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The bioavailability of trace metal contaminants in the Brunette River watershed Yuan, Yi

Abstract

Urban expansion with the development of our transportation network and its associated impervious surfaces, that readily transport surface runoff to our urban streams, has become the major non-point source of pollution in our urban watersheds. Trace metals and organic contaminants are of particular importance due to their potential toxic effects on aquatic organisms. The association of these contaminants with suspended solids and bed sediments, and the dynamic nature of the stream environment, has generated an interest in determining if these contaminants are available to aquatic organisms. The objective of this study was to investigate the potential bioavailability of the trace metals, copper and zinc, in stormwater runoff and the streams of an urban watershed. Two techniques, namely metal speciation and dialysis with resin receiver were employed in the Brunette River watershed located mainly in the metropolitan area of Great Vancouver, British Columbia. Stormwater and baseflow samples were collected along a hydrologic gradient and the trace metals partitioned into chelex resin extractable, dissolved and particulate components. The suspended solids, filtered from samples, were subjected to geochemical sequential partitioning into the exchangeable, easily reducible and residual trace metal phases. Chelex resin placed in dialysis tubes was deployed in streams and sediment traps for periods of a week to complex "available metals" during dry summer baseflow and wet winter stormrunoff periods. Manganese and iron were also investigated since they could affect the geochemical partitioning of the more toxic trace metals. Recent studies have demonstrated that manganese is accumulating in the Brunette watershed probably as a result of the gasoline additive MMT. The results of this study indicate that both Cu and Zn are at relatively high concentrations in this watershed, compared to the relevant water quality and toxicity criteria. The concentrations of these two metals decreased along the urban hydrological gradient, with highest level in the street surface runoff, decreasing in Still Creek and still further in the Brunette River as a result of sedimentation in Burnaby Lake. There was a high correlation of suspended sediment (SS) and the concentration of the four metals Cu, Zn, Mn and Fe in surface runoff, which indicated that the trace metals were mostly associated with the particles flushed from impervious surfaces and transported in the urban streams. Analysis of trace metal speciation demonstrates that only about 10 % of Cu was found in the immediate bioavailable form (resin extractable) at all sites. The particulate bioavailable fraction of Cu (exchangeable and reducible components) only accounted for 30% in street runoff and less than 10% in streams. Fifty percent of the Zn was observed in the immediate bioavailable form (resin extractable) in the watershed. The particulate bioavailable fraction of Zn accounted for 11 -24% along the watershed. Both these trace metals showed potential toxic effects to organisms in the Brunette River watershed, especially in streetrunoff and upstream in Still Creek when compared to the relevant criteria. Copper contributed a higher toxic effect to the aquatic life than zinc in this watershed since it was proportionally higher compared to the criteria. There was no clear relationship between Mn and the bioavailable fractions of Cu and Zn within the watershed observed in this study. However, there was an increase in particulate easily reducible Cu and Zn (manganese bound metal fraction) during stormflow conditions when compared to baseflow. In stream resin uptake experiments clearly demonstrated that the trace metal uptake rates were high during storm flow, especially when there was a long dry period before the rainfall. However, there was still considerable uptake of trace metals under baseflow conditions. The good correlation between bioavailable metal species in stream and metal uptake by the receiver resin suggested that both approaches are useful in evaluating the bioavailability of trace metals in the urban aquatic environment and can be used as tools to assess the impact of trace metal on the aquatic organisms in the urban receiving water.

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