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A proposed water-sharing agreement between Utah and Nevada is a raw deal for Utah that could deplete ancient aquifers under the West Desert's Snake Valley, ruining its fragile ecosystem and a local economy based on ranching and tourism.

That's according to a consortium of environmental groups and Snake Valley residents who urged Utah Gov. Gary Herbert on Tuesday to renegotiate the contract hammered out more than three years ago.

The agreement spells out how much water the Southern Nevada Water Authority may pump once it completes a 300-mile pipeline to deliver groundwater from the Snake and four other valleys to the rapidly growing Las Vegas metropolis.

But the agency has yet to acquire the Snake Valley water rights or the public right of way necessary to advance the $15.5 billion project, said Steve Erickson, a Salt Lake City activist who sits on the board of the Great Basin Water Network.

The agreement is predicated on assumptions about the valley's hydrology which new research suggests are erroneous, setting up the possibility that Southern Nevada's pumping scheme may actually "mine" water that could take decades to be recharged, according to the letter to Herbert signed by 18 environmental groups.

"We concur with [Utah's] Millard, Juab and Tooele counties that, contrary to [legal] advice you are receiving, the division of the shared aquifer is not fair or equitable," the letter states. "We would argue that Equitable Apportionment would favor Utah because the location of the headwaters or source of recharge is irrelevant in considering the equities involved, most of Snake Valley is in Utah and most of the current and historic water use is in Utah, the water supply is limited and water tables are already decreasing, and the potential for injury in Snake Valley is significant."

The governor welcomed the letter as "positive input," according to Herbert's environmental adviser Alan Matheson.

"We are reaching out to experts and affected citizens to ensure that any decision made is in the long-term interests of the state," he said. "The agreement is certainly not perfect, but it was the product of hard negotiations and there are positive things in there. We have to look at what would happen without it."

The Snake Valley lies to the east of the Snake Range, Nevada's highest, which contains Great Basin National Park and an a complex of caves. The scientific discussion centers on the amount of "interbasin flow" from Spring Valley, which sits to the west of and 1,000 feet higher than Snake Valley.

The environmental consortium contends that project planners overestimated the volume of inflow to the Snake Valley by as much as 30,000 acre-feet per year. More recent studies vary widely on their estimates, suggesting smaller or even negligible flows between basins.

"You have all these different flows, none of which are measurable. These estimates are going to be plus or minus 30 or 40 percent," said Reno-based hydrologist Tom Myers, who acts as a consultant for the Great Basin Water Network.

"This is an arid place and it gets drier every year," said Kirk Robinson of the Western Wildlife Conservancy. "All wildlife species depends on seeps and springs. If the water table drops, those seeps and springs will dry up."

Given such uncertainty it would be foolish to commit to a water-sharing agreement that could suck between 80,000 and 125,000 acre feet from the valley, Erickson said. The consortium proposes "a monitored, phased-in pumping program done in small increments over time."

"Why cede the water to Southern Nevada when this project is still decades off from being a reality," Erickson said. "We have plenty of time to do further science and assess future impacts."

The proposed interstate deal is fair to Utah, according to water authority spokesman J.C. Davis.

"The majority of recharge comes from the Nevada side. Nevada is not asking the 70-30 split in line with recharge values, but a 50-50 split," Davis said. "This isn't about waiting for additional data. This is about seeing if they [opponents] can put it off until infinity."

A panel of water-law experts has advised Herbert to sign the agreement to get protections locked in place, rather than risk losing them in future lawsuits.

"The agreement provides that there can be no action on the pipeline for 10 years after it is signed so additional studies can be done," Matheson said. "It can be modified over time to reflect new information generated."

Salt Lake County Mayor Peter Corroon said he appreciated the governor's "predicament," but signed the environmentalists' letter because of concerns that dust blowing off a dewatered Snake Valley could further compromise air quality on the Wasatch Front's urban corridor.

"If it's signed we lose the ability to stay strong on the concerns we've had," Corroon said. "Once you find out about significant impacts, it's too late you can't turn back the clock."