Lake Ontario: the predominant source of triazine herbicides in the St. Lawrence River

To estimate triazine herbicide concentrations and sources in the St. Lawrence River, water samples were collected at its two major inlets (from the Great Lakes, Cornwall station, and from the Ottawa River. Carillon station) and at the outlet (Quebec City station) of the fluvial section. Sampling was carried out over an 18-month period between 1995 and 1996. Triazines were detected only in the dissolved phase at concentrations ranging from 2 to 91, from <0.4 to 15. and from <0.4 to 13 ng . L-1 for atrazine, cyanazine, and simazine, respectively. Dilution models show that, despite the presence of sporadically high concentrations of herbicides in St. Lawrence tributaries during periods of their application, loading from the tributaries is minor. Mass balance calculations show that Lake Ontario is clearly the main source of triazines (similar to 90%) to the St. Lawrence River. During the 1995-1996 hydrological year, Lake Ontario contributed 15.1 x 10(3) of the 16.6 x 10(3) kg of atrazine outflowing the St. Lawrence River to the estuary. The difference (1.5 x 10(3) kg . year(-1)) can be attributed to tributaries in Quebec, which represent 0.75% of the amount of atrazine spread on farmlands. There is no evidence of the degradation of triazine compounds during their transit time in the river.