5 Conclusions
A series of geochemical profiles in both porewater and solid phases in the
freshwater sediments of major rivers in Germany (Neckar, Rhine, Elbe, Main, and
Weser), Lake Constance, Lean River (China), Moscow River and Oka River (Russia)
lead to the conclusion, that the mineralization of organic matter plays an important
role in the cycling of nutrients and heavy metals. First, heavy metals and nutrients
bound to organic particles (organic C, N, P) are released in inorganic forms (CO.,,
POa 3", NI'L', and heavy metal ions) into the porewater. They either diffuse into
overlying water or are precipitated with other ions, depending on the physicochemical
conditions. Second, the mineralization processes cause a change of redox potential,
adsorption capacity of solid phases (Fe oxide and Mn oxide), and concentration of
organic complexes. This can change binding forms of the nutrients and heavy metals
on particles, and indirectly influence their mobility.
Because of high input of organic material, anoxic conditions are dominating in
freshwater sediments. Low concentrations of dissolved Cu, Zn, Pb, and Cd in the
sediments, even in heavily polluted sediments (Neckar, Rhine, and Lean), are
associated with the precipitation of metal-sulfides. In the sediments of Neckar, Rhine,
Main, Weser, and Elbe, an acidification of the sediments during resuspension of the
sediments cannot be expected due to high carbonate contents (8-15 %) of the
sediments. In low acid neutralizing sediments such as from the Lean River (carbonate
contents < 2%), it is an open question whether heavy metals are released into the
supemating water. Furthermore, in sediments having low acid neutralizing capacity,
dredging and on-site or off-site disposal, v~ould minimize the controls imposed by the
reducing environment and mobilize metals into surface and groundwater systems.
The best management for such systems is the stabihzation of the structure retaining
the sedtments in their present condition and to minimize the variations in oxic-anoxic
conditlons in the sediments resulting from changes in water discharge.
A series of geochemical profiles in both porewater and solid phases in the
freshwater sediments of major rivers in Germany (Neckar, Rhine, Elbe, Main, and
Weser), Lake Constance, Lean River (China), Moscow River and Oka River (Russia)
lead to the conclusion, that the mineralization of organic matter plays an important
role in the cycling of nutrients and heavy metals. First, heavy metals and nutrients
bound to organic particles (organic C, N, P) are released in inorganic forms (CO.,,
POa 3", NI'L', and heavy metal ions) into the porewater. They either diffuse into
overlying water or are precipitated with other ions, depending on the physicochemical
conditions. Second, the mineralization processes cause a change of redox potential,
adsorption capacity of solid phases (Fe oxide and Mn oxide), and concentration of
organic complexes. This can change binding forms of the nutrients and heavy metals
on particles, and indirectly influence their mobility.
Because of high input of organic material, anoxic conditions are dominating in
freshwater sediments. Low concentrations of dissolved Cu, Zn, Pb, and Cd in the
sediments, even in heavily polluted sediments (Neckar, Rhine, and Lean), are
associated with the precipitation of metal-sulfides. In the sediments of Neckar, Rhine,
Main, Weser, and Elbe, an acidification of the sediments during resuspension of the
sediments cannot be expected due to high carbonate contents (8-15 %) of the
sediments. In low acid neutralizing sediments such as from the Lean River (carbonate
contents < 2%), it is an open question whether heavy metals are released into the
supemating water. Furthermore, in sediments having low acid neutralizing capacity,
dredging and on-site or off-site disposal, v~ould minimize the controls imposed by the
reducing environment and mobilize metals into surface and groundwater systems.
The best management for such systems is the stabihzation of the structure retaining
the sedtments in their present condition and to minimize the variations in oxic-anoxic
conditlons in the sediments resulting from changes in water discharge.