As part of the National Water-Quality Assessment
Program of the U.S. Geological Survey, an assessment of
60 volatile organic compounds (VOCs) in untreated,
ambient groundwater of the conterminous United States
was conducted based on samples collected from 2948 wells
between 1985 and 1995. The samples represent urban
and rural areas and drinking-water and nondrinking-water
wells. A reporting level of 0.2 μg/L was used with the
exception of 1,2-dibromo-3-chloropropane, which had a
reporting level of 1.0 μg/L. Because ambient groundwater
was targeted, areas of known point-source contamination
were excluded from this assessment. VOC concentrations
generally were low; 56% of the concentrations were
less than 1 μg/L. In urban areas, 47% of the sampled wells
had at least one VOC, and 29% had two or more VOCs;
furthermore, U.S. Environmental Protection Agency drinking-water criteria were exceeded in 6.4% of all sampled
wells and in 2.5% of the sampled drinking-water wells. In
rural areas, 14% of the sampled wells had at least one
VOC; furthermore, drinking-water criteria were exceeded
in 1.5% of all sampled wells and in 1.3% of the sampled
drinking-water wells. Solvent compounds and the fuel
oxygenate methyl tert-butyl ether were among the most
frequently detected VOCs in urban and rural areas. It was
determined that the probability of finding VOCs in untreated
groundwater can be estimated on the basis of a logistic
regression model by using population density as an
explanatory variable. Although there are limitations to this
national scale model, it fit the data from 2354 wells used
for model development and adequately estimated the VOC
presence in samples from 589 wells used for model
validation. Model estimates indicate that 7% (6−9% on
the basis of one standard error) of the ambient groundwater
resources of the United States probably contain at least
one VOC at a reporting level of 0.2 μg/L. Groundwater is used
in these areas by 42 million people (35−50 million based
on one standard error); however, human exposure to VOCs
from this ambient groundwater is uncertain because the
quality of the finished drinking water is generally unknown.
High quality water is more than the dream of the conservationists, more than a political slogan; high quality water, in the right quantity at the right place at the right time, is essential to health, recreation, and economic growth."
Assessment of temporal trends in national ground-water quality networks are rarely published in scientific journals. This is partly due to the fact that long-term data from these types of networks are uncommon and because many national monitoring networks are not driven by hypotheses that can be easily incorporated into scientific research. The U.S. Geological Survey (USGS) National Water-Quality Assessment Program (NAWQA) since 1991 has to date (2006) concentrated on occurrence of contaminants because sufficient data for trend analysis is only just becoming available. This paper introduces the first set of trend assessments from NAWQA and provides an assessment of the success of the program. On a national scale, nitrate concentrations in ground water have generally increased from 1988 to 2004, but trends in pesticide concentrations are less apparent. Regionally, the studies showed high nitrate concentrations and frequent pesticide detections are linked to agricultural use of fertilizers and pesticides. Most of these areas showed increases in nitrate concentration within the last decade, and these increases are associated with oxic-geochemical conditions and well-drained soils. The current NAWQA plan for collecting data to define trends needs to be constantly reevaluated to determine if the approach fulfills the expected outcome. To assist this evaluation, a comparison of NAWQA to other national ground-water quality programs was undertaken. The design and spatial extent of each national program depend on many factors, including current and long-term budgets, purpose of the program, size of the country, and diversity of aquifer types. Comparison of NAWQA to nine other national programs shows a great diversity in program designs, but indicates that different approaches can achieve similar and equally important goals.
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