Investigation of in situ leach (ISL) mining of uranium in New Mexico and post-mining reclamation

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 phosphateor lactateamended solutions. This hypothesis is consistent with limited mixing in an aquifer as a result of plug flow through the formation. peroxide) and a carbonate-complexing agent. It is injected into the aquifer to solubilize U by oxidizing it to more soluble phases, which then dissolve through a series of complexation reactions. The soluble U complexes are transported by groundwater flow to the extraction wells where they are pumped to the surface and are recovered in a mill. The barren lixiviant is then pumped back into the aquifer through injection wells and the process is repeated. The major advantages of ISL mining are that (U.S. Nuclear Regulatory Commission, 2009): 1) it produces little disturbance of the land surface, 2) construction costs are much less than conventional underground mining, 3) it is much safer for workers than underground mining, 4) very little solid waste is generated because there is virtually no excavation, and 5) there is little impact on groundwater resources due to mine dewatering. Often these advantages result in significant cost savings. However, there are also notable limitations of ISL mining. The ore deposit must be below the water table; the hydrogeology of the host formation must be amenable to circulation of lixiviant solutions through it; and the U minerals must be reactive with the leach solutions. Though ISL mining has been extensively practiced in other states, there is little experience with it in New Mexico. A major challenge facing the ISL mining industry is that groundwater must be returned to acceptable quality at the conclusion of mining. ISL mining alters the geochemistry of the ore body resulting in release of other constituents at concentrations that may exceed state groundwater standards and federal drinking water standards. In New Mexico, aquifer restoration is especially important because groundwater may be the sole water supply for drinking and other uses, such as in the Grants Mining District. The principal objective of this research was to evaluate two methods of stabilizing aqueous geochemistry and of restoring groundwater quality following ISL mining. A two-phase laboratory investigation was conducted in which different leach solutions were used to evaluate extraction of U from several low-grade ore samples. In contrast to other studies, which were done in batch systems, this investigation was performed using columns filled with U ore to better simulate ISL conditions. The purpose was to generate information regarding the chemistry of fluids that would be produced by ISL mining of NM U ores. The second phase of the investigation consisted of a series of column studies to evaluate two strategies for restoring groundwater quality. One method was based on chemical addition to stabilize minerals that release U and other constituents. This involved circulation of a phosphate (PO4) solution. The second method was based on re-establishing reducing conditions in the aquifer by stimulating growth of anaerobic microorganisms.