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Mexican wastewater plant threatens Tumacacori oasis

TUMACACORI – Lined with lush trees and a refuge for wildlife, the Santa Cruz River cuts a 16-mile oasis through the parched landscape near Nogales.

"When you put water and desert together, boy there's something about it that makes it pretty wonderful," said Grant Hilden, a volunteer at the Tumacacori National Historic Park.

Water is a blessing that Hilden knows not to take for granted. Deep wells driven by the area's thirst have historically strained the Santa Cruz's flow, drying it up completely during the 1970s.

A decade later, the river returned in this area, thanks in part to an agreement with Mexico that allows the International Wastewater Treatment Plant in Nogales, Ariz., to pump 15 million gallons per day of treated effluent into the river and sustain it year-round.

Much of that water comes from sewage flowing north across the border, but that will change in February 2012 with the completion of a new wastewater treatment plant in Mexico that will send its effluent into a south-flowing river.

The international agreement allows up to 9.9 million gallons daily from Nogales, Son., across the border and, when treated, into the Santa Cruz. In fact, rapid growth has caused the Mexican side to regularly exceed its allotment, reaching an annual average of 12.5 million gallons daily, according to plant certification documents.

But the new Los Alisos plant, under construction about 20 miles south of Nogales, will give Mexico the ability to control its own wastewater, including where it's reintroduced into the water table.

Tumacacori, known for its rugged landscape and three Spanish mission ruins, the oldest of which dates back to 1691, depends on the river for its picturesque appeal, Hilden said, adding that less water could hurt the area's aesthetics.

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"We don't know how long we're going to have all of this," he said. "We'll enjoy it while we can and hope there's still something left when that 10 million gallons goes south."

Sally Spener, a spokeswoman for the International Boundary and Water Commission, which manages water along the border, said Mexico has a right to the water currently flowing into Arizona.

"There's nothing in our existing agreements or treaties that requires Mexico to send water across into the Santa Cruz River," Spener said. "But as a practical matter, the water is a benefit to the Santa Cruz and the state."

The effluent contains high levels of the heavy metal cadmium and E. coli bacteria from ranching and is unsafe for human use, but even compromised water has economic and quality-of-life value in the desert, said Randy Serraglio, a conservation advocate with the Tucson-based Center for Biological Diversity.

"It draws a lot of people because of the beauty of the area, the trees, and to see what the West used to look like," he said.

Then there's the birds, which along with other wildlife depend on rivulets like the Santa Cruz as they migrate north. Water pumping and diversion has cost Arizona nearly 90 percent of its river habitat, making what's left of the Santa Cruz that much more important, Serraglio added.

"The Santa Cruz River used to be a beautiful riparian area right down through downtown Tucson," he said. "For the past 50 years, we've been over-mining the aquifer. Now people drive over the bridge on Congress Street [in downtown Tucson], and it's just a ditch."

While Hilden worries that Tumacacori and the Santa Cruz River will suffer from the new plant's construction, he said Mexico shouldn't be blamed for wanting to use its water in ways that benefit its people.

"It's produced in Mexico, and they have every right to it," he said. "But at the same time, it's going to cause some problems for us here."

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Click image to enlarge

Channing Turner/Cronkite News Service

This lush plant life is just part of the oasis created when a wastewater plant in Nogales, Ariz., began sending treated wastewater into the Santa Cruz River.

Comparing the plants

International Wastewater Treatment Plant (Nogales, Ariz.)

  • Total Capacity: 17.2 million gallons daily (MGD)
  • Current Usage: 15 MGD
  • Run-over from Mexico: 80 percent or 9.9 MGD (reported at closer to 12.5 MGD at times)

Los Alisos Wastewater Treatment Plant (Nogales, Son.)

  • Initial Capacity: 5 MGD
  • Planned Future Capacity: 10 MGD
  • Planned Completion: February 2012