Federal and local officials stepped up their criticism of BP PLC Thursday as the oil company's own data suggested that far more than 5,000 barrels a day of crude is leaking into the Gulf of Mexico from a well damaged by the destruction of the Deepwater Horizon rig.
Scientists, meanwhile, warned of environmental damage that could rival that caused by the 1989 Exxon Valdez oil-tanker spill in Alaska as globs of oil washed up for a second day in Louisiana's fragile coastal marshes.
"It's a very intimidating sight, the stark reality of black goop all over everything," said Ralph Portier, a professor of environmental sciences at Louisiana State University.
The oil spill could cause more of Louisiana's retreating marshland revert to open water, some scientists said.
"Louisiana has been losing wetlands for a century," said Christopher Craft, a wetlands ecologist at Indiana University in Bloomington, Ind. If a large amount of oil comes ashore and can't be quickly cleaned up, he added, "it could accelerate the rate of coastal loss."
Oil is reaching the shore despite frantic efforts to prevent its landfall: chemicals called dispersants sprayed to break up the spill; plastic piping, known as boom, laid along the shoreline; controlled burns of the slick at sea; and machines that skim the crude from the water's surface.
Doug Helton, an official with the National Oceanic and Atmospheric Administration (NOAA) who is monitoring the spill, said most of the oil is staying offshore. "But if you get 99% of it, you still have a lot of oil that could come ashore," Mr. Helton said.
Exactly how much oil is gushing from the ocean floor remained a point of contention. For weeks, BP and government officials have said they didn't know the amount, but pointed to a joint official estimate of 5,000 barrels a day.
But on Thursday, a BP spokesman said 5,000 barrels of oil a day are being diverted to a tanker through a mile-long tube the company inserted during the weekend into a shattered oil pipe called a rise. Yet more oil continues to spew into the Gulf.
Some scientists have estimated the leak could actually be 10 times the 5,000 barrel-a-day estimate. BP said the amount of oil escaping remains unclear, but is decreasing.
The federal government is trying to come up with its own estimate, said Jane Lubchenco, the NOAA's administrator. But efforts have been delayed in part because they would require more robots to collect data on the sea floor, raising concerns about an accident, she said.
Lawmakers posted on the Internet BP's videos of the leak—shot by the robots—showing a dark cloud of oil continuing to billow toward the surface.
The Obama administration on Thursday said BP isn't doing enough to keep the government and public informed about the spill, which occurred after the Deepwater Horizon rig exploded April 20, killing 11, and then sank some 50 miles offshore.
Environmental Protection Agency Administrator Lisa Jackson and Homeland Security Secretary Janet Napolitano said in a letter to the company's chief executive that BP's "efforts, to date, have fallen short in both their scope and effectiveness."
The EPA also said Thursday that it had ordered BP to find "a less toxic and more effective" chemical to break up the oil slick. The EPA ordered the British company to find an alternative to the chemical, Corexit 9500 made by Nalco Co. of Naperville, Ill., by late Thursday night and to begin using it by late Sunday night.
Of all the chemicals approved by the agency for use on oil spills, Corexit 9500 is among the most toxic to certain organisms, according to EPA tests. It also is among the least effective in breaking up the kind of oil that is prevalent in the area around the spill site, EPA tests concluded. Corexit 9500 was available in large quantities at the time of the accident, however, and was on the EPA's list of approved dispersants.
Earlier this month, BP placed an order for Sea Brat 4, another dispersant made by Alabaster Corp., of Pasadena, Texas. BP said Thursday afternoon it was studying Sea Brat 4 and other dispersants to decide which ones to choose as alternatives to Corexit.
The NOAA projects that oil will continue to wash ashore over the next few days, particularly in Louisiana's Plaquemines and Lafourche parishes, the two coastal areas that have borne the brunt of the onslaught in recent days.
The officials overseeing the spill response are sending cleanup crews to certain oiled areas before others, making judgments about which are more ecologically sensitive and which are likely to continue to be hit.
In the places most heavily affected by the oil, frustration flared at what many local leaders called an insufficient response to the spill by BP, which legally is responsible for the cleanup.
"This thing is not organized," said Billy Nungesser, president of Plaquemines Parish, where the Mississippi River empties into the Gulf and globs of oil have been found in the marshes. "They're trying," he said of BP officials, but "every day it keeps getting worse."
BP's response to the spill has been "unprecedented," said Toby Odone, a company spokesman. "The testimony to that is that very little oil has reached shore and that we have done everything in our powers to contain the oil out in the ocean and recover as much oil as possible."
The Department of Homeland Security said U.S. Coast Guard Adm. Thad Allen will continue to lead the federal government's response to the disaster after his appointment as Coast Guard commandant ends this month. Adm. Robert Papp will replace Adm. Allen as commandant.
Jared A. Favole and GautamNaik contributed to this article.