Salt water intrusion occurs when the water table is low and the ground water lacks sufficient water pressure to prevent the ocean from backing up into the ground water. In a damp area, the water table can be reached simply by digging. In this case the well walls are usually lined with brick, stone, or concrete in order to keep the sides from caving in on the well. A dug well can be up to 50 feet deep, and has the greatest diameter of any of the well types. Well water that contains a high number of dissolved minerals is called a mineral well.
Underground water is considered fairly clean because soils create a filter that remove large toxins.