The effect of bed load sediments on self-purification of river – Experimental approach

Assessing the profitability of river restoration requires knowledge of the importance of individual ecosystem elements for the value of specific ecosystem services. The aim of this study was to experimentally estimate the effect of the presence of bed load on the processes of water self-purification from suspended solids, organic matter and microplastic fibres. The experiment was carried out in channels with and without bed load, simulating conditions in a lowland river. The system was supplemented with artificial sewage containing FPOM and polypropylene fibres. Changes in the amount of suspended solids in the water, as well as organic matter and microplastics in the water and sediment, were recorded over the course of a week. The presence of a bed load increased the rate of suspension removal from the river water many times over. The decomposition rate of organic matter in the sediment was much higher than in the water, reaching 1 g of organic carbon per week. Bed load favoured the immobilisation of large microplastic fibres, facilitating their incorporation into bottom biofilms. The fibres formed a 'skeleton' that led to the formation of larger bed load biofilms. The study suggests that, even without restoring the connection between river water and groundwater, the supply of bed load to the river significantly increases the efficiency of natural processes in the river ecosystem, including self-purification.