Facilitated Transport of Contaminant Metals Through an Acidified Aquifer

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Abstract
A highly acidic aqueous waste containing metals was released into unlined seepage basins between 1955 and 1988 resulting in the contamination of the underlying aquifer. To provide insight about the mechanism (s) responsible for the facilitated movement of several of these contaminant metals, ground‐water samples were recovered from the aquifer along a 1.02‐km transect at approximately the rate of ground‐water flow. Facilitated contaminant transport was attributed primarily to the poor cation‐sorbing capacity of the aquifer matrix and the soluble nature of the metals in the acidic plume. Based on chemical equilibrium calculations of ground‐water ultrafiltrates which agreed with results from cationic and anionic resin‐exchange experiments, over 90% of each contaminant metal (Cr, Ni, Cu, Cd, Pb, and U) existed in cationic forms in the aquifer: either as soluble metals or as sorbates associated with positively charged ground‐water colloids. These cationic species were not retained by the aquifer because the pH of the aquifer matrix was slightly 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 ground‐water colloids and this association increased with the pH of the system. However, mobile colloids would likely play only a small role in the transport of contaminants through this aquifer because of their relatively low concentration.