Land use and trihalomethanes generation in water reservoir, a case study from Istanbul, Türkiye

Objective: The Büyükçekmece and Terkos drinking water reservoirs occur in the European part of Istanbul, have been created by isolation of two coastal lagoon lakes from the sea, which supply approximately 25% of water consumption of the Metropolitan city. Methods: Treated waters of these two reservoirs have quite different trihalomethanes (THMs) concentrations although being adjacent basins that range between 10-30 (mean 20.8) ppb for the Terkos and 20 – 65 (mean 41) ppb for the Büyükçekmece reservoir. THMs concentration in treated water for both Büyükçekmece and the Terkos reservoirs increase in the summer and decrease in the spring. Furthermore, THMs of the treated water of Büyükçekmece reservoir are so higher than those of the other reservoir waters of İstanbul. Results: The main reason of the higher value of THMs for the Büyükçekmece reservoir water could be related to ongoing agricultural activity in the vicinity of drainage basin. Another important indicator parameter for the THM formation mechanism is Specific UV absorbance (SUVA) value, that showed both of reservoir’s organics in water mostly composed with hydrophilic properties. Conclusion: Seasonal variations of low nitrate–high THMs in summer and high nitrate–low THMs in winter in treated lake waters can be explained by nitrogen run off from the land and consumption by photosynthesis when the water reach favourable temperature in summer. High nitrogen input into the Büyükçekmece reservoir could be associated with agricultural activity in the drainage basin which controls the high amount of organic carbon formation and high THMs generation during the treatment by chlorine.