Abstract
The metal mining industry has caused large quantities of heavy metals to enter countless river systems. The consequent spread of heavy metals is determined largely by how these metals bind with silt and soil particles and the transport pathways of these particles in the alluvial parts of river systems. Once
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the slib-bound metals have been taken up into one of the hydrological subsystems, such as the underwater soil and the banks, they continue to be subject to geomorphological, chemical and hydrological processes. After sedimentation, both the silt and the metals bound to it may be remobilized by physical disturbance of the river bed or banks. Chemical processes may contribute to changes in the form of binding of the metals or to solution of the metals, and to their entry into adjoining waters (groundwater, soil water or surface water) or absorption by plants. Remobilized metals, either bound to silt or in solution, can be transported again and redeposited, or may precipitate in both the river bed and the flood plain. Complex interactions take place between all these processes, which can also change in nature in the course of time. A better understanding of the dynamic conditions (i.e. transport, sedimentation and remobilization) that heavy metals are exposed to in a river system is extremely important in predicting the behaviour and the dangers of heavy metals in a fluvial environment.
The research was performed in the river Geul basin, which runs through Belgium and the Netherlands. The source of the Geul is in northwest Belgium, and the river flows through the South Limburg marl region to the river Meuse. Lead and zinc ores have been extracted and processed near the Belgian towns of Plombières and Kelmis from the late Middle Ages until the mid twentieth century. The discharge of process water and the storage of mine waste on the banks of the Geul have caused large quantities of heavy metals to enter the river system. The consequent spread has led to increased concentrations of heavy metals in the river silt and the banks. The presence of heavy metals is a potential threat to people and animals in the area. The natural meandering nature of the river Geul means that old, strongly contaminated sediments are transported repeatedly, and ultimately discharged into the river Maas. The Geul thus contributes to the pollution of this river, the second largest in the Netherlands and a source of drinking water.
The objective of this study is a quantitative survey of the spread of heavy metals in the Geul basin. Many of the processes deemed to be relevant were studied, with an emphasis on a cohesive, geographical approach. Ultimately a balance was drawn up of sediment and heavy metals, with reference to the insight gained and the database assembled. There were the following more specific objectives within this generally formulated objective:
to determine the position of the sources of heavy metals;
to analyse the hydrological factors that influence the transport of silt and heavy metals and to quantify the burden of these substances;
to map out flood areas and the contamination of the banks;
to determine the sedimentation velocity of contaminated silt and the flood plain capacity;
to analyse the forms of binding of heavy metals in silt and soil;
to estimate the environmental consequences of the presence of heavy metals.
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