SURFACE-GROUNDWATER MODELING IN THE SHEYENNE NATIONAL GRASSLAND, NORTH DAKOTA, USA

The Sheyenne National Grassland (SNG) is a part of the Dakota Prairie Grasslands managed by the US Forest Service. It contains the largest (70,000-acre) publicly-owned tract of the Northern Tallgrass Prairie ecosystem. The Forest Service identified two potential threats to the SNG related to groundwater. Increased groundwater withdrawals in the SNG area could potentially lower the groundwater table and endanger the tallgrass and wetland ecosystems on the grassland. Additionally, noxious weeds threaten the unique tallgrass ecological system, and herbicides (e.g. tordon and imazapic) are being used to contain them. Herbicide application has been restricted to limit the potential for groundwater contamination by forbidding herbicide application in areas where the water table is less than 10 feet below the ground surface in highly permeable soils or less than 6 feet below the ground surface in low permeable soils. A regional analytic element model (GFLOW) showed that current pumping in the SNG area, even if moderately increased, is not likely to negatively impact the SNG. However, if future groundwater withdrawals are implemented at their maximum capacity, appreciable lowering of the groundwater table in some parts of the SNG might threaten its unique ecology. A local groundwater model is used to develop depth-to-groundwater maps in the SNG that are valid for a particular time frame during which herbicide applications are planned. These maps are based on a modeled groundwater surface that best fits the monitoring well data just prior to this time frame. The accuracy of the depth-to-groundwater maps is enhanced by an interpolation scheme that reduces the residuals at the monitoring wells. These maps are used to decide where herbicides can or cannot be safely applied.