Aquifers are replenished as rainfall seeps down through the soil. Ground water travels through permeable soil on top of hard or impermeable layers. The level at which the soil is saturated is the water table. Pumping the well lowers the water table near it. These wells are particularly sensitive to seasonal changes and may dwindle during dry periods.
Artesian wells penetrate into ground water having confining layers above and below the aquifer.
Rainfall enters into the aquifer through permeable layers at high elevations causing the ground water to be under pressure at lower elevations. Because of this pressure, the water level in the well is higher than the aquifer. Since shallow wells penetrate into aquifers that are near the ground surface, they can become contaminated by barnyards, pastures, sewers, chemicals, or septic tank systems. Both rainfall and surface water runoff can carry pollutants down into shallow aquifers and well water. Since a hole penetrating an aquifer provides a direct route for contamination, wells must be designed to prevent pollution from entering and contaminating ground water.