Geochemical interactions between uranium-tailings fluids and subjacent bedrock. Canon City, Colorado: use of the computer model MINTEQ

Contamination of domestic water wells by Mo, Se, SO4 and U has been documented in the vicinity of a uranium mill near Canon City, Colorado. Fluids collected from the tailings ponds were passed through cores of the subjacent calcite-bearing sandstone to determine the effect of pH and Eh on the mobility of Al, Ca, Cu, Fe, Mn, Mo, Se, SO4, V, U, and Zn. During Experiment 27 the pH initially increased from 2.3 to 8.0 as calcite in the core dissolved. Concurrently, iron hydroxide precipitated in the micro-environment surrounding the carbonate grains, effectively reducing the area of calcite exposed to the acidic eluent. This led to a decrease in pH to 3.4. Experiment 27 was modeled using the mass transfer computer program, MINTEQ. The pH was modeled by dissolving decreasing amounts of calcite to simulate the acidification of the system, while Eh was set at the levels measured in the experiments. Mn was adequately described by the dissolution of manganiferous calcite, but an adequate model for dissolved Ca required both calcite dissolution and ion-exchange of Ca by Na. Al was simulated by the solubility constraint imposed by an amorphous aluminum hydroxide above a pH of 6.0, ad by AIOHSO4 in more acidic regimes. Fe was modeled by the precipitation of an amorphous hydroxide. Zn was modeled using triple-layer sorption routine with an amorphous iron hydroxide phase as the sobent, but Cu could not be modeled using the same values for the triple-layer parameters. Se sorption is affected by both the mass of sorbent in the system and by competition for surface sites with sulfate ion. The experiments suggest that Se may be the best tracer of the escape and movement of raffinate in the aquifer at Canon City