Water
1. Imagine slowly pouring water onto a pile of sand. Some water would flow off the sides, but most would seep into spaces between the grains of sand. Similarly, much of the water that falls onto Earth's surface becomes groundwater as it seeps into spaces between soil and rock. What would happen if you poured water onto a pile of silt instead? Would the water flow through the silt faster or slower than it does through sand?
2. Materials are permeable if they allow water to pass through. Soil, such as sand, that has large particles usually has large connected spaces, called pores. This soil has high permeability because water flows easily through the pores. Soil, such as silt, that has smaller particles has smaller connected pores. This soil has a lower permeability because water cannot flow as easily through the pores.
3. The rate at which water flows through the ground depends on permeability. Water may take an hour or less to flow through one meter of soil if it has high permeability. If the soil has low permeability, water may take a year or more to flow one meter. Water flows more slowly when you pour it onto silt than when you pour it onto sand.
4. If you could cut out a giant slice of ground, you would see different layers. Just below the surface is an unsaturated zone. The word saturated means "completely filled." water flows through the pores in the unsaturated zone because the zone is not completely filled with water.
5. Below the unsaturated zone is the zone of saturation. Water completely fills the pores of this layer. Groundwater is sometimes incorrectly described as any water under Earth's surface. In fact, groundwater is water in the zone of saturation. Beneath the zone of saturation is impermeable rock with unconnected pores. Water cannot flow into or through this layer.
6. The boundary between the unsaturated zone and the of saturation is called zone the water table. The depth of the water table in an area identifies how far below the surface groundwater is. The water table may be few meters below the surface in valleys or coastal areas, but hundreds of meters below the surface in mountainous regions. During rainy seasons, the water table is usually closer to the surface.
7. Water flows down ward in response to gravity. When water strikes rock that is not permeable, it moves at an angle. Rocks that are not permeable change the direction of water flow.
8. Some groundwater is in underground lakes and rivers. The photograph on the cover of this card shows a pool in an underground cave in Yucatan, Mexico. Most groundwater is in small cracks and pores of rock. The ground stores a tremendous amount of water. To understand how much, imagine a huge box filled with water. The box is a kilometer high, a kilometer wide and a kilometer deep. That amount of water is difficult to imagine. Now consider 23 million of those boxes of water. That is about how much water is stored under Earth's surface.
9. Groundwater is always moving. Its speed depends on the soil's permeability. Groundwater always returns to the surface. In some places, such as steep slopes on mountainsides, the water table touches Earth's surface. Here the water flows onto the surface as a natural spring.
10. In places where the water table is deep below the surface, people can bring this water to the surface. An aquifer is an underground layer of permeable rock or sediment that contains pore spaces through which water flows. People dig wells into an aquifer to obtain water. In some places, the pressure in an aquifer is so high that it causes the well water to rise to the surface. This type of well is called an artesian well.