| Metal contamination and acidity in mine drainage pose environmental threats to ecosystems and water supplies. Changes in metal contamination and acidity in acid mine drainage can be modeled in order to determine the main chemical processes controlling drainage chemistry. I applied this modeling technique to data taken from the Pecos Mine Operable Unit, New Mexico. Here, drainage from a waste rock pile varies in pH from three to five and carries high concentrations of zinc, sulfate, aluminum, copper, and other contaminants. As drainage flows toward the Pecos River, pH increases to greater than seven and heavy metal content decreases. In order to formulate a comprehensive process model of the drainage, I use published data on adsorption to aluminum oxide to include surface complexation to aluminum as a possible metal sink.;A process model of this drainage shows the main controls on the pH are reaction with local bedrock, limestone conglomerate, and concurrent mixing with tributary streams draining unmined regions. Models that account for both calcite dissolution and mixing of up to three kilograms background water per kilogram of mine drainage water ratio reproduce the observed decrease in aqueous metal concentrations with increasing pH. Contaminant concentrations attenuate via three different pathways: zinc, magnesium, manganese, and sulfate concentrations decrease primarily through dilution; aluminum, copper, and iron precipitate directly from solution as oxide and hydroxide phases; lead adsorbs to precipitating hydroxide surfaces. |
