A depth-resolved snapshot of microplastic abundances in riffle heads in a gravelbed river

To date, we know little about depth-resolved distributional patterns of microplastic particles (<630 µm) of streambed sediments. Reports in literature about the abundance of considerable amounts of microplastic particles in fluvial sediments indicate that the transport follows naturally occurring flow vectors into streambed sediments. We hypothesize, that locally occurring hydraulic conditions at the water-sediment interface which are characteristic for various naturally occurring riverbed morphological structures might influence the evolution of microplastic depth profiles.

As a first step of identifying morphologic feature specific microplastic depth profiles, we have carried out a series of six freeze core sampling campaigns at the centre of the longitudinal axis of two riffles located in a fourth order gravelbed river (Ruwer) contributing to the Moselle River in Trier, West Germany. The setup allows for sampling a 50 cm long undisturbed sediment freeze-core. Using dry ice as coolant and a diamond saw blade we obtained ten undisturbed sediment cube samples (125 ccm) cut from the vertical axis up to 50 cm depth from each freeze-core. Over the range from 5000 – 25 μm five size fractions were analysed with regard to mineralic sediment and microplastic particle distribution. In all sediment cube samples microplastic particles and fibres could be detected without showing distinct distributional patterns related to its depth. Although the samples represent only a small surface area (25 sq. cm), we could qualify and quantify 81 microplastic items plus 606 suspected items mainly composed of transparent fibres using Raman microspectroscopy. Furthermore, we observe an exponentially increase in microplastic abundance with decreasing size fraction by three orders of magnitude. Dominant plastic types are polypropylene, polyethylene, nitrile and polyether terephthalate representing 85% of our findings. Our first results imply that in riffle heads downward directed transport within the sediment layer affects rather the shape and size of the plastic than the absolute abundance. As a next step, typical depth patterns of microplastic at the upstream and downstream end of streambed riffles have to be identified.