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Chapter 15: Groundwater. Water In The Ground (1). Groundwater is defined as all the water in the ground occupying the pore spaces within bedrock and regolith. The volume of groundwater is 40 times larger than the volume of all water in fresh-water lakes or flowing in streams.
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Water In The Ground (1) • Groundwater is defined as all the water in the ground occupying the pore spaces within bedrock and regolith. • The volume of groundwater is 40 times larger than the volume of all water in fresh-water lakes or flowing in streams. • Less than 1 percent of the water on Earth is ground water. • Most ground water originates as rainfall.
Depth of Groundwater • Water is present everywhere beneath the land surface, but more than half of all groundwater, including most of what is usable, occurs above a depth of 750m. • Below a depth of about 750 m, the amount of groundwater gradually diminishes. • Russian scientists encountered water at more than 11 km below the surface.
The Water Table (1) • The zone of aeration (also called the unsaturated zone) is a layer of moist soil followed by a zone in which open spaces in regolith or bedrock are filled mainly with air. • Beneath the unsaturated zone is the saturated zone, a zone in which all openings are filled with water. • The upper surface of the saturated zone is the water table.
The Water Table (2) • Normally, the water table slopes toward the nearest stream or lake. • In fine-grained sediment, a narrow fringe as much as 60 cm thick immediately above the water table is kept wet by capillary attraction. • Capillary attraction is the adhesive force between a liquid and a solid that causes water to be drawn into small openings.
The Water Table (3) • In humid regions, the water table is a subdued imitation of the land surface above it. • It is high beneath hills and low beneath valleys because water tends to move toward low points in the topography under the influence of gravity.
How Groundwater Moves (1) • Groundwater operates continuously as part of the hydrologic cycle. • As rain seeps into the ground it enters the groundwater reservoir.
How Groundwater Moves (2) • Most of the groundwater within a few hundred meters of the surface is in motion. • Groundwater moves so slowly that velocities are expressed in centimeters per day or meters per year. • Groundwater must move through small, constricted passages, often along a tortuous route.
Porosity and Permeability (1) • Porosity is the percentage of the total volume of a body of regolith or bedrock that consists of open spaces, called pores. It is porosity that determines the amount of water that a given volume of regolith or bedrock can contain.
Porosity and Permeability (2) • The porosity of a sedimentary rock is affected by several factors: • The sizes and shapes of the rock particles. • The compactness of their arrangement. • The weight of any overlying rock or sediment. • The extent to which the pores become filled with the cement that holds the particles together. • The porosity of igneous and metamorphic rocks generally is low.
Porosity and Permeability (3) • Permeability is a measure of how easily a solid allows fluids to pass through it. • A high porosity does not necessarily mean a correspondingly high permeability. • An example of a sediment with high porosity and low permeability is clay. • Clay particles have diameters of less than 0,004 mm. • Clay may have a very high porosity because the percentage of pore space is high. • Because the pores are very small, the permeability is low.
Porosity and Permeability (4) • As the diameters of the pores increase, permeability increases. • Gravel, with very large pores, is more permeable than sand and can yield large volumes of water to wells.
Recharge and Discharge of Groundwater (1) • The process by which groundwater is replenished is called recharge. • The process by which groundwater reaches and flows from the surface is called discharge. • An area of the landscape where precipitation seeps downward beneath the surface and reaches the saturated zone is called a recharge area.
Recharge and Discharge of Groundwater (2) • The water moves slowly toward discharge areas, where subsurface water is discharged to streams or to lakes, ponds, or swamps. • The surface extent of recharge areas is invariably larger than that of discharge areas.
Recharge and Discharge of Groundwater (3) • In humid regions, recharge areas encompass nearly all the landscape beyond streams and their adjacent flood-plains. • In more arid regions, recharge occurs mainly: • In mountains. • In the alluvial fans that border them. • Along the channels of major streams that are underlain by permeable alluvium.
Recharge and Discharge of Groundwater (4) • The time water takes to move through the ground from a recharge area to the nearest discharge area depends on rates of movement and on the travel distance. • Movement may take from a few days to possibly thousands of years in cases where water moves through the deeper parts of a groundwater body.
Movement in the Zone of Aeration (1) • Water initially soaks into the soil, which usually contains clay resulting from the chemical weathering of bedrock. • The low permeability and the fine clay particles cause part of the water to be retained in the soil by forces of molecular attraction. • Some of this moisture evaporates directly into the air.
Movement in the Zone of Aeration (2) • Much of it is absorbed by the roots of plants, which later return it to the atmosphere through transpiration. • Because of the pull of the gravity, water that cannot be held in the soil by molecular attraction seeps downward until it reaches the saturated zone.
Movement by Percolation in the Saturated Zone (1) • Once in the saturated zone, groundwater moves by percolation. • Percolating water moves slowly through very small pores along parallel, thread-like paths. • Responding to gravity, water percolates from areas where the water is high toward areas where it is lowest. • It generally percolates toward surface streams or lakes.
Movement by Percolation in the Saturated Zone (2) • The velocity of groundwater flow increases as the slope of the water table increases. • Flow rates of groundwater tend to be very slow because percolating groundwater encounters a large amount of frictional resistance. • The highest rate yet measured in the United States, in exceptionally permeable material, was only about 250 m/yr.
How Fast Does Groundwater Flow? (1) • In 1856, Henri Darcy, a French engineer, concluded the velocity of groundwater must be related to: • The hydraulic gradient: the slope of the water table. • The permeability of the rock or sediment through which the water is flowing. • The coefficient of permeability: permeability, density, and viscosity of water, expressed as a coefficient (K). • Also called hydraulic conductivity.
How Fast Does Groundwater Flow? (2) • K(h1-h2) V = L where: K is a coefficient know as the “coefficient of permeability” or “coefficient of conductivity”; h1-h2 is the difference in altitude; L is the horizontal distance between two points; • Because discharge (Q) in streams varies as a function of both stream velocity (V) and cross-sectional area (A), Q = AV
Springs And Wells (1) • A spring is a flow of groundwater emerging naturally at the ground surface. • Small springs are found in all kinds of rocks, but almost all large springs issue from lava flows,limestone, or gravel.
Springs And Wells (2) • A vertical or horizontal change in permeability is a common explanation for the location of springs. • This change involves the pressure of an aquiclude, a body of impermeable or distinctly less permeable rock adjacent to a permeable one. • Springs may also issue from lava flows, especially where a jointed lava flow overlies an aquiclude, or along the trace of a fault.
Springs And Wells (3) • A well will supply water if it intersects the water table. • When water is pumped from a new well, the rate of withdrawal initially exceeds the rate of local groundwater flow. • This imbalance in flow rates creates a conical depression in the water table immediately surrounding the well called a cone of depression.
Springs And Wells (4) • The locally steepened slope of the water table increases the flow of water to the well, consistent with Darcy’s Law. • An impermeable layer of clayey sediment in the zone of aeration may produce a perched water body (a water body perched atop an aquiclude that lies above the main water table).
Aquifers (1) • An aquifer is a body of highly permeable rock or regolith that can store water and yield sufficient quantities to supply wells. • Bodies of gravel and sand generally are good aquifers, because they tend to be highly permeable and often have large dimensions. • Many sandstones are also good aquifers.
Aquifers (2) • Aquifers are of two types: • Confined (bounded by aquicludes). • Unconfined (an aquifer that is not overlain). • An example of an unconfined aquifer is the High Plains aquifer, which lies at shallow depths beneath the High Plains of the United States. • About 30 percent of the groundwater used for irrigation in the United States is obtained from the High Plains aquifer.
Aquifers (3) • In parts of Kansas, New Mexico, and Texas, the water table has dropped so much over the past half century that the thickness of the saturated zone has declined by more than 50 percent. • The Dakota aquifer system in South Dakota provides a good example of a confined aquifer. • Water that percolates into a confined aquifer flows downward under the pull of gravity. • As it flows to greater depths, the water is subjected to increasing hydrostatic pressure.
Aquifers (4) • Potentially, the water could rise to the same height as the water table in the recharge area. • Such an aquifer is called an artesian aquifer, and the well is called an artesian well. • A freely flowing spring supplied by an artesian aquifer is an artesian spring;
Aquifers (5) • The Floridian aquifer is a complex regional aquifer system in which both confined and unconfined units are present, and in which water locally reaches the surface by an artesian flow. • The aquifer system is restricted mainly to middle and late tertiary limestones. • The age of groundwater in the Floridian aquifer system has been determined by radiocarbon dating of carbonate molecules dissolved in the water. • Water in the well farthest from the recharge area is calculated to have been in the ground for at least 19,000 years.
Mining Groundwater And Its Consequences (1) • In the dry regions of western North America, groundwater is a major source of water for human consumption. • In many of these dry regions, withdrawal exceeds natural recharge. • Groundwater can be a nonrenewable resource.
Mining Groundwater And Its Consequences (2) • Natural recharge takes so long to replenish a depleted aquifer that vast underground water supplies have been lost to future generations. • When groundwater withdrawal exceeds recharge, the water table falls. • It can cause shallow wells to run dry and necessitate the drilling of still deeper wells.
Mining Groundwater And Its Consequences (3) • To halt the fall of the water table, groundwater sometimes can be artificially recharged by spraying biodegradable liquid wastes from food processing or sewage treatment plants over the land surface. • The pollutants are removed by biologic processes as the liquid percolates downward through the soil. • The purified water then recharges the groundwater system. • Runoff from rainstorms in urban areas can be channeled and collected in basins.
Subsidence of the Land Surface (1) • The water pressure in the pores of an aquifer helps support the weight of the overlying rocks or sediments. • When groundwater is withdrawn, the pressure is reduced, and the particles of the aquifer shift and settle slightly. • As a result, the land surface subsides.
Subsidence of the Land Surface (2) • The amount of subsidence depends on: • How much the water pressure is reduced. • The thickness and compressibility of the aquifer. • Land subsidence is widespread in the south-western United States. • It has caused structural damage to buildings, roads, cables, pipes, and drains.
Subsidence of the Land Surface (3) • Land subsidence can be especially damaging where water is pumped from beneath cities. • Mexico City. • Pisa, home of the famous Leaning Tower.
