Geochemistry of acid mine contamination-aquifer interactions

The Pinal Creek Basin near Globe, Arizona, is an example of a groundwater/surface water system contaminated by wastes from historic metal mining practices. Acidic mining wastes that have leached into the regional aquifer have come in contact with alluvial sediments, dissolving manganese oxides and precipitating iron sulfato-hydroxides. Manganese is reoxidized in sediments of a perennial stream (Pinal Creek) formed as the aquifer thins and groundwater is forced to land surface. Iron and manganese in the system are inextricably linked through redox couples. These interactions, which produce reaction rates varying by orders of magnitude and non-stoichiometric release of manganese, are poorly understood. We have designed a flow-through reaction cell to obtain in situ real time X-ray adsorption spectroscopy (XAS) data during geochemical reactions. By coupling solution chemistry data with spectroscopic measurements, we were able to show that the spinel mineral, jacobsite, forms as a metastable intermediate phase during the reductive dissolution of manganese oxide (MnO2) by ferrous iron (Fe(II)). The production of Fe(III) followed by the precipitation of ferric hydroxides and the formation of the jacobsite phase, were responsible for reducing the rate of the reaction as time progressed. Laboratory studies have determined that Mn-oxidation in stream sediments increases Co, Ni, and Zn loading through a variety of processes, including oxidation, co-precipitation, solid solution formation, structural exchange, and/or sorption. This information provides a more complete picture of the controls on metal transport and attenuation in the Pinal Creek Basin.