For example, flow systems of different sizes and depths can be present, and they can overlie one another, as indicated in Figure A-4. In a local flow system, water that recharges at a water-table high discharges to an adjacent lowland. Local flow systems can be underlain by intermediate and regional flow systems. Nevertheless, these deeper flow systems also eventually discharge to surface water, and they can have a great effect on the chemical characteristics of the receiving surface water. Ground-water flow systems can be local, intermediate, and regional in scale.
Permeability is a quantitative measure of the ease of water movement through aquifer materials.
For example, sand is more permeable than clay because the pore spaces between sand grains are larger than pore spaces between clay particles.