The Bay of Vidy is the most contaminated area of Lake Geneva: contamination is caused by the effluent of the sewage treatment plant (STP) of the City of Lausanne. The implementation of a chemical stage in the treatment plant to remove phosphorus using FeCl(3) in 1971 is indirectly recorded in the take sediments by a strong and sharp increase in the volume magnetic susceptibility (VMS) signal. A total of 43 sediment cores have been retrieved and measured for VMS. The synchronism of the VMS signal increase and its relation to the implementation of the P-removal stage in the STP has been shown in seven (137)Cs-dated sediment cores. The VMS signal has been used to date by stratigraphic correlation all of the 43 sediment cores and map the three-dimensional distribution of contaminated sediments. Over an area of 0.5 km(2) the sediment accumulation rates vary from 2.8 cm/y close to the effluent mouth, to 0.3 cm/y 1 km away. The total mass of contaminated sediments is estimated to 274,000 tons.