The 20th century brought new, more complicated forms of water pollution. Agricultural expansion led to the introduction of synthetic fertilizers, herbicides, pesticides, and concentrated animal wastes from large-scale livestock operations. All of these pollutants eventually wash into rivers, streams, and lakes, contaminating drinking water supplies and endangering aquatic life. Manufacturing plants have released persistent toxins such as PCBs, dioxins, and mercury into rivers across the country, and hospitals discharge large amounts of hazardous waste. Many of the most polluted rivers and lakes have been cleaned up since the 1970s, but a 1999 E.EA. water quality index reported that 21 percent of America’s 2,111 watersheds had serious problems, including pollution and loss of wetlands. Advisories were issued on fish consumption and water use across America.
Pollution also damages marine ecosystems and the people who depend on them for food, employment, and recreation. Contaminants including raw sewage and oil spread into the ocean through storm drain overflows. Although most dramatic when an oil well or cargo ship leaks oil as a result of an accident - like the catastrophic 2010 BP Deepwater Horizon explosion in the Gulf of Mexico -- most oil enters the ocean incrementally from municipal and industrial runoff, the cleaning of ships' bilges or tanks, and other routine occurrences. Because oil floats, it accumulates in large slicks on the water surface and on beaches. Although some of the most volatile components evaporate, the remaining oil is still toxic and can suffocate plants, birds, and animals.
Plastic is a serious danger in the ocean because it takes hundreds of years to decompose in water. Plastic bottles, bags, and nets are pushed by wind and ocean currents into massive floating waste clumps that kill animals like jellyfish, sea turtles, and seals who become entangled. The two largest of these clumps are known as the Western and Eastern Pacific Gyres -- together they are known as the Great Pacific Garbage Patch. Floating in a remote area of the Pacific Ocean, each is bigger than Texas and contains an estimated 3.5 million tons of trash. Since plastic particles can outnumber zooplankton by six to one, birds and fish mistake the smaller pieces for food. An international treaty prohibits disposal of plastic wastes in the ocean, but the treaty is very difficult to enforce, and the majority of plastics in the ocean are washed there from land. (The New York Times ‘Smarter by Sunday – 52 Weekends of Essential Knowledge for the Curious Mind’)
