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Urbanization poses increasing threats to aquatic ecosystems, including increased chemical loading. Of relatively recent concern is the potential of urban stormwater runoff to facilitate the spread of microplastics (MPs), including tire wear particles. Previous studies have demonstrated the effectiveness of bioretention treatment systems in treating runoff, thereby reducing chemical loading into surface waters and preventing acutely lethal and sublethal effects to aquatic organisms. In this study, we aimed to determine the effectiveness and longevity of bioretention soil media (BSM) at various infiltration depths, including the shallower depth currently required by the Washington Department of Ecology (18”). Experimental columns containing three different BSM depths were dosed with roadway runoff at an accelerated rate to simulate nine water years in approximately 30 calendar months. The chemical and biological effectiveness of the columns in treating runoff was assessed by analyzing influent/effluent chemistry and characterizing the health of juvenile coho salmon (Oncorhynchus kisutch). Bioretention treatment efficiently removed copper, zinc, total PAHs, and total suspended solids (> 70% removal). Influent stormwater runoff was acutely lethal to juvenile coho salmon (88, 90, 100, and 56.3% mortality in four exposures across the nine accelerated years). However, bioretention treatment was protective of coho, altogether preventing mortality for all treatment depths in three exposures and all but one depth in the last exposure, likely due to overflow when influent flow exceeded the ponding capacity of some of the columns. This study is ongoing and will continue to assess bioretention effectiveness through 10 accelerated years. Future research should consider the ability of bioretention systems to remove MPs and associated pollutants in runoff and explore the fate of MP-contaminant complexes in bioretention systems. Although contaminants themselves, MPs can also act as vectors of other contaminants of concern in aquatic ecosystems, including antibiotic resistance genes (ARGs). Contaminants co-occurring in runoff (e.g., heavy metals) can stimulate the selection or amplification of these ARGs. If left untreated, runoff carrying ARGs to surface waters could increase resistance in environmental bacteria and risks to human health. 

Abstract. Lagrangian particle tracking schemes allow a wide range of flow and transport processes to be simulated accurately, but a major challenge is numerically implementing the inter-particle interactions in an efficient manner. This article develops a multi-dimensional, parallelized domain decomposition (DDC) strategy for mass-transfer particle tracking (MTPT) methods in which particles exchange mass dynamically. We show that this can be efficiently parallelized by employing large numbers of CPU cores to accelerate run times. In order to validate the approach and our theoretical predictions we focus our efforts on a well-known benchmark problem with pure diffusion, where analytical solutions in any number of dimensions are well established. In this work, we investigate different procedures for “tiling” the domain in two and three dimensions (2-D and 3-D), as this type of formal DDC construction is currently limited to 1-D. An optimal tiling is prescribed based on physical problem parameters and the number of available CPU cores, as each tiling provides distinct results in both accuracy and run time. We further extend the most efficient technique to 3-D for comparison, leading to an analytical discussion of the effect of dimensionality on strategies for implementing DDC schemes. Increasing computational resources (cores) within the DDC method produces a trade-off between inter-node communication and on-node work.For an optimally subdivided diffusion problem, the 2-D parallelized algorithm achieves nearly perfect linear speedup in comparison with the serial run-up to around 2700 cores, reducing a 5 h simulation to 8 s, while the 3-D algorithm maintains appreciable speedup up to 1700 cores.