Enhanced monitored natural attenuation was conducted with molasses injections to an in situ trichloroethylene (TCE)-contaminated groundwater site. Three pilot-scale wells were conducted for monitoring and substrate injections. At most 97.0% of the TCE was effectively degraded after about 600 days. TCE detected in the injection wells attained Tier 2 Groundwater Regulation in Taiwan for non-drinking water sources. The reaction rates of Cl–VOCs in one of the injection wells explained the mechanism how these dechlorinating species transferred TCE to cis-DCE and VC in anaerobic conditions. The magnitude of the 1st order kinetic growth rates of Dehalococcoides (Dhc) were consistent with the associated TCE degradability. The qPCR and T-RFLP results concluded existence of crucial dechlorinating species including Dhc, which performed critical level of 106 copy L−1 to trigger the TCE degradation. The detected tceA and vcrA ranging from 2.07 × 103 to 6.57 × 105 copy L−1confirmed the pathway of TCE degradation. The two injection wells with the optimal TCE degradation showed increases of bacterial diversity estimated by the Shannon Index. The Shannon index explained the superiority of an increased bacterial diversity. The nonmetric multidimensional scaling results suggested that the bacterial dynamic was affected by the TCE degradation stages. The bacterial community structures were analogous when TCE was well degraded.
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