WATER MOVEMENT THROUGH THE UNSATURATED ZONE OF THE HIGH PLAINS AQUIFER IN THE CENTRAL PLATTE NATURAL RESOURCES DISTRICT, NEBRASKA, 2008-2012

In 2007, the U.S. Geological Survey, in cooperation with the Central Platte Natural Resources District and San Francisco State University, began a study to quantify transit rates of water and selected tracers in the unsaturated zone underlying the Central Platte Natural Resources District, Nebraska during 2008-12. Eight sites—four beneath groundwater irrigated land, three beneath rangeland, and one beneath non-irrigated farmland (dryland)—were located to assess the variability of water and tracer transit rates with respect to land use and east-west regional precipitation gradients of the study area. The sites were instrumented with heat-dissipation probes, suction lysimeters, and clustered monitoring wells; depths to groundwater ranged from 2.3 to 39.6 meters (m). Total water potential and unsaturated-zone profiles of tritium (³H), chloride (Cl^–), nitrate as N (NO₃^–), and bromide (Br^–), along with groundwater-age dates from environmental tracers—chlorofluorocarbons, sulfur hexafluoride, tritium/helium, tritium, and dissolved gases—were used to estimate rates of water movement in the unsaturated zone and groundwater recharge.

Measurements of total water potentials indicated that periodic wetting fronts reached greater maximum depths in the unsaturated zone beneath the irrigated sites than beneath the rangeland sites. The depth of the interface between the pre-bomb/post-bomb ³H and a ³H mass balance within the soil profiles were used to estimate water fluxes that ranged from 10 to 140 millimeters per year (mm/yr). The Cl^– mass-balance method yielded a range of water fluxes below the root zone of 4.3 to 21 mm/yr at rangeland sites. Concentrations of NO₃^– in pore water in the unsaturated zone were larger beneath the irrigated and dryland sites compared to beneath the rangeland sites; NO₃^– and Cl^– concentrations profiles were similar. Movement of Br^– through the unsaturated zone indicated greater water fluxes occur beneath irrigated lands than beneath rangeland. Recharge rates determined from age dates of samples from the shallowest wells generally were similar to those determined from the ³H and Cl^– mass-balance methods, whereas recharge rates determined from age dates from samples from the deeper wells likely resulted from a regional flow system that receives recharge from distant sources.