Effluent streams across the border from Mexico at Oso Wash on June 3.

A local river advocacy group says poorly treated effluent from a pair of sewage plants in Mexico is once again flowing across the border east of Nogales.

The problem in the area of Oso Wash, also known as Brickwood Canyon, was reported and subsequently mitigated in early 2020 and again in early 2021. But according to Ben Lomeli, a hydrologist and member of the Friends of the Santa Cruz River, he and fellow FOSCR member Connie Williams visited the site on June 3 in response to reports from hikers, and observed the flow to be heavier than during the previous two incidents.

They also thought it smelled stronger.

“The smelly effluent water flowing across the border now is murkier, with signs of algal and other lateral aquatic vegetative growth indicative of high nutrient content as compared to March 28, 2021,” Lomeli wrote in a report dated June 5 and distributed via email Monday to local, state and federal officials, among others.

“We could now smell the odor of sewage effluent from a distance as we approached the subject drainage,” he noted.

The on-again, off-again problem stems from a pair of sewage treatment plants at the Lomas de Anza housing development on the east side of Nogales, Sonora. Treated effluent is discharged from the plants and enters a creek that flows across a remote section of the U.S.-Mexico border just west of the Santa Cruz River. It’s several miles south of the Kino Springs Golf Course in an area long used for cattle ranching.

A spokesperson for the International Boundary and Water Commission told the NI in February 2020 that the effluent had been flowing across the border since the two treatment plants started operating several years prior. The plants were constructed by the builder behind the housing development, but in recent years have operated by OOMAPAS, the municipal water utility in Nogales, Sonora.


"The smelly effluent water flowing across the border now is murkier, with signs of algal and other lateral aquatic vegetative growth indicative of high nutrient content as compared to March 28, 2021," hydrologist Ben Lomeli wrote in a report dated June 5, two days after this photo was taken.

Treated effluent is not inherently bad for the environment; in fact, effluent from the Nogales International Wastewater Treatment Plant in Rio Rico is used to recharge the north-flowing Santa Cruz River. However, the concern with the effluent crossing the border from the Lomas de Anzas area is that it isn’t always treated properly before release.

In his report, Lomeli said a water sample taken from the Oso Wash on June 3 and tested for E. coli bacteria showed a high likelihood that it hadn’t been sufficiently treated or chlorinated before being released.

“Microorganisms such as algae, viruses, protozoa, worms, leeches, fungi, bacteria, rotifers and crustaceans that can occur in untreated or poorly treated water pose potential threats and risks to human health if they enter our water supply,” Lomeli wrote.

He added that FOSCR hopes U.S. agencies including the IBWC, Arizona Department of Environmental Quality “can once again coordinate with Mexican counterpart authorities to address and remedy this recurring transboundary contamination of improperly treated sewage effluent flows from the Lomas de Anza subdivision.”

In an email sent Tuesday, a spokesperson for the Arizona Department of Environmental Quality said the U.S. section of the IBWC regularly monitors the flow of effluent in the area, while ADEQ conducts more limited water quality sampling of the flow. 

Both agencies’ testing showed the presence of E. coli, ADEQ Communications Director Caroline Oppleman wrote in the email. In addition, DNA testing conducted by the University of Arizona at the behest of ADEQ showed that the sources of E. coli are both human and bovine waste – about half and half – Oppleman wrote, adding: “which we think is associated with the combination of stormwater flow, partially treated wastewater and cattle waste the flows pick up from nearby grazing areas.”

Oppleman described the two treatment plants in the Lomas de Anza area as "overburdened," and said the City of Nogales, Sonora has requested support from the North American Development Bank.

"While we expect the Nogales, Sonora/NADBANK infrastructure project to significantly reduce ongoing surface water impacts, ADEQ also has secured limited funding to design a wetlands area on the Arizona side of the border to provide natural treatment of flow in the area and improve surface water quality," she wrote.

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