Stormwater runoff in San Diego Creek, the primary tributary to Upper Newport Bay in Orange County, CA, has been found to be toxic to Ceriodaphnia and Mysidopsis with total acute 24 hr toxicities up to 16 acute toxic units (TUa). About half of this toxicity has been found to be due to the organophosphate (OP) pesticides diazinon and chlorpyrifos used in urban areas for structural termite and ant control, and lawn and garden pest control. The other half is due to unknown causes likely associated with the use of pesticides in commercial nurseries and agriculture. The non stormwater flow into Upper Newport Bay is non-toxic. Stormwater runoff events cause short-term pulses of toxic waters to enter this marine bay.
Studies on the fate of the toxic pulses of San Diego Creek water in Upper Newport Bay show that for many stormwater runoff events, the Creek freshwater forms a lens above the marine waters that is diluted by tidal mixing with the bay's marine waters. The regulatory issue that should be assessed is the water quality and ecological significance of the pulses of aquatic life toxicity that exist in the waters at the bottom of the freshwater lens where the Creek waters mix with the marine waters. For some stormwater runoff events, potentially toxic waters are present in a limited areal and volumetric extent in the bay for several days. It is unclear whether these toxic pulses of OP pesticides are significantly adverse to the beneficial uses of Upper Newport Bay because of their limited duration and the limited number/type of marine aquatic organisms that are exposed to toxic conditions associated with stormwater runoff events. Further studies need to be conducted to define the magnitude of the potential beneficial use impairment of bay waters due to this toxicity.