Watershed Transport Processes of Anthropogenic Litter

From abandoned vehicles to tiny plastic particles, anthropogenic litter ends up in the environment and damages ecosystems and economies. River ecosystems are highly impacted by anthropogenic litter and transport litter from land to the ocean. Managers, in particular, need information on watershed litter source and transport processes to make best management decisions. However, approaches to estimating anthropogenic litter fluxes are highly uncertain. In chapter two, we monitored litter accumulation on roadsides in the Inland Empire, CA. We determined that human transport was the primary process transporting litter from the sale location to the roadsides we studied. We quantified litter accumulation rates along roadsides as 1170 (917-1447) kg1km-1year-1 and established a harmonization tool in chapter three for comparing our results to other studies. Litter from roadsides is one source of macroplastics in river stormflow along with direct dumping and litter already within the channel. In chapter four, we investigated the sources of macroplastic in the Santa Ana River and tested the hypothesis that discharge controled macroplastic concentration. The particle size distribution of macroplastic particles (validated with the methodology in chapter five) did not respond to hydrologic transport mode (i.e. stormflow vs lowflow) suggesting that macroplastics in riverflow were primarily sourced from the stream channel. We found that the relationship between discharge and macroplastic concentration was nonmonotonic, and there were path-dependent effects causing variability. We estimated annual macroplastic flux in the Santa Ana as 18.2 (2.9-222.2) metric tonnes per year. Within the channel, microplastics (small plastic particles < 5 mm) can be influenced by turbulence to create concentration depth profiles. In chapter six, we tested the hypothesis that microplastics are transported via a predominant concentration depth profile, commonly assumed in other studies. We found that microplastics can be transported in any transport mode and that misapplication of transport mode assumptions to monitoring and modeling approaches increase uncertainty or systematic bias by multiple orders of magnitude. Overall, these studies support the advancement of the science and management of anthropogenic litter by bringing us closer to accurately monitoring and modeling a watershed mass balance of anthropogenic litter, and its plastics constituents