Case Study: Chemical and Biological Reduction of PCE by Pneumatic Fracturing and Injection of ZVI in Saprolite

This article presents a case study of the source-area treatment of tetrachloroethene (PCE) in a low-permeability formation using zero-valent iron (ZVI). Evidence of the stimulation of biological reduction processes within the treatment zone occurred. Pneumatic fracturing and injection of microscale ZVI slurry in the overburden and weathered bedrock zones was performed at a commercial brownfields redevelopment site in Maryland. A 20,000-square-foot source area impacted with PCE at concentrations greater than 15,000 g/L was treated at depths ranging from 10 to 70 feet bgs. An average ZVI dosage of 0.0024 iron-to-soil mass ratio within the overburden zone led to a 75 percent decrease in PCE mass in less than one year. For the weathered bedrock zone, an average 0.0045 iron-to-soil mass ratio resulted in a 92 percent decrease in PCE mass during the same period.  The reducing environment and hydrogen generated by the ZVI may have stimulated Dehalobacter populations, as evidenced by concentrations up to 104 cells per milliliter measured within the treatment area despite a groundwater pH as high as 9. The biological reductive dechlorination of the chlorinated ethenes explains the temporary increase in trichloroethene and cis-1,2-dichloroethene concentrations.

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