CHARACTERIZING AQUIFER PROPERTIES IN THE VICINITY OF A GROUNDWATER MOUND IN THE HIGH PLAINS AQUIFER, SOUTH-CENTRAL NEBRASKA

Several irrigation canal systems were constructed over the High Plains aquifer prior to widespread groundwater pumping and have produced four large groundwater mounds in Nebraska. One of these mounds is located in part under Phelps and Kearney counties, where the lateral extent is more than 2,000 km² and the maximum rise in the predevelopment water table is approximately 25 to 30 m. Elevated anthropogenic nitrate concentrations are widespread in Phelps County and also exceed 10 mg/L in parts of Kearney and Adams counties downgradient from the mound. Groundwater in this area is managed by two Natural Resources Districts using different regulations. The nitrate concentrations are high at depth in places, and aquifer heterogeneity further complicates management. Mapping aquifer properties using existing bore hole data indicates that: 1) the shape of the mound roughly matches the configuration of the canal system; 2) relatively thin (~3 m) but laterally extensive confining units exist within the aquifer; 3) gaps in these confining units may allow nitrate to reach deep parts of the aquifer; and 4) post-depositional deformation and erosion of the underlying Cretaceous surface and erosion on the Ogallala Group effect the transmissivity of the aquifer.