Contribution of municipal effluents to metal fluxes in the St. Lawrence river

The contribution of urban effluents to the total metal fluxes carried toward the sea by the St. Lawrence, a major world river, is 60% for Ag; 8-13% for Cu, Zn, Mo, Cd, and Bi; and less than 3% for all other measured elements (Al, V, Cr, Mn, Fe Co, Ni, As, Rb, Sr, Zr, Cs, Ba, W, Re, Pb,Th, U). This is inferred from measurements at the Montreal wastewater treatment plant. Except for Ag, municipal effluents do not weigh heavily on the St. Lawrence River metal budget, likely because of the physical-chemical primary treatment applied to most effluents. Compared to direct atmospheric deposition on the surface of the river, effluents would contribute half as much Pb and one-tenth as much Zn. In contrast, effluents deliver twice as much Cd and six times as much Cu as the atmosphere. Stable Pb isotope ratios ((206)pb/(207) Pb, Pb-206/Pb-208) in suspended particulate matter from the river indicate that the total Pb content in the river water is three times higher than the pristine level. The ratios of Cr, Ni, Cu, Zn, and Cd to Al in suspended particulate matter are high as compared to pre-industrial sediments, which suggests that trace element fluxes are higher today. To decrease metal levels in the St. Lawrence River further will be a challenge since the sources of metals are not well-known.