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Ground Water and Karstic Areas

GLY 2010 – Summer 2015

Lecture 19

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Ground Water

• Ground Water is found beneath the earth’s surface• It is found in two different zones:

Vadose Zone Water Table Phreatic Zone

• Gravity carries water from the surface downward

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Vadose Zone - Zone of Aeration

• Some is used by plants• Some evaporates• Some is absorbed by soil particles• The remainder continues past the water table• In this zone, much of the available pore space

is filled with air - Only a fraction is occupied by water

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Water Table Formation

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Water Table Fluctuations• Water table

follows the contour of the surface above it, but the water table is not confined to being below the ground

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Phreatic Zone - Zone of Saturation

• In this zone, all available pore spaces are filled with water

• This zone extends downward from the water table for some distance, generally less than a kilometer, because bedrock below this depth contains no pore spaces in most cases

What is Groundwater Used For?

• Thermoelectric power 510 Mgal/day (0.6%)

• Mining 1,020 Mgal/day (1.3%)

• Livestock 1,290 Mgal/day (1.6%)

• Aquaculture 1,910 (2.4%)

• Industrial 3,070 Mgal/day (3.9%)

• Domestic Use 3,740 Mgal/day (4.7%)

• Public Water Supplies 14,600 Mgal/day (18.3%)

• Irrigation 53,500 Mgal/day (67.2%)

• Total 79,600 Mgal/day

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Mgal/day = Million gallons per day

Current Groundwater

Levels

• Link to current GW information on Google earth: http://groundwaterwatch.usgs.gov/rtn/dws_maps/GoogleEarth/us_rtn.kmz

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Static image from 7-20-15

This is a real-time net with data transmitted every 1-4 hours

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Icebergs As a Source of Water

• Although thought is being given to using icebergs as a source of water, there are problems, including danger in shipping lanes, and potential climate change

Iceberg Calving in Antarctica

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Porosity

• The relative volume of pore (empty) space in a rock or sediment

• It determines how much water the rock or soil can holdWhite spaces are “pore” (empty)

spaces

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Porosity

• Primary porosity in rocks is the pore space that develops when the rocks form Examples include the space between grains

in lithified clastic rocks, or vesicles in extrusive igneous rocks

• Secondary porosity develops after the rock forms, as the result of fracturing or dissolution

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Porosity Types

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Permeability

• The ability of a rock or sediment to allow a fluid (water) to flow through it

• Permeability is greatly influenced by the connectiveness of the pore spaces

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Aquifer

• An aquifer is a layer(s) of rock or sediment from which fresh water may be obtained in quantity

• The word comes from the Latin for “to bear water”

• The best aquifers are porous and permeable, with adequate recharge

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Use of Aquifers

• Modern agricultural methods put heavy demands on water systems to supply the water needed to grow food and other crops

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Unconfined Aquifers

• No impermeable layer above them

• They are subject to rapid recharge from above, but may also be easily contaminated by spills, especially if they are shallow

• Most water supplies come from unconfined aquifers

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Ogallala Aquifer

• The Ogallala Aquifer underlies approximately 225,000 square miles

• A major unconfined aquifer

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Aquiclude

• Impermeable layers which prevent or greatly slow the flow of a fluid

• Clay layers are often aquicludes

• Artificial clay layers are used to line landfills, so that water from the landfill won’t leach into the aquifer

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Confined Aquifers

• Confined aquifers have aquicludes above, and sometimes below, them

• Less subject to pollution than unconfined aquifers

• May transport water for long distances

• The Dakota Sandstone, which originates in the Black Hills of western South Dakota, supplies water to much of the Dakotas and eastern Nebraska

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Artesian Well

• Wells drilled into a confined aquifer may lie at lower elevation than the recharge region of the aquifer

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Artesian Well Photo

• Water is virtually squeezed to the surface by underground pressure

• This picture shows how strong artesian pressure can be!

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Natural Spring• Places where

groundwater flows to the surface, issuing freely from the surface

• Impermeable subsurface layers, or erosion of the surface, may produce a spring

• Groundwater approximates the mean annual temperature in a region, unless if has been heated by a subsurface source of heat

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Hot Springs

• Thermal or hot springs are at least 6C warmer than the mean annual temperature

• Such heating is usually the result of contact or proximity to magma

• Found in many places, for example the Olympic, Sierra Nevada, Cascade, and Rocky Mountain ranges

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Geysers

• Intermittent hot springs, or geysers, are less common

• The word geyser comes from the Icelandic word geysa, meaning to gush, or geysir, meaning to rush forth

• Famous geyser fields: Yellowstone, New Zealand, northern California, and Iceland - all are associated with magma, either from subduction zones, spreading centers, or a hot spot

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Geyser Eruption,

Step 1

• Magma or hot rock heats groundwater in caverns• Water does not boil because of pressure

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Geyser Eruption,

Step 2

• Hot water rises, pressure is reduced

• Water begins to boil26

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Geyser Eruption,

Step 3

• Steam pushes water out of caverns as eruption starts• Pressure is reduced, water boils furiously, and eruption continues until

most water is gone

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Old Faithful Geyser

• Video of geyser erupting (with sound)

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Iceland and the Mid-Atlantic Ridge

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Icelandic Geothermal Power Plant

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Iceland

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Geothermal Energy and Pollution

• Geothermal fluids are often contaminated with salts, because hot water is an excellent solvent, and brings dissolved salts with it to the surface

The Svartsengi Geothermal Plant, Iceland; swimming in the Blue Lagoon

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Environmental Problems

• Numerous environmental problems are associated with the use of groundwater, and the use of thermal water (geothermal) energy sources

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Cone of Depression Animation

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Conditions for Cone of Depression Formation

• Rapid withdrawal of fluid from a well causes a localized lowering of the water table around the well

• May temporarily lower water below the bottom of the well, making further pumping impossible

• Most likely to occur in aquifers where horizontal flow rates are low

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Well Drawdown

• Wells are drilled into an aquifer• Water is redrawn at a rate higher than the natural

recharge rate• Result: water table is lowered• If continued, this will eventually make the well field

unusable

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Aquifer Pollution

• Effluent from the septic tank field seeps into the well field

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Subsurface Flow

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Subsidence

• Lowering the water table (or withdrawal of other fluids, such as petroleum) can cause the collapse of pore spaces in underlying rock or sediment, and the resulting collapse of the surface

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Subsidence Animation

• Data from satellite imagery over Pomona, California from June, 1992-January 1999

• Subsidence due to water pumping

• Red = subsidence• Blue = inflation

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Subsidence Continued

• This has happened in several localities: Las Vegas - more than a meter New Orleans - more than 2 meters Venice, Italy - more than 3 meters,

leading to frequent flooding Mexico City - more than 7 meters Central Valley, California - more

than 8 meters

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Salt-water Encroachment

• Coastal areas face another form of pollutant in their ground water - salt-water

• Salt-water is about 5% denser than fresh water

• In coastal areas, the freshwater floats like a bubble on top of the salt-water

• If the fresh water is over pumped, the size of the fresh water bubble decreases, and the salt-water rises (principle of isostasy)

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Salt-water Encroachment

• If salt-water rises high enough to invade the water-supply wells, they are permanently destroyed

• One of the functions of the water management district is to monitor salt-water levels, and regulate fresh water withdrawal, so that salt-water does not further encroach on fresh water wells

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Karst• Ground water is capable of dissolving limestone

layers, and creating cavities which slowly enlarge• These cavities may cause environmental problems• These cavities may form caverns, sinkholes, and

disappearing streams• Named for the Karst, Slovenia (part of the former

Yugoslavia) region where they were first described

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Dissolution of Limestone Underground

• When limestone is dissolved, the weight of overlying rock and sediment pushes down

• If the cavity is full of water, it may be strong enough to bear the weight

• If the cavity gets bigger, or water is removed by lowering of the water table, collapse is possible

• Such collapse is rapid

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Sinkholes

• A sinkhole is created

• Sinkholes are commonly found in broad areas underlain by fractured limestone

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Central Florida Karst• Black

regions are water in sinkholes

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Florida Karst

• Central Florida, in the area around Gainesville and Ocala, is another karstic region

• A very large sinkhole occurred in Winter Park, Fl. After a prolonged drought in 1981

Winter Park, Florida

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Winter Park Sinkhole Close-up

• Photograph by A. S. Navoy

• Sinkhole developed in one day

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Florida Sinkholes

• Sinkholes are most common when the non-limestone cover above limestone is 30-200’ thick (reddish color on map)

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Caverns

• Another common karst region feature is caverns

• Mammoth Cave in Kentucky is the largest cave system in the world

• Carlsbad Caverns in New Mexico, and Wind and Jewel Caves in South Dakota, also formed by the dissolutioning of limestone

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Cave Entrance

• Natural entrance to a cave

• Caves are dark, wet, and often have obstructed pathways

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Formation of Caverns

• Generally groundwater moves along fractures and between bedding planes in the limestone, and begins to dissolve the walls

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Disappearing Stream

• Surface streams may erode through the roof of a cavity, and the water simply disappears downward into the cavity

• Such disappearing streams are characteristic of karstic regions

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Cave Deposition

• Waters can precipitate limestone as well as dissolve it

• Most caves are wet, and deposition can occur if the water is saturated

• The resulting depositional features in caves are known as speleothems

• Two common types are stalactites and stalagmites

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Stalactites

• Water dripping slowly from the ceiling of caves may partially evaporate

• This concentrates the dissolved limestone, and may cause precipitation

• Very slowly, the limestone builds up in a downward direction

• After decades to centuries or longer, stalactites form (C=ceiling)

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Soda Straw Stalactites

• Thin soda straw stalactites on the ceiling of a cave• Stalagmites are much thicker, because they are

produced by water splashing

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Stalagmites

• When water from the tip of a stalactite drips, it hits the cave floor and spreads, often with some evaporation and subsequent precipitation of limestone

• The resulting formation is called a stalagmite, and builds from the ground up (g=ground)

• If the stalagmite meets the stalactite, a column is formed

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Cacahuamilpa Grottos, Mexico

• Guerrero, Mexico• Large stalagmite

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Shields

• Parachutes or shields, Lehman Cave, Great Basin National Park, Nevada

• A spectacular example of dripstone

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Groundwater – Surface Water Connection

• Groundwater and surface waters are strongly connected in many areas

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Gaining Stream

• Aquifer level is even with the stream level, and higher away from the stream

• Water flows from aquifer to stream

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Connected Losing Stream

• Aquifer level is even with the stream level, and lower away from the stream

• Water flows from stream to aquifer

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Disconnected Losing Stream

• Aquifer level is lower than the stream level• Water flows from stream to aquifer, and away

from the stream

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Movement Toward

Low Pressure Region

• Underground water is pulled downward by gravity• Pressure due to the weight of overlying fluid may

force water to flow back upward toward a low pressure zone, the stream

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Cliff-Face Springs

• When the water table above an aquiclude intercepts the surface on a cliff, a spring results

• If layers are sloping, this is a potential mass wasting location