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EM. - ' Natural Chromium Levels in Sods The chromium content of natural minerals and soils varies widely with the type and nature of the rocks or sediments The lowest natural chromium levels are generally found in granites (1 - 26 mg/kg), carbonates 0 - 16 mg/kg) and sandy sediments (16 - 36 mg/kg) The highest natural chromium levels are usually found in suspended river sediments (typical value 187 mg/kg), shales and (88-400 mg/kg), soils 0 - 3,000 mg/kg), and ultramafic igneous rocks (1,000 - 3,400 mg/kg) (Alloway 1990, Richard 1991) The highest chromium content tends to be associated with the finest grain size soils ' (Richard 1991) 5 eciation in Auatic Systems ' The median chromium concentration in most unpolluted fresh or seawater is usually less than 3 ug/L Natural chromium concentrations as high as 208 ug/L have been reported, however Elevated chromium levels are generally found in waters contacting soluble chromium minerals (Richard 1991) The EPA drinking water maximum contaminant level for chromium is 100 ug/L total chromium In environmental systems, chromium occurs in the hexavalent (Cr(VI)) and trivalent (Cr(III)) oxidation states Cr(VI) is a strong oxidant and can be reduced to Cr(ill) by reaction with ferrous iron (Fe(ll)), sulfides, and natural organic matter (Rai 1989, Richard 1991, Palmer 1991) Reduction of Cr(VI) by Fe(II) atoms released from clays and minerals and by organic matter has been reported even in the presence of dissolved oxygen (Eary 1991, Wittbrodt 1995) Cr(VI) reduction rates are pH dependent and increase under acidic conditions (Eary 1991, Wittbrodt 1996) In natural systems, oxidation of Cr(III) to Cr(VI) is efficient only in the presence of manganese oxide minerals (Bartlett 1979, Richard 1991, Palmer 1991) Diffusion of Cr(III) to the manganese oxide surface is required for the reaction, ' thus, Cr(III) minerals may persist in environments where they are thermodynamically unstable Cr(VI)-exists in natural waters as H2CrO4 with acid dissociation constants,_pKi = 0 86 and pK2 = 6 51 (Palmer 1991) Except under extremely acidic conditions, Cr(VI) will exist as an anion, referred to as the chromate anion The dichromate species, Cr2072 , is formed only at high chromium concentrations (> 500 ug/l_) which are generally not observed in environmental systems (Rai 1989) Chromate contaminated water has a characteristic yellow color observable at chromium concentrations above -1 mg/L(Palmer 1991) Cr(III) exists in natural waters as pnmanly as the hydroxide complex, Cr(OH)3(Rai 1989, Richard 1991, Palmer 1991) CrOH2+ is the dominant species at pH values from 3 8 to 6 3, and Cr(OH)3 is the dominant species at pH values from 6 3 to 11 5 (Rai 1989) Polymeric species (such as Cr2(OH)2"T) do not contribute significantly to total chromium solubility Cr(III) will form complexes with organic ligands as wells as with sulfate, ammonium, cyanide, sulphocyanide, fluoride and chloride (Richard 16- 1991) In natural systems, complexes of-Cr(III)-with-natural-organic-matter-and-with-other-organic C(Documents and 14 November 2004 Seiii ngslcgohnson\DesMop\UnocallGeochemEval_D oc doc