Chapter 15: Groundwater

Water In The Ground (1)

Groundwater is defined as all the water in the ground occupying the pore spaces within bedrock and regolith.

The volume of groundwater is 40 times larger than the volume of all water in fresh-water lakes or flowing in streams.

Less than 1 percent of the water on Earth is ground water.

Most ground water originates as rainfall.

Depth of Groundwater

Water is present everywhere beneath the land surface, but more than half of all groundwater, including most of what is usable, occurs above a depth of 750m.

Below a depth of about 750 m, the amount of groundwater gradually diminishes.

Russian scientists encountered water at more than 11 km below the surface.

The Water Table (1)

The zone of aeration (also called the unsaturated zone) is a layer of moist soil followed by a zone in which open spaces in regolith or bedrock are filled mainly with air.

Beneath the unsaturated zone is the saturated zone, a zone in which all openings are filled with water.

The upper surface of the saturated zone is the water table.

Figure 15.1

The Water Table (2)

Normally, the water table slopes toward the nearest stream or lake.

In fine-grained sediment, a narrow fringe as much as 60 cm thick immediately above the water table is kept wet by capillary attraction.

Capillary attraction is the adhesive force between a liquid and a solid that causes water to be drawn into small openings.

The Water Table (3)

In humid regions, the water table is a subdued imitation of the land surface above it.

It is high beneath hills and low beneath valleys because water tends to move toward low points in the topography under the influence of gravity.

How Groundwater Moves (1)

Groundwater operates continuously as part of the hydrologic cycle.

As rain seeps into the ground it enters the groundwater reservoir.

How Groundwater Moves (2)

Most of the groundwater within a few hundred meters of the surface is in motion.

Groundwater moves so slowly that velocities are expressed in centimeters per day or meters per year.

Groundwater must move through small, constricted passages, often along a tortuous route.

Porosity and Permeability (1)

Porosity is the percentage of the total volume of a body of regolith or bedrock that consists of open spaces, called pores. It is porosity that determines the amount of water that a given volume of regolith or bedrock can contain.

Porosity and Permeability (2)

The porosity of a sedimentary rock is affected by several factors:

The sizes and shapes of the rock particles.The compactness of their arrangement.The weight of any overlying rock or sediment.The extent to which the pores become filled with the cement that holds the particles together.

The porosity of igneous and metamorphic rocks generally is low.

Figure 15.2

Porosity and Permeability (3)

Permeability is a measure of how easily a solid allows fluids to pass through it.

A high porosity does not necessarily mean a correspondingly high permeability.

An example of a sediment with high porosity and low permeability is clay.

Clay particles have diameters of less than 0,004 mm.Clay may have a very high porosity because the percentage of pore space is high.Because the pores are very small, the permeability is low.

Porosity and Permeability (4)

As the diameters of the pores increase, permeability increases.

Gravel, with very large pores, is more permeable than sand and can yield large volumes of water to wells.

Recharge and Discharge of Groundwater (1)

The process by which groundwater is replenished is called recharge.

The process by which groundwater reaches and flows from the surface is called discharge.

An area of the landscape where precipitation seeps downward beneath the surface and reaches the saturated zone is called a recharge area.

Recharge and Discharge of Groundwater (2)

The water moves slowly toward discharge areas, where subsurface water is discharged to streams or to lakes, ponds, or swamps.

The surface extent of recharge areas is invariably larger than that of discharge areas.

Recharge and Discharge of Groundwater (3)

In humid regions, recharge areas encompass nearly all the landscape beyond streams and their adjacent flood-plains.

In more arid regions, recharge occurs mainly:In mountains.

In the alluvial fans that border them.

Along the channels of major streams that are underlain by permeable alluvium.

Figure 15.3

Figure 15.4

Recharge and Discharge of Groundwater (4)

The time water takes to move through the ground from a recharge area to the nearest discharge area depends on rates of movement and on the travel distance.

Movement may take from a few days to possibly thousands of years in cases where water moves through the deeper parts of a groundwater body.

Movement in the Zone of Aeration (1)

Water initially soaks into the soil, which usually contains clay resulting from the chemical weathering of bedrock.

The low permeability and the fine clay particles cause part of the water to be retained in the soil by forces of molecular attraction.

Some of this moisture evaporates directly into the air.

Movement in the Zone of Aeration (2)

Much of it is absorbed by the roots of plants, which later return it to the atmosphere through transpiration.

Because of the pull of the gravity, water that cannot be held in the soil by molecular attraction seeps downward until it reaches the saturated zone.

Movement by Percolation in the Saturated Zone (1)

Once in the saturated zone, groundwater moves by percolation.

Percolating water moves slowly through very small pores along parallel, thread-like paths.

Responding to gravity, water percolates from areas where the water is high toward areas where it is lowest.

It generally percolates toward surface streams or lakes.

Movement by Percolation in the Saturated Zone (2)

The velocity of groundwater flow increases as the

slope of the water table increases. Flow rates of groundwater tend to be very slow

because percolating groundwater encounters a large amount of frictional resistance.

The highest rate yet measured in the United States, in exceptionally permeable material, was only about 250 m/yr.

Figure 15.5

How Fast Does Groundwater Flow? (1)

In 1856, Henri Darcy, a French engineer, concluded the velocity of groundwater must be related to:

The hydraulic gradient: the slope of the water table.

The permeability of the rock or sediment through which the water is flowing.

The coefficient of permeability: permeability, density, and viscosity of water, expressed as a coefficient (K).

• Also called hydraulic conductivity.

How Fast Does Groundwater Flow? (2)

K(h1-h2)V =

L where:

K is a coefficient know as the “coefficient of permeability” or “coefficient of conductivity”;

h1-h2 is the difference in altitude;

L is the horizontal distance between two points; Because discharge (Q) in streams varies as a function of

both stream velocity (V) and cross-sectional area (A), Q = AV

Springs And Wells (1)

A spring is a flow of groundwater emerging naturally at the ground surface.

Small springs are found in all kinds of rocks, but almost all large springs issue from lava flows,limestone, or gravel.

Springs And Wells (2)

A vertical or horizontal change in permeability is a common explanation for the location of springs.

This change involves the pressure of an aquiclude, a body of impermeable or distinctly less permeable rock adjacent to a permeable one.

Springs may also issue from lava flows, especially where a jointed lava flow overlies an aquiclude, or along the trace of a fault.

Figure 15.6

Springs And Wells (3)

A well will supply water if it intersects the water table.

When water is pumped from a new well, the rate of withdrawal initially exceeds the rate of local groundwater flow.

This imbalance in flow rates creates a conical depression in the water table immediately surrounding the well called a cone of depression.

Figure 15.7

Springs And Wells (4)

The locally steepened slope of the water table increases the flow of water to the well, consistent with Darcy’s Law.

An impermeable layer of clayey sediment in the zone of aeration may produce a perched water body (a water body perched atop an aquiclude that lies above the main water table).

Figure 15.8

Aquifers (1)

An aquifer is a body of highly permeable rock or regolith that can store water and yield sufficient quantities to supply wells.

Bodies of gravel and sand generally are good aquifers, because they tend to be highly permeable and often have large dimensions.

Many sandstones are also good aquifers.

Aquifers (2)

Aquifers are of two types:Confined (bounded by aquicludes).

Unconfined (an aquifer that is not overlain).

An example of an unconfined aquifer is the High Plains aquifer, which lies at shallow depths beneath the High Plains of the United States.

About 30 percent of the groundwater used for irrigation in the United States is obtained from the High Plains aquifer.

Figure 15.9

Aquifers (3)

In parts of Kansas, New Mexico, and Texas, the water table has dropped so much over the past half century that the thickness of the saturated zone has declined by more than 50 percent.

The Dakota aquifer system in South Dakota provides a good example of a confined aquifer.

Water that percolates into a confined aquifer flows downward under the pull of gravity.

As it flows to greater depths, the water is subjected to increasing hydrostatic pressure.

Figure 15.10

Aquifers (4)

Potentially, the water could rise to the same height as the water table in the recharge area.

Such an aquifer is called an artesian aquifer, and the well is called an artesian well.

A freely flowing spring supplied by an artesian aquifer is an artesian spring;

Figure 15.11

Aquifers (5)

The Floridian aquifer is a complex regional aquifer system in which both confined and unconfined units are present, and in which water locally reaches the surface by an artesian flow.

The aquifer system is restricted mainly to middle and late tertiary limestones.

The age of groundwater in the Floridian aquifer system has been determined by radiocarbon dating of carbonate molecules dissolved in the water.

Water in the well farthest from the recharge area is calculated to have been in the ground for at least 19,000 years.

Figure 15.12

Mining Groundwater And Its Consequences (1)

In the dry regions of western North America, groundwater is a major source of water for human consumption.

In many of these dry regions, withdrawal exceeds natural recharge.

Groundwater can be a nonrenewable resource.

Mining Groundwater And Its Consequences (2)

Natural recharge takes so long to replenish a depleted aquifer that vast underground water supplies have been lost to future generations.

When groundwater withdrawal exceeds recharge, the water table falls.

It can cause shallow wells to run dry and necessitate the drilling of still deeper wells.

Mining Groundwater And Its Consequences (3)

To halt the fall of the water table, groundwater sometimes can be artificially recharged by spraying biodegradable liquid wastes from food processing or sewage treatment plants over the land surface.

The pollutants are removed by biologic processes as the liquid percolates downward through the soil. The purified water then recharges the groundwater system.

Runoff from rainstorms in urban areas can be channeled and collected in basins.

Subsidence of the Land Surface (1)

The water pressure in the pores of an aquifer helps support the weight of the overlying rocks or sediments.

When groundwater is withdrawn, the pressure is reduced, and the particles of the aquifer shift and settle slightly.

As a result, the land surface subsides.

Figure 15.14

Subsidence of the Land Surface (2)

The amount of subsidence depends on:How much the water pressure is reduced.

The thickness and compressibility of the aquifer.

Land subsidence is widespread in the south-western United States.

It has caused structural damage to buildings, roads, cables, pipes, and drains.

Subsidence of the Land Surface (3)

Land subsidence can be especially damaging where water is pumped from beneath cities.

Mexico City.

Pisa, home of the famous Leaning Tower.

Water Quality And Groundwater Contamination (1)

The chemistry of groundwater shows that the compounds dissolved in groundwater are mainly:

Chlorides.Sulfates.Bicarbonates of calcium.Magnesium.Sodium.Potassium.Iron.

Water Quality And Groundwater Contamination (2)

The composition of groundwater varies from place to place according to the kind of rock in which the water occurs.

In much of the central United States, for instance, the water is rich in calcium and magnesium bicarbonates dissolved from local carbonate bedrock (called hard water).

By contrast, in the northern United States, where the bedrock is often volcanic or graywacke sandstones, little dissolved matter and no appreciable calcium found in the groundwater (called soft water).

Water Quality And Groundwater Contamination (3)

Water circulating through sulfur-rich rocks may contain dissolved hydrogen sulfide (H2S) that has the disagreeable odor of rotten eggs.

The most common source of water pollution in wells and springs is sewage draining from septic tanks, privies, and barnyards.

Figure 15.15

Water Quality And Groundwater Contamination (4)

If sewage contaminated with bacteria passes through sediment or rock with large pores, such as coarse gravel or cavernous limestone it can remain polluted.

If the contaminated water percolates through sand or permeable sandstone, it become purified within short distances. Sand promotes purification by:

Mechanically filtering out bacteria.Oxidizing bacteria so they are rendered harmless.Placing bacteria in contact with other organisms that consume them.

Water Quality And Groundwater Contamination (5)

Excessive pumping that exceeds the natural flow of fresh groundwater toward the sea may eventually permit saline water to contaminate water supplies (seawater intrusion).

Toxic Wastes and Agricultural Poisons (1)

Vast quantities of human and industrial wastes are deposited each year in open basins or excavation at the land surface.

When rainwater seeps downward through the site, it carries away harmful soluble substances.

The pollutants often are toxic to humans as well as to plants and animals.

The pollution problems associated with landfill wastes involve tens of thousands of sites.

Figure 15.17

Toxic Wastes and Agricultural Poisons (2)

Pesticides and herbicides are sprayed over agricultural fields and suburban gardens to help improve quality and productivity.

Some of these chemicals have been linked with cancer and birth defects in humans, and some have led to disastrous population declines of wild animals.

Underground Storage of Hazardous Wastes (1)

Most studies concerning disposal of hazardous wastes—both toxic and radioactive—have concluded that underground storage is appropriate, provided safe sites can be found.

Safe sites for disposing of radioactive wastes must not be affected chemically by groundwater, physically by earthquakes or other disruptive events, or accidentally by people.

Underground Storage of Hazardous Wastes (2)

Geologists generally agree that the ideal underground storage site for radioactive wastes should possess the following characteristics:

The enclosing rock should have few fractures and low permeability.

The enclosing rock should have no present or future economic mineral potential.

Local groundwater flow should be away from plant and animal life.

Underground Storage of Hazardous Wastes (3)

Only very long paths of groundwater flow should be directed toward places accessible to humans.The area should have low rainfall.The zone of aeration should be thick.The rate of erosion should be very low.The probability of earthquakes or volcanic activity should be very low.Future climate change should be unlikely to affect groundwater conditions substantially.

Geologic Activity of Groundwater (1)

Of all the rocks in the Earth’s crust, the carbonate rocks are among the most readily attacked by the dissolution and hydrolysis.

Limestone, dolostone, and marble are the most common carbonate rocks and underlie millions of square kilometers of the Earth’s surface.

Carbonate minerals are readily dissolved by weak carbonic acid.

Figure 15.19

Geologic Activity of Groundwater (2)

The weathering attack occurs mainly along joints and other partings in the carbonate bedrock.

In temperate regions with high rainfall, a high water table, and a nearly continuous cover of vegetation, carbonate landscapes are being lowered at average rates of up to 10 cm/1000 years.

In dry regions with scanty rainfall, rates are far lower.

Geologic Activity of Groundwater (3)

The conversion of sediment into sedimentary rock is primarily the work of groundwater.

Substances in solution in the water are precipitated as cement in the spaces between rock and mineral particles of the sediment.

Calcite, quartz, and iron compounds (mainly hydroxides such as limonite) are the chief cementing substances.

Geologic Activity of Groundwater (4)

Less common than the deposition of cement between the grains of a sediment is replacement, the process by which a fluid dissolves matter already present and at the same time deposits from solution an equal volume of a different substance.

This process produces petrified wood.

Carbonate Caves And Caverns (1)

Caves come in many sizes and shapes. A very large cave or system of interconnected cave

chambers is often called a cavern:The Carlsbad Caverns in southeastern New Mexico include one chamber 1200 m long, 190 m wide, and 100 m high.

Mammoth Cave in Kentucky consists of interconnected caverns with an aggregate length of at least 48 km.

Carbonate Caves And Caverns (2)

The recently discovered Good Luck Cave on the tropical island of Borneo has one chamber so large that it could accommodate not only the world’s largest previously known chamber (in Carlsbad Caverns), but also the largest chamber in Europe (in Gouffre St. Pierre Martin, France) and the largest chamber in Britain (Gaping Ghyll).

Cave Formation (1)

Caves are produced mainly by a chemical process involving the dissolution of carbonate rock by circulating groundwater.

Limestone caves are generally believed to result from dissolution by carbonic acid.

Some caves, like Carlsbad, may have resulted from dissolution by sulfuric acid.

The rate of cave formation is related to the rate of dissolution.

A fully developed cave system may take 10,000 to 1 million years to produce.

Cave Formation (2)

The usual sequence of development involves these steps:

1. Initial dissolution along a system of interconnected open joints and bedding planes by percolating groundwater.2. Enlargement of a cave passage along the most favorable flow route by water that fully occupies the opening.3. Deposition of carbonate formations on the cave walls while a stream occupies the cave floor.4. Continued deposition of carbonate on the walls and floor of the cave after the stream has stopped flowing.

Cave Deposits

Some caves have been partly filled with insoluble clay and silt, originally present as impurities in limestone and gradually concentrated as the limestone was dissolved.

Flowstone is precipitated by flowing water. Dripstone is precipitated from dripping water.

Stalactites are icicle-like forms of dripstone hanging from the the ceilings of caves. Stalagmites are blunt mounds projecting upward from cave floor.Columns are stalactites joined with stalagmites.

Figure 15.23

Sinkholes

A sinkhole is a large dissolution cavity that is open to the sky.

Some sinkholes are formed when caves have collapsed.

Others are formed from dissolution.

Many sinkholes are located at the intersection of joints.

Sinkholes of the Yucatan peninsula in Mexico are called cenotes. • They have high vertical sides and contain water because their floors lie

below the water table.

New sinkholes are constantly forming because of the lowering of the water table resultant from excessive pumping of local wells.

Karst Topography (1)

Karst topography is a landscape in which caves and sinkholes are so numerous that they form a peculiar topography characterized by:

Many small, closed basins.Disrupted drainage pattern.Streams disappearing into the ground.Streams reappearing as large springs.

This topography was first described in the Karst region of the former Yugoslavia, extending from Slovenia to Montenegro.

Karst Topography (2)

Several distinctive kinds of Karst landscape are recognized:

The most common is sinkhole karst.• Examples occur in Indiana, south central Kentucky, central

Tennessee, and Jamaica.

Cone karst and tower karst occur in thick, well-jointed limestone that separates into isolated blocks as it weathers.

• Examples occur in part of Mexico, Central America, the Caribbean islands of Cuba and Puerto Rico, the South Pacific, and southeastern China.

Figure 15.26

Karst Topography (3)

Pavement karst consists of broad areas of bare limestone in which joints and bedding planes have been etched and widened by dissolution, creating a distinctive land surface.

• Examples occur in Spitzbergen, Greenland, and the Burren region of western Ireland.

Many paleokarsts hold valuable ore deposits.Zinc and lead deposits in Tennessee and Poland.

Fluorite in South Africa.

Antimony in China.

Bauxite in Jamaica.

Uranium in Australia and Kyrgyzstan.

Figure 15.28