U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3025

April 2008

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

Assessing the Vulnerability of Public-Supply Wells to 

Contamination: High Plains Aquifer Near York, Nebraska

Printed on recycled paper

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 ground water 

near the water table in urban areas across 

the Nation. Although contaminants were 

detected less frequently in deeper ground 

water 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, 

in 2001, an intensive study to assess the 

vulnerability of public-supply wells to 

contamination. As part of this study, 

the pathways and processes by which 

contaminants reach public-supply wells 

in nine aquifer systems across the country 

are being investigated. In addition to 

studying the processes that occur below 

land surface—whereby contaminants are 

mobilized or attenuated—scientists are 

also investigating how human activities 

can affect the vulnerability of publicsupply wells to contamination. 

This fact sheet highlights findings 

from the vulnerability study of a single, 

representative public-supply well in 

York, Nebraska (Clark and others, 

2008; Landon and others, in press). 

The selected high-capacity well typically 

produces more than 720,000 gallons per 

day from the upper confined aquifer of 

the High Plains aquifer. (See text box 

describing the aquifer.) A possible source 

of contamination to the well is intensive, 

irrigated agriculture, which can sometimes result in elevated concentrations 

of nitrate and pesticides in ground water. 

In addition, a sampling of the selected 

public-supply well by the USGS in 2002 

found low concentrations of the solvents 

trichloroethylene (TCE), tetrachloroethylene (PCE), and their degradation products, which may be linked to historical 

chemical use in urban and residential 

areas of York. Uranium and arsenic 

(which occur naturally in the sediments 

that make up the aquifers in the area) also 

were detected in 2002 at concentrations 

less than drinking-water standards but 

still of concern. 

Overall, the current NAWQA study 

found that three primary factors affect 

the movement and fate of contaminants 

and the vulnerability of the public-supply 

well in York: (1) timing of water entry 

(recharge) to the aquifer, (2) short-circuiting of natural flow paths through inactive wells, and (3) natural geochemical 

conditions of the aquifer. Study findings 

are intended to help water managers, 

drinking-water suppliers, policymakers, 

and scientists to 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, resource-development, 

and land-management scenarios.

The High Plains aquifer near York, Nebraska, 

is composed of layered alluvial deposits. The 

uppermost layer is loess (windblown dust), which 

is unsaturated in most locations. Below the 

loess is a layer of mostly sand and gravel that is 

referred to as the “unconfined aquifer.” Under 

typical flow conditions, some precipitation and 

irrigation water will infiltrate from land surface 

to the water table and then move downward and 

laterally over time through the unconfined aquifer. 

Upon reaching the base of the unconfined aquifer, 

water then travels almost vertically through the 

underlying layer of clayey silt till. This till is much 

less permeable than surrounding layers and acts 

as a confining unit, greatly slowing the movement 

of water into lower layers. Once through the 

confining unit, water enters the upper confined 

aquifer and resumes flowing in a primarily lateral 

direction and at a relatively rapid rate compared 

to that in the confining unit. The upper confined 

aquifer is the primary source of ground water for public-supply wells in the region. Beneath the upper confined aquifer is another 

confining unit underlain by discontinuous sand lenses (referred to as “lower confined lenses”). Many irrigation, commercial, and 

some older public-supply wells are screened in the unconfined aquifer, the upper confined aquifer, and the lower confined lenses. 

Recharge

Discharge

Water table

Generalized ground-water flow

High Plains aquifer

Lower confined lenses

 (sand and silt)

Unconfined aquifer

Confining unit 

Upper confined aquifer 

Lower confining unit

Loess

Not to scale

Inactive well

Irrigation well

Public-supply well

(sand and gravel)

(sand)

Shale

(clayey silt till)
Well Vulnerability Increased by 

Mixing of Recent Recharge With 

Older Water

“Ground-water age” refers to the 

elapsed time since water entered or 

“recharged” an aquifer at the water table. 

Because water in an aquifer typically 

flows downward and laterally over time, 

it is expected that “young” (recently 

recharged) water will be found near 

the water table and older waters will be 

found at greater depths. 

Ground-water samples in the York 

area were analyzed for modern age tracers (concentrations of chlorofluorocarbons, sulfur hexafluoride, and (or) ratios 

of helium and tritium). Age-tracer data 

indicate that the two primary aquifers 

near York—the unconfined aquifer and 

the upper confined aquifer—generally 

contain water of different age groups. 

The presence of modern age tracers 

in water from the unconfined aquifer 

indicates young water recharged within 

the last 60 years. In contrast, most of the 

water from the upper confined aquifer 

(which is a deeper unit and is the primary source of water for public supply) 

contains no detectable concentrations of 

modern age tracers and thus is old water 

recharged more than 60 years ago. 

Water entering the public-supply 

well at various depths also was analyzed 

for modern age tracers. As is typical 

of a production well, the long screened 

interval of the well (60 feet) captures a 

mixture of water that recharged at different times. However, the finding that 

young water is entering the well screen 

underneath older waters in the upper confined aquifer was surprising. Specifically, 

water with a mean age of about 1990 or 

younger enters the well through the bottom half of the screen and accounts for 

about 7 to 14 percent of the total amount 

of water produced by the well. Young 

water entering the well beneath older 

water suggests that short-circuiting of 

natural flow paths is occurring, allowing water from the unconfined aquifer to 

flow to depth below the confining unit 

more rapidly than expected. The presence 

and proportion of young water relative 

to old water is of concern because young 

ground water is more vulnerable to contamination from human activities associated with urban or agricultural land use 

than older ground water (which recharged 

before the use of manmade chemicals 

became prevalent).

Inactive Wells Serve as 

Short Circuits for Movement 

of Contaminants Across the 

Confining Unit

Study findings in York indicate that 

the mechanism for short circuits in the 

natural ground-water flow system is wellconstruction methods used in the area. 

Many irrigation wells, and some commercial and public-supply wells, are screened 

across both the unconfined and upper 

confined aquifers to maximize yields. 

Such wells are hereafter referred to as 

“multiscreened wells.” Multiscreened 

wells open to both aquifers offer conduits 

for downward movement of water (and 

associated contaminants) when the well 

is not in use—and movement along this 

short-circuit path is years to centuries 

faster than flow of water through the 

confining unit. During periods when a 

multiscreened well is not pumped (as 

usually occurs with irrigation wells during the fall, winter, and spring), contaminants that flow down the well and out of 

the well screen in the confined aquifers 

may travel beyond the capture zone of the 

well before pumping is resumed. 

Further evidence for multiscreened 

well leakage is the mixed chemical characteristics found in 6 of the 16 monitoring wells completed in the upper confined 

aquifer. These mixed waters are characterized by (1) stable-isotopic and majorion signatures intermediate between those 

found either in the unconfined aquifer or 

in most wells completed in the upper confined aquifer, (2) the anomalous presence 

of young waters, and (3) the presence of 

contaminants derived from human activity at the land surface (such as volatile 

organic compounds (VOCs) and pesticides) below the confining unit. For the 

six confined monitoring wells exhibiting 

mixed characteristics, the fraction of 

water from the unconfined aquifer was 

calculated to range from 41 to 94 percent, 

based on stable hydrogen isotope values. 

The occurrence of mixed water in only 

6 of the 16 wells completed in the upper 

confined aquifer suggests that the mixing 

is the result of flow from the unconfined 

aquifer to the confined aquifer(s) along 

localized short-circuit paths, consistent 

with the source being leakage through 

multiscreened wells.

Incorporating multiscreened well 

leakage into computer-model simulations 

of ground-water flow and solute transport produced results that correspond 

to the observed chemistry in the mixed 

monitoring wells and in the supply well. 

Modeled findings show that leakage 

through multiscreened wells accounts for 

about 25 percent of the total flow from 

the unconfined to the confined portion of 

the ground-water system. The majority of 

ground water (75 percent) follows slower, 

natural flow paths through the confining 

unit. The computer model was then used 

to determine the blend of water of different ages (or “age distribution of water”) 

produced by the public-supply well (fig. 

1). All water produced by the well that is 

less than about 60 years old travels to the 

upper confined aquifer by way of short 

circuits (shown in green). It is this young 

water that contains low levels of anthropogenic contaminants.

Study Design

A computer model was constructed to estimate the zone of contribution to the selected 

public-supply well (Landon and Turco, 2007). The zone of contribution is the threedimensional volume of the aquifer material through which ground water flows from the 

time it enters the ground-water system at the water table until it eventually discharges 

at the selected public-supply well. A network of 36 monitoring wells was installed in or 

near the simulated zone of contribution to the well to understand ground-water flow and 

geochemistry along apparent ground-water flow paths to the well. Water samples were 

collected from the monitoring wells and the supply well during October 2003 through April 

2005 and analyzed for naturally occurring contaminants (such as uranium and arsenic) 

and anthropogenic contaminants (caused by human activities). Other physical and 

chemical characteristics of water (such as major ions, age-dating tracers, and selected 

stable isotopes) were analyzed for, so that sources of water and reactions affecting 

the chemical composition of ground water can be better understood. In addition, 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.
Natural Geochemical Conditions 

Affect the Fate of Contaminants 

Short-circuiting of water through 

multiscreened wells not only speeds 

the arrival of contamination-susceptible 

young water to the upper confined aquifer; it also circumvents the contaminantattenuating capacity of the confining 

unit. Whether specific contaminants are 

attenuated or mobilized in the subsurface 

depends on the natural geochemical conditions encountered. 

The fate of several contaminants 

in the High Plains aquifer—particularly 

nitrate, TCE and PCE, and uranium—is 

greatly affected by the oxygen content 

of ground water. In the oxic conditions 

(dissolved oxygen concentrations greater 

than 2.5 milligrams per liter (mg/L)) 

found throughout most of the unconfined aquifer, nitrate, TCE and PCE, and 

uranium are stable and can be transported 

long distances. Following natural groundwater-flow paths, these constituents 

would eventually reach the generally 

anoxic environment (dissolved oxygen 

concentrations mostly less than 0.5 mg/L) 

of the confining unit and upper confined 

aquifer. Here, these constituents would 

be naturally attenuated, given enough 

residence time. Attenuation of nitrate is 

due to a process known as denitrification, 

in which nitrate is converted to harmless 

nitrogen gas in the absence of oxygen. 

Analysis of pore-water chemistry in 

the upper confining unit indicates that 

complete denitrification occurs across 

the upper 10–13 feet of this layer. TCE 

and PCE are degraded by microbes in the 

anoxic layers, as evidenced by greater 

concentrations of degradation products 

cis- and trans-1,2 dichloroethylene 

compared to parent compounds (TCE 

and PCE) in the upper confined aquifer 

than in the unconfined aquifer. Uranium 

is generally attenuated under anoxic 

conditions because it precipitates out of 

ground water as an insoluble mineral or 

sorbs onto aquifer sediments in anoxic 

conditions. 

When natural flow paths are not 

followed and young, oxic water shortcircuits through inactive wells, the time 

available for attenuating reactions is 

greatly reduced. Water that would normally spend years to centuries moving 

through the confining unit is now shunted 

directly to the upper confined aquifer, 

where it may spend relatively little time 

in an anoxic environment before it is captured and discharged by the supply well. 

Thus, attenuating reactions may not be 

carried to completion. For example, the 

presence of PCE and TCE in the selected 

supply well indicates that, once these 

contaminants reach the upper confined 

aquifer by short-circuit pathways, degradation rates are not fast enough relative 

to residence time to prevent all PCE and 

TCE from reaching the well.

Paradoxically, short-circuiting seems 

to override the expected attenuation of 

uranium in the upper confined aquifer. 

Instead, it actually mobilizes uranium: 

the greatest uranium concentrations 

were found in mixed waters in the upper 

confined aquifer that were the result of 

multiscreened well leakage. It is thought 

that uranium sorbed to tiny particles of 

organic carbon and iron oxides from 

the unconfined aquifer travels down the 

leaking wells. Once in the anoxic conditions of the upper confined aquifer, the 

iron dissolves, releasing uranium that 

then complexes with the calcium and 

bicarbonate in the upper confined aquifer. 

The uranium-calcium complex facilitates transport of uranium under anoxic 

conditions that would otherwise cause the 

uranium to precipitate. 

Figure 1. A ground-water flow model was used to simulate age distribution of water (the mixture of water of different ages) produced by 

the selected public-supply well in York, Nebraska. “Water age” refers to the time elapsed since water entered the ground-water system as 

recharge at the water table. The age distribution of water from this well is a combination of the age distribution of water that slowly flows 

across the confining unit, shown in blue, and the water contributed by leaking, inactive wells that are screened in both the unconfined and 

upper confined aquifers, shown in green. All water produced by the public-supply well that is less than about 60 years old travels to the upper 

confined aquifer by way of short circuits. Without short circuits, the well might not contain low levels of anthropogenic contaminants.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 

1 10 100 1,000 10,000

AGE, IN YEARS

PERCENT OF W

ATER ENTERING WELL

Water that flows through well bores of 

multiscreened wells

Water that flows through the 

upper confining unit
Human Activities Influence 

Public-Supply Well Vulnerability 

Well-construction methods cause 

vulnerability. Well-construction methods used in development of the High 

Plains aquifer to supply large quantities 

of irrigation and drinking water have, 

upon retrospect, introduced short-circuit 

pathways that adversely affect quality of 

water produced by public-supply wells 

near York. Currently, contaminated water 

entering the base of the selected publicsupply well by way of inactive wells 

that tap both aquifers is being diluted by 

enough old water from the upper confined aquifer that contaminant concentrations are less than Federal drinking-water 

standards. Uranium concentrations, in 

particular, in the mixed water entering 

the base of the well screen would cause 

the water-supply well to exceed the 

U.S. Environmental Protection Agency 

Maximum Contaminant Level allowed 

in drinking water if dilution by old water 

were not occurring. Human activities that 

might decrease the ratio of old to young 

water being produced by the public-supply well could have adverse impacts on 

drinking-water quality.

Pumping rate of the public-supply 

wells has little effect on movement of 

contaminants to the upper confined 

aquifer relative to pumpage of irrigation wells. Because the amount of water 

withdrawn by irrigation wells in the area 

is so much greater than withdrawals by 

public-supply wells, the pumping of 

public-supply wells generally has little 

effect on the magnitude of the downward 

flow between aquifers or the volume 

of young, contaminated water migrating down inactive wells. However, the 

pumping of a public-supply well may 

affect direction of contaminant movement 

once the young water reaches the upper 

confined aquifer because a pumping well 

captures water that would, under nonpumping conditions, flow past the well. A 

greater pumping rate will have a greater 

zone of contribution that is likely to 

capture leakage from more multiscreened 

wells. However, the overall effect of a 

greater zone of contribution is difficult to 

predict because a greater volume of old 

water also is likely to be intercepted. The 

impact on water quality would depend on 

the resulting ratio of old to young water 

that is produced. Another effect of greater 

pumping rate is to increase the rate of 

ground-water flow to the well, which 

shortens the time available for natural 

geochemical reactions to attenuate contaminants in the upper confined aquifer. 

Number and proximity of leaking 

wells to the public-supply well strongly 

influences vulnerability. The volume of 

water that leaks into the upper confined 

aquifer through multiscreened wells will 

depend on the number of leaking wells, 

their pumping rate (if they are seasonally 

pumped or pumped at a low rate that does 

not overcome downward flow between 

aquifers), and well construction (well 

diameter and length of screened or open 

interval). The difference in water level 

between the confined and unconfined 

aquifers also affects volume of leakage. 

A greater volume of water coming from 

leaking wells means that a greater proportion of young water to old water will 

be produced by the public-supply well, 

which will reduce the amount by which 

contaminants are diluted by old water. 

Figure 2 (next page) shows the 

simulated area contributing recharge to 

the selected public-supply well in York. 

The odd shape of the area contributing 

recharge—especially the isolated zones 

to the west of the primary contributing 

recharge area—is influenced by the location of wells simulated to be leaking as 

well as by the location of other pumping 

wells in the area. Figure 2 also shows that 

a public-supply well is 

more susceptible to contamination from a leaky 

multiscreened well located 

nearby than one located at 

a distance. For example, 

in the dark-blue zone in 

figure 2, a simulated 71 to 

80 percent of the recharge 

water to the multiscreened 

well in that area is captured by the public-supply 

well. Compared to this is 

one of the more distant, 

light-green zones, where 

less than 10 percent of 

the recharge water to the 

multiscreened well in that 

area reaches the publicsupply well. Note that the 

location and condition of 

multiscreened wells or 

boreholes may be difficult 

to determine (especially 

those that have been abandoned)—making it difficult to estimate the number and 

proximity of leaking wells to the publicsupply well.

All factors mentioned above, as well 

as the types of contaminants captured by 

the leaking wells and the pumping rate of 

the supply well relative to other pumping 

centers in the area, interact to influence 

the quality of water produced by a well. 

For instance, although nitrate concentrations in the public-supply well studied in 

York were very low (less than 0.2 mg/L), 

nitrate concentrations in other public-supply wells in York have exceeded drinking-water standards in the past, indicating 

that short-circuiting and pumping stress 

can overcome dilution and attenuation 

processes under some circumstances. 

Protection Effort Implications

Efforts to protect drinking-water 

supplies derived from ground water 

generally rely on restricting activities 

within the area contributing recharge 

to a public-supply well. For confined 

aquifers, the general area of protection 

around the wellhead may be defined by 

use of a time-of-travel calculation (U.S. 

Environmental Protection Agency, 1991). 

Although it has long been recognized that 

discrete short-circuit pathways through 

a confining unit can exist and that such 

areas should be afforded a higher level of 

Collecting samples at multiple depths in a pumping publicsupply well.
protection (U.S. Environmental Protection Agency, 1991), figure 2 shows the 

actual complexity that may arise when 

numerous short-circuit pathways (in this 

case, leaky multiscreened wells) are present in the surrounding aquifer. Because 

it is unlikely that all leaking wells can 

be accurately identified, the role of a 

confining unit as a hydrogeologic barrier to movement of contaminants may 

be compromised in ways that cannot be 

anticipated.

Land-use management within the 

area contributing recharge to a publicsupply well can affect the quality of 

water produced by the well. Changes 

such as converting land use from agricultural to urban, increasing the number 

of septic systems because of population 

growth, or restricting development as 

part of wellhead-protection efforts can 

all affect the concentration of anthropogenic chemicals in recharge. The lag 

time between changes in chemical input 

concentrations near the land surface and 

resulting changes in concentrations of 

the chemical in a public-supply well will 

depend upon the mix of water of different ages produced by the well; this mix 

is a direct consequence of the section of 

aquifer intersected by the well screen 

and the proximity of any short-circuiting 

feature. For example, using the simulated 

age distribution of water produced by the 

selected public-supply well in York (fig. 

1), the effects of changes in concentrations of nitrate in ground-water recharge 

were estimated. The calculations included 

leakage through multiscreened wells 

and denitrification in the upper confined 

aquifer. Projections of nitrate concentrations expected in the future indicate that 

the peak value of nitrate reaching the 

selected public-supply well lags behind 

the peak input concentration in recharge 

by approximately 30 years because of the 

particular mix of water of different ages 

produced by the well. Knowledge of the 

speed with which changes in land use 

would be expected to affect water quality 

in supply wells is critical when implementing and evaluating the effectiveness 

of management actions on ground-water 

quality.

References 

Clark, B.R., Landon, M.K., Kauffman, 

L.J., and Hornberger, G.Z., 2008, 

Simulations of ground-water flow, 

transport, age, and particle tracking 

near York, Nebraska, for a study of 

transport of anthropogenic and natural 

contaminants (TANC) to public supply 

wells: U.S. Geological Survey Scientific Investigations Report 2007–5068, 

47 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. 

Landon, M.K., Clark, B.R., McMahon, 

P.B., McGuire, V.L., and Turco, M.J., 

in press, Hydrogeology, chemical characteristics, and transport processes in 

the zone of contribution of a publicFigure 2. The shaded zones on the map show the simulated area contributing recharge to 

a supply well in York, Nebraska. These zones indicate where water that eventually reaches 

the selected supply well enters the ground-water system at the water table. Inactive wells 

in the surrounding aquifer system allow water in the unconfined aquifer to short-circuit the 

underlying confining unit and travel to the supply well in unexpected ways. This process 

results in a contributing area that is relatively complex and difficult to predict. Note that the 

area contributing recharge will be altered if the ground-water flow modeler simulates that a 

different set of inactive wells are leaking. 

 supply well in York, Nebraska: U.S. 

Geological Survey Scientific Investigations Report 2008–5050. 

Landon, M.K., and Turco, M.J., 2007, 

Hydrogeologic setting and groundwater flow simulation of the Eastern 

High Plains Regional Study Area, 

Nebraska: section 8 of Paschke, S.S., 

ed., Hydrogeologic settings and 

ground-water 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. 8-1 to 

8-28. 

U.S. Environmental Protection Agency, 

1991, Wellhead protection strategies 

for confined-aquifer settings: Washington, D.C., Office of Water, EPA 

570/9–91–008, 168 p.

—By Martha L. Jagucki, Matthew K. Landon, 

Brian R. Clark, and Sandra M. Eberts

Total extent of simulated

contributing recharge area York, Nebraska,

city limits

71 to 80 

51 to 60 

41 to 50

Recharge water moving through 

leaky multiscreened wells to the 

selected public-supply well. Percent 

of recharge to multiscreened well 

that reaches the public-supply well:

 

EXPLANATION

Recharge water moving through

upper confining unit to the 

selected public-supply well

21 to 30

11 to 20 

 1 to 10

Beaver

Creek *#

Selected

public-supply well

97°40'

97°40'

40°55'

97°35'

97°35'

40°55'

40°50' 40°50'

0 0.5 1 2 MILES

0 0.5 1 2 KILOMETERS
Central Valley aquifer system 

Aquifers composed of 

glacial deposits 

Floridan aquifer system

Surficial aquifer system

Basin and Range basin-fill and 

carbonate-rock aquifers 

Edwards-Trinity aquifer system

Rio Grande aquifer system

High Plains aquifer

Pomperaug River Basin, Connecticut 

Great Miami River Basin, Ohio 

Eagle Valley and Spanish Springs Valley, Nevada 

Salt Lake Valley, Utah 

PUBLIC-SUPPLY WELL STUDY AREA: 

4 

Eastern High Plains, Nebraska 1 

2 

5 

3 

6

7

Central-Northern Tampa Bay Region, Florida

Northeastern San Joaquin Valley, California 9

8

Middle Rio Grande Basin, New Mexico

South-Central Texas, Texas

2 

 9

5 6 4 

3 1 

7

 8

Regional-scale studies Additional local-scale studies 

The NAWQA Program

The study of public-supply well vulnerability is one of five national priority topics being 

addressed by the NAWQA Program in its second decade, which began in 2001. Other 

topics include effects of urbanization on stream ecosystems; ecological effects of nutrient 

enrichment; mercury in stream ecosystems; and sources, transport, and fate of agricultural 

chemicals. In addition, anthropogenic organic contaminants in source waters for many 

of the Nation’s largest community water systems are being assessed; concentrations 

in source waters are being compared to concentrations in finished waters. During the 

Program’s first decade, NAWQA scientists assessed surface- and ground-water chemistry, 

stream hydrology, habitat, and biological communities in 51 major river basins (“Study 

Units”; see map at http://water.usgs.gov/nawqa). Baseline assessments of pesticides, 

nutrients, VOCs, trace elements, dissolved solids, and radon, and of the condition of aquatic 

habitats and fish, insect, and algal communities are described in hundreds of reports, 

available at the Web site above. Reassessments planned in 42 of the Study Units in the 

Program’s second decade will determine trends at many of the streams and ground-water 

sites; fill critical gaps in the characterization of water quality; and build upon findings 

that show how natural features and human activities affect water quality and aquatic 

ecosystems.

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

Locations of previous or ongoing studies of public-supply well vulnerability to contamination from urban, agricultural, and natural sources.

Studies began in seven states in 2001 and in Texas and New Mexico in 2005; one study is scheduled for New Jersey in 2009.

Contacts

Sandra Eberts, Team Leader

(614) 430-7740, 

smeberts@usgs.gov

Brian Clark, Ground-Water Flow

Modeling

(501) 228-3655, 

brclark@usgs.gov

Matthew Landon, Geochemistry

(619) 225-6109, 

landon@usgs.gov