U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 1 of 3--Contamination From Hard-Rock Mining, Water-Resources Investigation Report 99-4018A

Geochemistry and Reactive Transport of Metal Contaminants in Ground Water, Pinal Creek Basin, Arizona

By James G. Brown, Pierre D. Glynn, and R.L. Bassett

This report is available in pdf format: Brown.pdf 179KB

ABSTRACT

Activities related to more than a century of large-scale copper mining in the Pinal Creek Basin in central Arizona have contaminated the regional alluvial aquifer and perennial streamflow with acidity and metals. Water-chemistry and solid-phase analyses and computer-aided geochemical modeling were used to understand the evolution of the ground-water plume between 1984 and 1998. The ground-water plume consists of three hydrochemical zones: (1) an acidic zone, which contains large concentrations of metals and has a pH that ranges from 3.6 to about 5; (2) a transition zone where carbonate-mineral dissolution causes pH to increase to above 5, which results in the precipitation of iron hydroxide and the adsorption of trace metals such as nickel and zinc; and (3) a neutralized zone, which contains large concentrations of manganese, calcium, and sulfate, and has a pH of about 6 to 7. Inverse geochemical modeling using NETPATH revealed that, in addition to calcite dissolution, silicate dissolution was required to account for the mass transfers of calcium and magnesium across the transition zone. Analysis of the measured changes in plume geochemistry was aided by PHREEQC reactive-transport modeling, which helped determine that oxidation-reduction reactions were significant in the acidic zone of the plume through the late 1980's. The local equilibrium assumption required by reactive-transport modeling probably was invalid for oxidation- reduction reactions that involved manganese and, to a lesser extent, neutralization reactions that involved calcite. Sensitivity analyses indicated that the rate of advance of the pH front was highly sensitive to the initial calcite concentration, and that ground water along a flow path near the base of the alluvium was in partial or indirect contact with the atmosphere, possibly through mixing with shallower water.