EVIDENCE OF YOUNG WATER IN THE TRANS-PECOS BASIN AQUIFERS

Since the 1950’s, nitrate (NO₃^-) concentrations in the basin aquifers of the Trans-Pecos region (Wild Horse and Michigan Flats, Lobo and Ryan Flats, Eagle Flats, and Red Light Draw) have generally been increasing. Of the 93 wells examined, 77 (82.8%) have documented variability in NO₃^- concentration of greater than 1 mg/L as NO₃^- and 65 (69.8%) have increased between 1 and 40 mg/L (as NO₃^-). In addition, recent samples collected from wells in the Eagle Flat, Wild Horse Flat, and Lobo Flat basins contain measurable amounts of CFCs, evidence of water younger than 70 years. These observations contradict the current models and previous estimates of groundwater age (~10,000 yrs) in the basins as well as the fundamental assumption that little or no recharge occurs on the basin floors. We hypothesize that the main sources of NO₃^- are mobilization of naturally occurring salts and synthetic fertilizers. Irrigation return flow and precipitation on the basin floors are the mechanisms by which the NO₃^- is transported to the underlying aquifers. Evidence of preferential flow paths including desiccation cracks and paleo-gravel channels in the basin floor sediments add further complexities to understanding flow and transport in the basin aquifers. In light of the NO₃^- and CFC data, we propose that the current models of flow and availability be re-evaluated to include these potentially significant sources of additional recharge. We recommend a re-examination of the current conceptual models of flow in similar systems around the world as these mechanisms of recharge may be occurring in other arid regions worldwide.