PRELIMINARY RESULTS OF HYDROGEOLOGICAL FRAMEWORK STUDIES OF SURFACE WATER - GROUND WATER SYSTEMS IN EASTERN NEBRASKA USING AIRBORNE AND GROUND GEOPHYSICS

Understanding the relationship between surface water and groundwater systems in Eastern Nebraska is critical to understanding water resources and developing management programs. The U.S. Geological Survey (USGS) conducted an airborne resistivity and magnetic (helicopter electromagnetic, HEM) survey and follow up ground surveys in order to characterize and map the hydrogeology in three glacial terrains in Eastern Nebraska during the spring of 2007. The project is part of the Eastern Nebraska Water Resources Assessment (ENWRA), a joint State of Nebraska and USGS study including the Conservation and Survey Division (University of Nebraska) and the following Natural Resource Districts: Lower Platte South, Lower Platt North, Lower Elkhorn, Lewis and Clark, Nemaha, and Papio-Missouri River. The three areas selected for the study, Ashland, Firth, and Oakland have glacial terrains and bedrock that typify different hydrogeologic settings for surface water and ground water in Nebraska. Approximately 600 line km were flown with HEM in each of the three glacial terrains with a line spacing of approximately 270 m and samples every three meters. Earth resistivities in the Oakland area are the lowest, ranging from 8 to 23 ohm-m, yielding a general depth of penetration of 50 - 60 m. The Ashland area centered on the Platte River exhibited earth resistivities from 6 to 100 ohm-m with an effective depth of penetration from 50 – 100 m. Within the Firth area, earth resistivities range from 5 to 40 ohm-m with a depth of penetration of 50 – 80 m. These HEM results correlate with ground time domain electromagnetic soundings and borehole geophysical logs. The total field magnetics in each area show significant anomalies (in cases, 1000 nT) from crystalline basement features such as mafic dikes and instrusives as well as structures. These may correlate with shallower features that could control the distribution of ground water. Three-dimensional models of the resistivity and magnetic field data indicate buried hydrogeologic features and structure that may control the surface- and ground-water interactions. Follow up drilling efforts coupled with ground-water modeling will use the airborne and ground geophysical models to build a better ground-water management program for Eastern Nebraska.