U.S.-Mexico Border Wastewater Transport Model Suggests Solutions to Reduce Coastal Contamination

A team of oceanographers and environmental engineers finds that prioritizing improvements to a sewage treatment plant south of the U.S.-Mexico border would result in the largest reduction in U.S. beach closures as well as swimmers who become ill from bathing in contaminated ocean water.

Raw sewage from Tijuana, Mexico, flows into the coastal ocean at the Tijuana River Estuary near Imperial Beach, Calif., and also 10 kilometers (6 miles) south of the border at Punta Bandera where the outfall of the San Antonio de los Buenos (SAB) treatment plant is located. This raw sewage leads to sickness among swimmers and beach closures primarily at Imperial Beach, just on the U.S. side of the border.

Researchers led by Falk Feddersen, a physical oceanographer at Scripps Institution of Oceanography at UC San Diego, developed a coupled coastal ocean, pathogen, and human-illness model for the border region that includes both sewage sources. They simulated the pathogen concentration and human-illness risk for various potential border infrastructure scenarios for the year 2017. They concluded that improving the capabilities of the SAB treatment plant led to the largest improvement in pathogen concentrations, reduction in ill swimmers, and fewer beach closures, and was more effective than curbing flows from the Tijuana River Estuary located very close to the border. The study appeared in the American Geophysical Union (AGU) journal GeoHealth.

“This study grew out of dialogue with various stakeholders including the City of Imperial Beach, State Parks, WILDCOAST, the Surfrider Foundation, the San Diego Regional Water Quality Control Board, and the US Environmental Protection Agency,” said Feddersen. “The study has had a significant impact on the decision making process for the border infrastructure funding.”

The recent United States, Mexico, and Canada trade agreements (USMCA) make funds available for potential infrastructure projects to reduce beach closures. The research, funded by the Environmental Protection Agency (EPA) through the North American Development Bank and the National Science Foundation (NSF), addresses a problem that results in an estimated $2 billion per year in medical costs, as well as lost revenue from beach closures, nationwide. This modeling study has helped guide how border infrastructure improvement funds are allocated by indicating which sewage infrastructure strategy results in the largest reduction in beach closures.

“This critical study helped us address beach water quality impacts caused by untreated sewage on both sides of the border more effectively and binationally,” said Lily Lee, manager of the EPA Region 9 Water Infrastructure Office, which includes California, Arizona, and Nevada in its jurisdiction. “Thanks to this effort, EPA proposed a series of projects, to be funded in part by USMCA funds, that will better address U.S. and Mexican sources of sewage. The North American Development Bank financed this study with funds provided by EPA’s Border 2020 Program.”

For the study, the research team estimated that under current conditions 3.8 percent of all yearly ocean swimmers at Imperial Beach would become sick from bathing in sewage contaminated waters. The estimate jumps to 4.5 percent during the summer. Addressing SAB flows reduces the percentage of summertime ill swimmers dramatically, to 0.5 percent.

The researchers acknowledge in the study that conditions in 2017 were not necessarily representative of conditions in other years where Tijuana sewage infrastructure issues can lead to flow during summers. Nevertheless, the study shows that the most effective mitigation scenario would remain unchanged if other years were considered.

The research further shows the potential for a water quality forecast system based on this model, which would provide 3-5 day forecasts of when beaches should be closed in order to keep swimmers safe. Scripps Oceanography is proposing this project to the California Environmental Protection Agency and California State Water Resources Control Board for funding identified within Senate Bill 170, the Budget Act of 2021.

Study co-authors include Alexandria Boehm from Stanford University, Sarah Giddings and Xiaodong Wu of Scripps Oceanography, and Doug Liden of the EPA.

More information on Scripps Oceanography research on the Tijuana River Valley cross-border region and future vision can be found here: https://scripps.ucsd.edu/crossborderpollution.