Ground-Water Quality in the Prairie du Chien-Jordan/Trempealeau Aquifer in Minnesota and Wisconsin, 1996
By Fong, A.L., Andrews, W.J., and Stark, J.R.
Abstract
Published in Proceedings of the 42st Annual Midwest Ground Water Conference, held at Coralville, IA; October 22-24, 1997.
The Prairie du Chien-Jordan aquifer (Prairie du Chien-Trempealeau aquifer in Wisconsin) in Minnesota and Wisconsin is the principle source of water for domestic wells and municipal wells in suburban communities surrounding the Twin Cities metropolitan area. Ground-water samples were collected from 50 domestic wells completed in this aquifer in the summer of 1996 as part of the U.S. Geological Survey's National Water-Quality Assessment Program. The purpose of this study is to characterize the quality of water in the aquifer and to characterize the differences in the unconfined and confined portions of the aquifer. Twenty-five wells were sampled in each portion of the aquifer. Water samples from the wells were measured for physical parameters and analyzed for major ions, trace metals, nutrients, pesticides, volatile organic compounds, radon, and tritium.
Wells in both unconfined and confined portions of the aquifer contained water that was hard. Dissolved oxygen concentrations and specific conductance was less in the unconfined portion as compared to the confined portion. Trace-metal concentrations were near or less than the reporting limit of 1 microgram per liter, except for iron, zinc, manganese, and barium. Iron and manganese concentrations commonly exceeded the secondary maximum contaminant levels set by the U.S. Environmental Protection Agency (300 and 50 micrograms per liter, respectively) and were generally greater in the confined portion of the aquifer. Nitrate concentrations were greater in the unconfined portion of the aquifer, where 10 samples exceeded 3 milligrams per liter, and 2 samples exceeded the maximum contaminant level of 10 milligrams per liter. In the confined portion of the aquifer, nitrate in 4 samples esceeded 3 milligrams per liter, but no samples exceeded the maximum contaminant level of 10 milligrams per
liter. Atrazine, and its metabolite deethylatrazine, were the most frequently-detected pesticides in water samples from both the unconfined (56 percent) and the confined (36 percent) portions of the aquifer. Volatile organic compounds were detected in all but 9 of the water samples, but one at concentrations exceeded 1 microgram per liter. Concentrations of volatile organic compounds were slightly greater in the unconfined portion of the aquifer. Carbon disulfide and methyl chloride were the most frequently-detected volatile organic compounds. Radon concentrations were greater in confined portions of the aquifer than in the unconfined portion, with medians of 500 and 340 picoCuries per liter, respectively. Sixty-six percent of the radon concentrations were greater than the suspended maximum contaminant level of 300 picoCuries per liter. Tritium concentrations were greater in the unconfined portion of the aquifer than in the confined portion of the aquifer, with medians of 6.87 and 1.87 tritium units,
respectively, indicating more recent recharge in the unconfined portion of the aquifer.
Water in the unconfined portion of the Prairie du Chien-Jordan aquifer has been affected to a greater degree by human aqctivity than water in the confined portion. Water in the confined portion of the Prairie du Chien- Jordan aquifer has longer residence times and greater concentrations of dissolution products of naturally-occurring materials in the aquifer. In general, wells (unconfined or confined) completed in the Prairie du Chien aquifer have greater frequencies of detection of nutrient, pesticides, and volatile organic compounds than wells completed in the Jordan or Trempealeau aquifers.
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U.S. Department of the Interior |
U.S. Geological Survey
