Long-Term Capacity of Plant Mulch to Remediate Trichloroethylene in Groundwater

Abstract

Passive reactive barriers (PRBs) are commonly used to treat groundwater that is contaminated with chlorinated solvents such as trichloroethylene (TCE). A number of PRBs have been constructed with plant mulch as the reactive medium. The TCE is removed in these barriers through adsorption, biological reductive dechlorination, and abiotic reactions with reduced iron minerals that are formed in the barrier. Generally speaking, adsorption has limited capacity for TCE removal and abiotic dechlorination is dependent on metal sulfides of biogenic origin. Therefore, the long-term performance of these barriers will be controlled by their capacity to support biological activity. Laboratory batch experiments were inoculated with an enrichment culture of dechlorinating microorganisms. Dechlorination of TCE to ethylene was achieved using plant mulch; however, neither water extractable nor organic-solvent extractable components of the mulch could sustain dechlorination of TCE. This indicates that biodegradation of organic wood fibers in the plant cell wall provides electron donors for dechlorination of TCE. Kinetic analysis of the methane production in the batch tests provides supporting evidence that the plant mulch is able to sustain long-term biological activity in a typical barrier constructed with plant tissues. The recognition of the intact plant tissues as a long-term electron donor expands the knowledge about the microbial dechlorination under natural conditions. In addition, the production of dissolved inorganic carbon (DIC) observed in a column study was used to estimate the life cycle of a full-scale biowall installed at Altus AFB, Oklahoma. Based on a consistent downward trend in DIC concentrations in the effluent and a stable concentration in the influent over time, the mulch in the biowall is expected to support microbial activity for 10 years.