Hazen Allred, 16, leads his mare near the northern reaches of the St. David Cienega. A new study indicates the Vigneto development in Benson will deplete the cienega’s water levels.

An environmental group has released a study that says a massive development proposed in Benson could deplete water levels under the federally protected St. David Cienega riparian area.

The study’s author points out that the results are preliminary, but he says they prove the development’s potential to impact the important wetland warrants more rigorous study.

The research was commissioned by the Center for Biological Diversity, which opposes the 28,000-home Villages at Vigneto project. The development could bring70,000 more people to Benson— a town of about 5,000 people, 45 miles east of Tucson.

The master planned community would be located northwest of the St. David Cienega, the only marshland left in the San Pedro River’s federally protected conservation area. The Bureau of Land Management calls 57,000-acre San Pedro Riparian National Conservation Area one of the most important riparian areas in the country.

The study found that in three of five tested scenarios — which vary by river-water levels and potential variations in underground geology — well pumping at Vigneto lowers the water table beneath the cienega by between 0.8 feet to 1.5 feet within 100 years.

That means Vigneto will most likely infringe on federal water rights protecting the San Pedro conservation area, said Robin Silver, co-founder of the Center of Biological Diversity.

Even though the study is preliminary, it’s an improvement over existing, outdated models of the potential impacts, Silver said.

“At this point, this is now the state-of-the-art, best available science,” he said.

Mike Reinbold, spokesman for developer El Dorado Holdings, declined to respond to emailed questions on the study. And the city of Benson doesn’t employ anyone with the expertise to weigh in on it, said City ClerkVicki Vivian.

City Councilman Chris Moncadasaid he’s leaving debate over water issues to agency experts. The City Council continues to work with El Dorado on finalizing a development agreement and the community master plan, though it’s too early to say when the council might vote on them, he said.

“The water issues are a state and federal issue,” Moncada said. “Until we hear further from the state or federal government, we’re going to continue with our process. If the state and federal government say they (the developers) have adequate water supply, we can’t legally say we think they’re wrong.”

The study’s author Robert Prucha, of the Colorado-based consulting company Integrated Hydro Systems, says the study was done on a “shoestring” budget. The Center for Biological Diversity paid about $4,000.

It’s a scaled-down version of a $1 million U.S. Geological Survey study that wasaxed in 2010 amid budget cuts. It would have been the last in a series of three studies funded by the USGS and Arizona Department of Water Resources that aimed to shed light on how groundwater pumping could affect the San Pedro River. Both agencies declined to comment on the new study.

EFFECTS ON SPRING

Stanley Leake, a semi-retired research hydrologist with the USGS, said the new study could be “a step in the right direction.” But he said, “It seems like more work needs to be done.”

Leake pointed out the underground workings of springs are often a mystery, and the study doesn’t prove that a major spring feeding the cienega would be affected by the pumping, because it’s not clear the spring is fed by groundwater.

Prucha agreed with the critique, but said even if the spring isn’t connected to the groundwater, the cienega would still be vulnerable to changes in the underground water table.

“The entire cienega could be leaking groundwater from below over a much broader area,” he said.

The study didn’t take into account the effects of climate change and increased local pumping, Silver said.

“We know it’s going to be much worse” than the study’s findings suggest, he said. “Why would anybody risk a national treasure for another development? The benefit of the doubt should go to the treasure.”

Supporters of the development say it will pump from a deep water supply that won’t affect the aquifer feeding the San Pedro, which Benson city officials have said is protected by an “impermeable clay layer.”

University of Arizona researcher Tom Maddocksaid for hydrologists, that’s “absolute nonsense.” A clay layer is not impermeable; it will only slow, not stop, the draining of the river water, he said. Maddock was advisor to the UA master’s student researcher who created the computer model the new study utilized.

LITIGATION

The study will be used in a lawsuit the Center for Biological Diversity plans to file against the Arizona Department of Water Resources, alleging the development will infringe on the federal water rights established when the San Pedro conservation area was created in 1988.

“It’s like you caught someone in the act of stealing something. What we’ve documented here is potential theft of federal water,” Silver said.

The center intends to file the suit if ADWR doesn’t withdraw a certificate of adequate water supply awarded to the Villages at Vigneto’s predecessor in 2008. At the time, ADWR didn’t consider the development’s potential impact on the San Pedro and federal water rights to areas that are federally protected, Silver said.

A similar case — involving the 7,000-home Tribute development proposed in Sierra Vista — is pending in court. A judge overruled the certificate of adequate water supply awarded to developer Castle & Cooke, arguing the state water agency did not consider competing federal claims to the groundwater. The developer is appealing.

Last week, Gov.Doug Duceyvetoed legislation that would have rendered the issue a moot point. SB 1268 would have allowed local municipalities to opt out of the water-protection ordinance adopted by Cochise and Yuma Counties requiring developers to get a certificate of adequate water supply for 100 years before construction can begin. And SB 1400 would have required county supervisors to reconsider adopting the ordinance every five years.

CALIBRATION

For the study’s results to be more defensible in a courtroom, the model should go through a formal process called “calibration,” Prucha said.

Calibration involves repeatedly adjusting inputs to a computer model to improve its ability to predict known values, such as water levels in particular wells. Prucha likens the process to calibrating a gun by shooting at an intended target and adjusting the gun’s sight — or the study’s parameters — until it reliably hits the target.

The original computer model he relied upon has been calibrated, but Prucha updated that model with new information from the USGS and so it should go through another calibration to improve accuracy, he said.

Until the model is calibrated, the results can only be viewed as an “indication” of Vigneto’s consequences, said Robert Mac Nish, a retired hydrologist who previously worked at the U.S. Geological Survey and was an adjunct professor at the UA.

“It may give an indication, but to look at it as any sort of a quantitative evaluation would be a mistake,” he said.”Without calibration, you can’t be very confident on the precision or accuracy of the projections.”

But Mac Nish said Vigneto’s well pumping is sure to impact water levels in the Benson area, including in the St. David Cienega and local residents’ wells.

“If you look at the actual water availability in the Benson sub-basin, there isn’t any water left for additional development,” he said. “The San Pedro is a classic example of a place that is running smack up against the availability of water as a limitation for growth.”

Contact reporter Emily Bregel at ebregel@tucson.com or 573-4233. On Twitter: @EmilyBregel