USING EITHER HYDROGEN OR DITHIONITE AS REDUCTANT IN URANIUM CONTAMINATED GROUNDWATER AT POSTLEACH URANIUM MINING SITES, SOUTH TEXAS

摘要

This paper summarizes the results of a summer research project conducted by a public high school teacher. The project focused on the impacts of in situ recovery (ISR) uranium mining on groundwater quality in South Texas. Uranium is a major groundwater constituent at ISR mining sites and there is need for more effective post-mining restoration strategies. One promising approach is to add reductants to reduce soluble U(VI) species to insoluble U(IV) species that are permanently immobilized within the aquifer formation. In this research a hydrogeochemical computer program from USGS (PHREEQC) was used to simulate how two different reductants, hydrogen gas (H_2) and sodium dithionite (Na_2S_2O_4), induce reduction of U(VI) to U(IV) and groundwater and associated changes in the hydrogeochemistry. The modeling simulation performed with H_2 as the reductant showed that significant reduction of uranium, iron and sulfate was achieved, resulting in very strong reducing conditions. In contrast, the modeling simulation with sodium dithionite as the reductant showed reduction of uranium and iron only. These results have significant implications with respect to the potential effectiveness of H_2 and dithionite as chemical reductants for restoring groundwater quality at uranium ISR mining sites. In particular, while addition of H_2 should achieve stronger reducing conditions near the injection point, the addition of dithionite will likely have a larger zone of influence because it will not be consumed via sulfate reduction reactions. A learning module based on the legacy cycle concept was developed that challenges the students to think through the processes and steps to determine the level of contamination and the amount of restoration required to return groundwater to safe levels. This learning module will be introduced into high school mathematics and science classes during spring semester 2011-2012 school year. A subsequent assessment of the students' performance will be carried out and reported.