Microbially-mediated chromate reduction in highly alkaline groundwater systems

Chromium ore processing residue (COPR) has been deposited at a site in the North of England, probably at the end of the nineteenth century. The site covers an area of approximately 2.2 ha, and is situated between a canal and a river that are about 150m apart. It is in a glacial valley underlain by millstone grit and in-filled with alluvial deposits (silt, clay and sand). The original surface deposit is a thin layer of sandy clay that was probably deposited during over-bank flow of the river. COPR has been tipped onto the hillside between the river and canal (which is ~7m above the river), possibly to support the canal bank. At some time in the past top-soil has been placed over the COPR, and the site is now covered with grass. Ground level on the tip is about 1.5m higher than the canal towpath. Currently the site is a cause for environmental concern because groundwater emerging from the waste is alkaline, visibly yellow and has an elevated Cr(VI) concentration.

This paper reports an investigation into the possible fate of any Cr(VI) that migrates downwards from the waste into the underlying soils. Sandy clay from immediately beneath the waste (assumed to be the topsoil layer prior to waste tipping) contains 30-70% acid extractable iron as reduced Fe(II), and between about 3,000 and 600 mg.kg 1 of Cr decreasing with depth. DNA fragments from soil bacteria were extracted from this soil, and microcosm experiments with this soil where the pH was reduced showed that it contains a viable bacterial population capable of iron-reduction. This sandy clay layer, despite a pH value of 10.5, appears to be acting as a natural reactive zone beneath the waste as it is accumulating chromium. It is thought that the mechanism of Cr(VI) reduction is most likely to be an abiotic reaction with the Fe(II) present in the soil, and that Fe(II) in the soil is being replenished by microbial iron reduction (albeit probably at a slow rate).