Field GPR monitoring of biostimulation in saturated porous media

Abstract

Abstract Time-lapse borehole radar was used to monitor a sandy aquifer with dielectric property changes hypothesized to result from biomass growth and biodegradation of hydrocarbons. A blend of gasoline and ethanol was released below the water table at Canadian Forces Base (CFB) Borden, Canada, and a source of oxygen was used for cyclic stimulation of microbial activity over a period of two years. A dense grid of fourteen borehole ground-penetrating radar (GPR) pairs monitoring the bioactive region showed 200 MHz radar wave velocity changes of ± 4 % and signal attenuation changes of ± 25 % during three cycles of variable biostimulation. GPR signal changes correlated spatially and temporally to independent measurements of groundwater velocity flow changes and geochemical variations that occurred in response to microbial activity. Greater relative changes in radar wave velocity of propagation were observed in the region of enhanced bacterial stimulation, adjacent to the oxygen release wells, and along the path of the hydrocarbon plume where biomass growth was the greatest. In the zone of maximum biological activity, radar wave velocity decreased during two cycles of enhanced biostimulation, whereas it increased during the intervening period of low-level biostimulation. Monitored regions at the fringe of the plume and farther down-gradient of the oxygen source, where biostimulation was less or absent, exhibited GPR signal changes of lesser magnitude and consistency. Spatial and temporal variabilities of GPR observations indicate a high degree of transient heterogeneity within the relatively homogeneous Borden aquifer as a result of hydrocarbon biodegradation. Mechanisms postulated to cause the dielectric property changes observed in the radar data are biogenic gas formation, mineral dissolution/precipitation, and direct biomass growth. Geochemical data, in-situ groundwater velocity measurements and biomass quantification suggest biomass formation in the pore space as a plausible mechanism for the changes observed in the aquifer. Therefore, at 200 MHz frequency, biomass formation is observed to cause a decrease of EM wave velocity of propagation suggesting an increase of the bulk dielectric constant of the water saturated medium. This finding is in agreement with published laboratory-scale GPR monitoring of microbial growth in porous media. This study shows that time-lapse GPR along with supporting biogeochemical observations can be used for monitoring of biological activity at the field scale.