Earth: Portrait of a Planet Chap 19 CR
Spring
A natural outlet from which groundwater flows up onto the ground surface. (pages 731, 733)
Aquitard
Sediment or rock that does not transmit water easily and therefore retards the motion of the water. (page 725) have low permeability. Shales, evaporites, and well-cemented sandstones serve as aquitards.
Water table
The boundary, that separates unsaturated zone above from saturated zone below
how deep down in the crust can we ind groundwater
liquid groundwater does circulate in basement igneous and metamorphic rock, perhaps to depths of over 15 km. Most of this deep circulation takes place along fractures. the ultimate base of ground-water in crust may be taken as the depth where water encounters such high temperatures and pressures that it becomes a hydrothermal fluid. it lies at a depth of between 15 and 25 km. So groundwater occurs only in the upper crust.
natural groundwater quality
—pure H2O. But as groundwater percolates downward, it reacts with the soil, sediment, or rock through which it passes, and these reactions transform it into a chemical solution. Deep in sedimentary basins, groundwater derived from rain or snow may mix with water left in pore spaces from the time of the original deposition of sediment millions of years ago. If the sediment was deposited in a marine environment, this ancient deep groundwater can be saline. Warmer groundwater, can hold more ions in solution than can cooler groundwater. If groundwater enters a new environment where it has the capacity to contain more ions, it may dissolve surrounding rock or sediment and produce secondary porosity. Deeper groundwater that has passed through salt-containing strata or has mixed with old saline pore water may become salty and unsuitable for irrigation or drinking.
when do springs form
- where the ground surface intersects the water table in a discharge area. near or on valley floors where they may add water to lakes or streams(a) -where flowing groundwater collides with a steeply dipping impermeable barrier, and pressure pushes it up the barrier to the ground (f) -where a perched water table intersects the surface of a hill (b) -where a network of interconnected fractures channels groundwater to the surface of a hill (e) -where downward-percolating water runs into a relatively impermeable layer (aquitard) and migrates along the top surface of the layer to a hillslope (c) - where the ground surface intersects a natural fracture (joint) that taps a confined aquifer in which the pressure drives the water to the surface = artesian spring
Each aquifer has a name, based on the region in which it occurs or on the stratigraphic unit that hosts the groundwater.
-Mahomet aquifer, Illinois -Phoenix Basin aquifer, Arizona -The Dakota Sandstone aquifer -The High Plains (Ogallala) aquifer: -Limestone aquifers of Florida
The permeability of a material depends on several factors:
-Number of available conduits: As the number of conduits increases, permeability increases - more routes to move through -Size of the conduits: Greater volumes of fluids can travel through wider conduits because frictional resistance to flow develops along the conduits' walls. -Straightness of the conduits: Water lows more rapidly through straight. In crooked conduits, the distance a water molecule actually travels may be many times the straight-line distance.
Formation of Karst Landscapes
-The establishment of a water table in limestone: The story of a karst landscape begins after the deposition and lithification of a thick interval of limestone. If erosion and uplift cause exhumation of the limestone, a water table can develop in the limestone below the ground surface. - The formation of a cave network: Once the water table has been established, dissolution begins, and a cave network develops. - A drop in the water table: If the water table later becomes lower, either because of a decrease in rainfall or because nearby rivers cut down through the landscape and drain the region, newly formed caves dry out. Downward-percolating groundwater emerges from the roofs of the caves; speleothems precipitate. - Roof collapse: If rocks fall off the roof of a cave for a long time, the roof eventually collapses. Such collapse yields sinkholes and troughs, leaving behind hills, ridges, and natural bridges of limestone.
Speleothem
A formation that grows in a limestone cave by the accumulation of travertine precipitated from water solutions dripping in a cave or flowing down the wall of a cave.
Geyser
A fountain of steam and hot water that erupts periodically from a vent in the ground in a geothermal region. Boiling point of water increases with growing pressure => hot groundwater at depth can remain in liquid form even if its temperature has become greater than the boiling point of water at the Earth's surface (100°C). Superheated water begins to rise, pressure on it decreases => superheated water becomes steam and this rises
Discharge area
A location where groundwater flows back up to the surface and may emerge at springs. (page 730)
Artesian spring
A location where the ground surface intersects a natural fracture (joint) that taps a confined aquifer in which the pressure can drive the water to the surface. (page 734)
Recharge area
A location where water enters the ground and infiltrates down to the water table. (page 730) As a rule, groundwater lows from regions where it has a higher hydraulic head to regions where it has a lower hydraulic head.
Perched water table
A quantity of groundwater that lies above the regional water table because an underlying lens of impermeable rock or sediment prevents the water from sinking down to the regional water table. (page 728)A perched water table occurs where a mound of groundwater becomes trapped above a localized aquitard that lies above the regional water table.
Karst landscape
A region underlain by caves in limestone bedrock; the collapse of the caves creates a landscape of sinkholes separated by higher topography, or of limestone spires separated by low areas. (page 748)
unwanted effects of rising water tables
A rising water table weakens the base of a hillslope or an underground failure surface triggers landslides and slump
Pore
A small, open space within sediment or rock. (page 722)
Hot spring
A spring that emits water ranging in temperature from about 30°C - 104°C. Occur where groundwater, as it slowly lows from recharge area to discharge area, follows a flow path that takes it many kilometers down into the crust, where bedrock, naturally warm due to the geothermal gradient, heats the ground-water. The curving path of flow eventually carries the deep groundwater back up to the surface. Second, hot springs develop in geothermal regions. In places where the hot water rises into soils rich in volcanic ash and clay, a viscous slurry forms and fills goopy mudpots. Bubbles of steam rising through the mud cause it to splatter about.
Disappearing stream
A stream that intersects a crack or sinkhole leading to an underground cavern, so that the water disappears into the subsurface and becomes an underground stream. (page 748)
Oasis
A verdant region surrounded by desert, occurring at a place where natural springs provide water at the surface. (page 734)
Artesian well
A well in which water rises on its own. (page 735) Penetrate confined aquifers in which pressure causes the water to rise on its own to a level above the top surface of the aquifer. If this level lies below the ground surface, we call the well a nonflowing artesian well. But if the level lies above the ground surface, the well is a lowing artesian well. Pressure in the confined aquifer pushes water up an artesian well or spring. Where the potentiometric surface lies underground, the well will be non- flowing, but where the surface lies above the ground, the well will be lowing.
Groundwater contamination
Addition of chemicals or microbes (e.g., from agricultural and industrial activities, and landfills or septic tanks) to the groundwater supply. (page 741)
Stalactite
An icicle-like cone that grows from the ceiling of a cave as dripping water precipitates limestone. (page 748)
Caves
An underground open space, most or all of which does not receive direct sunlight. May grow due to the dissolution of rock, or by the removal of rock by rockfalls or erosion. Most large cave networks, including Mammoth Cave, develop in limestone bedrock because limestone dissolves relatively easily in groundwater that has become a dilute solution of carbonic acid (H2CO3). Such acidic groundwater forms when water absorbs carbon dioxide (CO2). When carbonic acid comes in contact with calcite (CaCO3) in limestone, the two chemicals react to produce (HCO3)1- and Ca 2+ ions, which then dissolve. The reaction releases CO2 back into the air. Some limestone caves around the world formed due to reactions with sulfuric-acid-bearing water; develop where limestone overlies strata containing oil because microbes convert the sulfur in the oil to hydrogen sulfide (H2S) gas, which rises and reacts with oxygen and water to produce sulfuricacid. his acid, in turn, eats into limestone and reacts with it to produce gypsum and CO2 gas.
Stalagmite
An upward-pointing cone of limestone that grows when drips of water hit the floor of a cave. (page 748)
drawdown
As long as the rate at which groundwater ills the well exceeds the rate at which water is removed, the level of the water table near the well remains about the same. However, if people pump water out of the well too fast, then the water table sinks down around the well, in a process called drawdown
saturated zone (phreatic zone)
Below the unsaturated zone, water completely ills, or saturates, the pores, yielding tHe saturated zone.
life in caves
Caves that are open to the air provide a refuge for bats as well as for various insects and spiders. Similarly, ish and crustaceans enter caves where streams low in or out. Colonies of bacteria metabolize sulfur-containing minerals in this water and create thick mats of living ooze in the complete darkness of the cave.
Why do extensive cave networks, with large chambers and abundant passages, develop in some locations but not in others?
First, most caves form in limestone, so without a thick layer of limestone in the subsur- face, extensive networks can't form. Second, the dissolution that forms caves occurs primarily in freshwater at the water table, so unless the water table lies above sea level and below the land surface, extensive networks can't form. Third, for caves to develop, sufficient liquid water must be present, so extensive networks form preferentially in temperate or trop- ical regions drenched by rain. Finally, since percolation through organic matter provides the acidity that groundwater must have in order to dissolve limestone, most caves form faster beneath regions that have organic-rich soil.
different flows and their residence time
Groundwater close to Earth's surface = tens of meters to a few kilometers before returning to the surface. Such local low has a residence time = hours to weeks. Groundwater following paths of several kilometers to tens of kilometers constitutes intermediate flow and has a residence time of weeks to years. Groundwater following paths that carry it hundreds of kilometers across a large sedimentary basin constitutes regional flow and stays underground for centuries to millennia
Hard water
Groundwater that contains dissolved calcium and magnesium, usually after passing through limestone or dolomite. (page 731)
Wells
In an ordinary well, the base of the well penetrates an aquifer below the water table. Water from the pore space in the aquifer seeps into the well and ills it to the level of the water table
Geothermal region
Places where igneous activity increases the geothermal gradient substantially, so that hot rock lies close to the ground surface heats up groundwater, which may discharge at the surface in the form of hot springs and/or geysers. (page 737)In geothermal regions worldwide, steam provides a clean means of generating electricity, and hot water can circulate through pipes to provide home heating.
pressure in groundwater
Pressure in groundwater at a specific point underground in an unconfined aquifer comes from the weight of all the overlying water from that point up to the water table. A point at a greater depth below the water table feels more pressure than does a point at a lesser depth. At a location with a horizontal water table, the pressure acting on an imaginary horizontal reference plane at a specified depth below the water table is the same everywhere. But if the water table is not horizontal (like hills and valleys) the pressure at points on a horizontal reference plane at depth differs with location. Both the elevation of a volume of groundwater and the pressure within the water provide energy that, if given the chance, will cause the water to low. (potential energy)
Aquifer
Sediment or rock that transmits water easily. (page 725) have both high porosity and high permeability. Coarse gravels, poorly cemented sandstones, and highly fractured and partially dissolved limestones typically make good aquifers
where do limestone caves form
Some limestone dissolves above the water table, particularly along joints, which act as conduits for water flowing into the subsurface, and some limestone may dissolve deep below the water table. But it appears that most cave growth, or speleogenesis, takes place in limestone that lies just below the water table. Here groundwater acidity remains high, the mixture of groundwater and newly added rainwater is undersatu- rated (meaning that it has the capacity to dissolve more ions), and groundwater lows the fastest
Sinkhole
The bedrock beneath consists of limestone. Groundwater, the liquid water that resides in sediment or rock under the surface of the Earth, had gradually dissolved the limestone over time, carving open rooms, or caverns, undergroundA circular depression in the land that forms when an underground cavern collapses. (page 722)
Permeability
The degree to which a material allows fluids to pass through it via an interconnected network of pores and cracks. (page 724) A material containing isolated pores can have high porosity but low permeability. Cork exhibits this behavior—it has high porosity, but low permeability (so it can plug a bottle). In cork, a type of tree bark, woody cell walls isolate adjacent pores (empty cells) and prevent communication between them. Vesicular basalt, similarly, has high porosity but low permeability because rock surrounds each pore.
Cone of depression
The downward-pointing, cone-shaped surface of the water table in a location where the water table is experiencing drawdown because of pumping at a well. (page 735).Drawdown in a deep well can lower the water table over a broad region causing nearby shallower wells to go dry
Pore Collapse and Land Subsidence
The extraction of water from a pore eliminates the support holding the pore open because the air that replaces the water can be compressed. As a result, the grains pack more closely together. Such pore collapse permanently decreases the porosity and permeability of the rock or sediment and thus lessens its value as an aquifer. Pore collapse also decreases the volume of an aquifer, with the result that the ground above the aquifer sinks. Such land subsidence may cause fissures to develop at the surface and the ground to tilt - the flooding of Venice, Italy, accompanies land subsidence due to the withdrawal of groundwater beneath the city - directing surface water into recharge areas or by pumping surface water back into the ground
recharge and discharge area
The location where water enters the ground, meaning the region where the low has a down- ward trajectory, is the recharge area, and the location where groundwater lows back up to the surface is the discharge are
saline intrusion
The movement of seawater into freshwater aquifers, which can lead to contamination of drinking water sources and other consequences. Fresh groundwater lies in a layer above saline (salty) groundwater because of their differences in density. If pump water too quickly => boundary between the saline and the fresh groundwater rises. If the boundary rises above the base of a well, then the well will start to yield useless saline water.
Hydraulic head
The potential energy available to drive the flow of a given volume of groundwater at a location; it can be measured as an elevation above a reference. (page 729)
groundwater depletion
The proportion of a region's water supply that comes from groundwater depends on the local climate, the availability of other water supplies (such as lakes or rivers), local demand, and politics. Groundwater in many regions of the Earth must be viewed as a nonrenewable resource. The renewability of a particular groundwater reservoir depends on the balance between the rate of recharge and the rate of depletion. -growth of human populations -increasing crops / cultivating land / water consumption of farms -Global industrial growth also accelerates water use. => increased demand => groundwater withdrawal > groundwater recharge -changing climates, which tend to affect the distribution of rainfall, may decrease rates of groundwater recharge the most in places where it is needed the most. When we extract groundwater faster than it can be resupplied => water table becomes lower => existing wells, springs and rivers dry up. Notably, the water table can also drop when people divert surface water from the recharge area (Everglades) => Diversion of water from the Everglades' recharge area into canals has significantly lowered the water table, causing parts of the Everglades to dry up.
Hydraulic gradient
The slope of the water table. (page 732) A hydraulic gradient exists anywhere that the water table is not horizontal
Capillary fringe
The thin subsurface layer in which water molecules seep up from the water table by capillary action to fill pores. (page 727)
Porosity
The total volume of empty space (pore space) in a material, usually expressed as a percentage. (page 722)
Soil moisture
Underground water that wets the surface of the mineral grains and organic material making up soil, but lies above the water table. (page 726)
unconfined aquifer
Water can infiltrate directly into an unconfined aquifer from the Earth's surface.
Groundwater
Water that resides under the surface of the Earth, mostly in pores or cracks of rock or sediment. (page 722) subsurface water in the saturated zone.
formation of speleotherms
When the water table drops below the level of a cave that has developed, speleogenesis slows down or ceases, and the cave becomes an open space filled with air. In places where down- ward-percolating groundwater containing dissolved calcite emerges from the rock above the cave and drips from the ceiling, the surface of the cave gradually changes. As soon as this water re-enters the air, it evaporates a little and releases some of its dissolved carbon dioxide. As a result, calcite precipitates out of the water and produces a type of travertine called dripstone. The various intricately shaped formations that grow in caves by the accumulation of dripstone are called speleothems.
travertine terraces
Where geothermal waters that have passed through limestone bedrock spill out of natural springs and then cool, dissolved carbonate minerals in the water precipitate, forming travertine mounds or travertine terraces
contaminant plume
a cloud of contaminated groundwater that moves away from the source of the contamination
confined aquifer
an aquifer surrounded by a layer of impermeable rock or clay that impedes water flow
what can cause the flow direction of a nearby plume to change
development of a large cone of depression around a big well.
primary porosity
develops during sediment deposition and during rock formation. tends to decrease with increasing burial depth
Human-Caused Groundwater Contamination
groundwater contamination has increased as human activities; contaminants include agricultural waste (pesticides, fertilizers, and animal sewage); industrial waste (both organic and inorganic chemicals); effluent from landfills and septic tanks (including bacteria and viruses); petroleum products and other chemicals that do not dissolve in water; radioactive waste (from weapons manufacture, power plants, and hospitals); and acids leached from sulfide minerals in coal and metal mines.
unsaturated zone (vadose)
he region of the subsurface in which water only partially fills pores
Darcey's Law for groundwater flow
hydraulic gradient = (h1-h2) / j h1-h2: difference in head in m j: distance between the 2 points Q = (KA(h1-h2)) / j Q= velocity of flow ( discharge) K= hydraulic conductivity A= area Put in simpler terms, the flow rate of groundwater increases as the permeability increases and as the slope of the water table gets steeper.
Where does groundwater reside?
in the open space in rock and sediment (porosity)
groundwater flow paths
in unsaturated zone (between ground & water table): water percolates down. in saturated zones (region below water table): water flows downwards (pull of gravity) and can flow sideways or upwards due to pressure differences. Groundwater from regions where the water table sits high (under a hill) to regions where the water table sits low (below a valley), but because of low-path shape, some ground- water may low down into the crust along the first part of its path and then may low back up, toward the ground surface, along the final part of its path.
Position of the water table
its shape mimics, in a subdued way, the shape of the overlying topography (Fig. 19.8a). his means that the water table lies at a higher elevation beneath hills than it does beneath valleys.
avoid groundwater contamination
prevent contaminants from entering groundwater in the first place. This can be done by locating potential sources of contamina- tion on impermeable bedrock, isolated from aquifers. If such a site is not available, the storage area should be lined with a thick layer of clay or impermeable plastic. Clay not only acts as an aquitard, but can hold onto contaminants, and plastic liners keep contaminants from coming in contact with substrate. Stockpiling the containers in tunnels cut into salt domes, because salt is impermeable. Another option involves placing the containers in tunnels above the water table. natural processes can clean up ground water contamination. For example, chemicals may be absorbed by clay, oxygen in the water may oxidize them, and bacteria in the water may metabolize them, thereby turning them into harmless substance. drill test wells to determine which way and how fast the con- taminant plume is lowing. Once they know the low path, they can close wells in the path to prevent consumption of contaminated water. Clean groundwater with extraction wells. Remediate groundwater contamination=> bioremediation=> inject oxygen and nutrients into contaminated aquifer to foster the growth of bacteria that can consume and break down contaminant molecules. Insertion of permeable reactive barriers underground; subsurface walls of materials such as iron filings that react with contaminants chemically to transform them into safer, less soluble materials
secondary porosity
refers to new pore space produced in rocks some time after the rock first formed. Forms when groundwater passes through rock and dissolves minerals or cement, yielding small solution cavities. Secondary porosity also forms when rocks fracture, for the opposing walls of the fracture do not it together tightly
character of cave networks
rooms or chambers, and passages, which are tunnel- or slot-shaped corridors. May host underground lakes, conduits for underground streams. Shape depends on permeability and composition of rock=> Chambers develop where the lime- stone dissolves more easily, so in a sequence of strata, caves develop preferentially in the most soluble limestone beds. Chambers may also form where groundwater lows most rap- idly. Passages in cave networks typically follow pre-existing joints, which provide secondary porosity along which ground- water can low faster
consequences of groundwater use
saline intrusion, pore collapse and land subsidence
column of travertine
stalagmite merges with the overlying stalactite
Flowstone (travertine terrace)
structure formed by water flowing across cave's floor
rates of groundwater flow
typical low rates range between 0.01 and 1.4 m per day. Must percolate through a complex, crooked network of tiny conduits, so it must travel a much greater distance than it would if it could follow a straight path. Second, the surface tension of water makes it stick to the solid material around it and therefore slows its escape from pores. Velocity depends on slope of water table and permeability of material through which it moves =>ground water lows relatively quickly through high-permeability gravel under a steep hillslope, but it flows relatively slowly through a low-permeability, well-cemented sandstone
dry well
well that does not reach below the water table. Drilling into rock that lies above the water table, or into an aquitard, will not supply water and thus yields a dry well. Pumping groundwater from an big well can affect the water table and thus rendering the small well dry as the water table lowers.
