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A Concepts of Ground Water, Water Table, and Flow Systems SUBSURFACE WATER Water beneath the land surface occurs in two principal zones, the unsaturated zone and the saturated zone (Figure A-1),ln the unsaturated zone, the voids-that is, the spaces between grains of gravel, sand, silt, clay, and cracks within rocks-contain both air and water, Although a consider- able amount of water can be present in the unsaturated zone, this water cannot be pumped by wells because it is held too tightly by capillary forces, The upper part of the unsaturated zone is the soli-water zone. The soil zone is crisscrossed by roots, voids left by decayed roots, and animal and worm burrows, which enhance the infiltration of precipitation into the soil zone, Soil water is used by plants In life functions and transpiration, but it also can evaporate directly to the atmosphere, Unsaturated zone (," Sail. qSlJrf. ""'<iter <1cs <'Ol}e -_______. Water table ------------------ ---------- -----___ Surface water Saturated zone (ground water) Figure A-1, The water table is the upper surface of the saturated zone, The water table meets surface-water bodies at or near the shoreline of surface water if the surface-water body is connected to the ground-water system, In contrast to the unsaturated zone, the voids in the saturated zone are completely filled with water, Water in the saturated zone is referred to as ground water, The upper surface of the saturated zone is referred to as the water table, Below the water table, the water pressure is great enough to allow water to enter wells, thus permitting ground water to be withdrawn for use, A well is constructed by inserting a pipe into a drilled hole; a screen is attached, generally at Its base, to prevent earth materials from entering the pipe along with the water pumped through the screen, The depth to the water table is highly variable and can range from zero, when it is at land surface, to hundreds or even thousands of feet in some types of landscapes, Usually, the depth to the water table is small near permanent bodies of surface water such as streams, lakes, and wetlands, An important characteristic of the water table is that its configura- tion varies seasonally and from year to year because ground- water recharge, which is the accretion of water to the upper surface of the saturated zone, is related to the wide variation in the quantity, distribution, and timing of precipitation, THE WATER TABLE The depth to the water table can be determined by installing wells that penetrate the top of the saturated zone just far enough to hold standing water. Preparation of a water- table map requires that only wells that have their well screens placed near the water table be used, If the depth to water is measured at a number of such wells throughout an area of study, and if those water levels are referenced to a common datum such as sea level, the data can be contoured to indi- cate the configuration at the water table (Figure A-2), A 0 138.47 EXPLANATION 152.31 o 152.31 o LOCATION OF WELL AND ALTITUDE OF WATER TABLE ABOVE SEA LEVEL, IN FEET 131.42 o o 145.03 132.21 0126.78 0 -140- WATER-TABLE CONTOUR- Shows altitude of water table. Contour interval 10 feet. Datum is sea level o 137.90 121.34 o o 128.37 --GROUND-WATER FLOW LINE B o 138.47 152.31 '.s-o 0 131.42 o 132.21 o o 7...i"O 137.90 ~ ~0121.34 c ?o 0121.34 Figure A-2. Using known altitudes of the water table at indi- vidual wells (A), contour maps of the water-table surface can be drawn (B), and directions of ground-water flow along the water table can be determined (e) because flow usually is approxi- mately perpendicular to the contours, t I In addition to various practical uses of a water-table map, such as estimating an approximate depth for a proposed well, the configuration of the water table provides an indication of the approximate direction of ground-water flow at any location 6