Beach closures linked to toxic groundwater

Every summer, coastal communities from Maine to California are forced to close some of their most popular beaches because of unsafe levels of bacteria in the water. Typically, these sudden bacterial blooms disappear, only to return without warning later in the season. In many cases, health officials are unable to pinpoint the cause of the contamination, leading frustrated beachgoers to blame everything from offshore sewage pipes to passing cruise ships.

According to a study by scientists at Stanford (Calif.) University, the source of some of these unexplained pollution events may lie just a few feet below the sand, in contaminated groundwater that discharges into the surf zone, particularly during periods of extreme tides.

"Our study is the first to show that beach groundwater is a potential source of pollution," notes Alexandria B. Boehm, assistant professor of civil and environmental engineering. She points out that most beach pollution studies have focused on major infrastructure failures, such as inadequate sewage treatment plants or overwhelmed storm drain systems, while ignoring the possibility of groundwater contamination in the beach aquifer itself.

"Most people think that, once groundwater passes through sand and rock, the bacteria are filtered out," adds co-author Adina Paytan, assistant professor of geological and environmental sciences. "What we've found is that sand is really not a filter or a barrier."

A National Resources Defense Council survey of the more than 1,200 beach closings and advisories in the United States during a recent 12-month span notes that 62% were attributed to "unknown sources."

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