Factors controlling the heterogeneous distribution of Cr(VI) in soil, plants and groundwater: Evidence from the Assopos basin, Greece

The effective influence of industry or ultramafic rocks by natural processes to soil, plants and groundwater contamination by chromium, which is often a subject of debate, was investigated for the case of the Assopos basin, Greece. The Neogene Assopos basin, is mainly composed by Tertiary and Quaternary sediments of more than 400 m thick and is characterized by brittle type deformation (fault zones, faults). Chromium in soil, ranging from 67 to 204 ppm, is mostly hosted in chromite, Fe-chromite, Cr-bearing goethite and silicates.Special attention was given to the plants, which are a major source of organic matter that serves as the driving force for Cr(VI) reduction. The increase of the Fe, Mn and Ni contents, with the increasing Cr content in the plant-roots, in particular at the external parts of roots and those of bulb-type plants, suggest reduction and immobilization of Cr(VI) and that redox reactions play a significant role to the translocation processes from root to shoot.Groundwater samples from the Assopos aquifer showed a wide spatial variability, ranging from <2 to 180 ppb Cr_total content almost same to the Cr(VI)-values] despite their spatial association. The presence of Cr(VI)-contaminated groundwater at depths >200 m is attributed to a direct injection of Cr(VI)-rich industrial wastes at depth rather than that Cr(VI) is derived from the Assopos river or by the interaction between water and Cr-bearing rocks. The heterogeneous distribution of Cr in groundwater may be related with the intense neotectonic deformation, as is exemplified by several sharp tectonic contacts between sediment types, while the Cr content in soil is mostly depend on the transported chromite grains.