ABSTRACT
Wetlands, especially floodplains, offer a variety of ecosystem functions. One of the most important functions is their ability to act as water regulators by buffering the extremes associated with the discharge from rivers (de Groot et al., 2002; Golladay and Battle, 2002; Rupp et al., 2010). Floodplains also fulfil important retention functions in relation to the cycling and treatment of nutrients and contaminants that are transported through the river system (Costanza et al., 1997; Venterink et al., 2003; Rinklebe et al., 2007). Therefore, the contamination of floodplain soils by potentially toxic trace elements is of serious environmental concern, as many floodplain soils worldwide are polluted with trace elements (e.g., Rinklebe et al., 2007; Shaheen and Rinklebe, 2014; Shaheen et al., 2014a). In selected floodplain areas, hot spots emerge as a consequence of contaminated sediment deposition (Martin, 2000; Middelkoop, 2000; Krüger et al., 2005; Benson, 2006; Baborowski et al., 2007). Floodplain soils are often enriched with trace elements that originate from geogenic and anthropogenic sources (e.g., industrial discharge of waste into rivers or diffuse agricultural input from catchment areas) (Frohne et al., 2011, 2015). An example is the Elbe River, Germany, in which soils are contaminated mainly by wastes that originate from different industrial activities and that were discharged into the river during the previous centuries (e.g., Overesch et al., 2007; Rinklebe et al., 2007). Toxic trace elements such as arsenic (As) and chromium (Cr) are transported by the river water either in dissolved form or via suspended material, accumulate in floodplain soils during flooding, and exhibit low flow velocities (Du Laing et al., 2009c; Rennert and Rinklebe, 2009; Frohne et al., 2014). Over the decades, the Elbe River and its tributaries, which carry high concentrations of trace elements, have become one of the worst polluted, major river systems in Europe (e.g., Zimmer et al., 2011; Rinklebe and Shaheen, 2014; Shaheen and Rinklebe, 2014).
