Using Toxicity Identification Evaluations to Assist the TMDL for Aquatic Toxicity In Chollas Creek and San Diego Bay, California

Stormwater discharges from the mouth of Chollas Creek, a tributary of San Diego Bay, have been shown to be toxic to aquatic life. Although this section of the creek was added to the State's list of impaired waterbodies, the toxic constituent(s) were unknown and no linkage to the waterbody of concern (San Diego Bay) had been established. The primary objective of this study was to identify the toxicants and determine the areal extent of impairment in the waterbody of concern.The stormwater plume emanating from Chollas Creek was dynamic, covering areas up to 2.25 km2. Approximately half of the plume was estimated to be toxic to marine life, based upon results of purple sea urchin (Strongylocentrotus purpuratus) fertilization tests. The area nearest the creek mouth was the most toxic (No Effect Concentration, NOEC = 3 to 12% plume sample), and the toxicity decreased with distance from the creek mouth. The toxicity of plume samples was directly proportional to the magnitude of plume mixing and dilution until, once outside the plume margin, no toxicity was observed. Trace metals, most likely zinc, were responsible for the observed plume toxicity based upon toxicity identification evaluations (TIEs). Zinc was also the constituent identified from in-channel samples of Chollas Creek stormwater using TIEs on the storms sampled in this study, and in storms sampled during the previous storm season.