What defines groundwater?

Groundwater is specifically defined as water that is located in the zone of saturation, which is the area beneath the Earth's surface where all the pores in the soil and rock are filled with water. This underground water is held in aquifers and can be accessed through wells or springs. The zone of saturation is distinguished from the zone of aeration, where the spaces between soil particles may contain both air and water, but not completely saturated with water.

The other options reflect different states or forms of water that do not fit the definition of groundwater. Water that evaporates from lakes does not remain underground, rainwater collected in reservoirs is surface water stored for use, and water in the atmosphere refers to vapor or precipitation, rather than water stored in the ground. Thus, only the presence of water in the zone of saturation accurately encapsulates the concept of groundwater.