18_groundwater_soil_10_post

18_groundwater_soil_10_post - 18: Earth’s Resources 1 /...

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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 & irrigation 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 ____________. A. clouds B. glaciers C. groundwater C. oceans D. rivers and lakes undwater, Soil: 2 Groundwater and porosity • Groundwater resides in open spaces below the ground surface in either sediment or rock. either • Porosity: total volume of open space Porosity • Porosity includes fractures (cracks) and space between grains Porosity undwater, Soil: 3 Porosity Q2.The porosity of poorly Q2.The sorted rocks is ___ the porosity of well-sorted rocks. rocks. A. higher than B. lower than C. the same as Q3. A. True / B. False: Q3. Porosity always decreases as grain size of sedimentary rocks decreases. decreases. Q4. Which rock type has the Q4. lowest porosity? lowest A. igneous B. metamorphic C. sedimentary undwater, Soil: 4 Fig. 19.2 Porosity Porosity Porosity as cementation packing sorting # of fractures of undwater, Soil: 5 Fig. 19.2 Porosity and permeability • Permeability: measure of ease with which Permeability measure water moves through ground water Porosity: moderate I nterconnectedness: none none Permeability: none none Porosity: high high I nterconnectedness: high high Permeability: high high Fig. 19.4 undwater, Soil: 6 Porosity: none none Permeability: 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 low Groundwater 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 Spring water 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. H amblin undwater, Soil: 14 14 Springs and wells Fig. 19.13 • Well: hole drilled W ell hole below water table from which water is pumped to surface. pumped • Cone of depression: Cone depression of the water table produced by pumping pumping undwater, Soil: 15 15 Well-extraction problems • W ater removed from aquifer may cause particle compaction may --> ________________________. --> ________________________ 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 Fig. 19.21 • Saltwater encroachment: encroachment pumping of freshwater (which lies above heavier saltwater) may cause saltwater to move toward the well 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 Caves enlarged acidic calcite from regions underlain by limestone and marble calcite limestone marble Cave deposits in L uray Caverns, L uray, VA. Photo: R.W. Schlische undwater, Soil: 20 20 Caves • Caves form __________ __________ • Cave deposits form when __________ __________ Fig. 19.30 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 K arst region underlain by limestone (or marble) characterized by sinkholes and disappearing streams streams undwater, Soil: 24 24 Karst topography Sinkholes Fig. 19.29 Disappearing stream in China. Photo: W.K. Hamblin undwater, Soil: 25 25 Soil Formation Soil: loose weathered rock with organic matter that can support 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 most soluble minerals are leached out of the A-horizon, leaving behind lesssoluble iron-rich deposits (laterite) and aluminum-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 valleys. 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? A. column B. stalactite C. stalagmite 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 ____________. A. sandstone B. shale C. limestone D. granite 18-13. Topography dominated by sinkholes is termed ____________. A. valley and ridge B. karst 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 minerals? 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 unaltered sediment. 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 slopes. 18-17. Laterite soils are most commonly found in which type of environments? A. temperate forests B. tropical rain forests C. deserts D. grasslands undwater, Soil: 35 35 ...
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