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Eutrophication control by sediment treatment common assumptions and misconceptions T.P. Murphy1*, M. Kumagai2, K. Ishikawa2 1National Water Research Institute, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6 Canada. * 2Lake Biwa Research Institute, 1-10 Uchidehama, Otsu Shiga 520, Japan. Keywords: sediment dredging, sulphate, phosphorus, vivianite, lake Biwa Sediment processes influencing eutrophication Sediments are a major problem in aquatic ecosystem management and restoration . Sediments contain a much higher concentration of nutrients than the water column and can be a major source of nutrients to the water column. Recent studies in Sweden indicate that in summer, as much as 99% of the gross nutrient flux comes from the sediments (Rydin and Brunberg, 1998). Typhoon sediment agitation in Lake Biwa, Japan seems to be associated with the initiation of Microcystis blooms (Frenette et al., 1996). The southern basin of Lake Biwa has become more eutrophic and part of the problem appears to be related to sediments (Kurata, 1994). Unfortunately, when external sources of nutrients are controlled, this rapid seasonal reflux of nutrients from sediment can maintain eutrophication for decades (Sas, 1989). Improvements in the quality of the overlying water and associated components of the aquatic ecosystem often cannot be achieved without some form of sediment treatment. The control of excessive algal growth is still the biggest aquatic environmental problem, and the concern over algal toxins is growing (Carmichael, 1992). Advocates of environmental restoration often promote the hypothesis that directly treating a lake will enhance the utility of source control. Huge dredging projects to control eutrophication are taking place in Lake Biwa and other lakes in Japan. Sediment Quality Assessment and Management: Insight and Progress Edited by M. Munawar© 2003 Ecovision World Monograph Series Aquatic Ecosystem Health & Management Society Sediment redox reactions – iron phosphate reactions Before sediment treatment can be effectively implemented, the project manager must understand the basic processes influencing nutrients in sediment. Some concepts that pervade limnology are classic and in need of revision. For example, the work of Mortimer on iron phosphate reactions was profound in 1941, but if he had current geochemical analytical methods, he would be the first to amend his theories. Most sediment phosphorus studies have stressed the importance of ferric iron adsorption of phosphorus (Mortimer, 1941, 1971; Williams et al., 1971; Krom and Berner, 1981). Ferric iron is well known to adsorb and precipitate more phosphorus than ferrous iron. Thus, when eutrophication results in sediments becoming more reduced, phosphorus is dissolved which can eventually be released to the surface water (Murray, 1995). This scenario is important but overly simplistic. Although ferric reactions are probably mainly controlling the solubility of phosphorus in the surface sediments, the flux of phosphorus from deeper sediments in Lake Biwa, and presumably many other lakes, is controlled by vivianite [Fe3(PO4)2•8H2O] formation (Murphy et al., 2001, Fig. 1). Ferrous iron is well known to react with phosphorus to form the mineral vivianite. It can be an important sink for phosphorus in lakes. Usually its presence is detected indirectly using geochemical calculations and chemical analyses of sediment extractions or porewaters (Emerson and Widmer, 1978), but it has also been Fig. 1. Major sediment phosphorus reactions in Lake Biwa. 60 identified with microscopic and X ray diffraction methods (Cornwell, 1987). It has been demonstrated in lakes (Olsson et al., 1997, Hupfer et al., 1998) and groundwater (Griffioen, 1994) that the formation of vivianite is partly controlled by the ratio of iron to phosphate and that as long as the iron concentration exceeds sulphide concentration, vivianite will be stable. In Lake Biwa, vivianite was detected with a combination of magnetic separation, XRD and other procedures. There are important exceptions where geochemical calculaFig . 2. Concentration of total sulphur and acid volatile sulphide in Lake Biwa sediments. 61 tions indicate vivianite should precipitate, but it does not (Stauffer, 1987; Boers and de Bles, 1991). Interaction of deeper sediments with surface sediments The importance of ferrous iron reactions does not dispute the importance of the surface cap of oxic iron. The absence of an oxic cap reflects severe anoxia where vivianite is not stable in the subsurface sediments. The exchange of the surface 2 cm oxic layer of sediments with 20-30 cm of deeper sediments has been observed in Lake Biwa (Murphy and Kumagai, unpublished). In a Canadian lake with a much higher sedimentation rate than Lake Biwa, Carignan and Lean (1991) observed that 50-100 cm of sediment was involved with sediment phosphorus release. Although this concept of deeper sediments being...


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