Amounts less than 0.1 m3 or 100 kg are considered too small to have caused serious contamination because it is assumed that such quantities can only contaminate limited amounts of soil or groundwater and that this contamination, because of dilution, will never reach concentrations that are high enough to cause health impacts. The amount has been chosen arbitrarily and more on an empirical than a scientific basis.
The rate of degradation of pesticides is often expressed as half-life (DT50), expressed in years, months or days. Every pesticide has its own half-life value. After this period only half of the original amount of pesticide is left, the other half having been degraded away. Degradation occurs because of biological organisms (bacteria, fungi) and by physicochemical interactions.
The rate of degradation of various pesticides is influenced by external factors such as temperature, light and soil acidity. As a rule, degradation of a compound is considered complete after a period of five times the half-life of that compound. Consequently, a pesticides spill will not lead to soil contamination if the age of the spill exceeds five times the half-life of the pesticides.
A DT50 value of one-half year for soil will be used as the criterion for assessing whether or not a pesticide is relevant in causing contamination. Only those pesticides with a DT50 value of one-half year or less will be considered relevant. This value is also used in Appendix 3.
For a pesticide with a half-life of a six months or less, the risk of soil becoming contaminated only exists for a period of 2.5 years after the release of the pesticide. After this period, the pesticide will have been degraded. In this way, a differentiation is made between pesticide spills that are more urgent because of the low degradation of the pesticide, and those that are less urgent because the pesticides concerned are subject to fast degradation.