Effect of suspended solids on bioavailability of pyrethroid insecticides

Runoff and surface-water effluents commonly contain suspended solids. Adsorption to suspended particles and the associated dissolved organic matter (DOM) may significantly decrease the freely dissolved concentration of a hydrophobic compound and, hence, its availability to aquatic organisms. In the present study, we evaluated phase distribution and bioaccumulation of two synthetic pyrethroids, bifenthrin and permethrin, in water samples containing suspended solids from different source sediments. Uptake of [C-14]bifenthrin or [C-14] permethrin by Daphnia magna after 24 h consistently decreased with increasing levels of suspended solids in the range of 0 to 200 mg/L. The trend of decrease was closely mimicked by pesticide accumulation on polydimethylsiloxane (PDMS) fibers exposed under the same conditions, and the ratio of body residues in D. magna to the concentration detected in the PDMS fiber was consistently around 2.4. Regression analysis showed that the pesticide adsorbed on particles or DOM was completely unavailable to D. magna for uptake during the 24-h exposure. The relative contribution of particles and DOM to the reduced bioavailability depended on the organic matter content and the texture of the source sediment. The influence from particles was predominant for sandy sediments, but contribution from DOM became comparable to or even greater than particles when the organic matter content of the source sediment was 1% or greater. The inhibitory effects of suspended solids on bioavailability should be considered when monitoring runoff and surface-water effluents for synthetic pyrethroids. The proposed PDMS method is simple and inexpensive, and it may serve as an effective option for obtaining ecotoxicologically relevant concentrations.