Evidence for the natural origins of anomalously high chromium levels in soil of the Cecina Valley (Italy).

The problemof high levels of chromiumis one of themost important issues in soils of theMediterranean area,in particular those deriving fromophiolitic parent materials. Very often the chromiumconcentration is greaterthan the threshold values of legislation on soil pollution and the knowledge of the origin of contamination(natural or anthropogenic) is important to formulate risk characterization. This study evaluated the soilsfrom three coastal areas of the Cecina Valley (Tuscany, Italy) to understand the origin of chromium in thesoils, where high levels of hexavalent chromium were found in well and spring waters of the areas. Themain soil characteristics and the correlations among the values of chromium and nickel were determined.Chromium speciation was evaluated by synchrotron radiation X-ray absorption spectroscopy. The resultsshowed the presence of only trivalent chromium in soil and a positive linear correlation betweenchromium and nickel (e.g. r = 0.76 for the Marina di Bibbona-Bolgheri area), corroborating the hypothesisof a geogenic origin of contamination. This hypothesis was also supported by the low CRI index for thesoils with high total Cr content, indicating a higher presence of refractory minerals in the Marina diBibbona-Bolgheri area than Cecina and Collemezzano areas. The refractory material found in soils wasattributed to the presence of ophiolite outcrops in the surroundings and their sedimentary remnants. Theweathering of ultramafic-derived constituents and their regional-scale transport are believed to beresponsible for the enrichment of chromium and nickel in the investigated soils.