Dynamic root uptake model for neutral lipophilic organics.

In current European risk assessment, an equilibrium approach is used to estimate chemical uptake from soil into root vegetables. Here a dynamic model for uptake of neutral lipophilic compounds from soil into roots is presented. Using experimental results, it is compared with the equilibrium approach. Very lipophilic compounds (e.g., DDT) diffuse very slowly into plant tissue, so they are likely to remain in the peel of root vegetables. In addition, a dynamic (steady-state) flux model for uptake with transpiration water into thick roots is presented. The model considers input from soil and output to stem with the transpiration stream plus first-order metabolism and dilution by exponential growth. For chemicals with low or intermediate lipophilicity (log Kow < 2), there was no relevant difference between dynamic model and equilibrium approach. For lipophilic compounds, the dynamic model gave concentrations far below the thermodynamic equilibrium. The approach was tested against experimental uptake data of benzo[a]pyrene, polychlorinated biphenyls (PCBs), and chlorobenzenes from soil into carrots. Measured concentrations in carrot peels were up to 100 times higher than in the core. The equilibrium approach can predict concentrations in the peels, but for carrot cores and for the whole carrot, the flux model is superior and should be preferred for a more realistic risk assessment.