So far, 715,000 gallons of Corexit 9500 dispersant has been applied since the April 20 explosion of the Deepwater Horizon. (Patrick Semansky)

Oil washes ashore on 50 miles of Louisiana shoreline as tensions mount over how to treat the spill in the Gulf of Mexico.

On Thursday, the U.S. Environmental Protection Agency gave the London-based company 72 hours to replace the dispersant Corexit 9500 or to describe in detail why other dispersants fail to meet environmental standards.

The agency on Saturday released a 12-page document from BP, representing only a portion of the company's full response. Along with several dispersant manufacturers, BP claimed that releasing its full evaluation of alternatives would violate its legal right to keep confidential business information private.

But in a strongly worded retort, the EPA said that it was "evaluating all legal options" to force BP to release the remaining information "so Americans can get a full picture of the potential environmental impact of these alternative dispersants."

So far, 715,000 gallons of dispersant has been applied since the April 20 explosion of the Deepwater Horizon rig, mostly on the spill's surface. The chemical has also been released near the leaking pipe on the seafloor.

Government officials have justified both uses, saying that if oil reaches the shore, it would do more environmental harm than if it were dispersed off the coast.

Dispersants break oil into droplets that decompose more quickly. But scientists worry that extensive use of the chemicals in the BP spill is increasing marine life's exposure to the toxins in oil.

"While the dispersant BP has been using is on the agency's approved list, BP is using this dispersant in unprecedented volumes and, last week, began using it underwater at the source of the leak - a procedure that has never been tried before," the EPA noted last week, acknowledging that "much is unknown about the underwater use of dispersants."

In the company's May 20 letter to the EPA and the Coast Guard, responding to the EPA's directive, BP operations chief Doug Suttles wrote that only five products on the EPA's approved list meet the agency's toxicity criteria. And only one, besides Corexit, is available in sufficient quantities in the next 10 to 14 days, it said.

But that alternative product, Sea Brat #4, according to BP, contains a chemical that could degrade into an endocrine disruptor, a substance that creates hormonal changes in living creatures, and could persist in the environment for years.

As the tensions over how to treat the spill escalated, reddish-brown washes of oil, 2 inches thick in places, soiled Louisiana beaches. Hundreds of workers scooped up gooey piles of sand and stuffed them into plastic bags.

"It is worse today than on the past two days," said Darren Smith, 43, sweating from his work raking sand at a wildlife refuge on Elmer's Island. "There's definitely more oil, and it's just going to keep coming."

No booms protected the Elmer's Island beach because the National Guard had focused on building dams to divert oil from the wetlands behind the beach.

A few miles away at Port Fourchon, plastic barriers that looked like pompoms were strung together along the beach but did a poor job of keeping out the oil. More than 50 miles of Louisiana shoreline has been contaminated so far.