Water Under the Ground Chapter 12 Water On and Under the Ground Geology Today Barbara W. Murck Brian J. Skinner N. Lindsley-Griffin, 1999 Lutgens & Tarbuck,

Water Under the GroundWater Under the Ground

Chapter 12

Water On and Under the Ground

Geology Today

Barbara W. Murck

Brian J. Skinner

N. Lindsley-Griffin, 1999

Lutgens & Tarbuck, J.R. Griffin, 1999

Carlsbad Caverns, NM

The Water TableThe Water Table Water Table = Top surface

of the saturated zone Zone of aeration (unsaturated zone) - pores filled with air

Saturated zone - pores filled with water


Fig. 12.22

p. 353

How Groundwater MovesHow Groundwater Moves

N. Lindsley-Griffin, 1999Fig. 12.24, p. 355


A stream that lies above the water table loses water to the zone of saturation, will dry up at times.

A stream that lies below the water table receives water from zone of saturation, will flow all year.

Groundwater and StreamsGroundwater and Streams

Porosity and PermeabilityPorosity and PermeabilityPorosity - percentage of total volume of rock that consists of open spaces (pores). (how many pores, and how big they are)

Permeability - a measure of how easily a solid allows fluids to pass through it. (how well connected the pores are)


Fig. 12.23

p. 354

Porosity and PermeabilityPorosity and Permeability

IN GENERAL -

Porosity and permeability

are better in:

well sorted sediments

loosely packed sediments

spherical sediments

Cementation of sediment decreases both porosity and permeability


Low Porosity High Porosity

Poorly sorted Well sorted

Tightly packed Loosely packed

Nonspherical Spherical

AquifersAquifersAquifer - a body of water-saturated, porous and permeable, rock or regolith


Fig. 12.26

p. 358

AquifersAquifers

Confined aquifer: trapped between impermeable layers (aquicludes).

Unconfined aquifer: top of saturation zone is water table

N. Lindsley-Griffin, 1999 Fig. 12.26, p. 358

RechargeRecharge Recharge - replenishment of groundwater

Recharge zone - where groundwater is replenished by infiltration of rainfall and snowfall, which percolates downward to the saturated zone.


Fig. 12.26, p. 358

DischargeDischarge Discharge - process by which groundwater returns to the surface.

Examples: streams, springs, wells, the ocean.


WellsWells Pumping produces a cone of depression in the water table around the well.

Fig. 12.29, p. 360


ArtesianWellsArtesianWellsArtesian system: water pushed up to

water table by hydraulic pressure.

Requires a confined aquifer, and sufficient pressure.

Usually supplied by folded or faulted beds that surface at a higher elevation in the recharge area.


Fig. 12.27

p. 359

Sources of Groundwater ContaminationSources of Groundwater Contamination

Leaching of toxic chemicals from unlined landfills.

Leaking septic tanks. Agricultural chemicals.

LUST (Leaking Underground Storage Tanks)


Fig.12.30, p. 361

SpringsSprings

Springs form where a change in rock permeability forces groundwater out to the surface: a) limestone over shale.


SpringsSprings Springs form…

b) highly jointed permeable rock like a lava flow over impermeable limestone, or c) along a fault.


SpringsSprings

Springs form…

c) along a fault,

- where a permeable layer is blocked by being moved against an impermeable layer,

- or where the fault itself is impermeable.


Caves and Caverns

Caves and Caverns

Caves and caverns - underground open spaces - form by dissolution of carbonate rocks near the top of a seasonally fluctuating water table.

They are closed to the surface or have only a small opening.

Fig. 12.25 B, p. 357 N. Lindsley-Griffin, 1999

Caves and CavernsCaves and Caverns

Underground cavities form when water table is high, then fill with air when water table drops and water drains out.

Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999

Earlierwatertable

Presentwatertable

Caves and CavernsCaves and Caverns

After water table drops, percolation of acidic water from the surface dissolves ions which precipitate as calcite cave decorations in the open air of the cave.


Earlierwatertable

Presentwatertable

SinkholesSinkholes Sinkholes are dissolution cavities that are open to the sky. They form

by collapse of cave roofs or where rainwater is rich in carbon dioxide


Winter Park sinkhole, near Orlando FL - May 8-9, 1981

Grew to >100 m wide in 17 hours.

Fig. 12.25 C, p. 357

KarstKarstExtensive dissolution produces karst topography, with many small closed basins and sinkholes, and disrupted drainage.

(Fig. 12.25A, p. 356)


KarstKarstProlonged development of karst results in topography with jagged carbonate towers that served as the inspiration for Chinese scroll painting.


Limestone towers

Auilin, China

Cave - a natural cavity below

Earth’s surface

• Caves in limestone form by dissolution...

• below the water table.


“Soda Straws”

Cave decorations form by

precipitation above the water table

Sinkholes form when a cave roof collapses

FossilizationFossilizationGroundwater aids fossilization by leaching away organic matter and depositing silica or carbonate ions


Petrified Forest, AZ

Hot Springs and GeysersHot Springs and Geysers

Hot springs form where groundwater heated by subsurface magma rises to the surface.

Geysers (1) are eruptions of hot water caused when groundwater confined in subsurface fissures becomes superheated.



Hot Springs and GeysersHot Springs and Geysers

Geysers (2) Superheated water at depth expands, causing water to slop over the rim and releasing the confining pressure.

(3) Water “flashes” into

steam and the geyser erupts.

Hot spring deposits form by precipitation of dissolved ions as

the water cools

Documents

Water Under the Ground Chapter 12 Water On and Under the Ground Geology Today Barbara W. Murck Brian J. Skinner N. Lindsley-Griffin, 1999 Lutgens & Tarbuck,