Nitrate reduction during ground-water recharge, Southern High Plains, Texas

In arid and semi-arid environments, artificial recharge or reuse of wastewater may be desirable for water conservation, but NO3- contamination of underlying aquifers can result. On the semi-arid Southern High Plains (USA), industrial wastewater, sewage, and feedlot runoff have been retained in dozens of playas, depressions that focus recharge to the regionally important High Plains (Ogallala) aquifer. Analyses of ground water, playa-basin core extracts, and soil gas in an 860-km(2) area of Texas suggest that reduction during recharge limits NO3- loading to ground water. Tritium and Cl- concentrations in ground water corroborate prior findings of focused recharge through playas and ditches. Typical delta(15)N values in ground water (>12.5 parts per thousand) and correlations between delta(15)N and In CNO3--N suggest denitrification, but O-2 concentrations greater than or equal to 3.24 mg 1(-1) indicate that NO3- reduction in ground water is unlikely. The presence of denitrifying and NO3--respiring bacteria in cores, typical soil-gas delta(15)N values < 0 parts per thousand, and decreases in NO3-- N/Cl- and SO42-/Cl- ratios with depth in cores suggest that reduction occurs in the upper vadose zone beneath playas. Reduction may occur beneath flooded playas or within anaerobic microsites beneath dry playas. However, NO3--N concentrations in ground water can still exceed drinking-water standards, as observed in the vicinity of one playa that received wastewater. Therefore, continued ground-water monitoring in the vicinity of other such basins is warranted. (C) 2000 Elsevier Science B.V. All rights reserved.