U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3138

December 2012

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

Assessing the Vulnerability of Public-Supply Wells to 

Contamination: Rio Grande Aquifer System in Albuquerque, 

New Mexico

The Rio Grande aquifer system in New Mexico 

consists of a series of basins located along a 

generally north-south-trending zone called the Rio 

Grande Rift. Here, tectonic forces in the past forced 

the crust to begin to pull apart, creating downwardfaulted basins flanked by mountains. Over time, the 

basins gradually filled with alluvial sediments. Albuquerque is within the Middle Rio Grande Basin in 

central New Mexico. Sediments consisting of sand, 

silt, gravel, and clay in varying amounts have filled 

the basin to a maximum thickness of about 15,000 

feet. The public-supply well selected for study is 

screened from 351 to 1,179 feet below land surface. 

Like most public-supply and commercial wells in 

the Albuquerque area, it draws water from the part 

of the alluvial fill known as the Santa Fe Group.

In the Middle Rio Grande Basin, most water has 

historically entered, or recharged, the Rio Grande 

aquifer system as downward seepage from the Rio 

Grande and its tributaries (Plummer and others, 

2004) or as infiltration along the mountain fronts, 

where runoff of precipitation over relatively impermeable bedrock first encounters permeable basin-fill sediments. Some water also flows 

from the subsurface bedrock into the alluvial deposits through the sides of the filled basin. Since about the 1930s, manmade sources of 

recharge have been added, and directions of groundwater flow near Albuquerque have been greatly altered by human activities. The 

modern direction of groundwater flow on the east side of the Rio Grande is from the river corridor toward the public-supply wells centered 

primarily in the eastern part of Albuquerque, where large groundwater withdrawals have created a water-level depression. 

West

Supply

well

Recharge

Discharge

Urban area

Generalized

groundwater-

 flow direction

Water table

Coarser-grained

unconsolidated

alluvium

Finer-grained

unconsolidated

alluvium

FAULT

Bedrock

Mountain

front

Rio Grande aquifer system

Flood-plain 

deposits

Rio

Grande

Not to scale

East

Rio Grande

aquifer 

system

Albuquerque

The pathways and processes by 

which contaminants reach public-supply 

wells was intensively investigated 

during 2001–2012 in nine aquifer 

systems across the Nation as part of 

the U.S. Geological Survey’s National 

Water-Quality Assessment (NAWQA) 

Program. Specifically, scientists studied 

the processes that occur below land 

surface—whereby contaminants are 

mobilized or attenuated—as well as 

investigated 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 Albuquerque, 

New Mexico (Bexfield and others, 2012; 

Heywood, in press). The well selected for 

study (hereafter referred to as “the study 

well”) typically produces about 3,000 

gallons of water per minute from the Rio 

Grande aquifer system. In construction 

and operation, the study well is typical of 

many public-supply wells in the region 

(although not all wells in the region 

produce as much water as this well). 

Untreated water samples collected 

at the study well between 2007 and 

2009 contained some undesirable 

constituents, including contaminants 

that occur naturally and others that 

are anthropogenic (introduced by 

human activity). Arsenic, which occurs 

naturally in aquifer sediments, was 

found at concentrations ranging from 

12 to 18 micrograms per liter (μg/L), 

exceeding the Maximum Contaminant 

Level (MCL) of 10 μg/L established 

by the U.S. Environmental Protection 

Agency for drinking water. Arsenic is 

of particular concern to water managers 

in this region because concentrations in 

groundwater exceed the MCL in broad 

areas in and near Albuquerque (Bexfield 

and Plummer, 2003). The water utility 

for Albuquerque and Bernalillo County 

began using low-arsenic surface water 

as a major source of drinking water in 

2008 in response to declining water 

levels in the aquifer. Groundwater and 

surface water are now typically blended, 

which facilitates delivery to customers 

of water with arsenic concentrations 

below the MCL. The anthropogenic 

constituents trichloroethylene (TCE) 

and its degradation product cis-1,2-

dichlorethylene, as well as methyl tertbutyl ether (MTBE) and nitrate, also 

were detected in the study well, although 

all were at concentrations at least an 

order of magnitude less than established 

drinking-water standards, where such 
standards exist. TCE is a widely used 

solvent, and MTBE is a gasoline additive; 

documented releases of both chemicals 

have occurred in the urban area near the 

study well. Nitrate can be introduced by 

sewage sources in the Albuquerque area, 

although natural soil sources also are 

common in this area. 

Overall, the study findings point 

to four primary influences on the 

movement and (or) fate of contaminants 

and the vulnerability of the public-supply 

well in Albuquerque: (1) groundwater 

age (how long ago water entered, or 

recharged, the aquifer), (2) groundwater 

development (introduction of manmade 

recharge and discharge sources), (3) 

natural geochemical conditions of the 

aquifer, and (4) seasonal pumping 

stresses. Study findings are intended 

to help water managers, drinkingwater suppliers, policymakers, and 

scientists better understand how and 

why contamination of public-supply 

wells occurs and whether water quality 

may improve or degrade. Additionally, 

computer models developed for the 

study may be used to evaluate various 

pumping, resource-development, and 

land-management scenarios.

Mixing of Young Water with Older 

Water Increases Vulnerability of Well to 

Manmade Contaminants

“Groundwater age” refers to the 

elapsed time since water entered an 

aquifer at the water table. The age of 

groundwater older than 1,000 years was 

evaluated by using concentrations of the 

isotope carbon-14 (14C). To assess the 

presence of recently recharged water, 

concentrations of the age tracer tritium 

were primarily used. Concentrations 

of tritium in recharge waters increased 

substantially in the 1950s and 1960s 

as a result of nuclear weapons testing. 

Study Design

As a first phase of study in this area, a computer model of groundwater flow was constructed at a regional scale covering much of 

the Middle Rio Grande Basin (4,819 square miles) to estimate the 

“100-year zone of contribution” to the public-supply well selected 

for study (fig. 1) and also traveltimes from recharge areas to 

this and other nearby public-supply wells (Bexfield and others, 

2011). The 100-year zone of contribution is the three-dimensional 

volume of the aquifer material through which groundwater flows 

for the century prior to its eventual discharge at the study well. 

After the regional model was developed, a network of 9 existing 

monitoring wells was augmented by 13 additional wells. Most 

of the monitoring wells were “nested,” meaning that multiple 

monitoring wells tapping different depths in the aquifer were 

installed within a single borehole. The new monitoring wells were 

installed at four locations in or near the simulated 100-year zone 

of contribution to the study well to collect data on groundwater 

flow and geochemistry along likely paths of groundwater flow to 

the well. Water samples were collected from 22 monitoring wells 

and the 3 public-supply wells shown in figure 1 during June 2007 

through May 2009 and analyzed for naturally occurring contaminants (such as radon, uranium, and arsenic) and anthropogenic 

contaminants (such as volatile organic compounds and nutrients). Analyses for other water-quality constituents (such as 

major ions, dissolved organic carbon, age-dating tracers, and 

selected stable isotopes) were done to improve understanding of 

sources of water and reactions affecting the chemical composition of groundwater. 

To determine where water enters the screened interval of the 

study well, flow of water through the wellbore was measured 

during both pumping and nonpumping conditions. Similarly, water 

samples were collected from the study well at various depths 

while it was being pumped to determine where contaminants 

enter the well. By using these data, a more detailed, local-scale 

(9.3 square miles) computer model of groundwater flow and 

transport to the study well was constructed (Heywood, in press). 

The local model simulates a 108-year period to evaluate (1) 

changes in groundwater flow over time, (2) mixing of water of 

different ages within the groundwater-flow system, (3) changes 

in sources of water to the study well with seasonal changes in 

pumping rates, and (4) likely transport pathways and release 

times for anthropogenic contaminants that have been transported 

to monitoring and public-supply wells from known sources.

§ ̈¦25

106°39' 106°38' 106°37'

35°06'

35°05'

35°04'

35°03'

EXPLANATION

Local-scale study area

Eastern extent of river corridor 

(historical flood plain)

River or stormwater channel

River corridor

Other sampled public-supply wells

Study well

Monitoring well nest locations

PSW2

PSW1

PSW3

Rio Grand e

0 0.5 1 MILE

Base from New Mexico Geospatial Data 0 0.5 1 KILOMETER

Acquisition Coordination Committee, State of New Mexico, 

2005 imagery, 1:12,000 Projection: Lambert Conformal Conic 

 NEW MEXICO

Regional-scale 

 study area

Local-scale

 study area

 Middle Rio 

Grande Basin

Figure 1. The orange and green lines shown on the map represent two 

possible sets of flowpaths followed by water traveling through the aquifer 

system to the study well in eastern Albuquerque, based on two different 

assumptions about water-transmitting properties of the aquifer. The lines 

indicate the path traveled in the 100 years prior to reaching the well, as 

estimated by a computer model simulating regional groundwater flow, and 

show that the direction of groundwater flow in the area has changed from 

predevelopment (black arrow) to modern times (white arrow).
For the current study, groundwater 

having a tritium concentration above 

0.3 tritium units (TU) was considered 

to contain a component of young (post1950) recharge, whereas water with a 

tritium concentration below 0.3 TU was 

assumed to be native groundwater largely 

unaffected by young recharge. 

Analysis of age tracers indicates that 

groundwater from most sampled wells 

in the local study area is predominantly 

old (pre-1950) water. Native water had 

14C ages ranging from 6,500 years in 

some shallow monitoring wells (with 

screen midpoints within 60 feet of the 

water table) to more than 18,000 years 

old in deep monitoring wells (with screen 

midpoints more than 607 feet below the 

water table). Stable-isotope data indicate 

that the source of the native water was 

recharge from the Rio Grande. Most 

of this recharge likely occurred several 

miles to the north in the basin rather than 

near the local study area (Bexfield and 

others, 2011). 

In the Middle Rio Grande Basin, the 

youngest groundwater is expected to be 

found near the known sources of water 

infiltrating from land surface—the Rio 

Grande and the mountain front (fig. 2A). 

Because potential evapotranspiration 

(more than 51.2 inches per year) 

substantially exceeds the average rate 

of precipitation (8.7 inches per year), it 

has long been assumed that precipitation 

over most of the basin evaporates or 

is transpired by plants long before it 

infiltrates to water table, which is tens to 

hundreds of feet below land surface over 

much of the basin. Therefore, recently 

recharged groundwater is not expected 

to be found near the water table in most 

of the areas between the river and the 

mountain front. However, the presence of 

tritium at concentrations greater than 0.3 

TU in samples from many, but not all, of 

the shallow and intermediate monitoring 

wells (with screen midpoints between 

89 and 261 feet below the water table) 

means that these samples contained a 

fraction of post-1950 water in addition 

to a large component of water that was 

thousands of years old. Shallow-well 

samples containing tritium were estimated 

to consist of 14 to 36 percent young water. 

Samples from some intermediate wells 

consisted of an even greater fraction 

of young water—up to 46 percent. 

Groundwater from two of the three publicsupply wells sampled (including the study 

well) had a young fraction ranging from 3 

to 11 percent. Deep groundwater samples 

and the third public-supply well (PSW3, 

fig. 1) were devoid of young water.

Thus, age-tracer data from the study 

area indicate that recent recharge is 

reaching the aquifer across broad areas 

beneath Albuquerque, is migrating from 

shallow to intermediate depths of the 

aquifer (depths as great as 245 feet below 

the water table), and is traveling to some 

(but not all) of the public-supply wells. 

The movement of young water to publicsupply wells is of concern because young 

groundwater is much more likely to have 

accumulated manmade contaminants 

during the recharge process compared 

to old groundwater that recharged the 

aquifer thousands of years ago. The 

three major public-supply wells in the 

study area produce a large proportion of 

old groundwater (89 percent or greater) 

that has the potential to substantially 

dilute the anthropogenic contaminants 

that may be present in the young 

fraction; however, this old water can add 

contaminants that occur naturally within 

the aquifer, because old water from the 

deep zone of the aquifer is associated 

with elevated arsenic concentrations 

(exceeding the 10-μg/L MCL). 

Groundwater Development Draws 

Anthropogenic Contaminants Downward 

Into the Production Zone of the Aquifer

Chemical data provide clues as to 

the source of the young water. Stable 

isotopes of hydrogen and oxygen 

indicate that, unlike most groundwater 

in the study area, the young fraction 

of shallow water did not typically 

originate as seepage from the Rio 

Grande. Although the exact source and 

mechanism of recent recharge is not 

clear, it appears to be associated largely 

with human activities in urban areas for 

the following reasons: (1) Most shallow 

and intermediate monitoring wells with 

tritium concentrations greater than 0.3 

TU also contained manmade volatile 

organic compounds (VOCs) (primarily 

the industrial solvents perchlorethylene, 

TCE, or their degradation products at 

concentrations below the MCL), whereas 

the deep, old groundwater did not. (2) 

At least four of the six shallow water 

samples with a young component of 

water contained chloroform, a byproduct 

of drinking-water chlorination. The 

presence of a chlorination byproduct 

indicates that the recharge results at least 

in part from infiltration of chlorinated 

municipal-supply water.

The effect of manmade recharge 

sources and human water withdrawals on 

groundwater flow in the alluvial aquifer 

was initially examined by using a regional 

computer model of groundwater flow 

(Bexfield and others, 2011). The model 

simulates, from 1900 through 2008, both 

natural and manmade sources of recharge, 

as well as changes in groundwater 

withdrawals over time, based on available 

well records. Observed and simulated 

water levels indicate that the natural, 

predevelopment (pre-1930s) direction of 

groundwater flow in the eastern Middle 

Rio Grande Basin was from the northnortheast to the south-southwest, toward 

the Rio Grande (fig. 1). Groundwater was 

recharged from the Rio Grande and its 

tributaries and along the mountain front 

and flowed downvalley beneath the river 

(fig. 2A). Nearly all groundwater exited, 

or discharged, the system by way of 

evapotranspiration in the river corridor. 

Figure 2B shows how new, manmade 

sources of recharge and discharge 

have greatly altered groundwater flow 

and the regional groundwater budget. 

Flow has been redirected from the river 

corridor toward wells to the east, and 

horizontal and vertical hydraulic gradients 

have steepened, creating an increased 

downward component of flow. Water 

levels from predevelopment through 

2002 have likely declined by 20 to 120 

feet across much of the Albuquerque area 

(Bexfield and Anderholm, 2002). 

The mixing of young, often VOCcontaminated water from manmade 

recharge sources with older, native 

groundwater in eastern Albuquerque 

was investigated in more detail by use 

of a local-scale groundwater-flow model 

coupled to the regional model and a 

corresponding local-scale solute-transport 

model (Heywood, in press). Local-scale 

study boundaries are shown in figure 1. 

Additional sources of urban recharge 

were simulated at this local scale that 

are not shown in figure 2B. These 

sources include municipal waterline and 

sewerline leakage (the areal extent of 

which changed over time as Albuquerque 

grew and the service area increased), 

infiltration from constructed ponds and 

watering of turf areas, and seepage from 

a stormwater diversion channel that is 
unlined in places, allowing leakage from

the channel to the groundwater system. 

Local-scale simulations show that 

recent recharge from urban sources 

is pulled from the water table to 

intermediate zones of the aquifer when 

pumping stress is applied by publicsupply wells screened at intermediate 

and (or) greater depths (fig. 3A). 

Downward flow is nonuniform, because 

the young water travels preferentially 

(in larger volumes and at faster rates) 

through interconnected lenses of coarse 

sediments. This may explain why young 

groundwater was found beneath older 

groundwater at some locations in the 

study area. Such age inversions could 

also be the result of direct migration of 

young water down wellbores screened 

across large intervals of the aquifer (or 

down the gravel-filled annular space 

around such wells) that enables water 

and contaminants to rapidly bypass 

normal transport pathways through 

interconnected pore spaces in the aquifer

Transport simulations and chemical

data show that young water reaching 

the intermediate zone is mixed with old, 

native water. Once in the intermediate 

zone, this mixed water is transported 

laterally over relatively large distances t

public-supply wells. Samples collected 

from various depths in the study well 

while it was being pumped indicate 

that the largest fraction of young 

groundwater and trace amounts of TCE 

enter the supply well at depths between 

495 and 577 feet below land surface, 

corresponding to a particularly coarsegrained layer within the intermediate 

zone of the aquifer. 

Natural Geochemical Conditions in the 

Aquifer Affect the Fate of Contaminants

Geochemical conditions such as 

pH of groundwater control whether 

specific contaminants are attenuated or 

mobilized in the subsurface. In the local 

study area, pH generally increases with 

depth in the intermediate and deep zones 

of the aquifer. Water at depth has been in 

contact with aquifer sediments for a long 

time (more than 18,000 years in the deep 

zone of the aquifer). Greater contact time 

allows greater dissolution of minerals 

from the aquifer sediments, and these 

dissolution reactions tend to increase pH 

in groundwater. Above a pH of about 7, 

arsenic (present primarily in the form 

. 

o River (crop-producing)

 corridor

Riparian

evapotranspiration 

15 percent

Riverside

drains

 36 percent

River

infiltration 

58 percent

Santa Fe 

Group

Piedmont

Crystalline rock 

Pre-Santa Fe Group 

sedimentary rock

Tributary

recharge 

2 percent

Mountain-front

recharge

2 percent

Subsurface

recharge 

5 percent

Crop-irrigation

seepage 

6 percent Canal

seepage 

16 percent

Septic-field 

seepage 

0.7 percent Groundwater

withdrawal

26 percent

Inflow

of water from

aquifer storage

10 percent

Interior

drains 

23 percent

Rio

Grande

Shallow

alluvial 

aquifer

Water table

River

corridor

Riparian

evapotranspiration 

100 percent

River

infiltration 

60 percent

Santa Fe 

Group

Rio

Grande

Shallow

alluvial 

aquifer

Piedmont

Crystalline rock 

Pre-Santa Fe Group 

sedimentary rock

Tributary 

recharge 

7 percent Mountain-front

recharge

9 percent

Subsurface

recharge 

24 percent

Eastern mountains

Eastern mountains

Water table

Total flow, 187 billion gallons per year

B. Modern conditions

Total flow, 42 billion gallons per year

A. Predevelopment conditions

EXPLANATION

Direction of inflow

Direction of outflow

Direction of groundwater movement

West East

Figure 2. As population in the Albuquerque area has increased over the last 100 years, 

the addition of manmade features has greatly modified how groundwater enters and leaves 

the system. New manmade sources of recharge include an extensive network of irrigation 

canals developed in the early to mid-1900s that diverts water from the upstream Rio Grande 

and distributes the water to crops planted along the river. Residential development began 

by the 1930s, and today septic system leachfields contribute recharge to the groundwater 

system in unsewered areas of the basin. In urban areas, leaking water and sewer pipes are 

an additional source of recharge (not shown in figure). New sources of discharge include 

groundwater withdrawals from public-supply wells, which increased substantially beginning 

in the 1950s. In the river corridor, groundwater drains (riverside drains and interior agricultural drains) have been constructed to lower the water table beneath crops. The water 

captured by the drains is discharged to the Rio Grande.
of arsenate) is released from iron oxide 

coatings on sediments and dissolves into 

the deep groundwater. The amount of 

arsenic naturally present in the aquifer 

sediments also increases below a depth of 

820 feet in the vicinity of the supply well. 

Experiments on sediment cores found that, 

although some arsenic can be released 

from aquifer sediments to groundwater 

at all depths, as much as 10 times more 

arsenic can be released from the deepest 

sediment samples. Concentrations of 

arsenic exceeded the MCL of 10 μg/L 

in water from two deep wells, from one 

intermediate well, and from two supply 

wells (PSW3 and the study well). 

Operation of the Public-Supply Well 

Affects Drinking Water Quality 

Multiple lines of evidence indicate 

that seasonal changes in the pumping 

schedule of the study well affect the 

age and the quality of water produced 

by the well. Continuous water-level 

measurements in a monitoring well nest 

about 100 feet from the study well show 

that pumping of the study well affects 

vertical direction of flow in the nearby 

aquifer: when the pump is off, flow is 

upward throughout the studied thickness 

of the aquifer; when the pump is on, 

flow is downward from the shallow zone 

to the intermediate zone of the aquifer. 

Pumping for long durations is common 

in the summer, when water demand is 

high. Computer simulations show that 

increased pumping during the summer 

causes significant volumes of relatively 

young water to flow downward from the 

shallow to the intermediate zone of the 

aquifer (fig. 3A). Chemical and age-tracer 

data from the study well confirm that this 

downward flow of young water causes 

an increase in VOC detections (including 

TCE) in the summer, corresponding to 

a higher fraction of young water (about 

11 percent) produced by the well in 

the summer than in the winter months. 

Conversely, arsenic concentrations are 

lower in the summer (generally less than 

or equal to 12 μg/L) than in the winter 

because of a correspondingly lower 

fraction of old water being withdrawn 

from deeper parts of the aquifer (below 

820 feet), where arsenic concentrations 

are particularly high. 

In the winter, water demand is lower 

and the study-well pump typically is idle 

for several hours each day. Measurements 

of flow within the wellbore of the study 

well under nonpumping conditions 

indicate that the well effectively acts as 

a conduit for flow of groundwater—as 

much as 200 gallons per minute—from 

deep to intermediate parts of the aquifer 

when the pump is off. Measurements 

show that water primarily enters the 

wellbore from the aquifer below a depth 

of about 856 feet (from the high-arsenic 

part of the deep zone) and then flows out 

of the well into the intermediate zone of 

the aquifer (fig. 3B). 

When the study well is pumped for 

relatively short periods of time in the 

winter, the downward hydraulic gradient 

and the downward flow of shallow water 

occurs but is less than that observed 

during the summer. Water-quality data 

indicate that arsenic concentrations 

in water produced by the well can 

be higher when the pump is initially 

turned on than after several hours of 

pumping, suggesting that at least some 

of the high-arsenic water that migrated 

upward through the wellbore is stored 

near the well in the intermediate zone of 

the aquifer and is drawn back into the 

well once pumping resumes (fig. 3C). 

Computer simulation of groundwater 

flow to the well in the winter reveals 

that a smaller fraction of young water is 

produced by the well in the winter than 

in the summer, and a correspondingly 

greater fraction of old, deep water is 

produced. (Mixing calculations based on 

age-tracer concentrations estimate about 

6 percent of the water produced in winter 

is young, compared to 11 percent in the 

summer.) This mix of water results in 

higher arsenic concentrations (commonly 

more than 12 μg/L) in water produced 

in the winter than in the summer and a 

general absence of VOCs in the water 

produced by the study well in the winter.

A. PUMPING (Summer) 

Supply

well

Movement of recent 

recharge mixed with

native water 

Movement of 

native water 

Water table

Not to Scale

(shallow and

 intermediate 

 zones)

Shallow and

 zones

Movement of 

native water 

Deep zone

Movement of native, 

high-arsenic 

water 

B. NONPUMPING (Winter)

Supply

well

Movement of recent 

recharge mixed with

native water 

Movement of 

native water 

Water table

Not to Scale

Deep zone

(shallow and

 intermediate 

 zones)

Movement of 

native water 

Shallow and

 zones

Movement of native, 

high-arsenic 

water

C. PUMPING (Winter)

Supply

well

Movement of recent 

recharge mixed with

native water 

(shallow and

 intermediate 

 zones)

Movement of 

native water 

Movement of recent 

recharge mixed with

native water 

Water table

Not to Scale

Deep zone

Shallow and

 intermediate 

 zones

Movement of 

native water 

Movement of native, 

high-arsenic 

water

Figure 3. Water produced by the public-supply well is a mixture of native (old) water, high-arsenic water from the deep zone of the aquifer, 

and recent recharge from urban sources that can be contaminated with VOCs. The proportion of these three types of water entering the well 

changes with differing seasonal pumping rates. (A) During summer when the pump is on for long periods, recent recharge near the water 

table mixes with old, native water and is pulled steadily downward to the intermediate zone of the aquifer and to the public-supply well. 

(B) During winter when the pump is off for several consecutive hours, high-arsenic water travels up the well from the deep zone and exits into 

the intermediate zone of the aquifer. (C) When the pump is activated in the winter (for a relatively short time each day), some of the leaked, 

high-arsenic water is recaptured by the well. Having water with elevated arsenic concentrations simultaneously enter the well from both the 

intermediate and deep zones of the aquifer results in a larger proportion of high-arsenic water produced in the winter than in the summer.
Implications for Protection of DrinkingWater Quality

The screened interval of a publicsupply well affects its vulnerability to 

arsenic contamination. Eliminating 

deeper screened intervals in areas of the 

basin where arsenic concentrations are 

elevated at depth could reduce arsenic 

concentrations from supply wells 

both directly (by not pumping from 

higher-arsenic zones) and indirectly (by 

eliminating the potential for migration of 

high-arsenic water through wellbores into 

shallower parts of the aquifer).

VOCs such as TCE can travel 

rapidly downward to the intermediate 

zone of the aquifer and can be 

transported relatively long distances 

from the source of release. Urban 

sources of recharge (such as leaking 

water and sewer pipes, constructed ponds, 

turf irrigation, and unlined stormwater 

canals) cause substantial quantities of 

water to infiltrate to the water table even 

in the semi-arid climate of Albuquerque. 

Supply-well pumping enhances migration 

of this urban recharge containing low 

levels of VOCs both downward to the 

depth tapped by public-supply wells and 

horizontally. These findings indicate that 

source water protection (which guards 

against chemical releases at or near land 

surface) can favorably affect the quality 

of drinking water, even in areas where 

climate and hydrogeology have resulted 

in predominantly old groundwater. 

The presence of long-screened 

wells in an aquifer with a significant 

vertical component of groundwater flow 

is likely to alter the movement of water 

and contaminants through the aquifer 

even when those wells are not pumping. 

Two recent studies in two different 

aquifer systems have documented the 

flow of water through long-screened 

wells under nonpumping conditions. 

The study discussed herein measured 

the upward flow of water through a 

well from a high-arsenic, deep zone 

of the Rio Grande aquifer system to a 

shallower zone that generally has arsenic 

concentrations below the drinking-water 

standard. In contrast, Jurgens and others 

(2008) found that a long-screened well 

in the Central Valley aquifer system near 

Modesto, California, allows shallow 

water with elevated uranium and nitrate 

concentrations to migrate down the well 

and into the relatively pristine, deep 

zone of the aquifer when the well is not 

being pumped. Knowledge of vertical 

flow directions in the vicinity of a longscreened well (under both pumping 

and nonpumping conditions), coupled 

with understanding of variations in 

contaminant concentrations with depth 

in the aquifer, can help water managers 

predict the positive or negative effect that 

wellbore flow will have on water quality 

and can lead to development and testing 

of strategies to mitigate contamination 

(such as changes in pumping schedules, 

development of devices to inhibit 

wellbore flow when the pump is off, and 

eliminating intervals of well screens that 

are in zones of undesirable water quality).

References

Bexfield, L.M., and Anderholm, S.K., 

2002, Estimated water-level declines 

in the Santa Fe Group aquifer system 

in the Albuquerque area, central New 

Mexico, predevelopment to 2002: U.S. 

Geological Survey Water-Resources 

Investigations Report 02–4233, 1 sheet.

Bexfield, L.M., Heywood, C.E., 

Kauffman, L.J., Rattray, G.W., and 

Vogler, E.T., 2011, Hydrogeologic 

setting and groundwater flow 

simulation of the Middle Rio 

Grande Basin regional study area, 

New Mexico, section 2 of Eberts, 

S.M., ed., Hydrologic settings and 

groundwater-flow simulations 

for regional investigations of the 

transport of anthropogenic and natural 

contaminants to public-supply wells—

Investigations begun in 2004: U.S. 

Geological Survey Professional Paper 

1737–B, p. 2–1 to 2–61.

Bexfield, L.M., Jurgens, B.C., Crilley, 

D.M., and Christenson, S.C., 2012, 

Hydrogeology, water chemistry, and 

transport processes in the zone of 

contribution of a public-supply well in 

Albuquerque, New Mexico, 2007–9: 

U.S. Geological Survey Scientific 

Investigations Report 2011–5182, 114 p.

Bexfield, L.M., and Plummer, L.N., 

2003, Occurrence of arsenic in ground 

water of the Middle Rio Grande 

Basin, central New Mexico, chap. 

11 of Welch, A.H., and Stollenwerk, 

K.G., eds. Arsenic in ground water—

Geochemistry and occurrence: Boston, 

Kluwer Academic Publishers, 

p. 295–327.

Heywood, C.E., in press, Simulations of 

groundwater flow, transport, and age 

in Albuquerque, New Mexico, for a 

study of Transport of Anthropogenic 

and Natural Contaminants (TANC) to 

public-supply wells: U.S. Geological 

Survey Scientific Investigations Report 

2012–5242.

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.

Plummer, L.N., Bexfield, L.M., 

Anderholm, S.K., Sanford, W.E., and 

Busenberg, E., 2004, Geochemical 

characterization of ground-water 

flow in the Santa Fe Group aquifer 

system, Middle Rio Grande Basin, 

New Mexico: U.S. Geological Survey 

Water-Resources Investigations Report 

03–4131, 395 p.

By Martha L. Jagucki, Laura M. Bexfield, 

Charles E. Heywood, and Sandra M. Eberts

Contacts

Sandra Eberts, Team Leader, (614) 430-7740, smeberts@usgs.gov; Charles Heywood, Groundwater Modeling, (970) 626-9840, 

cheywood@usgs.gov; Laura Bexfield, Geochemistry, (505) 830-7972, bexfield@usgs.gov

For more information on the NAWQA topical study of contaminant transport to public-supply wells (TANC), see 

http://water.usgs.gov/nawqa/