QUANTITATIVE STRUCTURE-ACTIVITY-RELATIONSHIPS FOR PREDICTING THE TOXICITY OF PESTICIDES IN AQUATIC SYSTEMS WITH SEDIMENT

The toxicity of a series of organophosphorus (OP) and carbamate insect icides was measured against the midge Chironomus riparius in aquatic s ystems with and without sediment. Five molecular descriptors (molecula r volume, Henry's law constant, n-octanol/water partition coefficient (K(ow)), molecular connectivity, and linear solvation energy) were use d in regression analysis as potential predictors of insecticidal activ ity. The regressions were conducted for each descriptor against toxici ty values for the series of chemicals. Molecular volume and Henry's la w constant showed no relationship with toxicity. However, log K(ow) wa s moderately successful in describing the effect of sediment on toxici ty (r2 = 0.508). Prediction of toxicity was substantially improved whe n a linear solvation energy (LSE) or molecular connectivity (MC) model was used in regressions. In multiple regressions conducted on carbama tes and OPs separately, use of MC or LSE parameters explained up to 95 .8% of the variability in toxicity. Based on the results of regression analyses, sorptive interactions between these insecticides and sedime nt apparently dominate the processes affecting the toxicity of these c ompounds when sediment is present. In the absence of sediment, the reg ressions suggest that the molecular structure of the insecticides is m ore important than solubility or partitioning for determining toxicity .