MODELING THE EFFECTS OF GEOLOGIC HETEROGENEITY AND ANISOTROPY ON GROUNDWATER CONNECTIVITY BETWEEN POTHOLE WETLANDS AND DOWNSTREAM WATERS IN THE PRAIRIE POTHOLE REGION OF NORTH DAKOTA, USA

Hydrologic connectivity of prairie pothole wetlands with downstream waters has received increased scrutiny in recent years. In principle, groundwater flows from areas of high total head to low total head. Thus, groundwater flow from elevationally high pothole wetlands do downstream waters such as lakes and streams seems intuitive. However, the water table in the prairie pothole region (PPR) tends to follow the contours of the land surface and commonly forms an elevationally higher mound around pothole wetlands. These mounds form a zone of total head that is higher than the pothole wetland and creates a barrier to horizontal groundwater flow in the absence of mitigating factors.

In this study, we evaluated potential for groundwater to flow from pothole wetlands, under water table mounds, toward downstream waters in the North Dakota PPR. We created an idealized transect of our study area featuring a cluster of pothole wetlands and the typical slope, topographic character, and dimensions of the area. We then simulated groundwater flow along this transect using the USGS numerical models VS2DI and MODFLOW. Model boundaries were set to represent predominate landscape features. Groundwater flow simulations tested the effect of geologic heterogeneity, in the form of sand and gravel aquifers of various sizes, shapes, and positions, and anisotropy.

Results confirm water table mounds effectively disrupt groundwater flow from pothole wetlands to downstream waters. Nevertheless, simulations of isotropic, homogenous conditions showed groundwater flow under water table mounds was common between adjacent wetlands within the wetland cluster, where vertical gradients between wetlands were relatively high. Groundwater connectivity of the wetlands to more distant downstream waters was sensitive to ideal placement of sand and gravel units. Alternatively, increasing anisotropy to 1000:1 under homogenous geologic conditions produced relatively strong groundwater connectivity between some wetlands and downstream waters, but decreased connectivity within the cluster of wetlands. These simulations demonstrate a potential for geologic heterogeneity and anisotropy to overcome the effects of local water table mounds and connect groundwater to pothole wetlands on the Missouri Coteau in North Dakota to downstream waters.