Groundwater is that subsurface water contained in the interconnected pore spaces below the water table of an aquifer.
Groundwater will occur where there is an impermeable subsurface barrier that allows water flowing through the unsaturated zone to be collected and stored within the interconnected pore spaces at depth. This impermeable barrier may be bedrock or an impermeable layer of rock.
Once collected in the saturated zone, groundwater will flow from areas where the water table is highest toward areas where it is lowest. This percolation of water through the saturated zone is powered by gravity. Eventually, percolating water may leave the groundwater system and be discharged into streams or other surface water bodies.