Effect of a metal mixture (Cu, Zn, Pb, and Ni) on the bioavailability and bioaccumulation of cadmium in natural systems

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Stewart, A. Robin
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Molluscs were evaluated as biomonitoring tools for the Canadian mining industry. Bivalve molluscs were found to be appropriate biomonitors of metals in the aquatic environment, particularly for characterizing spatial and temporal trends in metal contamination. Further research was recommended to develop molluscs as biomonitors of tissue-residue based effects of metals. The effects of a metal mixture (Cu, Zn, Pb and Ni) on Cd bioavailability and accumulation by a freshwater unionid mussel, Pyganodon grandis, and the isoetid macrophyte, Eriocaulon septangulare, were examined in situ at the Experimental Lakes Area, Ontario, Canada. A limnocorral experiment was conducted during the summer of 1992, in which Cd was added alone to the water column in treatment 1 and with the metal mixture in treatments 2, 3 and 4 to raise background sediment Cd concentrations by 7 times. Copper, Zn, Pb and Ni were added to treatments 2, 3 and 4 to raise sediment concentrations by 3, 4 and 7 times, respectively. Treatments with themetal mixture had longer residence times for Cd in the water column than the treatment with Cd alone. Cadmium accumulation in mussels was significantly reduced in treatments with the highest concentration of the metal mixture compared to treatments with the lowest concentration of the metal mixture or with Cd alone. Tissue metallothionein levels were highest in the kidney and tended to decrease in treatments with increasing metal addition. The effect of competition on the partitioning of Cd in the water column appeared to be a less important phenomenon than competition at binding sites on the mussels in determining Cd uptake by the mussels. In the summer of 1995, littoral sediments were spiked with Cd alone and with the metal mixture at three increasing concentration levels (2, 4 and 6 times background). Eriocaulon septangulare was planted in the spiked sediment and placed at 0.5 m water depth in the littoral zone. The distribution of Cd among sediment fractions (easily-reducible, reducible and organic), in porewater, and in macrophytes was determined every second week for 10 weeks. The metal mixture had a significant affect on the distribution of Cd among geochemical fractions in the sediments after 2 and 8 eeks, but not after 10 weeks. At the highest concentration of the metal mixture, Cd shifted from the easily-reducible (Mn-oxide) fraction which is considered more "bioavailable" onto the less bioavailable reducible (Fe-oxide) and organic fractions. The highest Cd concentrations were found in the shoots of plants in the treatment with Cd alone and the treatment with the highest concentration of the metal mixture.