Detecting plastic items in the water column using an Acoustic Doppler Current Profiler (ADCP)

An Acoustic Doppler Current Profiler (ADCP) is commonly used to monitor flow velocity. An accurate method to obtain discharge in a river or a channel is to mount an ADCP to a boat and sail transects across the channel. Additionally, these surveys may also be used to obtain the amount of plastic items in the water column. The transport of plastic items suspended in the water column may be substantial and is more challenging to monitor than the transport of floating items. We carried out a feasibility test in a harbour of a river. We deployed the ADCP horizontally at 1.0 m depth and released plastic items (and similarly shaped organic items for comparison) 5 times at 1.0, 3.0 and 5.0 m from the ADCP. We compared the signal strength in a 5 s period after release with the background signal strength.

The item was steady within the detection volume for the majority of the 5 s periods. Three out of five plastic items had signal strengths a least 5 dB higher than the background strength (at several distances). We conclude that at least these items were detected. The similarly shaped organic items generally had a lower signal strength. Although the response of each item as a function of orientation, distance along and across the beam should be investigated further, the feasibility study shows the potential to additionally determine the amount of plastic items in the water column from ADCP observations.