chapter 17 groundwater. importance of groundwater groundwater is water found in the pores of soil...

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.