Why does water seep through the sand layer 10 m deep in a day, and only 1 mm through the clay layer?

Due to the density of the material. The sand is made up of tiny crystals. Due to the fact that these particles do not adhere tightly to each other, there is a very small free space between them, which is not visible to the normal eye. Even at a depth of 10 meters underground, the sand will not be compacted tightly enough. This is why water can seep through a large layer of sand. Clay itself is viscous and rather dense, the mass is homogeneous. Thus, the clay layer is a wall.