Chromium exists primarily in trivalent (Cr(III)) or hexavalent (Cr(VI)) oxidation states. Cr(VI) is a recognized environmental pollutant because it is a strong oxidant and much more toxic than Cr(III). Cr(III) occurs naturally (widely distributed in soils and plants) and is an essential nutrient. Cr(VI)is generally associated with industrial processes, although it does occur naturally in small amounts.
Currently there are few federal or state standards for Cr(VI). Most regulatory standards specify a limit on total chromium (the combined concentrations of Cr(III) and Cr(VI)). The EPA has recently recommended enhanced monitoring for Cr(VI) by water systems even if it isn’t a current regulatory standard.
For more information, please see the following links:
EPA’s Recommendations for enhanced monitoring for Hexavalent Chromium in Drinking Water