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Unformatted text preview: 18: Earth’s Resources 1 /
Groundwater & Soil • I n the United States,
groundwater is the source of
most water used for drinking
undwater, Soil: 1 Springs, M arble Canyon, Grand
Canyon National Park, Arizona.
Photo: L. Ulrich I mportance of groundwater
Table E.2 Q1. The majority of liquid, fresh water on Earth exists in ____________.
D. rivers and lakes
undwater, Soil: 2 Groundwater and porosity
• Groundwater resides in open spaces below the ground surface in
either sediment or rock.
• Porosity: total volume of open space
• Porosity includes fractures (cracks) and space between grains
Porosity undwater, Soil: 3 Porosity
Q2.The porosity of poorly
sorted rocks is ___ the
porosity of well-sorted
A. higher than
B. lower than
C. the same as
Q3. A. True / B. False:
Porosity always decreases
as grain size of
Q4. Which rock type has the
undwater, Soil: 4 Fig. 19.2 Porosity Porosity
Porosity as cementation
# of fractures
of undwater, Soil: 5 Fig. 19.2 Porosity and permeability
• Permeability: measure of ease with which
water moves through ground
water Porosity: moderate
I nterconnectedness: none
I nterconnectedness: high
high Fig. 19.4
undwater, Soil: 6 Porosity: none
none Porosity and permeability The sketch above shows three samples of sandstone. The pore spaces (gray)
between the sand grains (black) are completely filled in with cement. The
long, white features are open fractures (cracks).
Q5a. Which rock (A, B, or C) has the greatest porosity?
Q5b. Which rock (A, B, or C) has the greatest permeability? undwater, Soil: 7 Porosity and permeability
• Aquifer : porous, permeable unit that holds and releases water
• Aquitard: nonporous or nonpermeable unit that does not transmit water Fig. 19.5 Aquifer Aquifer
undwater, Soil: 8 Groundwater & Water Table
Fig. 19.6 Water table Q6. A. True / B. False: No water is present in the unsaturated
zone above the water table. undwater, Soil: 9 Groundwater & Water Table
Fig. 19.6 Water table • Unsaturated zone: pore (open) spaces partially filled with water.
• Saturated zone: all pore spaces filled with water.
• W ater table: the top of the saturated zone undwater, Soil: 10
10 Geometry of water table (WT)
• W T height mimics ground-surface height.
Elevation high, WT high Elevation low, WT low Fig. 19.7
• W T intersects ground surface at springs, marshes,
swamps, flowing streams and ___________.
undwater, Soil: 11
11 Geometry of water table (WT)
Elevation of water table changes with amount of precipitation. Wet period Dry period Fig. 19.6
undwater, Soil: 12
12 Controls on groundwater (GW)
fGroundwater flows from regions of high elevation to low elevation and from
regions of high pressure to regions of low pressure
r egions Fig. 19.9 • GW may flow down, sideways & up
down sideways up
undwater, Soil: 13
13 Springs and wells
• Spring: water issuing from
the ground, marking a place
where the water table
intersects the Earth’s surface
intersects Thousand springs,
Snake River Canyon,
I daho. Photo: W.K.
undwater, Soil: 14
14 Springs and wells
• Well: hole drilled
W ell hole
below water table
from which water is
pumped to surface.
pumped • Cone of depression:
depression of the
undwater, Soil: 15
15 Well-extraction problems
• W ater removed from aquifer
may cause particle compaction
--> ________________________ Fig. 19.21
undwater, Soil: 16
16 Well-extraction problems
Fig. 19.21 Leaning tower of Pisa & Venice, I taly
Photos: R.W. Schlische Also: New Orleans
undwater, Soil: 17
17 Well-extraction problems
to move toward
the undwater, Soil: 18
18 Well-extraction problems
• Excessive pumping --> __________________ 1 3 Fig. 19.13
2 undwater, Soil: 19
19 Groundwater activity
• Caves: enlarged solution cavities produced when acidic groundwater dissolves
calcite from regions underlain by limestone and marble
marble Cave deposits in
L uray Caverns,
L uray, VA.
Schlische undwater, Soil: 20
• Caves form
__________ • Cave
undwater, Soil: 21
21 Groundwater activity: cave
deposits Fig. 19.28
• Stalactite: deposit of calcite that forms on the ceiling of a cave as water
evaporates and precipitates calcite.
• Stalagmite: deposit of calcite on the ground of a cave produced by
evaporating groundwater that has dripped down from above.
• Column: produced when a stalactite and stalagmite grow toward each other.
undwater, Soil: 22
22 Groundwater activity
• Sinkhole: surface depression formed by the collapse of the roof of a cave
Sinkhole See Fig. 19.1,
19.29 Sinkhole in Winter Park, Florida. Photo: L. Skoogfors
undwater, Soil: 23
23 Groundwater activity
Plate, p. 682 • Karst topography: r egion
underlain by limestone (or
marble) characterized by
sinkholes and disappearing
undwater, Soil: 24
24 Karst topography
Sinkholes Fig. 19.29 Disappearing stream in
Photo: W.K. Hamblin
undwater, Soil: 25
25 Soil Formation
matter that can
vegetation. Fig. 7.12b undwater, Soil: 26
26 Fig. 7.13a Soil horizons Photo: Dept. of Primary Industries,
V ictoria, Australia undwater, Soil: 27
27 Soil profile: distinctive layers (horizons) within a soil
1. O horizon: top horizon consisting mostly of
decomposing organic matter.
2. A horizon: zone of leaching of soluble material.
3. B horizon: zone of accumulation of soluble material.
4. C horizon: partially weathered bedrock. Soil-forming factors
• Bedrock composition: thicker soil on
________________________ Fig. 7.13
undwater, Soil: 28
28 Soil-forming factors
• Slope: thicker soils on ____________ Fig. 7.13
undwater, Soil: 29
29 Soil-forming factors
• T ime: thicker soil after more weathering Fig. 7.13
undwater, Soil: 30
30 Soil-forming factors
Soils are better developed in ____________
Fig. 7.14d undwater, Soil: 31
31 Soils: M ineral Resources
I n tropical settings, the
minerals are leached
out of the A-horizon,
leaving behind lesssoluble iron-rich
deposits (bauxite) Fig. 7.14c
undwater, Soil: 32
32 Review Questions
18-1. Which statement concerning influences on porosity is least correct?
A. As the degree of cementation decreases, the porosity increases.
B. As the number of fractures increases, the porosity increases.
C. As sorting increases, the porosity decreases.
D. As the packing of particles increases, the porosity decreases.
18-2. A. True / B. False: Unconsolidated sediment typically has greater porosity
than lithified (compacted or cemented) rock which forms from it.
18-3. A. True / B. False: I f the porosity of a rock is high, the permeability must
also be high.
18-4. A. True / B. False: I f the permeability of a rock is high, the porosity must
also be high.
18-5. A. True / B. False: The water table is always a flat surface like a table-top.
18-6. The elevation of the water table in a given area is ____________.
A. is constant year-round
B. may rise during times of drought and sink during rainy periods
C. may rise during rainy periods and sink during droughts
undwater, Soil: 33
33 Review Questions
18-7. Which statement below is true?
A. The height of the water table decreases under hills and increases under
B. Swamps and lakes are regions where the water table lies at Earth’s surface.
C. Groundwater only flows downward.
18-8. Which of the following is not a cave deposit?
A. sinkhole B. column
C. stalagmite D. stalactite
18-9. Which of the following cave deposits only grow down from the roof of a cave?
18-10. Most dissolution of bedrock to form caves takes place ____________.
A. above the water table
B. below the water table
18-11. Most cave deposits form when the cave is located ____________.
A. above the water table
B. below the water table
18-12. Sinkholes are a concern in areas where the bedrock is ____________.
18-13. Topography dominated by sinkholes is termed ____________.
A. valley and ridge
C. horst and graben
undwater, Soil: 34
34 Review Questions
18-14. Which soil horizon is the zone of accumulation, so named because
dissolved matter, leached from other parts of the soil, precipitates to form new
A. A-horizon B. B-horizon
C. C-horizon D . O-horizon
18-15. Which statement below is false?
A. The C-horizon is chemically most similar to the underlying bedrock or
B. The O-horizon has the greatest proportion of organic matter.
C. The shallowest soil horizon is the A-horizon.
18-16. A. True / B. False: Soils are thickest in humid regions and on gentle
18-17. Laterite soils are most commonly found in which type of environments?
A. temperate forests
B. tropical rain forests
D. grasslands undwater, Soil: 35
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- Spring '09