HYDROGEOLOGIC FRAMEWORK BASED UPON GEOPHYSICAL STUDIES OF THE SAND HILLS OF WESTERN NEBRASKA

The Sand Hills of western Nebraska are part of an extensive terrain of dunes distributed across the Great Plains. These dune complexes provide unique ecosystems, enable large amounts of recharge to underlying aquifers including the Ogallala aquifer, and are very sensitive to climate changes. Understanding the hydrogeologic framework of these dune systems and their underlying aquifers is essential in management of groundwater resources and ecosystems. The hydrogeologic framework is a critical component in understanding implications of climate change for one of the most productive agricultural regions of the United States. Understanding how the climate history of dunes currently affects groundwater quality and flow will be essential in assessing impacts from dynamic climate changes.

The U.S. Geological Survey acquired airborne electromagnetic (AEM) resistivity surveys for areas of Crescent Lakes National Wildlife Refuge (CLNWR) and east of Bridgeport, Nebraska. Previous studies demonstrated that an ancestral dune dam blocked the now partially buried Blue Creek. This dune dam is defined by a resistive zone extending 45 meters in depth, suggesting that it now influences modern groundwater-flow paths and surface-water features. In addition, shallow lakes in the immediate area have highly variable salinity, which appears to be related to the lakes interaction with the regional groundwater system. The AEM resistivity depth sections show the subsurface distribution of dune sands and saline waters that act as a natural tracer for groundwater flow. Some saline waters are perched above more resistive zones that may contain fresher waters. Other conductive zones suggest downward flow from saline lakes. Groundwater-flow paths controlled by buried topography interpreted from new geophysical surveys are important in predicting how groundwater flow paths at different depths are interconnected. Groundwater–surface water relationships were interpreted along a flight line oriented north to south across the North Platte River and southwest of CLNWR, but includes a section of the Sand Hills related to this project.