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I hate to ruin your visit to the beach this weekend, but be warned: Multiple scientific tests have detected fecal bacteria in sand. That’s right — bacteria from human and animal feces. And while authorities routinely monitor beach water for high levels of bacteria, they don’t monitor the sand.

Perhaps they should as evidence suggests that contaminated sand can make you sick, causing diarrhea and other gastrointestinal illnesses.

Fecal bacteria have been detected at levels 10 to 100 times higher in sand than in the nearby water. Scientists recently discovered why that is: Fecal bacteria live longer in the sand than in the water, according to a study in the journal Environmental Science & Technology.

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Tao Yan and colleagues at the University of Hawaii at Manoa collected samples of beach sand and seawater on Oahu. In a laboratory, they spiked those samples with raw wastewater from a nearby plant, and then observed them for 24 days to see what would happen to the microbial communities within.

Two major strains of fecal bacteria, Enterococci and Clostridium perfringens, died off quickly in the water but not in the sand. In the sand, the bacteria lived on. The discovery appears to explain why scientists have long detected higher levels of bacteria in sand than water, says Yan.

We don’t yet know why the bacteria thrive in sand. It may be because sand contains cavities that lack oxygen, while seawater is full of oxygen. Most fecal bacteria, which come from the gut, prefer environments without oxygen.

Another possibility is that the bacteria are attaching to sand particles to form durable biofilms, which provide protection for the microbes.

Unfortunately, hand sanitizer won’t kill off these bacteria, says Yan. To avoid sickness from the sand, avoid contact with any open wounds or sores, he suggests.

And you might want to think twice about visiting a beach that has been closed due to high levels of bacteria in the water.

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“If a beach is closed, you shouldn’t go into the water, and you probably don’t want to mingle with the sand either,” says Yan. “The sand likely has even higher rates of bacteria.”