Sources of Fecal Indicator Bacteria and Nutrients to Malibu Lagoon and Near-Shore Ocean Water, Malibu, California

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Malibu Lagoon and near-shore ocean water in Malibu, Calif. have concentrations of fecal indicator bacteria (FIB) that occasionally exceed public health standards for recreational water.

Multiple graphs showing water level, salinity, streamflow over 12 months

Ocean tides, Malibu lagoon water levels, salinity and temperature, streamflow in Malibu Creek, and precipitation data, near Malibu, California, 2009.

(Public domain.)

Discharge of water from commercial and residential septic systems, and subsequent transport through shallow groundwater to the lagoon or near-shore ocean has been proposed as a possible source of FIB to the lagoon and the near-shore ocean. Other possible sources include direct deposition of fecal material and FIB from birds and other wildlife. The problem is complicated by the possibility of sustained survival or regrowth of FIB in the lagoon, especially during the summer months when water temperatures are warm.

The purpose of this study is to evaluate the occurrence, distribution, and sources of FIB and nutrients in shallow groundwater, Malibu Lagoon, and near-shore ocean water near Malibu, Calif.

The scope of the study includes detailed synoptic sample collection and time-series data collection in shallow groundwater, Malibu Lagoon, and the near-shore ocean. The study uses a combination of physical and isotopic hydrologic techniques coupled with state-of-the-art genetic, molecular, chemical, and optical tracers to determine the source of fecal contamination. Preliminary data collected during the summer and fall of 2009 were used to determine which techniques and tracers were suitable for use in this study. 

On the basis of the preliminary data, synoptic and time-series data collection strategies were developed. The synoptic data collection will include collection of traditional physical and isotopic hydrologic data coupled with genetic, molecular, and chemical tracers of fecal contamination. Data will be collected from shallow groundwater, Malibu Lagoon, and the near-shore ocean shortly after the rainy season to contrast with data collected during the dry summer season (summer 2009 data). Time-series data will be collected from selected wells, Malibu Lagoon, and the near-shore ocean at approximately bimonthly intervals for one year to provide information of groundwater quality and FIB concentrations under hydrologic conditions not sampled during synoptic data collection.

Isotopic data are proposed to trace the source of water (d18O and dD) and discharge of groundwater to Malibu Lagoon and the near-shore ocean (222Rn). Genetic (Terminal-Restriction Fragment Length Polymorphism, and human-specific Bacteroidales data), molecular (Phospholipid fatty acid data), and chemical data (wastewater indicator compounds) are proposed to trace the movement of bacteria and fecal contamination through the hydrologic flow system. Other tracers are proposed evaluate changes in the chemical composition of nutrients (d15N of nitrate and ammonia, and d18O of nitrate) and dissolved organic carbon (Ultraviolet absorbance and Excitation Emission Spectroscopy) as water flows through the system. No single hydrologic or bacteriological source tracking technique provides a truly unique identification of the source or hydrologic history of water sample or bacteria. As a consequence, interpretations from tracer data, used in both the synoptic and time-series data collection, are constrained by interpretations derived from traditional hydrologic and microbiological data.