HETEROGENEITY IN GROUNDWATER FLOW MODELING: SIMULATING FRACTURES IN A REGIONAL AQUITARD

Fractures in the Maquoketa Formation in southeastern Wisconsin, USA, are important in determining the vertical head distribution within this aquitard. Hydraulic head in the underlying aquifer is 120 m less than in the water-table aquifer due to deep municipal-well pumping. However, within the aquitard, the vertical head distribution shows no significant variation with depth, and the head gradient is steep only at its base, which is unfractured. The Maquoketa Formation is an Ordovician-age dolomitic shale of low hydraulic conductivity (10^-14 to 10^-12 m/s), measured on rock core. Field characterization involving slug, short-interval packer, and multiple-well pumping tests, showed that hydraulic conductivity ranges up to 10 orders of magnitude higher (10^-9 to 10^-4 m/s). We attribute the higher bulk conductivities to transmissive bedding-plane fractures, observed in numerous downhole geophysical logs.

Our conceptual model to explain the measured vertical head distribution required regional continuity of bedding-plane fractures extending to the Maquoketa Formation subcrop, less than 10 km to the west. Beyond the aquitard subcrop, hydraulic head is uniform with depth in the unlithified Pleistocene deposits and approximates the water elevation of a lake to the west of our study area. We hypothesized that only low horizontal head gradients could be sustained along such transmissive bedding-plane fractures. Therefore, a steep vertical head gradient would be present only at the unfractured base of the aquitard.

We constructed a finite-difference model in which the major bedding-plane fractures are simulated by thin high-transmissivity layers between low-transmissivity rock-matrix layers. Results were compared to those of a similar model that used an equivalent transmissivity for all layers. The model with explicit discrete-fracture layers reproduces the observed vertical head distribution better, whereas the model using equivalent transmissivity produces a more linear head loss with depth. In support of these findings, geochemical and isotopic analyses of water samples show no consistent trends with depth, and we found only non-tritiated water within the Maquoketa Formation. Our results emphasize the importance of incorporating discrete units of geologic heterogeneity in groundwater flow models.