If contaminated soil were flushed by pumping water through it, the pumped water under pressure would escape to the surroundings. This is unacceptable because the toxic wastes would be diluted to make treatment more difficult and dispersed to make collection more costly. A well to withdraw contaminated water should be in the region of highest concentration of contaminant. The following sketch shows a pumping scheme that might be acceptable for leaching with water pumped at the periphery to create higher pressure to prevent flow out of the site. The leachate is treated (a bioprocess is shown in this figure) and recycled. The above-ground treatment is tricky because the concentration of toxic material will usually be low. Both chemical and biological processes have troubles in dealing with dilute streams.
Toxic and hazardous wastes are often organic chemicals with poor water solubility. The most efficient way to extract them from soil would be to use organic solvents. However, adding solvents to soil would make the problem worse even if the solvents were not toxic. The expense would be unreasonable, and organics would represent very high BOD. One solvent that has been proposed is supercritical carbon dioxide in which many organic compounds are highly soluble. Lost solvent would merely escape to the air. The drawback is that the temperature and pressure to keep the carbon dioxide in its supercritical state would be a severe engineering challenge. There are only a few industrial extractions with supercritical fluids, and the technology is considered advanced and costly.
Volatile compounds can be extracted from soil simply by flushing with a gas. One attractive method for removing organic solvents from soil is aeration. The removal is somewhat slow but cost effective. Venting the spent air adds to air pollution, so some treatment is advisable. The air could be sent to a combustion unit where the contaminants burn along with the fuel. Another option is adsorption on activated carbon. The organic contaminants are burned off when the carbon is roasted to regenerate it for reuse.