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
§
99% of drinking water
for rural populations in
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
§
Desert
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 (
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.
3.
4.
VI. Hot Springs, Geysers and Hydrothermal
Systems
A.
1. Defined: Any spring that has waters over 37º C.
2. Associated with active volcanic regions and deep faults
3. "Mud pots" associated with
4.
B. Geysers
1. Defined:
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
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
2. Subsidence
§ Lowering of ground in elevation due to ground
settling and compaction
§ Significant damage to buildings and infrastructures
§ 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