Persistence of chromate in vadose zone and aquifer sediments in Hanford, Washington.

This study of vadose zone and aquifer sediments beneath a former dichromate spill site showed that the persistence of Cr in the sediments and the large differences in released mass and rate was caused by the dissolution of multiple Cr surface phases. Vadose zone sediments contained numerous 1 to 10 μm high solubility calcium chromate crystals, with lesser amounts of unidentified phases indicated by Ba/Cr association in weathered pyroxenes and Ca/Cr association in weathered Ca-rich plagioclase. Most (>90%) of the Cr mass in these vadose zone sediments was readily leached in laboratory columns at high concentrations (up to 187 mg/L Cr) likely from the highly soluble calcium chromate. Additional Cr associated with other Cr surface phases was additionally slowly released over 100 s of hours. The source of Ca and Ba for the Cr precipitates may be from mineral dissolution associated with the historical surface spills of Cr as an acidic dichromate solution. In contrast, aquifer sediments contained significantly less Cr, which was slowly released over 100 s of hours. Small-sized Cr-containing precipitates (<5 μm) were associated with Ca, Fe, and, to a lesser extent, Ba. Leaching with groundwater caused a decrease in ferrous iron surface phases. The observed leaching of Cr from vadose zone and aquifer sediments has created a continuous source of Cr to groundwater.