BIOLOGICAL GAS PRODUCTION DURING BIOSTIMULATION IN BENCH-SCALE FRACTURED ROCK SAMPLES

The application of biobarriers for remediation of contaminated fractured rock environments requires extensive consideration of growth conditions for bacterial populations. A limestone fracture-network sample (2b_H = 361 µm) was used to observe the effects of biogas production during biostimulation of a native bacterial population collected from local (Eastern Ontario) groundwater. Cycles of growth promotion with a nutrient/carbon source added to native groundwater, and starving where only native groundwater only was introduced were repeated three times, and gas production was measured at the end of the first feed phase and the beginning of the second feed phase to determine if this behavior changes with repeated cycles. Gas production was measured indirectly as the difference between the influent and effluent flow rates. Flow rates were measured every 1-3 minutes over several hours by mass using an analytical balance and a small graduated cylinder. Constant influent flow rates were maintained using a variable-speed peristaltic pump, and hydraulic head was measured approximately hourly using a measuring tape attached to the flow system. Velocities and fracture apertures were calculated from flow rate and hydraulic head measurements and interpreted with the cubic law. Volumes of gas produced were measured as changes in volume in the effluent tube as solution backed up into the lower reservoir due to the upward flow of gas bubbles from the rock fracture into the upper reservoir. Changes were marked along the effluent tube and recorded for every observed bubble event. Volumes of gas determined from observed bubbles and the differences in flow rates were converted to masses of two possible gases: methane, a commonly produced bacterial product, and air, for comparison purposes. Biogases produced by biobarriers may substantially contribute to clogging events in fractured rock, even if visible gas production is not observed. In the short term, biogases can also be responsible for large fluctuations in velocity, causing instabilities in biobarrier development. Larger, visually observable releases of biogas were consistently reproduced over the three test intervals, indicating that preferential flow paths are an important factor in flow behavior.