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From: Pixie A Hamilton [mailto:[log in to unmask]] 
Sent: Friday, June 26, 2009 5:13 PM
Subject: New USGS Release: Water Quality in Carbonate Aquifers Across
the Nation

***Please distribute to staff and other colleagues**** 

USGS released a new report, Factors affecting water quality in selected
carbonate aquifers in the United States, 1993-2005 (available at
http://water.usgs.gov/nawqa/pubs/carbonate/ with maps and other
companion materials). 

USGS scientists sampled for 151 chemical constituents or physical
properties in about 1,000 wells and springs across 20 states, mainly in
the eastern and central United States. The majority of the wells sampled
in the study are used as drinking water sources, either for domestic or
public supply. Therefore, these results are particularly relevant to
drinking-water quality issues. Other sampled wells not used for drinking
water included livestock wells, irrigation wells, and monitoring wells.
Carbonate aquifers are the largest sources of drinking water for public
supply of any bedrock aquifer, providing about 20 percent of the
groundwater supplied as drinking water to the Nation. 

In general, findings show that carbonate aquifers provide water of
acceptable quality for human use and consumption in the majority of
wells sampled across the U.S. With few exceptions, chemicals detected in
groundwater from carbonate aquifers were low, generally below
human-health benchmarks. Radon and nitrate were among the few
contaminants with elevated concentrations in samples from wells tapping
these important aquifers. 

Results showed that many of the carbonate aquifers have natural
features, such as confining clay layers, that protect the aquifer, and
thus the concentrations of contaminants can vary greatly. Carbonate
aquifers with features such as sinkholes, caves, and porous rocks are
particularly vulnerable to contamination, particularly aquifers located
in intensively farmed areas, and contaminant levels in a few of these
areas are among the highest in the Nation. 

Other selected highlights: 

Nitrate-mostly derived from man-made sources such as from fertilizer
applications, animal manure application, and septic tanks-was the most
commonly-detected contaminant at concentrations greater than the federal
drinking water standard for public-water supplies (10 parts per
million). Concentrations exceeded the federal drinking-water standard in
5 percent of the wells sampled. The vast majority of the samples that
exceeded the standard for nitrate were in the Piedmont and the Valley
and Ridge aquifers, which exceeded the standard in 63 and 14 percent of
the wells, respectively. The high levels were due to a combination of
the ease of contaminant transport and agricultural land use in those two
areas. 

USGS findings show that the types and concentrations of selected
contaminants in groundwater in carbonate aquifers are closely related to
land use, such as fertilizers, pesticides, and volatile organic
compounds (or VOCs). For example, concentrations of nitrate were
significantly higher in groundwater underlying agricultural land than in
groundwater underlying undeveloped or urban land. Herbicides were
detected more frequently in agricultural wells, whereas insecticides and
VOCs such as chloroform were more frequently detected in urban wells.
Only 2 of the 47 pesticides analyzed exceeded human-health benchmarks in
20 sites and 4 of the 59 VOCs in 5 sites analyzed exceeded federal
drinking-water standards. 

Findings also show that factors other than land use can affect
groundwater quality. For example, natural geochemistry is a factor
influencing radon occurrence. Radon concentrations exceeded the proposed
drinking-water standard of 300 picocuries per liter in 58 percent of the
samples where radon was analyzed. Natural factors controlling aquifer
confinement, groundwater residence times, and the presence of organic
carbon can help to minimize the transport of contaminants to an aquifer
or enhance degradation of contaminants to innocuous forms prior to
entering wells. 

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