EFFECT OF IN-SITU REDOX CONDITION ON ANOXIC CHLOROETHENE BIODEGRADATION IN TWO SHALLOW GROUNDWATER SYSTEMS

Mechanisms for anaerobic microbial degradation of chloroethene groundwater contaminants are highly sensitive to in situ redox conditions. Under anaerobic conditions, tetrachloroethene and trichloroethene are readily transformed via reductive dechlorination to the less chlorinated intermediates, dichloroethene (DCE) and vinyl chloride (VC). Because microbial reductions of DCE and VC are generally less favorable and apparently require highly reducing redox conditions, microbial reductive dechlorination of chloroethene contaminants often appears to “stall” under less-reducing, groundwater conditions (ex. Fe(III)/Mn(IV)-reducing conditions) leading to accumulation of DCE without apparent transformation to VC, ethene or ethane. However, some groundwater microbial communities can catalyze a net mineralization of DCE or VC to CO2 under anoxic redox conditions. Thus, the apparent “DCE stall” observed in groundwater systems characterized by relatively oxidized, anoxic redox conditions may reflect a shift from microbial reductive dechlorination to microbial oxidation rather than an general inhibition of chloroethene biodegradation. Such a shift in the mechanism of microbial degradation of groundwater chloroethene contaminants is illustrated for two low-temperature groundwater systems in Alaska.