Land-farm phytoremediation: Irrigating native desert shrubs with nitrate-contaminated ground water at a former uranium mill near Monument Valley, Arizona

Background/Question/Methods

The U.S. Department of Energy removed radioactive tailings from a former uranium mill near Monument Valley, Arizona, in 1994.  Elevated levels of nitrate, ammonia, and sulfate, waste products of the milling process, remain in a shallow alluvial groundwater plume spreading from a soil source where tailings were removed. Land farming was evaluated as an alternative to conventional pump-and-treat remedies for contamination in the alluvial aquifer.  Two native shrubs, Atriplex canescens and Sarcobatus vermiculatus, were transplanted and irrigated with water pumped from the aquifer.  Pilot study objectives included:  (1) reduce nitrate and ammonia concentrations in the aquifer by growing native shrubs on disturbed land, (2) sequester groundwater sulfate as calcium sulfate in the land-farm soil, analogous to natural gypsiferous soils in the area, and (3) produce crops of native plant seed.  Four nitrate levels in irrigation water were derived from the results of greenhouse studies: 250 mg/L, a level not likely toxic to crop plants or to livestock feeding on the crop; 500 mg/L, a level not likely toxic to crops but possibly toxic to livestock; 750 mg/L, a level possibly toxic to crops; and a clean water control.

Results/Conclusions

Soil profiles were significantly drier in A. canescens plots than in S. vermiculatus plots reflecting the higher survival, productivity, and leaf area of A. canescens.  Irrigation with plume water resulted in little if any accumulation of soil nitrate, due in part to plant uptake and denitrification in the A. canescens plots, and in part to leaching in S. vermiculatus plots where transpiration was inadequate.  Irrigation with plume water resulted in an accumulation of sulfate in the soil profile.  Overall, A. canescens was superior to S. vermiculatus as a phytoremediation crop.  For all treatment combinations, A. canescens had lower mortality rates, grew larger, transpired more water, and took up more nitrogen than S. vermiculatus.  Plant uptake and soil denitrification likely kept nitrate levels from building up in the land farm soil; plant transpiration limited recharge and leaching of nitrate and ammonia back into the aquifer; sulfate pumped from the plume remained in the soil profile, perhaps sequestered as calcium sulfate; and the land farm produced a native seed crop that could be used by the Navajo Nation for rangeland revegetation or mine land reclamation.