By Susan Daker Of DOW JONES NEWSWIRES HOUSTON (Dow Jones)--BP PLC (BP, BP.LN) was testing the blowout preventer on its relief well Tuesday, as the British oil company moves closer to finally killing its once-gushing well in the U.S. Gulf of Mexico, the federal spill response commander said. In the past two days, the company has stayed on schedule to start the "static kill" of the broken well on Aug. 2, retired Coast Guard Adm. Thad Allen said. In that procedure, the company will pump mud on top of the well. About a week after that the company will move forward with the "bottom kill," which is done via the relief well. The blowout preventer is an important piece of the equipment that is designed to prevent uncontrolled gushers. In the case of the Deepwater Horizon rig, the blowout preventer failed and played a role in causing the disaster. The rig exploded in April leading to one of the worst oil spills in U.S. history. The two procedures should work in conjunction to improve the operation's chance at succeeding in permanently sealing the well, which gushed between 3 million and 5.2 million barrels of oil into the gulf, Allen said in a teleconference. However, even though the well is no longer spewing oil thanks to a temporary cap put in place earlier this month, it is still uncertain how much damage has been done to the Gulf Coast's environment, said Jane Lubchenco, chief of the National Oceanic and Atmospheric Administration. The good news is that surface oil continues to break up into smaller parts, Lubchenco said on the call. "The sheer volume of oil that is out there will have to mean there will be significant impacts to the environment," Lubchenco said. "What we have yet to determine is the full impact and reach that the oil will have on not just the shoreline, not just the wildlife, but beneath the surface. We have a very aggressive research team to determine that." -By Susan Daker, Dow Jones Newswires; 713-547-9208; susan.daker@dowjones.com (END) Dow Jones Newswires July 27, 2010 15:10 ET (19:10 GMT)