From 1947 to 1971, the now out-of-business Montrose Chemical Corp. "discharged millions of pounds of DDT into Los Angeles County sewers, which empty two miles offshore in an area called the Palos Verdes Shelf." Over time, the site became the largest DDT deposit in the world, covering about 17 square miles of ocean floor, according to Environmental Health News. (The US banned the pesticide DDT in 1972 because it was majorly contanimating the food web, including fish, birds, and marine mammals--"until 2007 bald eagles on Santa Catalina Island ... had been unable to reproduce because DDT thinned their eggs." It can cause cancer and neurological effects; humans are advised not to eat potentially-contaminated animals.) The size of the deposit has been estimated to be as big as 110 tons, but very weirdly, the latest EPA tests now show it's down to 14 tons. No one can explain the sudden drop: "Scientists have no explanation for how almost 90 percent might have vanished in a mere five-year period -- between tests in 2004 and 2009 -- after decades of a slow, gradual decline." (The data was just made public.) Scientists sound completely baffled.
The shelf was declared a Superfund site back in 1996 (and because it's in 200-foot eep waters, "it is one of the most unusual and challenging sites on the nation's list of the worst dumpsites"). The EPA has a plan to cap part of the shelf with clean sand, but the latest drop-off brings DDT levels down to what they'd hoped to achieve anyway; the idea hasn't been abandoned yet, though.
So what's up with the sudden decline? Could be a highly-improbable statistical fluke in which testing just missed the worst pockets, or there could be something going on with the microbes that eat the pesticide (possibly more carbon in the water, which would fuel their multiplication). Then there's this: "Another possibility -- one that alarms some experts -- is that the chemicals diffused into the water so they are no longer on the ocean floor." That would mean more wildlife contamination, although there doesn't seem to be any evidence for more DDT in the water (fish are still contaminated at about the same levels). To help figure it out, the EPA is planning more testing in the fall.
· Going, going, gone? Vanishing DDT is big surprise at ocean deposit off Los Angeles [EHN]