Pulsed versus continuous exposures to evaluate the toxicity of sanitary sewer wet-weather overflows

Wet-weather overflows (WWOs) of raw untreated wastewater have the potential to introduce elevated concentrations of ammonia and metals into aquatic environments for brief periods, varying from a few hours to several days. Existing risk assessment tools for discharges typically compare toxicant concentrations with guideline values and use direct toxicity assessments assuming continuous rather than pulsed exposures. In this study, the water flea, Ceriodaphnia dubia, was used for both continuous (8-day, equivalent to 192-hr) and pulse (6-hr and 24-hr) chronic toxicity assessments of wet-weather, rain-ingress, and diluted influent, as well as water samples from the downstream receiving streams of Darling Mills Creek and Buffalo Creek in Sydney, Australia. Because partial responses were missing in some exposure scenarios, model fitting and the use of Effect Concentration causing 10% response values were deemed unreliable. Therefore, No Observed Effect Concentration values were used instead. No Observed Effect Concentration values for 6-hr and 24-hr pulse exposures were higher than those for continuous (8-day) exposure. When the concentrations of copper, zinc, and ammonia in pulse exposures were expressed as time-weighted average concentrations, they were found to be lower than those observed in continuous exposures to the same contaminants. The hazard quotient was below 1 during pulse exposures. The estimated required dilutions of influent were a high 1 in 4 during continuous exposure, but during 6-hr and 24-hr pulse exposures were considerably reduced to 1 in 1.25 to 2. This study enhances our comprehension of the toxicity associated with pulse contaminant exposures and contributes to the development of more effective approaches for the risk assessment and regulation of the more frequent (typical) short-duration sanitary sewer WWOs.