By Matt Davis

NEW ORLEANS, Louisiana, Aug 2, 2010 (AFP) - BP geared up Monday for its long-awaited "static kill," hoping to plug the Gulf of Mexico oil well and take a major step towards ending the region's worst ever environmental disaster.

Heavy drilling fluids, known in the trade as "mud," are to be pumped down into the well on Tuesday morning to plug the giant gusher that has threatened the Gulf's fishing, tourism and oil industries with financial ruin.

Engineers performed a dry-run of "injectivity tests" on Monday, but BP then said it had to delay the last-minute tests before its kill shot due to a hydraulic leak in the cap sealing the well.

The leak was not expected to significantly delay the planned Tuesday start of the kill operation.

"It is anticipated that the injectivity test and possibly the static kill will take place Tuesday," BP said in a statement.

Even as authorities aimed to shut down the Macondo well once and for all, they gave a more precise picture Monday of how much crude it spewed, saying an estimated 4.9 million barrels gushed from the ruptured well in the 87 days from the beginning of the disaster until the leak was finally capped on July 15.

Some 800,000 barrels were captured during containment operations, BP and the US government said in a statement. But the 4.1 million uncontained barrels now estimated to have flowed into the water make the spill the biggest accidental oil disaster in the history of the petroleum industry.

If the integrity of the well is intact, it should only be a matter of hours before it becomes evident that the "mud" is successfully holding down the oil.

But US spill response chief Thad Allen cautioned that the operation could take several days if the casing of the pipe has a leak through to the outer well bore.

"A decision on whether or not to put cement in after the mud will be completely dependent on the assessment of the integrity of the casing and the well bore," said Allen.

If well integrity has been compromised and the decision is taken to wait for the relief well that could mean a delay of more than a week before cementing can start.

BP says the relief well will not intercept the stricken well until some time between August 11 and 15, assuming no additional weather or procedural delays.

The Macondo well has gushed noxious crude into the sea since the Deepwater Horizon rig sank in April, devastating fragile habitats and bringing misery to many residents along the US Gulf Coast.

While locals are eager to see the well plugged for good, there are fears that a successful kill operation will prompt a mass exodus of officials brought into the region to respond to the crisis.

Crews have already begun collecting some of the millions of feet of protective boom after skimming vessels said they were having difficulty finding spilled crude on the sea surface anymore.

BP, which leased the Deepwater Horizon rig that exploded April 20, killing 11 workers and sparking the spill, has sought to reassure residents it will remain engaged and work to restore the area.

Many fishermen whose grounds were closed in the wake of the spill due to food safety concerns have found work assisting the clean-up effort but face an uncertain future.

They could soon lose their jobs again as there is less oil to mop up and there are no guarantees they will be able to return to fish soon in Gulf waters that could be contaminated for months or even years to come.

Concerns about chemical dispersants sprayed into the Gulf to help the oil dissipate deepened on Saturday when the US Congress released documents suggesting their use could have been more widespread than feared.

"BP carpet-bombed the ocean with these chemicals, and the Coast Guard allowed them to do it," said Democratic Representative Edward Markey, chairman of the House energy and environment subcommittee.

BP and the US response team have said more than 1.8 million gallons of dispersants have been used to break up the oil, but Markey said studies show the amount could be far higher.

But a study ordered by the US Environmental Protection Agency showed Monday that dispersants mixed with crude are no more toxic to marine life than oil alone and that the type of dispersant used in the Gulf was no worse than the alternatives.