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

In arid and semi-arid environments, artificial recharge or reuse of wastewa ter may be desirable for water conservation, but NO3- contamination of unde rlying aquifers can result. On the semi-arid Southern High Plains (USA), in dustrial wastewater, sewage, and feedlot runoff have been retained in dozen s of playas, depressions that focus recharge to the regionally important Hi gh Plains (Ogallala) aquifer. Analyses of ground water, playa-basin core ex tracts, and soil gas in an 860-km(2) area of Texas suggest that reduction d uring recharge limits NO3- loading to ground water. Tritium and Cl- concent rations in ground water corroborate prior findings of focused recharge thro ugh playas and ditches. Typical delta(15)N values in ground water (>12.5 pa rts per thousand) and correlations between delta(15)N and In CNO3--N sugges t 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 presenc e of denitrifying and NO3--respiring bacteria in cores, typical soil-gas de lta(15)N values < 0 parts per thousand, and decreases in NO3-- N/Cl- and SO 42-/Cl- ratios with depth in cores suggest that reduction occurs in the upp er vadose zone beneath playas. Reduction may occur beneath flooded playas o r within anaerobic microsites beneath dry playas. However, NO3--N concentra tions in ground water can still exceed drinking-water standards, as observe d in the vicinity of one playa that received wastewater. Therefore, continu ed ground-water monitoring in the vicinity of other such basins is warrante d. (C) 2000 Elsevier Science B.V. All rights reserved.