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 Pb-210 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 yr(-1) (0.47 g cm(-2) yr(-1)) near the head of the Trough to 0.04 cm yr(-1) (0.03 g cm(-2) yr(-1)) in the Gulf of St. Lawrence. Mercury (Hg) sediment profiles exhibit subsurface maxima that decrease from concentrations of 0.6 mu g g(-1) near the entrance to the Saguenay Fjord to 0.04 mu g g(-1) 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. Cs-137 and Pu-239,Pu-230 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.