Microplastic accumulation in streambed sediment downstream of a wastewater treatment plant in response to dynamic flow conditions

Wastewater treatment plants (WWTPs), are known sources of microplastics (1-1000 µm) to receiving streams. We analyzed surface water and streambed sediments at ~1km downstream of a WWTP in a rural stream near Birmingham, UK. To assess the temporal variation in microplastic transport and accumulation, we conducted a total of five sampling campaigns to sample both during high and low flow conditions. Point sampling was supported by semi-continuous measurements of flow and electrical conductivity to characterize the stream hydrologic conditions, especially in response to the WWTP effluent. We used the high frequency flow data as input to a mobile-immobile model for microplastic transport in streams that can account for the exchange between the surface water and streambed sediments, deposition and resuspension during baseflow and stormflow conditions. By combining the model with the less frequent microplastic measurements, we estimated inputs from the wastewater treatment plant and timescales of microplastic deposition and retention in the stream. Our findings advance the understanding of the interplay between microplastics depositing during low flows and resuspending during high flows, to improve predictions of microplastic fate and transport in river systems.