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 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.

Occurrence 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.

Minnesota 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.

Pesticide 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.

Pesticide 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

• Fallon, J.D., Fong, A.L., and Andrews, W.J., 1997, Water-quality assessment of part of the Upper Mississippi River Basin, Minnesota and Wisconsin-Pesticides in streams, streambed sediment, and ground water, 1974-94, 53 p.
• Kroening, S.E. and Andrews, W.J., 1997, Water-quality assessment of part of the Upper Mississippi River Basin, Minnesota, and Wisconsin-Nitrogen and phosphorus in streams, streambed sediment, and ground water, 1971-94:U.S. Geological Survey Water-Resources Investigations Report 97-4107, 61 p.
• Nolan, B.T., and Ruddy, B.C., 1996, Nitrate in Ground Waters of the United States-Assessing the risk: U.S. Geological Survey Fact Sheet 092-96, 4 p.
• Porcher, E.N., 1989, Ground water contamination susceptibility in Minnesota: Minnesota Pollution Control Agency, 1 sheet.
• Schmidt, R.R., 1987, Wisconsin's groundwater management plan report no. 5-Groundwater contamination susceptibility in Wisconsin: Wisconsin Department of Natural Resources, PUBL-WR-177-87, 27 p., 1 sheet.
• Stark, J.R.,Andrews, W.J., Fallon, J.D., Fong, A.L., Goldstein, R.M., Hanson, P.E., Kroening, S.E., and Lee, K.E., 1996, Water-quality assessment of part of the Upper Mississippi River Basin, Minnesota and Wisconsin-Environmental setting and study design: U.S. Geological Survey Water-Resources Investigations Report 96-4098, 62 p.

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