Carbonate aquifers provide water of acceptable quality for human use and consumption in the majority of wells sampled across the country, according to a recent U.S. Geological Survey (USGS) study.
With few exceptions, chemicals detected in ground water 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.
Carbonate aquifers with features such as sinkholes, caves
and porous rocks are vulnerable to contamination, particularly aquifers located
in intensively farmed areas. Contaminant levels in a few of these areas are
among the highest in the nation.
Carbonate aquifers are the largest sources of drinking water
for public supply of any bedrock aquifer, providing about 20 percent of the
ground water supplied as drinking water in the United States. Scientists
sampled for 151 chemical constituents or physical properties at about 1,000
wells and springs in these aquifers, which consist of rocks such as limestone
or dolomite, across 20 states, mainly in the eastern and central United States.
The report, “Factors affecting water quality in selected
carbonate aquifers in the United States, 1993-2005,” is available on-line at http://water.usgs.gov.
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).
“Nitrate is one contaminant to continue to monitor because
concentrations exceeded the federal drinking-water standard in 5 percent of the
wells sampled,” says USGS scientist Bruce Lindsey. “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.”
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.
USGS findings show that the types and concentrations of
selected contaminants in ground water in carbonate aquifers are closely related
to land use, such as fertilizers, pesticides and volatile organic compounds
(VOCs). For example, concentrations of nitrate were significantly higher in
ground water underlying agricultural land than in ground water 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 two of the 47 pesticides analyzed
exceeded human-health benchmarks in 20 sites, and four of the 59 VOCs in five
sites analyzed exceeded federal drinking-water standards.
Findings also show that factors other than land use can
affect ground water 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, ground water 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.
Carbonate aquifers sometimes have sinkholes and caves in karst
areas. Bedrock in karst aquifers dissolves relatively easily, providing voids
that can store large volumes of water and transmit large volumes of water to
wells. These aquifers, therefore, are highly productive, but also vulnerable to
contamination. Results showed that many of the aquifers have natural features,
such as confining clay layers, that protect the aquifer, and thus the
concentrations of contaminants can vary greatly.
The Edwards-Trinity aquifer system in Texas and the Floridan
aquifer system in several southeastern states are important water supplies, and
also are well known for their karst features.
benchmarks used in the study included drinking-water standards for contaminants
regulated under the Federal Safe Drinking Water Act and non-enforceable USGS
Health-Based Screening Levels (HBSLs) for unregulated contaminants. In relating
measured concentrations to health benchmarks, this study offers a preliminary
assessment of potential health concerns that identifies conditions that may
require further investigation, but the research is not a substitute for
comprehensive risk and toxicity assessments. USGS studies are ongoing in most
of the areas included in this report.
Contaminant Levels Generally Low in Most U.S. Carbonate Aquifers
July 13, 2009