Evaluation of Methods for Delineating Zones of Transport for Production Wells in Karst and Fractured-Rock Aquifers of Minnesota

Assessment of groundwater-flow conditions in the vicinity of production wells in karst and fractured-rock settings commonly is difficult due in part to the lack of detailed hydrogeologic information and the resources needed to collect it. To address this concern and to better understand the hydrogeology and aquifer properties of karst and fractured-rock aquifers in Minnesota, the U.S. Geological Survey, in cooperation with the Minnesota Department of Health, conducted a study to evaluate methods for delineating zones of transport for 24 production wells in karst and fractured-rock aquifers in Minnesota. Two empirical methods for delineating zones of transport around wells were applied to the 24 production wells that extract groundwater from karst and fractured-rock aquifers in nine Minnesota communities. These methods were the truncated-parabola and modified-ellipse methods, and both methods assume porous-media flow conditions. The 24 wells extracted water from a karst aquifer (Prairie du Chien-Jordan aquifer), porous aquifers interspersed with solution-enhanced fractures (Jordan and Hinckley aquifers), or fractured-bedrock aquifers (Biwabik Iron-Formation and Sioux Quartzite aquifers). Zones of transport delineated using these two empirical methods were compared with zones of transport previously delineated by Minnesota Department of Health hydrologists for the wells using the calculated-fixed-radius method and groundwater-flow models.

Large differences were seen in the size and shapes of most zones of transport delineated using the truncated-parabola and modified-ellipse methods compared with the zones of transport delineated by the Minnesota Department of Health. In general, the zones of transport delineated using the truncated-parabola and modified-ellipse methods were smaller in area than those delineated by the Minnesota Department of Health and included only small parts of the Minnesota Department of Health zones of transport. About two-thirds(67 percent) of the individual or composite truncated parabolas and modified ellipses covered less than 50 percent of the area included in zones of transport delineated by the Minnesota Department of Health. The shapes of some of the truncated parabola and modified ellipses did not closely match the zones of transport delineated by the Minnesota Department of Health using the calculated-fixed-radius method and groundwater-flow models. Differences between the zones of transport delineated by the truncated-parabola and modified-ellipse methods and those delineated by the Minnesota Department of Health can be explained by variations inherent to the methods and by the amount of complexity taken into account by different groundwater-flow models. Additional field hydrogeologic studies would be needed at specific sites to support the use of these zone-of-transport delineation methods. Application of the truncated-parabola and modified-ellipse methods to sites for which existing hydrogeologic information is limited can produce questionable results in karst and fractured-rock settings, particularly in areas where many high-capacity wells or active mining operations are nearby.