Flow and Solute Transport through a Single Fracture during the Biodegradation of Residual Toluene

A series of bench scale experiments were performed to assess the effects of biofilm and bio-enhanced toluene dissolution on flow and solute transport through a rough fracture. The fracture was cast from a real shale rock fracture. Sterilized artificial groundwater was used as the nutrition source to support the growth of microorganisms which were obtained from local groundwater. Hydraulic and tracer tests were carried out before and after the injection of toluene, and during the toluene biodegradation as well. The normalized hydraulic conductivity decreased sigmoidally during the experiment, and reached 0.02 at the end of the experiment. Biofilm growth was the main cause for hydraulic conductivity reduction after the injection of liquid toluene. The presence of separate phase toluene extended the tracer tailing compared to tracer tests without toluene. The longitudinal dispersion coefficient DL was proportional to the 1.5th power of the mean velocity in the rough fracture with or without residual toluene. Though a biofilm developed in the fracture during biotreatment, the effect of secondary biofilm-associated porosity on solute transport in the fracture had negligible effect on tracer transport due to its small thickness. It is also found that DL decrease exponentially with Pe reduction during the bio-treatment.