In the vicinity of sewage treatment plants located on Lake Geneva, two main geochemical associations were found to exist in the surficial sediments: 1) "Argilophilic" elements: Ni, Mn, B, V, (Co), associated to clay minerals. 2) "Organophilic" elements: Pb, Cu, Zn, Cd, Sn, (Ba, Cr, Hg} related to organic matter, the three last elements being under certain circumstances associated to clay. Background concentration values for metals were computed on the basis of ancient sediments data, allowing for a dilution effect due to CaCo₃. A contamination factor Cm was defined for a given element as the ratio of the concentration measured in modern sediments and the natural background. For a given sediment, or for a certain area, the pollution index IP, equal to the average of the 13 characteristic elements' Cm's, provides a measure of the extent of. polymetallic contamination. Mercury, Cd, Zn, Cu and Pb have a Cm>2. Sn, Cr, Ba, Ni and Co are found in sediments in concentrations slightly superior to background. Mn, B and V are lower than background, because of an unaccounted for dilution by quartz. Among the 12 sites investigated, all of them have an IP superior to unity, and reaching 7,5 in one instance.