The objective of this study is to examine the environmental impacts arising from the construction phase of a large conventional water treatment plant located in Istanbul by adopting a life cycle assessment methodology. The facility has a maximum flow rate of 400,000 m(3)/d and serves a population of about 2,600,000. A conventional treatment technology composed of rock and fine screens, aeration, coagulation-flocculation units, clarifiers, filters, chlorination, and sludge handling units, is used in the plant. The functional unit is 1,000 kg (1 m(3)) treated water. The investigated environmental impact categories are: global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), abiotic depletion potential fossil (ADP fossil), freshwater aquatic ecotoxicity potential (FAETP), human toxicity potential ( HTP), marine aquatic ecotoxicity potential (MAETP), and terrestrial ecotoxicity potential (TETP). The main contributors to GWP, AP, EP, and HTP are concrete and structural steel. FAETP and TETP are mostly arising due to the polyvinyl chloride pipelines and membranes, and the concrete used. The main shares in MAETP are concrete, aluminum, and structural steel. Transportation does not have a significant contribution to environmental impacts.