RESIDUES OF ORGANOCHLORINE PESTICIDES IN WATER SOURCES OF ISTANBUL ADNAN AYDIN 1 and TÜRKAN YURDUN 2 1 Environmental Research Center, Marmara University, Göztepe, Kadıköy 81040, Istanbul, Turkey; 2 Faculty of Pharmacy, Marmara University, Haydarpa¸ sa Kampüsü, Kadıköy 81040, Istanbul, Turkey (Received 30 October 1996; accepted in revised form 24 April 1998) Abstract. The concentrations of 9 chlorinated pesticides in the water sources and tap waters of Is- tanbul, Turkey, were determined by gas chromatographic methods following the enrichment through adsorption and elution techniques. The observed organochlorine pesticides were α- and γ -HCH and aldrin which had been banned from use. The contents of α- and γ -HCH in raw waters were in the range of 0.34–1.7 ppb and not detected-0.077 ppb, respectively. Aldrin was observed at the concentra- tion of 0.03 ppb in some samples. The residue levels of organochlorine pesticides in drinking water supplies of Istanbul were found significantly under the maximum permissible levels of standards. The effectiveness of potable water treatment processes and the importance of maintenance and back- washing of sand filters in pesticide removal were observed. Improper and delayed back-washings of filters caused an increase in the pesticide residues in the distributed water. In older water distribution lines, higher concentrations of some organochlorines were also observed. Keywords: drinking water, gas chromatography, lindane, organochlorine pesticide, residue analysis 1. Introduction Istanbul, Turkey, is a metropolis with twelve million inhabitants and is the largest urban settlement in Turkey. The average water demand for public and industrial use is 18 million cubic meters per day. The water demand of the city is mainly supplied from surface water sources, some of them have been in use for a thousand years. Total reservoir capacity of water sources is around 750 million cubic meters. The water requirements are met via several water sources on both sides of the Bosphorus. On the European side, the Terkos Lake, which was the main source until 1981, still meets 25% of the water demand of Istanbul, and the Büyükçek- mece Lake (15%), the Alibey Dam (7%) and the Foundation Reservoirs (1.5%) are also in service. In the Anatolian part of the city, the Ömerli-Darlık Dam Reservoir system meets almost half of the water resuirement of the city. The Elmalı Reservoir is another minor source (3%) on the eastern side (Figure 1). The water sources of Istanbul have encountered serious pollution problems. Unplanned new settlements, especially within the catchment areas of high capacity sources, such as Ömerli and Büyükçekmece Lake and the Alibey Dam, have no sewage systems and are important dangers. Besides the new settlements and some Water, Air, and Soil Pollution 111: 385–398, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.