How does permafrost block the downward drainage of water through soil?

Permafrost acts as an impermeable barrier to water because it consists of ground that remains perennially frozen, typically consolidated by ice. Because this subsurface layer is frozen, it prevents water from draining downward through the soil profile.123

This impermeability has significant environmental consequences, particularly during the summer thaw. As the surface layer of ground—known as the active layer—thaws, the water released is unable to penetrate the frozen ground beneath it. Consequently, the soil becomes saturated and boggy, often leading to the formation of wetlands in Arctic lowlands. Furthermore, because the permafrost layer is impermeable, the waterlogged soil in the active layer can easily slide down slopes, a process known as solifluction.123