Three-dimensional hydrostratigraphy of the Platte River Valley near Ashland, Nebraska: Results from Helicopter Electromagnetic (HEM) mapping in the Eastern Nebraska Water Resources Assessment (ENWRA) CB-2(NS) This study, part of the Eastern Nebraska Water Resources Assessment (ENWRA), used the results of a Helicopter Electromagnetic (HEM) geophysical survey in conjunction with subsurface data to map the hydrostratigraphy of the Platte River valley and adjacent uplands. Hydrostratigraphic units were interpreted using earth resistivity models from the HEM survey and subsurface data, including Conservation and Survey Division test holes, deep oil and gas well logs, and registered well logs. Thick (80-100 feet or 24-31 meter) deposits of highly conductive materials (clay and silt) limited the effectiveness of the HEM results for hydrostratigraphic mapping in the uplands. However, hydrostratigraphic units in recent and abandoned alluvial deposits of the Platte River, even where covered by 10-15 feet (3-5 m) of fine-grained sediments, could be mapped in detail using the HEM survey results. This study shows that the Platte River and Quaternary aquifer are apparently hydrologically connected throughout the study area, and that the Quaternary and Dakota aquifers are apparently hydrologically connected except where the discontinuous Cretaceous Dakota aquitard separates them. Three-dimensional modeling of hydrostratigraphic units using these methods permits calculation of unit volumes, saturated thicknesses, and estimates of drainable groundwater in the study area. Description: 58 pp, 2012. Paul R. Hanson, Jesse T. Korus, Dana P. Divine, Edited By David R. Larson. Print on demand, color copy available.