Elements influencing cost allocation in the Pinal Creek Aquifer, Arizona. USA. Part II: Geochemical controls on groundwater quality recovery

This is the second article in a three-part series (all published in this volume) describing the framework used to quantitatively allocate mass loading of metals to the Pinal Creek alluvial aquifer among multiple sources. This contribution describes the formation of, and zonations within, the 20 km plume, which consists of a Peripheral Margin (pH > 5.5), the Intermediate Zone (pH 4.2-5.5), and an Acidic Core (pH < 4.2), where metal-bearing iron oxyhydroxides and acid-bearing minerals (jarosite, A1[OH]SO4, and FeSO4 cements) have precipitated. While source remediation has improved groundwater quality, temporal reductions in solute concentrations at downgradient wells exhibit asymptotic behavior due to ongoing dissolution of acid-bearing precipitates, demonstrating that the total mass of metals released to the system over the last 75 years will dictate the ultimate remedial duration and cost. Column experiments, geochemical modeling, and analysis of empirical data indicate a minimum range of remedial time frames from about 20 years in the Peripheral Margin to more than 140 years in the Acidic Core. This analysis comprises a hitherto neglected element of cost allocation by causatively linking historical releases to the current and future environmental degradation that will incur response costs.