AN INTEGRATED APPROACH TO ASSESSING PUBLIC SUPPLY WELL VULNERABILITY IN FRACTURED SILICICLASTIC AQUIFER SYSTEMS

Approximately one third of the U.S. population uses public supply wells as their drinking water source and recently a greater focus has been placed on assessing the risk of contaminants entering these wells. A variety of methods can be used to better understand the primary mechanisms that control the transport of near-surface contaminants such as wastewater in bedrock multi-aquifer systems and to develop effective methods for assessing the vulnerability of wells in such settings. The City of Madison, Wisconsin obtains water from a fractured siliciclastic aquifer system using multiple wells that may be vulnerable to contamination.

Our experience in Madison shows that a combination of geomechanical, chemical, and microbiological methods may be necessary to assess the vulnerability of public supply wells to near-surface contaminants. Geomechanical methods involving the use of straddle packers and other borehole instruments suggest that fractures in siliciclastic bedrock are important transport pathways from the surface to the deep aquifer. In these settings, fractures may have an important role in the transport of near-surface contaminants into wells. Reverse water-level fluctuations (RWFs), a phenomenon in which water levels in wells open to one aquifer rise briefly in response to pumping from another aquifer, have been detected at radial distances of hundreds of meters from a pumping well. This suggests that the RWFs propagate rapidly through fractures to influence flow in the non-pumped aquifer. Groundwater sampling to detect chemical and microbiological wastewater indicators is a useful tool for characterizing transport within the fractured aquifer system. In order to accurately assess the vulnerability of public supply wells to near surface contaminants it is important to characterize fracture flow, contaminant transport, and the impact of well pumping using an integrated approach.