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Pesticide mixtures are common in streams with agricultural or urban influence in the watershed. The Pesticide Toxicity Index (PTI) is a screening tool to assess potential aquatic toxicity of complex pesticide mixtures by combining measures of pesticide exposure and acute toxicity in an additive toxic-unit model. The PTI is determined separately for fish, cladocerans, and benthic invertebrates. This study expands the number of pesticides and degradates included in previous editions of the PTI from 124 to 492 pesticides and degradates, and includes two types of PTI for use in different applications, depending on study objectives. The Median-PTI was calculated from median toxicity values for individual pesticides, so is robust to outliers and is appropriate for comparing relative potential toxicity among samples, sites, or pesticides. The Sensitive-PTI uses the 5th percentile of available toxicity values, so is a more sensitive screening-level indicator of potential toxicity. PTI predictions of toxicity in environmental samples were tested using data aggregated from published field studies that measured pesticide concentrations and toxicity to Ceriodaphnia dubia in ambient stream water. C. dubia survival was reduced to ≤50% of controls in 44% of samples with Median-PTI values of 0.1-1, and to 0% in 96% of samples with Median-PTI values >1. The PTI is a relative, but quantitative, indicator of potential toxicity that can be used to evaluate relationships between pesticide exposure and biological condition.

Transient and steady-state shear banding in a Laponite suspension were studied by particle imaging velocimetry in Couette and cone and plate flow geometries on samples with varying age. The time evolution of the velocity profile and shear stress was measured simultaneously as a fixed shear rate was imposed on the sample. By varying the aging time of the rejuvenated samples, we examined the effect of the stress overshoot on the shear banding behavior. Transient banding only occurred for aged samples that showed a stress overshoot. However, steady-state banding occurred when the imposed shear rate was below a critical value, even in the absence of a stress overshoot or transient banding. These results were consistent for independently prepared samples with vastly different overall ages, and in different flow geometries with varying intrinsic stress gradients. Different mechanisms for transient and steadystate shear banding will be discussed.

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