GEOCHEMICAL MODELING OF HARD-ROCK MINE DRAINAGE AND MINE-IMPACTED WATERS

Geochemical modeling is an important tool for the management of mine-impacted waters, particularly when the model is based on fundamental biological and chemical process information. Acid mine waters with low pH and high metal concentrations are particularly prone to microbial activity controlling metal-redox cycling and driving mineralogical reactions. We present two case studies that demonstrate how understanding coupled biotic-abiotic processes in mine-impacted waters improves geochemical model development, and how these models can then be used to guide site management. In the first case study, we investigated iron-rich precipitates that form in pipelines carrying acid mine drainage at five mine sites in California and Colorado. Microbial Fe(II) oxidation at pH values less than 5 drives precipitation of the Fe(III)-rich minerals schwertmannite and goethite, eventually causing flow restriction within the pipelines. We developed a geochemical model using PHREEQC that incorporated kinetic expressions for microbial Fe(II) oxidation and mineral precipitation based on laboratory and field studies. The model was used to evaluate various possible remediation strategies at Iron Mountain Mine, California. In addition, the influence of aqueous speciation on trace metals associated with the precipitates was elucidated using modeling and laboratory experiments. The second case study investigated the fate of Cu and other trace elements along Lower Spring Creek, a stream which transports metals downstream of Iron Mountain Mine. Sampling of water, sediment, and algae along with geochemical modeling were used to quantify attenuation of Al, Cu, Cd, Fe, and Zn along several reaches of the stream. The role of colloids and algae on metal mobility, transport, and attenuation is also currently being investigated at the site, as we strive to incorporate these processes into geochemical modeling.