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(022) Variability in WET results in a municipal wastewater effluent.
Armstrong, J1, Sakamoto, K1, Tremblay, D1, 1 Orange County Sanitation District, Fountain Valley, CA, USA
ABSTRACT- Inter- and intra-test variability in freshwater and marine acute and marine chronic WET test results was assessed in six species in a municipal wastewater effluent. The test species included the fathead minnow (Pimaphales promelas) and water flea (Ceriodaphnia dubia) freshwater acute tests, the Ease Coast mysid shrimp (Americamysis bahia) and inland silverside minnow (Menidia beryllina) marine acute tests, and the red abalone (Haliotis rufescens) larval development and purple sea urchin (Strongylocentrotus purpuratus) fertilization marine chronic WET tests. Inter-test variability was measured in tests conducted on separate samples collected between April and November 2002. Disinfection of the effluent with chlorination-dechlorination was initiated on August 1, 2002. The variability in the pre-and post-disinfection samples were analyzed separately. Intra-test variability was determined by conducting separate tests on split samples (two pre-disinfection samples only). Variability was measured as the coefficient of variation (cv) among test results. The degree of variability depended on the species and effluent type (disinfected or non-disinfected). In disinfected effluent, toxicity magnitude and variability was reduced by approximately 50% for all species, except C. dubia for which variability almost doubled while magnitude decreased slightly. Intra-test variability ranged from 0% (H. rufescens and S. purpuratus) - 116% (P. promelas) among test species. Intra-test variability was greatest in P. promelas ranging from 3% - 116% in the two samples tested and least in S. purpuratus, which was 0% in both samples. Correlation analysis indicated that test animal condition (as measured by reference toxicant test results) and differences in water quality parameters (pH and dissolved oxygen) can contribute significantly to variability in WET test results.
Key words: coefficient of variation, WET, acute, chronic
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