GROUNDWATER RECHARGE IN THE TEXAS HIGH PLAINS

Recharge is a critical issue for evaluation of groundwater resources and for assessment of groundwater vulnerability to contamination. We used a variety of approaches to estimate recharge in the Central High Plains near Amarillo, including soil physics, environmental tracers, and numerical modeling. Deep penetration of bomb pulse tritium, flushing of chloride, low calcium carbonate concentrations, and high water potentials indicate that playas focus recharge. Water fluxes estimated from tritium profiles in playas were up to 120 mm/yr. In contrast, chloride bulges, calcic soils, low water potentials and upward water potential gradients indicate negligible recharge in interplaya settings. The bulge shaped chloride profiles in interplaya settings indicate that water fluxes were higher during the Pleistocene (up to 5 mm/yr) and that chloride has been accumulating during the Holocene. Numerical simulations of nonisothermal liquid and vapor flow using the HYDRUS-1D code indicate that the water potential and chloride profiles can be reproduced by downward flux during the Pleistocene followed by an order of magnitude reduction in downward flux, zero flux, or upward flux during the Holocene. Groundwater data provide more integrated estimates of recharge. Spatial variability in groundwater chloride concentrations was examined relative to land use and potential sources of chloride. These data are used to provide regional estimates of recharge. Temporal variability in water table elevations is also used to evaluate variability in recharge relative to irrigation return flow and other land uses. Information on recharge from this study is extremely valuable in predicting groundwater resources during the next 50 yr and for delineating aquifer regions that are particularly susceptible to contamination.