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^VI in the sediments and the large differences in released mass and rate was caused by the dissolution of multiple Cr^VI 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^VI mass in these vadose zone sediments was readily leached in laboratory columns at high concentrations (up to 187 mg/L Cr^VI) likely from the highly soluble calcium chromate. Additional Cr^VI associated with other Cr^VI surface phases was additionally slowly released over 100 s of hours. The source of Ca and Ba for the Cr^VI precipitates may be from mineral dissolution associated with the historical surface spills of Cr^VI as an acidic dichromate solution. In contrast, aquifer sediments contained significantly less Cr^VI, which was slowly released over 100 s of hours. Small-sized Cr^VI-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^VI from vadose zone and aquifer sediments has created a continuous source of Cr^VI to groundwater.