Occurrence and transport of microplastics across the streambed interface during bank filtration

Riverbed sediments have been identified as temporary and long-term accumulation sites for microplastic particles (MPs), but the transport and retention mechanisms still need to be better understood. Here we assess the occurrence and spatial distribution of MPs in surface water, riverbed sediments, and groundwater of two German lowland rivers (Teltow Canal and Havel), under prevailing infiltrating conditions. Surface water and groundwater samples were collected at each site on a monthly and three monthly basis over the course of one year, respectively. At each site, three sediment freeze cores up to a depth of 100 cm were taken, and, together with the water samples, analysed by near-infrared spectroscopy (NIR) to infer the number of MPs (Ø > 100 µm), polymer types, and particle sizes.

The number of MPs detected varies considerably between the compartments (with concentrations in the groundwater being approximately one order of magnitude lower than in the river), the sampling sites, and also, but to a much lower extent, over the seasons. MPs were also detected throughout the entire depth of the sandy riverbed sediment, thereby highlighting the retention capacity of the riverbed sediments, but also the partial mobility of MPs from the river through the subsurface into the groundwater. These observations were supported by saturated column experiments with fluorescent polystyrene particles (fPS), which demonstrated that the vast majority of fPS were retained in the upper 20 cm or 15 cm of gravelly or sandy sediments for common filtration rates. However, it was also observed that approximately 0.3% of the introduced MPs, with sizes ranging from 100 µm to 500 µm, were transported throughout the entire column for a high filtration rate.

These results demonstrate that riverbed sediments have the capacity to retain MPs originating from surface water. Furthermore, they indicate that these sediments can also act as potential vectors for the infiltration of small MPs into local groundwater aquifers, especially under prevailing infiltrating conditions.