chapter 17 groundwater. importance of groundwater groundwater is water found in the pores of soil...
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Chapter 17 Groundwater
Importance of groundwater
Groundwater is water found in the pores of soil and sediment, plus narrow fractures in bedrock
Groundwater is the largest reservoir of fresh water that is readily available to humans
50% of drinking water comes from the ground
Importance of groundwater
Drinking Water!Geological role of groundwater
As an erosional agent, dissolving groundwater producesSinkholesCaverns
Groundwater serves as an equalizer of streamflow
SinkholesSinkhole in Guatemala
Sinkhole in Florida
Sinkholes Sink holes form when the groundwater
dissolves the layer of rock, usually limestone.
The water excavates or erodes a hole.This is the way caves are formed as well.In the US, sinkholes are common in areas
where limestone and other similar sedimentary rocks are prevalent such as Florida, Tenessee, Pennsylvania, New York,
West Virginia, California, Utah, and many other states.
Karst TopographyKarst topography is a landscape shaped
by the dissolution of a layer or layers of soluble bedrock,
usually carbonate rock such as limestone or dolomite
Rick Springs in Logan Canyon
Fresh water of the hydrosphere
Table 17.1
Distribution of groundwaterBelt of soil moisture – water held by molecular
attraction on soil particles in the near-surface zoneZone of saturation
FormationWater not held as soil moisture percolates
downward
Distribution of groundwaterZone of saturation
FormationWater reaches a zone where all of the open
spaces in sediment and rock are completely filled with water
Water within the pores is called groundwaterWater table – the upper limit of the zone of
saturation
Distribution of groundwaterCapillary fringe
Extends upward from the water tableGroundwater is held by surface tension in
tiny passages between grains of soil or sediment
Distribution of groundwaterZone of aeration
Area above the water tableIncludes the capillary fringe and the belt of
soil moistureWater cannot be pumped by wells
Distribution of groundwater
Figure 17.2
The water tableThe water table is the upper limit of the zone of
saturationVariations in the water table
Depth is highly variableVaries seasonably and from year to year
Shape is usually a subdued replica of the surface topography
The water tableVariations in the water table
Factors that contribute to the irregular surface of the water tableWater tends to “pile up” beneath high areasVariations in rainfallVariations in permeability from place to place
The water tableInteraction between groundwater and streams
Constitutes a basic link in the hydrologic cycle
Three types of interactionsGaining streams – gain water from the inflow of
groundwater through the streambedLosing streams – lose water to the groundwater
system by outflow through the streambed
Gainingand
losing streams
Figure 17.4
The water tableInteraction between groundwater and streams
InteractionsA combination of the first two – a stream gains in
some sections and loses in other areas
Storage and movement of groundwater
Porosity – percentage of total volume of rock or sediment that consists of pore spaces
Determines how much groundwater can be stored
Variations can be considerable over short distancesSandstone=11 - 25% porosity
Table 17.2
Storage and movement of groundwater
Permeability, aquitards, and aquifersPermeability – the ability of a material to
transmit a fluidAquitard – an impermeable layer that
hinders or prevents water movement (such as clay)
Aquifer – permeable rock strata or sediment that transmits groundwater freely (such as sands and gravels)
Storage and movement of groundwater
Movement of groundwaterExceedingly slow – typical rate of
movement is a few centimeters per dayEnergy for the movement is provided by the
force of gravityDarcy’s Law – if permeability remains
uniform, the velocity of groundwater will increase as the slope of the water table increases
Permeability vs. PorosityA sponge has high porosity but low
permeability
Explain this.
Aquitard –retards the flow of waterMay have porosity but the the permeability
is slowThe rock contains water but the water does
not move quickly enough through it.
Aquiclude – occluded water Water does not move through
Granite or other igneous rockUnweathered metamorphic rockChemical limestoneShales are the best
Aquifer – transfer water high porosity high permeabilityAn ideal Aquifer =
SandstoneCoalSands and gravelAll highly weathered rocks
The best wells are artesian wells- the pressure of the confining layers make the come to the surface on its own.
Storage and movement of groundwater
Movement of groundwaterDarcy’s Law – if permeability remains
uniform, the velocity of groundwater will increase as the slope of the water table increasesHydraulic gradient – the water table slope,
determined by dividing the vertical difference between the recharge and discharge points by the length of flow between these points
Hydraulic head – the vertical difference between the recharge and discharge points
Storage and movement of groundwater
The movement of groundwater is measured directly using
Various dyesCarbon-14
Hydraulic gradient
Figure 17.6
The Water Table never fills to the top of the soilThe area between the top of the water
table and the surface of the land is called the zone of aeration.
Rocks contain water up to about 2,500 feet.The less percipitation you have the farther
down you have to dig a well.
SpringsSprings
Occur where the water table intersects Earth’s surface
Natural outflow of groundwaterCan be caused by an aquitard creating a
localized zone of saturation which is called a perched water table
Utah Springs
Cascade Springs- UT
Hanging water table or perched water table
Spring resulting from a perched water table
Figure 17.8
Hanging water table in Moab-UTSandstone is the best aquifer!
Water is not a renewable resourceThe water at the top of the surface takes 870 years to have new water. Once it is drained or contaminated it is gone.
Hot springs and geysersHot springs
Water is 6-9oC warmer than the mean annual air temperature of the locality
The water for most hot springs is heated by cooling of igneous rock
Hot springs and geysersGeysers
Intermittent hot springsWater erupts with great forceOccur where extensive underground
chambers exist within hot igneous rockGroundwater heats, expands, changes to
steam, and erupts
Distribution of hot springs and geysers in the U.S.
Figure 17.9
Hot springs and geysersGeysers
Chemical sedimentary rock accumulates at the surfaceSiliceous sinter (from dissolved silica)Travertine (from dissolved calcium carbonate)
Siliceous sinter
Siliceous sinter
Siliceous sinter
WellsTo ensure a continuous supply of water, a well
must penetrate below the water tablePumping of wells can cause
Drawdown (lowering) of the water tableCone of depression in the water table
The average family 4 people uses 2,000 gallons of water a week!
Cone of depression in the water table
Figure 17.13
Wells
Artesian well – a situation in which groundwater under pressure rises above the level of the aquifer
Types of artesian wellsNonflowing – pressure surface is below ground
levelFlowing – pressure surface is above the ground
Not all artesian systems are wells, artesian spring also exist
Cone of depression of a well
Artesian well resulting from an inclined aquifer
Figure 17.15
Problems associated with groundwater withdrawal
Treating groundwater as a nonrenewable resourceIn many places the water available to
recharge the aquifer falls significantly short of the amount being withdrawn
SubsidenceGround sinks when water is pumped from
wells faster than natural recharge processes can replace it (San Joaquin Valley of California) Creates a sinkhole
Problems associated with groundwater withdrawal
Saltwater contaminationExcessive groundwater withdrawal causes
saltwater to be drawn into wells, thus contaminating the freshwater supply
Primarily a problem in coastal areas
Saltwater contamination
Figure 17.19
Groundwater contaminationOne common source is sewage
Extremely permeable aquifers, such as coarse gravel, have such large openings that groundwater may travel long distances without being cleaned
Sewage often becomes purified as it passes through a few dozen meters of an aquifer composed of sand or permeable sandstone
Groundwater contaminationSinking a well can lead to groundwater pollution
problemsOther sources and types of contamination include
substances such asHighway saltFertilizersPesticidesChemical and industrial materials
Geologic work of groundwater
Groundwater dissolves rockGroundwater is often mildly acidic
Contains weak carbonic acidForms when rainwater dissolves carbon dioxide
from the air and from decaying plantsCarbonic acid reacts with calcite in
limestone to form calcium bicarbonate, a soluble material
Geologic work of groundwater
CavernsMost caverns are created by acidic
groundwater dissolving soluble rock at or just below the surface in the zone of saturation
Features found within cavernsThey form in the zone of aeration
Geologic work of groundwater
CavernsFeatures found within caverns
Composed of dripstone (travertine)o Calcite deposited as dripping water evaporateso Collectively, they are called speleothemso Includes stalactites (hanging from the ceiling)
and stalagmites (form on the floor of a cavern)
Speleothems in Carlsbad
Caverns National Park
“Soda straws” in Carlsbad Caverns National Park
Geologic work of groundwater
Karst topographyLandscapes that have been shaped mainly
by the dissolving power of groundwaterSome common features include
Irregular terrainSinkhole or sinks (formed by groundwater slowly
dissolving the bedrock often accompanied by collapse)
Striking lack of surface drainage (streams)
Development
of karsttopography
Figure 17.26
End of Chapter 17
Quiz #3 10pts 1. What is the name of the Technology we used for the computer
lab? 2. Hilly land resulting from the dissolving power of groundwater
through limestone is called _________topography. 3.The greatest amount of our fresh water is stored in ______. 4. Sandstone is the best ___________, it has high porosity and high
permeability. 5. A well that has pressure and the water does not need to be
pumped out is called ____________. 6. Water is a renewable or nonrenewable resource? 7. the maximum size of a particle that a river can carry is called the
_______. 8. There are two types of flow in a river __________flow and
_______flow. 9. a stream that never reaches the ocean is called a/n
___________stream.