Sediment samples taken from Reynolds Channel and nearby waters in early 2013 to gauge the impact superstorm Sandy had on the environment yielded a radioactive surprise -- the element cesium-137.

Experts say the cesium deposits were likely from decades ago and that the powerful ebb tide Sandy produced kicked up the element and shifted it around. The original source of the cesium is unclear.

"It looks like there was a lot of motion . . . out of the rivers and into the estuary probably as the tide went out," said Cecilia McHugh, a professor at Queens College School of Earth and Environmental Sciences who took the samples.

While the levels are higher than would be expected, experts said the health risks are low. Human exposure, if possible, could come from eating fish contaminated with cesium-137, but that concentration would be minimal.

"I think it's highly unlikely that levels would be sufficiently elevated in fish to be a public health concern," said Nicholas S. Fisher, director of the Consortium for Inter-Disciplinary Environmental Research at Stony Brook University's School of Marine and Atmospheric Sciences.

Fisher said sampling fish, which would have lower contaminant levels than sediment, is a better way to determine if there is a danger.

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McHugh was part of a team from Queens College, Adelphi University and the University of Texas at Austin that examined offshore and inshore areas to see what impact the October 2012 storm may have had. She focused on metals and wastewater indicators in waters, such as Reynolds Channel, near Bay Park Sewage Treatment Plant, which went offline during the storm and sent raw and partially treated sewage into area waters.

McHugh detected increased levels of cesium-137 in the same locations where the concentration of lead, copper and zinc were also higher than what would be expected.

"Was it part of the marine environment already and resuspended by the storm or did it come from the plant?" she said. "This is not an easy black and white situation."

Cesium-137 is often found in nuclear weapons and reactors, but also is used in cancer treatments. It is most commonly associated with the fallout from atomic weapons testing dating from the 1950s. It was banned by treaty in 1963.

McHugh said the highest concentrations of cesium-137 were found in Reynolds Channel. Where one would normally see levels at 20 to 30 pico Curies per kilogram, McHugh detected the element at up to 141 pico Curies per kilogram.

"I think it's high but not terribly high," she said. "We do find that concentration elsewhere."

Professor J. Kirk Cochran, from the School of Marine and Atmospheric Sciences, said the levels are similar to what is found below the surface in Jamaica Bay. "I think the simple interpretation is that storms like Sandy can resuspend and redistribute lots of sediment," Cochran said.

The cesium-137 levels found are consistent with atmospheric weapons testing and it is not uncommon to find hot spots ranging from 1,000 to 10,000 pico Curies per kilogram, said Maureen Conley, a spokeswoman for the Nuclear Regulatory Commission.

"There is no indication to us that these cesium levels would come from anything else," she said.

The U.S. Environmental Protection Agency also collected sediment samples in the same area as McHugh this past December as part of a program to measure the impact of sewage plant failures in New York and New Jersey.

EPA spokesman John Martin said the agency was not looking for radioactive materials. "Sampling for such parameters is not common, usually only done if there's a reason to believe they're present, such as after a reported release," he said in an email.

It is routine for people to be exposed to small amounts of cesium-137 in soil and water due to atmospheric fallout, the agency said.

McHugh said she hopes to "fingerprint" the samples sometime this summer in an attempt to track down the source. She has also applied for a Department of Interior grant to test sediments annually, as well as grab core samples, perhaps reaching down 1 meter.

It's not easy to trace sources, as contaminants adhere to small particles, which can easily shift with the tides, said James Hunt, professor emeritus of environmental engineering at the University of California, Berkeley.

"It's hard to find them," he said. "When you find them, they're everywhere in different pockets. It's hard to find the source."

Nassau County, which operates Bay Park, did a series of water quality and sediment tests after the storm, spokesman Brian Nevin said.

He said the effects were found to be short-lived and caused by stormwater runoff, resuspension of sediments and discharge of raw and partially-treated sewage. "Based upon the data, the bay and tributary's water quality returned to pre-storm conditions by mid to late December," he said in an email.

The county has not tested for cesium-137 to see if it is moving through the plant because there are not documented "radioactivity issues with sludge, screenings or grit disposal," he said.

Hunt said the treatment plant was an unlikely source. "You wouldn't expect cesium-137 to be in a sewage treatment plant unless there was some kind of dumping upstream," he said.

A fish advisory for the area issued by the state Department of Health is already in effect, warning people to avoid crab and lobster paste and to limit intake of weakfish, bluefish, striped bass and American eel because of polychlorinated biphenyls, or PCBs.

The department does not have any current advisories related to cesium-137, spokesman Bill Schwarz said.