Laboratory Studies of Biobarrier Technology in Fractured Rock
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Experiments exploring transport and bio-containment of contaminants in fractured rock were completed using fractured-limestone samples obtained in eastern Ontario, Canada. Three single-fracture samples, a fracture-intersection sample and a fracture-network sample were set into vertical flow systems. Three phases of experiments focused on the transport and hydraulic properties of each sample, the effects of biobarriers on diffusion processes in fracture rock, and methods of improving biobarrier stability and survivability. Hydraulic apertures were determined from constant-flow measurements and transport properties were interpreted from Lissamine and KBr tracer experiments with velocities of up to 8500 m/d for all five samples. At Re > 16, linear to non-linear transitions were observed in enlarged single fracture A and the fracture intersection samples. Reversible increases in aperture were observed at Reynolds numbers (Re) of 7, 4, and 3 for single fractures A and B, and the fracture-network, respectively. Non-linear effects were not observed in these samples over the range of velocities studied (up to Re = 20). Results from the 1-D analytical transport model overestimated values of matrix porosity, suggesting that diffusion from dead zones and slow-flowing regions are also contributing to observed breakthrough curves. Methods of improving biobarrier stability in fractured rock were studied in two single-fracture samples and the fracture-network sample by stimulating naturally-occurring groundwater bacteria. Survivability was improved with successive cycles of feeding and starving and stimulating growth at lower temperatures. Modeled values of matrix porosity decreased by up to 50%, indicating that diffusion processes are strongly influenced by biofilm development. Back diffusion of Lissamine was measured using one single-fracture sample and the fracture-intersection sample. Lissamine was allowed to diffuse into the matrix of each sample and, following a suitable loading period, the back-diffusion of residual Lissamine concentrations were measured from the outflow. This was done in the presence and absence of biofilm, and following the introduction of biofilm onto the fracture surfaces, diffusion was no longer a governing process affecting transport and only advective transport was observed. Experiments were interpreted using a 3-D finite difference model with a three-layer porosity approach, and indicated a decrease in aperture and porosity following biostimulation.