Determining permissible concentrations

The calculations of permissible concentrations for direct contact are based on a daily intake of 2 mg of soil particles per kilograms body weight. Consequently, the maximum permissible levels, expressed in mg/kg soil (assuming no additional background exposure), are the TDI × 1 000/2 if the TDI is expressed in mg/kg body weight/day.

To calculate the maximum permissible levels of pesticides for a vegetable garden, the amount of contaminated vegetables ingested and the concentration of the pesticide in the edible parts of the plant must be considered. The quantity ingested depends on the daily ingestion of vegetables and the amount of vegetables growing at the contaminated site. It is assumed that all vegetables consumed come from a contaminated site. The quantity ingested on a daily basis is estimated to be 180 g leafy vegetables and tubers for adults of 60 kg.

The concentration of the contaminant in plants can be estimated using the
rate of transfer between the soil and the plant — the Bio Concentration Factor
(BCF). The BCF can be calculated on the basis of the log K_{OW} of the relevant
chemical, and soil parameters such as the percentage of humus and the
porosity of the soil defining the concentration in pure water. For these
calculations various mathematical models can be used. The model selected for
this manual was the Mackay model for the calculation of ground water
concentrations, and the estimation of the BCF by the model was based on the
data of Ryan et al., as presented by the HESP and Csoil model.

The critical concentrations can then be calculated by defining the concentration of the pesticides in soil, giving a concentration in vegetables and tubers that will lead to a daily intake of the contaminant equal to the ADI. The maximum permissible concentration (C) in tubers or vegetables is ADI/ consumption, the latter being 3 g/kg/day. The ADI needs to be expressed in mg/kg/day. The maximum permissible concentration in groundwater (Cgr max) is then C/BCF. Since C is expressed in mg/kg the Cgr max is also presented in mg/L. The Csoil max can then be calculated from Cgr max under equilibrium conditions.

An example has been worked out in Table A.

TABLE A

ADI[μg/kg.d] | BCF for tubers | BCF of leafy vegetables | Premissible exposure level for contaminated soils in which vegetables and tubers are grown [mg/kg soil] | |

DDE | 20 | 780 | 3.3 | 8 |

DDT | 20 | 1 700 | 1.9 | -* |

a-HCH | 1 | 31 | 5.7 | 0.1 |

b-HCH | 0.02 | 31 | 5.7 | 0.2** |

Endrin | 0.1 | 200 | 5.8 | 0.2 |

* No critical concentration thanks to the maximum solubility of DDT in water.