Downstream transfer of metal-contaminated sediments in the Geul river as a result of the extreme June 2021 flood event

Historic metal mining and smelting have greatly enhanced the levels and fluxes of heavy metals in and through the fluvial system of the Geul river, a nearly 60 km long transboundary meandering stream in the northeast of Belgium and southeast of the Netherlands. In this study, we examined the long-term (> 1 year) effects of the extreme June 2021 flood on the transfer of sediments and heavy metals through the Geul river system. For this, we quantified the volumetric sediment budget of the channel belt of Dutch part of the Geul river using 0.5 m resolution Lidar-derived DEMs (Algemeen Hoogtebestand Nederland - AHN) for the 2018-2022 period and compared that to the similarly derived sediment budget for the 2012-2018 period. Furthermore, samples of fine sediment from the river bed and the top of the point bars were collected at more or less regular downstream intervals in 2022 and 2023, respectively. These sediment samples were analysed for total zinc and lead concentrations.

During the 2012-2018 period, the sediment of the channel belt was generally negative with an average net erosion rate of about 130 m³ km^-1 y^-1. This implies that during this period, river cut-bank erosion was not fully compensated by pointbar accretion and that the surface level of the newly formed point bars of the meandering Geul river was in general lower than the former floodplain surface. During the 2018-2022 period, the sediment budget was close to zero in the first 22 km of the Dutch reach downstream from the Belgian-Dutch border. However, in the downstream portion of the channel belt, the net deposition rate increased strongly with an average of about 380 m³ km^-1 y^-1. This positive sediment budget indicates strong aggradation of the point bars, which can most likely be attributed to backwater effects during the 2021 flood event, which upstream from a culvert underneath a canal close to the confluence of the Geul river into the Meuse river.

The zinc and lead concentrations in the fine fractions of the bed sediments shows a gradually decreasing trend in downstream direction which can be attributed to dilution from less contaminated sediment inputs from soil erosion on the upstream hillslopes and bank erosion. This pattern cannot be directly linked to the June 2021 flood event. In the reach where the sediment budget was close to zero during the 2018-2022 period, the zinc and lead concentrations in the point bar sediments are comparable to those in the fine bed sediments and show similar decreasing downstream trend. However, in the downstream reach, where net aggradation occurred during the 2018-2022 period, the metal concentrations in the point bar sediments deviate from the generally decreasing trend and increase again by a factor of about four. This downstream pattern in metal concentrations denotes that during the 2021 flood event, sediments originating from the contaminated upstream reaches of the Geul river skipped a substantial reach the Geul channel belt and were mainly deposited in the downstream portion of the channel belt.