FACILITATED TRANSPORT OF CONTAMINANT METALS THROUGH AN ACIDIFIED AQUIFER

A highly acidic aqueous waste containing metals was released into unli ned seepage basins between 1955 and 1988 resulting in the contaminatio n of the underlying aquifer. To provide insight about the mechanism(s) responsible for the facilitated movement of several of these contamin ant metals, ground-water samples were recovered from the aquifer along a 1.02-km transect at approximately the rate of ground-water flow. Fa cilitated contaminant transport was attributed primarily to the poor c ation-sorbing capacity of the aquifer matrix and the soluble nature of the metals in the acidic plume. Based on chemical equilibrium calcula tions of ground-water ultrafiltrates which agreed with results from ca tionic and anionic resin-exchange experiments, over 90% of each contam inant metal (Cr, Ni, Cu, Cd, Pb, and U) existed in cationic forms in t he aquifer: either as soluble metals or as sorbates associated with po sitively charged ground-water colloids. These cationic species were no t retained by the aquifer because the pH of the aquifer matrix was sli ghtly below the measured point-of-zero charge, indicating the variable charge sites on the mineral surfaces within the aquifer likely had a net positive charge. Contaminants were associated with recovered groun d-water colloids and this association increased with the pH of the sys tem. However, mobile colloids would likely play only a small role in t he transport of contaminants through this aquifer because of their rel atively low concentration.