Nonaqueous phase liquids (NAPLs) are organic liquid contaminants that are found in aquifer environments and have the potential to contaminate groundwater. This project is designed to integrate information on phase behavior of multicomponent NAPLs with data on biodegradation rates to construct a comprehensive model for NAPLs containing polycyclic aromatic hydrocarbons (PAHs). This model simulates long term composition dynamics and can be used in combination with risk assessment protocols to predict human health risk and evaluate remediation options.

NAPL contaminants that are multicomponent PAH mixtures pose a challenge for remediation and regulation because of the large number of compounds involved, and the interdependence of their behavior. In-situ bioremediation is an attractive remediation option for these contaminants. Mathematical models that accurately predict multicomponent contaminant behavior over long time periods and translate this into risk are needed to predict the effectiveness of bioremediation, and whether remediation is even warranted.

The phase behavior of multicomponent NAPLs is being studied through experimental investigations of mixtures of PAHs. These are compounds that are solids in their pure states at ambient temperatures but they can exist as stable liquids when present in mixtures in the appropriate proportions. Experiments involve a combination of thermal and chemical analyses of multi-phase systems involving simple PAH mixtures, with inference based on thermodynamic theory of solid-liquid phase equilibrium. The biodegradation kinetics of mixtures of PAHs are being studied through experimental measurement of biodegradation rates of PAHs in sole-substrate and multi-substrate mixtures. The phase behavior and the biodegradation kinetics are incorporated into a numerical simulator that can describe the change in NAPL composition with time and space in an aquifer environment. The time-averaged aqueous concentrations are translated into human health risk using literature values of carcinogenic slope factors.

This project will be completed in January 1999. A numerical simulator has been developed that incorporates the mathematical models describing phase behavior and biodegradation kinetics. Simulations have been conducted describing a variety of remediation conditions including pump-and-treat, bioremediation, and solvent extraction. We have shown that NAPL solidification is likely to occur over time periods of years to decades and that this may have significant impacts on dissolution and transport processes. Most mathematical models describing NAPL contaminants do not account for solidification phase behavior. We have also shown that PAHs can contribute to risk for different reasons. A compound like benzo[a]pyrene has significant potential to contribute to risk because of its carcinogenicity. Despite continuous NAPL depletion, the concentration of benzo[a]pyrene increases due to the increase in its relative abundance in the NAPL phase. Naphthalene, which is much less carcinogenic than benzo[a]pyrene, can significantly contribute to risk because it may persist at high concentrations in groundwater. Compositional approaches to risk assessment lead to better risk predictions for PAHs than simple lumped metrics such as total petroleum hydrocarbon (TPH).

The following will be interested in this research: U.S. Environmental Protection Agency, State environmental regulatory agencies, environmental consulting firms focusing on hazardous wastes. Results have been presented at national meetings and published in engineering and science journals. In addition, a cumulative overview paper is in preparation which summarizes research findings and explores policy implications. This paper is targeted for the Policy Forum of Environmental Science and Technology, which has broad readership that includes a non-technical audience.