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

Abstract

In arid and semi-arid environments, artificial recharge or reuse of wastewater may be desirable for water conservation, but NO 3 − 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 NO 3 − loading to ground water. Tritium and Cl − concentrations in ground water corroborate prior findings of focused recharge through playas and ditches. Typical δ 15 N values in ground water (>12.5‰) and correlations between δ 15 N and ln C NO − 3–N suggest denitrification, but O 2 concentrations ≥3.24 mg l −1 indicate that NO 3 − reduction in ground water is unlikely. The presence of denitrifying and NO 3 −-respiring bacteria in cores, typical soil–gas δ 15 N values <0‰, and decreases in NO 3 −–N/Cl − and SO 4 2−/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, NO 3 −–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.