GEOL 446 Environmental Geology
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Transcript of GEOL 446 Environmental Geology
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GEOL 446Environmental Geology
PART IFOUNDATIONS OF ENVIRONMENTAL
GROLOGY
Chapter 1: philosophy and Fundamental Concepts
Chapter 2: Earth Material and Processes
Chapter 3Soils and Environment
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3. Soils and the Environment
Soil is one of the most important earth materials we encounter each day, but the definition of soil is difficult.
Soil Scientists (and most ordinary people):• fine-grained, well-weathered earth material that is able to
support plant growth• focus on the physical and chemical properties
Engineers:• any earth material that can be removed without blasting• focus on particle size and the amount of organic material• engineering applications
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Environmental Geologists:• must understand soil from many perspectives• characteristics affect agriculture, engineering,
hydrology, natural hazards and other aspects of land use• soil development and soil character is crucial to good
land use planning.
Read Table 3.1 (Soil Taxonomy)•
Understand the meaning of soil types, but do not memorize all of them.
Read Table 3.2 (Unified Soil Classification)• Learn the definition of each constituent that makes up soil.
3. Soils and the Environment
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Soil Development
Soil is an important part of the geologic cycle and soil characteristics are influenced by parent material, climate, topography, weathering, and the amount of time a particular soil has had to develop.
As soil develops, weathering creates distinct layers in soil. We call these layers soil horizons, and each soil horizon has distinctive characteristics. Every soil has a soil profile, a list of the horizons that describe a
particular soil .
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Soil HorizonsMaterials in a Soil System:
Vertical and horizontal movements create a soil profile made up of distinct layers parallel to the surface, which are called soil horizons.
Organic top layer (O)Zone of leaching (A and E)
Zone of accumulation (B)
Rock (weathered & unweathered) (C and R)
Soil
Rock
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Soil Horizons
O Mostly organic materials, decomposing leaves, and twigs. Often dark brown color.
A Mineral and organic materials, light black to brown. Leaching of clay, Fe and Ca.
E Light colored materials due to leaching of clay, Ca, Mg, and Fe to lower horizons. Horizons
A and E make up the Zone of Leaching.
B Enriched in clay, Fe oxides, Silica, carbonate and other material leached from above. This is
the Zone of Accumulation.
C Partially altered (weathered) parent material, which is either rock or loose sediment.
R Unweathered (unaltered) parent material = rock.
~3m
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Soil Development• A soil’s profile depends on its age and its conditions of
formation. Soil profile is the primary criteria for soil classification.
• Soils can be compared in terms of their relative development. Weakly developed soil profiles are generally younger and may have fewer horizons; well-developed soils are generally older and have more horizons.
Chronosequences
• Relative development of a series of soils allows their arrangement in a soil chronosequence. A soil chronosequence gives information about the history of the landscape. The relative development of the soils in a chronosequence tells the investigator about the climate and depositional history of the area.
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Soil Texture
Texture = relative proportion of sand, silt and clay.
Texture classes:
Coarsesands, loamy sand and sandy loams with less than 18 % clay, and
more than 65 % sand.
Mediumsandy loams, loams, sandy clay loams, silt loams with less than 35
% clay and less than 65 % sand; the sand fractions may be as high as 82 % if a minimum of 18 % clay is present.
Fineclays, silty clays, sandy clays, clay loams and silty clay loams with
more than 35 % clay.
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Clay(%) Silt(%)
Sand(%)
100 % CLAY
100 % SILT
100 % SAND
Clay
Sand Silt
Clay loam
LoamSandy loam Silt loam
See Figure 3.2in textbook
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Soil ClassificationSoils are often referred to as being sandy or clayey, or silty.
Different countries use different standards to define sand particle and silt particle sizes.
Particle sizes
Gravel, Cobbles, and Boulders particles greater than 2 mm diameter
Coarse and medium sand particles from 2 mm to 0.2 mm diameter
Fine and very fine sand particles from 0.2 mm to 0.074 mm diameter
Silt particles from 0.074 mm to 0.004 mm diameter
Clay particles less than 0.004 mm diameter
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Soil Classification
WELL SORTED WELL GRADED
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Unified Soil Classification SystemF
INE
-GR
AIN
ED
CO
AR
SE
-GR
AIN
ED
>50 % largerthan 0.074 mm
>50 % smallerthan 0.074 mm
Cla
ys
S
ilts
S
and
s
G
rave
ls GW = well-graded gravelGP = poorly graded gravelGM = silty gravelGC = clayey gravelSW = well-graded sandSP = poorly graded sandSM = silty sandSC = clayey sandML = siltMH = micaceous siltOL = organic siltCL = silty clayCH = high plastic clayOH = organic clayPT = peat and muckMostly Organics
Clean<)5 % fines(
Dirty>)12 % fines(
Clean<)5 % fines(
Dirty>)12 % fines(
Non-plastic
Plastic
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Water in Soils
Types of water:
Water on Earth is known by different terms, depending on where it is and where it came from.
• Meteoric water = water in circulation.
• Connate water = "fossil" water, often saline.
• Juvenile water = water from the interior of the earth.
• Surface water = water in rivers, lakes, oceans, etc
• Subsurface water = groundwater, connate water, soil, capillary water.
• Groundwater in the zone of saturation, may be fresh or saline.
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Water in Soils
Moisture Content of soil is calculated as follows:
W = weight, so that:
[(Wwet - Wdry)/Wdry] x 100 = H2O content (%)
Moisture content affects the engineering properties and stability of soils. A soil that is stable in dry conditions may become unstable to support the structures built on it when saturated with water.
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Engineering Properties of Soils
Plasticity• related to the water content
Liquid Limit (LL)
• water content above which the soil behaves like a liquid
Plastic Limit (PL)
• water content below which the soil is no longer plastic
Plasticity Index (PI), PI = LL - PL
• range of water contents that make the soil behave as a
plastic material
• Low PI (5 %): small change in water content, soil changes from solid to liquid
• High PI (< 35%): potential to expand and contract on wetting and drying
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Engineering Properties of Soils
Expansive Soils• high content of swelling clay (montmorillonite)• soils swell when water is incorporated between clay plates• shrinking occurs when soil is dried• damage to building and road foundations
Study Table 3.3 in textbook to understand more about soil descriptions and their significant properties.
• Study the Universal Soil Loss Equation (erosion) :
A = RKLSCP
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Universal Soil Loss Equation
• A = RKLSCP
A = long-term average annual soil loss for the site
R = long-term rainfall runoff erosion factor
K = soil erodibility index
L = hillslope/length factor
S = hillslope/gradient factor
C = soil cover factor (crop/vegetation and mgmt. factor)
P = erosion-control practice factor
• Used to predict the impact of sediment loss on local streams and other resources and to develop management strategies for minimizing impact.
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Water in Soils
FOREST Precipitation
Interception
Evapotranspiration
Soil
Rock
Water infiltrates andruns through soil
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Water in Soils
Farming Precipitation
Less Interceptionand EvapotranspirationSoil
Rock
Increased surface runoffand soil erosion fromfreshly plowed land
Increased sedimentin channel
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Water in Soils
CLEARCUT Precipitation
Little Interceptionand Evapotranspiration
Soil compaction
Rock
Increased surface runoffand sediment; weaker soil
More sedimentin channel
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Water in Soils
URBANIZATION Precipitation
Soil
Rock
Large increase in runofffrom urban surfaces andstorm sewers
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Key Terms to Review:
• weathering• soil horizons• soil profile development• soil chronosequence• soil fertility• unified soil classification• soil strength• soil sensitivity• liquefaction
• compressibility• erodibility• permeability• corrosion potential• shrink-swell potential• expansive soils• soil pollution• desertification• water table• soil plasticity index