The toxicity and accumulation of eleven moderately water-soluble metabolizable (MSM) or neutral lipophilic (NL) compounds were examined in the midge Chironomus riparius in tests without sediment (water only) and in treatments with sediment to which the test chemical was added directly to the water (spiked water) or directly to the sediment (spiked sediment). In general, the toxicity of NL compounds was significantly reduced when sediment was present regardless of whether the compound was added to the water or the sediment. The observed reduction in toxicity when sediment was present was attributable to sorption of the chemical to sediment, dissolved organic carbon (DOC), and particulates and was anticipated from literature Kow and measured Koc values. The toxicity and accumulation of MSM compounds in sediment and nonsediment systems were much more variable than for the NL compounds. For 1-naphthol, no significant difference in the distribution of parent chemical was seen in the water or sediment when used, although the toxicity of 1-naphthol was lowest when the compound was added directly to water in the presence of sediment. For the remaining MSM compounds, toxicity was significantly reduced only when the chemical was added directly to sediment. In general, partition coefficients were less useful in predicting the environmental behavior of MSM compounds. The most notable effect of sediment upon the MSM group was that the persistence of the parent compound was greatly increased in the sediments when the chemical was added to sediment.

The toxicity of a series of organophosphorus (OP) and carbamate insecticides was measured against the midge Chironomus riparius in aquatic systems with and without sediment. Five molecular descriptors (molecular volume, Henry's law constant, n‐octanol/water partition coefficient (Kow), molecular connectivity, and linear solvation energy) were used in regression analysis as potential predictors of insecticidal activity. The regressions were conducted for each descriptor against toxicity values for the series of chemicals. Molecular volume and Henry's law constant showed no relationship with toxicity. However, log Kow was moderately successful in describing the effect of sediment on toxicity (r2 = 0.508). Prediction of toxicity was substantially improved when a linear solvation energy (LSE) or molecular connectivity (MC) model was used in regressions. In multiple regressions conducted on carbamates 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 sediment apparently dominate the processes affecting the toxicity of these compounds when sediment is present. In the absence of sediment, the regressions suggest that the molecular structure of the insecticides is more important than solubility or partitioning for determining toxicity.

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