California state and federal water quality criteria have been established for ammonia, although the range of effects appear to vary under different water quality conditions such as water hardness, ionic strength (conductivity), total dissolved solids (TDS), and pH (Borgmann and Borgmann
1997; Ankley et al. 1995). A site specific objective (SSO) using a water effects ratio (WER) can be used to qualify existing criteria to identify a threshold effect level which has been scientifically tested and validated for the specific waterbody of concern. Our laboratory studies investigated
acute and chronic effects of ammonia exposure to the amphipod, Hyalella azteca to assist development of a WER for ammonia for two waterbodies of Los Angeles County (the Los Angeles and San Gabriel Rivers). A comprehensive series of toxicological testing was conducted during 4, 7, 14,
28, and 42-d tests with exposure concentrations ranging from < 1 to > 250 mg/L total ammonia. Associated water quality parameters such as water hardness, TDS, alkalinity, sodium (Na+), potassium (K+), and total organic carbon (TOC) were carefully monitored
and assessed as contributing factors. Results from the acute toxicity testing demonstrated a reduction in toxicity for ammonia-spiked river waters relative to the moderately hard laboratory water (MHLW) by a factor of >2; ammonia toxicity was also reduced by a factor of approximately 2-fold
when both EC50 and EC20 responses were compared. Results from chronic testing indicated that spiked ammonia concentrations <25 mg/L in SGR water did not cause adverse effects on the survival, growth, and reproduction of H. azteca over a 42-d exposure. Hormesis,
or improved survival and growth at low concentrations of ammonia, was observed in all exposure tests that utilized SGR water during chronic testing. Survival was greater at concentration >1 mg/L total ammonia with no influence on survival apparent at concentrations approaching 25 mg/L
total ammonia. A preliminary WER was calculated from data generated from the acute bioassay series ranged from 2.5 to 4.9 for the EC50 determinations. Chronic testing indicated the WER threshold was above our highest test concentration, 25 mg/L.
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