Sources and fluxes of mercury in the St. Lawrence river

A mass balance approach, based essentially on the reconstruction of daily f luxes and circumscribed by strict: error calculations, was designed to quan tify the main mercury sources for the St. Lawrence and its tributaries, whi ch constitute a large river system. High-frequency samplings were performed over an Is-month period (1995-1996) at the main water inputs and the mouth of the river. Minor tributaries and the Montreal effluent were also sample d. This strategy allowed models to be obtained that relate mercury concentr ations in solution and in particles to the hydrological regime. The calcula ted budget was balanced relative to the calculated errors of the estimates. Gross mercury export from the river was found to be 5.9 kmol yr(-1) (73% a s particulate). Tributaries and internal erosion of the river contributed e qually for a total of 75% of this gross load, whereas the Upper St. Lawrenc e River, which is almost exclusively composed of Lake Ontario waters, accou nted for less than 10%,and inventoried anthropogenic point sources accounte d for about 5%. Dissolved mercury was mainly from north shore tributaries, and particulate mercury was largely from erosion of the river bed and banks . On the basis of the present results as well as estimates of atmospheric d eposition from the literature it can be inferred that at least 88% of depos ited mercury was retained in the watersheds.