Sedimentation, bioturbation, and Hg uptake in the sediments of the estuary and Gulf of St. Lawrence

Sedimentation rates and mixing depths have been estimated from the application of a two-layer biodiffusion model to 210Pb profiles in sediment cores collected in the Laurentian Trough in the Estuary and Gulf of St. Lawrence. Sedimentation rates decrease exponentially with distance seaward from 0.70 cm yr21 (0.47 g cm22 yr21) near the head of the Trough to 0.04 cm yr21 (0.03 g cm22 yr21) in the Gulf of St. Lawrence. Mercury (Hg) sediment profiles exhibit subsurface maxima that decrease from concentrations of 0.6 mg g21 near the entrance to the Saguenay Fjord to 0.04 mg g21 in the central part of the Gulf. Hg release data for a chloralkali plant on the Saguenay River were used as the input function for the biodiffusion model and provided accurate simulation of measured profiles when mixing was permitted to extend into the deep (.10 cm) sediment layer. Estimates of Hg sediment inventories indicate that 100–130 tonnes (t) of Hg have been retained within the sediments of the St. Lawrence Estuary, which, combined with an Hg inventory of 30 t estimated for the sediments of the Saguenay Fjord, approximately balance the 136 t of Hg released from the chloralkali plant since 1947. 137Cs and 239,240Pu sediment-depth profiles were also simulated by the biodiffusion model using input functions based on atmospheric fallout modified by radionuclide transport through the St. Lawrence River drainage basin. During the early 1970s, fish and crustaceans in the Saguenay Fjord in Eastern Quebec were found to be severely contaminated by mercury, resulting in the closure of local fisheries. Geochronologies measured using 210Pb (Smith and Walton 1980) and fallout radionuclides (137Cs, 239,240Pu; Smith and Ellis 1982) in Hg-contaminated sediments indicated that the date (1947 6 3) for the initial contamination by Hg was synchronous with the construction of a chloralkali plant at Arvida on the Saguenay River (Smith and Loring 1981; Smith 1988). These and later studies (reviewed in Cossa 1990) confirmed that the source of the Hg pollution was the chloralkali plant, which, during its lifetime, released at least 136 tonnes of Hg into the environment prior to its eventual closure in 1976 (Smith and Loring 1981). An important question arising from these earlier studies is, to what extent have St. Lawrence Estuary sediments been contaminated by the same Hg source. This issue has commercial implications because the Estuary and Gulf of St. Lawrence collectively support a large fishery that is vulnerable to toxic chemical contamination through the food web (Gagnon et al. 1996), as evidenced by the closing of large areas to shellfish harvesting (Masse et al. 1986; Gagne and Sinclair 1991). The purpose of the present study was: (1) to determine sedimentation and mixing rates in the sediments of the Lauren-