Hydrogeochemical investigation of Cr in the ultramafic rock-related water bodies of Loutraki basin, Northeast Peloponnese, Greece

Abstract

This research aims to determine the hydrogeochemical characteristics, origin, and geochemical evolution of the Cr enriched alkaline waters in the ultramafic rock-related water bodies of Loutraki basin in Northeast Peloponnese, Greece. The significance of the particular research lies in the fact that Cr occurrence raises concerns with respect to health risks associated with the utilization of such aquifers for drinking water supply. Therefore, the investigation of water–rock interaction processes leading to Cr mobilization in such environments is of great importance. A total of 41 water samples including groundwater from the alluvial and fractured aquifers as well as surface water were collected and analyzed for major and trace elements during the wet and dry seasons of 2015. Solid samples including soils, sediments and rocks were also collected to provide evidence of elemental mobilization due to water–rock interaction, with special focus on Cr. Τhe main Cr-bearing minerals identified were Cr-spinel, ferrichromite, magnetite, clinopyroxene, chlorite and serpentine. The observed dissolution textures within the magnetite rim and across the magnetite–chromite boundary indicate that these latter can be also active sources of Cr(III). Both groundwater and surface waters are of Mg–HCO3 type due to CO2-driven dissolution of serpentine minerals and Mg-carbonates/hydroxides. The formation of hydromagnesite aggregates is attributed to precipitation from Mg2+-rich alkaline waters. The highest Cr(VI) concentrations were measured in the alluvial aquifer (6.7–74.3 μg L−1) and the lowest in the fractured ophiolitic aquifer (1.9–14.3 μg L−1); while in surface water, the maximum measured Cr(VI) concentration was up to 3.9 μg L−1. Finally, the identification of Mn-rich hematite indicates that the oxidation of Cr(III) to Cr(VI) is probably performed by manganese–iron oxides.