GLY 155 Introduction to Physical Geology, W. Altermann. Grotzinger Jordan. Understanding Earth. Sixth Edition

Transcription

1 Grotzinger Jordan Understanding Earth Sixth Edition Chapter 17: THE HYDROLOGIC CYCLE AND GROUNDWATER 2011 by W. H. Freeman and Company Chapter 17 The Hydrologic Cycle and Groundwater 1

2 About the Hydrologic Cycle Hydrology is the study of movements and characteristics of groundwater. The hydrologic cycle has a profound effect upon climate prediction. Water is vital so we must understand where to find water and how water supplies cycle through the Earth. 1. The Geologic Cycling of Water Water Reservoirs include all the places that water is stored in and on the Earth. Flows into a reservoir include inflows and outflows. Inflow equals outflow. 2

3 1. The Geologic Cycling of Water Reservoirs: 1. The Geologic Cycling of Water The hydrologic cycle is the movement of water through Earth s s crust, atmosphere and hydrosphere: precipitation infiltration and runoff evaporation, transpiration, and sublimation groundwater flow 3

4 1. The Geologic Cycling of Water The hydrologic cycle in 1000km 3 /a. 2. Hydrology and Climate Key climatic factors relative humidity rainfall landscape Key tectonic factors ocean-land relationships mountain rain shadows 4

5 2. Hydrology and Climate: The Rain Shadow Effect 2. Hydrology and Climate The runoff-precipitation relationship stronger in local areas less strong in regional areas The runoff is influenced by geological factors and by vegetation 5

6 2. Hydrology and Climate 2. Hydrology and Climate 6

7 Average annual precipitation for South Africa Average annual precipitation for Africa 7

8 South Africa has a mean annual precipitation (MAP) to mean annual runoff (MAR) ratio of 8.6%, that is, only 8.6% of the rainfall is available as surface water. This is one of the lowest conversion ratios in the world. Canada and Australia, which have similar MAP figures, have ratios of 65.7% and 9.8% respectively. 8

9 2. Hydrology and Climate Surface storage of water runoff lakes and reservoirs wetlands and swamps The annual runoff to annual precipitation ratio can be improved by artificial dams and by intensified vegetation cover The Hydrology of Runoff: Similarity of a dammed lake and a natural lake Increased surface area increases proportionally the infiltration area and time and improves groundwater reserves 9

10 The Hydrology of Runoff: Similarity of a dammed lake and a natural lake Increased surface area increases proportionally the infiltration area and time and improves groundwater reserves Coverage >96% Coverage 60-96% Coverage <60% Insufficient data In theory, some 34,000 cubic kilometres of freshwater are available globally for human use every year. If evenly distributed this would provide each person with roughly 8,000 cubic metres of water per year (based on the population in 2000). 10

11 3. The Hydrology of Groundwater Groundwater flow through soil and rock porosity and permeability groundwater table Groundwater: Porosity and the amount of open space in various materials 11

12 3. The Hydrology of Groundwater 3. The Hydrology of Groundwater Above and below the groundwater table (remember the diagenesis?) unsaturated (vadose) zone saturated (phreatic)) zone 12

13 3. The Hydrology of Groundwater Inflow and outflow of groundwater recharge (influent streams) discharge (effluent streams) 13

14 Groundwater: Effluent water headed for a stream 3. The Hydrology of Groundwater: Dynamics of the Groundwater Table The groundwater table follows the general shape of the surface topography but gentler. The heights of water table depends on the relationship between recharge and discharge. 14

15 3. The Hydrology of Groundwater 3. The Hydrology of Groundwater Types of aquifers unconfined has an aquiclude below confined has aquiclude above and below 15

16 3. The Hydrology of Groundwater Characteristics of some confined aquifers artesian (flowing) wells artesian flow (under pressure in a confined aquifer). At any point of the aquifer, the pressure is equivalent to the weight of the water above this point. 3. The Hydrology of Groundwater 16

17 3. The Hydrology of Groundwater Complex geological environments perched water tables unpredictable flow conditions 17

18 3. The Hydrology of Groundwater Balancing recharge and discharge balance = stable water table excess recharge = rising water table excess discharge = falling water table Groundwater: Excess discharge and the cone of depression 18

19 Groundwater: Excess discharge and the movement of salt water. Possible not only in coastal areas, but also where saline brines are part of the ground water. Salt water has higher density than fresh water! Groundwater: Excess discharge and the movement of salt water. Possible not only in coastal areas, but also where saline brines are part of the ground water. Salt water has higher density than fresh water! 19

20 4. Erosion by Groundwater Features of groundwater erosion caves and caverns stalactites and stalagmites karst features (karst topography) sinkholes 4. Erosion by Groundwater Characteristics of areas with karst high rainfall and abundant vegetation limestone bedrock with joints significant hydraulic gradient 20

21 4. Erosion by Groundwater: Karst Erosion by Groundwater: Carlsbad Caverns, New Mexico 21

22 Erosion by Groundwater: Sinkhole in Winter Park, Florida 5. Water Quality Contamination of the water supply rubbish dumps and agriculture lead pollution radioactive wastes microorganisms in water acidification of water (mining) other chemical contaminants 22

23 5. Water Quality: Human Contamination 23

24 5. Water Quality Reversing contamination easier if recharge rate is fast usually costly and very slow decontamination after pumping in-ground water treatments 5. Water Quality Dissolved materials in drinking water potable water has 150 ppm distilled water has < 1 ppm some elements have their limits example: arsenic, 0.05 ppm 24

25 6. Deep Water in the Crust Types of deep crustal groundwater meteoric water that seeps in magmatic water hydrothermal (hot spring) water Water Deep in the Crust: The Origin of Hot Springs and Geysers 25

26 Mammoth Hot Springs, Yellowstone National Park 6. Deep Water in the Crust Ancient microorganisms in deep aquifers active colonies of microbes that may have been there for millions of years are found in deep crustal groundwater they live by dissolving minerals 26

27 Thought questions for this chapter If global warming caused evaporation from the oceans to increase greatly, how would the hydrologic cycle of today be altered? How might the hydrologic cycle have been different 18,000 years ago, at the Wisconsin glacial maximum, when much of North America, Europe, and Asia were covered with ice? Thought questions for this chapter If you lived near the seashore and started to notice that your well water had a slightly salty taste, how would you explain the change in water quality? Why would you recommend against extensive development and urbanization of the recharge area of an aquifer that serves your community? Your new house is built on soil-covered granitic bedrock. Although you think that prospects for drilling a successful water well are poor, a well driller who is familiar with the area says he has drilled many good water wells in this granite. What arguments might each offer to convince the other? 27

28 Thought questions for this chapter You are exploring a cave and notice a small stream flowing on the cave floor. Where could the water be coming from? If you discovered that radioactive waste had seeped into groundwater from a nuclear processing plant, what kind of information would you need to predict how long it would take for the radioactivity to appear in well water 10 km from the plant? Why should communities ensure that septic tanks are maintained in good condition? Why are more and more communities in cold climates restricting the use of salt to melt snow and ice on highways? Key terms and concepts Aquiclude Aquifer Artesian flow Darcy s s law Discharge Drought Groundwater Groundwater table Hydraulic gradient Hydrologic cycle Hydrology Hydrothermal water Infiltration Karst topography Meteoric water 28

29 Key terms and concepts Permeability Potable Precipitation Rain shadow Recharge Relative humidity Runoff Saturated zone Sinkhole Unsaturated zone GLY 155 Introduction to Physical Geology, W. Altermann 29