Coastal waters contaminated with bacteria such as E. coli may lead to stomach aches, diarrhea and rashes for swimmers who accidentally swallow the water, but that’s not the the worst problem.

It turns out, sand is even more contaminated than water by fecal contamination, according to a new study from a University of Hawaii researcher.

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The research  shows that fecal bacteria in beach sand is often 10 to 100 times higher than in nearby seawater.

One explanation for the higher levels of contamination is that microbial communities decay more slowly on beach sand than they do in salt water.

“For a beach, the sand is as important as the water, but our monitoring efforts have been placed almost exclusively on the latter,” according to Associate Professor of Civil and Environmental Engineering Tao Yan. “Results of this study support the need for a holistic beach management approach that includes sand, which can help further advance our goal of public health protection.”