A Plant-Based Approach to Remediating a Nitrate-Contaminated Soil/aquifer System IN a Desert Environment

Groundwater nitrate contamination often occurs in association with historical uranium processing. We evaluated the potential use of plant-based mitigation of nitrate contamination in a shallow alluvial aquifer emanating from a former uranium mill (tailing piles, leach areas, an evaporation pond) near Monument Valley, AZ. We established a 1 ha irrigated planting of 4-wing saltbush (Atriplex canescens) and black greasewood (Sarcobactus vermiculatus) in the source area of a nitrate-contaminated alluvial aquifer and evaluated plant N-uptake and denitrification. Isotope signatures of N (¹⁵N/¹⁴N ratios) as well as direct assays of denitrification suggested that the biological reduction of nitrate occurred both in the source area and the plume. We believe that the establishment of native shrubs in the source area vadose zone has accelerated the rate of denitrification by changing the soil atmosphere conditions and biological activity. These processes combined with a control of drainage associated with plant transpiration reduced the rate of nitrate loading in the plume. We used ¹⁸O/¹⁶O and H/deuterium signatures from groundwater, vadose zone, and plant tissue to confirm that indigenous A. canescens and S. vermiculatus shrubs were extracting groundwater from the plume. Additionally, we estimated transpiration rates of A. canescens and S. vermiculates on a landscape scale from measurements of sap flow, vegetation cover, meteorological data, and Enhanced Vegetation Index (EVI) derived from MODIS sensors on the Terra satellite. Using a scaling model, we predicted that increasing vegetation cover would result in a reduction in the rate of nitrate transport and may also promote denitrification. Because ungrazed areas had higher vegetation cover, we believe that controlling grazing of these desert phreatophytes and selective planting in disturbed areas over the plume would be an effective way to mitigate the fate and transport of nitrate at this site.