GROUNDWATER IMPACTS OF URANIUM MINING AND MILLING

At least eight uranium deposit types are thought to exist in the United States (U.S.) Each are amenable to underground and (or) open pit mining, which require surface milling. In situ recovery (ISR), which is considered both subsurface mining and milling, was specifically developed to economically access low-grade, sandstone-hosted deposits. In the U.S., much of the open pit and underground uranium mining and milling took place prior to the promulgation of environmental laws in the late 1970s and resulted in groundwater impacts (e.g., aquifer drawdowns from dewatering, large consumptive water use, leakage of contaminants from pit lakes and evaporation ponds, leaching of mine waste and mill tailings piles into shallow groundwater, and migration of contaminant plumes in aquifers). Causes of these legacy groundwater impacts are now addressed by environmental regulations and industry best practices for current and future operations. ISR, which is the primary method of uranium mining in the U.S. today, eliminates mine and mill tailings and minimizes surface disturbances typical of underground and open pit mining. ISR employs wells to inject water enriched with an oxidant (usually oxygen gas) and a complexing agent (usually carbon dioxide gas or bicarbonate) into the saturated uranium deposit to dissolve the uranium in the aquifer and subsequently pump uranium-rich groundwater to ion exchangers to concentrate and precipitate the solid uranium oxide or “yellowcake”. Although ISR is considered to have a lower environmental impact due to its smaller surface footprint, groundwater concentrations of dissolved radionuclides (e.g., uranium, radium), trace elements (e.g., arsenic, lead, molybdenum, selenium) and major anions (e.g., sulfate, chloride) are often elevated after mining within the mining zone. Although current regulations require remediation of groundwater, restoring pre-mining water quality has proven challenging. The greatest concerns regarding ISR are the migration of contaminated groundwater from the mining zone into adjacent aquifers and large quantities of wastewater disposed of in deep wells. Regulators, mining companies and research institutions are working toward developing effective environmental remediation technologies and restoration strategies to mitigate groundwater impacts.