What is the zone of aeration?

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In groundwater systems, the zone of aeration is the region where pore spaces contain air and water, not fully saturated. This is the vadose or unsaturated zone, located between the land surface and the water table. Water there moves downward under gravity and capillary action, but the pores aren’t completely filled with water, so air remains in the pores. The boundary to the deeper groundwater is the water table, which marks the top of the saturated zone where pores are fully filled with water. The option describing a surface layer where evapotranspiration occurs refers to a surface process rather than the moisture state of the subsurface, so it doesn’t define the zone of aeration.