9/11 ash, and more found in river muck. (Earth Science).

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.

Those layers also included small rods and numerous bundles of fibers that ranged between 40 and 200 micrometers in length, she notes. The minuscule particles, chemically rich in calcium and silicon, probably came from construction materials such as drywall and fiberglass ceiling tiles.

Oktay and her colleagues also found that the sediments contain small but measurable quantities of iodine-131, a human-made radioactive isotope with a half-life of about 8 days. Total iodine concentrations were actually lower in the debris-filled layers, which means the source of the element probably isn't related to the attacks. Also, the iodine probably didn't leak from nuclear power plants upstream because other telltale radioactive isotopes didn't turn up.

Instead, says Oktay, the iodine--which is used in various medical treatments and sometimes carried home internally by patients--probably entered the river through local sewage systems. The researchers report their findings in the Jan. 21 Eos.--S.P.