Study of Plastic Accumulation in the Sediment of Three Southern California Rivers


Marcus Eriksen
Algalita Marine Research Foundation
148 Marina Dr.
Long Beach, Ca
562-598-4889


Introduction

The Algalita Marine Research Foundation has documented plastic floating in the North Pacific Ocean. Yet few studies look at the plastic marine debris that sinks. 60 billion pounds of resin was produced in the United States in 1992 (EPA, 1992), of which only 46.3% (polypropylene, high-density and low-density polyethylene) floats in either fresh water or seawater (Appendix A). It is difficult to assess how much of this plastic enters the marine environment, but studies of marine debris show that plastic consistently accounts for 60% - 80% of litter collected (Gregory and Ryan, 1997). If more than half of plastic made in the United States sinks in water, then how much plastic trash is embedded in the sediment of our lakes, rivers, wetlands, estuaries and near shore environment? There are a few studies that have documented plastic debris that sinks. Negatively buoyant plastics have been found in the marine environment worldwide, and represent the majority of marine debris on the seafloor (Galgani, 1996, 2000; Hess, 1999; Kanehiro, 1995; Stefatos, 1999), yet few studies have investigated the type of plastic found on the seafloor or in sediment. Srinivasa Reddy and others (2004) collected sediment near shipbreaking yards in Alang-Sosiya, India and found 81.43 + 4.03 mg of plastic per kg of sediment. Thompson and others (2004) examined 30 250ml samples of subtidal sediment along the coast of the United Kingdom in search of microscopic plastics, identifying nine types of synthetic polymers.

A recent study of pelagic plastics in the eastern region of the North Pacific Ocean found the density of small plastics to be 5114 g/km2 (Moore, 2001). Moore estimates that 5% of plastic produced over the last 50 years has migrated into the marine environment, resulting in approximately 250 million tons of plastic in the world’s oceans, of that estimate, 100 million tons originate from the United States (Moore, 2006). If 46.3% of this is positively buoyant, then 53.7%, or 134.25 million metric tons of plastic, could reside on the sea floor or trapped in the sediment of rivers, lakes and estuaries.

The purpose of this pilot study is to examine a method used to quantify plastic/polycarbonate in marine sediment in order to advise a more exhaustive study.

Materials and Methods
A total of 60 liters (.060 m3) of sediment was collected from three Southern California rivers: 15 liters from the Los Angeles River, 15 liters from the San Gabriel River, and 30 liters from the Ballona Creek. Samples were collected during October/November 2006, before the “First Flush” rain event. Using a 250cc metal scoop, sediment was dug from the river bottom up to 10 centimeters deep. Most of the length of these three rivers is lined with concrete walls and bottoms. Because sediment tends to be transported by river currents off of the concrete bottom, collection sites were chosen where the concrete bottom terminates and the natural bottom begins.

The sediment was passed through two sieves, with the collected debris representing objects greater than 5mm, and objects between 1mm and 5mm. All natural debris was removed from the samples, and the remaining debris was categorized into glass, plastic, metal, rubber, electronic components, road markings, and other. Plastic debris was divided into two subcategories: polycarbonates, other plastics (Table 1).

Results and Interpretation
The total mass of plastic was 28.44 grams, of which 7.5 grams was polycarbonate. The total mass of plastic represents a density of 474 g/m3 (Table 2). The quantity of polycarbonate was determined by visual inspection (pieces of car tail lights, broken CD’s or DVD’s, bicycle reflectors, particles of cigarette lighters, polycarbonate pre-production pellets).





In order to extrapolate the total mass of plastic in the sediment of the three rivers, the volume of sediment in the three rivers must be estimated. Using satellite images of the three rivers, the width of the river at the point where the concrete bottom ends, and the length to the Pacific Ocean was estimated. The total area of the three rivers was estimated to be .987 km2. In this study sediment was sampled up to 10 centimeters deep. This represents a total volume of 98,700 m3. With a plastic density of 474.000 g/m3, the extrapolated mass of plastic in the sediment of the three rivers is 46,783.800kg. The extrapolated mass of polycarbonate is 12,452.584kg (Table 3).

This estimate of the mass of plastic in the sediment of the three rivers assumes that the density of plastic found where the concrete bottom of the river ends is consistent with the density of plastic throughout the natural river bottom until the ocean is reached. This assumption is believed to be conservative because of the speed of the river current and the density of negatively buoyant plastics. It is likely that plastic, unlike pieces of metal and glass, will be transported downstream, perhaps into the ocean, before it is deposited because of its light density (1.05-1.4 g/cm3). The sample sites in the three rivers were selected next to the terminus of the concrete bottom where the current is the fastest before it reaches the ocean, therefore it is possible that less plastic has been deposited at this site than other locations in the river. Future studies should sample areas throughout the river and into the near shore environment.



Another way to estimate the quantity of plastic in the marine environment is to compare the estimated mass of plastic in the sediment of the three rivers to the human population in the watershed that is drained by those rivers. A per capita plastic footprint can be used to estimate the mass of plastic in other coastal marine environments based on population size.

In order to extrapolate the mass of plastic in the local marine environment for each resident within the watershed, the area of the three watersheds and the population density must be established. The land area in each watershed of the San Gabriel River, Los Angeles River, Ballona Creek, is 1658 km2, 2135 km2, and 337 km2, respectively (County of Los Angeles Public Works). The total area is 4130 km2. The population density of Los Angeles County is 905 people per km2 (U.S. 2000 Census).

Using the extrapolated mass of plastic from Table 3, the estimated mass of plastic in sediment per resident within the three watersheds studied is 12.517 g/person, of this estimate 3.332 g/person is polycarbonate (Table 4).



Conclusions and Recommendations
The findings in this pilot study, if accurate, indicate a significant volume of plastic in the sediment of urban rivers. If the estimated per/capita plastic footprint of 12.517 g/person were extrapolated to 300 million Americans, then the estimated mass of plastic in sediment of U.S. rivers would be 3,755,100 kilograms. The mass of polycarbonate would be 999,600 kilograms. This study only estimated plastic in the sediment within the area of the rivers themselves, not the area of the near shore environment. There is a need to gather a more accurate estimation of plastic in the sediment of rivers, lakes, estuaries and near shore environment, and to investigate the depositional patterns of low-density, negatively buoyant plastics, relative to fragments of glass, metal and other sediment.

Bisphenol A is a monomer with estrogenic activity used in the production of epoxy resins, dental sealants, and polycarbonate plastic. Bisphenol A can leach out of polycarbonate products when exposed to heat. Howdeshell and others (2003) have shown that Bisphenol A can leach out of polycarbonate animal cages at room temperature. It is unknown whether Bisphenol A can leach out of plastic debris in the marine environment, where water temperatures are usually cooler than ambient air temperatures. Degradation of polycarbonate does happen during transport in the marine environment due to tumbling downstream, which may release Bisphenol A, but further study is needed. Although Bisphenol A degrades readily in microbe rich environments (Dorn, et al. 1987) it’s presence as fragments of polycarbonate may create a point source for slow leaching.

The design of this pilot study should be modified to sample areas throughout the river basin. Samples were collected where the concrete bottom ends in each river. The river moves faster at that location than it does anywhere else before it reaches the ocean. A winnowing effect may have caused low density plastics, though negatively buoyant, to be transport out of the river to a point where the current diminishes. If these plastics follow the same depositional characteristics as other sediments, then an alluvial plume might be expected at the mouth of the river. Sampling sites should then continue into the near shore environment where the river current disappears.



Appendix A



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