Highly conservative behaviour of bed sediment-associated metals following extreme flooding

On December 26, 2015 (Boxing Day), an exceptional flood event occurred in the Irwell catchment, United Kingdom, when the neighbouring Mersey catchment experienced a much more typical winter run-off event. This provided an opportunity to examine the influence of high-magnitude hydrological processes on the behaviour of fine-grained metal-contaminated bed sediments. Forty sites across the two catchments were sampled for channel bed fine sediment storage and sediment-associated metal(loid) concentrations prior to, and following, the flooding. Sediments were analysed for total As, Cr, Cu, Pb, and Zn and then subjected to a five-step sequential extraction procedure. Despite a significant reorganisation of fine-grained (<63 mu m) sediment storage, metal(loid) concentrations demonstrated markedly conservative behaviour with no significant difference observed between pre-flooding and post-flooding values across both catchments. Estimates of the channel bed storage of sediment-associated metal(loid)s also showed minimal change as a result of the flooding. The metal partitioning data reveal only minor changes in the mobility of bed sediment-associated metal(loid)s, indicating that such flood events do not increase the availability of sorbed contaminants in these catchments. Post-flooding bed sediment metal(loid) loadings remain high, indicating persistent and long-lasting sources of contamination within the Irwell and upper Mersey fluvial network.