Groundwater Hydrogeology

1. Hydrogeology
• Collected by Jyoti Anischit

2. Groundwater Hydrology
• What is Groundwater?
• What is Groundwater Hydrology?
• The Geology of Groundwater
• Groundwater Recharge
• Aquifers
• Groundwater Movement
• Age of Groundwater
• Locating and Mapping Groundwater
• Drilling a Groundwater Well

3. contd

4. What is Groundwater?
• Found in the subsurface, inside pores within soil and
rock
• Spelled either as two words, Ground Water, or as one,
Groundwater
• Groundwater is the largest source of freshwater on
earth, and was little used until recently.
• With electricity and the modern pump, groundwater
has become very important to agriculture, cities, and
industries.
• It is usually much cleaner than surface water.

5. What is Groundwater Hydrology?
• It is the study of the characteristics, movement,
and occurrence of water found below the surface.
• Groundwater and aquifers are like surface water
and watersheds
– An aquifer is a geologic unit that transmits water.
– Piezometric surfaces are used to map water levels,
similar to topographic lines on maps.
– Each aquifer has its own piezometric surface.
– The water level elevation in wells are used to create the
piezometric surface.

6. The Geology of Groundwater
• Sedimentary Rocks
– sandstone, shale, limestone, conglomerate
• Glaciated Terrain
– large valleys and basins were carved out
– sediments (sands, clays) were left behind
• Alluvial Valleys and Fans
– along rivers and streams
• Tectonic Formations
– solid rock is fractured by pressures due to earth’s
movement

7. Wikipedia

8. Groundwater Recharge
• Water that replenishes aquifers
• Usually from surface water or precipitation that
infiltrates, and then percolates through the vadose zone
• Recharge happens when percolating water finally
reaches the water table, which is the top of the
saturated zone.
• Above the water table is the unsaturated zone where
water is held by capillary forces
• The root zone may capture some water that infiltrates
and lift it back to the atmosphere.

9. Figure 4.6 Lakes and wetland complexes often exist in areas with shallow
groundwater elevations that intercept the land surface..

10. Groundwater Recharge
Fetter, Applied Hydrology
Infiltration
Percolation
Saturated zone
Evapotranspiration
Overland flow

11. Aquifers
• Water-bearing geologic formation that can store
and yield usable amounts of water
• Aquifer types:
– unconsolidated, consolidated, fractured
– perched, unconfined, confined, artesian
– thermal springs
• Aquifer properties
– porosity = volume of pores (voids) per total volume of
aquifer
• n = Vv / Tt

12. Unconfined Aquifer

13. Figure 4.10 The Ogallala Aquifer provides water to irrigators, cities, and
other groundwater users in parts of South Dakota, Nebraska, Wyoming,
Colorado, Kansas, Oklahoma, Texas, and New Mexico.
Land surface elevation
in meters

14. Figure 4.7 Two conditions are necessary to create an artesian groundwater system: a
confined aquifer and sufficient pressure in the aquifer to force water in a well or other opening
to rise above the static water level of the aquifer.
Confined Aquifer

15. Aquifer Porosity

16. Example Porosity Calculation
• Take a 1000-mL beaker (1 liter)
• Fill it with sand to the top
• Measure how much water it takes to fill the beaker
to the top (say 300 mL)
• The porosity = (300 mL) / (1000 mL) = 30%

18. Groundwater Movement
• Water moves because of two factors
– The force pushing through the subsurface
– The permeability of the geologic media
• Darcy’s Law says that the flux of water (flow per
unit area) is calculated using these two factors:
– q = K i
– q = flux of water, ft / s
– K = hydraulic conductivity, ft / s
– i = hydraulic gradient, ft / ft
Note they both
have the same
units

20. • The hydraulic conductivity, K, is a measure of the
permeability of the aquifer
– gravels have large hydraulic conductivities
– clays and solid rock have small values
• The hydraulic gradient is a measure of the force acting on
the water
– it is like the slope of the land surface, water flows faster where it
is steep
– i = dh / dl = slope of the water surface
– h is the hydraulic head, or water level in a well
– dh is the change in water level between two wells
– dl is the distance between the wells
– determines the direction of flow.

21. Direction of Flow?

22. Soil surface
Water table

23. dh
dl

24. dl = 1,000 m
h = 50 m
h = 55 m
K = 5 m/day

25. dl = 1,000 m
h = 50 m
h = 55 m
K = 5 m/day dh = 5 m

26. dl = 1,000 m
h = 50 m
h = 55 m
K = 5 m/day dh = 5 m
dh/dl = 5/1,000 = 0.005

27. dl = 1,000 m
h = 50 m
h = 55 m
K = 5 m/day dh = 5 m
dh/dl = 5/1,000 = 0.005
q = K i = 5 x 0.005
q = 0.025 m/day

28. Karst Aquifers
Wikipedia

29. Underground Rivers?
Only in Karst aquifers!!
Wikipedia

30. Specific Yield
• Volume of water that can be removed per unit
volume of aquifer
– less than the porosity - hard to get the last drop!

31. Specific Yield Calculation
• Take a 1000-mL beaker (1 liter)
• Fill it with sand to the top
• Measure how much water it takes to fill the beaker
to the top (say 300 mL)
• The porosity = (300 mL) / (1000 mL) = 30%
• We pour the water out and 250 mL is collected
• What is the specific yield?
• (250 mL) / (1000 mL) = 25%
– Can’t get the last drop!

32. Age of Groundwater
• Time it takes for water to move through the
subsurface
• Maybe 1 to 25 years in aquifers near Athens
• Up to 30,000 years for water down on the
coast

34. Locating and Mapping Groundwater
• The first step is to generate a piezometric surface,
which maps water table elevation
– Wells are plotted on a map, and water levels in the wells
are indicated
– Lines of constant water level elevations are plotted (called
equipotentials)
– Flowlines (also call streamlines) are drawn so that they are
perpendicular to the equipotential lines
– Local rivers, lakes, and other surface water features are
plotted on the map.

35. Figure 3.45 Water levels (in feet above sea level) in monitoring wells and
contours of total potential (piezometric surface or water table surface) at a
contaminated site (Fetter, 1988).
Figure 3.45 Water levels (in feet above sea level) in monitoring wells and
contours of total potential (piezometric surface or water table surface) at a
contaminated site (Fetter, 1988).

36. Drilling a Groundwater Well
• Various methods are available for drilling a well
• A simple method is the auger method, which uses a
screw-like bit. This works in soft materials
• For solid rock, a simple technique is the hammer or
percussion method which pounds a hole in the rock
• Rotary methods uses a harden steel bit tipped with
diamonds to cut through the rock. Either water, air or
mud can be used to lubricate and to lift the cuttings.

38. Well Components
• A well pad is placed on the surface to hold up the
well.
• A blank casing is used from the surface down to the
aquifer. Clay or concrete fills the space outside the
casing.
• A screened casing is used in the aquifer. Sand or
gravel fills the space outside the casing
• A submerged turbine pump lifts the water to the
surface. The motor that drives the pump is either on
the surface or also submerged.

41. Cone of
depression in
potentiometric
surface near
Albany GA

43. Stream Depletion Factors
• Used to assess the effects of well pumping on
stream flow
• Depend on
– the distance to the stream (less effect with greater
distance)
– properties of the aquifer

44. Part II
• Jyoti Anischit

45. The Hydrologic Cycle

46. Where is the Water ?
Figure 16.2

47. 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

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

49. 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.

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

51. 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.

52. 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 flow discharge area
evaporation
recharge area
precipitation
condensation
runoff
transpiration
aquifer
water table
infiltration
Hydrologic Cycle

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

54. 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

55. Factors Influencing the Storage and
Movement of Groundwater
• Rock and sediment contain voids called pore
spaces.
• 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.

56. The Solution
• You started with 5 ml. of Sand; this includes both the
sand and the pore space, in other words this is the
TOTAL VOLUME
• 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.
• 1ml/5ml x 100% = 20% Porosity

57. Different Rock Types Have Different
Porosities
• Sediment might have porosities from 10 to 50%.
• The porosity of most igneous and metamorphic rocks
is less than 1%.
• 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).

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

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

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

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

62. Movement of Groundwater
• Water must migrate through the pore spaces of rock.
• Only occasionally are there "underground rivers of water".
• Underground water moves under a hydraulic gradient – a slope to the
water table.
• Hydraulic head – the difference in height of the water table between
the recharge and discharge points.
• 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.
• Velocities are typically several centimeters per day –
very slow!

63. Hydraulic Gradient
Figure 17.6

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

65. Springs

66. Springs: Desert Oases

67. Hot Springs and Geysers
• Water in hot springs is 6-9°C (10-15°F) warmer than
the mean annual air temperature.
• 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.
• Primarily found in young geological environments
such as western U.S.

68. Wells
• Where water is artificially withdrawn from the earth.
• Result in drawdown of the water table – a conical depression in
the water table known as a cone of depression.
• 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

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

71. Artesian Well
• 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.
• Friction reduces the artesian effect the further the
well is from the recharge area.
• A city water supply system is a good example of an
artificial artesian system.

72. Artesian Wells

73. Artesian Wells

74. Artificial Artesian System

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