NITRATE-NITROGEN LOSSES ALONG THE DECORAH SHALE SUBCROP, ROCHESTER MINNESOTA

In southeastern Minnesota, ground waters from the Upper Carbonate aquifer discharge along the subcrop of the underlying Decorah Shale, often forming wetlands. Ground water entering the wetlands often have nitrate-nitrogen (nitrate) concentrations exceeding the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 mg/L. As these waters flow through the wetlands, nitrate concentrations decrease. Total nitrate removal typically occurs before the water reaches the contact between the Decorah shale and the underlying Platteville Formation. Potential causes for nitrate loss include denitrification, ammonification, redox reduction, and dilution. The U.S. Geological Survey in cooperation with Rochester Public Utilities is evaluating the causes of these nitrate losses in a wetland lying on top of the Decorah shale in Rochester. Woodlands and former agricultural lands upgradient of the wetland are currently being developed for housing. Monitoring wells were installed upgradient and downgradient of the wetland and shallow piezometers were installed within the wetland to monitor changes in water quantity and quality of the Upper Carbonate aquifer, the Platteville Formation, and the St. Peter Sandstone aquifer. Water from the monitoring wells and piezometers collected in April, May, June, August, and October 2004 were analyzed for major chemical constituents, nutrients, and dissolved gases. Nitrate concentrations in water from the Upper Carbonate aquifer upgradient of the wetland were greater than 10 mg/L and decreased over the sampling period from 17.6 mg/L, December 2003 to 14.8 mg/L, October 2004. Nitrate concentrations within the wetland, along the upgradient edge, ranged from 0.5 mg/L to 10.5 mg/L. Nitrate concentrations along the entire downgradient edge of the wetland were less than 1 mg/L as nitrogen. Ambient denitrification rates in the soil samples in the wetland ranged from 0 to 0.45 µg nitrogen/cm2/day.