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Chp 16: Ground Water

GROUND WATER Definition= water that fills open spaces (pores=porosity) in rocks, soil, sediments beneath the Earth’s surface -soil, rocks, sediments act as filters removing impurities from water Terms: porosity - pore space between grains

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Chp 16: Ground Water

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  1. GROUND WATER Definition= water that fills open spaces (pores=porosity) in rocks, soil, sediments beneath the Earth’s surface -soil, rocks, sediments act as filters removing impurities from water Terms: porosity- pore space between grains permeability- measure of how connected these pores are with one another both are a function of grain size, grain shape, and any cements examples: detrital or clastic rocks have very good porosity and permeability limestones, igneous and metamorphic rocks have poorer porosity and permeability porosity can be enhanced by weathering-particularly in carbonates When first deposited, shales and siltstones have higher porosity than sandstones. Upon compaction shales and siltstones lose porosity faster…. Permeability: sandstones have greater permeability than other lithologies Test: pour water on sandstone, shale and siltstone-watch result…. Definition: aquifer=a permeable layer transporting groundwater The best aquifers are well sorted, well rounded sands and gravels Chp 16: Ground Water

  2. Porosity (pore space) is function of size, shape and arrangement of grains. • Well sorted • sedimentary rock • has good porosity • b. Poorly sorted • rock has low porosity • c. In soluble rocks, like • limestone, porosity can • be enhanced by • dissolution. • d. Fracturing in • Igneous and meta- • morphic rocks can • make them porous.

  3. Permeability= measure of how connected pores are…sands are more permeable than shales.

  4. Water Table: surface separating zone of aeration from zone of saturation a. zone of aeration: pore spaces contain air, water percolates down through this zone b. zone of saturation: pore spaces filled with water water table surface tends to mimic topography Gravity provides energy for downward movement of ground water. Rate of movement in zone of aeration is very slow. Spring: place where groundwater flows or seeps out of the ground. Usually flows along an impermeable layer (shale!) to location where meets surface. Perched water table: local trap is present where water trapped in zone of aeration; e.g. shale lens in sandstone Chp 16: Ground Water

  5. Subsurface divided into 2 zones: 1. aeration (water and air) 2. saturation (water only)

  6. Groundwater moves down through the zone of aeration to zone ofsaturation-top of which is known as water table. Some groundwatercan become trapped above water table if encounters impermeable shale.

  7. Springs form whenever laterally moving groundwater encountersEarth’s surface: 1. groundwater percolates laterally atop localimpermeable barrier until comes to surface.

  8. Water Wells: usually the result of drilling down to zone of saturation. Some are free flowing, most need pumps to bring water to surface. When groundwater is pumped from a well, water table in vicinity of well is lowered; this is called a cone of depression… Cone of depression forms because water withdrawal is faster than natural rate of replenishment Artesian Systems: system where groundwater is confined and builds up high hydrostatic (fluid) pressure. This requires: -aquifer must be bounded by non-porous layer (aquiclude) above and below. -rocks usually tilted and exposed at surface. -there is sufficient precipitation in recharge area to keep aquifer filled Chp 16: Ground Water

  9. Windmills used to pump water to surface from wells.

  10. Electric pumps more commonly used today to pump water.

  11. Over time, as water is withdrawn from a well the watertable changes locally around the well-it is depressed. Nearby wells can go dry if the deeper well removes water faster than itcan be replenished naturally.

  12. Artesian Systemmust have a porous rock confined above and below by less porous rocks; and it must be exposed at surface-usually in mountains- wherelots of rainfall can replenish water supply to porous layer. Recharge Area- wherewater is supplied to aquifer. Note the water table dips from the recharge area to the basin. Free-flowing wells are those above the projected water table.

  13. Does Groundwater erode and deposit material? -Groundwater reacts with rocks and minerals it comes in contact with-weathers them chemically, especially true with carbonates. -Carbonates can erode if carbonic acid (H2CO3) is present in groundwater. Most groundwater is slightly acidic because of reactions with CO2 in air and organic matter in soils, so….. -Groundwater dissolves carbonates, forming calcium bicarbonate, which is carried away in solution in groundwater. Effects of Carbonate Dissolution: Sinkholes and Karst Topography: a. Sinkholes: depressions where underlying soluble rock dissolves, leading to collapse of the surface These form in one of two ways- 1. solution valleys- merging of adjacent sinkholes 2. collapse of cave roof Karst topography: groundwater erosion is the cause…. Karst- surface characterized by numerous caves, springs, sinkholes, solution valleys, or disappearingstreams. Common theme here is that thick, soluble rocks occur just beneath the soil-usually carbonates. Chp 16: Ground Water

  14. Distribution of major limestone and karst areas of world:karst= irregular topography caused by groundwater erosion of soluble rocks (i.e. limestone, etc)

  15. Sinkhole formed in previously dissolved limestone as water table dropped.

  16. Water filled sinkhole-water enters from spring.

  17. Common features associated with Karst topography

  18. Stone Forest-125km south of Kunming, China. Formed by dissolution of carbonate rocks.

  19. Solution valleys, sinkholes and sinkhole lakes in Kentucky

  20. Chp 16: Groundwater Caves and Cave Deposits Caves form as groundwater moves through or along fractures and openings that are connected in subsurface. Cavern= a very large cave or system of connected caves Groundwater dissolves carbonates along fractures and bedding planes, eventually forming caves. 1.stalactite: icicle shaped structures hanging from ceiling 2. stalagmite: upward growing features from cave floor 3. column: when stalactite and stalagmite meet…

  21. Stalactites- icicle shaped features hanging from cave ceiling. Stalagmites- upward pointing features on cave floor. Columns form when they meet….

  22. Formation of Caves:A. as groundwater percolates thruzone of aeration and flows thruzone of saturation, it dissolves carbonate rocks, forms system ofpassageways.B. Groundwater moves alongwater table, carrying dissolvedmaterial, which enters surface streams thru springs.C. as surface streams erode deeper,the water table drops and exposes the passageways as caves.

  23. Geographic extent of the High Plains Aquifer and changes in water level frompre-development thru1993.

  24. Human impact upon Groundwater Systems: Use of groundwater for irrigation causes…. 1. lowering of water table 2. loss of pressure, causing free flowing wells to require pumping 3. saltwater incursion in coastal areas 4. subsidence: unconsolidated sediments compact as water removed 5. contamination: from sewage, landfills, toxic waste disposal sites, agriculture. Containment is very difficult once contaminated…..

  25. Saltwater incursion into water table:a. freshwater is less dense than saltwater, so it forms a lens shaped wedgeabove the salt water.B. If excessive pumping occurs, acone of depression can occur that is deep enough to allow the salty groundwater to flow into the well.C. Pumping water back into the fresh groundwater can restore the water tableand lower the interface between thefresh and salt groundwater.

  26. Excessive withdrawal of Ground Water beneath Mexico City-buildings tilt… Leaning Tower of Pisa is partly due to groundwater removal

  27. Withdrawal of petroleum from oilfield beneath Long Beach, CAresulted in up to 9m of ground subsidence because sediment compacted…

  28. Contamination of the water table:a. usually oxidation, bacterial degradation occurwithin aerated zone, BUTif septic system too close to water table, contamination can occur.B. Landfill: unless there isan impermeable layer between the base of landfill and water table, pollutants can percolate down into saturate zone and contaminate water table.

  29. Hydrothermal Activity Hydrothermal-meaning ‘hot water’ 1.Hot Springs: any spring in which water temperature is greater than 37 deg C 2.Mud Pot: chemically altered rocks yield clays that bubble as hot water and steam rise through them Heat comes from magma or cooling igneous rocks… 3. Geysers: hot springs that intermittently eject hot water and steam with tremendous force. -these are surface expression of an extensive underground system of interconnected fractures within hot igneous rocks. a. Water at bottom of fracture system is heated to very near boiling point. b. Eventually heated to where forms steam, blows out of the fracture system at the surface. -These commonly contain high percentages of dissolved minerals because minerals dissolve faster in very hot water. -When the mineral-rich hot water cools at the surface, the minerals in solution precipitate (i.e. they are deposited) as travertine or calcareous tufa. Geothermal Energy Energy produced from Earth’s internal heat….. -hot water and steam are used as energy: a well is drilled into fractured areas, steam is piped to electricity plants where it is converted to energy….

  30. Hydrothermal activity: Hot spring in Yellowstone Nat’l Park.

  31. Hydrothermal activity:water in this Hot spring at Bumpass Hell in Lassen Volcanic Park in CA is actually boiling naturally….

  32. Mud pot at Sulphur Works in Lassen Nat’l Park:

  33. Bathhouses in Bath, England:built by Romans over hot springs, after they conqueredEngland in AD 43

  34. Geysers in Yellowstone Nat’l Park: small geyser

  35. Eruption of a geyser:a.groundwater percolates down throughopenings in rocks, is heated by igneousrocks.B. When water is heated above boilingpoint, or pressure decreases, water changes to steam, which pushes water above it up and out of ground as geyser!

  36. Hot springs deposits in Yellowstone Nat’l Park: Minerva Terraceformed when calcium carbonate rich hot spring water cooled, precipitated travertine.

  37. Liberty Cap- geyseritemound formed by repeated geyser eruptions of a silicon-dioxide rich spring water

  38. Definition= water that fills open spaces (pores=porosity) in rocks, soil, sediments beneath the Earth’s surface -soil, rocks, sediments act as filters removing impurities from water porosity- pore space between grains permeability- measure of how connected these pores are with one another both are a function of grain size, grain shape, and any cements examples: detrital or clastic rocks have very good porosity and permeability limestones, igneous and metamorphic rocks have poorer porosity Definition: aquifer=a permeable layer transporting groundwater The best aquifers are well sorted, well rounded sands and gravels Water Table: surface separating zone of aeration from zone of saturation a. zone of aeration: pore spaces contain air, water percolates down through this zone b. zone of saturation: pore spaces filled with water water table surface tends to mimic topography Gravity provides energy for downward movement of ground water. Rate of movement in zone of aeration is very slow. Chp 16: Groundwater Summary

  39. Chp 16: Groundwater Summary Spring: place where groundwater flows or seeps out of the ground. Usually flows along an impermeable layer (shale!) to location where meets surface. Perched water table: local trap is present where water trapped in zone of aeration; e.g. shale lens in sandstone Water Wells: usually the result of drilling down to zone of saturation. Some are free flowing, most need pumps to bring water to surface. When groundwater is pumped from a well, water table in vicinity of well is lowered; this is called a cone of depression… Cone of depression forms because water withdrawal is faster than natural rate of replenishment Artesian Systems: system where groundwater is confined and builds up high hydrostatic (fluid) pressure. This requires: -aquifer must be bounded by non-porous layer (aquiclude) above and below. -rocks usually tilted and exposed at surface. -there is sufficient precipitation in recharge area to keep aquifer filled

  40. Chp 16: Groundwater Summary Does Groundwater erode and deposit material? -Groundwater reacts with rocks and minerals it comes in contact with-weathers them chemically, especially true with carbonates. -Carbonates can erode if carbonic acid (H2CO3) is present in groundwater. Most groundwater is slightly acidic because of reactions with CO2 in air and organic matter in soils, so….. -Groundwater dissolves carbonates, forming calcium bicarbonate, which is carried away in solution in groundwater. Effects of Carbonate Dissolution: Sinkholes and Karst Topography: a. Sinkholes: depressions where underlying soluble rock dissolves, leading to collapse of the surface These form in one of two ways- 1. solution valleys- merging of adjacent sinkholes 2. collapse of cave roof Karst topography: groundwater erosion is the cause…. Karst- surface characterized by numerous caves, springs, sinkholes, solution valleys, or disappearing streams. Common theme here is that thick, soluble rocks occur just beneath the soil-usually carbonates.

  41. Caves and Cave Deposits Caves form as groundwater moves through or along fractures and openings that are connected in subsurface. Cavern= a very large cave or system of connected caves Groundwater dissolves carbonates along fractures and bedding planes, eventually forming caves. 1.stalactite: icicle shaped structures hanging from ceiling 2. stalagmite: upward growing features from cave floor 3. column: when stalactite and stalagmite meet… Hydrothermal Activity- Hydrothermal-meaning ‘hot water’ 1.Hot Springs: any spring in which water temperature is greater than 37 deg C 2.Mud Pot: chemically altered rocks yield clays that bubble as hot water and steam rise through them Heat comes from magma or cooling igneous rocks… 3. Geysers: hot springs that intermittently eject hot water and steam with tremendous force. -these are surface expression of an extensive underground system of interconnected fractures within hot igneous rocks. Geothermal Energy Energy produced from Earth’s internal heat….. -hot water and steam are used as energy: a well is drilled into fractured areas, steam is piped to electricity plants where it is converted to energy…. Groundwater Summary

  42. Chp 16: Ground Water

  43. Chp 16: Ground Water

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