Principle
Besides active floating-layer removal, a floating layer can also be removed via skimming, in vertical filters, on a drain with drainage well or even with an open trench or gravel-filled trench.
In this case, a part of the NAPL liquid will flow towards the extraction area under the force of gravity, where it can be pumped away. As long as an NAPL layer remains, with a particular thickness, this process may continue. However, at a particular point, the remaining NAPL will stop flowing (e.g. when the soil’s so-called NAPL retention capacity is reached).
Implementation area and implementation conditions
Passive floating layer removal can be implemented in reasonably permeable soils which contain a considerable amount of (mobile) NAPL. The mobility of the NAPL must be tested in advance, e.g. via an NAPL-recovery test on a level indicator with filter, in the NAPL zone. For soils with less permeability, the technique is less effective and/or a more extraction resources need to be employed. One must also consider the influence of particular seasons and related water levels. It is possible, at a later stage in the remediation, to switch to a high-vacuum extraction.
If work is being carried out in open trenches or soil-filled trenches, then they must be covered and ventilated by an activated carbon filter.
Costs
The costs for such a remediation operation are lower than those for high-vacuum extraction. The means required for extraction are comparable to those required for ground-water extraction. The pumped NAPL product can be stored in surface tanks and can be transported to an external processor. If, besides NAPL, water is also pumped, then an oil/water separator must also be used, with a special process for the water fraction.
Environmental burden and measures to be implemented
The environmental burden of this technique is minimal and the energy requirement is low. The measures to be implemented are the same as for multi-phase extraction. If there is danger of subsidence, a stability survey is an additional requirement.