Sometimes after a storm, and sometimes for no apparent reason at all, “Beach Closed” signs appear at popular beaches and tourist spots. Beachgoers have become accustomed to the signs, and most people—though not all of them—avoid swimming and surfing when those signs are posted. But many still enjoy sunbathing on the beach, even if they’re not planning to go in the water. And that, a recent study shows, might be almost as much of a problem.
Researchers at the University of Hawaii studied areas and created models where the water had been contaminated by wastewater, such as might come from combined sewer overflows or leaking septic tanks. As this article reports, they discovered that although bacteria die relatively quickly in the water, they can remain for a longer period of time in the sand, and that the bacteria in the sand might recontaminate the water, potentially creating a “chronic source” of contamination at a given beach.
This article in the May/June 2005 issue of Stormwater magazine reported on similar findings, back when it was widely believed that beach sand was not a reservoir for bacteria. For that study, researchers studied areas in San Diego’s popular Mission Bay, which had experienced more beach postings and closures because of high bacterial levels than other beaches in San Diego County. They found that not only was the sand acting as a reservoir, but that the swimmers themselves—by disturbing the sand in areas with high concentrations of bacteria—were likely aiding in the recontamination process:
The results of the beach face transect assessment indicated there was a strong spatial pattern of bacterial densities along the beach face. Bacteria in beach face sediment samples collected in the upper intertidal zone were typically an order of magnitude greater than those in the lower intertidal zone. Thus, the beach face sands in the upper intertidal zone act as a reservoir for fecal coliform and enterococcus bacteria. The sediment resuspension assessment was designed to determine if bacteria associated with the upper intertidal beach face sediments were a source of bacteria to the receiving waters when the sediments are disturbed (e.g., by swimmer activity).
The results of the resuspension study indicate the bacterial reservoir maintained in the beach face sediments within the upper intertidal zone are released to the receiving waters when they are disturbed. This pattern was not observed when the experience was repeated in the lower intertidal zone.The question of what to do about the problem remains. Removing the source of the bacteria, of course, is the goal. In the meantime, as one infectious disease expert quoted in the article above recommended, if you’re sunbathing on the beach, it might be best to lie on a towel.