VARIABILITY OF NITRATE CONCENTRATIONS AS AN INDICATOR OF RAPID RECHARGE TO DEEP BASIN GROUNDWATERS, TRANS-PECOS TEXAS

In deep groundwater of the semi-arid and arid basins in the Trans-Pecos region of Texas, we have documented unexpectedly high levels of nitrate (NO₃^-) where 61% of tested water samples have > 5 mg/L NO₃^- as N and 22% of tested samples have >10 mg/L NO₃^- as N. The NO₃^- contamination is extensive; it affects 228 of 264 examined wells. However, its sources and pathways of transport are unclear. Additionally, the concentration of NO₃^- is changing over time in the majority (81.4%) of wells with multiple data points and generally increasing. These temporal changes raise additional questions about NO₃^- sources and the flow dynamics in the basins. Presence of NO₃^- and the temporal variability in its concentrations in deep groundwater has implications beyond contamination because it potentially indicates relatively rapid recharge (<60 yrs): a process that is not expected in this system based on previous estimates of groundwater age using chloride mass balance models and groundwater age dating techniques. For this project, data collected by previous academic research and several government agencies since the 1950’s were combined with our recently collected field data to document a multi-decadal trend of overall increasing NO₃^- concentration in deep basin groundwaters, indicating that there exist mechanism(s) of relatively rapid and wide spread recharge. We infer that significant recharge is occurring on the basin floors and that it may be spatially and temporally variable. We have also documented temporal changes in water chemistry in large irrigation wells during pumping that imply preferential flow channels not considered in existing numerical models. It is possible that in light of these observations, fundamental conceptual models of flow in these types of basins around the world need to be re-evaluated.