Chapter 13 - EES 1050 - Groundwater
Karst Topography
(Or simply karst), develops largely by groundwater erosion in many areas underlain by soluble rocks Karst topography is characterized by numerous caves, springs, sinkholes, solution valleys, and disappearing streams
Geothermal Energy
1-2% of the world's energy needs could be met with geothermal energy. Geothermal energy is currently used in Iceland, the US, Mexico, Italy, New Zealand, Japan, the Philippines, and Indonesia. Derived from mostly convergent zones and hot spots
For an artesian system to develop, three geologic conditions must be met:
1. The aquifer must be confined above and below by aquicludes, layers that are not permeable 2. The aquifer is usually tilted and exposed at the surface so it can be recharged. 3. Precipitation must be sufficient to keep the aquifer filled
Review Questions Answer: B
1. Which of the following is not an example of groundwater erosion? a. ____ karst topography. b. ____ stalactites. c. ____ sinkholes. d. ____ caves. e. ____ disappearing streams.
Review Questions Answer: C
2. The water table is a surface separating the a. ____ zone of porosity from the underlying zone of permeability. b. ____ capillary fringe from the underlying zone of aeration. c. ____ zone of aeration from the underlying zone of saturation. d. ____ capillary fringe from the underlying zone of aeration. e. ____ zone of saturation from the underlying zone aeration.
Review Questions Answer: C
3. The porosity of Earth materials is defined as a. ____ their ability to transmit fluids. b. ____ the depth of the zone of saturation. c. ____ the percentage of void space. d. ____ their solubility in the presence of weak acids. e. ____ the temperature of groundwater.
Review Questions Answer: D
4. A cone of depression forms when a. ____ a stream flows into a sinkhole. b. ____ water in the zone of aeration is replaced by water from the zone of saturation. c. ____ a spring forms where a perched water table intersects the surface. d. ____ water is withdrawn from a well faster than it can be replaced. e. ____ the ceiling of a cave collapses, forming a steep- sided crater
Review Questions Answer: E
5. Which of the following conditions must exist for an artesian system to form? a. ____ Groundwater must circulate near magma. b. ____ The water table must be at or very near the surface. c. ____ The rocks below the surface must be especially resistant to solution. d. ____ Water must rise very high in the capillary fringe. e. ____ An aquifer must be confined above and below by aquicludes.
Review Questions Answer:
6. Explain how groundwater weathers and erodes Earth materials.
Review Questions Answer:
7. Why should we be concerned about how fast the groundwater supply is being depleted in some areas?
Review Questions Answer: In a water well, groundwater rises to the top of the zone of saturation, or the water table, and from there, it must be pumped to the surface. In an artesian system, groundwater is confined to an aquifer by aquicludes above and below, the rock sequence is tilted so that it builds up high hydrostatic (fluid) pressure, and the aquifer is exposed at the surface, enabling it to be recharged. There is no intrinsic difference between water from a water well or from an artesian system. Any difference in quality is a function of dissolved minerals and any introduced substances.
8. What is the difference between an artesian system and a water well? Is there any difference between the water obtained from an artesian system and the water from a water well?
Review Questions Answer: In arid regions, such as Nevada, the water table is very low, in some cases tens and hundreds of meters below the surface. Furthermore, rainfall is scant, and what little rain does fall and percolate into the ground usually evaporates before it get very far. Thus, the likelihood of water reaching deeply buried canisters containing radioactive nuclear waste material where it could cor- rode the containers or become polluted by the radioac- tive material is virtually nonexistent for the foreseeable future. However, if the climate should change and the region become humid, say, in the next several thousand years, water from rainfall will percolate freely through the zone of aeration into the zone of saturation, and even- tually it will cause the water table to rise, possibly to the level of the buried radioactive nuclear waste material, where the groundwater system could then become pol- luted by the radioactive material. The average rate of groundwater movement during the next 5,000 years would have to be 8 cm per year to reach radioactive waste canisters buried at a depth of 400 m (400 m = 40,000 cm [100 cm/m X 400 m = 40,000 cm]; 40,000 cm/5,000 years = 8 cm/year).
9. One concern geologists have about burying nuclear waste in present-day arid regions such as Nevada is that the climate may change during the next several thousand years and become more humid, thus allowing more water to percolate through the zone of aeration. Why is this a concern? What would the average rate of groundwater movement have to be during the next 5,000 years to reach canisters containing radioactive waste buried at a depth of 400 m?
Aquifer
A permeable layer transporting groundwater
Aquifers are rocks, sediments, or soils that are permeable or capable of transmitting groundwater. Sand and gravel are often good aquifers
Aquicludes are impermeable rocks, sediments and soils that are incapable of transmitting groundwater. Unfractured shales, clays and many metamorphic and intrusive igneous rocks are aquicludes
Artesian System
Can be applied to any system in which groundwater is confined and builds up high hydrostatic (fluid) pressure Water in such a system is able to rise above the level of the aquifer if a well is drilled through the confining layer, thereby reducing the pressure and forcing the water upward An artesian system can develop when: (1) an aquifer is confined above and below by aquicludes (2) the rock sequence is (usually) tilted to build up hydrostatic pressure; and (3) the aquifer is exposed at the surface, thus enabling it to be recharged
Water from precipitation reacts with carbon dioxide (CO2) in air and organic-rich soils to produce carbonic acid (H2CO3)
Carbonic acid readily dissolves calcite(CaCO3) in limestones to produce caves Salt deposits and other water-soluble rocks are also susceptible to dissolution and the formation of caves
KEY Concepts Review
Caves form when groundwater in the zone of saturation weathers and erodes soluble rock such as limestone. Common cave deposits include stalactites, stalagmites, columns, drip curtains, and travertine terraces.
The precipitation of calcite within caves creates a variety of interesting depositional features.
Common cave deposits include: Stalactites Stalagmites Columns Drip Curtains Travertine Stalactites hang from the ceiling, stalagmites are on the "ground" (stalactites hold tight to the ceiling, stalagmites might reach the ceiling)
KEY Concepts Review
Contamination by humans is becoming a serious problem and can result from landfills, septic systems, toxic waste sites, and industrial effluents, all of which affect the quality of the groundwater.
Groundwater Quality
Excluding pollution from humans, groundwater quality is mostly a function of the: Kinds of materials that make up an aquifer Residence time of water in an aquifer Solubility of rocks and minerals in the aquifer These factors account for the amount ofdissolved materials in groundwater and areresponsible for such undesirable effects as hardwater and iron staining.
Karst topography largely develops by groundwater erosion in many areas underlain by soluble rocks, especially limestones
Features of karst topography include: Sinkholes, along with springs, solution valleys, disappearing streams, and caves
KEY Concepts Review
Geothermal energy is energy produced from Earth's internal heat and comes from the steam and hot water trapped within Earth's crust. It is a relatively nonpolluting form of energy that is used as a source of heat and to generate electricity.
Geothermal Energy
Geothermal energy is energy produced from Earth's internal heat. Comes from the steam and hot water trapped within Earth's crust It is a relatively clean form of energy that is used as a source of heat and to generate electricity.
Groundwater and the Hydrologic Cycle
Groundwater is part of the hydrologic cycle and an important natural resource. Groundwater is recharged by infiltration from precipitation, streams,lakes, and other surface waters. Groundwater exits the subsurface through springs and human use
KEY Concepts Review
Groundwater is part of the hydrologic cycle and an important natural resource. It consists of all subsurface water trapped in the pores and other open spaces in rocks, sediment, and soil.
Groundwater is all subsurface water that completely fills the pores and other open spaces in rocks, sediments, and soil.
Groundwater is responsible for forming beautiful caverns It is also an important source of freshwater
KEY Concepts Review
Groundwater moves slowly downward under the influence of gravity through the zone of aeration to the zone of saturation. Some of it then moves along the surface of the water table, and the rest moves from areas of high pressure to areas of low pressure.
KEY Concepts Review
Groundwater quality is mostly a function of the kinds of materials that make up an aquifer, the residence time of water in an aquifer, and the solubility of rocks and minerals.
KEY Concepts Review
Groundwater velocity varies greatly and depends on a number of factors. Generally, the average velocity of groundwater is a few centimeters per day.
Most groundwater moves from areas of high pressure to areas of low pressure,sometimes moving upward
Groundwater velocity varies greatly and depends on many factors. Generally, the average velocity of groundwater is only a few centimeters per day.
Fumeroles are steam discharges in volcanically active areas
Hot springs are springs where the water temperature is higher than 37°C (human body temperature)
Geysers
Hot springs that intermittently eject hot water and steam with tremendous force
Geysers
Hot springs which periodically eject hot water and steam with tremendous force
KEY Concepts Review
Hydrothermal refers to hot water, typically heated by magma, but also resulting from Earth's geothermal gradient as it circulates deeply beneath the surface. Manifestations of hydrothermal activity include fumaroles, hot springs, and geysers.
Cones of depression result when the rate of water removed from a well is greater than the rate flowing into the well from the aquifer
If the cone of depression reaches the bottom of the well, the well runs dry.
Groundwater Quality
If the total calcium and magnesium content of a water exceeds 120 mg/L, then the water is "hard." Hard water precipitates calcite scales in pipes, makes soap difficult to lather and may cause water heaters to overheat, produce hydrogen gas and explode. Water softeners remove calcium and magnesium from water and release sodium. Sodium may be a problem for those with high blood pressure.
KEY Concepts Review
In an artesian system, confined groundwater builds up high hydrostatic pressure. For an artesian system to develop, an aquifer must be confined above and below by aquicludes; the aquifer is usually tilted so that it can build up hydrostatic pressure; and the aquifer must be exposed at the surface so that it can be recharged.
Artesian systems - groundwater flows out of the well without pumping
In an artesian system, the aquifer is confined or"sandwiched" between two aquicludes. The confined groundwater builds up high hydrostatic pressure
Sinkholes
In regions underlain by soluble rock, the ground surface may be pitted with numerous depressions that vary in size and shape. These depressions, called sinkholes, or merely sinks, mark areas with underlying soluble rock
Capillary Fringe - a thin saturated zone found in some fine-grained materials
It extends a few centimeters or meters above the water table. The water in the capillary fringe does not flow as groundwater, but hangs between the closely space particles.
KEY Concepts Review
Karst topography develops by groundwater erosion in many areas underlain by soluble rocks. It is characterized by sinkholes, caves, solution valleys, and disappearing streams.
KEY Concepts Review
Modification of the groundwater system can cause serious problems such as lowering of the water table, saltwater incursion, subsidence, and contamination
As the world's population and industrial development expand, the demand for water,particularly groundwater, will increase
Most groundwater in the United States is used for irrigation of crops and public drinking water supplies
Hydrothermal refers to naturally occurring hot water
Most hydrothermal water results from groundwater infiltrating deep within the Earth and being heated by magmas or simply through the geothermal gradient. Fumaroles, hot springs, and geysers are all hydrothermal features.
Groundwater is a valuable natural resource that is being exploited rapidly
Over pumping and other modifications to the groundwater system can cause serious problems such as: Lowering of the water table Saltwater incursion Subsidence Contamination
Springs
Places where groundwater flows or seeps out of the ground. The water flows out of the ground for no apparent reason and from no readily identifiable source.
Groundwater is stored in open spaces in rocks called pores
Porosity is the percentage of a material's total volume that is pore space. Permeability is the capacity of rocks,soils or sediments to transmit fluids. Permeability is dependent on porosity,but also on the size of the pores and their interconnections. A porous rock with poorly connected pores would be impermeable
KEY Concepts Review
Porosity is the percentage of a material's total volume that is pore space. Permeability is the capacity to transmit fluids, and is dependent on porosity, but also on the size of the pores or fractures and their interconnections.
Water infiltrates downward under the influence of gravity through the zone of aeration to become groundwater in the zone of saturation
Some groundwater moves on the water table from high to lower elevations under the influence of gravity
KEY Concepts Review
Springs are found wherever the water table intersects the ground surface. Some springs are the result of a perched water table—that is, a localized aquiclude within an aquifer and above the regional water table.
Permeability
The capacity to transmit fluids
Caves and Cave Deposits
The dissolution of limestones by groundwater producesmany depositional and erosional features, including caves. Caverns are large caves or a system of interconnected caves
Porosity
The percentage of a material's total volume that is pore space. Porosity most often consists of the spaces between particles in soil, sediment, and sedimentary rocks, but other types of porosity include cracks, fractures, faults, and vesicles in volcanic rocks
Water Table
The surface that separates these two zones (aeration and saturation)
KEY Concepts Review
The water table is the surface separating the zone of aeration (in which the pores are filled with air and water) from the underlying zone of saturation (in which the pores are filled with water). The water table is a subdued replica of the overlying land surface in most places.
Hot springs
Travertine or calcareous tufa - Calcite that precipitates from supersaturated hot spring water Geyserite - Silica that precipitates from supersaturated hot spring water Yellowstone National Park
Groundwater
Water that fills open spaces in rocks, sediment, and soil beneath Earth's surface—is one reservoir in the hydrologic cycle.
KEY Concepts Review
Water wells are openings made by digging or drilling down into the zone of saturation. When water is pumped from a well, the water table in the area around the well is lowered, forming a cone of depression.
Cone of Depression
When groundwater is pumped from a well, the water table in the area around the well is lowered, forming a cone of depression
Water wells are human-made openings dug or drilled into the zone of saturation.
When the zone of saturation has been penetrated, water percolates into the well,filling it to the level of the water table Modern wells use pumps, which create cones of depression on the water table surrounding the well
The water table is located between the zone of aeration and the zone of saturation
Zone of aeration - where water initially infiltrates, most of the pores are filled with air Zone of saturation - a zone in which the pores are filled with water
Cave
a naturally formed, subsurface opening that is generally connected to the surface and is large enough for a person to enter. A cavern, on the other hand, is a very large cave or a system of interconnected caves.
Geothermal Energy
any energy produced from Earth's internal heat
Springs
are found wherever the water table intersects the surface. Water by itself flows out of the springs
Sinkholes
depressions in the ground formed by the dissolution of the underlying soluble rocks or the collapse of a cave roof.
Hot Spring
is any spring in which the water temperature is higher than 37 degrees Celsius, the temperature of the human body
Perched water tables
often have springs, are associated with localized perched aquifers within zones of aeration. Perched aquifers typically form on top of localized aquicludes
Water Wells
openings made by digging or drilling down into the zone of saturation. Once the zone of saturation has been penetrated, water percolates into the well, filling it to the level of the water table
Zone of Aeration
pore spaces contain mostly air
Zone of Saturation
pore spaces contain mostly water
Hydrothermal
referring to hot water. Some geologists restrict the meaning to include only water heated by magma, but here we use it to refer to any hot subsurface water and the surface activity that results from its discharge.
Types of Pores
some rock types have more porosity than others Notice that clays have abundant micropores. However, they tend to impermeable A well-sorted sedimentary rock has high porosity, whereas a poorly sorted one has lower porosity. In soluble rocks such as limestone, porosity can be increased by solution, whereas crystalline metamorphic and igneous rocks can be rendered porous by fracturing.
