Spatial and temporal variability in in-stream microplastic loads can impact downstream plastic export

Microplastic particles (MPs) are emerging contaminants of concern that have been isolated and described in various environmental compartments. River networks can not only act as major transport pathways of MPs to the world’s oceans, but also as intermediate and long-term sinks, as well as redistributors of MPs. MP in-stream concentration and load (concentration multiplied by discharge) are key parameters when quantifying MP downstream transport and provide an indication towards potential impacts on downstream ecosystem health. MP concentrations and loads within a catchment or river network presumably vary in space and time, yet extensive studies addressing the impact of anthropogenic factors (e.g., water management practices, point source release, landuse) in conjunction with such variability on downstream MP evolution are still scarce. Here we present key findings from two recent studies. The first study compares downstream MP concentrations and loads for the two neighboring catchments of Boulder Creek (BC) and South Boulder Creek (SBC), Colorado, USA, which vary in their population density and degree of urbanization. We collected 21 water samples (50 L, filtered through >63 µm mesh) from locations along both river channels. For each river segment we also obtained discharge information that helped us quantify MP in-stream loads and determine segment-wise load differences. Samples underwent digestion with Fenton’s reagent before potential MPs were characterized using fluorescent microscopy and Raman spectroscopy. We found that the degree of catchment urbanization influenced downstream MP patterns for both rivers, with BC (higher degree of urbanization and population density) expressing higher MP concentrations and loads than SBC. We also observed extensive downstream MP removal at certain locations where river flow was diverted for anthropogenic use in both streams. This caused a stepwise reduction of downstream MP loads and redistribution of MPs within the wider catchment. The second study looked at the temporal evolution of in-stream MP concentrations and loads about 1000 m downstream of a wastewater treatment plant (WWTP) at a sidearm of the River Blythe, UK. The WWTP represented a point-source and was the only major MP source to the stream at our sampling location. Water samples (3x 100 L, filtered through >63 µm mesh) were collected at different intervals (monthly over an entire year, weekly over two months, hourly over four days) to better relate possible variations in MP concentrations and loads to changes in WWTP effluent discharge and to study the representativeness of snap-shot sampling. Samples were digested with Fenton’s reagent before fluorescent microscopy and Raman spectroscopy. Results indicate that temporal variability in in-stream MP load could be based on both changes in stream discharge and changes in WWTP effluent concentration, individually or simultaneously. MP loads varied by up to an order of magnitude over the course of one hour, highlighting the importance of obtaining enough representative MP data when characterizing a river system. Our results show that spatial and temporal variability of MP concentrations and loads in rivers and river networks can be highly variable. This variability should be considered in large scale modeling exercises quantifying plastic fate and transport to the oceans.