Estimating floating macroplastic flux in the Santa Ana River, California

Study region: The Santa Ana River middle reach, a small coastal urban catchment in Southern California, USA experiences a Mediterranean climate and lowflows dominated by wastewater effluent.Study focus: River macroplastic flux can inform watershed management of plastic pollution. However, continuous macroplastic monitoring is not possible, so concentrations must be pre-dicted during unobserved periods. We monitored macroplastic concentration and aimed to improve macroplastic flux estimation using strategies commonly employed in estimating mineral sediment flux.New hydrological insights for the region: Floating macroplastic size distributions were statistically equivalent between lowflow and stormflow samples - evidence that channel processes controlled macroplastic size distribution or macroplastic size distributions outside the channel were the same as inside. Concentrations fell during the falling limb of one hydrograph and rose during the rising limb of another hydrograph. A generalized additive model (GAM) revealed that macro -plastic concentration increased in response to small discharge increases but decreased for the largest discharges. Macroplasitc depletion (relative to discharge) occurs at high flow magnitudes or during the falling limb. The annual mass flux of floating macroplastic was 27.4 (2.8-84.8) tonnes(1)yr(-1) or 18.2 (2.9-222.2) tonnes(1)yr(-1) as predicted using mean concentration or the GAM, respectively. With little data, the mean concentration approach may be appropriate but likely underestimates uncertainty - which will require extensive monitoring to reduce.

Automated summary

This study focuses on estimating the macroplastic flux in the Santa Ana River middle reach by monitoring the macroplastic concentration and using strategies commonly employed in estimating mineral sediment flux. The results show that channel processes control the size distribution of macroplastic, and different flow stages have different effects on plastic concentration. The annual flux of floating macroplastic is estimated to be 27.4 tonnes per year.