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

Szecsody, Jim E., Truex, Mike J., Qafoku, Nikolla P., McKinley, James P., Ivarson, Kristine A., Di Pietro, Silvina
The Science of the total environment 2019 v.676 pp. 482-492
aquifers, barium, calcium, chromium, crystals, groundwater, iron, leaching, plagioclase, sediments, solubility, vadose zone, Washington (state)
This study of vadose zone and aquifer sediments beneath a former dichromate spill site showed that the persistence of CrVI in the sediments and the large differences in released mass and rate was caused by the dissolution of multiple CrVI 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 CrVI mass in these vadose zone sediments was readily leached in laboratory columns at high concentrations (up to 187 mg/L CrVI) likely from the highly soluble calcium chromate. Additional CrVI associated with other CrVI surface phases was additionally slowly released over 100 s of hours. The source of Ca and Ba for the CrVI precipitates may be from mineral dissolution associated with the historical surface spills of CrVI as an acidic dichromate solution. In contrast, aquifer sediments contained significantly less CrVI, which was slowly released over 100 s of hours. Small-sized CrVI-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 CrVI from vadose zone and aquifer sediments has created a continuous source of CrVI to groundwater.