Reactive solute-transport simulation of pre-mining metal concentrations in mine-impacted catchments: Redwell Basin, Colorado, USA

With the increased importance of water resources in the western United States and many areas worldwide, the remediation of impacts from historical mining becomes ever more important. A possible process of making decisions about remediation for a catchment might include identification of principal sources of metals in the catchment, classification of the sources as natural or anthropogenic, and simulations to evaluate different options for removal of anthropogenic sources. The application of this process is based on understanding the pre-mining conditions in the catchment, so that remediation goals appropriately correct for the impacts of mining. A field experiment in Redwell Basin, Colorado, provided a setting to demonstrate this process and to evaluate pre-mining concentrations through reactive solute-transport modeling. The field experiment provided spatially detailed stream and inflow samples that were the basis for model calibration. Only two inflows along the study reach were affected by mining or mine exploration. To simulate pre-mining conditions, these inflows were removed from the model calibration; the result was a simulation of the stream with all the non-mining inputs. At a point downstream from the two mining inflows, the simulated pre-mining pH would have been 5.1, up from the measured 3.8. At the higher pH, the streambed likely would have been coated with Al precipitate. Simulated pre-mining Zn and Cu would have been 1300 µg/L and 18 µg/L, lower than the measured concentrations of 3340 and 93 µg/L. Despite these changes, the pre-mining conditions would not have met aquatic-life standards.