For many centuries soil has become contaminated by human activities, but only recently has soil pollution was considered a threat for man and environment based on increasing knowledge about the effects pollutants may have and on ways to detect them. Now, society is faced with an increasing number of polluted sites that are discovered and considered to need treatment. Although excavation and treatment of the excavated soil seems the most definitive measure, costs of such an operation and damage to buildings and infra structure prevents such an approach. For that reason (biological) in situ techniques have been developed and finally accepted. When carried out properly, in situ remediation leads to environmentally sound solutions in accordance with (the Netherlands) regulations. This does not mean that problems are not encountered. The principles of in situ treatment are discussed in this paper as well as the problems which are associated with certain types of contaminants and possible solutions for these problems. The paper is based on personal experience in the field of the development of in situ techniques, the performance of a large number of in situ projects and the problems to be solved in carrying out this task.
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