Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Robert C. Hale


The interactive effects of humic acids (HAs) and salinity on the bioavailability and toxicity of the pesticides chlorpyrifos and 4,4'-dichlorodiphenyltrichloroethane (DDT) were investigated. The effects of various HAs on the toxicity of chlorpyrifos were initially assessed with the chronic MicrotoxRTM test. Environmentally relevant concentrations of Aldrich, Peat, Leonardite and Suwannee River HAs had no significant effect on the toxicity of either chlorpyrifos or copper (Cu), the test's positive control. as reductions in contaminant toxicity had previously been reported for some contaminants by HAs, it appeared that salinity might be a mitigating factor. Thus salinity effects were further investigated with Aldrich HA only, in freshwater and at lower salinities. HA-pesticide associations were assessed through the pesticides' relative uptake by solid-phase microextraction (SPME) in the presence of Aldrich HA. Such binding has been hypothesized to reduce contaminant bioavailability. Increasing salinity (0--20 ppt) had no effect on the uptake of DDT by SPME, but generally enhanced that of chlorpyrifos. Aldrich HA alone greatly decreased the relative uptake of both pesticides at environmentally relevant DOM concentrations (∼0--20 mg C/l), and the effect was more pronounced for DDT. Increases in salinity reduced the effects of HA on uptake by SPME, and seemed to have an "effect threshold" between 1 and 5 ppt. to examine the biological consequences of possible HA-salinity interactions, the acute toxicities of chlorpyrifos and DDT were assessed using both freshwater (Ceriodaphnia dubia) and saltwater (Americamysis bahia) crustaceans. The DDT 24 hour LC50 for C. dubia was 1050 ng/l. No definitive value was obtained for A. bahia. CPF was more acutely toxic, with LC50s of 326 ng/l for A. bahia and 78.8 ng/l for C. dubia. Results of the acute toxicity experiments were in good agreement with the SPME data; i.e. while pesticide toxicities were reduced by HAs in freshwater, no reductions were seen in saline water (20 ppt). The toxicity reduction in freshwater was proportional to the HA concentration. The difference in toxicity mitigation is believed to be a function of salinity effects on HA-pesticide binding, likely due to conformational changes in the HA molecules, rather than organismal effects.



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