Richard Croft at RemedX talks through typical contaminants that occur at dry cleaning sites and describes various remediation solutions.
From our experience, the contamination found at dry cleaning sites is usually related to the solvents used in the cleaning process. Historically this was often ‘white spirit' and sometimes carbon tetrachloride. More recently perchloroethyene, a chlorinated hydrocarbon compound, has been the main solvent used on many sites.
Perchloroethylene can degrade to form daughter products in the subsurface. It is very common, if not our universal experience, to see perchloroethylene degrade in the subsurface through anaerobic bacterial action, into its daughter compounds of trichloroethylene, isomers of dichloroethene, and vinyl chloride.
On closer scrutiny, the further breakdown product of ethene can also be found. Unfortunately this degradation process often stalls partway through, leaving a predominance of one of the daughter products.
White spirit
White spirit is a generic term for a variety of refined or unrefined petroleum products in the approximate carbon range C8-C12. They can be a combination of aliphatic and aromatic components. Boiling range is from Octane @ 125oC to Dodecane @ 216oC.
Therefore, it does not act as a single compound but as a mixture of many. The aliphatic and aromatic compounds that make up white spirit do not readily degrade anaerobically in the subsurface, unlike PCE. However they can act as a carbon source for the anaerobic metabolism of certain bacteria that readily degrade PCE in the subsurface. Carbon tetrachloride does not tend to readily degrade anaerobically either.
Geology
Characteristics of these compounds can make their fate in the subsurface difficult to remediate. PCE and carbon tetrachloride are what is often termed ‘dense non-aqueous phase liquids' (DNAPLs), meaning simply that they sink in water.
When released in quantity to the sub-surface, they will pass vertically down through pore spaces in soils, or through fractures in rock, until an impermeable layer stops them, or the fractures cease. They do not pool in the capillary fringe as does a ‘light non-aqueous phase liquid' (LNAPL) such as white spirit. Thus for dry cleaning sites, the importance of fully understanding the site geology is fairly self-evident.
Site investigation considerations
During site investigation, the presence of PCE as a contaminant will probably be first seen in the groundwater as dissolved phase contamination. Usually, if the contamination is historic, this will be seen as the daughter products of PCE rather than high levels of PCE itself. It is often necessary to trace back the source of the dissolved chlorinated hydrocarbons to their source in the subsurface.
Only when this is done can the extent of the contamination be fully assessed. Given that the PCE ‘free phase' will continue to sink until its volume is residually saturated in pore spaces, or it hits a ‘permeability trap', the greatest concentrations of the contamination may exist at significant depth. This can make remediation very difficult.
Remediation options
There are commonly three techniques for remediation at laundry sites contaminated with the above compounds. These techniques can be applied singly or more often than not, combined in a ‘treatment train':
- Skimming techniques for removing phase separated, or 'free phase' hydrocarbon liquids related to white spirit contamination
- In-situ anaerobic bioremediation, used for remediation of PCE and its daughter compounds. This process utilises the natural bacterial degradation processes, but engineers an optimal environment in the subsurface to allow the natural process to be effective in completely degrading the PCE all the way to chloride, water and carbon dioxide
- In-situ aerobic bioremediation, used for remediation of white spirit contamination. This uses the naturally occurring aerobic bacteria in the subsurface, in a similar way to the anaerobic process mentioned above
- Vacuum extraction techniques. These can include high vacuum extraction, multi-phase extraction and soil vapour extraction. These vapour extraction techniques mainly take advantage of the physical properties of the various dry cleaning solvents to enable them to be recovered from the subsurface.
Many solvents have a relatively high vapour pressure and Henry's constant, which mean they will enter the vapour phase if in contact with air. By drawing air through the unsaturated zone, the volatile compounds will enter the vapour phase into the induced air flow through the soil pore matrix. This air is drawn under vacuum, to recovery wells and is extracted to a surface treatment plant.
For those solvents that are less volatile, such as many of the components of white spirit, the increased in oxygen in pore spaces in the unsaturated zone, as a result of vapour extraction, helps stimulate aerobic biodegradation.
Other techniques we have applied to the remediation of PCE and its daughter compounds are in-situ and ex-situ chemical oxidation. The chemical oxidation process uses an oxidant to chemically destroy the contaminant. The chemical reaction can be very rapid, depending on the choice of the oxidant, or mix of oxidants and/or catalysts. Reactive barriers, based on zero valent iron, can also be used to control off-site migration of dissolved phase chlorinated hydrocarbons.
Site-specific considerations
With all of the techniques mentioned, the success of the remediation is not automatic. Each technique is more or less effective in targeting the saturated or unsaturated zone. The importance of understanding the subsurface conditions and contaminant distribution is paramount.
Once these are understood, as far as is reasonably possible for any subsurface environment, the application of the specific remedial techniques must be considered at the site-specific level. Most, if not all remediation approaches should be tested before going to the full scale. This can take the form of laboratory-scale tests or field-scale pilot trials. Often both are required for any site of significant size or hydrogeological complexity.
Field-scale testing allows site-specific parameters such as radius of influence of wells and hydraulic conductivity to be determined to allow a robust treatment design to be prepared. Without such data, any such remediation design is guesswork. Contamination commonly encountered on dry cleaning sites can be usually remediated on site. The compounds are generally readily degraded, or recovered. The usual difficulty is the complexity of the specific site geology.
It is relatively unusual to find a site with significant DNAPL present at depth. Such a site would require a complex remedial approach that would be likely to result in the incorporation other remedial methods not discussed here, such as DNAPL skimming or thermal techniques.
R G Croft. RemedX Ltd
