A numerical investigation of metabolic reductive dechlorination in DNAPL source zones.

Abstract

Among the most intractable environmental remediation problems are those involving the release of dense non-aqueous phase liquids (DNAPLs), such as chlorinated solvents, to the subsurface. Research efforts have focused on the use of numerical models to investigate reductions in contaminant concentrations due to partial mass removal and improvements in the performance of complementary source zone remediation technologies. Previous numerical investigations, however, have been limited to two-dimensional systems. Furthermore, a lack of models capable of simulating the most promising complementary technology, metabolic reductive dechlorination, has limited its application. This work developed and applied compositional multiphase numerical simulators to examine the influence of dimensionality (two-dimensions versus three-dimensions) on DNAPL source zone simulations and to investigate the benefits of stimulating metabolic reductive dechlorination at a chlorinated ethene-DNAPL contaminated site. Results from the dimensionality investigation showed that the simulation of DNAPL migration, entrapment, and dissolution in two dimensions provided reasonable approximations to the behavior simulated in three dimensions. Commonly employed saturation distribution and mass recovery metrics were approximately equivalent. Flux-averaged concentrations simulated in two dimensions, however, tended to be three to four times higher than those simulated in three dimensions. This difference was attributed to dilution at the down gradient boundary. An alternative metric, mass flux reduction, however, yielded better agreement. To investigate the application of complementary mass removal technologies, an existing multiphase compositional simulator was modified to incorporate eight chemical components and four microbial populations: a fermentative population, two dechlorinating populations, and a competitor population. Monod kinetics, modified to incorporate electron donor thresholds, electron acceptor competition, and competitor inhibition, were used to simulate microbial growth and component degradation. This simulator was used to explore enhancements in DNAPL dissolution due to metabolic reductive dechlorination in one- and two-dimensional non-uniformly contaminated domains. Simulation results demonstrated the potential for this technology to accelerate interphase mass transfer. The level of enhancement was affected by the dechlorination rate, DNAPL zone length, and DNAPL zone configuration. The length of time dissolution enhancement persisted at the quasi-steady-state value (maximum), however, was generally less than 50 percent of the longevity of the source.