New United States Geological Survey (USGS) ground water studies explain what, when and how contaminants may reach public supply wells.
All wells are not equally vulnerable to contamination
because of differences in three factors – the general chemistry of the aquifer,
ground water age, and direct 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 ground water 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
The USGS tracked the movement of contaminants in ground
water and in public-supply wells in four 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
“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,”
says Sandra Eberts, USGS ground water study team leader.
Highlights of the Four Studies:
In the Central Valley aquifer system near Modesto, Calif., 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. 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, Conn., 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, Neb., 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
the Floridan aquifer system near Tampa, Fla., the USGS found that a large
percentage of young water and contaminants from a shallow sand aquifer travels
quickly along natural 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.
Studies Reveal Why Water Wells are Vulnerable to Contamination
February 12, 2010