Report No DWI0343

Dec 1993

Exceedances of the PCV for polycyclic aromatic hydrocarbons (PAHs) are recorded occasionally in some water supply zones in England and Wales. The exceedances are generally due to the presence of only one of the six PAHs which make up the PAH parameter, namely fluoranthene. The cause of these exceedances is believed to be leaching from the coal-tar pitch linings applied to the majority of iron water mains. This process appears to be prevalent in systems used to supply hard groundwaters.

The purpose of this project was to find factors linked to water quality parameters, including water temperature, and the design features of distribution systems such as the age of the linings, pipe size, retention times and location of sample points, which affect the levels of PAH leaching. To this end data from two Department of the Environment surveys and from two water undertakers were examined by statistical methods. In addition, laboratory experiments were carried out to test the effects of temperature, exposure time, surface area of lining to volume of water ratio and acidity on the leaching of PAH into water from samples of coal-tar pitch applied to metal plates.

Also investigated in laboratory tests was the possible effectiveness in reducing the rate of leaching of fluoranthene of a number of techniques which might be more readily applicable in practice than relining or replacing mains.

The main conclusions and recommendations arising from our investigations are:

The rate of fluoranthene leaching has a positive correlation with temperature, surface-to-volume ratio and retention time.

The retention time result can be linked to the finding of higher levels of fluoranthene in samples taken at sampling points near dead-ends on distribution systems. Therefore it is recommended that ways of eliminating dead-ends be investigated.

Of the remedial measures investigated only the partial covering of the lining material with woven nylon hose was shown to be effective. Thus it is recommended that ways of applying this technique, and similar techniques with other absorptive materials, in practice be investigated.

Similar tests using PVC and MDPE pipe materials, for which in real-life systems there is some evidence of their ability to absorb PAHs, failed to reduce fluoranthene leach rates.

It is thought that the possible effectiveness of forming phosphate and silicate deposits on coal-tar pitch lining material failed to be demonstrated because the tests were too short to allow the formation of deposits.

A change of pH from slightly acid to slightly alkaline and changes of the level of carbon dioxide had no demonstrable affect on fluoranthene leach rates.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.