APPLICATION OF GEOCHEMICAL MODELING FOR IN SITU REMEDIATION OF URANIUM

Solute speciation and residence times, reaction rates, adsorption/desorption and mineral precipitation/dissolution are important geochemical components of monitored natural attenuation (MNA). Uranium (U), Arsenic, molybdenum, selenium, vanadium, and radium-226 are important solutes of environmental concern at in-situ recovery uranium mining sites. Transport of these chemicals in groundwater is controlled by aqueous speciation influencing adsorption and/or precipitation, depending on site-specific redox conditions of aquifer systems. The computer code PHREEQC is used to quantify geochemical reactions important for evaluating the viability of MNA and bounding the extent to which they take place in mineralized aquifer systems. The aqueous geochemistry and mineralogy of the Smith Ranch-Highland U deposit, Wyoming provides an excellent example for modeling different water-mineral reactions taking place prior to (baseline) and during mining and after aquifer restoration. Hydrous ferric oxide is an important adsorbent for U(VI) and other trace elements associated with U rollfront deposits. This adsorbent is stable under relatively oxidizing conditions during both mining operations and post closure. Uranium(VI) complexing with bicarbonate and carbonate decreases the extent to which this actinide adsorbs onto HFO under oxidizing and circumneutral pH conditions. Chemical characteristics of upgradient groundwater control the long-term composition of post-remediated groundwater at the site. The long-term reductive capacity of an aquifer system and reaction rates are the two most important components controlling precipitation of U(IV, VI) minerals and other trace elements during and after groundwater remediation. Achieving baseline or pre-mining conditions at in-situ remediated U mining sites depends on site-specific geochemical and hydrological conditions, chemicals used for extracting U, and longevity of mining operations.