Chromium is a naturally occurring element that primarily exists in three states Chromium-0, Chromium III and Chromium VI (Hexavalent Chromium). Chromium-0 is metallic and is used in the production of steel. Chromium III readily forms many different compounds and is an essential human dietary element. However it is Chromium VI that is the most hazardous form of the element. Hexavalent chromium, Cr(VI), exists in ground water as
the relatively mobile and toxic anions HCrO4 -and CrO-42 . The reduced form of chromium, Cr(III), is a cation that under slightly acidic to alkaline conditions is fairly insoluble, readily adsorbed, and much less toxic than Cr(VI). Selenium also undergoes important redox transformations. Selenate (Se(VI)) is more mobile and less toxic than selenite (Se(IV)).