Source-Related Classification of St. Lawrence Estuary Sediments Based on Spatial Distribution of Adsorbed Contaminants

Abstract

The St. Lawrence River upper estuary is a very large and efficient mixing zone for contaminated industrial discharges originating upstream, yet elevated levels of various toxic contaminants have persistently been reported in local biota, such as eels and Beluga whales. As part of an initial assessment of the role of bottom sediments in the fate and pathways of contaminants through the estuary ecosystem, we examine the spatial distributions of particle-reactive trace and major elements in shallow-water accumulation zones. Concentrations of more than 30 elements were determined by ICP atomic emission spectrography. A number of these (Ce, Cr, Fe, La, Ni, and Zn) proved to be useful as fine sediment tracers, despite the complicating effects of intense sediment mixing and varying grain size and salinity. Prior normalization of the concentration values with reference to aluminum was necessary in most cases to minimize bias caused by grain-size variation between samples; however, there was no means of compensating for the effect of variable salinity on trace element concentrations. Through the use of a combination of two statistical approaches, linear regression and cluster analysis (ratio-matching), two distinct depositional zones for modern sediments were resolved, each associated with a different source. The largest zone covered the western end and along the south shore of the upper estuary, while the other was confined to the northeastern shore. The western zone is characterized by high-metal-content brought in by the St. Lawrence River. The northeastern zone is distinguished by the presence of lanthanide and rare earth elements (Ce, Y), indicating that its sediment source is probably the watersheds draining the monazite-bearing Grenville Province of the Canadian Shield to the north. The statistical separation of these two sediment groupings supports the conclusion that the St. Lawrence River sediment plume follows the southern shore of the estuary. The more sparsely distributed modern sediment deposits along the north shore tend to be derived from local sources.