Micro to macro investigation of biofilm impacts on the hydraulic parameters of porous and fractured media

Biofilm growth can affect the hydraulic properties of geological media in a number of ways. Precise quantification of these changes and extrapolation of laboratory data to field-scale problems has been impeded due to the complicated nature of completing combined biological-geochemical-engineering studies. In order to make progress in this area, an integrated programme of study has been designed to examine this problem at several different length scales and then combine results to produce a predictive model applicable to real-world problems. Growth and persistence of biofilms as well as the formation of mineral precipitates in the presence of a viable microbial community have been studied and a number of critical parameters quantified. At the sub-micron scale, X-ray photoelectron spectroscopy, Proton Induced X-ray Emission, and Confocal Scanning Laser Microscopy have been used to build up a picture of surface colonization, mineral precipitation, and 3-Dimensional biofilm structure and composition. Mesoscale results have determined how rapidly metal precipitates can be formed in the presence of microbes and how biofilm and precipitates are distributed within porous media under low-nutrient conditions. At the macro-scale, a geotechnical centrifuge model has been used to determine changes in bulk hydraulic parameters that occur after a robust microbial community has been established in both porous and fractured media. In situ hydraulic conductivity (k) in a natural porous medium (Congleton sand) was reduced by approximately 50% after colonization by P. aeruginosa under low-nutrient conditions. These results allow scaling up to field conditions, while the direct microscopic observations explain exactly how this reduction in conductivity occurs. The data also show that in an experiment using an Fe-bearing solution there is at least a 20% reduction in k. Furthermore, the centrifuge experiments also determined that above approximately 28 G shear forces become high enough such that the biofilm no longer maintains its structural integrity and/or surface adhesion. Knowledge of this limit is critical in designing the next generation of centrifuge models as well as in predicting how natural biofilms will behave under conditions of high fluid flow.