A Review of Flood-Related Storage and Remobilization of Heavy Metal Pollutants in River Systems

Abstract

Recently observed rapid climate changes have focused the attention of researchers and river managers on the possible effects of increased flooding frequency on the mobilization and redistribution of historical pollutants within some river systems. This text summarizes regularities in the flood-related transport, channel-to-floodplain transfer, and storage and remobilization of heavy metals, which are the most persistent environmental pollutants in river systems. Metal-dispersal processes are essentially much more variable in alluvia than in soils of non-inundated areas due to the effects of flood-sediment sorting and the mixing of pollutants with grains of different origins in a catchment, resulting in changes of one to two orders of magnitude in metal content over distances of centimetres. Furthermore, metal remobilization can be more intensive in alluvia than in soils as a result of bank erosion, prolonged floodplain inundation associated with reducing conditions alternating with oxygen-driven processes of dry periods and frequent water-table fluctuations, which affect the distribution of metals at low-lying strata. Moreover, metal storage and remobilization are controlled by river channelization, but their influence depends on the period and extent of the engineering works. Generally, artificial structures such as groynes, dams or cut-off channels performed before pollution periods favour the entrapment of polluted sediments, whereas the floodplains of lined river channels that adjust to new, post-channelization hydraulic conditions become a permanent sink for fine polluted sediments, which accumulate solely during overbank flows. Metal mobilization in such floodplains takes place only by slow leaching, and their sediments, which accrete at a moderate rate, are the best archives of the catchment pollution with heavy metals.

Keywords: Flood; Heavy metals; Mobilization; Pollution; River; Sediment.

Fig. 1

Changes of heavy metals concentrations, losses on ignition, content of fine fraction and water depth in the same place of the channel are related to channel bar migration during the 2 years period (from Ciszewski , modified)

Fig. 2

Distribution of metal-contaminated sediments in zones along the channelized reach of the Odra River is related to the width of the pre-regulations channel and repeated channel training works in 19^th and 20^th century (from Ciszewski and Turner 2009)

Fig. 3

Depth profiles of Fe, U and Pb in redox-altered and polluted overbank sediments of the Ploučnice River, the Czech Republic; EFs (enrichment factors) are concentrations normalized to global (Fe) or local (Pb and U) background values (EF=1); Fe concentrations was normalized by Al concentrations and divided by mean upper crustal global reference Fe/Al ratio; U and Pb concentrations were divided by local background concentrations for the overbank sediments of the studied river (according to Matys Grygar et al. ; Matys Grygar et al. 2016a)

Fig. 4

Distribution of U in top 0–10 cm in floodplain of the Ploučnice River, typical for rivers dominated by metal pollution of fine grained sediments mixed with unpolluted parent coarser grained sediment. Data obtained by in situ (portable, handheld) XRF mapping. Lower concentrations of U (<90 ppm) are in coarser sediments in areas covered by less polluted post-mining sediments in proximal floodplain, higher concentrations are in finer distal floodplain deposits and in certain parts of levee (J. Elznicová, unpublished results)