The effect of biostimulation on geochemical and microbiological conditions in an isolated dolostone fracture
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A biostimulation field trial was conducted to determine the effect of nutrient addition on microbial populations in a fractured rock environment. The ultimate goal of this research is to induce bioclogging in rock fractures as a method of in situ containment and remediation of contaminated groundwater. This trial focused on biostimulation of indigenous bacteria in a single fracture through the addition of bioavailable carbon, nitrogen and phosphorus sources. Bench-scale experiments were conducted to determine the optimal source and concentration of nutrients for microbial growth. The final mixture selected for the field trial consisted of sodium lactate plus two liquid fertilizers, resulting in a 100:9:4 molar solution of bioavailable carbon, nitrogen, and phosphorus with a carbon source concentration of 8.9 g/L. The field trial was conducted in an uncontaminated area adjacent to an abandoned quarry in southern Ontario, Canada. The geology of the site consists of flat-lying dolostone pervaded by bedding plane fractures, with minimal overburden. An arrangement of three boreholes isolated a single fracture at a depth of 17m using straddle packer systems. A groundwater recirculation system was created with groundwater withdrawal at BH7 and reinjection of amended water at BH9. Throughout the three-week biostimulation experiment, general groundwater parameters, including temperature, dissolved oxygen and electrical conductivity, were monitored frequently. Geochemical and microbiological conditions including available electron acceptors, biochemical oxygen demand, heterotrophic plate counts, and microbial diversity were evaluated before and after the experiment. Monitoring results for the withdrawal well confirmed that nutrient delivery was occurring, albeit with substantial mass loss due to incomplete flow field development. Numerical modelling of the system estimated a nutrient mass loss of 29%. Geochemical monitoring of key electron acceptors suggested that redox conditions in the isolated fracture were greatly affected by nutrient addition. Biological data indicated significant changes in the microbial populations, with heterotrophic plate counts increasing significantly in the isolated fracture. Changes in microbial diversity were also observed through 16S rDNA analysis. Denaturing gradient gel electrophoresis results indicated substantial diversification and growth of the microbial community following biostimulation. Further research will investigate the potential for bioclogging at a NAPL-contaminated fractured bedrock site.