During the remediation using two-phase extraction HMVT noticed up to 10 times higher concentrations of oil components in groundwater were removed during sonic drilling (200 Hz). This led to the idea that with vibrations in the soil, NAPL phases could be mobilized. The potential technique based on this mechanism to could improve the removal of contaminants was given the name “Acoustic Remediation”.
The purpose of this HIP-pilot establishing the “Proof of Principle” by testing the “Acoustic Remediation” approach. This technique aims to use acoustic waves to mobilize NAPL droplets in the soil. Here, with mobilization is meant: “Bringing undissolved NAPL droplets in a groundwater flow field so that they can be transported with the groundwater flow”
Full report available in Dutch (see section Conclusion and Recommendations)
 NAPL: Non-Aqeous Phase Liquid.
The purpose of this HIP-pilot establishing the “Proof of Principle” by testing the “Acoustic Remediation” approach.
The study was conducted in two phases:
1. In the first phase, the propagation and radius of influence of vibrations and the enhancement on contaminant availability was tested.
2. In the second phase, a number of parameters was varied, allowing to establish a more detailed understanding of the method of application (first stage optimization and effectiveness).
Goal of this HIP pilot has been the application of vibration tests at contaminated sites with differences in soil structure and contamination type. Experiments have been conducted on two locations: a site with fine sand with a shallow mineral oil contamination (Botlek location) and a site with coarse sand with a VOCl contamination at depth.
A vibrating needle is connected through an inverter to a standard 240V – 50 Hz power supply. The needle is generaly used at a frequency of 200 Hz. The vibrator consists of a rotating clapper in a cylindrical enclosure. The clapper rotates at high speed against the housing and generates a strong vibration in the acoustic range. Seismic receivers were installed to study the behaviour of the vibrations in the soil.
The results on both sites (Doetinchem and Botlek) indicate that no NAPL phase was mobilized. It is likely that at both sites NAPL dissolution was enhanced. Based on new insights from the results of the vibration experiment at Doetinchem, it is expected that the optimal vibration frequency for enhanced dissolution is likely less than 200 Hz. This is due to the lower resonance frequency of soil in most locations in the Netherlands.
As regards the mechanism of mobilization, it seems unlikely that vibration of the small droplets themselves caused NAPL mobilization. The smaller an object, the higher the resonance frequency, which would require much higher frequencies for droplets. Simultaneously, the influence radius of the vibration in the ground goes down with higher frequencies. As expected the resonance frequency for a NAPL droplet on a grain scale is thousands of times smaller than the soil as a whole, the radius of influence of such high frequencies is expected to be minimal (mm). A disruptive influence on NAPL phases by acoustic vibration is therefore most likely due due to resonance of the soil as a whole. As shown in this study, the frequency at which this occurs also has a much greater radius of influence (10s of meters).
At the Doetinchem location likely a slightly enhanced dissolution (3%) of NAPL phase was observed. This mass removal is small compared to the potential of mobilizing NAPL phases in the original hypothesis of this HIP pilot. The potential for enhanced remediation of sites with NAPL using acoustic vibrations by increasing the dissolution rate is therefore not very effective. For actually increased mass removal the mobilization and generation of small droplets NAPL as well as transport by sufficient groundwater flow is required.
The extent to which acoustic vibrations can disturb a NAPL, depends, in addition to the oscillation frequency, also on the energy with which they reach the NAPL. This is also the position of the vibration source relative to the spatial distribution of NAPL phases in the soil of interest. The frequency during sonic drilling (200 Hz) is used, although not optimal, but given the high energy level at which this occurs will NAPL near the drill site probably affected. This probably explained the practical observations that led to this investigation. The spatial distribution of NAPL phases, especially those of DNAPL, are often the biggest unknown in practice. Improved understanding of the distribution locations, can assist in the consideration of the (planned) sonic drilling location (s) possible disturbance of the NAPL phases can result. The impact of this disruption is also expected to depend on the degree of saturation in which the NAPL is present in the soil. For DNAPL phases with high pore saturation degrees, including in particular DNAPL pools, possible adverse effects should take into account, such as the further sinking of the DNAPL phase.
For residual pore saturations it is most likely that the acoustic disturbance will solely lead to increased access of NAPL phase. This can speed up the dissolution. Based on the results of this study this is of low impact, and it is not cost effective to solely introduce vibrations in the soil for that purpose. Possibly, however, improved accessibility for remedial agents may improve remediation efficiency. The potential application, operation and cost of this approach would have to be further investigated.
Figure: example of praph relating frequency and concentration of CVOC compounds.
To benefit from possible positive effects sonic drilling should be performed, where possible, to coincide with the active remediation measures (eg NAPL extraction or ISCO). For an improved understanding under what conditions vibration does cause mobilization of NAPL phases, remediation conditions (e.g. the amount of mass removal) before and during active sonic drilling can be compared.