NAPL Trapping Cap
Posted by Mark Bartlett on Thu, Apr 15, 2010 @ 12:03
See the recent article in the Civil Engineering Journal, concerning a "Dense Non-aqueous Phase Liquid (DNAPL) Trapping Cap" that passively vents gases related to the decomposition of organic matter, while retaining the coal tar that was mobilized by the gas bubbles.
This reminded me of a similar cap designed and implemented by Norfolk years ago at a site that was in a flood plain for an abutting river in the Boston Metro-West area.
Norfolk's system was designed to control, capture, and treat DNAPL related contaminants. The Norfolk site was different in the following ways:
1. Instead of coal tar, we had chlorinated solvents.
2. Instead of permeable gravel, we used sand.
3. Instead of a clay cap, we used a plastic membrane (anchored with soil on top).
4. We encouraged removal by air sparging below the cap.
5. We included a soil vapor extraction system to capture and treat the solvent gases trapped by the cap.