The report appears in the Aug. 1 issue of the American Chemical Society's journal Environmental Science & Technology. ACS is the world's largest scientific society.
Coastal water quality is controlled by a number of complex physical and biological factors, including tidal cycles and seasonal rainfall. This complexity makes beach water monitoring difficult, with levels of bacteria in a certain area changing in just a few minutes.
For the new study, the researchers examined monitoring data compiled for beaches throughout Southern California, keeping track of tidal patterns and analyzing them for concentrations of enterococci -- bacteria that allow scientists to estimate the risk of illness from swimming in marine waters. "This is the largest array of beaches examined at the same time for a similar pattern," says Alexandria Boehm, Ph.D., an environmental engineer at Stanford University and lead author of the study.
She and her colleagues at the Southern California Coastal Water Research Project found that in the full and new phases of the moon, levels of enterococci were higher at the vast majority of the beaches studied. Boehm found that during so-called "spring tides," when water levels vary the most between high and low tides, a beach is twice as likely to be out of compliance with water quality standards. Spring tides are exceptionally high or low tides that take place during the full and new moons, but have nothing to do with the season of the year.
The results are of immediate practical use to swimmers and beach managers alike, according to Boehm. "The general public can use the phase of the moon and the tide stage to assess the relative risk of illness," she says. "It is riskier to swim during spring-ebb tides [receding tide] compared to all other tidal conditions."
Beach managers can now use tides as they currently use rainfall to assess warnings, Boehm suggests. When it rains, managers recommend that swimmers not enter the water for three days. "They could also suggest that during spring tides -- and especially spring-ebb tides -- water quality is more likely to be impaired, and those who are risk-averse should avoid swimming," Boehm says.
The results might also help managers identify potential sources of pollution at beaches. "Most sources of enterococci at beaches are unknown," Boehm says. "Because we found tidal signals in enterococci densities at beaches with no obvious point source, like storm drains and creeks, this suggests that there is a widespread tidally forced source of enterococci at beaches."
Boehm suggests several candidates for this "mystery" source, including beach sands, decaying plant material and polluted groundwater. "Beach sands and wrack [piles of seaweed and animal remains that wash ashore] have been shown at freshwater beaches to harbor fecal indicator bacteria and even pathogenic bacteria," Boehm says. "Beach managers who want to improve water quality at their beaches should investigate the potential of these sources to be contributors of enterococci to marine waters."
Boehm cautions that enterococci from beach sands and wrack may not correlate with health risk the same way as enterococci from runoff or sewage. "We just don't know for sure, since no one has done an epidemiological study to connect human illness to enterococci from non-point sources other than runoff," Boehm says. "We need to do additional work to understand the source of enterococci at all these beaches."
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-- Jason Gorss
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