MOBILITY OF CHROMATE CONTAMINATION IN THE VADOSE ZONE AT THE U. S. DEPARTMENT OF ENERGY'S HANFORD SITE, WASHINGTON

Extensive areas of chromate-contaminated groundwater are found in the reactor operation areas of the U.S. Department of Energy’s Hanford Site in south-central Washington State. Sodium dichromate was used as a corrosion inhibitor in the nuclear reactors and for decontamination purposes. The persistent contamination, shallow excavations, and limited deep vadose zone sampling indicate ongoing chromate flux from the vadose zone.

Chromate is considered to be highly mobile in the low organic-carbon oxic vadose zone and aquifer sediments, however, a number of physical and geochemical factors, potentially affect the mobility and lead to long tailing in the dissolved concentrations. This may have implications for the long-term effectiveness of remediation options.

Column leaching experiments and micro-scale characterization on sediments from 5 locations of vadose zone contamination illustrate different transport behaviors. The majority of the chromate is removed in the first pore volume of saturated flow in most samples, but variability of the tailing is related to factors such as the co-disposal of acidic waste, iron content, and sediment lithology. The pH of the initial sediment leach is a good predictor of the extent of tailing with low pH sediments exhibiting longer tails. Sediments from an acid-disposal facility contained a large fraction of tightly-bound chromium. This chromium was found to be mixed Cr(VI)/Cr(III) and iron was found as mixed Fe(II)/Fe(III) by X-ray photoelectron spectroscopy. Extensive reduction to Cr(III) limits mass flux from this source. Preliminary data from samples collected in the vicinity of a major contaminant source showed initial alkaline pH and only minor tailing.