Hydrogeology and Groundwater Geochemistry in Fractured Dolomite, Northeastern Wisconsin

Abstract: The fractured Silurian dolomite of northeastern Wisconsin falls about midway along a continuum between diffuse-flow fractured aquifers and true karst and exhibits some characteristics of each environment, as shown by a field study of spatial and temporal variations in hydraulic head and geochemical parameters in Door County, Wisconsin.

At a regional scale, repeated sampling of 65 wells showed rapid and sometimes unpredictable spatial and temporal variations in concentrations of indicator parameters. Horizontal groundwater movement can be rapid, and indicator parameters can be traced up to 2.7 mi (4.4 km) following a large contamination event.

At a local scale, data from standpipe piezometers along a potentiometric divide suggest that the common practice of constructing water-table maps using existing supply wells in fractured dolomite may lead to incorrect interpretations of the configuration of the water table and the thickness of the unsaturated zone. Vertical hydraulic gradients are relatively large. Aquifer tests suggest that the aquifer is highly anisotropic at the piezometer site, but tritium and ¹⁸0 data indicate fairly rapid recharge and vertical groundwater movement. Temperature logs appear to be particularly useful in delineating zones of rapid groundwater movement in boreholes.