GROUND WATER is any water below the earth's surface resulting from seepage of surface water. It extends to a layer of impervious material. Because of increasing pressure with increasing depth it is believed that six miles is the greatest depth attained through seepage, crevices and pores being there nonexistent. The upper surface of ground water is called the water table. The depth of this surface may vary down to a certain depth where it is known as the permanent water table. This is not level but slopes in common with the impervious surface below.