4.3. Recommended water quality guideline trigger values for ammonia
The derived guideline values used to date by regulatory agencies have all been based on NOECs. The limitations of NOEC values have been widely discussed (e.g. Chapman et al., 1996) and it is generally agreed that EC10 or EC20 values be reported in toxicity
testing and used in future guideline derivation. These values were,however, not reported in the earlier studies of ammonia toxicity, or in the more recent papers used in this guideline revision, so NOEC values had to be used.In terms of aqueous guidelines, an ammonia concentration 460 lg total NH3–N/L is recommended as representing a low risk of acute or chronic toxic effects in slightly to moderately disturbed systems, i.e. the PC95 value that is protective of 95% of species. The PC90 concentration of 750 lg total NH3–N/L represents a low to moderate risk of toxic effects, with possible chronic toxicity to a few sensitive organisms. A guideline concentration of 7.9 mg total NH3–N/L (the concentration below the LC50 for 95% of species)represents a major risk of acute toxicity to sensitive species