Development and application of a screening model for evaluating bioenhanced dissolution in DNAPL source zones.

In-situ bioremediation, a widely applied treatment technology for source zones contaminated with dense non-aqueous phase liquids (DNAPLs), has proven economical and reasonably efficient for long-term management of contaminated sites. Successful application of this remedial technology, however, requires an understanding of the complex interaction of transport, mass transfer, and biotransformation processes. The bioenhancement factor, which represents the ratio of DNAPL mass transfer under microbially active conditions to that which would occur under abiotic conditions, is commonly used to quantify the effectiveness of a particular bioremediation remedy. To date, little research has been directed towards the development and validation of methods to predict bioenhancement factors under conditions representative of real sites. This work extends an existing, first-order, bioenhancement factor expression to systems with zero-order and Monod kinetics, representative of many source-zone scenarios. The utility of this model for predicting the bioenhancement factor for previously published laboratory and field experiments is evaluated. This evaluation demonstrates the applicability of these simple bioenhancement factors for preliminary experimental design and analysis, and for assessment of dissolution enhancement in ganglia-contaminated source zones. For ease of application, a set of nomographs is presented that graphically depicts the dependence of bioenhancement factor on physicochemical properties. Application of these nomographs is illustrated using data from a well-documented field site. Results suggest that this approach can successfully capture field-scale, as well as column-scale, behavior. Sensitivity analyses reveal that bioenhanced dissolution will critically depend on in-situ biomass concentrations.

Experimental and numerical validation of the total trapping number for prediction of DNAPL mobilization.

The total trapping number (N(T)), quantifying the balance of gravitational, viscous, and capillaryforces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual,flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 x 10(-3)-1.10 x 10(-2)), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing.