Ground‐penetrating radar observations of enhanced biological activity in a sandbox reactor

Abstract

[1] In this study, we evaluate the use of ground-penetrating radar (GPR) to investigate the effects of bacterial activity in water saturated sand. A 90-day laboratory-scale controlled experiment was conducted in a flow-through polycarbonate sandbox using groundwater from the Kansas River alluvial aquifer as inoculum. After 40 days of collecting baseline data, bacterial growth was stimulated in the sandbox by the addition of a carbon and nutrient solution on a weekly basis. Radar signal travel time and attenuation were shown to increase downgradient of the nutrient release wells relative to upgradient locations. After 60 days, the frequency of nutrient injections was increased to twice per week, after which gaseous bubbles were visually observed downgradient of the nutrient release wells. Visual observation of active gas production correlated spatially and temporally with a rapid decrease in radar signal travel time, confirming that GPR can monitor the generation of biogenic gases in this system. Analysis of the sediments indicated microbial lipid biomass increased by approximately one order of magnitude and there were no changes in the inorganic carbon content of bulk sediment mineralogy. These findings suggest that the increase in biomass and gas production may be responsible for the observed changes in radar signal travel time reported in this study. Therefore, this study provides evidence that GPR can be used to monitor biological activity in water saturated sand.

Application of ground‐penetrating radar to measure near‐saturation soil water content in peat soils

“…Ideally we would have recorded transmission mode measurements of the direct wave across the peat block; however, this measurement would require two antennas and multiple GPR electronics not at our disposal. We therefore used the reflection mode as previously successfully applied by Comas and Slater [2007] and Schillig et al [2010]. The excellent data quality we obtained indicates that the reflection mode approach sufficiently satisfied our objectives.…”

Field GPR monitoring of biostimulation in saturated porous media

“…GPR has been used successfully in the field to detect biogenic gas (Comas et al, 2005;Parsekian et al, 2010Parsekian et al, , 2011. Formation of a separate gas-phase in the pore space would decrease the bulk dielectric constant of the medium and hence increase the velocity of propagation of the radar signal, as observed in the laboratory by Schillig et al (2010). During this experiment, biogenic gas formation is likely to occur during time periods of enhanced biostimulation near the GPR boreholes closest to the ORC.…”

“…Although the exact value of the dielectric constant of the biomass at Borden still remains unclear, Schillig et al (2010) reported a decrease in 1.2 GHz GPR wave velocity observed during enhanced biostimulation and biomass growth in a sandbox laboratory experiment. The sandbox Fig.…”

“…For example, another study measured the dielectric constant (and conductivity) for two common bacteria, E. Coli and Micrococcus sp., at ε r~5 5 and~80 respectively for 200 MHz (Carstensen, 1967). In a saturated sandbox laboratory experiment, Schillig et al (2010) reported 1.2 GHz radar wave velocity propagation reduction as a function of biomass formation which suggested microbial dielectric constant values slightly greater than pore water (ε r water .~80). Österreicher-Cunha et al (2004) reported GPR velocity decrease while monitoring bioventing of unsaturated gasoline-ethanol contaminated soils and suggested that additional studies are necessary to understand the changes in GPR signals.…”

“…These by-products may include biogenic gasses (Comas et al, 2005;Delozada et al, 1994;Schillig et al, 2010), secondary minerals (DeJong et al, 2006;Williams et al, 2005), or even mineral dissolution (Lane et al, 2006;Welch et al, 2002). Some of these may occur simultaneously.…”