Impacts of baseflow and flooding on microplastic pollution in an effluent-dependent arid land river in the USA

Effluent discharge from wastewater treatment plants can be a substantial source of microplastics in receiving water bodies including rivers. Despite growing concern about microplastic pollution in freshwater habitats, the literature has not yet addressed effluent-dependent rivers, which derive 100% of their baseflow from effluent. The objective of this study was to document and explore trends in microplastic pollution within the effluent-dependent lower Santa Cruz River near Tucson, Arizona (USA). We examined microplastic concentrations in the water column and benthic sediment and microplastic consumption by mosquitofish ( Gambusia affinis ) at 10 sites along a ~40 km stretch of the lower Santa Cruz River across two time periods: baseflow (effluent only) and post-flood (effluent immediately following urban runoff). In total, across both sampling periods, we detected microplastics in 95% of water column samples, 99% of sediment samples, and 6% of mosquitofish stomachs. Flow status (baseflow vs post-flood) was the only significant predictor of microplastic presence and concentrations in our models. Microplastic fragment concentrations in the water column were higher post-flood, microplastic fiber concentrations in benthic sediment were lower post-flood, and mosquitofish were more likely to have consumed microplastics post-flood than during baseflow. The additional microplastics detected after flooding was likely due to a combination of allochthonous material entering the channel via runoff and bed scour that exhumed microplastics previously buried in the riverbed. Effluent-dependent urban streams are becoming increasingly common; more work is needed to identify microplastic pollution baselines and trends in effluent rivers worldwide.