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Mahar, Amy M.

Water Resources and Lake Studies Center

2004

M.S.

This thesis examines joint trace metal-organophosphate toxicity and looks to identify potential causes of intralaboratory variability associated with the USEPA's chronic Ceriodaphnia dubia toxicity test. Both topics raise critical issues about the appropriate use or current reliability of the traditional, single toxicant, dose-response toxicity tests in the monitoring and regulatory process. Copper and zinc and the organophosphate insecticide diazinon, have been widely detected, often in combination, in urban stormwater runoff and adjacent surface waters at levels often exceeding USEPA and IJC chronic freshwater criteria. The traditional single-toxicant test does not provide a complete picture of types of interactions that may occur between multiple chemicals, with the consequence that the resulting water quality standards often do not provide adequate protection of aquatic life when effluent contains multiple pollutants. Additionally, the C. dubia test has been associated with a high degree of intralaboratory variability. In chronic tests, inconsistencies in the amount and timing of food introduction, thus the degree of toxicant adsorption and accessibility may be, in part, responsible for varying degrees of toxicity.
Differences in organism sensitivity within a single species, as a result of varying ages of brood stock, was also investigated as a possible source of variability. To gain insight into mixture toxicity and to increase control of variability, two separate sets of 7 day C. dubia toxicity experiments were conducted. In the first experiment, the joint toxicity of mixtures of copper, zinc and diazinon was determined using the concentration additivity model. Treatments included all combinations of the chemicals at 0, 25, 50, 75 and 100% of their individual LC50s, adding to equal one toxic unit (TU). The TU was then used to classify the response in each mixture as additive, greater-than or less-than-additive. For survival, additive interactions were observed in 75% zinc + 25% diazinon and 50% copper + 25% zinc + 25% diazinon treatments. For reproductive inhibition, additive responses were detected in 75% copper + 25% zinc, 75% copper + 25% diazinon, and 75% zinc + 25% diazinon treatments.
The less-than-additive interactions found for all remaining mixture treatments may be explained by the fact that diazinon causes toxicity through AChE inhibition and metals act through other modes. It is unlikely that such metal-organophosphate mixtures would be more toxic than predicted from the chemicals individually. In the second experiment, the effects of maternal age (7 or 14d) and the presence and amount of YCT food solids on toxicity were evaluated. Food solids, containing either 2.54 ug/L or 1.75 ug/L, were added 5 minutes before or 15 minutes after organism transfer.
Results showed no effect of maternal age on neonate survival or reproduction when the neonates were exposed to copper or zinc. Copper toxicity decreased consistently, and often significantly, with increases in suspended solids and/or delayed addition of food solids. Zinc toxicity also decreased, although not significantly, with increased suspended solids. Diazinon toxicity was unaffected by increases in food solids. This pattern of differing degrees of toxicity for each toxicant can be explained by how readily each toxicant adsorbs to the suspended solids, with positively charged copper adsorbing most readily and diazinon adsorbing least readily to particulates. In such static tests, food solids with adsorbed toxicants may settle out of suspension, changing the concentration of bioavailable toxicant to which the filter-feeding organism is exposed, altering the resulting toxicity.