1. Chapter 13 Dynamic Earth Eric H Christiansen
Groundwater Systems
Chapter 13
Dynamic Earth
Eric H Christiansen

2. Major Concepts
Groundwater is an integral part of the hydrologic system, and it is intimately related to surface water drainage.
The movement of groundwater is controlled largely by the porosity and permeability of the rocks through which it flows.
The water table is the upper surface of the zone of saturation.
Groundwater moves slowly through the pore spaces in rock.

3. Major Concepts
The natural discharge of groundwater is generally into springs, streams, marshes, and lakes.
Aquifers are saturated permeable rocks; they may be confined between impermeable layers or unconfined and open to the surface.
Erosion by groundwater produces karst, with caves, sinkholes, solution valleys, and disappearing streams. Precipitation of minerals from groundwater creates deposits in caves and along fractures and cements many kinds of clastic sedimentary rocks.
Alteration of the groundwater can produce many unforeseen problems, such as pollution, subsidence, collapse, and disruption of ecosystems.

4. Groundwater Systems Groundwater is simply water below Earth’s surface.
Two physical properties of a rock largely control the amount and movement of groundwater.
Porosity, the percentage of the total volume of the rock consisting of voids.
Permeability, the capacity of a rock to transmit fluids.

5. Groundwater Systems Groundwater flows in an open, dynamic system.
Gravity is primary driving force.
Water enters the system at the ground surface through recharge, and generally flows slowly through connected pores in soil and rock.
Locally, it dissolves soluble rocks deposits minerals along fractures and in caves.
Leaves the system through discharge.
Figure 13.01: The groundwater system is an open system of water flowing below the surface but still under the influence of gravity.

6. Porosity Percent of the total volume that is open space.
Affected by the size and shape of particles.
Increased by fracturing or dissolution.
Decreased by compaction and cementation.
Well sorted sediments have higher porosity than poorly sorted sediments .
Figure 13.02: Various types of pore spaces in rocks permit the flow of groundwater.

7. Permeability
A measure of how well pores are connected and how straight a path a fluid follows
Permeability is a property of the rock
Other liquids such as oil flow through rock
Density and viscosity influence flow rate
Figure 13.02: Various types of pore spaces in rocks permit the flow of groundwater.

8. The Water Table and Aquifers
Figure 13.04: The movement of groundwater in an unconfined aquifer is directed toward areas of least pressure.
The water table is the upper surface of the zone of saturation.
Aquifers are saturated permeable rocks; they may be open or confined.

9. The Water Table Water is found at some depth almost everywhere
Figure 13.06: The base of an unconfined groundwater reservoir is not an abrupt surface like the water table.
Water is found at some depth almost everywhere
The water table is the boundary below which all pore spaces are filled with water
The volume of pore spaces decreases with depth
Usable water is restricted to a few hundred meters below the surface

10. The Water Table Forms the boundary between the Saturated Zone
Figure 13.03: The water table is the upper surface of the zone of saturation. Water seeps into the ground through pore spaces in rock and soil.
Forms the boundary between
the Saturated Zone
the Unsaturated Zone
The unsaturated zone = zone of aeration

11. Unconfined Aquifers
An aquifer that has highly permeable material extending from ground surface downward to an aquitard at its base.
Recharge is from seepage, lateral flow of groundwater, or upward leakage through the aquitard.
Also called a water table aquifer.
Figure 13.04: The movement of groundwater in an unconfined aquifer is directed toward areas of least pressure.

12. Confined Aquifers
Figure 13.10: Flowing (or artesian) wells occur only when the top of the well is below the potentiometric surface and require no pumping.
Confining layers (aquitards) above and below a permeable zone
Recharge at high elevation where permeable beds are exposed.

13. Major Aquifers in the United States
Figure 13.05: The major aquifers of the United States are shown on this map. Each aquifer consists of permeable rocks.
Data from: U.S. Geological Survey

14. Flow of Groundwater Groundwater flows due to the force of gravity
Recharge flows downward to the water table
Gravity creates water pressure in the aquifer
Groundwater then flows from high pressure to low pressure
3D model and image provide courtesy of ctech.com

15. Flow of Groundwater: Unconfined Aquifers
Figure 13.04: The movement of groundwater in an unconfined aquifer is directed toward areas of least pressure.
Recharge flows downward to the water table which usually mimics the ground surface
Gravity creates water pressure in the aquifer
Groundwater then flows from high pressure to low pressure

16. Flow of Groundwater: Unconfined Aquifers
Figure 13.04: The movement of groundwater in an unconfined aquifer is directed toward areas of least pressure.
Groundwater pressure can be measured as hydraulic head-- the elevation of the water table at a given point
Flow is from high to low pressure or hydraulic head
Hydraulic gradient is the difference in head between two points—the driving force

17. Flow of Groundwater: Confined Aquifers
Figure 13.10: Flowing (or artesian) wells occur only when the top of the well is below the potentiometric surface and require no pumping.
Because water pressure builds in the confined zone, the groundwater level (potentiometric surface) may be above the top of the aquifer
Artesian aquifers have a potentiometric surface above ground surface

18. Natural and Artificial Discharge
Groundwater discharge occurs to lakes, streams and wetlands
Maintains flow in streams during dry periods
Springs include any natural flow of water from the ground surface
Intersection of water table- ground surface
Figure 13.08B: The springs issue from the north wall of the canyon and are fed by water that flowed underground.
© Charles Knowles/ShutterStock, Inc.

19. Natural Discharge: Springs
Figure 13.07A: A line of springs develops on valley walls where impermeable beds cause groundwater in permeable layers to migrate.
Figure 13.07B: Springs form along valley slopes where cavernous limestone permits the free flow of groundwater to the surface.
Figure 13.07C: Many faults displace rocks so that impermeable beds are placed next to permeable beds.
Figure 13.07D: Surface water readily seeps into vesicular and jointed basalt flows.

20. Artificial Discharge: Wells
Before Pumping
Water table is nearly horizontal
During Pumping
Water table is depressed as water is withdrawn from the aquifer
Forms cone of depression
Figure 13.09: A cone of depression in a water table results if water is withdrawn from a well faster than it can be replenished.

21. Thermal Springs and Geysers
Figure 13.13: Hot springs are common where groundwater is heated in regions with young volcanism or deep faulting.
In areas of recent igneous activity, rocks near magma chambers can remain hot for hundreds of thousands of years.
Groundwater  migrating through these areas of hot rock becomes heated and, when discharged to the surface, produces thermal springs and geysers.

22. Geysers
Figure 13.12A: Groundwater circulating through hot rocks in an area of recent volcanic activity collects in caverns and fractures.
Figure 13.12B: The expanding steam forces water upward until it is discharged at the surface vent.
Figure 13.12C: The preliminary discharge of water reduces the pressure on the water lower down.
Figure 13.12D: Eruption ceases when the pressure from the steam is spent and the geyser tubes are empty.

23. Erosion by Groundwater
Figure 13.14: Importance of fractures in the evolution of a cave system is revealed in the Redwall Limestone exposed on walls of Grand Canyon.
Slow-moving groundwater can dissolve huge quantities of soluble rock and carry it away in solution.
Subsurface dissolution forms:
Caves
Sinkholes
Karst topography.

24. Groundwater dissolution of salt in formation, Utah

25. Figure 13.20C: Dissolution is expressed vividly by the abundant cavities so that the limestone outcrops resemble Swiss cheese.
Caves
Shallow groundwater dissolves carbon dioxide to form a weak acid.
The slightly acidic water percolates through the fractures and bedding planes, slowly dissolving the limestone and enlarging the openings to form caves
Dissolution is expressed vividly by the abundant cavities so that the limestone outcrops resemble Swiss cheese. From Guilin area of China.

26. Figure 13. 18A: Initial stage
Figure 13.18A: Initial stage. Scattered sinkholes dotting the landscape grow in size and number as caverns enlarge and their roofs collapse.
Figure 13.18B: Intermediate stage. Individual sinks enlarge and merge with those in adjacent areas to form solution valleys.
Figure 13.18C: Late stage. Solution activity removed most of the limestone formation. Only isolated knolls remain as remnants of the former surface.
Karst Topography
Carbonate, sulfate, or salt beds at or near the ground surface
Humid climate
Solution valleys with remnant walls and towers
Disappearing streams
Sinkholes

27. Karst Topography
Figure 13.19: Major areas of karst topography of the world are restricted to regions where outcrops of limestone occur in humid climatic conditions.

28. Figure 13.17A: Sinkhole karst, Kentucky.
Figure 13.17B: Sinkhole in a karst terrain in Florida.
Figure 13.17C: In some karst regions, streams disappear into subsurface caverns like this one in China.
Figure 13.17D: Groundwater solution enlarged these fractures in limestones in New Zealand.
Courtesy of John S. Shelton
Courtesy of USGS
Sinkhole karst, Kentucky. (Courtesy of John S. Shelton)
Sinkhole in a karst terrain in Florida. (Courtesy of GeoPhoto Publishing Co.)
In some karst regions, streams disappear into subsurface caverns like this one in China.
Groundwater solution enlarged these fractures in limestones in New Zealand.

29. Karst from Space
Figure 13.20D: Karst terrain. Karst processes form distinctive regional landscapes as shown on this map of southern China.
Karst processes form distinctive regional landscapes as shown on this map of southern China. The fractures that control groundwater flow become etched into the landscape as linear valleys.

30. Deposition by Groundwater
The mineral matter dissolved by groundwater can be deposited in a variety of ways.
Groundwater commonly deposits mineral matter as cement between grains in permeable deposits such as sandstone and conglomerates.
The most spectacular deposits are stalactites and stalagmites, which are found in caves.
Figure 13.24: Calcite deposited by groundwater cements the rounded quartz sand grains together, as shown in this thin section of sandstone.

31. Deposition by Groundwater
Groundwater also carries dissolved silica
Locally it can replace plant material to make spectacular fossils.
Petrified trees formed this way in of the Petrified Forest Member of the Chinle Formation, Arizona. Weathering and erosion later exposed them.
Figure 13.25: Petrified trees litter the area, piled like giant jackstraws about a rolling landscape on the Petrified Forest Member.

32. Deposition by Groundwater
Mammoth Hot Springs, Yellowstone National Park, was formed by the deposition of travertine (CaCO3) as the warm spring water evaporated and lost carbon dioxide.
Figure 13.26: Mammoth Hot Springs, Yellowstone National Park.

33. Caves Solution caves form by groundwater erosion
Figure 13.22: Many varieties of cave deposits are shown in this idealized diagram.
Solution caves form by groundwater erosion
Some partially fill by deposition from groundwater
Stalagmites
Stalactites
Travertine
Drip stones
Many varieties of cave deposits are shown in this idealized diagram. Most are composed of calcite deposited by water that seeps into the open cave and then loses carbon dioxide as the water evaporates

34. Groundwater Resources
Groundwater is a valuable resource that is being exploited at an ever-increasing rate.
Ancient groundwater systems have also produced valuable mineral resources.
Geothermal waters could become a significant energy source. Forms near shallow magma systems and along faults.
Geothermal energy is stored in heated groundwater and is extracted in facilities such as this one in New Zealand to generate electricity.
Typically where magma is relatively close to the surface and heats groundwater or where water can circulte deepley to high temperature parts of the crust.
Figure 24.15: Geothermal energy stored in heated groundwater is extracted in facilities such as this one in New Zealand to generate electricity.
© Graham Prentice/ShutterStock, Inc.

35. Alteration of Groundwater Systems
Figure 13.31: Dam constructed on permeable limestone in western Wyoming never functioned; the surface water seeped into the subsurface.
A variety of problems resulting from human activities alter the groundwater system:
Pollution
Saltwater encroachment,
Changes in the position of the water table
Subsidence following withdrawl.
A dam constructed on permeable limestone in western Wyoming never functioned because the surface water seeped into the subsurface. The dam lies at the beginning of the gorge. The light-colored sediment behind the dam marks the fraction of the reservoir that formed before water was lost through seepage.

36. Waste Disposal and Groundwater
Figure 13.27A: Permeable sand and gravel overlying impermeable shale creates potential pollution problems: contaminants move with groundwater.
Figure 13.27B: An impermeable shale confines pollutants and prevents significant infiltration into the groundwater system in the limestone below.
Figure 13.27C: A fractured rock body provides a zone where pollutants can move readily in the general direction of groundwater flow.
Figure 13.27D: An inclined, permeable aquifer below a disposal site permits pollutants to enter a confined aquifer and move down the dip of the beds.
The effects of waste disposal or leaking storage tanks on a groundwater system depend on the geologic setting. In many cases, water seeping through the disposal site enters and pollutes the groundwater system.
A. A permeable layer of sand and gravel overlying an impermeable shale creates a potential pollution problem because contaminants are free to move with groundwater.
B. An impermeable shale (or clay) confines pollutants and prevents significant infiltration into the groundwater system in the limestone below.
C. A fractured rock body provides a zone where pollutants can move readily in the general direction of groundwater flow.
D. An inclined, permeable aquifer below a disposal site permits pollutants to enter a confined aquifer and move down the dip of the beds, so that they contaminate the system.

37. Saltwater Encroachment
Figure 13.28A: A lens of fresh groundwater beneath the land is buoyed up by denser saltwater below.
Figure 13.28B: Excessive pumping causes a cone of depression in the water table on top of the freshwater lens.
Figure 13.28C: Fresh water pumped down an adjacent well can raise the water table; this lowers the interface between altwater and fresh water.
The relationship between fresh water and saltwater on an island or a peninsula is affected by the withdrawal of water from wells.
Excessive pumping causes a cone of saltwater encroachment.
A lens of fresh groundwater beneath the land is buoyed up by denser saltwater below.
Excessive pumping causes a cone of depression in the water table on top of the freshwater lens and a cone of saltwater encroachment at the base of the freshwater lens.
Fresh water pumped down an adjacent well can raise the water table around the well and lower the interface between the fresh water and the saltwater

38. Modified Groundwater Drainage Systems
Figure 13.29A: Natural drainage of southern Florida in 1871 spread southward from Lake Okeechobee in a broad sheet only a few centimeters deep.
Figure 13.29B: Canals diverted the natural flow of surface water across the Everglades.
Modification of the natural drainage system of the Everglades in southern Florida.
Natural drainage of southern Florida in 1871 spread southward from Lake Okeechobee in a broad sheet only a few centimeters deep. This sheet maintained swampy conditions in the Everglades and established a water table very close to the surface.
Canals diverted the natural flow of surface water across the Everglades. The water table was lowered, the swamp was destroyed in some areas, and saltwater encroached in wells along the coast.

39. Subsidence
Figure 13.30: Subsidence of buildings in Mexico City resulted from compaction after groundwater was pumped from unconsolidated sediment.
Subsidence of buildings in Mexico City resulted from compaction after groundwater was pumped from unconsolidated sediment beneath the city.
Subsidence has caused this building to tilt and sink more than 2 m.

40. Summary of Major Concepts
Groundwater is an integral part of the hydrologic system, and it is intimately related to surface water drainage.
The movement of groundwater is controlled largely by the porosity and permeability of the rocks through which it flows.
The water table is the upper surface of the zone of saturation.
Groundwater moves slowly through the pore spaces in rock.

41. Summary of Major Concepts
The natural discharge of groundwater is generally into springs, streams, marshes, and lakes.
Aquifers are saturated permeable rocks; they may be confined between impermeable layers or unconfined and open to the surface.
Erosion by groundwater produces karst, with caves, sinkholes, solution valleys, and disappearing streams. Precipitation of minerals from groundwater creates deposits in caves and along fractures and cements many kinds of clastic sedimentary rocks.
Alteration of the groundwater can produce many unforeseen problems, such as pollution, subsidence, collapse, and disruption of ecosystems.