Geochemical constraints on the sources of Cr(VI) contamination in waters of Messapia (Central Evia) Basin

Abstract

The present study aims to define geochemical constraints on contamination source(s) by Cr(VI) in Central Evia, through a compilation of new and literature data on an integrated set of approaches, including mineralogy and (isotope) geochemistry; leach experiments on rocks, soils and Ni-laterites; and comparison with literature data on groundwater.The concentrations of Cr(VI) in leachates from various geological materials decrease in the following order: Ni laterites from Kastoria > soils > altered ultramafic rocks > Ni laterites from Evia. A salient feature is the diversity in the Cr(VI) concentrations and δ53Cr values in (a) groundwater, ranging from <2 to 850 μg/L Cr(VI) and 0.60–1.99‰ δ53Cr, and (b) leachates from Ni-laterites, ranging from 0.6 to 2.0 μg/L and 1.01‰ in Evia samples and from 750 to 1200 μg/L and negative to slightly positive (−0.21 to 0.03‰) δ53Cr in Kastoria samples. A positive correlation (R2 = 0.514) between the δ53Cr values and depth of the aquifers, potentially reflects the decrease in soluble O2 with increasing depth. The relatively high Cr, Mn and the abundance of fine-grained Mn-(hydr)oxides in the Kastoria laterite ore, may be major controlling factors for the existence of high Cr(VI) concentrations in the water leachates. In contrast, the low Cr(VI) concentrations and positively fractionated Cr isotopes in the Ni-laterite leachates from Evia are potentially related to redox and multistage transportation/redeposition processes.The positive correlation between Cr concentrations and Ca/Mg ratios (R2 = 0.647) for rock leachates and the negative correlation (R2 = −0.694) for soil leachates suggest that Cr in groundwater may be released mainly from Cr-bearing Mg-silicates in altered ultramafic rocks and soils. The plot of δ53Cr values versus Cr(VI) concentrations for the contaminated waters from Central Evia and Assopos Basin fall within both fields defined from a global database for natural waters and waters contaminated by human activities.