Mineralogical and geochemical characterization of a chromium contamination in an aquifer - A combined analytical and modeling approach

Abstract

This study presents a detailed mineralogical and chemical characterization of a chromium contamination in alluvial deposits underlying a leather tannery in Austria. Drill cores revealed that the contamination is represented by a black layer that exhibits sharp boundaries with adjacent brown alluvial sediments. This black layer is located below the groundwater table and begins at the source of the contamination. The black layer thickens with increasing distance from the source and the depth of the upper boundary remains at the same level relative to the surface. Mineralogical and chemical investigations showed that the black layer contains reduced Cr phases, a Cr(III) hydroxide and a Cr(III)-Ca-containing hydrocalcite, which are present as discrete grains, in vein-like pore spaces and/or as a coating around mineral grains. Cr(VI) is nearly only present above the groundwater table. Our 2-D model predicts a longitudinal spread of the black layer of approximately 37 m downgradient from the source and a maximum transversal spread of approximately 10 m. This study shows that, if the source of the contamination is not eliminated, the plume will increase continuously farther downgradient.