BP Americas chief operating officer Doug Suttles says the ‘top kill’ operation initiated on Wednesday to stanch the Gulf oil spill is “performing as expected” and could be completed within 24 hours.

But U.S. Coast Guard Rear Admiral Mary Landry, who spoke with Suttles at an early-evening media briefing, took a more reserved tone. “I do not want to express optimism until I know for sure that we’ve secured the well and that the leak has stopped,” he said.

By 5pm Houston time on Wednesday BP had already pumped over 7,000 barrels of heavy drilling mud into the damaged blowout preventer on the wellhead created by the Deepwater Horizon rig whose destruction last month unleashed the spill. Much of that mud appears to be flowing up into the Gulf.

BP’s live video feed showed that the flows from the rig’s broken riser has strikingly changed in color: until this morning crude and natural gas poured forth in a smoky black and grey plume, but now a tan material can be seen billowing forth into the Gulf’s icy depths (see image to left). “What you’ve been observing out of the top of that riser is most likely mud,” confirms Suttles.

For the top kill to stanch the spill, of course, plenty of mud must also flow down into the 13,000-foot deep well. The hope is that a column of mud, roughly twice as dense as water, will create a ‘hydrostatic head’ of sufficient force to hold down the high-pressure oil and gas pushing up. Suttles says that when and if pressure measurements confirm that the flow is on hold, BP will pour cement into the line to create a seal. And if mud alone doesn’t work BP is poised to add a ‘junk shot’, tossing rubber shreddings and other materials into the blowout preventer to gum up its exit paths.

However, as we reported last week, the junk shot carries the added risks of clogging and thus rupturing the riser. If the top kill works–and Landry is right to be cautious given BP’s series of tech failures over the last month–it won’t have come too soon. Suttles confirmed today that its spill had impacted 100 miles of coastline, 35 of which will require cleanup, including 30 acres of marshland for which ‘cleanup’ will likely fall far short of ‘clean’. Then there’s the thousands of barrels of crude oil that’s been dispersed into the Gulf of Mexico ecosystem.

Landry said yesterday that none of the chemicals previously used to disperse oil had been used today, reflecting rising concern about the toxicity of both the chemical dispersants and the oil itself. Should the top kill fail, BP has a new set of procedures and ad-hoc equipment in the works. And whether it works or not, BP will continue drilling the two relief wells that Suttles says are required to definitively close off the well.