U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004–5149

Assessing the Susceptibility to Contamination of Two Aquifer 

Systems Used for Public Water Supply in the Modesto and 

Fresno Metropolitan Areas, California, 2001 and 2002

Prepared in cooperation with the

CALIFORNIA STATE WATER RESOURCES CONTROL BOARD
Cover photo:

Photograph taken from the 9th floor of the Double Tree Hotel in Modesto, California on 9/09/2004. Photo by Cathy Munday, U.S. Geological Survey.
Assessing the Susceptibility to 

Contamination of Two Aquifer Systems 

Used for Public Water Supply in the 

Modesto and Fresno Metropolitan Areas, 

California, 2001 and 2002

By Michael T. Wright, Kenneth Belitz, and Tyler Johnson 

Prepared in cooperation with the California State Water Resources Control Board

Scientific Investigations Report 2004–5149

U.S. Department of the Interior

U.S. Geological Survey 6438-15
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Suggested citation:

Wright, M.T., Belitz, Kenneth, and Johnson, Tyler, 2004, Assessing the susceptibility to contamination of two aquifer 

systems used for public water supply in the Modesto and Fresno metropolitan areas, California, 2001 and 2002: U.S. 

Geological Survey Scientific Investigations Report 2004–5149, 35 p.
iii

Contents

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

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

Hydrogeologic Setting .................................................................................... 2

Approach ................................................................................................... 6

Methods ................................................................................................... 8

Well Selection ....................................................................................... 8

Sample Collection ................................................................................. 8

Quality Control ....................................................................................... 9

Blank Data .................................................................................... 9

Surrogate Data .............................................................................. 9

Determination and Use of Categorical Variables ................................................ 10

Well Depth .................................................................................... 10

Land Use ....................................................................................... 10

Leaking Underground Fuel Tanks ......................................................... 10

Determining the Presence of Young Ground Water ....................................... 10

Determining the Presence of High Nitrate Concentrations .............................. 10

Censoring of Data in the Fresno Study Area ................................................... 10

Stable Isotopes and Source of Ground-Water Recharge .................................... 11

Statistical Analysis and the Use of Boxplots ................................................... 11

Results ...................................................................................................... 12

VOC Detections in Quality-Control and Associated Environmental Samples ............... 12

Occurrence of VOCs, Tritium, and Nitrate ...................................................... 12

Detection Frequency of Individual VOCs ......................................................... 13

Relation between VOC Occurrence, Young Ground Water, and Nitrate Concentration ... 15

Depth to the Top of the Uppermost Perforation as an Explanatory Variable ............... 17

Land Use as an Explanatory Variable ............................................................ 18

LUFT Density as an Explanatory Variable ...................................................... 19

Stable Isotopic Content of Ground Water as an Explanatory Variable ..................... 19

Summary and Conclusions .............................................................................. 23

References Cited .......................................................................................... 23

Appendixes ................................................................................................ 27
iv

Figures

Figure 1. Map showing location of Fresno and Modesto study areas, the Kings and 

Modesto ground-water basins, and alluvial deposits in California .................. 3

Figure 2. Map showing study area showing urban land use areas and locations of public 

supply wells that were sampled in July and August 2002 in metropolitan Fresno, 

California .................................................................................... 4

Figure 3. Map showing study area showing urban land use areas and locations of public 

supply wells that were sampled in March and April 2001 in metropolitan Modesto, 

California .................................................................................... 5

Figure 4. Graph showing detection frequencies of volatile organic compounds, classes of 

volatile organic compounds, and tritium for wells sampled in the Fresno and 

Modesto study areas, California, in 2002 and 2001, respectively ..................... 13

Figure 5. Graph showing detection frequencies of volatile organic compounds, divided by 

class, in samples collected in the Fresno and Modesto study areas, California, 

in 2002 and 2001, respectively ............................................................ 14

Figure 6. Box plots showing the number of volatile organic compounds (VOCs) detected in 

wells categorized by relative ground-water age in the Fresno and the Modesto 

study areas, California, in 2002 and 2001, respectively ................................. 15

Figure 7. Box plots showing the number of volatile organic compounds (VOCs) detected in 

wells categorized by nitrate concentration in the Fresno and the Modesto study 

areas, California, in 2002 and 2001, respectively ....................................... 16

Figure 8. Graphs showing detection frequencies of volatile organic compounds (VOCs), 

tritium, and nitrate in shallow and deep wells sampled in the Fresno and the 

Modesto study areas, California, in 2002 and 2001, respectively ..................... 17

Figure 9. Graphs showing the number of volatile organic compounds (VOCs) detected in 

shallow and deep wells sampled in the Fresno and the Modesto study areas, 

California, in 2002 and 2001, respectively ................................................ 18

Figure 10. Box plots showing distribution of depth to the top of the uppermost perforation, 

percentage urban land use, nitrate concentration, and tritium levels in the Fresno 

and Modesto study areas, California, in 2002 and 2001, respectively ............... 20

Figure 11. Graphs showing detection frequencies of volatile organic compounds, tritium, 

and nitrate by urban land use category in wells sampled in the Fresno, and the 

Modesto study areas, California, in 2002 and 2001, respectively ..................... 21

Figure 12. Scatter plot showing delta oxygen-18 versus delta deuterium for samples with 

and without volatile organic compounds (VOCs) collected in the Fresno and the 

Modesto study areas, California, in 2002 and 2001, respectively ..................... 22

Tables

Table 1. Volatile organic compounds, primary source or use, Chemical Abstract Service 

number, and laboratory reporting limits for the U.S. Geological Survey National 

Water Quality Laboratory analytical schedule 2020 .................................... 7

Table 2. Median value for the number of volatile organic compound detections, tritium 

levels, and nitrate concentrations in samples collected in shallow and deep wells 

in Fresno and Modesto, California, in 2002 and 2001, respectively .................. 18

Table 3. Median value for the number of volatile organic compound detections, tritium 

levels, and nitrate concentrations by degree of urbanization for samples 

collected in the Fresno and Modesto study areas, California, in 2002 and 2001, 

respectively ................................................................................. 21
v

Appendixes

Appendix 1. Identification and construction information for sampled wells in the Fresno 

and Modesto study areas, California, in 2002 and 2001, respectively ............ 28

Appendix 2. Summary of volatile organic compounds detected in quality-control blanks 

and associated environmental samples for the Fresno and Modesto study 

areas, California, in 2002 and 2001, respectively .................................... 29

Appendix 3a. Isotopic content of environmental samples collected in the Fresno and 

Modesto study areas, California, in 2002 and 2001, respectively .................. 30

Appendix 3b. Nitrate concentrations obtained from the California Department of Health 

Services database for wells sampled ................................................ 31

Appendix 4a. Wells with volatile organic compound detections in the Fresno study area, 

California, 2002 ........................................................................... 32

Appendix 4b. Wells with volatile organic compound detections in the Modesto study area, 

California, 2001 ........................................................................... 34

Conversion Factors, Water-Quality Information, and 

Abbreviations and Acronyms

Inch/Pound to SI

Multiply By To obtain

acre 4,047 square meter (m2)

acre-foot (acre-ft) 1,233 cubic meter (m3)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/yr)

foot (ft) 0.3048 meter (m)

inch (in.) 2.54 centimeter (cm)

mile (mi) 1.609 kilometer (km)

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L)

SI to Inch/Pound 

Multiply By To obtain

liter (L) 0.2642 gallon (gal)

meter (m) 3.281 foot (ft)

milliliter (ml) 0.0338 ounce (oz)

Water-Quality Information

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

micrograms per liter (μg/L). Milligrams per liter is approximately equivalent to parts per million. 

Micrograms per liter is approximately equivalent to parts per billion.

Data for the isotopes oxygen-18 and deuterium are reported in delta (δ) notation as per mil 

(parts per thousand); tritium data are reported in picocuries per liter (pCi/L).
vi

Abbreviations and Acronyms

bls below land surface

BTEX benzene, toluene, ethyltoluene, xylenes

CAS California Aquifer Susceptibility

DBCP 1, 2-dibromo-3-chloropropane

DHS Department of Health Services

DWR Department of Water Resources

EPA U.S. Environmental Protection Agency

GAMA Ground-Water Ambient Monitoring

GEIMS Geographic Environmental Information Management System

GMWL global meteoric water line

LLNL Lawrence Livermore National Laboratory

LRL laboratory reporting level

LT-MDL long-term method detection limit

LUFT leaking underground fuel tank

MCL maximum contaminant level

MDL method detection limit

MTBE methyl tert-butyl ether

NAWQA National Water Quality Assessment Program

NWIS National Water Information System

NWQL National Water Quality Laboratory

PCE tetrachloroethylene

QC Quality Control

SWRCB California State Water Resources Control Board

TCE trichloroethylene

THM trihalomethane

VOC volatile organic compound

USGS U.S. Geological Survey
Assessing the Susceptibility to Contamination of Two 

Aquifer Systems Used for Public Water Supply in the 

Modesto and Fresno Metropolitan Areas, California, 

2001 and 2002

By Michael T. Wright, Kenneth Belitz, and Tyler Johnson 

Abstract

Ground-water samples were collected from 90 active 

public supply wells in the Fresno and Modesto metropolitan 

areas as part of the California Aquifer Susceptibility (CAS) 

program. The CAS program was formed to examine the 

susceptibility to contamination of aquifers that are tapped by 

public supply wells to serve the citizens of California. The 

objectives of the program are twofold: (1) to evaluate the 

quality of ground water used for public supply using volatile 

organic compound (VOC) concentrations in ground-water 

samples and (2) to determine if the occurrence and distribution of low level VOCs in ground water and characteristics, 

such as land use, can be used to predict aquifer susceptibility 

to contamination from anthropogenic activities occurring at, 

or near, land surface. An evaluation was made of the relation 

between VOC occurrence and the explanatory variables: depth 

to the top of the uppermost well perforation, land use, relative 

ground-water age, high nitrate concentrations, density of leaking underground fuel tanks (LUFT), and source of recharge 

water.

VOCs were detected in 92 percent of the wells sampled 

in Modesto and in 72 percent of the wells sampled in Fresno. 

Trihalomethanes (THM) and solvents were frequently detected 

in both study areas. Conversely, the gasoline components—

benzene, toluene ethylbenzene, and xylenes (BTEX)—were 

rarely, if at all, detected, even though LUFTs were scattered 

throughout both study areas. The rare occurrence of BTEX 

compounds may be the result of their low solubility and labile 

nature in the subsurface environment.

Samples were analyzed for 85 VOCs; 25 were detected 

in at least one sample. The concentrations of nearly all VOCs 

detected were at least an order of magnitude below action 

levels set by drinking water standards. Concentrations of four 

VOCs exceeded federal and state maximum contaminant levels (MCL): the solvent trichloroethylene (TCE) and the fumigant 1, 2-dibromo-3-chloropropane (DBCP) in Fresno, and 

the solvents TCE and tetrachloroethylene (PCE) in Modesto. 

Chloroform, which is a by product of water disinfection and a 

constituent used in industrial processes since the 1920s, was 

the most frequently detected compound, whereas the gasoline 

oxygenate methyl tert-butyl ether (MTBE), which has been 

in widespread production and use only since the 1990s, was 

detected in only 2 percent of the samples.

Downward migration of contaminants appears to be a 

viable pathway of contamination in the unconfined and semiconfined aquifers underlying the Fresno and Modesto study 

areas. Within the individual study areas, VOCs were detected 

more frequently and in greater numbers in shallower wells 

than in deeper wells. Additionally, VOCs were detected more 

frequently and in greater numbers in Modesto than in Fresno. 

Wells sampled in Modesto were significantly shallower than 

the wells sampled in Fresno; the other explanatory variables examined in this report were not significantly different 

between the two study areas.

VOCs occurred more frequently in younger ground water 

(water recharged after 1952) than in older ground water (water 

recharged prior to 1952). Additionally, wells withdrawing 

younger ground water had a higher number of VOCs detected 

per well than did wells withdrawing older ground water. 

Younger ground water was at or near the land surface during a 

period when VOCs came into widespread production and use. 

Therefore, wells from which younger ground water is withdrawn may be more susceptible to contamination.

Of the explanatory variables examined in this study, 

land use was the best predictor of aquifer susceptibility in the 

Fresno and Modesto study areas. VOCs were detected more 

frequently in wells located in heavily urbanized areas. The 

number of VOCs detected in ground water was positively correlated to the degree of urbanization. VOCs are produced and 

used primarily in urban land use settings; therefore, aquifers 

underlying urban areas may be more susceptible to contamination from these compounds.

Other variables had little or no predictability. Overall, the 

presence of high nitrate concentrations was only marginally 

useful in predicting aquifer susceptibility to VOC contamination. In Fresno, nitrate concentrations had a moderate correlation to VOC occurrence in ground water, whereas in Modesto, 

nitrate concentrations did not predict VOC occurrence. The 

density of LUFTs and the stable isotopic content of ground 
2 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

Introduction

California depends heavily on ground water to supply its 

growing population. In 1995, California withdrew 4,450 acreft of fresh ground water per day from its aquifers (Solley and 

others, 1998), which is approximately 19 percent of the total 

ground water withdrawn in the United States each day. Out of 

the 4,450 acre-ft, 827 acre-ft were withdrawn by public supply 

wells and used primarily for domestic purposes. It is in urban 

areas where the majority of ground water is withdrawn by 

public supply wells. Aquifers located beneath urban areas can 

be susceptible to contamination from volatile organic compounds (VOCs) from point and non-point sources (Grady and 

Casey, 2001). VOCs are a class of anthropogenic compounds 

that are used widely in urban areas and that have contaminated 

ground-water supplies. Recently, public supply wells contaminated with the gasoline oxygenate MTBE have been closed in 

Santa Monica, Sacramento, and many other areas in California 

(California State Water Resources Control Board, 2001).

Because anthropogenic compounds were detected in 

ground water used for public supply, the California State 

Water Resources Control Board (SWRCB) created the 

Ground-Water Ambient Monitoring (GAMA) Program. The 

objectives of the program are twofold: (1) to evaluate the 

quality of ground water used for public supply by using VOC 

concentrations found in ground-water samples and (2) to 

determine if the occurrence and distribution of low level VOCs 

in ground water and characteristics, such as land use, can be 

used to predict aquifer susceptibility to contamination from 

anthropogenic activities occurring at or near land surface. The 

California Aquifer Susceptibility (CAS) assessment is the 

component of GAMA that is examining the susceptibility of 

aquifers serving the citizens of California by way of production wells. The U.S. Geological Survey (USGS) is partnering with the California State Water Resources Control Board 

(SWRCB), the California Department of Health Services 

(DHS), the California Department of Water Resources (DWR), 

and Lawrence Livermore National Laboratory (LLNL) to 

complete the CAS assessment.

This report presents the results and analyses of the occurrence and distribution of VOCs, tritium, nitrate, and stable isotopes in two aquifer systems in the San Joaquin Valley. These 

two aquifer systems serve the San Joaquin Valley metropolitan 

areas of Fresno and Modesto. Forty public supply wells were 

sampled in Modesto in the spring of 2001, and 50 public 

supply wells were sampled in Fresno in the summer of 2002. 

USGS laboratories analyzed ground-water samples for stable 

isotopes and VOCs; the tritium content of the samples was 

determined by LLNL. Nitrate values were obtained from the 

California DHS database. The California DHS regularly monitors public supply wells for a variety of chemical constituents 

in order to assure compliance with federal and state drinking 

water standards.

water were not good predictors of VOC occurrence in Fresno 

and Modesto ground water. LUFT density was not useful 

because gasoline components comprised less than 2 percent 

of the VOCs detected in the study areas. Source of recharge 

water is indicated by stable isotope ratios. The presence or 

absence of VOCs in ground-water samples was not correlated 

with stable isotope values. Therefore, source of recharge water 

was not important in predicting aquifer susceptibility in the 

Fresno and Modesto study areas.

Hydrogeologic Setting

The Fresno and Modesto metropolitan areas are within 

the Kings and Modesto ground-water basins, respectively 

(fig. 1). The Kings ground-water basin is part of the Tulare 

Lake Hydrologic Region, and the Modesto ground-water 

basin is part of the San Joaquin Hydrologic Region (California 

Department of Water Resources, 2003). These two hydrologic 

regions are within the San Joaquin Valley, which is bordered 

by the Coast Ranges to the west and the Sierra Nevada to the 

east. The San Joaquin Valley is a structural trough that is filled 

with sediments up to 6 mi thick that consist of intercalated 

lenses of gravel, sand, silt, and clay (Page, 1986).

 The Fresno metropolitan area (fig. 2) is located within the 

Kings ground-water basin. The Kings Basin is bordered by the 

San Joaquin River to the north and partially by the Kings River 

to the south (California Department of Water Resources, 2003). 

The local climate consists of hot, dry summers and wetter, mild 

winters. The mean annual precipitation is approximately 10 

inches, with rainfall increasing in an easterly direction toward 

the higher elevations of the Sierra Nevada (Page and LeBlanc, 

1969). More than 90 percent of the precipitation falls between 

October and April. Fresno is the most populated area within the 

Kings ground-water basin with a population of approximately 

440,000 in 2002 (City of Fresno, 2003a). The sole source of 

public water supply for the city of Fresno is ground water (City 

of Fresno, 2003b). In 2002, the city of Fresno withdrew over 

167,220 acre-ft of ground water (City of Fresno, 2003c). 

The Kings ground-water basin is composed of unconsolidated deposits of continental origin. The main water-bearing 

unit of the aquifer consists of older alluvium that extends from 

the base of the Sierra Nevada to beyond the western boundary of the city of Fresno. These sedimentary deposits are an 

intercalated fill of permeable sand and gravel strata layered 

with relatively impermeable lenses of silt and clay. Ground 

water is mostly unconfined, although there are areas where 

partially confined conditions exist because of the lenticular 

nature of the aquifer matrix (Muir, 1977). The base of the 

fresh water aquifer (fresh water being defined as having less 

than 2,000 mg/L dissolved solids) ranges from approximately 

900 ft below land surface (bls) near the Sierra Nevada to 

approximately 2,000 ft bls near the city of Fresno. The movement of ground water in the Kings Basin generally is to the 
Figure 1. Location of Fresno and Modesto study areas, the Kings and Modesto ground-water basins, and 

alluvial deposits in California.

DWR, California Department of Water Resources.

Hydrogeologic Setting 3

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Figure 2. Study area showing urban land use areas and locations of public supply wells that were sampled in 

July and August 2002 in metropolitan Fresno, California.

4 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

southwest towards the axis of the valley. However, a large 

cone of depression exists under the city of Fresno owing to 

ground-water withdrawals; this change in hydraulic gradient 

causes a shift in ground-water movement around the Fresno 

metropolitan area to a northwesterly direction. Recharge in the 

ground-water basin is from precipitation; seepage from rivers, 

streams, and irrigation canals; and deep percolation of irrigation 

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

Kings ground-water basin is also replenished through engineered recharge that occurs by way of a 200-acre spreading 

basin located on the eastern portion of the city of Fresno; this 

recharge water is composed of runoff from the nearby Sierra 

Nevada.

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Figure 3. Study area showing urban land use areas and locations of public supply wells that were sampled in March 

and April 2001 in metropolitan Modesto, California.

Hydrogeologic Setting 5

The Modesto metropolitan area (fig. 3) is within the 

Modesto ground-water basin. The Stanislaus River forms 

the northern border of the basin; the Tuolumne River flows 

parallel to, and just north of, its southern border. The local 

climate is very similar to the climate in Fresno, although 

Modesto receives a couple more inches of rainfall annually 

(California Department of Water Resources, 2003). Modesto 

is the most populated area within the Modesto ground-water 

basin. In 2002, the population of Modesto was 203,300 (City 

of Modesto, 2004a). Ground water provides the majority of 

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6 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

domestic, public water supply in the Modesto metropolitan 

area, with over 49,660 acre-ft per year withdrawn from the 

Modesto ground-water basin (City of Modesto, 2004b).

Ground water in the Modesto basin primarily is in the 

unconfined to partially confined part of the aquifer that is 

composed of unconsolidated to semi-consolidated deposits 

(Karen Burow, U.S. Geological Survey, written commun., 

2004). Consolidated deposits of continental origin lie in the 

eastern portion of the basin and generally yield low quantities of water to wells; these deposits consist of poorly sorted 

gravel, sand, silt, and clay. Continental deposits are near the 

surface in the eastern part of the basin and at depths of over 

400 ft bls in the western part of the basin. In the western half 

of the basin, older alluvium composed of intercalated layers of 

gravel, sand, silt, and clay is up to 400 ft thick (Davis and others, 1959). Confined conditions occur in the southwestern part 

of the basin where the aquifer lies beneath the Corcoran Clay, 

which extends from approximately 150 to 250 ft bls (California Department of Water Resources, 2003). The rest of the 

aquifer system is mostly unconfined, except where clay and 

silt lenses create partially confined conditions. The base of the 

fresh water aquifer is approximately 700 to 900 ft bls just west 

of the city (Page and Balding, 1973); at the time this report 

was written, no data had been found that delineates the base 

of the fresh water aquifer underlying the city proper. Groundwater flow in the Modesto Basin generally is to the southwest, 

towards the axis of the valley. Aquifer recharge in the basin is 

from precipitation; seepage from rivers, streams, and irrigation 

canals; and deep percolation of irrigation water.

Approach

The CAS assessment has been utilizing low-level VOC 

analyses to identify public supply wells that are already 

affected by contamination, but at concentrations well below 

federal and state maximum contaminant levels (MCL). The 

MCL is the highest level of a contaminant that is allowed in 

drinking water and is an enforceable standard (U.S. Environmental Protection Agency, 2002). Table 1 shows the suite of 

85 VOCs analyzed for at the USGS National Water Quality 

Laboratory (NWQL), along with their primary use or source. 

In this report, all detections of VOCs, whether above or below 

laboratory reporting level (LRL), were used in calculating 

detection frequencies and in statistical tests.

The NWQL uses LRLs for reporting non-detections, but 

is also able to semi-quantitatively detect compounds below the 

LRL. The LRL and long-term method detection limit (LTMDL) are calculated based on quality control experiments 

conducted by the NWQL. The LT-MDL mitigates the reporting of false positive readings. The risk of reporting a false 

positive concentration equal to or greater than the LT-MDL 

when the compound is not present is 1 percent or less. The 

LRL (LRL = 2 × LT-MDL) is used to minimize the chance of 

reporting a false negative reading. The risk of reporting a false 

negative is 1 percent or less when the actual concentration is 

equal to or greater than the LRL (Childress and others, 1999).

The CAS assessment also has been utilizing the tritium 

content of samples to identify young ground water. Young 

ground water is generally thought to be more susceptible to 

contamination owing to the prevalent use of regulated chemicals (VOCs) during the last approximately 50 years. Additionally, natural processes of attenuation have more time to reduce 

contaminant concentrations in ground water that remains in 

the aquifer system for longer periods. Ground-water samples 

collected during the CAS assessment were analyzed for the 

stable isotopes of water—deuterium and oxygen-18. Stable 

isotopes can help determine the source of recharge water to an 

aquifer system. Knowing the source of recharge water may aid 

in identifying pathways of aquifer contamination.

In addition to the aforementioned chemical components, 

this study also examined nitrate concentrations found in 

ground water underlying the Fresno and Modesto study areas. 

Nitrate analyses required no extra sampling since a large and 

readily accessible amount of water-quality data for public 

supply wells is available in the California DHS public supply 

well database. High nitrate concentrations were examined to 

determine the ability to predict the occurrence of VOCs. Relatively high nitrate concentrations may occur in ground water 

due to anthropogenic activities at or near the land surface and 

thus may be a useful tool in predicting the occurrence of other 

anthropogenic compounds.

Besides examining the chemical composition of ground 

water, the explanatory variables of well depth, which is 

defined as the depth to the top of the uppermost perforation, 

and land use were studied in order to determine their ability 

to predict ground-water contamination and aquifer susceptibility. This is an approach used by the USGS National Water 

Quality Assessment (NAWQA) Program. One objective of the 

NAWQA Program is to examine how factors such as depth to 

the top of the uppermost perforation and land use are related 

to ground-water quality and aquifer susceptibility (Gilliom 

and others, 1995). Previous NAWQA studies of unconfined 

aquifers have shown a negative correlation between depth to 

the top of the uppermost perforation and VOC occurrence 

and distribution (Clawges and others, 1999). The shallower a 

well’s perforations are, the shorter the distance a contaminant 

must travel from its source at or near the land surface.

NAWQA studies have also shown that type of land 

use—for example, urban or agriculture—can be correlated 

with VOC occurrence and distribution in unconfined aquifers 

(Squillace and others, 2002). Wells located in urban areas may 

have more detections of anthropogenic compounds, such as 

VOCs, compared to wells located in less urbanized or more 

agricultural/rural areas. This may occur because VOCs generally are manufactured and used more frequently in urbanized 

settings. Because this study was done using only public supply 

wells located in primarily urban areas, only the degree of 

urbanization was used to examine the relation between land 

use and ground-water quality.
VOC Primary use or source CAS number

Laboratory reporting limits

Fresno study

μg/L

Modesto study

μg/L

1,1-Dichloroethane Solvent 75-34-3 0.035 0.035

1,1-Dichloroethylene (DCE) Organic synthesis 75-35-4 .04 .04

1,1-Dichloropropene Organic synthesis 563-58-6 .05 .026

1,1,1,2-Tetrachloroethane Solvent 630-20-6 .03 .03

1,1,2,2-Tetrachloroethane Solvent 79-34-5 .09 .09

1,1,1-Trichloroethane (TCA) Solvent 71-55-6 .032 .032

1,1,2-Trichloroethane Solvent 79-00-5 .06 .06

1,1,2-Trichlorotrifluoroethane (CFC-113) Refrigerant 76-13-1 .06 .06

1,2-Dibromo-3-chloropropane (DBCP) Fumigant 96-12-8 .5 .21

1,2-Dibromoethane Solvent 106-93-4 .036 .036

1,2-Dichlorobenzene Solvent 95-50-1 .031 .031

1,2-Dichloroethane Solvent 107-06-2 .13 .13

1,2-Dichloropropane Solvent 78-87-5 .029 .029

1,2,3,4-Tetramethylbenzene Hydrocarbon 488-23-3 .23 .23

1,2,3,5-Tetramethylbenzene (isodurene) Hydrocarbon 527-53-7 .2 .2

1,2,3-Trichlorobenzene Organic synthesis 87-61-6 .27 .27

1,2,3-Trichloropropane Solvent 96-18-4 .16 .16

1,2,3-Trimethylbenzene Gasoline 526-73-8 .12 .12

1,2,4-Trichlorobenzene Solvent 120-82-1 .07 .19

1,2,4-Trimethylbenzene Organic synthesis 95-63-6 .056 .056

1,3-Dichlorobenzene Solvent 541-73-1 .03 .03

1,3-Dichloropropane Organic synthesis 142-28-9 .12 .12

1,3,5-Trimethylbenzene Gasoline 108-67-8 .044 .044

1,4-Dichlorobenzene Fumigant 106-46-7 .05 .05

2,2-Dichloropropane Organic synthesis 594-20-7 .05 .05

2-Butanone Solvent 78-93-3 5 1.6

2-Chlorotoluene Solvent 95-49-8 .026 .026

2-Hexanone Solvent 591-78-6 .7 .7

3-Chloropropene Organic synthesis 107-05-1 .07 .07

4-Chlorotoluene Solvent 106-43-4 .05 .06

4-Isopropyl-1-methylbenzene Organic synthesis 99-87-6 .07 .07

4-Methyl-2-pentanone Solvent 108-10-1 .37 .37

Acetone Solvent 67-64-1 7 7

Acrylonitrile Organic synthesis 107-13-1 1.2 1.2

Benzene Gasoline 71-43-2 .035 .035

Bromobenzene Solvent 108-86-1 .036 .036

Bromochloromethane Organic synthesis 74-97-5 .07 .044

Bromodichloromethane Disinfection by-product 75-27-4 .048 .048

Bromoethene Fire retardant 593-60-2 .11 .1

Bromoform (tribromomethane) Disinfection by-product 75-25-2 .06 .06

Bromomethane Fumigant 74-83-9 .26 .26

Butylbenzene Organic synthesis 104-51-8 .19 .19

Carbon disulfide Organic synthesis 75-15-0 .07 .07

Chlorobenzene Solvent 108-90-7 .028 .028

Chloroethane Solvent 75-00-3 .12 .12

Chloroform (trichloromethane) Disinfection by-product 67-66-3 .024 .024

Chloromethane Refrigerant 74-87-3 .17 .25

cis-1,2-Dichloroethylene Solvent 156-59-2 .038 .038

cis-1,3-Dichloropropene Fumigant 10061-01-5 .09 .09

Dibromochloromethane Disinfection by-product 124-48-1 .18 .18

Dibromomethane Solvent 74-95-3 .05 .05

Dichlorodifluoromethane (CFC-12) Refrigerant 75-71-8 .18 .27

Dichloromethane (methylene chloride) Solvent 75-09-2 .16 .16

Diethyl ether Solvent 60-29-7 .17 .17

Diisopropyl ether Gasoline 108-20-3 .1 .1

Ethyl methacrylate Organic synthesis 97-63-2 .18 .18

Ethyl tert-butyl ether (ETBE) Gasoline 637-92-3 .05 .054

Table 1. Volatile organic compounds, primary source or use, Chemical Abstract Service number, and laboratory reporting limits for 

the U.S. Geological Survey National Water Quality Laboratory analytical schedule 2020.

[VOC, volatile organic compound; CAS, Chemical Abstract Service; μg/L, microgram per liter]

Approach 7
VOC Primary use or source CAS number

Laboratory reporting limits

Fresno study

μg/L

Modesto study

μg/L

Ethylbenzene Gasoline 100-41-4 0.03 0.03

Hexachlorobutadiene Organic synthesis 87-68-3 .14 .14

Hexachloroethane Solvent 67-72-1 .19 .19

Isopropylbenzene Organic synthesis 98-82-8 .06 .032

m- and p-Xylene Gasoline 108-38-3/106-42-3 .06 .06

Methyl acrylate Organic synthesis 96-33-3 2 1.4

Methyl acrylonitrile Organic synthesis 126-98-7 .6 .6

Methyl iodide Organic synthesis 74-88-4 .25 .12

Methyl methacrylate Organic synthesis 80-62-6 .35 .35

Methyl tert-butyl ether (MTBE) Gasoline 1634-04-4 .17 .17

Naphthalene Organic synthesis 91-20-3 .5 .25

n-Propylbenzene Solvent 103-65-1 .042 .042

o-Ethyl toluene Hydrocarbon 611-14-3 .06 .06

o-Xylene Gasoline 95-47-6 .07 .038

sec-Butylbenzene Organic synthesis 135-98-8 .032 .032

Styrene Organic synthesis 100-42-5 .042 .042

tert-Amyl methyl ether Gasoline 994-05-8 .08 .11

tert-Butylbenzene Organic synthesis 98-06-6 .048 .06

Tetrachloroethylene (PCE) Solvent 127-18-4 .027 .1

Tetrachloromethane (carbon tetrachloride) Solvent 56-23-5 .06 .06

Tetrahydrofuran Solvent 109-99-9 2.2 2.2

Toluene Gasoline 108-88-3 .05 .05

trans-1,2-Dichloroethylene Solvent 156-60-5 .032 .032

trans-1,3-Dichloropropene Fumigant 10061-02-6 .09 .09

trans-1,4-Dichloro-2-butene Organic synthesis 110-57-6 .7 .7

Trichloroethylene (TCE) Solvent 79-01-6 .038 .038

Trichlorofluoromethane (CFC-11) Refrigerant 75-69-4 .09 .09

Vinyl chloride Organic synthesis 75-01-4 .11 .11

Table 1. Volatile organic compounds, predominant source or use, Chemical Abstract Service number, and laboratory reporting limits 

for the U.S. Geological Survey National Water Quality Laboratory analytical schedule 2020.—Continued

[VOC, volatile organic compound; CAS, Chemical Abstract Service; μg/L, microgram per liter]

8 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

Another explanatory variable examined for its ability to 

predict ground-water contamination was the density, or number, of leaking underground fuel tanks (LUFTs) located near 

public supply wells. LUFTs are sites where chemicals, primarily gasoline and diesel fuel, are known to have been released 

into the environment. LUFTs can introduce gasoline components such as benzene, toluene, ethylbenzene, and xylenes 

(BTEX), as well as the gasoline oxygenate methyl tert-butyl 

ether or MTBE into the subsurface environment (Moran 

and others, 1999; Johnson and others, 2000). Therefore, the 

density of LUFTs within a prescribed distance of each well 

sampled was studied to determine if this is an effective indicator of ground-water contamination by VOCs.

Methods

Well Selection

Wells for both CAS studies were selected by using a gridbased random sampling design (Scott, 1990) with grids consisting of 50 equal area cells. Suitable wells were chosen using 

an inventory obtained from the USGS National Water Information System (NWIS) and information provided by the water 

purveyors for the cities of Modesto and Fresno. Wells were 

randomly selected in each cell to statistically represent the 

zone of ground water tapped by the majority of public supply 

wells in the area. Those wells were then ranked based on the 

total number of wells chosen for a particular cell. Only wells 

that had construction information (screened intervals, depth of 

perforations, and date constructed) available were considered 

for sampling. Depth to the top of the uppermost perforation of 

the wells sampled ranged from 74 to 248 ft (median = 124.5 

ft) and 116 to 445 ft (median = 188.5 ft) for the Modesto and 

Fresno study areas, respectively. Selected construction information for the wells sampled can be found in appendix 1.

Sample Collection

Forty wells in the Modesto area were sampled during a 

2-week period in March and April, 2001. Fifty wells in the 

Fresno area were sampled during a 2-week period in July 2002. 

Each of the wells was sampled for VOCs, stable isotopes of 

water, and tritium. Procedures utilized by the USGS 

NAWQA Program were implemented in this study to mitigate 

the potential for airborne contamination of samples and (or) 
Methods 9

cross contamination between wells (Koterba and others, 1995; 

U.S. Geological Survey, 1998; U.S. Geological Survey, 1999). 

These procedures also helped assure that a representative 

sample of ground water would be collected at each site. Each 

public supply well was pumped for at least 20 minutes in order 

to purge a minimum of three casing volumes before sampling. 

Samples were collected by affixing tubing to the sampling port 

closest to the well head. All samples were unfiltered and collected before any type of chemical treatment, such as chlorination, was done to the well water.

Either tygon or copper tubing was used for collecting 

VOC samples for this study. Tygon tubing was used because 

it is flexible and available in different diameters, which is 

important because sampling ports have spigots with different 

diameters. However, tygon tubing is not a preferred sampler 

material when collecting VOCs because it may leach plasticizers and thus contaminate the sample (U.S. Geological Survey, 

1998). To avoid this problem, only high purity, plasticizerfree tygon tubing was used in this study (Cole Parmer, 2004). 

Before going into the field, sample tubing was cleaned according to procedures implemented by the NAWQA Program 

(Koterba and others, 1995).

Volatile organic compounds were collected in 40-ml 

sample vials that were purged using three vial volumes of 

sample water and then bottom filled to eliminate headspace. 

Drops of one-to-one (1:1) hydrochloric acid were added to 

the sample to decrease the pH to 2 or less, and the sample was 

put on ice and shipped for analysis. To collect stable isotopes 

of water, a 60-ml clear glass bottle was filled with unfiltered 

water, sealed with a conical cap, and secured with electrical 

tape to prevent leakage and evaporation. Tritium samples were 

collected by filling 1-L amber bottles with unfiltered water, 

closing each using a cap with a conical insert and then securing it with electrical tape. VOCs, stable isotopes, and tritium 

concentrations were determined by analytical methods similar 

to those used by the NAWQA Program (Coplen and others, 

1991; Conner and others, 1998; Hudson and others, 2002). 

VOC analyses were done at the NWQL in Denver, Colorado; 

stable isotopic analyses were done at the USGS stable isotope 

laboratory in Reston, Virginia; and tritium analyses were done 

at LLNL in Livermore, California.

Quality Control

Blank Data

Blank samples were collected at 10 percent of the well 

sites to determine if contaminants were introduced during 

sample collection, processing, storage, transportation, and 

laboratory analysis. Only VOC-free water (blank water) was 

used in the preparation of blanks. Four types of quality-control 

blanks were collected in Modesto: equipment, trip, field, and 

source solution. Only field and source blanks were collected 

in Fresno. Equipment blanks were collected to determine 

if the sampling equipment introduced contamination. Trip 

blanks were collected to determine if shipping, handling, and 

intermittent storage of sample containers produced bias. Field 

blanks were processed at the sampling location to determine 

if procedures used during sample collection and laboratory 

analysis introduced contamination and if cleaning procedures 

were effective. Source solution blanks were prepared to verify 

that the blank water used for the field, trip, and equipment 

blanks had no concentrations of VOCs detectable at the laboratory-reporting limit (LRL).

Field blanks were collected at the sampling site by pouring blank water directly from its original storage container 

through clean sampling equipment and into sample vials. 

Source-solution blanks also were collected at the sampling site 

by pouring blank water directly into sample vials that were 

then stored, shipped, and analyzed in the same manner as the 

environmental samples. Equipment blanks were collected in 

the same manner as field blanks, but were processed under 

controlled conditions away from the sampling site. Trip blanks 

consisted of blank water in a sealed sample vial that was kept 

with the other sample vials from the start of the field activities 

through laboratory analysis.

 A VOC detected in an environmental sample was 

considered a result of contamination if the following criteria were met: (1) the VOC was detected in a quality-control 

blank, but not in the associated source solution blank, (2) the 

VOC detected in the quality-control blank was also detected 

in environmental samples, and (3) the minimum concentration detected in environmental samples was lower than the 

maximum concentration detected in quality-control blanks. 

Contamination was indicated when the concentration of VOCs 

in quality-control blanks was as high as or higher than the concentration in the environmental samples (Dawson and others, 

2003). Samples that were determined to have a detection of 

one or more specific VOCs resulting from contamination were 

censored, and subsequently any VOC in question was reported 

as not being detected for the sample in question.

Surrogate Data

Surrogate data were used to calculate percentage recoveries for VOCs detected in the CAS study. Surrogate compounds 

are not normally found in the environment and are used to 

identify potential problems associated with laboratory analyses. Potential problems include matrix interferences (such 

as high levels of dissolved organic carbon) that produce a 

positive bias, and incomplete laboratory recovery (such as 

improper maintenance and calibration of analytical equipment) that produces a negative bias. In the laboratory, three 

surrogates (1,2-dichloroethane-d4, toluene-d8, and p-bromofluorobenzene) were added to each quality control (QC) and 

environmental sample, and their percentage recoveries were 

reported along with targeted VOC concentrations. A 70- to 

130-percent recovery of surrogates is considered acceptable; 

values outside this range indicate possible problems with the 

analytical procedure (Conner and others, 1998).
10 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

Determination and Use of Categorical Variables

Well Depth

Wells were put into one of two depth categories, shallow 

or deep, depending on the depth to the top of their uppermost 

perforation. Shallow and deep categories were demarcated 

by the median depth to the top of the uppermost perforation 

for each particular study area; wells having a depth less than 

the median value were categorized as shallow; wells having a 

depth greater than the median value were categorized as deep. 

Information on the depth of well perforations was obtained 

from the city water purveyors for Fresno and Modesto, as well 

as the California Department of Water Resources and USGS 

databases.

Land Use

In order to assign a land-use category to a public supply 

well, a circular buffer (radius = 500 m) was placed around 

each well sampled and the percentages of land-use types 

(urban, agriculture, or native vegetation) within that buffer 

zone were calculated. The land use category was then based 

on the dominant type within each well’s buffer area (Koterba, 

1998). Because the predominant land use type within the 

buffered areas for both Fresno and Modesto was urban, wells 

were categorized as either high or low in their degree of 

urbanization. The median percentage of urban land use for 

all the buffered areas in a particular basin was used to divide 

wells into high and low urban land use categories. The median 

percentage of urban land use in Modesto was 100 percent 

and in Fresno, 95 percent. Land use data were obtained from 

the California Department of Water Resources for the Fresno 

(California Department of Water Resources, 2000a) and the 

Modesto (California Department of Water Resources, 2000b) 

study areas.

Leaking Underground Fuel Tanks

Wells were classified into one of two LUFT categories, 

high density or low density, depending on the number of 

LUFTs contained within the buffer zone associated with each 

well. The number of LUFTs assigned to each well sampled 

was determined in much the same way as the amount of urban 

land use assigned to each well; circular buffers (radius = 500 

m) were placed around each well and the number of LUFTs 

within the buffer zone was tabulated. The location and chemical content of LUFTs for each of the study areas was obtained 

from the SWRCB Geographic Environmental Information 

Management System (GEIMS), a data warehouse which tracks 

regulatory data about underground fuel tanks, fuel pipelines, 

and public drinking water supplies (California State Water 

Resources Control Board, 2004).

Determining the Presence of Young Ground 

Water

Tritium concentrations were used to determine whether 

ground-water samples contained a component of young 

ground water (water recharged within the last approximately 

50 years) or the sample consisted only of old ground water 

recharged prior to that 50 year period. Tritium is introduced 

into ground water when atmospheric tritium replaces hydrogen atoms in rain and snowfall, and this precipitation then 

infiltrates the soil. The primary source of atmospheric tritium 

is cosmic rays bombarding water vapor in the atmosphere 

(Thatcher, 1962). However, thermonuclear weapons testing in 

the 1950s and 1960s produced elevated levels of tritium in the 

atmosphere and thus elevated tritium levels in ground water; 

by 1963, the extensive testing of thermonuclear weapons 

ended. Atmospheric tritium levels in the northern hemisphere 

reached a maximum in 1963, and levels of tritium in the atmosphere and ground water are now approaching background 

levels.

Young ground water that has been at the land surface 

within the last 50 years is thought to be more susceptible to 

contamination due to the widespread production and use of 

VOCs during this period. A tritium concentration of 1.0 picocurie per liter (pCi/L) was used as the demarcation between 

young and old ground water (Michel, 1989; Michel and Schroeder, 1994). Ground water with tritium concentrations above 

the 1.0-pCi/L threshold is referred to as young, and ground 

water with tritium concentrations that fall below the threshold 

is referred to as old.

Determining the Presence of High Nitrate 

Concentrations

Nitrate concentrations were analyzed as predictors of 

aquifer contamination from VOCs. Relatively high nitrate 

concentrations in ground water may indicate that an aquifer 

has been influenced by anthropogenic activities taking place 

at or near the land surface. Therefore, ground water that is 

relatively high in nitrate may also contain other anthropogenic 

contaminants, such as VOCs. The median nitrate value for all 

wells sampled in each study area was used to place wells into 

one of two categories: wells with high nitrate concentrations 

or wells with low nitrate concentrations. Each study area was 

evaluated individually and assigned its own median nitrate 

value. Nitrate values for individual wells were determined by 

using the median nitrate concentration of analyses performed 

within the previous 12 months of this study. Nitrate-as-nitrogen concentrations were used for the analyses in this report, 

but for simplicity are referred to as nitrate concentrations.

Censoring of Data in the Fresno Study Area

The explanatory variables of well depth and land use had 

a statistically significant negative correlation to one another 
Methods 11

in the Fresno study area (Kendall’s tau test, p = 0.05). Deeper 

wells tended to be located in less urbanized areas, whereas 

shallower wells tended to be located in more urbanized 

areas. Because of this correlation, a meaningful analysis of 

the relation between VOC occurrence, tritium levels, nitrate 

concentrations, and the explanatory variables of well depth 

and land use was not possible. To solve this problem, wells 

were systematically censored (removed from the data set) until 

there was no statistically significant correlation between the 

explanatory variables of well depth and land use. A correlation 

was not considered statistically significant if the p-value for a 

Kendall’s tau test was greater than 0.1.

The following steps were used to censor wells: (1) wells 

were sorted in ascending order based on depth to the top of the 

uppermost perforation; (2) wells were divided into two depth 

categories (see determination of well depth categories above), 

shallow and deep; (3) the shallowest well, from the shallow 

depth category, with the highest percentage of urban land use 

(see determination of land use category above) was censored; 

(4) the deepest well, from the deep depth category, with the 

lowest percentage of urban land use was censored; and (5) 

well depth and land use were again compared to determine if a 

statistically significant correlation existed in the new censored 

data set. If a statistically significant correlation between well 

depth and land use was not found, no additional wells were 

censored. If a statistically significant correlation still existed, 

then steps 3–5 were repeated until there was no significant 

correlation between well depth and land use in the Fresno 

study area.

Four out of 50 wells were censored in order to reduce 

the correlation between well depth and land use in the Fresno 

study area (Kendall’s tau test, p-value = 0.12). The censored 

data set was used only to examine the relation between the 

explanatory variables of well depth and land use, and the 

occurrence and distribution of VOCs, tritium, and nitrate.

Stable Isotopes and Source of Ground-Water 

Recharge

The source of ground-water recharge can be inferred 

by determining the stable isotopic content of local ground 

and surface waters. Two naturally occurring stable isotopes 

of hydrogen and oxygen are deuterium and oxygen-18, 

respectively. Deuterium and oxygen-18 are heavier than the 

more abundant protium and oxygen-16 isotopes and readily substitute for these lighter isotopes in water molecules. 

The mass differences created by isotopic substitution in the 

water molecule allow for a measurable fractionation during 

physical hydrological processes (Clark and Fritz, 1997). The 

relative abundance of these isotopes is expressed as a ratio in 

delta notation, which is often plotted next to the linear global 

meteoric water line (GMWL). The GMWL defines the relation 

between deuterium and oxygen-18 in the global fresh surface 

waters and is an average of many local and regional water 

lines (Craig, 1961). Owing to physical hydrological processes, 

such as evaporation, mixing, and precipitation, deviations from 

the GMWL will occur. These deviations can provide insight 

into the source of the water recharging an aquifer system and, 

thereby, possibly identify pathways of contamination.

Statistical Analysis and the Use of Boxplots

Nonparametric statistics were used in this study to examine the relations between explanatory variables, such as degree 

of urbanization, and response variables, such as number of 

VOC detections. For ground-water samples that had nondetectable concentrations of tritium or nitrate, a value of one-half 

the method detection limit (MDL) was used to compute test 

statistics and create plots. Scatter plots were the first step used 

to analyze the existing correlations between variables. Scatter plots are used to visually examine the data because relying 

solely on the value of the correlation coefficient to measure 

relation strength can be misleading (Helsel and Hirsch, 1992). 

The correlation coefficient is used to indicate the strength of 

the association between two continuous variables 

Kendall’s tau test was the rank order statistical test used 

to examine the correlation between occurrence and distribution of chemical components and the explanatory variables of 

depth to the top of the uppermost perforation, land use, tritium 

levels, nitrate concentrations, and LUFT density. Explanatory 

variables were also grouped into categories (see Determination 

and Use of Categorical Variables above) and compared to one 

another using the Wilcoxon rank sum test. The Wilcoxon rank 

sum test is a nonparametric, median test statistic that compares 

two independent data groups (categories) to determine if one 

group contains larger values than the other. For example, if 

wells are grouped based on depth to the top of the uppermost 

perforation, this test could determine whether wells grouped in 

the shallow category or the deep category tend to have a higher 

number of VOCs detected per well.

The results of Kendall’s tau and the Wilcoxon rank sum 

statistical tests were considered significant if the null hypothesis was rejected at least 95 percent of the time (p-value = 

0.05). The null hypothesis for Kendall’s tau test is that there is 

no significant correlation between the two continuous variables being tested. The null hypothesis for the Wilcoxon rank 

sum test is that there is no significant difference between the 

observations of the two independent data groups being tested.

Boxplots were used to analyze and display data in this 

report. A boxplot is a convenient way to look at the characteristics of a data set. The boxed portion of a boxplot contains 50 

percent of the data points, that is, the values located between 

the 25th and 75th percentiles of the data set. The median 

value is identified by a solid line through the box. The lines 

or “whiskers” extending from the end of the boxes expand out 

to the extreme values of the data set that are no more than 1.5 

times the length of the interquartile range. Any values beyond 

1.5 times the interquartile range (outliers) are marked with a 

solid line not connected by a whisker to the box (Venables and 

Ripley, 1999).
12 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

Results

VOC Detections in Quality-Control and 

Associated Environmental Samples

A summary of the results of the quality control (QC) and 

associated environmental samples can be found in appendix 2. 

Three VOCs (toluene, tetrachloroethylene (PCE), and dichloromethane were detected in 7 QC blanks (4 field, 1 source 

solution, 1 equipment, and 1 trip) collected for the Modesto 

study. Dichloromethane was detected in 1 field blank (0.02 

μg/L) and the associated source-solution blank (0.02 μg/L), 

but not in the paired environmental sample. Consequently, 

environmental data for dichloromethane were not censored. 

Tetrachloroethylene was detected in two field blanks, but not 

in the associated source solution blanks. Both field blanks had 

PCE concentrations of 0.02 μg/L. Six environmental samples 

had PCE concentrations less than or equal to 0.02 μg/L. These 

PCE detections were censored and counted as nondetections. 

Toluene was detected in 3 blanks: 1 field blank (0.01 μg/L), 

1 trip blank (0.02 μg/L), and 1 equipment blank (0.03 μg/L). 

Toluene was detected in two environmental samples (both at 

0.01 μg/L). Because the equipment and trip blanks had higher 

concentrations of toluene than either of the environmental 

samples, both environmental toluene detections were counted 

as nondetections. None of the QC blanks collected as part of 

the Fresno study had VOC detections.

All but two samples (one environmental sample and 

one field blank) analyzed for the Fresno study had surrogate 

recoveries within acceptable limits (70 to 130 percent). The 

environmental sample had a high recovery (133 percent) of 

the surrogate 1, 2-dichloroethane-d4. The VOCs chloroform 

and PCE were detected in this environmental sample. Therefore, these compounds may have been present in the sample 

at a lower concentration than is reported. Both of these VOC 

concentrations were listed as estimated values in the USGS 

database and, on the basis of the QC data, were not censored 

further. The field blank also had a high recovery (138 percent) of the surrogate 1,2-dichloroethane-d4

. No VOCs were 

detected in this QC sample. All surrogate recoveries for the 

Modesto CAS data were within the accepted limits.

Occurrence of VOCs, Tritium, and Nitrate

Detection frequencies for VOCs, class of VOCs, and 

tritium at levels greater than or equal to 1.0 pCi/L in the Fresno 

and the Modesto study areas are shown in figure 4. Overall, 

VOCs were detected more frequently in Modesto (92 percent) 

than in Fresno (72 percent). The class of VOCs known as trihalomethanes (THMs) was detected more frequently in Modesto 

(92 percent) than in Fresno (56 percent). Solvents also were 

detected more frequently in Modesto (70 percent) than in 

Fresno (58 percent). In Fresno, BTEX compounds were rarely 

detected (2 percent), whereas MTBE was not detected at all. 

Conversely, MTBE was detected in Modesto (5 percent), 

whereas BTEX compounds were not detected at all.

The prevalence of THMs in ground water used for public 

supply has been documented (Stackelberg and others, 2000; 

Shelton and others, 2001). A review of the occurrence and distribution of VOCs in ground-water supplies (Westrick, 1984) 

stated that the most frequently detected VOC class was THMs. 

THMs are formed when water containing organic matter, 

including drinking water and wastewater, is disinfected. These 

compounds may be introduced into aquifers through landscape 

irrigation, leaking drinking water and storm drain infrastructure, and disposal of treated wastewater. In a national survey 

of drinking-water sources, THMs were detected frequently in 

samples taken from public water systems that relied strictly 

on ground water; the larger the system, the more frequently 

THMs were detected (Grady, 2003). Therefore, it is reasonable 

to expect THMs to be the class of VOCs detected most frequently in the ground-water supplies of the large public water 

systems serving the Fresno and Modesto metropolitan areas.

 Solvents are used in a wide variety applications ranging 

from metal degreasing to dry cleaning (Thiros, 2000). These 

compounds and their degradation products have been detected 

frequently during previous studies of ground-water quality 

(Stackelberg and others, 2000; Hamlin and others, 2002). In 

a national study that analyzed the VOC content of untreated, 

ambient ground water in nearly 3,000 wells, solvents were 

detected more frequently in wells associated with urban use 

than with agricultural land use (Squillace and others 1999).

In contrast, the VOCs commonly referred to as BTEX 

compounds and the gasoline oxygenate MTBE are not 

frequently detected in the wells sampled in the Fresno and 

Modesto study areas. The low detection frequencies for the 

BTEX compounds may be a synergistic effect that is due to 

their relatively low solubility and labile nature, whereas the 

low detection frequencies of MTBE may be due to the fact 

that compound has been in heavy production and use only 

since the early 1990s (Schwarzenbach and others, 1993; Grady 

and Casey, 2001). Therefore, it may not have reached the 

depths of the aquifers sampled in this study in any appreciable 

quantities. The detection frequencies for BTEX and MTBE 

compounds in this study were similar to the frequencies 

reported by Moran and others (2003), who analyzed the results 

obtained by the USGS NAWQA Program (1993−2002) and 

source-water survey (1999-2000).

Tritium at levels greater than 1.0 pCi/L was frequently 

detected in both the Fresno (92 percent) and the Modesto (90 

percent) study areas. The median tritium levels found in Fresno 

wells was 10.9 pCi/L and 11.9 pCi/L for Modesto wells (appendix 3a). These tritium levels show that some fraction of the 

ground water withdrawn from these basins has been recharged 

since the 1950s. The median concentration of nitrate found in 

Fresno was 3.3 mg/L and in Modesto, 3.4 mg/L (appendix 3b). 

The similarity in nitrate detections in the two basins may be due 

to the similarity in land uses. Land use has been shown to be an 

important explanatory variable in predicting the concentrations 
Figure 4. Detection frequencies of volatile organic compounds, classes of volatile 

organic compounds, and tritium for wells sampled in the Fresno and Modesto study 

areas, California, in 2002 and 2001, respectively.

VOC, volatile organic compound; THM, trihalomethane; BTEX, benzene, toluene, 

ethyltoluene, xylenes; MTBE, methyl tert-butyl ether. ≥, less than or equal to; pCi/L, 

picocurie per liter.

Results 13

of nitrate in ground water (Eckhardt and Stackelberg, 1995; 

Tesoriero and Voss, 1997).

Detection Frequency of Individual VOCs

Figure 5 shows the detection frequencies for individual 

VOCs in the Fresno and Modesto study areas. Twenty-five 

different VOCs were detected in Fresno and Modesto (appendixes 4a and 4b). Seven of the 25 VOCs identified in Modesto 

samples were frequently detected and 3 of the 19 VOCs 

identified in Fresno samples were frequently detected. VOCs 

were considered frequently detected if they were identified 

in at least 20 percent of the wells sampled. In Modesto, the 

disinfection by-products chloroform, bromodichloromethane, 

bromoform, and dibromochloromethane, commonly referred 

to as THMs, were among the frequently detected VOCs, as 

were the solvents PCE, TCE, and dibromomethane. Frequently 

detected VOCs in Fresno included the THM chloroform, as well 

as the solvents PCE and 1, 2-dichloropropane. In contrast, the 

soil fumigant 1, 2-dibromo-3-chloropropane (DBCP) was rarely 

detected in either the Modesto (2 percent) or the Fresno (2 percent) study area. DBCP is a pesticide that was used to control 

nematodes in the agricultural areas of California. This carcinogenic compound has been detected frequently in drinking water 

wells in the San Joaquin Valley (Burow and others, 1999).

Chloroform was also the most frequently detected VOC 

in untreated, ambient ground water that was studied as part of 

the NAWQA Program from 1985 to 1995. The production of 

chloroform has been documented in the United States since 

the 1920s and is primarily used in the synthesis of the refrigerant hydrochlorofluorocarbon or HCFC-22 (IARC, 1979). 

Chloroform, bromodichloromethane, bromoform, and dibromochloromethane are commonly formed during the chlorination process used to disinfect drinking and reclaimed water 

(Grady and Casey, 2000). This process has been used for much 

of the 20th century (White, 1972; Chlorine Chemistry Council, 2004). Therefore, activities like landscape irrigation and 

disposal of reclaimed water may increase chloroform concentrations in ground water.

The solvents PCE and TCE have been detected frequently 

during previous studies of ground-water quality (Grady and 

Casey, 2000; Hamlin and others, 2002). These solvents are 

among the most widely produced chlorinated organics and 

have many commercial and industrial uses (Stackelberg and 

others, 2000). These chlorinated organics are hydrophobic 

and denser than water; thus, they primarily move through the 

unsaturated zone as a distinct liquid phase. When these compounds migrate downward, they frequently become trapped in 

soil pores only later to be slowly dissolved by water percolating 

through the soil profile. Because of these chemical properties, 

solvents like PCE and TCE can bring about large and persistent plumes of ground-water contamination (Broholm and 

others, 1999).

Almost all VOC detections in this study were at least an 

order of magnitude (factor of 10) below maximum safe concentrations set by federal and state drinking water regulations. Only 

four wells had a detection that exceeded drinking water standards; one TCE detection (59.6 μg/L) and one DBCP detection 

(0.6 μg/L) in Fresno, and one TCE detection (6.1 μg/L) and one 

PCE detection (5.6 μg/L) in Modesto (appendix 4b). The MCL 

set by the U.S. Environmental Protection Agency (EPA) and 

the state of California for TCE and PCE is 5 μg/L; the MCL for 

DBCP is 0.2 μg/L. 

TCE concentrations in the DHS database for the well in 

Fresno also exceeded the MCL; however, DBCP concentrations did not. The ground water withdrawn by the two wells 

in Fresno with TCE and DBCP concentrations that exceeded 

the MCL are treated so that federal and state regulations are 

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Figure 5. Detection frequencies of volatile organic compounds, divided by class, in 

samples collected in the Fresno and Modesto study areas, California, in 2002 and 2001, 

respectively.

Not detected where not shown.

14 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

met before delivery to the public (Bob Little, city of Fresno, 

oral commun., 2004). The TCE and PCE concentrations in the 

DHS database for the wells in Modesto did not exceed the MCL 

prior to sampling. After sampling, the TCE concentration did 

exceed the MCL in the DHS database; the PCE concentration 

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did not. Ground water from the two wells in Modesto with TCE 

and PCE concentrations that exceeded the MCL are not being 

used; these wells will not be used until the ground water can be 

treated to meet federal and state regulations (Lanora E. Hill, oral 

commun, 2004). 
Results 15

Figure 6. The number of volatile organic compounds (VOCs) detected in 

wells categorized by relative ground-water age in (A) the Fresno and (B) the 

Modesto study areas, California, in 2002 and 2001, respectively.

n is the number of wells.

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Relation between VOC Occurrence, Young 

Ground Water, and Nitrate Concentration

In Fresno, wells withdrawing young ground water had a 

higher detection frequency of VOCs (68 percent) than wells 

withdrawing old ground water (40 percent). Modesto wells 

withdrawing young ground water also had a higher detection 

frequency of VOCs (94 percent) than wells withdrawing older 

ground water (75 percent). The distribution of the number of 

VOCs detected in wells withdrawing young versus old ground 

water in the Fresno and the Modesto study areas are shown 

in figures 6A and 6B, respectively. There was a significantly 

higher number of VOC detections in Fresno wells withdrawing 

young ground water than in wells withdrawing old ground water 

(p = 0.05, Wilcoxon rank sum test). In Modesto, there was 

a significant positive correlation between number of VOCs 

detected and tritium levels (p = 0.04, Kendall’s tau test). Of the 

wells that had at least one VOC detection, 94 percent of the 

wells in Fresno and 92 percent of the wells in Modesto were 

also withdrawing young ground water.

These results indicate that young ground water is more 

susceptible to contamination than old ground water. Young 

ground water has been at or near the land surface while anthropogenic compounds like VOCs have been in widespread production and use. Additionally, the younger the ground water, 

the less time that processes of natural attenuation have had to 

reduce contaminant concentrations. Even though the results 

indicate that young ground water is more susceptible than old 

ground water to VOC contamination, these data do show that 

old ground water can be contaminated.
Figure 7. The number of volatile organic compounds (VOCs) detected in wells 

categorized by nitrate concentration in (A) the Fresno and (B) the Modesto study 

areas, California, in 2002 and 2001, respectively.

n is the number of wells. ≥, greater than or equal to; <, less than.

16 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

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Contamination of old ground water may be caused by 

point source contamination and (or) preferential flow paths 

that may aid in transporting liquids relatively quickly from the 

land surface to the water table. Point sources of contamination may include hazardous materials spills, leaking storage 

tanks, or improper disposal of VOC laden waste. Preferential 

flow paths in these ground-water basins may arise from the 

improper sealing of abandoned and destroyed wells, and (or) 

the improper placement, or lack, of sanitary seals in active 

public supply wells.

In Fresno, VOCs were detected more frequently in wells 

with high nitrate concentrations (80 percent) than in wells with 

low nitrate concentrations (68 percent). In Modesto, wells 

with high nitrate concentrations had slightly higher detection 

frequencies of VOCs (95 percent) than wells with low nitrate 

concentrations (90 percent). The distribution of the number 

of VOC detections for samples with high and with low nitrate 

concentrations are shown in figure 7. In Fresno, the statistical 

significance of the difference between the number of VOCs 

detected in wells with high nitrate and wells with low nitrate 

was borderline (p = 0.07, Wilcoxon rank sum test). In Modesto, 

there was no statistical significance between the number of 

VOCs detected in the high and the low nitrate wells (p = 0.33, 

Wilcoxon rank sum test).
Results 17

Figure 8. Detection frequencies of volatile organic compounds (VOCs), tritium, and nitrate in shallow 

and deep wells sampled in (A) the Fresno and (B) the Modesto study areas, California, in 2002 and 2001, 

respectively.

n is the number of wells. ft, foot; mg/L, milligram per liter; pCi/L, picocurie per liter; ≥, greater than or equal to. �������������������

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The absence of a significant positive correlation between 

nitrate concentrations and the number of VOCs detected 

in ground water may be due to the labile nature of nitrate 

under certain natural conditions. Anaerobic denitrification by 

bacteria has been shown to be a viable process for removing 

nitrate in ground water (Korom, 1992). A study of the terminal 

electron accepting processes in several San Joaquin Valley 

ground-water basins showed that favorable conditions existed 

for nitrate removal by denitrification (Chapelle and others, 

1995). Therefore, the process of denitrification may remove 

nitrate from ground water while VOC concentrations are left 

largely unchanged.

Depth to the Top of the Uppermost Perforation as 

an Explanatory Variable

The detection frequencies for VOCs, tritium at 

levels greater than or equal to 1.0 pCi/L, and high nitrate 

concentrations in shallow and deep wells in Fresno are 

shown in figure 8A. VOCs were detected more frequently 

in shallow wells (86 percent) than in deep wells (67 percent). 

Figure 9A shows the distribution of the number of VOC detections for shallow and deep wells in the Fresno study area. The 

number of VOCs detected had a statistically significant negative correlation to well depth (table 2). These results indicate 

that depth to the top of the uppermost perforation is a good 

predictor of VOC occurrence in the Fresno study area.

Tritium at levels greater than or equal to 1.0 pCi/L was 

detected more frequently in shallow wells (95 percent) than in 

deep wells (87 percent) in Fresno. A statistically significant 

negative correlation was found between depth to the uppermost 

perforation and tritium levels (table 2). High nitrate concentrations also were detected in shallow wells (54 percent) more 

frequently than in deep wells (42 percent). The median value 

of nitrate concentrations was greater in shallow wells than in 

deep wells; however, the difference in concentrations is not 

statistically significant (table 2). Tritium levels and nitrate concentrations detected in Fresno show that shallow wells contain 

younger water with higher concentrations of nitrate than deep 

wells, which indicates that wells perforated at shallow depths 

are more susceptible to influences occurring at or near the land 

surface than wells screened at deeper depths.

The detection frequencies for VOCs, tritium at levels 

greater than or equal to 1.0 pCi/L, and high nitrate concentrations in shallow and deep wells in Modesto are shown in 

figure 8B. VOCs were detected more frequently in shallow 

wells (95 percent) than in deep wells (90 percent), and the 
Figure 9. The number of volatile organic compounds (VOCs) detected in shallow 

and deep wells sampled in (A) the Fresno and (B) the Modesto study areas, 

California, in 2002 and 2001, respectively.

n is the number of wells. ft, foot.

18 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

Fresno Modesto

Shallow

(135–187 ft)

Deep

(190–445 ft) p-value Shallow(74–121 ft)

Deep

(128–248 ft) p-value

VOCs (detections) 2 1 10.04 5.5 4 20.04

Tritium (pCi/L) 14 8.4 1.02 10.3 14.7 2.4

Nitrate-N (mg/L) 4 3.1 2.1 3.6 3.5 2.5 1

Statistical test: Kendall’s tau.

2

Statistical test: Wilcoxon rank sum.

Table 2. Median value for the number of volatile organic compound detections, tritium levels, and nitrate concentrations in samples 

collected in shallow and deep wells in Fresno and Modesto, California, in 2002 and 2001, respectively.

[Depth categories are demarcated by the median depth to the top of the uppermost perforation for all wells in each study area. VOC, volatile organic compound. Nitrate-N, nitrate nitrogen. ft, foot; pCi/L, picocurie per liter; mg/L, milligram per liter]

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Results 19

Land Use as an Explanatory Variable

For wells in areas of high and low urban land use in 

Fresno, the detection frequencies for VOCs, tritium at levels 

greater than or equal to 1.0 pCi/L, and high nitrate concentrations are shown in figure 11A. VOCs were detected more 

frequently in high urban land use wells (96 percent) than in 

low urban land use wells (56 percent). The number of VOCs 

detected showed a statistically significant positive correlation 

with urban land use (table 3). The history of a compound’s 

production and use may be the best predictor of its occurrence 

in the environment. Therefore, it is reasonable to expect VOCs 

to occur more frequently in wells that are located in more 

urbanized areas, where these compounds are most often used 

number of VOCs detected was significantly greater in shallow 

wells (table 2). Figure 9B shows the distribution of the number 

of VOC detections for shallow and deep wells in the Modesto 

study area. These results indicate that depth to the top of the 

uppermost perforation is a good predictor of VOC occurrence 

in the Modesto study area. Tritium at levels greater than or 

equal to 1.0 pCi/L were detected as frequently in deep wells as 

in shallow wells (90 percent); the median levels were higher 

for the deep wells, but the difference was not statistically 

significant (table 2).

In Modesto, high nitrate concentrations were detected just 

as frequently in deep wells as in shallow wells (45 percent) 

(fig. 8B). Median nitrate concentrations in shallow wells were 

higher than those in deep wells, but the difference was not statistically significant (table 2). The fact that the occurrence and 

distribution of tritium and nitrate were not significantly correlated to depth in Modesto indicates young water has infiltrated 

the depths of the aquifer sampled in this study; even so, VOCs 

occurred more frequently in the shallow wells of Modesto. 

This finding may be a result of the partially confined nature 

of the aquifer system in the Modesto study area and (or) the 

hydrophobic properties of VOCs, which allow contaminants to 

adsorb to the aquifer matrix and then only slowly be released 

into aqueous phase over time.

The differences in VOC occurrence and distribution 

between the Modesto and Fresno study areas are likely due to 

differences in the depth to the top of the uppermost perforations 

of the wells. Figure 10 shows that land use, tritium levels, and 

nitrate concentrations in Fresno and Modesto were very similar, whereas the depth to the top of the uppermost perforation 

was significantly less for the wells in Modesto than for the 

wells in Fresno. The number of VOCs detected per well was 

greater in Modesto than in Fresno (p < 0.01, Wilcoxon rank 

sum test), indicating that depth to the top of the uppermost 

perforation is an important explanatory variable in predicting the occurrence and distribution of VOCs. Other studies 

of ground-water quality in partially confined and unconfined 

aquifers also have shown that wells screened at shallower 

depths tend to have a higher frequency of VOC detections than 

wells screened at deeper depths (Clawges and others, 1999).

LUFT Density as an Explanatory Variable

Analysis of LUFT data showed that 99.6 percent of the 

underground storage tanks located within the study areas 

contained gasoline, diesel fuel, or other petroleum distillates 

(California State Water Resources Control Board, 2004). A 

total of three gasoline components were detected in 3 of the 90 

wells sampled in the Fresno and Modesto study areas. Because 

gasoline components were rarely detected, LUFT density was 

not analyzed as a explanatory variable (appendix 4).

(Moran and others, 2002). Analysis of ground-water quality 

data collected from across the nation by the USGS NAWQA 

Program has shown that VOCs are detected more frequently 

in wells that are located in densely populated, urbanized areas 

than in less populated, rural areas (Squillace and others, 1999). 

Tritium was detected at levels greater than or equal to 

1.0 pCi/L almost as often in low urban land use wells (83 

percent) as in high urban land use wells (95 percent) in Fresno 

(fig.11A). Tritium levels did not have a significant statistical 

correlation with urban land use (table 3). High nitrate concentrations were detected more frequently in high urban land use 

wells (61 percent) than in low urban land use wells (35 percent) 

(fig. 11A); the difference in concentration was statistically 

significant (table 3). In urban areas, nitrate can be introduced 

into the subsurface environment by several pathways, such as 

leaking wastewater infrastructure, disposal of wastewater via 

percolation ponds, fertilization of residential lawns and gardens, and septic tank systems. High nitrate values in urbanized 

areas may also be a legacy of agricultural activities.

The detection frequencies for VOCs, tritium at levels 

greater than or equal to 1.0 pCi/L, and nitrate in high and low 

urban use wells in Modesto are shown in figure 11B. VOCs 

were detected in 100 percent of the wells located within high 

urban land use areas and in 84 percent of the wells located 

within low urban land use areas. VOC detections showed a 

significant positive correlation to urban land use in Modesto 

(table 3). Tritium was detected at levels greater than or equal 

to 1.0 pCi/L less frequently in low urban land use wells (84 

percent) than in high urban land use wells (95 percent), but the 

difference in the median tritium level between high and low 

urban land use wells was not statistically significant (table 3). 

High nitrate concentrations were detected more frequently in 

low urban land use wells (53 percent) than in high urban land 

use wells (48 percent). However, the difference in median 

nitrate concentrations between the two land use categories was 

not statistically significant (table 3).

Stable Isotopic Content of Ground Water as an 

Explanatory Variable

Ground-water recharge in Fresno and Modesto occurs 

through several processes: infiltration of local precipitation, 

percolation from streams and rivers, percolation from urban and 
20 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

Figure 10. Distribution of depth to the top of the uppermost perforation, percentage urban land use, nitrate concentration, and 

tritium levels in the Fresno and Modesto study areas, California, in 2002 and 2001, respectively.

p-values indicate the statistical significance of the difference in distribution between the study areas. n is the number of wells.

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Figure 11. Detection frequencies of volatile organic compounds, tritium, and nitrate by urban land use category in wells 

sampled in (A) the Fresno, and (B) the Modesto study areas, California, in 2002 and 2001, respectively.

n is the number of wells in each category; ft, foot; mg/L, milligram per liter; pCi/L, picocurie per liter; ≥, greater than or equal to.

Fresno Modesto

Urban low

(< 95 percent)

Urban high

(≥ 95 percent) p-value Urban low(< 100 percent)

Urban high

(100 percent) p-value

VOCs (detections) 1 2 1<0.01 4 5 10.05

Tritium (pCi/L) 10.5 10.9 2.30 17.6 9.6 2.14

Nitrate-N (mg/L) 2.7 4.1 1.04 3.5 3 2.30

1

Statistical test: Kendall’s tau.

2

Statistical test: Wilcoxon rank sum.

Table 3. Median value for the number of volatile organic compound detections, tritium levels, and nitrate concentrations by degree 

of urbanization for samples collected in the Fresno and Modesto study areas, California, in 2002 and 2001, respectively.

[Degree of urbanization is demarcated by the median percentage of urbanized land occupying in a circular buffer zone around each well in each study area. 

VOC, volatile organic compound. Nitrate-N, nitrate nitrogen. pCi/L, picocurie per liter. mg/L, milligram per liter. <, less than; ≥, greater than or equal to]

Results 21

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Figure 12. Scatter plot of delta oxygen-18 (δ18O) versus delta deuterium (δD) for samples with and without volatile 

organic compounds (VOCs) collected in (A) the Fresno and (B) the Modesto study areas, California, in 2002 and 

2001, respectively.

22 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002

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agricultural irrigation, and engineered recharge by spreading 

basins (Fresno study area only). Generally speaking, regional 

precipitation becomes isotopically lighter as it moves eastward 

from the low elevations of the San Joaquin Valley trough to the 

high elevations of the Sierra Nevada (Clark and Fritz, 1997). 

Ground water recharged by local precipitation, which falls at 

low elevations is more isotopically enriched (less negative) 

than ground water recharged by rivers, agricultural irrigation, 

and engineered recharge operations because the source of 

water recharged by the latter is mostly from precipitation that 

has fallen in the higher elevations of the Sierra Nevada and is 

therefore isotopically depleted (more negative). Figure 12 shows 

plots of the isotopic composition of ground-water samples collected from the Fresno and Modesto study areas. Samples are 

differentiated based on whether or not VOCs were detected.
Summary and Conclusions 23

Summary and Conclusions

A total of 90 public supply wells were sampled in the 

Fresno and Modesto study areas; 50 wells were sampled in the 

Fresno study area, and 40 wells were sampled in the Modesto 

study area. Thirty-six of the 50 wells sampled in Fresno had 

at least one volatile organic compound (VOC) detection; one 

well contained trichloroethylene (TCE), the concentration of 

which was nearly 12 times the maximum contaminant level. 

Another well in Fresno contained the fumigant 1, 2-dibromo3-chloropropane (DBCP) at 3 times the MCL. Thirty-seven 

of the 40 wells sampled in Modesto had at least one VOC 

detection; 2 of these wells had one detection each that just 

exceeded the MCL. The exceedances were concentrations of 

the solvents TCE and tetrachloroethylene (PCE).

Twenty-five of the 85 VOCs analyzed for in this study 

were detected in ground-water samples. Eight of these 25 

were frequently detected (more than 20 percent) in the wells 

sampled. Trihalomethanes (THM), primarily chloroform, 

accounted for half of these frequently detected compounds; 

solvents, primarily PCE, accounted for the other half. In contrast to THMs and solvents, the gasoline components BTEX 

(benzene, toluene, ethyltoluene, xylenes) and MTBE (methyl 

tert-butyl ether) were rarely detected. Two wells in Modesto 

had detections of MTBE, and one well in Fresno had a 

detection of the BTEX compound toluene, suggesting that 

these are not contaminants of concern in these study areas.

The explanatory factors of relative ground-water age, 

depth to the top of the uppermost perforation, and land use are 

good predictors of aquifer susceptibility in both the Fresno and 

the Modesto study areas. VOCs were detected more frequently 

in wells withdrawing young ground water (water recharged 

after 1952) than in wells withdrawing old ground water (water 

recharged before 1952). Wells withdrawing young ground 

water also have a higher number of VOCs detections than 

wells withdrawing old ground water. The detection of VOCs in 

wells withdrawing old ground water implies that point sources 

of contamination and (or) preferential flow paths may be, or 

may have been, pathways of contamination in these study 

areas. These results indicate that wells withdrawing young 

ground water are more susceptible to contamination than wells 

withdrawing old ground water.

VOCs were detected more frequently in shallow than in 

deep wells in the Fresno and the Modesto study areas, indicating that the downward migration of contaminants is a viable 

pathway for aquifer contamination. These results suggest that 

depth to the top of the uppermost perforation is an important 

variable to consider when determining aquifer susceptibility. 

This is most likely especially true in unconfined and partly 

confined aquifer systems that are not only found in these study 

areas, but in many areas throughout California where ground 

water is an important source of public supply.

Land use showed the strongest correlation to VOC occurrence of all the explanatory variables analyzed in this study. 

Urban land use was the major type of land use in both the 

Fresno and the Modesto study areas. The history of production and use of a compound may be the ultimate predictor of 

whether or not it will be found in the environment. In urban 

areas, activities are diverse and require the use of many VOCs; 

these chemicals can be introduced into the subsurface environment through spills, storage tank leaks, and improper disposal. 

Therefore, it is reasonable to expect VOCs to be primarily 

associated with urban land use, just as it may be reasonable to 

expect pesticides to be primarily associated with agricultural 

land use.

Overall, nitrate concentration was only a marginally 

useful predictor of VOC occurrence in this study; VOCs were 

not significantly correlated to higher nitrate concentrations in 

either study area. The relation between VOCs and nitrate may 

be a reflection of the labile nature of nitrate under the anaerobic conditions that often occur in aquifer systems. Stable isotopic content of ground water and LUFT density were not useful 

in the prediction of aquifer susceptibility in the Fresno and the 

Modesto study areas. Stable isotopic content of samples did 

not provide insight into the susceptibility to contamination; 

samples with VOC detections spanned the range of isotopic 

values that composed the local meteoric water lines in Fresno 

and Modesto, and therefore did not implicate a specific source 

of water that may be more susceptible to contamination than 

other water sources. The density of LUFTs was not important 

simply because the compounds stored in these tanks were 

rarely detected in ground-water samples. Gasoline components 

accounted for less the 2 percent of the VOCs detected.

Stable isotopic composition of ground-water samples 

in Fresno plots over a wide range of values that are nearly 

parallel and close to the GMWL (fig. 12A), indicating a 

mixture of recharge from local precipitation and recharge from 

Sierra Nevada runoff. Samples that have no VOC detections 

plot within the same range of isotopic values as samples with 

VOC detections. In addition, the regression lines for samples 

with VOC detections and samples without VOC detections 

are nearly coincident with one another and with the local 

meteoric water line. This suggests that the source of groundwater recharge is not an important factor in predicting VOC 

occurrence and distribution in Fresno.

In Modesto, the isotopic composition of ground water 

also plots over a wide range of values that are parallel and 

close to the GMWL (fig. 12B), indicating a mixture of 

recharge water from local precipitation and Sierra Nevada 

runoff. Samples with detections of VOCs and samples without 

detections of VOCs plot over a similar range of values. The 

regression lines for these two sample types are almost coincident, indicating that the source of ground water is not an 

important factor in predicting the occurrence and distribution 

of VOCs in Modesto.
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26 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002
Appendixes

Appendixes 27
Appendix 1. Identification and construction information for sampled wells in the Fresno and Modesto study areas, California, in 

2002 and 2001, respectively.

[CAS, California Aquifer Susceptibility program; LSD, land surface datum; FRCAS, Fresno study area; MODCAS, Modesto study area. ft, foot; --, no data]

CAS identification 

No.

Well depth

(ft below LSD)

Top perforation

(ft below LSD)

Bottom 

perforation, 

(ft below LSD)

FRCAS-01 450 180 450

FRCAS-02 510 235 500

FRCAS-03 760 200 400

FRCAS-04 447 225 447

FRCAS-05 430 190 430

FRCAS-06 770 180 760

FRCAS-07 420 150 420

FRCAS-08 332 192 248

FRCAS-09 156 220 257

FRCAS-10 610 250 600

FRCAS-11 312 176 --

FRCAS-12 681 213 670

FRCAS-13 480 180 470

FRCAS-14 270 152 268

FRCAS-15 332 116 220

FRCAS-16 500 175 500

FRCAS-17 397 294 387

FRCAS-18 410 160 400

FRCAS-19 580 350 570

FRCAS-20 660 400 640

FRCAS-21 550 445 540

FRCAS-22 420 150 240

FRCAS-23 409 168 409

FRCAS-24 402 172 402

FRCAS-25 355 135 345

FRCAS-26 440 200 440

FRCAS-27 390 180 390

FRCAS-28 203 160 168

FRCAS-29 262 226 231.5

FRCAS-30 650 320 640

FRCAS-31 440 270 440

FRCAS-32 213.5 208 213.5

FRCAS-33 540 150 510

FRCAS-34 430 210 430

FRCAS-35 294 154 294

FRCAS-36 280 146 259

FRCAS-37 396 200 301

FRCAS-38 520 190 510

FRCAS-39 450 150 450

FRCAS-40 640 150 630

FRCAS-41 500 265 500

FRCAS-42 270 200 260

FRCAS-43 -- 158 266

FRCAS-44 510 280 500

FRCAS-45 735 400 725

CAS identification 

No.

Well depth

(ft below LSD)

Top perforation

(ft below LSD)

Bottom 

perforation, 

(ft below LSD)

FRCAS-46 340 195 202

FRCAS-47 302 140 300

FRCAS-48 630 190 620

FRCAS-49 420 140 420

FRCAS-50 371 187 246

MODCAS-01 92 92 118

MODCAS-02 136 92 138

MODCAS-03 231 137 225

MODCAS-04 109 109 --

MODCAS-05 152 104 148

MODCAS-06 296 200 292

MODCAS-07 133 133 --

MODCAS-08 104 104 332

MODCAS-09 100 100 --

MODCAS-10 234 175 234

MODCAS-11 244 104 232

MODCAS-12 294 144 294

MODCAS-13 242 104 194

MODCAS-14 277 106 277

MODCAS-15 391 91 366

MODCAS-16 295 200 292

MODCAS-17 255 92 216

MODCAS-18 260 160 210

MODCAS-19 263 180 --

MODCAS-20 189 121 149

MODCAS-21 312 74 274

MODCAS-22 204 116 120

MODCAS-23 100 100 112

MODCAS-24 172 132 164

MODCAS-25 138 197 226

MODCAS-26 128 128 --

MODCAS-27 123 94 123

MODCAS-28 302 96 278

MODCAS-29 415 110 410

MODCAS-30 280 164 276

MODCAS-31 322 130 320

MODCAS-32 305 145 305

MODCAS-33 268 228 268

MODCAS-34 256 248 256

MODCAS-35 275 139 271

MODCAS-36 221 105 213

MODCAS-37 255 180 255

MODCAS-38 233 97 229

MODCAS-39 245 129 241

MODCAS-40 265 180 260

28 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002
Appendix 2. Summary of volatile organic compounds detected in quality-control blanks and associated environmental samples for 

the Fresno and Modesto study areas, California, in 2002 and 2001, respectively.

[VOC, volatile organic compound; FRCAS, Fresno study area; MODCAS, Modesto study area; μg/L, microgram per liter; na, not applicable]

Study/VOC

Quality-control blanks Maximum 

concentration 

(μg/L detected in 

blank samples)

Minimum 

concentration

(μg/L detected in 

environmental 

samples)

Number of 

potentially 

affected 

environmental 

samples

Number of 

environmental 

samples 

flagged

Number of 

equipment blank 

detections/

analyses

Number of 

trip blank 

detections/

analyses

Number of 

field blank 

detections/

analyses

FRCAS

No detections na na 0/5 na na na na

MODCAS

Methylbenzene (toluene) 1/1 1/1 1/4 0.03 0.01 2 2

Tetrachloroethene (PCE) 0/1 0/1 2/4 .02 .01 6 6

Dichloromethane, μg/L 

(Methylene chloride)

0/1 0/1 1/14 .02 .05 3 0

1

VOC also detected in associated source solution blank.

Appendixes 29
CAS identification 

No.

Deuterium/

protium, 

ratio per mil 

(82082)

Oxygen-18/

oxygen-16,

ratio per mil

(82085)

Tritium level 

(pCi/L)

FRCAS-01 -61.5 -7.88 5.14

FRCAS-02 -74.3 -9.77 4.09

FRCAS-03 -81.1 -10.88 6.83

FRCAS-04 -85.5 -11.53 22.00

FRCAS-05 -87.4 -11.72 8.35

FRCAS-06 -90.1 -12.11 <1.0

FRCAS-07 -81.7 -10.71 16.53

FRCAS-08 -58.9 -7.59 16.90

FRCAS-09 -65.7 -8.52 11.09

FRCAS-10 -74.2 -9.83 2.69

FRCAS-11 -84 -11.11 10.90

FRCAS-12 -71.5 -9.56 14.63

FRCAS-13 -73 -9.87 16.19

FRCAS-14 -80.7 -11.02 18.01

FRCAS-15 -77.8 -10.48 21.89

FRCAS-16 -58 -7.41 10.49

FRCAS-17 -86.8 -11.69 19.59

FRCAS-18 -78.4 -10.38 2.12

FRCAS-19 -71.8 -9.46 <1.0

FRCAS-20 -59.8 -7.62 3.76

FRCAS-21 -74.2 -10.06 <1.0

FRCAS-22 -77.9 -10.13 4.10

FRCAS-23 -93.6 -12.5 1.78

FRCAS-24 -93.9 -12.4 12.13

FRCAS-25 -82.5 -11.21 19.10

FRCAS-26 -81.4 -10.77 <1.0

FRCAS-27 -77.3 -10.55 6.06

FRCAS-28 -82.1 -11.1 2.36

FRCAS-29 -77.9 -10.56 4.64

FRCAS-30 -81.1 -10.93 <1.0

FRCAS-31 -75.8 -9.85 2.40

FRCAS-32 -65.9 -8.47 28.32

FRCAS-33 -91.5 -12.21 24.55

FRCAS-34 -78.6 -10.44 11.31

FRCAS-35 -74.3 -9.84 18.20

FRCAS-36 -64.6 -8.17 16.53

FRCAS-37 -90.9 -12.01 --

FRCAS-38 -73.4 -10.03 13.11

FRCAS-39 -87.1 -11.63 45.73

FRCAS-40 -86.6 -11.37 9.28

FRCAS-41 -69.9 -9.13 19.83

FRCAS-42 -82.5 -11.02 15.75

FRCAS-43 -73.3 -9.61 15.92

FRCAS-44 -90.3 -11.75 15.90

FRCAS-45 -87.6 -11.94 1.08

CAS identification 

No.

Deuterium/

protium, 

ratio per mil 

(82082)

Oxygen-18/

oxygen-16,

ratio per mil

(82085)

Tritium level 

(pCi/L)

FRCAS-46 -82 -11.1 9.64

FRCAS-47 -63 -8.38 22.81

FRCAS-48 -68.4 -8.85 6.82

FRCAS-49 -63.2 -8.45 18.25

FRCAS-50 -84 -11.03 9.87

MODCAS-01 -73.9 -9.98 2.22

MODCAS-02 -81.3 -11.01 9.54

MODCAS-03 -82.8 -11.26 22.52

MODCAS-04 -79.4 -10.88 17.48

MODCAS-05 -74.8 -9.93 9.68

MODCAS-06 -72.3 -9.56 18.71

MODCAS-07 -71.9 -9.7 5.27

MODCAS-08 -77.7 -10.37 31.69

MODCAS-09 -92.6 -12.15 9.18

MODCAS-10 -76.5 -10.16 12.96

MODCAS-11 -72 -9.61 14.03

MODCAS-12 -66.8 -9.17 16.36

MODCAS-13 -69.7 -9.44 3.58

MODCAS-14 -66.3 -8.96 7.34

MODCAS-15 -67.4 -8.94 3.33

MODCAS-16 -71.7 -9.82 17.63

MODCAS-17 -74.5 -10.01 16.32

MODCAS-18 -67.8 -9.26 1.79

MODCAS-19 -67.3 -9.15 5.34

MODCAS-20 -80.1 -10.4 17.77

MODCAS-21 -64.5 -8.75 10.92

MODCAS-22 -71.7 -9.76 <1.0

MODCAS-23 -71.3 -9.73 <1.0

MODCAS-24 -79.8 -10.8 31.70

MODCAS-25 -83.1 -10.89 1.53

MODCAS-26 -84.6 -11.38 21.21

MODCAS-27 -83.5 -11.38 39.44

MODCAS-28 -70.7 -9.48 26.25

MODCAS-29 -76.9 -10.7 1.00

MODCAS-30 -82 -11.2 <1.0

MODCAS-31 -79.1 -10.74 5.29

MODCAS-32 -78.3 -10.41 20.74

MODCAS-33 -77 -10.49 45.40

MODCAS-34 -71.8 -9.89 70.97

MODCAS-35 -74.2 -10.24 5.92

MODCAS-36 -79.5 -10.6 71.71

MODCAS-37 -69.6 -9.42 9.59

MODCAS-38 -74.7 -10.09 68.14

MODCAS-39 -69.6 -9.04 37.87

MODCAS-40 -76.6 -10.32 <1.0

Appendix 3a. Isotopic content of environmental samples collected in the Fresno and Modesto study areas, California, in 2002 and 

2001, respectively.

[Number in parentheses is the data parameter code, a five-digit code used in the U.S. Geological Survey’s computerized data system, National Water Information System, to uniquely identify a specific constituent or property. Tritium levels were determined by Lawrence Livermore National Laboratory. CAS, 

California Aquifer Susceptibility program; FRCAS, Fresno study area; MODCAS, Modesto study area. per mil, parts per thousand; pCi/L, picocurie per liter; 

<1.0, below method detection limits (MDL) (0.5 pCi/L was used to compute statistical analysis of tritium data that was below MDL). --, no data]

30 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002
Appendix 3b. Nitrate concentrations obtained from the California Department of Health Services database for wells sampled.

[Nitrate concentration is the median value of all analyses completed in the 12-month period before sampling. CAS, California Aquifer Susceptibility program; Nitrate-N, nitrate nitrogen; FRCAS, Fresno study area; MODCAS, Modesto study area. mg/L, milligram per liter]

CAS identification No. Nitrate-N, mg/L

FRCAS-01 4.07

FRCAS-02 2.71

FRCAS-03 2.03

FRCAS-04 2.26

FRCAS-05 1.81

FRCAS-06 1.81

FRCAS-07 1.58

FRCAS-08 3.16

FRCAS-09 4.07

FRCAS-10 4.75

FRCAS-11 1.58

FRCAS-12 1.81

FRCAS-13 .68

FRCAS-14 3.84

FRCAS-15 9.94

FRCAS-16 2.94

FRCAS-17 5.88

FRCAS-18 8.36

FRCAS-19 2.71

FRCAS-20 2.94

FRCAS-21 .90

FRCAS-22 5.42

FRCAS-23 4.97

FRCAS-24 4.97

FRCAS-25 6.55

FRCAS-26 3.39

FRCAS-27 3.39

FRCAS-28 4.52

FRCAS-29 5.42

FRCAS-30 4.52

FRCAS-31 7.46

FRCAS-32 2.94

FRCAS-33 4.52

FRCAS-34 2.03

FRCAS-35 1.13

FRCAS-36 6.33

FRCAS-37 5.42

FRCAS-38 .68

FRCAS-39 4.52

FRCAS-40 2.26

FRCAS-41 3.16

FRCAS-42 .90

FRCAS-43 6.33

FRCAS-44 2.03

FRCAS-45 4.07

CAS identification No. Nitrate-N, mg/L

FRCAS-46 5.65

FRCAS-47 3.84

FRCAS-48 3.84

FRCAS-49 3.16

FRCAS-50 3.16

MODCAS-01 1.75

MODCAS-02 6.38

MODCAS-03 4.15

MODCAS-04 2.75

MODCAS-05 3.53

MODCAS-06 3.45

MODCAS-07 .74

MODCAS-08 2.01

MODCAS-09 1.84

MODCAS-10 3.73

MODCAS-11 3.62

MODCAS-12 9.29

MODCAS-13 2.99

MODCAS-14 .88

MODCAS-15 8.28

MODCAS-16 4.14

MODCAS-17 2.15

MODCAS-18 1.00

MODCAS-19 6.63

MODCAS-20 .23

MODCAS-21 4.03

MODCAS-22 5.02

MODCAS-23 9.51

MODCAS-24 4.75

MODCAS-25 .23

MODCAS-26 4.00

MODCAS-27 5.74

MODCAS-28 6.00

MODCAS-29 1.84

MODCAS-30 2.77

MODCAS-31 2.53

MODCAS-32 1.31

MODCAS-33 3.61

MODCAS-34 1.95

MODCAS-35 3.47

MODCAS-36 4.72

MODCAS-37 1.95

MODCAS-38 4.04

MODCAS-39 1.08

MODCAS-40 5.40

Appendixes 31
Appendix 4a. Wells with volatile organic compound detections in the Fresno study area, California, 2002. 

[CAS, California Aquifer Susceptibility program. ymd, year, month, day. Percentage values are detection frequencies. Number in parentheses is the data parameter code, a five-digit code used in the U.S. Geological Survey computerized data system, National Water Information System, to uniquely identify a specific constituent or property. LRL, laboratory reporting limit; FRCAS, Fresno study area. μg/L, microgram per liter; VOC, volatile organic compound; E, estimated value; --, not detected]

CAS 

identification 

No.

Date

(ymd) Time

Disinfection byproduct

(56 percent)

Solvent

(58 percent)

Trichloromethane 

(Chloroform), 

μg/L

(32106) 

Bromodichloromethane,

μg/L 

(32101)

Bromoform 

(Tribromomethane),

μg/L 

(32104)

Tetrachloroethylene 

(PCE),

μg/L 

(34475)

1,2-Dichloropropane,

μg/L

(34541)

Trichloroethylene 

(TCE),

μg/L

(39180)

cis-1,2-

Dichloroethylene, 

μg/L

(77093)

Tetrachloromethane 

(Carbon 

tetrachloride), 

μg/L

(32102)

1,1-

Dichloroethane, 

μg/L

(34496)

1,2-

Dibromoethane, 

μg/L

(77651)

1,2-

Dichlorobenzene, 

μg/L

(34536)

1,2,3-

Trichloropropane, 

μg/L

(77443)

1,2-

Dichloroethane,

μg/L

(32103)

LRL <0.024 <0.048 <0.06 <0.027 <0.029 <0.038 <0.038 <0.06 <0.035 <0.036 <0.031 <0.16 <0.13

FRCAS-01 20020715 0800 E0.01 -- -- E0.01 E0.04 -- -- -- -- -- -- -- --FRCAS-02 20020715 0930 -- -- -- -- E.01 -- -- -- -- -- -- -- --FRCAS-03 20020715 1100 -- -- -- -- E.02 -- -- -- -- -- -- -- --FRCAS-04 20020715 1140 E.01 -- -- -- E.01 -- -- -- -- -- -- -- --FRCAS-05 20020715 1240 E.02 -- -- -- -- -- -- -- -- -- -- -- --FRCAS-06 20020715 1340 -- -- -- -- E.02 -- -- -- -- -- -- -- --FRCAS-07 20020715 1440 E.03 -- -- -- E.01 -- -- -- -- -- -- -- --FRCAS-08 20020716 0820 E.09 E0.01 -- E.05 E.02 E0.06 -- -- -- -- E0.01 -- --FRCAS-09 20020716 0940 E.03 -- -- E.02 E.02 -- -- -- -- -- -- -- --FRCAS-10 20020716 1040 E.07 -- -- .20 E.02 .18 1.11 -- -- -- -- -- --FRCAS-11 20020716 1140 E.02 -- -- E.03 -- -- -- -- -- -- -- -- --FRCAS-12 20020716 1300 -- -- -- -- E.02 -- -- -- -- -- -- -- --FRCAS-14 20020716 1530 .16 -- -- 1.42 -- 59.55 E.09 -- -- -- -- -- --FRCAS-15 20020717 0830 E.02 -- -- -- -- -- -- -- -- -- -- -- --FRCAS-16 20020717 0950 -- -- -- E.01 E.04 -- -- -- -- -- -- -- --FRCAS-18 20020717 1140 -- -- -- -- -- -- -- -- -- -- -- -- --FRCAS-23 20020718 1030 E.02 -- -- E.02 -- E.09 -- -- -- -- -- -- --FRCAS-25 20020718 1130 .21 E.04 E0.07 .38 E.02 E.05 .11 -- E0.04 E0.05 -- -- --FRCAS-26 20020718 1250 E.02 -- -- -- -- -- -- -- -- -- -- -- 0.14FRCAS-27 20020718 1340 E.03 -- -- .79 -- -- -- E0.05 -- -- -- -- --FRCAS-28 20020718 1500 E.07 -- -- E.05 -- -- -- E.07 -- -- -- -- --FRCAS-29 20020719 0810 E.04 -- -- E.03 -- -- -- -- -- -- -- -- --FRCAS-31 20020719 1020 -- -- -- E.02 -- -- -- -- -- -- -- -- --FRCAS-32 20020729 0820 E.09 -- -- .42 -- -- -- -- -- -- -- -- --FRCAS-33 20020729 1120 E.01 -- -- -- -- -- -- -- -- -- -- -- --FRCAS-34 20020729 1250 E.10 -- -- -- -- -- -- -- -- -- -- -- --FRCAS-35 20020729 1410 E.08 -- -- E.04 -- -- -- -- -- -- -- -- --FRCAS-36 20020730 0800 E.06 -- -- -- .16 E.04 -- -- -- -- -- 0.35 --FRCAS-37 20020730 0850 E.04 -- -- .11 -- E.03 -- -- E.03 -- -- -- --FRCAS-40 20020730 1240 -- -- -- .20 -- -- -- -- -- -- -- -- --FRCAS-42 20020730 1450 E.03 -- -- .10 -- -- -- -- -- -- -- -- --FRCAS-43 20020731 0810 E.05 -- -- -- -- -- -- -- -- -- -- -- --FRCAS-46 20020731 1210 E.04 -- -- E.06 -- -- -- -- -- -- -- -- --FRCAS-47 20020731 1340 E.03 -- -- -- -- -- -- -- -- -- -- -- --FRCAS-49 20020801 0810 E.05 -- -- E.02 -- -- -- -- -- -- -- -- --FRCAS-50 20020801 0920 E.02 -- -- E.03 E.03 -- -- -- -- -- -- -- --

Wells with VOC detections 28 2 1 21 14 7 3 2 2 1 1 1 1Detection frequency (percentage) 56 4 2 42 28 14 6 4 4 2 2 2 2

32 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002
CAS 

identification 

No.

Date

(ymd) Time

Refrigerant

(10 percent)

Organic synthesis

(2 percent)

Fumigant

(2 percent)

Gasoline

(2 percent)

Number 

of VOCs 

detected

Dichlorodifluoromethane 

(CFC-12),

μg/L

(34668)

1,1,2-Trichlorotrifluoroethane 

(CFC-113),

μg/L

(77652) 

Trichlorofluoromethane 

(CFC-11), 

μg/L

(34488)

1,1-Dichloroethylene 

(DCE),

μg/L

(34501)

1,2-Dibromo-3-

chloropropane 

(DBCP),

μg/L

(82625)

Methylbenzene 

(Toluene),

μg/L

(34010)

LRL <0.18 <0.06 <0.09 <0.04 <0.5 <0.05

FRCAS-01 20020715 0800 E0.08 -- -- -- -- -- 4FRCAS-02 20020715 0930 -- -- -- -- -- -- 1FRCAS-03 20020715 1100 -- -- -- -- -- -- 1FRCAS-04 20020715 1140 -- -- -- -- -- -- 2FRCAS-05 20020715 1240 -- -- -- -- -- -- 1FRCAS-06 20020715 1340 -- -- -- -- -- -- 1FRCAS-07 20020715 1440 -- -- -- -- -- -- 2FRCAS-08 20020716 0820 -- -- -- -- -- -- 6FRCAS-09 20020716 0940 -- -- -- -- -- -- 3FRCAS-10 20020716 1040 E17.97 -- -- -- -- -- 6FRCAS-11 20020716 1140 -- -- -- -- -- -- 2FRCAS-12 20020716 1300 -- -- -- -- -- -- 1FRCAS-14 20020716 1530 E.16 -- -- -- -- -- 5FRCAS-15 20020717 0830 -- -- -- -- -- -- 1FRCAS-16 20020717 0950 -- -- -- -- -- -- 2FRCAS-18 20020717 1140 -- -- -- -- 0.65 -- 1FRCAS-23 20020718 1030 -- -- -- E0.04 -- -- 4FRCAS-25 20020718 1130 -- -- -- -- -- -- 9FRCAS-26 20020718 1250 -- -- -- -- -- -- 2FRCAS-27 20020718 1340 -- -- -- -- -- -- 3FRCAS-28 20020718 1500 -- -- -- -- -- -- 3FRCAS-29 20020719 0810 -- -- -- -- -- -- 2FRCAS-31 20020719 1020 -- -- -- -- -- -- 1FRCAS-32 20020729 0820 -- -- -- -- -- -- 2FRCAS-33 20020729 1120 -- -- -- -- -- -- 1FRCAS-34 20020729 1250 -- -- -- -- -- -- 1FRCAS-35 20020729 1410 -- -- -- -- -- -- 2FRCAS-36 20020730 0800 -- E0.07 -- -- -- -- 5FRCAS-37 20020730 0850 -- -- -- -- -- -- 4FRCAS-40 20020730 1240 -- -- -- -- -- -- 1FRCAS-42 20020730 1450 -- -- -- -- -- -- 2FRCAS-43 20020731 0810 -- -- -- -- -- -- 1FRCAS-46 20020731 1210 -- -- E0.08 -- -- E0.01 4FRCAS-47 20020731 1340 -- -- -- -- -- -- 1FRCAS-49 20020801 0810 -- -- -- -- -- -- 2FRCAS-50 20020801 0920 -- -- -- -- -- -- 3

Wells with VOC detections 3 1 1 1 1 1Detection frequency (percentage) 6 2 2 2 2 2

Appendix 4a. Wells with volatile organic compound detections in the Fresno study area, California, 2002. —Continued[CAS, California Aquifer Susceptibility program. ymd, year, month, day. Percentage values are detection frequencies. Number in parentheses is the data parameter code, a five-digit code used in the U.S. Geological Survey computerized data system, National Water Information System, to uniquely identify a specific constituent or property. LRL, laboratory reporting limit; FRCAS, Fresno study area. μg/L, microgram per liter; VOC, volatile organic compound; E, estimated value; --, not detected]

Appendixes 33
Appendix 4b. Wells with volatile organic compound detections in the Modesto study area, California, 2001.

[CAS, California Aquifer Susceptibility program. ymd, year, month, day. Percentage values are detection frequencies. Number in parentheses is the data parameter code, a five-digit code used in the U.S. Geological Survey computerized data system, National Water Information System, to uniquely identify a specific constituent or property. LRL, laboratory reporting limit; MODCAS, Modesto study area. μg/L, microgram per liter; VOC, volatile organic compound; E, estimated value; --, not detected]

CAS

identification

No.

Date

(ymd) Time

Disinfection byproduct

(92 percent)

Solvent

(70 percent)

Chloroform

(Trichloromethane ),

μg/L

(32106)

Bromodichloromethane,

μg/L

(32101)

Bromoform 

(Tribromomethane),

μg/L

(32104)

Dibromochloromethane, 

μg/L

(32105)

Tetrachloroethylene 

(PCE),

μg/L

(34475)

Trichloroethylene 

(TCE),

μg/L

(39180)

Dibromomethane, 

μg/L

(30217)

Tetrachloromethane 

(Carbon

tetrachloride),

μg/L 

(32102)

1,2-Dichloropropane,

μg/L

(34541)

cis-1,2-

Dichloroethylene, 

μg/L

(77093)

1,1-Dichloroethane,

μg/L

(34496)

1,1,1-Trichloroethane

(TCA),

μg/L

(34506)

LRL <0.024 <0.048 <0.06 <0.18 <0.1 <0.038 <0.05 <0.06 <0.029 <0.038 <0.035 <0.032

MODCAS-01 20010327 0830 E0.09 -- -- -- -- -- -- 0.12 -- -- -- --MODCAS-02 20010327 1030 1.20 E0.08 -- -- 0.81 E0.01 -- .32 -- -- -- --MODCAS-03 20010327 1230 .59 E.06 -- -- E.04 E.03 -- -- -- -- -- --MODCAS-04 20010327 1330 E.03 -- -- -- .22 E.01 -- -- E0.04 -- -- --MODCAS-05 20010328 0800 .21 E.02 E0.09 -- E.04 -- -- E.02 -- -- -- --MODCAS-06 20010328 0940 -- -- E.07 -- -- -- -- -- -- -- -- --MODCAS-07 20010328 1050 E.02 E.02 -- -- -- -- -- -- -- -- -- --MODCAS-08 20010328 1230 E.02 -- -- -- .31 E.01 -- -- -- -- -- --MODCAS-09 20010328 1340 E.02 -- -- -- -- -- -- -- -- -- -- --MODCAS-10 20010329 0800 .31 E.03 -- -- -- -- -- -- -- -- E0.05 --MODCAS-11 20010329 0920 1.86 .25 E.08 E0.13 E.03 E.02 E0.03 -- -- -- -- --MODCAS-12 20010329 1020 2.58 .16 -- -- 5.60 -- -- -- -- -- -- --MODCAS-13 20010330 0820 .97 .21 E.07 E.10 -- -- E.05 -- -- -- -- E0.01MODCAS-14 20010330 0920 4.53 .61 .43 .41 E.04 -- E.05 -- -- -- -- --MODCAS-15 20010330 1020 .45 E.04 -- -- .13 -- -- -- -- -- -- --MODCAS-16 20010330 1110 1.19 .19 .21 E.16 E.04 6.13 -- -- -- E0.02 -- E.09MODCAS-17 20010403 0820 2.14 .29 .91 .18 .18 E.03 E.08 .99 -- -- -- --MODCAS-18 20010403 0930 E.08 -- -- -- -- -- -- -- -- -- -- --MODCAS-19 20010403 1130 .15 -- -- -- .71 -- -- -- -- -- -- --MODCAS-20 20010403 1330 18.14 1.04 -- E.07 3.29 .21 E.03 -- -- .25 -- --MODCAS-21 20010404 0820 1.85 .18 .40 E.12 1.57 -- -- E.05 -- -- -- E.01MODCAS-22 20010404 1020 5.25 .73 .23 .34 .50 -- E.05 -- -- -- -- --MODCAS-23 20010404 1230 .17 -- -- -- .13 E.01 -- -- -- -- -- --MODCAS-24 20010404 1350 E.07 -- -- -- E.04 -- -- -- -- -- -- --MODCAS-25 20010405 0820 1.19 E.09 -- -- -- -- -- -- -- -- -- --MODCAS-26 20010405 1030 1.98 .34 .47 E.16 .15 E.10 -- -- E.08 -- -- --MODCAS-27 20010405 1130 1.59 .23 1.29 .31 .17 E.02 E.05 -- -- -- E.02 --MODCAS-28 20010406 0830 3.85 .49 .54 .44 .12 -- .13 -- -- -- -- --MODCAS-30 20010406 1100 .48 .11 .19 E.12 -- -- -- -- -- -- -- --MODCAS-31 20010406 1220 5.04 .93 .37 .63 -- -- -- -- -- -- -- --MODCAS-32 20010410 0840 .67 .12 .32 E.15 1.96 3.01 E.06 -- -- E.03 E.02 --MODCAS-33 20010410 1000 1.50 .32 1.19 .35 -- -- -- -- E.03 -- -- --MODCAS-34 20010410 1110 1.79 .39 1.64 .45 -- -- E.10 -- -- -- -- --MODCAS-35 20010410 1240 E.08 E.03 -- -- -- -- -- E.09 -- -- -- --MODCAS-36 20010410 1420 E.09 -- .21 E.05 -- E.01 -- -- -- -- -- --MODCAS-37 20010411 0830 6.12 .56 .10 E.18 -- -- -- -- -- -- -- --MODCAS-38 20010411 0940 E.04 -- -- -- -- -- -- -- -- -- -- --

Wells with VOC detections 36 26 19 18 21 13 10 6 3 3 3 3Detection frequency (percent) 90 65 48 45 52 32 25 15 8 8 8 8

34 Assessing Susceptibility to Contamination of Two Aquifer Systems, Modesto and Fresno, California, 2001 and 2002
CAS

identification

No.

Date

(ymd) Time

Solvent

(70 percent)

Refrigerant

(25 percent)

Organic 

Synthesis

(2 percent)

Gasoline

(5 percent)

Fumigant

(2 percent)

Number 

of VOCs 

detected

Dichloromethane 

(Methylene 

chloride),

μg/L 

(34423)

1,2-Dibromoethane,

μg/L

(77651)

1,2-Dichlorobenzene,

μg/L

(34536)

trans-1,2-

Dichloroethylene, 

μg/L

(34546)

Dichlorodifluoromethane 

(CFC-12), 

μg/L 

(34668)

1,1,2-Trichlorotrifluoroethane 

(CFC-113),

μg/L

(77652)

Trichlorofluoromethane 

(CFC-11), 

μg/L 

(34488)

1,1-Dichloroethylene 

(DCE),

μg/L

(34501)

Methyl tertbutyl ether 

(MTBE),

μg/L

(78032)

1,2-Dibromo3-chloropropane

(DBCP),

μg/L

(82625)

LRL <0.16 <0.036 <0.031 <0.032 <0.27 <0.06 <0.09 <0.04 <0.17 <0.21

MODCAS-01 20010327 0830 -- -- -- -- -- -- 0.27 -- -- -- 3MODCAS-02 20010327 1030 -- -- E0.01 -- E0.74 -- .15 -- -- -- 8MODCAS-03 20010327 1230 -- -- -- -- -- -- -- -- -- -- 4MODCAS-04 20010327 1330 -- E0.03 -- -- -- -- -- -- -- -- 5MODCAS-05 20010328 0800 -- -- -- -- -- -- -- -- -- -- 5MODCAS-06 20010328 0940 -- -- -- -- -- -- -- -- -- -- 1MODCAS-07 20010328 1050 -- -- -- -- -- -- -- -- -- -- 2MODCAS-08 20010328 1230 -- -- -- -- -- 0.25 -- E0.028 -- -- 5MODCAS-09 20010328 1340 -- -- -- -- E.06 -- -- -- -- -- 2MODCAS-10 20010329 0800 -- -- -- -- -- -- -- E.049 -- -- 4MODCAS-11 20010329 0920 -- -- -- -- -- -- -- -- -- -- 7MODCAS-12 20010329 1020 -- -- -- -- -- -- -- -- E0.06 -- 4MODCAS-13 20010330 0820 -- -- -- -- -- -- -- -- -- -- 6MODCAS-14 20010330 0920 -- -- -- -- -- -- -- -- -- -- 6MODCAS-15 20010330 1020 -- -- -- -- -- -- -- -- -- -- 3MODCAS-16 20010330 1110 -- -- -- E0.02 -- 15.49 -- E.095 -- -- 11MODCAS-17 20010403 0820 -- -- -- -- E.92 -- -- -- -- -- 9MODCAS-18 20010403 0930 -- -- -- -- -- -- -- -- -- -- 1MODCAS-19 20010403 1130 -- -- -- -- E.07 -- -- -- -- -- 3MODCAS-20 20010403 1330 0.23 -- -- -- -- -- -- E.021 E.07 -- 10MODCAS-21 20010404 0820 -- -- -- -- -- -- -- -- -- -- 7MODCAS-22 20010404 1020 -- -- -- -- E.14 -- -- -- -- -- 7MODCAS-23 20010404 1230 -- -- -- -- -- -- -- -- -- -- 3MODCAS-24 20010404 1350 -- -- -- -- -- -- -- -- -- -- 2MODCAS-25 20010405 0820 -- -- -- -- -- -- -- -- -- -- 2MODCAS-26 20010405 1030 E.06 -- -- -- -- -- -- -- -- -- 8MODCAS-27 20010405 1130 E.05 -- -- -- -- -- -- -- -- -- 9MODCAS-28 20010406 0830 -- -- -- -- E.07 -- -- -- -- -- 7MODCAS-30 20010406 1100 -- -- -- -- -- -- -- -- -- -- 4MODCAS-31 20010406 1220 -- -- -- -- -- -- -- -- -- -- 4MODCAS-32 20010410 0840 -- -- -- -- E.06 E.01 -- E.036 -- -- 12MODCAS-33 20010410 1000 -- -- -- -- -- -- -- -- -- 0.26 6MODCAS-34 20010410 1110 -- -- -- -- -- -- -- -- -- -- 5MODCAS-35 20010410 1240 -- -- -- -- -- -- -- -- -- -- 3MODCAS-36 20010410 1420 -- -- -- -- -- -- -- -- -- -- 4MODCAS-37 20010411 0830 -- -- -- -- -- -- -- -- -- -- 4MODCAS-38 20010411 0940 -- -- -- -- -- -- -- -- -- -- 1

Wells with VOC detections 3 1 1 1 7 3 2 5 2 1Detection frequency (percent) 8 2 2 2 18 8 5 12 5 2

Appendix 4b. Wells with volatile organic compound detections in the Modesto study area, California, 2001.—Continued[CAS, California Aquifer Susceptibility program. ymd, year, month, day. Percentage values are detection frequencies. Number in parentheses is the data parameter code, a five-digit code used in the U.S. Geological Survey computerized data system, National Water Information System, to uniquely identify a specific constituent or property. LRL, laboratory reporting limit; MODCAS, Modesto study area. μg/L, microgram per liter; VOC, volatile organic compound; E, estimated value; --, not detected]

Appendixes 35
Wright and others—Assessing Susceptibility to Contamination of Aquifer Systems in the Modesto and Fresno, California—Scientific Investigations Report 2004–5149 

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