Seasonal groundwater monitoring for trace-element distribution and CR(VI) pollution in an area affected by negligible anthropogenic effects

Water quality concerns have motivated the investigation of toxic trace element levels in natural systems and the weathering of outcrops of ultramafic rocks is a potential source of toxic trace elements. The Pollino Massif (southern Italy) is included in the area of the Pollino National Park, where ophiolite-bearing terrains outcrop. In this area anthropogenic pollution is negligible and a proper groundwater chemical characterization furnish the chance of evaluating solely natural metal pollution. With this in mind we assessed the distribution of a large set of trace elements, including Cr as total dissolved Cr and Cr(VI), in spring water sampled both in the wet season and in the dry season. The water have either a Mg-HCO3 or Ca-HCO3 type composition and during the dry season there is a significant increase in the number of the Mg-HCO3 water. A critical scenario of contamination from natural mineralization has been observed for Cr(VI) in springs of both Mg-HCO3 and Ca-HCO3 type water, and in both the seasons. These water flow in serpentinite, gneiss, conglomerate, and slate. The Cr(VI) concentration is, for each water, higher in the dry season, suggesting that natural Cr(VI) pollution may be enhanced during drought periods. Finally, R-mode factor analysis indicates the water, in both seasons are characterized by: i) the release of Si, Sc, and Cr, due to the dissolution of Cr-rich silicic rocks, ii) a common fate of high solubility for As, Ba, Sr, and, iii) a common fate of low solubility for A1, Fe, and Mn.