Source investigation of non-rain induced freshwater inputs to the San Pedro shelf, California

While conducting routine water quality monitoring a low salinity signal was observed during the summer, almost three miles offshore at the surface, inshore and upcoast of the Orange Sanitation District´s, California ocean outfall. During this time, state shoreline bacteriological standards for total coliform bacteria were exceeded. A study was conducted in the summer of 1997 to determine the origin of this low salinity water using salinity and total and fecal coliform bacteria as primary indicators. Following hypothesis (H₀) was tested: the low salinity surface water and elevated shoreline bacteria were due to effluent discharged from the District´s ocean outfall. Results showed (1) lower salinity water upcoast and inshore of the outfall with elevated levels of bacteria; and (2) an area downcoast and inshore with elevated levels of bacteria not associated with lower salinity water. However, since water column stratification (e.g., thermocline/pycnocline) is strongest in the summer and there was no indication of advection of the outfall plume, H₀ was rejected. The authors concluded that the most likely sources of the shoreline microbiological contamination was due to dry weather urban runoff from the San Gabriel River and storm drains upcoast and the Talbert Marsh downcoast; the later assertion is still problematic and is the subject of continuing study