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Mixing, removal and mobilization of trace metals in the Rhine estuary
Duinker, J.C.; Nolting, R.F. (1978). Mixing, removal and mobilization of trace metals in the Rhine estuary. Neth. J. Sea Res. 12(2): 205-223
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579, more
Peer reviewed article  

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  • Duinker, J.C.
  • Nolting, R.F.

    Trace metal concentrations in suspension and solution were measured in the Rhine estuary in a survey and at three fixed stations with different salinity regimes during a complete tidal cycle at one hour intervals. The concentrations in suspension are determined in the first place by the mixing of fluviatile and marine derived particles almost in the same proportion as the waters in which they were suspended. The field data in plots of dissolved concentrations of Fe, Zn, Cu and Cd against salinity are below the ideal (straight) mixing line, indicating the precipitation of originally dissolved river-borne species during estuarine mixing. The corresponding data for dissolved manganese are above the ideal mixing line at low salinities (below 15‰ S) and below this line at higher salinities. This is due to the response of Mn (II)-Mn (IV) equilibria to the rapid changes in important parameters during estuarine mixing (ionic strength, chlorinity, pH, Eh). Manganese is cycled continuously through dissolved and particulate forms. The overall effect for Mn, however, is also removal from solution. A series of laboratory experiments involving short-term (1-hour) interaction between sea water and a natural freshwater suspension did not show evidence for mobilization of Fe, Mn, Al, Mg, Zn, Cu, Cd, Cr, Pb, Si and Ti. Mg appeared to be taken up from sea water. It is argued that extrapolation to estuarine conditions of these and other laboratory experiments on sorption of trace metals may be not justified. Arguments are given why existing models that suggest the mobilization of trace metals from river-borne suspended matter once in contact with sea water fail to represent actual concentration data on dissolved species. Trace metal concentrations in bottom sediment of the Rhine estuary reflect the mixing and removal processes that take place in the water column. The estuary acts as a sink for dissolved and particulate trace metals. Of the total river-borne load of several trace metals only a limited fraction may reach the ocean.

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