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

The frequency and duration of flooding events are 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

Climate variability during the past 2,000 years and past economic and irrigation activities in the Aral Sea basin

International audienceThe lake level history, here based on the relative abundance of Ca (gypsum), is used for tracing past hydrological conditions in Central Asia. Lake level was close to a minimum before approximately A.D. 300, at about A.D. 600, A.D. 1220, A.D. 1400 and since 1960s it is lowering again. Lake water level was lowest during the fourteenth or early fifteenth centuries as indicated by a coeval settlement, which today is still under water near the well-dated mausoleum of Kerderi. Pollen data from riparian vegetation indicate generally wet conditions between A.D. 400 and A.D. 900, intermitted by short intervals with drier conditions (AD 550–600; A.D. 650–700) and riverbanks were again dry from A.D. 900–1150, A.D. 1450–1550, and from A.D. 1970 onward moisture decreased steadily. Irrigation activities were at a maximum between 300 B.C. and A.D. 300 (Classical Antiquity) and between A.D. 800 and A.D. 1300 (Medieval Age) and after A.D. 1960