From: Andy Soos, ENN
Published February 12, 2010 11:23 AM

How Ground Water Contamination Spreads

Why are some wells contaminated and some are not?

All wells are not equally vulnerable to contamination because of differences in three factors: the general aquifer chemistry, groundwater age, and paths within aquifer systems that allow water and contaminants to reach a well. More than 100 million people in the United States receive their drinking water from public groundwater systems, which can be vulnerable to naturally occurring contaminants such as radon, uranium, arsenic, and man made compounds, including fertilizers, septic tank leachate, solvents and gasoline hydrocarbons.


An aquifer is an underground bed or layer of permeable rock, sediment, or soil that yields water. Some aquifers are potable (drinkable) and some are not based on their chemistry (What is dissolved in the water).

Ground water age is a factor of how fast does the water recharge in the aquifer. The recharge rate will affect the aquifer water chemistry.

The path that the ground water takes underground may bypass sources of contamination or it may go through them.

The US Geological Service (USGS) has tracked in a recent study the movement of contaminants in groundwater and in public supply wells in several aquifers in California, Connecticut, Nebraska and Florida. The importance of each factor differs among the various aquifer settings, depending upon natural geology and local aquifer conditions, as well as human activities related to land use and well construction and operation.

“Our findings can help public supply well managers protect drinking water sources by prioritizing their monitoring programs and improving decisions related to land use planning, well modifications or changes in pumping scenarios that might help to reduce movement of contaminants to wells,” said Sandra Eberts, USGS groundwater study team leader.

Highlights of the four USGS studies include:

In the Central Valley aquifer system near Modesto, California, the USGS found that agricultural and urban development have enabled uranium to move from sediments to water in the upper part of the aquifer. This water can drain down the well when it is not pumping and enter the lower aquifer (direct path to the aquifer). When pumping resumes, contaminant concentrations can be temporarily elevated in water pumped from the well. As a result of USGS findings, public supply well managers have changed their pumping schedule, which has reduced the amount of contaminated water pumped from the well.

In the glacial aquifer system in Woodbury, Connecticut, the USGS found that the young age of the water throughout the aquifer makes it vulnerable to contamination from man made compounds. The USGS also found that dry wells used in Woodbury to capture storm water runoff reroute the potentially contaminated water directly into the aquifer used as a drinking water source. This direct transfer prevents soil and unsaturated sediments near the land surface from filtering out some of the contaminants.

In the High Plains aquifer near York, Nebraska, the USGS found some contaminants in a public supply well that seems protected by overlying clay. Nearby irrigation wells have allowed water containing nitrate and volatile organic compounds to leak down from an overlying shallow aquifer into the aquifer that serves as the drinking water source for the public supply well. A direct path to the aquifer from a source of contamination had been made.

In the Floridan aquifer system near Tampa, Florida, the USGS found that a large percentage of young water and contaminants from a shallow sand aquifer travels quickly along natural paths or conduits until it reaches a supply well in a lower rock aquifer that serves as a drinking water source. Because of these natural conduits, the supply well is vulnerable to the man made contaminants in the upper aquifer, and the mixing of waters from the two aquifers has caused arsenic concentrations to increase in water reaching the supply well.

What the USGS studies have shown is how easy it is to contaminate the underlying aquifers. Another analogy is how a roof leak will move from the original ingress point and show up as a drippage a long way from the original leak. It takes a careful study of all of the factors and access paths to find the root cause of ground water contamination.

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