Numerous studies have demonstrated that microbial reductive dechlorination (MRD) in close proximity to free-phase tetrachloroethene (PCE) and trichloroethene (TCE) can lead to enhanced dissolution (1.5- to 14-fold enhancement over abiotic dissolution). However, markedly fewer studies have examined the microbial contribution to contaminant diffusion within, or back-diffusion from, low-permeability media. To take advantage of these microbial processes influencing mass release and plume longevity, it is essential to understand the growth and distribution of the organohalide-respiring microbial species, Dehalococcoides mccartyi (Dhc), which are characterized by reductive dehalogenase (RDase) genes (e.g., bvcA or vcrA) associated with the transformation of TCE and dichloroethene to vinyl chloride to ethene. This study combines the sequential evaluation of abiotic TCE diffusion and biodegradation in a 2-D aquifer cell packed with four soil lenses of differing permeability (1.0x10-12 to 1.2x10-11 m2) and organic carbon (OC) content (&lt;0.1 to 2%) within a background of ASTM 20/30 sand underlain with a clay layer. The system was saturated with TCE (60 mg/L), flushed with background solution without TCE, re-saturated with TCE, and bioaugmented with the PCE-to-ethene dechlorinating microbial consortium, KB-1® (SiREM). This design and approach allowed for differentiation of biologically enhanced diffusion over aqueous diffusion alone from low-permeability porous media. A numerical model was used to investigate the coupling of desorption, diffusion, and contaminant mass distribution under abiotic and biotic conditions. Experimental measurements of chlorinated ethenes and ethene during the biotic phase were compared with numerical simulations of desorption and diffusion in the absence of MRD to quantify biologically-enhanced diffusion from low permeability media. At the conclusion of the experiment, measured total Dhc cells were uniformly distributed throughout the background sand, lower permeability lenses, and clay. Dhc 16SrRNA and RDase gene abundance in all samples were 4 times greater than in the inoculum, demonstrating that cells were capable of penetrating low-permeability porous media. However, cells harboring the bvcA gene were more abundant in the clay and around the two lenses of highest OC, indicating that the distribution of specific Dhc strains was influenced by the availability of electron acceptors released from soils of differing physical properties.

Bioenhanced diffusion and dynamics of Dehalococcoides mccartyi strains in heterogeneous porous media

256th National Meeting (2018)

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