James W. Mercer

A review of immiscible fluids in the subsurface: Properties, models, characterization and remediation

Abstract In the past few years, as hazardous waste sites have been studied more often and in more detail, immiscible fluids have been encountered in the subsurface with greater frequency. These nonaqueous phase liquids (NAPLs) behave differently than dissolved solutes in the subsurface. This behavior depends on fluid properties such as interfacial tension, viscosity and density. In addition, mass transfer produces vapor transport in the vadose zone and solute transport in groundwater. Mass transfer depends on properties associated with volatilization and aqueous solubility. As a consequence, characterization techniques as well as remediation efforts must be modified at sites where NAPLs are present. Although considerable research is necessary before NAPL problems are well understood, sufficient work has been performed to permit a review of NAPL properties and behavior in the subsurface.

DNAPL Site Characterization Issues at Chlorinated Solvent Sites

Past releases of chlorinated solvents generally occurred as dense nonaqueous phase liquids (DNAPLs). Because DNAPLs are heavier than water and are sparingly soluble, they pose difficult characterization, remediation and long-term management challenges. At such sites, a distinction is made between the source zone, which includes portions of the subsurface where DNAPL is or was present as a separate phase, and the downgradient plume of dissolved contamination resulting from groundwater flow through the source zone. Site management frequently involves attempting to contain or deplete contamination in both of these areas. During the past two decades, several promising in situ technologies (e.g., chemical oxidation, thermal extraction and cosolvent/surfactant flushing) have been applied at many sites to remove or destroy contaminants in DNAPL source zones. Yet the U.S. Environmental Protection Agency (USEPA) Expert Panel on DNAPL Remediation (USEPA, 2003) concluded that:

Remedial Action Assessment for Hazardous Waste Sites Via Numerical Simulation

Abstract A common remedial action at many hazardous waste sites consists of some type of hydraulic control which generally takes the form of one or a combination of the following: (1) drains, (2) wells and (3) permeability barriers such as walls and caps. All of these hydraulic controls have been tested in construction projects where workers need dry conditions. Under these circumstances, time frames are relatively short (a few years or less) and monitoring is very accurate (workers identifying leaks within days). For hazardous waste sites, however, design criteria for hydraulic controls must consider time frames of many years and monitoring systems that include only a few observation points. Because the design criteria for construction projects and remediation are so different, there is no assurance that hydraulic controls will work equally well for both applications. For hazardous waste sites, uncertainty in the effectiveness of hydraulic controls has led to the combined use of modelling and monitoring. This usage of modelling in assessing various remedial actions is demonstrated by presenting simulation results from four hazardous waste sites. Groundwater flow modelling is used to evaluate various combinations of drains, walls and caps at the Lipari site in New Jersey. A variably-saturated flow model is applied to various combinations of a wall, cap and drains considered for the Love Canal site in Niagara Falls, NY. A flow and transport model is applied to a purge well system to recover a chlorinated hydrocarbon spill in New England. Finally, an immiscible flow model is applied to the S-Area site, also in Niagara Falls, to evaluate the remedial action necessary to prevent the downward movement of dense non-aqueous liquid. In all four applications, emphasis is placed on the groundwater system response to the remedial action. The results of such simulations may be used in an iterative fashion to improve both remedial design and monitoring.

Use of Groundwater Models and the Settlement Process at the Chem-Dyne Hazardous Waste Site

As part of a hazardous waste negotiation settlement process, groundwater flow and transport models were developed for the Chem-Dyne site in Hamilton, Ohio. These models were subsequently used for a preliminary evaluation of a proposed remedial action using extraction/injection wells. As a result of this modelling evaluation, a large portion of the settlement concerned performance measures, monitoring, and contingencies. This ultimately led to a remediation that is goal-oriented rather than duration-oriented. This represented the first settlement of a major groundwater clean-up under the Superfund law. In addition to the use of models in the settlement process, the consent decree includes the use of models throughout the clean-up process and evaluation of the response of the groundwater flow system to the remedial action. The consent decree also requires the use of modelling prior to termination of the extraction/injection wells.