Hexavalent Cr(VI) exists primarily as HCrO4, (bichromate) or CrO4 (chromate), which is relatively soluble over much of the environmental pH range. CrO4 is an oxidant, carcinogenic, mutagenic and is toxic to a variety of plants and animals even at concentrations below 50 μg kg-1 (Turner and Rust, 1971). Trivalent Cr(III) is relatively non-toxic and forms sparingly soluble hydroxide and mixed (Fe,Cr) hydroxide precipitates at circum neutral pH and above. Cr(III) is weakly mobile in groundwater because of low solubility, and precipitated Cr(OH)3 reacts slowly with molecular oxygen.
Ion Chromography is typically used. Here are a couple of papers to consider:
http://www.state.nj.us/dep/dsr/chromium/cr-soils-wastefinal32609.pdf
http://www.thermoscientific.com/content/dam/tfs/ATG/CMD/CMD%20Documents/AU-165-Separation-Chromium-III-and-IV-AN70416-E.pdf
It is a hard work to deal with Cr(VI) and Cr(III) in soils samples due to risks of oxidation/reduction due to pH and Eh variations. However, you can find methods for extraction and quantification at the US-EPA web page, including methods called 7196A or 3060A
Not easy since Chromium can move back and forth from CrIII to CrVI. Brian Buckley of my Institute might be able to help with more details
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