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A plan to pipe water to Las Vegas from near the Utah border could nearly double monthly residential water bills in Nevada's largest city, according to a report prepared by the sponsoring utility's contractor.

The study, submitted to Silver State officials to demonstrate construction feasibility as part of the Southern Nevada Water Authority's water-rights application, says the proposed pipeline from Snake Valley and other points north of Las Vegas could add more than $31 to the typical monthly bill of $36.

Taken with other system upgrades, including a new and deeper outtake in Lake Mead, the expenses could push average bills above $90, according to the report by Las Vegas-based Hobbs, Ong & Associates.

Pipeline foes said the report's total cost figure — $15.5 billion with finance charges, compared with the $3.6 billion or less that the utility previously stated — makes the plan unaffordable.

"We have long considered the numbers provided by the water authority highly suspect," said Launce Rake, a pipeline opponent and Las Vegas resident who served on a Clark County citizens' committee addressing water rates in 2007.

"Our concern here, beyond the irreversible environmental damage," he said, "is that working families who are already having a very tough time making ends meet would be hit hard by massive increases in their water bills."

But officials with the water authority say the report changes nothing. It was written to determine whether the utility can afford the project, and spokesman J.C. Davis said it shows that it can.

The lower costs that the utility has quoted in the past were for current construction prices without financing, he said. The report assumes a phased and financed construction period from 2016 to 2050.

The hypothetical rate increase makes several conservative assumptions in reaching a higher rate for individuals. One is that Las Vegas would never resume growing and that all costs for the pipeline would be borne by existing ratepayers. Others are that the utility could not collect sales taxes or new connection fees in the future, as it does now.

And even if the average bill does grow to $90, Davis said, that's still cheaper than in many Western cities, including San Diego and Colorado Springs. "It doesn't change anything," he said. "[The projected rate] is really about what would be expected."

Rural Nevadans around Ely have complained that groundwater pumping for the pipeline would draw down their water table, while Utah officials as far away as Salt Lake County have complained of environmental consequences that they fear would include dust storms.

The utility says it needs the water as a hedge against drought and climate change in the Colorado River Basin.