By Rob Barry, Dionne Searcey and John Carreyrou

Hurricane Sandy’s environmental impact is still being assessed, but the worries for residents of New York and New Jersey are crystallized by one fact: Of the two states’ 198 Superfund toxic-waste sites, 45 are within a half-mile of coastal areas vulnerable to storm surge.

The Environmental Protection Agency, which oversees cleanup of those sites, was unable to say how many of them flooded on the night of Oct. 29. But the agency said its initial appraisals show that several “were impacted by the storm,” including a site contaminated by lead near Sayreville, N.J., and the Gowanus Canal and Newtown Creek sites in New York City.

The 45 Superfund sites vulnerable to coastal flooding were identified by The Wall Street Journal using data from the EPA and the U.S. Army Corps of Engineers. Many of the sites are concentrated in northern New Jersey in a blighted industrial zone west of Manhattan, 11 flank the Delaware River and a half-dozen are scattered across New York’s Long Island.

Superfund sites are generally considered the most hazardous toxic-waste sites in the country. Congress established the program in 1980 following the Love Canal environmental disaster, which ravaged a community of several hundred families that had settled over a former chemical dump in Niagara Falls, N.Y.

Once the EPA has given a site the Superfund designation, the agency has the power to force the sites’ polluters to pay for its cleanup costs. Today, there are 1,313 active Superfund sites nationwide on the EPA’s so-called National Priorities List. New Jersey has the most, at 111. New York is fourth, at 87.

The EPA said it tested water samples from Brooklyn’s Gowanus Canal and nearby flooded buildings, but found only “low levels” of potentially cancer-causing pollutants, which it said may be “related to spilled fuel and runoff from asphalt.” New York state officials say they think the floodwaters probably traveled over the Gowanus and Brooklyn’s other Superfund site, Newtown Creek, without disturbing the pollutants that line the bottoms of both waterways.

But Thomas Burke, a professor and associate dean at Johns Hopkins School of Public Health, said the Gowanus and Newtown Creek — whose cleanups haven’t begun in earnest yet — are more vulnerable to flooding risks than sites in more advanced stages of remediation, where caps and liners have already been placed over bottom-lying toxic material.

“There really has to be a careful evaluation of whether there has been any disturbing of the waste,” Mr. Burke said.

New Jersey officials downplayed any problems. “There was no major flooding in North Jersey. Superfund sites were not inundated by tidal surges,” said Larry Ragonese, a spokesman for the state environmental agency.

In Sandy’s wake, one New York neighborhood group is taking matters into its own hands. Kate Zidar, a member of the Newtown Creek Alliance, said her organization hired a consultant to do some testing after the EPA declined to take samples of the floodwater inside buildings close to the creek. “There’s an information gap that we need to fill,” she said.

In the Gowanus neighborhood of Brooklyn, which lies between the gentrified enclaves of Carroll Gardens and Park Slope, residents and business owners said water overran several blocks.

In New Jersey, one site may have been affected by the storm: the Raritan Bay Slag Superfund Site in Sayreville. A seawall and jetty along the bay’s southern shore were contaminated with lead slag, a byproduct of metal smelting, which has tainted the surrounding area with lead and other heavy metals. On a flyover to survey damage, a U.S. Coast Guard member spotted an overturned 10,000-gallon fuel tank near the sea wall, but it didn’t appear to harm it.

The EPA said it is collecting samples from the site “to determine the extent of flooding damage and its impacts on lead contamination.”