Each summer, coastal communities from Maine to California are forced to temporarily close 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 ahave been unable to pinpoint the cause of the contamination, leading frustrated beachgoers to blame everything from offshore sewage pipes to passing cruise ships.
But according to a new study by Stanford University scientists, the source of some of these unexplained pollution events may lie just a few feet below the sand, in contaminated groundwater that moves into the surf zone especially during periods of extreme tides. The findings were published May 20 on the Environmental Science & Technology (ES&T) website.
"Our study is the first to show that beach groundwater is a potential source of pollution," said Alexandria B. Boehm, the Clare Boothe Luce, assistant professor of civil and environmental engineering at Stanford and co-author of the ES&T paper. She explained 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," added co-author Adina Paytan, an assistant professor of geological and environmental sciences at Stanford. "What we've found is that sand is really not a filter or a barrier."
The discovery that contaminated groundwater can discharge into the ocean and mix with coastal seawater may help explain some of the mysterious bacterial infestations that plague American beaches every year.
According to a National Resources Defense Council survey, of the more than 12,000 beach closings and advisories in the United States in 2002, 62 percent were attributed to "unknown sources."