Organic mulch biowall treatment of chlorinated solvent-impacted groundwater

To enhance biological reductive dechlorination of trichloroethylene (TCE)-impacted groundwater at Offutt AFB, Nebraska, an in situ pilot-scale permeable reactive treatment wall (100 ft [L] x 1 ft [W] x 23 ft [D]) was constructed under a contract with the Air Force Center for Environmental Excellence. The biowall was installed using a continuous trenching and backfill method and filled with a 50:50 mixture of organic mulch and concrete sand. The mulch served as an inexpensive source of fermentable organic matter, which produces hydrogen to promote reductive dechlorination. A comparison of the aquifer conditions before the emplacement of the biowall and during the 18-month test showed a decrease in the oxidation-reduction potential, dissolved oxygen, and pH of the groundwater. After the first six months, there was significant production of cis-1,2-dichloroethene (cis-DCE) downgradient of the biowall, indicating that the mulch was promoting reductive dechlorination. During the 18 months of operation, TCE removals as high as approximately 74-89% were achieved. Low concentrations of vinyl chloride were produced downgradient (<0.0025 mg/L). Production of ethene and ethane, final reductive dechlorination products, was observed. In summary, the mulch biowall was effective in removing chlorinated ethenes from groundwater without significant vinyl chloride production.