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SOURCES, FATES, AND EFFECTS OF SHIPBORNE GARBAGE 50 lateral movement along the beaches. Thus, items leaving the beach areas probably floated away in surface waters or sank to the sea floor following accumulation of dense debris. Beaches that were cleared completely accumulated about 50 percent of the original amount of plastic in about three months, and 61 percent of the original amount in six months. On the other hand, the persistence of debris on beaches of Padre Island, off the southeastern coast of Texas, appears to be on the order of days, judging by daily collections and observations of marked debris (Miller, 1993). The area is subject to intense tidal movements. Thus, the time frames of debris retention on beaches, before removal to the adjacent waters or sediments, ranges from days to months. Debris also collects in other places, such as in "backshore" areas behind sand dunes on undeveloped barrier beaches. Debris buried in such areas on Padre Island includes materials deposited well over a decade ago (Miller, 1993). Coastal sediments constitute an important long-term sink for litter, but few investigations have explored the quantitative aspects of submerged debris. To date, concern about this issue has been limited primarily to fishermen, who often catch debris in their nets. This was evident in a recent British study of the Swansea Bay area, which receives discards from commercial fishing as well as wastes from four rivers and 99 stormwater outfalls (Williams et al., 1993). Macerated industrial discharges and screened sewage also are introduced into the bay. Litter was collected from 30 static gillnets, which varied in length from 57 to 732 meters (62 to 801 yards), with a mesh size of 15 to 20 centimeters (6 to 8 inches). Plastics accounted for 66 percent of the articles. Twenty-four percent of the 3,670 litter items were found to be of sewage origin, a finding attributed to ineffective screening and to inputs from stormwater outfalls. Sources of the remainder of the debris could not be identified. Implications for Special Areas The fate of garbage discharged in or near special areas is of particular concern, because these areas need extra protection. The behavior of waste materials may vary significantly among these areas; factors that warrant study include the proximity to shore of discharges, winds and currents, water column stratification, biological production, oxygen levels at depth, and whether a seasonal pycnocline develops. Some areas, such as parts of the Gulf of Mexico, have low oxygen levels at great depth, a situation that generally reduces the rate of decomposition of garbage on the bottom (Swanson et al., 1994). Key characteristics of each special area are summarized in Appendix E. Regardless of the features of a special area, the zero-discharge rules do not protect these areas fully, because marine debris (including vessel garbage) can be transported over long distances. For example, debris originating from the Hudson/ Raritan Estuary has been stranded nearly 100 kilometers (more than 50 miles)