The impact of increased flooding occurrence on the mobility of potentially toxic elements in floodplain soil - A review

Jessica Ponting; Thomas J Kelly; Anne Verhoef; Michael J Watts; Tom Sizmur

doi:10.1016/j.scitotenv.2020.142040

The impact of increased flooding occurrence on the mobility of potentially toxic elements in floodplain soil - A review

Sci Total Environ. 2021 Feb 1:754:142040. doi: 10.1016/j.scitotenv.2020.142040. Epub 2020 Sep 1.

Authors

Jessica Ponting¹, Thomas J Kelly², Anne Verhoef³, Michael J Watts⁴, Tom Sizmur⁵

Affiliations

¹ Department of Geography and Environmental Science, University of Reading, Reading RG6 6DW, UK; Inorganic Geochemistry, Centre for Environmental Geochemistry, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK.
² Department of Geography and Environmental Science, University of Reading, Reading RG6 6DW, UK; Department of Geography, Queen Mary University of London, Mile End Road, London E1 4NS, UK.
³ Department of Geography and Environmental Science, University of Reading, Reading RG6 6DW, UK.
⁴ Inorganic Geochemistry, Centre for Environmental Geochemistry, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK.
⁵ Department of Geography and Environmental Science, University of Reading, Reading RG6 6DW, UK. Electronic address: t.sizmur@reading.ac.uk.

PMID: 32916489
DOI: 10.1016/j.scitotenv.2020.142040

Abstract

The frequency and duration of flooding events is increasing due to land-use changes increasing run-off of precipitation, and climate change causing more intense rainfall events. Floodplain soils situated downstream of urban or industrial catchments, which were traditionally considered a sink of potentially toxic elements (PTEs) arriving from the river reach, may now become a source of legacy pollution to the surrounding environment, if PTEs are mobilised by unprecedented flooding events. When a soil floods, the mobility of PTEs can increase or decrease due to the net effect of five key processes; (i) the soil redox potential decreases which can directly alter the speciation, and hence mobility, of redox sensitive PTEs (e.g. Cr, As), (ii) pH increases which usually decreases the mobility of metal cations (e.g. Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, Zn²⁺), (iii) dissolved organic matter (DOM) increases, which chelates and mobilises PTEs, (iv) Fe and Mn hydroxides undergo reductive dissolution, releasing adsorbed and co-precipitated PTEs, and (v) sulphate is reduced and PTEs are immobilised due to precipitation of metal sulphides. These factors may be independent mechanisms, but they interact with one another to affect the mobility of PTEs, meaning the effect of flooding on PTE mobility is not easy to predict. Many of the processes involved in mobilising PTEs are microbially mediated, temperature dependent and the kinetics are poorly understood. Soil mineralogy and texture are properties that change spatially and will affect how the mobility of PTEs in a specific soil may be impacted by flooding. As a result, knowledge based on one river catchment may not be particularly useful for predicting the impacts of flooding at another site. This review provides a critical discussion of the mechanisms controlling the mobility of PTEs in floodplain soils. It summarises current understanding, identifies limitations to existing knowledge, and highlights requirements for further research.

Keywords: Climate change; Contamination; Flooding; Floodplain soil; Mobility; Potentially toxic elements.

Publication types

Review