Fish Study Abstract

Title : Decamethylcyclopentasiloxane (D5): Prolonged Toxicity to the Rainbow Trout ) Oncorhynchus mykiss) Under Flow-Through Test Conditions
Author : Drotter, Durt R., Woodburn, Kent B.

The purpose of this study was to determine the prolonged toxicity of decamethylcyclopentasiloxane (D5) to the rainbow trout (Oncorhynchus mykiss) during a 45-day exposure under flow-through test conditions. The test was performed based on procedures in OECD Guideline 204, Fish, Prolonged Toxicity Test: 14-day Study. The results of the study were based on nominal test concentrations. The in-life phase of the test was performed from March 16, 2009 to April 30, 2009.

Juvenile rainbow trout (average total length 27 mm) were exposed to five test concentrations, a negative (dilution water) control and a solvent control (0.1 mL dimethylformamide/L). Ten rainbow trout were exposed in each test chamber and two replicates were tested per treatment group for a total of 20 fish per test concentration. Dilution water was dechlorinated municipal water. Each replicate received approximately 7.2 volume additions per day. Nominal test concentrations were 1.1, 2.1, 4.3, 8.5 and 17 µg/L. The highest test concentration chosen was the published water solubility of D5. Analytical determination of test concentrations was not performed during this study.

Visual observations of the test solutions during the test indicated that the solutions were clear and colorless. Three times during the test a stock solution delivery pump stalled. However, due to the prolonged (45 day) exposure, these temporary (≤ 6 hour) interruptions in test substance delivery were not considered to influence the overall exposure.

Manual measurements of temperature during the test ranged from 11.4 to 12.4°C. Measurements of dissolved oxygen remained ≥ 8.0 mg/L (76% of saturation) and pH ranged from 7.1 to 7.5. Dilution water characteristics at test initiation were as follows: hardness -- 120 mg/L as CaCO₃, alkalinity -- 64 mg/L as CaCO₃ and conductivity -- 337 µmhos/cm. Daily measurements of total residual chlorine were <10 µg/L.

For this study, the target exposure period was approximately 80% of steady-state conditions, in order to maximize body burden residues of D5 in the young fish and therefore provide a worst-case assessment of potential toxicity following long-term exposure. A long-term fish bioconcentration study on D5 has been conducted and the depuration rate constant (k₂) from that work was used to calculate this exposure timeframe; this period was determined to be approximately 45 days, using methods from OECD Guideline 305.

After 45 days of exposure (approximately 80% of steady-state conditions), all surviving rainbow trout appeared normal and healthy. Mortality in the negative control, solvent control, 1.1, 2.1, 4.3, 8.5 and 17 µg/L treatment groups was 5, 5, 0, 5, 25, 20 and 5%, respectively. Fisher’s exact test showed that mortality was significantly different in the 4.3 µg/L treatment group in comparison to

the pooled controls (p ≤ 0.05). However, due to the lack of significant mortality in the 8.5 and 17 µg/L treatment groups, this mortality was not considered to be treatment-related. The 45-day LC50

value for rainbow trout (Oncorhynchus mykiss) exposed to decamethylcyclopentasiloxane (D5) was determined to be >17 µg/L; the limit of water solubility. There were no statistically significant reductions in mean wet weight or total length of rainbow trout exposed to D5 at concentrations ≤ 17 µg/L in comparison to the pooled controls (p > 0.05). Consequently, the NOEC for rainbow trout in a prolonged (45-day) exposure to D5 was 17 µg/