The origins and loadings of Cl−, Na+, and Ca2+to ionically polluted Onondaga Lake, NY, are determined based on monitoring data collected for surface inflows over the 1973–1993 interval. The analysis delineates for lake managers the central role a chemical company has played in the loading of these constituents to the lake and the resulting ionic enrichment of the lake. The recurring, but disparate, ratios of Cl−, Na+, and Ca2+concentrations maintained in the two largest tributaries to the lake depict contrasting industrial and geologic origins of these constituents. Concentrations of the three ionic species are inversely related to flow in these tributaries. The average annual combined loading of Cl−, Na+, and Ca2+to the lake over the last 12y of operation of the chemical plant was 1.2 million metric tons. The plant closed in 1986, and by 1989 the annual loads of Cl−, Na+, and Ca2+had decreased by about 79, 67, and 70% respectively. The residual annual input of the ionic waste in 1989, from the area of the most recently abandoned waste beds, was about 12% of the pre-closure load.