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

   Most igneous and metamorphic rocks = poor

   Fine-grained sedimentary rock & limestone = poor

   Coarse-grained sedimentary rock = fair to good

   Fractured volcanic rock and limestone = fair/good

   Loose unconsolidated sediments = good

 

4. Permeability

       Defined: The capacity to transmit fluids

     Depends on several factors concerning the material

   Porous

   Size of pore spaces and/or fractures

   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

   Shales and very well cemented sedimentary rocks

   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

   Caves and caverns are above the water table

   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 37 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

                  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