Feasibility of in situ implementation of vibrations to mobilize NAPL ganglia

A growing number of incidents of nonaqueous phase liquid (NAPL) spills in the recent past have warranted development of innovative and cost‐effective remediation technologies. Of particular concern is the entrapment of LNAPL (NAPL lighter than water) in the form of ganglia or blobs near the water table by virtue of strong capillary forces. The residual ganglia are the leftover component after pumping of free product and typically occupy 20 to 60% of the pore space. Mobilization of these ganglia would require unrealistically high hydraulic gradients and is often beyond the scope of pump‐and‐treat processes. This paper deals with the feasibility of in situ implementation of localized vibrations for controlled mobilization and collection of LNAPL ganglia. Specifically, the paper covers three components. First, the principles involved in soil‐water‐NAPL interactions under the influence of vibrations are discussed. The effects of vibrations on a soil‐NAPL‐water medium are postulated in terms of pore structure and relative density changes, changes in the permeability of the medium as a result of the changes in pore structure, and development of cyclic pore pressures. Second, results from bench‐scale experiments are presented that involved vibrating contaminated soils under the simultaneous influence of hydraulic gradients. A bench‐scale model consisting of a vibrator integrated with an injection and pumping system was found to be successful in these experiments. The results from the tests showed that up to 85% removal of ganglia can be achieved using this process. Third, the principles involved in the vibratory mobilization were applied to in situ conditions to develop a methodology to estimate the zone of influence of the process. The analogy between this process and an existing geotechnical process known as vibroflotation is exploited to demonstrate the methodology.