Report No DWI0772
Permeation of Benzene, Trichloroethene and Tetrachloroethene through Plastic Pipes An Assessment for Drinking Water Inspectorate
The following survey of contamination of drinking water found reported contamination incidents across six countries spanning l956 to 1994. Very high levels of contamination by aromatics and chlorinated solvents in drinking water after a period of stagnation of the water have been recorded, exceeding proposed new standards for benzene, trichloroethene, and tetrachloroethene. Incidents involving service pipes occurred more often than those involving mains supplies; contamination was usually detected by consumer complaints of taste and smell. Polybutylene (PB), polyethylene (PE) and to a lesser extent polyvinylchloride (PVC) pipes were involved. Few incidents were reported concerning gasket materials even though these are more readily permeable, this may be because these materials have a much smatter contact area with drinking water. Permeation through polyethylene has been reported for quite low levels of soil contamination. A high number of reported permeation events took place where the soil was uncontaminated when the pipe was laid, often involving gross spillage of petroleum products or solvents. Many cases took place in residential areas caused by leakage from cars or spillage of paint thinners onto soil directly surrounding the plastic pipe. Early incidents often concerned leaking gas mains, but this cause seemed to disappear in later years, possibly due to new practices stopping routing plastic water pipes in locations likely to be contaminated by leaking gas mains.
The permeation of these pollutants through polyethylene and polyvinylchloride has been reviewed via the literature. The two polymers have different permeation properties for organic materials. Organic compounds diffuse through polymeric materials at rates dependent on the polymer, the molecular dimensions of the pollutant, the attractive forces between them, and the temperature. Permeation is dependent on the diffusion through the material and the solubility of the organic in the polymer, the latter is determined by the partition coefficient of the organic between the external medium and the pipe material. Diffusion rates through the amorphous regions of unswollen polyethylene are higher than those through unswollen polyvinylchoride (a glassy polymer). Hence materials which are soluble in polyethylene can permeate the polymer even when present at low concentrations. The mass flux is controlled by the concentration gradient between the external medium and the water in the pipe the greater the concentration gradient the greater the mass flux. Excessive swelling and dissolution of polyethylene in the organic pollutant is limited by the crystalline regions of the polymer.
The lower diffusion coefficients through unswollen polyvinylchloride mean that permeation rates are so low that they will not result in contamination of the water supply (permeation would be expected to take thousands of years). However in the presence of high levels of organics which are good solvents or good swelling agents for the PVC the polymer can be become swollen. Diffusion rates increase markedly through the swollen material and the pipe can be rapidly permeated within months, weeks or days. This is most likely to happen in instances where gross spillage of solvents has occurred. Permeation in the intervening region between unswollen and greatly swollen polymer is poorly researched, and impossible to predict from theory.
Permeation in any particular circumstances, and hence the levels of contaminants in the water supply, cannot be predicted with certainty because of the number of factors and unknowns. Permeation will be dependent on the nature and chemical activity of the organic contaminant in the soil; the level of ground water in the soil, which determines whether permeation takes place from the aqueous or vapour phase; the distribution of the chemical contaminant between aqueous, solid or gas phases of the soil, which determines the mobility of the contaminant through the soil; the soil type especially the organic carbon content, the time of exposure, and temperature. Where permeation takes place from the vapour phase at low ground water levels, the soil pollutant vapour concentrations are difficult to measure.
Permeation incidents often involve complex mixtures of organic chemicals whose permeation characteristics may differ widely from those of single organic chemicals studied in the laboratory. Swelling as a result of contact with one contaminant can influence the subsequent transport of other organic penetrants through the pipe wall. However there is little in the literature which deals with this problem.