EFFECTS OF OCTAMETHYLCYCLOTETRASILOXANE (OMCTS) ON FRESH-WATER AND MARINE ORGANISMS

The toxicity of octamethylcyclotetrasiloxane (OMCTS) to representative freshwater and marine fish and invertebrates was investigated. Testin g procedures followed Toxic Substance Control Act (TSCA) guidelines wi th chemical-specific adaptations as presented in the OMCTS Testing Con sent Order (Docket OPTS-42071A). The physicochemical properties and be havior of OMCTS (i.e., high volatility, low water solubility) preclude d the use of conventional experimental practices and exposure systems. Procedures and systems developed during these investigations provided a mechanism that produced exposure levels equal to the maximum achiev able (i.e., ''functional'') solubility of OMCTS in natural dilution wa ters. The toxicant delivery systems and exposure chambers were designe d to minimize volatilization by elimination of the air/water interface . The flow-through systems maintained consistent exposure concentratio ns and adhered to U.S. Environmental Protection Agency (EPA) Guideline test performance criteria. The ''functional'' water solubility of OMC TS in freshwater and seawater ranged from 14 to 30 mu g/L and from 6.0 to 9.0 mu g/L, respectively. Functional water solubility appeared to vary slightly with test conditions and dilution water characteristics. Continuous exposures of 2 to 93 d were conducted during these investi gations with daphnids (Daphnia magna), rainbow trout (Oncorhynchus myk iss), mysids (Mysidopsis bahia), and sheepshead minnow (Cyprinodon var iegatus). The rainbow trout was determined to be the most sensitive sp ecies to OMCTS (14-d LC50 = 10 mu g/L). At levels equal to the functio nal water solubility, OMCTS was not acutely toxic to D. magna, mysids, or sheepshead minnow. The survival of D. magna was reduced by 16%, re lative to the control organisms, after 21-d exposures to 15 mu g/L OMC TS; exposure to 7.9 mu g/L OMCTS or less had no effect on daphnid surv ival or reproduction. No toxicity was observed at the highest concentr ation tested in a 93-d exposure of rainbow trout early life stages. Th e no-observed-effect concentration for this study was 4.4 mu g/L, the same as determined in a 14-d extended acute study.