Impacts of biofilm on diffusion in fractured rock.

摘要

Fractured rock aquifers consist of complex flow systems that impose several constraints on cleanup efforts. Remedial techniques in such aquifers are influenced by diffusion of contaminants into the rock matrix and the subsequent back diffusion into the fracture. In particular, the back diffusion process can release a low concentration of contaminant into the groundwater for an extended period of time alter the main source of contamination is removed from the fracture network.The main objective of this study was to assess the potential of a biofilm to limit diffusion between the host rock and fracture. The impacts of different types of microorganisms on the diffusion rate were compared, and the performances of the tracers employed in diffusion experiments were evaluated. P. putida, E. coli and an indigenous groundwater population were used to grow biofilm on porous ceramic disks with nominal pore diameter of 6 mum. Disks were installed into stainless steel double-reservoir cells. The source reservoirs were spiked to obtain initial concentrations of 1500 ppm of bromide and 5000 ppb of Lissamine. In total, 17 experiments were conducted to assess the diffusion. Finally, a semi analytical model was used to interpret the geometric factors (tortuosity) of the porous media and the impacts of the biofilm on the effective diffusion coefficient.Results, estimated from bromide and Lissamine concentration profiles, did not suggest a significant impact of biofilm on the diffusion through the ceramic disks. This could be due to heterogeneity of biofilm structures, loose structures of developed biofilms, lack of biofilm penetration into the pores inside the disks as a result of nutrient overloading or biofilm decay/detachment during the test. In addition, Lissamine did not appear to perform as a conservative tracer in some cases where mass balance calculations indicated a loss of dye.It was recommended to characterize the biofilm structure as well as the biofilm impregnated zone, and to adjust the nutrient loading in order to obtain a more packed structure for the biofilm developed by small size bacteria. Recommendations also included a thorough evaluation of Lissamine behaviour as a conservative tracer.Biofilms have been defined as cells immobilized on a solid surface and embedded in a gel matrix of extra-cellular polymeric substances (EPS) excreted by microorganisms. Biofilms can be stimulated on rock surfaces to act as a barrier in groundwater systems, which influences hydraulic properties of the fractured media as well as the rate of contaminant transport between the rock matrix and the fracture.