Phase Change and Remobilisation of Heavy Metals in Floodplain Sediments of UK River Systems after Frequent Flooding
Metal mining used to be a major source of heavy metal contamination for river systems and affected areas exceed 4000 km2 in England and Wales. Under flooding conditions, heavy metals stored in riverbeds and floodplains can be remobilised and become secondary sources of diffuse pollution. During remobilisation, heavy metal species and their association with soil/sediment particles have been changed. This paper investigates heavy metals in floodplain sediments of the Rivers Severn, Manifold and Derwent, UK. Results show further accumulation of heavy metals in recent years, especially with frequent large magnitude floods, even though mines closed and mining activities ceased more than a century ago. Deposits from recent flooding events contain increased levels of Pb, Zn, Cu and Cd, especially within a relatively short distance from the channel. Investigation into total metal concentrations in these floodplain soils reveals that the concentrations of Cd, Cu, Pb and Zn are substantially higher than their background levels and national average values. All nine floodplains examined are classified from uncontaminated/moderately contaminated (class 1) to strongly/extremely contaminated levels (class 5) with Cd, Cu, Pb and Zn. Metal distributions across the floodplains are significantly related to distance from the river channels. Patterns of metal distribution on the floodplains are different. The levels of Cd, Cu, Pb and Zn decrease with the distance away from the river channel, and decline downstream on the Manifold and Severn floodplains; whereas the distributions of Cd and Zn show a relatively unchanged pattern throughout the Derwent floodplains. Speciation of heavy metals in floodplain sediments is investigated and a significant alteration is observed. Although the concentration of total heavy metals along vertical profiles decreases, the bio-available fraction has increased in sediments deposited in recent years. This poses a potential toxic danger to livestock and humans through the food chain. The increase of the bio-available fraction is associated with remobilisation of sediments from floodplains, which act as a diffuse source of contamination during flooding. Prolonged inundation of floodplains alters the Eh/pH condition of the previously deposited sediments/soils and causes changes in heavy metal speciation. Reworking of the floodplain surface by flood water facilitates transportation and re-deposition of sediments and sediment-associated heavy metals.
flooding floodplain heavy metal contamination heavy metal speciation
Yingkui ZHAO Xiuqing ZHANG Susan MARRIOTT
School of Built & Natural Environment,University of Central Lancashire,Preston PR1 2HE,UK School of Built & Natural Environment,University of the West of England,Bristol BS16 1QY,UK
国际会议
2009 International Symposium on Environmental Science and Technology(2009环境科学与技术国际会议)
上海
英文
979-985
2009-06-02（万方平台首次上网日期，不代表论文的发表时间）