RISKS ASSOCIATED WITH REGIONAL REDOX-CONTROLLED ROLL-FRONTS IN AGRICULTURAL AREAS

A multi-pathway, probabilistic risk model related to "undiscovered" uranium deposits in an agricultural area in Southern Colorado is compared to known uranium districts in which in situ leach (ISL) mining activities are underway or planned. The control area is a portion of the Alamosa Basin designated by a National Uranium Resource Evaluation (NURE) study as favorable for regional redox-controlled, roll-front sandstone uranium deposits (Johnson, V.C., McCarn, D.W. et al, 1982). Favorability for occurrence of uranium deposits is based on depositional environment, alteration fronts, coincident geochemical fronts, and the presence of elevated concentrations of uranium in ground water proximal to the favorable zone.

Near-field, high-volume, artesian, agricultural water wells in the 60 km long favorable zone are remobilizing uranium, probably radium, possibly other redox-sensitive metals (As, Se, Mo, V, etc.), and radon out of the redox zone and distributing this material as contaminants onto agricultural crops using large, central-pivot irrigation systems. Previous limited geochemical sampling has demonstrated concentrations as high as 140 µg / L U₃O₈from these wells. High-volume pumping in near-field wells in the reduced zone or in the redox zone is postulated to cause oxidizing waters to flow across the redox zone thereby mobilizing uranium in a manner similar to commercial In-Situ-Recovery (ISR) uranium mining. Sampling of the surficial aquifer demonstrates high correlation of elevated uranium values with the favorable zone in the confined, artesian aquifer further suggesting that remobilization may be occurring.

Because of decades-long water use in the area, significant contaminants are likely to have built-up in soils. Identification and closure of the relatively few near-field wells responsible for remobilization of environmentally sensitive metals and radionuclides would result in a reduction of future contamination of agricultural soils and crops and consequent reduction of risk.

Regional redox-controlled roll-fronts have a unique geochemical signature. Many basins in the Western USA are candidates for regional redox-controlled, roll-fronts.

Based on measurements in the San Luis Valley, an attempt is made to assess probabilistic risks based on distributions of critical data.