 — — 0.92 14.0 — *1,660 — 14.8 ≤0.15 — ≤3 — DM-U-03 — E0.19 3.3 84.0 — *1,640 0.19 5.2 ≤0.15 — ≤3 — DM-U-04 E2.6 0.76 5.1 69.0 — 543 0.02 0.95 ≤0.05 — — ≤0.02 DM-U-05 — 0.66 7.4 73.0 — 751 0.02 5.2 ≤0.06 — ≤5 ≤0.02 DM-U-06 E2.0 0.98 9.7 57.0 — 639 0.04 1.8 ≤0.06 — — —

DM-U-07 — — *12.8 22.0 — *1,740 0.25 — ≤0.12 — 10 —

DM-U-08 — E0.28 7.9 42.0 — *2,550 E0.13 E0.58 ≤0.20 — 8 —DM-U-09 — — 0.42 17.0 — *2,610 — 8.4 ≤0.14 — — —

DM-U-10 — E0.32 7.1 26.0 — *1,460 0.27 — ≤0.10 — — —

DM-U-11 9.8 0.22 9.5 31.0 — 311 0.18 ≤0.09 0.39 ≤0.82 76 ≤0.06

DM-U-12 — E0.29 *18.1 31.0 — 743 — — ≤0.06 — 10 —DM-U-13 E3.1 0.13 7.2 53.0 — 257 0.05 0.89 ≤0.09 — — ≤0.02

DM-U-14 E1.9 0.17 1.8 39.0 0.01 464 0.05 10.4 0.38 — — —

DM-U-15 E2.1 0.09 1.3 33.0 E0.01 919 0.04 6.6 ≤0.24 — — —DM-U-16 — E0.09 1.8 27.0 — 492 E0.05 17.3 ≤0.27 — — —

Table 8. Trace elements collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed. Information about the constituents given in table 3E. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, 

laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; ≤, less than or equal to; *, value is at a concentration greater than the benchmark level]
Table 8 59

GAMA well 

identification number

Lithium 

(μg/L) 

(01130)

Manganese 

(μg/L) 

(01056)

Molybdenum 

(μg/L) 

(01060)

Nickel 

(μg/L) 

(01065)

Selenium 

(μg/L) 

(01145)

Silver 

(μg/L) 

(01075)

Strontium 

(μg/L) 

(01080)

Thallium 

(μg/L) 

(01057)

Tungsten 

(μg/L) 

(01155)

Uranium 

(μg/L) 

(22703)

Vanadium 

(μg/L) 

(01085)

Zinc 

(μg/L) 

(01090)

Benchmark type na SMCL-CA HAL-US MCL-CA MCL-US SMCL-CA HAL-US MCL-US na MCL-US NL-CA SMCL-CA

Benchmark level na 50 40 100 50 100 4,000 2 na 30 50 5,000

[LRL or SRL] [0.44] [0.52] 2 [0.028] [0.36] 1 [0.040] [0.010] 2 [0.40] [0.020] [0.11] 1 [0.008] [0.16] [4.8] 1

WSJV grid wells (39 wells sampled)Delta-Mendota subbasin study area (29 grid wells sampled)DM-01 16.4 *93.1 28.1 ≤0.07 — ≤0.005 140 — 43.8 2.07 0.42 —

DM-02 26.3 *842 7.7 0.75 0.13 — 920 — 3.5 0.13 0.48 23.0

DM-03 14.7 — 0.70 0.47 0.95 — 812 — ≤0.03 3.13 12.1 —

DM-04 9.7 *583 2.8 0.41 — — 688 — 1.0 1.66 0.18 12.9

DM-05 31.1 *414 2.7 ≤0.30 E0.03 — 477 — 0.34 0.01 0.37 —

DM-06 58.1 E1.0 1.0 E0.54 8.1 — 1,750 — — 6.33 6.1 —

DM-07 25.6 E1.1 1.4 E0.47 0.69 E0.025 803 — — 1.90 9.7 53.8

DM-08 20.8 — 2.3 E0.43 0.36 — 1,150 — — 2.99 4.8 —

DM-09 41.3 ≤0.50 2.2 0.83 1.8 E0.018 1,390 — — 5.05 5.8 10.6

DM-10 36.6 25.6 7.6 — 1.6 — 916 — ≤0.07 1.81 5.7 —

DM-11 27.3 ≤0.40 0.80 1.2 10.5 E0.017 898 — — 2.63 6.1 —

DM-12 52.9 — 1.2 ≤0.33 4.5 — 1,040 — — 3.83 2.2 13.2

DM-13 24.8 *237 4.1 E0.53 — — 1,850 — 0.36 0.08 E0.52 —

DM-14 42.3 — 1.3 1.1 1.6 0.032 2,740 — ≤0.06 12.7 4.4 ≤4.4

DM-15 79.2 *493 39.1 2.3 E0.38 — 3,060 — 0.88 5.30 — —DM-16 20.0 *872 7.1 0.76 — — 1,060 — 0.97 0.98 3.3 —DM-17 71.0 E3.9 0.90 — — — 415 — 44.0 — 9.0 —

DM-18 151 *163 1.5 1.6 — — *5,610 — — — 2.1 —

DM-19 106 E1.4 *67.1 1.6 7.9 — 1,280 — — 4.04 5.6 —

Table 8. Trace elements collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed. Information about the constituents given in table 3E. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, 

laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; ≤, less than or equal to; *, value is at a concentration greater than the benchmark level]
60 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 GAMA well identification number Lithium (μg/L) (01130) Manganese (μg/L) (01056) Molybdenum (μg/L) (01060) Nickel (μg/L) (01065) Selenium (μg/L) (01145) Silver (μg/L) (01075) Strontium (μg/L) (01080) Thallium (μg/L) (01057) Tungsten (μg/L) (01155) Uranium (μg/L) (22703) Vanadium (μg/L) (01085) Zinc (μg/L) (01090) Benchmark type na SMCL-CA HAL-US MCL-CA MCL-US SMCL-CA HAL-US MCL-US na MCL-US NL-CA SMCL-CA Benchmark level na 50 40 100 50 100 4,000 2 na 30 50 5,000 [LRL or SRL] [0.44] [0.52] 2 [0.028] [0.36] 1 [0.040] [0.010] 2 [0.40] [0.020] [0.11] 1 [0.008] [0.16] [4.8] 1 WSJV grid wells (39 wells sampled)—Continued Delta-Mendota subbasin study area (29 grid wells sampled)—Continued DM-20 69.1

— 0.80 2.4 28.9

— 2,130

—

— 2.81 4.5

—

DM-21 82.8 6.8 3.6 1.0 2.2

— 798

—

— 2.83 2.6 84.0

DM-22 194 *3,190 11.0 3.8 0.09 E0.014 *4,090

— 1.8 4.11 3.9 ≤2.9

DM-23 24.2 4.6 2.6 0.46 5.8

— 802

— 0.19 4.85 19.7 ≤1.6

DM-24 27.2 *2,680 3.0 1.9 E0.04

— 1,300

— 0.23 0.46 1.2 ≤4.7

DM-25 9.1 *617 9.3 0.42 0.04

— 413

— 2.0 1.61 0.81 ≤2.0

DM-26 28.5 1.0 1.2 1.5 0.83 ≤0.005 1,120

—

— 4.98 6.6 7.3

DM-27 31.3 *363 *47.7 0.65 E0.05

— 643

— 1.6 0.38 2.8

—

DM-28 70.9 ≤0.40 0.90 3.4 *65.8 0.029 3,170

—

— 6.54 5.6 ≤3.3

DM-29 8.9 *4,750 3.0 1.7 0.09

— 1,000

— 0.40 0.90 0.4 ≤1.7

Westside subbasin study area (10 grid wells sampled)WS-01 30.8 2.9 4.2 0.54 10.1

— 1,250

— ≤0.04 3.55 6.2 88.5

WS-02 2.6 18.0 10.1 1.1 0.06

— 132

— 86.4 0.64 3.5

—

WS-03 33.9 6.9 12.4 5.0 34.9 0.016 1,970

— ≤0.03 2.23 7.9 18.2

WS-04 12.5 40.3 15.3 2.7 0.39

— 745

— 0.41 1.04 5.9 ≤2.9

WS-05 10.6 29.3 31.4 1.3 E0.04 0.016 357

— 19.8 0.14 2.6 ≤2.8

WS-06 8.3 32.5 38.6 21.7 0.53

— 2,670

— 0.15 0.03 8.0 ≤4.3

WS-07 24.1 *60.1 22.0 0.56 E0.02

— 525

— 12.8 0.08 0.34

—

WS-08 11.1 11.6 12.6 ≤0.25 1.9

— 147

— 0.62 0.15 33.5 16.7

Table 8. Trace elements collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed. Information about the constituents given in table 3E. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, 

laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; ≤, less than or equal to; *, value is at a concentration greater than the benchmark level]
Table 8 61

GAMA well 

identification number

Lithium 

(μg/L) 

(01130)

Manganese 

(μg/L) 

(01056)

Molybdenum 

(μg/L) 

(01060)

Nickel 

(μg/L) 

(01065)

Selenium 

(μg/L) 

(01145)

Silver 

(μg/L) 

(01075)

Strontium 

(μg/L) 

(01080)

Thallium 

(μg/L) 

(01057)

Tungsten 

(μg/L) 

(01155)

Uranium 

(μg/L) 

(22703)

Vanadium 

(μg/L) 

(01085)

Zinc 

(μg/L) 

(01090)

Benchmark type na SMCL-CA HAL-US MCL-CA MCL-US SMCL-CA HAL-US MCL-US na MCL-US NL-CA SMCL-CA

Benchmark level na 50 40 100 50 100 4,000 2 na 30 50 5,000

[LRL or SRL] [0.44] [0.52] 2 [0.028] [0.36] 1 [0.040] [0.010] 2 [0.40] [0.020] [0.11] 1 [0.008] [0.16] [4.8] 1

WSJV grid wells (39 wells sampled)—ContinuedWestside subbasin study area (10 grid wells sampled)—ContinuedWS-09 29.4 *144 9.4 0.54 0.04 — 138 — 13.9 0.10 2.3 26.5

WS-10 18.7 *144 30.9 1.4 0.09 — 1,170 — 3.6 0.42 0.65 ≤2.4

WSJV understanding wells (19 wells sampled)DM-U-01 25.9 *335 3.7 0.52 — — 1,460 — 0.77 13.0 0.40 9.4

WS-U-01 15.3 *85.5 11.7 0.38 0.07 — 321 — 2.1 0.18 9.0 —

WS-U-02 13.7 40.2 13.4 ≤0.32 0.19 — 348 — 0.71 0.49 9.3 ≤1.6WS-U-03 14.9 *91.0 24.8 0.97 0.75 — 550 — 0.40 0.52 4.7 ≤3.7Monitoring wells (15 wells sampled)DM-U-02 40.0 1.5 1.2 1.1 1.7 E0.028 2,390 — ≤0.09 *34.7 3.6 —

DM-U-03 7.1 — 29.3 2.8 6.4 E0.031 1,910 — ≤0.07 *56.6 8.0 —

DM-U-04 12.0 4.0 1.4 0.55 0.61 — 605 — 0.17 1.11 6.2 —

DM-U-05 12.8 3.2 1.7 0.54 0.47 — 561 — 0.15 1.79 7.2 ≤1.4DM-U-06 11.4 5.4 2.5 0.65 0.61 — 455 — 0.36 1.97 6.7 —

DM-U-07 10.9 *66.2 *55.0 1.1 — — 163 — 1.9 0.42 E0.63 —

DM-U-08 52.8 *130 22.6 E0.93 2.5 — 857 — 0.77 4.95 8.0 —

DM-U-09 86.0 25.1 9.0 1.0 6.0 — *4,930 — ≤0.10 3.25 2.0 —

DM-U-10 16.6 *112 38.9 — — — 308 — 6.5 4.65 3.0 —DM-U-11 10.5 *366 30.0 2.0 0.12 0.014 292 — 5.3 6.39 1.9 —

DM-U-12 13.9 *52.7 0.90 E0.67 — — 101 — 26.6 0.77 7.3 —DM-U-13 2.2 5.5 5.8 0.83 0.69 — 436 — 0.33 0.45 21.2 —

Table 8. Trace elements collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed. Information about the constituents given in table 3E. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, 

laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; ≤, less than or equal to; *, value is at a concentration greater than the benchmark level]
62 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 GAMA well identification number Lithium (μg/L) (01130) Manganese (μg/L) (01056) Molybdenum (μg/L) (01060) Nickel (μg/L) (01065) Selenium (μg/L) (01145) Silver (μg/L) (01075) Strontium (μg/L) (01080) Thallium (μg/L) (01057) Tungsten (μg/L) (01155) Uranium (μg/L) (22703) Vanadium (μg/L) (01085) Zinc (μg/L) (01090) Benchmark type na SMCL-CA HAL-US MCL-CA MCL-US SMCL-CA HAL-US MCL-US na MCL-US NL-CA SMCL-CA Benchmark level na 50 40 100 50 100 4,000 2 na 30 50 5,000 [LRL or SRL] [0.44] [0.52] 2 [0.028] [0.36] 1 [0.040] [0.010] 2 [0.40] [0.020] [0.11] 1 [0.008] [0.16] [4.8] 1 WSJV understanding wells (19 wells sampled)—Continued Monitoring wells (15 wells sampled)—Continued DM-U-14 23.1 12.9 2.4 2.8 8.1 0.013 1,720

— 0.15 3.46 6.6

—

DM-U-15 62.1 1.1 3.2 2.1 1.4 ≤0.005 841

— ≤0.08 13.2 3.5

—

DM-U-16 88.1 4.0 3.5 2.7 11.6 E0.021 1,310

— ≤0.06 4.88 5.3

—

1 The SRL was defined by Olsen and others (2010). Values measured at concentrations less than or equal to the SRL have been flagged with a ≤ symbol and reclassified as less than or equal to the value 

reported by the laboratory.

2 The SRL was defined based on the highest concentration detected in WSJV blanks. Values measured at concentrations less than or equal to the SRL have been flagged with a ≤ symbol and reclassified as less 

Table 8.

than or equal to the value reported by the laboratory.

Trace elements collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to 

July 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells were analyzed. Information about the constituents given in table 3E. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, 

laboratory reporting level; SRL, study reporting level; μg/L, micrograms per liter; na, not available; —, not detected; ≤, less than or equal to; *, value is at a concentration greater than the benchmark level]
Table 9 63

Table 9. Nutrients and dissolved organic carbon (DOC) detected in samples collected for the Western San Joaquin Valley (WSJV) 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells 

were analyzed for nutrients. Samples from 30 wells were analyzed for DOC. Information about the constituents given in table 3F. GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study 

area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark 

type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is 

lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health advisory level; MCL-US, USEPA 

maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, laboratory 

reporting level; SRL, study reporting level; mg/L, milligrams per liter; na, not available; —, not detected; nc, not collected; *, value is at a concentration greater 

than the benchmark level; ≤, less than or equal to; NWQL, USGS National Water Quality Laboratory]

GAMA well 

identification number

Ammonia 

(as nitrogen) 

(mg/L) 

(00608)

Nitrite 

(as nitrogen) 

(mg/L) 

(00613)

Nitrite plus nitrate 

(as nitrogen) 1

(mg/L) 

(00631)

Total nitrogen 

(ammonia + nitrate + nitrite 

+ organic nitrogen) 

(mg/L) 

(62854)

Phosphate, 

orthophosphate 

(as phosphorus) 

(mg/L) 

(00671)

DOC 

(mg/L) 

(00681)

Benchmark type HAL-US MCL-US MCL-US na na na

Benchmark level 2

 24.7 1 10 na na na

[LRL or SRL] [0.016] 3 [0.001] 3 [0.04] [0.10] [0.006] [0.66]

WSJV grid wells (39 wells sampled)

Delta-Mendota subbasin study area (29 grid wells sampled)

DM-01 0.12 — — 0.13 0.083 nc

DM-02 0.25 — — 0.30 0.061 1.5

DM-03 — — 4.66 4.89 0.050 E0.5

DM-04 0.13 — — 0.16 0.120 nc

DM-05 0.18 — — 0.24 0.026 1.2

DM-06 — — 4.56 4.62 0.026 —

DM-07 — — 4.08 4.13 0.044 nc

DM-08 — — 2.58 2.50 0.055 nc

DM-09 — — 4.84 4.79 0.030 E0.3

DM-10 0.06 0.002 2.14 2.21 0.037 —

DM-11 — — *14.5 15.9 0.041 nc

DM-12 — — 6.02 6.57 0.027 nc

DM-13 0.24 — — 0.24 0.055 nc

DM-14 — — 2.60 2.58 0.042 nc

DM-15 0.30 — E0.03 0.32 0.050 nc

DM-16 0.29 — — 0.27 0.107 nc

DM-17 3.8 — — 3.85 E0.05 nc

DM-18 1.1 — — 1.05 0.009 nc

DM-19 — — 1.22 1.24 0.022 —

DM-20 — — *20.0 19.9 0.027 nc

DM-21 — ≤0.001 4.56 4.27 0.036 nc

DM-22 0.66 — — 0.62 0.088 nc

DM-23 — — 2.66 2.64 0.050 nc

DM-24 0.32 — — 0.39 0.076 nc

DM-25 0.13 — — 0.11 0.151 E0.6

Table 9
64 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

GAMA well 

identification number

Ammonia 

(as nitrogen) 

(mg/L) 

(00608)

Nitrite 

(as nitrogen) 

(mg/L) 

(00613)

Nitrite plus nitrate 

(as nitrogen) 1

(mg/L) 

(00631)

Total nitrogen 

(ammonia + nitrate + nitrite 

+ organic nitrogen) 

(mg/L) 

(62854)

Phosphate, 

orthophosphate 

(as phosphorus) 

(mg/L) 

(00671)

DOC 

(mg/L) 

(00681)

Benchmark type HAL-US MCL-US MCL-US na na na

Benchmark level 2

 24.7 1 10 na na na

[LRL or SRL] [0.016] 3 [0.001] 3 [0.04] [0.10] [0.006] [0.66]

WSJV grid wells (39 wells sampled)—Continued

Delta-Mendota subbasin study area (29 grid wells sampled)—Continued

DM-26 — — 7.06 7.56 0.029 E0.5

DM-27 0.36 ≤0.001 — 4

 0.31 0.051 E0.4

DM-28 ≤0.01 — *23.8 23.9 0.031 1.1

DM-29 0.48 0.002 — 0.55 — nc

Westside subbasin study area (10 grid wells sampled)

WS-01 — — 7.16 7.42 0.023 nc

WS-02 0.52 0.002 — 0.73 0.149 nc

WS-03 E0.02 0.007 8.70 8.52 0.022 —

WS-04 — 0.006 0.05 E0.08 0.017 nc

WS-05 0.76 — — 0.77 0.017 1.5

WS-06 0.09 — 0.27 0.37 0.011 nc

WS-07 0.26 ≤0.001 — 0.29 0.028 nc

WS-08 ≤0.01 0.086 0.57 4

 0.56 0.022 nc

WS-09 0.48 ≤0.001 — 0.53 0.170 1.9

WS-10 0.24 — — 0.22 0.035 E0.6

WSJV understanding wells (19 wells sampled)

DM-U-01 E0.02 — — E0.05 0.100 nc

WS-U-01 0.70 — — 0.72 0.046 E0.5

WS-U-02 0.81 0.008 0.13 1.02 0.030 E0.6

WS-U-03 E0.02 ≤0.001 — — 0.011 —

Monitoring wells (15 wells sampled)

DM-U-02 — — 1.57 1.57 0.039 1.3

DM-U-03 — — *13.0 12.7 0.848 2.1

DM-U-04 — — 1.98 1.97 0.104 0.7

DM-U-05 — — 2.61 2.48 0.047 E0.4

DM-U-06 — 0.004 1.47 1.51 0.172 E0.4

DM-U-07 0.25 — — 0.36 0.139 2.8

DM-U-08 0.07 — — E0.08 0.099 E0.4

Table 9. Nutrients and dissolved organic carbon (DOC) detected in samples collected for the Western San Joaquin Valley (WSJV) 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells 

were analyzed for nutrients. Samples from 30 wells were analyzed for DOC. Information about the constituents given in table 3F. GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study 

area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark 

type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is 

lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health advisory level; MCL-US, USEPA 

maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, laboratory 

reporting level; SRL, study reporting level; mg/L, milligrams per liter; na, not available; —, not detected; nc, not collected; *, value is at a concentration greater 

than the benchmark level; ≤, less than or equal to; NWQL, USGS National Water Quality Laboratory]
Table 9 65

GAMA well 

identification number

Ammonia 

(as nitrogen) 

(mg/L) 

(00608)

Nitrite 

(as nitrogen) 

(mg/L) 

(00613)

Nitrite plus nitrate 

(as nitrogen) 1

(mg/L) 

(00631)

Total nitrogen 

(ammonia + nitrate + nitrite 

+ organic nitrogen) 

(mg/L) 

(62854)

Phosphate, 

orthophosphate 

(as phosphorus) 

(mg/L) 

(00671)

DOC 

(mg/L) 

(00681)

Benchmark type HAL-US MCL-US MCL-US na na na

Benchmark level 2

 24.7 1 10 na na na

[LRL or SRL] [0.016] 3 [0.001] 3 [0.04] [0.10] [0.006] [0.66]

WSJV understanding wells (19 wells sampled)—Continued

Monitoring wells (15 wells sampled)—Continued

DM-U-09 — 0.002 *12.7 13.9 0.032 —

DM-U-10 0.05 — — E0.07 0.068 E0.5

DM-U-11 0.45 — — 0.60 1.99 6.8

DM-U-12 0.08 — — E0.10 0.289 nc

DM-U-13 E0.02 0.048 *14.1 14.8 0.237 E0.4

DM-U-14 ≤0.01 0.184 *11.6 11.8 0.023 7.5

DM-U-15 — 0.037 *13.4 13.8 0.088 0.9

DM-U-16 E0.02 0.018 *12.9 12.8 0.022 E0.6

1

 Nitrite plus nitrate, (as nitrogen) is reffered to as nitrate in the text for clarity.

2

 The HAL-US is 30 mg/L “as ammonia.” To facilitate comparison to the analytical results, we have converted and reported this HAL-US as 24.7 mg/L “as 

nitrogen.”

3

 The SRL was defined based on a detection in a WSJV blank. Values measured at concentrations less than or equal to the SRL have been flagged with a 

≤ symbol and reclassified as less than or equal to the value reported by the laboratory.

4

 Total nitrogen concentrations in these samples are less than the sum of the filtered nitrogen constituents and fall outside the USGS NWQL acceptance 

criterion of a 10 percent relative percent difference. However, the absolute difference is ≤0.011 mg/L, thus, the unacceptable replication is unlikely to affect 

interpretation of the data.

Table 9. Nutrients and dissolved organic carbon (DOC) detected in samples collected for the Western San Joaquin Valley (WSJV) 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells 

were analyzed for nutrients. Samples from 30 wells were analyzed for DOC. Information about the constituents given in table 3F. GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study 

area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark 

type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is 

lower than the MCL-US or no MCL-US exists. HAL-US, U.S. Environmental Protection Agency (USEPA) lifetime health advisory level; MCL-US, USEPA 

maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; E, estimated or having a higher degree of uncertainty; LRL, laboratory 

reporting level; SRL, study reporting level; mg/L, milligrams per liter; na, not available; —, not detected; nc, not collected; *, value is at a concentration greater 

than the benchmark level; ≤, less than or equal to; NWQL, USGS National Water Quality Laboratory]
66 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Table 10. Major and minor ions, silica, and total dissolved solids (TDS) detected in samples collected for the Western San Joaquin 

Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells 

were analyzed. Information about the constituents given in table 3G. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; 

DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding 

well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US 

when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California 

Department of Public Health (CDPH) maximum contaminant level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, 

U.S. Geological Survey; LRL, laboratory reporting level; mg/L, milligrams per liter; SiO2, silicon dioxide; E, estimated or having a higher degree of uncertainty; 

na, not available; —, not detected; *, value is at a concentration greater than the recommended benchmark level; **, value is at a concentration greater than the 

upper benchmark level]

GAMA well 

identification 

number

Bromide 

(mg/L) 

(71870)

Calcium 

(mg/L) 

(00915)

Chloride 

(mg/L) 

(00940)

Fluoride 

(mg/L) 

(00950)

Iodide 

(mg/L) 

(71865)

Magnesium 

(mg/L) 

(00925)

Potassium 

(mg/L) 

(00935)

Sodium 

(mg/L) 

(00930)

Sulfate 

(mg/L) 

(00945)

Silica 

(as SiO2

) 

(mg/L) 

(00955)

Total 

dissolved 

solids (TDS) 

(mg/L) 

(70300)

Benchmark type na na SMCL-CA MCL-CA na na na na SMCL-CA na SMCL-CA

Benchmark level na na 1

 250 (500) 2 na na na na 1 250 (500) na 1 500 (1,000)

[LRL] [0.02] [0.044] [0.12] [0.08] [0.002] [0.016] [0.064] [0.10] [0.18] [0.058] [10]

WSJV grid wells (39 wells sampled)

Delta-Mendota subbasin study area (29 grid wells sampled)

DM-01 0.25 15.0 56.7 0.85 0.112 1.55 2.29 205 *319 65.2 *772

DM-02 0.97 105 *320 0.18 0.180 25.6 5.12 386 **606 38.0 **1,680

DM-03 0.28 52.0 92.5 0.12 0.004 28.6 2.37 76.1 77.4 48.7 *537

DM-04 0.18 46.9 79.9 0.10 0.025 18.3 1.98 52.8 42.5 29.5 397

DM-05 0.27 25.0 89.0 — 0.034 16.4 3.10 64.8 71.3 30.2 381

DM-06 0.63 88.1 204 0.24 0.004 69.6 1.95 126 *338 28.4 **1,090

DM-07 0.67 44.7 197 0.28 0.038 29.9 1.49 141 74.0 29.3 *729

DM-08 0.79 63.6 *258 0.36 0.035 41.0 1.98 185 143 22.6 *959

DM-09 0.97 87.9 *256 0.20 0.081 45.8 2.83 144 170 27.8 *946

DM-10 1.41 59.9 *326 0.18 0.340 34.8 2.32 255 225 27.1 **1,140

DM-11 1.08 67.5 *306 0.14 0.009 129 2.52 110 235 27.0 **1,130

DM-12 0.38 70.5 105 0.17 0.005 26.8 2.04 118 210 26.8 *722

DM-13 0.94 87.8 *264 0.17 0.293 43.2 2.92 179 *334 32.2 **1,130

DM-14 1.19 158 *429 0.24 0.030 112 3.86 314 **737 24.4 **2,130

DM-15 0.54 214 124 0.18 0.439 72.6 3.36 207 **1,080 32.0 **1,940

DM-16 1.98 76.2 **535 0.13 0.369 39.9 2.98 248 84.2 23.3 **1,100

DM-17 3.93 40.9 **1,350 0.45 2.64 E0.04 5.66 741 5.33 65.6 **2,290

DM-18 2.78 162 **846 0.24 0.672 114 5.78 344 **509 20.9 **2,100

DM-19 0.59 83.5 147 0.34 0.657 39.6 1.97 218 **545 32.6 **1,200

DM-20 0.80 168 164 0.15 0.005 86.7 2.45 99.6 **555 26.4 **1,300

DM-21 0.33 52.9 107 0.48 0.012 46.2 2.51 142 *258 19.1 *765

DM-22 3.62 310 **1,050 0.12 0.674 135 8.17 594 **1,000 49.9 **3,380

DM-23 0.24 41.6 82.0 0.20 0.021 26.8 1.85 90.3 102 43.3 *521

DM-24 1.04 101 *390 0.09 0.093 57.1 4.08 220 213 33.4 **1,260

DM-25 0.39 35.8 149 0.15 0.074 18.2 1.58 128 77.4 26.0 *537

Table 10
Table 10 67

GAMA well 

identification 

number

Bromide 

(mg/L) 

(71870)

Calcium 

(mg/L) 

(00915)

Chloride 

(mg/L) 

(00940)

Fluoride 

(mg/L) 

(00950)

Iodide 

(mg/L) 

(71865)

Magnesium 

(mg/L) 

(00925)

Potassium 

(mg/L) 

(00935)

Sodium 

(mg/L) 

(00930)

Sulfate 

(mg/L) 

(00945)

Silica 

(as SiO2

) 

(mg/L) 

(00955)

Total 

dissolved 

solids (TDS) 

(mg/L) 

(70300)

Benchmark type na na SMCL-CA MCL-CA na na na na SMCL-CA na SMCL-CA

Benchmark level na na 1

 250 (500) 2 na na na na 1 250 (500) na 1 500 (1,000)

[LRL] [0.02] [0.044] [0.12] [0.08] [0.002] [0.016] [0.064] [0.10] [0.18] [0.058] [10]

WSJV grid wells (39 wells sampled)—Continued

Delta-Mendota subbasin study area (29 grid wells sampled)—Continued

DM-26 0.43 88.5 136 0.28 0.018 47.8 1.59 99.0 139 25.4 *764

DM-27 1.82 51.7 *473 0.22 0.400 18.4 3.27 535 **610 29.3 **1,770

DM-28 2.20 220 **600 0.17 141 3.02 444 **918 24.2 **2,750

DM-29 0.63 114 235 0.13 0.063 42.3 3.31 151 216 27.0 *966

Westside subbasin study area (10 grid wells sampled)

WS-01 0.45 94.8 96.2 0.23 — 52.1 2.55 137 *492 34.2 **1,110

WS-02 0.23 11.1 47.7 0.99 0.165 1.28 1.13 160 8.33 23.3 470

WS-03 0.50 134 111 0.18 0.081 52.9 2.02 188 **706 24.4 **1,360

WS-04 0.21 85.5 56.9 0.27 0.194 8.82 1.30 195 **533 18.5 *954

WS-05 1.15 38.4 250 0.49 0.375 0.97 1.09 287 *312 17.6 *991

WS-06 0.29 570 89.3 0.26 0.290 3.55 1.62 201 **1,730 12.3 **2,780

WS-07 0.15 65.1 39.0 0.29 0.095 9.78 1.83 200 *487 26.6 *909

WS-08 0.14 22.6 27.3 0.22 0.029 1.83 1.07 153 *291 30.9 *569

WS-09 0.46 16.8 95.8 0.53 0.236 2.63 2.17 203 162 59.5 *672

WS-10 0.26 173 75.5 0.20 0.219 19.6 1.95 268 **817 26.2 **1,480

WSJV understanding wells (19 wells sampled)

DM-U-01 0.41 85.7 194 0.18 0.162 54.1 2.15 110 *264 37.3 *911

WS-U-01 1.53 37.9 *319 0.10 0.491 4.87 1.05 259 111 28.6 *855

WS-U-02 0.97 31.7 188 0.16 0.224 5.50 1.06 198 207 27.3 *805

WS-U-03 0.34 72.2 93.0 0.14 0.102 19.7 2.80 276 **634 11.5 **1,140

Monitoring wells (15 wells sampled)

DM-U-02 1.35 127 *429 0.20 0.128 129 3.22 321 **696 25.1 **2,110

DM-U-03 1.57 164 *467 0.27 0.025 131 2.95 331 **802 21.9 **2,300

DM-U-04 0.23 46.6 63.1 0.24 0.007 28.0 2.60 47.9 75.1 37.3 432

DM-U-05 0.29 51.3 79.9 0.20 0.006 24.3 2.27 57.2 62.9 31.8 430

DM-U-06 0.18 42.6 47.1 0.21 0.010 20.2 2.99 55.7 67.7 29.2 392

DM-U-07 0.53 15.5 112 0.26 0.499 2.72 1.14 302 *316 29.8 **1,020

DM-U-08 0.90 47.9 *256 0.36 1.95 18.1 2.81 217 152 41.1 *827

Table 10. Major and minor ions, silica, and total dissolved solids (TDS) detected in samples collected for the Western San Joaquin 

Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells 

were analyzed. Information about the constituents given in table 3G. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; 

DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding 

well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US 

when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California 

Department of Public Health (CDPH) maximum contaminant level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, 

U.S. Geological Survey; LRL, laboratory reporting level; mg/L, milligrams per liter; SiO2, silicon dioxide; E, estimated or having a higher degree of uncertainty; 

na, not available; —, not detected; *, value is at a concentration greater than the recommended benchmark level; **, value is at a concentration greater than the 

upper benchmark level]
68 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

GAMA well 

identification 

number

Bromide 

(mg/L) 

(71870)

Calcium 

(mg/L) 

(00915)

Chloride 

(mg/L) 

(00940)

Fluoride 

(mg/L) 

(00950)

Iodide 

(mg/L) 

(71865)

Magnesium 

(mg/L) 

(00925)

Potassium 

(mg/L) 

(00935)

Sodium 

(mg/L) 

(00930)

Sulfate 

(mg/L) 

(00945)

Silica 

(as SiO2

) 

(mg/L) 

(00955)

Total 

dissolved 

solids (TDS) 

(mg/L) 

(70300)

Benchmark type na na SMCL-CA MCL-CA na na na na SMCL-CA na SMCL-CA

Benchmark level na na 1

 250 (500) 2 na na na na 1 250 (500) na 1 500 (1,000)

[LRL] [0.02] [0.044] [0.12] [0.08] [0.002] [0.016] [0.064] [0.10] [0.18] [0.058] [10]

WSJV understanding wells (19 wells sampled)—Continued

Monitoring wells (15 wells sampled)—Continued

DM-U-09 1.13 240 *322 0.24 1.13 89.3 2.59 223 **916 31.8 **2,050

DM-U-10 1.77 34.4 *453 0.48 0.411 4.13 2.75 462 *474 24.0 **1,650

DM-U-11 0.32 33.9 96.4 0.40 0.074 13.1 4.51 136 81.7 20.9 *568

DM-U-12 0.87 10.1 206 0.62 0.259 1.09 3.67 280 185 57.5 *861

DM-U-13 0.11 21.6 23.7 0.35 0.011 10.7 4.33 47.5 21.7 75.5 337

DM-U-14 0.53 87.7 118 0.17 0.020 55.1 3.28 109 *296 24.8 *907

DM-U-15 0.27 64.1 62.0 0.53 0.006 29.2 1.05 87.0 121 25.2 *593

DM-U-16 0.74 100 135 0.52 0.006 62.2 2.19 142 *422 25.6 **1,070

1

 The SMCL-CA benchmarks for chloride, sulfate, and TDS have recommended and upper benchmark levels. The upper benchmark level is shown in 

parentheses.

Table 10. Major and minor ions, silica, and total dissolved solids (TDS) detected in samples collected for the Western San Joaquin 

Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Samples from all 58 wells 

were analyzed. Information about the constituents given in table 3G. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; 

DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding 

well. Benchmark type, benchmark level, and LRL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US 

when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California 

Department of Public Health (CDPH) maximum contaminant level; SMCL-CA, CDPH secondary maximum contaminant level. Other abbreviations: USGS, 

U.S. Geological Survey; LRL, laboratory reporting level; mg/L, milligrams per liter; SiO2, silicon dioxide; E, estimated or having a higher degree of uncertainty; 

na, not available; —, not detected; *, value is at a concentration greater than the recommended benchmark level; **, value is at a concentration greater than the 

upper benchmark level]
Table 11 69

Table 11. Total arsenic and iron (unfiltered) and arsenic, chromium, and iron species (filtered) detected in samples collected for the 

Western San Joaquin Valley Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. Information about the constituents given in table 3H. 

Samples for total arsenic and iron (unfiltered) and arsenic and iron species (filtered) were collected at all 58 wells. Samples for chromium species were 

collected at 33 slow wells. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study 

area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark 

level, and MDL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are 

identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health (CDPH) 

maximum contaminant level; MCL-US, U.S. Environmental Protection Agency maximum contaminant level; SMCL-CA, CDPH secondary maximum 

contaminant level. Other abbreviations: USGS, U.S. Geological Survey; MDL, method detection limit; na, not available; μg/L, micrograms per liter; *, value 

is at a concentration greater than the benchmark level; nc, not collected; —, not detected; NWQL, National Water Quality Laboratory]

GAMA well 

identification number

Arsenic 

(total, 

unfiltered) 

(μg/L) 

(na)

Arsenic-III 

(filtered) 

(μg/L) 

(99034)

Arsenic 

(total, 

filtered) 

(μg/L) 1

(99033)

Iron 

(total, 

unfiltered) 

(μg/L) 

(na)

Iron-II 

(filtered) 

(μg/L) 

(01047)

Iron 

(total, 

filtered) 

(μg/L) 

(01046)

Chromium 

(total, 

filtered) 

(μg/L) 

(01030)

Chromium-VI 

(filtered) 

(μg/L) 

(01032)

Benchmark type MCL-US na MCL-US SMCL-CA na SMCL-CA MCL-CA na

Benchmark level 10 na 10 300 na 300 50 na

[MDL] [0.5] [1] [0.5] [2] [2] [2] [0.1] [0.1]

WSJV grid wells (39 wells sampled)

Delta-Mendota subbasin study area (29 grid wells sampled)

DM-01 *13.3 *10.8 *13.2 42.9 25.0 41.4 nc nc

DM-02 — — — *535 *490 *507 — —

DM-03 5.4 — 5.2 — — — 31.6 31.9

DM-04 8.2 8.4 8.4 45.5 27.6 45.6 nc nc

DM-05 2.9 2.9 2.9 *809 *408 *410 — 0.1

DM-06 0.94 — 1.1 35.9 4.2 8.6 30.5 27.4

DM-07 1.4 — 1.4 7.0 — 9.5 nc nc

DM-08 1.7 — 1.6 23.0 — 2.9 nc nc

DM-09 0.67 — 0.65 14.7 — — nc nc

DM-10 1.7 — 1.5 6.3 — 4.0 1.7 1.3

DM-11 1.1 — 1.1 6.3 — 4.0 nc nc

DM-12 — — — 5.7 — 5.6 7.5 3.8

DM-13 2.2 1.8 1.8 *752 55.1 65.4 nc nc

DM-14 0.94 — 0.90 2.2 — 2.8 nc nc

DM-15 1.9 — 1.9 47.9 — 2.0 nc nc

DM-16 3.9 3.7 3.8 87.7 83.4 91.0 nc nc

DM-17 — — — 36.1 2.8 3.0 nc nc

DM-18 1.0 1.0 1.0 *3,310 *3,065 *3,453 nc nc

DM-19 0.56 — — 10.5 — — 5.2 5.1

DM-20 0.64 — 0.64 4.5 — — nc nc

DM-21 0.78 — 0.54 *790 26.7 70.9 nc nc

DM-22 7.6 7.5 7.5 206 154 182 nc nc

DM-23 8.3 — 8.5 — — — nc nc

DM-24 3.2 3.2 3.2 *1,135 *1,067 *1,133 nc nc

DM-25 6.9 6.7 6.7 272 266 275 — —

Table 11
70 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

GAMA well 

identification number

Arsenic 

(total, 

unfiltered) 

(μg/L) 

(na)

Arsenic-III 

(filtered) 

(μg/L) 

(99034)

Arsenic 

(total, 

filtered) 

(μg/L) 1

(99033)

Iron 

(total, 

unfiltered) 

(μg/L) 

(na)

Iron-II 

(filtered) 

(μg/L) 

(01047)

Iron 

(total, 

filtered) 

(μg/L) 

(01046)

Chromium 

(total, 

filtered) 

(μg/L) 

(01030)

Chromium-VI 

(filtered) 

(μg/L) 

(01032)

Benchmark type MCL-US na MCL-US SMCL-CA na SMCL-CA MCL-CA na

Benchmark level 10 na 10 300 na 300 50 na

[MDL] [0.5] [1] [0.5] [2] [2] [2] [0.1] [0.1]

WSJV grid wells (39 wells sampled)—Continued

Delta-Mendota subbasin study area (29 grid wells sampled)—Continued

DM-26 0.86 — 0.87 8.0 — 7.4 13.4 12.1

DM-27 6.7 4.8 6.5 81.5 16.8 68.0 — —

DM-28 — — — 12.0 — 11.0 23.2 17.0

DM-29 *14.5 *14.2 *14.2 *11,700 *10,900 *11,200 nc nc

Westside subbasin study area (10 grid wells sampled)

WS-01 0.89 — 0.80 141 4.9 26.4 nc nc

WS-02 — — — 10.3 — — nc nc

WS-03 1.0 — 1.0 159 — 3.0 — —

WS-04 1.9 — 1.9 79.6 14.6 62.1 nc nc

WS-05 4.0 — 3.9 14.6 6.9 6.5 — —

WS-06 1.7 — 1.8 24.8 — 4.0 nc nc

WS-07 2.6 2.6 2.6 41.8 35.5 43.2 nc nc

WS-08 7.7 — 7.5 *326 19.1 41.3 nc nc

WS-09 0.66 — 0.64 2.4 85.9 87.9 0.10 —

WS-10 3.2 2.0 3.2 36.0 23.8 24.3 0.40 —

WSJV understanding wells (19 wells sampled)

DM-U-01 *12.8 7.6 *12.6 143 111 126 nc nc

WS-U-01 *12.4 *11.6 *12.5 2 23.8 20.9 20.8 — —

WS-U-02 *10.2 — *10.4 7.5 4.2 6.1 1.0 0.29

WS-U-03 3.5 2.3 3.2 277 16.2 45.1 — —

Monitoring wells (15 wells sampled)

DM-U-02 0.79 — 0.73 56.9 — 2.1 15.0 15.0

DM-U-03 2.7 — 2.7 3.9 — — 6.1 5.2

DM-U-04 6.5 — 4.3 10.8 3.1 — 0.92 0.77

DM-U-05 6.4 — 6.2 4.4 — — 5.4 5.2

DM-U-06 8.0 — 8.4 13.2 — — 1.8 1.3

DM-U-07 *12.5 *11.7 *11.7 6.0 — — 0.48 0.55

DM-U-08 6.6 5.8 6.4 58.1 — 5.9 0.42 0.17

Table 11. Total arsenic and iron (unfiltered) and arsenic, chromium, and iron species (filtered) detected in samples collected for the 

Western San Joaquin Valley Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to 

June 2010.—Continued

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. Information about the constituents given in table 3H. 

Samples for total arsenic and iron (unfiltered) and arsenic and iron species (filtered) were collected at all 58 wells. Samples for chromium species were 

collected at 33 slow wells. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study 

area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark 

level, and MDL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are 

identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health (CDPH) 

maximum contaminant level; MCL-US, U.S. Environmental Protection Agency maximum contaminant level; SMCL-CA, CDPH secondary maximum 

contaminant level. Other abbreviations: USGS, U.S. Geological Survey; MDL, method detection limit; na, not available; μg/L, micrograms per liter; *, value 

is at a concentration greater than the benchmark level; nc, not collected; —, not detected; NWQL, National Water Quality Laboratory]
Table 11 71

GAMA well 

identification number

Arsenic 

(total, 

unfiltered) 

(μg/L) 

(na)

Arsenic-III 

(filtered) 

(μg/L) 

(99034)

Arsenic 

(total, 

filtered) 

(μg/L) 1

(99033)

Iron 

(total, 

unfiltered) 

(μg/L) 

(na)

Iron-II 

(filtered) 

(μg/L) 

(01047)

Iron 

(total, 

filtered) 

(μg/L) 

(01046)

Chromium 

(total, 

filtered) 

(μg/L) 

(01030)

Chromium-VI 

(filtered) 

(μg/L) 

(01032)

Benchmark type MCL-US na MCL-US SMCL-CA na SMCL-CA MCL-CA na

Benchmark level 10 na 10 300 na 300 50 na

[MDL] [0.5] [1] [0.5] [2] [2] [2] [0.1] [0.1]

WSJV understanding wells (19 wells sampled)—Continued

Monitoring wells (15 wells sampled)—Continued

DM-U-09 — — — 6.0 — — 9.7 8.5

DM-U-10 6.5 4.2 6.4 6.7 — — — —

DM-U-11 8.3 7.4 8.0 95.7 63.2 75.0 0.25 —

DM-U-12 *15.9 3.0 *16.0 4.1 2.5 3.1 — —

DM-U-13 6.5 — 6.5 14.0 — — 1.6 1.1

DM-U-14 1.3 — 1.3 2.1 — — 10.8 10.2

DM-U-15 1.1 — 1.1 2.0 — — 7.2 6.7

DM-U-16 1.4 — 1.4 18.5 — — 19.5 19.7

1

 Reported values are for filtered samples acidified with hydrochloric acid (HCl). Filtered unacidified samples also were collected and analyzed for comparison 

with the acidified samples because of potential concerns about arsenic loss under sulfate-reducing conditions in some cases using the HCl-preservation method 

(Planer-Friedrich and others, 2007). However, unless otherwise noted, all HCl-preserved samples had higher arsenic concentrations than unacidified samples and 

values closer to those measured by the USGS NWQL in filtered samples preserved with nitric acid.

2

 Arsenic concentration measured in a filtered unacidified sample was 15.0 μg/L.

Table 11. Total arsenic and iron (unfiltered) and arsenic, chromium, and iron species (filtered) detected in samples collected for the 

Western San Joaquin Valley Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to 

June 2010.—Continued

[The five-digit USGS parameter code is used to uniquely identify a specific constituent or property. Information about the constituents given in table 3H. 

Samples for total arsenic and iron (unfiltered) and arsenic and iron species (filtered) were collected at all 58 wells. Samples for chromium species were 

collected at 33 slow wells. GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study 

area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type, benchmark 

level, and MDL as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are 

identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health (CDPH) 

maximum contaminant level; MCL-US, U.S. Environmental Protection Agency maximum contaminant level; SMCL-CA, CDPH secondary maximum 

contaminant level. Other abbreviations: USGS, U.S. Geological Survey; MDL, method detection limit; na, not available; μg/L, micrograms per liter; *, value 

is at a concentration greater than the benchmark level; nc, not collected; —, not detected; NWQL, National Water Quality Laboratory]
72 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table 12. Isotopic tracers detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010. [The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Information about the constituents given in table 3I. Samples from all 58 wells were analyzed for stable isotopes of hydrogen and oxygen in water and tritium activities. Samples from 19 wells were analyzed for stable isotopes of nitrogen in dissolved nitrate. Samples from 33 wells were analyzed for stable isotopes of sulfur in dissolved sulfate. Samples from 34 wells were analyzed for stable isotopes of boron and strontium in water. Samples from 57 wells were analyzed for stable isotopes of carbon in dissolved inorganic carbon and carbon-14 abundance. Stable isotope ratios of hydrogen, oxygen, nitrogen, sulfur, boron, and carbon are reported in the standard delta notation (δ), the ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material. Stable isotope ratios of strontium are reported as the abundance of atoms of the heavier isotope to the lighter isotope of the element. Carbon-14 is reported as percent modern carbon. Tritium values less than the sample-specific critical level (ssLC) are reported as non-detections (—). GAMA well identification number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; H, hydrogen; O, oxygen; N, nitrogen; S, sulfur; B, boron; Sr, strontium; C, carbon; CSU, 1-sigma combined standard uncertainty; pCi/L, picocurie per liter; na, not available; pmc, percent modern carbon; —, not detected; ±, plus or minus; nc, not collected] GAMA well identification number d2H of water (per mil) (82082) d18O of water (per mil) (82085) Tritium (pCi/L) (07000) d15N of nitrate (per mil) (82690) d18O of nitrate (per mil) (63041) d34S of sulfate (per mil) (49932) d11B (per mil) (62648) Strontium isotope ratio (87Sr/86Sr) (atom ratio) (75978) d13C (per mil) (82081) Carbon-14 (pmc) (49933) Benchmark type na na MCL-CA na na na na na na na Benchmark level na na 20,000 na na na na na na na Result ± CSU ssLC WSJV grid wells (39 wells sampled) Delta-Mendota subbasin study area (29 grid wells sampled) DM-01 –65.8 –9.00 — 0.35 nc nc nc nc nc –19.28 2.160 DM-02 –77.6 –10.2 15.8 ± 0.64 0.35 nc nc –11.22 36.6 0.7066 –22.12 89.77 DM-03 –61.3 –8.36 7.2 ± 0.61 0.41 4.90 0.83 –4.22 13.1 0.7063 –14.12 89.38 DM-04 –84.4 –11.2 11.2 ± 0.54 0.38 nc nc nc nc nc –15.25 106.8 DM-05 –72.6 –9.87 9.9 ± 0.73 0.41 nc nc –5.18 24.0 0.7072 –14.62 101.6

DM-06 –58.9 –7.88 3.0 ± 0.45 0.38 6.86 2.58 –9.25 15.0 0.7074 –13.33 59.94

DM-07 –58.1 –7.82 3.8 ± 0.45 0.41 nc nc nc nc nc –14.31 71.37

DM-08 –60.9 –8.20 7.6 ± 0.61 0.41 nc nc nc nc nc –14.95 93.67

DM-09 –66.4 –8.75 10.6 ± 0.54 0.38 7.04 1.47 –7.03 25.1 0.7071 –14.50 76.89

DM-10 –59.8 –7.80 3.6 ± 0.41 0.35 8.86 2.40 –8.02 29.8 0.7068 –14.70 41.63

DM-11 –65.1 –8.42 8.4 ± 0.51 0.38 nc nc nc nc nc –13.81 84.16

DM-12 –61.3 –7.94 3.3 ± 0.41 0.35 nc nc nc nc nc –11.98 35.08

DM-13 –61.3 –8.20 — 0.38 nc nc nc nc nc –12.86 17.37

DM-14 –67.9 –8.90 10.7 ± 0.51 0.32 nc nc nc nc nc –13.15 89.18

DM-15 –65.7 –8.80 — 0.32 nc nc nc nc nc –13.29 5.890

Table 12
Table 12 73

GAMA well 

identification number

d2H of water

(per mil) 

(82082)

d18O of water 

(per mil) 

(82085)

Tritium 

(pCi/L) 

(07000)

d15N of 

nitrate 

(per mil) 

(82690)

d18O of 

nitrate 

(per mil) 

(63041)

d34S of 

sulfate 

(per mil) 

(49932)

d11B 

(per mil) 

(62648)

Strontium isotope 

ratio 

(87Sr/86Sr) 

(atom ratio) 

(75978)

d13C 

(per mil) 

(82081)

Carbon-14 

(pmc) 

(49933)

Benchmark type na na MCL-CA na na na na na na naBenchmark level na na 20,000 na na na na na na na

Result ± CSU ssLC

WSJV grid wells (39 wells sampled)—ContinuedDelta-Mendota subbasin study area (29 grid wells sampled)—ContinuedDM-16 –85.8 –11.4 — 0.32 nc nc nc nc nc –30.80 33.70DM-17 –54.0 –5.49 0.90 ± 0.32 0.32 nc nc nc 22.6 0.7047 –12.45 7.900

DM-18 –63.5 –7.80 — 0.32 nc nc nc nc nc –6.03 5.360DM-19 –68.4 –9.07 — 0.32 28.56 15.97 –13.46 7.9 0.7071 –9.94 1.310

DM-20 –55.7 –7.36 6.8 ± 0.48 0.35 nc nc nc nc nc –11.88 63.08DM-21 –58.7 –7.68 7.4 ± 0.41 0.32 nc nc nc nc nc –12.17 90.62DM-22 –67.4 –8.88 — 0.38 nc nc nc nc nc –27.69 13.84DM-23 –53.2 –7.07 — 0.41 nc nc nc nc nc –10.72 49.12DM-24 –75.9 –9.89 17.0 ± 0.64 0.38 nc nc nc nc nc –14.59 114.6

DM-25 –88.0 –11.6 1.8 ± 0.41 0.38 nc nc –5.21 26.2 0.7072 –17.28 82.07

DM-26 –66.4 –8.98 8.4 ± 0.41 0.32 5.84 1.66 –3.90 19.2 0.7070 –15.80 100.3

DM-27 –67.8 –8.84 — 0.35 nc nc –7.05 19.6 0.7063 –18.01 1.300

DM-28 –66.9 –8.62 10.6 ± 0.48 0.32 4.89 2.04 –6.98 29.0 0.7067 na 1 na 1

DM-29 –74.6 –9.58 9.2 ± 0.41 0.32 nc nc nc nc nc –15.78 107.9

Table 12. Isotopic tracers detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Information about the constituents given in table 3I. Samples from all 58 wells were analyzed for stable isotopes of hydrogen and oxygen in water and tritium activities. Samples from 19 wells were analyzed for stable isotopes of nitrogen in dissolved nitrate. Samples from 33 wells were analyzed for stable isotopes of sulfur in dissolved sulfate. Samples from 34 wells were analyzed for stable isotopes of boron and strontium in water. Samples from 57 wells were analyzed for stable isotopes of carbon in dissolved inorganic carbon and carbon-14 abundance. Stable isotope ratios of hydrogen, oxygen, nitrogen, sulfur, boron, and carbon are reported in the standard delta notation (δ), the ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material. Stable isotope ratios of strontium are reported as the abundance of atoms of the heavier isotope to the lighter isotope of the element. Carbon-14 is reported as percent modern carbon. Tritium values less than the sample-specific critical level (ssLC) are reported as non-detections (—). GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; H, hydrogen; O, oxygen; N, nitrogen; S, sulfur; B, boron; Sr, strontium; C, carbon; CSU, 1-sigma combined standard uncertainty; pCi/L, picocurie per liter; na, not available; pmc, percent modern carbon; —, not detected; ±, plus or minus; nc, not collected]
74 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 GAMA well identification number d2H of water (per mil) (82082) d18O of water (per mil) (82085) Tritium (pCi/L) (07000) d15N of nitrate (per mil) (82690) d18O of nitrate (per mil) (63041) d34S of sulfate (per mil) (49932) d11B (per mil) (62648) Strontium isotope ratio (87Sr/86Sr) (atom ratio) (75978) d13C (per mil) (82081) Carbon-14 (pmc) (49933) Benchmark type na na MCL-CA na na na na na na na Benchmark level na na 20,000 na na na na na na na Result ± CSU ssLC WSJV grid wells (39 wells sampled)—Continued Westside study subbasin area (10 grid wells sampled) WS-01 –51.0 –7.25 — 0.41 nc nc nc nc nc –8.07 63.77 WS-02 –77.2 –10.7 1.5 ± 0.35 0.35 nc nc nc nc nc –14.39 0.340 WS-03 –52.2 –7.13 — 0.32 10.60 9.47 –8.40 13.4 0.7064 –12.00 12.62 WS-04 –57.6 –7.98 — 0.32 nc nc nc nc nc –12.07 8.520 WS-05 –63.7 –8.69 — 0.38 nc nc –3.82 4.7 0.7068 –26.92 0.830

WS-06 –58.4 –7.97 — 0.41 nc nc nc nc nc –13.02 11.54

WS-07 –64.3 –8.56 — 0.38 nc nc nc nc nc –12.95 5.350

WS-08 –66.2 –9.11 — 0.35 nc nc nc nc nc –12.37 2.650

WS-09 –70.0 –9.39 — 0.32 nc nc 5.43 26.6 0.7077 –24.29 1.050

WS-10 –59.7 –8.23 — 0.32 nc nc –6.62 6.9 0.7067 –15.83 5.080

WSJV understanding wells (19 wells sampled)DM-U-01 –54.8 –7.37 3.2 ± 0.41 0.35 nc nc nc nc nc –11.93 72.58

WS-U-01 –66.5 –9.00 — 0.38 nc nc –1.92 5.4 0.7068 –24.05 0.900

WS-U-02 –67.1 –9.03 — 0.38 20.90 6.37 –4.17 9.9 0.7064 –12.27 1.520

WS-U-03 –61.0 –8.07 — 0.32 nc nc –8.68 15.6 0.7059 –11.19 1.570

Table 12. Isotopic tracers detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Information about the constituents given in table 3I. Samples from all 58 wells were analyzed for stable isotopes of hydrogen and oxygen in water and tritium activities. Samples from 19 wells were analyzed for stable isotopes of nitrogen in dissolved nitrate. Samples from 33 wells were analyzed for stable isotopes of sulfur in dissolved sulfate. Samples from 34 wells were analyzed for stable isotopes of boron and strontium in water. Samples from 57 wells were analyzed for stable isotopes of carbon in dissolved inorganic carbon and carbon-14 abundance. Stable isotope ratios of hydrogen, oxygen, nitrogen, sulfur, boron, and carbon are reported in the standard delta notation (δ), the ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material. Stable isotope ratios of strontium are reported as the abundance of atoms of the heavier isotope to the lighter isotope of the element. Carbon-14 is reported as percent modern carbon. Tritium values less than the sample-specific critical level (ssLC) are reported as non-detections (—). GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; H, hydrogen; O, oxygen; N, nitrogen; S, sulfur; B, boron; Sr, strontium; C, carbon; CSU, 1-sigma combined standard uncertainty; pCi/L, picocurie per liter; na, not available; pmc, percent modern carbon; —, not detected; ±, plus or minus; nc, not collected]
Table 12 75

GAMA well 

identification number

d2H of water

(per mil) 

(82082)

d18O of water 

(per mil) 

(82085)

Tritium 

(pCi/L) 

(07000)

d15N of 

nitrate 

(per mil) 

(82690)

d18O of 

nitrate 

(per mil) 

(63041)

d34S of 

sulfate 

(per mil) 

(49932)

d11B 

(per mil) 

(62648)

Strontium isotope 

ratio 

(87Sr/86Sr) 

(atom ratio) 

(75978)

d13C 

(per mil) 

(82081)

Carbon-14 

(pmc) 

(49933)

Benchmark type na na MCL-CA na na na na na na naBenchmark level na na 20,000 na na na na na na na

Result ± CSU ssLC

WSJV understanding wells (19 wells sampled)—ContinuedMonitoring wells (15 wells sampled)DM-U-02 –65.0 –8.39 11.0 ± 0.73 0.32 13.93 7.83 –6.87 19.7 0.7079 –13.37 98.56

DM-U-03 –61.5 –8.10 7.9 ± 0.48 0.38 7.43 –0.41 –6.69 21.1 0.7078 –13.14 102.8

DM-U-04 –43.3 –5.15 6.3 ± 0.51 0.32 5.82 2.62 –2.68 10.7 0.7064 –12.97 106.1

DM-U-05 –49.8 –6.61 5.4 ± 0.45 0.38 3.63 1.73 –2.60 17.2 0.7068 –12.70 90.55

DM-U-06 –44.9 –6.00 5.0 ± 0.48 0.32 5.16 2.39 –3.10 15.6 0.7064 –12.77 96.81

DM-U-07 –68.2 –9.27 — 0.35 nc nc 7.05 22.6 0.7063 –19.21 0.900

DM-U-08 –65.5 –8.71 — 0.35 nc nc –7.23 8.1 0.7069 –11.63 5.130

DM-U-09 –61.9 –8.04 — 0.35 14.66 14.76 –12.53 15.9 0.7069 –11.77 13.11

DM-U-10 –65.6 –8.72 — 0.35 nc nc –6.86 60.7 0.7065 –39.64 0.660

DM-U-11 –68.7 –9.00 8.0 ± 0.38 0.32 nc nc 0.21 18.1 0.7077 –14.95 85.62

DM-U-12 –75.7 –10.0 — 0.32 nc nc –3.37 58.1 0.7067 –37.47 4.200

DM-U-13 –53.1 –6.67 0.30 ± 0.38 0.35 3.04 2.24 3.10 12.4 0.7072 –13.18 54.59

DM-U-14 –61.9 –8.09 14.6 ± 0.61 0.35 3.37 0.95 –10.29 17.0 0.7074 –13.85 61.69

DM-U-15 –65.3 –8.75 11.1 ± 0.51 0.32 2.69 0.72 –8.17 7.2 0.7079 –15.82 93.00

DM-U-16 –63.5 –8.26 9.7 ± 0.48 0.32 2.20 0.38 –10.10 12.9 0.7075 –13.01 70.56

1 Sample was ruined during preparation at the laboratory. 

Table 12. Isotopic tracers detected in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to June 2010.—Continued[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Information about the constituents given in table 3I. Samples from all 58 wells were analyzed for stable isotopes of hydrogen and oxygen in water and tritium activities. Samples from 19 wells were analyzed for stable isotopes of nitrogen in dissolved nitrate. Samples from 33 wells were analyzed for stable isotopes of sulfur in dissolved sulfate. Samples from 34 wells were analyzed for stable isotopes of boron and strontium in water. Samples from 57 wells were analyzed for stable isotopes of carbon in dissolved inorganic carbon and carbon-14 abundance. Stable isotope ratios of hydrogen, oxygen, nitrogen, sulfur, boron, and carbon are reported in the standard delta notation (δ), the ratio of a heavier isotope to more common lighter isotope of that element, relative to a standard reference material. Stable isotope ratios of strontium are reported as the abundance of atoms of the heavier isotope to the lighter isotope of the element. Carbon-14 is reported as percent modern carbon. Tritium values less than the sample-specific critical level (ssLC) are reported as non-detections (—). GAMA well identification 

number: DM, Delta-Mendota subbasin study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside subbasin study area understanding well. Benchmark type and benchmark level as of March 1, 2010. Benchmark type: Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower than the MCL-US or no MCL-US exists. MCL-CA, California Department of Public Health maximum contaminant level. Other abbreviations: USGS, U.S. Geological Survey; H, hydrogen; O, oxygen; N, nitrogen; S, sulfur; B, boron; Sr, strontium; C, carbon; CSU, 1-sigma combined standard uncertainty; pCi/L, picocurie per liter; na, not available; pmc, percent modern carbon; —, not detected; ±, plus or minus; nc, not collected]
76 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Table 13. Results for analyses of dissolved standard gases detected in samples collected for the Western San Joaquin Valley (WSJV) 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Information about the 

constituents given in table 3I. Samples from 33 wells were analyzed for dissolved standard gases in water. GAMA well identification number: DM, DeltaMendota study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside 

subbasin study area understanding well. Other abbreviations: USGS, U.S. Geological Survey; μg/L, micrograms per liter; mg/L, milligrams per liter; na, not 

available; —, not detected]

GAMA well 

identification number

Carbon dioxide 

(mg/L) 

(00405)

Dissolved oxygen 

(mg/L) 

(62971)

Argon 

(mg/L) 

(82043)

Dissolved nitrogen gas 

(mg/L) 

(00597)

Methane 

(μg/L) 

(76994)

Benchmark type na na na na na

Benchmark level na na na na na

WSJV grid wells (15 wells sampled)

Delta-Mendota subbasin study area (11 grid wells sampled)

DM-02 8.59 0.45 0.68 21.07 0.012

DM-03 13.47 5.06 0.68 21.22 —

DM-05 8.48 4.37 0.55 15.53 0.016

DM-06 10.02 3.83 0.60 17.24 —

DM-09 14.22 0.84 0.66 20.83 —

DM-10 7.46 0.90 0.65 22.54 —

DM-19 3.83 0.28 0.65 22.23 —

DM-25 12.04 0.29 0.68 20.94 0.007

DM-26 27.56 2.99 0.66 19.45 —

DM-27 2.04 0.26 0.70 21.88 0.002

DM-28 21.30 6.90 0.73 23.27 —

Westside subbasin study area (4 grid wells sampled)

WS-03 2.77 0.22 0.60 19.07 —

WS-05 0.32 0.24 0.68 21.12 0.058

WS-09 6.11 0.26 0.71 21.72 0.009

WS-10 1.71 0.25 0.73 24.18 0.008

WSJV understanding wells (18 wells sampled)

WS-U-01 0.99 0.22 0.64 17.18 6.638

WS-U-02 0.78 0.29 0.84 28.37 0.091

WS-U-03 0.57 0.23 0.57 16.65 0.777

Monitoring wells (15 wells sampled)

DM-U-02 34.69 0.28 0.67 20.54 —

DM-U-03 54.31 6.26 0.60 16.58 —

DM-U-04 11.75 3.01 0.84 25.75 —

DM-U-05 7.55 5.10 0.73 21.39 —

DM-U-06 7.23 1.49 0.78 23.63 —

DM-U-07 2.97 0.27 0.75 22.98 0.008

DM-U-08 5.36 0.28 0.69 24.86 —

DM-U-09 7.22 0.27 0.62 20.40 —

DM-U-10 2.22 0.28 0.74 23.42 0.001

DM-U-11 22.39 0.23 0.64 19.05 0.612

Table 13
Table 13 77

GAMA well 

identification number

Carbon dioxide 

(mg/L) 

(00405)

Dissolved oxygen 

(mg/L) 

(62971)

Argon 

(mg/L) 

(82043)

Dissolved nitrogen gas 

(mg/L) 

(00597)

Methane 

(μg/L) 

(76994)

Benchmark type na na na na na

Benchmark level na na na na na

WSJV understanding wells (18 wells sampled)—Continued

Monitoring wells (15 wells sampled)—Continued

DM-U-12 22.94 0.26 0.71 22.94 0.001

DM-U-13 15.51 4.79 0.59 17.22 —

DM-U-14 9.08 1.35 0.74 23.95 —

DM-U-15 14.80 1.63 0.68 19.28 —

DM-U-16 12.33 5.02 0.77 25.25 —

Table 13. Results for analyses of dissolved standard gases detected in samples collected for the Western San Joaquin Valley (WSJV) 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued

[The five-digit USGS parameter code below the constituent name is used to uniquely identify a specific constituent or property. Information about the 

constituents given in table 3I. Samples from 33 wells were analyzed for dissolved standard gases in water. GAMA well identification number: DM, DeltaMendota study area grid well; DM-U, Delta-Mendota subbasin study area understanding well; WS, Westside subbasin study area grid well; WS-U, Westside 

subbasin study area understanding well. Other abbreviations: USGS, U.S. Geological Survey; μg/L, micrograms per liter; mg/L, milligrams per liter; na, not 

available; —, not detected]
78 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Appendix 

This appendix includes discussions of the methods used 

to collect and analyze groundwater samples and report the 

resulting water-quality data. These methods were selected to 

obtain representative samples of the groundwater from each 

well and to minimize the potential for contamination of the 

samples or bias in the data. Procedures used to collect and 

assess QC data, and the results of the QC assessments also are 

discussed. 

In the WSJV study unit, groundwater samples were 

collected and QA/QC procedures were implemented using 

standard and modified USGS protocols from the NFM 

(U.S. Geological Survey, variously dated; Wilde and others, 

1999, 2004) and the NAWQA Program (Koterba and others, 

1995). The QA plan followed by the NWQL, the primary 

laboratory used to analyze samples for this study, is described 

in Maloney (2005) and Pirkey and Glodt (1998).

Sample Collection and Analysis

Prior to sampling, each well was pumped continuously 

to purge at least three casing-volumes of water from the well 

(Wilde and others, 1999). Production wells were sampled by 

using Teflon® tubing with brass and stainless-steel fittings 

attached to a sampling point on the well discharge pipe as 

close to the well head as possible. The sampling point was 

located upstream from water-storage tanks or well-head 

treatment systems (if present). If a chlorinating system was 

attached to the well, the chlorinator was shut off, when 

possible, before the well was purged and sampled, in order 

to clear all chlorine out of the system. The absence of free 

chlorine was verified using a Hach® field test kit. Monitoring 

wells were sampled by using a Grunfos® submersible sampling 

pump with Teflon® tubing and stainless-steel fittings. For 

the intermediate schedule, samples were collected at the 

well head using a foot-long length of Teflon® tubing. For the 

slow schedule, the samples were either collected in the same 

manner as the intermediate schedule or collected inside an 

enclosed chamber located inside a mobile laboratory and 

connected to the well head by a 10–50 ft length of the Teflon®

tubing (Lane and others, 2003). All fittings and lengths of 

Teflon® tubing were cleaned between samples (Wilde, 2004).

For the field measurements, groundwater was pumped 

through a flow-through chamber (attached to the sampling 

point) fitted with a multi-probe meter that simultaneously 

measures the field water-quality indicators—dissolved 

oxygen, temperature, pH, and specific conductance. Turbidity 

was measured in the field with a calibrated turbidity meter. 

Field measurements were made in accordance with protocols 

in the NFM (Radtke and others, 2005; Wilde and Radtke, 

2005; Lewis, 2006; Wilde, 2006; Wilde and others, 2006). 

All sensors on the multi-probe meter were calibrated daily. 

Measured turbidity, dissolved oxygen, temperature, pH, 

and specific conductance values were recorded at 5-minute 

(min) intervals, and when these values remained stable for 

a minimum of 30 min, samples for laboratory analyses then 

were collected. 

Field measurements and instrument calibrations were 

recorded on field record sheets and electronically on field 

laptop computers using the Personal Computer Field Form 

(PCFF) program. Analytical service requests for the NWQL 

were generated by PCFF, whereas analytical service requests 

for non-NWQL analysis were entered into laboratory-specific 

spreadsheets. Information from PCFF was uploaded directly 

into the USGS NWIS database at the end of every week of 

sample collection. 

Prior to sample collection, polyethylene sample bottles 

were pre-rinsed three times using deionized water, and then 

once with native sample water before sample collection. 

Samples requiring acidification were acidified to a pH of 

between 2 and 1 with the appropriate acids using ampoules 

of certified, traceable concentrated acids obtained from the 

NWQL.

VOC and low-level fumigant samples were each 

collected in three 40-mL sample vials that were purged with 

three vial volumes of groundwater before bottom-filling 

to eliminate atmospheric contamination. One to one (1:1) 

hydrochloric acid to water (HCl/H2

O) solution was added 

as a preservative to the VOC samples, but not to the lowlevel fumigant samples. 1,2,3-TCP samples were collected 

by top-filling two 40-mL sample vials (provided by Weck 

Laboratories, Inc. [Weck], City of Industry, California) that 

were pre-treated with 0.5 mL 6 Normal (N) HCl acid as a 

preservative. Each sample to be analyzed for perchlorate 

was collected in a 125-milliliter (mL) polystyrene bottle and 

then filtered in two or three 20-mL aliquots of groundwater 

through a 0.20-micrometer (μm) pore-size Corning® syringetip disk filter into a sterilized 125-mL bottle. Tritium samples 

were collected by bottom-filling one 1-L polyethylene bottle 

and one 1-L glass bottle with unfiltered groundwater, after 

first overfilling the bottles with three volumes of unfiltered 

groundwater. Total arsenic and iron (unfiltered) samples were 

collected by top filling a 250-mL polyethylene bottle that was 

covered with tape to prevent light exposure with unfiltered 

groundwater and acidified with 6-N HCl as a preservative. 

Samples for analysis of stable isotopes of hydrogen and 

oxygen in water were collected in a 60-mL clear glass bottle 

filled with unfiltered groundwater, sealed with a conical 

cap, and secured with electrical tape to prevent leakage and 

evaporation.

Pesticides and pesticide degradates and NDMA samples 

were collected in 1-L baked amber glass bottles. Samples for 

pesticides and pesticide degradates were filtered through a 

0.7-μm nominal pore-size glass fiber filter during collection, 

whereas the NDMA samples were filtered at Weck prior to 

analysis. NDMA sample containers, treated with 0.05 gram 

(g) of sodium thiosulfate (Na2

S2O3) as a preservative, were 

provided by Weck.
Appendix 79

Trace element, major and minor ion, silica, and TDS 

samples required filling one 250-mL polyethylene bottle with 

unfiltered groundwater and one 500-mL and one 250-mL 

polyethylene bottle with filtered groundwater (Wilde and 

others, 2004). Filtration was done using a 0.45-μm pore-size 

PALL® unvented capsule filter that was pre-rinsed with 2 L 

of deionized water, and then rinsed with 1 L of groundwater 

prior to sampling. The 250-mL filtered sample then was 

preserved with 7.5-N nitric acid. Samples for nutrients and 

stable isotopes of nitrogen and oxygen in dissolved nitrate 

each were filtered into 125-mL brown polyethylene bottles. 

Samples for arsenic and iron species were filtered into a 

250‐mL polyethylene bottle that was covered with tape to 

prevent light exposure and preserved with 6-N hydrochloric 

acid. Samples for stable isotopes of sulfur in dissolved sulfate 

were filtered into 1-L polyethylene bottles. Samples for stable 

isotopes of boron and isotopic ratios of strontium in water 

were filtered into one 250-mL polyethylene bottle and secured 

with electrical tape to prevent leakage and evaporation. 

Samples for stable isotopes of carbon in dissolved inorganic 

carbon and carbon-14 abundance were filtered and bottom 

filled into 500-mL glass bottles that first were overfilled with 

three bottle volumes of filtered groundwater. These samples 

had no headspace and were sealed with conical caps to avoid 

atmospheric contamination. Samples for field alkalinity 

titrations were collected by filtering groundwater into a 

500‐mL polyethylene bottle.

Chromium species samples were collected at the hose 

bib at the well head using a 10-mL syringe with an attached 

0.45-μm pore-size MILLEX®HA disk filter. After the syringe 

was rinsed thoroughly and filled with groundwater, 4 mL 

of sample water was forced through the disk filter; the next 

2 mL of the groundwater were filtered slowly into a small 

centrifuge vial for analysis of total chromium. Chromium-VI 

then was collected by attaching a small cation-exchange 

column to the syringe filter after conditioning the column with 

2 mL of sample water; an additional 2 mL of sample water 

was collected in a second centrifuge vial. Both vials were 

preserved with 10 microliters (μL) of 7.5-N nitric acid (Ball 

and McCleskey, 2003a, b).

DOC samples were collected from the hose bib at the 

well head using a Teflon® filtration apparatus attached to a 

foot-long length of Teflon® tubing. For each sample, 100 mL 

of groundwater and (or) certified inorganic-free blank water 

was first filtered to waste through the baked 0.7-μm nominal 

pore-size glass-fiber filter, then 100 mL of groundwater was 

filtered into a 125-mL baked amber glass bottle (Wilde and 

others, 2004). Each sample then was preserved immediately 

by lowering the pH to between 2 and 1 with 4.5-N sulfuric 

acid.

Dissolved standard gases were collected by using a 

copper tube sampling apparatus attached to the hose bib at 

the well head. Each sample was collected by bottom filling 

two 150-mL glass serum bottles that were first overfilled with 

approximately 5 gallons of unfiltered groundwater. These 

samples had no headspace or air bubbles inside the bottles and 

were sealed underwater to avoid atmospheric contamination. 

Dissolved noble gases were collected in 3⁄8-inch-diameter 

copper tubes using reinforced nylon tubing connected to 

the hose bib at the well head. Groundwater was flushed 

through the tubing to dislodge bubbles before the flow was 

restricted with a back-pressure valve. Clamps on either side 

of the copper tube then were tightened, trapping a sample of 

groundwater for analyses of dissolved noble gases (Weiss, 

1968). 

Field alkalinity was measured in the mobile laboratory 

at the well site. Alkalinity was measured on filtered samples 

by Gran’s titration method (Gran, 1952). Titration data 

were entered directly into PCFF, and the concentrations of 

bicarbonate (HCO3

–

) and carbonate (CO3

2–) automatically 

were calculated from the titration data using the advanced 

speciation method (http://or.water.usgs.gov/alk/methods.html) 

with pK1 = 6.35, pK2 = 10.33, and pKW = 14. Concentrations 

of HCO3

–

 and CO3

2– also were calculated from the laboratory 

alkalinity and laboratory pH measurements. 

Temperature-sensitive samples were stored on ice prior 

to, and during daily shipping to the various laboratories. The 

non-temperature- or non-time-sensitive samples for chromium 

species, tritium, stable isotopes of hydrogen and oxygen in 

water, stable isotopes of sulfur in dissolved sulfate, stable 

isotopes of boron, isotopic ratios of strontium in water, and 

dissolved noble gases were shipped monthly. Temperatureor time-sensitive samples for VOCs, low-level fumigants, 

pesticides and pesticide degradates, perchlorate, NDMA, 

1,2,3-TCP, trace elements, nutrients, DOC, major and minor 

ions, silica, and TDS were shipped daily. The temperaturesensitive samples for total (unfiltered) arsenic and iron, filtered 

arsenic and iron species, and dissolved standard gases were 

stored on ice and shipped weekly. Samples for stable isotopes 

of nitrogen and oxygen in dissolved nitrate and stable isotopes 

of carbon in dissolved inorganic carbon and carbon-14 

abundance were stored on ice, archived in a laboratory 

refrigerator and (or) freezer, and shipped after results for 

nitrate and alkalinity were received from the NWQL.

Nine laboratories performed chemical analyses for 

this study (table A1), although most of the analyses were 

performed at the NWQL or by laboratories contracted by the 

NWQL. The NWQL maintains a rigorous QA program (Pirkey 

and Glodt, 1998; Maloney, 2005). Laboratory QC samples, 

including method blanks, continuing calibration verification 

standards, standard reference samples, reagent spikes, 

external certified reference materials, and external blind 

proficiency samples are analyzed regularly. Method detection 

limits are tested continuously, and laboratory reporting 

levels are updated accordingly. NWQL maintains National 

Environmental Laboratory Accreditation Program (NELAP) 

and other certifications (http://www.nelac-institute.org/accredlabs.php). In addition, the USGS Branch of Quality Systems 

(BQS) maintains independent oversight of QA at the NWQL 

and laboratories contracted by the NWQL. The BQS also 
80 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

runs the National Field Quality Assurance Program (NFQA) 

that includes annual testing of all USGS field personnel for 

proficiency in making field water-quality measurements 

(http://internalbqs.cr.usgs.gov/NFQA/NFQA.php). Results 

for analyses made at the NWQL or by laboratories contracted 

by the NWQL are uploaded directly into the USGS NWIS 

database. Results of analyses made at other laboratories are 

compiled in a project database and uploaded from there into 

the USGS NWIS database. Some laboratory QC data are 

stored in the USGS NWIS database also.

Data Reporting

Laboratory Reporting Conventions

The USGS NWQL uses different conventions for 

reporting results for organic, inorganic, and radioactive 

constituents. The USGS NWQL uses the LRL for reporting 

analytical results for most organic constituents. The LRL is set 

to minimize the reporting of false negatives (not detecting a 

compound when it actually is present in a sample) to less than 

1 percent (Childress and others, 1999). The NWQL updates 

LRL values regularly, and the values listed in this report were 

in effect during the period that groundwater samples from the 

WSJV study unit were analyzed (March to July 2010).

The LRL usually is set at two times the long-term method 

detection level (LT-MDL). The LT-MDL is derived from 

the standard deviation of at least 24 method detection level 

(MDL) determinations made over an extended period of time. 

The MDL is the minimum concentration of a substance that 

can be measured and reported with 99-percent confidence 

that the concentration is greater than zero (at the MDL there 

is less than 1 percent chance of a false positive). LT-MDLs 

continually are monitored and updated (Childress and 

others, 1999; U.S. Environmental Protection Agency, 2002). 

Concentrations less than the LT-MDL are reported as nondetections with a dash (–) in the data tables.

Concentrations between the LRL and the LT-MDL are 

reported as having a higher degree of uncertainty (coded by 

the letter “E” preceding the values in the tables and text). For 

information-rich methods, detections less than the LT-MDL 

have high certainty of presence, but the precise concentration 

is uncertain. These values are also E-coded. Informationrich methods are those that utilize gas chromatography or 

high-performance liquid chromatography (HPLC) with mass 

spectrometry detection, such as those methods used to analyze 

VOCs and pesticides. Compounds are identified by presence 

of characteristic fragmentation patterns in their mass spectra 

in addition to being quantified by measurement of peak areas 

at their associated chromatographic retention times. E-coded 

values also may result from detections outside the range of 

calibration standards, from detections that did not meet all 

laboratory QC criteria, and from samples that were diluted 

prior to analysis (Childress and others, 1999).

Some constituents in this study are reported by using 

minimum reporting levels (MRLs) or method uncertainties 

(MU). The MRL is the smallest measurable concentration of 

a constituent that may be reliably reported when using a given 

analytical method (Timme, 1995). The MU generally indicates 

the precision of a particular analytical measurement; it gives a 

range of values wherein the true value will be found. 

Results for most constituents are given using the LRL, 

MDL, or MRL values provided by the analyzing laboratories. 

Results for some organic and inorganic constituents are 

presented using SRLs derived from assessing results from 

QC samples associated with groundwater samples collected 

as part of the GAMA-PBP [see the appendix section titled 

“Methods for Determining Study Reporting Levels (SRLs)”]. 

The methods used for analysis of tritium measure 

activities by counting techniques (table A1). Activity often 

is used instead of concentration for reporting the presence 

of radioactive constituents. Activity of these constituents 

in groundwater is measured in units of picocuries per liter 

(pCi/L), and 1 pCi/L is approximately equal to two atoms 

decaying per minute. The number of atoms decaying is equal 

to the number of alpha or beta particles emitted. 

The reporting limit for tritium is based on sample-specific 

critical levels (ssLC) (McCurdy and others, 2008). The critical 

level is analogous to the LT-MDL used for reporting analytical 

results for organic and non-radioactive inorganic constituents. 

Here, the critical level is defined as the minimum measured 

activity that indicates a positive detection of the radionuclide 

in the sample with less than a 5-percent probability of a false 

positive detection. Sample-specific critical levels are used for 

radioactive measurements because the critical level is sensitive 

to sample size and sample yield during analytical processing 

and is dependent on instrument background, on counting times 

for the sample and background, and on the characteristics of 

the instrument being used and the nuclide being measured. An 

ssLC is calculated for each sample, and the measured activity 

in the sample is compared to the ssLC associated with that 

sample. Measured activities less than the ssLC are reported as 

non-detections with a dash (–) in the data tables.

The analytical uncertainties associated with measurement 

of activities also are sensitive to sample-specific parameters, 

including sample size, sample yield during analytical 

processing, and time elapsed between sample collection and 

various steps in the analytical procedure, as well as parameters 

associated with the instrumentation. Therefore, measured 

activities of tritium are reported with sample-specific 

combined standard uncertainties (CSU). The CSU is reported 

at the 68-percent confidence level (1-sigma).

Stable isotopic compositions of hydrogen, oxygen, 

nitrogen, sulfur, boron, and carbon are reported as relative 

isotope ratios in units of per mil using the standard delta 

notation (Coplen and others, 2002):
Appendix 81

i sample

reference

E

R

R = − x

















1 1,000 per mil

(A1)

where

i is the atomic mass of the heavier isotope of 

the element,

E is the element (H for hydrogen, O for oxygen, 

N for nitrogen, S for sulfur, B for boron, 

and C for carbon),

Rsample is the ratio of the abundance of the heavier 

isotope of the element (2H, 18O, 15N, 34S, 11B, 13C) to the lighter isotope of the 

element (1H, 16O, 14N, 32S, 10B, 12C) in the 

sample, and

Rreference is the ratio of the abundance of the heavier 

isotope of the element to the lighter isotope 

of the element in the reference material.

The reference material for oxygen and hydrogen is 

Vienna Standard Mean Ocean Water (VSMOW), which is 

assigned δ18O and δ2H values of 0 per mil (note than δ2H is 

sometimes written as δD because the common name of the 

heavier isotope of hydrogen, hydrogen-2, is deuterium). The 

reference material for nitrogen is atmospheric nitrogen gas, 

which is assigned a δ15N value of 0 per mil. The reference 

material for sulfur is Vienna Canon Diablo Troilite (VCDT), 

which is assigned a δ34S value of 0 per mil. The reference 

material for boron is the National Institute of Standards and 

Technology (NIST) reference material SRM 951 boric acid, 

which is assigned a δ11B value of 0 per mil (Coplen and others, 

2002). The reference material for carbon is Vienna Pee Dee 

Belemnite (VPDB), which is assigned a δ13C value of 0 per 

mil. Positive values indicate enrichment of the heavier isotope, 

and negative values indicate depletion of the heavier isotope, 

compared to the ratios observed in the standard reference 

material. Isotopic ratios of strontium in water are presented 

as the abundance of atoms of the heavier isotope (87Sr) to the 

lighter isotope (86Sr) of the element.

Constituents Determined by Multiple Methods 

or Laboratories

Ten constituents targeted in this study were measured 

by more than one analytical method or by more than one 

laboratory (table A2). The preferred methods for these 

constituents were selected on the basis of the procedure 

recommended by the NWQL (http://wwwnwql.cr.usgs.gov/

dyn.shtml?Preferred_method_selection_procedure). 

The water-quality indicators—alkalinity, pH, and specific 

conductance—were measured in the field and at the NWQL. 

The field measurements are the preferred method for all 

three constituents; however, both measurements are reported 

(table 4). Field values are generally preferred because field 

conditions are considered more representative of groundwater 

conditions (Hem, 1985).

DBCP and EDB each appear on the following NWQL 

analytical schedules: VOCs (Schedule 2020; table 3A) 

and low-level fumigants (Schedule 1306; table 3B). For 

constituents collected on Schedules 2020 and 1306, the 

preferred method was Schedule 1306. 

1,2,3-TCP was measured at the NWQL (reporting level 

was an LRL of 0.12 μg/L) and Weck (reporting level was an 

MRL of 0.005 μg/L) (table 3A, D). Both sets of results are 

reported for this constituent.

For total arsenic, chromium, and iron (filtered) 

concentrations, the approved method, Schedule 1948 

(table 3E), used by the NWQL is preferred over the research 

methods used by the NRP-TML (table 3H); however, 

both measurements are reported (tables 8 and 11). The 

concentrations measured by the NRP-TML are only used to 

calculate ratios of redox species for arsenic, chromium, and 

iron.

For example, 

( )

( )

( ) ( )

( )

Cr III Cr T Cr VI

Cr VI Cr VI

− =

(A2)

where 

Cr(T) is the total chromium concentration 

(measured),

Cr(VI) is the concentration of hexavalent chromium 

(measured), and

Cr(III) is the concentration of trivalent chromium 

(calculated). 

Tritium also was measured at two laboratories: LLNL 

and USGS Stable Isotope and Tritium Laboratory, Menlo 

Park, California (SITL) (table 3I). Only tritium data from the 

SITL was available for reporting at the time of this publication 

(table 12).

Quality-Assurance Methods and Results

The purpose of QA is to identify which data best 

represent environmental conditions and which may have been 

affected by contamination or bias during sample collection, 

processing, storage, transportation, and (or) laboratory 

analysis. Four types of QC measurements were evaluated in 

this study: (1) blank samples were collected to assess positive 

bias as a result of contamination during sample handling 

or analysis, (2) replicate samples were collected to assess 

variability, (3) matrix-spike tests were done to assess positive 

or negative bias, and (4) surrogate compounds were added to 

samples analyzed for organic constituents to assess potential 

bias from laboratory analytical methods. Results that were 
82 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

found to have significant bias, on the basis of the QC data 

collected from this study and previous studies, were flagged 

with an appropriate remark code (described in subsequent 

sections) and were not used to calculate detection frequencies 

or other statistical analyses.

Blanks

The primary purposes of collecting blanks are to evaluate 

the magnitude of potential contamination of samples with 

compounds of interest during sample handling or analysis, and 

to identify and mitigate the sources of sample contamination. 

Methods of Blank Collection and Analysis

Blanks were collected using blank water certified by the 

NWQL to contain less than the reporting levels for selected 

constituents investigated in the study (http://wwwnwql.cr.usgs.

gov/USGS/OBW/obw.html). Nitrogen-purged, organic-free 

blank water was used for blanks of organic constituents, and 

inorganic-free blank water was used for blanks of inorganic 

constituents. Three types of blanks were collected:

• Field blanks were collected to assess potential 

contamination of samples during collection, 

processing, transport, and analysis. To collect field 

blanks, blank water either was pumped or poured 

through the sampling equipment (fittings and tubing) 

used to collect groundwater samples, then processed 

and transported using the same protocols used for the 

groundwater samples. Typically, 12 liters of blank 

water were pumped or poured through the sampling 

equipment before each field blank was collected. Field 

blanks were analyzed for VOCs, low-level fumigants, 

pesticides and pesticide degradates, perchlorate, 1,2,3-

TCP, NDMA, trace elements, nutrients, DOC, major 

and minor ions, silica, TDS, total arsenic and iron 

(unfiltered), and arsenic, chromium, and iron species 

(filtered). 

• Source-solution blanks are collected at the beginning 

of a study or when using a new lot of blank water 

to assess potential contamination of samples during 

transport and analysis, and potential contamination of 

the certified blank water obtained from the NWQL. 

Source-solution blanks were collected by pouring 

blank water directly into sample containers that were 

preserved, stored, shipped, and analyzed in the same 

manner as the groundwater samples. Source-solution 

blanks were analyzed for VOCs, low-level fumigants, 

pesticides and pesticide degradates, perchlorate, 

NDMA, 1,2,3-TCP, trace elements, major and minor 

ions, silica, TDS, total arsenic and iron (unfiltered), 

and arsenic, chromium, and iron species (filtered). 

• An equipment blank was collected at the beginning of 

the study unit at the USGS Sacramento Field Office 

to assess whether the Grunfos® submersible sampling 

pump was sufficiently clean to be used for collection 

of samples from monitoring wells in the WSJV study 

unit. The equipment blank was analyzed for VOCs, 

pesticides and pesticide degradates, perchlorate, 

NDMA, 1,2,3-TCP, trace elements, nutrients, DOC, 

major and minor ions, silica, and TDS. 

Blanks were not collected for stable isotopes. Stable 

isotopes of hydrogen, oxygen, nitrogen, sulfur, boron, 

strontium, and carbon are an intrinsic property of any of these 

elements; therefore, the concept of a blank does not apply 

to these ratios. In addition, blanks were not collected for 

tritium or dissolved gases. Tritium and dissolved gases are in 

the atmosphere and would dissolve into any solution used in 

collecting a blank, making it impractical to collect a blank for 

these constituents.

Methods for Determining Study Reporting Levels (SRLs)

SRLs for selected VOCs were defined by Fram and 

others (2012) on the basis of the assessment of results from 

field blanks, source-solution blanks, laboratory instrument 

blanks, and groundwater samples from the first 32 GAMA 

PBP study units (May 2004 through September 2010). SRLs 

were established for those VOCs that had evidence of potential 

contamination due to field or laboratory processes. Detections 

of VOCs having concentrations less than or equal to the SRLs 

were reclassified as non-detections.

SRLs for trace elements detected in field blanks collected 

in the first 20 GAMA PBP study units (May 2004 through 

January 2008) were defined by Olsen and others (2010) on the 

basis of statistical assessment of results from the field blanks. 

The assessment used order statistics and binomial probabilities 

to construct an upper confidence limit (Hahn and Meeker, 

1991) for the maximum concentration of constituents possibly 

introduced while groundwater samples were collected, 

transported, and analyzed. Detections of trace elements 

having concentrations less than or equal to the SRLs were 

reported with a ≤ symbol preceding the value to indicate that 

the true value may be less than or equal to the reported value 

(including the possibility of being a non-detection). 

SRLs for other constituents collected for the WSJV study 

unit were defined as equal to the highest concentration 

measured in the blanks. Detections of organic constituents 

in groundwater samples at concentrations less than or 

equal to the SRLs have been reclassified as non-detections. 

Detections of inorganic constituents in groundwater samples 

at concentrations less than or equal to the SRLs have been 

reclassified with a ≤ symbol preceding the reported value 

to indicate that the true value may be less than or equal 

to the reported value (including the possibility of being a 

non-detection). 

In the USGS NWIS database, data affected by SRLs (if 

any) are accompanied with the following comment: Result is 

< or = reported value, based on QC data.
Appendix 83

Detections in Blanks and Application of SRLs

Table A3 presents a summary of detections in the blanks 

(eight field blanks, three source-solution blanks, and one 

equipment blank) and the SRLs that were applied for the 

WSJV study unit. Blanks were collected at up to 17 percent of 

the sites sampled in the WSJV study unit. 

Four VOCs (acetone, carbon disulfide, chloroform, and 

toluene) were detected in blanks collected for the WSJV 

study unit, and each also have SRLs defined by Fram and 

others (2012). Detections of VOCs in groundwater samples 

with concentrations less than or equal to the SRL (defined 

by the blank detections or by Fram and others, 2012) were 

flagged with a ≤ symbol and reclassified as non-detections. An 

additional four VOCs (1,2,4-trimethylbenzene, ethylbenzene, 

m- plus p-xylene, and o-xylene) have SRLs defined by Fram 

and others (2012) and were detected in groundwater samples 

(table 5).

Acetone was detected in one field blank at a 

concentration of E0.3 μg/L. Acetone was not detected in 

groundwater samples; therefore, definition of an SRL was not 

necessary. 

Carbon disulfide was detected in one field blank at a 

concentration of 0.06 μg/L. This concentration is greater than 

the SRL of E0.03 μg/L defined by Fram and others (2012). 

The SRL of 0.03 μg/L was defined on the basis of statistical 

assessment of results from field blanks collected during 

32 GAMA study units; therefore, it was considered appropriate 

to use this value, rather than the higher concentration 

measured in the field blank, as the SRL. 

Chloroform was detected in the equipment blank at a 

concentration of E0.02 μg/L. This concentration is equal 

to the defined SRL for groundwater samples collected with 

monitoring well equipment (Fram and others, 2012). 

1,2,4-Trimethylbenzene has an SRL of 0.56 μg/L as 

defined by Fram and others (2012). 1,2,4-Trimethylbenzene 

was detected at a concentration less than the SRL in one 

groundwater sample (WS-07: E0.02 μg/L); therefore, this 

result is not presented in table 5. 

Toluene was detected in three field blanks and one 

source-solution blank at concentrations ranging from 

E0.01 μg/L to 0.04 μg/L. Toluene has an SRL of 0.69 μg/L 

as defined by Fram and others (2012). The SRL of 0.69 μg/L 

was used on the basis on these results. Toluene was detected 

at concentrations less than or equal to the SRL in two 

groundwater samples (WS-U-02: 0.03 μg/L; DM-U-11: 

E0.02 μg/L); the latter of which (DM-U-11) was reclassified 

as a non-detection.

Ethylbenzene, m- plus p-xylene, and o-xylene were not 

detected in blanks collected for the WSJV study unit, but 

were detected in samples from wells WS-U-01 and WS-U-02. 

Additionally, toluene was detected in groundwater samples 

from WS-U-02 (see previous paragraph). The concentrations 

of ethylbenzene and o-xylene in WS-U-01 and of m- plus 

p-xylenes, o-xylene, and toluene in WS-U-02 were less than 

the SRLs established by Fram and others, 2012). However, 

multiple lines of evidence suggested that the detections of 

the hydrocarbons ethylbenzene, m- plus p-xylene, o-xylene, 

and toluene are more likely to be the result of environmental 

occurrence of these compounds in the groundwater sampled 

from these wells; therefore, SRLs were not applied and the 

data were not reclassified in table 5. 

First, WS-U-02 was previously sampled by the GAMAPBP in 2005 (well TLR-07, Southeast San Joaquin Valley: 

Burton and others, 2008); ethylbenzene, m- plus p-xylene, 

o-xylene, and toluene and high concentrations of benzene were 

all detected in this well in 2005 and in the sample collected 

for the WSJV study unit. Such repeated detections of the same 

compounds in the same well sampled 5 years apart are more 

likely to reflect actual presence of these compounds in the 

well rather than random contamination. Second, data from the 

CDPH database indicated historical detections of benzene in 

WS-U-01 and WS-U-02, often at concentrations greater than 

the MCL-CA of 1 μg/L. These results indicate that there is a 

history of hydrocarbon presence in the sampled groundwater 

in these wells. Because the LRLs of the analyses reported in 

the CDPH database are much higher than those used by the 

GAMA-PBP and because concentrations of ethylbenzene, 

m- plus p-xylene, o-xylene, and toluene measured by the 

GAMA-PBP were much lower than the concentrations of 

benzene, it is not surprising that these other hydrocarbons 

have not generally been detected in the CDPH analyses. Third, 

it is not unexpected that groundwater that is highly affected 

by hydrocarbon contamination (as evidenced by the high 

concentrations of benzene in these wells) would also have 

detections of other relatively soluble hydrocarbons, including 

ethylbenzene, m- plus p-xylene, o-xylene, and toluene.

Seven trace elements (cobalt, copper, lead, manganese, 

molybdenum, silver, and tungsten) were detected in blanks 

collected for the WSJV study unit. An additional four trace 

elements (chromium, iron, nickel, and zinc) were detected in 

groundwater samples at concentrations less than or equal to 

the SRLs defined by Olsen and others (2010). Detections of 

trace elements in groundwater samples with concentrations 

less than or equal to the SRL were flagged with a ≤ symbol in 

table 8.

Cobalt was detected in all eight field blanks and in the 

equipment blank at concentrations ranging from 0.10 μg/L to 

0.33 μg/L. An SRL of 0.33 μg/L was defined on the basis of 

the blank results. 

Copper was detected in two field blanks at concentrations 

of E0.59 μg/L and E0.84 μg/L, respectively. Copper has an 

SRL of 1.7 μg/L as defined by Olsen and others (2010). The 

SRL of 1.7 μg/L was used on the basis of these results.

Lead was detected in all three field blanks at 

concentrations ranging from E0.02 μg/L to 0.04 μg/L. Lead 

has an SRL of 0.65 μg/L as defined by Olsen and others 
84 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

(2010). The SRL of 0.65 μg/L was used on the basis of these 

results.

Manganese was detected in all eight field blanks and the 

equipment blank at concentrations ranging from 0.21 μg/L to 

0.52 μg/L. An SRL of 0.52 μg/L was defined on the basis of 

the blank results. 

Molybdenum was detected in a field blank and the 

equipment blank, both at concentrations of E0.02 μg/L. 

Molybdenum was not detected in groundwater samples at 

concentrations less than or equal to those measured in the 

blanks; therefore, application of an SRL was not necessary 

(table 8).

Silver was detected in a field blank and the equipment 

blank at concentrations of 0.010 μg/L and E0.005 μg/L, 

respectively. An SRL of 0.010 μg/L was defined on the basis 

of the blank results. 

Tungsten was detected in a field blank at a concentration 

of 0.03 μg/L. Tungsten has an SRL of 0.11 μg/L as defined 

by Olsen and others (2010). The SRL of 0.11 μg/L was used 

based on these results. 

Two nutrients were detected in one field blank; 

ammonia (as nitrogen) (E0.02 μg/L) and nitrite (as nitrogen) 

(E0.001 μg/L). SRLs were defined for each nutrient on the 

basis of the blank results. 

Alkalinity and TDS each were detected in one field blank 

at concentrations of E4 mg/L and 34 mg/L, respectively. 

Alkalinity and TDS were not detected in groundwater samples 

at concentrations less than or equal to those measured in 

the field blanks; therefore, application of an SRL was not 

necessary (tables 4 and 10).

Replicates

Sequential replicate samples were collected to assess 

the precision of the water-quality data. Estimates of data 

precision are needed to assess whether differences between 

concentrations in samples are because of differences in 

groundwater quality or because of variability that may result 

from collecting, processing, and analyzing the samples. 

Methods of Assessment of Replicate Samples

Three methods for measuring variability were used to 

assess precision over the range of measured concentrations 

found in groundwater samples. The variability between results 

in the pairs of sequential replicate samples was represented 

by the standard deviation (SD) for low concentrations and 

by relative standard deviation (RSD) for high concentrations 

(Anderson, 1987; Mueller and Titus, 2005). The RSD is 

defined as the SD divided by the mean concentration for 

each replicate pair of samples expressed as a percentage. The 

boundary between concentrations of each metric was used was 

defined as five times the selected reporting level (RL) for each 

constituent. The RL may be an LRL, SRL, MDL, or MRL for 

each constituent; therefore pairs with mean concentrations 

< 5 RL used the SD, and pairs with mean concentrations 

≥ 5 RL used the RSD. 

Replicate pairs of analyses of all constituents except for 

radiochemical constituents were evaluated as follows:

• If both values were reported as detections, the SD or 

RSD was calculated. Acceptable precision is defined 

as an SD of less than 1⁄2 RL or an RSD of less than 

10 percent. For comparison, an RSD of 10 percent is 

equivalent to a relative percent difference (RPD) of 

14 percent. 

• If both values were reported as non-detections, the 

variability was set to zero by definition. 

• If one value was reported as a non-detection, and the 

other value was reported as a detection less than the 

RL, then a value of zero was substituted for the nondetection, and the SD was calculated. Substituting 

zero for the non-detection would yield the maximum 

estimate of variability for the replicate pair. 

• If one value for a sample pair was reported as a 

non-detection and the other value was flagged with a 

≤ symbol (less than or equal to the SRL), or if both 

values were flagged with a ≤ symbol, the SD was not 

calculated because the values may be analytically 

identical. The ≤ symbol indicates that the value is 

a maximum potential concentration and the true 

concentration could be less than the RL for the other 

sample in the pair.

• If one value was reported as a non-detection and the 

other value was reported as a detection greater than 

the RL, the variability for the pair was considered 

unacceptable.

Replicate pairs of analyses of radiochemical constituents 

were evaluated using the following equation (McCurdy and 

others, 2008):

( )

1 2

2 2

1 2

R R z

CSU CSU

− =

+

(A3)

where 

z is the test statistic,

R1 and R2 are the results for the two samples in the 

replicate pair, and

CSU1 and CSU2 are the combined standard uncertainties 

associated with the results.

Values of z < 1.65 correspond to significant levels (p) 

< α, where α =0.05, and thus indicate replicate pairs with 

acceptable precision. Replicate pairs with values that are 

statistically indistinguishable at a confidence level (α) of 

α = 0.05 are defined as acceptable.
Appendix 85

If results from replicate sample pairs indicate that 

variability is unacceptable for a constituent, then this greater 

variability must be considered when interpreting the data. If 

measured concentrations are slightly greater than a waterquality benchmark, then actual concentrations could be less 

than that benchmark. Similarly, if measured concentrations 

are slightly less than a water-quality benchmark, then actual 

concentrations could be greater than a benchmark. If a 

constituent has high variability in replicate sample pairs, then 

a larger difference between concentrations measured in two 

samples is required to conclude that the two samples have 

significantly different concentrations. 

Variability in Replicate Samples

Table A4A−B summarize the results of replicate analyses 

for constituents detected in groundwater samples collected in 

the WSJV study unit. Replicate samples were collected for up 

to 10 percent of the samples collected. Of the 1,370 replicate 

pairs of constituents analyzed, 93 were for constituents 

detected in at least one groundwater sample. Of these 93 pairs, 

none were outside the limits for acceptable variability. 

Six replicate pairs of samples were analyzed for the 

VOCs, and all pairs were composed of two values reported 

as non-detections with the exception of one replicate pair for 

chloroform and one replicate pair for MTBE. The replicate 

pairs all yielded two values reported as detections, and all 

replicate pair analysis resulted in SDs within acceptable 

precision. 

Six replicate pairs of samples were analyzed for the lowlevel fumigants, and all pairs were composed of two values 

reported as non-detections.

Six replicate pairs of samples were analyzed for the 

pesticide and pesticide degradate compounds, and all pairs 

were composed of two values reported as non-detections 

with the exception of two replicate pairs each for metolachlor 

and one replicate pair for simazine, deethylatrazine, atrazine, 

hexazinone, S-ethyl-dipropylthiocarbamate (EPTC), and 

3,4-dichloroaniline. The replicate pairs all yielded two values 

reported as detections, and all replicate pair analysis resulted 

in SDs within acceptable precision. 

Six replicate pairs for perchlorate and 1,2,3-TCP and 

four replicate pairs for NDMA were analyzed at Weck for 

variability. Five of the replicate pairs for perchlorate and 

1,2,3‐TCP and all of the replicate pairs for NDMA were 

composed of two values reported as non-detections. The 

remaining replicate pairs for perchlorate and 1,2,3-TCP 

yielded two values reported as detections, and each replicate 

pair analysis resulted in an SD or RSD within acceptable 

precision. 

Six replicate pairs of samples were analyzed for DOC, 

and one of these pairs was composed of two values reported as 

non-detections. The five remaining replicate pairs yielded two 

values reported as detections, and all replicate pair analyses 

resulted in SDs within acceptable precision.

Replicate pairs of samples were analyzed for trace 

elements, nutrients, major and minor ions, silica, TDS, total 

arsenic and iron (unfiltered), and arsenic, chromium, and iron 

species (filtered). All replicate pairs were either composed of 

two values reported as non-detections or two values reported 

as detections. The analyses for all replicate pairs reported 

as detections resulted in SDs and RSDs within acceptable 

variability.

Six replicate pairs for tritium were analyzed for 

variability. All replicate pairs yielded statistically similar 

results (p ≤ 0.05) and were, therefore, considered acceptable.

Matrix Spikes

Addition of a known concentration of a constituent 

(spike) to a replicate groundwater sample enables the 

laboratory to determine the effect of the matrix, in this case 

groundwater, on the analytical technique used to measure the 

constituent. The known compounds added in matrix spikes are 

the same as those analyzed in the groundwater samples. This 

enables an analysis of matrix interferences on a compoundby-compound basis. For this study, matrix spikes were added 

by the laboratories performing the analysis rather than in the 

field. Low and high matrix-spike recoveries may be a potential 

concern if the concentration of a compound in a groundwater 

sample is close to the benchmark; a low recovery could result 

in a falsely measured concentration less than the benchmark, 

whereas a high recovery could result in a falsely measured 

concentration greater than the benchmark.

The GAMA-PBP defined the data-quality objective 

range for acceptable median matrix-spike recoveries as 70 

to 130 percent. Only constituents with median matrix-spike 

recoveries outside of this range were flagged as having 

unacceptable recoveries. Matrix spikes were performed 

for VOCs, low-level fumigants, pesticides and pesticide 

degradates, 1,2,3-TCP, and NDMA because the analytical 

methods for these constituents may be susceptible to matrix 

interferences. 

Matrix-Spike Recoveries

Table A5A–D presents a summary of matrix-spike 

recoveries for the WSJV study unit. Spiked samples were 

collected at up to 16 percent of the wells sampled. 

Six groundwater samples were spiked with VOCs to 

calculate matrix-spike recoveries. Median matrix-spike 

recoveries for all 85 spike compounds were between 70 and 

130 percent. Two spike compounds (hexachlorobutadiene and 

styrene) had at least one matrix-spike recovery < 70 percent. 

Two spike compounds (acetone and DBCP) had at least one 

matrix-spike recovery > 130 percent, and DCBP was detected 

in groundwater samples (table A5A). 

Eight groundwater samples were spiked with low-level 

fumigants to calculate matrix-spike recoveries. Median 

matrix-spike recoveries for both spike compounds were 
86 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

between 70 and 130 percent. DBCP had at least one matrixspike recovery > 130 percent and two recoveries < 70 percent. 

EDB had three matrix-spike recoveries < 70 percent. 

Eight groundwater samples were spiked with pesticide 

and pesticide degradate compounds to calculate matrixspike recoveries. Median matrix-spike recoveries for 61 of 

the 83 spike compounds were between 70 and 130 percent. 

Twenty of the spike compounds had median matrix-spike 

recoveries < 70 percent, and two had median matrix-spike 

recoveries > 130 percent. Of the 16 compounds detected in 

groundwater samples, 14 had median matrix-spike recoveries 

within the acceptable range (table A5C). 

The median matrix-spike recovery for hexazinone was 

less than the acceptable range (64.7 percent). Hexazinone 

was detected in three DM study area grid wells (0.029 μg/L, 

0.009 μg/L, and 0.045 μg/L) and four DM-U wells 

(0.012 μg/L, 0.027 μg/L, 0.024 μg/L, and 0.076 μg/L; table 6). 

The minimum detected concentration (E0.009 μg/L) is slightly 

greater than the LRL of 0.008 μg/L. Because hexazinone was 

detected at a concentration of 0.001 μg/L, greater than the 

LRL, it was determined that the less than acceptable median 

matrix-spike recovery did not affect the NWQL analysis and 

results.

The median matrix-spike recovery for tebuthiuron was 

greater than the acceptable range (138.3 percent). Tebuthiuron 

was detected in one understanding well (DM-U-01) at a 

concentration of E0.01 μg/L (table 6). Because tebuthiuron 

was detected at a concentration an order of magnitude less 

than the LRL of 0.028 μg/L (table 6), it was determined that 

the less than acceptable median matrix-spike recovery did not 

affect the NWQL analysis and results. 

At least one matrix-spike recovery for 44 pesticide 

and pesticide degradate spike compounds was < 70 percent. 

Of these spike compounds, deethylatrazine, 2-ethyl-6-

methylaniline, and hexazinone were detected in groundwater 

samples. One pesticide and pesticide degradate spike 

compound (fipronil sulfide) had at least one matrix-spike 

recovery < 70 percent and one matrix-spike recovery 

> 130 percent, but it was not detected in groundwater samples. 

At least one matrix-spike recovery for seven pesticide and 

pesticide degradate spike compounds was > 130 percent. 

Of these spike compounds, tebuthiuron was detected in 

groundwater samples. 

Four groundwater samples were spiked with NDMA, 

and nine groundwater samples were spiked with 1,2,3-TCP to 

calculate matrix-spike recoveries at Weck. All median matrixspike recoveries were between 70 and 130 percent. 

Surrogate Compounds

Surrogate compounds are added to groundwater samples 

in the laboratory prior to analysis to evaluate the recovery 

of similar constituents. Surrogate compounds were added 

in the laboratory to all groundwater and QC samples that 

were analyzed by the NWQL for VOCs and pesticides and 

pesticide degradates. Surrogates are used to identify general 

problems that may arise during laboratory sample analysis 

that could affect the analysis results for all compounds in 

that sample. Potential problems include matrix interferences 

(such as high levels of DOC) that produce a positive bias or 

incomplete laboratory recovery (possibly because of improper 

maintenance and calibration of analytical equipment) that 

produces a negative bias. A 70 to 130 percent recovery of 

surrogates, in general, is considered acceptable; values outside 

of this range indicate possible problems with the processing 

and analysis of samples (Connor and others, 1998; Sandstrom 

and others, 2001).

Surrogate Compound Recoveries

Table A6 presents a summary of the surrogate recoveries 

for the WSJV study unit. The table lists the surrogate, the 

analytical schedule on which it was applied, the number of 

analyses for blank and groundwater samples, the number of 

surrogate recoveries < 70 percent, and the number of surrogate 

recoveries > 130 percent for the blank and groundwater 

samples. Blank and groundwater samples were considered 

separately to assess whether or not the matrixes present in 

groundwater samples affect surrogate recoveries. 

Most surrogate recoveries for the blank and groundwater 

samples were within the acceptable range of 70 to 130 percent. 

In total, 92 percent of the blank and 93 percent of the 

groundwater sample surrogate recoveries for VOC analyses 

were within the acceptable range. In addition, 92 percent of 

the blank and 94 percent of the groundwater sample surrogate 

recoveries for pesticide and pesticide degradate analyses 

were within the acceptable range. There were no significant 

differences between VOC and pesticide and pesticide 

degradate surrogate recoveries in blank and groundwater 

samples (Wilcoxon rank sum test, p < 0.05).
Appendix 87

Laboratory Quality-Control Results for 

Inorganic Constituents

Laboratory bias as indicated from internal laboratory 

QC tests was investigated to determine whether or not the 

WSJV study unit data were affected by laboratory procedures. 

The BQS operates the Inorganic Blind Sample Project (IBSP) 

to monitor and evaluate the quality of results for analyses of 

trace elements, nutrients, major and minor ions, silica, and 

TDS by the NWQL. The IBSP submits standard reference 

samples consisting of natural matrix water samples spiked 

with reagent chemicals that contain known concentrations of 

the inorganic constituents (Farrar and Long, 1997). The IBSP 

data are readily available on the BQS website, and the BQS 

issues monthly summaries of the results, reporting the amount 

of bias observed in the results (http://bqs.usgs.gov/IBSP/).

The BQS monthly summaries were examined for March 

through July 2010, the period of time during which samples 

were analyzed for the WSJV study unit. During this time 

period, the BQS reported that four inorganic constituents 

showed evidence of bias: a positive bias for barium, cadmium, 

and molybdenum and a negative bias for magnesium. 

Examination of the results for the IBSP samples for these four 

constituents indicated that the analytical biases reported by the 

BQS were not significant. 

The IBSP samples for barium had concentrations ranging 

from 26.0 μg/L to 63.0 μg/L. The average difference between 

the measured and expected concentrations was 1.0 μg/L 

(standard deviation = 2.0 μg/L), and the average relative 

percent difference between the measured and expected 

concentrations was 3 percent (standard deviation = 4 percent). 

The MCL-CA for barium is 1,000 μg/L; thus, the estimate of 

bias from the ISBP samples would be relevant for assessment 

of whether groundwater samples have barium concentrations 

greater than or less than the MCL-CA concentration or greater 

than or less than one-half of the MCL-CA concentration. 

However, the maximum concentration of barium in samples 

from the WSJV study unit was 346 μg/L (table 8); thus, a 

potential positive bias of 1.0 μg/L, or 3 percent, would not 

result in a measured concentration greater than either threshold 

when the true concentration would have been below the 

threshold.

The IBSP samples for cadmium had concentrations 

ranging from 0.18 μg/L to 1.7 μg/L. The average difference 

between the measured and expected concentrations was 

0.12 μg/L (standard deviation = 0.08 μg/L), and the 

average relative percent difference between the measured 

and expected concentrations was 24 percent (standard 

deviation = 38 percent). The MCL-US for cadmium is 

5 μg/L, thus the estimate of bias from the ISBP samples 

would be relevant for assessment of whether groundwater 

samples have cadmium concentrations greater than or less 

than the MCL-US concentration or greater than or less 

than one-half of the MCL-US concentration. However, the 

maximum concentration of cadmium in samples from the 

WSJV study unit was 0.34 μg/L (table 8); thus, a potential 

positive bias of 0.12 μg/L, or 24 percent, would not result in a 

measured concentration above either threshold when the true 

concentration would have been below the threshold.

The IBSP samples for molybdenum had concentrations 

ranging from 1.8 μg/L to 33.4 μg/L. The average difference 

between the measured and expected concentrations was 

0.50 μg/L (standard deviation = 0.79 μg/L), and the average 

relative percent difference between the measured and 

expected concentrations was 5 percent (standard deviation 

= 4 percent). The HAL-US for molybdenum is 40 μg/L, 

thus the estimate of bias from the ISBP samples would be 

relevant for assessment of whether groundwater samples 

have molybdenum concentrations greater than or less than the 

HAL-US concentration or greater than or less than one-half of 

the HAL-US concentration. Taking into account the potential 

positive bias of 0.5 μg/L or 5 percent for molybdenum 

concentrations reported by the NWQL, there would have been 

no significant change in whether the reported values were 

greater than or less than the HAL-US concentration or greater 

than or less than one-half of the HAL-US concentration. 

The IBSP samples for magnesium had concentrations 

ranging from 0.05 mg/L to 7.3 mg/L. The average difference 

between the measured and expected concentrations was 

–0.17 mg/L (standard deviation = 0.22 mg/L), and the 

average relative percent difference between the measured and 

expected concentrations was –5 percent (standard deviation 

= 4 percent). A negative bias of 5 percent or 0.17 mg/L can 

only potentially affect measured concentrations of less than 

7.3 mg/L in WSJV study unit groundwater samples (table 10). 

Magnesium does not currently have an established regulatory/

non-regulatory health-based or aesthetic benchmark; 

therefore, this negative bias will not affect the assessments of 

groundwater quality being made by the GAMA-PBP.
88 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table A1. Analytical methods used by the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) and contract laboratories. [Laboratory entity codes in the USGS National Water Information System (NWIS) for laboratories other than the USGS National Water Quality Laboratory (NWQL) are given in parentheses after the laboratory names. Abbreviations: USGS, U.S. Geological Survey; UV, ultraviolet; USEPA, U.S. Environmental Protection Agency; VOC, volatile organic compound; DOC, dissolved organic carbon; NRP, USGS National Reasearch Program] Constituent Analytical method Laboratory and analytical schedule Citation(s) Water-quality indicators Field parameters Calibrated field meters and test kits USGS field measurement U.S. Geological Survey, variously dated Organic constituents VOCs Purge and trap capillary gas chromatography/mass spectrometry NWQL, Schedule 2020 Connor and others, 1998 Low-level fumigants Micro-extraction and gas chromatography/electron- capture detection (USEPA method 504.1) NWQL, Schedule 1306 Eichelberger, 1993 Pesticides and pesticide degradates Solid-phase extraction and gas chromatography/mass spectrometry NWQL, Schedule 2033 Zaugg and others, 1995; Lindley and others, 1996; Sandstrom and others, 2001; Madsen and others, 2003 Constituents of special interest Perchlorate Liquid chromatography with mass spectrometry/mass spectrometry (USEPA Method 331.0) Weck Laboratories, Inc. [Weck], City of Industry, California (CA-WECK), standard operating procedure ORG099.R01 U.S. Environmental Protection Agency, 2005 1,2,3-Trichloropropane (1,2,3-TCP) Isotopic dilution purge and trap/gas chromatography/ mass spectrometry Weck Laboratories, Inc. [Weck], City of Industry, California (CA-WECK), standard operating procedure ORG083 Okamoto and others, 2002

N-Nitrosodimethylamine 

(NDMA)

Isotopic dilution with gas chromatography and 

chemical-ionization mass spectrometry (USEPA Method 1625 modified)

Weck Laboratories, Inc. [Weck], City of Industry, 

California (CA-WECK), standard operating procedure ORG065.R10

U.S. Environmental Protection Agency, 1989; 

Plomley and others, 1994

Inorganic constituents

Major and minor ions and trace 

elements 

Atomic absorption spectrometry, colorimetry, ionexchange chromatography, inductively coupled 

plasma atomic emission spectrometry, and mass spectrometry

NWQL, Schedule 1948 Fishman and Friedman, 1989; Faires, 1993; Fishman, 

1993; McLain, 1993; American Public Health Association, 1998; Garbarino, 1999; Garbarino and others, 2006

Nutrients Alkaline persulfate digestion, Kjedahl digestion NWQL, Schedule 2755 Fishman, 1993; Patton and Kryskalla, 2003

DOC UV-promoted persulfate oxidation and infrared 

spectrometry

NWQL, Laboratory Code 2613 Brenton and Arnett, 1993

Total arsenic and iron (unfiltered) 

and arsenic, chromium, and iron species (filtered)

Various techniques of ultraviolet visible (UV-VIS) 

spectrophotometry and atomic absorbance spectroscopy

USGS NRP Trace Metal Laboratory [TML], Boulder, 

Colorado (USGSTMCO)

Stookey, 1970; To and others, 1998; Ball and 

McCleskey, 2003a and 2003b; McCleskey and others, 2003

Stable isotopes of hydrogen and 

oxygen in water

Gaseous hydrogen and carbon dioxide-water 

equilibration and stable-isotope mass spectrometry

USGS Stable Isotope Laboratory, Reston, Virginia 

(USGSSIVA), NWQL Schedule 1142

Epstein and Mayeda, 1953; Coplen and others, 1991; 

Coplen, 1994

Stable isotopes of nitrogen and 

oxygen in dissolved nitrate

Denitrifier method and mass spectrometry USGS Stable Isotope Laboratory, Reston, Virginia 

(USGSSIVA), NWQL Laboratory Code 2900

Révész and Casciotti, 2007

Table A1
Table A1 89

Constituent Analytical method Laboratory and analytical schedule Citation(s)

Stable isotopes

Stable isotopes of sulfur in dissolved 

sulfate

Vacuum extraction and mass spectrometry USGS Stable Isotope Laboratory, Reston, Virginia 

(USGSSIVA), NWQL Laboratory Code 1951

Carmody and others, 1998

Stable isotopes of boron in water Negative thermal-ionization mass spectrometry USGS NRP Metals Isotope Research Laboratory, 

Menlo Park, California (USGSMICA)

Vengosh and others, 1989; Dwyer and Vengosh, 2008

Isotopic ratio of strontium in water Chemical separations and thermal-ionization mass 

spectrometry

USGS NRP Metals Isotope Research Laboratory, 

Menlo Park, California (USGSMICA)

Bullen and others, 1996

Stable isotopes of carbon in dissolved 

inorganic carbon and carbon-14 abundance

Accelerator mass spectrometry Woods Hole Oceanographic Institution, National 

Ocean Sciences Accelerator Mass Spectrometry Facility [NOSAMS], Woods Hole, Massachusetts (MA-WHAMS), NWQL Schedule 2255

Vogel and others, 1987; Donahue and others, 1990; 

McNichol and others, 1992; Gagnon and Jones, 1993; McNichol and others, 1994; Schneider and others, 1994

Radioactivity and dissolved gases

Tritium Electrolytic enrichment-liquid scintillation USGS Stable Isotope and Tritium Laboratory [SITL], 

Menlo Park, California (USGSH3CA), NWQL Schedule 1565

Thatcher and others, 1977

Dissolved standard gases (argon, 

carbon dioxide, methane, nitrogen, and oxygen)

Gas chromatography/thermal conductivity detector 

and flame ionization detector 

USGS Chlorofluorocarbon Laboratory, Reston, 

Virginia (USGSCFCVA)

Busenberg and others, 2001

Dissolved noble gases (argon, 

helium-4, krypton, neon, and xenon), tritium, and helium isotope ratios

Helium-3 in-growth and mass spectrometry Lawrence Livermore National Laboratory [LLNL], 

Livermore, California (CA-LLNL)

Moran and others, 2002; Eaton and others, 2004

Table A1. Analytical methods used by the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) and contract laboratories.—Continued[Laboratory entity codes in the USGS National Water Information System (NWIS) for laboratories other than the USGS National Water Quality Laboratory (NWQL) are given in parentheses after the laboratory names. Abbreviations: USGS, U.S. Geological Survey; UV, ultraviolet; USEPA, U.S. Environmental Protection Agency; VOC, volatile organic compound; DOC, dissolved organic carbon; NRP, USGS National Reasearch Program]
90 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table A2. Preferred analytical methods or laboratories for selected constituents collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010. [Preferred analytical schedules/methods are selected on the basis of the procedure recommended by the NWQL (http://wwwnwql.cr.usgs.gov/dyn.shtml?Preferred_method_selection_procedure). Abbreviations: USGS, U.S. Geological Survey; NWQL, USGS National Water Quality Laboratory, Denver, Colorado; DBCP, 1,2-Dibromo-3-chloropropane; EDB, 1,2-Dibromoethane; NRP, USGS National Research Project; VOC, volatile organic compound; 1,2,3-TCP, 1,2,3-Trichloropropane; Weck, Weck Laboratories Inc., City of Industry, California; TML, USGS NRP Trace Metal Laboratory, Boulder, Colorado; LLNL, Lawrence Livermore National Laboratory, Livermore, California; SITL, USGS Stable Isotope and Tritium Laboratory, Menlo Park, California; np, no preference] Constituent (synonym or abbreviation) Primary constituent classification Analytical methods or laboratory Preferred analytical method or laboratory Results from preferred method reported Low-level DBCP Fumigant Schedule 1306 (NWQL), Schedule 2020 (NWQL) Schedule 1306 Low- level EDB Fumigant Schedule 1306 (NWQL), Schedule 2020 (NWQL) Schedule 1306 Results from both methods reported pH Water-quality indicator field, Schedule 1948 (NWQL) field Specific conductance Water-quality indicator field, Schedule 1948 (NWQL) field Alkalinity Water-quality indicator field, Schedule 1948 (NWQL) field 1,2,3-TCP VOC Schedule 2020 (NWQL), Weck Weck Arsenic (total, filtered) Trace element Schedule 1948 (NWQL), TML Schedule 1948 Chromium (total, filtered) Trace element Schedule 1948 (NWQL), TML Schedule 1948 Iron (total, filtered) Trace element Schedule 1948 (NWQL), TML Schedule 1948 Tritium Inorganic tracer SITL, LLNL np

Table A2
Table A3 91

Table A3. Constituents detected in blanks and the study reporting level (SRL) analysis for the Western San Joaquin Valley (WSJV) 

Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.

[The SRLs were defined by: (1) the highest concentration in blanks collected for this study unit, (2) Fram and others (2012), or (3) Olsen and others (2010). 

Abbreviations: LRL, laboratory reporting level; volatile organic compound, VOC; ≤, less than or equal to; E, estimated or having a higher degree of 

uncertainty; μg/L, micrograms per liter; mg/L, milligrams per liter; nv, no value in category; —, not detected; TDS, total dissolved solids]

Constituent

Number of blank detections / 

total number of blanks

LRL

Concentrations 

detected 

in blanks

SRL 

Number of groundwater 

samples ≤ -coded / 

total number of 

detections

Field 

blanks

Sourcesolution 

blanks

Equipment 

blank

VOCs (μg/L)

Acetone 1/8 0/3 0/1 0.8 E0.3 nv 0/0

Carbon disulfide 1/8 0/3 0/1 0.04 0.06 0.06 2/6

Chloroform (trichloromethane) 0/8 0/3 1/1 0.03 E0.02 0.03 0/5

Ethylbenzene 0/8 0/3 0/1 0.036 — 0.1 0/2

Toluene 3/8 1/3 0/1 0.018 E0.01, E0.03, E0.04, 0.04 0.69 1/2

1,2,4-Trimethylbenzene 0/8 0/3 0/1 0.032 — 0.56 1/1

m- plus p-Xylene 0/8 0/3 0/1 0.08 — 0.4 0/1

o-Xylene 0/8 0/3 0/1 0.032 — 0.1 0/2

Trace elements (μg/L)

Chromium 0/8 0/2 0/1 0.12 — 0.42 14/41

Cobalt 8/8 0/2 1/1 0.010 0.10, 0.16, 0.18, 0.21, 0.22, 

0.27, 0.28, 0.33, 0.33, 0.33

0.33 48/58

Copper 2/8 0/2 0/1 1.0 E0.59, E0.84 1.7 5/9

Iron 0/8 0/2 0/1 6 — 6 8/43

Lead 3/8 0/2 0/1 0.030 E0.02, E0.02, 0.04 0.65 27/38

Manganese 8/8 0/2 1/1 0.26 E0.21, E0.23, 0.30, 0.37, 0.45, 

0.46, 0.49, 0.51, 0.52

0.52 3/52

Molybdenum 1/8 0/2 1/1 0.028 E0.02, E0.02 nv 0/58

Nickel 0/8 0/2 0/1 0.12 — 0.36 5/55

Silver 1/8 0/2 1/1 0.010 E0.005, 0.01 0.01 3/17

Tungsten 1/8 0/2 0/1 0.020 0.03 0.11 10/46

Zinc 0/8 0/2 0/1 2.8 — 4.8 14/26

Nutrients and major ions (mg/L)

Ammonia (as nitrogen) 1/8 0/0 0/1 0.02 E0.02 E0.02 3/36

Nitrite (as nitrogen) 1/8 0/0 0/1 0.002 E0.001 nv 0/18

Alkalinity 1/8 0/2 0/1 8 E4 nv 0/58

TDS 1/8 0/2 0/1 10 34 nv 0/58

Table A3
92 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010 Table A4A. Quality-control summary for replicate analyses of organic constituents, constituents of special interest, and dissolved organic carbon (DOC) in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010. [Constituents for which all replicate pairs were non-detections are not listed. Abbreviations: SD, percent standard deviation; RSD, percent relative standard deviation; ≤, less than or equal to; >, greater than; <, less than; RL; reporting level; nv, no value in category] Constituent Number of non- detections / number of replicates Number of 

≤-coded 

replicates

Number of SDs 

> 1⁄2 RL / number 

of replicates with 

concentration 

< 5 times the RL 

Concentrations of 

replicates with SDs 

> 1⁄2 RL 

(groundwater 

sample, replicate) 

Number of RSDs 

> 10 percent / 

number of replicates 

with concentration 

> 5 times the RL

Concentrations 

of replicates with 

RSDs > 10 percent 

(groundwater 

sample, replicate) 

Volatile organic compounds (VOCs) Chloroform (Trichloromethane) 5/6 nv 0/1 nv nv nv

Methyl tert-butyl ether (MTBE) 5/6 nv 0/1 nv nv nv

Pesticides and pesticide degradatesSimazine 5/6 nv 0/1 nv nv nv

Deethylatrazine (2-Chloro-4-isopropylamino-6-amino-s-triazine) 5/6 nv 0/1 nv nv nv

Atrazine 5/6 nv 0/1 nv nv nv

Hexazinone 5/6 nv 0/1 nv nv nv

S-Ethyl-dipropylthiocarbamate (EPTC) 5/6 nv 0/1 nv nv nv

3,4-Dichloroaniline 5/6 nv 0/1 nv nv nv

Metolachlor 4/6 nv 0/2 nv nv nv

Constituents of special interestPerchlorate 5/6 nv nv nv 0/1 nv

Dissolved organic carbon (DOC)DOC 1/6 nv 0/5 nv nv nv

Table A4A
Table A4B 93

Table A4B. Quality-control summary for replicate analyses of inorganic constituents in samples collected for the Western San Joaquin 

Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.

[Constituents for which all replicate pairs were non-detections are not listed. Abbreviations: SD, percent standard deviation; RSD, percent relative standard 

deviation in percent; RL, reporting level; >, greater than; <, less than; ≤, less than or equal to; nv, no value in category]

Constituent

Number of nondetections or 

<-coded replicates / 

number of replicates 

Number of SDs 

> 1⁄2 RL / number 

of replicates with 

concentration 

< 5 times the RL

Concentrations 

of replicates 

with SDs > 1⁄2 RL 

(groundwater 

sample, replicate) 

Number of RSDs 

> 10 percent / number 

of replicates with 

concentration 

> 5 times the RL

Concentrations 

of replicates with 

RSDs > 10 percent 

(groundwater 

sample, replicate) 

Trace Elements

Aluminum 3/6 0/3 nv nv nv

Antimony 3/6 0/3 nv nv nv

Arsenic 0/6 nv nv 0/6 nv

Barium 0/6 nv nv 0/6 nv

Beryllium 5/6 0/1 nv nv nv

Boron 0/6 nv nv 0/6 nv

Cadmium 1/6 0/1 nv 0/4 nv

Chromium 4/6 nv nv 0/2 nv

Cobalt 5/6 0/1 nv nv nv

Copper 4/6 0/2 nv nv nv

Iron 3/6 nv nv 0/3 nv

Lead 4/6 nv nv 0/2 nv

Lithium 0/6 nv nv 0/6 nv

Manganese 0/6 nv nv 0/6 nv

Molybdenum 0/6 nv nv 0/6 nv

Nickel 2/6 0/1 nv 0/3 nv

Selenium 2/6 0/1 nv 0/3 nv

Silver 4/6 0/1 nv 0/1 nv

Strontium 0/6 nv nv 0/6 nv

Tungsten 0/6 0/1 nv 0/5 nv

Uranium 0/6 0/1 nv 0/5 nv

Vanadium 0/6 0/1 nv 0/5 nv

Zinc 5/6 0/1 nv nv nv

Nutrients 

Ammonia (as nitrogen) 1/6 0/3 nv 0/2 nv

Nitrate plus nitrite (as nitrogen) 1 3/6 0/1 nv 0/2 nv

Nitrite (as nitrogen) 4/6 0/1 nv 0/1 nv

Total nitrogen (ammonia + nitrite + 

nitrate + organic nitrogen)

0/6 0/4 nv 0/2 nv

Phosphate, orthophosphate 

(as phosphorus)

0/6 0/4 nv 0/2 nv

Major and minor ions, silica, and total dissolved soilds (TDS) 

Bromide 0/6 nv nv 0/6 nv

Calcium 0/6 nv nv 0/6 nv

Chloride 0/6 nv nv 0/6 nv

Fluoride 0/6 0/4 nv 0/2 nv

Iodide 0/6 0/1 nv 0/5 nv

Magnesium 0/6 nv nv 0/6 nv

Potassium 0/6 nv nv 0/6 nv

Sodium 0/6 nv nv 0/6 nv

Table A4B
94 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Constituent

Number of nondetections or 

<-coded replicates / 

number of replicates 

Number of SDs 

> 1⁄2 RL / number 

of replicates with 

concentration 

< 5 times the RL

Concentrations 

of replicates 

with SDs > 1⁄2 RL 

(groundwater 

sample, replicate) 

Number of RSDs 

> 10 percent / number 

of replicates with 

concentration 

> 5 times the RL

Concentrations 

of replicates with 

RSDs > 10 percent 

(groundwater 

sample, replicate) 

Major and minor ions, silica, and total dissolved soilds (TDS) —Continued

Sulfate 0/6 nv nv 0/6 nv

Silica (as SiO2

) 0/6 nv nv 0/6 nv

TDS 0/6 nv nv 0/6 nv

Total arsenic and iron (unfiltered) and arsenic, chromium, and iron species (filtered) 

Arsenic (total, unfiltered) 0/6 nv nv nv nv

Iron (total, unfiltered) 1/6 5/6 nv nv nv

Arsenic (total, filtered) 0/6 nv nv 0/6 nv

Arsenic-III (filtered) 2/6 nv nv 0/4 nv

Chromium (total, filtered) 1/4 0/1 nv 0/2 nv

Chromium-VI 0/4 0/2 nv 0/2 nv

Iron (total, filtered) 1/6 5/6 nv nv nv

Iron-II (filtered) 1/6 5/6 nv nv nv

1

 Nitrite plus nitrate (as nitrogen) is referred to as nitrate in the text for clarity.

Table A4B. Quality-control summary for replicate analyses of inorganic constituents in samples collected for the Western San Joaquin 

Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.—Continued

[Constituents for which all replicate pairs were non-detections are not listed. Abbreviations: SD, percent standard deviation; RSD, percent relative standard 

deviation in percent; RL, reporting level; >, greater than; <, less than; ≤, less than or equal to; nv, no value in category]
Table A5A 95

Table A5A. Quality-control summary for matrix-spike recoveries of volatile organic compounds (VOCs) in samples collected for 

the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to 

July 2010.

[Acceptable recovery range is between 70 and 130 percent]

Constituent Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

Median recovery 

(percent)

Acetone 6 101 150 123.4

Acrylonitrile 6 99 119 111.9

tert-Amyl methyl ether (TAME) 6 90 103 94.7

Benzene 1 6 96 111 100.9

Bromobenzene 6 90 103 94.6

Bromochloromethane 6 102 112 106.5

Bromodichloromethane 6 94 108 104.3

Bromoform (Tribromomethane) 1 6 84 109 99.4

Bromomethane (Methyl bromide) 6 92 118 106.4

n-Butylbenzene 6 79 106 90.1

sec-Butylbenzene 6 85 109 93.7

tert-Butylbenzene 6 89 114 101.2

Carbon disulfide 1 6 75 91 77.5

Carbon tetrachloride (Tetrachloromethane) 6 94 110 97.5

Chlorobenzene 6 92 102 95.5

Chloroethane 6 101 112 103.3

Chloroform (Trichloromethane) 1 6 101 116 112.9

Chloromethane 6 103 116 105.7

3-Chloropropene 6 107 121 110.2

2-Chlorotoluene 6 90 114 95.8

4-Chlorotoluene 6 87 114 97.1

Dibromochloromethane 1 6 88 110 102.5

1,2-Dibromo-3-chloropropane (DBCP) 1 6 94 132 105.2

1,2-Dibromoethane (EDB) 6 94 110 101.0

Dibromomethane 6 95 111 100.3

1,2-Dichlorobenzene 6 92 112 98.8

1,3-Dichlorobenzene 6 88 113 97.1

1,4-Dichlorobenzene 6 86 112 95.7

trans-1,4-Dichloro-2-butene 6 84 113 94.9

Dichlorodifluoromethane (CFC-12) 6 70 99 87.6

1,1-Dichloroethane (1,1-DCA) 1 6 100 116 105.2

1,2-Dichloroethane (1,2-DCA) 6 101 117 109.0

1,1-Dichloroethene (1,1-DCE) 6 91 104 97.3

cis-1,2-Dichloroethene (cis-1,2-DCE) 6 97 109 101.8

trans-1,2-Dichloroethene (trans-1,2-DCE) 6 97 116 101.1

1,2-Dichloropropane 1 6 95 104 98.9

1,3-Dichloropropane 6 97 110 103.6

2,2-Dichloropropane 6 89 97 92.0

1,1-Dichloropropene 6 89 100 92.5

cis-1,3-Dichloropropene 6 82 97 88.4

trans-1,3-Dichloropropene 6 82 95 89.6

Diethyl ether 6 104 121 105.9

Diisopropyl ether (DIPE) 6 91 113 96.3

Table A5A
96 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Constituent Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

Median recovery 

(percent)

Ethylbenzene 2 6 87 100 94.0

Ethyl tert-butyl ether (ETBE) 6 88 101 94.2

Ethyl methacrylate 6 90 100 94.6

o-Ethyl toluene 1 6 79 106 91.5

Hexachlorobutadiene 6 65 89 80.0

Hexachloroethane 6 78 110 96.1

2-Hexanone (n-Butyl methyl ketone) 6 82 107 96.2

Iodomethane (Methyl iodide) 6 106 126 110.7

Isopropylbenzene 1 6 82 109 92.8

4-Isopropyl-1-methyl benzene 1 6 77 110 95.7

Methyl acrylate 6 92 119 109.3

Methyl acrylonitrile 6 96 123 113.3

Methyl tert-butyl ether (MTBE) 1 6 100 116 104.8

Methyl iso-butyl ketone (MIBK) 6 86 104 95.5

Methylene chloride (Dichloromethane) 6 94 108 100.0

Methyl ethyl ketone (2-butanone, MEK) 6 94 111 109.0

Methyl methacrylate 6 88 101 92.2

Naphthalene 6 82 111 100.5

Perchloroethene (PCE, Tetrachloroethene) 6 92 108 104.6

n-Propylbenzene 1 6 84 100 93.8

Styrene 6 32 105 87.6

1,1,1,2-Tetrachloroethane 6 90 112 96.3

1,1,2,2-Tetrachloroethane 6 93 109 99.8

Tetrahydrofuran 6 88 122 109.8

1,2,3,4-Tetramethylbenzene 1 6 74 108 95.1

1,2,3,5-Tetramethylbenzene 1 6 79 122 104.9

Toluene 2 6 93 107 96.6

1,2,3-Trichlorobenzene 6 87 114 98.5

1,2,4-Trichlorobenzene 6 78 100 87.5

1,1,1-Trichloroethane (1,1,1-TCA) 6 95 107 102.0

1,1,2-Trichloroethane (1,1,2-TCA) 6 96 112 105.5

Trichloroethene (TCE) 1 6 92 107 95.6

Trichlorofluoromethane (CFC-11) 6 86 115 100.3

1,2,3-Trichloropropane (1,2,3-TCP) 6 96 115 101.1

Trichlorotrifluoroethane (CFC-113) 6 79 101 87.9

1,2,3-Trimethylbenzene 1 6 83 121 102.0

1,2,4-Trimethylbenzene 6 85 118 100.1

1,3,5-Trimethylbenzene 6 87 109 95.6

Table A5A. Quality-control summary for matrix-spike recoveries of volatile organic compounds (VOCs) in samples collected for 

the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to 

July 2010.—Continued

[Acceptable recovery range is between 70 and 130 percent]
Table A5B 97

Constituent Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

Median recovery 

(percent)

Vinyl bromide (Bromoethene) 6 99 110 101.2

Vinyl chloride (Chloroethene) 6 104 125 113.2

m- plus p-Xylene 2 6 86 111 96.8

o-Xylene 2 6 86 101 91.9

1

 Constituent detected in groundwater samples.

2

 The SRLs were defined by Fram and others (2012). Values measured at concentrations less than or equal to the SRL have been flagged with a ≤ symbol, 

reclassified as non-detections, and not included in the calculations of detection frequencies (with the exception of wells WS-U-01 and WS-U-02 [see the 

appendix section titled “Detections in Blanks and Application of SRLs” for details]).

Table A5A. Quality-control summary for matrix-spike recoveries of volatile organic compounds (VOCs) in samples collected for 

the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to 

July 2010.—Continued

[Acceptable recovery range is between 70 and 130 percent]

Table A5B. Quality-control summary for matrix-spike recoveries of low-level fumigants in samples collected for the Western 

San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.

[Acceptable recovery range is between 70 and 130 percent]

Constituent Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

Median recovery 

(percent)

1,2-Dibromo-3-chloropropane (DBCP) 1 8 42 115 95.7

1,2-Dibromoethane (EDB) 8 48 112 84.4

1

 Constituent detected in groundwater samples.

Table A5B
98 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Table A5C. Quality-control summary for matrix-spike recoveries of pesticides and pesticide degradates in samples collected for 

the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to 

July 2010.

[Acceptable recovery range is between 70 and 130 percent]

Constituent Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

Median recovery 

(percent)

Acetochlor 8 86 117 102.0

Alachlor 8 90 115 103.0

Atrazine 1 8 89 117 102.0

Azinphos-methyl 8 55 137 104.5

Azinphos-methyl oxon 8 41 89 68.1

Benfluralin 8 56 85 71.2

Carbaryl 8 83 144 105.5

Carbofuran 8 83 150 111.6

2-Chloro-2,6-diethylacetanilide 8 88 116 96.6

4-Chloro-2-methylphenol 8 59 93 79.3

Chlorpyrifos 8 72 101 89.1

Chlorpyrifos oxon 8 18 47 33.5

Cyanazine 8 64 125 104.9

Cyfluthrin 8 36 93 64.1

λ-Cyhalothrin 8 28 73 54.9

Cypermethrin 8 37 84 65.8

DCPA (Dacthal) 1 8 98 116 110.9

Deethylatrazine (2-Chloro-4-isopropylamino-6-amino-s-triazine) 1 8 24 112 89.8

Desulfinylfipronil 8 74 129 101.0

Desulfinylfipronil amide 8 38 107 84.3

Diazinon 8 80 104 95.6

Diazinon oxon 8 71 107 94.7

3,4-Dichloroaniline 1 8 74 98 90.2

3,5-Dichloroaniline 1 8 86 103 97.1

Dichlorvos 8 13 38 22.0

Dicrotophos 8 8 48 32.7

Dieldrin 8 70 106 85.6

2,6-Diethylaniline 1 8 81 101 95.6

Dimethoate 8 16 70 57.9

Disulfoton 8 58 86 75.2

Disulfoton sulfone 8 74 103 85.2

α-Endosulfan 8 63 108 78.3

Endosulfan sulfate 8 63 115 93.2

Ethion 8 45 89 69.2

Ethion monoxon 8 53 99 76.3

Ethoprophos 8 87 118 100.5

S-Ethyl-dipropylthiocarbamate (EPTC) 1 8 76 108 94.4

2-Ethyl-6-methylaniline 1 8 66 99 88.7

Fenamiphos 8 74 101 93.0

Fenamiphos sulfone 8 60 113 92.5

Fenamiphos sulfoxide 8 17 75 39.0

Fipronil 8 74 144 95.1

Fipronil sulfide 8 67 143 99.1

Fipronil sulfone 8 55 99 81.1

Fonofos 8 80 100 92.8

Hexazinone 1 8 46 85 64.7

Table A5C
Table A5C 99

Constituent Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

Median recovery 

(percent)

Iprodione 8 60 126 84.0

Isofenphos 8 80 133 104.0

Malaoxon 8 69 106 85.6

Malathion 8 79 111 96.3

Metalaxyl 1 8 92 117 110.9

Methidathion 8 66 108 87.2

Metolachlor 1 8 83 114 104.9

Metribuzin 8 64 105 92.6

Molinate 1 8 89 109 99.5

Myclobutanil 8 75 109 78.2

1-Naphthol 8 12 52 33.7

Oxyfluorfen 8 44 83 62.7

Paraoxon-methyl 8 51 98 68.3

Parathion-methyl 8 72 125 87.5

Pendimethalin 8 72 118 82.3

cis-Permethrin 8 33 107 59.3

Phorate 8 54 80 78.0

Phorate oxon 8 75 119 102.3

Phosmet 8 22 76 56.8

Phosmet oxon 8 16 64 45.1

Prometon 1 8 91 113 103.2

Prometryn 1 8 91 117 105.6

Pronamide 8 78 110 96.1

Propanil 8 83 124 104.0

Propargite 8 44 94 68.1

cis-Propiconazole 8 105 260 131.9

trans-Propiconazole 8 59 113 84.4

Simazine 1 8 88 112 100.4

Tebuconazole 8 51 103 80.6

Tebuthiuron 1 8 89 210 138.3

Tefluthrin 8 41 60 51.3

Terbufos 8 59 95 84.7

Terbufos oxon sulfone 8 42 105 75.2

Terbuthylazine 8 97 117 107.1

Thiobencarb 8 88 114 102.3

Tribufos 8 33 68 56.7

Trifluralin 8 58 89 73.4

1

 Constituent detected in groundwater samples.

Table A5C. Quality-control summary for matrix-spike recoveries of pesticides and pesticide degradates in samples collected for 

the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to 

July 2010.—Continued

[Acceptable recovery range is between 70 and 130 percent]
100 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010

Table A5D. Quality-control summary for matrix-spike recoveries of constituents of special interest in samples collected for the 

Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, 

March to July 2010.

[Acceptable recovery range is between 70 and 130 percent]

Constituent Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

Median recovery 

(percent)

1,2,3-Trichloropropane (1,2,3-TCP) 1 9 95 117 110

N-Nitrosodimethylamine (NDMA) 4 86 131 113.0

1

 Constituent detected in groundwater samples.

Table A5D
Table A6 101

Table A6. Quality-control summary for surrogate recoveries of volatile organic compounds (VOCs) and pesticides and pesticide degradates in samples collected for the Western San Joaquin Valley (WSJV) Groundwater Ambient Monitoring and Assessment (GAMA) study unit, California, March to July 2010.[Abbreviations: NWQL, U.S. Geological Survey National Water Quality Laboratory; VOC, volatile organic compound]

Surrogate

NWQL 

analytical 

schedule

Constituent 

class 

analyzed

Number 

of blanks 

analyzed

Median 

recovery 

in blanks 

(percent)

Number of 

surrogate 

recoveries less 

than 70 percent 

in blanks

Number of surrogate 

recoveries greater 

than 130 percent 

in blanks

Number of 

groundwater 

samples 

analyzed

Median 

recovery 

in samples 

(percent)

Number of 

surrogate 

recoveries less 

than 70 percent 

in samples

Number of 

surrogate 

recoveries greater 

than 130 percent 

in samples

1-Bromo-4-fluorobenzene 2020 VOC 13 90 0 0 58 89 0 0

1,2-Dichloroethane-d4 2020 VOC 13 127 0 3 58 125 0 13

Toluene-d8 2020 VOC 13 94 0 0 58 93 0 0

Diazinon-d10 2033 Pesticide 12 81 2 0 58 84 7 0α-HCH-d6 2033 Pesticide 12 89 0 0 58 94 0 0

Table A6
102 Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010
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For more information concerning the research in this report, 

contact the

 Director, California Water Science Center

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Mathany and others—Groundwater-Quality Data in the Western San Joaquin Valley Study Unit, 2010: Results from the California GAMA Program—Data Series 706