Remediation of NAPL Source Zones: Lessons Learned from Field Studies at Hill and Dover AFB

Innovative remediation studies were conducted between 1994 and 2004 at sites contaminated by nonaqueous phase liquids (NAPLs) at Hill and Dover AFB, and included technologies that mobilize, solubilize, and volatilize NAPL: air sparging (AS), surfactant flushing, cosolvent flooding, and flushing with a complexingsugar solution. The experiments proved that aggressive remedial efforts tailored to the contaminant can remove more than 90% of the NAPL-phase contaminant mass. Site-characterization methods were tested as part of these field efforts, including partitioning tracer tests, biotracer tests, and mass-flux measurements. A significant reduction in the groundwater contaminant mass flux was achieved despite incomplete removal of the source. The effectiveness of soil, groundwater, and tracer based characterization methods may be site and technology specific. Employing multiple methods can improve characterization. The studies elucidated the importance of smallscale heterogeneities on remediation effectiveness, and fomented research on enhanced-delivery methods. Most contaminant removal occurs in hydraulically accessible zones, and complete removal is limited by contaminant mass stored in inaccessible zones. These studies illustrated the importance of understanding the fluid dynamics and interfacial behavior of injected fluids on remediation design and implementation. The importance of understanding the dynamics of NAPL-mixture dissolution and removal was highlighted. The results from these studies helped researchers better understand what processes and scales are most important to include in mathematical models used for design and data analysis. Finally, the work at these sites emphasized the importance and feasibility of recycling and reusing chemical agents, and enabled the implementation and success of follow-on full-scale efforts

In Situ Mixed Region Vapor Stripping in Low-Permeability Media. 2. Full-Scale Field Experiments

This paper is the second in a three-part series that describes mixed region vapor stripping (MRVS) for in situ treatment of fine-grained soils contaminated by volatile organic compounds (VOCs) including trichloroethene (TCE), 1, 1, 1-trichloroethane (TCA), and related halocarbons. As described in this paper, MRVS processes were studied during full-scale field experiments wherein ambient or heated air was injected at high volumetric flow rates during in situ soil mixing, and VOCs were volatilized and advectively removed from the subsurface, captured in a shroud covering the mixed region, and then treated on-site. The field test was conducted at an inactive land disposal site in southern Ohio where dense silty clay soils were contaminated by VOCs at concentrations in the 10500 mg kg−1 range. During the field studies, seven columns, each 3.0 m diameter and 4.6 or 6.7 m deep, were treated with ambient air (~15-25°C) or heated air (~120-130°C) injected at flow rates of 28°40 m3 min−1. Intensive monitoring and measurement activities defined contaminant behavior and key MRVS operation and performance parameters. The field testing revealed that MRVS could rapidly reduce the concentrations of VOCs (i.e., TCE, TCA, …) in dense silty clay soil by 88–98%. The rate and extent of reduction was somewhat higher with the injection of heated air as compared to ambient air. Regardless of injection air temperature, as treatment progressed, the rate of VOC removal became increasingly mass transfer limited. © 1995, American Chemical Society. All rights reserved