Environmental impacts and issues of In-Situ Leach uranium mining in the United States of America

Full text: In-situ leach (ISL) mining of sandstone-hosted deposits has been steadily producing uranium in the U.S. since the late 1970s. In Texas, 38 mines have extracted uranium along the southwest Texas coastal plain. Wyoming production has come from three mines developed in the Powder River Basin, and Nebraska has one active ISL facility in the Crawford Basin. Twenty new or expanded ISL mines are expected to submit license applications within the next two years. Increasing concern over groundwater degradation resulting from uranium ISL mining has resulted in more extensive state regulation and prompted the U.S. Geological Survey to review restoration data for these mines to quantify environmental impacts and critical issues. ISL mines in the United States inject oxygen or hydrogen peroxide enriched ground water into reduced roll-front deposits to extract uranium via carbonate-uranyl complexes in ground water. Some operators add carbon dioxide, carbonate or bicarbonate to the lixiviant as a carrying agent, although sandstones in the U.S. commonly contain enough native carbonate that this is unnecessary. Other lixiviants utilized in past operations including sodium and ammonium bicarbonate and sulphuric acid are not used in modern U.S. operations because of difficulties in restoring ground water to pre-mining conditions. Baseline studies show Ra, U, As, Pb, Cd, Se, and Hg in groundwater before ISL mining are typically elevated above U.S. Environmental Protection Agency (EPA) drinking water Maximum Contaminant Levels (MCL). Cl, Mn, Fe, total dissolved solids, and sulfate are commonly elevated above secondary EPA standards. Newly-raised challenges to ISL mining have questioned the validity of statistical methods used when calculating baseline values in ISL operations. Restoration techniques used in U.S. ISL well fields are dominated by ground water sweep and treatment of ground water by reverse osmosis. Additionally, two well fields have been restored using bioremediation. In other well fields reductants (hydrogen sulfide gas and sodium sulfide) have been used to reverse the effects of oxidizing lixiviants. Ra, U, As, Pb, Se, Cd and Hg are sometimes elevated above MCL after restoration as are Mn, Fe, Cl, total dissolved solids and sulfate. Criticism about inadequate monitoring has led to proposed regulation changes lengthening post-restoration monitoring. None of the restoration techniques identified to date have been successful in returning all ground water parameters to baseline. However, some well fields have been restored to 'class of use' which generally satisfies state requirements. (author)