Brief summary: from the New York Times, July 30, 2010—
Even before the recent BP oil spill disaster, the Gulf of Mexico has been America’s dumping grounds for decades dating back to the 1930′s. In part, the dumping of old weapons ordinances, yearly oil spills from nearby land seepage, and refinery operations, along with agricultural run-off, hundreds of miles away, have greatly contributed towards its steep decline.
Jobs, and money, finite resources too precious for that region to risk losing, have long dampened enthusiasm for vigilant regulatory enforcement, and environmental support by local politicians has been historically lacking.
In the case of agricultural run-off of nitrogen fertilizer, from states as far away as Iowa that grow corn, 1.5 million tons of nitrogen flow into the Gulf each year. A dead zone, 6000-7000 square miles has resulted from this non-point source of pollution.
The federal Clean Water Act specifically exempts agricultural run-off from regulatory action under its charter.
For more information: Gulf of Mexico Has Long Been Dumping Site