"The authority's position is it is the state engineer's responsibility to make sure that doesn't happen," Reimer said.
King's approval for the proposal in March 2012 is being challenged separately, in Nevada state court.
Attorney Simeon Herskovits, representing the Great Basin Water Network, told Gordon that the water underground dates to ice ages and isn't replenished today. He called environmental damage inevitable.
"The project is designed to take the maximum amount of water from these basins in perpetuity," he said.
Environmental studies took eight years before the bureau in December 2012 granted permission for the pipeline to cross 263 miles (423 kilometers) of federal land from sparsely populated White Pine and Lincoln counties to the Las Vegas area, home to 2 million residents and host to 40 million tourists a year.
In response to another question from the judge, government lawyer Luther Hajek said the federal Bureau of Land Management "just issues the right-of-way. It doesn't have final say on how much water is pumped."
State and federal go-aheads brought challenges in state and federal courts from environmentalists, activists, local governments in rural towns in the two counties, plus the Duckwater and Ely Shoshone tribes and the Confederated Tribes of the Goshute Reservation in Utah.
No tribe in the area was properly consulted, and none signed off on the plan, Rovianne Leigh, attorney for the Goshutes, said Monday.
She described tribal elders' fears that a meadow wetland in Spring Valley will go dry. The site is revered and used in sacred ceremonies, and remembered as the site of massacres 150 years ago.
"When the water is gone, it is gone forever," Leigh said.
Water agency officials concede it will cost billions of dollars to build a pipeline to carry 75,000 gallons of water daily a distance comparable to a drive from Los Angeles to Las Vegas. But they say it may become essential if drought keeps shrinking the Lake Mead reservoir on the Colorado River, which supplies 90 percent of Las Vegas drinking water.
Project planners say that once wells are drilled and water begins flowing, engineers can measure the effects on the environment and correct, or mitigate, any damage that might result.
"Can the (Bureau of Land Management) stop this project before water is drawn?" the judge mused from the bench. "Isn't this a case of momentum at this point? A ball rolling downhill?"
Gordon, who also has more than three years' worth of written court filings from all sides, said he expects to issue a written decision within several weeks.
That would be before King holds hearings in late September on a state judge's order that he reconsider whether there is enough underground water to supply the pipeline.