Water that falls to the earth in the form of rain, snow, sleet, or hail continues its journey in one of three ways:
- It might land on a body of water
- It may become surface runoff and eventually travel into a body of water
- It might seep into the ground
Water that seeps into the ground moves in a downward direction because of gravity, passing through pore spaces between the soil particles, until it reaches a soil depth where the pore spaces are already filled, or saturated with water.
When water enters the saturated zone it becomes groundwater. The top of this saturated zone is called the water table. A water-bearing soil or rock formation that is capable of collecting and yielding enough water for human use is called an aquifer.
When infiltrating water reaches the water table, it begins to move along with the groundwater flow, which tends to follow a downhill or downward slope direction. Compared to waters in a stream or river, groundwater flows very slowly, from as little as inches in a day through clay to as much as 3-4 feet each day through sand and gravel.
The speed at which water passes through, or infiltrates, the soil depends on the size and shape of soil particles, and the amount of room between the soil particles. Sand and gravel have larger spaces between particles than clay does, so water travels faster through sand and gravel. Clay particles are flattened and have a lot of surface area to which water can cling, which means that clay deposits hold more water.
In time, groundwater resurfaces by:
- Intersecting with a nearby water body like a river or stream. About 40% of river flow nationwide (on average) depends on groundwater.
- Emerging from a hillside as a spring.
- Withdrawal from the ground through a well. Approximately 50% of Americans obtain all or part of their drinking water from groundwater.
In New England, the average depth of groundwater ranges from 8-20 ft. In the southwestern part of the US, the average depth of groundwater ranges from 200-600 ft.
When pollutants leak, spill, or are carelessly discarded on the ground, they, like water, move through the soil. If there is a well near the pollution, that well runs the risk of being contaminated by polluted groundwater. Because it is located deep in the ground, groundwater pollution is generally difficult and expensive to clean up. In some cases people have to find alternative sources of water because their wells became contaminated.