Incorporation of coprecipitation reactions in predictive geochemical models

The geochemical model used to predict the composition of water in the pit lake formed after cessation of mining at Echo Bay Mineral’s Cove pit (Lander County, Nevada) included reactions to describe the coprecipitation of cadmium into calcite. Models that include coprecipitation reactions provide more realistic estimates of water compositions than do models that only use mineral precipitation and adsorption reactions. At Cove, coprecipitation reactions reduce the predicted concentration of cadmium to below regulatory limits. Programs that are typically used to perform predictive calculations (e.g., PHREEQC and MINTEQA2) do not explicitly contain routines to assess coprecipitation, but such reactions can be easily included in the conceptual geochemical model. For the coprecipitation of cadmium into calcite, values for the distribution coefficient (D) in the range of 70 to 1500 were obtained from the literature. The values for D are large and indicate that coprecipitation is an important process in controlling concentrations of cadmium in systems that also contain calcite. Initial calculations used the Doerner and Hoskins equation. However, the large values for D resulted in final concentrations of cadmium that were unrealistically low. An alternative equation developed by Riehl was employed in the final estimates. For large values of D, the assumptions in the Doerner and Hoskins equation are not valid and the estimated dissolved concentrations of cadmium become extremely small. The estimates using the Riehl equation are reasonable, and agree with observed conditions.