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 Final 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 mo deling 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, copp er, aluminum and other metals. From 1984 to 1994, concentrations of co ntaminants in the alluvial aquifer in Final Creek Basin, Arizona, decr eased as a result of mixing, recharge, remedial pumping and chemical r eactions. For reactions involving gypsum and rhodochrosite, the equili brium modeling assumption of a local geochemical equilibrium was gener ally valid. From 1984 to 1990, water along the simulated flow path was at equilibrium or slightly supersaturated with gypsum, and gypsum equ ilibria controlled dissolved concentrations of calcium and sulfate. Be ginning in 1991, water in the acidic part of the plume became increasi ngly undersaturated with respect to gypsum, indicating that the gypsum available for dissolution in the aquifer may have been completely con sumed by about 1991. Rhodochrosite precipitation was thought responsib le for the measured attenuation in dissolved manganese in the neutrali zed zone. For reactions involving calcite, the assumption of a local g eochemical equilibrium was generally not valid. Dissolution of calcite in the transition zone was not sufficient to establish equilibrium al though, following neutralization, the calcite saturation index decreas ed to -1.2 in 1986. Calcite undersaturation decreased along the flow p ath in the neutralized zone, and equilibrium was attained about 7 km d owngradient of the transition zone. The assumption of a local geochemi cal equilibrium was not valid for oxidation-reduction reactions that i nvolved iron oxides and manganese oxides. Kinetically controlled oxida tion-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 c ontaminant transport at Final Creek, future work will require a kineti c modeling approach to more accurately simulate selected reactions bet ween the plume and aquifer materials. (C) 1998 Elsevier Science B.V. A ll rights reserved.