Requests for Alternate Concentration Limits (ACL’s) of contamination in groundwater at uranium mill tailing sites must evaluate the potential harm to human health and the environment. At Gas Hills, Wyoming, the multi-component reactive transport code PHREEQC supported the ACL application in demonstrating that milling-related constituents in groundwater will not pose a hazard to human health and the environment at designated Points of Exposure (POE’s). In the conceptual geochemical model for the site, acidic tailing water containing elevated sulfate concentrations mixes with native groundwater and then reacts with calcite along the flow path. Gypsum precipitates, and metals and radionuclides are attenuated by both adsorption to iron oxyhydroxides and by ion exchange with clay minerals. As groundwater continues to migrate into more reduced zones downgradient, precipitation of metal sulfide and other reduced mineral phases occur at the redox boundary, similar to the formation of the original uranium roll-front deposits mined in the Gas Hills.

In the reactive transport model, inflows consisted of representative tailing water containing hazardous constituents at the proposed ACL concentrations. Inflow concentrations were allowed to decline during model runs to simulate the expected decrease in tailing water contribution to groundwater over time. Water quality data from downgradient monitoring wells were used to define initial conditions along the flow path. In addition, surface adsorption and ion exchange parameters for the metals and radionuclides were based on a combination of site-specific and literature data. Mineral phases such as calcite, gypsum, uraninite, radium sulfate and amorphous thorium hydroxide were allowed to precipitate if they became oversaturated. Although pyrite, gypsum, and calcite are known to occur in the aquifer, groundwater data along the flow path were undersaturated with respect to these minerals and therefore no mineral phases were assumed to be initially present in the model. Consequently, surface adsorption and ion exchange were the dominant mechanisms controlling the modeled concentrations of hazardous constituents. The modeled concentrations were within the range of background concentrations at the POE’s throughout the 1,000 year compliance period.