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As part of the U.S. Geological Survey's National Water-Quality Assessment of the lower Tennessee River Basin, 32 wells and 2 springs were sampled in the summer of 1999 to characterize the water quality of the Mississippian carbonate aquifer in the Eastern Highland Rim. This karst aquifer is unconfined and relatively shallow (mean depth of sampled wells 90 feet), making it susceptible to contamination from surface sources. Withdrawals for water supply in 1995 totaled about 40 million gallons per day, of which about 5 million gallons per day were withdrawn for domestic use.
Field water-quality data and inorganic constituent data indicate that a variety of ground-water flow systems were represented in the sampled wells. Wells completed in shallow flow systems in gravels overlying bedrock or in interconnected conduits were typified by samples with low specific conductance, 40 microsiemens per centimeter at 25 degrees Celsius (µS/cm), and high dissolved oxygen concentrations, 8 milligrams per liter (mg/L). Wells completed in deeper flow systems or parts of the aquifer with longer ground-water residence times were typified by samples with high specific conductance (1,290 µS/cm) and low dissolved oxygen (1 mg/L). Fecal-indicator bacteria were present in samples from 12 wells. The highest concentration of 340 colonies per hundred milliliters occurred in a well with a potential point source of bacteria nearby. Nitrate concentrations ranged from less than 0.05 mg/L to about 16 mg/L. Twelve wells with a range of depths had nitrate concentrations less than 1 mg/L, suggesting that ambient concentrations of nitrate in this aquifer are low. More than half of the wells had nitrate concentrations above 2 mg/L, which may indicate a contribution of nitrogen from anthropogenic sources at these wells. Samples from two wells, located near potential point sources of nitrogen, exceeded the drinking water standard of 10 mg/L.
U.S. Department of the Interior, U.S. Geological Survey
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