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Biodegradation of [ring‐14C] mecoprop (2‐(4‐chloro‐2‐methylphenoxy)propionic acid) was determined in surface and sub‐surface soil at concentrations of 0·0005, 0·05, 0·5, 5, 50, 500, 5000 and 25000 mg kg‐1. The kinetics of mineralisation were evaluated from the mineralisation rates as a function of time and by non‐linear regression analysis. In the sub‐surface soil, degradation was 6–8 times slower than in surface soil, but the shape of the curves was the same in both layers. At concentrations between 0·0005 and 0·5 mg kg‐1, in both surface and sub‐surface soil, degradation was initially zero‐order followed by first‐order kinetics. At 5 to 500 mg kg‐1 in surface soil and 5 to 50 mg kg‐1 in sub‐surface soil the degradation rate was initially either constant or decreasing followed by exponential degradation indicating increasing populations of mecoprop decomposers in the soil. At 5000 and 25000 mg kg‐1 in the surface soil and at 500, 5000 and 25000 mg kg‐1 in the sub‐surface soil, the degradation was negligible, as determined by the percentage [14C] carbon dioxide evolved. By non‐linear regression, the three‐half order model was found to describe the mineralisation. © 1998 SCI

A series of physiologically based toxicokinetic (PBTK) models for tebuconazole were developed in four species, rat, rabbit, rhesus monkey, and human. The developed models were analyzed with respect to the application of the models in higher tier human risk assessment, and the prospect of using such models in risk assessment of cumulative and aggregate exposure is discussed. Relatively simple and biologically sound models were developed using available experimental data as parameters for describing the physiology of the species, as well as the absorption, distribution, metabolism, and elimination (ADME) of tebuconazole. The developed models were validated on in vivo half-life data for rabbit with good results, and on plasma and tissue concentration-time course data of tebuconazole after i.v. administration in rabbit. In most cases, the predicted concentration levels were seen to be within a factor of 2 compared to the experimental data, which is the threshold set for the use of PBTK simulation results in risk assessment. An exception to this was seen for one of the target organs, namely, the liver, for which tebuconazole concentration was significantly underestimated, a trend also seen in model simulations for the liver after other nonoral exposure scenarios. Possible reasons for this are discussed in the article. Realistic dietary and dermal exposure scenarios were derived based on available exposure estimates, and the human version of the PBTK model was used to simulate the internal levels of tebuconazole and metabolites in the human body for these scenarios. By a variant of the models where the R(-)- and S(+)-enantiomers were treated as two components in a binary mixture, it was illustrated that the inhibition between the two tebuconazole enantiomers did not affect the simulation results for these realistic exposure scenarios. The developed models have potential as an important tool in risk assessment.

: Biodegradation of [ring-14C] mecoprop (2-(4-chloro-2-methylphenoxy) propionic acid) was determined in surface and sub-surface soil at concentrations of 0É0005, 0É05, 0É5, 5, 50, 500, 5000 and 25 000 mg kg~1.The kinetics of mineralisation were evaluated from the mineralisation rates as a function of time and by non-linear regression analysis.In the sub-surface soil, degradation was 6È8 times slower than in surface soil, but the shape of the curves was the same in both layers.At concentrations between 0É0005 and 0É5 mg kg~1, in both surface and subsurface soil, degradation was initially zero-order followed by Ðrst-order kinetics. At 5 to 500 mg kg~1 in surface soil and 5 to 50 mg kg~1 in sub-surface soil the degradation rate was initially either constant or decreasing followed by exponential degradation indicating increasing populations of mecoprop decomposers in the soil. At 5000 and 25 000 mg kg~1 in the surface soil and at 500, 5000 and 25 000 mg kg~1 in the sub-surface soil, the degradation was negligible, as determined by the percentage [14C] carbon dioxide evolved.By non-linear regression, the three-half order model was found to describe the mineralisation.1998 SCI ( Pestic. Sci., 52, 126È132, 1998