Who's responsible — IB Water Pollution

One reason this problem is so hard to follow is that no single agency is in charge. The pipes and plants are split across two countries, several levels of government, and a treaty body that sits between them. This page shows who controls what.

The short answer

If the question is about...	Who's responsible
Tijuana's sewers, pump stations, and the Punta Bandera plant	CESPT, the Baja California state water utility for Tijuana, with Mexico's state and federal governments funding and building major projects
The PB-CILA river diversion	Built under a U.S.–Mexico agreement in 1991; operated and maintained by CESPT³
The South Bay treatment plant and the canyon collectors	The U.S. Section of the International Boundary and Water Commission (USIBWC)
The South Bay Ocean Outfall	Jointly owned by USIBWC and the City of San Diego; operated and maintained by the City
Agreements between the two countries	The International Boundary and Water Commission, through its U.S. Section and its Mexican Section (CILA)
Federal funding and the 2025 agreement	The U.S. Environmental Protection Agency and Mexico's environment ministry (SEMARNAT)
Beach advisories, warnings, and closures	San Diego County's Department of Environmental Health and Quality
Hydrogen sulfide monitoring and air alerts	San Diego County Air Pollution Control District
Health studies and guidance	San Diego County public health, with the CDC
Water quality permits and enforcement on the U.S. side	San Diego Regional Water Quality Control Board and the State Water Board
The estuary and Border Field State Park	California State Parks, the U.S. Fish and Wildlife Service, and NOAA's research reserve program
The river valley park, Saturn Boulevard fix, and proposed U.S. river diversion	San Diego County, with the City of San Diego on some land
The river trash boom	Alter Terra and the Rural Community Assistance Corporation, funded by the State Water Board

In Mexico: a state utility runs Tijuana's sewers

Tijuana's sewer system isn't run by the city. It's run by CESPT, a Baja California state utility.¹² A May 2026 op-ed in Times of San Diego⁴ compared it to San Diego's sewers being run out of Sacramento, and noted that CESPT gets about 96% of its revenue from service fees, which don't cover what the system needs. Mexico's federal water agency and the state government design and pay for major projects, often with U.S. grants.

That arrangement helps explain a pattern: repairs tend to happen after failures, rather than on a regular replacement schedule.

In the U.S.: a treaty agency runs the border plant

The South Bay International Wastewater Treatment Plant and the five canyon collectors belong to the U.S. Section of the International Boundary and Water Commission. The South Bay Ocean Outfall, which also carries treated water from a City of San Diego plant, is jointly owned by USIBWC and the City.¹¹¹² It's a federal agency created to carry out water treaties with Mexico, which is why a plant in San Diego answers to a treaty body rather than a local utility.

The EPA provides much of the money. In January 2020, Congress set aside $300 million through the U.S.–Mexico–Canada trade agreement for infrastructure in the Tijuana River watershed.⁵⁹ EPA and USIBWC are jointly funding the South Bay plant's rebuild, which is now fully funded at about $600 million.¹³¹⁴ EPA also funds projects in Mexico through its Border Water Infrastructure Program, run with the North American Development Bank.

Local and state agencies handle the impacts

San Diego County issues beach closures, monitors the air, studies the health effects, runs the regional park, and is designing a U.S.-side river diversion. The Regional Water Board oversees water quality on the U.S. side, including the permit for the South Bay plant, and tracks transboundary flows. State and federal wildlife agencies protect the estuary.¹⁶

These agencies deal with the consequences, but none of them can order repairs in Mexico.

The agreements that tie it together

1944 Water Treaty. The foundation for U.S.–Mexico water cooperation. The two countries add to it through agreements called "minutes."
Minute 283 (1990). Mexico committed not to let wastewater cross the border into the Tijuana River Valley. It led to the PB-CILA diversion and the South Bay plant.¹
Minute 320 (2015). Set up binational work on sediment, trash, and water quality in the Tijuana River watershed.¹
Minute 328 (2022). Listed specific projects for 2022 to 2027, including expanding the South Bay plant, replacing the Punta Bandera plant, and repairing key pipes and pump stations.¹
2018 lawsuits and 2022 settlement. In 2018, Imperial Beach, Chula Vista and the Port of San Diego sued the U.S. Section of the IBWC over sewage flows. The City of San Diego, the Regional Water Board, the State Lands Commission and the Surfrider Foundation filed separate suits. All settled in April 2022: USIBWC agreed to reduce cross-border flows and report its progress publicly for seven years, while EPA put the $300 million from the trade agreement toward a long-term fix.⁹¹⁰⁷
July 2025 memorandum of understanding. EPA and SEMARNAT agreed on a package of projects and funding commitments on both sides, with quarterly public updates.⁶⁸
A cease and desist order (2025). The Regional Water Board issued one to USIBWC over the South Bay plant's permit violations, so the plant is under a state enforcement order while it is being rebuilt.¹¹
Minute 333 (December 2025). Set up joint work on Punta Bandera, future treatment needs, and maintenance funding. It doesn't commit any new U.S. funding, and its actions depend on money being found.¹⁵

Why accountability is hard

Most of the sources are in Mexico, and the U.S. can't compel repairs there; it can negotiate, share costs, and tie funding to milestones. On the U.S. side, the agency running the border plant answers to federal treaty obligations, not to local residents or the county. Knowing who controls which piece is the first step to asking the right people for the right thing. Speak up covers how.

Who is responsible for what