"In conventional septic systems, wastewater treatment tends to be inefficient for certain contaminants," said Nick de Sieyes, an engineering graduate student working with Boehm. "As a result, untreated sewage can end up polluting nearby groundwater."
Prior to this study, scientists had never observed in detail a plume of contaminated groundwater flowing from a septic system to the sea. To track groundwater pollution at Stinson Beach, the research team obtained a permit from the National Park Service to install a network of 120 monitoring wells near a large septic system close to a beach parking lot that collects wastewater from nearby homes and public toilets.
The wells were placed in parallel rows on the beach separating the septic system from the ocean a distance of several hundred yards. This network of wells allowed researchers to collect groundwater samples and assess the degree of contamination flowing from the septic system, through the beach and out to sea.
The results were encouraging in one respect, de Sieyes said. Tests revealed low concentrations of fecal indicator bacteria - microbes that are used by health officials to evaluate water quality for beach closures.
"The septic system appeared to be treating fecal indicator bacteria to a relatively high degree, so the chance of triggering a water-quality advisory in the surf zone during our study was low," he said.
Although few microbes made it out of the leach field alive, the scientists discovered a plume of nitrogen-enriched groundwater flowing through the sand toward the ocean. Studies have shown that excess nitrogen can cause harmful blooms of phytoplankton and other algae that choke off oxygen in coastal waters.'/>"/>
|Contact: Mark Shwartz|