Proceedings - Teaching and Learning Centre - Simon Fraser ...

Fraser River Action Plan 3rd Research Workshop
Linking Sediment Geochemistry in the Fraser River Estuary
to Metal Bioaccumulation in Lower Trophic Levels
C. Thomas and L. Bendell-Young
Department of Biology
Simon Fraser University
The Fraser River estuary is the largest estuary on the Pacific coast of Canada. It provides a vital habitat for a
wide variety of birds, fish and wildlife, as well as playing an integral role in the lives of many people in B.C.
Metals released into the Fraser River ultimately end up in the estuarine sediments. Once in the sediment, these
metals represent a potential source of metals to benthic-dwelling organisms. The objectives of this study were to
contrast the geochemistry of trace metals (Cd, Pb, Zn, Mn, Fe and Hg) at three sites along the mudflats of the
Fraser River estuary (Sturgeon Bank, Roberts Bank and Boundary Bay) and to relate differences in trace metal
geochemistry to metal availability in Macoma balthica (a deposit-feeding bivalve). These three sites were
chosen to reflect a range in grain size, concentration of organic matter and metal contamination. The most
northerly site is Sturgeon Bank, which, before 1989, received primary-treated sewage directly onto its foreshore.
To its south is Roberts Bank, which is strongly influenced by the main arm of the Fraser River. At the far end of
the estuary is Boundary Bay which has very little freshwater input and consists of coarse-grained sand.
The fate and bioavailability of a metal depends on the geochemistry of the sediment, as well as the partitioning
of the metal among the different geochemical components. Sediment geochemistry is defined as concentrations of
organic matter, reducible Fe (Fe oxides) and easily reducible Mn (Mn oxides). A selective chemical extraction
procedure was employed to assess the partitioning of metals onto the different geochemical fractions in the
sediment. Porewater samples were collected to assess the contribution of diagenesis to concentrations of
particulate Mn and Fe at the sediment-water interface. To determine the bioavailable fraction of the sediment, a
correlation analysis will be done between levels of metals in M. balthica and concentrations of metals recovered
in the different sediment fracions.
Conclusions
Sediment matrix vs. metal concentration results revealed extreme variation in concentrations of Fe and Mn in the
surficial sediments. Conversely, the range of organic matter was considerably less than expected (1.5% - 6.5%),
with a surprisingly low percentage (4%) measured by the sewage treatment plant (STP). Boundary Bay was
found to be distinctly different from the other two sites in terms of low to non-existent concentrations of Fe and
Mn oxides. The highest concentrations of Hg (0.22 µg/g) were found closest to the STP where they exceeded the
maximum acceptable value set by the Ministry of Environment (0.15 µg/g). A strong association between
organic matter and Hg was also found between all sites. Maximum concentrations of Zn were found at Roberts
Bank, which points to the Fraser River as an important source of metals. Porewater results indicated that the
sediments were a source of Fe and Mn to the surficial sediments at Sturgeon and Roberts banks; however, at
Boundary Bay, this was not the case.
Objectives
Long-Term
1. To provide baseline information on the geochemical components and processes influencing metal
availability in the mudflats of the Fraser River estuary; and,
2. Apply this information in developing predictive models to assess which sites pose a greater risk of metal
bioaccumulation.
Short-Term
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