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Wet weather discharges from urban, highway, and combined sewer overflow (CSO) systems are often evaluated in the context of traditional “steady-state” conditions and dry-weather oriented water quality standards. Of course, wet weather discharges are highly variable in concentration, load, and duration; and the traditional tools for evaluating these discharges for their potential toxic effects do not account for this variability. An innovative method to assess time-variable toxicity of wet weather discharges has been developed. The time-variable bioassay generally consists of standard techniques, but with the key exception being that test organisms are exposed to a sequence of discrete wet weather samples collected over the duration of an event. The time-variable method has been tested and compared to traditional toxicity testing using multiple wet weather discharges from two highway bridge decks and several CSOs in Atlanta.

For the bridge sites, the time-variable tests showed that acute and chronic toxicity was either not significant or minor under a variety of wet weather event conditions, particularly when receiving water dilution was considered. In contrast, most of the traditional bioassays resulted in a significant toxicity response. The biological surveys of the receiving waters reinforced the conclusion of the time-variable testing. For the CSO sites, there was no observed mortality in 48-hour acute tests in 100 percent effluent (CSO) at any of the seven CSOs with dechlorinated CSO. Therefore, the time-variable tests became a confirmation of the standard 48-hour acute tests. Additional toxicity tests conducted in conjunction with water-effect ratio testing of the effluent have continued to confirm this lack of an acute toxic response to CSO.

These results illustrate the importance of developing and implementing a decision-solution process that integrates biological and water quality information in a way that properly addresses the time-scales associated with wet weather events and related potential effects. Incorporation of “weight of evidence” analysis and “strength of evidence” analysis into this overall decision-making process will contribute further to sound regulatory and management decisions related to wet weather discharges.
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Document Type: Research Article

Publication date: 2003-01-01

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