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Transcript
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 GEOL g406 Environmental Geology S. Hughes, 2003 Soils and the Environment 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. GEOL g406 Environmental Geology S. Hughes, 2003 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. Unsurprisingly, variations in climate, parent material, type of weathering and amount of time produce distinct soils that express these variations. 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. GEOL g406 Environmental Geology S. Hughes, 2003 Soil Horizons Materials 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) Soil Rock GEOL g406 Environmental Geology Zone of accumulation (B) Weathered rock (C and R) S. Hughes, 2003 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. ~3m 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. S. Hughes, 2003 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. GEOL g406 Environmental Geology S. Hughes, 2003 Soil Taxonomy Entisols - soils with little or no morphological development Vertisols - clayey soils with high shrink/swell capacity Inceptisols - soils with weakly developed subsurface horizons Aridisols - CaCO3-containing soils of arid environments with moderate to strong development Mollisols - grassland soils with high base status Andisols - soils formed in volcanic ash Spodosols - acid soils with a subsurface accumulation of metal-humus complexes Alfisols - soils with a subsurface zone of silicate clay accumulation and >35% base saturation Ultisols - soils with a subsurface zone of silicate clay accumulation and <35% base saturation Oxisols - intensely weathered soils, tropical and subtropical Histosols - organic soils (peak, bog, muck) Gelisols - soils with permafrost within 2 m of the surface S. Hughes, 2003 Soil Texture Texture = relative proportion of sand, silt and clay. Texture classes: Coarse sands, loamy sand and sandy loams with less than 18 % clay, and more than 65 % sand. Medium sandy 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. Fine clays, silty clays, sandy clays, clay loams and silty clay loams with more than 35 % clay. GEOL g406 Environmental Geology S. Hughes, 2003 See Figure 3.2 in textbook 100 % CLAY Clay Clay (%) Silt (%) Clay loam Sandy loam Loam Silt loam Sand Silt 100 % SAND 100 % SILT Sand (%) S. Hughes, 2003 Soil Classification Soils are often referred to as being sandy or clayey, or sometimes 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 S. Hughes, 2003 Soil Classification WELL SORTED GEOL g406 Environmental Geology WELL GRADED S. Hughes, 2003 Gravels Sands >50 % larger than 0.074 mm GW = well-graded gravel Clean (<5 % fines) GP = poorly graded gravel Dirty GM = silty gravel (>12 % fines) GC = clayey gravel SW = well-graded sand Clean (<5 % fines) SP = poorly graded sand Dirty SM = silty sand (>12 % fines) SC = clayey sand Silts ML = silt Non-plastic MH = micaceous silt OL = organic silt >50 % smaller CL = silty clay than 0.074 mm CH = high plastic clay Plastic OH = organic clay PT = peat and muck Mostly Organics Clays FINE-GRAINED COARSE-GRAINED Unified Soil Classification System 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 and so on. • Subsurface water = groundwater, connate water, soil, capillary water. • Groundwater exists in the zone of saturation, and may be fresh or saline. GEOL g406 Environmental Geology S. Hughes, 2003 S. Hughes, 2003 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 unable to support the structures built on it when saturated with water. Be sure to read the sections of your text describing the engineering properties of soil. GEOL g406 Environmental Geology S. Hughes, 2003 Adhesion and Cohesion GEOL g406 Environmental Geology S. Hughes, 2003 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 S. Hughes, 2003 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 GEOL g406 Environmental Geology S. Hughes, 2003 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 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. GEOL g406 Environmental Geology S. Hughes, 2003 Water in Soils FOREST Precipitation Evapotranspiration Interception Soil Water infiltrates and runs through soil Rock GEOL g406 Environmental Geology S. Hughes, 2003 Water in Soils CLEARCUT Precipitation Soil compaction Little Interception and Evapotranspiration Increased surface runoff and sediment; weaker soil Rock GEOL g406 Environmental Geology More sediment in channel S. Hughes, 2003 Water in Soils Farming Precipitation Less Interception and Evapotranspiration Soil Increased surface runoff and soil erosion from freshly plowed land Rock GEOL g406 Environmental Geology Increased sediment in channel S. Hughes, 2003 Water in Soils URBANIZATION Precipitation Soil Large increase in runoff from urban surfaces and storm sewers Rock GEOL g406 Environmental Geology S. Hughes, 2003 Effect of Land Use on Sediment Yield, eastern U.S. Piedmont Region. GEOL g406 Environmental Geology S. Hughes, 2003 Soils and the Environment Key Terms to Review: • weathering • soil horizons • soil profile development • soil chronosequence • soil fertility • unified soil classification • soil strength • soil sensitivity • liquefaction GEOL g406 Environmental Geology • compressibility • erodibility • permeability • corrosion potential • shrink-swell potential • expansive soils • soil pollution • desertification • water table • soil plasticity index S. Hughes, 2003