MONITORING WELL DESIGN IN FRACTURED SILURIAN DOLOMITE IN DOOR COUNTY, WI

Fractured rock poses special problems when designing monitoring systems. In the Silurian dolomite in Door County, WI, groundwater flow and contaminant transport are dominated by interconnected fracture networks. A good monitoring system should intersect the dominant flows and fractures without creating new connections between different fracture networks.

We are studying potential indicators of agricultural and septic waste in groundwater that discharges to wetlands in Door County, WI. One of our sites has a monitoring well located midway between potential sources and a large spring complex that discharges into the Mink River wetland. We found that the water chemistry as indicated by fluid conductivity of the wetland spring discharge (675-613 microS/cm) was significantly different from the well water (490 microS/cm). This led us to consider whether the monitoring well was in the same flow path as the spring discharge. We used borehole geophysics, flow logging, and measurements of water chemistry to improve design of the monitoring well. We found that there were two dominant fracture flow systems, one shallow and one deep. The deep fracture system had higher head than the shallow system so that groundwater flowed out of the deep fracture system, upwards in the well bore and into the shallow fracture system. We rebuilt the well so that the two fracture systems were separated. We placed two piezometers in the well boring, one screened across the shallow fracture system and one screened across the deep fracture system and separated by grout. The fluid conductivity in the shallow piezometer (659 microS/cm) now more closely matches that seen in the spring discharge (675-613 microS/cm) while the deep piezometer remains significantly different (544 microS/cm). We conclude that the shallow piezometers are now more likely to represent flows midway between the sources and the springs.

This study illustrates the need to consider that there might be different fracture networks intersected by a single boring when installing monitoring wells. Tools such as borehole geophysics, flow logging, and water chemistry measurements should be applied when designed monitoring wells in fractured rock.