APPENDIX I Toxicity Identification Evaluation Reports for Chollas ...

Toxicity Identification Evaluation of Sweetwater
River Stormwater Using Selenastrum
capricornutum August 2006
5. DISCUSSION
Results of this TIE indicate that one or more cations that comprise the total dissolved solids
(TDS) found in stormwater samples from Sweetwater River may be responsible for the reduced
growth of S. capricornutum. First, the results of the 8 mg/L EDTA test allowed for the increased
growth of S. capricornutum or demonstrated that toxicity was reduced through the addition of
EDTA to the 100% stormwater sample. This indicates that the causative agent of toxicity was a
cation, such as a free metal ion (e.g., cadmium, mercury, lead), a dissolved inorganic salt (e.g.,
sodium, calcium, magnesium), or a cationic organic compound such as a cationic surfactant,
because EDTA is a chelating agent that is well known to bind inorganic and organic cationic
chemicals (USEPA 1991).
In addition to TIE tests, chemical analyses of Sweetwater stormwater samples indicate that
cations comprising the TDS may be the causative agent(s) of toxicity. Concentrations of TDS
measured in the undiluted stormwater sample were 2640 mg/L (see results in complete
stormwater report), a concentration that exceeds the water quality objectives for this measure,
or 1500 mg/L, in the lower Sweetwater River (RWQCB 1994). In contrast, no organophosphate
pesticides were detected in stormwater samples and the only metals detected were arsenic,
nickel, and zinc, all of which were found at low concentrations, or concentrations far below water
quality objectives.
TIE test results also indicate that the causative agents of toxicity in the Sweetwater River
stormwater samples did not share similar physicochemical properties to those of many other
classes of chemicals. The lack of toxicity reduction in the SPE and PBO tests indicates that the
causative agent was likely not a non-polar organic, or a common-use pesticide (e.g. pyrethroid
or organophosphate), respectively.
Statistical analyses of TIE results also provide some evidence that the causative agent could be
a ion imbalance; growth was diminished in the undiluted 100% stormwater sample, relative to all
treatments while growth was elevated in the 50% and 75% stormwater sample relative to the
dilution water (0% stormwater) sample. This result indicates that the causative agent was toxic
in the undiluted stormwater sample but significantly less toxic in the diluted stormwater samples.
In contrast, in the dilution water control, the lack of ions may have led to diminished growth
relative to the diluted stormwater samples. This result is not surprising because others have
shown that elevated growth in TDS-enriched natural waters as compared to TDS-poor synthetic
mediums (LeBlond and Duffy 2001).
It is well known that elevated concentrations of ions comprising the TDS, or an imbalance of the
ions comprising the TDS may be toxic to aquatic organisms; however, the concentration of TDS
that causes toxicity is highly dependent on the species and types of ions comprising the TDS.
While most species are tolerant of concentrations of TDS that exceed 1000 mg/L, some
spawning or larval fishes are highly sensitive to TDS. Concentrations of 350 mg/L TDS reduced
spawning of striped bass (Morone saxatilis) in the San Francisco Bay-Delta (Kaiser Engineers,
1969). In contrast the LC 50 for TDS for fathead minnows (Pimphales promelas) has been shown
to range from 2,000 mg/L to 15,000 mg/L in natural waters, depending in part on the types of
ions comprising the TDS (Mount et al. 1997). To the best of our knowledge, little is known about
the effects of TDS on S. capricornutum.
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