Monte Carlo evaluation of microbial-mediated contaminant reactions in heterogeneous aquifers |
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| Authors: | Mohamed MA Mohamed Kirk Hatfield Ahmed E Hassan |
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| Institution: | 1. Groundwater Protection and Restoration Group, Civil and Structural Engineering Department, University of Sheffield, Mappin Building, Mappin Street, Sheffield S1 3JD, UK;2. Irrigation and Hydraulics Department, Faculty of Engineering, Cairo University, P.O. Box 12211, Giza 12613, Egypt;3. Civil and Coastal Engineering Department, University of Florida, P.O. Box 116580, Gainesville, FL 32611, USA;4. Division of Hydrologic Sciences, Desert Research Institute, Las Vegas, NV 89119, USA |
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| Abstract: | Monte Carlo simulations are conducted to evaluate microbial-mediated contaminant reactions in an aquifer comprised of spatially variable microbial biomass concentrations, aquifer hydraulic conductivities, and initial electron donor/acceptor concentrations. A finite element simulation model is used that incorporates advection, dispersion, and Monod kinetic expressions to describe biological processes. Comparisons between Monte Carlo simulations of heterogeneous systems and simulations using homogeneous formulation of the same two-dimensional transport problem are presented. For the assumed set of parameters, physical aquifer heterogeneity is found to have a minor effect on the mass of contaminant biodegraded/transformed when compared to a homogeneous system; however, it noticeably changes the dispersion, skewness, and peakness of contaminant concentration distributions. Similarly, for low microbial growth rate, given favorable microbial growth characteristics, biological heterogeneity has minor effect on the mass of contaminant biodegraded/transformed when compared to a homogeneous system. On the other hand, when higher effective growth rates are assumed, biological heterogeneity and spatial heterogeneities in essential electron donor/acceptors reduce the efficiency of biotic contaminant reactions; consequently, model simulations derived from heterogeneous biomass distributions predict remediation time scales that are longer than those simulated for homogeneous systems. When correlations between physical aquifer and biological heterogeneities are considered, the assumed correlation affects predicted mean and variance of contaminant concentration and biomass distributions. For example, an assumed negative correlation between hydraulic conductivity and the initial biomass distribution produces a plume where less efficient biotic contaminant reactions occur at the leading edge of the plume; this is consistent with less degradation/transformation occurring over regions of higher groundwater velocities. However, the presence and absence of these correlations do not appear to affect the efficiency of microbial-mediated contaminant attenuation. |
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| Keywords: | Stochastic Subsurface Transport Biodegradation Monte Carlo |
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