Sediment-Water Interchange of Copper and Nickel in Experimental Aquaria

Lake and stream sediments often reflect recent additions of heavy metals before elevations of such elements are detectable in the overlying water. Concentrations of metals in sediments of lakes may be three to four orders of magnitude greater than in the water. While tests of water quality may, therefore, indicate no elevated metal levels in the soluble phase, a water body may still be heavily polluted, with organic and inorganic material in sediments acting as sites for precipitation and adsorption of the metals. The possibility of reentry of sedimented material into the water, due to physical, chemical or biological processes in natural situations, always exists. Experimental work was carried out on nickel and copper in the sediments of Middle Lake (recently limed to pH 6.5, and fertilized) and of Clearwater Lake (untreated), Sudbury, Ontario, under a variety of physical and biological conditions in aquaria. Results showed that both copper and nickel increased in concentration in water overlying the sediments, eg. in “live” aerobic systems over 28 days total copper in water increased from 0.15 to 0.85 ppm, and total nickel from 0.3 to 3.0 ppm. The pH of the water, initially 6.5, dropped to below 4.0. In sterile anaerobic conditions, total copper decreased slightly, and total nickel remained constant, while pH in this condition initially dropped only slightly, and at 28 days was 6.0. The “live” anaerobic system was intermediate in all these respects. “Free” copper at lower pH values (measured by differential pulse anodic stripping voltammetry) was predictably equal to total copper (measured by flame atomic absorption or by anodic stripping), but in the sterile anaerobic treatments where pH was 6.0, “free” copper was less than 50% of the total copper. Since ionic copper is believed to be the form toxic to biota, the lowered pH and increased ionic copper in water close to the sediments appears to have serious implications.