By Timothy Gardner

WASHINGTON (Reuters) - The Keystone XL oil pipeline would be vulnerable to attacks threatening water supplies for millions of homeowners and farmers, according to a report by NextGen Climate, a political group led by billionaire activist Tom Steyer.

Attackers could wreck remote pump stations along the pipeline's route in the northern Great Plains with just 4 pounds (1.8 kg) of readily available explosives, Dave Cooper, a former Navy Seal and a senior operative on the mission that killed Osama bin Laden, said in the 14-page NextGen report released Wednesday.

Under Cooper's most likely scenario, coordinated bomb attacks could cause a spill of 68,095 barrels of oil that would be difficult to clean up from an important aquifer.

That amount of oil would be more than has ever spilled from a U.S. pipeline. Under Cooper's worst case scenario, more than 172,000 barrels would be released. The Exxon Valdez tanker released about 250,000 barrels of oil into Prince William Sound.

"A coordinated attack at several critical points would not only wreak havoc ... it would likely overwhelm the existing engineering capability needed to clean it up," Cooper said in the report, some of which was redacted for public readership.

Plots to attack on U.S. pipelines have been rare. In 2007 the Department of Justice arrested several people it said were planning to blow up fuel pipelines at John F. Kennedy Airport. A 2012 Congressional Research Service report found pipeline computer systems were vulnerable and hackers could infiltrate them to cause spills, explosions or fires.

Cooper's report was the latest move by Steyer, a San Francisco-based investor and climate change activist, to fight TransCanada Corp's Keystone XL pipeline, which critics say would increase emissions linked to global warming.

Supporters of Keystone, which would link Canada's vast oil sands patches with refineries in Texas, say it would bolster North American energy security and provide thousands of jobs.

Earlier this year, Steyer pledged to spend $100 million to back pro-environment candidates in congressional campaigns ahead of the Nov. 4 U.S. elections.

Cooper, who looked at details of the pipeline available on the Internet and made a stealth visit to another TransCanada oil pipeline called Keystone 1, said he reached a pump station on the line in Nebraska not far from a major aquifer. "I was not approached, questioned or even noticed at any point," he said.

The State Department has delayed its recommendation to President Barack Obama on whether the Keystone XL pipeline should be approved pending a legal matter.

TransCanada spokesman Shawn Howard asked why Cooper only looked at one pipeline that would only be a tiny fraction of the total length of other U.S. pipelines. He said TransCanada regularly works with U.S. agencies to improve safety and security plans.

(Reporting by Timothy Gardner; Editing by Bernadette Baum, Steve Orlofsky and Tom Brown)