Monitoring and assessment

Obtaining representative samples of macro and microplastics in rivers can be problematic. For surface sampling of microplastics stationary or towed nets have been used. Alternatively, an underwater pump can be used to collect water which is then passed through a net (van der Wal et al. 2015). A floating sampler has been developed in Europe, for larger items (> 3.2 mm), by the organisation Waste Free Water. This is in two parts, with a surface net and a suspension net collecting at a depth of 0.2 to 0.7 m (van der Wal et al. 2015). Measuring the transport of material along the river bed has been undertaken using bottom nets designed for fishing (Mirrit et al. 2014). In addition floating booms have been deployed in rivers, harbours and other waterways to serve as litter traps. River flows can be very episodic, and the quantities of material transported may vary considerably on an hourly, weekly, seasonal or multi-year basis. In addition, flows are not constant across the cross-section of the river.