HYDROGEOLOGIC CHARACTERIZATION OF FRACTURED DOLOMITE FOR REGULATORY MONITORING – A MULTIPLE METHOD APPROACH

Development of long-term monitoring plans for carbonate aquifers requires evaluation of flow in highly permeable fractures and in low permeability matrix. The feasibility study for expanding a solid waste landfill in NE Wisconsin used bedrock coring, down-hole geophysical and flow-meter logging, single- and multiple-well, aquifer-response tests, and head measurements at multiple-well nests to evaluate groundwater flow in the ~140’ thickness of Silurian dolomite aquifer present at the site. Results from the study indicate high permeability at the top of the dolomite, with relatively minor permeability deeper in the dolomite.

Bedrock cores and geophysical logs indicate extensively fractured and weathered dolomite just below the contact with the overlying glacial sediments. Except for the uppermost rock, the geophysical logs (spontaneous potential, single point and normal resistivity) indicate little variability in the rock, which is compatible with observations from cores and optical televiewer logs. Gamma readings are low in the uppermost (oxidized or otherwise geochemically altered) dolomite, and are higher in deeper more shaley zones or zones with more abundant styolites.

The estimated transmissivity for the upper 25’ of dolomite, based on the results of the multiple-well pumping test, is 23,000 ft²/day. The estimated transmissivity for the upper 25’ of dolomite based on the borehole flow logging (geometric mean for 4 boreholes) is 11,000 ft²/day. The higher result for the pumping test likely reflects the larger scale of the test. Flow metering at the 4 boreholes indicates upward flow within the uppermost 25’ of dolomite. The uppermost 25’ of dolomite was selected for installation of wells for the long-term monitoring program, including “D” wells screened in the top of the dolomite and “E” wells installed about 10’ deeper. Upward flow in the upper bedrock and a shallow base of the local flow system are also indicated by water quality data from the “D” and “E” wells, which are significantly different for parameters related to agricultural land use and road salt, such as nitrates and chloride. The multiple investigative methods produced scale-dependent but compatible results indicating the highly transmissive uppermost dolomite as an important focus for long-term monitoring of bedrock groundwater quality.