Implications of pulsed chemical exposures for aquatic life criteria and wastewater permit limits.

Subacute effects of pulsed copper, zinc, or ammonia exposures were examined, including a range of pulse concentrations, durations, frequencies, and recovery times between pulses, using short-term chronic Pimephales promelas and 21-d Daphnia magna tests. Sublethal effects were rarely observed independent of mortality. Effects were observed only at concentrations near the species continuous exposure 48 h LC50 for each chemical. Daphnia often rebounded from temporary reproduction effects, meeting or exceeding control responses by the end of the test. Effects of 24 h ammonia or copper pulses were diminished soon after the pulse was removed, while 24 h zinc pulses caused continued effects for several days following removal of the pulse, indicating a slower uptake and/or depuration rate for zinc. D. magna exhibited less mortality as copper pulses were spaced further apart, while fish were equally or more affected with longer recovery times between copper pulses, indicative of different adaptation mechanisms between the two species. Responses were not predictable based on either average concentration or a combination of duration and concentration. Chronic water quality criteria and effluent permit limits, expressed as a 4- or 30-d average concentration, respectively, may not be appropriate for protecting against effects of pulsed exposures, depending on the frequency, magnitude, and duration of pulses, as well as the recovery period between events.