Groundwater is part of the hydrologic cycle. This cycle is in constant operation, moving water from the earth to the atmosphere by evaporation and back to the earth's surface as precipitation. Precipitation not lost to evaporation, transpiration, or runoff slowly seeps through the pores and fractures of the rocks and loose material of the earth's surface. This infiltration (or recharge) is the process that moves water into the ground.

When water reaches the zone that is saturated (water occupies all the spaces) it becomes groundwater. If there is enough groundwater in these subsurface materials to yield a reasonable supply of water to a well or spring, this supply of water is called an aquifer.

Two properties control whether an aquifer will supply a good quantity of water: the ability to store water (porosity) and the ability to transmit water (permeability). There is great variability in aquifers: for example, fine-grained deposits made of silt or clay, may have high porosity and hold a large volume of water, but interconnections may be so small that little water can pass through them. On the other hand, a sand and gravel deposit may have a lower porosity but because the open spaces are connected, this deposit transmits water freely and yields large amounts of water to wells.

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