Biomarkers associated with the biodegradation of chlorinated ethenes (CEs), such as RDase functional gene DNA, mRNA, and enzymes, are increasingly monitored in contaminant plumes to assess reductive dechlorination (RD). These biomarkers provide direct proof of RD and are used to evaluate biodegradation potential and possibly infer degradation rates. However, aquifers are inherently heterogeneous, and local hydrodynamic and biogeochemical conditions likely influence the behavior and efficiency of organohalide-respiring bacteria. Moreover, identifying the key factors shaping the observed spatiotemporal patterns of chemicals and biomarkers remains a challenge.
At the Grindsted site (Denmark), a 1500-m long CE plume has developed from a former pharmaceutical factory. Extensive monitoring of both chemicals and biomarkers has been conducted at this site. To aid in the data interpretation, we developed new enzyme-based kinetics linking the reductive dechlorination of cis-1,2-dichloroethene to vinyl chloride and ethene with the expression of the vcrA and bvcA genes in Dehalococcoides. These kinetics were integrated into a 1D reactive transport model simulating the Grindsted plume, also including the estimated variable flow rates. Fitting the observed chemicals and biomarkers (i.e., functional gene DNA and mRNA) required to calibrate the kinetic parameters with a zonal approach.
As a result, the model pointed out different ecological strategies of vcrA- and bvcA-carrying Dehalococcoides, which affected the RD efficiency within the plume core. Moreover, CE biodegradation efficiency appeared further decreased under high flow rate conditions, because of the shorter residence time.
This modeling endeavor is the first attempt of field application and allowed enhancing the mechanistic interpretation of observed chemical and biomarker patterns in the CE plume. Consequently, it proved to be a reliable support for designing of future monitoring and remediation strategies.