To discuss water-supply issues, you must understand how ground water moves through or is contained by rock underground. Ground water is stored in small cracks and voids in soil and bedrock. Porosity is the proportion of a volume of rock or soil that consists of open spaces. Igneous and metamorphic rocks, such as granite and schist, have low porosities unless they are fractured. However, many sedimentary rocks can be quite porous. Loose sediments, such as Cape Cod's glacial deposits, can be highly porous: 40 percent in sand, and even 90 percent in clay.
Porosity tells us how much water rock or soil can retain. Permeability is a measure of how easily water can travel through porous soil or bedrock. Soil and loose sediments, such as sand and gravel, are porous and permeable. They can hold a lot of water, and it flows easily through them. Although clay and shale are porous and can hold a lot of water, the pores in these fine-grained materials are so small that water flows very slowly through them. Clay has a low permeability.
Remember these water-holding and water-moving characteristics when you do Activity 1 - A Model Aquifer.
|The recharge to and discharge from the Cape Cod aquifer under natural conditions (Ryan, 1980).