The Effect of Thermally and Chemically Enhanced Biostimulation on the Hydraulic Properties of a Discrete Fracture Network in a Bedrock Aquifer
Smith, Reid T.
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The impact of thermally and chemically enhanced biostimulation of indigenous bacteria in a fractured rock aquifer and the resulting changes in hydraulic properties of the discrete fracture network were investigated at the field scale in this study. A field trial was conducted using five 30 m deep vertical boreholes drilled into limestone and granite geological units in a 100 m2 section of a field in Kingston, Ontario. Prior to a 14 day biostimulation experiment, pulse interference tests and tracer experiments were conducted between the various boreholes to characterize the fracture permeability and connections. Biostimulation methods were applied using a semi-passive injection withdrawal flow field. During periods of injection withdrawal, groundwater was recirculated at 15 ±2 Lpm through an aboveground reservoir (460 L) and gravity drainage system. Recirculating groundwater temperature was raised to 20°C - 25°C and a 4.5 L sodium lactate based nutrient solution was injected once daily. During biostimulation the groundwater temperature, geochemistry, microbiology and fracture hydraulic properties between the recirculating borehole pair were monitored. Hydraulic testing results showed that borehole transmissivity was reduced by up to 92% (injection borehole) of pre-biostimulation values and transmissivity of multiple borehole connections had been reduced by up to five orders of magnitude. The results of the tracer experiments showed an increase in solute tortuosity and arrival time and a decrease in peak concentration following biostimulation. The changes in transport observed in the tracer experiments are corroborated by heat transport measurements in the recirculation borehole pair. Microbiological and geochemical evidence of biological growth were observed in recirculating groundwater, but absent in the groundwater samples analyzed. Visual observations confirmed the increase in biological growth, although no direct characterization of the microbial community was performed. This study indicates the semi-passive operation of thermally and chemically enhanced biostimulation can provide a successful method for bioclogging a discrete fracture network. Pulse interference tests and tracer experiments were necessary to effectively evaluate the growth and distribution of the biobarrier, which developed beyond the influence of the injection well. Additional research is required to develop a better understanding of the factors governing biobarrier formation and longevity prior to industrial application.