OFR14-04, Hydrogeologic Properties of the St. Lawrence Aquitard, Southeastern Minnesota

Abstract

To better understand the properties of bedrock aquitards, we initiated a project that utilizes widely ranging methods to gain insights into the properties of the St Lawrence Formation and adjacent strata of the upper Tunnel City Group (Mazomanie and Lone Rock Formations) and lower Jordan Sandstone. The St Lawrence Formation has been traditionally regarded as an aquitard in the Paleozoic bedrock hydrogeologic system.Our investigation addresses hydraulic properties at site-specific as well as sub-regional (square miles) scales. Specific activities included drilling, testing and instrumentation of a borehole in the eastern Twin Cities Metro area (TCMA), the Afton multilevel system (MLS) hole, that has provided us with detailed multi-level measurements of hydraulic characteristics above, below and within the St. Lawrence Formation. We also analyzed the distribution of fractures in bedrock outcrops, in an effort to understand the controls these fractures might have on borehole hydraulic conditions. Borehole and outcrop scale data are augmented with ground-water tracer experiments that provide horizontal and vertical travel times at the sub-regional scale. Compilation of existing published and unpublished hydraulic and water chemistry data provide additional insights into the properties of the St Lawrence Formation and adjacent units across a wider extent of southeastern Minnesota.