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.