Acute toxicity of monochloramine to juvenile chinook salmon (Oncorhynchus tshawytscha Walbaum) and Ceriodaphnia dubia

A comprehensive data set for the acute toxicity of monochloramine to juveni le chinook salmon, Oncorhynchus tshawytscha, and the aquatic invertebrate C eriodaphnia dubia are presented. For exposures up to 10 days, the equation LC50 = 7244t-(0.4525) (where LC50 = monochloramine concentration in mg/L fo r 50% lethality and t = time in minutes) can predict the LC50 value for mon ochloramine in juvenile chinook salmon and accounts for 94.4% of the variab ility in the experimental data. Predictions of acute toxicity were less rel iable for concentrations in the range 0.5 mg/L to 0.8 mg/L since the LT50 v alue varied substantially in this range. Post-exposure mortality occurred o nly with exposures of >0.67 mg/L monochloramine. Feeding fish immediately b efore and during acute monochloramine exposure created a large chloramine d emand in test chambers. In all tests where fish were fed, measured monochlo ramine concentrations were below detectable levels (<10 mg/L) and no fish m ortality occurred. Monochloramine toxicity to the freshwater invertebrate Ceriodaphnia dubia w as studied with time-to-lethality tests at 26 different concentrations. The equation LC50 = 61600t-(1.0748) predicted the LC50 value for chloramine in C. dubia and accounted for 94.8% of the variability in the experimental da ta. The mathematical equations that describe the LC50 for juvenile chinook salm on and C. dubia were used as reference lines for the evaluation of relative monochloramine toxicity Both juvenile chinook salmon and C. dubia were amo ng the most sensitive fish and aquatic invertebrate species, respectively, C. dubia being more sensitive than juvenile chinook salmon for exposure dur ations >30 minutes.