4.1: Introduction- How Water Gets to Be Groundwater
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I suppose I don’t need to tell you that groundwater is the term used for the liquid water at’s present beneath the land surface in the pore spaces in regolith and in cracks in bedrock in virtually all areas except in very cold climates where the subsurface water is permanently frozen in the form of what’s called permafrost.
Groundwater is invisible until it emerges at the surface from springs and wells. And there’s nothing spectacular about groundwater and its movement, the way there is with, say, catastrophic landslides or floods. But much of the world’s water supply is from groundwater wells, so it’s a topic of great importance.
I think it’s true that the general public is aware of the existence of freshwater underground. I suspect, however, that a clear understanding of the environment and movements of that water is not widespread. Where does groundwater come from? Where does it reside? How does it move? What happens to it? How deep down do we have to drill or dig to find it? How deep in the Earth does it extend? It’s questions of this kind that we need to address in this chapter.
How Water Become Groundwater
The main way that groundwater is replenished is from the Earth’s surface, by infiltration of surface water down through the soil to become groundwater. As a prelude to our study of groundwater, this section deals with this first step in the process of groundwater flow: infiltration of surface water through the uppermost layer of the solid Earth.
You all know that some of the rain that falls on the soil surface runs off into streams and rivers and some sinks down into the soil. I’ll define infiltration as the downward movement of water across the upper surface of the soil layer. The infiltration rate is the rate at which the surface water moves downward across the upper surface of the soil layer. The infiltration rate is measured in depth of water per unit time, the same as precipitation. The infiltration capacity is the maximum rate of infiltration at a given point on the soil surface and under a given set of conditions.
Once the water infiltrates into the soil, it tends to continue its downward movement into deeper layers. The term percolation is used for the continued downward movement of the water that infiltrates the upper surface of the soil layer. Not all of it keeps on moving downward, however: if the regolith layer is sufficiently dry beforehand, some of the downward-percolating water is left behind in the pore spaces between the regolith particles, mainly in two forms: little fillets at grain contacts, and absorbed by porous materials like plant residues.