Actually hexavalent chrome is also produced under oxidizing conditions in the presence of manganese oxides. However such conditions would not exist in the ROF and any organic matter ( such as can be found in a swamp) would immediately reduce any hexavalent Cr that there might be to harmless Cr(III). The only documented cases of natural Cr(VI) are in tropical climates where you have deep and extensively weathered soils over chromite deposits. The whole Cr(VI) issue is a red herring.