Determination of critical body residue values for three current-use pesticides in Hyalella azteca: Predictive techniques versus direct tissue residue measurement

Scope and Method of Study: Extensive pesticide application has contributed to the contamination of surface water, placing aquatic organisms at an elevated level of risk of toxicant exposure. Most aquatic toxicity assessments use water or sediment chemical concentrations as a "dose metric" to indicate the level of exposure experienced by organisms in aquatic systems. While measurement of the amount of chemical present at the site of toxic action would result in the most accurate dose metric, such a measurement is not typically feasible. As an alternative, whole body residue (or critical body residue) analysis should give a better estimate of the amount of compound able to reach the site of toxic action than water or sediment analysis. This is especially true for lipophilic, non-reactive compounds. Although the critical body residue (CBR) approach has been used in previous studies, very little body residue data is available for current use pesticides such as the three used in this study: trifluralin, pendimethalin, and bifenthrin. In this study, three approaches were used to predict CBR values for the three pesticide for Hyalella azteca. Approach 1 (using predicted BCF and empirical LC50 values) and 2 (using empirical BCF and empirical LC50 values) were calculated for each compound and compared to the directly measured (LBR 50) tissue concentration.;Findings and Conclusions: Approach 1 was the least reliable CBR determination method and resulted in an overestimate of CBR values for all three compounds. Approach 2 gave overall better estimates of CBR, since both LC50 and BCF values were empirically determined for each compound. The CBR values determined for bifenthrin, trifluralin, and pendimethalin all fall below the range expected for compounds whose mode of action is non-polar narcosis (1.7--8 mmol/kg) or baseline toxicity. This could indicate that all three compounds either have a specific mode of action or that transformation products may be contributing to toxicity. The toxicity of many pesticides, such as pendimethalin and trifluralin, could be assessed by the CBR approach within standard toxicity testing protocols. However, the approach may not be feasible for compounds that are highly toxic such as bifenthrin.