Use of radon-222 to evaluate the influence of groundwater discharge on fecal indicator bacteria concentrations in the near-shore ocean, Malibu, California

To protect beach-goers from waterborne disease, California requires water-quality monitoring for fecal indicator bacteria (FIB) at beaches having more than 50,000 visits annually. The source(s) of FIB in ocean beaches in excess of marine recreational water standards is often not known, or may be incorrectly identified. Onsite wastewater treatment systems (OWTS) used to treat residential and commercial sewage have been implicated by regulatory agencies as a possible source of FIB to recreational ocean beaches, near Malibu, California. For this to occur, treated wastewater must first move through groundwater prior to discharge at the ocean. Groundwater discharge to the ocean near Malibu Lagoon (the estuary of Malibu Creek) is complicated by seasonally changing water levels in the lagoon. The lagoon is isolated from the ocean by a sand berm that develops across the mouth of the lagoon during the dry season. Higher water levels in the lagoon during the dry season, and lower water-levels during the wet season, cause seasonal changes in the direction of groundwater flow and the magnitude of discharge from the adjacent small (3,400 hectare), alluvial aquifer. Radon-222, an indicator of groundwater discharge, was measured in Malibu Lagoon, in the near-shore ocean adjacent to the lagoon, and in the near-shore ocean adjacent to unsewered residential development to determine the timing and magnitude of groundwater discharge. During the dry season, when the berm of the lagoon was closed and the lagoon was isolated from the ocean, radon-222 concentrations in the near-shore ocean during low tide increased as water discharged from the lagoon through the berm. Enterococcus concentrations in the near-shore ocean increased to almost 600 Most Probable Number (MPN) per 100 milliliter at this time. Radon-222 concentrations also increased at low tide as groundwater discharged to the ocean from the adjacent alluvial aquifer underlying the unsewered residential development, but there was no corresponding increase in FIB concentrations. In contrast, during the wet season when the berm of the lagoon was open, radon-222 data show groundwater discharge from the alluvial aquifer was primarily into the lagoon at low tide. The water in the lagoon was subsequently discharged directly to the near-shore ocean as a result of tidally driven circulation through the open berm. Radon-222 data showed little, if any, groundwater discharge to the near-shore ocean adjacent to the unsewered residential development at this time, implying that FIB in the near-shore ocean would not be derived from OWTS in the unsewered residential areas. The absence of FIB in recreational beaches adjacent to unsewered residential development at low tide suggests that mechanisms other than groundwater discharge may be responsible for high FIB concentrations in this area. For example, microbial communities sampled in the near-shore ocean were genetically similar to those present in kelp and different from OWTS samples-suggesting that high FIB concentrations, especially at high tide, may result from wave action on the beach releasing FIB entrained in kelp that was derived from birds and other non-human sources.