TECHNICALLY DEFENSIBLE CAPTURE ZONES IN CARBONATE AQUIFERS ON A LIMITED BUDGET - EXAMPLES FROM WISCONSIN

Fractured carbonate and karst aquifers present particular challenges for hydrogeologists developing groundwater protection plans. These environments are often exceedingly vulnerable to contamination, yet many of the standard methods for delineation of capture zones rely on porous-medium approximations (such as calculated fixed radius, uniform flow equation, WHPA or WHAEM numerical models) and may not be technically defensible. In addition, most groundwater protection efforts have limited budgets that do not allow for the collection of new field data. So what methods can be used in carbonate aquifers that are both technically defensible and cost effective?

Hydrogeologic mapping, combined with a review of existing water-quality data, can be a cost-effective, technically-defensible method of delineating capture zones in carbonate aquifers. A recently completed statewide project used existing data to define source water areas for municipal and community wells in six counties where 1) carbonate rock was the uppermost bedrock unit, 2) Pleistocene materials were < 50 ft in thickness, and 3) a 1:1,000,000 water-table map was available.

The statewide project used a combination of geologic and water-quality criteria to identify karst wells. Capture zones for each well were delineated using published 1:100,000 water-table maps. Karst development within the capture zones was assessed by 1) reviewing soil surveys and air photos and 2) conducting a site visit.

We compare the results of this method to more sophisticated studies near Sturgeon Bay, WI, where a complex, 3-D numerical model, which incorporates extensive data on the regional fracture system, indicates that capture zones for the city's wells are several km in length and travel times to the wells are approximately 1 to 2 years. Geochemical and isotopic data suggest that these travel times are realistic. If we apply the above method to wells in Sturgeon Bay, we 1) correctly identify all wells as karst wells and 2) delineate capture zones, which compare well to those determined from the detailed wellhead protection project.

Comparison of these projects suggests that flow-system mapping, vulnerability mapping, and a review of existing water-quality data provide a technically-defensible, cost-effective method of determining capture zones in carbonate aquifers.