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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 …