SCALING UP SUBSURFACE BACTERIAL TRANSPORT FROM THE COLUMN TO THE FIELD SCALE: IMPORTANCE OF ECOSYSTEM-LEVEL EFFECTS

Bacterial transport at the field-scale can involve a number of poorly understood controls that are difficult to study at the column-scale. Consequently, environmental controls are not typically considered in models that describe subsurface bacterial transport. However, because bacteria are an integral part of subsurface ecosystems, factors such as the environmental carrying capacity can be critical determinants of their subsurface fate and transport. The environmental carrying capacity of shallow aquifers subject to carbon limitation can be related to the amount of dissolved organic matter (DOM) that can be readily utilized by the microbial populations and, consequently, can vary both spatially and temporally. Unattached bacteria advected downgradient in carbon-limited aquifers are particularly sensitive to the level of DOM present. For a shallow, glacial-outwash aquifer contaminated with treated sewage (Cape Cod, MA), ~ 2/3 of the spatial variability for unattached bacteria within the upgradient 3 km of the contaminant plume was explained statistically by the abundance of organic wastewater contaminant DOM, even though both sorptive-filtration and predation by groundwater nanoflagellates (protists) of unattached bacteria vary spatially. Additional consideration of the age of the DOM explained ~98% of the variability of unattached bacteria along the longitudinal axis of the plume. Sets of replicate, down-well dialysis-chambers, initially seeded with undifferentiated, laboratory-grown aquifer bacteria at abundances ranging from 100,000 to 10,000,000 bacteria/mL, came to chemical equilibrium with surrounding groundwater within ~1 day. Within one week, the disparate bacterial abundances converged on a value that was consistent with the in-situ bacterial abundance within that part of the plume. Incubation for a longer (21 day) period resulted in final bacterial abundances that were not statistically different than those observed for the shorter (7-day) incubations. Results from the aforementioned study suggest that DOM-determined environmental carrying capacity may be useful in predicting subsurface concentrations of bacteria advecting downgradient from contaminant sources in certain aquifers and in bioaugmentation applications.