U.S-Mexico Border Water Quality

• On June 10, the Agency for Toxic Substances and Disease Registry (ATSDR) shared the following fact sheet outlining their effort to evaluate potential health risks in the Tijuana River Valley.
  • ATSDR Fact Sheet (English)
  • ATSDR Fact Sheet (Español)
  • Sign up to receive email updates from ATSDR by contacting Ben Gerhardstein (bgerhardstein@cdc.gov) in the ATSDR Region 9 office.

• Tijuana River Contamination from Urban Runoff and Sewage: A Public Health Crisis at the Border
• Health Impacts in Residents Exposed to Tijuana River Pollution: A Community-Based Assessment of Environmental and Public Health Risks

• Centers for Disease Control and Prevention (CDC) and County of San Diego Community Assessment for Public Health Emergency Response (CASPER)
• CDC, Agency for Toxic Substances and Disease Registry (ATSDR), and County of San Diego ATSDR Exposures Survey (ACE)
• SDSU School of Public Health “Healthy Water, Healthy Air” Community Survey

• Visit the SDAPCD Tijuana River Valley Webpage for more information regarding Air Quality Complaints and Odor Investigations, Hydrogen Sulfide Monitoring, and Air Improvement Relief Effort (Free Air Purifiers).

• USEPA conducted a preliminary evaluation of the Lower Tijuana River Valley for eligibility to be added to the National Priorities List (NPL) under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), also known as Superfund. This was in response to an October 24, 2024, petition from local elected officials to investigate the Tijuana River Valley for Superfund designation with the goal of obtaining federal resources to clean up hazardous substances and protect the health of people and sensitive environments.

On January 3, 2025, USEPA responded that further evaluation of the Tijuana River Valley was not warranted, in part, because contaminants detected in water and sediment samples do not exceed USEPA’s screening levels for protection of human health.

Transboundary flows in the main channel of the Tijuana River and those that bypass canyon collectors in the Tijuana River Valley severely impact areas in and around Border Field State Park and the Tijuana River National Estuarine Research Reserve, including closure of access to Border Field State Park, invasive plant infestations, significant increases in mosquito populations, critically low dissolved oxygen levels in the Tijuana River Estuary, and reduced fish species and overall presence of fish.

• Tijuana River Estuary Monitoring
• Marine Science Institute, University of California Santa Barbara 2023 Annual Report
• Tijuana River National Estuarine Research Reserve (TRNERR) Oneonta Slough Station Water Quality

The U.S. International Boundary and Water Commission (USIBWC) collection and treatment facility consists of the following:

• South Bay International Wastewater Treatment Plant (SBIWTP)
• Five diversion structures (also referred to as canyon collectors) located at:
• Stewart's Drain
• Silva Drain
• Canyon del Sol
• Smuggler's Gulch
• Goat Canyon
• Two Pump stations:
• Hollister Street Pump Station
• Goat Canyon Pump Station
• Two junction boxes:
• Junction Box 1
• Junction Box 2
• South Bay Land Outfall
• South Bay Ocean Outfall
• Other associated infrastructure, such as the pipes and conveyances between the diversion structures, pump stations, and the wastewater treatment plant.

Together, these comprise a federally-owned treatment works and are collectively referred to as the Facility.

The SBIWTP may receive domestic and industrial wastewater from the following sources:

• City of Tijuana’s municipal collection system;
• Five canyon collectors: Stewart’s Drain, Canyon del Sol, Silva Drain, Smuggler’s Gulch, and Goat Canyon; and
• Transboundary flows from other locations collected by vacuum trucks.

SBIWTP influent is unlike influent at other wastewater treatment plants because the SBIWTP receives wastewater, sediment, and debris from a collection system in Mexico that USIBWC does not control. Lack of control over influent quality and flow rates puts the SBIWTP in a vulnerable state, necessitating ongoing corrective maintenance.

The Facility is regulated under San Diego Water Board Order No. R9-2021-0001 as amended by Order No. R9-2023-0009, National Pollutant Discharge Elimination System No. CA0108928, Waste Discharge Requirements for the United States Section of the International Boundary and Water Commission South Bay International Wastewater Treatment Plant Discharge to the Pacific Ocean Through the South Bay Ocean Outfall (Order). The Order and reports required by the Order are available on the California Integrated Water Quality System Project (CIWQS) web page.

In August 2024, USIBWC awarded a contract for a SBIWTP rehabilitation and expansion project. USIBWC has set a self-imposed deadline of December 2027 to complete the SBIWTP rehabilitation and expansion. The plant will remain in operation throughout design and construction. The project includes essential rehabilitation of existing infrastructure and capacity expansion to double treatment capacity of 25 MGD to reach 50 MGD average flow, with a peak hydraulic flow capacity of 75 MGD.

On May 20, 2025, USIBWC and USEPA announced a planned 10-MGD expansion of the SBIWTP treatment capacity from 25 MGD to 35 MGD and projected completion and start-up within 100 days, by August 28, 2025. On August 27, 2025 the San Diego Water Board adopted Cease and Desist Order No. R9-2025-0139, which established interim effluent limitations for USIBWC’s 10-MGD expansion and requires compliance with USIBWC’s final permitted effluent limitations by Juny 30, 2026. In September 2025, USIBWC began treating up to 35 MGD of influent wastewater through advanced primary treatment followed by 25 MGD of secondary treatment using existing activated sludge processes. The advanced primary treatment consists of chemically-enhanced primary treatment (CEPT), using coagulants to increase solids removal. USIBWC blends up to 10 MGD of advanced primary effluent with the 25 MGD secondary effluent prior to discharge to the Pacific Ocean via the SBLO and SBOO.

The July 2022 Minute 328 of the 1944 U.S.-Mexico treaty entitled Utilization of the Colorado and Tijuana Rivers and of the Rio Grande (1944 Water Treaty) and the USIBWC-U.S. Environmental Protection Agency (USEPA) June 2023 Joint Record of Decision (ROD) for the United States-Mexico Canada Agreement (USMCA) Mitigation of Contaminated Transboundary Flows Project both include the SBIWTP rehabilitation and expansion project, but the minute and ROD have different roles. Minute 328 is the agreement between Mexico and the U.S. that includes project responsibilities, such as operation and maintenance (O&M) and cost sharing. The ROD is what enables USEPA and USIBWC to proceed to the design phase for proposed projects.

• Minute 320

The October 2020 International Boundary and Water Commission (IBWC) Minute 320 of the 1944 U.S.-Mexico treaty entitled Utilization of the Colorado and Tijuana Rivers and of the Rio Grande (1944 Water Treaty) establishes a framework of binational collaboration to address trash, sediment, and water quality issues in the Tijuana River Watershed. IBWC leads a Minute 320 binational core group and binational workgroups represented various stakeholder agencies, non-governmental organizations, and academia. These groups work to advance binational coordination/collaboration and projects to improve water quality and reduce trash and sediment loading in the Tijuana River Valley.

• Minute 328

The July 2022 IBWC Minute 328 of the 1944 Water Treaty is an international agreement between Mexico and the U.S. that outlines 18 projects planned for 2022-2027 as well as seven potential projects for the unspecified future. The projects in Mexico include improvements to Tijuana’s wastewater collection and treatment systems to better manage wastewater and eventually reuse treated wastewater. Completion of this suite of infrastructure projects will provide benefit on both sides of the border, including reductions in wastewater in the Tijuana River Valley and Pacific Ocean. The public can access the U.S. International Boundary and Water Commission (USIBWC) Geographic Information System (GIS) web portal to view the location, description, and status of projects included in Minute 328.

• USMCA Mitigation of Contaminated Transboundary Flows Project

The June 2023 USIBWC-USEPA Joint Record of Decision (ROD) for the United States-Mexico-Canada Agreement (USMCA) Mitigation of Contaminated Transboundary Flows Project contains ten proposed projects in Mexico and the U.S. aimed at reducing transboundary water pollution; four core project and six supplemental projects. The four core projects underwent a detailed analysis in the November 2022 USEPA-USIBWC USMCA Programmatic Environmental Impact Statement (PEIS). The six supplemental projects are still early in their planning phase and were not yet ready for detailed environmental review in the PEIS. To date, funding has not been identified for most of the ROD projects. Without full implementation of the projects needed to control transboundary flows, polluted water and trash are likely to continue to impact the Tijuana River Valley, Tijuana River Estuary, and coastal waters from the international border to the City of Coronado.

Several of the projects specified in the ROD are the same as, or similar to, those included in Minute 328, but the minute and ROD have different roles. Minute 328 is the agreement between the two countries that includes project responsibilities, such as operation and maintenance (O&M) and cost sharing. The ROD is what enables USEPA and USIBWC to proceed to the design phase for proposed projects. Section 2.8 of the PEIS includes a comparison of USMCA and Minute 328 projects.

Three projects in the Tijuana River Valley were funded by Senate Bill 170 through the State Water Resources Control Board Division of Financial Assistance:

• Tijuana River Flood Control Trash Control Structure ($4.73 million – Rural Community Assistance Center)
• Smuggler’s Gulch Dredging Project ($4.25 million – County of San Diego)
• Tijuana River Valley Hydrology and Habitat Restoration ($2 million – County of San Diego)

Each of the three projects are deeply rooted in the 13 years of coordinated federal, State of California, local agency, and non-governmental organization efforts in the Tijuana River Valley Recovery Team to restore and protect water quality. They were originally proposed in the 2012 Tijuana River Valley Recovery Strategy: Living with the Water and refined and analyzed in the 2020 Tijuana River Needs and Opportunities Assessment Report.

The Tijuana River trash control structure project involves the design, construction, operation, and maintenance of a floating trash boom system for one storm season in the main channel of the river, immediately downstream of the international border. The Rural Community Assistance Center deployed the trash boom system and hosted a ribbon cutting ceremony on December 17, 2024. This is a demonstration project. The information gathered will be used to develop permanent trash control infrastructure (Project J of the June 2023 U.S. International Boundary and Water Commission-U.S. Environmental Protection Agency [USIBWC-USEPA] Joint Record of Decision (ROD) for the United States-Mexico-Canada Agreement (USMCA) Mitigation of Contaminated Transboundary Flows Project).

The Smuggler’s Gulch dredging project involved the removal of approximately 30,000 cubic yards of accumulated sediment, trash, and debris in Smuggler’s Gulch and the Tijuana River Pilot Channel. The accumulated sediment, trash, and debris contributed to flooding, which threatens public and private properties and critical habitats. In March 2024, the County of San Diego initiated trash and sediment removal activities, which concluded in late 2024. This dredging is necessary prior to installation of permanent sediment and trash capture infrastructure at Smuggler’s Gulch, which will be funded by a separate grant.

The Tijuana River Valley hydrology and habitat restoration project consists of remediating a contaminated seven-acre property adjacent to the Tijuana River and restoring it to native upland coastal sage scrub habitat. Demolition of on-site structures, soil preparation, irrigation installation, and planting are complete. Habitat restoration is underway with successful natural recruitment and minimal non-native plant species present.

Proposition 4, which includes $50 million for border river pollution grants, was approved by California voters in November 2024. Funding will start in Fiscal Year 25-26.

The U.S. International Boundary and Water Commission (USIBWC) owns the South Bay International Wastewater Treatment Plant (SBIWTP). Discharges from the SBIWTP are regulated by San Diego Water Board Order No. R9-2021-0001 as amended by Order No. R9-2023-0009, National Pollutant Discharge Elimination System No. CA0108928, Waste Discharge Requirements for the United States Section of the International Boundary and Water Commission South Bay International Wastewater Treatment Plant Discharge to the Pacific Ocean Through the South Bay Ocean Outfall (Order). The Order is available on the California Integrated Water Quality System Project (CIWQS) web page.

When the San Diego Water Board adopted Order No. R9-2021-0001 on May 12, 2021, it also issued corresponding Cease and Desist Order No. R9-2021-0107 (CDO) to require USIBWC to address shortcomings, inadequacies, and maintenance issues at the SBIWTP to ensure consistent compliance with Order No. R9-2021-0001.

On December 21, 2021, the San Diego Water Board adopted Order No. R9-2021-0220, amending the CDO to revise certain time schedules and require USIBWC to submit quarterly compliance assurance reports.

On March 8, 2023, the San Diego Water Board adopted amendments to Order No. R9-2021-0001.

On December 18, 2023, the San Diego Water Board issued Time Schedule Order No. R9-2023-0189 (TSO) for USIBWC to achieve compliance with secondary effluent limitations by August 15, 2024. USIBWC did achieve compliance with secondary effluent limitations by that date. However, the SBIWTP has been in compliance with secondary effluent limitations since November 2024.

Additional enforcement actions related to the Order, including notices of violation (NOVs), are available in CIWQS.

The San Diego Water Board developed the Lower Tijuana River Indicator Bacteria and Trash Advance Restoration Plan (ARP) to address water quality impairments through an implementation plan with actions to restore and maintain water quality standards.

The ARP implementation plan proposes a memorandum of understanding between the San Diego Water Board, USIBWC, and USEPA to establish agreements, roles, and responsibilities to control transboundary sources of pollution within specified timeframes, respective jurisdictions, and respective funding allocation.