U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3036

April 2009

National Water-Quality Assessment, Transport of Anthropogenic and Natural Contaminants (TANC) to Public-Supply Wells

Assessing the Vulnerability of Public-Supply Wells to 

Contamination: Central Valley Aquifer System near Modesto, 

California

Printed on recycled paper

Near Modesto, the primary water-bearing units of 

the Central Valley aquifer system are a sequence of 

unconsolidated, interlayered lenses of sand, gravel, 

silt, and clay that originated as alluvial deposits. The 

sediments consist largely of granitic detritus and 

weathering products from the Sierra Nevada. The 

aquifer in the study area is at least 400 feet thick and is 

unconfined in the shallow part of the system, meaning 

that it can freely receive water percolating down from 

land surface. With depth, however, the aquifer becomes 

partially isolated from sources of vertical recharge 

by discontinuous lenses of fine-grained silt and clay. 

The public-supply well selected for study is screened 

from 91 to 366 feet below land surface. This interval 

spans the zone typically tapped by public-supply wells 

in the region, although the majority of public-supply 

wells have a shorter screened interval and do not 

extend to the depth of the selected well. Groundwater 

flow near Modesto has been greatly altered by human 

activities. Most notably, large amounts of groundwater withdrawals and application of irrigation water have created a significant 

downward component of flow. These water-use practices also control local horizontal movement of groundwater. Irrigation has 

created a recharge rate of 23.6 to 27.6 inches per year in the agricultural areas northeast of Modesto. Groundwater flows from these 

agricultural areas towards the center of the city to the southwest, where large groundwater withdrawals paired with low recharge 

(estimated at 11.8 inches per year) have created a regional water-level depression (Burow and others, 2004).

Flood-plain 

deposits

Silt and clay

Unconsolidated alluvium

Not to scale Semiconsolidated alluvium

Discharge Recharge

Water table Generalized groundwater flow

Central Valley aquifer system

Public-supply

well

Irrigation

well

Agricultural area Urban area

Monitoringwell

cluster

The U.S. Geological Survey’s 

National Water-Quality Assessment 

(NAWQA) Program found, in studies 

from 1991 to 2001, low levels of mixtures of contaminants in groundwater 

near the water table in urban areas across 

the Nation. Although contaminants were 

detected less frequently in deeper groundwater typically developed for public 

supply (Hamilton and others, 2004), the 

proximity of contaminant mixtures to 

underlying public water-supply sources 

prompted the NAWQA Program to begin 

intensive studies in 2001 to assess the 

vulnerability of public-supply wells to 

contamination. Specifically, the pathways 

and processes by which contaminants 

reach public-supply wells in nine aquifer 

systems across the country are being 

investigated. Scientists are studying the 

processes that occur below land surface—whereby contaminants are mobilized or attenuated—as well as investigating how human activities can affect the 

vulnerability of public-supply wells to 

contamination. 

This fact sheet highlights findings 

from the vulnerability study of a public-supply well in Modesto, California 

(Burow and others, 2008; Jurgens and 

others, 2008). The well selected for study 

pumps on average about 1,600 gallons per 

minute from the Central Valley aquifer 

system during peak summer demand. 

Water samples were collected at the public-supply well and at monitoring wells 

installed in the Modesto vicinity (see 

“Study Design” sidebar on next page). 

Samples from the public-supply wellhead 

contained the undesirable constituents 

uranium, nitrate, arsenic, volatile organic 

compounds (VOCs), and pesticides, 

although none were present at concentrations exceeding drinking-water standards. 

Of these contaminants, uranium and 

nitrate pose the most significant waterquality risk to the public-supply well 

because human activities have caused 

concentrations in groundwater to increase 

over time. Some of the contaminants 

found in untreated water from the publicsupply well are anthropogenic (introduced 

by human activity); others occur naturally. 

Specifically, elevated concentrations of 

nitrate and the presence of pesticides are 

commonly linked with intensive, irrigated 

agriculture. VOCs are commonly used as 

solvents or gasoline components in urban 

and residential areas. Uranium and arsenic 

occur naturally in alluvial sediments. 
Study Design

A computer model of groundwater flow and transport was constructed at a regional scale 

to estimate the “zone of contribution” to the selected public supply well (Phillips and others, 

2007). The zone of contribution is the three-dimensional volume of the aquifer material through 

which groundwater flows from the time it enters the groundwater system at the water 

table until it is eventually discharged at the public-supply well. A network of 23 monitoring 

wells was installed in or near the simulated zone of contribution to the well to understand 

groundwater flow and geochemistry along apparent groundwater flow paths to the well. 

Multiple monitoring wells completed at different depths were installed within a single 

borehole. Water samples were collected from the monitoring wells and the supply well during 

October 2003 through June 2005 and analyzed for naturally occurring contaminants (such as 

uranium and arsenic) and anthropogenic contaminants (such as volatile organic compounds, 

pesticides, and nutrients). Other water-quality constituents were analyzed (such as major 

ions, age-dating tracers and selected stable isotopes) to improve understanding of sources of 

water and reactions affecting the chemical composition of groundwater. In addition, well-bore 

flow was measured and water samples were collected from the supply well at various depths 

while it was being pumped to determine where water and contaminants enter the well. 

Finally, a more detailed, local-scale computer model of groundwater flow and transport to the 

selected public-supply well was constructed to calculate the traveltime of water particles 

entering the public-supply well and to simulate long-term nitrate and uranium concentrations 

reaching the well. 

Overall, NAWQA findings point 

to four primary factors that affect the 

movement and (or) fate of contaminants 

and the vulnerability of the publicsupply well in Modesto: (1) groundwater 

age (how long ago water entered, or 

recharged, the aquifer); (2) irrigation and 

agricultural and municipal pumping that 

drives contaminants downward into the 

primary production zone of the aquifer; 

(3) short-circuiting of contaminated water 

down the public-supply well during the 

low-pumping season; and (4) natural 

geochemical conditions of the aquifer. 

Study findings are intended to help water 

managers, drinking-water suppliers, 

policymakers, and scientists better understand how and why contamination of 

public-supply wells occurs and whether 

water quality may improve or degrade. 

Additionally, study findings may be used 

to evaluate various pumping, resourcedevelopment, and land-management 

scenarios.

Mixing of Young Water with 

Older Water Increases Well 

Vulnerability 

“Groundwater age” refers to the 

elapsed time since water entered an 

aquifer at the water table. Because water 

in an aquifer typically flows downward 

and laterally over time, it is expected 

that “young” water (water that reached 

the water table less than 50 years ago) 

will be found closest to the water table 

and older water (water that recharged 

50 or more years ago) will be found at 

greater depths. The presence and proportion of young water relative to old water 

produced by the public-supply well is of 

interest because young groundwater is 

more vulnerable to contamination from 

human activities in urban and agricultural 

areas than is older groundwater, which 

recharged before manmade chemicals 

became prevalent.

As is typical of a production well, 

the long screened interval of the publicsupply well (275 feet) captures a mixture 

of water that recharged at different times. 

To understand the resulting blend of 

water of different ages (or “age distribution of water”) produced by the publicsupply well, a computer-model simulation of groundwater flow and transport 

was used to calculate the traveltime (or 

age) of water particles entering the well 

along the length of the well screen. Modeled findings show that simulated ages 

of water particles reaching the publicsupply well range from 9 years to more 

than 30,000 years. Most of the water is 

at the young end of this range (fig. 1), 

resulting in a median age of 54 years. The 

age of water particles entering the well 

generally increases with depth; however, 

there are small perturbations in this trend 

where younger water enters beneath 

slightly older water, due primarily to the 

heterogeneity of the aquifer sediments. 

For example, overlapping lenses of 

coarse-grained sediments may transmit 

water to depth more rapidly than a flow 

path that intersects silt and clay lenses. 

Figure 2 shows areas where water that 

eventually reaches the public-supply well 

enters the groundwater system, along 

with the traveltimes associated with the 

water particles. The shortest traveltimes 

are not always associated with particles 

that recharged closest to the well, due in 

part to the heterogeneity of the aquifer 

sediments. 

Groundwater-quality data also were 

used to interpret water ages. Concentrations of modern age tracers (sulfur 

hexafluoride and tritium) increased in 

the atmosphere—and also in precipitation—within the last four or five decades 

as a result of human activities. Thus, 

groundwater containing these tracers is 

considered to be young, having entered 

the groundwater system within the last 

50 years. Results of water sampling for 

modern age tracers and other chemical 

constituents revealed that water quality 

in the aquifer is stratified into shallow, 

intermediate, and deep zones having 

distinct chemistry (fig. 3). Water in the 

shallow zone—defined as less than 164 

feet (50 meters) below land surface—has 

a mean age between 30 and 40 years 

based on modern age-tracer data and has 

been highly affected by human activities. 

Specifically, water samples from shallow 

monitoring wells and the shallow depthdependent sample from the public-supply 

well have much higher specific conductance and concentrations of alkalinity, 

nitrate, sulfate, and uranium than samples 

collected from the intermediate and deep 

zones, and also more frequent detections 

and higher concentrations of VOCs and 

pesticides. The uranium concentration 

(35.8 micrograms per liter (μg/L)) in the 

sample of shallow water entering the 

public-supply well exceeded the 30-μg/L 

Maximum Contaminant Level (MCL) 

allowed in drinking water by the U.S. 

Environmental Protection Agency, and 

the nitrate concentration (9.7 milligrams 

per liter (mg/L)) was near the MCL of 

10 mg/L. Most contaminants that reach 

the public-supply well enter from the 

shallow zone of the aquifer. Measurements of the flow rate of fluids entering the well at different depth intervals 

indicate that water from the shallow 
Figure 1. A groundwater flow model was used to 

simulate age distribution of water (the mixture of 

water of different ages) produced by the selected 

supply well in Modesto, California. “Age” refers 

to the time elapsed since water entered the 

groundwater system as recharge at the water table. 

0

0

0.005

0.010

0.015

0.020

0.025

100 200 300

SIMULATED PARTICLE AGE, IN YEARS

ca3427_Figure 12

FRACTION OF FLOW TO WELL

400 500+

Figure 2. The colored zones on the map show the simulated area contributing 

recharge to a supply well in Modesto, California. These zones indicate where water 

that eventually reaches the selected supply well enters the groundwater system at 

the water table. The particle traveltimes indicate the number of years that it takes 

for a water particle to complete the journey from the water table to the well. 

EXPLANATION

Boundary of local groundwater-flow model 

Road or highway 

Monitoring-well cluster 

Other pumping wells in the area

Public-supply well selected for study 

0 0.5 1 1.5 Miles

0 0.5 1 1.5 Kilometers

8 to 10

> 10 to 20

> 20 to 30

> 30 to 40

> 40 to 50

> 50 to 60

> 60 to 70

> 70 to 80

> 80 to 90

> 90 to 100

> 100

Traveltime to public-supply well from

 contributing recharge area, in years.

 (> is greater than)

Study

well

zone supplies less than 20 percent of the 

groundwater entering the well (fig. 3). 

The remaining 80 percent of water 

entering the well comes from the deep 

and intermediate zones of the aquifer. 

Water in the deep zone of the aquifer was 

estimated to be thousands of years old and 

contained no anthropogenic contaminants. 

Nitrate, uranium, and alkalinity were at 

concentrations consistent with recharge 

under more natural conditions (fig. 3). 

Water in the intermediate zone is a mixture of young, anthropogenically affected 

water and older, more pristine water. 

Groundwater Development Drives 

Contaminants Downward Into 

Production Zone of Aquifer

Downward groundwater flow exists 

year-round in the Modesto area, but the 

summertime application of irrigation 

water at land surface (for agricultural 

crops and urban landscapes) combined 

with water withdrawal at depth by well 

intakes has intensified this downward 

flow component, as indicated by the 

greater vertical gradient measured during 

the growing season (0.11) than during 

the winter (0.04) at a monitoring well 

in the Modesto area. The downwardmoving irrigation waters contain manmade contaminants, and the particular 

types of contaminants in those waters 

are influenced by land-use practices. For 

example, findings from wells screened 

near the water table show that nitrate 

and sulfate concentrations were higher 

beneath land that was being farmed or 

had recently been farmed (within the last 

10 to 20 years) than beneath urban areas. 

This is attributed to common agricultural 

practices near Modesto that include applications of nitrogen fertilizers to increase 

crop yields and gypsum soil amendments to promote infiltration. In contrast, 

VOCs—which have many commercial 

and household uses—were typically 

found only beneath urban areas. 

Agricultural development near 

Modesto has been accompanied by an 

increase in alkalinity and uranium in 

shallow groundwater. Alkalinity is a 

measure of the capacity of water to resist 

changes in pH; in the study area this 

capacity comes primarily from bicarbonate in groundwater as a result of mineral 

dissolution. In 1910, when precipitation and seepage from streams near the 

mountain front were still the dominant 

forms of recharge to the aquifer and 

natural vegetation predominated, shallow 

groundwater in the area had an alkalinity 

of about 110 mg/L as calcium carbonate (Mendenhall and others, 1916). In 

contrast, in 2003 and 2004—when the 

primary source of recharge was percolating excess irrigation water and natural 

vegetation had been replaced by crops 

(Burow and others, 2004)—the median 

alkalinity of shallow groundwater was 

309 mg/L. Because mineral dissolution is 

enhanced by plant and microbial processes in the soil zone and by increased 

moisture, increased crop production 

coupled with increasing amounts of 

irrigation in the 1940s and 1950s likely 

began to move high-alkalinity water from 

the soil zone, into the aquifer, and downward through the shallow groundwater 

flow system. A statistically significant 

correlation between alkalinity and nitrate 

concentrations supports this hypothesis. 

Below the shallow zone of the aquifer, 

alkalinity concentrations are roughly 

equivalent to those before agricultural 
Figure 3. The portion of the aquifer tapped by the public-supply well was divided 

into shallow, intermediate, and deep zones based on water chemistry. The shallow 

zone has been highly affected by irrigation (both agricultural and urban) that is driving 

anthropogenic contaminants downward. Water-quality in the deep zone represents 

water recharged under more natural conditions.

Adjacent

monitoring

well

Publicsupply

well

EXPLANATION

Mud

Gravel

Sand

Muddy sand

Direction of flow within 

casing during pumping 

conditions

Depth below land surface, in meters

110

100

90

80

70

60

50

40

30

20

10

0

120

Flow rate = 1,600 gallons 

 per minute

0

164

366

230

Depth below land surface, in feet

Shallow aquifer

zone—Less than

20% of total flow

Intermediate 

aquifer zone—

About 55% of 

total flow

Deep aquifer

zone—About

25% of total flow

95

Water source Nitrate Uranium

Alkalinity

Shallow aquifer 

zone 12 24 309

Intermediate 

aquifer zone 4.0 4 146

Deep aquifer 

zone 1.6 0.5 85

Concentrations in the aquifer are median 

values, from monitoring wells completed 

in each zone, sampled in 2003–5 (nitrate 

as nitrogen and alkalinity as calcium 

carbonate equivalent, both in milligrams 

per liter; uranium in micrograms per liter). 

development, with median concentrations 

in the intermediate and deep zones of 146 

and 85 mg/L, respectively (fig. 3). 

Downward movement of highalkalinity water is of concern to water 

managers because alkalinity (which has 

no health-related drinking water standard) 

was strongly correlated with uranium 

(which has a MCL of 30 μg/L, based 

on human health concerns). The current 

study found that uranium concentrations 

in groundwater beneath urban and agricultural land near Modesto were similar, 

suggesting concentrations are not related 

to land use. Instead, the downwardmoving front of high-alkalinity water 

liberates naturally occurring uranium 

that is adsorbed to aquifer sediments. 

This causes uranium concentrations 

in shallow groundwater to be elevated 

(median concentration of 24 μg/L). 

Concentrations of bicarbonate ions that 

chemically bond with uranium decrease 

below 164 feet because the groundwater 

at these depths recharged before intensive 

irrigation began. Below 164 feet, lowalkalinity conditions favor adsorption of 

uranium to aquifer sediments, resulting 

in much lower uranium concentrations in 

the aquifer’s intermediate zone (median 

of 4 μg/L) and deep zone (median of 0.5 

μg/L) than in the shallow zone (fig. 3).

Currently, the shallow water entering 

the public-supply well contains elevated 

concentrations of uranium and nitrate, 

but these concentrations are diluted within 

the well by the older water that makes up 

80 percent of the flow to the well; so, 

concentrations at the wellhead are less 

than drinking-water standards. As irrigation and agricultural and municipal pumping continue to drive the movement of 

anthropogenically affected water downward to greater depths in the aquifer, the 

ratio of affected water to older, unaffected 

water will increase in the future, reducing 

dilution and thereby adversely influencing 

drinking-water quality. 

Natural Geochemical Conditions 

in Aquifer Affect Fate of 

Contaminants 

Geochemical conditions such as 

oxygen content of groundwater control whether specific contaminants are 

attenuated or mobilized in the subsurface. Dissolved oxygen is present in all 

groundwater samples from the Modesto 

area at concentrations greater than 1.5 

mg/L, indicating that most of the aquifer 

is oxic. Oxic conditions can be attributed 

to the scarcity of organic carbon in the 

aquifer—when organic carbon is available, microbial respiration consumes 

oxygen—and also to the input of oxygenated irrigation water. In oxic conditions, 

nitrate is mobile in the groundwater 

system. Because of the mostly downward movement of water in the study 

area, nitrate in shallow groundwater near 

Modesto is likely to continue moving 

to greater depths in the aquifer without 

significant attenuation. 

Construction and Operation 

of Public-Supply Well Affects 

Drinking Water Quality 

Water samples were collected from 

five depths in the public-supply well. 

Surprisingly, the deepest sample from the 

public-supply well (320 feet below land 

surface) contained higher concentrations 

of nitrate and uranium and had a higher 

specific conductance than did most 

samples from monitoring wells screened 

at a similar depth. Because the screen of 

the public-supply well extends through 

all three zones of the aquifer and because 

a downward component of groundwater 

flow exists year-round in the Modesto 

area, NAWQA researchers determined 

that anthropogenically influenced water 

from the shallow zone can migrate down 

the public-supply well and into the deep 

zone of the aquifer when the well pump 

is turned off (fig. 4). This type of leakage 

is prevalent during the winter, when the 

public-supply well is pumped for only a 

short duration each day. During the several hours that the pump is off, large volumes of anthropogenically affected water 

accumulate in the deep aquifer. When 

the pump is briefly activated, the lift of 

the pump overcomes the downward flow 

gradient, and the contaminated water that 

is stored in the deep zone of the aquifer 

is drawn back toward the well. The result 

is that, during winter, anthropogenically 

affected water simultaneously enters the 

well from both the shallow and the deep 

zone of the aquifer, composing up to 

45 percent of flow to the well compared 

to about 20 percent (from the shallow 

zone alone) during summer. When the 

public-supply well is pumped more 

frequently and for longer periods once 

the growing season starts, down-well 

leakage of water from the shallow zone 

is minimized, and affected water that has 

accumulated near the well in the deep 

aquifer over the winter is eventually 
Figure 4. During winter when the 

pump is off for several consecutive 

hours, contaminated water from 

the shallow zone of the aquifer 

travels down the well or gravel 

pack and exits at the base of the 

well screen into the deep aquifer. 

When the pump is activated for a 

short time each day, some of this 

leaked water is recaptured by the 

well. Prolonged pumping during 

summer evacuates contaminated 

water from the deep zone, and 

water quality of the public-supply 

well improves.

NONPUMPING

Supply

well

Monitoring

well

Movement of high-nitrate

and -uranium water

Water table

Low nitrate

 and uranium

 (intermediate

 and deep zones)

High nitrate

 and uranium

 (shallow zone)

Not to Scale 

PUMPING

Supply

well

Monitoring

well

Movement of high-nitrate

and -uranium water

Movement of lownitrate and -uranium 

water

Water table

Low nitrate

 and uranium

 (intermediate

 and deep zones)

High nitrate

 and uranium

 (shallow zone)

Not to Scale

evacuated, allowing clean water to once 

again enter the well from the deep aquifer. Thus, water quality produced by the 

well improves during the summer.

Supporting evidence for the well 

leakage theory is (1) an examination of 

the seasonal fluctuations in the historic 

water-quality record for the supply well, 

with higher concentrations of nitrate, 

uranium, specific conductance and other 

anthropogenic compounds (chloroform, 

perchloroethylene, and deethylatrazine) 

produced during winter; (2) similar 

seasonal fluctuations in water quality of 

a deep monitoring well within 130 feet of 

the public-supply well that is apparently 

sampling the leaked water stored in the 

deep zone, and (3) seasonal fluctuations 

in the age of water discharged by the 

public-supply well, with a greater proportion of young water produced in winter. 

Because about a third of the irrigation and 

public-supply wells in the Modesto area 

have screens that span from the shallow 

to the deep zone of the aquifer, it is likely 

that leakage of anthropogenically affected 

water down production wells occurs at 

other locations in the Modesto area. 

Protection Effort Implications 

Pumping schedule of the publicsupply well affects water quality. 

Well-field operators in Modesto noted 

that pumping for longer periods of time 

during winter months (with less downtime between pumping events) improved 

water-quality during these months. Previously, when the well was pumped for 

only a short duration each day in winter, 

it captured contaminated water that had 

migrated into the deep zone of the aquifer 

and resulting wellhead concentrations of 

nitrate and uranium sometimes exceeded 

regulatory levels. Pumping for longer 

periods minimizes the flow of anthropogenically affected water down the borehole. It also evacuates the contaminated 

water from the deep zone of the aquifer 

and allows clean water to enter the well 

and dilute high nitrate and uranium 

concentrations from the shallow zone, 

thereby bringing the concentrations back 

within drinking water standards. 

Screened interval of a publicsupply well affects its vulnerability. The 

median depth of public-supply wells in 

the Modesto area is 268 feet. Because the 

particular supply well selected for study 

is deeper (366 feet) and has a screen that 

intersects a greater portion of the deep 

aquifer zone than most of these wells, 

it produces a greater proportion of old 

groundwater than most other publicsupply wells. It is likely that concentrations of uranium and nitrate will be 

higher in other wells that have shallower 

screened intervals and thus have a large 

proportion of relatively young, anthropogenically affected water contributing to the well and a correspondingly 

smaller proportion of dilute, older water. 

Historically, water from some publicsupply wells in Modesto has periodically 

exceeded the MCL for nitrate. Several 

public-supply wells had to be abandoned 

for drinking-water use because uranium 

concentrations exceeded the MCL.

Past land use within the contributing recharge area may affect quality 

of water produced by the public-supply 

well for decades. Most of the contributing recharge area (CRA) for the selected 

public-supply well lies beneath land that 

is currently urban. However, urbanization 

of the area is relatively recent, beginning in about 1960; water older than 

about 40 years was affected primarily 

by agricultural practices at the time of 

recharge. The effect of changing land-use 

practices over time on long-term nitrate 

concentrations in the public-supply well 

was estimated from the simulated age 

distribution of water produced by the 

public-supply well to determine proportions of water recharged in different 

years. Assumptions in the simulation 

were (1) all land use was agricultural 

until 1960 and (2) between 1960 and 

2004, the proportion of agricultural 

input in the CRA decreased and the 

proportion of urban input increased at 

an estimated rate of about 1 to 2 percent 

per year. Nitrate input concentrations 

in agricultural recharge water over time 

were estimated from nitrogen fertilizer 

application rates in the Modesto area, 

which steadily increased from the 1940s 

through the mid-1970s. Nitrate input 

concentrations estimated this way for the 

year 2004 were consistent with measured 

nitrate concentrations in wells screened 

near the water table beneath agricultural areas (median of 15 mg/L). Nitrate 

concentrations in recharge beneath the 

urban area was assumed to be 3.1 mg/L, 

based on water samples collected near 

the water table in 2004. Results of the 

simulation indicate that concentrations 

in the public-supply well peaked in the 

late 1990s and will decrease slightly from 

current levels of 5.5 mg/L to near 5 mg/L 

in the public-supply well during the next 

100 years (fig. 5). This prediction is valid 

only if nitrate input beneath the growing 

urban area remains low and nitrate input 

beneath the agricultural area does not 

increase significantly (owing to increase 

in application rates of nitrogen fertilizer). 
Simulated concentration in public-supply well

Estimated concentration in recharge

YEAR

1945 1965 1985 2005 2025 2045 2065

25

20

15

10

5

0

Nitrate MCL

2085 2105

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER, AS NITROGEN

ca3427_Figure 21. 

Lagtime between concentration peaks

Figure 5. Temporal changes in nitrogen concentrations in recharge water were 

estimated from changes in land use and application rates of nitrogen fertilizer 

in the Modesto area. Resulting changes in the concentration of nitrate in water 

discharged from the well over time were then simulated. Simulation results 

indicate that nitrate concentrations in the well water peaked in the late 1990s, 

lagging about 25 years behind the peak nitrate concentration in recharge water.

By Martha L. Jagucki, Bryant C. Jurgens, Karen R. Burow, and Sandra M. Eberts

For more information on the Transport of Anthropogenic and Natural Contaminants to Supply Wells (TANC) topical study, see

http://oh.water.usgs.gov/tanc/NAWQATANC.htm

Wells installed beneath a high proportion 

of agricultural land or recently converted 

agricultural land will likely have higher 

concentrations of nitrate than the publicsupply well in this investigation. 

The simulated long-term nitrate 

concentrations in the public-supply 

well reflect a lag time of 20 to 30 years 

between peak concentrations in recharge 

and peak concentrations in the well. This 

apparent delay between input at land surface and output at the well is the result of 

the wide range of ages of water reaching 

the public-supply well combined with the 

changing nitrogen input concentrations 

over time. This indicates that protection efforts in the Modesto area that rely 

on changes in land-use management to 

improve water quality may take 20 to 30 

years to bring about results in drinkingwater quality. 

Uranium concentrations will 

continue to increase unless new management strategies are developed. 

High-alkalinity, high-uranium water is 

expected to continue moving deeper in 

the groundwater system over time, gradually composing an increasing proportion 

of the water entering the public-supply 

well. The effect of the downward-moving 

water on long-term uranium concentrations in the public-supply well was estimated by using traveltimes derived from 

the groundwater flow model to determine 

the proportions of water entering the 

supply well that recharged in different 

years. Then, an alkalinity-input history 

was estimated based on the assumption 

that alkalinity concentration in recharge 

was 110 mg/L prior to 1945 and then 

increased linearly to current concentrations of about 450 mg/L at the water 

table. Alkalinity input in the future was 

assumed to remain at current conditions. 

The resulting simulation indicates that 

concentrations of uranium in the publicsupply well (based on a defined relation 

between uranium and alkalinity concentrations) will likely approach the MCL 

over time; however, it will take more than 

100 years because of the contribution of 

old water at depth in the public-supply 

well that dilutes uranium concentrations 

in shallower water entering the well. 

This allows time to evaluate management 

strategies and to alter well-construction 

or pumping strategies to prevent concentrations from exceeding the drinkingwater standard. 

References 

Burow, K.R., Jurgens, B.C., Kauffman, 

L.J., Dalgish, B.A., Phillips, S.P., and 

Shelton, J.L., 2008, Simulations of 

ground-water flow and particle pathline 

analysis in the zone of contribution of a 

public-supply well in Modesto, eastern 

San Joaquin Valley, California: U.S. 

Geological Survey Scientific Investigations Report 2008–5035, 41 p.

Burow, K.R., Shelton, J.L., Hevesi, J.A., 

and Weissmann, G.S., 2004, Hydrogeologic characterization of the Modesto 

area, San Joaquin Valley, California: 

U.S. Geological Survey Scientific 

Investigations Report 2004–5232, 54 p.

Hamilton, P.A., Miller, T.L., and Myers, 

D.N., 2004, Water quality in the 

Nation’s streams and aquifers—Overview of selected findings, 1991–2001: 

U.S. Geological Survey Circular 1265, 

20 p. 

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.

Mendenhall, W.C., Dole, R.B., and 

Stabler, Herman, 1916, Ground water 

in San Joaquin Valley, California: U.S. 

Geological Survey Water-Supply Paper 

398, 310 p.

Phillips, S.P., Burow, K.R., Rewis, D.L., 

Shelton, J.L., and Jurgens, B.C., 2007, 

Hydrogeologic settings and groundwater flow simulations of the San 

Joaquin Valley regional study area, 

California, sec. 4 in Paschke, S.S., ed., 

Hydrogeologic settings and groundwater flow simulations for regional 

studies of the transport of anthropogenic 

and natural contaminants to public-supply wells—Studies begun in 2001: U.S. 

Geological Survey Professional Paper 

1737–A, p. 4–1 through 4–31.