GROUNDWATER MOVEMENT THROUGH THE UNSATURATED ZONE OF THE HIGH PLAINS AQUIFER IN THE CENTRAL PLATTE NATURAL RESOURCES DISTRICT, NEBRASKA

The U.S. Geological Survey, in cooperation with the Central Platte Natural Resources District and San Francisco State University, instrumented eight sites with heat-dissipation probes, suction lysimeters, and clustered monitoring wells to quantify transit rates of water and selected tracers in areas of the High Plains aquifer unsaturated zone underlying the Central Platte Natural Resources District, Nebraska during 2008-10. The eight sites were located to assess the variability of water and tracer transit rates primarily resulting from land use and east-west regional precipitation gradients of the study area. Depths to groundwater exceeded 10 meters at seven sites. Continuous soil cores were analyzed for particle size, bulk density, and pore-water concentrations of nitrate (NO₃^–) as N, chloride (Cl^–), and tritium (³H). Wells were installed just below the water table to determine the chemical and isotopic composition of the most recently recharged groundwater. Soil samples at seven sites were silt to silty clay and fine sand to sand at the remaining site. Analysis of environmental tracers in shallow groundwater indicate most residence times greater than 50 years at sites outside the Platte River Valley, and less than 10 years at sites within the Platte River Valley. Peaks of pore-water ³H concentrations in soil profiles from the atmospheric nuclear bomb testing in the 1960s could not be determined at some locations probably because of ³H being flushed through the unsaturated zone since the nuclear bomb tests. Using available ³H peak profiles, recharge rates of about 58 mm/yr were computed, which is consistent with other studies in the High Plains aquifer. Pore-water NO₃^– and Cl^– concentrations in soil cores indicated peaks similar in shape and depth to the ³H profiles. Preliminary data indicate that water and chemical fluxes in the unsaturated zone are a function of irrigation patterns, regional precipitation gradient, and winter precipitation events.