Groundwater
The water found underground, simply referred to as groundwater, is nearly 60 times as plentiful as the fresh water found in surface lakes and rivers (excluding water found in glaciers). Groundwater is an important and valuable commodity, providing most of the water used by communities living in arid regions of the world. Groundwater is often less polluted than water taken from rivers, making in an important source of fresh water. In our study of the Hydrologic Cycle, we learned that a portion of the water that falls to the earth as precipitation percolates into the ground where it is slowly cycled back to the oceans and returns to the hydrologic cycle. How easily and quickly the water moves through the ground is dependent on two important factors, porosity and permeability. Porosity is the percentage of open (or pore) space in a volume of rock. It is often a measurement of a rocks ability to hold water. Permeability is the capacity of a rock to transmit a fluid, like water or petroleum. It determines how easily fluids flow through the rock. As you can see from the table, different rocks and sediments have different values of porosity and permeability. Knowing the type of rock, its porosity and permeability is important when looking for places where groundwater might be found. It's also important to understand the difference between these two terms. A rock can be porous, holding lots of water, but not permeable (often referred to as impermeable), so the water cannot travel through the rock. Rocks, such as shale, and sediment, like clay, are examples of highly porous but impermeable rock and make for poor places to look for groundwater.
The copyright of the article Groundwater in Everyday Geology is owned by Geoff Habiger. Permission to republish Groundwater in print or online must be granted by the author in writing.
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