Mineralogical and Physicochemical Methods to Differentiate between Natural and Anthropogenic Sources of Chromium in Soils

This study evaluates pollution from a former tannery by examining soils containing high concentrations of chromium (between 138 and 720 mg/kg dry matter according to the sample). This study presents an original methodology for characterizing chromium according to its two possible origins in soils: natural or anthropogenic. Conducting both a mineralogical study and a physicochemical study made it possible to clearly understand these origins. Natural chromium is particularly linked to pyroxenes, a primary mineral of the Massif Central, France, and found in large quantities in the samples studied. Anthropogenic chromium is adsorbed by vermiculites on their interfoliary sites. These clays possess a high capacity of cationic exchange and can thus attract chromium easily. In all of the samples studied, the naturally chromiferous mineral contribution was high (97 to 519 mg/kg dry matter), whereas that of the clays was low (41 to 201 mg/kg dry matter), indicating a smaller proportion of chromium introduced by humans. Natural chromium values are approximately 70 to 80%, whereas those for anthropogenic chromium are approximately 20 to 30%. This approach makes it possible to set realistic objectives for the cleanup of chromium-contaminated soils.