Abstract
Floodplains downstream of urban catchments are sinks for potentially toxic trace elements. An intensification of the hydrological cycle and changing land use will result in floodplains becoming inundated for longer durations in the future. We collected intact soil cores from a floodplain meadow downstream of an urban catchment and subjected them to an inundation/drainage cycle in the laboratory to investigate the effect of flood duration on trace element concentrations in the soil porewater. The porewater concentrations of Ni, Cr, and Zn increased, whereas Cu and Pb decreased with flood duration. All the Cr present in porewaters was identified as Cr(III). Copper concentrations increased after drainage but Pb mobility remained suppressed. Both pH and dissolved organic carbon (DOC) increased with flood duration but were lower in treatments that were drained for the longest duration (which were also the treatments flooded for the shortest duration). The porewater concentrations of Cr and Ni decreased after drainage to levels below those observed before inundation, mirroring the DOC concentrations. We concluded that the duration of floodplain inundation does have an influence on the environmental fate of trace elements but that flooding does not influence all trace elements in the same way. The implications of an intensification of the hydrological cycle over the coming decades are that floodplains may become a source of some trace elements to aquatic and terrestrial ecosystems. Environ Toxicol Chem 2020;39:2124-2135. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.
Highlights
The deposition of sediments on floodplain soils during a low flow velocity flooding event often results in floodplains becoming a sink for potentially toxic trace elements such as Ni, Cr, Cu, Pb, and Zn (Walling and Owens 2003; Overesch et al 2007; Rinklebe et al 2007; Du Laing et al 2009; Hurley et al 2017)
The samples that were drained for the longest period of time had mobile Ni and Cr concentrations that were at a level lower than those observed prior to inundation (Figure 2), indicating that the influence of flooding on Ni and Cr mobility in this floodplain soil is transient
The present study investigated soil porewater concentrations and further work is required to establish whether high concentrations of trace elements such as Zn, relative to those found in rivers such as the Loddon, will translate into elevated concentrations in watercourses during periods of floodplain inundation
Summary
The deposition of sediments on floodplain soils during a low flow velocity flooding event often results in floodplains becoming a sink for potentially toxic trace elements such as Ni, Cr, Cu, Pb, and Zn (Walling and Owens 2003; Overesch et al 2007; Rinklebe et al 2007; Du Laing et al 2009; Hurley et al 2017) These trace elements originate from solid and dissolved inputs from either the natural erosion of soils upstream or, in urban catchments, anthropogenic sources of contaminants including transport emissions, industrial waste, mining, landfills, sewage treatment facilities, and diffuse runoff from fields impacted by agricultural applications or aerial deposition. Climate model projections show an increased flood risk in many regions across Europe caused by an expected increase in the magnitude, duration, and frequency of fluvial flooding (Wilby et al 2008; Blöschl et al 2017) The effect of these climatic changes will be influenced by land management within individual catchments. Drastic changes in management (e.g., a 50% expansion in forest cover) may be required to maintain the status quo in the face of future
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