In the summer of 1999, high levels of sewage-loving bacteria shut down much of the shoreline at Huntington State Beach, south of Los Angeles. Scientists still have no idea what caused the upswing in bacteria. Now, a team of environmental scientists suggests that contaminated water beneath the beach might be the source.
Scientists have been scrambling to find the origin of the coliform bacteria. Although harmless themselves, these bacteria signal the presence of feces, and their presence has been linked to sickened beachgoers and associated with the microorganisms responsible for dysentery, cholera, and typhoid. In Huntington's case, however, the usual sources--sewage outflow, hot water discharge from power plants, or bird feces from marshlands--have been ruled out.
Now, a trio of environmental scientists at Stanford led by Alexandria Boehm is pointing fingers at the groundwater in beaches. To investigate seepage that could be contaminating the surf zone, the team collected water from shallow holes dug on the beach. Because groundwater readily sucks up radium isotopes from the soil and seawater has little radium, the team was able to estimate the amount of groundwater leaking into the surf at any given time by measuring radium levels.
In an article published online 20 May in Environmental Science and Technology, the team reports that unusually high levels of radium were associated with high bacteria counts in the surf zone. But the mystery is far from solved. When the team tested 26 groundwater samples for bacteria, only one turned up positive. That suggests at least two possibilities, says Boehm. There still might be an elusive but concentrated groundwater source of bacteria, or the groundwater--which they found to be rich in nutrients--may sustain bacteria that get to the surf zone from some other source.
Still, others aren't convinced the groundwater is a player in the pollution problem. The team hasn't shown cause and effect, says Leslie Rosenfeld, a physical oceanographer at the Naval Postgraduate School in Monterey. Groundwater flow and bacteria counts would coincide if both are tied to the tides, she points out. And Marlene Noble, an oceanographer at USGS in Menlo Park, adds that no one really knows what impact nutrients would have on bacteria in the surf zone.