VALIDATING THE USE OF RADIOTRACERS AS ANALOGS OF STABLE METAL BEHAVIOR IN ENCLOSED AQUATIC ECOSYSTEM EXPERIMENTS
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Radiotracer experiments were carried out in enclosures in two lakes at the Experimental Lakes Area, Northwestern Ontario, to study the pathways of trace metal removal from the shallow water columns to the sediments of these lakes. As part of the larger experiment, stable carriers of Fe, Mn, and Zn were added individually to each of three enclosures. Rates of removal to sediments at pH 6.3 and backdiffusion after HCl addition to pH 4.8 of radiotracer 59 Fe, 54 Mn, and 65 Zn were compared to the respective rates of their stable counterparts. Tracer removal and backdiffusion rates in enclosures which received stable carrier additions were further compared with the tracer behaviour in enclosures which received no carrier additions. Radiotracers and stable carriers were removed from the water column at similar rates at pH 6.3, and were remobilized at similar rates from the sediments when the pH of the water column was lowered to 4.8. The presence of the added stable carriers had no effect on the removal or remobilization of Mn or Zn. However, the removal of 59 Fe from the water column was slowed slightly by the addition of Fe(III) carrier compared to removal in the other tubes, possibly due to the formation of colloidal phases. These results show that information on the rates of trace metal transport obtained using radiotracers is applicable to stable metal behavior in these freshwater systems. The use of radiotracers to study trace metal geochemistry under natural conditions is preferred to the addition of stable metals which may alter the physical and chemical properties of the suspended particulate matter as occurred after the addition of stable iron. Speciation studies showed that adsorption to ion exchange resins does not provide a reliable indication of the ionic form of metals in lake water.
