The United States and Mexico disagree on the source of a weeks-long sewage spill at the border, but an investigation into the cause demonstrated what those working on the border already know — the South Bay International Wastewater Treatment Plant in San Diego is in desperate need of repair.

As of Friday, U.S. officials at the International Boundary and Water Commission — a binational agency that works on border water treaties — believed the millions of gallons of raw sewage that escaped the border wastewater system earlier this month came from a crack in a big concrete pipe in Mexico called the International Collector, which carries sewage from Tijuana wastewater mains to the U.S. treatment plant. (That pipeline is old and by now has surpassed its useful life, according to a 2019 report by the North American Development Bank.)

Mexican officials, from the state of Baja California’s water department called CESPT, denied there’s a rupture in that pipe and that the water is flowing through the system where it normally should. The problem, CESPT director Gonzalo López López said, is somewhere between a working pump in Mexico and a troubled critical piece of infrastructure on the U.S. side where 95 percent of the wastewater passes to be treated.

“We still don’t have a certain result on whether it’s a problem in the U.S. or in Mexico,” López said on Friday.

The two sides agree something is clogging the system and causing wastewater to flow where it shouldn’t, spilling instead into a border drain and into the Tijuana River. Both countries sent cameras through their respective systems to try and locate that clog but haven’t yet revealed what the footage showed.

Typically fingers point to Tijuana whenever there’s a border spill like this, but there is a piece of aging and broken infrastructure in the U.S. that’s under investigation.

Meet Junction Box 1, located just over the border line on the U.S. side. It’s basically a trench in the ground with a valve or a set of gates that control the flow of wastewater passing from Mexico into the U.S.-side treatment plant. (IBWC officials were checking that box for an obstruction but as of Monday hadn’t found any evidence of a clog there, confirmed Lori Kuczmanski, a spokeswoman for the agency.)

One gate is broken and the other has a damaged track and is stuck, Xochitl Aranda, acting principal engineer of operations at IBWC, told the San Diego Regional Water Quality Control Board back in March.

IBWC has until the end of the month to submit plans for its repair. But trying to get the job done on an international boundary has its challenges.

Lowering workers into the box to make repairs requires coordination with Mexico to shut down the continuous flow of wastewater each day.

The U.S. Can’t Control the Tijuana Sewage Faucet

The plant treats 25 million gallons of Tijuana wastewater per day, water that would otherwise flow untreated into the Tijuana River and the Pacific Ocean, further polluting the coastline. But the problems at Junction Box 1 mean the plant can’t control how much wastewater Mexico sends through the pipe to the plant. At times, the plant got much more water than it could treat.

That meant IBWC was on the cusp of violating its permit to discharge treated water into the Pacific Ocean through a five-mile-long pipe called the South Bay Ocean Outfall, according to a cease-and-desist order from the Regional Water Quality Control Board. The agency specifically called-out Junction Box 1.

“Compliance with the 2021 permit effluent limitations is unlikely until the sluice gates in Junction Box 1 have been repaired and flows into the (plant) are reduced to a monthly average of 25 (million gallons per day),” the order reads.

The agency racked-up dozens of permit violations from November 2020 through January 2021 for discharging millions more gallons of dirtier wastewater beyond what the plant is permitted to treat. That puts extra strain on the aging plant.

“If you’re accepting too much (water) you’re knocking out of balance the other systems in the plant,” said Chris Helmer, director of environment and natural resources for Imperial Beach, the San Diego city closest to the border and typically most affected by these Tijuana River sewage spills. “It sets off a chain reaction and other things can break quickly if you don’t quickly repair and have control of these basic elements.”

Water Quality board members were surprised.

“I just had no idea the problems were so numerous and substantial,” said board chair Celeste Cantú at the March meeting. “We’ve been aware of upsets and problems and challenges historically, but it seems like what you have described here is a very difficult plant to operate under the circumstances.”

The plant is old and many of the components require complete replacement, Aranda said. But plant operators have to fix things temporarily instead.

“Many of our pumps and components wear down much faster … the life spans are much shorter due to the nature of the quality of the influent we treat,” Aranda said, in other words, the wastewater received from Tijuana is so bad that it wears the plant down faster than its estimated lifespan.

Aranda said the agency has been asking Congress for more money to make those repairs.

The U.S. Environmental Protection Agency recently announced it would double the plant’s size with $300 million granted by the U.S.-Mexico-Canada Agreement, allocated under President Donald Trump’s Administration. But construction is at least a year away.

Meanwhile, the gates at Junction Box 1 are still broken.