CORRELATION ISSUES IN MODELING STRUCTURE AND FLOW IN GLACIAL-DRIFT AQUIFERS AND AQUITARDS

Correlation of aquifer and aquitard materials poses a challenge in 3D mapping in glaciated terrain because of complexity in the thickness and lateral extent of the depositional units and a general scarcity of data. Geostatistical methods provide a means for exploring that correlation structure and providing input to groundwater flow models.

In this study, geostatistical methods were initially applied to nearly 300 km of drilling data derived from 11,000 private well drilling logs and 200 high-quality monitoring well logs. Analyses were focused on a 12,000-km² region in central Minnesota dominated by the Rainy, Superior, and Wadena lobes.

The drilling data were categorized according to the logged sediment descriptions and analyzed in a variety of geomorphologically distinct zones using binary indicator geostatistics and transition probability geostatistics. A lack of stationarity was observed, consistent with a fundamental complexity of glacial depositional and erosional processes. Correlation generally varied as much between geographically distinct zones of like geomorphology as it did between zones of different geomorphology.

These geostatistical methods were then tested to evaluate their relative effectiveness in supporting numerical flow modeling within a model domain containing the higher quality data. For the indicator variography and kriging approach, the flow model relied on the kriging results by grading the hydraulic conductivity (K) values between high and low values on the basis of the indicator output. Calibration was performed by systematic testing of various plausible end members of the K range. The transition probability runs were calibrated by stochastic inverse modeling. A baseline single-K analysis was also performed and calibrated through inverse modeling. Resulting K values from all four approaches were comparable, with the indicator kriging providing a very slight edge in terms of the calculated errors at calibration targets. The modeling suggests that in complex glaciated terrain, in which lateral correlation of hydrostratigraphic units is small relative to grid spacing and drilling data spacing, a single-K model provides a suitable approach to determining regional groundwater flow, despite the large contrasts in K prevalent in glacial drift materials.