Infiltration Begins as PrecipitationWater that falls as rain and snow may eventually move downward (due to gravity) into the subsurface soil and rock, and become what is known as infiltration. How much water infiltrates into the soil, and then to the aquifer depends greatly on a number of factors. Infiltration of precipitation falling on the ice cap of Greenland might be very small, where a stream disappearing into a cave can act as a direct funnel right into groundwater. Eventually it might enter a stream by seepage into the stream bank. Some of the water may infiltrate deeper, recharging groundwater aquifers. If the aquifers are shallow or porous enough to allow water to move freely through it, you can drill wells into the aquifer and utilize the water from them. Water may travel long distances or remain in groundwater storage for long periods before returning to the surface or seeping into other water bodies, such as streams and the oceans. Subsurface WaterAs precipitation penetrates into the subsurface soil, it generally forms an unsaturated zone and a saturated zone. In the unsaturated zone, the voids — the spaces between grains of gravel, sand, silt, clay, and cracks within rocks — contain both air and water. Although a considerable amount of water can be present in the unsaturated zone, this water cannot be pumped by wells because it is held too tightly by capillary forces. The upper part of the unsaturated zone is the soil-water zone. The soil zone is criss-crossed by roots, openings left by decayed roots, and animal and worm burrows, which allow the precipitation to infiltrate into the soil zone. Water in the soil is used by plants in life functions and transpiration, but it also can evaporate directly to the atmosphere. Below the unsaturated zone is a saturated zone where water completely fills the voids between rock and soil particles. In places where the water table is close to the land surface and where the water can move through the aquifer at a high rate, aquifers can be replenished artificially. For instance, when the Edwards aquifer gets low, and San Antonio's water supply is in danger, water might be pumped from the Trinity aquifer to recharge the Edwards. 
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Some water that infiltrates will remain in the shallow soil layer, where it will gradually move vertically and horizontally through the soil and subsurface material. 
