Abstract
Soil and groundwater pollution by chlorinated solvents, such as tetrachloroethylene (PCE) are common throughout the industrialized world. These so called DNAPLs pose environmental health risks and sites contaminated with DNAPLs often require remediation. One of the most effective and economically feasible methods for NAPL cleanup is in-situ bioremediation. Early bioremediation
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activities were directed at contamination plumes. However, recently, interest is growing more and more in the application of biodegradation to source zones. This thesis strengthens current knowledge and adds valuable insights into biodegradation within a DNAPL source zone. We have combined batch, column, and sandbox experiments and numerical modeling. All experiments were carried out under anaerobic conditions with groundwater containing a dechlorinating microbial assemblage originating from a tetrachloroethene (PCE)-contaminated site. In batch experiments, no dechlorination was found in solutions with initial PCE concentrations higher than 0.5 mM. In column experiments, however, dechlorination was observed not only in columns with dissolved PCE, but even in columns containing PCE liquid. This difference in biodegradation behavior between batch and column arrangements is believed to be due do the interplay of biodegradation, advection, and diffusion. These processes create a pore-scale distribution of PCE concentration, varying from solubility limit, next to a blob, to lower-than-toxicity values within a short distance. These processes also lead to enhanced dissolution of PCE. In columns with residual PCE, we found a bioenhanced dissolution factor of 5, which is comparable to values found in the literature. A bioenhanced dissolution factor higher than one indicates that the longevity of the source zone can be shortened significantly. Similar results were found in experiments carried out in a 2D tank with inner dimensions of 2000 x 940 x 45 mm. In the tank, horizontal groundwater at rates comparable to field conditions was established and a zone of residual PCE over the height and a pool at the bottom of the tank were created. Biodegradation in the residual source zone was established. Bio-enhanced dissolution is believed to have occurred as the total chlorinated ethene concentration was about four times higher than the solubility limit of PCE. The bioenhanced dissolution factor resulted from a higher concentration gradient, higher PCE solubility limit, and an increased dissolution rate coefficient. Model calculations with a traditional constant dissolution rate coefficient and solubility limit will lead to an underestimation in mass removed. In both column and tank experiments, we have established that mobilization of residual PCE occurred as a result of dechlorination activities. Mobilization and the observation of an increase in dissolution rate constant in the presence of dechlorination are probably effects of the production of surfactants by the dechlorination microbial assemblage. Modeling studies from the various experiments were performed. From the batch experiments, biotic parameters were estimated with a sensitivity analysis to determine significance of other biological reactions influencing the dechlorination process. From the column experiments parameters to describe DNAPL dissolution and dechlorination were estimated for porous media. We concluded that the dissolution rate coefficient increased with time. Parameters obtained in the column experiment were used to model the tank experiment. Computations closely matched the total mass that left the tank from the laboratory experiment
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