USING WATER-QUALITY DATA AS A SCREENING TOOL FOR IDENTIFYING “KARST” WELLS IN WISCONSIN

The 1996 Amendments to the Safe Drinking Water Act require states to conduct source-water assessments for all public water-supply wells. These assessments have focused renewed attention on the problems in delineating source-water areas for wells in fractured carbonate and karst settings. The dual-porosity nature of these aquifers-- high-permeability fractures or conduits transmit the majority of the water while the lower-permeability matrix blocks provide the storage capacity -- make them exceedingly susceptible to contamination, challenging to characterize, and difficult to model. The U.S. Environmental Protection Agency's Technical Guidance documents on wellhead protection in fractured rock (Bradbury and others, 1991) and karst settings (Schindel and others, 1997) suggest that many of the standard methods for delineation of capture zones that rely on the porous-medium approximation (such as calculated fixed radius, uniform flow equation, WHPA or WHAEM numerical models) are inappropriate for these settings. Water resource managers, tasked with developing source-water protection plans for 1000's of wells, must determine those wells for which “standard porous-medium methods” are appropriate and those wells that are “karstic” and require alternate approaches.

In Wisconsin, it is not always clear which wells qualify as “karstic”. In many areas of the state, unconsolidated Pleistocene deposits overlie carbonate bedrock, which in turn is often underlain by a productive sandstone aquifer. This geology gives rise to several questions. If the uppermost bedrock aquifer consists of carbonates, what thickness of unconsolidated sediment is necessary in order to consider it a porous-medium dominated system? If the uppermost bedrock is a karstified carbonate unit, but a well is completed in the underlying sandstone aquifer, can one treat it as a porous-medium aquifer?

We have recently begun using water-quality data as a screening tool to answer these and similar questions. This talk will use both existing nitrate-nitrogen data as well as project-specific tritium and fluid temperature/conductivity data to illustrate how water-quality data can be used to assess the “karstic” character of water-supply wells in Wisconsin.