Published on January 16, 2008
Ground Water: Ground Water Introduction: Introduction Distribution of Water: Oceans 97.2 Glaciers and other ice 2.15 Ground Water 0.61 Fresh Water Lakes 0.009 Saltwater Lakes 0.008 Soil Moisture 0.005 Atmosphere 0.001 Rivers 0.0001 Ground water is a very important natural resource that is becoming increasing difficult to manage. Growing cities in arid climates are removing ground water faster than it’s be replenished. Pollution of ground water by industrial wastes, agricultural pesticides, etc… Growing population and improvements in lifestyles impact ground water supplies. Porosity and Permeability: Porosity and Permeability Ground water – the water that lies beneath the ground surface, filling the pore space between grains in bodies of sediment and clastic sedimentary rock, and filling cracks and crevices in all types of rock. It accounts for approximately 22% of the world’s supply of freshwater In order to understand how water gets underground, where it’s stored, and how it moves while underground, we must first understand Porosity and permeability. Porosity – the percentage of rock or sediment that consists of voids or openings; is a measurement of a rock’s ability to hold water. Permeability – refers to the capacity of a rock to transmit a fluid such as water or petroleum through pores and fractures. Indicates the relative ease of water flow and indicates the degree to which openings in a rock interconnect. Porosity and Permeability: Porosity and Permeability The Water Table: The Water Table Several kilometers down percolation stops due to increased cementation of sediment and increasing weight of the overlying rock, both of which lead to decrease pore spaces. The subsurface zone in which all rock openings are filled with water is called the saturated zone. The upper surface of the saturated zone is called the water table. The water level at the surface of most lakes and rivers coincide with the water table. “Click to view animation” Water, responding to gravity, percolates down into the ground through the soil and through cracks and pores in the rock. The Water Table: The Water Table Above the water table is a zone that is generally unsaturated and is referred to as the unsaturated zone (zone of aeration) Within the unsaturated zone, surface tension causes water to be held above the water table. The capillary fringe is a transition zone with higher moisture content at the base of the vadose zone just above the water table. This zone is generally less than a meter thick. A perched water table is the top of a body of ground water separated from the main water table beneath it by a zone that is not saturated. Forms as ground water collects above a lens of less permeable shale within a more permeable rock, such as sandstone. Aquifers: Aquifers An aquifer is a body of saturated rock or sediment through which water can move easily. Aquifers are both highly permeable and saturated with water. Good aquifers include sandstone, conglomerate, well-jointed limestone, bodies of sand and gravel, and some fragmental or fractured volcanic rocks. Wells drilled in shale beds are not usually very successful because shale is relatively impermeable. Crystalline rocks, such as granite, gabbro, gneiss, schist, and some types of limestone are also not good aquifers. Shale and crystalline rock are sometimes called aquitards because they retard flow. Crystalline rocks that are highly fractured, however, may be porous and permeable enough to provide a good well. Aquifers: Aquifers An unconfined aquifer, is located at the surface overlying an impermeable bed, or aquitard. Has a water table because it is only partly filled with water. Recharged rapidly by precipitation. Has a rising and falling water table during wet and dry seasons. Relatively rapid movement of ground water through it. A confined aquifer is completely filled with water under pressure and is usually separated from the surface by a relatively impermeable confining bed, or aquitard. Is recharged slowly through confining shale beds. Very slow movement of ground water; may have no response at all to wet and dry seasons. The Movement of Ground Water: The Movement of Ground Water Ground water moves relatively slow in response to differences in water pressure and elevation. Water moves from a region of high hydraulic head to a region of low head. Hydraulic head = elevation + pressure. In fig. A, the pressure at both A and B are the same, but the elevation difference causes flow towards B. In fig. B, the pressure head is higher at point C is higher than at point D, so water will move towards D even thought they have the same elevation. In fig. C, water moves up towards point G from point F. Springs and Streams: Springs and Streams A spring is a place where water flows naturally from rock onto the land surface. Some springs discharge where the water table intersects the land surface. They also occur where water flows out from caverns or along fractures, faults, or rock contacts that come to the surface. Slide12: Spring issuing from a limestone cavern Jasper National Park Alberta, Canada (Photo by David McGeary) Permeable sandstone, Southern Utah (Photo by David McGeary) Streams and Groundwater: Streams and Groundwater Climate determines the relationship between stream flow and the water table. In rainy regions, most streams are gaining streams; they receive water from the saturated zone. The surface of these streams coincide with the water table. The discharge of these streams increases downstream. Where the water table intersects the land surface over a broad area, ponds, lakes, and swamps are found. In drier climates, rivers tend to be losing streams; they are losing water to the saturated zone. The channels lie above the water table. Water percolating into the ground beneath the stream may cause the water table below the stream to rise This ground water remains beneath the stream even when the streambed is dry. Wells: Wells A Well is a deep hole, generally cylindrical, that is dug or drilled into the ground to penetrate an aquifer within the saturated zone. The addition of new water to the saturated zone is called recharge. When water is pumped from a well, the water table is typically drawn down around the well into a depression shaped like an inverted cone known as a cone of depression. This local lowering of the water table, called drawdown, tends to change the direction of groundwater flow by changing the slope of the water table. “Click to view animation” Wells: Wells In unconfined aquifers, water rises in shallow wells to the level of the water table In confined aquifers, the water is under pressure and rises in wells to a level above the top of the aquifer Such a well is called an artesian well, and confined aquifers are also called artesian aquifers In some artesian wells, the water rises above the land surface, producing a well spout “Click to view animation” Groundwater Erosion and Deposition: Groundwater Erosion and Deposition Most caves probably are formed by ground water circulating below the water table. If the water table drops or the land is elevated above the water table, the cave may begin to fill in again by calcite precipitation. H2O + CO2 + CaCO3 ↔ Ca++ + 2HCO3- development of caves → ← development of flowstone and dripstone Ground water with high concentration of calcium (Ca++) and bicarbonate (HCO3-) ions may drip slowly from the ceiling of an air-filled cave. Caves, Sinkholes, and Karst Topography Caves ( or caverns) are naturally formed underground chambers. Most develop when slightly acidic ground water dissolves limestone along joints and bedding planes, opening up cavern systems as calcite is carried away in solution. Natural ground water is commonly slightly acidic because of dissolved carbon dioxide from the atmosphere. Groundwater Erosion and deposition: Groundwater Erosion and deposition Deposits of calcite built up in caves by dripping water are called dripstone or speleothems. Stalactites are icicle like pendants of dripstone hanging from cave ceilings. Stalagmites are cone-shaped masses of dripstone formed on cave floors, generally directly below stalactites. Stalactites and stalagmites may eventually join to form a Column. Karst Topography: Karst Topography Solution of limestone underground may produce features that are visible on the surface. Extensive cavern systems can undermine a region so that roofs collapse and form depressions in the land surface above. Sinkholes are closed depressions found on land surfaces underlain by limestone. They form either by the collapse of a cave roof or by solution as descending water enlarges a crack in limestone. Limestone regions in Florida, Missouri, Indiana, and Kentucky are heavily dotted with sinkholes. Sinkholes can also form in regions underlain by gypsum or rock salt. An area with many sinkholes and cave systems beneath the land surface is said to have Karst topography. Slide20: A Sinkhole in Winter Park, FL (© AP/Wide World Photos) Effects of Groundwater Action: Effects of Groundwater Action Other Effects Ground water is important in the preservation of fossils such as petrified wood. Petrified wood develops when porous buried wood is either filled in or replaced by inorganic silica carried by ground water. Calcite or silica carried by ground water can also replace the original material in marine shells and animal bones. Sedimentary rock cement, usually silica or calcite, is carried into place by ground water. When a considerable amount of cementing material precipitates locally in a rock, a hard rounded mass called a concretion develops, typically around an organic nucleus such as a leaf, tooth, or other fossil. Geodes are partly hollow, globe-shaped bodies found in some limestones and locally in other rocks. The outer shell is amorphous silica, and well-formed crystals of quartz, calcite, or other minerals project inward toward a central cavity. The origin of geodes is complex but clearly related to ground water. How do Humans Affect Groundwater: How do Humans Affect Groundwater Modifications of the groundwater system may have many consequences, including: lowering of the water table, causing wells to dry up loss of hydrostatic pressure, requiring water to be pumped saltwater incursion land subsidence contamination Lowering the Water Table: Lowering the Water Table results when groundwater is withdrawn at a significantly greater rate than it is replaced Saltwater Incursion: Saltwater Incursion results from excessive pumping of groundwater in coastal areas Subsidence: Subsidence When excessive amounts of groundwater are withdrawn from poorly consolidated sediment and sedimentary rocks the water pressure between grains is reduced and the weight of the overlying materials causes the grains to pack closer together Contamination of Ground Water: Contamination of Ground Water Ground water in its natural state is relatively free of contaminants in most areas, and is why it is a widely used source of drinking water. This is why contamination of ground water can be a serious problem. Pesticides and herbicides applied to agricultural crops can find their way into ground water when rain or irrigation water leaches the poisons downward into the soil. Fertilizers are also a concern because nitrate is harmful in even small quantities in drinking water (e.g. “Blue Baby Syndrome”). Rain can also leach pollutants from city landfills into groundwater supplies. i.e. Ant poison, broken thermometer, lead-base paint, cadmium, oven cleaner, etc… Contamination of Ground Water: Contamination of Ground Water Contamination of Ground Water: Contamination of Ground Water Contaminated ground water is extremely difficult to clean up. Wells are installed to pump contaminated water out of the ground to treat it, or replace it with clean water. This process can take decades and tens of millions of dollars to complete. Groundwater contamination can be largely prevented with careful thought and considerable expense. Landfills sited well above the water table and flood levels, in a region of discharge rather than recharge. Site sealed below by impermeable clay barriers and plastic liners and sealed off from rainfall. Installation of dikes to divert surface runoff through or from the site. Pumping wells can cause or aggravate ground-water contamination. “Well drawndown” can increase the slope of the water table locally, thus increasing the rate of groundwater flow and giving the water less time to be purified underground before it is used. “Drawdown” can even reverse the original slope of the water table, perhaps leading to the contamination of wells. Heavily pumped wells near a coast can be contaminated by saltwater intrusion. Hydrothermal Activity: Hydrothermal Activity Hot springs are springs in which the water is warmer than human body temperature (370C). Water can gain heat in two ways while it is underground. 1) Circulation of ground water near a magma chamber or a warm igneous rock. In the U.S., most hot springs are associated with recent volcanism (e.g. Yellowstone National Park). 2) Circulation deep in the Earth’s crust can also heat ground water. A geyser is a type of hot spring that periodically erupts hot water and steam. Eruptions are most likely caused by a constriction in the underground “plumbing” of a geyser. As hot water comes to the surface and cools, it may precipitate some of its dissolved ions as minerals. Travertine is a deposit of calcite that often forms around hot springs When silica precipitates around a geyser, it’s called geyserite. “Click to view animation” Slide33: “Old Faithful” geyser in Yellowstone National Park, WY Slide34: Precipitation of calcite in the form of Travertine terraces Mammoth hot springs, Yellowstone National Park, WY Slide35: Geyserite deposits around the vent of Castle geyser Yellowstone National Park (photo by David McGeary) Geothermal Energy: Geothermal Energy Geothermal Energy Electricity can be generated by harnessing naturally occurring steam and hot water in areas that are exceptionally hot underground. In such areas, wells can tap steam that is then piped to a powerhouse, where it turns a turbine that spins a generator, creating electricity.