Alm speculated that contaminated sand may therefore be the source of bacteria that cause beach closings.
"Often a beach is closed because of high counts of fecal indicators in the water, but we often can't identify a source for itno sewage overflow or storms, just unexplained high levels of bacteria in the water," she said.
Research now suggests that these waters may become contaminated as they lap up on shores rich with sand bacteria.
But since the bacteria themselves are not harmful to humans, Alm added, many of these beach closings may be unnecessary.
"Another possibility that hasn't been explored yet is, if fecal indicators can persist in the sand, could other fecal microorganisms, ones that are disease-causing, persist also? We don't have any information on that yet," she said.
According to McLeod, researchers are also uncertain about the sources of sand contamination, although humans are at least partly to blame. For example, he said, scientists have confirmed a link between fecal contamination from animals and human food waste.
"When people eat at the beach and leave food litter or leave food unattended, and you see a seagull or other birds eat the food, they tend to defecate right into the sand, and that contaminates the sand immediately," he said.
Alm noted that the sand is also full of naturally occurring microorganisms. Studies show these bacteria are efficient at exchanging genetic information with each other.
She is interested in finding out if this genetic exchange carries over to foreign bacteria. If so, this could potentially lead to antibiotic resistance among disease-causing bacteria.
"It gets kind of tricky when you start thinking about all the possibilities," she said.
McLeod said the report is meant to raise awareness of beachgoers, prompting them to "leave no trace" when visiting the beach and to wash after playing in the sand.
"People generally tend to go to the beach to flop out and have a good time," he said. "We want them to do that, but we want them to be aware and be smart. We think smart, educated beachgoers will be healthy beachgoers."
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