Under the 1993 general discharge permit for produced waters on the Outer Continental Shelf, discharges must be tested periodically to determine chronic toxicity to marine organisms. If toxicity is observed, regulators are likely to require the permittee to undertake Toxicity Identification Evaluation (TIE) studies to determine the cause of toxicity. Information on the toxicity of total salinity or individual ions is essential for guiding TIE studies with produced waters. The Gas Research Institute is sponsoring the development of such information for three common marine test species: mysid shrimp (Mysidopsis bahia),sheepshead minnow (Cyprinodon variegatus), and inland silverside minnow (Menidia beryllina). Based on test results, Salinity-Toxicity Relationship (STR) models are being constructed that will predict the survival of these three species upon exposure to a produced water of known ionic composition. Similar models for freshwater organisms are already in use. When routine biomonitoring indicates that a sample of produced water is toxic to one of the test organisms, the STR model can be used to determine whether the ionic composition of the sample is a contributing factor. If the predicted toxicity of the sample is similar to the measured toxicity, the effect can be presumed to be due to an excess or deficiency of one or more ions, or of total salinity. The STR model can then be used to identify the ion or ions in excess or deficiency, and to suggest strategies for eliminating the sample toxicity by restoring the appropriate ion balance. If the STR model predicts that the sample should not be toxic based on its ionic composition, other sources of toxicity (e.g., trace metals) are implied.

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