Modeling joint toxicity of mixtures of organic chemicals to microorganisms in soil using quantitative structure activity relationship (QSAR) techniques

A laboratory procedure was developed to measure the toxicity of different organic chemicals in soil medium for a surrogate microbial culture. The toxicities of 35 organic chemicals acting individually and jointly in a mixture were determined by respirometry. Mixture toxicity was examined with 10- and 8-component mixtures. The assays were replicated on selected chemicals under identical conditions and at different soil moisture.; Quantitative Structure Activity Relationship (QSAR) approach was used to develop models for congeneric group of chemicals for a randomly selected training set. The training set was part of the 35 chemicals used in this study, These predictive QSAR models were validated with the chemicals in the testing set, i.e., the remaining chemicals. In this research it is hypothesized that the toxic effect on the microorganisms is caused by the concentration of the chemical available in the dissolved form in the soil moisture. The extent of partitioning of the chemicals within the soil-water matrix, and their biosorption to the cells of the surrogate organisms were determined by isotherm studies. In addition, the volatilization of the chemical into the headspace of the respirometer reactor was considered in the mechanistic modeling to develop QSAR models.; It can be concluded that the use of respirometer to assess microbial toxicity in soil medium is a rapid and reliable method. Mixture toxicity results indicate that the joint effects of chemicals are simply additive. Correlation between toxicity tests done in aqueous medium for test culture and in soil medium with the same microbial culture was statistically significant. In addition, the reproducibility tests yielded statistically comparable results.; The predictive models developed are useful to determine the toxicity of chemicals from congeneric groups to organisms in soil medium, acting either individually or jointly in a mixture. This research is useful to flag new chemicals for toxicity in soil medium without extensive testing, and to evaluate different remedial options to reclaim the soil when single or multiple chemicals are involved. Details of the laboratory procedure and test results are presented together with toxicity predictions for chemicals not assayed in the laboratory. A few practical applications are discussed.