Transport of fine suspended sediment and the effect on the distribution pattern of heavy metals in the upstream part of the Geul river system
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Heavy metal mining and its ancillary waste-water is an important source for contaminants in the environment. Pollutants are often transported and stored in fluvial systems. The Geul river is an example of a river system which distributes heavy metals (mainly zinc & lead) from former mining sites. Sediment storage takes place in the gravel bed, which makes the bed an important source and sink for attached metals. In this study, the factors are studied that control the fine sediment infiltration (FSI) in the river bed and determine the sediment exchange flux. Knowledge about the factors for the downstream part of catchment area was already available due to previous research. However, the upstream and most contaminated part of the system have been studied less and fluxes are unknown, as well as the distribution pattern of heavy metals in the bed sediments. Two types of sediment traps were installed in the river bed to study the FSI: Gravimetric traps and metal-based traps, whereby the gravimetric traps measure the direct FSI and the metal-based traps rely on a change in metal concentration of the sediment placed prior to burial in the traps. Bed samples were collected and analyzed for heavy metals using a handheld XRF device to map the distribution pattern of heavy metals. Due to the presence of both suspended transport and bed load transport, the gravimetric method appeared to be working the best. Suspended sediment and bed load sediment have a different metal concentration, which causes errors in the calculations with the metal-based traps, leading to an overestimation during normal flow conditions. The FSI is dependent on the discharge, as the suspended sediment concentration is exponentially related to it. Furthermore, the gravel size, gravel size distribution and the sediment storage are important factors controlling the FSI rate. Based on the infiltration measurements, a very rough estimate could be made of a sediment residence time of 1.7 days. Peak zinc & lead concentrations of bed sediments appeared to be multiple times higher than the maximum permissible values. The peak concentrations shifted approximately 1 kilometer downstream of the mining sites and did not broaden much. This offers opportunities to tackle the contamination problem relatively easy in any case of desirable remediation.