Elements Influencing Cost Allocation in the Pinal Creek Aquifer,Arizona, USA: An Introduction and Synopsis of Findings

This article provides context for the ensuing three-part study published in this volume that describes quantitative allocation of mass metal loading to the 20 km groundwater plume in the Pinal Creek alluvial aquifer. The plume resulted from >75 years of copper ore leaching by ferric sulfate and sulfuric acid in the Globe-Miami mining district, Arizona. Geochemical fingerprinting, followed by spatial and temporal analysis of Pinal Creek monitoring well data, identified three distinct source areas and plumes. Each exhibited a unique chlorine-copper-iron chemical signature that resulted from differences in process geochemistry, ore mineralogy, and solution handling. As the acid plume advanced, carbonate buffering capacity was consumed, with concomitant precipitation of metal oxyhydroxides that evolved into acid-bearing aluminum and iron cements. Column experiments, geochemical modeling, and empirical data indicate that dissolution of the residual acidic precipitates will result in asymptotic reductions in metal concentrations, which will affect response costs for up to 140 years after initiation of remedial pumping in the late 1980s. Finally, metal loading to the alluvial aquifer was quantified for each source area using Darcy's Law or flow data combined with the sum of aluminum, copper, iron, manganese, and zinc, which constitute >99% of the total metal mass. Based on this analysis, to date Webster Gulch contributed 94% of the loading, Upper Bloody Tanks Wash contributed 5%, and the Miami Unit contributed 1%. A sensitivity analysis that varied all parameters in the loading calculation by ±20% resulted in only small differences in allocation (±1%) because the large mass released from Webster Gulch (618 kt of metal) dominates the overall allocation.