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ABIOTIC DECHLORINATION OF 1,1,1 TCA AND TCE IN A HIGH pH PLUME

Sami A. Fam, Ph.D., P.E. L.S.P., Stewart Mountain, P.E., L.S.P., Innovative Engineering Solutions, Inc.,
Sam Fogel, Ph.D., Margaret Findlay, Ph.D., Donna Smoler, Bioremediation Consulting, Inc., George Sylvester, Univar USA Inc.

This paper describes a site whose groundwater is impacted by trichlorethene (TCE), 1,1,1-trichloroethane (1,1,1 TCA) at concentrations in the 300-ppm range, and has elevated pH from historic releases of alkali. Portions of the VOC plume that are in the neutral pH zone show active anaerobic dechlorination to ethene. The portions of the VOC plume within the elevated pH (up to pH 12.7) area demonstrate almost complete abiotic dechlorination of the VOCs. 

Microcosm studies using site groundwater were conducted to evaluate the potential to enhance anaerobic dechlorination and abiotic degradation of VOCs. Groundwater samples were collected from four monitoring wells and were tested in the laboratory under a variety of conditions. Groundwater from well MW-2 had normal pH and was not colored. In the live microcosms amended with sodium lactate, 85% of the TCE and cis-1,2-DCE were dechlorinated, 74 % to ethene, and 11 % to vinyl chloride (VC) within the study period. 

Groundwater from well MW-9 had an elevated pH and was opaque black in color, due to the presence of humic material extracted from the soil by the high pH water. Twenty-three microcosms of varying conditions were set up to evaluate the dechlorination mechanism in this high pH water. There are two rapid abiotic processes affecting 1,1,1-TCA, one in which 1,1,1-TCA is reductively dechlorinated to 1,1-DCA, and the other in which 1,1,1-TCA is rapidly converted to a non-chlorinated product, probably acetate. 

The dechlorination of 1,1,1-TCA by both abiotic processes was strongly enhanced by adding cysteine and the co-factor vitamin B12. The addition of FeS2, (iron pyrite) also increased the abiotic dechlorination of 1,1,1-TCA, but only the process leading to non-chlorinated product. 

The other contaminant, TCE, was degraded in unamended groundwater approximately 50 % in 105 days, significantly slower than the 1,1,1-TCA dechlorination rates, with 10 % recovered as acetylene

Based on the results of the microcosm studies, the site remedies that have been selected are: Enhanced bioremediation for the area with neutral pH, and monitored natural attenuation for VOCs within the high pH plume. The pH of the groundwater will be adjusted to more neutral levels prior to its flow past the property boundary.

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