Development and Sensitivity Analysis of a Fully Kinetic Model of Sequential Reductive Dechlorination in Groundwater

A fully kinetic biogeochemical model of sequential reductive dechlorination (SERD) occurring in conjunction with lactate and propionate fermentation, iron reduction, sulfate reduction, and methanogenesis was developed. Production and consumption of molecular hydrogen (H2) by microorganisms have been modeled using modified Michaelis–Menten kinetics and has been implemented in the geochemical code PHREEQC. The model have been calibrated using a Shuffled Complex Evolution Metropolis algorithm to observations of chlorinated solvents, organic acids, and H2 concentrations in laboratory batch experiments of complete trichloroethene (TCE) degradation in natural sediments. Global sensitivity analysis was performed using the Morris method and Sobol sensitivity indices to identify the most influential model parameters. Results show that the sulfate concentration and fermentation kinetics are the most important factors influencing SERD. The sensitivity analysis also suggests that it is not possible to simplify the model description if all system behaviors are to be well described

Development and Sensitivity Analysis of a Fully Kinetic Model of Sequential Reductive Dechlorination in Groundwater

A fully kinetic biogeochemical model of sequential reductive dechlorination (SERD) occurring in conjunction with lactate and propionate fermentation, iron reduction, sulfate reduction, and methanogenesis was developed. Production and consumption of molecular hydrogen (H2) by microorganisms have been modeled using modified Michaelis–Menten kinetics and has been implemented in the geochemical code PHREEQC. The model have been calibrated using a Shuffled Complex Evolution Metropolis algorithm to observations of chlorinated solvents, organic acids, and H2 concentrations in laboratory batch experiments of complete trichloroethene (TCE) degradation in natural sediments. Global sensitivity analysis was performed using the Morris method and Sobol sensitivity indices to identify the most influential model parameters. Results show that the sulfate concentration and fermentation kinetics are the most important factors influencing SERD. The sensitivity analysis also suggests that it is not possible to simplify the model description if all system behaviors are to be well described

Evaluation of Bioaugmentation with Entrapped Degrading Cells as a Soil Remediation Technology

Soil augmentation with microbial degraders immobilized on carriers is evaluated as a potential remediation technology using a mathematical model that includes degradation within spatially distributed carriers and diffusion or advection-dispersion as contaminant mass transfer mechanisms. The total volume of carriers is a critical parameter affecting biodegradation performance. In the absence of advection, 320 and 20 000 days are required to mineralize 90% of the herbicide linuron by Variovorax sp. SRS16 encapsulated in 2 mm beads with 5 and 20 mm spacings, respectively. Given that many pesticide degraders have low intrinsic degradation rates and that only limited carrier to soil volume ratios are practically feasible, bioaugmented soils are characterized by low effective degradation rates and can be considered fully mixed. A simple exponential model is then sufficient to predict biodegradation as verified by comparisons with published experimental data. By contrast, the full spatially distributed model is needed to adequately model the degradation of faster degrading contaminants such as naphthalene and benzene which can be mass-transfer limited. Dimensionless Damköhler numbers are proposed to determine whether the spatially distributed model is required. Results show that field scale applications of immobilized degraders will be limited by the amount of carriers required to reach acceptable degradation rates

Field Evaluation of Biological Enhanced Reductive Dechlorination of Chloroethenes in Clayey Till

The performance of enhanced reductive dechlorination (ERD) for in situ remediation of cis-1,2-dichloroethene (cDCE) and vinyl chloride in clayey till was investigated in a pilot test. A dilute groundwater solution containing emulsified soybean oil and Dehalococcoides bacteria was injected into a sand-filled hydraulic fracture. Fermentation of the ERD solution caused the establishment of a dechlorinating bioactive zone in the fracture within 1 month of injection. By 148 days, all the cDCE in the fracture was dechlorinated to ethene. Analysis of a clay core from Day 150 indicated that electron donor and fermentation products diffused from the fracture at least 10 cm into clay and that stimulated dechlorination occurred in the clay in the presence of Dehalococcoides (7.9·104 cells g−1). Comparison of chloroethene profiles in the Day 150 core to modeled diffusion profiles indicated degradation occurred in a bioactive zone extending approximately 5 to 6 cm into the clay matrix. These data suggest that a bioactive zone established in a sand-filled fracture can expand into the adjacent clayey till matrix and facilitate mass transfer from the matrix to the bioactive zone. These findings offer promise for ERD and support further development of methods for deploying ERD in clayey till and other low-permeability deposits