scope:

1.3 Scope of Study

The American Petroleum Institute contracted with the Institute of Applied Sciences at North Texas State University to assess the role of suspended solids in regulating the bioavailability of petroleum refinery effluent chemicals to fathead minnows (Pimephales promelas) and Daphnia magna. Bioavailability of the test chemicals was determined by measurement of acute toxicity in the presence or absence of suspended solids. A chemical that was not available to cause acute toxicity was operationally defined as not bioavailable. The research consisted of six interrelated activities:

1. Selection of four (4) petroleum refinery effluent chemicals representing a broad spectrum of physical-chemical properties to be used in laboratory toxicity experiments on bioavailability

2. Selection of three (3) sediments representing a broad range of physical-chemical characteristics to be used in laboratory bioavailability experiments

3. Determination of the sorption partition coefficients and Freundlich isotherms for the four selected chemicals and the three selected sediments

4. Determination in the laboratory of how the three sediments acting as suspended solids affect the acute toxicities of the four chemicals over the range of concentrations of suspended solids typically found in freshwater systems

5. Development of correlations between suspended solids levels and acute toxicities for the test chemicals

6. Evaluation of experimental results to determine the efficacy of the Site-Specific Water Quality Criteria modification process to petroleum refinery effluent chemicals

The objectives and scope of this study imposed certain restrictions that were not apparent prior to initiation of the project. These restrictions became known as the "box" and required considerable experimental design and effort to overcome. The "box" also frames the results and conclusions and limits generalization of the results of this project.

Certain limitations comprise the sides of the "box." Contrary to our expectations, the list of refinery effluent chemicals did not include many chemicals that met specific selection criteria. We needed to use chemicals that both sorb and are acutely toxic. Selection of test sediments also required extensive activity. We anticipated that the organic carbon content would be one of the critical variables regulating the amount of chemical sorbed and thus the amount bioavailable. Careful, examination of the literature and other information revealed that organic carbon contents of suspended or bed sediments in "natural" situations do not vary an order of magnitude. Therefore, the sediment organic carbon content axis was compressed from about 0 to 2% and was not as suitable an independent variable for prediction of bioavailable (in solution) test chemical as we initially believed. Estimation of sorption partition coefficients (Kp) for the petroleum refinery effluent chemicals was hampered by the lack of accurate solubility values or previous laboratory measurement of partitioning potential. Sorption tests with chemicals that partition to solids almost totally (i.e., high Kp) are difficult to conduct due to the problems in determining the aqueous concentration of a relatively water-insoluble chemical. Kp values at low solids level (<5 mg/L) are also difficult to perform because these are usually determined by the difference between total and centrifuged sample analyses. Sorption tests of chemicals that do not sorb appreciably are also difficult to analyse in the solid phase.

The toxicity of the test chemicals forms another wall of the "box." Apparently, most chemicals that sorb to a great extent are not acutely toxic. The principal problem with highly insoluble chemicals appears to be chronic toxicity. Another consideration that comprised a side of the "box" was the shape of the dose-response curve for a candidate chemical. In order to measure reduction in toxicity due to chemical sorbed onto suspended sediments, the decrease in soluble phase chemical (dose) must yield a large reduction in toxicity (response). At the "real world" levels of suspended solids used in this project, the expected test chemical removal from solution was a relatively small fraction of the total mass. Given the number of test animals and other constraints, reduction in toxicity could be observed in this case only if the dose-response curve for a test chemical was relatively steep.