Transport of Microparticles and Microplastics in San Francisco Bay and Adjacent National Marine Sanctuaries

Microplastic pollution is a growing concern in many parts of the world, but the pathways between terrestrial sources and particle fates spanning the ocean patches and burial in the benthos are poorly understood. In an estuarine setting these particles, characterized by a wide range of settling and rising rates, are transported by three-dimensional circulation, subjected to energetic mixing, and face distinct and divergent fates. We combined field and laboratory data with a multi-scale three-dimensional hydrodynamic model to simulate particles discharged into San Francisco Bay, USA, via stormwater and wastewater, to distributions in surface water and sediments at estuary and coastal scales. The rising and settling characteristics of particles were estimated based on laboratory analysis of field samples, including composition, shape, and size. Predicted export of particles to the coastal ocean was highly sensitive to particle buoyancy, and even minimal settling rates led to retention of particles within the Bay. Comparisons between predicted abundance and field data showed good agreement at large scales, for both surface water and sediment measurements. Small scale discrepancies were often indicative of localized or transient flow features, with implications for future sampling methodologies. From both the model results and field data, we conclude that microplastics originating in San Francisco Bay do, on occasion, reach the majority of the nearby National Marine Sanctuaries. However, only buoyant particles are likely to travel any notable distance beyond the Golden Gate. The buoyant particles are efficiently transported in the freshwater plume leaving the Bay, often taking them northward along the coast, or dispersed south and west by regional winds and coastal currents.