A preliminary study of heavy metal distribution on the floodplain of the River Severn, UK by a single flood event

An important feature of floodplain deposits is the spatial variability of sediment grain size. Study of the variability can improve understanding of the transport and deposition of sediments and particle-associated heavy metals. Heavy metal depositional processes are associated with those of sediment particles and these have been generally inferred from the surface and subsurface distribution of heavy metals in floodplain sediments, but have not been documented during individual flood events. Since subsurface distribution of heavy metals can be altered through pedological processes, deposits from a single flood event can give more realistic information on heavy metal distribution across the floodplain by floodwater. Prolonged heavy rainfall across the Severn catchment resulted in severe flooding in early January 1998. The floodwater left a layer of sediment (approx. 0.7 cm thick at the riverbank near the Severn channel) across the floodplain of the River Severn at Tewkesbury. This provided an opportunity for examination of heavy metal deposition and distribution across the floodplain and its association with sediment particles during a single flood event. Results of sediment grain size analysis show a pattern of decreasing grain size with distance from the channel. A sharp decrease at approximately 27 m from the channel was also observed and considered a result of transporting capacity loss. Results of heavy metal analysis indicate that Pb, Zn, Co and Cu concentrations increase with distance from the channel, but Cd concentration shows a sediment-related enrichment within a distance of 20 m from the channel. The correlation coefficients between grain size and heavy metal concentration show that Pb, Zn, Cu and Co are associated with silt and clay fractions while Cd is not significantly associated with any particular sediment grain size. Particle-associated heavy metals can be transported further away downstream. (C) 1999 Elsevier Science B.V. All rights reserved.