Mercury distribution in relation to iron and manganese in the waters of the St. Lawrence river

Waters entering the St. Lawrence River from Lake Ontario and from the Ottawa River have been sampled on a quaternary basis between March 1995 and October 1996. Mercury, iron, manganese, organic carbon and other chemical parameters have been determined in the filtered water (< 0.5 micron) and suspended particles (> 0.5 micron). Average total mercury concentrations (+/- standard deviation, number of determinations) in filtered samples were 1.3 +/- 0.9 pM (39) at the mouth of Lake Ontario and 4.0 +/- 2.3 pM (54) at the outlet of the Ottawa River. The respective average concentrations in suspended particles were 1.1 +/- 0.4 (39) and 0.6 +/- 0.2 (55) nmol g-1. Partial correlation analyses reveal that in the fraction < 0.5 micron--which seems to consist of a significant fraction of colloids--mercury was significantly linked to iron, while in the particulate phase, mercury was related to manganese. Mercury was not correlated with organic carbon. A sorption model on iron and manganese hydroxides is proposed to explain these observations.