Louisiana has closed a 12-mile section of Gulf coastal waters and beaches after Hurricane Isaac washed up large areas of oil and tar balls at the location of one of the worst inundations of BP oil during the Deepwater Horizon disaster of 2010.

Robert Barham, secretary of the Louisiana Department of Wildlife and Fisheries, said crews discovered large sections of viscous oil and tar balls floating from the beach to one mile offshore between Elmer’s Island Wildlife Refuge, just west of Grand Isle, to Pass Fourchon.

“It’s a very large mass that is viscous but hasn’t coalesced into tar mats yet,” Barham said. “But the Elmer’s Island beaches are littered with tar balls of every size, from eraser size to the size of baseballs.”

Samples will be analyzed by the LDWF and the state Department of Environmental Quality to determine if it originated from the Deepwater Horizon, Barham said.

BP quickly put out a press release pointing out “it is premature to make any claims about possible oiling there – whether it is from the Deepwater Horizon accident or any other source. We are awaiting test results on residual oil reported in an area west of Grand Isle, Louisiana. As state officials have made clear, it is important to fingerprint the residual oil to determine its origin. If any of it is connected to the Deepwater Horizon accident, BP stands ready to remove it under the direction of the Coast Guard’s federal on-scene coordinator.”

Of course, the source of the pollution could be one of the other 4,000 rigs planted off the Louisiana coast over the decades–many of which have been the cause of thousands of other leaks–small-, large- and medium-sized.

Indeed, the U.S. Coast Guard this week was investigating as many as 90 reports of oil and chemical releases associated with the storm in Louisiana’s coastal oil corridor.

Louisiana officials and environmentalists were not surprised oil (whether BP’s or someone else’s) was spilled or old oil resurfaced, because this has been happening for decades.

When spilled oil approaches the coast it mixes with sand, becomes too heavy to float, and sinks to the silty floor near the surface line. It then often is quickly covered by more sand and sediment carried by the water from the Mississippi River. But heavy weather regularly dredges it up from the soft bottom, and waves carry it to the beach and even push it inside the marsh. That’s why surf anglers and beach bathers in Louisiana long ago grew used to stepping on tar balls.

The persistence of the oil has kept clean-up crews working along the coast since the April 2010 spill.

Unfortunately, these tar balls and mats are more than a messy inconvenience. While the most toxic parts of raw oil–aromatic hydrocarbons–quickly dissipate into the atmosphere, the tar mats, tar balls and viscous sludge that reappear after storms remain a threat to fish, wildlife and humans, state authorities said. They can contain polycyclic aromatic hydrocarbons (PAHs), components that can persist in the environment for 50 years or more, disrupting endocrine systems in both humans and wildlife.

While constant testing since Deepwater Horizon has shown no amounts of PAHs in Louisiana coastal fish and seafood to cause concern to humans, research already has indicated even these low level amounts could be affecting important fish in the food chain.

Secretary Barham cautioned the current discovery along the coast may not be the last. Crews are still inspecting other known Deepwater Horizon hot spots. “Our people are still out conducting a thorough examination of the entire coast to check for storm impacts, including coast line erosion and oil, and we probably won’t be finished for several days,” he said. “Yes, we expected this could happen, but it’s still very troubling.”

Louisiana charter operators and anglers know that while offshore drilling accidents may create only temporary impacts for investors, they pose decades of hardships for the environment and industries that rely on it.