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A bio-partitioning tracer technique for the estimation of microbial biomass effects on the transport of partitioning solutes in saturated subsurface media is presented. Bioaugmentation is a remedial approach under evaluation for dense nonaqueous phase liquid (DNAPL) sources at sites with extreme geological complexities, such as fractured rock. Biomass accumulates in the subsurface following bio-stimulation activities and provides an expanded organic phase affecting the behavior of the solutes and tracers. If the magnitude of partitioning into the biomass is significant relative to the presence of residual DNAPL, the DNAPL saturation determined from traditional partitioning tracer techniques could be overestimated after bio-stimulation.
For traditional partitioning tracer studies, such as those performed in conjunction with pump-and-treat or cosolvent flooding, microbial growth conditions remain relatively constant both before and after the remedial action. Accordingly, accounting for microbial biomass changes has been unnecessary historically. Because partitioning tracer studies are typically conducted both before active remediation and after to characterize changing DNAPL saturation, it is now apparent that it is necessary to understand the influence of the changing biomass on the partitioning solute tracers used at sites following bioaugmentation.
To assess the biomass effect on solute transport, a series of sand packed columns are injected with dissolved phase tetrachloroethylene (PCE) followed by varying amounts of a commercially available DHC-containing consortia and lactate to simulate a range of microbial growth similar to the effect of bio-stimulation activities at a field site. The columns are exposed to a partitioning tracer suite both before and after enhanced bioaugmentation. Following the tracer tests, biomass is estimated in the columns by analyzing the fatty acids of the total microbial community attached to the sand. The breakthrough of the tracer suite is used to quantify retardation and assess the effect of the biomass on the tracer studies. |