Experimental studies of the influence of grain size, oxygen availability and organic carbon availability on bioclogging in porous media.

Changes in the hydraulic properties of porous material due to bioclogging have been observed in many laboratory simulations and field studies. Because such changes in hydraulic properties influence the movement of fluids and contaminants, microbial ecology data are required for improved transport modeling. Here we investigate the effects of environmental variables previously shown to influence bioclogging, specifically oxygen availability, sediment grain size, and organic carbon (nutrient) concentration on the hydraulic properties of simulated subsurface environments. Our study provides evidence of a different clogging mechanism for aerobic and anaerobic microbial communities under high organic carbon concentrations (400 mg L(-1)). This work also suggests that the clogging mechanism operating in anaerobic microbial communities is more sensitive to carbon availability than that in the aerobic microbial communities. We found that grain size does have an effect on clogging, but it appears that there is a threshold carbon concentration, and therefore biomass, below which these effects are insignificant. Differences between the microbial communities that developed under different oxygenation conditions were detected using 16s rRNA analysis.

Microbial controls on metal mobility under the low nutrient fluxes found throughout the subsurface.

Laboratory simulations and field studies of the shallow subsurface have shown that microbes and their extracellular products can influence the mobility of toxic metals from waste disposal sites. Modelling the transport of contaminants in groundwater may, therefore, require the input of microbial ecology data in addition to geochemical data, thus increasing the costs and the uncertainty of predictions. However, whether microbial effects on contaminant mobility occur extensively in the natural subsurface is unknown because the conditions under which they have been observed hitherto are generally unrepresentative of the average subsurface environment. Here, we show that microbial activity affects the mobility of a toxic trace metal (Cu) under the relatively low nutrient fluxes that dominate subsurface systems. More particularly, we show that under these low nutrient conditions, microbes and microbial products can immobilize metal but may themselves be subject to subsequent mobilization, thus complicating the pattern of metal storage and release. Our results show that the capability of microbes in the subsurface to change both the capacity of porous media to store metal, and the behaviour of metal that is released, is not restricted to the well researched environments close to sites of waste disposal. We anticipate our simulations will be a starting point for generating input data for transport models, and specifying the mechanism of metal remobilisation in environments more representative of the subsurface generally.