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National Water-Quality Assessment Program Pesticides and Nitrate in Surficial Sand and Gravel Aquifers as Related to Modeled Contamination Susceptibility in Part of the Upper Mississippi River Basin
Map
showing ground-water susceptibility (80 kb)
Map showing pesticides in samples from wells completed in surficial sand and gravel aquifers (121 kb) Map showing nitrate in samples from wells completed in surficial sand and gravel aquifers (127 kb) Bar chart showing percent of land cover types in high contamination susceptibility areas (21 kb) The occurrence of pesticides and nitrate (nitrite plus nitrate as nitrogen) in surficial sand and gravel aquifers in parts of Minnesota and Wisconsin was summarized as part of an analysis of historical water-quality data for the Upper Mississippi River Basin study unit of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program. Water samples were collected by State and Federal agencies for pesticides (366) and nitrate (410) between 1971 and 1994 from wells completed in surficial sand and gravel aquifers. State agencies in Minnesota and Wisconsin have developed models to determine areas where ground water is susceptible to contamination based on geologic and hydrologic conditions (Schmidt, 1987, and Porcher, 1989). Water-quality data is evaluated with respect to the ground-water susceptibility models. The results also are evaluated with respect to overlying land use and land cover. Samples from wells with detectable levels of one or more pesticides or nitrate concentrations exceeding the U.S. Environmental Protection Agency's (USEPA) Maximum Contaminant Level (MCL) of 10 milligrams per liter (mg/L) generally coincided with areas of high contamination susceptibility. Furthermore, samples from wells located in areas of high contamination susceptibility had pesticide detection frequencies and nitrate concentrations that correlated to overlying land use and land cover. Samples from wells located in high susceptibility areas that were surrounded by cropland had greater pesticide detection frequencies and greater nitrate concentrations than wells located in similar susceptibility areas but surrounded by different land use and land cover types such as forest, urban, and wetlands. Ground Water as a Source of Drinking WaterGround water is the source
of drinking water for 75 and 70 percent of the populations of Minnesota
and Wisconsin, respectively (Kroening and Andrews, 1997). For many rural
communities, ground water is the sole source of drinking water (Nolan and
Ruddy, 1996).
Ground water exists in pores
between particles in subsurface sediment or rock units. Sediment or rock
units that yield significant quantities of water are termed "aquifers".
Permeable
sand and gravel, deposited as glacial outwash or as alluvium, can serve
as important reservoirs for ground water (sand and gravel aquifers). If
sand and gravel aquifers are not overlain by a confining layer, they are
termed "surficial" sand and gravel aquifers. Surficial sand and gravel
aquifers provide much of the drinking water for the Upper Mississippi River
Basin study unit. These aquifers are relatively shallow and easily accessible.
Surficial sand and gravel aquifers are also areas in which contamination
susceptibility is high because both water and contaminants can move quickly
through these permeable materials (Schmidt, 1987, and Porcher, 1989). Identifying
potential areas of high contamination susceptibility can increase water-resource
managers' and planners' awareness of potential ground-water contamination.
The Upper Mississippi River Basin NAWQA Study UnitOccurrence of pesticides and
nitrate in ground water was summarized as part of an analysis of historical
water-quality data (Fallon and others, 1997, and Kroening and Andrews,
1997) for the Upper Mississippi River Basin study unit of the U.S. Geological
Survey's National Water-Quality Assessment (NAWQA) Program. The water-quality
data were collected and are maintained by Federal, State, and local agencies
(Minnesota Department of Agriculture, Minnesota Department of Health, Minnesota
Pollution Control Agency, Wisconsin Department of Natural Resources, and
U.S. Geological Survey).
The Upper Mississippi River
Basin study unit encompasses the drainage of the Mississippi River upstream
of the outlet of Lake Pepin (Stark and others, 1996). The analysis of historical
water-quality data for ground water was limited to wells completed in the
principal aquifers in a 19,500-square mile area in the eastern part of
the study unit: unconfined sand and gravel aquifers, buried sand and gravel
aquifers, and the Prairie du Chien-Jordan aquifer. The evaluation presented
by this Fact Sheet is based on data for wells completed in surficial unconfined
sand and gravel aquifers.
Ground-Water Contamination Susceptibility ModelsMinnesota and Wisconsin have
developed ground-water contamination susceptibility models to determine
areas where ground water is susceptible to contamination based on geologic
and hydrologic conditions such as soil permeability, subsurface material,
and depth to water table (Schmidt, 1987, and Porcher, 1989). The models
were designed to access the ease with which the infiltration of contaminants
could occur between the land surface and the water tables. The susceptibility
models may help agencies develop suitable plans and coordinate regulatory
and monitoring efforts to aid in maintaining clean water supplies. Although
the geologic and hydrologic variables and model-development methods differed
between the Minnesota and Wisconsin models, the indicated relative susceptibilities
for similar settings appear to be comparable.
ImplicationsPesticide detections and nitrate
concentrations found in water samples from wells completed in surficial
sand and gravel aquifers relate to land cover and land-use activities.
Although geologic and hydrologic conditions influence the water quality
of surficial aquifers, contamination in the highest susceptibility areas
appear to be associated with human activities. Pesticide detections and
nitrate concentrations exceeding USEPA's MCL increase in areas of agricultural
land cover (cropland) compared to areas of other land use/land cover types.
The dominant source of pesticides and nitrates detected in ground water
are from the application of pesticides and fertilizers (respectively) on
agricultural
crops (Fallon and others, 1997, and Kroening and Andrews, 1997).
Although land cover defined
as mostly cropland constitutes less than 10 percent of all land cover overlying
high risk areas, nearly 40 percent of all pesticide detections and increased
nitrate concentrations occur in these areas. Thus, ground-water contamination
is greatest in agricultural areas where geologic and hydrologic conditions
indicate high susceptibility to contamination.
SummaryPesticide detections and nitrate
concentrations in water samples from surficial sand and gravel aquifers
in part of the Upper Mississippi River Basin were evaluated with respect
to modeled areas of ground-water contamination susceptibility and to land
use/land cover. In general, water samples with detectable levels of one
or more pesticides or nitrate concentrations greater than the USEPA's MCL
of 10 milligrams per liter, coincided with areas of high ground-water contamination
susceptibility. Wells located in high susceptibility areas surrounded by
mostly cropland had relatively high detection frequencies of pesticides
and nitrate concentrations exceeding 10 milligrams per liter compared to
wells located in areas of other land cover types. Although geologic and
hydrologic variables influence the susceptibility of ground water to contamination,
land cover and human land-use activities also influence the susceptibility.
References
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