The role of floodplains in attenuating contaminated sediment fluxes in formerly mined drainage basins

Many upland river catchments in the UK have been historically mined for metals such as lead (Pb) and zinc (Zn), and as part of the mining process large quantities of metal contaminated sediment were released into the river system. The levels of sediment associated heavy metal contamination in river systems are largely controlled by the volumes of contaminated sediment released into the river and fluvial processes (e.g. erosion and deposition). As a consequence, the contamination patterns are often highly variable, which can make it difficult to create accurate assessments of the volumes of contaminated sediment remaining within the system. This paper uses a combination of techniques to establish the volumes of metal contaminated sediment remaining within the River Swale, UK. Firstly, using detailed field sampling and a geographical information system (GIS), it estimates the volumes of sediment remaining within one formerly mined tributary (Gunnerside Beck) which is then extrapolated to represent the contaminant volumes on other tributaries of the River Swale. Secondly, combining fresh field data with a range of existing data, volumes of contaminated sediment on the main stream of the River Swale are established. This two tier approach shows that significant volumes of contaminated sediment remain within the River Swale, with over 32 000 tonnes of Pb within the mined tributaries and 123 000 tonnes within the main channel belt of the River Swale itself. This represents approximately 28% of the Pb produced in the Swale catchment. Given these volumes and present day rates of removal, it may take over 5000 years for all of the metal rich sediment to be removed from the catchment. If the contaminated sediment is used as a tracer, present day rates of reworking of floodplain sediment can be calculated to be 0·02% per year. Copyright © 2008 John Wiley & Sons, Ltd.