Porosity and Permeability
You have already seen that some rocks are better than others at holding water. Almost all rocks have some amount of holes, or pores, for holding water. The total amount of pores in a rock is called the rock’s porosity. This value may be near zero or as much as 30% of the rock’s volume. Depending on how close the pores are and how big they are, some rocks will transmit water better than others. A rock’s permeability determines how well that rock can transmit water. If a rock layer’s permeability is very low or near zero, then it can serve as a confining bed that bounds a confined aquifer. On the other hand, rock layers with high permeability can transmit water more freely and are sources of unconfined aquifers.
Take a look at the rock pictures. Which ones do you think are best for transmitting water? You can roll over each image with your mouse to learn about it.
