CSULA - Geology: Groundwater

The Hydrologic Cycle

Water Basics
- Sea water 97.2%
- Fresh Water 2.8%
- Polar ice & glaciers 2.15%
- Groundwater 0.62%
- Lakes and Rivers 0.01%
The Three Fates of Precipitation
- Infiltration
  - The movement of water into rocks or soil through cracks and pore spaces
- Runoff
  - Water that flows over the land
- Transpiration/Evapotranspiration
  - the release of water vapor to the atmosphere by plants The Hydrologic Cycle
Infiltration capacity of the soil is controlled by:
- Intensity and duration of rainfall
- Soil saturation
- Soil texture
- Slope of the land
- Nature of the vegetative cover
Main Sources of Water
- Surface Water
  - Lakes, rivers, reservoirs
- Ground Water
  - In the Earth, flows through fractures and pores

What is Groundwater?

Water found in the pores and fractures of soil and bedrock
Largest reservoir of fresh water
Tends to be less polluted than surface water
An important erosional agent
- Dissolves limestone to form
  - Sinkholes
  - Caves
Equalizer of streamflow
- Most water percolates down into the Earth
- Forms underground "streams

Groundwater Terminology

Belt of soil moisture
- Water held by molecular attraction on soil particles in the near-surface zone
- Used by plants
- Excess water percolates down to the zone of saturation
Zone of saturation
- Water not held as soil moisture percolates downward
- 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
Zone of aeration
- Area above the water table & below zone of saturation
- Includes the capillary fringe and the belt of soil moisture
Capillary fringe
- Extends upward from the water table
- Groundwater is held by surface tension in tiny passages between grains of soil or sediment
Water table
- The upper limit of the zone of saturation
- Variations can occur in the water table
  - Depth is highly variable
  - Varies seasonably and from year to year
- Shape is usually a subdued replica of the surface topography
- 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
How does water get from the stream to the water table?
- 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
  - Interactions - A combination of the first two

Groundwater Movement & Storage

Factors
- 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
  - Specific retention
    - The portion whish is retained as a film on particles, rock surfaces, and pore spaces
  - Specific yield
    - The portion which will drain under gravity
- 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)
- Groundwater basins
So how does groundwater move?
- Exceedingly slow
  - Typical rate: 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
  - Hydraulic gradient
    - the water table slope
  - Hydraulic head
    - The vertical difference between the recharge and discharge points
Measuring groundwater flow
- Primarily done with
  - Dyes
  - Carbon-14

Getting Groundwater Out of the Ground

Extraction Methods
- "Natural" Methods
  - 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
  - Hot Springs
    - Water is 6-9 °C warmer than the mean annual air temperature of the locality
    - The water for most hot springs is heated by cooling of igneous rock
  - 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
      - Mechanisms for the entire process are not well understood
    - Chemical sedimentary rock accumulates at the surface
      - Siliceous sinter (from dissolved silica)
      - Travertine (from dissolved calcium carbonate)
- Man-made Methods
  - Wells
    - Must penetrate below the water table
    - Pumping of wells can cause
      - Drawdown (lowering) of the water table
      - Cone of depression in the water table
- Artesian well/spring
  - 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
Problems with groundwater removal
- Non-renewable resource
- Subsidence
- Contamination
Natural Sources
- Biologic activity / organisms
  - Bacteria and viruses
- Naturally occurring elements
  - Uranium, radium, arsenic, and fluoride
    - Often naturally occurring in rock formations
Human Caused
- Acid Rain
  - Acid rain is caused by:
    - CO2 from cars and power plants
    - NOx from cars
    - SO2 from power plants

Agricultural / Industrial Run-off
Accidental chemical spills.
Leakage from underground storage tanks.
Improper disposal of household wastes such as cleaning fluids, paint, and motor oil.

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
- Forms caverns at or just below the zone of saturation
Caverns
- Composed of dripstone (travertine)
  - Calcite deposited as dripping water evaporates

Collectively, they are called speleothems
- Includes:
  - Stalactites (hanging from the ceiling)
  - Stalagmites (form on the floor of a cavern)

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)

The Hydrologic Cycle

What is Groundwater?

Groundwater Terminology

Groundwater Movement & Storage

Getting Groundwater Out of the Ground

Geologic Work of Groundwater

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