Effect of Point and Diffuse Source Loadings on Mercury Concentrations in the Wabigoon River: Evidence of a Seasonally Varying Sediment–Water Partition

Abstract

Ten years after major mercury loadings from a chlor-alkalai plant were sharply reduced, mercury levels in the surface sediments, water, and suspended sediments of the Wabigoon River downstream of Dryden, Ontario, remain markedly elevated. During 1979, a typical water year, average mercury concentrations in water throughout an 80-km reach were nearly five times the mean value at the control site. Elevated mercury levels in the Wabigoon River are primarily due to the chemical and biological remobilization of mercury from sediments. Lesser amounts are due to the erosion of mercury-enriched sediments during high flows and to current releases from the mill site at Dryden. In the absence of resuspended sediment during high flows, mercury levels in contaminated waters can be best described by a sediment – water – suspended solids partition. This condition was established within half a day in the most contaminated portion of the system. Mercury concentrations in water can fluctuate seasonally by an order of magnitude. Highest concentrations were associated with elevated summer temperatures.