Limited dispersal of riverine litter onto nearby beaches during rainfall events

Abstract

Rivers are assumed to be one of the major conduits for plastics entering the sea, but little is known about the interchange of plastic litter between rivers and coastal waters. We measured the amount of litter stranding on the beach around the mouth of a seasonal river in Cape Town, South Africa, to infer litter dispersal distances. Prior to the mouth opening, litter was fairly uniformly distributed along the 2.4 km study beach. Once the mouth was opened for the first major winter storm, most litter stranded within 0.5 km of the river mouth. Beach litter loads peaked during rain events and decreased after the first rainfall event, presumably due to flushing of accumulated litter in the coastal wetland. However, litter loads within the wetland remained high, indicating that only a proportion of litter flushes during floods. Experimental deployments of marked litter items confirmed that most floating litter strands close to the mouth, although less buoyant items travelled farther than more buoyant items. Exchanges occurred in both directions, with marine litter found up to 1.2 km inland in the estuary. On a rising tide, more litter items moved into the estuary than into the sea, despite strong offshore winds. Our results illustrate the complex interplay of litter between rivers and the sea, and suggest that a large proportion of floating litter carried down small rivers washes ashore shortly after entering the sea. Cleaning beaches around river mouths, especially after rain events, will help to reduce leakage of plastic from land-based sources into the sea.