Coronado should ask whether the county’s beach-water testing accurately distinguishes between safe and unsafe water. Public officials should scrutinize any method that may unduly restrict access to public beaches. But when untreated sewage is known to be present, the burden of proof should favor protecting swimmers.
The dispute concerns two ways of measuring enterococci, bacteria used as indicators that fecal contamination and disease-causing organisms may be present.
Enterolert, the older method, places a water sample in a growth medium and waits for living bacteria to multiply. Results usually require 24 hours or longer. Droplet digital polymerase chain reaction, or ddPCR, identifies and measures bacterial DNA and can produce results within approximately three hours.
That difference in time is the principal reason rapid DNA-based testing was developed.
Traditional culture tests were not designed to provide immediate warnings about rapidly changing coastal conditions. If a sample is collected Monday and the result does not arrive until Tuesday, swimmers may spend all Monday in contaminated water. By Tuesday, currents may have carried the pollution elsewhere, making the laboratory result an accurate description of Monday’s sample but potentially irrelevant to Tuesday’s conditions.
Congress recognized this problem when it enacted the Beaches Environmental Assessment and Coastal Health Act in 2000 and directed the EPA to develop rapid methods for measuring fecal-indicator bacteria. The result is the ddPCR test, which replaces yesterday’s water-quality report with information useful today.
The EPA has noted that elevated bacterial conditions at California beaches frequently last less than a day. A method requiring 24 hours or longer can therefore report a hazard after it has disappeared or fail to warn the public until after exposure has occurred. Same-day ddPCR results allow officials to warn swimmers while contamination is present and to remove restrictions promptly after the danger passes.
The ddPCR method also addresses another limitation of culture testing. Environmental stresses can injure bacteria so that they do not reproduce in a laboratory tray. Enterolert counts only organisms capable of growing under those conditions. ddPCR detects genetic material from living, injured and recently dead bacteria without waiting for them to multiply.
That greater sensitivity is one reason the two methods sometimes disagree, as ddPCR’s greater sensitivity may mean that the culture method failed to detect bacteria that could not grow in the laboratory.
Before adopting ddPCR, San Diego County along with the California Department of Public Health and the Southern California Coastal Water Research Project conducted a two-year analysis involving more than 3,000 samples from 51 sites covering the county’s entire 70-mile coastline. Samples were collected during wet and dry weather and ddPCR results were compared side by side with Enterolert and an EPA-approved qPCR Method.
The study found that ddPCR met federal requirements for equivalence leading to approval by the EPA and the state and establishment of the present threshold for contamination of 1,413 enterococci gene copies per 100 milliliters.
Coronado conducted a much narrower comparison in 2023 that reported disagreement between Enterolert and ddPCR and characterized 56.3% of ddPCR excessive results as false positives when Enterolert was treated as the standard. The finding deserves consideration, but it does not overturn the county’s far larger, geographically broader, and seasonally varied validation.
The county identified significant limitations in Coronado’s work: samples were collected by different organizations, the study was too short, its sample was comparatively small, and there were errors in the data output. Moreover, calling a ddPCR result a “false positive” because Enterolert did not agree assumes that Enterolert is correct, which is precisely the point at issue.
Neither test method ordinarily identifies every pathogen in the water. Enterococci are indicators warning that viruses, bacteria, and protozoa may also be present. The absence of culturable enterococci does not establish the absence of pathogens, which may be present even when damaged indicator bacteria no longer reproduce in a culture tray.
San Diego County warns that contact with fecally contaminated recreational water can result in many diseases including meningitis and paralysis. Children, older adults, and people with compromised immune systems may face greater risks.
When someone may become seriously ill, prudent policy is to err on the side of caution. An unnecessary advisory is inconvenient and has economic consequences. An erroneous declaration that sewage-contaminated water is safe exposes people to a hazard they cannot evaluate for themselves.
Why is Enterolert still used more frequently nationally?
The answer is practical and institutional. It is less expensive, requires simpler equipment and less-specialized personnel, and is embedded in regulations and laboratory procedures. Most public laboratories already own the necessary equipment. Moving to ddPCR requires expensive instruments, molecular testing expertise, laboratory accreditation, local validation, and sometimes changes to laws and regulations.
Entrenched use of Enterolert simply demonstrates how difficult and expensive it can be for government agencies to replace an established procedure.
Under California law, a known untreated-sewage release reaching recreational water requires closure. County officials have explained that ddPCR is then used to determine when contamination has cleared sufficiently to reopen the water. The issue is therefore not simply whether ddPCR is closing otherwise safe beaches. In many cases, it determines when beaches affected by confirmed sewage can safely reopen.
The county’s experience supplies additional evidence. It uses ddPCR along more than 50 locations, yet the major increase in excessive readings has occurred primarily in sewage contaminated South County beaches. Comparable increases have not occurred along the remainder of the county coastline.
A technical dispute should not divert attention from solving the real problem. The contamination originates with inadequate sewage collection and treatment infrastructure in Mexico. The solution is not to encourage the use of a test that produces fewer warnings. It is to eliminate the sewage so that neither test detects a hazard.
Editor’s note: John Tato graduated from Coronado High School in 1965. He retired from the U.S. Department of State in 2005.
