PRACTICAL METHODS FOR WELLHEAD PROTECTION AREA DELINEATION IN KARST AND FRACTURED AQUIFERS IN MINNESOTA

The process of delineating capture zones for wells completed in fractured and solution-weathered aquifers is complicated by the physical characteristics of these aquifer settings. Public water suppliers that pump from such aquifers rarely have the resources needed to conduct extensive field studies that can help define groundwater flow characteristics. As a result, it is important that wellhead protection area (WHPA) delineation methods exist that are both practical and technically defensible.

The Minnesota Department of Health (MDH) is in the process of establishing guidelines for WHPA delineations in karst and fractured aquifer settings in Minnesota. These guidelines incorporate a menu of options that relate to specific aquifer settings. They range from deeply buried settings where hydraulic gradients are low or unknown to shallow, water table settings. In addition, the guidelines address dual porosity settings and cases where a karst or fractured aquifer is hydraulically connected with adjacent porous media aquifers. Emphasis is placed on using existing data and relatively simple calculations combined with hydrogeologic mapping for the delineation of conservative capture zones. However, the methods are scaleable so that the results of more sophisticated fracture-flow investigations can be readily incorporated to provide more accurate delineations.

The United States Geological Survey (USGS) has recently entered into an agreement with the MDH to evaluate the proposed guidelines by comparing them with other delineation methods at a series of example sites in Minnesota. The results of this analysis will be incorporated into the MDH guidance document, at which time it will become a reference for WHPA delineation in Minnesota.