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Biogeochemical processes in subsurface can dramatically alter the behavior of multiphase fluid flow and the hydrodynamics of porous media by bio-clogging. There exist analytical solutions such as soilsuction – saturation equations which can be used to predict the water retention curve relations in unsaturated soils. However, due to the complexity of various biogeochemical products, their pore-scale behavior and their interplay with pore structure, such analytical solutions would not provide accurate predictions. In this study, a large database of pore-networks were generated by adjusting the statistical and spatial pore and tube size distribution of the networks resembling various levels of bio-clogging. Numerical simulations including the evolution of pore structure, water retention relationship, and air invasion dynamics during desaturationwere explored. The numerical simulations verified that local pore-clogging leads to the development of isolated pore clusters and impermeable zones. The evolution of impermeable zones results in the formation of preferential flow paths towards the mobile zones for the multiphase flow problems. Using parallel computation, the critical predictors of water retention curves in bio-clogged porous media are found.

Microbially Induced Desaturation and Precipitation (MIDP) through denitrification is an emerging ground improvement method in which indigenous nitrate reducing bacteria are stimulated to introduce biogas, biominerals and biomass in the soil matrix. In this study, a numerical model is developed to evaluate the effect of biogas, biominerals and biomass on the hydraulic properties of soils treated with MIDP. The proposed model couples the biochemical conversions to changes of porosity and water saturation and predicts changes in permeability through two separate power law equations. Experimental studies from the literature are used to calibrate the model. Comparing the results with other studies on bioclogging or biomineralization in porous media reveals that the combined production of biogas, biomass, and biominerals results in efficient clogging, in the sense that only a small amount of products leads to a substantial permeability reduction. Based on this comparison, the authors postulate that biogenic gas bubbles preferably form within the larger pore bodies. The presence of biogenic gas in the larger pore bodies forces calcium carbonate minerals and biomass to be formed mainly at the pore throats. The interaction between the different phases results in more efficient clogging than observed in other studies which focus on a single product only. [ABSTRACT FROM AUTHOR]

We are developing a multiphase reactive transport model for denitrification-driven ground improvement to account for the dominating influence of the complexities expected in the field, including biochemical process competition, microbial inhibition, and complicated soil conditions. Denitrifying bacteria are ubiquitous in most subsurface environments, and denitrification is a common process in anaerobic environments. Since denitrification is feasible regardless of location, microbially induced desaturation and precipitation via denitrification (MIDP) has widespread potential for ground improvement, particularly earthquake-induced liquefaction. Inorganic carbonate precipitates as calcium carbonate minerals in presence of dissolved calcium ions, shown to increase strength and stiffness and reduce permeability in the subsurface. Biogenic gas production reduces the degree of saturation and permeability and increases the compressibility of the pore fluid for liquefaction mitigation. Previous MIDP models have represented the biogeochemistry well, but the flow conditions have been very simple -- batch or homogeneous 1-D transport -- or did not consider biochemical reactions seen under field conditions. Field testing has shown that these simplistic models do not account for natural field conditions experienced in the field that influence the uniformity of MIDP treatment and the spatial distribution of MIDP products, both of which are crucial for ground improvement. Our model considers stratified soil conditions, competing biochemical reactions, groundwater composition, and by-product accumulation. The influence of these mechanisms will be addressed so that predictions of the performance of field-scale MIDP treatment is comprehensive and treatment designs account for the complex soil environment. [ABSTRACT FROM AUTHOR]