Natural bioremediation of a nitrate-contaminated soil-and-aquifer system in a desert environment

Mining activities are a major source of land degradation in arid regions, and remediation methods developed for mesic sites may not be appropriate for arid sites. In climates where potential evapotranspiration exceeds precipitation, it might be possible to prevent the migration of contaminants away from a mine site by controlling the site water balance through vegetation, and allowing natural attenuation processes to reduce pollutant levels over time. We investigated the feasibility of remediating a nitrate-contaminated source-plume system in a desert environment using biological methods. The study site was a former uranium mill in Monument Valley, Arizona, where NO3− used in ore processing had leaked from the soil beneath a tailings pile (referred to as the source area) into an alluvial aquifer (referred to as the plume) spreading away from the source area. We used 15N/14N ratios and direct assays of denitrification to show that biological denitrification occurred in both the source area and plume, and could reduce NO3− levels over time. Denitrification in the source area could be stimulated by providing additional moisture to the soil through irrigation, whereas providing a carbon source (ethanol) stimulated denitrification in samples from the plume. We used 18O/16O and H/deuterium signatures in water and plant samples to show that the native saltbush (Atriplex canescens) and black greasewood (Sarcobatus vermiculatus) plant community was extracting water from the plume, offering a possible means of preventing migration of the plume by controlling grazing over the site. We conclude that biological remediation is a feasible alternative to pump/treat solutions at this type of site.