New Mexico Geology

The purpose of this study was to consider the effectiveness of two methods of restoring groundwater quality in a subsurface uranium-bearing formation following in situ leach (ISL) mining. To accomplish this it was necessary to develop an understanding of the geochemical characteristics of an aqueous solution that might be produced by an ISL mine. Samples of material from three different uranium (U) mines were collected and their acid leachable elemental concentration determined. Additional samples were then leached with aerated sodium bicarbonate (NaHCO₃) solutions at concentrations ranging from 1 mM to 500 mM. The fraction of the acid-leachable U and other trace elements released by this leaching process depended on NaHCO₃ concentration, U mineralogy, and the amount of solid organic matter in the samples. Less than 5% U was leached from samples with high organic matter using the NaHCO₃ leach solution. Groundwater restoration methods were then investigated using column experiments. Two methods were evaluated, a chemical stabilization method based on addition of phosphate (PO₄) and a microbial method in which lactate was added to stimulate growth of dissimilatory sulfate and metal reducing organisms. Neither method was effective. This was believed to be due to sweeping of the leach solution from the columns by the phosphate- or lactate-amended solutions. This hypothesis is consistent with limited mixing in an aquifer as a result of plug flow through the formation.