TARGET VALUES AND BACKGROUND LEVELS IN THE NETHERLANDS: HOW TO DEFINE GOOD SOIL QUALITY

FORMULATION OF THE PROBLEM The target values in the Netherlands are based on (natural) background levels and on negligible risk concentrations [l] . The relation with the background levels is variable for the individual compounds. For the heavy metals the relation is relatively well known, while only little information is available on background levels for organic compounds. The target levels for the heavy metals are high percentiles of natural occurring concentrations, varying between 83 and 94% [2, 3) . Thus there is a 6 to 17% probability that the target level for a heavy metal is exceeded in a natural soil. As the evaluation of "good" soil quality is based on each individual compound analysed, the probability of exceeding a target value, and therefore exceeding "good" soil quality, will rise significantly when analysing for instance 8 heavy metals. This could be solved by simply raising the target values. However, due to the relation with negligible risk, for most compounds this is · not acceptable. Furthermore, the original data were measured 15 years ago and should preferably be updated.