Investigating of bioclogging in homogenous and heterogeneous uncontaminated and contaminated sands

Bioclogging can be defined as the reduction of hydraulic conductivity and porosity of a saturated porous medium due to microbial growth. Wastewater disposals, artificial groundwater recharge, in-situ bioremediation of contaminated aquifers, construction of water reservoirs, or secondary oil recovery are all affected by this process. The potential for soil and groundwater contamination may increase by the rapid movement of the solutes through soil due to the presence of preferential flow which resulted in increasing bypassing of soil matrix and increasing pore water velocities. On the other hand the presence of preferential flow could affect the clean-up process of the contaminated land by extending the remedial time. The reason behind that is the relatively quick contaminant clean-up in the high permeable zones compared to the slow contaminant clean-up in the low permeable layers. Therefore, this study aims to investigate the bioclogging process in porous media and the factors that can affect this process, also to understand how all aspects of flow are affected by the clogging process, and finally to investigate the potential of biological growth to control direction and location of subsurface hydraulic flow to overcome the problems of preferential flow. The bioclogging process was investigated through a series of sand column experiments in homogeneous and heterogeneous porous media. Six sand fractions ranging from 63-1180 μm were selected as a porous media. Two bacterial strains (P. putida mt-2 and B. indica) were used in this study. Different analytical methods such as loss on ignition and the total number of cells were used to analyse the soil samples. iv The outcomes of this study showed that the growth of bacteria in porous media can reduce the heterogeneity of the porous media, thereby reducing the impact of the preferential flow which could affect the clean-up process of the contaminated land. Pore throat model with the incorporation of different bioclogging models such as the biofilm or plugs (Vandevivere et al., 1995), micro-colony (Okubo and Matsumoto, 1979), and macroscopic (Clement et al., 1996) models were applied to evaluate the results of the experimental work in heterogeneous porous media. The changes in hydraulic conductivity and the porosity of porous media were modelled by assuming that the bioclogging occurs in the small pores which connect the large pores of the porous media. Generally amongst the three bioclogging models, the current study showed that the measured values of the hydraulic conductivity relatively coincide with the predicted values obtained by using Vandevivere et al. (1995) model. Nevertheless, the predicted values of the hydraulic conductivity coincided to some extent with the measured values of the hydraulic conductivity for the large sand fractions. This corresponds with the findings of several previous studies which also confirmed that bioclogging models can only predict the change of the hydraulic conductivity for the large sand fractions. The failure of these models could be related to the assumptions made by each model, which could be less appropriate in fine-textured materials than they are in coarse textured ones. The second possible reason for the disparities between observations and model predictions is related to the assumption made in some of these models that the microorganisms which are responsible for clogging form biofilms of constant thickness which uniformly coated the surface of soil particles.