Modeling of Complex, Multi-Component NAPL Remediation for Decision Support

In situ remediation of nonaqueous phase liquid (NAPL)-impacted sites is difficult and costly. Even with enhancements (e.g., chemical and thermal) and partial NAPL recovery, mass transfer constraints associated with partitioning from residual NAPL typically control the depletion rate of sources and attainment of cleanup goals. Practical methodologies are needed to support strategic evaluation, planning, and implementation of cost-effective remedial approaches that are considered protective. A modeling system averaging over the NAPL-impacted saturated soil volume was developed and demonstrated at the former Williams Air Force Base. The system combines upscaled, physically based mass transfer coefficients for multi-component NAPL dissolution with theoretical enhancements specific to multiple remediation processes. Dissolution increases in the presence of aqueous phase reactions, according to first- or second-order kinetics, by increasing concentration gradients. Slow biological processes are considered first order in modeling natural attenuation and enhanced biological degradation. Fast reactions associated with chemical oxidation are considered second order. These enhancement models are equally applicable to numerical simulations of NAPL remediation. Pump and treat enhances dissolution in proportion to increases in the characteristic velocity associated with dissolution. The demonstration yielded realistic predictions, with greater certainty, for outcomes from of multiple technologies intended to reduce remedial timeframes and life cycle costs. The enhanced dissolution modeling provides a site-specific, quantitative assessment of changes in NAPL source discharge concentration and mass discharge over time for various remedial options that is equivalent to assessments from complex numerical transport models, given typical input data limitations.