Ground water restoration at abandoned uranium mills on the Navajo Nation using native, desert phreatophytes

The U.S. Department of Energy Office of Legacy Management is evaluating natural and enhanced attenuation processes in search of sustainable remedies for soil and ground water at a former uranium mill near Monument Valley, Arizona. During the 1960s, ammonia and ammonium nitrate were used to precipitate uranium following an acid heap-leaching process. Ammonium and nitrate remaining in the vadose zone after site cleanup are a continuing source for an alluvial plume spreading north of the site. In 1999, Atriplex canescens and Sarcobatus vermiculatus were planted and irrigated (0.32 to 0.36 m yr^-1) at a 2-m spacing in the source area. Monitoring of soil moisture profiles and percolation flux indicate that the planting has curtailed recharge and leaching and has removed nitrate from the source soil. By 2004, vigorous growth (2 m² per plant canopy cover) was maintaining a deficit soil water balance and had removed approximately 190 kg of N. Plant uptake alone, however, cannot account for the loss of soil nitrate (> 50%) and ammonium (> 25%) from the vadose zone during the period. Enrichment of ¹⁵N and greater N₂O flux in the irrigated soil support the hypothesis that N loss is primarily a response to microbial denitrification and then nitrification when source soils are wet and dry, respectively. Except for a long history of overgrazing, A. canescens and S. vermiculatus populations would also dominate the natural vegetation of the plume area. Exclosure studies suggest that restricting grazing combined with replanting of these native phreatophytes could increase the phytoremediation capacity for the plume from 25 to 181 kg ha^-1 yr^-1 N, greatly increase transpiration, and thereby slow plume migration. Management of large-scale plantings in both the source and plume areas could also, in time, improve rangeland condition and produce a valuable native seed crop.