1. In the investigated area a large-scale drainage work, the so-called Kasumigaura basin drainage work, is under construction.
2. Recently, several troubles (decline of groundwater level, the muddiness and contamination of groundwater etc) have been brought about with the construction.
3. It has been estimated that the main cause of the troubles were owing to the special method which was applied in the shield work (compressed air method, dewatering by deep well, chemical grouting etc).
4. The following became clear after the investigation on the above mentioned problem. Because the shield was situated at the basal bed of shallow groundwater, contaminated water leaked down through the broken impermeable layer which was cut by the work and spread out in the deep groundwater.