Chlorinated hydrocarbons (organochlorides) are a very large and diverse group of hydrocarbon molecules that also have at least one covalently bound chlorine atom chemically bonded to them. The number of carbon atoms and its consequential three dimensional arrangements as well as the number of chlorine atoms attached will determine the chemical and physical properties of the chlorinated hydrocarbon formed. This class of compounds has an extensive range of applications and is frequently found on contaminated sites.
Chlorinated hydrocarbons are used predominantly as solvents and have historically been used as industrial degreasers, dry cleaning solvents, anaesthetic agents and as refrigerants. They are colourless, volatile liquids with a moderately sweet aroma and partially soluble in but denser than water. They are the most common DNAPL.
The more common forms of chlorinated solvent contamination of soils and ground waters include;
Tetrachloroethene (PCE, Tetrachloroethylene)
Carbon tetrachloride (Tetrachloromethane or carbon tet)
1,1,1 - Trichloroethane (TCA, methyl chloroform, chlorothene, Solvent 111)
Dichloromethane (DCM or methylene chloride)
QROS currently utilises two methods to rapidly quantify/semi-quantify their presence in soils and water on site. The first of these is the ColorTec method which can detect the presence of chlorinated solvents at ppb levels. The analysis takes approximately 10-15 minutes and gives a semi-quantitative result, with a Minimum Detection Limit of 3 ppb in water and in soil. The method is unable to differentiate between chlorinated solvent types and the data produced assume a given 'typical' mixture of solvents is present. If the contaminate is known to be a single Chlorinated hydrocarbon then quantification can be based on that assumption and a more accurate concentration determined, and like wise for a mixture of chlorinated hydrocarbons if the relative ratio has previously been determined.
A further rapid onsite technique currently under development within QROS utilises a positive ion heated diode system and is sensitive to levels as low as 0.005 ppm and not susceptible to interferences from BTEX or other volatile hydrocarbons. The sensor is also unaffected by moisture levels that would quickly destroy a PID lamp. The unit can be operated as either a "sniffer" providing screening data that can be used to identify gross contamination, or as a discreet analyser to provide semi-quantitative results providing the type of chlorinated solvent is known. The analyser is completely portable and can operate for a day before battery recharge is required.
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