Ocean currents model to be evaluated as part of study examining Tijuana pollution flows

SCCWRP and its partners will use field data from an ongoing study tracking persistent fecal contamination at Imperial Beach near the U.S.-Mexico border to evaluate a computer model that predicts the movement of nearshore waters in the area.

The ocean currents forecasting model, which was developed by the Scripps Institution of Oceanography, covers an approximately 12-mile stretch of nearshore waters spanning the U.S.-Mexico border. The forecasts may explain how primary-treated effluent being discharged into coastal waters by a Tijuana wastewater treatment plant disperses and travels north to San Diego County’s Imperial Beach during south swell events.

Imperial Beach beachgoers and lifeguards have reported persistent sewage odors at the beach consistent with these south swells, which prompted researchers to launch a source tracking study in 2018 to determine if the Tijuana treatment plant is the source.

To validate the Scripps model, SCCWRP and its partners are using DNA-based field samples being collected during ongoing sampling efforts along the effluent plume’s predicted south-to-north route, from the San Antonio de Los Buenos treatment plant in Tijuana to Silver Strand, which is just north of Imperial Beach.

Samples are being collected not only in the wadeable zone closest to the shoreline, but also in the surf zone where surfers commonly line up to catch waves.

The paired sampling effort, which spans the U.S. side of the border only, will provide insights into the relative magnitude of fecal pollution exposure for U.S. beachgoers along the shoreline vs. in the line-up area where surfers wait for the next wave.

Researchers will analyze the water samples using DNA-based microbial source tracking methods, marking one of the first applications of the technology to track the dispersal of human sources of fecal contamination across a large swath of the Southern California coast, as well as to link fecal contamination to a specific location.

Researchers are applying this same DNA-based source tracking technology to a separate study that will determine whether human fecal contamination found in San Diego and Orange County waterways can be linked to sanitary sewer pipes via the microbial community living inside the pipes. The goal is to investigate whether the microbial community found inside sewage pipes – referred to as biofilm – has a genetic signature that is unique to these sewage pipe environments.

Likewise, researchers in the Imperial Beach study are examining whether the microbial community found at the Tijuana treatment plant has a genetic signature unique to the plant, and whether that signature can be reliably detected as the Tijuana wastewater plume moves up the coast – and mixes with other human and non-human fecal contamination sources.

For the Imperial Beach study, researchers also will work to associate other major
potential human fecal contamination sources at Imperial Beach with unique genetic signatures, including water flowing into the Tijuana River estuary from canyons on the Mexican side of the border. The Tijuana River estuary also terminates a few miles south of Imperial Beach; although a sand berm reduces the flow of estuary water into Imperial Beach coastal waters for much of the year, the berm is intermittently breached as a result of high surf and tidal conditions.

Multiple Mexican beaches dot the coast between the Tijuana treatment plant and Imperial Beach, underscoring the value of investigating the plant’s role in potentially contributing to beach fecal contamination. Additionally, Imperial Beach already is on a federal 303(d) listing of impaired water bodies for high fecal indicator bacterial levels.

The San Antonio de Los Buenos sewage treatment plant, located about 10 miles south of Imperial Beach, uses primary treatment methods to treat sewage, then discharges the effluent to a stream that terminates at the coastline.

The Imperial Beach study includes cooperation and participation by SCCWRP member agencies, environmental organizations on both sides of the U.S.-Mexico border, and U.S. lifeguards. Results of the study could help inform future management interventions to more effectively control and mitigate cross-border fecal contamination issues

For more information, contact Dr. John Griffith.