The U.S. Section of the International Boundary and Water Commission (USIBWC) announced it is investigating the cause of the transboundary flow of wastewater mixed with heavy stormwater that has been entering the U.S.
USIBWC stated that the flow entered the U.S. at the San Diego, California, and Tijuana, Baja California international boundary since the early morning of March 9, 2025.
The flow is in the vicinity of Stewart’s Drain, which leads to the South Bay International Wastewater Treatment Plant (SBIWTP).
The volume of the flow exceeded the capacity of Stewart’s Drain, which led to the transboundary flow. The flow caused an accumulation of debris that blocked SBIWTP’s Junction Box #1, resulting in partial flows reaching the plant.
According to USIBWC, flows increased later in the day, and Stewart’s Drain had returned to normal.
USIBWC is meeting with experts from the U.S. Environmental Protection Agency (EPA), U.S. Department of States, and Mexico to determine the cause of the excessive flow and whether it was related to Mexico’s work on a new international collector.
The USIBWC requested a binational meeting with Mexico.
