Geochemistry of an acidic chromium sulfate plume

The historical disposal of acidic chromium sulfate solutions into unlined lagoons between 1953 and 1970 at an industrial site resulted in formation of a dense aqueous phase liquid (DAPL) plume [specific gravity 1.11 g/cm(3), pH 3, up to 4700 mg/L Cr(III), and up to 90,000 mg/L SO4]. The DAPL sank through the shallow glacial till aquifer to an underlying impermeable gneissic bedrock from where it migrated downgradient along buried channels incised in the bedrock. Because of its high density, the plume chemistry is sharply stratified vertically. Chromium(III) predominates in the DAPL because excess Cr(Vl) not reduced in the original process has been reduced by Fe(H) derived from silicates, while Cr(OH)(3)(am) occurs as surface coatings on silicate minerals and as discrete particles mixed with Fe(OH)(3)(am) and Al(OH)(3)(am). The solubility of Cr(OH)(3)(am) accurately describes Cr(III) concentrations in the plume and nearby groundwater, while Al and Fe in solution are also consistent with solubility-controlling oxyhydroxides. Because of these solubility controls, metal cations are attenuated relative to more mobile Cl and SO4, resulting in a chromatographic separation of solutes downgradient from the plume origin. The good agreement between predicted and observed solution concentrations illustrates the utility of equilibrium modeling when interpreting metal transport characteristics and in determining the efficacy of natural attenuation in subsurface systems. (c) 2007 Elsevier Ltd. All rights reserved.