MAPPING REGIONAL FLOW SYSTEMS IN MINNESOTA WITH NATURAL AND ANTHROPOGENIC TRACERS -- IMPLICATIONS FOR SUSTAINABLE GROUND WATER RESOURCE MANAGEMENT

For a number of years, the Minnesota Department of Natural Resources in collaboration with the Minnesota Geological Survey, has conducted ground water studies in support of the County Geologic Atlas and Regional Hydrogeologic Assessment series of publications. To better understand the physical hydrogeology and ground water flow systems of the study areas, about 80 to 100 water chemistry samples per study area are analyzed for general chemistry, trace elements, tritium, and at selected sites, carbon-14. At some locations, stable isotopes of oxygen and hydrogen are analyzed to assess the interaction of ground water with surface water.

Using GIS techniques, the data are plotted both in plan view and on numerous geologic cross sections. Both natural and anthropogenic chemical constituents are utilized as ground water flow system tracers, especially tritium, chloride, nitrate, bromide, and boron. With these data, local or shallow systems with detectable tritium and evidence of anthropogenic impacts can be differentiated from regional or deeper systems with no detectable tritium and no evidence of anthropogenic impact. The regional or deeper flow systems that have been sampled for carbon-14 have ages that vary in range from a few hundred to a few tens of thousands of years before present. Old ground waters are widely used for individual and community water supplies.

The hydrologic conditions interpreted by these studies can be directly used by water resource managers to understand aquifer systems and potential constraints and to help develop long-term (i.e., sustainable) management strategies.