BIOISOLATION OF NON-AQUEOUS PHASE LIQUID POOL MIXTURES IN THE SUBSURFACE: THE IMPORTANCE OF NON-IDEAL CHEMICAL BEHAVIOR AND POOL CHEMICAL HETEROGENEITY ON DISSOLUTION AND BIOAVAILABILITY

Groundwater contamination by subsurface non-aqueous phase liquid (NAPL) pools comprised of poly-aromatic hydrocarbons (PAHs) continues to be a critical topic for remediation and containment engineers. Bioisolation, whereby the NAPL pools are left in place and engineered bioremediation is used to isolate contamination from downstream receptors, is one alternative for minimizing gross migration of these toxic organics.

Our focus for the present discussion is the effect of non-ideality and chemical heterogeneity within a pool on the mass transfer of soluble substrate and the subsequent implication for bioavailability. We postulate that these influences are significant and thereby warrant consideration in the design or analysis of a scheme for NAPL pool bioisolation.

To develop the conceptual framework from which we examine our hypothesis we present, in general, a new numerical research model for investigating coupled phenomena applicable to bioisolation. The model, NPDTbio, incorporates saturated/unsaturated flow, multi-species solute transport, biomass transport, competitive or noncompetitive organic solute sorption to soil and/or biomass, dissolution of non-ideal chemical pool mixtures, nutrient-limited biodegradation with numerous competition or induction options and that permits alterations in porosity, permeability and dispersivity due to biomass growth and accumulation.

To support our postulate, we present experimental results for the dissolution of a non-ideal binary NAPL mixture, naphthalene and hexachlorobutadiene, from pools of various lengths. With this data we demonstrate, (a) the role of chemical non-ideality on overall naphthalene dissolution, (b) the transient development of spatial chemical heterogeneity within the pool, and (c) the viability of the numerical model to simulate multi-component non-ideal NAPL pool dissolution.

We then present an electron acceptor-rich bioisolation scenario where the sole source of contamination is from a subsurface NAPL pool mixture and, via numerical modeling and sensitivity analysis, demonstrate a causal relationship between a downstream effective contamination metric and the transient development of spatial chemical heterogeneities within the source due to non-uniform dissolution and chemical non-ideality.