Chlorinated solvent contamination is an extensive problem in the UK due to their wide use throughout the 20th century in industries such as dry cleaners and metal processing plants.
Chlorinated solvents are organic compounds that contain chlorine atoms. The properties of chlorinated solvents make them ideal for many industrial-cleaning applications such as degreasing oils and fats. The properties of different chlorinated solvents needs to be carefully assessed when selecting the most appropriate remedial solution as some are more recalcitrant than others. Common solvents include Tetrachloroethene (PCE) and Trichloroethene (TCE), used extensively in the dry-cleaning industry, and 1,1,1 - Trichloroethane (TCA) and Methylene Chloride used as industrial degreasers.
Chlorinated Solvents when released into the subsurface will tend to sink through the saturated zone as they are denser than water. As a result small droplets (ganglia) get trapped in the soil ‘pore-space’ as a Non-Aqueous Phase Liquid (NAPL), which in turn can act as a long-term source of dissolved phase contamination. These NAPL source zones can hamper any site remediation effort, as they are difficult to treat and detect. Often correlation of groundwater sample concentrations against saturation solubility’s for specific solvent compounds is required to determine the location of NAPL source areas.
Chlorinated Solvents are volatile and relatively soluble, the latter resulting in the quick formation of a dissolved phase plume down gradient of the original spillage. Additionally, as chlorinated solvents are dense non-aqueous phase liquids (DNAPL) they will tend to partition deeper into an aquifer than other compounds, pooling on impermeable layers such as clay, which in turn become saturated as the solvent seeps through the soil matrix. Positively however they will be subject to natural attenuation processes, the most important being bio-degradation. Although the process of bioremediation will ultimately lead to the destruction of the solvent incomplete metabolism can result in the generation of more mobile or toxic by-products e.g. Vinyl Chloride.
As with most organic based compounds a larger percentage of the Chlorinated Solvents mass will be held in the soil phase albeit to a lesser extent than TPH. Relative to TPH a higher mass percentage will be partitioned in the dissolved phase.
A number of approaches can be used to address Chlorinated Solvent contamination in-situ.
For contamination trapped in the unsaturated zone, Vacuum extraction techniques can be used to extract the contaminant in the vapour phase, such as:
Soil Vapour Extraction (SVE)
Dual Phase Vacuum Extraction (DPVE)
The treatment of NAPL source zones can be achieved using:
Steam enhanced remediation (SER)
Longer-term options include:
Intensive in-situ bioremediation
Air Sparge/Vacuum Extraction.
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One of the most cost effective means of addressing the down gradient dissolved phase is the use of in-situ bioremediation, reactive barriers or Air Sparge/Vacuum Extraction. Go to Remediation Technology sections for review of remedial techniques.