Spatial Variability and Combined Risk of Metal elements and Microplastics along the River Severn (UK)

In order to better understand the processes of transport and accumulation of microplastics with other co-pollutants, we developed a large-scale study along the UK's longest river, the River Severn (354 km). Our objective is to quantify the concentrations and loads of MPs and metal elements in sediments and surface water, and also to discuss their origin by considering tributary inputs and land-use changes. River Severn successively flows through a diversified landscape with forested uplands and pasturelands, where former mining areas (lead, silver and zinc) are found (Shropshire and Plymlimon). It also presents small towns which originally developed thanks to wool production and heavy industries. Downstream, the river receives inputs from tributaries draining urban and industrial areas (Birmingham Black Country and Coventry), as well as wastewater treatment plant inputs (around Worcester).

Streambed sediments and surface water from 16 sites located along the river were collected during low water conditions to take into account this spatial feature. MP concentrations and polymer types were quantified by using µFT-IR. Metal concentrations (Cd, Cu, Cr, Fe, Ni, Pb, Zn) as well as hydrological and sediment properties (grain size, organic matter – OM, carbonates) were also acquired during the sampling campaign. Pollution Load (PLI) and Polymer Risk (PRI) indices, as well as daily MP fluxes were assessed.

Results highlight multi- contaminated hotspots in the upstream section (PLI>2), mostly because of lead (Pb), zinc (Zn) and cadmium (Cd) due to diffuse pollution from historic mining areas, while MP hotspots were distinct and limited. A gradual growth of metals and MP concentrations and loads as well as an increase of polymer diversity occurred in the downstream direction, in both surface water and sediments. A major multi-pollution hotspot was found south of Worcester (PLI>5), which seems polluted by both diffuse pollution from tributaries (such as the Avon River coming from Coventry) and by local sewage inputs.

The composition of pollution hotspots greatly contrasts along the river, as underlined by various metal and MP concentration and types, most likely coming from point-sources, as well as brought by some tributaries draining urban-industrial areas in the downstream direction. This knowledge gain will help to shape future river water quality management.