The sediment has been polluted over the years with industrial chemicals and heavy metals, said Subra, who tested the sediment for the Southern Mutual Help Association, a nonprofit organization in New Iberia, Louisiana.

　　"These water bodies have received industrial wastes for decades," she said. "This material has toxic chemicals, metals, and organic petrochemicals(石化产品)."

　　Matters have only been made worse by multiple oil spills caused by Katrina and Rita. According to the U.S. Coast Guard, 11 oil spills have occurred in southern Louisiana, totaling 7.4 million gallons (28 million liters) of oil, most of which has been contained.

　　Bacteria levels are also especially high in the Plaquemines sludge, said Rodney Mallett, spokesperson for the Louisiana Department of Environmental Quality.

　　"The sewage treatment plants were underwater," he explained. "Between the animal waste and the human waste, you've got a lot of bacteria."