Ch. 17 - Groundwater

Class: GEOL-101

Notes:

17.1 The Importance of Groundwater

Describe the importance of groundwater as a source of freshwater and the role of groundwater as a geologic agent.

Groundwater

Presentation content:

• Groundwater is water found in the pores of soil and sediment, plus narrow fractures in bedrock
• Groundwater and the Hydrosphere
  • 3/5 of 1% of the hydrosphere is groundwater
  • Groundwater is the largest reservoir of freshwater that is readily available to humans
    • Groundwater makes up 14% of all freshwater reservoirs (most occurs as glacial ice)
    • Groundwater actually makes up 30.1% of all liquid freshwater reservoirs

Notes from the lecture:

• Water can whether and erode
• Example: Erosion of the underlying limestone
  • Below surface -> a connected network of caverns where water is flowing
• Freshwater makes only 2.5%
• Surface water and other freshwater: 1.3%

Geological Importance of Groundwater

Presentation content:

• As an erosional agent
  • Dissolving soluble bedrock such as limestone
    • Formation of caves and sinkholes
• Equalizer of stream flow
  • Storage that sustains streams during dry periods

Notes from the lecture:

• Example: Carlsbad Caverns
• Groundwater can shape our ground and surface topography

A Basic Resource

Presentation content:

• Every day in the United States we use ~350 billion gallons of freshwater
  • ~23 percent comes from groundwater
  • Groundwater exists almost everywhere—an advantage in places that lack available surface water sources
• Used primarily for irrigation

Notes from the lecture:

• Very important for drinking and agricultural purposes
• 77% Surface water
• 23% Groundwater
  • 68.4% Irrigation
  • 19.3% Public supply
  • Others.
• In general we use a lot of water, and a lot of it comes from groundwater

17.2 Groundwater and the Water Table

Prepare a sketch with labels that summarizes the distribution of water beneath Earth’s surface. Discuss the factors that cause variations in the water table and describe the interactions between groundwater and streams.

Distribution of Groundwater

Presentation content:

• Most Groundwater Soaks into the Ground from Precipitation

  • Zone of soil moisture is a zone where water is held by molecular attraction on soil particles in the near-surface zone
    • Used by plants
    • Evaporates directly back to the atmosphere
  • Water not held in this zone percolates further downward

• Zone of saturation is a zone where all of the pore spaces are completely filled with water

  • Also called the phreatic zone
  • Water in the zone of saturation is groundwater
  • The water table is the upper limit of the zone of saturation
  • Extending upward from the water table is the capillary fringe

• The unsaturated zone (vadose zone) is the area above the zone of saturation

  • Pore spaces include both air and water
  • Includes the zone of soil moisture
  • Includes the capillary fringe—a region where groundwater is held in pore spaces by surface tension

Notes from the lecture:

• Water is trapped into grains of sediment in the sub surface
  • Held in place because of the pressure between materials
  • Not big lakes of water underground
• Most of it will migrate downward and reach our zone of saturation, which is where the water table is located
• The overlying zone is referred to as the unsaturated zone

Variations in the Water Table

Presentation content:

• Depth is highly variable
• Varies seasonally and from year to year
  • Precipitation variations affect the depth of the water table
• Shape is usually a subdued replica of the surface topography
• Except where it is at the surface, it cannot be observed directly

Notes from the lecture:

• It is not a flat surface, there is probably topography
• Seasonality also affects the process
• The shape of the topography also plays a role
• Monitoring and Mapping the Water Table
  • How do we measure groundwater?
  • We use a floating box
    • If water levels goes up or down it is recorder, so we measure the buoyancy of that little box
    • Water is infiltrated into the device

Interaction Between Groundwater and Streams

Presentation content:

• Constitutes a basic link in the hydrologic cycle
  • Gaining streams
    • Gain water from the inflow of groundwater through the streambed
    • Water table is higher than the stream surface
  • Losing streams
    • Lose water to the groundwater system by outflow through the streambed
    • Water table is lower than the stream surface
  • Combination streams
    • A stream can gain in some sections and lose in others

Notes from the lecture:

• Gaining stream -> If you have a very high water table, and as a result it is in count activity with the surface -> the river is at the same level of the water table
• Loosing stream -> surface water is feeding the groundwater

17.3 Storage and Movement of Groundwater

Summarize the factors that influence the storage and movement of groundwater. Discuss how groundwater movement is measured and the different scales of movement.

Porosity

Presentation content:

• The percentage of pore (open) spaces in a rock or sediment is called porosity
  • Depends on the size and shape of the grains, how well they are sorted, and how tightly they are packed
    • Poorly sorted sediments have a low porosity
    • Most crystalline rocks only gain porosity through fractures
• Determines how much groundwater can be stored

Notes from the lecture:

• We can know approximately how much water it fits
• Determines your total storage of water

Permeability, Aquitards, and Aquifer

Presentation content:

• Permeability is the ability of a material to transmit a fluid
  • Depends on the connectivity between pores
• An aquitard is an impermeable layer that hinders or prevents water movement
  • Example: Clay
• An aquifer is permeable rock strata or sediment that transmits groundwater freely
  • Example: sands and gravels

Notes from the lecture:

• The ability of water to flow through the aquifer is dictated through volume -> more pressure to push it through
• Example: Limestone is far less permeable -> need to apply more pressure to get the same amount of water

Groundwater moves slowly

Presentation content:

• Groundwater moves very slowly
  • Average rate is 4 cm per day
• Underground rivers are rare, and a common misconception
• A Simple Groundwater Flow System
  • The force of gravity and pressure differences move groundwater
    • Groundwater is replenished in recharge areas
    • Groundwater flows back to the surface in discharge areas

Notes from the lecture:

• driven by Gravity + Pressure

• Most of our recharge areas are through river systems
• In terms of the hydrological cycle:
  • Water is going to runoff and feed into the river system (into the discharge area)
  • Most of the time gravity will do the heavy-lifting
  • But also water infiltrates

Measuring Groundwater Movement

Presentation content:

• Darcy’s law is a measure of the volume of water that flows through an aquifer
  • Uses the hydraulic gradient, conductivity, and cross-sectional area
  • Hydraulic gradient is the water table slope
  • Hydraulic conductivity takes into account the permeability of the aquifer and viscosity of the liquid to determine how fast water will flow through a medium

Notes from the lecture:

• Measuring the differences of the height of the water table (h1 - h2)

Different Scales of Movement

Presentation content:

• The area of groundwater flow systems vary from a few square kilometers to tens of thousands of square kilometers
• Regional groundwater systems interact with deeper, larger groundwater systems

Notes from the lecture:

Localized systems, subregional systems, and deep regional systems

17.4 Wells and Artesian Systems

Discuss water wells and their relationship to the water table. Sketch and label a simple artesian system.

Well

Presentation content:

• A well is a hole bored into the zone of saturation—significantly below the water table
  • Most common methods for removing groundwater
    • More than 16 million water wells in the United States
    • More than 13 million belong to private households

Notes from the lecture:

• In Michigan every hose has its own water well
• You do not want to drink it because is very salty water
• IncCertain cases it is so salty that you can use it for agricultural purposes

Drawdown

Presentation content:

• Drawdown—As water is withdrawn from the well, the surrounding water table is lowered
  • A cone of depression (cone-shaped depression in the water table) forms around a well
  • Hydraulic gradient increases near wells with a cone of depression

Notes from the lecture:

• Pulls the water table down -> a cone of depression

Perched Water Table

Presentation content:

• Forms where an aquitard is situated above the main water table

Artesian Well

Presentation content:

• An artesian well or spring is a system where groundwater under pressure rises above the level of the aquifer
• Two conditions must be met to form an artesian system:
  • Water must be confined to an inclined aquifer
  • Aquitards must exist above and below the aquifer to confine the aquifer
    • An aquifer confined by aquitards is called a confined aquifer

Notes from the lecture:

• Think of it as pushing water so the water in front has to go somewhere, so it goes up.
• In this excample, this is avegetative area, but in other places these are wazies

Types of Artesian Wells

Presentation content:

• Types of Artesian Wells

  • Nonflowing artesian well—Pressure surface is below ground level
  • Flowing artesian well—Pressure surface is above the ground

• Not all artesian systems are wells; artesian springs also exist

  • Groundwater may reach the surface through a fracture
  • Sometimes responsible for forming desert oases

• Some artesian systems transmit water a great distance

  • Example: South Dakota

Water towers

Presentation content:

• Municipal water towers create artificial artesian systems
  • Tower acts as a recharge area
  • Pipes confine the “aquifer”
  • Faucets are the flowing artesian wells

Notes from the lecture:

• As gravity pushes it downward it creates artificial pressure that helps water flow through your hose

17.5 Springs, Hot Springs, and Geysers

Distinguish among springs, hot springs, and geysers.

Springs

Presentation content:

• A natural outflow of water from the intersection of the water table and the ground surface is called a spring
• Many geologic situations lead to the formation of springs, not just perched water tables
  •- Permeable zones exist as fractures or solution channels

Notes from the lecture:

• Intersection of the water table and the ground surface
  • Creates a waterfall

Hot Springs

Presentation content:

• Water in a hot spring is 6ºC to 9ºC warmer than the mean annual air temperature of the locality
  • More than a 1000 in the United States!
• The water for most hot springs is heated by the cooling of igneous rock
  • Most hot springs (more than 95%) are heated this way
• Some hot spring water is warmed by the geothermal gradient
  • Example: Warm Springs, GA

Notes from the lecture:

• Examples:
  • Yellowstone National park
  • Hot Springs, Arkansas
  • Warm Springs, Georgia
• A system where the temperature of the groundwater is higher than the surface water
• As you drill down with depth temperature increases, in these locales temperature elevates at a grater rate

Gaysers

Presentation content:

• Geysers are intermittent hot springs in which columns of water erupt with force
• How Geysers Work
  • These occur where extensive underground chambers exist within hot igneous rock
  • Groundwater heats under great pressure from overlying water (but does not boil)
  • The super heated water expands, and some is forced to the surface reducing the pressure on water below
  • Water in the chamber then changes to steam, and erupts

Notes from the lecture:

• Result of more complex underground chambers

How a Geyser Works

Presentation content:

Notes from the lecture:

• There is a network of different chambers that helps that water eventually go upward
• Great example: Yellowstone national park
• You have an elevated geothermal gradient
• Pressure builds up in the system, and starts to be released -> upward motion

Geyser deposits

Presentation content:

• Chemical sedimentary rock accumulates at the surface, precipitated from minerals dissolved in the groundwater
• The type of precipitate reflects the chemical makeup of the bedrock that the groundwater interacted with
  • Siliceous sinter or geyserite form from dissolved silica
  • Travertine or calcareous tufa forms from dissolved calcium carbonate
    • Example: Mammoth Hot Springs, Yellowstone

Notes from the lecture:

• Color is also due to microbes living in these environments (extreme temperature)
• Travertines are calcites that form through these hot springs

17.6 Environment Problems

List and discuss important environmental problems associated with groundwater.

Mining Groundwater

Presentation content:

• We should be treating groundwater as a nonrenewable resource
  • In many places, the water available to recharge the aquifer is significantly short of the amount being withdrawn
  • Example: High Plains Aquifer
    • Underlies 111 million acres
    • One of the largest and most agriculturally significant aquifer
    • Accounts for 30 percent of all groundwater used for irrigation in the United States

Notes from the lecture:

• We use a lot of groundwater! we can run into challenges of supply
• One of the major aquifers is the High Plains aquifer
  • Extends from up Texas up to Kansas, Nebraska, Iowa, Indiana, etc.
  • We can see a decrease in the water level -> we are using water at a greater rate that the system can recharge
• Rises can be due to long periods of precipitation.

Subsidence

Presentation content:

• The ground sinks when water is pumped from aquifers faster than natural recharge processes can replace it

• Particularly pronounced in areas underlain by thick layers of unconsolidated sediments

  • Example: San Joaquin Valley of California
    • Subsidence approached 9 m!

• Prolonged drought can induce subsidence

• Example: California 2016 Drought

  • 64% of the state experienced drought
  • 60% of the state’s water needs were met by groundwater
  • Lowering of the water table led 0.6 m (2 feet) of subsidence in a little over a year

Notes from the lecture:

• Example: Sierra Nevada and Coastal range
  • Very fructile agriculture region were there are been many subsidences
  • In 1977 we have seen a fall of about 30 feet subsidence
  • There is still a large amount of prolific water here.
  • If you keep lowering this how does that change stream systems?
    • You can actually start to get saltwater intrusion.
      • Once that salt water enters the system you can't get rid of it, it starts contaminating the whole system.
• Keep in mind subsidence is variable, the rate of subsidence can be increased.
• There is research that says that this has allowed for mountains to lift.

Saltwater Intrusion

Presentation content:

• Excessive groundwater withdrawal causes saltwater to be drawn into wells, thus contaminating the freshwater supply
• Fresh water is less dense than salt water, so it naturally floats as a “lens” shaped body above it
• The base of a fresh water body can extend well below sea level
• Pumping out the fresh water faster than it recharges can result in salt water being drawn up with it
  • Primarily a problem in coastal areas
  • Can correct the problem with recharge wells

Notes from the lecture:

• Pumped water for a large amount of time -> Cone of depression:
  • Pulls in a large volume of water
• It is very important to keep this in mind when we are pumping water near coastal areas because this can contaminate our fresh groundwater.

Preventing Saltwater Contamination with Recharge Basins

• Recharge basins intercept surface runoff and allow the water to infiltrate. this helps to maintain the water table and prevent saltwater intrusion. recharge basins are used in many places, not just coastal areas.
• There is a lot of effort into trying to prevent these kinds of saltwater contamination

Groundwater Contamination

Presentation content:

• One common source is sewage

  • Extremely permeable aquifers (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

  

• Sinking a well can lead to groundwater pollution problems

  • Cone of depression will locally increase or reverse the slope of the water table
  • Contributing to contamination of other nearby wells that had no been polluted prior to drawdown

• Other sources and types of contamination include highway salt, fertilizers, pesticides, chemicals, and industrial materials

  • Similarly, contamination from leaking holding ponds can enter the groundwater supply

  

Notes from the lecture:

• Limestones undergo disclusion and create caverns
• Keep in mind groundwater moves very slowly
• Keep in mind near gas stations wells that can be contaminated very easily
• If we add a new well that could also cause a problem, it changes the depression and changes the movement of the flow of the water.

17.7 The Geologic Work of Groundwater

Explain the formation of caverns and the development of karst topography.

Groundwater Dissolves Rock

Presentation content:

• Most 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 that can be carried away in solution as dissolved load

Notes from the lecture:

• Slightly lower PH water
  • A little bit of carbon dissolved in it
  • SO it can react with calcite and limestone
• This can generate large caverns

Caverns

Presentation content:

• The most spectacular results of erosion by groundwater

• Most caverns are created by acidic groundwater dissolving soluble rock

• About 17,000 caves have been discovered in the United States

  • Famously large examples: Mammoth Cave in Kentucky and Carlsbad Caverns in New Mexico

• Cavern development

  • Developed as acidic groundwater dissolves limestone bedrock
  • Development occurs at several levels
    • Most active erosion is at or just below the zone of saturation

• How dripstone forms

  • Calcium carbonate deposited as dripping water evaporates is called travertine

• Dripstone features—speleothems

  • Speleothem is the general name for all dripstone features
    • Includes stalactites (hanging from the ceiling) and stalagmites (form on the floor of a cavern)
    • These may join together to form a column

Notes from the lecture:

• This is all created through the dissolution of rock through groundwater
• Most caverns are generated in limestone bed rock
• They form at different layers, and can be connected through faults and fractures
• These usually form through infiltration of groundwater.
• Often you have different types of structures
  • Drip stones
• Partial pressure of the fluid changes so it allows liquid to start to precipitate downwards
  • They can also grow together to form large groups of stalactites or stalagmites

Karst Topography

Presentation content:

• Karst topography is a landscape that has been shaped mainly by the dissolving power of groundwater

  

• Sinkholes or sinks (formed by groundwater slowly dissolving the bedrock often accompanied by collapse)

• Striking lack of surface drainage (streams)

• Tower karst: forms where thick limestone is highly fractured and jointed, groundwater dissolves along these fractures and leaves behind residual towers

Notes from the lecture:

• Groundwater is doing a lot of activity but we can't see it because it is all underground
• Karst towers -> very curvasive little mountains
  • There is a lot of rock between
  • Hills rather than huge towers

End of Chapter 17 - Concept Checks

17.1 The Importance of Groundwater

1. What percentage of Earth’s total freshwater supply is groundwater?
2. What share of Earth’s liquid freshwater is groundwater?
3. List two geologic roles that groundwater plays.
4. What share of U.S. freshwater is provided by groundwater? What is most groundwater used for?

17.2 Groundwater and the Water Table

1. When rain falls on land, what factors influence the amount of water that soaks in?
2. Define groundwater and relate it to the water table.
3. A kitchen table is flat. Is this usually the case for a water table? Why?
4. Contrast a gaining stream and a losing stream.

17.3 Storage and Movement of Groundwater

1. Distinguish between porosity and permeability.
2. What is the difference between an aquifer and an aquitard?
3. What factors cause water to follow the paths shown in Figure 17.10?
4. Relate groundwater movement to hydraulic gradient and hydraulic conductivity.

17.4 Wells and Artesian Systems

1. Define drawdown and relate this term to the term cone of depression. Sketch a simple cross section of an artesian system with a flowing well. Label aquitards, aquifers, and the pressure surface.
2. Why do some artesian wells not flow at Earth’s surface?

17.5 Springs, Hot Springs, and Geysers

1. Describe some circumstances that lead to the formation of a spring.
2. What warms the waters that flow at Hot Springs National Park, Arkansas, and at Warm Springs, Georgia?
3. What is the source of heat for most hot springs and geysers? How is this reflected in the distribution of these features?
4. Describe what occurs to cause a geyser to erupt.

17.6 Environmental Problems

1. Describe the problem associated with pumping groundwater for irrigation in the southern High Plains.
2. Explain why the ground may subside after groundwater is pumped to the surface.
3. Which aquifer would be most effective in purifying polluted groundwater: coarse gravel, sand, or cavernous limestone?
4. Describe a significant problem that may arise when groundwater is heavily pumped at a coastal site.

17.7 The Geologic Work of Groundwater

1. How does groundwater create caverns?
2. What causes cavern formation to stop at one level (depth) but continue or begin at a lower level?
3. How do stalactites and stalagmites form?
4. Describe two ways in which sinkholes form.