Wetenschap - 18 april 2002

PhD student samples mud along the Rhine for heavy metals

PhD student samples mud along the Rhine for heavy metals

The large rivers that flow into the Netherlands are important for trade with Germany and other European countries. But unfortunately they also carry toxic chemicals into our country. PhD student Corine van Griethuysen carried out fieldwork in river floodplain lakes, where toxic chemicals accumulate.

Van Griethuysen works at the Aquatic Ecology and Water Quality Management Group and is interested in the risks of heavy metals in floodplain sediment. She explored a number of lakes in the floodplains of the Waal, Rhine and IJssel by boat. She took samples of the mud on the bottom of the lakes using Perspex tubes. "During these trips you can really see the dynamic nature of the river in different seasons. Sometimes the floodplain lakes are almost completely dry, at other times the water in the floodplain is four metres deep."

This episodic flooding has led to accumulation of heavy metals, says Van Griethuysen. In the field she measured the presence and chemical availability of copper, lead, zinc and other metals, in particular, metals in pore water and those bound to sulphides. The metals can affect plants and animals living on the floodplains. Badgers for example have been found to have reproduction problems due to intake of cadmium.

Pollution

"In the sixties and seventies especially, a lot of polluted mud was deposited in the Dutch floodplains. The heavy metals originate mainly from industry although mining areas are also a major source, such as the valley of the river Geul, a tributary of the Meuse in the south of Holland."

At the moment, the risk of new pollution sources is small, because industrial management has improved, believes Van Griethuysen. "We don't have disasters like in Romania where in January 2000 the dam of a mining lagoon broke." Cyanide waste and other toxic effluent affected over 300 km of rivers in Romania and Hungary, leaving them 'ecologically dead'.

Environmental risk

Danger is not imminent in the Netherlands, but it does lurk beneath the surface. Van Griethuysen points to the fact that there are plans to dig into the floodplain soils in order to widen the riverbed and decrease the flooding risk. This carries an environmental risk, she says "When you stir up the floodplain sediments, chemical conditions change and sulphides can be broken down, releasing heavy metals to the ground water or surface water." The polluted soil that is dug up, also must be stored somewhere. "Research has been done on whether the sediment can be stored in deep holes in the floodplain. Depending on the sediment type and chemical and hydrological conditions, it might be possible to store toxic chemicals in this way with only a small risk of them being released into the environment."

During her fieldwork along the Dutch rivers, Van Griethuysen noticed that chemical conditions in the sediments varied a lot. This affects the environmental risk of pollution. For example clayey soils bind chemicals better than sandy soils. Van Griethuysen thinks that carefully examining the local conditions is crucial when dealing with polluted floodplains.

Hugo Bouter

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