High concentrations of H{sub}2O{sub}2 are being tested for in situ oxidation and remediation of buried organic contaminants in soils and groundwater. Peroxide is being considered as a direct chemical oxidant in Fenton-type reactions or as a source of oxidizing equivalents in bioremediation schemes. How H{sub}2O{sub}2 affects the oxidation state and solubility of Cr(III) and Cr(VI), common co-contaminants with organic chemicals, is explored here in four chemically diverse soils containing elevated levels of Cr. Soil contaminated with soluble Cr(VI) from chromite ore processing residue and soil containing high levels of recently reduced Cr (III) from electroplating waste both released dissolved Cr(VI) after single applications of up to 24 mM H{sub}2O{sub}2. In no case was there evidence that H{sub}2O{sub}2 reduced preexisting Cr(VI) to Cr(III), even though this would be allowed thermodynamically. Chromate in the leachates exceeded the U.S. EPA drinking water standard for total dissolved Cr (2μM) by a factor of 10-1000. Anaerobic conditions in an organic-rich, tannery waste-contaminated soil protected Cr(III) from oxidation and mobilization. Mineral forms of Cr in serpentinitic soil near a former chromite mine also resisted oxidation on the time scale of days. Mobilization of Cr(VI) could be a hazardous consequence of using H{sub}2O{sub}2 for in situ remediation of chemically complex wastes, but H{sub}2O{sub}2 could prove attractive for ex situ treatment (i.e., soil washing). This paper demonstrates marked differences among Cr-contaminated soils in their capacity to release Cr(VI) upon chemical treatment with H{sub}2O{sub}2.
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