Geology Lecture Outline

     Groundwater – (Ch 17) 

 

 

I. Lecture Content

     The Hydrologic Cycle

     Nature of Groundwater

     Water Tables and Aquifers

     Groundwater Movement

Springs, Water Wells, and Artesian Systems

     Erosion and Deposition by Groundwater

Hydrothermal Systems

     Groundwater Pollution and Conservation          

 

II. Introduction

     A. Freshwater Moving Under our Feet

1. Where the hydrosphere overlaps the lithosphere

§      Water table

§      Aquifers

§      Springs

§      Geysers

 

             2. Vital source of freshwater for many parts of the world

§      Nearly 90% of Earth's liquid freshwater is subsurface

§      22% of all freshwater supplies in the world

§      50% of all public drinking water supplies in U.S.

§      99% of drinking water for rural populations in U.S.

 

             3. Alarming increases in contamination and over-pumping

 

     B. Groundwater and the Hydrologic Cycle

             1. Groundwater comes from infiltration of precipitation and

                        runoff into the permeable subsurface

 

            2. Groundwater is water that resides in permeable, porous

                       spaces, fractures and cavities within subsurface rock

                       and unconsolidated material that are called aquifers

 

            3. Rock layers that block groundwater flow = aquicludes

 

4. Groundwater moves slowly and eventually recycles back

                       into the ocean via rivers to complete the cycle

 

III. Nature of Groundwater

     A. Aquifers - Natural groundwater conduits and reservoirs

1. Defined: Permeable subsurface material that holds and

                     transmits groundwater

 

2. Unique properties of material that make a good aquifer

§      Good porosity

§      Good permeability

§      Thick, laterally extensive layer

 

3. Porosity

·     Defined: Percentage of a material's total volume that

                   consists of pore (open) space

·     Porosity varies greatly among different rock types

ü   Most igneous and metamorphic rocks = poor

ü   Fine-grained sedimentary rock & limestone = poor

ü   Coarse-grained sedimentary rock = fair to good

ü   Fractured volcanic rock and limestone = fair/good

ü   Loose unconsolidated sediments = good

 

4. Permeability

·       Defined: The capacity to transmit fluids

·     Depends on several factors concerning the material

ü   Porous

ü   Size of pore spaces and/or fractures

ü   Interconnectedness pore spaces and/or fractures

 

5. The world's most productive aquifers are of two types

·     Well-sorted and well-rounded sands and gravels

·     Highly fractured limestones

 

6. Top of saturated surface of aquifer called the water table

 

B. Aquicludes - Natural barriers to groundwater

1. Defined: Materials that block movement of groundwater

 

2. Controlling properties associated with aquicludes

·     Low permeability (impermeable)

·     Typical of certain unfractured rock types

ü   Shales and very well cemented sedimentary rocks

ü   Most igneous and metamorphic rocks

 

C. Water Table

1. Defined: Top of the saturation zone of a shallow aquifer

 

2. The water table marks the surface between the zone of

      saturation and the zone of aeration

·     See Figures 16.3 and 16.4

 

3. Zone of saturation - zone of subsurface where all the

      pore space is filled by water

·     Subsurface region occupied by groundwater

 

4. Zone of aeration - zone between ground surface and the

       water table where most pore space is filled by air

·     Subsurface region situated over groundwater

 

5.  The surface of a water table typically reflects the

       topography of the overlying ground surface.

 

6. The level of a water table can fluctuate up and down, in

      response to several factors

·     Seasonal and long-term climate changes

·     River and lake water recharge and removal

·     Water well pumping

 

7. Possibility of having several stacked water tables

·      Perched water table

·      See Figure 16.4

 

     D. Groundwater Movement

            1. Groundwater movement propelled by gravity

·     Movement from high elevation to low elevation

·     Movement from high pressure to low pressure

·     Just like that for rivers and glaciers

 

2. Velocity of groundwater flow depends on a few things

·     Aquifer permeability

·     Water table gradient

                       

3. Typical velocities of groundwater range from 250 m/day

         to 1 cm/year          Average =  cm's/day

 

     E. Groundwater Recharge and Discharge

         1. Groundwater systems are dynamic & always in state of flux

·     Water is continually being simultaneously added and

                            removed from many different sources and localities

 

·     The term used to describe the flux of a groundwater

       system is termed the groundwater budget

  

     2. Recharge occurs when water is added to the system

·      Rainfall or snowmelt percolation (vertical recharge)

 

·      River, lake, or catch basin input (lateral recharge)

 

·      Injection well pumping (direct recharge)

 

              3. Withdrawal occurs when water is removed from system

·      Rivers and lakes

 

·      Springs and swamps

 

·      Ocean

 

·      Water well pumping

 

      4.  In most regions of world today, groundwater budgets are

          negative; i.e. more is taken out than put back in.

 

F. Groundwater Quality

     1. Quality of groundwater is a function of several factors:

·      Type of material (minerals) that make up the aquifer

 

·      Total time spent in the aquifer

 

·      Solubility of the rocks and minerals

 

·      Human impacts

 

2. Difference between hard water and soft water

·      Hard water = high in Ca+2 and Mg+2 dissolved ion

 

·      Soft water = high in Na+ and K+

 

IV. Springs, Water Wells, and Geysers

A. Springs

      1. Defined: Places on the ground surface where groundwater

                             seeps or gushes out.

 

      2. Typically, a spring is a place where the water table

            intersects the earth's surface.

·     Can be a normal or perched water table

·     Usually situated on, or at the base, of a hillside

·     Lateral downslope movement of water

 

              3. Spring water can exit the Earth having a wide range of

                    temperatures

·     From near freezing to near boiling

 

      4.  Many famous places and resorts are situated at a spring

§      Palm Springs

§      Desert Hot Springs

           

B. Water Wells

      1. Defined: Dug or drilled openings in the ground down to

                               the zone of saturation

 

      2. Groundwater percolates into the well opening until it

           is level with the surrounding water table

 

      3. Well water is either lifted or pumped out to the surface

·      Simply bucketed

·      Manual, mechanical or electrical pump

·      Natural artesian = free flowing

 

      4.  When wells pump out water faster than the surrounding

            groundwater can recharge the well, the surrounding

 water table lowers to form a cone of depression around

 the well.

·      Water table drops in level around well = drawdown.

·      If pumping continues, eventually the well will go dry.

·      A cone of depression can become so extensive that neighboring wells will get affected

 

5. Some water wells are designed to be injection wells

·      Water is pumped into the well (recharge)

·      Well injection is done for various reasons

 

6. Water wells are the major source of farm irrigation and 

        drinking water for many parts of the world (U.S. too)

 

C. Artesian Groundwater Systems

     1. Defined: Any groundwater system that has a confined

                 aquifer and builds a high hydrostatic (fluid) pressure

 

      2. Groundwater in these systems is able to rise under its

           own pressure above the height of its aquifer if a well is

         drilled down into it.

·      Artesian wells flow freely without need for pumping

·      Natural artesian springs also exist

 

3. Three geologic conditions to create an artesian system

·     Aquifer must be confined above and below by a pair  of aquicludes

 

·     Rock layering is tilted so that the high end of the aquifer is exposed at the surface for recharge

 

·     Sufficient sources of recharge (precip.) to keep filled

 

 

V. Erosion and Deposition in Groundwater Systems

A. Chemical Weathering and Erosion of Aquifers

      1. Susceptible in regions rich in carbonate rocks

·      Predominantly in extensive limestone

 

·      Mild to hot climate with Mod to High Rainfall

 

      2. Groundwater rich in carbonic acid as an erosion agent

 

·      REACTION #1

                Water + carbon dioxide = carbonic acid

                              (H2O + CO2 H+ + HCO3-)

 

·      REACTION #2

  Carbonic acid +Calcite = Calcium ion +Carbonic acid ion

             (H+ + CaCO3 Ca+2 + HCO3-)

·      Process known as Carbonation

 

            3. Erosion of subsurface carbonate rock creates a variety

                     of unique surface and underground features

·     Sinkholes and solution valleys

 

·     Caves and caverns

           

B. Chemical Deposition in Groundwater Systems

     1.  Loss of CO2 in cave groundwater to the air causes the

            opposite reaction of erosion chemical reaction #2 above

                              (Ca+2 + HCO3- H+ + CaCO3)

            2.  Deposits of chemically precipitated carbonate minerals

                  develop within the underground caves and caverns

                  collectively termed "dripstone" formations

·     Form under certain conditions

ü   Caves and caverns are above the water table

ü   Dissolved CO2 is released from water into air

·     Stalactites and stalagmites

·     Columns and drip curtains

 

C. Development of Karst Topography

     1. Associated with regions having extensive shallow

              carbonate rock layers and moist, temperate climates

    

     2.  Karst terrain characterized by undulating surfaces with

               numerous pock-mark depressions called sinkholes

 

      3.  Cave and cavern systems form beneath karst terrains

 

      4.  Many sinkholes are the result of collapsed caverns

           

      5.  Other features of karst terrains include disappearing

               streams, springs, solution valleys, and lakes

 

D. Major Karst Regions Around the World

           1. Eastern U.S.A

            2. Southern Australia

            3. Southeastern China

4. Eastern Europe

 

 

VI. Hot Springs, Geysers and Hydrothermal Systems

     A. Hot Springs

          1. Defined: Any spring that has waters over 37º C.

 

            2. Associated with active volcanic regions and deep faults

 

            3. "Mud pots" associated with hot springs

 

            4. Hot springs waters are typically mineral-laden

 

     B. Geysers

1. Defined: Hot springs that intermittently eject hot water

            and steam into the air at high velocity

 

2. Geysers have unique underground plumbing systems

§      See Figure 16.30 for an illustration

 

3. Travertine deposits typically deposited around vents

 

     C. Hydrothermal Systems and Geothermal Energy

          1. Defined: Underground hot water and/or steam circulation

                       systems associated with hot rocks and/or magma

·      Includes hot springs and geysers

 

            2. Hydrothermal systems potentially hold tremendous

                      amounts of geothermal energy in several forms

·      Hot water and steam

·      Hot dry rock

·      Hot magma

 

                3. Geothermal energy plants generate power in two ways

·     Directly in the form of piped hot water and steam

·     Indirectly as electricity using steam turbines

 

4.     Geothermal energy is relatively clean and renewable

 

VII. Negative Modifications of Groundwater Systems

     A. Excessive Groundwater Withdrawal Creates Several

            Negative Effects

 

      1. Dropping Water Tables

§      Wells dry up

§      Rivers and lakes affected

§      Vegetation suffers

§      Mid Western U.S. hard hit

 

      2. Subsidence

§      Lowering of ground in elevation due to ground

           settling and compaction

 

§      Significant damage to buildings and infrastructures

 

§      Central Valley and Los Angeles heavily affected

 

§      See Table 16.2 for worldwide subsidence localities

 

      3. Saltwater Incursion

§      Shrinking fresh groundwater layer floating on top of

    seawater groundwater causes salty groundwater to

    slowly rise and displace the freshwater layer

 

§      Coastal areas that pump lots of groundwater at risk

 

     B. Increasing Amounts of Pollutants Are Entering Many

             Vital Groundwater Systems Around the World

 

          1. Types of Contaminants

·     Sewage

 

·     Landfill effluent

 

·     Toxic chemicals

 

·     Nuclear wastes

 

·     Hydrocarbons and petrochemicals

 

·     Fertilizers, herbicides and pesticides

           

          2. Sources of Contaminants

·     Leaking sewers, septic systems and treatment plants

·     Landfills and garbage dumps

 

·     Urban runoff and illegal dumping

 

·     Leaking underground fuel tanks

 

·     Toxic waste sites and industrial plants

 

·     Abandoned mines and tailing piles

 

·     Runoff from farms and ranches

 

 

VIII. Groundwater Remediation and Conservation  

     A. Techniques Used to Clean up Polluted Groundwater

          1. Coordinated multi-well pumping system

·      Set up an array of monitoring wells

 

·      Use of radioactive tracers to track water movement

 

·      Use of both injection and withdraw pumping

ü                  Containment and elimination of pollution plume

                                         

2. Injection of hydrocarbon-eating bacteria into aquifers

      that are contaminated by hydrocarbons like gasoline

 

     B. Conservation Methods for Maintaining Healthy Aquifers

          1. Maintain sustainable rates of pumping (withdraw)

·      Closely monitor the water table

 

·      Heavily fine individuals and corps that over pump

 

            2. Maximize groundwater recharge rates

·      Build and maintain runoff catch basins

 

·      Build and operate runoff injection wells

 

               3.  Actively monitor and test groundwater for quality

·     Stop individuals and/or corporations that are polluting

 

·     Heavily fine individuals and corps that pollute aquifers

 

IX. Groundwater Vocabulary - Ch 16

Aquiclude

Aquifer

Artesian system

Cave/cavern

Carbonation

Cone of depression

Drawdown

Dripstone formations

Geothermal energy

Geyser

Groundwater

Hot spring

Hydrothermal

Injection well

Karst topography/terrain

Permeability

Porosity

Recharge

Saltwater incursion

Sinkhole

Spring

Subsidence

Water well

Water table

Withdrawal

Zone of aeration

Zone of saturation