Abstract
•Chrysotile and chromite minerals confirmed the ultramafic origin of Vergina soil.•Cr(VI) concentration in Vergina soil decreased continuously with increasing depth.•Mn(III)–Mn(IV) oxides responsible for Cr oxidation are present in the tested soils.•Cr(VI) and Crtot concentrations in groundwater decrease linearly with depth.•Increasing soil pH is considered responsible for decreased Cr oxidation with depth.
In this paper the origin and concentration of chromium (Cr) in an ophiolitic aquifer in Vergina, northern Greece were investigated. The study area has only agricultural activity so that industrial Cr contamination was precluded. Soil sampling included topsoil and drillcore samples collected down to 98m depth. Groundwater samples were collected from three existing wells and a spring at the area and from different depths of the soil boring using the discrete sampling method. Mineralogical analysis of soils confirmed the presence of ultramafic minerals, including chrysotile and chromite. Soil elemental analysis showed significant concentration of total chromium (Crtot; max 12,000mg/kg) and hexavalent chromium (Cr(VI); max 7.5mg/kg). Significant Crtot (91μg/L) and Cr(VI) (64μg/L) concentrations exceeding the drinking water limit of 50μg/L were also detected in groundwater. In both the discrete soil and groundwater samples a decreasing trend of Cr(VI) concentration was observed with increasing depth, while Crtot increased. The increasing trend in Crtot is attributed to the increasing contribution of unweathered ultramafic minerals with depth, while the decreasing Cr(VI) may be related to the increasing soil pH that does not favor Cr(III) oxidation by Mn-oxides.