Aquatic toxicity testing was employed over a three year period to test the effectiveness of a secondary impoundment system in reducing biological toxicity of an industrial wastewater discharge. A two tiered approach was used to determine the effects of the wastewater on two cladoceran species, Daphniamagna and Ceriodaphniadubia, and two sensitive life stages of a vertebrate, Pimephalespromelas. Endpoints measured were both acute (lethality) and chronic (growth and reproduction). Results from the first year of testing, conducted on wastewater collected from the inflow to the secondary impoundment system, indicated both lethal and sublethal effects. Results from the second year of testing, conducted on the outflow of the secondary impoundment system, showed reduced chronic toxicity and complete absence of acute toxicity. Minor modifications were made to the existing treatment system and toxicity testing was conducted for the second consecutive year on the outflow of the secondary impoundment system. Results from the third year of testing showed no acute or chronic toxicity, indicating improved wastewater treatment.

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