CONFINING LAYERS Springs Types Water that naturally exits on the crust of the earth is called a spring. The water in a spring can originate from a water table aquifer or from a confined aquifer. When a spring comes from a confined aquifer, it is commonly the result of a geological fault (a break in the confining layer). Only water from a confined aquifer spring is considered desirable for a public water system. 78 Chapter 3 Introduction to Water Sources

Water Movement Through an Aquifer Composition of an Aquifer An aquifer is made up of a combination of solid material, such as rock and gravel, and open spaces called pores. Regardless of the type of aquifer, the water in the aquifer is in motion. This motion is caused by gravity or by pumping. The flow of water through the aquifer is influenced by the size of the material, the number of pores, and the connection between the pores. 24 Porosity The ratio of pore space to total volume. That portion of a cubic foot of soil that is air space and therefore can contain moisture. Volume of Water The volume of water in an aquifer is dependent upon the amount of space available between the various grains of material that make up the aquifer. The amount of space available is called porosity24. All grains approximately the same size. High Porosity Mixed grain sizes. Low Porosity

Interconnecting pores High Porosity Ease of Movement Some material, such as clay, can hold a lot of water and has a high porosity, but the pores are not connected. It is therefore difficult for the water to move through the clay. The ease of movement through an aquifer is dependent upon how well the pores are connected. Below are three sections of different aquifers. The porosity of all three is the same. Section three (3) offers far less friction to the water and thus allows greater flow. The ability of an aquifer to pass water is called permeabillty25. 25 Permeability The property of a material that permits appreciable movement of water through it when it is saturated and the movement is actuated by hydrostatic pressure of the magnitude normally encountered in natural subsurface water. Low Permeability Medium Permeability

High Permeability Water Quality The most ideal public water system is one that offers high quality and high quantity. This is commonly an aquifer composed of a mixture of sand and gravel. This provides adequate quantity and filters out unwanted material.