Sediment cores pulled from the Hudson River near the World Trade Center site just a month after the Sept. 11, 2001, terrorist attacks contain a thin layer of metal-rich ash and pulverized debris. That's not surprising. What did surprise researchers was the discovery of radioactive iodine–a substance unrelated to the attacks–in the top few centimeters of river silt.
On Oct. 12, 2001, scientists obtained samples of river sediment from two sites within 1.5 kilometers upstream of where the World Trade Center's twin towers once stood. The top 3 cm of silt contained layers with unnaturally high concentrations of copper, strontium, and zinc from the towers, says Sarah D. Oktay, a geochemist at the University of Massachusetts in Boston.