• Water Shortage
• Stream Flow Depletion
• Land Sudsidence
• Saltwater Infiltration
• Increased Pumping Costs
• Groundwater Pollution

Confusion about groundwater (table 11.1)

• Typically can't see it other than in caves and mines
• Exists in pore spaces in sedimentary rocks
• Exists in joints or fractures in the bedrock

94% of fresh water is groundwater

Additional Importance

• Sinkholes -- erosional process
• Storage -- areas of large erosion will hold groundwater

Distrubution of Underground Water

Infuential Factors

• Steepness of slope
• Nature of Surface Material
• Intensity of Rainfall
• Type and Amount of Vegetation

Belt of soil moisture -- near surface zone

Zone of saturation -- all of the open spaces that are completely filled by water -- Groundwater

Zone of aeration -- area above the water table that includes the capillary fringe and belt of soil moisture.

Water Table

• predicting the productivity of wells
• explaining the changes in the flow of springs and streams
• accounting for fluctuation in the levels of lakes

Variations

• depth can be 0 to 100s of meters
• seasonal and from year to year
  • quantity
  • distrubution
  • timing of precipitation
• can be mapped using wells (figure 11.3)
• tends to replicate the surface topography

Factors of irregular surfaces

• groundwater moves slowly and at varying rates
• water as a result tens to pile up beneath high areas
• level can drop during drought
• variations in rainfall
• permeability

Interactions between Groundwater and Streams
Basic link of hydrologic cycle

Gaining Streams

• gain water from the inflow of groundwater through the streambed
• water has to be higher than stream bed

Losing Streams

• lose water by outflow through streambed
• elevation lower than surface of the stream

Combination -- gains in some sections and losses in others.

Losing streams can be connected by continuous saturated zone or disconnected from the groundwater system by an unsaturated zone. Disconnected may have a bulge.

Flow directions can change

• over short-time. ie., storms
• temporary flood peaks

Factors Influencing the Storage and Movement

• Porosity
  • quantity of groundwater that can be stored in pore spaces
  • % total volume of rock or sediment that consists of pore spaces
  • typically sedimentary rocks, but can also be:
    • joints
    • faults
    • cavities
    • vesicles
    • Pore Spaces Depends
      • size and shape of grains
      • packing
      • degree of sorting
      • amount of cementing material
• Permeability
  • ability to transmit fluid
  • for water to flow pores need to be connected and large enough
  • smaller spaces, slower movements
  Categories
  • Specific Yield -- the portion that will drain under the influence of gravity - what is availble for use.
  • Specific Retention -- the part retained as a film on particles and rock surface and in tiny openings - how much stays behind.
• Aquitards - impermeable layers that hinder or prevent water movement - e.g., clay
• Aquifers - permeable rock strata or sediment that transmit groundwater freely - e.g., sands and gravels.

Movement of Groundwater

• very slow -- centimeters per year
• moves by gravity -- from high to low
• typically follows a long, curving path
• can go against the force of gravity
  • the deeper you go into the zone of saturation, the greater the water pressure
  • tends to migrate towards points of lower pressure

Darcy's Law

• basics to understanding of groundwater movement
• found if permeability remainx uniform, the velocity of groundwater will increase as the slope of the water table increases

• Hydraulic Gradient -- water slope -- determined by dividing the vertical difference between the recharge and discharge point (head) by the length of flow between these points.
• Hydraulic Head -- the vertical difference between recharge and discharge

Measuring Movements

• Introducing Dye
• Carbon-14 isotopes

Springs

• Occur where the water table intersects the ground surface
• Natural outflow of water
• Perched water table -- where an aquitard creates a localized zone of saturation - figure 11.7

Hot Springs and Geysers

• Hot Springs
  • The water is 6-9 degrees C warmer than the mean annual air temperature of the local area
  • The water for most hot springs is heated by the cooling of igneous rocks
• Geysers -- figure 11.10
  • Intermittent hot springs
  • Water erupts with great force -- figure 11.9
  • Occur where extensive underground chambers exist within hot igneous rock
  • Groundwater heats, expands, changes to steam, and erupts

  Material deposited - chemical sedimentary rock accumulates at the surface

  • Geyserite or Siliceous sinter -- water contains dissolved silica
  • Travertine or Calcareous tufa -- water contains dissolved calcium carbonate (a form of limestone)

Wells

• To ensure a continuous supply of water, a well must be drilled below the water table
• Pumping wells can cause:
  • Drawdown or lowering of the water table
  • Cone of depression in the water table -- figure 11.12 This is formed because the hydraulic gradient is increased

Artesian Wells -- where the gorundwater is under pressure rises above the level of the aquifer

To exist and artesian well must: -- figure 11.14

• water must be confined to an aquifer that is inclined so that one end can recieve water
• aquitards, both above and below the aquifer, must be present to prevent the water from escaping

• Types of Artesian wells -- figure 11.14
  • Nonflowing -- pressure surface is below the surface
  • Flowing -- pressure surface is above the ground
• Not all artesian systems are wells, artesian springs also exist -- the groundwater reaches the suface by rising though fractures

Water can be transmitted long distances either naturals or man-made -- figures 11.15 and 11.16.

Problems with groundwater withdrawal

• Nonrenewable resource -- the amount in the aquifer is typically a smaller amount then what is being pumped.
• Subsidence -- the ground sinks as a result of pumping an area to quickly -- p. 317
  • San Joaquin Valley
  • Las Vegas
  • New Orleans and Baton Rouge
  • Houston - Galveston area
  • Mexico City
• Saltwater Contamination -- figure 11.18
  • excessive pumping causes saltwater to be dranw into the wells, thereby comtaminating the groundwater supply
  • primarily in coastal areas

Groundwater Contamination

• Sewage -- septic tanks, inadequate or broken sewer systems, and/or farm wastes
  • Permeable aquifers, the material can travel long distance without being cleaned
  • oftern becomes purified as its flows thru a few dozen meters of aquifer composed of sand or permeable sandstone
• Sinking a well -- changes the slope of water table
• Other sources
  • Highway salt
  • Fertilizers
  • Pesticides
  • Chemical and industrial materials - some of which can be classfied as hazardous
    • pipelines
    • storage tanks
    • landfills
    • holding ponds

Once discovered the well is abondoned and the most cost effective is to allow the system to clean itself, a quicker but more costly solution is to treat the water.

Geologic Work of Groundwater

Groundwater dissolves rock

• Mildly acidic
  • contains weak carbonic acid
  • forms when rainwater dissolves carbon dioxide from the air and from decaying plants
• Calcium bicarbonate - carbonic acid reactes with the calcite in the limestone -- a soluable material

• Acidic groundwater dissolves soluble rock at or just below the surface in the zone of saturation
• Features:
  • form in the zone of aeration
  • composed of dripstone(travetine)
    • as dripping water evaporates, calcite is deposited
    • called speleothems
      • stalactites -- hanging from the ceiling
      • stalagmites -- form on the floor of the cavern

• Mammonth caves, Kentucky -- 540 kms of interconnected passages
• Carlsbad Caverns, New Mexico -- Big Room - can fit 14 football fields and enough high to fit the U. S. Capitol Building.

Typically areas unlain by limestone.

Features include:

• Irregular terrain
• sinkholes or sinks -- groundwater dissolving the bedrock and is often associated by collapse
  Formation can occur:
  • The limestone just below the surface is dissolved by downward seeping rainwater recharged with carbon dioxide. As time goes by the bedrock is lowered and fractures increase in size. Eventually the surface is shallow and gentle.
  • Abrubtly where the roof of the cavern collapses. These tend to be steep-sided and deep.
  • striking lack of streams or surface drainage -- the runoff is quickly funneled below the surface.

Advanced Karst Topography

Tower Karst - typically forms in wet tropical and subtropical environments

• SE China - Guilin District
• Puerto Rico
• Cuba
• Northern Vietnam

Questions?

1. Where is groundwater found?
2. What does overuse result in?
3. What is the role of groundwater?
4. What are the influential factors for groundwater?
5. What is the distribution of groundwater?
6. What is the water table?
7. What are the interactions between groundwater and streams?
8. What factors influence the storage and movement of groundwater?
9. What is Darcy’s law?
10. How can you measure the movement of water?
11. What is a spring?
12. What is a hot spring?
13. What are geysers?
14. What factors determine a successful well?
15. What happens in pumping wells?
16. What is an artesian well?
17. What can happen as a result of groundwater withdrawal?
18. What causes groundwater contamination?
19. How are caverns formed?
20. What features are associated with caverns?
21. What is karst topography?

Volcabulary Terms

• Aquifer
• Aquitard
• Artesian
• Belt of soil moisture
• Capillary Fringe
• Cavern
• Cone of depression
• Darcy's Law
• Drawdown
• Flowing arterian well
• Gaining stream
• Geyser
• Groundwater
• Head
• Hot spring
• hydraulic gradient
• Karst topography
• Losing stream
• Nonflowing artesian well
• Perched water table
• Permeability
• Porosity
• Sinkhole
• Sink
• Speleothem
• Spring
• Stalacite
• Stalagmite
• Water Table
• Well
• Zone of aeration
• Zone of saturation

Return to the Lecture Page
Return to the Class Page