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

Jim E Szecsody; Mike J Truex; Nikolla P Qafoku; James P McKinley; Kristine A Ivarson; Silvina Di Pietro

doi:10.1016/j.scitotenv.2019.04.173

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

Sci Total Environ. 2019 Aug 1:676:482-492. doi: 10.1016/j.scitotenv.2019.04.173. Epub 2019 Apr 19.

Authors

Jim E Szecsody¹, Mike J Truex², Nikolla P Qafoku², James P McKinley², Kristine A Ivarson³, Silvina Di Pietro⁴

Affiliations

¹ Pacific Northwest National Laboratory, Richland, WA 99354, United States of America. Electronic address: jim.szecsody@pnnl.gov.
² Pacific Northwest National Laboratory, Richland, WA 99354, United States of America.
³ CH(2)M Hill Plateau Remediation Contractor, Richland, WA 99354, United States of America.
⁴ Applied Research Center, Florida International University, Miami, FL 33023, United States of America.

PMID: 31048177
DOI: 10.1016/j.scitotenv.2019.04.173

Abstract

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.

Keywords: CaCrO(4); Cr leaching from sediments; Slow Cr release from sediments.