Onderzoek naar het gedrag van koper, zink, mangaan en ijzer in de westelijke Waddenzee

Water, suspended and bottom sediment samples in the western part of the Dutch Wadden Sea have been analyzed for their metal content (Cu, Zn, Mn, Fe). For sediment samples the total amount, the exchangeable and leachable (according to Chester and Hughes) fractions of these metals were determined. For suspended samples only the leachable fraction was determined. 0.1 N HCl appeared to be the most preferable leaching agent to determine the leachable fraction. It caused 5% clay mineral lattice breakdown. The levels of metals in water show a non-conservative character. The levels of metals in suspended matter cannot be explained by particle size differences (fraction<16µm according to De Groot)

Suspended matter in the Scheldt estuary

The Scheldt estuary is characterised by a specific energy pattern resulting from the interaction of wave energy, tidal energy and river energy. It divides the estuary into three parts and governs suspended matter transport and distribution pattern. Observation of suspended matter transport shows the existence of three estuarine turbidity maxima (ETM), a marine-dominated ETM in the lower estuary at the river mouth, a river-dominated ETM in the upper estuary with suspended matter concentration reaching up to 300 mg/l, and the most important tide-dominated ETM in the middle estuary with suspended matter concentrations from several hundred milligrams per litre up to a few grams per litre. Resuspension is the dominant phenomenon in this last ETM due to the tidal related bottom scour, which is initiated when a critical erosion velocity of 0.56 m/s is exceeded. An assessment of residual current along the axis of the estuary shows distinctive pattern between the surface water flow and the near bottom water flow. Also the local morphology of the river, natural or man-made, has a prominent effect on the orientation and strength of the residual currents flowing along either side of the river or river bend. Evaluation of suspended matter concentration in relation to the current flow shows no systematic correlation either because of phenomena as scour lag and settling lag mainly in the middle estuary, or because of the current independency character of uniform-suspension mainly in the upper and lower estuary. Quantification of suspended matter load exhibits a net downstream transport from the upper estuary, a near-equilibrium sustainable status in the middle estuary and a net upstream transport of suspended matter from the lower estuary. The characteristic of suspended matter is induced by and is a function of e.g. tidal phase, spring-neap tide, longitudinal and vertical distribution mechanisms, seasons, short and long terms of anthropogenic influence and/or estuarine maintenance. Suspended matter is dominated by complex and cohesive organo-mineral aggregates. It consists of a variable amount of an inorganic fraction (average of 89%) and an organic fraction and occurs largely as flocs, the size of which is remarkably larger in the upper estuary and smallest within the ETM in the middle estuary. Independent time series measurements (1990–2000) of suspended matter property show an increasing sand fraction, a decreasing organic matter content, a rise in d13C as well as a decrease in water transparency. These independent measurements exhibit coherent consequences of estuarine maintenance operations. Maintenance dredging of the shipping channel and harbours and dumping operation in the Scheldt strengthen marine influence further landward, resulting in a sustained tidal range increment and upstream flow and transport of suspended matter

Water quality modelling of the western Scheldt estuary

1n 1987 the National Institute for Coastal and Marine Management (the Netherlands) initiated the SAWES project (Systems Analysis WEstern Scheldt). The main goals of the project were to acquire knowledge with respect to the pollution problems in the Scheldt estuary and to apply this knowledge to management issues of the Scheldt on an operational level. The main interest was to understand quantitatively the relation between inputs of polluting substances and effects on the ecosystem. In the framework of the project, a water quality model for the Scheldt estuary was made. In 1995, the model area was expanded to include the Belgian coastal region at the mouth of the estuary. The model calculates the fate of discharged substances and establishes the relation between pollution inputs and the resulting water quality, including general water quality (oxygen, pH, alkalinity, major ion chemistry and nutrients) as well as pollutant concentrations. The model incorporates all chemical processes which affect these concentrations, including the precipitation/dissolution of metal sulphides which to a large extent controls the fate of trace metals. Based on calibration and verification exercises, it has been shown that the model provides a good presentation of the physical and chemical processes taking place within the estuary and can therefore be used to support policy development for the estuary. For example, the model can compute how the water and sediment quality in the estuary will respond to reduced inputs of waste or to such human activities as dredging