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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
Importance of groundwater
Geological role of groundwater
• As an erosional agent
• Groundwater serves as an equalizer of
streamflow
Fresh water of the Hydrosphere
Distribution of groundwater
Zone of saturation
• Formation
– Water reaches a zone where all of the open
spaces in sediment and rock are completely
filled with water
– Water within the pores is called groundwater
• Water table – the upper limit of the zone
of saturation
Distribution of groundwater
Capillary fringe
• Extends upward from the water table
• Groundwater is held by surface tension in
tiny passages between grains of soil or
sediment
Distribution of groundwater
Zone of aeration
• Area above the water table
• Includes the capillary fringe and the belt
of soil moisture
• Water cannot be pumped by wells
Distribution of
Groundwater
The water table
The water table is the upper limit of the
zone of saturation
Variations in the water table
• Depth is highly variable
– Varies seasonably and from year to year
• Shape is usually a replica of the surface
topography
The water table
Variations in the water table
• Factors that contribute to the irregular
surface of the water table
– Water tends to “pile up” beneath high areas
– Variations in rainfall
– Variations in permeability from place to
place
The water table
Interaction between groundwater and
streams
• Constitutes a basic link in the hydrologic
cycle
• Three types of interactions
– Gaining streams – gain water from the inflow
of groundwater through the streambed
– Losing streams – lose water to the
groundwater system by outflow through the
streambed
Gaining and losing streams
The water table
Interaction between groundwater and
streams
• Interactions
– A combination of the first two – a stream
gains in some sections and loses in other areas
Factors influencing the 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
distances
Groundwater 1
Factors influencing the storage and
movement of groundwater
Permeability, aquitards, and aquifers
• Permeability – the ability of a material to
transmit a fluid
• Aquitard – 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)
Movement of groundwater
Exceedingly slow – typical rate of
movement is a few centimeters per day
Energy for the movement is provided by
the force of gravity
Darcy’s Law – if permeability remains
uniform, the velocity of groundwater will
increase as the slope of the water table
increases
Movement of groundwater
Darcy’s Law
• Hydraulic 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
Features associated with
groundwater
Springs
• Occur where the water table intersects
Earth’s surface
• Natural outflow of groundwater
• Can be caused by an aquitard creating a
localized zone of saturation which is called
a perched water table
Springs may result from
a perched water table
Features associated with
groundwater
Hot springs
• Water is 6-9o
C warmer than the mean
annual air temperature of the locality
• The water for most hot springs is heated
by cooling of igneous rock
Features associated with groundwater
Geysers
• Intermittent hot springs
• Water erupts with great force
• Occur where extensive underground
chambers exist within hot igneous rock
• Groundwater heats, expands, changes to
steam, and erupts
Distribution of hot springs and
geysers in the United States
Old Faithful geyser in
Yellowstone National Park
Features associated with groundwater
Wells
• To ensure a continuous supply of water, a
well must penetrate below the water table
• Pumping of wells can cause
– Drawdown (lowering) of the water table
– Cone of depression in the water table
Formation of a cone of
depression in the water table
Features associated with groundwater
Artesian well – a situation in which
groundwater under pressure rises above
the level of the aquifer
• Types of artesian wells
– Nonflowing – pressure surface is below
ground level
– Flowing – pressure surface is above the
ground
• Not all artesian systems are wells, artesian
spring also exist
An artesian well resulting
from an inclined aquifer
Problems associated with
groundwater withdrawal
Treating groundwater as a nonrenewable
resource
• In many places the water available to
recharge the aquifer falls significantly
short of the amount being withdrawn
Subsidence
• Ground sinks when water is pumped from
wells faster than natural recharge
processes can replace it (San Joaquin
Valley of California)
Problems associated with
groundwater withdrawal
Saltwater contamination
• Excessive groundwater withdrawal causes
saltwater to be drawn into wells, thus
contaminating the freshwater supply
• This is primarily a problem is coastal
areas
Saltwater contamination due
to excessive well pumping
Groundwater contamination
One 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 contamination
Sinking a well can lead to groundwater
pollution problems
Other sources and types of
contamination include substances such as
• Highway salt
• Fertilizers
• Pesticides
• Chemical and industrial materials
Geologic work of groundwater
Groundwater dissolves rock
• Groundwater is often mildly acidic
– Contains weak carbonic acid
– Forms when rainwater dissolves carbon
dioxide from the air and from decaying plants
• Carbonic acid reacts with calcite in
limestone to form calcium bicarbonate, a
soluble material
Geologic work of groundwater
Karst topography
• Landscapes that to a large extent have
been shaped by the dissolving power of
groundwater
• Some common features include
– Irregular terrain
– Sinkhole or sinks (formed by groundwater
slowly dissolving the bedrock often
accompanied by collapse)
– Striking lack of surface drainage (streams)
Development of karst topography

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Groundwater 1

  • 2. 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
  • 3. Importance of groundwater Geological role of groundwater • As an erosional agent • Groundwater serves as an equalizer of streamflow
  • 4. Fresh water of the Hydrosphere
  • 5. Distribution of groundwater Zone of saturation • Formation – Water reaches a zone where all of the open spaces in sediment and rock are completely filled with water – Water within the pores is called groundwater • Water table – the upper limit of the zone of saturation
  • 6. Distribution of groundwater Capillary fringe • Extends upward from the water table • Groundwater is held by surface tension in tiny passages between grains of soil or sediment
  • 7. Distribution of groundwater Zone of aeration • Area above the water table • Includes the capillary fringe and the belt of soil moisture • Water cannot be pumped by wells
  • 9. The water table The water table is the upper limit of the zone of saturation Variations in the water table • Depth is highly variable – Varies seasonably and from year to year • Shape is usually a replica of the surface topography
  • 10. The water table Variations in the water table • Factors that contribute to the irregular surface of the water table – Water tends to “pile up” beneath high areas – Variations in rainfall – Variations in permeability from place to place
  • 11. The water table Interaction between groundwater and streams • Constitutes a basic link in the hydrologic cycle • Three types of interactions – Gaining streams – gain water from the inflow of groundwater through the streambed – Losing streams – lose water to the groundwater system by outflow through the streambed
  • 13. The water table Interaction between groundwater and streams • Interactions – A combination of the first two – a stream gains in some sections and loses in other areas
  • 14. Factors influencing the 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 distances
  • 16. Factors influencing the storage and movement of groundwater Permeability, aquitards, and aquifers • Permeability – the ability of a material to transmit a fluid • Aquitard – 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)
  • 17. Movement of groundwater Exceedingly slow – typical rate of movement is a few centimeters per day Energy for the movement is provided by the force of gravity Darcy’s Law – if permeability remains uniform, the velocity of groundwater will increase as the slope of the water table increases
  • 18. Movement of groundwater Darcy’s Law • Hydraulic 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
  • 19. Features associated with groundwater Springs • Occur where the water table intersects Earth’s surface • Natural outflow of groundwater • Can be caused by an aquitard creating a localized zone of saturation which is called a perched water table
  • 20. Springs may result from a perched water table
  • 21. Features associated with groundwater Hot springs • Water is 6-9o C warmer than the mean annual air temperature of the locality • The water for most hot springs is heated by cooling of igneous rock
  • 22. Features associated with groundwater Geysers • Intermittent hot springs • Water erupts with great force • Occur where extensive underground chambers exist within hot igneous rock • Groundwater heats, expands, changes to steam, and erupts
  • 23. Distribution of hot springs and geysers in the United States
  • 24. Old Faithful geyser in Yellowstone National Park
  • 25. Features associated with groundwater Wells • To ensure a continuous supply of water, a well must penetrate below the water table • Pumping of wells can cause – Drawdown (lowering) of the water table – Cone of depression in the water table
  • 26. Formation of a cone of depression in the water table
  • 27. Features associated with groundwater Artesian well – a situation in which groundwater under pressure rises above the level of the aquifer • Types of artesian wells – Nonflowing – pressure surface is below ground level – Flowing – pressure surface is above the ground • Not all artesian systems are wells, artesian spring also exist
  • 28. An artesian well resulting from an inclined aquifer
  • 29. Problems associated with groundwater withdrawal Treating groundwater as a nonrenewable resource • In many places the water available to recharge the aquifer falls significantly short of the amount being withdrawn Subsidence • Ground sinks when water is pumped from wells faster than natural recharge processes can replace it (San Joaquin Valley of California)
  • 30. Problems associated with groundwater withdrawal Saltwater contamination • Excessive groundwater withdrawal causes saltwater to be drawn into wells, thus contaminating the freshwater supply • This is primarily a problem is coastal areas
  • 31. Saltwater contamination due to excessive well pumping
  • 32. Groundwater contamination One 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
  • 33. Groundwater contamination Sinking a well can lead to groundwater pollution problems Other sources and types of contamination include substances such as • Highway salt • Fertilizers • Pesticides • Chemical and industrial materials
  • 34. Geologic work of groundwater Groundwater dissolves rock • Groundwater is often mildly acidic – Contains weak carbonic acid – Forms when rainwater dissolves carbon dioxide from the air and from decaying plants • Carbonic acid reacts with calcite in limestone to form calcium bicarbonate, a soluble material
  • 35. Geologic work of groundwater Karst topography • Landscapes that to a large extent have been shaped by the dissolving power of groundwater • Some common features include – Irregular terrain – Sinkhole or sinks (formed by groundwater slowly dissolving the bedrock often accompanied by collapse) – Striking lack of surface drainage (streams)
  • 36. Development of karst topography