It requires that any shipment going directly from one port in the United States to another be carried on vessels built in the United States and operated by an American crew.
While there have been waivers granted in the past, they are rare, according to a government official familiar with maritime regulations who spoke on condition of anonymity because she was not involved in the matter involving the salt shipment.
There is a provision that allows the law to be waived, but that was designed to deal with matters of national security, according to the official.
As with other maritime laws, this one involves a tangle of federal agencies.
Technically, the law says the federal Department of Transportation can grant a waiver, the official said. But there are other agencies that play a role in the decision, a process that has grown more complicated since Sept. 11, 2001. For instance, the Coast Guard, which was part of the Transportation Department, is now part of Homeland Security. Another central authority controlling activity at American ports is the United States Maritime Administration.
"In practice, what is happening today is that it is a joint decision with the D.O.T. and Homeland Security and, when appropriate, the Department of Defense and Energy can be brought in," the official said.
Limited waivers were granted after Hurricane Katrina and Hurricane Sandy to expedite the movement of fuel and oil to areas affected by the storms.
But, the official said, even after Hurricane Sandy, there were so many hurdles to obtaining a waiver for cargo that state officials never even applied.
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For New Jersey communities, the intricacies of maritime law are of little interest.
They just know their salt sheds are barren and the situation is unlikely to improve soon.
Even before the most recent storms, 373,000 tons of salt had been used on state roads as of Feb. 11, according to state officials. Over the course of all of last year, they used only 258,000 tons.
Mayor Steven Fulop of Jersey City, speaking to reporters over the weekend, said side streets were frozen over because of the shortage.
"We were told that we're getting today a total of 500 tons of salt, which will get us through hopefully one storm," he said. "We use about 800 tons per storm, so it's not adequate."