Scientific Investigations Report 2008–5156

National Water-Quality Assessment Program

Transport of Anthropogenic and Natural

Contaminants (TANC) to Public-Supply Wells 

U.S. Department of the Interior

U.S. Geological Survey

Hydrogeology, Water Chemistry, and Factors 

Affecting the Transport of Contaminants in the 

Zone of Contribution of a Public-Supply Well in 

Modesto, Eastern San Joaquin Valley, California

water table

ALLUVIAL

MORE CONSOLIDATED DEPOSITS

FAN DEPOSITS

Regional flow

FLOOD-PLAIN

DEPOSITS

Confining unit

AGRICULTURAL RECHARGE

public-supply wells irrigation wells

river

Aquifer

URBAN RECHARGE

NOT TO SCALE

Hydrogeology, Water Chemistry, and Factors 

Affecting the Transport of Contaminants in 

the Zone of Contribution of a Public-Supply 

Well in Modesto, Eastern San Joaquin 

Valley, California

By Bryant C. Jurgens, Karen R. Burow, Barbara A. Dalgish, and Jennifer L. Shelton

National Water-Quality Assessment Program

Transport of Anthropogenic and Natural Contaminants (TANC) 

to Public-Supply Wells

Scientific Investigations Report 2008–5156

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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

reproduce any copyrighted materials contained within this report.

Suggested reference:

Jurgens, B.C., Burow, K.R., Dalgish, B.A., and Shelton, J.L., 2008, Hydrogeology, water chemistry, and factors affecting the transport of contaminants in the zone of contribution of a public-supply well in Modesto, eastern San Joaquin 

Valley, California: U.S. Geological Survey Scientific Investigations Report 2008–5156, 78 p. Available at http://pubs.

usgs.gov/sir/2008/5156
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with credible scientific information that helps to 

enhance and protect the overall quality of life and that facilitates effective management of water, biological, energy, and mineral 

resources (http://www.usgs.gov/). Information on the Nation’s water resources is critical to ensuring long-term availability of 

water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish and wildlife. Population growth and 

increasing demands for water make the availability of that water, now measured in terms of quantity and quality, even more 

essential to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support national, regional, State, 

and local information needs and decisions related to water-quality management and policy (http://water.usgs.gov/nawqa). The 

NAWQA Program is designed to answer: What is the condition of our Nation’s streams and ground water? How are conditions 

changing over time? How do natural features and human activities affect the quality of streams and ground water, and where 

are those effects most pronounced? By combining information on water chemistry, physical characteristics, stream habitat, and 

aquatic life, the NAWQA Program aims to provide science-based insights for current and emerging water issues and priorities. 

From 1991–2001, the NAWQA Program completed interdisciplinary assessments and established a baseline understanding 

of water-quality conditions in 51 of the Nation’s river basins and aquifers, referred to as Study Units (http://water.usgs.gov/

nawqa/studyu.html).

Multiple national and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA Program as 42 of 

the 51 Study Units are reassessed. These assessments extend the findings in the Study Units by determining status and trends 

at sites that have been consistently monitored for more than a decade, and filling critical gaps in characterizing the quality of 

surface water and ground water. For example, increased emphasis has been placed on assessing the quality of source water 

and finished water associated with many of the Nation’s largest community water systems. During the second decade, NAWQA 

is addressing five national priority topics that build an understanding of how natural features and human activities affect water 

quality, and establish links between sources of contaminants, the transport of those contaminants through the hydrologic system, 

and the potential effects of contaminants on humans and aquatic ecosystems. Included are topics on the fate of agricultural 

chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient 

enrichment on aquatic ecosystems, and transport of contaminants to public-supply wells. These topical studies are conducted in 

those Study Units most affected by these issues; they comprise a set of multi-Study-Unit designs for systematic national assessment. In addition, national syntheses of information on pesticides, volatile organic compounds (VOCs), nutrients, selected trace 

elements, and aquatic ecology are continuing.

The USGS aims to disseminate credible, timely, and relevant science information to address practical and effective waterresource management and strategies that protect and restore water quality. We hope this NAWQA publication will provide you 

with insights and information to meet your needs, and will foster increased citizen awareness and involvement in the protection 

and restoration of our Nation’s waters.

The USGS recognizes that a national assessment by a single program cannot address all water-resource issues of interest. 

External coordination at all levels is critical for cost-effective management, regulation, and conservation of our Nation’s water 

resources. The NAWQA Program, therefore, depends on advice and information from other agencies—Federal, State, regional, 

interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, and other stakeholder groups. Your 

assistance and suggestions are greatly appreciated.

 Matthew C. Larsen

Acting Associate Director for Water
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v

Contents

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

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

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

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

Summary of Previous Water-Quality Studies...................................................................................5

Study Design...................................................................................................................................................6

Public-Supply Well Selection .............................................................................................................6

Monitoring Well Network ....................................................................................................................6

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

Drilling, Coring, and Well Installation ................................................................................................9

Sampling Periods and Collected Analytes .....................................................................................10

Water Sample Collection...................................................................................................................10

Laboratory Analytical Methods........................................................................................................11

Quality-Control Sample-Collection Methods .................................................................................12

Blanks. .........................................................................................................................................12

Replicates....................................................................................................................................12

Spikes. .........................................................................................................................................12

Quality-Control Summary .........................................................................................................13

PHREEQC Calculations.......................................................................................................................13

Hydrogeologic Setting.................................................................................................................................13

Geology.................................................................................................................................................13

Hydrofacies..........................................................................................................................................14

Ground-Water Occurrence and Movement...................................................................................16

Public-Supply Well Construction and Well-Bore Flow Characteristics ....................................16

Ground-Water Age ............................................................................................................................16

Carbon-14....................................................................................................................................18

Tritium. .........................................................................................................................................19

Tritium/Helium-3 ........................................................................................................................22

Sulfur Hexafluoride....................................................................................................................25

Combined Tritium/Helium-3 and SF6 Interpreted Ages ........................................................26

Aquifer and Public-Supply Well Vulnerability.......................................................................27

Water Chemistry .........................................................................................................................................27

Field Measurements...........................................................................................................................28

Reduction/Oxidation Conditions.......................................................................................................28

Stable Isotopes of Hydrogen and Oxygen ......................................................................................30

Pesticides and Volatile Organic Compounds .................................................................................30

Pesticides and Volatile Organic Compounds in Storm Runoff ...........................................31

Seasonal Variability of Volatile Organic Compound Occurrence......................................32

Nitrate and Sulfate .............................................................................................................................33

Stable Isotopes of Nitrate and Sulfate............................................................................................33

Arsenic..................................................................................................................................................38

Uranium ................................................................................................................................................39
vi

Uranium Isotopes................................................................................................................................41

Composition of Ground Water and Surface Water.......................................................................42

Depth-Dependent Sampling..............................................................................................................44

Well Construction and Operation Effects on Water-Quality........................................................47

Factors Affecting the Transport of Contaminants to the Public-Supply Well....................................49

Contaminants Reaching the Public-Supply Well...........................................................................49

Factors Affecting the Transport of Nitrate and Uranium..............................................................49

Historical and Long-term Vulnerability to Nitrate and Uranium Contamination ......................50

Summary and Conclusions.........................................................................................................................52

Acknowledgments.......................................................................................................................................53

References ...................................................................................................................................................54

Appendix A: Collection and Analysis of Aquifer Solids. ....................................................................61

Mineralogical Analyses and Carbon Content ................................................................................61

Arsenic and Uranium Extractions ...........................................................................................63

Uranium Adsorption Experiments ..........................................................................................65

Appendix B: Quality-Control Analysis...................................................................................................66

Major Ions, Nutrients, Trace elements, Dissolved Organic Carbon and Ultra-Violet 

Absorbance............................................................................................................................67

Blanks. .........................................................................................................................................67

Replicates....................................................................................................................................67

Pesticides and Volatile Organic Compounds (VOC)......................................................................67

Blanks. .........................................................................................................................................67

Replicates....................................................................................................................................69

Spikes (Surrogates and Matrix Spikes) .................................................................................69

Miscellaneous Compounds......................................................................................................70

Appendix C: Major ion composition, pH, and calcite saturation in ground water from 

monitoring wells......................................................................................................................................71

Appendix D: Volatile organic compounds (VOC) detected in ground water from 

monitoring wells......................................................................................................................................74

Appendix E: Pesticide compounds detected in ground water from monitoring wells and the 

public-supply well...................................................................................................................................77

Figures

Figure 1. Map showing study area near Modesto in the eastern San Joaquin 

Valley, California ........................................................................... 4

Figure 2. Map showing regional model simulation of 100-year contributing recharge 

area and contours of areas of recharge with higher probabilities of reaching the 

public-supply well ........................................................................... 7

Figure 3. Map showing location of monitoring wells installed for a study of the Transport 

of Anthropogenic and Natural Contaminants to a public-supply well in Modesto, 

eastern San Joaquin Valley, California.................................................... 8

Figure 4. Graph showing distribution of hydrogeologic facies and approximate location 

of formation boundaries from boreholes drilled for a study of the Transport of 

Anthropogenic and Natural Contaminants to a public-supply well in Modesto, 

eastern San Joaquin Valley, California ................................................... 15
vii

Figure 5. Map showing water-level elevations in water-table wells in October–

November 2004 in Modesto, eastern San Joaquin Valley, California. ............... 17

Figure 6. Graph showing water-level elevation in monitoring wells at an urban and 

agricultural land-use setting in Modesto, eastern San Joaquin Valley, 

California .................................................................................... 18

Figure 7. Graph showing percentage of cumulative flow at 6.1-meter (20-foot) intervals 

and approximate distribution of flow contributed from the shallow, intermediate, 

and deep aquifer zones to a public-supply well in Modesto, eastern San Joaquin 

Valley, California ........................................................................... 19

Figure 8. Graph showing tritium concentrations expected in Modesto area ground water 

in 2004 that originated as precipitation between 1953 and 2003 ..................... 22

Figure 9. Graph showing relation between tritium concentrations in ground-water 

samples collected during October and November of 2003 and 2004 and depth

to the top perforated interval of monitoring wells in Modesto, eastern San 

Joaquin Valley, California. .................................................................. 22

Figure 10. Graph showing relation between delta helium-3 (δ3He) and percentage of 

terrigenic helium in ground-water samples from Modesto, eastern San Joaquin 

Valley, California ........................................................................... 24

Figure 11. Graph showing relation between total tritium (tritium plus tritiogenic helium-3) 

in ground-water samples and public-supply well samples in Modesto, eastern 

San Joaquin Valley, California ............................................................ 25

Figure 12. Graph showing estimated mean age of ground water in the local study area in 

Modesto, eastern San Joaquin Valley, California ....................................... 27

Figure 13. Graph showing isotopic composition of ground-water and surface-water 

samples collected in October and November of 2003 and 2004 in Modesto, 

eastern San Joaquin Valley, California ................................................... 31

Figure 14. Graph showing relations among sulfate, nitrate, and isotopes of nitrogen 

and oxygen in dissolved nitrate in ground water collected from monitoring wells 

in October and November of 2003 and 2004. (A) Relation between sulfate and 

nitrate. (B) Relation between delta nitrogen-15 (δ15N) and nitrate. (C) Relation 

between sulfate and delta sulfur-34. (D) Relation between δ15N and delta 

sulfur-34. (E) Relation between δ15N and delta oxygen-18 (δ18O) ..................... 36

Figure 15. Graph showing relation of delta nitrogen-15 in dissolved nitrate and chloride 

to bromide ratios (Cl/Br) in ground-water samples collected from monitoring 

wells in October and November of 2003 and 2004 in Modesto, eastern San 

Joaquin Valley, California .................................................................. 37

Figure 16. Graph showing relation between arsenic concentrations and orthophosphate 

in ground-water samples collected from monitoring wells in October and 

November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California ... 39

Figure 17. Map showing aerial gamma-ray survey of uranium concentrations in soils 

in the San Joaquin Valley .................................................................. 40

Figure 18. Graph showing relation between uranium concentrations and alkalinity 

measurements from ground-water samples collected from monitoring wells 

in October and November of 2003 and 2004 in Modesto, eastern San Joaquin 

Valley, California ........................................................................... 41

Figure 19. Relation between uranium and calcium concentrations in ground-water samples 

collected in October and November of 2003 and 2004 in Modesto, eastern San 

Joaquin Valley, California .................................................................. 41
viii

Figure 20. Graph showing relation between total uranium activity and the uranium 

activity ratio in ground-water, depth-dependent samples from the publicsupply well and ground-water samples from monitoring wells collected in 

August, October, and November of 2004 in Modesto, eastern San Joaquin 

Valley, California ........................................................................... 42

Figure 21. Trilinear graph showing major ion composition of ground-water samples 

collected in October and November 2003 and 2004 in Modesto, eastern San 

Joaquin Valley, California .................................................................. 43

Figure 22. Graph showing uranium concentrations in and specific conductance 

measurements for ground water, depth-dependent samples collected from 

the public-supply well, and ground-water samples collected from monitoring 

wells at site FPA in August 2004 in Modesto, eastern San Joaquin Valley, 

California .................................................................................... 48

Figure 23. Graph showing isotopic composition of ground water from public-supply 

well depth-dependent samples and ground-water and surface-water samples

in Modesto, eastern San Joaquin Valley, California .................................... 49

Figure 24. Graph showing application of nitrogen fertilizers in Stanislaus County 

since 1945..................................................................................... 50

Figure 25. Graph showing nitrate and uranium concentrations in ground water from 

the public-supply well in Modesto, eastern San Joaquin Valley, California, 

between 1966 and 2005 ..................................................................... 51

Figure 26. Graph showing movement of anthropogenically influenced ground water 

and approximate ground-water age distribution in Modesto, eastern San 

Joaquin Valley, California .................................................................. 52

Appendix

Figure A1. Graph showing relation between uranium concentrations and uranium 

adsorbed to aquifer sediment collected at various depths in Modesto, eastern 

San Joaquin Valley, California ............................................................ 65

Tables

Table 1. Construction characteristics and grouping variables for the installed well 

network in Modesto, eastern San Joaquin Valley, California ........................ 9

Table 2. Water-quality sample periods and schedule of analytes collected from 

monitoring wells, the pubic-supply well, and surface-water sites in Modesto, 

eastern San Joaquin Valley, California, 2003–2005 .................................... 10

Table 3. List of analytes for schedules listed in table 2 .......................................... 11

Table 4. Carbon-14, tritium/helium-3, and sulfur hexafluoride measurements and 

interpreted ages of ground water collected from monitoring wells in October 

and November of 2003 and 2004 and in August 2004 in Modesto, eastern San 

Joaquin Valley, California .................................................................. 20

Table 5. Summary of well characteristics and water chemistry field measurements of 

depth categories for monitoring wells sampled from October through November 

of 2003 and 2004, for the public-supply well sampled from October 2003 through 

June 2005, and for surface-water sites sampled in February and June 2005, in 

Modesto, eastern San Joaquin Valley, California ....................................... 29
ix

Table 6. Summary of pesticides and volatile organic compounds (VOC) collected from 

monitoring wells sampled from October through November of 2003 and 2004 in 

Modesto, eastern San Joaquin Valley, California ....................................... 32

Table 7. Comparison of selected volatile organic compounds (VOC) in ground water 

collected from water-table wells during winter and summer seasons from 

winter 2003 through summer 2005 in Modesto, eastern San Joaquin Valley, 

California .................................................................................... 32

Table 8. Selected inorganic constituents and isotopes in ground water from monitoring 

wells sampled from October through November of 2003 and 2004 in Modesto, 

eastern San Joaquin Valley, California ................................................... 34

Table 9. Volatile organic compounds, pesticides, and selected inorganic constituents 

and isotopes in ground water from depth-dependent public-supply well samples 

and samples collected at adjacent monitoring wells during August and 

September 2004 in Modesto, eastern San Joaquin Valley, California ............... 45

Appendixes

Table A1. Relative abundance of bulk minerals and minerals in particles less than

1 micrometer from core samples collected during monitoring well installation in 

 2003 and 2004 in Modesto, eastern San Joaquin Valley, California .................. 62

Table A2. Percentage of non-magnetic and slightly magnetic heavy minerals separated 

 from core samples collected during monitoring well installation in 2003 and 

 2004 in Modesto, eastern San Joaquin Valley, California .............................. 63

Table A3. Total Carbon and organic carbon content of sediments from core samples 

 collected during monitoring well installation in 2003 and 2004 in Modesto, 

 eastern San Joaquin Valley, California................................................... 63

Table A4. Selected trace element concentrations from extractions performed on core and

 cutting samples collected during monitoring well installation in 2003 and 2004 in

 Modesto, eastern San Joaquin Valley, California .................................... 64

Table B1. Summary of mean relative standard deviations for replicate sample pairs of

 inorganic and nutrient analytes in ground-water samples collected in October 

 and November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, 

 California .................................................................................... 68

Table C1. Major ion composition, pH, and calcite saturation in ground water from 

 monitoring wells sampled in October and November of 2003 and 2004 in 

 Modesto, eastern San Joaquin Valley, California .................................... 72

Table D1. Volatile organic compounds (VOC) detected in ground water from monitoring 

wells sampled from October through November of 2003 and 2004 in Modesto, 

eastern San Joaquin Valley, California ................................................... 75

Table E1. Pesticide compounds detected in ground water from monitoring wells and 

the public-supply well sampled in October and November of 2003 and 2004 in 

Modesto, eastern San Joaquin Valley, California ....................................... 78
x

Conversion Factors, Spatial Datums, Abbreviations, and 

Acronyms

SI to Inch/Pound

Multiply By To obtain

Length

micrometer (μm) 0.00003937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre 

square kilometer (km2

) 247.1 acre

Volume

cubic centimeter (cm3

) 0.06102 cubic inch (in3)

cubic kilometer (km3) 0.2399 cubic mile (mi3

)

milliliter (mL) 0.00211 ounce, US, liquid

Flow rate

cubic meter per day (m3

/d) 0.000264 million gallons per day (Mgal/d)

cubic meter per day (m3

/d) 35.31 cubic foot per day (f3/d) 

cubic meter per minute (m3

/min) 264.2 gallon per minute (gal/min)

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

meter per year (m/yr) 3.281 foot per year (ft/yr) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Hydraulic gradient

meter per kilometer (m/km) 5.27983 foot per mile (ft/mi)

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

°F=(1.8×°C)+32

Spatial Datums

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

(NAVD 88).

Elevation, as used in this report, refers to distance above the vertical datum.

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

Water-Quality Information

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

micrograms per liter (μg/L). Specific conductance (SC) is given in microsiemens per centimeter 

at 25°C (μS/cm). The activity concentration of uranium isotopes is given in picocuries per liter 

(pCi/L); 1 pCi is equivalent to 2.2 disintegrations per minute. The concentration of dissolved tritium 

is expressed in tritium units (TU); 1 TU is 1 tritium atom per 1,018 hydrogen atoms or is equivalent 

to 7.1 disintegrations per minute per liter.

The saturation index (SI = IAP/Ksp) is the ratio of the ion activity product (IAP) in solution and 

the solubility equilibrium constant (Ks) of a mineral. It is a measure of a mineral’s tendency to 

dissolve or precipitate. A positive value indicates that the water is supersaturated with respect 

to the mineral considered and that the mineral would tend to precipitate. A negative value 

indicates that the water is undersaturated with respect to the mineral considered and that 

mineral, if present, would tend to dissolve.
xi

Abbreviations and Acronyms

Ar argon

atm atmosphere 

bls below land surface

°C degree Celsius

CaCO

3 calcium carbonate

CH

4 methane

CIAT 2-Chloro-4-isopropylamino-6-aminos-triazine

cm

3 cubic centimeter

cm3 @ STP/g

 of water

cubic centimeters at standard temperature and pressure per gram of water

CO

2 carbon dioxide

CO2 (aq) aqueous carbon dioxide

CRA contributing recharge area

DEA deethylatrazine

DHS Department of Health Services

DO dissolved oxygen

DOC dissolved organic carbon

EM Electromagnetic

emm exponential mixing model

epm exponential-piston flow model

Fe iron

ft foot

gpm gallons per minute 3H tritium 3

He helium-3

3

Hetrit tritiogenic helium-3

4

He helium-4

ICP-MS inductively-coupled plasma–mass spectrometry

kg/ha kilogram per hectare

mg/kg milligram per kilogram

mg/L milligram per liter

pCP

2

 (g) partial pressure of carbon dioxide gas

pmC percent modern carbon

ppm parts per million

pptv parts per trillion by volume

CIAT 2-Chloro-4-isopropylamino-6-aminos-triazine

CRA contributing recharge area

DEA deethylatrazine

DHS Department of Health Services

DO dissolved oxygen

DOC dissolved organic carbon

EM Electromagnetic 3H tritium

MCL U.S. Environmental Protection Agency maximum contaminant level

MCL-CA California maximum contaminant level

MTBE methyl tert-butyl ether

NAWQA National Water-Quality Assessment N nitrogen

NURE National Uranium Resource Evaluation

NWIS National Water Information System

NWQL National Water Quality Laboratory O oxygen
xii

PCE tetrachloroethene

PSW public-supply well

PVC polyvinyl chloride

QC quality control

RSD relative standard deviation

SC specific conductance

SF6 sulfur hexafluoride

STP standard temperature and pressure

TANC transport of anthropogenic and natural contaminants

TU tritium unit

USEPA U.S. Environmental Protection Agency

VCDT Vienna Canyon Diablo Troilite

VSMOW Vienna Standard Mean Ocean Water

USGS U.S. Geological Survey

U(VI) hexavalent uranium, a stable valence of uranium

UAR uranium activity ratio of uranium-234 to uranium-238

VOC volatile organic compound

ZOC zone of contribution

δD delta deuterium

δ15N delta nitrogen-15

δ18O delta oxygen-18

δ34S delta sulfur-34

μg/L microgram per liter

μm micrometer
Abstract

Ground-water chemistry in the zone of contribution of 

a public-supply well in Modesto, California, was studied 

by the U.S. Geological Survey National Water Quality 

Assessment (NAWQA) Program’s topical team for Transport 

of Anthropogenic and Natural Contaminants (TANC) to 

supply wells. Twenty-three monitoring wells were installed 

in Modesto to record baseline hydraulic information and to 

collect water-quality samples. The monitoring wells were 

divided into four categories that represent the chemistry of 

different depths and volumes of the aquifer: (1) water-table

wells were screened between 8.5 and 11.7 m (meter) (28 and 

38.5 ft [foot]) below land surface (bls) and were within 5 m 

(16 ft) of the water table; (2) shallow wells were screened 

between 29 and 35 m (95 and 115 ft) bls; (3) intermediate

wells were screened between 50.6 and 65.5 m (166 and 

215 ft) bls; and (4) deep wells are screened between 100 to 

106 m (328 and 348 ft) bls. Inorganic, organic, isotope, and 

age-dating tracers were used to characterize the geochemical 

conditions in the aquifer and understand the mechanisms of 

mobilization and movement of selected constituents from 

source areas to a public-supply well. 

The ground-water system within the study area has 

been significantly altered by human activities. Water levels 

in monitoring wells indicated that horizontal movement of 

ground water was generally from the agricultural areas in the 

northeast towards a regional water-level depression within 

the city in the southwest. However, intensive pumping and 

irrigation recharge in the study area has caused large quantities 

of ground water to move vertically downward within the 

regional and local flow systems. 

Analysis of age tracers indicated that ground-water age 

varied from recent recharge at the water table to more than 

1,000 years in the deep part of the aquifer. The mean age 

of shallow ground water was determined to be between 30 

and 40 years. Intermediate ground water was determined 

to be a mixture of modern (post-1950) and old (pre-1950) 

ground water. As a result, concentrations of age tracers were 

detectable but diluted by older ground water. Deep ground 

water generally represented water that was recharged under 

natural conditions and therefore had much older ages. Ground 

water reaching the public-supply well was a mixture of older 

intermediate and deep ground water and young shallow

ground water that has been anthropogenically-influenced to a 

greater extent than intermediate ground water.

Uranium and nitrate pose the most significant threat to 

the quality of water discharged from the public-supply well. 

Although pesticides and VOCs were present in ground water 

from the public-supply well and monitoring wells, currently 

concentrations of these contaminants are generally less than 

one-hundredth the concentration of drinking water standards. 

In contrast, both uranium and nitrate were above half the 

concentration of drinking water standards for public-supply 

well samples, and were above drinking water standards for 

several water-table and shallow monitoring wells. Shallow

ground water contributes roughly 20 percent of the total flow 

to the public-supply well and was the entry point of most 

contaminants reaching the public-supply well.

Naturally-occurring uranium, which is commonly 

adsorbed to aquifer sediments, was mobilized by oxygenrich, high-alkalinity water, causing concentrations in some 

monitoring wells to be above the drinking-water standard 

of 30 μg/L (microgram per liter). Adsorption experiments, 

sediment extractions, and uranium isotopes indicated uranium 

in water-table and shallow ground water was leached 

from aquifer sediments. Uranium is strongly correlated to 

bicarbonate concentrations (as measured by alkalinity) in 

ground water. Bicarbonate can effectively limit uranium 

adsorption to sediments. As a result, continued downward 

movement of high-alkalinity, oxygen-rich ground water will 

likely lead to larger portions of the aquifer having elevated 

uranium. 

Hydrogeology, Water Chemistry, and Factors Affecting 

the Transport of Contaminants in the Zone of Contribution 

of a Public-Supply Well in Modesto, Eastern San Joaquin 

Valley, California

By Bryant C. Jurgens, Karen R. Burow, Barbara A. Dalgish, and Jennifer L. Shelton
2 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Nitrate concentrations were above the drinking water 

standard of 10 mg/L (milligram per liter) in two watertable wells influenced by agricultural practices. In contrast, 

concentrations in most water-table wells influenced by 

urban practices were less than 5.0 mg/L. All three shallow

monitoring wells which were located beneath urban land had 

nitrate concentrations above the drinking water standard. 

Co-occurrence of sulfate with nitrate indicated that agricultural 

practices mainly were responsible for high nitrate in ground 

water. Denitrification can occur in localized areas; however, 

attenuation of nitrate was not significant enough to decrease 

contaminant concentrations throughout the aquifer. As a result, 

nitrate concentrations in ground water were mostly influenced 

by nitrogen fertilizer application rates and dispersion 

processes. 

The water chemistry of the public-supply well was 

strongly influenced by well-bore leakage. During static 

conditions, anthropogenically-influenced shallow ground 

water migrated down the gravel pack or through the well 

screen and down the well into the deep part of the aquifer 

where it was stored until pumping was resumed. Because 

deep ground water contributes approximately 25 percent of 

the total flow, the amount of shallow ground water which 

could contribute to the total flow could increase from about 

20 percent to approximately 45 percent. The relative amount 

of shallow ground water would then decrease as migrated 

shallow ground water was pumped from the deep part of 

the aquifer. As a result, historical water-quality data shows 

seasonal fluctuations, with higher concentrations of nitrate and 

uranium during winter months. Increasing the amount pumped 

during the winter months may improve water-quality during 

these months; however; continued downward migration of 

uranium and nitrate will likely negatively affect the long-term 

sustainability of the local ground-water aquifer as a source of 

drinking water, as well as the public-supply well itself.

Introduction

In 2000, ground water provided 37 percent of the 

public drinking-water supplies used by 242 million people 

nationwide (Hutson and others, 2004). Dependence on ground 

water for public supply has increased five fold over the last 50 

years, causing increased concern about the quality of water 

pumped by public-supply wells. California provides more 

ground water for public supply than any other state in the 

Nation, serving more than 30 million people and accounting 

for 18 percent of the total ground water withdrawals for public 

supply in 2000 (Hutson and others, 2004). 

Ground-water withdrawals from the Central Valley 

Principal aquifer are the second largest in the Nation, 

accounting for 13 percent of total withdrawals (Maupin and 

Barber, 2005). Most of the ground water use is in the eastern 

San Joaquin Valley where irrigation is the primary water use; 

however, the rapidly increasing population and periods of 

drought are expected to increase reliance on ground water 

for drinking water. Both agricultural and urban practices 

have affected the quality of ground water in the eastern San 

Joaquin Valley. In shallow ground water beneath farmed 

land, pesticides were detected in 69 percent of domestic 

wells sampled during 1993–95, and nitrate concentrations 

were above the maximum contaminant level (MCL) of 

10 mg/L (U.S. Environmental Protection Agency, 2003) in 

24 percent of domestic wells (Dubrovsky and others, 1998). 

Low concentrations of volatile organic compounds (VOC) 

were detected in more than 70 percent of the public-supply 

wells sampled in Fresno and in more than 90 percent of the 

wells sampled in Modesto in 2001–2002 (Wright and others, 

2004). In addition to contaminants derived from land-use 

practices, natural contaminants such as arsenic and uranium 

were frequently near the top of the list of inorganic and 

radionuclide contaminants that have the highest number of 

MCL exceedences in public-supply wells throughout the 

State (California Department of Public Health, 2006). In the 

Modesto and Turlock areas, arsenic concentrations greater 

than 10 μg/L were detected in 12 percent of the wells (Belitz 

and others, 2003) and uranium concentrations greater than 

30 μg/L have caused the abandonment of at least 11 publicsupply wells in the Modesto area (city of Modesto, written 

comm., 2003). Multiple sources of contaminants and the 

generally permeable soils throughout the eastern San Joaquin 

Valley present a difficult challenge to water managers trying 

to provide a reliable, clean source of drinking water to an 

expanding population. 

To improve understanding of the relations between the 

sources of contaminants, their transport through hydrologic 

systems, and the effects of contaminants on the quality 

of drinking water, the U.S. Geological Survey’s National 

Water-Quality Assessment (NAWQA) program began several 

water-quality investigations within representative aquifers and 

geographic areas across the Nation to study the transport of 

anthropogenic and natural contaminants (TANC) to publicsupply wells (Wilber and Couch, 2002). The TANC studies 

investigate the transport of contaminants within that part of 

the aquifer that contributes water to public-supply wells at 

multiple scales in different hydrogeologic settings (Eberts and 

others, 2005). Seven regional studies (hundreds to thousands 

of square kilometers) were completed in a range of settings, 

using regional ground-water flow models and water-quality 

data from multiple public-supply wells (Paschke and others, 

2007). Following the regional analysis, more detailed localscale studies (less than 100 km2

) were implemented in 4 of 

the 7 study areas: the Central Valley Aquifer in the eastern 

San Joaquin Valley near Modesto, California; the Floridian 

Aquifer near Tampa, Florida; the Pomperaug River Basin near 

Woodbury, Connecticut; and the High Plains Aquifer near 

Lincoln, Nebraska (Eberts and others, 2005). The local-scale 

studies were designed to enhance the understanding of specific 

physical and chemical processes affecting the vulnerability of 

public-supply wells in the regional aquifer. 
Introduction 3

The public-supply well that is the focus of this report 

is in the eastern San Joaquin Valley in Modesto, California, 

and typifies many public-supply wells throughout the eastern 

San Joaquin Valley that have both urban and agricultural 

land uses within the area contributing recharge. Therefore, a 

detailed analysis of the sources and processes that affect the 

vulnerability of this well will likely elucidate factors common 

to other wells that are susceptible to contamination throughout 

the eastern San Joaquin Valley.

Purpose and Scope

This report is one of two reports that document the 

local-scale TANC investigations in the eastern San Joaquin 

Valley near Modesto, California. This report describes 

the hydrogeology, water chemistry, and factors affecting 

the transport of contaminants in the zone of contribution 

of a public-supply well in Modesto, California. This 

characterization was used to develop hypotheses regarding 

the long-term vulnerability of the public-supply well to nitrate 

and uranium concentrations. A local-scale ground-water 

flow and transport model was used to test the hypotheses. 

The second report (Burow and others, 2008) documents the 

local-scale ground-water flow model and presents results of 

the tests of the hypotheses related to the transport and fate of 

contaminants of interest outlined in this report. These reports 

serve as a foundation for future comparisons among the four 

local-scale TANC study areas. 

This report describes the design of the local monitoring 

well network and documents the methods used to collect the 

hydrogeologic and water-quality data. Water chemistry data 

and tracers of ground-water age collected from the monitoring 

wells and sediment analyses collected during monitoring well 

installation were used to understand the sources, distribution 

and processes controlling the transport of contaminants 

within the zone of contribution of the public-supply well. 

Because ground-water samples collected at the well head of 

the public-supply well were a composite of water contributed 

from different parts of the aquifer, water chemistry and age 

tracers were collected at different depths within the pumping 

public-supply well in order to determine where contaminants 

enter the well. General water-chemistry characteristics such 

as dissolved ions, and oxidation-reduction conditions are 

described. Organic constituents and nitrate, which are mainly 

derived from anthropogenic sources, were used to identify 

the influence of different land-uses on the water-quality of 

the aquifer. Sediment extractions and adsorption experiments 

were used to identify sources and processes affecting the 

mobilization and occurrence of arsenic and uranium in ground 

water. Several isotopes were also used to determine possible 

sources of water, and contaminant sources and elemental 

transformations. The report describes the interpretative steps 

in estimating ground-water ages using atmospheric tracers 

and describes the distribution of apparent ground-water ages. 

Also, the report describes construction and well-bore flow 

characteristics of the public-supply well and how operation of 

the well affects the chemistry of water discharging from the 

well head.

Description of Study Area

The local-scale study area is in the northeastern part 

of the San Joaquin Valley (fig. 1). The San Joaquin Valley 

occupies the southern two-thirds of the Central Valley 

of California. The Central Valley is a large, northwesttrending, asymmetric structural trough filled with marine 

and continental sediments up to 10 km thick (Page, 1986; 

Gronberg and others, 1998). The San Joaquin Valley is a level 

depression more than 400 km long and 30 to 90 km wide. 

East of the Valley, the Sierra Nevada rise to an elevation of 

more than 4,200 m; west of the Valley are the Coast Ranges, 

a series of parallel ridges of moderate elevations (Mendenhall 

and others, 1916). Streams in the northern part of the San 

Joaquin Valley drain through the San Joaquin River northward 

to the San Francisco Bay; the southern part of the Valley is 

hydrologically closed. 

The local-scale study area is in the north part of Modesto 

and occupies 50 km2 (fig. 1). The study area is nested within 

the TANC regional study area. The TANC regional-scale study 

area is about 2,700 km2, and extends from the Stanislaus River 

on the north to the Merced River on the south and bounded on 

the east by the Sierra foothills and the west by the San Joaquin 

River (Phillips and others, 2007). 

The land surface in the regional study area slopes 

downward from the Sierra Nevada foothills to the San Joaquin 

River; gradients range from about 5 m/km near the foothills 

to less than 1 m/km near the river. The climate is semiarid, 

characterized by hot summers and mild winters; rainfall 

averaged 315 mm annually from 1931 to 1997 and normally 

occurs during late fall through early spring (Western Regional 

Climate Center, 2008). Mean annual air temperature was 

17 degrees Celsius between 1971 and 2000 (Western Regional 

Climate Center, 2008).

The San Joaquin River (fig. 1) is the central drainage for 

the northern San Joaquin Valley and is the only major surfacewater outlet from the valley. The Stanislaus, Tuolumne, and 

Merced Rivers drain the Sierra Nevada and are tributaries 

to the San Joaquin River. All rivers in the study area have 

been significantly modified from natural conditions. Each 

has multiple reservoirs for irrigation and power generation, 

which delays discharge of large amounts of snowmelt runoff. 

Imprinted over this hydrology is an extensive network of 

canals used to deliver water for irrigation. 
4 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Figure 1. Study area near Modesto in the eastern San Joaquin Valley, California.

Hydrofacies model grid extent 

Boundary containing active area of local model (Burow and others, 2008)

Road or highway 

Boundary of regional model (Phillips and others, 2007) 

0 5 1 0 Kilometers

0 5 10 Miles

reviRniuqaoJnaS

reviRdecreM

reviRsualsinatS

reviRenmulouT

Modesto

YTNUOCSUALSINATS

YTNUOCDECREM

YTNUOCNIUQAOJNAS

keerCyrD

adaveNarreiS

San Joaquin segnaRtsaoC

Valley

Study Area

Central

Valley

Urban area

121°14’ 120°44’

37°45’ 37°21’

EXPLANATION
Introduction 5

Dry wells have been used extensively throughout the 

regional study area since the 1950s as means to rapidly route 

and distribute storm water to the subsurface in agricultural 

and urban areas. In the Modesto urban area, dry wells were 

drilled using large-diameter bits (about 1 m) to depths of 15 

to 25 m and were filled to the surface with rock aggregate. A 

perforated casing approximately 6 m long and 15 cm wide 

was placed in the center of the borehole for the first 6 m below 

land surface (bls) to promote infiltration (city of Modesto, oral 

commun. 2004). Adjacent to each dry well, a catch basin was 

constructed to capture surface runoff and deliver water to the 

dry well when the catch basin overflowed. Over 11,000 dry 

wells have been constructed in the Modesto incorporated area 

(city of Modesto, oral commun. 2004) and could affect the 

quality of shallow ground water in the local study area. 

About 65 percent of the regional study area is planted 

in irrigated crops. Dominant crops include almonds, walnuts, 

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

Department of Water Resources, 2001). The local study 

area contains about 54 percent urban land and 46 percent 

agricultural land. During the last 4 decades, the urban area has 

expanded into the agricultural area as a result of increasing 

population. In Stanislaus County, the estimated population 

in 2005 was more than 500,000 people, an increase of more 

than 10 percent since 2000 (California Department of Finance, 

2006). 

Agricultural irrigation began in the regional study area in 

the early 1900s, and as of 2000, accounts for about 95 percent 

of the total water use (Burow and others, 2004). Surfacewater supplies from the Stanislaus and Tuolumne Rivers are 

managed by irrigation districts and delivered to agricultural 

users through hundreds of kilometers of lined canals. Most 

irrigation districts and private agricultural users pump ground 

water for irrigation. Some districts also pump ground water 

to lower the water table in areas where it has risen too close 

to the land surface to support agriculture without active 

management. Private agricultural ground-water pumpage 

is not measured in the study area, but is estimated at about 

32 percent of total agricultural water use in water year 2000 

(Burow and others, 2004; Phillips and others, 2007). 

Urban water demand is met by surface-water and 

ground-water supplies. Before 1995, drinking water for the 

city of Modesto was supplied by ground water only. In 1994, 

a surface-water treatment plant was completed, which now 

provides about half of Modesto’s municipal and industrial 

water supply. In water year 2000, about 55 percent of the 

urban water requirement was met with ground water (Burow 

and others, 2004; Phillips and others, 2007).

The aquifer in the local study area is unconfined, 

although water-bearing layers of sand and gravel become 

semi-confined with depth owing to numerous, overlapping, 

discontinuous clay lenses. Before the ground-water basin 

was developed, most ground-water recharge occurred in the 

upper parts of the alluvial fans where streams entered the 

basin. Ground water generally moved southwest, toward the 

axis of the San Joaquin Valley, and discharged primarily to 

the San Joaquin River and surrounding marshlands. However, 

diversion of surface water from streams and intensive pumping 

and recharge resulting from irrigated agriculture and urban 

growth have significantly altered the natural flow system. 

Following development of the ground-water basin, percolating 

irrigation water became the primary form of ground-water 

recharge, and irrigation pumping became the primary form 

of ground-water discharge (Davis and others, 1959; Page and 

Balding, 1973; Londquist, 1981). As a result, ground water has 

been driven vertically downward within the regional and local 

flow systems, and water moving laterally may be pumped and 

reapplied at the surface multiple times. In addition to irrigation 

pumping, focused ground-water pumping and lower recharge 

within the urban area have caused a water level depression 

within the city (Burow and others, 2004).

Summary of Previous Water-Quality Studies

In general, previous studies found the quality of ground 

water in the regional study area to be good, although the 

chemistry of the water has been affected by agricultural and 

urban activities. Ground water in the unconfined system 

was typically a mixed cation-bicarbonate type water, and 

trends from a calcium-bicarbonate or calcium-magnesiumbicarbonate type water in the east to a calcium-sodiumbicarbonate or sodium-bicarbonate type water near the San 

Joaquin River, according to a study by Page and Balding 

(1973). Total dissolved solids and chloride concentrations 

varied laterally and vertically, but generally increased along 

the direction of regional ground-water flow from less than 200 

mg/L and 20 mg/L east of Modesto to more than 500 mg/L 

and 100 mg/L near the San Joaquin River (Page and Balding, 

1973). 

Nitrate concentrations in public-supply wells sampled in 

Modesto in 2001 ranged from 0.23 to 9.5 mg/L as nitrogen; 

the median was 3.5 mg/L (Wright and others, 2004). Nitrate 

concentrations were elevated (greater than 3.5 mg/L as 

nitrogen) more frequently in public-supply wells that were 

within 500 m of agricultural or undeveloped areas than in 

wells surrounded by urban areas only (Wright and others, 

2004). In the eastern San Joaquin Valley, nitrate concentrations 

in ground water from domestic wells were significantly higher 

beneath almond orchards than beneath corn, alfalfa, and 

vegetable crops and vineyards (Burow and others, 1998). 

In contrast to nitrate, VOCs were related to urban 

land-use practices: at least one VOC was detected in 37 out 

of 40 public-supply wells in Modesto (Wright and others, 

2004). The most frequently detected VOCs were disinfection 

byproducts, such as chloroform, and solvents, such as PCE, 

TCE, and dibromomethane; these VOCs were detected in 

more than 25 percent of the wells.
6 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

In addition to anthropogenic contaminants, naturallyoccurring trace elements and radionuclides in ground water, 

such as arsenic and uranium, are of concern in the Modesto 

area. Arsenic concentrations commonly exceed 5 μg/L 

(Belitz and others, 2003) and uranium has exceeded the U.S. 

Environmental Protection Agency (USEPA) MCL of 30 μg/L 

in several public-supply wells, causing the operation of 

these wells for drinking water use to stop (city of Modesto, 

written commun. 2003). Because these naturally-occurring 

contaminants are ubiquitous in the alluvial sediments, 

identifying the processes which control their mobilization 

and transport could help water managers meet compliance 

standards.

Study Design

To investigate the sources and processes affecting 

public-supply well vulnerability, hydrologic modeling and 

water chemistry data were used to select a single well from 

60 public-supply wells. A monitoring well network was 

installed at multiple depths within the contributing recharge 

area (CRA) and the zone of contribution (ZOC) of the selected 

public-supply well. Hydrogeologic and water-quality data 

were collected during and after installation of monitoring 

wells and used to characterize the aquifer. Water-quality data 

were collected from the public-supply well and monitoring 

wells multiple times over the course of two and half years to 

determine seasonal variations in water chemistry.

Public-Supply Well Selection

A regional ground-water flow model was used to simulate 

the ZOC and CRA for 60 public-supply wells (Phillips and 

others, 2007). The ZOC is the three-dimensional volumetric 

part of the aquifer through which ground water flows to the 

discharging well from the CRA (Alley, 2003). The CRA is 

defined as the surface area on the three-dimensional boundary 

of the ground-water system that delineates the location of 

water entering the ground-water system that eventually flows 

to the well from the aquifer. Particle tracking analysis was 

used to compute the extent and percentages of different land 

uses within the CRA of each well and to relate the occurrence 

of contaminants in the wells to each land use. 

The public-supply well selected for this study is in 

northeast Modesto (fig. 2), and was selected from a subset of 

15 wells that were sampled by the USGS in 2003 for a wide 

range of analytes. These 15 wells had annual pumpage that 

ranked in the top 25 percent of public-supply wells in Modesto 

for 2000, and most wells had longer screened intervals and 

a larger CRA than the other public-supply wells in the study 

area. The selected well was approximately 120 meters deep 

(391 ft), was screened from 27.7 (91 ft) to 111.6 m (366 ft) 

bls, and had an open bottom to a depth of about 115 m (377 ft) 

(table 1). This well withdrew water at an average rate of 

about 6.1 cubic meters per minute (1,600 gpm). The CRA

for the selected well had an areal extent of 4.2 km2 (fig. 2, 3), 

consisting of about 65 percent agricultural land and 32 percent 

urban land. 

Monitoring Well Network

Monitoring wells were constructed in the CRA and ZOC 

of the selected public-supply well and adjacent areas to define 

hydrologic boundaries and characterize chemical compounds 

present beneath the different land-use settings. Uncertainty 

in defining the areal extent of the CRA necessitated finding 

locations at the water table that were most likely to contribute 

recharge to the public-supply well. Therefore, a probabilistic 

method was used to create a particle density map (fig. 2) 

to reveal areas within the ZOC that were more likely to 

contribute recharge that would reach the public-supply well. 

A more detailed description of this method is outlined in the 

accompanying TANC model report (Burow and others, 2008). 

Candidate sites for monitoring wells were prioritized on the 

basis of their relative location within the CRA boundaries 

and on the particle density map; however, other factors, such 

as drilling permission, accessibility, and civilian and worker 

safety were also considered. 

Monitoring wells were installed in two phases. During 

the first phase, thirteen wells were installed at four sites 

(FPA[4 wells], FPB [3 wells], FPC [3 wells], and FPD [3 

wells]) during the summer of 2003 (fig. 3). During the second 

phase, 10 additional monitoring wells were installed between 

February and March 2004 at 6 sites (FPE [3 wells], OFPA 

[3 wells], OFPB [1 well], SA [1 well], SB [1 well], SC [1 

well]) to target more probable recharge sites and specific depth 

zones within the aquifer. The wells were completed at depths 

ranging from within less than 5 m (16 ft) below the water table 

to more than 100 m (328 ft) bls (table 1). The screen length for 

each monitoring well was 1.5 m (5 ft).

The 23 monitoring wells installed during this study were 

divided into four categories that represent the chemistry of 

different depths and volumes of the aquifer: (1) water-table

wells were screened between 8.5 and 11.7 m (28 and 38.5 ft) 

bls and were within 5 m (16.4 ft) of the water table; (2) 

shallow wells were screened between 29 and 35 m (95 and 

115 ft) bls and represent the chemistry of ground water below 

the water-table and down to 50 m (164 ft) bls; (3) intermediate

wells were screened between 50 and 65.5 m (166 and 215 ft) 

bls and represent the chemistry of ground water between 50 m 

and 70 m (164 and 230 ft) bls; and (4) deep wells are screened 

between 100 to 106 m (328 and 347 ft) bls and represent the 

chemistry of ground water below 70 m (230 ft) (table 1). 

The wells were also assigned a land-use category on 

the basis of the surrounding land use. Urban land consisted 

primarily of residential land with commercial land along major 

streets. Agricultural land was farmed land growing primarily 

citrus and almond orchards. Recently Urbanized land was land 

that had been agricultural within the last 10 to 20 years, but 

has been or is being currently developed, mostly for residential 

housing. Some parts of this land may still be farmed.
Study Design 7

Figure 2. Regional model simulation of 100-year contributing recharge area (CRA) and contours of areas of 

recharge with higher probabilities of reaching the public-supply well (Phillips and others, 2007). Water level 

elevation contours from spring 2000 water-level measurements.

120°57’

37°42’ 37°39’

121°

18 (59)

20 (66)

16 (52)

22 (72)

24 (79)

14 (46)

26 (85)

14 (46)

0 0.5 1 2 Miles

0 1 2 0.5 Kilometers

EXPLANATION

Active area of local-scale model grid (Burow and others, 2008)

Hydrofacies model grid extent (Burow and others, 2008)

Public-supply well

Simulated 100 year contributing recharge area (CRA)

Road or highway

Lines of equal water level elevation,

in meters (feet in parentheses)

Regional model grid (Phillips and others, 2007)

20 (66)

Agricultural land use area

Urban land use area

Dry Creek

Lines of equal particle density, 

in number of particles per model cell

< 500

501 – 1,000

1,000 – 1,500

1,501 – 2,000

2,001 – 3,000
8 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Figure 3. Location of monitoring wells installed for a study of the Transport of Anthropogenic and Natural 

Contaminants (TANC) to a public-supply well in Modesto, eastern San Joaquin Valley, California. 37°43’ 37°40’

121° 120°57’

0 0.5 1 2 Miles

0 0.5 1 2 Kilometers

EXPLANATION

Active area of local-scale model grid boundary (Burow and others, 2008)

Public-supply well

Simulated 100 year contributing recharge area (CRA) (Phillips and others, 2007)

Road or highway

Agricultural land use area

Urban land use area

Monitoring Well Sites (number of wells at 

location in parentheses)

FPA (4)

SA (1)

SC (1)

SB (1)

OFPB (1)

FPC (3)

FPB (3)

FPD (3)

FPE (3)

OFPA (3)

Irrigation canal

A

A‘

Monitoring well transect (section shown in figure 4)

Catch basin used for storm water and runoff sampling
Methods 9

Methods

Drilling, Coring, and Well Installation

Two drilling methods were used to install the monitoring 

wells. Mud-rotary methods were used to install twenty of the 

23 monitoring wells at seven sites (FPA, FPB, FPC, FPD, 

FPE, OFPA, OFPB); multiple 5.1-cm (2.0-in.) diameter 

monitoring wells were installed in a single borehole at each 

of these sites. Monitoring wells were constructed using 

schedule-80 polyvinyl chloride (PVC) with a sand pack that 

usually spanned 0.61 m (2 ft) below the screen to 0.61 m (2 

ft) above the screen; the annular space between each well was 

sealed with bentonite and the well was finished flush with the 

land surface. Hollow-stem auger methods were used to install 

the three water-table wells (SA, SB, SC). Construction of 

the water-table wells was the same as that of wells installed 

using the mud-rotary method, although only a single well was 

installed in each borehole. All drilling equipment was steamcleaned or pressure-washed near each site before being moved 

to the next site.

Cuttings were rinsed and archived by 1.5-m (5 ft) depth 

intervals in order to determine approximate lithology in each 

borehole. Core samples were collected from various depths 

and boreholes to obtain representative vertical, lateral, and 

textural profiles of the aquifer material. The core collection 

procedures and analysis methods and results are detailed in 

Appendix A at the end of this report. Electromagnetic (EM) 

induction, resistivity, and natural-gamma geophysical logs 

were collected at each borehole and were correlated with the 

drill cuttings and core descriptions. The geophysical logs were 

used to help place monitoring well screens and help identify 

the formation boundaries and the textural categories from 

which the hydrofacies were determined. 

Table 1. Construction characteristics and grouping variables for the installed well network in Modesto, eastern San Joaquin Valley, 

California.

[NAVD 1988, North American Vertical Datum 1988; no., number; PSW, public-supply well]

Well 

identification 

no.

Depth 

category/

well type

Land-use 

setting

Altitude above 

mean sea level, 

NAVD 1988 

(meter)

 Depth below 

land surface 

to top of open 

interval 

(meter)

Depth below land 

surface to bottom 

of open interval 

(meter)

Well depth 

(meter)

Depth below 

land surface to 

water 

(meter)

FPA-1 Water-table Urban 27.1 9.1 10.7 11.3 7.8

FPA-2 Shallow Urban 27.1 35.1 36.6 37.2 7.9

FPA-3 Intermediate Urban 27.1 65.5 67.1 67.7 10.1

FPA-4 Deep Urban 27.1 104.6 106.1 106.7 10.5

FPB-1 Water-table Urban 28.3 9.1 10.7 11.9 7.8

FPB-2 Intermediate Urban 28.3 57.0 58.5 59.2 10.9

FPB-3 Deep Urban 28.3 100.0 101.5 102.2 11.5

FPC-1 Water-table Recently urbanized 29.6 11.7 13.3 13.9 8.5

FPC-2 Intermediate Recently urbanized 29.6 61.9 63.4 64.1 11.8

FPC-3 Deep Recently urbanized 29.6 105.8 107.3 108.0 12.2

FPD-1 Water-table Agricultural 31.1 8.5 10.1 10.7 9.3

FPD-2 Intermediate Agricultural 31.1 50.9 52.4 53.1 13.0

FPD-3 Deep Agricultural 31.1 101.8 103.3 109.5 13.5

FPE-1 Water-table Urban 28.7 9.1 10.7 11.7 8.8

FPE-2 Shallow Urban 28.7 29.9 31.4 32.3 8.8

FPE-3 Intermediate Urban 28.7 61.9 63.4 64.3 11.9

OFPA-1 Water-table Urban 28.0 9.1 10.7 11.6 9.2

OFPA-2 Shallow Urban 28.0 29.0 30.5 32.0 9.2

OFPA-3 Intermediate Urban 28.0 57.3 58.8 60.8 11.9

OFPB-2 Intermediate Agricultural 31.1 50.6 52.1 53.2 11.3

SA Water-table Urban 29.6 9.1 10.7 11.7 8.7

SB Water-table Urban 31.1 9.1 10.7 10.8 9.3

SC Water-table Urban 29.6 9.1 10.7 12.3 10.2

PSW PSW Urban 27.1 27.7 111.6 114.9 10.2
10 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Sampling Periods and Collected Analytes 

Wells were sampled between one and five times during 

the study (from October 2003 through June 2005), depending 

on the depth and the time each well was drilled and installed 

(table 2). Most wells were sampled once for an extensive list 

of constituents, including major ions and trace elements, radon 

and radionuclides, dissolved gases, age tracers, stable isotopes, 

volatile organic compounds, and pesticides (table 3). Some 

wells were screened near the water table so that contaminant 

sources could be characterized and temporal variability could 

be assessed. These wells and the public-supply well were 

sampled two or three additional times for an abbreviated list 

of analytes. To characterize the water-quality of different 

parts of the aquifer that contribute water to the public-supply 

well, samples were collected for an extensive list of analytes 

at multiple depths within the pumping public-supply well in 

August 2004 (tables 2, 3). Because of difficulty collecting 

depth-dependent samples, not all analytes were collected 

from the public-supply well in August 2004. The adjacent 

monitoring wells (at site FPA) also were sampled with a more 

limited schedule at that time. 

 “Dry wells” are underground structures that receive 

water from precipitation events during the winter and from 

landscape irrigation and other residential uses during the 

summer. Over 11,000 dry wells are located in the incorporated 

areas of Modesto and are an important pathway for recharge. 

To identify constituents contributed to the dry wells, urban 

runoff samples were collected during a winter storm and a 

summer sprinkler irrigation that produced enough runoff to 

permit sampling. Because of flow conditions, summer runoff 

samples were only collected for major ions, nutrients, and 

stable isotopes. 

Water Sample Collection

Ground-water samples were collected following 

methods developed by the NAWQA program to minimize 

contamination and to promote the collection of high-quality, 

consistent ground-water data among NAWQA study units 

(major hydrologic systems that are the focus of NAWQA 

studies) throughout the Nation (Koterba and others, 1995). 

Ground-water samples were collected using a submersible 

impeller pump connected to polytetrafluoroethylene (PTFE 

or “Teflon”) tubing to minimize cross-contamination between 

successive sampling locations. A minimum of 3 casing 

volumes was removed to purge each well, and water samples 

were collected after readings of pH, dissolved oxygen 

concentration (DO), specific conductance (SC), turbidity 

and temperature had become stable (as defined by Koterba 

and others, 1995). Water-quality samples from the publicsupply well were collected near the well head using Teflon 

tubing connected to a stainless-steel ball valve that tapped the 

discharge pipe. Samples from the public-supply well were 

collected before chlorination or treatment. Sample-collection 

and preservation chambers were used to reduce airborne 

contamination. All equipment was cleaned following each 

sampling event with dilute phosphate-free liquid soap and 

methanol and rinsed repeatedly with de-ionized water.

Five depth-dependent and one composite sample was 

collected from the public-supply well in August 2004. 

Sampling the public-supply well at different depths required 

different sampling techniques and methods. One depthdependent sample was collected at 45.7 m (150 ft) bls using 

the submersible impeller pump and methods described above; 

however, restrictions between the pump bowls and well casing 

prevented using this equipment at depths greater than 45.7 m. 

Table 2. Water-quality sample periods and schedule of analytes collected from monitoring wells, the public-supply well, and surfacewater sites in Modesto, eastern San Joaquin Valley, California, 2003–2005.

[DDS, depth-dependent sample. PSW, public-supply well. Dry wells are underground structures that receive water from precipitation events during the winter 

and from landscape irrigation and other residential uses during the summer]

Date Seasonal 

category Sites/wells Depth levels Schedule

Ground water sampling

October–November 2003 Winter FPA, FPB, FPC, FPD, PSW All Flow-path 1

June 2004 Summer FPA, FPB, FPC, FPD, FPE, OFPA, SA, SB, SC, PSW Water table only Source

August 2004 Summer FPA, PSW (DDS) All Flow-path 1

October–November 2004 Winter FPA, FPB, FPC, FPD, PSW Water table only Source

October–November 2004 Winter FPE, OFPA, OFPB, SA, SB, SC All Flow-path 2

June 2005 Summer FPA, FPB, FPC, FPD, FPE, OFPA, SA, SB, SC, PSW Water table only Source

Surface water sampling

February 2005 Winter Surface urban runoff - FPA, FPE catch basin 2 sites Dry well

June 2005 Summer Surface urban runoff - FPE catch basin 1 site Dry well
Methods 11

Below this depth, a modified small-diameter gas displacement 

pump, which is operated by repeated pressure and release 

cycles of compressed gas, was used to collect water-quality 

samples at 51.8, 57.9, 88.4, and 97.5 m bls (Izbicki, 2004). 

Because this type of sampler does not provide a continuous 

flow of sample water, samples were collected periodically 

in conjunction with water-quality samples after successive 

pressure cycles to monitor pH, SC, DO, and temperature 

to ensure sample consistency. The composite sample was 

collected by connecting teflon tubing to a ball-valve on the 

side of the well head.

Two dry wells at sites FPA and FPE were sampled during 

this study. Samples were collected from both sites in February 

2005, and one sample was collected from site FPE in June 

2005. Storm water and runoff from residential watering is 

collected in catch basins in the street gutter, and overflow from 

the catch basins is routed to dry wells beneath the sidewalk. 

Dry wells were sampled at either the catch basin or within the 

dry well using a peristaltic pump with tygon tubing. VOCs 

were sampled directly from the peristaltic pump to avoid 

losing compounds to volatilization. Approximately three liters 

of water was retrieved and used to process filtered samples for 

pesticides, major ions, nutrients, and trace elements. 

Laboratory Analytical Methods

Samples collected for VOCs, gasoline oxygenates, 

pesticides, DOC, major ions, trace elements, nutrients, and 

radon-222 were analyzed by the USGS National Water Quality 

Laboratory (NWQL) in Lakewood, Colo. Gasoline oxygenates 

(NWQL schedule 4024) are VOCs added to gasoline to 

aid in the combustion of fuel (Rose and Sandstrom, 2003). 

Eight compounds are analyzed on this schedule and include 

the oxygenates themselves and their main degradates (Rose 

and Sandstrom, 2003). The VOCs referred to throughout 

this report are a set of 85 compounds that includes 5 of the 

compounds on the gasoline oxygenate schedule. Samples 

collected for VOCs, including gasoline oxygenates, were 

unfiltered and determined by gas chromatography and mass 

spectrometry using methods described by Connor and others 

(1998) and Rose and Sandstrom (2003). Pesticide samples 

were collected using a 0.7-μm glass-fiber filter and analyzed 

by C-18 solid-phase extraction and capillary column gas 

chromatography and mass spectrometry (Zaugg and others, 

1995; Lindley and others, 1996; Madsen and others, 2003; 

Sandstrom and others, 2001). Water samples collected 

for DOC were pushed through a 0.45-μm glass-fiber filter 

using high-purity nitrogen gas and analyzed by ultra-violetpromoted persulfate oxidation and infrared spectrometry 

(Brenton and Arnett, 1993). Samples collected for major 

ions, trace elements, arsenic speciation, and nutrients were 

filtered through a 0.45-μm capsule filter and analyzed using 

inductively-coupled plasma atomic emission spectrometry 

(ICP-AES), graphite furnace atomic absorption spectrometry, 

and ion-exchange chromatography and colorimetry equipment 

according to methods described by Fishman and Friedman 

(1989), Faires (1993), Fishman (1993), McLain (1993), 

Garbarino (1999), Garbarino and others, 2002, and Patton 

and Kryskalla (2003). Chemical concentrations in water 

are reported in milligrams per liter (mg/L) or micrograms 

per liter (μg/L). Samples to be analyzed for radon-222 were 

collected in a syringe before the water came into contact with 

the atmosphere and were injected into a mineral-oil-based 

scintillation solution in a 30-ml glass scintillation vial. The 

vial was tightly capped, shaken vigorously, and shipped within 

24 hours of collection to the laboratory for analysis by liquidscintillation counting (Prichard and Gesell, 1977); results are 

reported in picocuries per liter (pCi/L).

Table 3. List of analytes for schedules listed in table 2.

[DO, dissolved oxygen concentration; SC, specific conductance. Dry wells 

are underground structures that receive water from precipitation events during 

the winter and from landscape irrigation and other residential uses during the 

summer]

Analyte

Sampling schedule of analytes

Flowpath 1Flowpath 2Source 

well

Dry 

well

Water-quality field measurements

(Temperature, pH, turbidity, 

DO, SC, alkalinity)

X X X X

Volatile organic compounds X X X X1

Gasoline oxygenates X X X1

Pesticides X X X1

Herbicide metablolites X

Major inorganic constituents X X X X

Trace elements X X X

Nutrients X X X X

Arsenic speciation X X

Dissolved organic carbon X X X

Ultra-violet absorbance X X X

Nitrogen and oxygen isotopes in 

dissolved nitrate

X X

Stable isotopes of water X X X

Carbon isotopes X

Sulfur isotopes X X

Uranium isotopes X X

Radium isotopes X X

Radon-222 X X

Gross alpha/gross beta radiation 

(72-hour/30-day)

X X

Gross alpha/gross beta radiation X

Dissolved gases X X

Tritium2 X

Tritium/helium-33 X

Sulfur hexaflouride X

 1February 2005 samples only.

2

 University of Miami, Tritium Laboratory.

3

 Lamont-Doherty Earth Observatory.
12 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Ground-water samples collected for stable isotopes 

of hydrogen, oxygen, sulfur, and nitrogen and oxygen in 

dissolved nitrate were sent to the USGS Stable Isotope 

Laboratory in Reston, Virginia. Stable isotopes are reported 

as ratios of heavier elemental isotopes to their corresponding 

lighter elemental isotopes in per mil (‰ or per thousand) 

relative to defined isotopic standards. Delta-deuterium (δD) 

and delta-oxygen-18 (δ18O) in water were measured by 

a continuous-flow isotope ratio-mass spectrometer using 

hydrogen-gas/water equilibration and carbon-dioxide-gas/

water equilibration techniques, respectively (Coplen and 

others, 1991; Epstein and Mayeda, 1953; Coplen, 1994). 

The results were reported relative to Vienna Standard Mean 

Ocean Water (VSMOW). Dissolved sulfate samples were 

collected and prepared for isotopic analysis using the methods 

of Carmody and others (1997), and the results were reported 

relative to the Vienna Canyon Diablo Troilite (VCDT). 

Isotopes of nitrogen and oxygen in nitrate samples were 

analyzed by bacterial conversion of nitrate to nitrous oxide 

and the results were reported relative to nitrogen gas in air for 

nitrogen and to the VSMOW for oxygen (Sigman and others, 

2001; Casciotti and others, 2002; Coplen and others, 2004). 

Ground-water samples were analyzed for radioactive 

isotopes of uranium and radium by Eberline Services in 

Richmond, California, and are reported in pCi/L. Eighteen 

ground-water samples were analyzed for uranium isotopes 

by standard methods using alpha spectrometry (ASTM D 

3972). Radium-226 and radium-228 were measured by radon 

emanation and beta counting, respectively, following standard 

USEPA methods (EPA 903.1; EPA 904.0—modified). Gross 

alpha and gross beta in ground-water samples were determined 

after 72 hours and 30 days of sample collection. Gross alpha 

and gross beta were determined by evaporating an aliquot of 

sample water and counting alpha and beta emissions from the 

residue (EPA 900.0—modified). 

Ground-water samples were analyzed for dissolved gases 

(N2

, Ar, CO2, CH4, O2) and sulfur hexafluoride (SF6) at the 

USGS Chlorofluorocarbon Laboratory in Reston, Virginia. 

Dissolved gases were measured by gas chromatography after 

being extracted from headspaces of glass samplers (Busenberg 

and others, 1998); sulfur hexafluoride was determined by 

methods described by Busenberg and Plummer (2000).

 Samples collected for tritium and helium-3 were 

submitted to the Noble Gas Laboratory of Lahmont-Doherty 

Earth Observatory for analysis following methods described 

by Clark and others (1976) and Schlosser and others (1989). 

Tritium samples collected in October and November 2004 

were analyzed at the University of Miami Tritium Laboratory 

in Miami, Florida. Tritium from these samples was determined 

by electrolytic enrichment and gas counting as described by 

Östlund, 1987.

Some organic compounds were analyzed more than once 

using different methods. Methyl tert-butyl ether and acetone 

were the only analytes detected in ground-water samples that 

were analyzed by multiple methods. Only the results from 

preferred analytical methods are presented in this report.

Quality-Control Sample-Collection Methods

Quality-control samples were collected to assess 

the validity of ground-water data used for interpretation, 

and to describe any bias and variability in data associated 

with sample collection, processing, transport, storage, and 

laboratory analysis. Three types of quality-control samples 

were collected in the field: blanks, replicates, and spikes. 

Blanks

All blanks were processed using nitrogen-purged 

“universal” blank water that was certified for use in collecting 

blanks for VOCs, pesticides, low-level nutrients, DOC, major 

ions, and trace elements. Three types of blanks were collected: 

equipment, field, and source-solution. Equipment blanks were 

used to determine if the sampling equipment, such as tubing, 

pumps, and fittings, used to collect ground-water samples 

had introduced contamination. Before ground-water samples 

were collected from monitoring well sites, equipment blanks 

were collected at the USGS Sacramento Field Office for 

two sets of ground-water sampling equipment. Field blanks 

were collected to determine if the methods used for sample 

collection, processing, transport, storage, and laboratory 

analysis had introduced contamination and to determine if 

the decontamination procedures used to clean the equipment 

between sampling sites were effective. Source-solution blanks 

were collected to verify that the universal blank water had no 

detectable concentrations of interest. Source-solution blanks 

were only collected for VOCs because they are volatile and 

ubiquitous and therefore can potentially contaminate the blank 

water used for quality-control samples. For this study, field 

blanks were collected for 10 percent of the samples.

Replicates

Replicates were collected sequentially; that is, after all 

of the regular sample containers were filled (and processed, if 

necessary) for each type of analysis, a replicate set of sample 

containers was filled (and processed, if necessary) for that type 

of analysis, and so on for the different types of analyses, until 

all of the regular samples and corresponding replicates were 

collected for one site. For this study, approximately 15 percent 

of the samples had corresponding replicates. 

Spikes

Spikes included surrogates and matrix spikes. Surrogates 

are special performance-monitoring compounds that routinely 

are added to samples for certain types of analytical methods 

that involve extractions. Surrogate compounds are usually 
Hydrogeologic Setting 13

deuterated or fluoridated analogs of the analytes of interest, 

but do not exist in the environment. For this study, surrogates 

were measured in all of the samples for VOCs and pesticides. 

Spike samples were replicate samples to which known 

concentrations of all of the analytes of interest for a method 

were added for the purpose of verifying analyte recoveries in 

the ground water being studied (after subtracting any amounts 

present in the corresponding environmental sample). For this 

study, about 5 percent of VOC and pesticide samples had 

corresponding matrix spikes.

Quality-Control Summary

Analysis of the quality-control samples indicated that 

the quality of most data collected throughout this study is 

good. Detailed discussions of quality-control samples are 

presented at the back of this report in Appendix B. Not all 

data collected for this study is presented or analyzed in this 

report. Data from this study can be retrieved online from 

the NAWQA Data Warehouse at http://infotrek.er.usgs.gov/

traverse/f?p=NAWQA:HOME:2677137420746277. Some 

water-quality results from this study not in the text of this 

report are given in Appendixes C (major ions), D (VOCs), and 

E (pesticides). 

Only one compound (toluene), out of 88 VOCs and 

gasoline oxygenates, had concentrations in water samples 

that could have been the result of contamination during 

sample handling or analysis. Consequently, toluene was not 

used in the data analysis of ground-water samples. Groundwater samples from FPC-1 and FPD-1 had concentrations of 

gasoline compounds (MTBE, m- and p-xylenes) that could 

have been the result of incidental contamination and therefore 

were removed from analysis. A “V” is marked next the values 

for the compounds from FPC-1 and FPD-1 (Appendix D), 

indicating the concentration could be the result of 

contamination. 2-Chloro-4-isopropylamino-6-amino-s-triazine 

(CIAT, a metabolite of atrazine, also known as deethylatrazine 

or DEA) had the lowest recovery of analytes in a spiked 

solution. The analytical method used to determine CIAT 

concentrations in samples has performed poorly, and therefore, 

CIAT concentrations are reported as estimated concentrations 

(Appendix E). Bromide, fluoride, and manganese were 

detected in blanks,which indicates that concentrations in 

ground-water samples could be partly due to contamination 

introduced during sampling or analysis. Fluoride was not 

frequently detected and was not used in the data analysis 

sections of this report. Bromide concentrations less than 0.031 

mg/L (the mean blank concentrations plus three standard 

deviations) and manganese concentrations less than 4.3 μg/L 

(three standard deviations of the mean blank concentration) 

were removed from analysis. As a result, 1 bromide sample 

and 28 manganese samples were excluded. Zinc had an 

average relative standard deviation (RSD) greater than 60 

percent within replicate pairs and was detected in field blanks 

at concentrations similar to environmental concentrations. 

Zinc was not included in the data analysis section of this 

report. Radium-226 had an RSD greater than 20 percent but 

was not used in the data analysis section of this report.

PHREEQC Calculations

Speciation of dissolved uranium and saturation indices 

of uranium minerals and calcite in ground water was 

determined using PHREEQC, version 2.12.5 (Parkhurst and 

Appelo, 1999). The Lawrence Livermore National Laboratory 

thermodynamic database, included with PHREEQC, was 

modified for speciation calculations. Formation constants for 

uranium complexes were updated using thermodynamic data 

reported by Grenthe and others (1992) and Langmuir (1997), 

and two uranium species (Ca2UO2(CO3)

3

0

 and CaUO2(CO3)

3

–2) 

reported by Bernhard and others (2001) were added to the 

database. Some species whose existence has been questioned 

were excluded or modified to represent a more likely species. 

The resulting thermodynamic database is consistent with 

uranium species and formation constants used by Davis and 

Curtis (2003).

Hydrogeologic Setting

Geology

The local-scale study area is on the belt of coalescing 

alluvial fans of low relief that lie between the dissected 

uplands to the east and the nearly flat surface of the valley 

trough (Burow and others, 2004). The aquifer in the 

local study area consists of approximately 120 meters of 

unconsolidated Pliocene-Pleistocene sediments overlying 

Tertiary sedimentary units. The sediments dip to the 

southwest, so progressively older units outcrop at the surface 

to the northeast. All wells in this study were completed in the 

Pliocene-Pleistocene sediments, although some wells in the 

Modesto area penetrate the water-bearing Tertiary Mehrten 

Formation. 

The unconsolidated Pliocene-Pleistocene deposits are 

separated into four units: Laguna, Turlock Lake, Riverbank, 

and Modesto Formations. The Turlock Lake, Riverbank, and 

Modesto Formations form a sequence of overlapping terrace 

and alluvial fan systems (Davis and Hall, 1959; Marchand 

and Allwardt, 1981) indicating cycles of alluviation, soil 

formation, and channel incision that were influenced by 

climatic fluctuations and resultant glacial stages in the Sierra 

Nevada (Bartow, 1991; Weissmann and others, 2005). The 

sediments were deposited by the Stanislaus and Tuolumne 

Rivers as a heterogeneous mixture of unconsolidated, 

interlayered lenses of gravel, sand, silt, and clay (Marchand 

and Allwardt, 1981; Burow and others, 2004). The sediments 

of the Pleistocene formations are primarily derived from 

igneous intrusive rocks of the Sierra Nevada, with minor 

amounts of metavolcanic and metasedimentary rocks derived

from the western metamorphic belt (Gronberg and others, 

1998).
14 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

As part of the TANC study, sediments collected from 

drill cuttings and cores were characterized to depths greater 

than 110 m bls. The lithology of the cuttings and cores was 

primarily silt and clay, which comprises more than 60 percent 

of the sediments collected, although some boreholes 

contained more coarse materials than other boreholes. Optical 

mineralogy of coarse sediments (sands) primarily consisted of 

quartz, feldspar, and biotite, which is consistent with the x-ray 

diffraction results for core samples presented in Appendix A. 

Iron- and manganese-oxide stains were on many surfaces 

of mineral grains, and sand grains were commonly weaklycemented at depths greater than 70 m. 

Recognition of buried soils, or paleosols, has been 

used to identify formation and sequence boundaries in the 

San Joaquin Valley and was used in this study to define 

approximate boundaries of formations and of units within 

formations (fig. 4: Davis and Hall, 1959; Arkley, 1964; 

Marchand and Allwardt, 1981; Weissmann and others, 2002a). 

Between glacial episodes, long periods of landscape stability 

allowed the formation of mature, well-developed soils having 

distinct features that were often preserved after burial by 

successive glacial cycles. These paleosols are texturally clayrich and have distinctive color and structure owing to intensive 

pedogenic alteration. Oxidation of iron during weathering of 

the soil when it was exposed at the surface results in a reddish 

color (Munsell hue 5YR-7.5YR) throughout the soil horizon, 

and evidence of clay translocation, a common pedogenic 

process, can be inferred from coatings on grains and ped 

surfaces (soil aggregates). 

The Modesto Formation, of Pleistocene age, is the 

uppermost formation interpreted in the subsurface and is 

exposed at the surface across most of the local-scale study 

area. The formation is usually unsaturated and comprises 

approximately the top six meters of the subsurface, with 

moderately to poorly sorted unconsolidated arkosic sand, 

gravel, clay, and silt. The soils developed on these sediments 

are young and typically have a poorly defined B-horizon. As a 

result, infiltration is moderate to moderately rapid (Burow and 

others, 2004).

The Riverbank Formation, also of Pleistocene age, 

underlies the Modesto Formation in much of the local-scale 

study area, but is exposed at the surface to the east. Sediments 

are moderately sorted, comprised of unconsolidated arkosic 

sand, gravel, and silt, and less coarse-grained materials than 

underlying formations. Soils developed on the exposed portion 

of the formation are moderately developed with a clay-rich 

B-horizon that is commonly cemented (duripan) (Burow and 

others, 2004). 

Locally, the Turlock Lake Formation is a relatively thick 

Pleistocene formation (up to 90 m thick in places) and is 

divided into lower and upper units by a distinct 

well-developed paleosol. Although the sediments of the 

Turlock Lake formation are commonly indistinguishable 

from those of the older Laguna Formation or of the younger 

Riverbank and Modesto Formations, the sediments are 

characterized by coarsening upward sequences of gravel and 

coarse sand overlying well sorted fine-grained sand, silt and 

clay. 

The Laguna Formation consists of Pliocene age fluvial 

deposits of gravel, sand, silt, and clay that coarsen upwards. 

The sediments are primarily granitic in origin but have a 

higher mafic content in some deposits because of erosion and 

redeposition of volcanic sediments from the older Tertiary 

Mehrten Formation. Sediments from the Laguna Formation 

are more consolidated than those from the younger formations 

and sand grains are commonly cemented. 

Hydrofacies

Sediments were classified into hydrofacies units to 

facilitate development of a spatial correlation model of 

hydraulic conductivity used in the ground-water flow 

model (Burow and others, 2008). Hydrofacies are defined 

here as sediments having similar or predictable hydraulic 

properties on the basis of geologic attributes such as texture 

or depositional environment. The alluvial sediments in the 

study area consist primarily of interlayered lenses of gravel, 

sand, silt, and clay deposited by the Stanislaus and Tuolumne 

Rivers. The sediments were deposited in cyclic aggradation 

sequences linked to Pleistocene glacial episodes and the local 

base level of the San Joaquin River (Davis and Hall, 1959; 

Weissmann and others, 2002a; Burow and others, 2004; 

Weissmann and others, 2005). 

For this study, sediments were classified into four 

hydrofacies categories: gravel, sand, muddy sand, and 

mud (fig. 4). The occurrence and distribution of these four 

hydrofacies for each well site was projected along a groundwater transect. The gravel hydrofacies was characterized as 

cobbles and pebbles in a coarse sand matrix, representing 

channel lag deposits. The gravel hydrofacies was expected to 

have the highest hydraulic conductivity. The sand hydrofacies 

was characterized as moderately well sorted medium to 

coarse sand, representing primary channels of the Stanislaus 

or Tuolumne Rivers or secondary tributary channels. The 

muddy sand hydrofacies was characterized as finely laminated 

fine-grained sand in a matrix of clay and silt, typically 

representing floodplain deposits near channels. The mud

hydrofacies was characterized as massive to laminated silt 

and mud, representing floodplain deposits more distal than 

the muddy sand hydrofacies. The term mud is defined here as 

undifferentiated silt and (or) clay. The mud hydrofacies was 

expected to have the lowest hydraulic conductivity. A more 

detailed discussion of hydraulic properties of the hydrofacies 

presented in this report is given in the companion report by 

Burow and others (2008).
Hydrogeologic Setting 15 Figure 4. Distribution of hydrofacies and approximate location of formation boundaries from boreholes drilled for a study of the Transport of Anthropogenic and Natural Contaminants to a public-supply well in Modesto, eastern San Joaquin Valley, California. Locations of the well sites were projected onto the plane of the transect and do not represent the true distances between neighboring sites. 

0 40

80

0 40 80

FPE

FPA FPB FPC FPD OFPA OFPB

GravelSandMuddy sandMud

-100

-20-40-60-80

0

4020

Elevation, in meters relative to datum

A

SW

A‘

NE

Resistivity - 16-in,in OHM-Meters

Vertical scale greatlyexaggerated

Datum is North AmericanVertical Datum, 1988

0 0.5 1 Kilometer

1 Mile

0.25

0.25 0.5 0

Approx. Sequence Boundary

Paleosol

FPA Borehole / Well Identification

Well casing and screeninterval of well

0

20406080

100120

0 40 80

2040

0 40 80

0

20406080

100120

0 40 80 0 40 80

PSW

80

100120

Ohm-Meters Ohm-Meters

Ohm 

Meters

Ohm-Meters

Ohm-Meters Ohm-Meters

MODESTO FM

RIVERBANK FM

UPPER TURLOCK LAKE FM

LOWER TURLOCK LAKE FM

LAGUNA FM

UPPER TURLOCK LAKE FM

EXPLANATION

0

204060

0

20406080

204060

0

6080

100120

0

Ohm-Meters

0 40 80

0

20406080
16 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Ground-Water Occurrence and Movement

Locally, ground water flows from the northeast, beneath 

agricultural land, towards the urban area to the southwest 

(fig. 5), although a large component of ground-water flow is 

downward. Seasonal fluctuations in pumping and recharge 

caused the water-table elevation to vary between 0.5 and 

0.7 m (1.5 to 2.3 ft) between winter (January 2004) and 

summer (June 2005). The water-table elevation beneath 

urban land (FPA-1) was lowest during peak pumping season 

(August–September), approximately 18.9 m (62 ft) above sea 

level, and was highest during the winter when pumping was 

at a minimum, approximately 19.8 m (65 ft) (fig. 6). Similar 

patterns of maximum and minimum water-table elevation 

changes were expected in the agricultural area. Only one 

agricultural well (FPD-1) had continuous recorders in this 

study. This well is located near an irrigation canal that was 

filled with water between March and October each year. As 

a result of irrigation and possibly seepage from the canal, 

the water table at site FPD was highest during the summer 

irrigation season, approximately 22.5 m (74 ft) in 2004, 

and lowest during the winter, approximately 21.3 m (70 ft). 

Continuous water-level monitoring of deep monitoring wells 

(FPA-4 and FPD-3) indicated a significant response to cyclic 

and seasonal pumping, which created strong vertical gradients 

during the summer months (fig. 6). Vertical gradients in the 

study area ranged from 0.009 to 0.04 during the winter to 0.01 

to 0.11 during the summer of 2004. Steep vertical gradients 

were caused by inter-bedded lenses of clays and silts and 

pumping stress, resulting in semiconfined conditions at depths 

greater than 50 meters (164 feet) bls. Horizontal movement of 

deep ground water was primarily the same direction as that of 

the unconfined aquifer, and typical horizontal gradients were 

about 0.0006. 

Public-Supply Well Construction and Well-Bore 

Flow Characteristics

The public-supply well in this study was drilled in 1961 

to a depth of approximately 115 m (377 ft) bls. It is perforated 

from 27.7 m (91 ft) to 111.6 m (366 ft) bls and has an open 

bottom. On the basis of the monitoring well data and the long 

open interval, it was determined that the water discharged 

from the well was a mixture of water from the three depth 

zones identified in this report: shallow, intermediate, and deep.

 Well-bore flow at different depths within the pumping 

public-supply well was calculated from two sets of tracerinjection data in order to quantify the proportion of water 

contributed to the well-bore from each zone (fig. 7). Tracer 

injections were done every 20 ft and each set of tracer 

injections were offset by 3 m (10 ft) to provide greater detail 

in the vertical direction. Less than 20 percent of the flow 

was from the shallow zone, about 55 percent was from the 

intermediate zone, and 25 percent was from the deep zone. 

Because the pump intake was located at approximately 47 m 

(155 ft) bls, water entering the well above this depth flowed 

downward, while water below the pump intake flowed upward 

(fig. 7). Although perforations started at 27.7 m (91 feet) bls, 

the velocity profile showed that most of the shallow ground 

water did not enter the well until a depth of 39.6 m (130 ft) 

bls. A review of a television log of the well provided by the 

city of Modesto showed the well casing was encrusted with 

mineral deposits, which likely decreased the ability of the 

perforations less than 39.6 m (130 ft) to transmit water from 

the aquifer. Most of the mineral deposit likely consisted of 

calcite, which is supersaturated in shallow ground water; 

discussed later in this report. 

Ground-Water Age 

The age of ground water is often estimated in order 

to characterize ground-water flow paths and determine 

susceptibility of an aquifer to contamination. The age of 

ground water is the time elapsed since a water mass was 

isolated from the atmosphere. Water that enters an unsaturated 

zone moves downward to the water table as “recharge” to the 

ground-water flow system. As additional recharge continues to 

enter the ground-water flow system, older recharge is pushed 

deeper by the newer recharge, resulting in a trend of increasing 

ground-water age with depth. This trend can be perturbed 

locally in areas of ground-water pumping or artificial recharge, 

in areas of above-average hydraulic conductivity, and at sites 

where well-bore leakage can cause hydraulic short-circuiting, 

or as a combination of any or all of these factors. 

In this study, ground-water ages were determined 

by measuring tritium, helium-3, sulfur hexafluoride and 

carbon-14 in ground water. Not all constituents were measured 

at all wells. Each tracer has a timescale for which the age of 

ground water can be deteremined. Tritium, its decay product 

helium-3, and sulfur hexafluoride are tracers of “modern” (less 

than 50 years) ground water because their concentrations in 

the atmosphere increased within the last four or five decades 

as a result of human activities. Sulfur hexafluoride has a more 

recent datable time period than tritium. Carbon-14 is usually 

used to identify ground water that was recharged more than 

1,000 years ago, although carbon-14 concentrations increased 

in the atmosphere within the last 50 years, also as a result of 

nuclear bomb testing. In this study, carbon-14 was collected 

only at deep wells and was used as an indicator of old ground 

water.
Hydrogeologic Setting 17

37°43’ 37°40’

121° 120°57’

EXPLANATION

20 (66)

18 (59)

22 (72)

16 (52)

24 (79)

20.4 (67)

19.8 (65)

21.0 (69)

19.2 (63)

21.6 (70)

Active area of local-scale model grid (Burow and others, 2008)

Public-supply well

Road or highway

Lines of equal water-level elevation (NAVD 88),

in meters (feet in parentheses), spring 2000

20 (66)

Agricultural land-use area

Urban land-use area

0 0.5 1 2 Miles

0 0.5 1 2 Kilometers

Monitoring Well Sites (number of wells at 

location in parentheses)

Irrigation canal

Lines of equal water-level elevation (NAVD 88) - dashed where estimated,

in meters (feet in parentheses), October–November 2004

20 (66)

Generalized direction of ground-water flow

Figure 5. Water-level elevations in water-table wells in October–November 2004 in Modesto, eastern San Joaquin Valley, California.
18 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Carbon-14

Carbon-14 is a naturally occurring radioactive isotope of 

carbon having a half-life of 5,730 years. Because of its long 

half-life, carbon-14 can be used to estimate mean groundwater ages ranging from 1,000 to less than 30,000 years 

before present (Clarke and Fritz, 1997). Carbon-14 dating or 

radiocarbon dating is considered to be an interpretive agedating tool because carbon-14 is measured from dissolved 

carbon in ground water and is not part of the water molecule; 

therefore, one must infer the source of carbon to the ground 

water. For age-dating purposes, the source of carbon was 

presumed to be solely the recharge water that entered the 

ground-water flow system, assuming negligible contribution 

from old organic carbon that might have been in the sediment. 

Radiocarbon dating is based on measuring the loss of the 

parent radionuclide carbon-14 in a given sample, assuming 

the initial carbon-14 concentration is known and that losses or 

gains of carbon-14 are minimal or can be quantified (Clarke 

and Fritz, 1997). The activity of carbon-14 in a sample is 

reported in percent modern carbon (pmC), which indicates 

the carbon-14 activity of the sample relative to that of modern 

carbon, defined as 95 percent of the carbon-14 activity of the 

National Bureau of Standards oxalic acid in 1950 (Clarke and 

Fritz, 1997). 

Carbon-14 activities for the four deep wells in order 

of lowest to highest were 32, 37, 43, and 58 pmC (table 4); 

the highest carbon-14 activity was from FPA-4, which was 

determined to be influenced by shallow water that migrated 

through the public-supply well to deep parts of the aquifer 

during static conditions (discussed later in this report). As 

a result, the FPA-4 sample represents a mixture of young 

and old ground water and does not accurately reflect native 

carbon-14 activities of the deep aquifer zone. Excluding 

this sample, the carbon activities corresponded to a median 

carbon-14 age of approximately 8,300 years (table 4). The 

ages of water from wells FPD-3, FPC-3, and FPB-3 increase 

from north to south, suggesting the source of recharge of old 

ground water could be from the Stanislaus River. Sodium 

concentrations also increased along this transect indicating 

that cation exchange processes may be causing sodium 

concentrations to increase as ground-water ages. 

The carbon-14 ages presented here assume minimal 

dilution from water and rock interactions. Dissolution and 

precipitation of carbonate and oxidation of old organic carbon 

within the aquifer causes dilution of dissolved carbon-14 in 

ground water. Carbon-14 ages that do not account for sources 

of carbon-14 are older than the true carbon-14 age . The extent 

of these reactions have not been rigorously analyzed for this 

report, but water chemistry data indicate these processes are 

probably not significant enough to affect carbon-14 activities 

by more than about 10 percent modern carbon. As a result, 

the calculated carbon-14 ages are interpretive and subject to 

considerable uncertainty, but indicate that the age of the deep 

ground water is thousands of years old. 

Water level elevation from site FPA, 

Urban

Jan Feb Mar MayApr June July Aug Sept Oct Nov Dec

January 2004–December 2004

Jan Feb Mar MayApr June July Aug Sept Oct Nov Dec

January 2004–December 2004

5 5

10

15

20

25

10

15

20

25

Water table

Shallow

Deep

Water level elevation from site FPD, 

Agricultural

Water table

Deep

Elevation of piezometric surface,

in meters above NAVD 1988 

Elevation of piezometric surface,

in meters above NAVD 1988 

Figure 6. Water-level elevation in monitoring wells at an urban and agricultural land-use setting in Modesto, eastern San Joaquin 

Valley, California. 

Wells located beneath agricultural land are at the head of the flow path while the urban wells are at the toe of the flow path.
Hydrogeologic Setting 19

Tritium

Tritium (3H) is a radioactive isotope of hydrogen having a 

half-life of 12.3 years (Lucas and Unterweger, 2000). Tritium 

is produced naturally in the upper atmosphere as a result of 

cosmogenic radiation and in the subsurface from the neutron 

bombardment of lithium-6, although the amount of tritium 

produced from this reaction is often negligible (Clark and 

Fritz, 1997). Large quantities of tritium were introduced into 

the atmosphere from above-ground nuclear bomb testing 

between 1952 and the early 1960s (Michel, 1989). As a 

result, concentrations of tritium in precipitation since 1952 

were substantially elevated and reached a peak in 1963. 

Since that time tritium in precipitation has been gradually 

declining. Because tritium is a radioactive isotope, tritium 

concentrations measured in ground water in 2004 would be 

lower than the concentration would have been at the moment 

when that water first precipitated from the atmosphere. As a 

result, tritium concentrations in ground water were compared 

with the concentration of tritium in precipitation minus the 

amount of tritium that would have decayed between the time 

of precipitation and time of sampling (fig. 8), 

Approximate ages of ground water can be inferred from 

concentrations of tritium using the following criteria: (1) 

ground water containing tritium concentrations less than 1 

tritium unit (TU) is considered to have been recharged before 

1950; (2) ground water containing tritium concentrations 

greater than 1 TU is considered to have been recharged after 

1950. High concentrations of tritium (>10 TU) indicate post1950s ground water that was likely recharged around the time 

of the 1963-bomb peak. 

FPA

-100

-80

-60

-40

-20

0

20

40

Elevation, in meters

0 40 80

Publicsupply 

well

0

20

40

60

80

100

120

Ohm-Meters

Mud

Gravel

Sand

Muddy sand

EXPLANATION

Resistivity - 16-in,

in OHM-Meters

Vertical 

exaggeration X 27

Datum is National Geodetic

Vertical Datum, 1929

Direction of flow within casing 

during pumping condtions

Depth below land surface, in meters

0 20 40 60 80 100

110

100

90

80

70

60

50

40

30

20

10

 0

120

Intermediate

aquifer zone -

Shallow aquifer 

zone - less than

20% of total flow

Deep aquifer

zone - approx.

25% of

total flow

Cummulative flow profile #1

Cummulative flow profile #2

Percent of total flow

Flow rate = 6.1 cubic meters per minute or 1,600 gpm

approx. 55%

of total 

flow 

Figure 7. Percentage of cumulative flow at 6.1-meter (20-foot) intervals and approximate distribution of flow contributed from the 

shallow, intermediate, and deep aquifer zones to a public-supply well in Modesto, eastern San Joaquin Valley, California.
20 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Well 

identi

fication 

no.

Depth 

category

Sample 

date

Carbon-14 dating Tritium and tritium/helium-3 dating

Carbon-14, 

water, 

filtered, 

percent 

modern

Uncorrected 

carbon-14 

age 

(years)

Counting 

error

(± years)

Tritium 

(TU)

Tritium 

age 

category

[

3

He]tritiogenic

 (TU)

Terrigenic 

helium-4

(x 10–8 cm3

@ STP/g of 

water)

[

3

H]/[3He]tritiogenic

 apparent

pistonflow age

(years)

FPA-1 Water-table 10/29/2003 nc na na 3.3 Post-1950 — — Not datable

FPB-1 Water-table 11/19/2003 nc na na 2.5 Post-1950 — — Not datable

FPC-1 Water-table 10/20/2003 nc na na 4.7 Post-1950 — 2.5 Not datable

FPD-1 Water-table 10/21/2003 nc na na 4.6 Post-1950 — — Not datable

FPE-1 Water-table 10/27/2004 nc na na 2.6 Post-1950 nc na na

OFPA-1 Water-table 11/03/2004 nc na na 3.4 Post-1950 nc na na

FPA-2 Shallow 11/04/2003 nc na na 17.3 Circa 

1963

115 — 36.6

FPE-2 Shallow 10/27/2004 nc na na 11.2 Circa 

1963

nc na na

OFPA-2 Shallow 11/03/2004 nc na na 5.2 Post-1950 nc na na

FPA-3 Intermediate 11/05/2003 nc na na 4.0 Post-1950 40.2 8.1 43.1

FPB-2 Intermediate 11/18/2003 nc na na 8.3 Post-1950 68.5 0.5 40.2

FPC-2 Intermediate 11/12/2003 nc na na 7.7 Post-1950 44.1 0.3 34.3

FPD-2 Intermediate 11/13/2003 nc na na 0.9 Post-1950 7.8 0.2 40.2

FPE-3 Intermediate 10/27/2004 nc na na 2.5 Post-1950 nc na na

OFPA-3 Intermediate 11/03/2004 nc na na 1.1 Post-1950 nc na na

OFPB-2 Intermediate 11/02/2004 nc na na 14.2 Circa 

1963

nc na na

FPA-4 Deep 11/06/2003 57.6 4,600 100 1

1.1 Pre-1950 — 101.6 Pre-1950

FPB-3 Deep 11/18/2003 32.3 9,300 100 0.5 Pre-1950 — — Pre-1950

FPC-3 Deep 11/12/2003 36.7 8,300 100 <0.1 Pre-1950 — 20.1 Pre-1950

FPD-3 Deep 11/17/2003 42.6 7,100 100 0.4 Pre-1950 — 15.5 Pre-1950

PSW Flowpath 11/13/2003 98.6 <100 100 6.9 Post-1950 41.4 4.6 34.9

PSW DDSComposite08/17/2004 nc na na 4.6 Post-1950 32.7 37.0 37.6

PSW DDS-45.7 08/17/2004 nc na na 8.5 Post-1950 56.6 — 36.6

PSW DDS-51.8 08/24/2004 nc na na 2.5 Post-1950 nc na Not datable

PSW DDS-57.9 08/25/2004 nc na na 1.4 Post-1950 nc na Not datable

PSW DDS-88.4 08/23/2004 56.7 4,700 100 1

2.7 Pre-1950 nc na Not datable

PSW DDS-97.5 08/19/2004 59.4 4,300 100 1

1.5 Pre-1950 nc na Not datable

Table 4. Carbon-14, tritium/helium-3, and sulfur hexafluoride measurements and interpreted ages of ground water collected from 

monitoring wells in October and November of 2003 and 2004 and in August 2004 in Modesto, eastern San Joaquin Valley, California. 

[DDS-X, depth-dependent sample and depth below land surface; emm, exponential mixing model; epm, exponential piston-flow model; 3H, hydrogen-3; 3He, 

helium-3; pptv, parts per trillion by volume; SF6, sulfur hexafluoride; STP standard temperature and pressure; TU, tritium units. cm3, cubic centimeter; g, gram; 

na, not applicable; nc, not collected; no., number. <, less than; —, not detected]
Hydrogeologic Setting 21

Well 

identification 

no.

Depth 

category

Sample 

date

SF6

 dating Combined tritum/helium-3 

and SF6

 dating

SF6

 (pptv)

SF6

apparent

piston-flow

age

(years)

Lumpedparameter

model

used for

interpreted

mean age

Combined

[

3

H]/[3He] tritiogenic

and SF6 

interpreted

mean age

(year)

FPA-1 Water-table 10/29/2003 3.92 7.5 SF6

, 3H epm 6

FPB-1 Water-table 11/19/2003 3.43 9.25 SF6

, 3H epm 10

FPC-1 Water-table 10/20/2003 2.47 13.5 SF6

, 3H epm 15

FPD-1 Water-table 10/21/2003 2.82 12 SF6

, 3H epm 12

FPE-1 Water-table 10/27/2004 nc na na na

OFPA-1 Water-table 11/03/2004 nc na na na

FPA-2 Shallow 11/04/2003 1.09 22 SF6

, 3H/3He epm 32

FPE-2 Shallow 10/27/2004 nc na na na

OFPA-2 Shallow 11/03/2004 nc na na na

FPA-3 Intermediate 11/05/2003 nc na 3

H/3He epm 81

FPB-2 Intermediate 11/18/2003 0.51 27.75 3

H/3He epm 53

FPC-2 Intermediate 11/12/2003 1.10 22 3

H/3He epm 32

FPD-2 Intermediate 11/13/2003 0.75 25 SF6

, 3H/3He emm 90

FPE-3 Intermediate 10/27/2004 nc na na na

OFPA-3 Intermediate 11/03/2004 nc na na na

OFPB-2 Intermediate 11/02/2004 nc na na na

FPA-4 Deep 11/06/2003 nc na na na

FPB-3 Deep 11/18/2003 nc na na na

FPC-3 Deep 11/12/2003 nc na na na

FPD-3 Deep 11/17/2003 nc na na na

PSW Flowpath 11/13/2003 nc na 3

H/3He epm 35

PSW DDS-Composite 08/17/2004 0.73 25 3

H/3He epm 55

PSW DDS-45.7 08/17/2004 1.73 17.5 3

H/3He epm 35

PSW DDS-51.8 08/24/2004 0.48 28 SF6, 3H epm 38

PSW DDS-57.9 08/25/2004 0.48 28 SF6

, 3H epm 38

PSW DDS-88.4 08/23/2004 nc na na na

PSW DDS-97.5 08/19/2004 nc na na na

Table 4. Carbon-14, tritium/helium-3, and sulfur hexafluoride measurements and interpreted ages of ground water 

collected from monitoring wells in October and November of 2003 and 2004 and in August 2004 in Modesto, eastern San 

Joaquin Valley, California.—Continued

[DDS-X, depth-dependent sample and depth below land surface; emm, exponential mixing model; epm, exponential piston-flow model; 3H, 

hydrogen-3; 3He, helium-3; pptv, parts per trillion by volume; SF6, sulfur hexafluoride; STP standard temperature and pressure; TU, tritium 

units. cm3, cubic centimeter; g, grams; na, not applicable; nc, not collected; no., number. <, less than; —, not detected]

1

Native ground water was generally pre-1950, but samples are, at times, influenced by wellbore flow and storage of water from the shallow 

part of the aquifer to the deep part of the aquifer during non-pumping periods of the public-supply well.
22 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Tritium concentrations in ground water in the study area 

varied with depth (fig. 9), but generally resembled a diluted 

“piston-flow” profile (simple aging of the ground water as 

it moves along a flow path, without mixing or preferential 

flow), mirroring the tritium atmospheric source curve with 

increasing depth below the water table. Water-table wells had 

concentrations of tritium ranging from 2.5 to 4.7 TU and a 

median of 3.4 TU (table 4, fig. 9). These concentrations are 

consistent with recent tritium concentrations in precipitation 

(fig. 8). The well having the highest tritium concentration 

at each site was either the shallow or the intermediate well, 

which suggests that the tritium bomb peak in the aquifer 

was between 30 and 50 m (about 95 and 165 ft) bls (fig. 9). 

However, tritium concentrations in this zone were not as high 

as bomb-peak concentrations (fig. 8). The concentrations 

predicted in ground water were based on the assumption 

of piston-flow transport with no mixing or dispersion of 

the concentration front. Under this assumption, tritium 

concentrations in ground water (fig. 9) would be similar to, 

but not less than, the recharge input shown in figure 8. The 

lower-than-expected concentrations observed in ground water 

was likely caused by mixing and dispersion within the aquifer. 

It is also likely that the wells were not screened in the precise 

depth intervals that would capture the maximum of the tritium 

peak. 

The concentrations of tritium in intermediate wells 

varied widely, from less than 1 TU to more than 14 TU. Some 

intermediate wells containing tritium greater than 3 TU were 

perforated in deeper sediments than wells with less than 3 TU, 

indicating that heterogeneous aquifer materials have affected 

the vertical migration of water and led to earlier arrival of 

the tritium pulse in the more vertically connected parts of the 

aquifer. In other parts of the aquifer, clays and more-flowrestrictive units likely hindered the vertical movement of the 

tritium pulse. 

The concentration of tritium in deep monitoring wells 

was generally less than 1 TU and suggests that water was 

recharged before 1950, which is consistent with carbon-14 age 

results. One deep well, FPA-4, had concentrations higher than 

1 TU (fig. 9), but does not represent concentrations of the deep 

aquifer zone since the sample was affected by shallow water 

that flowed through the public-supply well to the deep aquifer 

zone during static conditions (discussed later in this report). 

As a result, this sample is labeled “pre-1950” ground water. 

Tritium/Helium-3 

Tritium decays to the stable isotope helium-3 (3He). If 

helium-3 is measured in a ground-water sample along with 

its parent radionuclide, 3

H, then, assuming closed-system 

behavior, the time (t) elapsed since recharge is determined by 

(Plummer and others, 1993)

Figure 8. Tritium concentrations expected in Modesto area 

ground water in 2004 that originated as precipitation between 

1953 and 2003. 

Expected concentrations were calculated by subtracting the 

amount of tritium that decayed between the time precipitation fell 

and the time ground water was sampled from the original tritium 

in precipitation. Modified and updated from Michel, 1989.

Post-1950 ground water

likely recharged around 

1963 bomb peak

Post-1950 ground water

Pre-1950 ground water

1950 1960 1970 1980 1990 2000

Year

0

20

40

60

80

100 Tritium, in tritium units

10

TU

1

TU

1–0

TU

0 5 10 15

Tritium, in tritium units

90

60

30

Depth to top of open interval, in meters

Ground water recharged 

around 1963

0

120

20

Post-1950 ground water

likely recharged around 

the 1963 bomb peak

Post-1950 

ground water

Pre1950 

ground 

water

FPA-4

Water-table

Shallow

Intermediate

Deep

PSW

Figure 9. Relation between tritium concentrations in groundwater samples collected during October and November of 2003 

and 2004 and depth to the top perforated interval of monitoring 

wells in Modesto, eastern San Joaquin Valley, California.
Hydrogeologic Setting 23

t = (t /ln(2)) ln[1 + He / H], (1)

where

t is the appar

1/2

3

trit

3

ent ground-water age in years before 

time of sampling,

t 1/2

3

trit

is the half-life of tritium (12.3 years),

He is the tritiogenic concentration of helium-3 in the 

sample (in tritium units), and

H is the sample tritium concentration in tr 3 itium units

Although this calculation is straightforward, determining 

the concentration of helium-3 that directly resulted from the 

radioactive decay of tritium, or the “tritiogenic” concentration 

of helium-3 (3Hetrit), can be complicated because helium in 

ground water can have several sources. Helium concentrations 

in ground water often exceed the concentration predicted from 

air-water equilibrium. Most of the additional helium is usually 

from excess air. Excess air is introduced to ground water when 

entrapped air bubbles dissolve during fluctuations of the water 

table. In older ground water, helium concentrations also can 

exceed concentrations from air-water equilibrium and excess 

air. This additional component of helium is termed “terrigenic 

helium” because the source of the helium is generated from 

geologic sources in the subsurface. Terrigenic helium is 

comprised of helium generated from the decay of uranium 

and thorium in subsurface sediments or from the introduction 

of mantle helium into the ground-water system either by 

mineral weathering of volcanic sediments or from mantle fluid 

migration along faults. Since there are no known faults in the 

study area, mantle fluid migration is not thought to occur in 

the study area. In order to derive an equation for 3

Hetrit (1), a 

mass-balance equation for helium-3 is needed.

3

trit

3

tot

3

eq

3

exc

3

terr

3

tot

He = He He He He

where,

He

− − − ( ) 2

is the total helium-3 concentration in the

sample,

He is th 3

eq e air-water equilibrium (see below)

concentration of helium-3 in water,

He is the concentration of helium-3 from 3

exc the 

entrainment of excess air during recharge, and

He 3

terr is the concentration of helium-3 from terrigenic 

sources, such as mantle derived helium-3, 

or from the decay of lithium-6 (Schlosser 

and others, 1989).

In this study, helium concentrations are reported in cubic 

centimeters at standard temperature and pressure per gram of 

water (cm3 @ STP/g of water). Helium-3 was analyzed using 

isotopic methods and is reported in delta notation as the ratio 

of helium-3 to helium-4 in the sample (Rs) relative to the ratio 

of helium-3 to helium-4 in the atmosphere (Ratm; 1.384 x 10–6; 

[Clarke and others, 1976]):

δ 3He = (R / s a R 1 tm − ⋅) 100 (3)

In order to obtain the tritiogenic concentration of 

helium-3 (3Hetrit), the isotope ratios for other sources of helium 

and the concentration of helium-4 must be known. This 

requires a mass-balance equation for helium-4:

4

tot

4

eq

4

exc

4

terr

4

tot

He = He + He + He (4)

where,

He is the total helium-4 concentration in the sample,

He is the concent 4

eq ration of helium-4 in water at 

equilibrium with the atmosphere,

He is the concentration of helium-4 from the 

entr

4

exc

ainment of excess air during recharge, and

He is the co 4

terr ncentration of helium-4 from terrigenic 

sources, such as mantle derived helium-4, or 

from radioactive decay of uranium-thorium 

series elements (Schlosser and others, 1989).

Finally, equations 2, 3, and 4 were used to derive the 

concentration of tritiogenic helium-3 (3Hetrit):

3

trit

4

tot S

4

tot

4

tot atm

4

terr atm

4

He = He R He He R +

He R + 

× − × ×

× He R (1 ) He R (5)

where 

is the He/ He 

eq atm

4

terr terr

3 4

× − α − ×

α isotope fractionation 

between air and water (0.983) and 

Rterr is the ratio of helium-3 to helium-4 of terrigenic 

helium ( He / He ) (Schlosser and 

others, 1989).

3

terr

4

terr

Calculations of concentrations of tritiogenic helium-3 

(

3

Hetrit) were evaluated for two potential sources of error 

associated with 4Heeq and 4Heterr. The air-water equilibrium 

concentration of helium-4 (4Heeq) was determined from 

neon concentrations in the sample to correct for helium-4 

concentrations from excess air (4Heexc). The neon concentration 

in the atmosphere was calculated using recharge temperatures 

determined from argon and nitrogen concentrations in ground 

water. The mean recharge temperature determined from argon 

and nitrogen concentrations in ground-water samples was 

16.5 degrees Celsius, which is similar to the mean annual air 

temperature of 17 degrees Celsius, suggesting minimal error 

in 4Heeq. 

Ground water containing terrigenic helium (4Heterr) that is 

greater than 10 percent of the total dissolved helium-4 (4Hetot) 

complicates the determination of age because the Rterr must be 

known to calculate the age accurately (Plummer and others, 

2000). The percentage of terrigenic helium in total dissolved 

helium was more than 3 percent in ten of the fifteen samples 
24 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

and more than 20 percent in seven of the fifteen (table 4). 

Ground-water samples that had less than 10 percent terrigenic 

helium were insensitive to changes in the terrigenic helium-3 

to helium-4 ratio (Rterr), while samples that had the largest 

percentage of terrigenic helium-4 were most sensitive. Ages 

for samples having greater than 10 percent were calculated 

using an Rterr value of 6.0 × 10–7 (fig. 10) and indicates a 

small amount of mantle helium (~5 percent) is present in 

ground water. The source of mantle helium is likely from the 

weathering of volcanic sediments associated with the Mehrten 

formation. Varying the value of Rterr by ± 2.0 × 10-7 caused 

ages to differ by as much as 20 years. Helium could not be 

measured in samples from FPB-3 and in depth-dependent 

samples deeper than 50 meters bls; these samples were “not 

datable” (table 4). 

Of the 13 monitoring wells where samples for tritium/

helium-3 dating were collected, only five wells could be dated 

using the tritium/helium-3 method (FPA-2, FPA-3, FPB-2, 

FPC-2, FPD-2: table 4). In addition, the composite sample 

from August 2004, the flow-path sample from October 2003, 

and the shallowest depth-dependent sample (August 2004) 

from the public-water-supply (PSW) well were datable 

(table 4; fig. 10). Four tritium/helium-3 samples collected 

from monitoring wells screened near the water table could 

not be dated because the tritiogenic component of helium-3 

could not be computed. Young waters require very accurate 

determinations of excess air and helium-3 because the 

helium-3 produced from tritium decay is such a small part 

of the total helium-3 in ground water. Diffusion of helium-3 

across the water table could also prevent the determination 

of age using the tritium/helium-3 method. Three of the 

four deep monitoring wells (FPB-3, FPC-3, FPD-3) had 

tritium concentrations below 1 TU and contained terrigenic 

helium in excess of 70 percent (fig. 10; helium could not be 

determined from FPB-3), indicating the ground water from 

these wells was recharged before atomic weapons tests. The 

samples from these wells are categorized as “pre-1950” on 

table 4. This finding is consistent with carbon-14 results that 

suggested deep ground water was recharged prior to nuclear 

bomb testing. The deep monitoring well at FPA-4 had tritium 

slightly above 1 TU (1.1 TU); however, this sample was 

influenced by chemistry of shallow ground water that migrated 

through the public-supply well and recharged the deep aquifer 

during static conditions. This finding and the data for the other 

deep monitoring wells were interpreted as indicating that the 

native ground water at FPA-4 was recharged before 1950. 

The spatial variability in calculated apparent tritium/

helium-3 ages indicate that aquifer heterogeneity may be 

important in controlling the transport of dissolved species. The 

apparent age of the sample from shallow well FPA-2 was 37 

years as of 2004 or the apparent year of recharge was 1967 

(table 4). Apparent ages of samples from the four intermediate

wells ranged from 34 to 43 years. The youngest sample was 

from the deepest intermediate well, FPC-2 and was younger 

than ground water at FPA-2, whose screen was about 25 m 

shallower. This suggests that the vertical movement of water 

is not uniform throughout the study area. At some sites, the 

vertical connection of highly permeable hydrofacies (fig. 

4) permits faster migration of dissolved species to deeper 

parts of the aquifer. In addition, most of these intermediate

samples contained water that was recharged near the time of 

the 1963 bomb-peak, but tritium plus tritiogenic helium-3 

concentrations in these samples were much less than would 

be expected from a piston-flow input of the tritium in 

precipitation (fig. 11). This suggests that dispersion and 

mixing with pre-1950 water diluted the concentrations of the 

atmospheric age tracers. 

0 20 40

400

300

200

100

-100

-200

0

60 80 100

Terrigenic Helium, percent of helium-4

DHelium-3, percent

FPA-3

2003

Air 2004

FPB-3 FPA-4 FPC-3 FPD-3

FPA,B,D-1 FPC-1

FPB-2

FPC-2

FPA-2

FPD-2

Water-table

Shallow

Intermediate

Deep

PSW

Figure 10. Relation between δ3He (delta helium-3) and 

percentage of terrigenic helium in ground-water samples from 

Modesto, eastern San Joaquin Valley, California. 

The dashed line, which diverges from the solubility of helium 

in water at 16.5 degrees Celsius and 33 meters in elevation, 

corresponds to the addition of excess helium with 3

He/4He of 6.0 

× 10–7. The solid line for air corresponds to the addition of helium 

with a 3

He/4He ratio of 1.384 × 10–6.
Hydrogeologic Setting 25

Sulfur Hexafluoride

Sulfur hexafluoride (SF6) is a colorless, odorless, 

nonflammable, nontoxic, stable gas that is used as an electrical 

insulator in high-voltage switches and transformers, and 

as a blanket gas in the melting operations of magnesium 

metal production (Busenberg and Plummer, 2000). SF6 in 

the atmosphere has increased from nearly zero before SF6-

filled electrical switches were introduced in 1953 to more 

than 85,700 tons in 1995 (Busenberg and Plummer, 2000). 

Because of its high rate of increase in the atmosphere, its 

known atmospheric history, and its apparent resistance to 

biodegradation and adsorption onto sediments and organic 

matter, SF6 can be used as a conservative tracer of groundwater age where sediments and rocks do not contribute 

significant natural background concentrations (Busenberg and 

Plummer, 2000). The use of SF6 as a tracer of ground-water 

age has a practical dating limit from the present to 1973, 

when atmospheric concentrations were 3 times the natural 

background concentration and the age uncertainty for waters 

recharged in that year was less than ±3 years (Busenberg and 

Plummer, 2000). 

In the absence of terrigenic sources, SF6

 concentrations 

in ground water result from the solubility of SF6 in water at 

the time of recharge and from the entrainment of excess air. 

Uncertainty in SF6 concentrations in ground-water results from 

estimations of solubility conditions at the time of recharge, 

such as recharge temperature, elevation, salinity, and the 

amount of excess air in ground water. However, an uncertainty 

of ±1 to 2°C in the recharge temperature or ±100 m in the 

recharge elevation usually results in errors of less than one 

year in the estimated age (Busenberg and Plummer, 2000). 

Salinity was not considered a source of error in samples 

from the study area, since specific conductance was less 

than 2,000 microsiemens per centimeter (μS/cm). Higher 

uncertainty can result from underestimating excess air, which 

can cause the ground-water age to be underestimated by 1 

to 2 years for waters recharged between 1973 and 1990, and 

by 1 to 2.5 years for waters recharged after 1990 (Busenberg 

and Plummer, 2000). For this study, recharge temperature and 

excess air were determined from dissolved argon and nitrogen 

concentrations in ground-water samples using an average 

recharge elevation of 30.5 m (100 ft) above sea level (North 

American Vertical Datum, 1988). 

Granitic rocks can contain SF6 in amounts large enough 

to preclude the use of SF6 as a dating tool (Busenberg 

and Plummer, 2000) in certain ground-water systems. 

For example, ground-water samples collected from wells 

perforated in pre-Cambrian granitic sediments in the Middle 

Rio Grande Basin, New Mexico, contained SF6

 concentrations 

in excess of modern concentrations in the atmosphere, which 

prevented the use of SF6 to date ground water in that study 

area (Busenberg and Plummer, 2000). The study area in this 

report contains sediments derived mainly from granitic rocks 

of the Sierra Nevada. Concentrations of SF6 in some groundwater samples were higher than would be indicated by tritium/

helium-3 apparent ages but far below concentrations found 

in other granitic settings. Consequently, granitic rocks in the 

study area were not considered a source of SF6 in samples.

Concentrations of SF6 in eight monitoring wells ranged 

from 0.51 to 3.4 pptv (parts per trillion by volume); they were 

highest at the water table and generally decreased with depth. 

SF6 samples were not collected from the deep wells since it 

was expected that the water from these wells were recharged 

before the introduction of SF6 in the atmosphere (table 4). 

SF6 concentrations in water–table wells corresponded to an 

apparent age range of 7.5 to 13.5 years and a median apparent 

age of 10.5 years. Ground-water samples from shallow and 

intermediate wells had apparent ages ranging from 22 to 

28 years. FPA-2 and FPC-2 had the youngest apparent age 

(22 years) of the four deeper samples; the apparent ages for 

FPB-2 and FPD-2 were 28 and 25 years, respectively. As the 

tritium/helium-3 age dates indicate, vertical ground-water 

velocities are not uniform in space and vertically connected 

pathways can allow contaminants to migrate locally to deeper 

parts of the aquifer.

SF6 samples from the public-supply well indicate 

that some ground water reaching the well first entered the 

ground-water system less than 30 years before sampling. The 

composite public-supply well sample indicated that ground 

water discharging from the well has an apparent age of 

25 years. The apparent ages of depth-dependent samples are 

less than 18 years for the shallowest sample at 45.7 m bls and 

28 years for samples from intermediate depths of 51.8 and 

57.9 m bls (table 4). 

Figure 11. Relation between total tritium (tritium plus tritiogenic 

helium-3) in ground-water samples and public-supply well 

samples in Modesto, eastern San Joaquin Valley, California.

1950 1960 1970 1980 1990 2000

Year

0

100

200

300

400

Tritium, in tritium units

Shallow

Intermediate

October 2003 sample

Composite sample at well head

45.7 meters

Tritium in ppt, (updated from Michel, 1989)

Monitoring well samples

Public-supply well (PSW) samples

PSW depth-dependent samples
26 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Combined Tritium/Helium-3 and 

SF6

 Interpreted Ages

The interpretation of ground-water ages from tracer 

concentrations given above is based on an assumption of 

piston-flow, whereby little dispersion or mixing of water 

having different tracer concentrations (or ages) has taken place 

within the aquifer or well. This assumption allows the direct 

computation of time elapsed since the water was isolated from 

the atmosphere, as if the water has a single age. Although this 

assumption may be valid in certain instances, dispersion and 

mixing is often significant enough to affect the mean age of 

the ground water in wells, even in wells that are screened over 

short intervals (<1.5 m), such as the monitoring wells installed 

for this study (Weissmann and others, 2002b). This leads 

to deviations of the apparent tracer-derived age and the true 

mean age. Inconsistencies between the apparent age of a water 

sample interpreted using different environmental tracers can 

indicate that the sampled ground water represents a mixture 

of water with different ages. The use of multiple tracers, 

therefore, can help account for effects of dispersion and 

mixing on apparent mean ground-water age and can provide 

some insight into the distribution of ages in a ground-water 

sample. 

For this TANC study, the apparent ages calculated 

using the tritium/helium-3 method did not agree well with 

those apparent ages calculated using the sulfur hexafluoride 

method in those samples where both tritium/helium-3 and 

sulfur hexafluoride were collected (table 4). The tritium/

helium-3 ages for FPA-2, FPB-2, FPC-2, and FPD-2 were 

on average about 14 years older than the ages based on 

SF6

 concentrations. In addition, the combined tritium and 

tritiogenic helium-3 (3Hetrit) concentrations in at least two of 

the intermediate wells were lower than expected for water 

affected by fallout from the 1963 bomb-peak, which suggests 

the tritium was diluted with pre-1950 water (fig. 11). Because 

of these inconsistencies, several lumped parameter models, 

which are simple mathematical functions that describe age 

distributions for various combinations of recharge area and 

aquifer geometry (Maloszewski and Zuber, 1982; Zuber, 

1986), were tested as alternatives to account for the age 

distributions represented by the ground-water samples. 

Mean ages of ground-water samples were determined 

from an age distribution described by the exponential-piston 

flow model using software developed by Böhlke (2006). This 

software permits comparison of tracer concentrations with 

predicted concentrations for a variety of lumped parameter 

models. The epm calculation is a fractional summation of the 

exponential mixing and piston-flow model. As a result, the 

epm distribution varies for different fractions of exponential 

mixing and piston-flow. The fractions are usually expressed 

as a ratio of piston-flow to exponential (xP/xE). The epm

used to determine mean ages for monitoring wells had equal 

fractions of piston-flow and exponential mixing (xP/xE = 

0.5/0.5 = 1). A ratio of 0.20 (xP/xE = 0.17/0.83 = 0.20) was 

used for public-supply well samples. Generally, these model 

distributions best represented the tracer concentrations in this 

study.

Exponential-piston flow (epm) mean ages for four watertable wells was based on tritium and SF6 concentrations. 

Ground water from deep monitoring wells was excluded from 

this analysis because the water was more than 1,000 years old 

and did not contain tritium from atomic weapons testing.

Mean ages estimated by the exponential-piston flow 

model for water-table wells ranged from 6 to 15 years and 

were very close to the piston-flow ages calculated from 

SF6 concentrations (table 4). The deeper (shallow and 

intermediate) wells had mean ages that ranged from 32 to 

90 years and were either between the tritium/helium-3 and 

SF6 apparent ages or older than both of these ages. The 

sample from FPD-2 had the oldest mean age of those from the 

monitoring wells containing young tracer concentrations. A 

large portion of the water in this well and in FPA-3 was from 

water recharged before the 1950s. Therefore, the modernwater tracer concentrations were diluted and the mean ages of 

the well samples were greater. 

The public-supply well withdraws water from a large 

volume of aquifer and captures water that varies in age from 

less than 30 years old near the top of the well screen to more 

than 1,000 years old near the bottom. The concentrations 

of tracers of ground-water age in composite samples from 

2003 and 2004 were affected by well-bore leakage and (or) 

the mixture of young and very old ground water within the 

borehole. The mean ages of the public-supply well samples 

was 35 years in 2003 and 55 years in 2004. These ages show 

that the age of ground water pumped from the public-supply 

well depended on the time of year, pumping history, and 

hydrologic conditions. The younger mean age was influenced 

by wellbore leakage, whereas the older mean age in 2004 

more accurately reflects the influence of older ground water 

contributed to the well under normal operation during the 

summer. Identifying the amount and influence of old ground 

water in the public-supply well is an important factor affecting 

the vulnerability of the well to anthropogenic and perhaps 

natural contaminants. While modern tracers are appropriate 

for identifying the most vulnerable water entering the well, 

wells containing a significant amount of older water can dilute 

contaminants contained within the young fraction. 
Water Chemistry 27

Aquifer and Public-Supply Well Vulnerability

The mean ages and carbon-14 ages were used to 

delineate approximate ages of ground water within the aquifer 

(fig. 12) and identify parts of the aquifer that are vulnerable to 

anthropogenic contaminants. Both urban and agricultural landuse activities are potential sources of contaminants within the 

study area. Consequently, parts of the aquifer recharged within 

the last 50 years are particularly susceptible to contaminants 

such as nitrate and volatile organic compounds because the use 

of these compounds has increased since their introduction into 

the environment. It was expected that contaminants associated 

with agricultural activities would be more prevalent in ground 

water located in the northeast, whereas urban contaminants 

would be more prevalent in ground water near the publicsupply well in the southwest.

The contributing recharge area of the public-supply well 

encompasses urban and agricultural land-use settings. The 

proportion of water within the public-supply well that is less 

than 50 years is at least 20 percent. As a result, the water 

chemistry discharging from this well is expected to contain 

anthropogenic contaminants.

Water Chemistry 

The chemical composition of ground water in the study 

area reflects chemical reactions with sediment, directions and 

rates of ground-water flow, and natural and anthropogenic 

constituent sources. On the basis of chemical analyses of 

aquifer material and water sampled within the study area, 

the sections below include (1) a description of groundwater and surface-water chemistry, (2) a description of 

constituents of special interest, and (3) an interpretation of 

water chemistry data together with ground-water flow data to 

help indicate ground-water flow patterns and pathways. This 

characterization allows for improved understanding of groundwater chemistry in different parts of the aquifer system and 

chemical transport to the public-supply well.

FPE

FPA FPB FPC FPD OFPA OFPB

-100

-80

-60

-40

-20

0

20

40

A

SW

A‘

NE

0 10.5 Kilometer

1 Mile

0.25

0 0.25 0.5

Meters

Vertical scale greatly 

exaggerated

20

0 – 20 years

20 – 40 years

40 – 100s years

> 1,000 years

Approximate water-table

 elevation

Approximate land-surface 

 elevation

Well screen interval

Depth below land 

 surface, in meters

Approximate mean age of 

 ground water

EXPLANATION

20

40

60

80

100

20

40

60

80

100

PSW

20

40

60

80

100

Urban Recently urbanized Agricultural

Figure 12. Estimated mean age of ground water in the local study area in Modesto, eastern San Joaquin Valley, California.
28 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Field Measurements

Specific conductance, alkalinity, and pH at different well 

depths indicate the water chemistry of the aquifer is vertically 

stratified (table 5). Specific conductance, which is related 

to the total dissolved ions in solution, was highest near the 

water table (median = 886 μS/cm) and decreased with depth 

to a median value of 201 μS/cm in deep monitoring wells. 

Similarly, alkalinity (reported as calcium carbonate; CaCO3) 

exceeded 450 mg/L near the water table and decreased with 

depth to a minimum of 79 mg/L in deep monitoring wells. pH 

ranged from 6.8 near the water table to a maximum of 8.1 in 

deep monitoring wells and was inversely related to alkalinity 

(Spearman’s rho = –0.63, p-value <0.001) and specific 

conductance (Spearman’s rho = –0.61, p-value <0.001). 

Field measurements of pH, specific conductance, and 

alkalinity of one deep well, FPA-4, were distinctly different 

from those of other deep wells (table 5). The water chemistry 

of this well, which is less than 30 m away from the publicsupply well, was affected by inter-borehole flow within 

the public-supply well during a prolonged period of nonoperation. During static conditions, strong downward vertical 

gradients allowed ground water from the shallow part of the 

aquifer to travel through the public-supply well and recharge 

the surrounding deep portion of the aquifer. Well FPA-4 likely 

received some this water. This effect, which was recognized 

after sampling was completed, influenced the chemistry of 

all water samples collected from FPA-4 during October 2003, 

and details of this process are presented later in the Well 

Construction and Operation Effects on Water-Quality section.

Hydraulic gradients, along with the stratified water 

chemistry, suggest ground water from the water table moved 

downward to depths of at least 35 meters (tables 1, 4). Below 

50 m, specific conductance measurements of intermediate

wells (334 μS/cm) suggest the vertical movement of the 

overlying shallow ground water may be hindered by finegrained strata. However, some intermediate wells have higher 

specific conductance values, indicating the presence of more 

vertically connected pathways that allow water to migrate to 

deeper parts of the aquifer. The lower specific conductance 

values of deep wells likely reflects the chemistry of recharge 

before the area was developed. Recharge prior to development 

was primarily from precipitation and seepage from streams 

near the mountain front. These sources of recharge would 

likely yield low specifice conductance water to the groundwater system. 

Dissolved ions in ground water originate from the 

recharge source itself (for example, precipitation) and from the 

geochemical reactions that release additional ions as the water 

infiltrates down through the soil and unsaturated zone to the 

water table. The primary geochemical reactions in the soil are 

mineral dissolution and cation exchange. The concentration 

of dissolved ions in water reaching the water table can be 

increased by evaporation, transpiration, and application of 

fertilizers or soil amendments during infiltration. 

Dissolved ion concentrations in ground water were 

strongly correlated to alkalinity measurements; Spearman’s 

rho = 0.94, p-value <0.001). Alkalinity is a measure of the 

capacity of water to resist changes in pH. The principal source 

of alkalinity in the study area is bicarbonate, which is the 

primary product of mineral dissolution. Mineral dissolution is 

enhanced by biological activity in the unsaturated zone. The 

partial pressure of carbon dioxide gas (pCO2 (g)) can be as 

much as two-orders of magnitude higher in soils than in the 

atmosphere because of plant root respiration and microbial 

respiration from the decomposition of organic matter. The 

acidity derived from carbon dioxide gas dissolution to form 

aqueous carbon dioxide, CO2 (aq), is buffered by mineral 

dissolution causing the release of ions and the formation of 

bicarbonate in solution. Neglecting processes that tend to 

increase or concentrate dissolved ions, mineral dissolution is 

the primary process affecting the ion composition of water 

reaching the water table. 

The calculated pCO2

 (g) of ground water from watertable and shallow wells averaged about 10–1.6 atm, indicating 

recharge water is strongly affected by biological activity 

beneath both urban and agricultural land (Appendix C). 

Concentrations of carbon dioxide in all ground water in the 

study area were higher than concentrations in the atmosphere, 

but generally decreased with depth. As ground water moves 

below the water table, the fluid loses contact with atmospheric 

gases and with time (hundreds to thousands of years), the 

dissolution of primary minerals will cause the pCO2 (g) to 

increase and the pH to increase in ground water. The chemistry 

of ground water in deep wells likely reflects this process.

Reduction/Oxidation Conditions

Reduction/oxidation (redox) conditions in ground water 

were evaluated using field measurements of dissolved oxygen 

and laboratory measurements of nitrate, manganese, iron, 

and sulfate according to the redox classification methodology 

described by Paschke and others (2007). All groundwater samples were consistent with an oxygen-reducing 

environment. Dissolved oxygen concentrations ranged from 

1.8 to 6.6 mg/L; the median concentration was 4.7 mg/L 

(table 5). 

Dissolved oxygen concentrations measured in water-table

wells varied more than those in deeper wells. This is likely 

because organic carbon content varies with sediment texture, 

and some water-table wells may have been screened in more 

fine-grained strata. The water-table well FPA-1 showed some 

indication of reducing conditions where concentrations of 

manganese and iron were close to significant levels used in the 

redox classification. This well also had the lowest dissolved 

oxygen concentration (1.8 mg/L), but nitrate concentration 

was greater than 0.5 mg/L, indicating this well is likely a 

mixture of water from different redox environments. 
Water Chemistry 29

Table 5. Summary of well characteristics and water chemistry field measurements of depth categories for monitoring wells sampled from October through November of 2003 and 2004, for the public-supply well sampled from October 2003 through June 2005, and for surface-water sites sampled in February and June 2005, in Modesto, eastern San Joaquin Valley, California. [Values reported as ranges with median values in parentheses unless specified otherwise. CaCO3, calcium carbonate; mg/L, milligrams per liter; °C, degrees Celsius; μS/cm, microsiemens per centimeter; PSW, public supply well; na, not applicable]

Site type or 

depth category

Depth to top 

of screened 

interval 

(screen length) 

(meter)

Water-level elevation at 

time of sampling 

(meter)

Number of 

wells/samples Temperature, °C

Specific 

conductance, 

μS/cm @ 25°C

Dissolved 

oxygen, 

mg/L

pH, 

standard units

Alkalinity,

 dissolved, 

mg/L as CaCO3

Surface-water chemistryCatch basin surface sample surface sample 3 12–13 (13) 54–463 (59) 4.9–10.2 (8.4) 7.0–7.6 (7.0) 17–110 (22)

Ground-water chemistryWater-table 8.5–11.7 (1.5) 18.9–21.8 (20.5) 9/9 19.8–23.2 (22.4) 529–1060 (886) 1.8–6.6 (4.8) 6.8–7.4 (7.1) 208–477 (418)

Shallow 29.0–35.1 (1.5) 18.8–19.8 (19.2) 3/3 20.3–21.0 (20.8) 704–1080 (784) 3.6–5.0 (3.9) 7.1–7.4 (7.4) 285–384 (298)

Intermediate 50.6–65.5 (1.5) 16.1–19.8 (17.5) 7/7 20.6–22.4 (21.6) 288–492 (334) 3.4–5.4 (5.0) 7.4–7.8 (7.6) 110–187 (133)

Deep 100.0–105.8 (1.5) 16.6–17.6 (17.1) 4/4 22.7–23.0 (23.0) 175–329 (201) 2.2–3.0 (2.7) 7.4–8.1 (8.1) 79–139 (84)

PSW 27.7 (91.4) na 1/4 19.4–21.2 (20.2) 414–667 (500) 4.4–5.1 (4.6) 7.1–7.5 (7.4) 171–288 (204)
30 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

In general, deep ground water was consistently lower 

in dissolved oxygen than ground water from shallow and 

intermediate depths. Although the aquifer is primarily oxic, 

reducing microenvironments within aquifer material may 

cause localized declines in oxygen concentrations over long 

time scales. 

Stable Isotopes of Hydrogen and Oxygen

Stable isotopes of hydrogen and oxygen were used to 

characterize the isotopic composition of local ground water, 

to identify sources and mixtures of water, and to characterize 

fractionation processes that affect ground-water chemistry. 

The isotopic composition of ground water is usually similar 

to the mean isotopic composition of recharge water from 

precipitation, streamflow, or other sources that infiltrate into 

the ground and the unsaturated zone to reach the water table 

(Kendall and McDonnell, 1998). Precipitation is formed from 

water vapor from the atmosphere that condenses as a result of 

adiabatic cooling (cooling due to decreased pressure) of the 

air mass. Precipitation that condenses at colder temperatures, 

such as snow in the Sierra Nevada, is enriched in the lighter 

isotopes compared with precipitation in the Valley, which 

condenses at warmer temperatures. The Global Meteoric 

Water Line (GMWL), which defines the relation between δ18O 

and δ2H in worldwide fresh surface waters (as determined by 

Rozanski and others, 1993), was used as a reference line to 

compare with local ground-water and surface-water trends 

(fig. 13).

Ground water from FPD-1 and surface water from the 

Stanislaus and Tuolumne Rivers and the canal near FPD-1 had 

the lightest isotopic composition of water in the study area 

(fig. 13). The isotope values reflected values for precipitation 

and runoff from the Sierra Nevada, which is collected behind 

dams and distributed through a network of canals for use 

as irrigation water during the summer growing season. The 

isotopic composition of ground water from FPD-1 was similar 

to the canal and other surface-water compositions (fig. 13), 

indicating water from this well originated from canal leakage 

or surface water used for irrigation.

Most of the ground water sites in the study area had 

heavier isotopic compositions than the surface-water sites 

(fig. 13). However, three samples (from FPA-2, FPC-2, FPE-2) 

had isotope values between those of the surface water and 

most ground water, indicating a mixture of native ground 

water with surface water. Ground water from OFPA-2 and 

FPB-2 could also have been affected by irrigation with local 

surface water, although to a lesser extent. 

The isotopic composition of ground water from deep

wells, except from FPA-4, generally varied less than that of 

ground water from shallower depths. Isotope values for FPA-4 

were lighter than those for the other deep wells because some 

ground water from the shallow zone, which was isotopically 

lighter, recharged the deep zone through the public-supply 

well during static (non-pumping) conditions.

Water used for agricultural and residential irrigation in 

the summer can be partially evaporated during infiltration. 

Evaporation of water fractionates the isotopes of hydrogen 

and oxygen and increases the concentration of dissolved ions 

in solution. Fractionation of the isotopes during evaporation 

is highly dependent on the relative humidity (Clark and Fritz, 

1997). If the relative humidity is 100 percent, values for the 

isotopic composition of water during evaporation forms a 

line having a slope of about 8, similar to the slope of 8.17 

that characterizes the GMWL. If relative humidity is less 

than 100 percent, evaporation forms a line having a slope 

less than 8, which plots to the right of the GMWL. Previous 

investigations in the San Joaquin Valley found slopes between 

4.1 and 5.7 for evaporated ground water (Fujii and Swain, 

1995). 

The isotopic composition of ground water in the study 

area indicates that evaporation did not have a major effect on 

ground-water quality. Consequently, higher concentrations 

of dissolved ions the shallow aquifer were not the result 

of significant evaporative concentration. However, plant 

transpiration can also concentrate solutes in the subsurface 

while leaving the residual water isotopically unchanged (Clark 

and Fritz, 1997). Using ground water for irrigation could also 

increase the concentrations of dissolved ions, but its affect on 

ground-water quality is difficult to quantify. Therefore high 

concentrations of dissolved ions in the shallow part of the 

ground-water system (<50 m bls) may, in part, be a result of 

both transpiration and application of ground water used for 

irrigation. 

Pesticides and Volatile Organic Compounds

Pesticides and volatile organic compounds (VOC) usually 

do not occur naturally, and therefore, the presence of these 

compounds in ground water is the direct result of human 

activities. Pesticide is a generic term for organic compounds 

that are used as fungicides, herbicides, insecticides, and 

rodenticides, and for the degradation products of these 

compounds. VOCs are in paints, solvents, gasoline-related 

compounds, refrigerants, fumigants, and disinfected water. 

VOCs were categorized by compound class in Appendix D to 

help identify likely sources of contamination to the aquifer.

Ten pesticide compounds were detected in 23 groundwater samples from monitoring wells (Appendix E). 

2-Chloro-4-isopropylamino-6-amino-s-triazine (CIAT, 

atrazine deethylated metabolite, also known as deethylatrazine 

or DEA) and simazine were the most frequently detected 

pesticides, both occurring in 39 percent (9 of 23) of the 

monitoring wells. Atrazine was detected in 35 percent (8 of 

23) of the ground-water samples, including all but 2 of the 

samples in which CIAT was detected. Atrazine and simazine 

have similar chemical properties and both have been used in 

rights-of-way and agricultural applications in the eastern San 

Joaquin Valley; although only simazine is currently used in 

agricultural applications (Burow and others, 1998). None of 
Water Chemistry 31

the ground-water samples had pesticide concentrations above 

regulatory drinking-water standards; concentrations were 

typically at least 100 times below regulatory standards.

Results from sampling ground-water indicated 

that pesticides were detected more widely and at higher 

concentrations in water-table and shallow wells and decreased 

with depth in deeper wells (table 6). All of the water-table

wells had at least 1 detected pesticide and up to 4 detected 

pesticides per ground-water sample. About 66 percent of 

pesticides detected in all ground-water samples were detected 

in the water-table wells, indicating that land activities have 

directly affected the chemistry of the water recharging the 

aquifer. Of the 9 water-table wells, 2 wells (FPD-1, FPC-1) 

were in land-use settings that were being farmed or recently 

urbanized (within the last 10 to 20 years). The diuron 

degradate, 3,4-dichloroanaline, was detected in only these two 

wells (Appendix E). This pesticide compound is normally 

associated with agricultural landscapes, whereas pesticide 

compounds detected in wells located beneath urban landscapes 

could not be attributed solely to either urban or agricultural 

land-use settings because the detected pesticides have both 

agricultural and urban uses. Each of the shallow wells and 

three intermediate wells (FPC-2, FPD-2, OFPB-2) had at least 

one detected pesticide, indicating anthropogenic compounds 

that entered the aquifer were transported to deeper depths 

below the water table. One deep well, FPA-4, had a detection 

of CIAT; however, the detection is the result of inter-borehole 

flow within the public-supply well during a prolonged period 

of non-operation and does not accurately reflect the chemistry 

of deep ground water.

VOCs also were most widely detected in water-table

and shallow wells (table 6). None of the VOCs detected in 

this study were above drinking water regulatory standards. 

Trichloromethane (chloroform) was the most frequently 

detected compound and was present in all but one of the 

water-table wells. Previous USGS studies determined that 

chloroform, often formed as a byproduct of chlorinating 

drinking water, is commonly associated with urban landscapes 

where leaky distribution lines or residential lawn watering 

allow treated water to infiltrate to the ground-water system 

(Squillace and others, 1999; Shelton and others, 2001). 

The water-table well where chloroform was not present, 

FPD-1, was located beneath an agricultural field. Therefore, 

chloroform from urban sources was not expected in this well. 

Chloroform was also detected in all of the shallow wells, but 

in only 3 of the 11 deeper wells (Appendix D).

Solvents were the second most widely detected 

compound class in this study. Tetrachloroethene (PCE) was 

the most widely detected solvent and was detected in watertable and shallow wells. This compound was detected at 

concentrations that were within one-tenth of the drinking 

water regulatory limit of 5 μg/L at two shallow wells (OFPA-2, 

FPA-2). All other VOCs detected in this study were less than 

one-tenth of a drinking water standard and were generally less 

than one-hundredth of a drinking water standard. 

Gasoline-related compounds include benzene, toluene, 

ethylbenzene, and xylene (BTEX) compounds and fuel 

additives such as methyl tert-butyl ether (MTBE). These 

compounds were detected in 6 of the 9 water-table wells, 

whereas they were detected in only 3 of the 14 deeper wells. 

No gasoline-related compounds were detected in deep wells.

Pesticides and Volatile Organic Compounds in 

Storm Runoff

Surface-water samples were collected for pesticides and 

VOCs during a storm-water runoff event to characterize the 

composition of precipitation-derived recharge to urban dry 

wells. Nine different pesticide compounds were detected in 

two storm-water runoff samples from dry wells near sites FPA 

and FPE. These compounds were consistent with compounds 

detected in past studies of pesticides in storm-water runoff 

(Kratzer, 1998). Although pesticide compounds were prevalent 

in runoff, only 2 of the 9 pesticide compounds, DCPA and 

pendimethalin, were detected in ground-water samples; the 

other 7 compounds may have been degraded or the runoff 

may have been diluted by native ground water free of these 

compounds after infiltration into the ground-water system.

delta Oxygen - 18, in per mil

delta Hydrogen - 2, in per mil

Stanislaus River

Tuolumne River

Canal near FPD

FPD-1

FPE-2 FPA-2

FPC-2

FPA-4 FPB-2

OFPA-2

-12 -11 -10 -9 -8 -7

-80

-90

-70

-60

-50

Water-table

Shallow

Intermediate

Deep

PSW

Surface Water

Global Meteoric Water

Line (Rozanski & others, 1993)

EXPLANATION

Figure 13. Isotopic composition of ground-water and surfacewater samples collected in October and November of 2003 and 

2004 in Modesto, eastern San Joaquin Valley, California.
32 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Four VOCs, mostly gasoline-related compounds, 

were detected in the two storm-water runoff samples. Mand p-xylene were detected in both samples and benzene, 

4-isopropyltoluene, and methyl ethyl ketone were detected in 

the sample from the dry well near FPE. Methyl ethyl ketone 

is used as a solvent for lacquers, but is also a byproduct of 

cigarette smoke and gasoline exhaust (Delzer and others, 

1996). These compounds are common in urban environments, 

and could have been entrained in precipitation from the 

atmosphere or entrained in runoff from petroleum compounds 

on the ground. As a result, some gasoline constituents in 

ground water near the water table may have originated 

in recharge from precipitation-derived runoff. Leaky 

underground fuel storage tanks likely also contribute gasolinerelated compounds to ground water. 

Seasonal Variability of Volatile Organic 

Compound Occurrence

Concentrations and detection frequencies of VOCs in 

water-table wells varied for samples collected in the winter 

(October–November samples) compared with samples in the 

summer (June and August samples) (table 7). The variability 

is likely related to sources of recharge and the presence or 

absence of VOCs in the atmosphere and their susceptibility to 

degradation. More than 90 percent of the annual precipitation 

falls during the winter months (November thru April). BTEX 

compounds were detected most widely during the winter when 

these compounds may be entrained in precipitation falling 

on urban landscapes or captured by runoff from compounds 

on the ground. In residential areas, runoff containing these 

compounds is routed to dry wells, which allow water to 

immediately enter the ground-water system. Because these 

compounds are readily degradable in aerobic environments 

(Zogorski and others, 2006), their concentrations and the 

number of detections were lower in summer ground-water 

samples than in winter ground-water samples. In contrast, the 

number of detections of compounds such as chloroform and 

PCE, which are more persistent and not normally detected 

in runoff, was lower for the winter ground-water samples, 

as a result of dilution by storm runoff than for summer 

ground-water samples.

Table 6. Summary of pesticides and volatile organic compounds (VOC) collected from monitoring wells sampled from October through 

November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California. 

[%, percent]

Well type Number 

of wells

Pesticides Volatile Organic Compounds (VOC)

Total 

number of 

detections

Number of 

wells with 

one or more 

detections

Number of 

detected 

compounds

Percentage 

of wells 

with one 

or more 

detections

Total

number of 

detections

Number of 

wells with 

one or more 

detections

Number of 

detected 

compounds

Percentage 

of wells 

with one 

or more 

detections

Water-table 9 25 9 8 100% 31 8 11 89%

Shallow 3 4 2 4 67% 12 3 9 100%

Intermediate 7 5 3 3 43% 4 3 3 43%

Deep 4 1 1 1 25% 3 2 3 50%

Table 7. Comparison of selected volatile organic compounds (VOC) in ground water collected from water-table wells during winter and 

summer seasons from winter 2003 through summer 2005 in Modesto, eastern San Joaquin Valley, California. 

[BTEX, benzene, toluene, ethylbenzene, and xylene (gasoline-related compounds); μg/L, microgram per liter]

Seasonal 

sample set

BTEX Compounds Trichloromethane (Chloroform), Tetrachloro-ethene (PCE)

 Detection 

frequency 

(percent)

Number of 

detected BTEX 

compounds

 Detection 

frequency 

(percent)

Median 

concentration 

(μg/L)

Detection 

frequency 

(percent)

Median 

concentration 

(μg/L)

Winter 46.2 5 84.6 0.12 38.5 0.15

Summer 27.8 3 88.9 0.25 61.1 0.04
Water Chemistry 33

Nitrate and Sulfate

Dissolved nitrate was detected in all 23 ground-water 

samples and ranged from 1.0 to 17.4 mg/L as nitrogen (as 

N); the median concentration was 4.0 mg/L (table 8). Nitrate 

was analyzed as nitrite plus nitrate, but only two samples 

had detectable levels of nitrite and both concentrations were 

below the reporting level (0.008 mg/L). Consequently, nitrite 

plus nitrate is referred to as “nitrate” expressed as elemental 

nitrogen throughout the report. Six monitoring wells had 

nitrate concentrations that were above the USEPA maximum 

contaminant level (MCL) of 10 mg/L (U.S. Environmental 

Protection Agency, 2003). Although adults are usually not 

affected by nitrate toxicity, infants and children may develop 

methemoglobinemia from consuming high concentrations 

of nitrate (Comly, 1945). Monitoring wells are not used 

for drinking water and, therefore, are not required to meet 

drinking water standards. None of the four samples from the 

public-supply well had concentrations above the MCL. The 

median nitrate concentration in four samples collected from 

the public-supply well was 5.5 mg/L. Nitrate ranged from 4.4 

to 7.2 mg/L. 

Nitrate concentrations exceeded 3.0 mg/L in 35 percent 

(8 of 23) of the ground-water samples. Concentrations of 

nitrate less than 3 mg/L in oxic ground water could reflect 

natural background concentrations (Mueller and Helsel, 1996).

Dissolved nitrate concentrations may be decreased by 

reducing nitrate to nitrogen gas and, to lesser extent, ammonia. 

Ammonia was detected below the laboratory reporting limit 

in two wells (FPA-1, OFPA-2), and excess nitrogen gas was 

not significant enough to indicate denitrification in most wells. 

Analysis of redox-sensitive species indicated the aquifer 

is primarily oxic, although anoxic conditions may occur in 

microenvironments. As a result, nitrate is expected to persist in 

ground water in the study area.

In water-table wells, nitrate concentrations ranged from 

1.5 to 17.4 mg/L (table 8), and the land use above these wells 

suggests multiple sources of nitrate. Seven out of nine watertable wells are located beneath urban land and most had low 

concentrations of nitrate; the median concentration for these 

wells was 3.1 mg/L. However, two of these wells, SB and 

SC, are located in or near unsewered subdivisions. Nitrate 

concentrations for these two wells were 6.5 and 11.1 mg/L, 

respectively. The two highest concentrations of nitrate, 17.4 

and 12.6 mg/L, were in samples from water-table wells 

located beneath land that was being (FPD-1) or was previously 

(within the last 10–20 years) being farmed (FPC-1) (table 8). 

In samples collected from shallow, intermediate, and 

deep monitoring wells, nitrate concentrations ranged from 1.0 

to 12.9 mg/L and generally decreased with increasing depth. 

Nitrate concentrations in all three shallow monitoring wells 

were above the MCL of 10 mg/L, ranging from 11.8 to 12.9 

mg/L. Nitrate concentrations in intermediate wells ranged 

from 1.6 to 8.7 mg/L. The variability of nitrate in these wells 

indicate that some parts of the aquifer are more vertically 

connected to overlying shallow ground water than others. 

The deep wells had the lowest nitrate concentrations, ranging 

from 1.0 to 2.4 mg/L, which likely reflects natural background 

concentrations.

Nitrate concentrations were positively correlated to 

specific conductance (Spearman’s rho = 0.63, p = 0.003), 

which is consistent with past studies of agricultural landscapes 

throughout the eastern San Joaquin Valley (Nightingale, 1972; 

Burow and others, 1998; Burow and others, 1999). Nitrate 

occurs naturally at typically low concentrations (<3 milligrams 

per liter as N) in ground water; however, in agricultural areas, 

application of nitrogen fertilizers to increase crop yield has 

elevated concentrations of nitrate in shallow ground water in 

the eastern San Joaquin Valley (Nightingale, 1972; Owens 

and others, 1992; Burow and others, 1998). Dissolution of 

fertilizers in surface water or ground water used for irrigation, 

and transpiration during plant growth will increase the 

concentration of dissolved ions in solution. The correlation 

between nitrate and specific conductance in this study likely 

reflects this process. 

All of the major ions, except sodium and potassium, 

were significantly correlated with nitrate concentrations. 

Nitrate is most strongly correlated with sulfate concentrations 

(Spearman’s rho = 0.78, p ≤ 0.001), which is consistent with 

adding gypsum (CaSO4

·2H2O) soil amendments and fertilizer 

during farm operations (fig. 14A). Because the build-up 

of salts and the compaction of soils decrease permeability, 

gypsum is often added to soils to improve infiltration of 

irrigation water. The highest concentrations of sulfate were 

from water-table wells beneath land that was being (FPD-1) 

or was previously (within the last 10–20 years) being 

farmed (FPC-1) and from shallow wells, similar to nitrate 

concentrations. This suggests that the main anthropogenic 

sources of nitrate and sulfate concentrations in the aquifer 

were fertilizers and soil amendments. 

Stable Isotopes of Nitrate and Sulfate

The source of nitrate and, to lesser extent, the source of 

sulfate was further evaluated using stable isotopes of nitrogen 

and oxygen in dissolved nitrate and sulfur in dissolved 

sulfate. The isotopic composition of dissolved nitrate can also 

be an indicator of denitrification in ground water. Isotopic 

composition typically is expressed as a ratio of the less 

abundant isotope (usually the heavier isotope) to the widely 

abundant isotope (usually the lighter isotope), in parts per 

thousand (per mil); this ratio is expressed using standard delta 

notation indicated by the delta (δ) symbol followed by the 

formula for the less abundant isotope. 
34 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California Well identifi- cation no. Depth category/ well type Sample date Nitrite plus nitrate, dissolved (mg/L as N) Sulfate, dissolved (mg/L) Arsenic, dissolved (μg/L) Arsenate (HAsO4–2), dissolved (μg/L as As) Ortho- phosphate, dissolved (mg/L as P) Alkalinity, dissolved, as CaCO3 (mg/L) Uranium (natural), dissolved (μg/L) Uranium-234 (pCi/L) Uranium-238 (pCi/L) Parameter code 00631 00945 01000 62453 00671 29802 22703 22610 22603 LRL 0.06 0.18 0.2 1.2 0.006 0.04 0.04 0.1 0.1 FPA-1 Water-table 10/29/2003 1.49 16.9 15.9 14.2

— 363 24.5 10.9 12.3

FPB-1 Water-table 11/19/2003 3.15 15.4 6.4 7.5 0.755 318 11.1 3.9 4.7

FPC-1 Water-table 10/20/2003 12.6 36.2 3.5 3.0 0.300 2444 23.1 4.8 5.8

FPD-1 Water-table 10/21/2003 17.4 75.5 3.9 3.3 0.590 248 23.4 8.2 8.9

FPE-1 Water-table 10/27/2004 1.77 9.81 9.3 8.8 2.31 208 4.76 1.4 1.7

OFPA-1 Water-table 11/03/2004 3.10 40.0 11.9 12.4 1.47 427 24.9 7.5 8.7

SA Water-table 10/25/2004 2.40 49.3 2.3 0.040 477 29.9 9.1 10.0

SB Water-table 10/25/2004 6.46 38.7 3.9 0.033 427 32.2 9.4 11.0

SC Water-table 10/26/2004 11.1 47.8 6.8 0.044 418 28.5 8.7 10.6

FPA-2 Shallow 11/04/2003 11.8 35.9 10.4 10.4 0.930 285 21.5 7.3 10.9

FPE-2 Shallow 10/27/2004 12.9 63.7 9.1 8.3 0.383 298 21.5 6.0 8.8

OFPA-2 Shallow 11/03/2004 12.5 56.4 8.8 9.8 1.60 384 45.3 13.1 18.1

FPA-3 Intermediate 11/05/2003 3.02 14.4 6.6 6.7 0.737 110 2.56 nc nc

FPB-2 Intermediate 11/18/2003 3.96 23.4 7.6 7.8 1.33 144 6.57 nc nc

FPC-2 Intermediate 11/12/2003 8.72 24.9 5.9 5.9 0.75 187 9.99 nc nc

FPD-2 Intermediate 11/13/2003 3.98 16.4 9.7 9.7 0.507 126 3.37 nc nc

FPE-3 Intermediate 10/27/2004 5.58 22.5 4.9 4.5 0.281 133 4.33 nc nc

OFPA-3 Intermediate 11/03/2004 4.42 9.61 5.2 5.5 0.271 129 3.74 nc nc

OFPB-2 Intermediate 11/02/2004 1.62 6.89 6.7 6.4 2.75 146 4.51 nc nc

FPA-4 Deep 11/06/2003 2.39 11.1 6.5 6.4 1.31 139 1.52 0.1 0.3

FPB-3 Deep 11/18/2003 1.02 2.73 5.8 6.2 0.592 80.4 0.32 nc nc

FPC-3 Deep 11/12/2003 1.45 E0.17 5.5 5.5 0.120 79 0.24 nc nc

FPD-3 Deep 11/17/2003 1.78 5.69 4.4 4.6 0.197 88 0.59 0.1 0.3

MOD025 PSW 11/13/2003 7.23 28.3 6.2 6.0 0.022 288 23.9 nc nc

Table 8. Selected inorganic constituents and isotopes in ground water from monitoring wells sampled from October through November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California. [The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent or property. LRL, laboratory reporting level; mg/L, milligram per liter; μg/L, microgram per liter; pCi/L, picocurie per liter; per mil, per thousand; E, estimated; —, not detected; nc, not collected. N, nitrogen, P, phosphorous; As, arsenic; CaC O3, calcium 

carbonate]
Water Chemistry 35

Well identification no. Depth category/ well type Sample date Uranium-234 to uranium-238 (ratio)

Deuterium/

protium ratio, 

water, 

unfiltered, 

per mil

Nitrogen-15 /

Nitrogen-14 

ratio in nitrate 

fraction, 

water, 

filtered, per mil

Oxygen-18 /

Oxygen-16 

ratio in nitrate 

fraction, 

water, 

filtered, 

per mil

Oxygen-18 /

Oxygen-16 

ratio, 

water, 

unfiltered, 

per mil

Sulfur-34 /

Sulfur-32 

ratio in sulfate, 

water, 

filtered, 

per mil

Parameter code 82082 82690 63041 82085 49932

LRLFPA-1 Water-table 10/29/2003 1.13 –64.8 8.6 2.8 –9.0 2.1

FPB-1 Water-table 11/19/2003 1.19 –62.9 4.8 1.4 –8.7 3.9

FPC-1 Water-table 10/20/2003 1.19 –65.9 6.8 –0.2 –9.1 7.0

FPD-1 Water-table 10/21/2003 1.09 –83.8 7.3 5.6 –11.2 5.8

FPE-1 Water-table 10/27/2004 1.22 –57.3 4.4 0.5 –8.1 2.0

OFPA-1 Water-table 11/03/2004 1.16 –61.9 4.5 0.5 –8.6 3.3

SA Water-table 10/25/2004 1.10 -66.6 4.6 0.1 –9.0 nc

SB Water-table 10/25/2004 1.17 –59.9 6.5 0.9 –8.4 nc

SC Water-table 10/26/2004 1.22 –65.1 6.9 –0.5 –8.8 nc

FPA-2 Shallow 11/4/2003 1.49 1–70.9 6.4 –1.1 1–9.8 7.3

FPE-2 Shallow 10/27/2004 1.46 –72.9 5.1 1.4 –10.0 7.6

OFPA-2 Shallow 11/03/2004 1.38 –68.0 7.1 0.0 –9.3 7.2

FPA-3 Intermediate 11/05/2003 nc –63.3 4.3 0.2 –8.6 5.3

FPB-2 Intermediate 11/18/2003 nc –68.4 4.8 0.3 –9.2 4.9

FPC-2 Intermediate 11/12/2003 nc –73.5 5.1 –0.7 –9.9 6.0

FPD-2 Intermediate 11/13/2003 nc –61.3 5.4 1.3 –8.3 2.9

FPE-3 Intermediate 10/27/2004 nc nc 7.0 6.1 nc 7.5

OFPA-3 Intermediate 11/03/2004 nc nc 4.3 3.9 nc 2.5

OFPB-2 Intermediate 11/02/2004 nc nc 6.0 –0.1 nc 3.9

FPA-4 Deep 11/06/2003 1.98 –68.2 5.7 0.1 –9.4 4.3

FPB-3 Deep 11/18/2003 nc –66.4 3.4 0.2 –9.1 1.8

FPC-3 Deep 11/12/2003 nc –65.1 3.2 0.3 –8.9 nc

FPD-3 Deep 11/17/2003 2.11 –64.8 4.3 1.0 –9.0 1.1

MOD025 PSW 11/13/2003 nc –66.7 5.8 –0.2 –9.2 7.45

Table 8. Selected inorganic constituents and isotopes in ground water from monitoring wells sampled from October through November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California. —Continued[The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent or property. LRL, laboratory reporting level; mg/L, milligram per liter; μg/L, microgram per liter; pCi/L, picocurie per liter; per mil, per thousand; E, estimated; —, not detected; nc, not collected. N, nitrogen, P, phosphorous; As, arsenic; CaC O3, calcium carbonate]

1Value from sample collected on 8/15/2006.2Laboratory value.
36 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Deep

Intermediate

PSW

Shallow

Water-table

EXPLANATION

denitrification

FPA-1

OFPA-2

SC

SB

FPD-1

FPC-1 OFPA-2

FPA-1

FPA-2

FPE-2

FPD-1

Gypsum dissolution

FPA-2

FPE-2

SC

SB

MCL MCL

Transpirative

concentration?

Median D15N 

and D18O

0 1 2 3 4 5 6 7 8 9 10

delta Nitrogen - 15, in per mil

-2

-1

0

1

2

3

4

5

6

7

8

delta Oxygen - 18, in per mil

E

0 1 2 3 4 5 6 7 8 9 10

delta Nitrogen - 15, in per mil

0

1

2

3

4

5

6

7

8

9

10

delta Sulfur - 34, in per mil

D

0 1 2 3 4 5 6 7 8 9 10

delta Nitrogen - 15, in per mil

0

2

4

6

8

10

12

14

16

18

20

Nitrate, in milligrams per liter

B

0 10 20 30 40 50 60 70 80 90 100

Sulfate, in milligrams per liter

0

1

2

3

4

5

6

7

8

9

10

delta Sulfur - 34, in per mil

C

0 10 20 30 40 50 60 70 80 90 100

Sulfate, in milligrams per liter

0

2

4

6

8

10

12

14

16

18

20

Nitrate, in milligrams per liter

A

Figure 14. Relations among sulfate, nitrate, and isotopes of nitrogen and oxygen in dissolved nitrate in ground water collected from 

monitoring wells in October and November of 2003 and 2004. (A) Relation between sulfate and nitrate. (B) Relation between delta 

nitrogen-15 (δ15N) and nitrate. (C) Relation between sulfate and delta sulfur-34 (δ34S). (D) Relation between δ15N and δ34S. (E) Relation 

between δ15N and delta oxygen-18 (δ18O). MCL, U.S. Environmental Protection Agency maximum contaminant level; PSW, public-supply 

well.
Water Chemistry 37

δ15N values for dissolved nitrate in ground water ranged 

from 3.2 to 8.6 per mil and δ18O values for dissolved nitrate 

ranged from –1.1 to 6.1 per mil (table 8). Nitrate from FPA-1 

had the heaviest δ15N value, 8.6 per mil and had a δ18O value 

of 2.8 per mil (fig. 14E). The isotopic composition of nitrate 

for this sample was consistent with denitrification of nitrate 

having an initial composition similar to the median isotopic 

composition of ground water (δ15N = 5.1 per mil, δ18O = 

0.3 per mil) in the study area (Kendall, 1998). Measurements 

of redox-sensitive species from this water, such as manganese 

and iron, suggest this water was partially reduced or was 

a mixture of water having different redox conditions. 

Denitrification causes enrichment in nitrogen-15 as well 

as oxygen-18 in the residual nitrate because less energy is 

required to break molecular bonds between lighter isotopes 

(nitrogen-14, oxygen-16) during microbial reduction. As 

denitrification proceeds, the plotted isotopic composition of 

the residual nitrate forms a line having a slope of about 0.5 

(Kendall, 1998) (fig. 14E). Although denitrification removed 

some nitrate in ground water from this well (FPA-1), as a 

whole, nitrate concentrations in ground water in the study area 

were not significantly affected by denitrification. 

The isotopic composition of nitrate in ground water 

is consistent with multiple sources of nitrogen that include 

inorganic fertilizers, soil nitrogen, and septic waste (Kendall, 

1998). Fogg and others (1998) reported δ15N values derived 

from inorganic fertilizers that were less than 7 per mil 

nationwide and typically less than 5 per mil in California. 

Fogg and others (1998) reported also that δ15N values for 

septic sources of nitrate are typically greater than 7 per mil 

nationwide; however, δ15N values below 7 per mil for septic 

sources are not uncommon.

δ15N values generally increased with nitrate concentrations (fig. 14B). High concentrations of nitrate in ground water 

in the eastern San Joaquin Valley are usually the result of 

agricultural applications of nitrogen fertilizers, although dairy 

farms and septic leachate contribute nitrate to ground water 

in localized parts of the Valley. The absence of dairy farms in 

the study area suggests nitrate concentrations in local ground 

water are not influenced by dairy farms. Some older parts of 

the city remain unsewered, and therefore, septic leachate could 

be a source of nitrate in the local ground water.

Anion ratios of chloride to bromide (Cl/Br) were used to 

evaluate the influence of septic sources on nitrate in ground 

water. Panno and others (2006) report Cl/Br for septic sources 

within a range of 65 to 5,404 and a median value of 769. As a 

whole, within the study area, the ratios of chloride to bromide 

for ground water were low (generally less than 250). However, 

three wells—SC, SB, and OFPA-2—yielded Cl/Br ratios 

above 300 and δ15N values greater than 6 per mil (fig. 15). 

Jagucki and Darner (2001) considered a Cl to Br ratio of 400 

to be the theoretical upper limit for natural water, but ratios 

between 110 and 400 could indicate anthropogenic sources. 

Because wells that yielded Cl to Br ratios between 110 and 

250 included wells that had low nitrate concentrations (FPE-1, 

SA, FPC-3) and deep ground water that was recharged before 

the area was developed, a more conservative range is 250 to 

400 for wells that may be influenced by septic sources in the 

study area. However, the anion ratios derived for our study 

area were significantly lower than the median value reported 

by Panno and others (2006). This indicates that septic leachate 

may be in some ground water, but probably is not the most 

significant source of nitrate in ground water in the study area. 

High nitrate concentrations (greater than 10 mg/L) in 

ground water beneath land that was being farmed (FPD-1) 

or was previously (within the last 10–20 years) being farmed 

(FPC-1) yielded two of the highest δ15N values (7.25 and 6.8, 

respectively); the two wells in these areas had oxic water and 

yielded low Cl/Br ratios (fig. 15). This suggests that fertilizer 

applied during farm operations was enriched in nitrogen-15; 

however, δ15N values for other wells containing nitrate 

concentrations greater than 10 mg/L varied between about 5 

and 7 per mil. The lowest nitrate concentrations (less than 3 

mg/L) and lightest δ15N values (less than 5 per mil) were from 

deep wells and are consistent with natural nitrogen sources in 

soil reported by Fogg and others (1998). This indicates that the 

source of nitrate for concentrations between 3 and 10 mg/L is 

likely a mixture of natural and fertilizer sources of nitrogen.

δ34S values for naturally-occurring sulfur compounds 

throughout the environment range from less than -50 per mil 

to greater than 50 per mil (Krouse and Mayer, 2000). The 

wide range in δ34S is due to the wide range in valence states 

–2 to +6) and its many functional groups (Krouse and Mayer, 

2000). The isotopic composition of sulfur in marine evaporite 

deposits reflects the isotopic composition of seawater at 

the time of deposition. The isotopic composition of sulfate 

Figure 15. Relation of delta nitrogen-15 in dissolved nitrate 

and chloride to bromide ratios (Cl/Br) in ground-water samples 

collected from monitoring wells in October and November of 2003 

and 2004 in Modesto, eastern San Joaquin Valley, California.

PSW, public-supply well.

0 2 4 6 8 10

delta Nitrogen 15, in per mil

0

100

200

300

400

Chloride to Bromide, in mass ratio

OFPA-2

SC

SB

FPA-1

FPD-1

FPC-1

FPE-1

FPC-3

Water-table

Shallow

Intermediate

Deep

PSW
38 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

in seawater has varied between 10 per mil and 35 per mil 

through geologic time (Krouse and Mayer, 2000). The isotopic 

composition of sulfate in modern seawater is 21 per mil (Clark 

and Fritz, 1997).

Values of δ34S for dissolved sulfate in ground water in 

the study area ranged from about 1 per mil for deep ground 

water to almost 8 per mil for many wells that had sulfate 

concentrations greater than 20 mg/L (fig. 14C, table 8). The 

lower δ34S values (less than about 2.0 per mil) for low sulfate 

samples were interpreted as indicating natural sources of 

sulfate, such as the atmosphere. Low sulfate concentrations 

and low δ34S values could result from sulfate reduction also, 

although no hydrogen sulfide was detected in the ground 

water. The highest δ34S values are most likely from application 

of gypsum soil amendments, which is enriched in 34S. Ground 

water in some wells had higher sulfate concentrations than 

could be explained by the application of gypsum. Sulfate 

concentrations from these wells could have been affected by 

transpiration and recycling of ground water. 

The isotopic composition of nitrate is positively 

correlated to the isotopic composition of sulfate in oxic 

ground water (Spearman’s rho = 0.72, p-value = 0.003; note: 

FPA-1 was removed because of theorized denitrification; 

fig. 14D). This correlation indicates that sources of sulfate and 

nitrate in ground water were likely derived from agricultural 

practices. Some of the major forms of fertilizer in the study 

area are calcium-nitrate, urea, and ammonium-sulfate (Vance 

Kennedy, retired USGS and local farmer, oral commun. 

2005). In addition, gypsum is added annually to promote 

infiltration (Vance Kennedy, retired USGS and local farmer, 

oral commun. 2005). The dissolution of these compounds 

during irrigation is consistent with the strong correlations 

among dissolved nitrate, sulfate, and the isotopic composition 

of nitrogen and sulfur in ground water. 

Arsenic

Arsenic, a naturally-occurring trace element, is common 

in drinking water, but can be hazardous to human health at 

high concentrations. The MCL for arsenic was lowered from 

50 to 10 μg/L as of January 23, 2006. Arsenic was detected in 

all ground-water samples and ranged from 2.3 to 15.9 μg/L: 

the median concentration was 6.5 μg/L (table 8). The publicsupply well was sampled twice; arsenic concentrations from 

these samples were 6.2 and 5.8 μg/L. 

Granitic rocks of the Sierra Nevada contain minerals that 

have arsenic as a minor constituent (Welch and Lico, 1998), 

and arsenopyrite is a common mineral associated with sulfide 

deposits within the western metamorphic belt of the Sierra 

Nevada (Diggles and others, 1996). Arsenic in these minerals 

can be released into solution by weathering and then removed 

from the water because of strong attractions to mineral grain 

surfaces and organic material. The sediment and organic 

material in the eastern San Joaquin Valley was transported and 

deposited there by streams draining the Sierra Nevada during 

a series of glacial advances and retreats in the Pleistocene and, 

in lesser amounts, during deposition of fluvial sediments in the 

Holocene. Arsenic concentrations from soils of the eastern San 

Joaquin Valley are generally less than 4 mg/kg (Tidball and 

others, 1986). 

Within the study area, concentrations of HNO3 

extractable iron and arsenic from aquifer sediments were 

weakly correlated (Appendix A; Spearman’s rho = 0.75, 

p-value = 0.1, n = 6), which suggests arsenic is adsorbed to 

ferric oxyhydroxides and that concentrations in ground water 

are predominately controlled by adsorption processes. The 

adsorption of arsenic is a function of the interrelationship of 

mineral surfaces, dissolved arsenic species, pH, and competing 

ions (Stollenwerk, 2003). 

Arsenic in ground water is normally a protonated 

inorganic oxyanion of trivalent arsenite (H3AsO3

0

) or 

pentavalent arsenate (HAsO4

2-). Arsenate predominates in 

oxidizing environments, whereas arsenite predominates in 

reducing environments. In alkaline ground water, both arsenate 

and arsenite are more weakly adsorbed as pH increases 

because deprotonation of the mineral surface causes sorption 

sites to be less abundant (Stollenwerk, 2003). Previous 

investigations found that, in general, arsenic is more weakly 

adsorbed to aquifer sediments when pH values are greater than 

7.5 (Manning and Goldberg, 1997; Welch and others, 2000).

Under reducing conditions, ferric oxyhydroxides can 

dissolve causing arsenic to be liberated from adsorption 

sites, leading to increased dissolved arsenic concentrations in 

solution. The species of arsenic liberated by this process may 

be either arsenate or arsenite, depending on which species was 

adsorbed. Because arsenate species generally predominate in 

the range of redox potential at which iron oxyhydroxides are 

stable, the primary form of adsorbed arsenic in the aquifer 

is expected to be arsenate (Hem, 1989; Stollenwerk, 2003). 

Therefore, the presence of dissolved iron and arsenate in 

ground water may indicate reductive dissolution. The presence 

of arsenite and absence of dissolved iron and sulfide may 

indicate reductive desorption.

Speciated arsenic samples were collected sequentially 

with non-speciated arsenic samples to determine the dominant 

form of arsenic in ground water (table 8). Arsenite was not 

detected above the reporting limit of 1 μg/L. This finding 

suggests the redox state of ground water was above the redox 

potential for arsenate reduction and was consistent with an 

oxic environment. However, FPA-1 had the highest nonspeciated arsenic concentration (15.9 μg/L) and the highest 

dissolved iron and manganese concentrations (Appendix E) 

of all ground-water samples. Although oxygen was present 

in the ground-water sample, the presence of dissolved iron 

and manganese was inconsistent with an oxic redox state and 

suggests this water was partially reduced. Because arsenic 

was dissolved as arsenate, reductive dissolution of iron 

oxyhydroxides and subsequent release of adsorbed arsenic was 

likely responsible for the elevated arsenic concentrations in 

this ground water.
Water Chemistry 39

Arsenic was weakly correlated to dissolved manganese 

(spearman’s rho = 0.40, p-value = 0.06) and iron 

concentrations (Spearman’s rho = 0.38, p-value = 0.07) 

in ground water. Both metals form oxyhydroxides and are 

potential sorption sites for many trace elements. However, 

both metals are generally insoluble in oxidizing ground water 

and the weak correlation reflects the redox condition of the 

aquifer more than the source of arsenic. Therefore, dissolved 

arsenic in local ground water may be controlled by other 

processes that affect arsenic adsorption to aquifer sediments.

A previous investigation of arsenic concentrations in 

the San Joaquin Valley found arsenic concentrations were 

positively correlated with pH, indicating that higher pH 

inhibit dissolved arsenic from adsorbing to aquifer sediments 

(Belitz and others, 2003). In that study, 42 percent of wells 

with arsenic concentrations greater than 10 μg/L had pH 

values greater than 8. In this study, arsenic and pH were 

not correlated (spearman’s rho = –0.13, p-value = 0.5), 

suggesting pH was not a dominant control on dissolved 

arsenic concentrations in the study area. However, only three 

wells had pH values that were greater than or equal to 8 and 

the highest pH value was 8.1. The number of ground-water 

samples having a pH higher than 8 may have been too small to 

indicate a correlation with arsenic concentrations.

Anions may effectively compete for sorption sites with 

arsenic if the number of sorption sites in aquifer sediments 

is limited. Orthophosphate was positively correlated with 

arsenic concentrations, suggesting that phosphate anions 

may prevent the adsorption of arsenic (Spearman’s rho = 

0.5, p-value = 0.02; fig. 16). Phosphate and arsenate form 

similar surface species, and arsenate adsorption generally 

decreases with increasing phosphate concentrations for several 

adsorbents (Manning and Goldberg, 1996; Stollenwerk, 

2003). Orthophosphate concentrations ranged from 0.03 to 

2.75 mg/L; the median concentration was 0.59 mg/L (table 8). 

Neither arsenic nor orthophosphate were related to land use or 

specific depth zones within the aquifer. 

Uranium

Uranium concentrations in ground-water samples ranged 

from 0.2 to 45.3 μg/L; the median concentration was 10.0 μg/L 

(table 8). Samples were collected from the public-supply 

well twice. Concentrations of dissolved uranium from these 

samples were 23.9 μg/L and 11.2 μg/L. Two ground-water 

samples from monitoring wells had uranium concentrations 

that were above the MCL of 30 μg/L, although these wells 

were not required to meet drinking water standards. Uranium 

is a kidney toxin (National Academy of Sciences, 1980; U.S. 

Environmental Protection Agency, 2003) and a radionuclide 

(U.S. Environmental Protection Agency, 2003). 

Granitic rocks of the Sierra Nevada contain minerals that 

have uranium as a minor constituent, and weathering of these 

minerals releases uranium to water (Otton and others, 1989; 

Thomas and others, 1993). Dissolved uranium that adsorbed 

to local sediment was transported to the eastern San Joaquin 

Valley by glacial and fluvial processes during the Pleistocene 

and Holocene. Soil samples collected south of the study area 

were analyzed for uranium as part of the National Uranium 

Resource Evaluation (NURE) study (Smith, 2001) and had 

an average abundance of 3.1 mg/kg soil. Average uranium 

abundance in soils in the study area were not analyzed during 

the NURE program; however, aerial gamma-ray surveys of 

uranium in the San Joaquin Valley (fig. 17) have shown similar 

concentrations (Phillips and others, 1993). Concentrations 

of extractable uranium in aquifer sediments in the study area 

increased with the depth of the sediments (Appendix A; 

Spearman’s rho = 0.75, p-value <0.03, n = 10). Uranium 

adsorption experiments showed adsorption of uranium is not 

limited in intermediate and deep sediments. However, uranium 

did not adsorb to shallow and water-table sediments in the 

experiments, suggesting that geochemical conditions in the 

shallow parts of the aquifer (<50 m bls) may favor the release 

of uranium from sediments.

Water-table

Shallow

Intermediate

Deep

PSW

0 2 4 6 8 10 12 14 16 18 20

Arsenic, micrograms per liter

0

1

2

3

4

5

Orthophosphate, in milligrams per liter

FPA-1

MCL-US

Figure 16. Relation between arsenic concentrations and 

orthophosphate in ground-water samples collected from 

monitoring wells in October and November of 2003 and 2004 in 

Modesto, eastern San Joaquin Valley, California.

MCL, U.S. Environmental Protection Agency maximum 

contaminant level; PSW, public-supply well.
40 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Uranium (VI) is the most stable valence under oxidizing 

conditions, and in the absence of complexing ions in solution, 

the uranyl cation (UO2

+2) is the predominate species of 

uranium in alkaline ground water (Langmuir, 1978). If 

ground water was under these conditions, mobilization and 

transport of uranium (VI) would be limited by its strong 

adsorption to mineral surfaces (Waite and others, 1994; 

Kohler and others, 1996; Pabalan and others, 1998; Davis 

and others, 2004; Catalano and Brown, 2005). However, 

carbonate and phosphate form strong neutral and anionic 

complexes with uranyl, which can inhibit the attraction of 

uranyl to mineral grain surfaces and lead to higher dissolved 

uranium concentrations in ground water (Langmuir, 1978; 

Davis and others, 2004). In addition, Bernhard and others 

(2001) reported the existence of two calcium-uranyl-carbonate 

species, Ca2UO2(CO3)

3

0

 and CaUO2(CO3)

3

–2, and subsequent 

studies reported that these species may predominate in many 

ground-water systems (Brooks and others, 2003).

Uranium concentrations in ground water were highest in 

water-table and shallow wells and decreased with increasing 

depth below the water table (Spearman’s rho = -0.84, p-value 

≤0.001; table 8). Uranium concentrations in ground water 

beneath urban and agricultural land were similar, indicating 

that present-day land use is not a major factor controlling 

dissolved uranium in ground water (tables 1, 8).

37° 38°

EXPLANATION

Dry Creek

Stanislaus River

STANISLAUS COUNTY

MERCED COUNTY

Tuolumne River

Kings River

0 10 205 Kilometers

0 10 20 5 Miles

SAN JOAQUIN COUNTY

FRESNO COUNTY

San Joaquin River

Merced River

Boundry of regional model

Active area of local-scale model grid

High: 5.4

Low: 0.0

Uranium, ppm

121° 120°

Figure 17. Aerial gamma-ray survey of uranium concentrations in soils in the San Joaquin Valley (Phillips and others, 1993).

ppm, parts per million.
Water Chemistry 41

The redox state of ground water within the study area is 

predominantly oxic; dissolved oxygen concentrations were 

above 1.0 mg/L in all samples. This suggests that uranium was 

in hexavalent form.

Uranium concentrations were strongly correlated to 

alkalinity (as CaCO3) (Spearman’s rho = 0.94, p-value ≤0.001) 

(fig. 18), but were not correlated with concentrations of 

phosphate in ground water (Spearman’s rho = –0.49, p-value 

= 0.60). The principal source of alkalinity is bicarbonate. 

Because there has been some question as to which uranium 

complexes, phosphate or carbonate, predominate in oxic 

ground water (Langmuir, 1978; Grenthe and others, 1992; 

Sandino and Bruno, 1992), uranium was speciated using 

PHREEQC version 2.12.5 (Parkhurst and Appelo, 1999). 

Uranium speciation was calculated using inorganic data, 

field measurements of pH, dissolved oxygen, and alkalinity 

from the monitoring wells. Speciation results indicate that 

Ca2

UO2(CO3)3

0

 is the dominant form of dissolved uranium in 

ground-water from the study area, accounting for more than 

90 percent of uranium species. This indicates that carbonate 

complexes predominate in ground water in the study area 

and that phosphate complexes were minimal. Additional 

calculations of saturation indices for uranium minerals 

indicated that dissolved uranium is not likely to precipitate 

into solid-phases.

Uranium was strongly correlated with calcium also 

(spearman’s rho = 0.96, p-value <0.001; fig. 19). The 

correlations support speciation calculations that indicate 

uranium in ground water in the study area forms a complex 

that includes calcium. 

Uranium, alkalinity, and calcium concentrations decrease 

with depth. This suggests that calcium-uranyl-carbonate 

complexes have caused or have been causing adsorbed 

uranium in water-table and shallow sediments to dissolve 

into ground water. In deep ground water, the concentrations 

of uranium were always less than 2 μg/L and usually less than 

1 μg/L. Although bicarbonate and calcium are present in deep 

ground water, concentrations of these constituents are not 

high enough to dissolve uranium from the mineral surface. 

Consequently, geochemical conditions in deep ground water 

favor uranyl adsorption. 

Uranium Isotopes

Natural uranium is composed of three radioactive 

isotopes: uranium-234, uranium-235, and uranium-238. 

The concentration of each isotope is reported as an activity 

concentration, which is the number of disintegrations per 

minute per liter of water. The total activity concentration of 

uranium (uranium-234 plus uranium-235 plus uranium-238) 

has a California MCL for public water systems of 

20 picocuries per liter (pCi/L). 

Figure 18. Relation between uranium concentrations and 

alkalinity measurements from ground-water samples collected 

from monitoring wells in October and November of 2003 and 2004 

in Modesto, eastern San Joaquin Valley, California.

MCL, U.S. Environmental Protection Agency maximum 

contaminant level; PSW, public-supply well.

0 100 200 300 400 500

Alkalinity, in milligrams per liter as calcium carbonate

0

10

20

30

40

50

Uranium, in micrograms per liter

Spearman’s rho = 0.94

p-value = <0.001

MCL

Water-table

Shallow

Intermediate

Deep

PSW

Water-table

Shallow

Intermediate

Deep

PSW

0 10 20 30 40 50

Uranium, in micrograms per liter

0

20

40

60

80

100

120

140

Calcium, in milligrams per liter

 MCL

Figure 19. Relation between uranium and calcium 

concentrations in ground-water samples collected in October 

and November of 2003 and 2004 in Modesto, eastern San Joaquin 

Valley, California.

MCL, U.S. Environmental Protection Agency maximum 

contaminant level; PSW, public-supply well.
42 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

The decay of uranium-238 produces two short-lived 

daughters, thorium-234 and protactinium-234, which decay to 

the long-lived uranium-234 isotope. Given enough time, the 

activity ratio of uranium-234 to uranium-238 (UAR) will reach 

secular equilibrium (UAR = 1) provided the mineral remains 

undisturbed. However, as uranium-238 decays, an alpha 

particle is ejected and the daughter product, thorium-234, 

recoils in the opposite direction, damaging the crystal 

structure of the mineral. This can lead to the preferential 

loss of thorium-234. Thorium can be displaced as far as 

550 angstroms from the mineral, which may be enough to 

be ejected completely from the mineral or to be concentrated 

near the mineral rind (Osmond and Cowart, 2000). As thorium 

decays, uranium-234 accumulates and leads to an increase 

of uranium-234 activity relative to uranium-238 activity 

(UAR >1). Auto-oxidation during thorium-234 decay and 

subsequent exposure of the rind to water can also lead to the 

complexation and removal of uranium-234 (Clark and Fritz, 

1997). As a result, the uranium activity ratio can be used to 

infer mixing, leaching, alpha-recoil, dilution, and (or) decay 

along a ground-water flow path. However, because some or 

all of these processes may occur simultaneously, determining 

which of these processes is most responsible for the activity 

ratio for ground water is difficult. In this study, geochemical 

conditions in ground water which favor the mobility of 

uranium were most likely influenced by leaching and mixing 

processes, whereas geochemical conditions in ground water 

which favor uranium adsorption were more likely influenced 

by alpha-recoil. 

Uranium isotopes were collected from 14 monitoring 

wells to help understand the mobilization and movement 

of high concentrations of uranium in ground water within 

the study area. Twelve samples were from water-table and 

shallow monitoring wells and two were from deep wells. The 

total activity concentration of uranium from these samples 

ranged from 0.4 to 32.0 pCi/L and the median was 15.9 pCi/L 

(table 8). 

Although the water-table and shallow ground-water 

samples had similar uranium concentrations, the UAR for 

shallow samples were slightly higher (median = 1.46) than 

those for the water-table samples (median = 1.17 [table 8; fig. 

20]). Because dissolved uranium likely maintains equilibrium 

with the population of adsorbed uranium in the surrounding 

sediment, the UAR of dissolved ground water should reflect 

the UAR of adsorbed uranium. This would suggest that the 

adsorbed uranium in the shallow sediments had undergone 

alpha-recoil to a greater extent prior to being mobilized than 

the sediments near the water table. The highest activity ratios 

were associated with deep ground water, which also had 

the lowest total activity concentration of uranium (fig. 20). 

Because geochemical conditions favor the adsorption or 

immobility of uranium in deep ground water, uranium-234 can 

accumulate in sediment as a result of alpha-recoil to a much 

greater extent than water-table and shallow sediments where 

uranium is mobile. 

Composition of Ground Water and 

Surface Water

The chemical composition of ground water in the 

Modesto study area is primarily the result of weathering of 

granitic sediment and of applying natural and anthropogenic 

chemicals associated with land-use practices. A trilinear or 

“Piper” diagram is a useful tool for evaluating the relative 

proportions of major cations and anions in ground water 

(fig. 21). Bicarbonate (from alkalinity measurements) was 

the dominant anion in ground water (fig. 21), comprising 

more than 60 percent in all ground water. Most ground water 

contained less than 30 percent sulfate plus chloride plus 

nitrate, but wells affected by agricultural fertilizers and soil 

amendments contained larger percentages of nitrate and sulfate 

(fig. 21, process 2). The cation composition was generally 

a mix of calcium, magnesium, and sodium; however, the 

percentage of magnesium was higher in ground water from 

Uranium, total, in picocuries per liter

Uranium-234 to uranium-238, activity ratio

2.2

MCL-CA

0

EXPLANATION

5 10 15 20 25 30 35 40 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Water-table

Shallow

Deep

Surface discharge at well head

45.7 meters

57.9 meters

97.5 meters

Monitoring well samples

Public-supply well, 

depth-dependent sample

Figure 20. Relation between total uranium activity and the 

uranium activity ratio (UAR) in ground-water, depth-dependent 

samples from the public-supply well and ground-water samples 

from monitoring wells collected in August, October, and November 

of 2004 in Modesto, eastern San Joaquin Valley, California.

MCL-CA, California maximum contaminant level.
Water Chemistry 43

the water-table and shallow monitoring wells than from the 

intermediate and deep wells (fig. 21).

The cation composition of the water in intermediate and 

deep wells suggest ground water evolves from a calciummagnesium-sodium type composition to a composition 

dominated by sodium (fig. 21, process 1). The percentage of 

sodium plus potassium increases and calcium plus magnesium 

decreases along a north-south transect (fig. 3) in deep 

monitoring wells: FPD-3, FPC-3, FPB-3, and FPA-4 (collected 

in 2004). The major-ion chemistry and carbon-14 data for 

wells along this transect are consistent with cation exchange 

and indicate that recharge may have originated from the 

Stanislaus River before ground-water development.

The dissolution of primary minerals (mainly plagioclase), 

calcite precipitation, and cation exchange are the principal 

reactions affecting the major-ion chemistry as ground water 

moves from recharge areas to deeper parts of the aquifer. 

Dissolution of plagioclase can cause the pH of ground water to 

increase and the partial pressure of carbon dioxide to decrease. 

Both calcite precipitation and cation exchange will remove the 

cations of calcium and magnesium from solution, and calcite 

precipitation will also remove bicarbonate and lower the pH 

of ground water. Calcite was determined through calculations 

to be saturated mostly in water-table and shallow ground 

water, which suggests calcite precipitation is the dominant 

reaction controlling the composition of young ground water 

Percent

Calcium

 0 100 

 80 

 60 

 40 

 20 

 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

Percent

Percent

2 1

Irrigation return and land-use 

effects: Addition of nitrogen 

fertilizers, soil amendments 

(gypsum), and increased dissolved

 ions.

Dissolution of primary minerals, 

calcite precipitation, oxidation of 

organic matter and cation 

exchange

1

2

Processes

FPD-1

FPA-4, 2003

FPD-3

FPC-3

FPB-3

1 FPA-4, 2004

Deep

Intermediate

PSW

Tuolumne River

Stanislaus River

Shallow

Water-table

Figure 21. Major ion composition of ground-water samples collected in October and November 2003 and 2004 in Modesto, eastern San 

Joaquin Valley, California. 

The Tuolumne and Stanislaus river samples are averaged compositions from previous studies. Major ion composition of recharge that 

occurred under natural conditions (pre-development) is represented by the shaded circle. 
44 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

(<100 years) (Appendix C). Plagioclase dissolution is 

very slow once contact with the CO2-rich soil atmosphere 

is removed. Ground water is normally separated from the 

atmosphere as ground water moves downward below the 

water table. For this reason, plagioclase dissolution and cation 

exchange are most likely the major reactions controlling the 

composition of old ground water, although minor amounts of 

calcite precipitation is still likely to occur. 

Although the cation composition of ground water from 

some water-table wells appears to follow a geochemical trend 

similar to that for deeper wells, this ground water is generally 

a mixture of surface water (Tuolumne River), precipitation 

recharge, and (or) deep ground water applied as irrigation in 

urban and agricultural areas. Stable isotopes of water indicate 

that surface water is not a major component of recharge in 

urban water-table wells. Consequently, ground water in watertable wells within the urban area having larger percentages of 

sodium likely originated not from surface water, but from deep

ground water applied to the surface. 

Two water-table wells and three shallow wells had higher 

percentages of calcium, sulfate, and nitrate than other wells 

within in the study area (fig. 21, process 2). The water-table

well, FPD-1, is located in an agricultural land-use setting 

where fertilizers and soil amendments are applied annually. 

Some of the major forms of fertilizer used in the study area 

are calcium-nitrate, urea, and ammonium-sulfate (Vance 

Kennedy, retired USGS and local farmer, oral commun. 

2005). In addition, gypsum (CaSO4

·2H2O) is added annually 

to promote infiltration (Vance Kennedy, retired USGS and 

local farmer, oral commun. 2005). The dissolution of these 

chemicals in oxic irrigation water is consistent with the higher 

concentrations of dissolved calcium, nitrate, and sulfate 

relative to other dissolved constituents in the two water-table

wells (including FPD-1) and the three shallow wells. Although 

the three shallow wells are located beneath urban land, most 

urban water-table wells had less than 5 mg/L of nitrate, which 

suggests that at least part of the shallow ground water was 

recharged under agricultural land.

Since development of the Valley for agricultural 

production, the source of recharge in the study area has no 

longer been dominated by precipitation and recharge along 

streams and rivers. The composition of ground water that 

was recharged naturally is likely similar to the composition 

represented by the shaded circle on figure 21. Many of the 

water-table wells in the study area appeared to have chemical 

compositions that were altered from natural conditions. The 

compositions varied as a result of applying surface water and 

pumped ground water to the surface and adding fertilizers and 

soil amendments to irrigation water. The changes in recharge 

and chemical compositions are an important concern because 

applying irrigation water to urban and agricultural landscapes 

and adding fertilizers and soil amendments tend to increase the 

concentrations of dissolved ions in recharge water reaching the 

ground water.

Dissolved ion content of water-table wells and shallow 

wells is significantly higher than that in deep ground water 

(table 5). The dissolved ion content of deep ground water was 

likely similar to the dissolved ion content of natural recharge. 

Dissolved ions in water-table and shallow ground water likely 

increased in response to the application of irrigation water 

for urban and agricultural purposes during summer months 

and subsequent geochemical reactions in the biologically 

active unsaturated zone. The predominant shifts in major-ion 

chemistry resulting from this process have been increases in 

magnesium, calcium, and bicarbonate. 

Depth-Dependent Sampling

To characterize the water-quality of different parts of the 

aquifer that contribute water to the public-supply well, groundwater samples were collected at multiple depths within the 

pumping public-supply well in August 2004. For comparison 

of results, ground-water samples also were collected at the 

adjacent monitoring wells (at site FPA) at that time. 

Depth-dependent samples collected from the pumping 

public-supply well indicate that the highest concentrations 

of uranium, nitrate, arsenic, and organic constituents entered 

the well at shallow depths (table 9). Even though about 

80 percent of the water discharged from the well was from 

the intermediate and deep zones having low concentrations of 

uranium, nitrate and arsenic, the shallow ground water had a 

significant effect on the overall quality of water. 

The chemistry of shallow ground water (DDS-45.7, 

depth-dependent sample from 45.7 m bls) within the 

public-supply well was consistent with the type, number, 

and concentrations of inorganic constituents and organic 

compounds found in the adjacent monitoring well (table 9) 

and the other wells screened in the shallow zone (tables 5, 

6, 8). The shallow depth-dependent sample (DDS-45.7) had 

the highest specific conductance, alkalinity measurement, 

and nitrate, sulfate, arsenic, and uranium concentrations of 

the public-supply well depth-dependent samples (table 9). 

The uranium concentration (35.8 μg/L) in this sample was 

above the MCL, and nitrate was detected at a concentration 

(9.7 mg/L) near the MCL of 10 mg/L. This sample also 

had the highest number of detections and concentrations of 

VOCs (4) and pesticides (3) of the depth-dependent samples 

(table 9). None of the VOCs or pesticides were detected at 

concentrations above an MCL and most concentrations were 

less than one-hundredth of the MCL.
Water Chemistry 45

Table 9. Volatile organic compounds, pesticides, and selected inorganic constituents and isotopes in ground water from depth-dependent public-supply well samples and samples collected at adjacent monitoring wells during August and September 2004 in Modesto, eastern San Joaquin Valley, California. [The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent or property. VOC, volatile organic compound; CaCO3, calcium carbonate; N, nitrogen; E, estimated; LRL, laboratory reporting level; DDS-X, depth-dependent sample and depth below land surface. m, meter; μS/cm @ 25°C, microsiemens per centimeter 

at 25 degrees Celsius; μg/L, microgram per liter; mg/L, milligrams per liter; —, not detected]

Well identification no. Depth category Sample date

Field parameters VOCs—Disinfection byproducts and solvents

Sampling 

depth 

(meter)

Specific 

conductance, 

(μS/cm

@ 25°C)

Alkalinity 

(mg/L 

as CaCO3)

Trichloromethane 

(chloroform) 

(μg/L)

Bromodichloromethane 

(μg/L)

Tetrachloroethene 

(μg/L)

Tetrachloromethane 

(μg/L)

Parameter code 00003 00095 29802 32106 32101 34475 32102

LRL 0.024 0.028 0.06 0.06

DDS-Composite Composite 8/17/2004 0 414 174 0.14 — E0.06 —DDS-45.7 Shallow 8/17/2004 45.7 852 378 0.54 E0.03 0.23 E0.02

DDS-51.8 Intermediate 8/24/2004 51.8 292 113 E0.04 — — —DDS-57.9 Intermediate 8/25/2004 57.9 311 118 E0.04 — — —DDS-88.4 Deep 8/23/2004 88.4 270 108 E0.04 — — —DDS-97.5 Deep 8/19/2004 97.5 336 133 E0.09 — — —

Well identification no. Depth category Sample Date

Pesticides Inorganic constituents

2-Chloro-4-

isopropylamino6-amino-striazine 

(CIAT), 

(μg/L)

Atrazine, 

(μg/L)

Simazine, 

(μg/L)

Sulfate, 

dissolved 

(mg/L)

Arsenic, 

dissolved 

(μg/L)

Uranium 

(natural), 

dissolved 

(μg/L)

Nitrite 

plus nitrate, 

dissolved 

(mg/L as N)

Parameter code 04040 39632 04035 00945 01000 22703 00631

LRL 0.006 0.007 0.005 0.18 0.2 0.04 0.06DDS-Composite Composite 8/17/2004 E0.007 E0.006 0.006 15.5 5.8 11.2 4.50

DDS-45.7 Shallow 8/17/2004 E0.03 0.024 0.020 41.1 7.0 35.8 9.73

DDS-51.8 Intermediate 8/24/2004 — — — 9.47 5.6 5.41 3.10

DDS-57.9 Intermediate 8/25/2004 — — — 10.2 5.2 6.02 3.02

DDS-88.4 Deep 8/23/2004 E0.003 — E0.004 5.33 5.5 3.20 1.91

DDS-97.5 Deep 8/19/2004 E0.005 E0.006 0.005 8.79 5.2 4.91 2.30
46 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California Table 9. Volatile organic compounds, pesticides, and selected inorganic constituents and isotopes in ground water from depth-dependent public-supply well samples and samples collected at adjacent monitoring wells during August and September 2004 in Modesto, eastern San Joaquin Valley, California.—Continued [The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent or property. VOC, volatile organic compound; CaCO3, calcium carbonate; N, nitrogen; E, estimated; LRL, laboratory reporting level; DDS-X, depth-dependent sample and depth below land surface. m, meter; μS/cm @ 25°C, microsiemens per centimeter at 25 degrees Celsius; μg/L, microgram per liter; mg/L, milligrams per liter; —, not detected] Well identifi- cation no. Depth category Sample date Field parameters VOCs—Disinfection byproducts and solvents Specific conductance, (μS/cm @ 25°C) Alkalinity (mg/L as CaCO3

)

Trichloromethane 

(chloroform) 

(μg/L)

Bromodichloromethane 

(μg/L)

Tetrachloroethene, 

(μg/L)

Trichloroethene 

(μg/L)

cis-1,2-

Dichloroethene 

(μg/L)

Parameter code 00095 29802 32106 32101 34475 39180 77093

LRL 0.024 0.028 0.06 0.38 0.24

FPA-1 Water-table 9/1/2004 830 434 E0.08

— 1.90 0.11 E0.02

FPA-2 Shallow 9/1/2004 766 333 0.13

— 0.81

—

—

FPA-3 Intermediate 9/1/2004 275 98

—

—

—

—

—

FPA-4 Deep 9/1/2004 195 82

—

—

—

—

—

Well 

identification no. Depth category Sample Date

Field parameters Pesticides Inorganic constituents

Specific 

conductance, 

(μS/cm

@ 25°C)

Alkalinity 

(mg/L 

as CaCO3

)

2-Chloro4-isopropylamino-6-

amino-striazine 

(CIAT), 

(μg/L)

Atrazine, 

(μg/L)

Simazine, 

(μg/L)

3,4-

Dichloroaniline 

(μg/L)

Desufinyl-

 fipronil

(μg/L)

Prometon 

(μg/L)L

Sulfate, 

dissolved 

(mg/L)

Arsenic, 

dissolved 

(μg/L)

Uranium 

(natural), 

dissolved 

(μg/L)

Nitrite 

plus 

nitrate, 

dissolved 

(mg/L 

as N)

Parameter code 00095 29802 04040 39632 04035 61625 62170 04037 00945 01000 22703 00631

LRL 0.006 0.007 0.005 0.0045 0.012 0.005 0.18 0.2 0.04 0.06

FPA-1 Water-table 9/1/2004 830 434 E0.008 0.009 0.009 E0.003 E0.007 0.001 19.6 8.8 39.5 2.21

FPA-2 Shallow 9/1/2004 766 333P E0.007

—

—

—

—

— 37.8 9.0 25.8 14.46

FPA-3 Intermediate 9/1/2004 275 98 E0.005

—

—

—

—

— 13.4 5.5 2.39 3.01

FPA-4 Deep 9/1/2004 195 82

—

—

—

—

—

— 2.23 5.5 0.43 0.90
Water Chemistry 47

Chloroform was the only VOC detected in the two 

intermediate depth-dependent public-supply well samples 

(DDS-51.8, DDS-57.9; table 9). Pesticides were not detected 

in either sample. The two samples from this zone were 

composite samples of water from the deep zone and water 

from the intermediate zone. As a result, the concentration 

and detection of compounds in these samples were a flowweighted average of the concentrations of constituents 

contributed from deep depths plus intermediate depths. 

Chloroform was detected at a concentration of 0.37 μg/L and 

was the lowest concentration among the depth-dependent 

samples (table 9). Inorganic constituents in intermediate

depth-dependent samples were generally at or below 

concentrations detected in the intermediate monitoring wells. 

Two deep depth-dependent samples (DDS-88.4, 

DDS-97.5) had higher concentrations and more detections of 

organic compounds than intermediate samples (table 9). This 

suggests that the organic compounds in the intermediate zone 

could be the result of the introduction of water from the deep

zone. Chloroform, simazine and CIAT were detected in both 

deep samples. These compounds were found in the shallow

sample also, but at higher concentrations. 

The deep aquifer zone can be further divided into an 

upper and lower part on the basis of borehole flow data, each 

contributing approximately 13 percent of the total flow to 

the public-supply well. The sample from 97.5 m (320 ft) bls 

represents the average chemistry of water that enters the well 

below 97.5 meters (320 ft) bls, and the sample from 88.4 m 

(290 ft) bls is the average chemistry of water that enters the 

well below 88.4 m (280 ft) bls, and therefore includes the 

water chemistry at 97.5 m (320 ft) bls. On the basis of specific 

conductance (SC) values at 88.4 m and 97.5 m (table 9), water 

entering the well between 88.4 m and 97.5 m (280 to 320 ft) 

bls, must be more dilute (lower SC) than the water entering the 

well below 97.5 m.

Uranium and nitrate were detected at higher 

concentrations in the deepest sample (DDS-97.5) than in 

the sample collected at 88.4 m bls. These concentrations 

were greater than concentrations in deep monitoring wells, 

excluding the sample from FPA-4 in November 2003 (tables 

8,9). The sample collected at 97.5 m bls had the same three 

pesticide compounds as those collected from the shallow

depth. The concentrations were lower in this sample than 

in the shallow sample but were higher than the sample 

collected at 88.4 m bls. On the basis of the observation that 

the concentrations and number of detections of organic and 

inorganic constituents in the deepest sample also were higher 

than those detected in the adjacent monitoring wells and 

other monitoring wells screened in the deep aquifer zone, the 

water chemistry below 97.5 m was likely affected by well 

construction and hydraulic conditions of the aquifer.

Well Construction and Operation Effects on 

Water-Quality

Because the public supply well is perforated over a 

long interval (fig. 4, table 1), during static conditions the 

well can act as a conduit for flow from the shallow aquifer, 

where hydraulic head is high, to the deep aquifer, where the 

hydraulic head is low. As a result, the chemistry of the water 

in the deep aquifer zone surrounding the well is strongly 

influenced by the water chemistry of the shallow zone and by 

well operation schedules. Figure 22 shows the concentration 

of uranium and specific conductance measurements for 

depth-dependent samples taken within the PSW and results 

from the nearby monitoring wells at site FPA. Figure 22 also 

shows the effects of well-bore leakage through the PSW on the 

nearby deep monitoring well (FPA-4). FPA-4 is approximately 

40 m (130 ft) upgradient of the public-supply well and is 

perforated between 104.5 and 106.1 m (343–348 ft) bls. 

The specific conductance and hydraulic head in FPA-4 

was at a minimum during summer (maximum pumping), 

whereas specific conductance and hydraulic head in FPA-4 

increased dramatically during winter (minimum pumping) and 

downward flow from the high specific conductance shallow

aquifer zone was greatest. 

The effects of well operation and construction on 

contaminant transport to the deep part of the aquifer were first 

identified in samples collected from FPA-4 in October, 2003. 

At that time, the public-supply well had been offline for over a 

month while a new sediment filtration system was installed on 

the well. Although the public-supply well became operational 

just before sampling, chloroform, tetrachloroethene, and CIAT 

were detected in samples from FPA-4 (Appendices D, E). The 

following year, the deep monitoring well (FPA-4) was sampled 

a second time as part of the depth-dependent sampling. At 

that time, the public-supply well had been pumped regularly 

for most of the summer. VOCs and pesticide compounds 

were not detected in this sample (table 9) and concentrations 

of inorganic species were lower than those in the flow-path 

sample (taken in October 2003) listed in table 8. This suggests 

that a significant portion of the water that migrated into the 

deep aquifer zone from the shallow zone through the publicsupply well had been evacuated as a result of prolonged 

pumping. Ground water from the shallow (DDS-45.7) and 
48 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

the deep depth-dependent samples (DDS-88.4, DDS-97.5) 

was isotopically lighter (deuterium, oxygen-18) than that 

in samples from the intermediate depth-dependent samples 

(fig. 23). Surface water had the lightest isotopic composition 

of water in the study area. The shallow sample (DDS-45.7) 

had the lightest composition of the depth-dependent samples 

and was likely influenced by surface water used for irrigation, 

although much less than other shallow wells such as well 

FPA-2. Ground water from the deep depth-dependent samples 

was a mixture of shallow and native deep ground water. The 

ground-water sample collected from the public-supply well at 

97.5 m bls was lighter than the sample from 88.4 m bls, which 

indicates shallow ground water was preferentially stored in 

more permeable hydrofacies below 97.5 m bls during static 

conditions (table 9, fig. 23). 

Uranium activity ratios indicated that the source of 

uranium in deep depth-dependent samples originated from 

shallow ground water. Uranium concentrations and activity 

ratios for the shallow depth-dependent sample (DDS-45.7) 

was similar to those for the shallow monitoring wells (fig. 20). 

Uranium concentrations from the deep depth-dependent 

sample was higher than those from the deep monitoring wells 

and had uranium activity ratios similar to those for shallow

ground water. The combination of higher uranium activity 

ratios and lower concentrations of uranium in the intermediate

depth-dependent sample was similar to that for ground water 

from deep monitoring wells. This suggests the intermediate

aquifer was not significantly affected by ground water from 

the shallow zone.

The percentage of terrigenic helium in samples from 

the public-supply well increased from 43 percent in October 

2003 to more than 85 percent in August 2004 (fig. 10). This 

increase likely was the result of increased contributions of 

more native, deep ground water entering the well in August 

2004. This change is consistent with storage of shallow water 

stored in the deep aquifer at the time ground-water samples 

were collected in October 2003. 

Figure 22. Uranium (U) concentrations in and specific conductance (SC) measurements for ground water, depth-dependent samples 

collected from the public-supply well, and ground-water samples collected from monitoring wells at site FPA in August 2004 in Modesto, 

eastern San Joaquin Valley, California. 

Piezometric head and specific conductance (SC) measured in well FPA-4 from September 2004 through May 2005. in., inch; PSW, publicsupply well.

FPA

-100

-80

-60

-40

-20

0

20

40

Elevation, in meters

Composite sample

 at well head

U

U

39.5

25.8

0.9

35.8

6.0

3.2

4.9

11.2

5.4

2.4

SC

830

766

195

275

SC

852

311

270

336

414

High U,

SC

Low U,

SC

Mud

Gravel

Sand

Muddy sand

292

EXPLANATION

Sep Oct Nov Dec Jan Feb Mar Apr May

2004 - 2005

180

200

220

240

260

280

300

Specific Conductance (SC), in microsiemens per centimeter

8

10

12

14

16

18

Elevation above NAVD 1988, in meters

SC

Piezometric Head

0 40 80

PSW

0

20

40

60

80

100

120

Ohm-Meters

Resistivity - 16-in,

in OHM-Meters

Vertical 

exaggeration X 27

Datum is North American

Vertical Datum 1988

Direction of flow during static condtions

Boundary between high uranium and specific 

conductance above line and low uranium and SC 

below line.
Factors Affecting the Transport of Contaminants to the Public-Supply Well 49

Factors Affecting the Transport of 

Contaminants to the Public-Supply 

Well

Contaminants Reaching the Public-Supply Well

Contaminants reaching the public-supply well that pose 

the greatest drinking water risk are uranium, nitrate, and 

to a lesser extent, arsenic. None of the composite samples 

collected from the public-supply well had concentrations 

of uranium, nitrate or arsenic above the MCL, but each of 

these constituents was detected at concentrations above half 

the MCL (tables 8,9). Shallow depth-dependent samples 

contained concentrations of uranium (35.8 μg/L) above the 

MCL of 30 μg/L and concentrations of nitrate (9.7 mg/L) 

near the MCL of 10 mg/L. This part of the aquifer contributes 

approximately 20 percent of the total flow to the well. Arsenic 

has an MCL of 10 μg/L and also was elevated (7.0 μg/L) 

in the shallow depth-dependent sample. However, arsenic 

concentrations are not sufficiently high in this sample 

or any other depth-dependent sample to threaten the 

public-supply well given current geochemical conditions. 

Since the geochemical conditions that might favor widespread 

arsenic desorption, such as increases in phosphate and pH 

or changes in redox states, are not likely to occur, arsenic 

in ground water from the public-supply well is expected 

to remain elevated but unlikely to exceed the MCL. 

Consequently, uranium and nitrate pose the most imminent 

threat to the public-supply well as a source of drinking water. 

Organic constituents entering the public-supply well are 

below current MCLs, but their presence indicates the well is 

vulnerable to point-source contamination. Three VOCs were 

detected, most notably chloroform and tetrachloroethene 

(PCE), but the concentration of each were below one-tenth 

of any MCL. Three pesticides (CIAT or DEA, atrazine, 

and simazine) were detected at concentrations below 

one-hundredth the concentration of each corresponding 

MCL (CIAT or DEA does not have an MCL). Because the 

concentrations necessary for these constituents to exceed an 

MCL were not found in any of the depth-dependent samples 

or monitoring wells, these contaminants were not expected 

to pose an imminent contamination risk to the public-supply 

well.

Factors Affecting the Transport of 

Nitrate and Uranium

Elevated levels of uranium and nitrate in the publicsupply well are primarily related to five factors: ground-water 

age, well construction, source strength, redox conditions, and 

alkalinity. These factors are described individually below and 

then presented together as they relate to uranium and nitrate 

transport in the following section.

Ground-water age—Ground-water pumping throughout 

the study area for irrigation and drinking-water supply has 

increased the rates of downward movement of shallow ground 

water. The distribution of mean ages interpreted from tracers 

collected at monitoring wells indicate that young (generally 

less than 50 years) ground water migrated to depths of up to 

about 60 m bls in parts of the aquifer (fig. 12). The age of 

most ground water below this depth was generally on the order 

of 100s to1000s of years old. Young ground water contains 

elevated levels of uranium (>20 μg/L) and nitrate (>5 mg/L), 

whereas most older ground water contained background 

concentrations of nitrate (<3 mg/L) and low concentrations 

of uranium (<2 μg/L). Continued downward movement of 

shallow ground water will likely cause the amount of aquifer 

containing young ground water to increase. 

-12 -11 -10 -9 -8 -7

-85

-90

-80

-75

-70

-65

-60

-55

-50

Global meteoric water line,

 (Rozanski et al., 1993)

DDS-45.7

DDS-51.8

DDS-57.9

DDS-88.4

DDS-97.5

DDS-Composite

Canal near FPD

Stanislaus Riv er

Tuolumne Riv er

delta Oxygen-18, in per mil

delta Deuterium, in per mil

FPA-2

FPD-1

Figure 23. Isotopic composition of ground water from publicsupply well depth-dependent samples (depth of sample after 

dash) and ground-water and surface-water samples in Modesto, 

eastern San Joaquin Valley, California.
50 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Well Construction—The public-supply well is screened 

from 27.7 m (91 ft) to 111.6 m (366 ft) bls and has an open 

bottom. During static conditions, strong vertical hydraulic 

gradients cause shallow ground water to migrate from the 

upper part of the well screen to the near bottom of the well 

screen and into the deep part of the aquifer, where it was 

stored until the next pumping cycle. Because the well was 

operated on a limited schedule during the winter (wet season), 

uranium and nitrate have been historically high in winter 

samples in comparison to summer samples. 

Source Strength—The concentration of nitrate in ground 

water is primarily affected by the amount of fertilizers applied 

on agricultural land. The amount of fertilizers applied over 

agricultural land has tripled over the last 50 years (Ruddy and 

others, 2006; fig. 24). In contrast to nitrate, uranium exists 

naturally in aquifer sediments and is unrelated to land use. 

Geochemical conditions which favor desorption of uranium 

can cause uranium concentrations in ground water to exceed 

the MCL.

Redox Conditions—Ground water within the study area 

was predominately oxic, although there were some localized 

reducing microenvironments. Uranium and nitrate are mobile 

in oxic ground water. Although some dissolved nitrate 

may be depleted in localized, reduced microenvironments, 

denitrification is not a significant factor affecting high 

concentrations of nitrate in ground water. Sulfidic reducing 

conditions, which may occur in localized, microenvironments, 

could facilitate the formation of uranium minerals, but like 

nitrate, is not a significant factor affecting high concentrations 

of uranium in ground water. 

Alkalinity—Alkalinity is a measure of the capacity 

of water to resist changes in pH. The principal source of 

alkalinity in the study area is bicarbonate, which is the primary 

product of mineral dissolution. Bicarbonate liberates adsorbed 

uranium from sediment, leading to higher concentrations of 

dissolved uranium in ground water. Applying irrigation water 

to crops in agricultural areas and lawn grasses in urban areas 

increases bicarbonate concentrations, and hence alkalinity, 

in water reaching the ground-water system. Alkalinity above 

250 mg/L (as CaCO3) in ground water in the study area had 

elevated uranium; alkalinity above 400 mg/L in some ground 

water in the study area had uranium above the MCL of 

30 μg/L. Nitrate is unaffected by alkalinity in ground water.

Historical and Long-term Vulnerability to Nitrate 

and Uranium Contamination

Historically, seasonal water-quality samples collected 

from the public-supply well have been significantly affected 

by the water chemistry of shallow ground water entering the 

well (fig. 25). Nitrate concentrations in samples collected 

from the public-supply well since 1966 were significantly 

higher during the winter season (October through May) than 

during summer (p-value ≤ 0.001, Kruskal-Wallis test). The 

median nitrate concentration in PSW samples collected during 

the winter season was 8.1 mg/L compared with 3.0 mg/L in 

samples collected during summer. Similar effects are indicated 

in public-supply well records of uranium concentrations 

in samples collected since 1989; the median uranium 

concentration for the winter season samples (19.6 pCi/L) was 

higher than the median for the summer samples (6.8 pCi/L) 

(p-value = 0.001, Kruskal-Wallis test). 

Results from this study indicate that nitrate and uranium 

pose the greatest threat to the long-term sustainability of the 

ground-water resource in the study area and perhaps in other 

areas within the eastern San Joaquin Valley (fig. 26). If water 

management practices and land use do not change, dispersion 

and continued downward movement of ground water will 

cause the fraction of anthropogenically affected ground water 

that has high concentrations of nitrate and uranium to increase 

in the public-supply well, and possibly other wells in the 

eastern San Joaquin Valley.

1940 1950 1960 1970 1980 1990 2000

Year

0

20

40

60

80

100

Kilograms per hectare

Figure 24. Application of nitrogen fertilizers in Stanislaus County 

since 1945 (Ruddy and others, 2006).
Factors Affecting the Transport of Contaminants to the Public-Supply Well 51

Nitrate concentrations in ground water mostly depend 

on the amount of nitrate applied on agricultural lands and the 

amount of recharge contributed to the aquifer within urban 

areas. If application rates of nitrogen fertilizers increase as 

indicated by nitrogen sales data (Ruddy and others, 2006; 

fig. 24), concentrations in the aquifer could increase. On 

the other hand, if the amount of urban recharge, which had 

comparatively low nitrate, increases as a result of urbanization 

of agricultural lands, concentrations could level off and 

decline over time. 

Nitrate in ground water is expected to behave 

conservatively and persist in the study area. The persistence of 

nitrate in ground water is limited mainly by redox conditions. 

In oxic ground water, nitrate does not degrade, has limited 

adsorptive properties, and does not tend to form minerals. In 

contrast, reducing redox conditions in ground water usually 

facilitate the removal of nitrate by denitrification. However, 

reducing conditions in the study area occurred in localized, 

microenvironments only and did not appear to significantly 

affect nitrate concentrations in ground water.

The mobility of uranium (VI) in ground water depends on 

the presence and concentration of strong aqueous complexes 

and the tendency of uranium to form solid-phase minerals or 

adsorb to aquifer sediments. Results from uranium extractions, 

adsorption experiments, and analysis of uranium isotopes 

indicated that uranium was leached from shallow sediments 

and transported to deeper parts of the aquifer (fig. 26). 

Mobilized uranium was prevalent in ground-water samples 

collected from near the water table to depths of at least 

50 m bls, likely owing to the downward movement of highalkalinity (dominately bicarbonate anion), oxygen-rich ground 

water. At depths below 50 m bls, geochemical conditions 

favor adsorption processes that limit dissolved uranium 

concentrations. 

Uranium concentrations in the aquifer are not expected 

to decrease over the long-term. Uranium adsorbed on 

intermediate and deep aquifer sediments represents a 

lasting source of uranium available for transport. Continued 

downward migration of oxygenated, high-alkalinity ground 

water from shallower depths is likely to mobilize the available 

fraction of uranium from these deeper sediments. The 

rate at which uranium concentrations may increase in the 

intermediate and deep parts of the aquifer will depend on the 

vertical rate of ground-water movement, which varies spatially 

owing to aquifer heterogeneity and differences in hydraulic 

stresses. 

Nitrate as nitrogen, in mg/L

Uranium, in pCi/L

Summer nitrate

Summer uranium

Winter nitrate

Winter uranium

1965 1970 1975 1980 1985 1990 1995 2000 2005

0

2

4

6

8

10

0

5

10

15

20

25

30

Figure 25. Nitrate and uranium concentrations in ground water from the public-supply well in Modesto, eastern San Joaquin Valley, 

California, between 1966 and 2005. Nitrate and uranium data were separated on the basis of seasonal pumping cycles.

mg/L, milligram per liter; pCi/L, picocurie per liter.
52 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Summary and Conclusions

The hydrogeology, water chemistry, and factors affecting 

the transport of contaminants in the zone of contribution 

of a public-supply well was investigated in Modesto, 

eastern San Joaquin Valley, California, as part of the U.S. 

Geological Survey’s National Water Quality Assessment 

(NAWQA) Program’s Transport of Anthropogenic and 

Natural Contaminants (TANC) to public-supply wells topical 

study. Beginning in 2003 and ending in 2005, twentythree monitoring wells were installed within the zone of 

contribution of a public-supply well, and the chemistry of 

aquifer materials and ground-water samples were collected 

and analyzed to characterize the local ground-water system. 

The local study area was in the northeastern part of the 

San Joaquin Valley in Modesto, California. The aquifer is 

mostly made-up of unconsolidated deposits of interlayered 

lenses of sand, gravel, silt, and clay that were deposited by 

streams draining the Sierra Nevada during glacial cycles. 

Aquifer sediments are predominantly granitic in origin and 

contain arsenic and uranium.

High rates of recharge in the agricultural area 

surrounding the city of Modesto and large ground-water 

withdrawals within the city have caused ground water near 

the water table to move downward and laterally towards the 

city. As a result, the chemistry of ground water near the water 

table has affected the chemistry of ground water deeper in the 

aquifer than would be expected under natural conditions. 

The chemistry of ground-water samples collected 

near the water table has been affected by overlying landuse practices. Pesticides were present in water-table wells 

beneath agricultural and urban land-use settings; however, 

VOCs generally were detected beneath urban land only. 

Nitrate and sulfate concentrations beneath urban land were 

generally lower than would be expected beneath agricultural 

land. In contrast to anthropogenically influenced constituents, 

uranium is ubiquitous in sediments, and dissolved uranium 

concentrations in ground-water samples in water-table wells 

were high beneath both land-use settings. Alkalinity was 

strongly correlated with uranium concentrations. The principal 

source of alkalinity is bicarbonate, which forms a complex 

Figure 26. Movement of anthropogenically influenced ground water and approximate ground-water age distribution in Modesto, 

eastern San Joaquin Valley, California.

20

100

40

80

20

100

80

High nitrate,

Low nitrate, high uranium

high uranium

PSW

20

100

40

80

Urban Recently urbanized Agricultural

FPE FPA FPB FPC FPD

OFPA OFPB

-100

-80

-60

-40

-20

20

 0

40

A

SW

A‘

NE

0 10.5 Kilometer

1 Mile

0.25

0 0.25 0.5

Meters

Vertical scale greatly 

exaggerated

Leading edge of agriculturallyaffected water

20

0 – 20 years

20 – 40 years

40 – 100s years

> 1,000 years

Direction of ground 

 water flow

Approximate water-table

 elevation

Approximate land-surface 

 elevation

Well screen interval

Depth below land 

 surface, in meters

Approximate mean age of 

 ground water

EXPLANATION

60 60

60

Leading edge of 

urban-affected

water Peak agriculturallyaffected water

40
Acknowledgments 53

with uranium and increases its solubility in water. Alkalinity 

concentrations in water reaching the water table can be 

increased by irrigation practices in both agricultural and urban 

(residential) areas.

The ground-water chemistry in shallow wells (between 

25 and 35 m bls [meters below land surface]) resulted 

from a mixture of ground water that was influenced by 

both agricultural and urban land-use practices. Because the 

amount of recharge from agricultural irrigation is greater 

than urban recharge and because the majority of the study 

area was farmed prior to about 1960, the chemistry of ground 

water at these depths—where water can be up to 40 years 

old—is likely influenced more by agricultural practices than 

by urban practices. Nitrate concentrations were generally 

high in the shallow ground water throughout the study area. 

Concentrations of PCE, however, indicated that this ground 

water also was locally influenced by urban practices. In 

addition, concentrations of uranium in shallow wells were 

elevated, and the concentration of uranium in the shallow 

depth-dependent sample from the public-supply well was 

above drinking water standards. Because the age of the 

shallow ground water was generally post industrial, the water 

quality in this part of the aquifer has largely determined, and 

will continue to determine, the vulnerability of the publicsuppy well to contamination.

The quality of the ground water from intermediate wells 

(between 45 and 70 m bls) is generally good, although VOCs 

and pesticides were detected in 3 out of 7 wells. Nitrate 

concentrations were higher than background concentrations 

(3 mg/L) in 5 out of the seven monitoring wells. Because 

much of the land was farmed before urbanization, intermediate 

ground water was more affected by agricultural practices than 

by urban practices. The lower specific conductance, higher pH 

values, and older ground-water ages indicate that intermediate

ground water is a mixture of water affected by agricultural 

and, to lesser extent, urban practices and older more pristine 

water that was recharged under more natural conditions. As 

a result, ground-water from intermediate monitoring wells 

was generally older than 40 years and likely older than postindustrial atmospheric age tracers.

Deep ground water (between 100 and 120 meters bls) 

has been unaffected by land-use practices, and reflected 

ground water that evolved under natural conditions. As a 

result, VOCs and pesticides were not detected in the deep part 

of the aquifer except where well construction and operation 

allowed contaminants to migrate from shallow ground water. 

Nitrate concentrations were generally less than 3 mg/L, 

which are indicative of natural background concentrations. 

Concentrations of uranium were generally less than 1 μg/L. 

Ground-water samples from the deep monitoring wells were 

“tritium dead.” Uncorrected carbon-14 ages indicated the age 

of ground water from this part of the aquifer was greater than 

1,000 years.

Well-bore flow data within the pumping public-supply 

well showed water that discharged from the well head 

consisted of 20 percent shallow water, 55 percent intermediate

water, and 25 percent deep water. However, during static 

conditions, shallow ground water moved downward through 

the public-supply well borehole and into the deep part of the 

aquifer. Because the chemistry of water from the shallow

zone was of poorer-quality than that from the deep zone, 

prolonged static conditions during the winter allowed large 

volumes of contaminated water to accumulate in the deep part 

of the aquifer near the public-supply well. Historical waterquality data show that the quality of water that is pumped from 

the public-supply well is substantially affected by seasonal 

changes in pumping. The concentrations of tracers of groundwater age in composite samples from the public-supply well 

were affected by well-bore leakage and the mixture of young 

and very old ground water and concentrations of contaminants 

within the borehole. As a result, the age of ground water 

pumped from the public-supply well depended on the time of 

year, pumping history, and hydrologic conditions. 

Although increased pumping during winter months could 

cause winter water-quality samples to be below drinking water 

standards, continued downward migration of contaminated 

ground water to deeper parts of the aquifer threatens the 

long-term sustainability of the aquifer as a source of clean 

drinking water. Application rates of nitrogen fertilizers leveled 

off during the mid-1970s to late-1990s, but recently may be 

increasing again, which could cause nitrate concentrations 

in the aquifer to rise. Increases in urban recharge or loss of 

agricultural recharge as urban areas expand into farmed land 

may significantly decrease the nitrate concentrations at the 

public-supply well. Alkalinity measurements were relatively 

high beneath both land-use settings, and movement of high 

alkalinity water to deeper depths is likely to increase uranium 

concentrations throughout the aquifer and eventually at the 

well unless flow patterns change. The quantitative effects 

of both these hypotheses were tested and are reported in the 

companion report describing the local-scale ground-water flow 

model. 

Acknowledgments

This study could not have been completed without the 

help and cooperation of the city of Modesto, particularly 

employees Lanora Hill and Jeremy Damas, who allowed 

the USGS access to the city public-supply well several 

times and altered the pump base of the public-supply well 

to accommodate equipment needed for the depth-dependent 

part of this study. Their patience and contributions while this 

investigation was completed are greatly appreciated. Sandy 

Eberts, the TANC team leader, provided critical support 

and facilitated many fruitful discussions among scientists 

throughout this study. Craig Brown, USGS Connecticut Water 

Science Center organized, distributed and helped interpret 

the solid-phase analyses, which proved to be an important 

component in the local-scale study. Neil Dubrovsky, USGS 

NAWQA Nutrient and Trace Element National Synthesis 
54 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Chief, provided assistance and important insight to the 

analysis of uranium and arsenic throughout this study. Ken 

Stollenwerk, USGS National Research Program, completed 

the sorption experiments and extractions and assisted in the 

interpretation of this data. 

The interpretation of ground-water ages in alluvial 

settings was aided by numerous discussions with Dr. Graham 

Fogg at the University of California at Davis. Dr. Robert 

Zierenberg at the University of California at Davis provided 

guidance and insight into the major-ion composition and 

geochemical evolution of water described in this report. 

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Appendix A: Collection and Analysis of Aquifer Solids. 61

Appendix A: Collection and Analysis of 

Aquifer Solids.

Core samples were collected from various depths and 

boreholes to obtain representative vertical, lateral, and textural 

profiles of the aquifer material. Eight cores were collected by 

the mud-rotary drilling method using a wireline coring device 

with a split-barrel sampler lined with a 5.1-cm-diameter 

acrylic sleeve. Two cores were collected by the hollow stem 

auger method using the same coring device but with a slightly 

larger diameter sleeve. After retrieval from the borehole, core 

sleeves were capped and taped at each end to prevent exposure 

to the atmosphere. Core sleeves were then transferred to a 

nitrogen-filled plastic bag, and subsamples of the core were 

cut, capped, and taped. Core samples were analyzed for heavy 

minerals, mineralogical content, particle size, bulk density, and 

carbon content. Acid extractions were performed to determine 

concentrations and the distribution of trace elements, iron 

content and iron-reducible sulfides in aquifer material. 

In addition, adsorption experiments and another series of 

extractions were done to understand the source and movement 

of arsenic and uranium in the aquifer. 

Mineralogical Analyses and Carbon Content

The mineralogy of samples was determined by 

using powder X-ray diffraction, for bulk mineralogy and 

clay separates. Heavy mineral separations and magneticsusceptibility separations were done on sediment samples 

to identify minerals that may contain sulfides, oxides, and 

other more reactive minerals that may be relevant to aquifer 

weathering. The sediment core subsamples were small (10 g) 

and, therefore, may not adequately represent the lithology, but 

they provided some information that was useful in assessing 

geochemistry and provenance. Samples for heavy minerals 

were sieved to <60 mesh, separated using heavy liquids 

(density >2.7 g/cm3), and then by magnetic susceptibility. Half 

of the sample was ground for emission spectroscopy, which 

provided semi-quantitative analysis for 35 elements, and the 

other half was examined with light microscopy and powder 

XRD to help identify the grains. The nonmagnetic fraction, 

which includes most sulfides, oxides, siderite, and silicates, 

and the slightly magnetic fraction were analyzed to target the 

more reactive minerals of interest (Arbogast, 1990). 

The relative abundances of the bulk minerals and clay 

composition determined by X-ray diffraction were consistent 

with granitic source rocks of the Sierra Nevada and optical 

mineralogic descriptions of previous investigations (table A1). 

Quartz was the most abundant mineral and was typically 

followed in order of decreasing abundance by plagioclase 

and potassium feldspar, and minor amounts of hornblende. 

The most abundant clay mineral was smectite, which was 

usually followed in sequence by illite and kaolinite, and 

lesser amounts of chlorite, hornblende, and goethite. Smectite 

is the primary weathering product in arid and semi-arid 

environments, where less rain increases soil residence times 

and the concentrations of dissolved ions (Appelo and Postma, 

1996).

Heavy minerals were separated from four core samples, 

and the slightly-magnetic and non-magnetic fractions were 

analyzed (table A2). The heavy mineral assemblage was 

consistent with common accessory minerals found in granitic 

environments. Amphiboles/pyroxenes were the most abundant 

mineral followed by lesser amounts of sphene, epidote, zircon, 

and biotite. 

Carbon in aquifer sediments was extracted and converted 

to CO2(g) using an elemental analyzer and measured with a 

continuous-flow isotope-ratio mass spectrometer (Brenna, 

1997; Revesz and Qi, 2003). Carbon isotopic results are 

reported in parts per thousand (per mil) relative to the VPDB 

(Vienna Peedee belemnite) standard by assigning a value 

of +1.95 per mil to National Bureau of Standard 19 CaCO3

(Coplen, 1994). Total carbon content of aquifer sediments 

ranged from 0.1 g/kg, less than the detection level, to a 

maximum of 0.6 g/kg. Carbon content was greater in shallow 

sediments than in deep sediments, but differences between 

concentrations were usually within analytical precision. The 

primary form of carbon in sediments was organic carbon. 

Inorganic carbon was not detected above the detection level 

of 0.2 g/kg (table A3). Small differences between total carbon 

and organic carbon content may suggest some inorganic 

carbon is present in aquifer sediments, although the difference 

between total and organic carbon was within the analytical 

error. Carbonate minerals, such as calcite, are likely the only 

source of inorganic carbon in the aquifer. Calcite was not 

detected by X-ray diffraction analysis, but calcite can form 

authigenically (through in-place precipitation) on the surfaces 

of clay minerals and may not be detected by X-ray diffraction 

methods (Izbicki and others, 2002). 

Although organic carbon content of the sediments 

analyzed in this study was not high, the amount of organic 

carbon generally decreased in deeper sediments (Spearman’s 

rho = –0.65, p-value = 0.05; table A3). This could indicate 

that reduction of organic carbon may have caused the lower 

dissolved oxygen levels measured in deep monitoring wells 

(median = 2.7). Microbes utilize organic carbon as a source 

of energy by changing the valence state of carbon and another 

element or compound. Where sources of carbon are high, 

microbial reactions can significantly affect the reductionoxidation (redox) conditions of ground water. Depending 

on the redox state, oxygen and nitrate can be removed, and 

dissolved manganese and iron concentrations may become 

elevated. However, the presence of dissolved oxygen in deep

ground water indicated that ground water remained oxic and 

could indicate that organic carbon content in aquifer materials 

was not sufficient to cause substantial reduction of nitrate, and 

oxides of manganese and iron.
62 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California Table A1. Relative abundance of (I) bulk minerals and (II) minerals in particles less than 1 micrometer from core samples collected during monitoring well installation in 2003 and 2004 in Modesto, eastern San Joaquin Valley, California. [Lower integers indicate higher relative abundance. μm, micrometer; <, less than] I. Relative abundance of bulk minerals Borehole ID Depth to top of core segment (meters) Depth to bottom of core segment (meters) Clay (percent) Silt (percent) Sand (percent) Quartz Plagioclase K-Spar Total clay Hornblende SA 8.5 8.8 6 47 47 1 2 3 4 SB 8.5 8.8 4 26 70 1 2 3 4 FPA 8.8 10.4 3 21 76 1 2 3 4 OFPA 18.3 19.8 3 12 85 1 2 3 4 FPA 42.4 43.9 7 48 46 1 2 3 5 4 OFPA 57.0 58.5 2 8 90 1 3 2 5 FPD 63.1 64.6 12 38 50 1 2 4 3 FPA 75.9 77.4 13 48 39 1 2 3 4 FPD 107.9 109.4 12 51 38 1 2 3 4 5 FPA 109.4 111.0 8 34 57 1 2 3 5 trace II. Relative abundance of minerals in particles <1μm Borehole ID Depth to top of core segment (meters) Depth to bottom of core segment (meters) Clay (percent) Silt (percent) Sand (percent) Smectite Illite Chlorite Kaolinite Hornblende Goethite SA 28 29 6 47 47 1 2 5 3 4 SB 28 29 4 26 70 1 2 4 3 5 FPA 29 34 3 21 76 1 2 trace 3 trace 4 OFPA 60 65 3 12 85 1 2 trace 3 FPA 139 144 7 48 46 1 2 trace 3 4 OFPA 187 192 2 8 90 2 1 4 3 5 FPD 207 212 12 38 50 2 1 trace 3 trace FPA 249 254 13 48 39 1 2 3 trace 4 FPD 354 359 12 51 38 1 2 trace 3 4 FPA 359 364 8 34 57 1 4 trace 2 trace 3
Appendix A: Collection and Analysis of Aquifer Solids. 63

Arsenic and Uranium Extractions

Most arsenic (As) and uranium (U) in aquifer sediments 

that are available for transport in ground water are adsorbed 

to surfaces of coatings on sediment grains. The abundances of 

As and U in grain coatings or possibly other mineral phases 

from core samples were evaluated using a 10-percent nitric 

acid (HNO3

) extraction. The nitric acid extraction method 

was primarily intended to target ferric oxyhydroxides and 

their associated trace elements, but the extraction can also 

isolate other oxides, clays, and carbonate and sulfide phases to 

varying degrees. The extraction was done on wet sediments, 

but the moisture content was determined for a separate 

subsample and used to correct the analytical concentrations 

to dry weight. The sample/solution ratio was 5 g ± 1 g wet 

weight to 15 mL of acid solution. The sediment-acid alliquots 

were set on a shaker table for 6 hours, then centrifuged, 

filtered with 0.45 μm-pore filters, and analyzed for several 

minor elements by inductively-coupled-plasma opticalemission spectroscopy. Dilution to a 5-percent nitric acid 

solution was required for analysis of As and U, which yielded 

lower detection limits.

Table A2. Percentage of non-magnetic and slightly magnetic heavy minerals separated from core samples collected during monitoring 

well installation in 2003 and 2004 in Modesto, eastern San Joaquin Valley, California.

Borehole ID

Depth to 

top of core 

segment 

(meter)

Depth to 

bottom of core

segment 

(meter)

Non-magnetic

Weight 

percentage 

(combined)

Slightly 

magnetic

Weight 

percent

SB 8.5 8.8 Amphibole/pyroxene 50 Hematite 90

Sphene 40 Sphene trace

Zircon, biotite 10 epidote trace

biotite trace

OFPA 57.0 58.5 Sphene 70 Hematite 100

Amphibole/pyroxene 20 Sphene trace

Hematite, zircon 10 Amphibole trace

FPA 75.9 77.4 Amphibole/pyroxene 60 Amphibole/ pyroxene 70

Sphene 20 Sphene trace

Zircon, epidote, biotite 10 Epidote trace

Biotite trace

FPD 107.9 109.4 Amphibole/pyroxene 20 Amphibole/ pyroxene 90

Epidote trace Sphene and epidote 10

Zircon trace

Sphene trace

Table A3. Total Carbon and organic carbon content of sediments from core samples collected during monitoring well installation in 

2003 and 2004 in Modesto, eastern San Joaquin Valley, California.

[—, not detected; δ13C, delta carbon-13. m, meter; g/kg, gram per kilogram; per mil, per thousand; <, less than]

Borehole ID

Depth to 

top of core 

segment 

(m)

Depth to 

bottom of core 

segment 

(m)

Total carbon 

(inorganic 

plus organic) 

(g/kg)

δ13C of 

Total carbon 

(per mil)

Organic 

carbon, 

(g/kg)

δ13C of 

Organic carbon 

(per mil)

SA1 28 29 0.7 –19 0.6 –27.2

SB1 28 29 0.4 –25.1 — —

FPA2 29 34 0.3 — 0.3 —

OFPA1 60 65 0.2 –25.9 — —

FPA2 139 144 <0.1 — <0.2 —

OFPA1 187 192 0.3 –26.1 0.3 –27.1

FPD2 207 212 0.2 — <0.2 —

FPA2 249 254 <0.1 — <0.2 —

FPD2 354 359 0.3 — 0.2 —

FPA2 359 364 <0.1 — <0.2 —

 1Analyzed at the Reston Stable Isotope Laboratory in Reston, Virginia.

2

Analyzed at the National Water Quality Laboratory in Lakewood, Colorado.
64 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Poorly-crystalline iron oxyhydroxides isolated from 

the nitric acid extracts yielded concentrations ranging from 

950 to 2,600 mg/kg (mg of Fe per kg of bulk sediment). The 

extracts also contained 0.23 to 1.2 mg/kg of uranium and 

0.12 to 0.30 mg/kg of arsenic, and generally represent ironreducible and loosely bound concentrations in sediments, 

although iron, arsenic, and uranium associated with organic 

matter could also be liberated by this extraction. Arsenic and 

iron concentrations in the sediment extracts were weakly 

correlated (spearman’s rho = 0.75, p-value = 0.1, n = 6), 

but suggest that arsenic in ground water is controlled by 

adsorption to ferric oxyhydroxides. Uranium concentrations 

in the sediment extracts significantly increased with sample 

depth (spearman’s rho = 0.75, p-value <0.03, n = 10) and were 

not correlated to either iron or arsenic extractions. Adsorption 

of dissolved species to mineral surfaces depended on the 

availability of adsorption sites and the geochemical conditions 

of the solution. Although ferric oxyhydroxides varied spatially 

within the study area, iron concentrations were several orders 

of magnitude greater than the extractable concentrations of 

uranium, indicating adsorption sites for uranium likely were 

not limited. Therefore, uranium may have been leached from 

adsorption sites in shallow sediments because of geochemical 

conditions different from the deeper conditions which support 

uranium adsorption.

The extractions can overestimate the fraction of 

uranium and arsenic that can be mobilized by ground water. 

Hypothetically, if all of the extractable uranium and arsenic 

had been dissolved in ground water, their concentrations could 

have ranged from about 900 to 2,000 μg/L of arsenic and from 

about 1,600 to 8,100 μg/L of uranium, based on a porosity of 

0.28 (table A4). These hypothetical concentrations suggest 

that neither arsenic nor uranium in ground water are limited 

by their abundance in sediments. However, the sediment 

is not likely to come into contact with water of a similar 

geochemical make-up as the acid extractions used in this 

experiment and; therefore, is an overestimate of the fraction of 

uranium and arsenic available for transport in sediments.

 The fraction of uranium in sediments available for 

transport was determined using an alkaline (pH = 9.45), 

sodium bicarbonate extraction (Kohler and others, 2004). The 

extraction was performed on nine core sediments and drill 

cutting samples collected from soils and from various depths 

throughout the aquifer. The extraction was done by combining 

35 mL of sodium-bicarbonate solution and 2 g of sediment in 

40-mL polypropylene centrifuge tubes. The sediment-solution 

was mixed continuously for five days and then centrifuged to 

separate the extractant. Uranium was measured as U(VI) by 

inductively coupled plasma–mass spectrometry (ICP-MS). 

The sodium bicarbonate extractions indicate uranium 

may be liberated from intermediate and deep sediments. 

In contrast, uranium concentrations from the shallowest 

sediments were not detected, which suggests that the fraction 

of uranium available for transport had already been leached 

from adsorption sites in these sediments or that less uranium 

(all fractions) had been in the shallow sediments than in the 

deeper sediments. These findings are consistent with the 

results of the 10-percent nitric acid extractions.

Table A4. Selected trace element concentrations from extractions performed on core and cutting samples collected during monitoring 

well installation in 2003 and 2004 in Modesto, eastern San Joaquin Valley, California. 

[HCO3, bicarbonate; mg/L, milligram per liter; mg/kg, milligram per kilogram; μg/L, microgram per liter; na, not analyzed; —, not detected; %, percent

Borehole ID

Depth to

top of core

segment

Depth to

bottom of core 

segment

Extractant: 10% nitric acid (1.5 normality) solution Extractant: 1,050 mg/L HCO3

; 

pH 9.45

Iron Arsenic Equivalent Uranium Equivalent Uranium Equivalent 

SA 0.9 1.5 na na na na na — —

SA 8.5 8.8 na 0.3 2,000 0.23 1,600 — —

SB 8.5 8.8 na 0.15 1,000 0.47 3,200 na na

FPA 8.8 10.4 2,000 0.24 1,600 0.56 3,800 — —

OFPA 18.3 19.8 na 0.19 1,300 0.76 5,200 0.02 120

FPA 42.4 43.9 2,600 0.21 1,400 0.99 6,700 0.05 370

OFPA 57.0 58.5 na 0.13 900 0.34 2,300 0.05 320

FPD 63.1 64.6 1,300 0.12 800 0.95 6,500 0.25 1,700

FPA 75.9 77.4 1,900 0.15 1,000 0.9 6,100 na na

FPD 107.9 109.4 1,000 0.14 900 0.96 6,500 0.11 800

FPA 109.4 111.0 950 0.14 900 1.2 8,100 0.08 500

 1The median porosity from 6 cores was 0.28. This value was used to estimate hypothetical ground water concentrations.
Appendix A: Collection and Analysis of Aquifer Solids. 65

Uranium Adsorption Experiments 

Batch reactor experiments were used to quantify 

adsorption of U(VI) onto sediments from core and drillcutting samples collected from surface soils and at various 

depths throughout the aquifer. Experiments were done by 

combining 35 mL of deionized water with 2 g of sediment 

in 40-mL polypropylene centrifuge tubes. Adsorption 

isotherms for U(VI) were determined by diluting U(VI) 

stock solutions prepared from uranium oxide (UO3), to 

obtain initial concentrations ranging from 0 to 600 μg/L. 

Sodium bicarbonate was added to each set of samples to 

approximate the native concentrations of the bicarbonate ion 

(HCO3

–

) in ground water associated with each core sample. 

Initial pH values were adjusted with 0.1N sulfuric acid to the 

approximate pH of native ground water.

The suspensions were mixed continuously on an endrotating mixer at two revolutions per minute (rpm). An 

equilibration time of five days was used. It was determined 

in preliminary experiments that U(VI) adsorption was rapid; 

90 percent was adsorbed within a few hours, and equilibrium 

was attained within two days. Solutions were separated from 

the sediment by centrifuging at 15,000 rpm for 15 minutes and 

filtering with a 0.2-μm pore filter. The pH was then measured, 

and the bicarbonate concentration was determined by titration 

using 0.2 N sulfuric acid to a pH 4.5 endpoint. U(VI) in 

solution was determined by ICP-MS.

Adsorption-test samples consisting of drill cuttings and 

sediment material from depths ranging from the soil zone 

(1 meter) to the shallow part of the aquifer system (40 m) 

showed that the final uranium concentration was greater 

than the initial concentration (fig. A1). This indicates that 

the introduced uranium was not adsorbed to sediments and 

an additional amount of native uranium was released from 

sediments. The amount of native uranium also increased 

with increasing amounts of introduced uranium. To ensure 

the result was not due to experimental error, the adsorption 

experiments were rerun. Results verified the initial experiment. 

Only sediments from the intermediate and deep zones of the 

aquifer showed any appreciable adsorption of uranium. Linear 

isotherms for these sediments indicate uranium adsorption was 

not limited by the concentrations used in the experiment. The 

amount adsorbed or desorbed followed a general trend from 

deepest to shallowest, with the most adsorbed from the deepest 

and the most desorbed from the shallowest sample (fig. A1). 

The adsorption isotherm for site OFPA, 57.0–58.5, appeared 

to mark the boundary between adsorption and desorption. 

Native uranium and alkalinity measurements in ground water 

were high above this depth, generally greater than 20 μg/L 

and 250 mg/L, respectively, and decreased below this depth, 

less than 10 μg/L and 200 mg/L. These results suggest that 

desorption or dissolution of uranium into ground water is not 

solely related to its presence or concentration in the sediments, 

but also to the presence of bicarbonate.

Figure A1. Relation between uranium concentrations and uranium adsorbed to aquifer sediment collected at various depths in 

Modesto, eastern San Joaquin Valley, California.

100 300 500 700 900

-6

-4

-2

0

2

4

6

8

Adsorption

Desorption

Uranium adsorbed, in micrograms per kilogram

Uranium, final, in micrograms per liter

FPA

FPD

FPD

OFPA

FPA

OFPA

FPA

SA

SA

109.4 – 111.0

107.9 – 109.4

63.1 – 64.6

57.0 – 58.5

42.4 – 43.9

18.3 – 19.8

8.8 – 10.4

8.5 – 9.1

0.9 – 1.5
66 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Appendix B: Quality-Control Analysis

Quality-control analysis was done for all data 

collected during the TANC study. Not all data collected 

for this study is presented or analyzed in this report. Data 

from this study can be retrieved online from the NAWQA 

Data Warehouse at http://infotrek.er.usgs.gov/traverse/

f?p=NAWQA:HOME:2677137420746277

Quality-control samples (blanks, replicates, and spikes) 

were collected to monitor for bias and variability in the 

environmental data. Bias can be either negative or positive 

error caused by loss of analyte (for example, by degradation) 

or adding an analyte (for example, by cross-contamination). 

Variability is an expression of the difference between 

comparable measurements; since ground-water quality did not 

vary much during the short time when samples were collected, 

variability between replicate samples were likely mainly due 

to differences in sample handling or analyses.

Blanks were used to monitor for positive bias caused by 

sample contamination. A ground-water sample was believed 

to have been contaminated during collection, handling, or 

processing if (1) a constituent was detected in an equipment 

or field blank, (2) that constituent was detected in groundwater samples, and (3) the minimum concentration detected 

in the ground-water samples was less than the maximum 

concentration detected in field blanks. If a constituent was 

detected in an equipment or field blank, then the concentration 

detected in the blank was compared with that in the associated 

source-solution blank to determine if the contaminant came 

from the blank water itself. 

Field blank detections that could not be attributed 

to contamination by blank water, laboratory methods or 

procedures, or storage were compared with detections 

of constituents in associated ground-water samples and 

evaluated with respect to criterion 3 above. An associated 

ground-water sample is defined as any ground-water sample 

collected immediately before or after a field blank. Detecting 

a constituent in the field blank that was also detected in an 

associated ground-water sample may indicate carry-over 

contamination. Carry-over contamination occurs when a 

constituent is introduced into a sample from the sampling 

equipment in spite of cleaning procedures. If carry-over 

contamination was identified as the cause of the detection 

in the field blank, subsequent ground-water samples and 

field blanks also was evaluated for evidence of carry-over 

contamination. A constituent that was detected in a field blank 

but was not detected in an associated ground-water sample 

was not the result of carry-over contamination. 

Ground-water results were censored when detection of 

a constituent was caused by incidental contamination of the 

sample from equipment or field procedures. The threshold 

concentration for censoring data was determined by adding 

the blank concentration to the long-term method detection 

level (LT-MDL) for the constituent in question. For example, 

if toluene was detected in a field blank at a concentration of 

0.02 μg/L, and the LT-MDL for toluene is 0.02 μg/L, then 

the concentration of toluene in the associated ground-water 

sample would have to be greater than or equal to 0.04 μg/L to 

be reported as a detection. 

If a constituent was detected in multiple blanks 

(equipment or field) and the constituent could not be attributed 

to the source solution or to carry-over contamination, then 

any ground-water sample that had the constituent in question 

was censored at a threshold concentration. The threshold 

concentration was determined by averaging the blank 

concentrations and adding three times the standard deviation 

of blank samples. All ground-water samples containing 

constituent concentrations less than or equal to the threshold 

concentration were censored. Censored values are indicated by 

a “V” preceding the value in the tables, and are excluded from 

the summary statistics.

Replicate samples, collected sequentially, were used to 

assess variability caused by differences in sample handling 

or analysis. At least one replicate sample pair was collected 

for each schedule of analytes sampled during this study. 

Multiple replicate pairs were collected for analysis of VOCs, 

major ions, nutrients, dissolved organic carbon, ultra-violet 

absorbance, and trace elements, the most frequently sampled 

constituents. Relative standard deviation (RSD) was used as a 

measure of variability between replicate pairs and is defined 

as the absolute value of the standard deviation divided by 

the mean concentration for each replicate pair, multiplied 

by 100 percent. RSD values were calculated using raw, 

unrounded values of concentrations. If a compound was 

detected in the environmental sample but was not detected in 

the duplicate sample or vice versa, the concentration used to 

calculate the RSD for the compound in the replicate pair was 

assigned a value of one-fourth the minimum detection level. 

RSD values less than 20 percent were considered acceptable in 

this study. 

Spikes, including surrogates and matrix spikes, were 

collected by adding solutions containing known amounts of 

selected pesticides or VOCs to some replicate ground-water 

samples. The concentration of each compound in solution was 

compared with the spike solution concentration and expressed 

as a percentage of the spiked concentrations. Recoveries of 

surrogate and matrix-spike compounds were used to evaluate 

bias for pesticide or VOC analyses. Recoveries between 70 

and 130 percent were considered acceptable in this study.
Appendix B: Quality Control Analysis 67

Major Ions, Nutrients, Trace elements, 

Dissolved Organic Carbon and Ultra-Violet 

Absorbance

Blanks

Four field blanks and three equipment blanks were 

collected during this study and analyzed for major ions. (One 

field blank was removed from quality-control analysis because 

a bottle mix-up led to the partial analysis of an environmental 

sample in place of the blank.) Calcium, magnesium, sodium, 

silica, bromide, fluoride and manganese were detected in 

one or more blanks at concentrations lower than those in 

ground-water samples. Only bromide, fluoride and manganese 

concentrations were within two orders of magnitude of their 

concentrations in ground-water. These results indicate that 

concentrations found in ground-water samples may partly be 

due to contamination introduced during sampling or analysis. 

Fluoride was not frequently detected and was not used in 

the data-analysis part of this report. Bromide concentrations 

less than 0.031 mg/L (three standard deviations of the mean 

blank concentration) and manganese concentrations less 

than 4.3 μg/L (three standard deviations of the mean blank 

concentration) were removed from analysis. As a result, one 

bromide sample and 28 manganese samples were censored, as 

indicated by a “V” next to the value reported in Appendix E.

Four field blanks, one source-solution blank, and nine 

equipment blanks were analyzed for nutrients. Nutrients 

were not detected in the field blanks and the source-solution 

blank. Orthophosphate was detected in one equipment blank 

at a concentration one-fifth of the lowest concentration in all 

ground-water samples. Since eight of the blank samples did 

not contain detectable orthophosphate, this compound was not 

considered to significantly contribute to ground-water sample 

concentrations and was not censored. 

Eleven trace elements were detected in field blanks and 

equipment blanks. Seven of these trace elements—barium, 

cadmium, cobalt, copper, molybdenum, nickel, and zinc 

—were not censored because the constituent would not be 

used for data analysis. Barium, cadmium, and molybdenum 

concentrations in blank samples were generally one-tenth 

of the concentration in a ground-water sample. Therefore, 

censoring values for these constituents would not cause the 

censoring of ground-water samples. Censoring values for 

cadmium, cobalt, copper, nickel and zinc were 1.2, 0.1, 0.5, 

2.7, 7.8 μg/L, respectively. Aluminum was detected in one 

equipment blank at a concentration similar to that in the 

following ground-water sample. This ground-water sample 

was censored. A field blank done immediately following that 

ground-water sample did not contain detectable concentrations 

of aluminum. Arsenic, strontium, and vanadium were detected 

in blanks but were not censored because the detections were 

at least ten times below the lowest concentration detected in 

ground water.

Four field blanks were analyzed for dissolved organic 

carbon and three were analyzed for ultra-violet absorbing 

organic constituents (UV absorbance). Dissolved organic 

carbon was detected in all four field blanks. The median 

concentration for blanks was greater than the median 

concentration for ground-water samples. This result could 

have been due to analytical difficulties or to a lack of total 

removal of methanol used to clean the equipment between 

sampling ground-water and collecting field blanks. As a result, 

dissolved organic carbon concentrations in ground water are 

unreliable and were not used for data analysis. Ultra-violet 

absorbing organic constituents were not detected in any of the 

blanks.

Replicates

Five replicate pairs of ground-water samples were 

analyzed for major ions, four replicate pairs were analyzed for 

nutrients, two replicate pairs were analyzed for trace elements, 

four replicate pairs were analyzed for dissolved organic 

carbon, and three replicate pairs were analyzed for ultraviolet absorbance at 254 nanometers and 280 nanometers. 

Fifty out of fifty-three analytes (94 percent) had a mean 

relative standard deviation less than 15 percent and forty-one 

(77 percent) had RSDs less than 5 percent (table B1). The 

mean relative standard deviation for zinc was 62 percent. Zinc 

was not used for data analysis in this report.

Pesticides and Volatile Organic Compounds 

(VOC)

Blanks

One field blank and three equipment blanks were 

collected and analyzed for pesticides. Pesticides were not 

detected in these blanks.

Toluene was the most frequently detected compound 

among 14 equipment, 9 field, and 7 source-solution blanks; the 

combined detection frequency was 63 percent. Toluene was 

detected more frequently in the equipment (78 percent) and 

the field (71 percent) blanks than in the source-solution blanks 

(43 percent). Because of the widespread low-level detection of 

this compound in quality-control samples, toluene detections 

in the environmental data was censored at 0.19 μg/L, which 

is the average toluene concentration in blank samples plus 3 

times the standard deviation of the toluene concentrations. 

The highest toluene concentration was 1.97, μg/L, which is 

over 7 times higher than any other sample. Toluene from this 

sample was considered an outlier and was removed from the 

calculation of the censored value. Since the censoring protocol 

would reduce the number of detections in ground water to one, 

toluene was left out of the VOC data analysis. 
68 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Table B1. Summary of mean relative standard deviations for replicate sample pairs of inorganic and nutrient analytes in ground-water 

samples collected in October and November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California. 

[The number of pairs used is in parentheses. Duplicate non-detect values were not compared. RSD, relative standard deviation; UV, ultraviolet; %, percent; 

>, greater than]

<5% <10% <15% >20%

Calcium (5) Fluoride1

 (2) UV absorbance (280nm)2 Zinc (2)

Magnesium (5) Iron (1) Dissolved organic carbon Methane recoverable (1)

Potassium (5) Manganese3

 (4) Uranium-235 (4) Radium-226

Sodium (5) UV absorbance (254nm)

Bromide (4) Gross beta radioactivity, Cs-137 curve (1)

Chloride (5) Dissolved oxygen lab (1)

Silica (5)

Sulfate1

 (5)

Residue on evaporation (4)

Ammonia plus organic nitrogen (1)

Nitrite plus nitrate (4)

Orthophosphate (4)

Total nitrogen1

 (3)

Aluminum1

 (2)

Antimony (1)

Arsenic (2)

Barium (2)

Boron (2)

Chromium (2)

Cobalt1

 (2)

Lithium (2)

Molybdenum (2)

Nickel (2)

Selenium (1)

Strontium (2)

Vanadium (2)

Uranium (2)

Alpha radioactivity 2-sigma combined 

uncertainty, Th-230 curve (1)

Beta radioactivity 2-sigma combined 

uncertainty, Cs-137 curve (1)

Deuterium/Protium ratio (1)

Oxygen-18/Oxygen-16 ratio (1)

Nitrogen-15/Nitrogen-14 ratio in nitrate 

fraction (1)

Oxygen-18/Oxygen-16 ratio in nitrate 

fraction (1)

Radon-222 2-sigma combined uncertainty (1)

Radon-222 (1)

Sulfur-34/Sulfur-32 ratio in sulfate (1)

Uranium-234 (4)

Uranium-238 (4)

Dissolved nitrogen gas (1)

Argon (1)

Tritium4

 1One sample pair exceeded mean RSD but was less than 20%.

2

 One sample pair exceeded mean RSD but was less than 25%.

3

 One sample pair exceeded 30%.

4

 Lamont-Doherty Earth Observatory.
Appendix B: Quality Control Analysis 69

Gasoline compounds (MTBE, benzene, ethylbenzene, 

xylene, and excluding toluene) were detected in 4 equipment 

and 4 field blanks. These detections and gasoline compounds 

detected in two ground-water samples (FPC-1, FPD-1) were 

caused by contamination from one of two sources: (1) the use 

of contaminated VOC vials or (2) equipment and field blank 

processing procedures. Concentrations of gasoline compounds 

from FPC-1 and FPD-1 (samples from October—November, 

2003) and two equipment blanks and one field blank were 

likely caused by VOC vial contamination, indicated by a “V” 

next to the value (Appendix D). VOC vials used for these 

samples were contaminated only by gasoline compounds 

because of exposure of the vials to exhaust fumes and 

gasoline. The other two equipment and three field blanks were 

contaminated because of equipment blank and field blank 

processing errors. The processing of equipment and field 

blanks requires nitrogen-purged, VOC-free blank water to 

flow through sampling equipment before collection of quality 

control (QC) samples. Because the amount of VOC-free water 

was limited, care had to be given to ensure that the timing of 

the QC sample coincided with the arrival VOC-free water. 

Sampling before the arrival of VOC-free water caused the 

gasoline compounds to contaminate the equipment and field 

blanks. To correct the contamination, more time was given 

for blank water to flow through the equipment before the 

blanks were collected. This problem was discovered late in the 

project, and no gasoline compounds, excluding toluene, were 

detected in the last equipment and field blanks collected in this 

study.

Replicates

One of the 36 environmental pesticide samples was 

collected in replicate and used to calculate RSD values for 

detected pesticides. The same four compounds were detected 

in both the environmental and the replicate samples. The 

mean RSD value for the four detected compounds was 

2.9 percent, and the RSD did not exceed 5 percent for any of 

the compounds detected. 

Four of the 59 VOC samples (environmental plus 

QC) were collected in replicate and used to calculate 

RSD values for detected VOCs. The mean RSD value for 

tetrachloroethene, trichloromethane, bromodichloromethane, 

and MTBE was 7.3, 5.9, 11.1, and 105, respectively. The 

RSD for MTBE was over 105 because MTBE was detected 

in the environmental sample but not in the duplicate sample. 

Consequently, the concentration used to calculate the RSD 

for MTBE in the replicate pair was one-fourth the minimum 

detection level. Four analytes were detected in this replicate 

pair, and RSD values for the other three compounds were 

greater than 10 percent but less than 15 percent. The higher 

overall RSD values from this replicate pair likely indicate 

problems with analytical instruments or sample processing. 

Spikes (Surrogates and Matrix Spikes)

Surrogate compounds were added to all of the samples 

that were analyzed for pesticides and VOCs, including 

gasoline oxygenates. Surrogate compounds are constituents 

that behave like pesticide or VOC analytes but are not 

normally found in the environment and are added to each 

sample just before analysis. Surrogates are used to evaluate 

the performance of the analytical methods in detecting similar 

compounds in each sample, and to determine whether the 

chemistry of each sample would interfere with the pesticide 

or VOC analysis (referred to as “matrix interference”). The 

results of surrogate compounds are reported as a percentage 

of the total amount of surrogate added to each sample and 

are referred to as a “recovery.” A 70- to 130-percent recovery 

is considered acceptable — although this range varies for 

individual compounds — and values outside this range 

indicate possible problems with the processing and analysis of 

samples. 

Two surrogates, α-HCH-d6 and diazinon-d10, were added 

to each of the 42 pesticide samples (environmental plus QC 

samples) in this study. The mean recovery was 88 percent 

(standard deviation = 8.8) for alpha-HCH-d6 and 87 percent 

(standard deviation = 15.6) for diazinon-d10. Only five of the 

42 samples of the diazinon-d10 surrogate had recoveries less 

than 70 percent. Four of those five samples were collected at 

site FPA as part of the depth-dependent sampling. Pesticide 

samples collected from these wells during the previous 

year (October 2003) had acceptable surrogate recoveries, 

which suggests that matrix interference was unlikely and 

that the lower recovery was probably the result of laboratory 

processing or equipment problems. 

Three surrogates were added to each of the 94 VOC 

samples (environmental plus QC samples), and one 

surrogate was added to each of the 40 gasoline oxygenate 

samples (environmental plus QC samples) that were 

collected for this study. Mean VOC surrogate recoveries 

for 1,2-dichloroethane-d4, 1-bromo-4-fluorobenzene, and 

toluene-d8 were 111 percent (standard deviation = 7.9), 91 

percent (standard deviation = 9.4), and 98 percent (standard 

deviation = 4.6), respectively. Only one sample had a recovery 

of 1-bromo-4-fluorobenzene that was lower than 70 percent, 

and 3 samples had recoveries of 1,2-dichloroethane-d4 greater 

than 130 percent. The mean gasoline oxygenate recovery for 

isobutyl alcohol-d6 was 95 percent (standard deviation = 11.5) 

and all samples had recoveries between 70 and 130 percent.
70 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

A spike solution containing 31 pesticides was added 

to one replicate ground-water sample and analyzed for 

pesticides. The average matrix-spike recovery was 73 percent 

and the standard deviation was 21 percent. 2-Chloro-4-

isopropylamino-6-amino-s-triazine (CIAT, a metabolite of 

atrazine, also known as deethylatrazine or DEA), which is a 

degradation product of atrazine and is frequently detected in 

ground-water, had the lowest spike recovery at 11 percent. 

Such a low recovery suggests the possibility of false negatives 

and that CIAT could be more prevalent than was detected and 

(or) exist at higher concentrations than were reported.

A spiked solution containing 86 VOCs was added to 

three duplicate ground-water samples that were collected 

sequentially with environmental samples and analyzed for 

VOCs. No compound had recoveries below 70 percent. Five 

compounds had one spike recovery greater than 130 percent 

in three spike samples. An average recovery was calculated 

for each of these compounds. The average minimum recovery 

of the three spiked samples was 99.8 percent (standard 

deviation = 7.8), and the average maximum recovery was 

114 percent (standard deviation = 8.8). The average median 

recovery was 105.5 percent (standard deviation = 7.6). 

Miscellaneous Compounds

Replicate samples were collected for stable isotopes, 

radioactive isotopes, gross-alpha radioactivity, gross-beta 

radioactivity, dissolved gases, and tritium (table B1). Most 

analytes had relative standard deviations (RSD) less than 

five percent (table B1). Methane and radium-226 had an RSD 

greater than 20 percent and were not used in the data analysis. 

A RSD for radium-228 was not calculated because the 

certainty that the concentration of radium-228 in the replicate 

pair were greater than zero was less than 95 percent. Four 

replicate samples of uranium isotopes were used to evaluate 

the reproducibility of uranium isotope values. The average 

RSDs for replicate pairs for uranium-238 and uranium-234 

were less than 3 percent, whereas the average RSD for 

uranium-235 was14 percent. 

Arsenic speciation samples were collected immediately 

after the trace element samples were collected. The sums of 

arsenate and arsenite were compared to the non-speciated 

arsenic results from the trace element sample, and RSDs were 

calculated between the two methods. Differences between the 

two methods was expected to be low because any additional 

arsenic species, such as organic ligands, was likely negligible. 

Thirty-one arsenic speciation samples were used to calculate 

the RSDs between schedules. The average RSD among all 

samples was less than 5 percent, and the highest RSD was less 

than 15 percent. 
Appendix C: Major ion composition, pH, and calcite saturation in ground water from monitoring wells 71

Appendix C: Major ion composition, pH, and calcite saturation in ground water from monitoring wells
72 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Well identification no.

Depth 

category/

well type

Sample date

Residue on 

evaporation, 

dried at 180°C, 

dissolved 

(mg/L)

Calcium, 

dissolved 

(mg/L)

Magnesium, 

dissolved 

(mg/L)

Potassium, 

dissolved 

(mg/L)

Sodium, 

dissolved 

(mg/L)

Bromide, 

dissolved 

(mg/L)

Chloride, 

dissolved 

(mg/L)

 Parameter code 70300 00915 00925 00935 00930 71870 00940

 LRL 10 0.02, 0.011 0.008 0.16 0.2, 0.1 0.02, 0.0161 0.2

FPA-1 Water-table 10/29/2003 466 51.7 23.0 3.25 82.3 0.28 11.8

FPB-1 Water-table 11/19/2003 412 59.3 26.5 2.33 44.6 0.14 7.33

FPC-1 Water-table 10/20/2003 626 88.9 37.6 3.90 86.8 0.24 18.1

FPD-1 Water-table 10/21/2003 486 77.6 36.7 2.11 24.4 1.56 11.7

FPE-1 Water-table 10/27/2004 328 42.1 24.2 1.57 33.3 0.13 29.9

OFPA-1 Water-table 11/03/2004 578 77.7 27.8 3.33 93.3 1.06 24.6

SA Water-table 10/25/2004 668 120 54.9 2.91 45.1 0.12 18.6

SB Water-table 10/25/2004 642 92.7 46.0 2.87 68.7 — 32.6

SC Water-table 10/26/2004 698 106 53.5 2.93 62.4 E0.12 43.4

FPA-2 Shallow 11/04/2003 484 70.0 30.3 4.04 37.1 0.10 13

FPE-2 Shallow 10/27/2004 536 81.4 34.0 3.09 45.2 0.21 14.3

OFPA-2 Shallow 11/03/2004 676 118 43.5 4.01 58.5 0.21 76.7

FPA-3 Intermediate 11/05/2003 280 23.7 7.13 4.70 22.5 0.09 6.37

FPB-2 Intermediate 11/18/2003 279 34.2 11.3 4.88 29.7 0.17 8.25

FPC-2 Intermediate 11/12/2003 338 50.6 15.8 6.82 34.0 0.15 9.61

FPD-2 Intermediate 11/13/2003 239 23.5 8.48 3.42 34.8 0.23 9.57

FPE-3 Intermediate 10/27/2004 276 38.4 10.8 5.67 21.6 0.06 8.59

OFPA-3 Intermediate 11/03/2004 245 33.4 9.15 3.81 22.9 0.07 9.36

OFPB-2 Intermediate 11/02/2004 223 28.4 7.21 3.43 31.4 0.04 5.1

FPA-4 Deep 11/06/2003 245 18.2 6.89 6.58 39.6 0.18 8.93

FPB-3 Deep 11/18/2003 175 10.6 3.71 4.86 20.6 0.13 4.75

FPC-3 Deep 11/12/2003 137 12.0 3.95 4.83 18.1 0.04 4.45

FPD-3 Deep 11/17/2003 173 15.9 5.33 5.39 17.6 0.39 5.81

MOD025 PSW 11/13/2003 437 64.5 25.7 3.86 42.3 0.08 16.7

Table C1. Major ion composition, pH, and calcite saturation in ground water from monitoring wells sampled in October and November 

of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California. 

[The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent or 

property. °C, degree Celsius; CO2(g), carbon dioxide gas; LRL, laboratory reporting level; mg/L, milligram per liter; μg/L, microgram per liter; g, gram; ppm, 

parts per million; —, not detected; E, estimated; V, some or all of concentration could be due to contamination; PSW, public supply well. Calculated partial 

pressure of carbon dioxide gas is reported as a log concentration. Saturation index for calcite is the ratio of the calculated reaction quotient to the equilibrium 

constant; Alkalinity, sulfate and nitrate are reported in table 8] 
Appendix C: Major ion composition, pH, and calcite saturation in ground water from monitoring wells 73

Well identification no.

Depth 

category/

well type

Sample date

Residue on 

evaporation, 

dried at 180°C, 

dissolved 

(mg/L)

Silica, 

dissolved 

(mg/L)

Iron, 

dissolved 

(μg/L)

Manganese, 

dissolved 

(μg/L)

pH, field, 

standard 

units

Calculated 

partial 

pressure of

 CO2

(g) 

(ppm, log 

concentration)

Saturation 

index of 

calcite

 Parameter code 70300 00955 01046 01056 00400

 LRL 10 0.04 6, 6.4 0.6, 0.8

FPA-1 Water-table 10/29/2003 466 63.1 47 43.3 7.1 –1.48 –0.05

FPB-1 Water-table 11/19/2003 412 67.1 E6 4.9 7.4 –1.84 0.26

FPC-1 Water-table 10/20/2003 626 21.2 — V0.6 7.2 –1.51 0.33

FPD-1 Water-table 10/21/2003 486 45.1 — V1.8 7.3 –1.84 0.16

FPE-1 Water-table 10/27/2004 328 59.0 11 10.5 7.0 –1.64 –0.48

OFPA-1 Water-table 11/03/2004 578 71.3 — V1.0 6.8 –1.12 –0.16

SA Water-table 10/25/2004 668 74.7 E4 V2.5 7.0 –1.28 0.27

SB Water-table 10/25/2004 642 73.4 — V2.5 7.0 –1.33 0.12

SC Water-table 10/26/2004 698 71.1 10 V1.7 7.1 –1.46 0.23

FPA-2 Shallow 11/04/2003 484 64.9 9 6.6 7.4 –1.91 0.23

FPE-2 Shallow 10/27/2004 536 66.8 — VE0.1 7.4 –1.89 0.31

OFPA-2 Shallow 11/03/2004 676 68.0 — V0.9 7.1 –1.49 0.25

FPA-3 Intermediate 11/05/2003 280 62.7 13 8.3 7.4 –2.29 –0.53

FPB-2 Intermediate 11/18/2003 279 63.3 20 11.5 7.7 –2.49 0.00

FPC-2 Intermediate 11/12/2003 338 66.0 E4 4.9 7.6 –2.28 0.16

FPD-2 Intermediate 11/13/2003 239 57.6 — 6.7 7.8 –2.64 –0.10

FPE-3 Intermediate 10/27/2004 276 61.0 — VE0.1 7.6 –2.42 –0.08

OFPA-3 Intermediate 11/03/2004 245 63.6 — — 7.4 –2.23 –0.33

OFPB-2 Intermediate 11/02/2004 223 50.7 E4 13.8 7.4 –2.18 –0.35

FPA-4 Deep 11/06/2003 245 63.4 19 23.7 7.4 –2.19 –0.54

FPB-3 Deep 11/18/2003 175 63.6 E3 8.8 8.0 –3.02 –0.38

FPC-3 Deep 11/12/2003 137 63.7 — VE0.1 8.1 –3.13 –0.22

FPD-3 Deep 11/17/2003 173 62.4 — V0.3 8.1 –3.09 –0.08

MOD025 PSW 11/13/2003 437 64.5 — VE0.1 7.5 –2.01 0.30

1

 Laboratory reporting levels (LRL) are revised annually based on method performance. The first LRL was applied to samples collected in 2003, the second LRL 

was applied to samples in 2004.

Table C1. Major ion composition, pH, and calcite saturation in ground water from monitoring wells sampled in October and November 

of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California.—Continued

[The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent or 

property. °C, degree Celsius; CO2(g), carbon dioxide gas; LRL, laboratory reporting level; mg/L, milligram per liter; μg/L, microgram per liter; g, gram; ppm, 

parts per million; —, not detected; E, estimated; V, some or all of concentration could be due to contamination; PSW, public supply well. Calculated partial 

pressure of carbon dioxide gas is reported as a log concentration. Saturation index for calcite is the ratio of the calculated reaction quotient to the equilibrium 

constant; Alkalinity, sulfate and nitrate are reported in table 8]
74 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Appendix D: Volatile organic compounds (VOC) detected in ground water from monitoring wells
Appendix D: Volatile organic compounds (VOC) detected in ground water from monitoring wells 75

Table D1. Volatile organic compounds (VOC) detected in ground water from monitoring wells sampled from October through 

November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California.

[The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent 

or property; LRL, laboratory reporting level; μg/L, micrograms per liter; —, not detected; E, estimated; V, some or all of concentration could be due to 

contamination; NC, not collected; PSW, public supply well]

Compound class: Trihalomethanes Solvents

Well 

identification no.

Depth 

category/

well type

Sample 

date

Number 

of 

detections

Trichloromethane 

(chloroform)

(μg/L)

Bromo

dichloromethane

(μg/L)

Tetrachloroethene 

(PCE)

(μg/L)

Trichloroethene 

(TCE)

(μg/L)

cis-1,2-

Dichloroethene

(μg/L)

1,1-

Dichloroethene

(μg/L)

1,1,1-

Trichloroethane

(μg/L)

1,1-

Dichloroethane

(μg/L)

 Parameter code 32106 32101 34475 39180 77093 34501 34506 34496

 LRL 0.024 0.028 0.03, 0.061 0.038 0.024 0.024 0.032 0.035

FPA-1 Water-table 10/29/2003 3 0.12 — — — — — — —

FPB-1 Water-table 11/19/2003 3 0.11 — 0.12 — — — — —

FPC-1 Water-table 10/20/2003 4 3.53 0.12 E0.01 — — — E0.02 —

FPD-1 Water-table 10/21/2003 0 — — — — — — — —

FPE-1 Water-table 10/27/2004 1 0.13 — — — — — — —

OFPA-1 Water-table 11/03/2004 5 0.21 — E0.05 — — — — —

SA Water-table 10/25/2004 4 0.15 — — — — — — —

SB Water-table 10/25/2004 3 1.85 E0.04 — — — — — —

SC Water-table 10/26/2004 8 2.75 E0.04 — — — — — —

FPA-2 Shallow 11/04/2003 2 E0.06 — 0.56 — — — — —

FPE-2 Shallow 10/27/2004 3 1.00 E0.05 — — — — — —

OFPA-2 Shallow 11/03/2004 7 0.47 — 1.21 E0.03 E0.02 E0.03 — 0.17

FPA-3 Intermediate 11/05/2003 0 — — — — — — — —

FPB-2 Intermediate 11/18/2003 1 E0.02 — — — — — — —

FPC-2 Intermediate 11/12/2003 1 — — — — — — — —

FPD-2 Intermediate 11/13/2003 0 — — — — — — — —

FPE-3 Intermediate 10/27/2004 0 — — — — — — — —

OFPA-3 Intermediate 11/03/2004 0 — — — — — — — —

OFPB-2 Intermediate 11/02/2004 2 E0.01 — — — — — — —

FPA-4 Deep 11/06/2003 2 E0.08 — E0.04 — — — — —

FPB-3 Deep 11/18/2003 0 — — — — — — — —

FPC-3 Deep 11/12/2003 0 — — — — — — — —

FPD-3 Deep 11/17/2003 1 — — — — — — — —

PSW 2,3 PSW 11/13/2003 3 1.65 0.24 0.2 — — — — —

Number of wells with detections 14 4 6 1 1 1 1 1

Detection frequency (percent) 61 17 26 4 4 4 4 4

Minimum detection 0.01 0.04 0.01 0.03 0.02 0.03 0.02 0.17

Median detection 0.1423 0.05 0.08 0.03 0.02 0.03 0.02 0.17

Maximum detection 3.534 0.12 1.21 0.03 0.02 0.03 0.02 0.17
76 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Compound class: Gasoline-related compounds Refrigerants

Well 

identification no.

Depth 

category/

well type

Sample 

date

Number 

of 

detections

Methyl 

tertbutyl 

ether 

(μg/L)

m-Xylene 

plus 

p-xylene 

(μg/L)

Ethylbenzene 

(μg/L)

1,2,4-

Trimethylbenzene 

(μg/L)

o-Xylene 

(μg/L)

1,2-

Dichlorobenzene 

(μg/L)

Trichlorofluoromethane 

(μg/L)

1,1,2-

Trichloro1,2,2-

trifluoroethane 

(μg/L)

 Parameter code 78032 85795 34371 77222 77135 34536 34488 77652

 LRL 0.1, 0.171 0.06 0.03 0.056 0.038 0.048 0.08, 0.161 0.038

FPA-1 Water-table 10/29/2003 3 E0.2 — — — — — 0.27 —

FPB-1 Water-table 11/19/2003 3 E0.1 — — — — — — —

FPC-1 Water-table 10/20/2003 4 V0.2 VE0.09 V0.02 — V0.03 — — —

FPD-1 Water-table 10/21/2003 0 — V0.02 — — — — — —

FPE-1 Water-table 10/27/2004 1 — — — — — — — —

OFPA-1 Water-table 11/03/2004 5 1.0 E0.06 E0.01 — — — — —

SA Water-table 10/25/2004 4 — E0.08 E0.02 — E0.03 — — —

SB Water-table 10/25/2004 3 0.2 — — — — — — —

SC Water-table 10/26/2004 8 0.1 E0.09 E0.02 E0.03 E0.03 — — E0.03

FPA-2 Shallow 11/04/2003 2 — — — — — — — —

FPE-2 Shallow 10/27/2004 3 — — — — — E0.02 — —

OFPA-2 Shallow 11/03/2004 7 — — — — — — E0.04 —

FPA-3 Intermediate 11/05/2003 0 — — — — — — — —

FPB-2 Intermediate 11/18/2003 1 — — — — — — — —

FPC-2 Intermediate 11/12/2003 1 — — — E0.07 — — — —

FPD-2 Intermediate 11/13/2003 0 — — — — — — — —

FPE-3 Intermediate 10/27/2004 0 — — — — — — — —

OFPA-3 Intermediate 11/03/2004 0 — — — — — — — —

OFPB-2 Intermediate 11/02/2004 2 — E0.04 — — — — — —

FPA-4 Deep 11/06/2003 2 — — — — — — — —

FPB-3 Deep 11/18/2003 0 — — — — — — — —

FPC-3 Deep 11/12/2003 0 — — — — — — — —

FPD-3 Deep 11/17/2003 1 — — — — — — E0.08 —

PSW2,3 PSW 11/13/2003 3 — — — — — — — —

Number of wells with detections 5 4 3 2 2 1 3 1

Detection frequency (percent) 23 19 13 8.7 8.7 4.3 13 4.3

Minimum detection 0.12 0.04 0.01 0.03 0.03 0.02 0.04 0.03

Median detection 0.16 0.07 0.02 0.05 0.03 0.02 0.08 0.03

Maximum detection 0.97 0.09 0.02 0.07 0.03 0.02 0.27 0.03

 1 Laboratory reporting levels (LRL) are revised annually based on method performance. The first LRL was applied to samples collected in 2003; the second 

LRL applies to samples in 2004.

2 PSW was not included in compound statistical summary.

3

 Dibromochloromethane and tribromomethane were detected in the PSW but were not included in the compound list because these compounds were not 

detected in ground-water samples.

Table D1. Volatile organic compounds (VOC) detected in ground water from monitoring wells sampled from October through 

November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California.—Continued

[The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent 

or property; LRL, laboratory reporting level; μg/L, micrograms per liter; —, not detected; E, estimated; V, some or all of concentration could be due to 

contamination; PSW, public supply well]
Appendix E: Pesticide compounds detected in ground water from monitoring wells and the public-supply well 77

Appendix E: Pesticide compounds detected in ground water from monitoring wells and the public-supply well
78 Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants...Zone of Contribution, Public-Supply Well in Modesto, California

Table E1. Pesticide compounds detected in ground water from monitoring wells and the public-supply well sampled in October and November of 2003 and 2004 in Modesto, eastern San Joaquin Valley, California. [The five-digit parameter code is used in the U.S. Geological Survey National Water Information System (NWIS) to uniquely identify a specific constituent or property. DCPA, Dacthal; LRL, laboratory reporting level; E, estimated; μg/L, microgram per liter; —, not detected; PSW, public supply well]

Well 

identification 

no.

Depth 

category/

well type

Sample 

date

Number of 

detections

2-Chloro-4-

isopropylamino-6-

amino-striazine 

(μg/L)

Simazine 

(μg/L)

Atrazine

(μg/L)

Prometon

(μg/L)

3,4-

Dichloroaniline

(μg/L)

Pendimethalin

(μg/L)

Metolachlor

(μg/L)

Diazinon

(μg/L)

Dieldrin

(μg/L)

DCPA 

μg/L

 Parameter code 04040 04035 39632 04037 61625 82683 39415 39572 39381 82682

 LRL 0.006 0.005 0.007 0.01, 0.0051 0.0045 0.022 0.006, 0.0131 0.005 0.009 0.003

FPA-1 Water-table 10/29/2003 4 E0.007 0.012 0.009 0.01

—

—

—

—

—

—

FPB-1 Water-table 11/19/2003 4 E0.004 0.020 0.013

—

— E0.014

—

—

—

—

FPC-1 Water-table 10/20/2003 4 E0.011 0.108 0.044

— 0.013

—

—

—

—

—

FPD-1 Water-table 10/21/2003 4 E0.096 0.048 0.013

— 0.015

—

—

—

—

—

FPE-1 Water-table 10/27/2004 3 — 0.013

— E0.010

—

—

— 0.005

—

—

OFPA-1 Water-table 11/03/2004 2 E0.008 0.006

—

—

—

—

—

—

—

—

SA Water-table 10/25/2004 1 —

—

—

—

—

—

—

—

— E0.002

SB Water-table 10/25/2004 2 — 0.018 0.017

—

—

—

—

—

—

—

SC Water-table 10/26/2004 1 — 0.023

—

—

—

—

—

—

—

—

FPA-2 Shallow 11/04/2003 0 —

—

—

—

—

—

—

—

—

—

FPE-2 Shallow 10/27/2004 3 E0.010 0.007 0.009

—

—

—

—

—

—

—

OFPA-2 Shallow 11/03/2004 1 —

—

—

—

—

—

—

— E0.005

—

FPA-3 Intermediate 11/05/2003 0 —

—

—

—

—

—

—

—

—

—

FPB-2 Intermediate 11/18/2003 0 —

—

—

—

—

—

—

—

—

—

FPC-2 Intermediate 11/12/2003 2 E0.005

— 0.008

—

—

—

—

—

—

—

FPD-2 Intermediate 11/13/2003 1 —

—

—

—

—

— E0.004

—

—

—

FPE-3 Intermediate 10/27/2004 0 —

—

—

—

—

—

—

—

—

—

OFPA-3 Intermediate 11/03/2004 0 —

—

—

—

—

—

—

—

—

—

OFPB-2 Intermediate 11/02/2004 2 E0.013

— 0.059

—

—

—

—

—

—

—

FPA-4 Deep 11/06/2003 1 E0.004

—

—

—

—

—

—

—

—

—

FPB-3 Deep 11/18/2003 0 —

—

—

—

—

—

—

—

—

—

FPC-3 Deep 11/12/2003 0 —

—

—

—

—

—

—

—

—

—

FPD-3 Deep 11/17/2003 0 —

—

—

—

—

—

—

—

—

—

PSW2 PSW 11/13/2003 3 E0.006 0.009 0.013

—

—

—

—

—

—

—

Number of wells with detections2 9 9 8 2 2 1 1 1 1 1

Detection frequency (percent) 39.1 39 35 9 9 4 4 4 4 4

Minimum detection 0.004 0.006 0.008 0.010 0.013 0.014 0.004 0.006 0.006 0.002

Median detection 0.0079 0.018 0.013 0.010 0.014 0.014 0.004 0.006 0.006 0.002

Maximum detection 0.096 0.108 0.059 0.010 0.015 0.014 0.004 0.006 0.006 0.002

1 PSW was not included in compound statistical summary. 2 Laboratory reporting levels (LRL) are revised annually based on method performance. The first LRL was applies to samples collected in 2003, the second LRL applies to samples in 2004.

Jurgens—Hydrogeology, Water Chemistry, and Factors Affecting the Transport of Contaminants in the Zone of Contribution

of a Public-Supply Well in Modesto, Eastern San Joaquin Valley, California

Scientific Investigations Report 2008–5156

Printed on recycled paper 

Science Center Director

USGS California Water Science Center 

6000 J Street, Placer Hall

Sacramento, CA 95819

http://ca.water.usgs.gov

Modesto, CA York, NE

Woodbury, CT

Tampa, FL

Central Valley aquifer system

High Plains aquifer system

Glacial aquifer system

Floridan aquifer system

Study

Area