Effects of octamethylcyclotetrasiloxane (OMCTS) on freshwater and marine organisms

Abstract

The toxicity of octamethylcyclotetrasiloxane (OMCTS) to representative freshwater and marine fish and invertebrates was investigated. Testing procedures followed Toxic Substance Control Act (TSCA) guidelines with chemical‐specific adaptations as presented in the OMCTS Testing Consent Order (Docket OPTS‐42071A). The physicochemical properties and behavior of OMCTS (i.e., high volatility, low water solubility) precluded 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 achievable (i.e., “functional”) solubility of OMCTS in natural dilution waters. The toxicant delivery systems and exposure chambers were designed to minimize volatilization by elimination of the air/water interface. The flow‐through systems maintained consistent exposure concentrations and adhered to U.S. Environmental Protection Agency (EPA) Guideline test performance criteria. The “functional” water solubility of OMCTS in freshwater and seawater ranged from 14 to 30 μg/L and from 6.0 to 9.0 μg/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 investigations with daphnids (Daphnia magna), rainbow trout (Oncorhynchus mykiss), mysids (Mysidopsis bahia), and sheepshead minnow (Cyprinodon variegatus). The rainbow trout was determined to be the most sensitive species to OMCTS (14‐d LC50 = 10 μg/L). At levels equal to the functional water solubility, OMCTS was not acutely toxic to D. magna, mysids, or sheepshead minnow. The survival of D. magna was reduced by 16%, relative to the control organisms, after 21‐d exposures to 15 μg/L OMCTS; exposure to 7.9 μg/L OMCTS or less had no effect on daphnid survival or reproduction. No toxicity was observed at the highest concentration tested in a 93‐d exposure of rainbow trout early life stages. The no‐observed‐effect concentration for this study was 4.4 μg/L, the same as determined in a 14‐d extended acute study.

Temperature Dependence of the Organic Carbon/Water Partition Ratios (KOC) of Volatile Methylsiloxanes

“…The NOEC from this study was 16.9 μg/l based on measured concentrations. It is possible that this study is one of the supplementary studies referred to by Sousa et al (1995). IUCLID (2005) also reports the results of an 18-day prolonged acute toxicity study with rainbow trout of various sizes.…”

“…Much of this information has now been published in review articles in the open literature (e.g. Sousa et al, 1995;Chandra, 1997). The essential details of the tests are reported here as many of the tests were carried out using modified test systems (e.g.…”

“…Sousa et al (1995) investigated the toxicity of D4 to a freshwater fish (rainbow trout, O. mykiss) and a marine fish (sheepshead minnow, Cyprinodon variegatus) in a prolonged acute toxicity study (14 days) using a flow-though system with no head space (to prevent loss from volatilisation). The D4 tested was >99 per cent pure and stock solutions of it were prepared daily by slow-stirring dilution water with a floating layer (approximately 6 mm thick) of D4.…”

“…The LC 0 for the larger fish (5 g) was >31 μg/l, but small fish were more sensitive (LC 80 = 23 μg/l). No further details of this test are currently available, but it is possible that this study is one the supplementary studies referred to by Sousa et al (1995).…”

“…Following on from the results from the prolonged acute study with O. mykiss (see Section 0), Sousa et al (1995) carried out a 93 day early lifestage study was with the same species. The stock solution preparation and test system used were similar to the prolonged acute study, but the flow rate was increased to give approximately 7.6 volume replacements per day.…”