CHARACTERIZATION OF LATERALLY EXTENSIVE, HIGH PERMEABILITY ZONES WITHIN THE TUNNEL CITY GROUP IN SOUTHERN WISCONSIN

Laterally extensive, high permeability zones in the Tunnel City Group appear to cause preferential groundwater flow in the Cambrian sandstone aquifer in southern Wisconsin. These zones are thought to exert a major influence on groundwater discharge to spring complexes and may be the cause of extensive and rapid contaminant plume migration at a chemical spill site. The thin zones (less than approximately one meter) within the Tunnel City Group are attributed to conglomeratic beds, bedding plane partings, erosional contacts, and/or horizontal fractures in the glauconitic sandstone. We are using geophysical logs, permeability measurements, and borehole flowmeter logs to describe the extent and quantify the properties of the high permeability features within the aquifer system. Comparison of geophysical logs from multiple bedrock monitoring wells in the region shows correlations in the natural gamma, single point resistance, and spontaneous potential logs. This analysis demonstrates that lithologic properties of the Tunnel City Group are laterally continuous; individual zones, which are as thin as 0.3 to 0.6 meters, can be identified among wells that are located tens of kilometers apart. Because lithologic properties are laterally continuous within the unit, hydrogeologic properties may also be laterally continuous. A straddle packer assemblage was used to conduct slug tests at discrete intervals (approximately one meter) within the Tunnel City Group and the Wonewoc Formation at several sites within Dane County. Test results show that zones of high hydraulic conductivity exist adjacent to zones of relatively low hydraulic conductivity without zones of transition. While both the Tunnel City Group and the Wonewoc Formation have high hydraulic conductivity zones, the features within the Tunnel City Group appear to result in preferential groundwater flow. Many of the high hydraulic conductivity zones in the Tunnel City Group correlate with sharp changes in fluid temperature and fluid resistivity and/or sharp changes in flow rate in flowing wells. High resolution borehole flowmeter logging has similarly identified gradient reversals in non-flowing wells. These changes can be qualitative indicators of zones of higher relative permeability or discrete features that allow preferential groundwater flow.