Woods Hole researcher says beach sand may harbor harmful bacteria

In some cases, when high levels of harmful bacteria are present, beach sand may represent the same public health risk as water.
File photo by Susan Safford

In some cases, when high levels of harmful bacteria are present, beach sand may represent the same public health risk as water.

An article in Oceanus, a magazine published by the Woods Hole Oceanographic Institute (WHOI), suggests that, like the water people swim in, beach sand may sometimes contain elevated levels of dangerous bacteria which could constitute a public health risk. However, except in extreme case of contamination such as from a sewage spill, common sense safeguards will avoid illness.

WHOI physical oceanographer Britt Raubenheimer was alarmed by the experience of a colleague who contracted an infection from a “particularly nasty strain of antibiotic-resistant bacteria” that he was exposed to on a beach (he had a small cut on his leg) and spent three days in hospital. Dr. Raubenheimer and her colleague invited biologist Rebecca Gast, who specializes in microbial ecology, to join them in a project to find out whether beach sand might serve as a reservoir of microbes. Oceanus reports, “At the time, there had been no studies of how many microbes are in the sand, how long they live, or how far they could move from their initial location.”

The WHOI scientists sampled DNA from Enterococcus faecalis, a species that comes from feces and can cause illness in people, such as diarrhea, nausea, stomach ache, upper respiratory illness, rash, eye ailments, earache, or infected cuts.

One of the study’s interesting findings is that even sand below the surface at the high water mark was not free of bacterial DNA. “Dry” sand is not completely dry. It contains enough moisture to support bacteria. “The dry sand is worse than wet sand, in some ways,” said Dr. Raubenheimer. If the water is clean, “you’re better off being . . . where the sand has been washed by the waves quite a bit.”

The Times contacted Elizabeth Halliday, who works with Dr. Gast. Ms. Halliday is a doctoral candidate in biological oceanography, now in her fifth year in a program offered jointly by WHOI and MIT. She explained: “Just like soil, sand has a rich bacterial community that is generally harmless. The fecal bacteria in dry sands results primarily from the deposition of fecal bacteria directly on the beach, for example from the poop of dogs, birds, wildlife, or people. Foot traffic can spread it around very quickly. Likewise, fecal bacteria from poop on land — including pets, agriculture, or human waste from leaky infrastructure or combined sewage overflow — may be washed onto the beach with rain. Unlike the wet sands that are in constant flux with the water, bacteria from all these different sources can accumulate in the dry sand and sometimes grow. That’s why we see apparently higher amounts of fecal bacteria in dry sand than wet sand and probably more bacteria in the top few inches of sand than from deeper down – sand that is buried one foot deep is likely not in contact with all these sources.”

People, especially children, interact differently with sand than they do with water. Ms. Halliday told The Times, “The relative risks associated with getting ill from contaminated water correspond to specific exposures that occur with typical bathing behavior, like whether you put your head under water, how much time you spend in the water, etc. These influence how many bacteria we accidentally swallow/get in our ears/etc. The way we interact with sands is different; even if we assumed that days with elevated bacteria in the water corresponded to elevated bacteria in the sands, we have no evidence that the standard used for water recreation is appropriate for sand activities too. We don’t have studies to show whether interacting with sands on these days would result in more illness.”

However, the risk is low. Ms. Halliday went on to say that the more you interact with the sand (sitting on the sand, digging in it, getting buried in it), the more the risk of illness, “but in total, the incidence of these (generally minor) illnesses is very low . . . one or two extra illnesses per thousand above the baseline ‘normal’ rate.

“The good news is that we don’t tell people to stay off the beach, but by understanding this slight increase in risk, we can take steps to reduce our exposure – like taking time to wash our hands before eating or showering off after being buried in the sand,” Ms. Halliday concluded.