Analysis and simulation of reactive transport of metal contaminants in ground water in Pinal Creek Basin, Arizona

Large-scale mining activities have generated a plume of acidic ground water more than 15 km long in the regional aquifer of the Pinal Creek Basin. A one-dimensional reactive-transport model was developed using PHREEQC to aid in the analysis of transport and chemical processes in the plume and to determine the uses and limitations of this type of modeling approach. In 1984, the acidic part of the plume had a pH as low as 3.4 and contained milligram-per-liter concentrations of iron, copper, aluminum and other metals. From 1984 to 1994, concentrations of contaminants in the alluvial aquifer in Pinal Creek Basin, Arizona, decreased as a result of mixing, recharge, remedial pumping and chemical reactions. For reactions involving gypsum and rhodochrosite, the equilibrium modeling assumption of a local geochemical equilibrium was generally valid. From 1984 to 1990, water along the simulated flow path was at equilibrium or slightly supersaturated with gypsum, and gypsum equilibria controlled dissolved concentrations of calcium and sulfate. Beginning in 1991, water in the acidic part of the plume became increasingly undersaturated with respect to gypsum, indicating that the gypsum available for dissolution in the aquifer may have been completely consumed by about 1991. Rhodochrosite precipitation was thought responsible for the measured attenuation in dissolved manganese in the neutralized zone. For reactions involving calcite, the assumption of a local geochemical equilibrium was generally not valid. Dissolution of calcite in the transition zone was not sufficient to establish equilibrium although, following neutralization, the calcite saturation index decreased to −1.2 in 1986. Calcite undersaturation decreased along the flow path in the neutralized zone, and equilibrium was attained about 7 km downgradient of the transition zone. The assumption of a local geochemical equilibrium was not valid for oxidation–reduction reactions that involved iron oxides and manganese oxides. Kinetically controlled oxidation–reduction reactions continued in the acidic part of the flow path for years following the passage of the transition zone. Although the equilibrium approach helped to provide an increased understanding of contaminant transport at Pinal Creek, future work will require a kinetic modeling approach to more accurately simulate selected reactions between the plume and aquifer materials.