One factor in assessing metal mobility at the Frontier Hard Chrome industrial site is the degree of chromium reduction and secondary mineralization in a silt unit and underlying sand and gravel aquifer that extends from the site toward the Columbia River. Samples were analyzed by ICP-AES for metals concentration, scanning electron microscopy/electron microprobe for mineralogical texture and microanalysis, powder x-ray diffraction for mineral identification, and optical microscopy for textural observations. Microprobe analysis showed that chromium occurred in metallic particles originating in an uppermost Fill Unit, and in iron oxides, iron-titanium oxides and fine-grained iron aluminun silicates in the both the Silt and Aquifer Units. X-ray diffraction analysis showed that the fine-grained fraction of the Silt and Aquifer Units contained an abundant suite of detrital clay minerals including primarily illite, chlorite, and smectite, and lesser kaolinite. The presence of chlorite and smectite is consistent with the microprobe observation of iron-bearing aluminum silicates in the fine grained fraction of the samples. Chromium concentration in the fine-grained material was elevated to a level consistent with chromium in the bulk material and about 10-20 times nearby background concentrations, suggesting that an important contributor to the bulk chromium content resides in the clay minerals.

"EPA 910-R-99-015." "August, 1999." Bibliographic references included.