Optimizing Quantitative Structure-Activity Relationships for the toxicity of chlorinated compounds to soil organisms taking into account the dynamics of bioavailable concentrations

Recently, we developed quantitative structure-activity relationships (QSARs) for the toxicity to Folsomia candida of eight chlorobenzenes (CB) and four chloroanilines (CA). These QSARs used freely dissolved soil interstitial water concentrations determined by solid-phase microextraction (SPME), and corresponding with the EC10 and EC50 for effects on reproduction. Chemical concentrations however, may change over time and so may the available concentration, although not necessarily proportional to the change in the total concentration. QSARs relating toxicity to initial concentrations therefore may not be accurate. In this study, for the same chemicals SPME-extracted available concentrations in standardized natural LUFA2.2 soil were determined at three time points over the 28-day duration of the F. candida reproduction test. Toxicity based on actual available concentrations was negatively correlated with lipophilicity for the CB but showed no consistent trend for the CA. Over time the significance of the QSARs for the CB decreased due to differences in loss rate of the interstitial water concentrations of different compounds, which was highest for 1,3,5trichlorobenzene (80%). 3,5-Dichloroaniline also showed a strong decrease (30%) of available concentrations. Interstitial water concentrations of compounds with high logKow were stable. For the CB, optimized QSAR models were developed based on the geometric mean of measured concentrations, representing actual exposure over the 28-day test period. This study shows that measured available concentrations over time can be used to develop and optimize QSARs and identify compounds with deviating exposure patterns and consequent deviating risk potentials.