Assessment of potential for natural attenuation of chlorinated ethenes and ethanes in ground water at a petrochemical reclamation site, Harris County, Texas

Redox conditions in the numerous sand channels zone beneath a petrochemical reclamation site in Harris County, Texas, range from sulfate reducing to methanogentic as indicated by the presence of methane in ground water and the range of molecular hydrogen concentrations. Assessment of the potential for reductive dechlorination using BIOCHLOR as a screening tool indicated conditions favoring anaerobic degradation of chlorinated organic compounds in the numerous sands channels zone. Evidence supporting reductive dechlorination includes apparently biogenic cis-1,2 dichloroethene; an increased ratio of 1,2 dichloroethane or 1,1,2 trichloroethane down gradient from the assumed containment source area; ethane and methane concentrations greater than background concentrations within the area of the containment plume; and a positive correlation of the ratio of ethane to vinyl chloride as a function of methane concentrations. The body of evidence presented in this report argues for hydorgenolysis of trichloroethane to cis-1,2 dichloroethene; of 1, 1,2, trichloroethane to 1,2 dichloroethane; and of vinyl chloride to ethane within the numerous sand channels zone. Simulations using BIOCHILOR yielded apparent first order decay constants for reductive dechlorination in the sequence Tetachloroethene trichloroethene Cis-1,2 dichloroethene vinyl chloride ethene with the range of literature values reported for each compound and apparent first order decay constants for reductive dechlorination in the sequence 1,1,2, trichloroethane 1,2 dichloroethane slightly greater than literature values reported for each compound along the upgradient segment of a simulated ground water flow path. Except for vinyl chloride, apparent rates of reductive dechlorination for all simulated species show a marked decreases along the downgradient segment of the simulated ground eater flowpath. Evidence for reductive dechlorination of chlorinated ethenes within the numerous sand channels zone indicates potential for natural attenuation of chlorinated ethenes. Reductive dechlorination of chlorinated ethenes apparently occurs to a lesser extent, including relatively less potential for natural attenuation of chlorinated ethenes. Additional data are needed on the concentrations and distribution of chlorinated ethenes and ethenes in individual fine minimum reporting levels for future chloroethane analyses, might enable a more complete and quantitative assessment of the potential for natural attenuation at the site.