Groundwater
Introduction
• Definition—Water stored in the open space within rocks
and sediments beneath the ground surface
• Less than 1% of the water on Earth is stored as
groundwater
• Groundwater represents about 22% of the world’s supply
of fresh water
– 40 times the water in lakes and rivers
– 1/3 of the world’s glacial and polar ice
Depth of Groundwater
• Most groundwater originates as rain water that percolates into the
ground
• Most occurs at depths less than 750 meters
– The amount of groundwater would cover all the land areas to a
depth of 55 meters
– Water is found deeper, but the pressures are so great that the
pore spaces are very small
– Amount decreases downward at irregular rates
Basic Terms
• Porosity—the percentage of open space called pores
• Permeability—how easily fluids can pass through a solid
• Aquifer—“water-bearer” or a material water can move
through easily
– Must be both porous and permeable
• Aquiclude—material through which water moves very
poorly
Basic Terms
• Water Table
– Separates the zone of aeration from the zone of saturation
– Tends to be parallel to the land surface but to have less relief
• Rises under hills
• Descends under valleys
– Rises in wet seasons and falls in dry seasons
Basic Terms
• Zone of saturation—all of the open space is filled with
water
• Zone of aeration
– Belt of soil moisture—individual soil particles are coated with a
thin film of water
– Intermediate belt—drier, with some soil particles lacking water
coating
– Capillary fringe—immediately above the water table where water
is drawn upward by capillary action
Movement of Groundwater
• In the zone of aeration, water tends to percolate downward
under the influence of gravity
– Its flow is impeded by clay particles to which it adheres
– Its flow can be blocked by particles closing off pore space
• In the zone of saturation, water migrates from where the
water table is high to where it is lower
Movement of Groundwater
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Velocity (V) is proportional to the hydraulic gradient V  (h1-h2)/l
The proportionality constant is K, which is known as the coefficient of permeability or the
hydraulic conductivity
Thus, V = K(h1-h2)/l
Discharge is Velocity x Area so Darcy’s Law is
Q = AV = AK (h1-h2)/l
Wells
Cone of Depression
• Water table is depressed in area where pumping occurs
• Size of cone of depression depends on rate of pumping and the permeability
of the material
Productive & Unproductive Wells
Springs
• Springs develop whenever the water table intersects the
ground surface
– This doesn’t happen everywhere because the water table tends
to be parallel to the ground surface
– It does happen when the ground surface is over-steepened,
such as at fault scarps
– It happens when fractures intersect the water table at a higher
elevation than they intersect the ground surface
– It also happens when rainwater is stopped in its downward
percolation
Springs
Aquifers
• An aquifer bounded by aquicludes is referred to as a confined aquifer
• An aquifer not overlain by an aquiclude is an unconfined aquifer
• Shallow, surface aquifers tend to be unconfined
– The Cohansey aquifer locally
– These tend to be easy to contaminate
• Deeper aquifers tend to be confined
– They are more likely to be bounded by impermeable layers
– The are more difficult to contaminate
– They may produce artesian conditions
The High Plains Aquifer—
An Unconfined Aquifer
The Dakota Aquifer System—
A Confined Aquifer
Artesian Conditions
Caves
• Water moving along fractures dissolves limestone
– Acids accelerate dissolution of limestone
– Water and carbon dioxide produce carbonic acid
– Other acids could also work
• As fractures widen, underground open space develops
– At what depth this happens is of some debate
– Your book argues that caves form in the upper part of the zone of
saturation
Caves
• Deposition of the cave formations
– Must happen when the cave is above the water table
– Your book says most formations develop while a stream is still flowing
through the cave
• Formations
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Stalactite—hangs tight from the ceiling
Stalagmite—grows mighty-big from the ground
Column—joined stalagmites and stalactites
Drapery—sheets of dripstone
Cave Formation
• Water seeps into sinkholes and flows along fractures
dissolving limestone
Cave Formation
• Erosion of stream causes general water table to drop,
drying out some fractures
Cave Formation
• Cave forms above local water table, due to water from above
seeping in through fractures
Cave Formations
Dripstone
Drapery
Flowstone
Soda Straws
Karst Formation
• In areas underlain by limestone the surface is shaped by
solution features
– Streams disappear underground through fractures and reappear
far away
– Caves collapse producing sinkholes
Karst Features
Water Hardness
• Water hardness is determined by the cations in solution
– Divalent cations, such as Ca, Fe, Mg make water hard
– Monovalent cations, such as Na and K make water soft
• Hard water problems
– Poor sudsing of detergents
– Scale in pipes
– Scale on surfaces
Groundwater Temperatures
• Groundwater is warmer in warm climates and cooler in
cold climates
• At depths of approximately 50 feet groundwater is about
50° F year round
• As you go deeper than 50 feet, temperature increases 1°F
for every 50 to 75 feet
• Water from significant depth reaches springs at elevated
temperatures
Geysers
• Rain water enters fractures in rock
• Proximity to hot, igneous rock provides heat to warm groundwater
• The pressure of the overlying water raises boiling point
Geysers
• When water begins to boil, some sloshes out lowering pressure
• Water is now above its boiling point and flashes into steam
• This causes a geyser eruption
Groundwater Problems
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Saltwater Intrusion
Land Subsidence
Septic Systems
Contamination of Groundwater
Contamination of Surface Water
Saltwater Intrusion
• An issue in coastal areas
• Freshwater lens sits on top of salt water because it is less dense
• Excessive pumping expands the cone of depression and causes saltwater
intrusion
• The freshwater lens can also be contaminated by pollutants leaking down from
the surface
Land Subsidence
• Withdrawal of groundwater can cause the land to subside
– Pore water occupies the space between grains and helps
support the weight of overlying layers
– Withdrawal removes this support causing subsidence
Septic Systems
• Septic systems are a way of handling human waste
• Components
– Septic tank in which solids sit and decompose
• Tank must be large enough to allow solids time to decompose
• Bacteria must be active to decompose solids
– Leach field—liquid from the tank flows out to the leach field and percolates
downward
• By the time the effluent has reached the water table, it is clean
Septic Tanks
• Septic tank stores sewage allowing time for solids to be decomposed by
bacteria
• Liquids flow out into leach field
• Liquids seep through permeable material and are filtered clean before they
reach water table
Septic Problems
• The tank is too small so waste does not have time to decompose
• The bacteria in the tank have been killed off
• The material in the leach field is to permeable and dirty water
reaches the water table
• The material in the leach field is too impermeable, and the syustem
backs up
• The tank is too close to a well
• Pressure-dose system to combat some problems