GEOCHEMISTRY OF METALS IN SURFACE AND GROUND WATER FROM AN ABANDONED MINE IN THE NORTH FORK OF THE AMERICAN FORK RIVER, UTAH

We investigated the aqueous geochemistry and hydrology of the North Fork of the American Fork River, its tributaries, and ground water in the vicinity of the Pacific mine to determine the impact ground water has on the relatively high levels of toxic metals (As, Cd, Cu, Fe, Mn, Pb, and Zn) in the river. Brown and cutthroat trout have absorbed As, Pb, and Cd into their tissues at concentrations that are hazardous to human health if consumed. Additionally, ground water flows through the mine tailings before entering the river, producing an acidic plume with high metals concentrations.

We used surface-water discharge measurements and metals concentrations in surface and ground water to determine metals loading rates in the North Fork and its tributaries. The results suggest that dissolved metals enter the river when it is gaining water from ground water during the late spring and early summer. However, the total metals load generally decreases through the reach of the river adjacent to the mine due to the decrease in suspended metals. Cadmium and Mn travel as dissolved species in the North Fork and its tributaries, whereas Cu, Fe, Pb, and Zn travel as suspended solids. Arsenic seems to be associated with both suspended solids and dissolved species.

The geochemical modeling program PHREEQC, incorporating the diffuse double layer surface complexation application module, was used to investigate the chemistry that controls the mobility and attenuation of metals in surface and ground water at the site. Based on the PHREEQC results, the most important reaction in these waters, with respect to metal attenuation, is the precipitation of hydrous ferric oxide. The metals that sorb onto the hydrous ferric oxide are Cu, Pb, most importantly Zn, and to a lesser degree As.