The contribution of stream bank erosion to downstream dilution of mining-contaminated stream bed sediment in the Geul river in the Netherlands

The Geul river, a nearly 60 km long transboundary stream in the northeast of Belgium and southeast of the Netherlands, suffers from excessive fine sediment inputs. This causes the gravel bed to become clogged, which, in turn, hampers the reintroduction of salmonids in the river. To examine the occurrence and origin of fine sediments on the gravel bed of the Geul river, the fine sediment layer was mapped and sampled along the entire reach of the river from near the source to the mouth in late Spring 2021. The bed sediment samples were analysed for trace metal concentrations. The results from the trace metal analysis show that the diffuse ‘uncontaminated’ sediment inputs from the catchment resulting from soil erosion cannot fully explain the declining pattern of the zinc and lead concentrations in the bed sediments of the Dutch reach of the Geul river, downstream from the historic mining sites in the Belgian part of the catchment. This implies that additional sources of fine sediment that is less contaminated than the bed sediment exist. Assuming a model of a constant area-specific sediment yield from the catchment and a constant bank erosion rate per unit river length, the contribution of bank erosion in the Geul river was estimated to increase from 35% at the Belgian-Dutch border to about 67% at the river mouth. This model explains about 56% of the variance of measured zinc and lead concentrations in the bed sediments. The residual concentrations (modelled – measured) correlate negatively with the rubidium concentration in the bed sediment, which is a proxy for the clay mineral content of the sediment. The rubidium concentration explains about three quarters of the variance unexplained by the above sediment delivery model. The model assumptions and results are discussed against independent observations of suspended sediment transport and bank erosion in the Geul river.