Ecotoxicity of poorly water-soluble substances

Acute toxicity tests were performed with Daphnia and two radiolabelled pesticides, lindane (water solubility 7 mg/L, logPow 3.76) and Pendimethalin (water solubility 0.3 mg/L LogPow 5.18) comparing different solubilizing agents and saturated solutions with respect to stability of exposure and bioavailability. Concentrated stock solutions were prepared with micelles (Tween-80), liposomes (negative and positive charged surfaces), and acetone (applied to the test vessels and evaporated before addition of water and daphnids). Saturated solutions were prepared in water and water amended with 20 mg/L and 100 mg/L humic acids. Finally, the test substances were adsorbed to algae (Scenedesmus subspicatus) and applied to the test water. Daphnids reacted most sensitively to lindane and pendimethalin adsorbed to algae (48h-EC50s: lindane: 107 μg/L; pendimethalin: 78 μg/L), whereas humic acids decreased toxicity. Micelles, liposomes and acetone did not alter the bioavailability of lindane compared to a saturated solution in water (EC50s: water: 552 μg/L; micelles, liposomes and acetone: 406–793 μg/L). For pendimethalin saturated solution in water led to only 25% mortality. Low toxicity was also observed for pendimethalin applied as acetone stock solution and resuspended after evaporation of the acetone whereas liposomes increased toxicity of pendimethalin. Application with micelles gave a clear concentration-effect relationship with an EC50 of 763 μg/L thus confirming that the EC50 lies above the water solubility. The results demonstrate, that (1) exposure above water solubility may be important when dealing with poorly water-soluble substances and should therefore be considered in ecotoxicity testing, (2) exposure routes other than the water phase, i.e. uptake through the digestive system, are highly relevant for daphnids and should be integrated also in acute ecotoxicological tests.