Effect of Biomass Accumulation on Biologically Enhanced PCE DNAPL Dissolution: A Numerical Simulation
Abstract
Sites contaminated by dense nonaqueous phase liquids (DNAPLs) are among the most difficult to remediate. Recently, several batch and column experiments have shown the capability of using dechlorinating bacteria to accelerate the dissolution rate of DNAPL containing tetrachloroethene (PCE). Although some mathematical models have been proposed to describe the mechanisms of biologically enhanced dissolution, none of them has incorporated the effect of biomass accumulation on the reduction of local hydraulic conductivity of the porous medium. This study has developed a mathematical model that describes advection, dispersion, and reactions in a two-dimensional domain with groundwater flowing past the interface of a PCE DNAPL pool. As to the relationship between biomass and hydraulic conductivity, we have adopted published empirical correlations, including biofilm growth and plug-like growth. The inhibitory effect of PCE to bacteria and the effect of competition for a common electron donor within a microbial community have also been evaluated. The equations are solved numerically. Under the condition of no competition, our model shows that the PCE concentration profile is restricted to the vicinity of the PCE source (less than 0.5cm) when the dechlorinating activity is high. The impact of biomass accumulation is twofold: 1. Bioclogging reduces the local hydraulic conductivity significantly and thus decreases the flow past the water-DNAPL interface, slowing dissolution. 2. A high biotransformation rate of PCE increases the concentration gradient of PCE on the water-DNAPL interface, enhancing dissolution. When the mechanism of bioclogging is excluded, the mixing of the reactants is carried out by the advection of the electron donor and the transverse dispersion of PCE, overestimating the extent of enhanced dissolution. In contrast, when bioclogging is considered, the mixing of PCE and electron donor mainly depends on diffusion and/or transverse dispersion in biologically active zones. The competition study shows that the effectiveness of bio-enhanced dissolution will diminish quickly if no measure is taken to control the growth of other populations that compete for electron donor without biodegrading PCE.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.H52E..09C
- Keywords:
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- 1800 HYDROLOGY;
- 1831 Groundwater quality