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Volatile Organic Compounds in Surface and Ground Water in the Upper Mississippi River Basin, Minnesota and Wisconsin, 1978-94

By Andrews, W.J.

As part of a retrospective analysis of water-quality data for the Upper Mississippi River Basin study area of the National Water-Quality Assessment Program, the U.S. Geological Survey collected and summarized analyses of volatile organic compounds in ground and surface water from water-quality data bases maintained by federal, state, and local agencies. The principal purposes of the Program are to define ambient water quality on regional and national scales and to evaluate the effects of land use on water quality and aquatic biota. The Upper Mississippi River Basin study area includes all of the Twin Cities Basin, the northern part of the Hollandale Embayment, and the westernmost part of Wisconsin in the St. Croix River Basin. Water-quality data collected form 1978-94 by the U.S. Geological Survey, the Minnesota Department of Health, the Minnesota Pollution Control Agency, Metropolitan Council Wastewater Services, and the Wisconsin Department of Natural Resources were included in this retrospective analysis.

Analysis of the concentrations of volatile organic compounds in water samples collected form streams and wells in the study area led to the following conclusions:

• Trace amounts of chlorinated alkanes, such as trichloroethylene, were detected sporadically in samples from the Mississippi, Minnesota, and Vermillion Rivers.
  
• Alkyl benzenes, such as ethyl benzene and meta-xylene, were detected sporadically in samples from the chain of lakes used as the municipal supply for St. Paul, Minnesota.
  
• Commonly used chlorinated alkanes and alkyl benzenes were detected at low concentrations, generally near detection limits from less than 1 to 3 micrograms per liter, in 5 percent or less of the samples.
  
• Volatile organic compounds generally had slightly greater frequencies of detection and greater concentrations in samples from wells completed in glacial-drift aquifers than in samples from wells completed in bedrock aquifers, which were generally deeper.
  
• Chlorination of water significantly increases the frequency of detection of trihalomethane volatile organic compounds, such as chloroform and bromodichloromethane.

The small frequency of detection of volatile organic compounds in surface and ground water sampled form widely-distributed networks of sampling stations and wells in the study area indicates that, although there are thousands of sites which potentially emit these compounds to water and to the atmosphere, these compounds have not substantially affected the quality of surface and ground water in the study area.