For many years the water supply systems in Greece have been disinfected mainly by chlorination. A consequence of chlorination is the formation of trihalomethanes (THM). The aim of this investigation was to determine their levels in the Athens water supply system over a period of time. The sources for this system are the lakes Marathon and Iliki and a number of boreholes. Because of the high bromide concentration, bromoform is the dominant THM formed. Samples from different points of the Galatsi Treatment Plant (GTP) and the distribution network of Athens were taken and analysed for THMs. It was shown that the THM concentrations were lower in the spring and winter than in the summer and autumn. All THM levels measured were lower than the EPA standard. It was also shown that when the water supply from the boreholes was stopped, the concentration of the brominated halomethanes decreased. Bearing in mind that the boreholes were the main source of bromide ions, this behavior was expected.
