Groundwater
17
Groundwater opens with a discussion of the importance of underground water as the largest reservoir of
freshwater that is readily available to humans. Following an examination of the distribution and movement of
groundwater, springs and wells are discussed. The chapter closes with investigations of the environmental
problems of groundwater, groundwater contamination, and the geologic work of groundwater.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Briefly discuss the importance of groundwater.
Explain the distribution of underground water including the concept of the water table.
Understand the interaction between groundwater and surface streams.
List and briefly discuss those factors influencing the storage and movement of groundwater.
Understand the basic mathematics involved in measuring the movement of groundwater.
Compare and contrast springs, hot springs, and geysers.
Discuss the basic details of a well and an artesian well.
List and discuss in some detail the major problems associated with groundwater withdrawal.
Give geographic examples in the United States where groundwater withdrawal problems have
occurred.
Briefly discuss groundwater contamination.
Explain briefly the geologic work accomplished by groundwater.
Discuss the main features associated with karst development.
Chapter Outline___________________________________________________________________
I.
II.
Belt of soil moisture – water held by
molecular attraction on soil particles in
the near-surface zone
B. Zone of saturation
1. Formation
a. Water not held as soil moisture
percolates downward
b. Water reaches a zone where all
the open spaces in sediment and
rock are completely filled with
water
c. Water within the pores is called
groundwater
Importance of groundwater
A. Groundwater is water found in the pores
of soil and sediment, plus narrow joints
and fractures in bedrock
B. Largest reservoir of fresh water that is
readily available to humans
C. Geological roles
1. As an erosional agent, dissolving
groundwater produces
a. Sinkholes
b. Caverns
2. An equalizer of streamflow
A.
Distribution of underground water
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CHAPTER 17
Water table – the upper limit of the
zone of saturation
C. Capillary fringe
1. Extends upward from the water table
2. Groundwater is held by surface
tension in tiny passages between
grains of soil or sediment
D. Zone of aeration
1. Area above the water table
2. Includes the capillary fringe and the
belt of soil moisture
3. Water cannot be pumped by wells
2.
III.
IV.
The water table
A. Upper limit of the zone of saturation
B. Variations in the water table
1. Depth is highly variable
a. Varies seasonally and
b. From year to year
2. Shape is usually a subdued replica of
the surface topography
3. Factors that contribute to the
irregular surface of the water table
a. Water tends to “pile up” beneath
high areas
b. Variations in rainfall
c. Variations in permeability from
place to place
C. Interaction between groundwater and
streams
1. A basic link in the hydrologic cycle
2. Three interactions
a. Gaining streams – gain water
from the inflow of groundwater
through the streambed
b. Losing streams – lose water to the
groundwater system by outflow
through the streambed
c. A combination of the first two –
stream gains in some sections and
loses in others
Factors influencing the storage and
movement of groundwater
A. Porosity
1.
Percentage of the total volume of
rock or sediment that consists of pore
spaces
2 Determines how much groundwater
can be stored
3. Variations can be great
B. Permeability, aquitards, and aquifers
1. Permeability – the ability of a
material to transmit a fluid
2. Aquitard – an impermeable layer that
hinders or prevents water movement
(e.g., clay)
3. Aquifer – permeable rock strata or
sediment that transmits groundwater
freely (e.g., sands and gravels)
V.
Movement of Groundwater
A. Exceedingly slow – typical rate of
movement is a few centimeters per day
B. Energy for the movement is provided by
the force of gravity
C. Darcy’s Law – if permeability remains
uniform, the velocity of groundwater
will increase as the slope of the water
table increases
1. Hydraulic gradient – the water table
slope, determined by dividing the
vertical difference between the
recharge and discharge points by the
length of flow between these points
2. Head – the vertical difference
between the recharge and discharge
points
D. Measured directly using
1. Dyes
2. Carbon-14
VI.
Features associated with groundwater
A. Springs
1. Water table intersects Earth’s surface
2. Natural outflow of groundwater
3. Can be caused by an aquitard
creating a localized zone of saturation
and a perched water table
B. Hot springs
Groundwater
1.
Water is 6–9ºC warmer than the
mean annual air temperature of the
locality
2. The water for most is heated by
cooling of igneous rock
C. Geysers
1. Intermittent hot springs
2. Water erupts with great force
3. Occur where extensive underground
chambers exist within hot igneous
rock
4. Groundwater heats, expands, changes
to steam, and erupts
5. Chemical sedimentary rock
accumulates at the surface
a. Siliceous sinter (from dissolved
silica)
b. Travertine (from dissolved
calcium carbonate)
D. Wells
1. To ensure a continuous supply of
water, a well must penetrate below
the water table
2. Pumping can cause
a. Drawdown (lowering) of the
water table and a
b. Cone of depression in the water
table
E. Artesian wells
1. Applied to any situation in which
groundwater under pressure rises
above the level of the aquifer
2. Types of artesian wells
a. Nonflowing – pressure surface is
below ground level
c. Flowing – pressure surface is
above the ground
3. Not all artesian systems are wells,
artesian springs also exist
VII. Problems associated with groundwater
withdrawal
A. Treating groundwater as a nonrenewable
resource
B. In many places the water available to
recharge the aquifer falls significantly
short of the amount being withdrawn
143
1.
Ground sinks when water is pumped
from wells faster than natural
recharge processes can replace it
2. e.g., San Joaquin Valley of California
C. Saltwater contamination
1. Excessive groundwater withdrawal
causes saltwater to be drawn into
wells, thus contaminating the
freshwater supply
2. Primarily a problem in coastal areas
VIII. Groundwater contamination
A. One common source is sewage
1. Extremely permeable aquifers, such
as coarse gravel, have such large
openings that groundwater may travel
long distances without being cleaned
2. Sewage often becomes purified as it
passes through a few dozen meters of
an aquifer composed of sand or
permeable sandstone
B. Sinking a well can lead to groundwater
pollution problems
C. Other sources and types of
contamination include
1. Highway salt
2. Fertilizers
3. Pesticides
4. Chemical and industrial materials
leaking from
a. Storage tanks
b. Landfills
c. Holding ponds
IX.
Geologic work of groundwater
A. Groundwater dissolves rock
1. Groundwater is often mildly acidic
a. Contains weak carbonic acid
b. Forms when rainwater dissolves
carbon dioxide from the air and
from decaying plants
2. Carbonic acid reacts with calcite in
limestone to form calcium
bicarbonate, a soluble material
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B.
Caverns
Most are created by acidic
groundwater dissolving soluble rock
at or just below the surface in the
zone of saturation
2. Features found within caverns
a. Form in the zone of aeration
b. Composed of dripstone
(travertine)
1. Calcite deposited as dripping
water evaporates
2. Features, collectively called
speleothems, include
a. Stalactites hanging from
the ceiling, and
b. Stalagmites, which form
on the floor of a cavern
and reach upward
C. Karst topography
1. Landscapes that to a large extent have
been shaped by the dissolving power
of groundwater
1
2.
Common features
a. Irregular terrain
b. Sinkholes, or sinks
1. Surface depressions
2. Formed by
a. Slowly dissolving bedrock
as the groundwater moves
downward
b. Sudden cavern collapse
c. Striking lack of surface drainage
(streams)
3. Tower karst
a. Southern China
b. Region of steep sided hills
c. Forms in tropical and subtropical
regions with thick beds of highly
jointed limestone
d. Large volumes of limestone have
been dissolved leaving only
residual towers
Answers to the Review Questions
1. According to Table 7.1, groundwater comprises about 14 percent of all freshwater. This quantity
significantly exceeds water contained in rivers, lakes, unsaturated soils, and the atmosphere. In as much
as water stored in glaciers and ice caps accounts for 85 percent of all freshwater, groundwater comprises
about 94 percent of all liquid freshwater.
2. Groundwater inflow sustains flow in perennial streams and accounts for most, if not all stream discharge
during extended time intervals between precipitation events. Thus groundwater contributes to the
geological work of streams.
3. The aeration and saturation zones are defined by the status of their pore space. In the saturated zone,
water completely fills all pore space over an indefinitely long period of time; this water constitutes the
groundwater. In the aerated zone, pores are normally filled or partly filled with air (aerated) and soil
gases. Temporary saturation may occur in the aerated zone following heavy rains or snowmelt. For
unconfined groundwater conditions, the water table marks the upper, boundary surface of the saturated
zone. Water in the aerated zone is commonly referred to as soil moisture to differentiate it from
groundwater in the saturated zone.
4. The water table (the upper boundary of the saturated zone) is a two dimensional feature (surface) but it is
rarely flat. For unconfined aquifer conditions in humid areas, the water table mimics the surface
topography. In dry lands, the water table domes upward beneath an influent stream.
Groundwater
145
Relative highs in the water table indicate recharge, and lows associated with effluent streams and
pumping wells (cones of depression) indicate that water is being discharged from the groundwater
system.
5. Meteorological drought deals with the degree of dryness based on the departure of precipitation from
normal values and the duration of the dry period. Hydrological drought refers to deficiencies in surface
and subsurface water supplies. It is measured as streamflow and as lake, reservoir, and groundwater
levels. Therefore, precipitation values may return to normal values, but streamflow and levels in lakes,
reservoirs, and groundwater may still be below normal, signifying a hydrological drought still exists.
6. Gaining streams are those streams that gain water from the inflow of groundwater through the streambed.
This situation occurs when the elevation of the water table is higher than the level of the surface of the
stream. A losing stream is the opposite situation where a stream loses water to the groundwater by
outflow of water through the streambed. This results from the elevation of the water table being lower
than the level of the stream surface.
7. Both describe important hydraulic characteristics of soil and rock. Porosity is defined as the volume
percentage of open space (voids, pores, cracks, etc.) in a given volume of soil or rock. Highly porous
materials can hold abundant water when saturated; low porosity materials can hold only small amounts of
water. Permeability refers to how easily water will flow from opening to opening through a porous
material. To be permeable, a porous material must have openings and cracks (pore spaces) that connect
with one another and are large enough for water to flow freely between pores.
8. Both terms describe bedrock or unconsolidated deposits in terms of their hydraulic properties. An
aquitard is composed of impermeable material (water will not flow through it); thus an aquitard (an
impermeable stratum or layer) can stop water percolating downward from the surface or prevent water
from moving upward or downward from a saturated zone (an aquifer or aquifers). An aquifer is a general
term to describe any saturated, water-bearing, subsurface, geologic stratum or deposit of porous,
permeable bedrock or unconsolidated material.
9. If the pore spaces and interpore connections are very small, the material will have a low permeability
despite having a high porosity. A water-saturated, mud layer would be a good example. It has a
substantial water content (porosity) but the pores and connections are very small; thus water moves with
great difficulty and the mud has a very low permeability.
10. Figure 17.5 shows the flow paths (streamlines) in an isotropic, unconfined aquifer. The water always
moves toward regions of lower pressure (the downslope direction of the water table), and the slope and
orientation of a tangent line to any point on a streamline indicates the direction and magnitude of the
pressure gradient (hydraulic gradient) force pushing the water through the saturated media. The average
magnitude of the hydraulic gradient is found by dividing the elevation difference between the initial
(recharge) and final (discharge) points by the path length of the streamline. Note that recharge points are
on the water table at elevations above the common elevations of the discharge points (the surface of the
effluent stream). Although the local, upward flow of groundwater beneath the effluent stream might at
first glance appear to defy the law of gravity, the water is being pushed “uphill” by the weight of water
laterally above it along the same streamline.
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Such curved, “looping” streamlines and orthogonal, equipotential lines are forms of solutions to potentialflow problems, groundwater being but one example. In such cases, matter or energy moving through
some physical media are driven by potential-field gradients (forces) and scaled by a media property
(permeability in the case of porous media flow). Thus Darcy’s Law is formulated as V = K(h/l) (V is
velocity, a vector; K is the permeability, a property of the porous media; and h/l is the hydraulic gradient
force, a vector).
11. Henry Darcy was a nineteenth century French engineer and hydrologist who, in 1856, formulated the
basic equation describing groundwater flow on the basis of his theoretical and observational studies of
groundwater in the area around Dijon, France. This equation, V = K(h/l), is now known as Darcy’s Law.
12. This situation results in a perched water table. Water seeping downward from the surface is stopped at the
top of the aquitard and accumulates, forming a gently sloping, mound-shaped, local, saturated zone in an
aquifer above the aquitard. This saturated zone has its own water table “perched” above the elevation of
the regional water table.
13. Most geothermal waters are heated by geologically young, hot, igneous bodies at depth; thus they are
concentrated in areas of active or recent volcanism in the western states. Warm springs also occur in nonvolcanic areas, such as those in the Appalachian Mountains. In these situations, the groundwater
circulates deep below the surface and is heated by the warmer rocks at depth; being less dense than cold
water, it then rises back to the surface as a warm spring.
14. This situation could arise for many different reasons. First, a perched water table may be intersected by
one well and not the other. In other areas, the natural slope of the water table or a cone of depression from
another well could be involved. In karst areas, solution cavities, collapse breccias, or other highly porous
zones may alternate locally with relatively impermeable, non-porous bedrock, resulting in a prolific well
in one location and a dry hole nearby. In areas of complex bedrock or regolith geology, neighboring wells
drilled to the same depths may penetrate units with greatly differing porosities and permeabilities. In areas
underlain by massive, non-porous bedrock such as granite and gneiss, a single, fortuitous fracture
intersection may make the difference between a productive well and a dry hole.
15. Under unconfined conditions, the water in a well rises to the exact level of the local water table. In
artesian aquifers, the groundwater is confined and under pressure. In a well drilled into such an aquifer,
the water will rise above the elevation of the top of the saturated zone, and the excess pressure may be
high enough for the well to flow freely at the surface (no pumping). An artesian aquifer must be sealed by
an overlying aquitard and saturated laterally to elevations above the aquifer-aquitard boundary where the
well penetrates into the aquifer. Lateral saturation at higher elevations and confined hydraulic conditions
are necessary to generate the excess pressure. This typically involves inclined strata such as porous and
permeable sandstone with shale aquitards above and below.
16. Artesian aquifers are typically inclined, distinctive strata or lithologic units. First, they must be bounded
above and below by impervious strata. Second, the aquifer must be saturated below its unconfined water
table in the recharge area, typically along a mountain front. At any point in an artesian aquifer, the water
is under a pressure generated by the weight of the water in the overlying, saturated part
Groundwater
147
of the aquifer. If a well penetrates the aquifer, the water rises to the elevation of the pressure surface, but
the well will flow freely (without pumping) only if the elevation of the pressure surface exceeds the
elevation of the well head.
17. Early wells drilled into the Dakota Sandstone (late 1800s) were strongly pressurized and some were
“gushers”. After over a century of continuous withdrawals, aquifer pressures have substantially declined
and many wells that once flowed freely now require pumping.
The Dakota Sandstone (Cretaceous) is a very important source of water in western and central South
Dakota. Recharge begins in the Black Hills. Water from streams and snowmelt infiltrates an inclined,
highly porous, Mississippian limestone unit stratigraphically below the Dakotas. East of the Black Hills,
the tilted strata flattens beneath the western plains. In central South Dakota, the aquitard between the two
units is breached and water is recharged upward into the Dakota aquifer and spreads laterally beneath the
central and western parts of the state.
18. The area is fairly dry and there is little natural recharge to the aquifer. Thus continued pumping depletes
the groundwater and causes the water table to drop. In some areas, the water table in the Ogallala aquifer
has declined over 200 feet since large-scale pumping for agricultural irrigation was started.
19. The aquifer here is composed of unconsolidated sands and silts that shrink or compact when dewatered
(when they change from a water-saturated to an unsaturated condition). Compaction is accomplished by
permanent closing of some of the original pore space in the aquifer; thus the land surface subsides.
20. Freshwater floats on the denser, salty water. The general rule of thumb is that the freshwater extends
downward a distance below sea level equal to 40 times the distance that the water table is above sea level.
Thus the freshwater lens extends to a depth of 160 meters below sea level and 164 meters below the water
table. This analysis assumes that a reasonably permeable unconfined aquifer extends indefinitely
downward from the water table.
21. Urbanization is accompanied by pavements, roofs, storm sewers, concrete-lined stream channels, and
other impermeable ground coverings that intensify runoff and prevent water from infiltrating into the
subsurface soil and bedrock. Therefore, natural recharge in an urban area is greatly reduced.
22. The sand aquifer would be most effective. The water would move more slowly, and the pollutants would
be more likely to contact grain surfaces where they could be adsorbed or chemically degraded.
23. Toxic, flammable, explosive, and corrosive substances are classified as hazardous. These would include
pesticides, gasoline, jet fuel, and chemicals such as sulfuric acid and benzene.
24. Two common speleothems (dripstone features) are stalactites and stalagmites. Both are composed of
calcium carbonate precipitated from water dripping from the roofs of caverns. Stalactites grow (hang)
down from the ceiling; they are slender and pointed like icicles. Stalagmites grow up from the floor; they
are stout with blunt tips and rippled surfaces.
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CHAPTER 17
Speleothems grow only when the cavern is aerated and above the water table. Water dripping from the
cave roof is moving downward through the unsaturated zone; obviously if the cave roof were below the
water table, the cave would be filled with water!
25. Karst topography. The term was coined in reference to the distinctive landforms developed on limestone
bedrock in Slovenia, a small country that was once a province in the northeastern part of the former
Yugoslavia.
26. Sinkholes develop only in areas underlain by soluble bedrock such as limestone, anhydrite, and gypsum.
When a cavern suddenly collapses, a circular to elliptical, closed depression forms as the rocks and soil
above the cavern subside. Also, sinkholes may slowly subside and enlarge as intersecting vertical
fractures are gradually widened and enlarged into a pipe-like channel by solution and removal of the
soluble bedrock.
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