THE ROLE OF GEOSTATISTICS IN WELLHEAD PROTECTION ZONE DELINEATION IN GLACIAL DRIFT

The hydrogeology of glacial drift settings generally presents a high degree of complexity because of glacial depositional and erosional processes. The resulting stratigraphic package may include materials with a wide distribution of permeabilities, along with such variability that hole-to-hole correlations may be difficult to determine.

Wellhead protection (WHP) programs rely on various approaches for delineating zones of protection. One method involves detailed numerical groundwater flow modeling of an area surrounding a well or wellfield, including justifiable boundary conditions, accurately estimated input parameter values, and a suitable subsurface model. But for WHP modeling in glaciated terrain, the complicated stratigraphy may preclude the creation of a valid subsurface model.

Geostatistical analysis of high-quality and/or high-density drilling data from the vicinity of the wellfield may provide the necessary means for stochastic modeling of stratigraphy and groundwater flow. This presentation is focused on a case study in central Minnesota, where a wellfield is located in an area where several ice lobes converged to create a highly heterogeneous hydrogeologic framework. Two methods under investigation are indicator kriging and transition probability geostatistics. The results describe the 3-D correlation structure of the aquifer and aquitard units, and multiple, equally probable realizations provide a probabilistic WHP model.