EVAPOTRANSPIRATION AND GEOCHEMICAL CONTROLS ON GROUNDWATER PLUMES AT ARID SITES: LESSONS FROM ARCHETYPE URANIUM MILLING SITES

Data from several former uranium milling sites in the western United States affirm a conceptual model in which the climate and geomorphology, and the associated geochemical and hydrological conditions, control the subsurface fate and transport of contaminants. In arid settings, typical for milling sites, shallow groundwater is transferred into the vadose zone and atmosphere via evaporation, transpiration and diffuse surface seepage. During these transfers, dissolved constituents precipitate as evaporite minerals (e.g., blödite, thenardite, and halite) along with accessory minerals (e.g., carnotite) containing trace elements such as uranium. In locations where the water table is relatively deep (> 2m), these precipitates will accumulate as nonpedogenic intervals in the deep vadose zone near the capillary fringe, around the roots of phreatophyte plants, and near surface seeps. In areas where the water table is shallow, precipitates will also accumulate at the soil surface as a result of capillarity and evaporation. The accumulation and distribution of constituents associated with milling/extraction impacted groundwater is analogous to natural evaporite ore deposits in North America, Australia, Europe and Africa.

The sites in Tuba City Arizona and Riverton Wyoming are archetype mill sites in arid settings, representing deep and shallow water table cases, respectively. Available hydrological, geochemical and radiological (aerial gamma) data from these sites provide key insights related to contaminant fate and transport. At Tuba City, hydrological and geochemical processes limit the size of the groundwater plume and reduce the potential for contaminated groundwater to crop out at Moenkopi Wash, while eolian processes have resulted in limited surficial dispersal of milling related constituents along the primary wind vectors. At both sites, milling-related evaporite minerals that have formed in the near-field and mid plume area will sustain elevated groundwater concentrations of anthropogenic constituents such as sulfate and uranium for an extended timeframe.