SUBSURFACE HETEROGENEITIES, INTERFACES AND BIODEGRADATION: DEFINING THE LIMITS ON IN SITU BIOREMEDIATION
Seagren, Eric A.
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Subsurface heterogeneities and the associated interfacial processes impact in situ bioremediation by affecting the availability of substrates to the microorganisms. This research hypothesized that using the scales of subsurface heterogeneities as an organizational principle, a quantitative framework based on a set of dimensionless numbers could be developed to capture the effects of the competing interfacial and biokinetic processes and define the limits for successful application of in situ bioremediation. The overall goal of this study was to use an integrated experimental and numerical modeling approach to evaluate the developed quantitative framework under different simulated scenarios relevant to the subsurface. Three experimental scenarios were selected to simulate field sites limited by either (1) macro-scale vertical transverse dispersion (Scenario #1), (2) micro-scale biokinetics (Scenario #2), or (3) meso-scale sorption/desorption (Scenario #3). Experiments were performed in a saturated, heterogeneous intermediate-scale flow cell (ISFC) with two layers of contrasting hydraulic conductivities and monitored the transport of a naphthalene plume through two phases: Phase I, simulating an intrinsic biodegradation; and Phase II, simulating an engineered bioremediation, with selected system perturbations. In the first Phase II perturbation, nitrogen (N) and phosphorus (P) amendments in excess of stoichiometric requirements were made, while the second perturbation was selected based on the rate-limiting process identified via the quantitative framework. A numerical model was used to simulate the Phase I experiments and verify the independently determined mass transport and biokinetic parameters, which were then used in the dimensionless parameters of the proposed quantitative framework. Scenario #3 was not completed due to the time constraints, but Scenarios #1 and #2 successfully demonstrated application of the quantitative framework. In Scenario #1, Phase I, vertical dispersion was identified as the overall rate-limiting process. Correspondingly, increased advection and mechanical dispersion in Phase II increased naphthalene biodegradation by ~ 2.7 times, whereas the N and P addition had no effect. In Scenario #2, Phase I, dispersion and biokinetics were identified as rate-limiting processes. Thus, in Phase II, N and P addition moderately improved biodegradation, but removal of inhibitory, high salinity conditions to improve the biokinetics increased naphthalene mass loss ~2.7 times. These results demonstrate the potential for application of the proposed quantitative framework to predict the rate-limiting process for in situ bioremediation and aide in the appropriate selection of any system perturbations for enhancing in situ bioremediation.