To reduce the flow of groundwater current under a disposal area by shutting off with an impervious wall and providing a water permeable layer inside or outside this impervious wall.

An impervious structure 20, which surrounds a stratum 12 at a definite wide site under a disposal area 11, is provided. The impervious structure 20 is formed by providing a permeable water layer 20b inside an impervious wall 20a while the permeable layer 20b is constructed by throwing gravel or cobble stones into a ditch cut deeply in the stratum. A groundwater current 16, even if it flows into the impervious wall 20a once, is taken into the permeable layer 20b and flows in the downstream direction and is discharged outside the impervious wall 20a on the downstream side. The level of underwater is slanted like a segment 22 in a natural state, but it turns into a horizontal state like a segment 23 based on a compound impervious wall comprising the impervious wall 20a and the permeable layer 20b. Therefore, the groundwater level in the area surrounded by the compound impervious wall is horizontal and its hydraulic gradient is 0 so that the flow of the groundwater may be restrained.