Natural and anthropogenic impacts on the geochemical composition and metal speciation of fine sediment in a glacier-fed Canadian river basin

Purpose
Surveillance monitoring programs can provide fast, relatively low-cost, synoptic information on key water quality drivers and help inform land management decisions. Here, we evaluate longitudinal changes in the geochemical composition and metal speciation of deposited sediment over a 967 km reach of a large glacier-fed river and its key tributaries in central Alberta, Canada. In particular, the work provides a basis to understand how a major urban conurbation influences the geochemical composition and trace element properties of deposited sediment.

Methods
The concentrations and spatial distribution of major elements and sediment-associated metals (As, Cd, Cr, Cu, Pb, Hg, Ni, Zn) were determined and compared to consensus-based threshold effect concentrations (TEC). Metal partitioning to geochemical properties of sediment was evaluated by sequential extraction.

Results
Levels of Cr exceeded the TEC in 28% and 20% of the samples in the North Saskatchewan River and tributary samples, respectively. One Ni sample (3%) also exceeds the TEC on the main stem of the North Saskatchewan River. No consistent downstream increase in major element or trace metal concentrations was observed. The majority of Cr was bound to the largely non-bioavailable silicate phase thus highlighting the importance of parent geology as a source of metals to receiving streams.

Conclusion
The results of this surveillance monitoring provide preliminary data on the distribution and speciation of sediment-associated metals in the North Saskatchewan River and thereby address an evidence gap common to many large river basins in Canada.