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Why Some Public-Supply Wells are More Vulnerable to Contamination Than Others

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00:00 Why Some Public-Supply Wells are More Vulnerable to Contamination Than Others

00:01 ( USGS CoreCast Movie and Music )


U.S. Geological Survey

National Water-Quality

Assessment Program

00:20 (video of Sandra Eberts speaking)

Hi, I’m Sandra Eberts, a hydrologist with the USGS and I lead a team of scientists who have conducted groundwater-quality studies on public-supply wells in aquifers across the Nation.

More than 100 million people in the United States, or about 35 percent of the population, receive their drinking water from public groundwater systems, which can be vulnerable to naturally occurring contaminants such as radon, uranium and arsenic, as well as manmade compounds, including fertilizers, septic-tank leachate, solvents and gasoline hydrocarbons.

00:51 (Video of drilling, photos of sampling, borehole geophysical logging, and public-supply wells)

As you might expect, tracking contaminant movement in groundwater and in wells can be challenging; but because of our studies, we have a better understanding of why some wells are more vulnerable to contamination than others. We also have gained insights into ways that this information can be used by public-supply well managers to protect their drinking water sources.

01:13 (map of United States showing four study areas and four fact sheets)

We studied four very different aquifers and wells in California, Connecticut, Nebraska and Florida so that our findings can be applied to similar types of aquifer settings and wells throughout the Nation. Our results are published in four USGS fact sheets and described in this video.

01:33 (Video of Sandra speaking)

01:36 (Photos of public-supply wells, drilling, checking monitoring wells, sampling, measuring water levels in monitoring wells, and public-supply wells)

The vulnerability of an individual public-supply well depends on the vulnerability of the groundwater in the surrounding aquifer and the position of the well within the aquifer. This is because groundwater vulnerability generally varies throughout an aquifer and individual wells produce unique mixtures of the water. Recognizing what contributes to each can help us do a better job of protecting our drinking water resources and predicting the vulnerability of our public wells to contamination.

02:01 (video of Sandra speaking)

My team found that three key factors—the geochemical conditions in the aquifer and supply well, the age of the groundwater in the well, and the presence of preferential flow pathways—helped to explain why we observed the contaminants that we did in each well we studied.

02:17 (Music)

(Animated slide of Geochemical Conditions. Photos of water sampling and core samples of rocks)

Geochemical conditions, for example, the amount of dissolved oxygen, iron, or pH affect the fate of contaminants in the groundwater.

A contaminant like arsenic may be released from the aquifer material and increase in groundwater when there is little or no oxygen dissolved in the water, while nitrate may be degraded or breakdown in similarly anoxic conditions. The term “anoxic” is generally used to describe water with less than 0.5 mg/L of dissolved oxygen. “Oxic” is used to describe water with greater amounts of dissolved oxygen.

Because geochemical conditions can vary within an aquifer, the position of a well screen in the aquifer may affect whether a contaminant can persist in the groundwater all the way to the well screen.

03:11 (Music)

(Animated slide of Groundwater Age and Age Mixing. Photos of public-supply wells, types of land use and possible sources of contaminates, drilling, water sampling and core samples of rocks)

Water from a public-supply well is frequently a mixture of water from a range of time periods associated with different land uses and potential sources of contaminants. “Young” water is more vulnerable to contamination from human sources. Old water is not necessarily free from contaminants because the water can dissolve minerals from the aquifer material.

The position of a well screen affects the age mixture of water pumped by the well, and therefore, whether or not contaminants present in the aquifer are also in the well.

03:52 (Sound of a rain falling)

(Animated slide of Preferential Flow Pathways. Photos of a center-pivot irrigation system, sinkholes, sign along road with words, “We Buy Sink Hole Homes”, house that is falling into a sinkhole, and a turbine well.)

Features such as nearby wells and natural karst features such as sinkholes frequently allow water to bypass or “short-circuit” much of the aquifer material. Such preferential flow pathways can allow contaminants to move quickly through an aquifer.

04:21 (Video of Sandra speaking)

As I share our findings from the four areas that we studied, you’ll see that the importance of each of the three factors that I just described differs for each type of aquifer and well combination.

04:34 (Music)

(Map of United States, with the Central Valley aquifer system in California highlighted in yellow and the City of Modesto labeled)

04:37 (Animated slide of a cross-section through the earth showing the movement of groundwater and how contaminants enter and move through the aquifer and to a public-supply well.)

The first aquifer-well combination is an aquifer made up of hundreds of feet of sands, gravel, silt, and clay with oxic conditions and a long well screen. The groundwater age is tens to thousands of years old. We use the term young water to describe water that entered an aquifer less than 60 years ago.

Our example comes from Modesto, California, where agricultural development followed by more recent urban activity has introduced low concentrations of man-made contaminants into the shallow part of the aquifer. Irrigation combined with water withdrawal at depth has intensified the downward movement of this relatively young, affected water in the aquifer.

Irrigation has also changed the geochemical conditions in the shallow part of the aquifer creating an environment that promotes the leaching of uranium from the aquifer material into the water.

During times of little or no pumping, water in the shallow part of the aquifer drains down the inside of the well and replaces old, unaffected water in the lower part of the aquifer that would otherwise dilute contaminants in the well. When the pump is briefly turned on, affected water simultaneously enters the well from both the shallow and deep part of the aquifer, temporarily increasing concentrations of contaminants.

As a result of USGS findings, public-supply well managers changed their pumping practices, which reduced the amount of young, affected water that is pumped by the well.

06:04 (Music)

(Map of United States, with the glacial aquifer system highlighted in red and the City of Woodbury, Connecticut labeled)

06:10 (Animated slide of a cross-section through the earth showing the movement of groundwater and how contaminants enter and move through the aquifer and to a public-supply well.)

The second aquifer-well combination is an aquifer made up of sand with mostly oxic conditions and a short well screen.

Our example comes from Woodbury, Connecticut; where water in the aquifer is generally less than 10 years old and therefore vulnerable to contamination from man-made chemicals. The public-supply well is similarly vulnerable because it doesn’t pump much old water that could dilute any man-made contaminants that reach the well.

Many homes in the area have septic systems, and leachate from the septic systems has higher concentrations of nitrate than would naturally occur in the aquifer. Because the groundwater is oxic, nitrate can persist once introduced into the aquifer.

06:53 (Video of Sandra speaking)

We also found that dry wells used in Woodbury to reduce stormwater runoff serve as preferential flow pathways that route potentially contaminant-laden runoff water directly into the aquifer, bypassing soil and unsaturated sediments that otherwise could filter out some of the contaminants. The groundwater in the supply well currently meets drinking-water criteria, although there have been some exceedences in the past.

07:18 (Music)

(Map of United States, with the High Plains aquifer highlighted in green and the City of York, Nebraska labeled)

07:25 (Animated slide of a cross-section through the earth showing the movement of groundwater and how contaminants enter and move through the aquifer and to a public-supply well.)

The third aquifer-well combination is an aquifer made up of mostly sand with anoxic conditions, and a well screen beneath a confining unit composed of clay. This clay layer slows the flow of water between an overlying sand aquifer and this deeper, confined aquifer. The water in the confined aquifer is generally hundreds to thousands of years old, whereas the water in the overlying sand aquifer is tens of years old.

Our example comes from York, Nebraska, where many nearby wells have screens open to both the confined aquifer and the overlying aquifer. These nearby wells have allowed young water containing nitrate and volatile organic compounds, or VOCs, in the shallow unconfined aquifer to leak down into the confined aquifer that serves as the drinking water source.

Anoxic conditions in the clay and confined aquifer breakdown of some of the contaminants, like nitrates and VOCs. But if the source of the leaked water is close to a public-supply well, there may not be enough time for contaminants to breakdown on the way to the well.

08:29 (Video of Sandra speaking)

Water from the well that we studied had very low concentrations of contaminants, but other nearby wells had high nitrate concentrations that exceeded drinking water standards.

Without these multi-screened wells acting as “short circuits” in the aquifer system, the water in the confined aquifer would be too old to contain man-made contaminants.

08:50 (Music)

(Map of United States, with the Floridan aquifer system highlighted in blue and the City of Tampa, Florida labeled)

08:57 (Animated slide of a cross-section through the earth showing the movement of groundwater and how contaminants enter and move through the aquifer and to a public-supply well.)

Our final aquifer-well combination is carbonate, or limestone rocks with oxic and anoxic conditions and an open-hole well design. The age of groundwater ranges from less than one year to tens of years

Our example comes from an area near Tampa, Florida. Here we found that when the public-supply well is pumped, the movement of water in sinkholes, conduits, and other natural preferential flow pathways in the rock is accelerated.

As a result, the water in the well is much younger and more vulnerable to manmade contaminants than the water in most of the surrounding carbonate rock aquifer.

We also found that pumping pulls geochemically different water from an overlying sand aquifer through these preferential flow pathways towards the supply well.

This causes naturally occurring arsenic in the rock to be released into the groundwater and reach the well.

09:49 (Video of Sandra speaking)

09:52 (Animated slides of Geochemical Conditions, Groundwater Age and Age Mixing, and Preferential Flow Pathways.)

09:58 (Video of Sandra speaking)

As you have seen, knowledge of geochemical conditions, the mixture of groundwater ages, and the presence of preferential flow pathways can help explain whether and how contaminants in an aquifer reach a public-supply well. But what’s really important is how to translate this knowledge into action that ultimately protects our drinking water sources.

10:13 (Photo of public-supply well) (Words on the slide, Translating Knowledge Into Action

10:16 (Photo of water sampling) (Words on the slide, Anticipate which contaminants might reach a public-supply well)

Measuring inexpensive water-quality parameters like dissolved oxygen, iron, manganese, and sulfate can help characterize the geochemical environment in an aquifer and public-supply well can help a public-supply well manager anticipate which contaminants, once in the aquifer, would be more or less likely to reach the well.

10:35 (Photo of sign with the words, Groundwater sensitive area in case of spill call 911) (Words on the slide, Forecast how responsive a well might be to changes that take place at or near the land surface)

Having an idea of the mixture of ground-water age in a public-supply well can help a resource manager forecast how responsive a well might be to changes that take place at or near the land surface.

10:49 (Photo of drilling) (Words on the slide, Recognize whether a public-supply well could benefit from a deeper completion)

Knowing that widespread pumping by large-capacity wells from a common depth can drive contaminants downward into the production zone can help resource managers recognize when a public-supply well could benefit from a deeper completion. Of course, the possibility of contamination from deep, natural sources should be considered.

11:10 (Photo of water sampling) (Words on the slide, Identify public-supply wells that may be particularly vulnerable to downward-migrating contaminants)

Comparing the chemistry of water from confined aquifer public-supply wells to the chemistry of water in an overlying aquifer can help public-supply well managers identify those wells that may be particularly vulnerable to downward migrating contaminants.

11:25 (Video of Sandra speaking)

The more the science behind public-supply well vulnerability to contamination is factored into decision making, the more likely desired outcomes will be achieved.

Thanks for watching.

11:38 (Music)

(Photo of sun setting behind a drilling rig)

11:44 (Music)

(Photo of kids filling water bottles at a kitchen sink)

(Ralph Haefner, USGS Supervisory Hydrologist speaking)

The quality of drinking water from the Nation’s public-water systems is regulated by the U.S. Environmental Protection Agency under the Safe Drinking Water Act. USGS studies on public-supply well vulnerability to contamination complement drinking water monitoring required by federal, state, and local programs, which primarily focus on post-treatment monitoring.

12:03 (Music)

Visit our website

To learn more about USGS groundwater-quality research and for links to a fact sheet and other publications about each of the studies, visit our Website

12:13 ( USGS CoreCast Movie and Music )

12:21 End


Title: Why Some Public-Supply Wells are More Vulnerable to Contamination Than Others

Description: This video discusses how scientists have tracked what, when, and how contaminants may reach public-supply wells in four aquifers in California, Connecticut, Nebraska, and Florida.

Location: USA

Date Taken: 2/11/2010

Length: 12:21

Video Producer: Donna Runkle , U.S. Geological Survey

Note: This video has been released into the public domain by the U.S. Geological Survey for use in its entirety. Some videos may contain pieces of copyrighted material. If you wish to use a portion of the video for any purpose, other than for resharing/reposting the video in its entirety, please contact the Video Producer/Videographer listed with this video. Please refer to the USGS Copyright section for how to credit this video.

Additional Video Credits:
Produced by Donna Runkle, Stephanie Janosy, and Sandra Eberts

Details, factsheets, publications and more can be found by visiting the Understanding Contaminant Occurence in Public-Supply Wells site.

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