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

The problem of high levels of chromium is one of the most important issues in soils of the Mediterranean area, in particular those deriving from ophiolitic parent materials. Very often the chromium concentration is greater than 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 soils from three coastal areas of the Cecina Valley (Tuscany, Italy) to understand the origin of chromium in the soils, where high levels of hexavalent chromium were found in well and spring waters of the areas. The main 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 results showed the presence of only trivalent chromium in soil and a positive linear correlation between chromium and nickel (e.g. r = 0.76 for the Marina di Bibbona-Bolgheri area), corroborating the hypothesis of a geogenic origin of contamination. This hypothesis was also supported by the low CRI index for the soils with high total Cr content, indicating a higher presence of refractory minerals in the Marina di Bibbona-Bolgheri area than Cecina and Collemezzano areas. The refractory material found in soils was attributed to the presence of ophiolite outcrops in the surroundings and their sedimentary remnants. The weathering of ultramafic-derived constituents and their regional-scale transport are believed to be responsible for the enrichment of chromium and nickel in the investigated soils.