Detection of suspended macroplastics in the field using acoustic backscatter

Suspended plastic transport is one of the big unknowns in present plastic research. It is either neglected or measured using expensive and labour-intensive underwater net-measurements. In search for additional measurement methods, the Acoustic Doppler Current Profiler (ADCP) has recently been explored as a tool for detecting plastic debris. ADCPs emit a high-frequency acoustic signal, which is scattered back to the transducer by particles in the water column. The frequency shift and the strength of the returned signal (backscatter) are used for respectively flow velocity and suspended sediment concentration estimates. Large objects like fish and organic material can be recognised in the acoustic signal by a strong local increase in backscatter. It has been shown that ADCPs can also detect plastic debris during controlled tests. The characteristics of the acoustic signal of plastics in an uncontrolled setup under varying conditions is still understudied.

To develop knowledge on in situ plastic detection, we first deployed an ADCP in a small flowing system into which we introduced plastic litter varying in size, composition and shape. Secondly, a test was undertaken with a simultaneous ADCP and net measurement in a large Dutch river. The uncontrolled plastic transport of the river is estimated based on ADCP data, and calibrated and validated using the net measurement. The tests gave novel insight into the signature of flowing plastics in the backscatter signal, and the present possibilities and challenges of in situ plastic detection using ADCPs. Overall, plastic detection using ADCPs has the potential to provide valuable information about the spatial and temporal variability of suspended macroplastic transport in rivers, aiding efforts to mitigate the negative impacts of plastic pollution. More research is needed to create a complete overview of backscatter characteristics of litter and to develop effective automated algorithms and methods for accurately distinguishing plastic.