A Coronado resident is defending San Diego County's beach water testing method, arguing it better protects swimmers from Tijuana River sewage contamination.

John Tato, a 1965 Coronado High School graduate who retired from the U.S. Department of State in 2005, published an op-ed in The Coronado News on Tuesday, Sept. 8. He made the case for the county's use of droplet digital polymerase chain reaction (ddPCR) testing, which detects bacterial DNA in beach water and delivers results in about three hours. The older Enterolert culture-based test requires 24 hours or longer.

Tato wrote that when untreated sewage is known to be present, the burden of proof should favor protecting swimmers.

The op-ed follows Mayor John Duncan's challenge to the county's ddPCR protocol at the Aug. 18 City Council meeting. Duncan cited a January 2026 peer-reviewed study by researchers Matthew Verbyla and Rosanna Lacarra that characterized 56.3% of ddPCR results exceeding the contamination threshold as false positives when Enterolert was treated as the benchmark, using Coronado's summer 2023 beach data. City officials at that meeting pushed to raise the county's threshold from 1,413 gene copies per 100 milliliters to 6,791, which they said would align ddPCR with the older EPA standard, as The Coronado News reported.

Tato challenged that framing directly. Labeling ddPCR results as "false positives" because Enterolert did not agree assumes Enterolert is correct, he argued, and that assumption is the core of the dispute.

He pointed to the county's pre-adoption validation: a two-year analysis of more than 3,000 samples from 51 sites across San Diego County's 70-mile coastline, conducted with the California Department of Public Health (CDPH) and the Southern California Coastal Water Research Project.

That study found ddPCR met federal equivalence requirements, leading to EPA approval in October 2020 and CDPH sign-off. San Diego County became the first federally and state-approved coastal county to use ddPCR on May 5, 2022, according to the county's Department of Environmental Health and Quality. The EPA's approval document puts the ddPCR false positive rate at 6%.

Tato also noted that the county uses ddPCR at more than 50 locations, yet the spike in elevated readings has occurred primarily at sewage-contaminated South County beaches. The rest of the coastline has not seen comparable increases, he wrote, suggesting the test is detecting real contamination rather than producing false alarms.

At the Aug. 18 meeting, San Diego County Public Health Officer Dr. Sayone Thihalolipavan explained that beach closures are triggered by confirmed sewage events such as spills or visible contamination, not by test results alone. The ddPCR test determines when contamination has cleared enough to reopen beaches. The current threshold was set by CDPH, and the county does not have authority to change it, Thihalolipavan said.

A county letter to the city dated Aug. 18 included a September 2024 attachment warning that less stringent water quality requirements could pose a "substantial public health risk," according to The Coronado News.

The real problem is inadequate sewage infrastructure in Mexico, Tato concluded.

"The solution is not to encourage the use of a test that produces fewer warnings," he wrote. "It is to eliminate the sewage so that neither test detects a hazard."

No follow-up council discussion on the testing dispute has been publicly scheduled. Residents can check current beach water quality conditions on the county's Beach and Bay monitoring page.