Biogeochemical modelling to assess the effect of bioclogging on multiple electron acceptor-mediated petroleum hydrocarbon bioremediation in vadose zone.

Bioremediation is an economically viable and sustainable clean-up strategy. Hydrodynamic, as well as transport characteristics of the porous medium, can evolve over the period as a result of biological clean-up activities. The present study proposes a 2-D numerical framework to simulate the effect of bioclogging on multiple electron acceptor-mediated petroleum hydrocarbon bioremediation in the vadose zone. For modelling, a spill of BTEX (benzene, toluene, ethylbenzene and xylene) is assumed near source zone. The developed model results are validated using three previously published datasets on flow, transport and biodegradation in the vadose zone. Simulations are performed for three types of soil, including clay, sand and loam. The analysis shows that sand has a maximum infiltration rate and clay has a minimum. Hydraulic conductivity and saturation profile peaks reach their minimal value at a shallower depth (around four times) when bioclogging is present compared to when it is absent. The migration depth and concentration of BTEX are observed to be restricted to a shallower depth in aquifers with the presence of microbial clogging. The outcome shows that electron acceptor consumption is more (around sevenfold for oxygen, fourfold for nitrate and threefold for sulphate) in the presence of bioclogging at the shallower zone. Zeroth order spatial moment and sensitivity analyses show that biological clogging, number of electron acceptors and inhibition constant substantially affect BTEX bioremediation in the vadose zone.

Biogeochemical modelling to assess benzene removal by biostimulation in aquifers containing natural reductants.

Biostimulation of aquifers contaminated with gasoline spills is vigorously affected by the biogeochemical environment existing there. In this study, biostimulation of benzene is simulated using a 2D coupled multispecies biogeochemical reactive transport (MBRT) model. The model is implemented at an oil spill site near a hypothetical aquifer containing natural reductants. Multiple electron acceptors are introduced to promote faster biodegradation rate. However, after reaction with natural reductants, it reduces the number of available electron acceptors, acidifies the subsurface environment, and inhibits bacterial growth. These mechanisms are assessed using seven coupled MBRT models sequentially. The finding of the present analysis reveals that biostimulation has caused a substantial drop in concentration of benzene and is efficient in reducing its penetration depth. The results also shows that the intervention of natural reductants in the biostimulation process is slightly diminished by pH adjustment of aquifers. When the pH level in aquifer changes from acidic pH 4 to neutral pH 7, it is observed that the biostimulation rate of benzene as well as microbial activity increases. Electron acceptors consumption is more at neutral pH. Overall, it can be inferred from zeroth-order spatial moment and sensitivity analyses that retardation factor, inhibition constant, pH, and dispersivity in vertical direction significantly affect benzene biostimulation in aquifers.

Numerical modeling to assess the effect of soil texture on transport and attenuation of petroleum hydrocarbons in unsaturated zone.

Soil texture in the unsaturated zone is a critical factor affecting the transport, accumulation, and attenuation rate of petroleum hydrocarbons (PHCs) in unsaturated conditions. The scope of this study is to investigate the soil texture impact on the fate of PHCs in unsaturated zones. The main objective is to formulate a coupled flow and multicomponent transport model for simulating the PHC plumes in various soil textures. Zeroth spatial moment (ZSM) of simulated PHC plumes is estimated to quantify the transient effect of soil textures on the dissolved PHC mass in the system. A BTEX (benzene, toluene, ethylbenzene, and xylene) spill is considered at the source zone for modeling. Simulations are carried out for clay, sand, and loam textures. The outcome of the study suggests that the infiltration rate in the unsaturated zone is minimal in clay texture. Wetting front depths and BTEX source depletion rates are found to be in the following order: clay < loam < sand. The migration depth of BTEX components in the sand texture is approximately twice the depth for clay and loam after 50 days. An increment in the BTEX source zone length by twofold enhances the dissolved BTEX mass in the unsaturated system by approximately 33% in all soil textures. Overall, the modeling and sensitivity studies conclude that the soil texture, vertical dispersivity, source zone length and composition, sorption characteristics, and volatility critically affect the depth and extent of BTEX migration in unsaturated zones.