Chapter 14 - Groundwater

• Importance of Groundwater

  Groundwater is water stored belowe Earth's surface, mainly in tiny pore spaces between rock or sediment grains.

  • represents the largest reservoir of freshwater that is readily available to humans. and is a critical resorce for human civilization.
  • dissolves rocks to make sinkholes and caverns and supplies streams with additional water.
  • e.g. US - 306 billion gallos of freshwater per year
• Groundwater and Water Table

  Distribution of Ground water

  • some of the water that soaks into the ground does not travel far because it is held by molecular attraction as a surface film on soil particles.
  • this near surface zone is zone of soil moisture
  • criscrossed by roots, voids left by decayed roots and animal and worm burrows that enhance the infiltration of rainwater into the soil.
• Groundwater and Water Table


  Distribution of Ground water

  • water that is not held as soil moisture percolates downward until it reaches a zone where all the open spaces in sediment and rock are completely filled with water.
  • this is the zone of saturation (also called phreatic zone)
  • water in this zone is also called ground water
  • the upper limit of this zone is known as the water table.
  • extending upward from the water table is the capillary fringe.
  • here groundwater is held by surface tension in tiny passages between grains of soil or sediment.
• Groundwater and Water Table

  Distribution of Ground water

  • unsaturated zone (vadose zone)
  • the area above the water table that includes the capillary fringe and the zone of soil moisture.
  • pore spaces in this zone contain both air and water.
  • although a considerable amount of water can be present in this zone, this water cannot be pumped by wells because it clings two tightly to nick and soil particles.
  • By contrast, below the water table the water pressure is great enough to allow water thus permitting groundwater to be withdrawn for use to enter wells
• Groundwater and Water Table

  Variations in the Water Table

  • water table is highly variable and can range from zero, when the tone of saturation is at the surface, to hundreds of meters below the surface in some places.
  • an important characteristic of the water table:
  • its configuration varies seasonally and from year to year because the addition of water to the groundwater system is closely related to the quantity, distribution and timing of precipitation.
  • water table is rarely level.
  • its shape is usually a subdued replica of the surface topography.
• Groundwater and Water Table

  Variations in the Water Table

  • In a wetland (swamp) the water table is right at the surface.
  • Lakes and streams generally occupy areas low enough that the water table is above the land surface.
  • Several factors contribute to the irregular surface of the water table.
  • groundwater moves very slowly and at varying rates under different conditions.
  • because of this, water tends to "pile up" beneath high areas between stream valleys.
  • if rainfall were to cease completely, these water table "hills" would slowly subside and approach the level of valleys.
• Groundwater and Water Table


  Interactions between Groundwater Table

  • basic link in the hydrologic cycle
  • this interaction can happen in 1 of 3 ways.
  • streams may gain water from the inflow of ground water thru the streambed (gaining streams)
  • for this to occur, the elevation of the water table must be higher than the level of the surface of the stream.
  • streams may lose to the groundwater system by outflow thru the streambed (losing stream)
  • when this happens, the elevation of the water table must be lower than the surface of the stream.
• Groundwater and Water Table


  Interactions between Groundwater Table

  • A combination of the 1st two
  • a stream gains in some sections and loses in others.
• Storage & Movement of Groundwater

  Influential Factors

  • porosity and permeability

  1. Density

  • water soaks into the ground because bedrock, sediment and soil contain countless voids called pore spaces.
  • quantity of groundwater that can be stored depends on porosity of material, which is the percentage of total volume of rock or sediment.
  • voids most often found on spaces between sedimentary particles, but also common in joints, faults, cavities formed by the dissolving of soluble rock like limestone and vesicles (voids left by gases escaping from lava)
• Storage & Movement of Groundwater

  Influential Factors

  • porosity and permeability

  1. Density

  • pore spaces depends on the size and shape of grains, how packed, degree of sorting, and amount of cementing in sedimentary rocks.
  • where sediments are poorly sorted, the porosity is reduced because the finer particles tend to fill the openings among larger grains.
  • in igneous and metamorphic rocks, fractures must provide the porosity.
• Storage & Movement of Groundwater

  Influential Factors

  • porosity and permeability

  2. Permeability

  • rock or sediment may be very porous but still not allowed for water to pass thru.
  • pores must be connected to allow water flow, and they may be large enough to allow flow.
  • groundwater moves by twisting and turning thru small interconnected openings.
  • the smaller the pore spaces, the more slowly the water moves.
  • e.g. clay deposit may greatly store water, but its pore spaces are so small.
  • significant in determining the rate of groundwater movement & quantity of water that might be pumped from a well.
• Storage & Movement of Groundwater

  Influential Factors

  • porosity and permeability

  3. Aquitards & Aquifers

  • Aquitards - impermeable layers that hinder water movement (clay)
  • Aquifers - permeable rock strata/sediment that transmit groundwater freely (sand, gravels)
• Storage & Movement of Groundwater

  Groundwater flow system

  • 3D body of Earth material saturated with moving groundwater
  • shows groundwater moving along flow paths from recharge areas where groundwater is being replenished.
  • to discharge areas along streams, where groundwater is flowing back to surface
• Storage & Movement of Groundwater

  Groundwater flow system

  • Different Scales of Movement
  • Red arrows
  • show water movement in a somewhat deeper system in which groundwater does not discharge into the nearest body but to a more distant one.
  • Black arrows
  • groundwater movement in a deep regional system that lies beneath the more shallow ones and is connected to them.
• Walls and Artesian Systems


  • Well
  • most common method for removing groundwater by boring a hole into the zone of saturation.
  • serves as small reservoirs into which groundwater migrates and from which it can be pumped to the surface.
  • Whenever water is withdrawn from a well, the water table around the well is lowered.
  • this is called drawdown and decreases with increasing distance from the well.
  • result is a cone of depression which is a depression in the water table that is conical in shape.
• Walls and Artesian Systems


  • Drilling a successful well is a familiar challenge in areas where groundwater is primary source of supply,
  • one well is successful at a depth of 10 meters but a deeper depth may be needed for others.
  • when subsurface materials are heterogenous, the amount of water that a well can provide may vary a great deal over short distance.
  • e.g. when 2 nearby wells are drilled to the same level, only 1 is successful because there may be a perched water table beneath.
• Walls and Artesian Systems

  • Perched table
  • forms when an aquitard is situated above the main water table.
  • Artesian System
  • in most wells, water cannot raise on its own.
  • if it's at 30 meters, it remains at that level.
  • in some wells, water rises without being pumped.
  • abundant in the region of Northern France.
  • Artesian
  • applied to any situation in which groundwater under pressure rises above the level of aquifer.
• Walls and Artesian Systems?

  Artesian

  • 2 conditions are needed for its existence:
  • water is confined to an aquifer that is inclined so that one end can receive water.
  • aquitards, both above and below, must be present to prevent the water from escaping.
• Walls and Artesian Systems

  Artesian

  • confined aquifer
  • when such a layer is tapped, the pressure created by the weight of the water above forces the water to rise.
  • if there were no friction, the water in the well would rise to the level of the water at top of aquifer.
  • friction reduces height of pressure surface
  • the greater the distance from recharge area (where water enters aquifer), the greater the friction and the less the rise of water.
• Walls and Artesian Systems


  Artesian

  • Non-flowing artesian well
  • pressure surface is below ground level.
  • Flowing artesian well
  • pressure surface is above ground and a well is drilled into the aquifer.
• Walls and Artesian Systems

  Not all artesian systems are wells

  • Artesian springs
  • exist where groundwater reaches the surface by rising along a natural fracture such as fault, rather than artificial hole.
  • in deserts, artesian springs sometimes create oases
  • oases - act as conduits, often transmitting water with great distances from recharge to point of discharge.
• Springs, Geysers & Geothermal Energy

  Springs

  • whenever the water table intersects Earth's surface, a natural outflow of groundwater results called spring.
  • often form when an aquitard blocks the downward movement of groundwater and causes the water to move laterally, when the permeable bed crops out, a spring results.
• Springs, Geysers & Geothermal Energy

  Springs

  • are not confined to places where a perched table creates a flow at the surface.
  • even in areas underlain by impermeable crystalline rocks, permeable zones may exist in the form of fractures or solution channels.
  • if these openings fill with water and intercel the ground surface along a slope, a spring results.
• Springs, Geysers & Geothermal Energy


  • Hot Springs
  • frequently used definition is the water in a hot spring is 6 to 9° warmer than the mean annual air temperature for the locality where it occurs.
  • temperatures in deep mines and oil wells usually rise with increasing depth, an average of about 25°C per km. (geothermal gradient)
  • when groundwater circulates at great depths, it becomes heated.
  • if hot water rises rapidly to the surface, it may emerge as a hot spring.
• Springs, Geysers & Geothermal Energy

  Geysers

  • intermittent hot springs or fountains in which columns of water are ejected with great force at various intervals, often rising 30 to 60 meters into the air.
  • after the jet of water ceases, a column of steam rushes out with a thunderous roar.
  • e.g. Old Faithful Geyser in Yellowstone National Park
  • came from the lcelandic word geysa = gush
• Springs, Geysers & Geothermal Energy


  Geyser deposits

  • when groundwater from hot springs and geysers flow out at the surface, material in solution is often precipitated, producing an accumulation of chemical sedimentary rock.
  • when the water contains dissolved silica, a material called siliceous sinter or geyserite is deposited around the spring.
• Springs, Geysers & Geothermal Energy


  Geyser deposits

  • Travertine or calcareous tufa is deposited when the water contains dissolved calcium carbonate.
  • term is used if the material is spongy and porous.
  • e.g. Mammoth Hot Springs in Yellowstone Nationa'l Park
  • Supersaturated with CaCO3 which then precipitates.
  • contains dissolved silica and CaCO3
  • some has sulfur which gives a poor taste and unpleasant odor.
• Springs, Geysers & Geothermal Energy


  Geothermal Energy

  • harnessed by tapping natural reservoirs of steam and hot water.
  • occur where subsurface temperatures are high, due to relatively recent volcanic activity.
  • used in 2 ways: space heating and generate electricity
  • e.g. In Reykjavik, Iceland - steam and hot are pumped into buildings throughout the city for space heating.
• Springs, Geysers & Geothermal Energy

  Geothermal Energy

  These geologic factors favor a geothermal reservoir of commercial value:

  1. potent source of heat

  • such magma chambers are most likely in regions of recent volcanic activity.
  • large magma chamber deep enough to ensure adequate pressure and slow cooling but not so deep that the natural water circulation is inhibited.
  • 2. Large and porous reservoirs w/ channels connected to the heat source
  • near which water can circulate and then be stored in the reservoir.
• Springs, Geysers & Geothermal Energy

  Geothermal Energy

  These geologic factors favor a geothermal reservoir of commercial value:

  
  

  3. A cap of low-permeability rocks that

  • inhibits the flow of water and heat to the surface.
  • a deep well-insulated reservoir contains much more stored energy than a similar but uninsulated reservoir.
• Environmental Problems

  • Groundwater can be "mined" by being extracted at a rate that is greater than the rate of replenishment.
  • When a groundwater is a nonrenewable resource, the water table drops.
  • e.g. in parts of High Plains aquifer, the water table has fallen by more than 150 ft.
  • The extraction of groundwater can cause pore spces to decrease in volume and grains of loose Earth materials to pack more closely together.
  • this overall compaction of sediment volume can result in subsidence.
• Environmental Problems

  • Saltwater contamination is a common environmental problem in coastal areas.
  • fresh groundwater "floats" on salty groundwater.
  • If sufficient freshwater is pumped out to lower the water table by some amount, the base of the freshwater lens will rise about 40x that amount.
  • deep wells may begin to access the deeper, salty water instead.
  • Once groundwater is contaminated, expensive remediation or abandonment of aquifer may happen.
• Geologic Work of Groundwater

  • Caverns
  • most spectacular results of groundwater's erosional hardwork.
  • Cavern Development
  • most caverns are created at or just below the water table, in the table of zone of saturation.
  • acidic groundwater follows lines of weakness in the rock such as joints & bedding planes.
  • as time passes, the dissolving process slowly creates cavities and enlarge them into caverns.
• Geologic Work of Groundwater

  1. Cavern Development

  • in many caves, development has occurred at several levels with the current caverns-forming activity occurring at the lowest elevation.
  • as streams cut their valleys deeper, the water table drops as the elevation of the river drops.
  • consequently, during periods when surface streams are rapidly down cutting, groundwater level drops rapidly and cave passages are abandoned by the water while the passages are still small in cross-sectional area.
  • when the entrenchment of stream is slow, there is time for large cave passages to form.
• Geologic Work of Groundwater

  2. How Dripstone Forms?

  • these are depositional features created by the endless dripping of water over great spans of time.
  • cave deposits = dripstones
  • although the formation of caverns takes place in the zone of saturation, the deposition of dripstone is not possible until the caverns are above the water table in the unsaturated zone.
• Geologic Work of Groundwater

  3. Spleothems

  • various dripstones features found in caverns
  • Stalactites
  • Icicle-like pendants that hang from the ceiling of cavern
  • Form where water seeps thru cracks above.
  • Deposition occur as a ring around the edge of the water drop. As drop after drop follows, each leaves an infinite trace of calcite and a hollow limestone tube is created.
• Geologic Work of Groundwater

  3. Spleothems

  • Water then moves thru the tube, remains suspended for a moment, contributes a tiny ring of calcite and falls to the caver floor.
  • this stalactite is called a soda straw.
  • often the hollow tube becomes plugged or supply of water increases.
  • either case, the water is forced to flow and deposit along the outside of the tube.
• Geologic Work of Groundwater

  3. Spleothems

  • Stalagmites
  • spleothems that form on the floor of a cavern and reach upward the ceiling.
  • the water supplying the calcite for stalagmite growth falls from the ceiling & splatters over the surface.
  • as a result, these don't have a central tube and are more massive & rounded on their upper ends.
  • a downward growing stalactite nd upward-growing stalagmite may join to form a column.
• Karst Topography

  landscape that have been shaped by dissolving power of groundwater.

  remnants of a time when rainier conditions prevail.

  1. Sinkholes

  • karst areas have irregular terrain punctuated with many depression called sinkholes/sinks,
  • form in 2 ways:
  • develop gradually over many yrs without any physical disturbance to the rock.
  • limestone below the soil is dissolved by downward-sweeping rainwater that is freshly charged with CO2.
  • with time, the bedrock surface is lowered and the fractures into which the seeps are enlarged.
• Karst Topography

  1. Sinkholes

  • as the fractures grow in size, soil subsides into the widening voids, from which it is removed by groundwater flowing in the passages below.
  • these depressions are usually shallow and have gentle slopes.
  • can form abruptly and without warning when the roof of a cavern collapsed under its own weight.
  • depressions here are steep-sided & deep.