Groundwater flows through rock with a high porosity and a high permeability at a fast rate. The denser and less permeable the rock is, the slower the groundwater moves. The greater the permeability of a rock, the faster the groundwater flows through it. Groundwater is water that is found in spaces and fractures in rocks, as well as in the sediment between the earth's surface. Groundwater starts off as rain, hail or snow. It seeps down through the sand and soil into the groundwater system. The groundwater eventually flows into rivers, lakes or oceans.