1. GROUNDWATER Chapter 17

2. The Hydrologic Cycle

3. Where is the Water ? Where is the Water ? Figure 16.2

4. Groundwater is a Resource  The amount of groundwater is vast but not unlimited.  About 0.6% of the world's water found underground.  It provides:  50% of the world's drinking water  40% of the water used for irrigation  25% of industry's needs

5. Nevertheless, in many places overuse and misuse has resulted in:  streamflow depletion  land subsidence  saltwater intrusion  increased pumping costs from ever deeper supplies  contamination

6. What is Groundwater?  Groundwater is water that is found underground in the cracks and spaces in soil, sand, and rocks.  Groundwater is stored in—and moves slowly through—geologic formations called aquifers.

7. Distribution of Groundwater n n Groundwater is typically misunderstood: n n Underground “lakes” and “rivers” are rare n n Most underground water exists in spaces between grains (in “pore spaces”)

8. Distribution of Groundwater  Zone of Aeration – The area above the water table that includes the zones soil moisture and capillary fringe.  Soil Moisture – Groundwater held by molecular attraction as a surface film on soil particles. Used by plants for life functions including transpiration.  Capillary Fringe – Immediately above the water table, where groundwater is held by surface tension in the spaces between the grains of soil or sediment.  Zone of Saturation – Zone where all of the open spaces between the soil/sediment grains is completely filled with water.  Water Table – The upper limit of the zone of saturation.  Groundwater – Water held within the zone of saturation.

9. How Does Groundwater Move?  Underground, water slowly moves from an aquifer’s recharge areas (areas where water seeps into the aquifer from rain fall, snow melt, etc.) to it’s discharge area (like streams, springs and lakes).  Groundwater is always moving (this is called groundwater flow) and moves very slowly--only inches per year. groundwater flowdischarge area evaporation recharge area precipitation condensation runoff transpiration aquifer water table infiltration Hydrologic Cycle

10. Water Table Formation Water Table Formation Animation #95

11. The water table is rarely level; it is a subdued replica of the surface topography. This reflects: n Variations in rainfall (seasonal). n The slow rate at which water moves through the subsurface. n The effects of gravity. n Where a stream, lake or swamp is found, the water table coincides with the surface of the body of water.

12. Gaining Streams – Gain water from the inflow of groundwater through the streambed (the elevation of the water table must be higher that the elevation of the surface of the stream). Losing Streams – Streams that lose water to the groundwater system by outflow through the streambed (the elevation of the water table is lower than the elevation of the surface of the stream). Figure 17.4

13. Factors Influencing the Storage and Movement of Groundwater n Rock and sediment contain voids called pore spaces. n The porosity is the percentage of the total volume of rock that consists of pore spaces. The quantity of water that can be stored depends on the porosity.

14. How is Porosity Measured? Porosity(%)= Vol. of Pore Space x 100% Total Volume Total Volume See the following example

15. Assume you have two graduated cylinders, one with 5 ml. of sand, the other with 5 ml. of water.

16. Pour the water into the sand

17. Suppose that after letting the water settle, there remains 4 ml of water above the sand; what is the porosity?

18. The Solution n You started with 5 ml. of Sand; this includes both the sand and the pore space, in other words this is the TOTAL VOLUME n You poured in 5 ml of water, 4 ml remained above the sand, 1 ml went into the sand. Where did it go? Into the pores. The PORE SPACE VOLUME = 1 ml. n 1ml/5ml x 100% = 20% Porosity

19. Different Rock Types Have Different Porosities n Sediment might have porosities from 10 to 50%. n The porosity of most igneous and metamorphic rocks is less than 1%. n Porosity in sediments tends to be:  Higher if sediment is well sorted  Lower if sediment is well packed  Lower if sediment is well cemented  Fractures can increase the porosity in igneous, metamorphic rocks, and certain sedimentary rocks (especially limestones).

20. The quantity of groundwater that can be stored depends on the porosity of the material.

21. The Permeability of Rock is It’s Ability to Transmit Water n Depends on porosity and interconnectedness. n Molecular attraction or surface tension inhibits flow particularly in small pore spaces (shales). n Aquifers are rocks with high permeability (easily transmit water). n Aquitards are impermeable (barrier to water flow).

22. If pore spaces are too small, surface tension keeps water from moving Aquitards Hinder or Prevent Groundwater Movement

23. Aquifer Permeability http://serc.carleton.edu/NAGTWorkshops/visualization/collections/groundwater.html Aquifer Speed Animation

24. Movement of Groundwater n Water must migrate through the pore spaces of rock. n Only occasionally are there "underground rivers of water". n Underground water moves under a hydraulic gradient – a slope to the water table. n Hydraulic head – the difference in height of the water table between the recharge and discharge points. n The flow rate or velocity is governed by Darcy's law: V = K h/l where V is the velocity, K is the is the permeability coefficient, h is the hydraulic head, and l is the horizontal distance between the recharge and discharge points. n Velocities are typically several centimeters per day – very slow!

25. Hydraulic Gradient Figure 17.6

26. Springs n Outflow of groundwater that occurs where the water table intersects the earth's surface. n Or where there is a perched aquifer

27. Springs

28. Springs: Desert Oases

29. Hot Springs and Geysers n Water in hot springs is 6-9°C (10-15°F) warmer than the mean annual air temperature. n Water in hot springs and geysers moves up from greater depths where it has been heated by the natural geothermal gradient, or by cooling igneous bodies. n Primarily found in young geological environments such as western U.S.

30. Distribution of Hot Springs and Geysers in U.S.

31. Geysers are intermittent hot springs which erupt violently due to the "flashing" of water to steam.

32. Geysers http://serc.carleton.edu/NAGTWorkshops/visualization/collections/groundwater.html

33. Deposits associated with geysers and hot springs at Yellowstone NP are travertines or silica-rich geyserites

34. Wells n Where water is artificially withdrawn from the earth. n Result in drawdown of the water table – a conical depression in the water table known as a cone of depression. n Cone of depression increases the hydraulic gradient in the vicinity of the well. This may be good over the short-run, but has long-term adverse consequences.  Lowers the water table overall  Cone of depression becomes larger  Reduces the column of water in contact with the well

35. The presence of springs and the productivity of wells depend on the permeability and distribution of subsurface materials.

36. Wells and the Cone of Depression

38. The Cone of Depression Cones of Depression Animation #12

39. Artesian Well n Where the hydraulic gradient causes the water in a well to move to a level above the aquifer. n Non-flowing artesian well – water does not come to the surface. n Flowing artesian well – water rises to the surface and flows freely.

40. Artesian Well n Must have the following conditions:  Water must be confined to an inclined aquifer.  Aquitards must be present both above and below the aquifer to keep the water from escaping. n Friction reduces the artesian effect the further the well is from the recharge area. n A city water supply system is a good example of an artificial artesian system.

41. Artesian Wells

43. Artificial Artesian System

44. Problems associated with Groundwater Withdrawal n Continued pumping (withdrawal) can lead to a drop in the water table. n Groundwater becomes a nonrenewable resource n Subsidence n Saltwater Intrusion

45. Map showing decline in Ogallala aquifer due to over-pumping

46. Subsidence in the San Joaquin Valley, CA due to the withdrawal of groundwater such that the water pressure drops and the weight of the overburden is transferred to the sediment causing closer packing of the sediments and thus subsidence. 9 meters from 1925-1977 Animation

47. Subsidence from pumping out water faster than it can replenish.

48. Saltwater Intrusion n Occurs along coastlines where saltwater replaces freshwater which has been withdrawn. n Fresh water floats on top of salt water. n A cone of depression in the top of the zone of freshwater will produce an inverted cone in the interface between freshwater and saltwater. n When the fresh water table is lowered by 1 meter the bottom of the fresh water zone will rise by 40 meters!

49. Saltwater Intrusion

50. Groundwater Contamination n Sewage, liquid wastes such as solvents, highway salt, fertilizers, and pesticides are some of the worst offenders. n These enter the groundwater through septic tanks, holding ponds, landfills, leaking underground tanks and pipelines, and infiltration.

51. Sources of Groundwater Contamination Sometimes agricultural chemicals and livestock wastes find their way into the groundwater

52. Sources of Groundwater Contamination Sometimes materials leached from landfills and other waste sites find their way into the groundwater.

53. Groundwater Contamination Woburn, MA Animation

54. Groundwater can be remediated (in some cases) in the subsurface. But it takes time and cleanup will always require some degree of natural attenuation. 1. By natural filtration in low- permeable aquifers. 2. By chemical oxidation or other bio-chemical processes. 3. Bio-remediation – through the work of bacteria (biotechnology).

55. Karst Topography and Caves Karst Topography forms in limestone terrain and is characterized by: n Caves/Caverns, n Sinkholes, n Solution Valleys n Tower Karst n Disappearing Streams, n Springs

56. Aerial Photos of Karst Terrain

57. Sinkhole, Winter Park, FL

58. Sinkhole, Frostproof, FL

59. General View of Karst Topography (Shenandoah Valley)

60. Tower Karst

61. Caves and caverns typically form in limestone n As soon as a chamber is filled with air, the stage is set for the decoration phase of cavern building to begin n Speleothems are cave formations n Speleothems are made of calcite  Form a rock called travertine – dripstone  Soda Straws  Stalactites - hang from ceiling  Stalagmites - on the ground  Columns – joined stalagmites and stalactites

62. Speleothems in Carlsbad Caverns National Park