Leaking water mains

Leaking mains water can often be identified as it may have its own built-in tracer. Mains water is treated using chlorine gas (or compounds such as sodium hypochlorite (NaOCl) that release chlorine gas) to ensure that any bacteria it contains are killed. If the water contains any organic material such as humic acids a reaction will occur forming a series of compounds known collectively as trihalomethanes (THMs). The THM chemicals are simple single carbon compounds with the general formulae CHX3 where X is any halogen (i.e. chlorine, bromine, fluorine and iodine) or any combination of these. The most frequently detected THM compounds are chloroform (trichloromethane), bromodichloromethane, dibromochloromethane, and bromoform (tribromomethane). THMs can only be detected by laboratory analysis that typically includes a total THM measurement and tests for the specific THM compounds. Detection limits are usually in the range of 0.2 - 0.4 µg/l. Tests for any free chlorine present are usually carried out on site using a kit comprising a small test tube that is filled with a water sample to which a tablet of indicator chemical is added. The colour of the water is compared to a standard chart to determine the presence and concentration of the free chlorine.

THMs are completely artificial and do not occur in nature. If any are detected in water there are only two possible origins: mains water treatment and leaks from chemical plants. Their presence usually provides unambiguous proof that a water main is leaking and shows a link between the local water distribution network and the site where the water sample was taken. The use of these chemicals as tracers is restricted as both the THMs and chlorine gas are volatile and gradually dissipate from the water. Consequently the method may not work at large distances from the leaking water main. The loss of the chemicals from the water is time related and so the distance depends of the permeability of the ground that controls the flow rates from the leak.

Copyright © 2007 by Rick Brassington

Eur Geol Eur Ing Professor F.C. Brassington BSc MSc CGeol FGS CEng MICE FCIWEM