WP9 Pesticide Mixtures
Wednesday, 16 November 2005: 8:00 AM - 6:30 PM in Exhibit Hall

(CAR-1116-956681) Assessing persistence and aquatic toxicity of a xylene-emulsifier herbicide to salmonids and daphnids.

Cardwell, R.¹, van Genderen, E.¹, Buhler, G.², Caldwell, R.², Edgington, A.¹, Power, L.¹, Smith, C.¹, ¹ Parametrix Environmental Research Laboratory, Albany, OR, USA² Northwest Aquatic Sciences, Newport, OR, USA

ABSTRACT- A mixture of xylene and an emulsifier (the adjuvant) is used as an herbicide to control some of the aquatic plants that can clog irrigation fixtures and retard flow in eastern Washington irrigation canals. Because the treated water cannot be released downstream before it is considered acutely nontoxic to aquatic life, acute toxicity bioassays of rainbow trout fry and Daphnia magna were used to define the risks posed by the emulsifier, xylene, and herbicide mixture. Bioassays rather than chemical analysis were judged to be the most cost effective means for assessing net bioavailability and toxicity of the chemical mixtures. Because the emulsifier, a mixture of seven chemicals, was acutely toxic and affected mixture toxicity, its persistence was measured using bioassay, as was that of the herbicide mixture. Although the emulsifier constituted only 2% of the formulated herbicide (98% xylene), mixture toxicity was considered because definitive toxicity tests with both organisms suggested the emulsifier was up to seven times more toxic than xylene alone on a volume per volume basis. In fact, rainbow trout were equally sensitive to the emulsifier and the formulated herbicide while D. magna appeared to be most sensitive to the herbicide exposure. Whether the interaction was additive or more-than-additive could not be determined. Based on slow stirring in beakers to simulate mixing in flowing canals, the emulsifier was less persistent (half-life 3 hr) than the xylene (half-life 7.5 hr), but both dissipated rapidly via volatilization. There was evidence that some of the emulsifier's degradation intermediates were acutely toxic if the treated water was held long enough (≥75 hr @ 20 C) before release to natural streams. Toxicity bioassays proved to be a cost-effective and technologically effective means for assessing toxicity, persistence and toxicological interactions of this complex mixture.

Key words: Xylene, Aquatic, Toxicity, Persistence