New Sonora sewage plant has binational ramifications

Nogales Sonora Mayor Jose Angel Hernandez Barajas, left, gets a tour of construction work at the site of the new Los Alisos sewage treatment plant. The new plant will mean less sewage being pumped across the border into Arizona, but it also means less treated effluent in the Santa Cruz River. Contributed photo.

Nogales, Sonora is expected to have its own sewage plant by the end of 2012, significantly reducing the flow of waste across the border and treated effluent into the Santa Cruz River north of Rio Rico.

Currently, about 10 million gallons of wastewater flows downhill each day from Nogales, Sonora, and across the border to the Nogales International Wastewater Treatment Plant (NIWTP) in Rio Rico. But after February 2012, some of that sewage will be pumped about 4-1/2 miles uphill from a compression station before it is sent flowing more than 10 miles downhill to the new Los Alisos plant, said Francisco Gastelum Ceballos, director general of the Nogales, Sonora water and sewer department, known by its Spanish acronym as OOMAPAS.

Once the new plant is completed, an estimated 5 million gallons per day (MGD) of effluent will begin to flow into Los Alisos River, about 20 miles south of the border. Like the facility in Rio Rico, the Sonoran plant will use ultraviolet technology to kill bacteria before the effluent is discharged.

OOMAPAS Sewage Plant Director Cesar Campas called the project “totally positive,” and said it’s “a great replicable model for the state of Sonora and a step forward.”

What’s more, he said, the Los Alisos plant will quickly be able to boost capacity.

Within a relatively short time after it opens in 2012, Campas said, “we would hope to expand the plant to 7.5 MGD.”

Additionally, the uphill pumping compression station and the pipeline to the plant are being constructed to potentially handle a volume of up to 15 MGD of influent to Los Alisos.

The construction of the plant, and its potential to expand, has ramifications for riverways and their basins on both sides of the border.

Arizona effects

From a U.S. perspective, the Sonoran sewage has been a blessing and a curse since it started flowing across the border following an international agreement in 1958.

The effluent augmented surface flow in the Santa Cruz River north of the Rio Rico plant, but sewage ran into the Nogales Wash, regularly causing environmental concerns. The sewage “influent” also contained pollutants that were not treated properly – such as cadmium, which showed up in high levels last year in NIWTP sludge, some of which was used to fertilize farmland.

Sonora, meanwhile, has had to pay about $350,000 annually for operating costs of treatment at the Rio Rico plant. Once the amount of waste it pumps to the plant decreases, so will its share of the operating costs.

“The limit for Nogales, Sonora sewage crossing the line is supposed to be 9.9 MGD,” said Tom Konner, border environmental engineer with the Environmental Protection Agency. “And up to now all anybody cared about was not exceeding that number in the future, not how flows might decline in the future.”

Konner said past discussions surrounding Los Alisos included a verbal agreement that Nogales, Sonora would guarantee an 8.8 MGD flow to the Rio Rico plant, but he didn’t think it was ever formalized.

However, as part of a 1967 agreement that accompanied the opening of the original treatment plant in Rio Rico, International Boundary and Water Commissioners in Mexico and the United States agreed that Mexico could dispose part or all of the Nogales, Sonora sewage in its own country whenever it decided.

“If I were a border resident concerned with long-term Santa Cruz river flow and lived in the area, I’d consider proposing a binational, city-to-city, state-to-state and federal-to-federal agreement that guaranteed some flow to the NIWTP and look for the funds to pay the Mexican side to do so,” Konner said.

Sherry Sass, founder of the organization Friends of the Santa Cruz River, said that historic maps show that the river has about a 13-mile flow downstream of the NIWTP. “But I think it’s closer to 15-18 miles depending on the season, precipitation patterns, etc.,” she said.

Alejandro Barcenas, former director of the Santa Cruz Active Management Area, a now-defunct entity that helped regulate water use in the county, said he doesn’t think reduced influent from Sonora will have significant short-term effects on the river.

“I think the river will survive Los Alisos sewage diversions pretty well,” Barcenas said. “Our concern should be groundwater in the basin which is likely to drop” as climate-change prediction are less winter rains and more flash-flood surges.

Sonora effects

Los Alisos River currently runs intermittently over much of its southerly course toward Magdalena, Sonora. At one point about 2 miles south of the site of the new sewerage plant, well fields that supply the majority of the city’s potable water are a contributor to the lack of surface water.

Barcenas said water levels at the well fields “have dropped about 100 feet and are a cause of complaints by farmers to the south who say they can no longer use (nearby) wells to irrigate.”

The sewage effluent from the new Los Alisos plant will provide surface flow downstream to farmers along the river, and may help to recharge the well fields. However, Barcenas said, “The water flowing on the surface will not efficiently percolate into groundwater.”

The fact that the plant will discharge north of the potable well fields and agricultural areas will make water quality critical in the future, said Campas, the OOMAPAS sewage plant director

“We will be testing the water quality constantly for months after we begin operation to understand just how effective our pretreatment is and what further steps may be needed,” he said.

At the same time, along Los Alisos, Campas said, “There will be tree-planting projects, particularly native oaks in the area, to make the river a more riparian body.”

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