Plastic debris is a common persistent pollutant in marine and coastal environments worldwide. Questions naturally arise among those dealing with the debris as to its source. Here we quantify the contribution of a large urban area, the Los Angeles basin, to the problem via the two main rivers draining its watershed. Samples of river water were taken with a variety of nets during both rainy and dry conditions from mass emission sites above the reach of tidal influence in the Los Angeles and San Gabriel Rivers. The samples were from different strata in the flowing rivers using nets with less than 1mm mesh. Laboratory analysis of the samples divided the plastic debris into two size classes: >=1mm, <4.75mm, and >=4.75mm. A reason for the two size classes, in addition to providing a distinction between macro and micro debris, was that under California law, debris less than 5mm is not considered trash subject to regulation. The debris was first sorted into natural, plastic, and non-plastic manmade items. The plastic debris was then classified by type; either fragments, foams (mainly polystyrene), pre-production resin pellets, line, or films. Results extrapolated for 24 hours of flow from one moderate and one heavy rain day and one dry day when added together produced the following for 72 hours: Total number of plastic objects and fragments, 2,333,871,120.0 (2.3 billion), total weight of plastic objects and fragments 30, 438.52 kg (30 metric tons). In terms of the number of pieces, the majority, 71% were foams, with miscellaneous fragments at 14%, pre-production resin pellets at 10% and whole items at 1%. In terms of weight, however, whole items were the heaviest, at 37% of the total, followed by fragments at 29%, pellets at 13% and foamed polystyrene at 11%.
