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© 2011 Pearson Education, Inc. Lecture Outlines Chapter 15 Environment: The Science behind the Stories 4th Edition Withgott/Brennan.

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Presentation on theme: "© 2011 Pearson Education, Inc. Lecture Outlines Chapter 15 Environment: The Science behind the Stories 4th Edition Withgott/Brennan."— Presentation transcript:

1 © 2011 Pearson Education, Inc. Lecture Outlines Chapter 15 Environment: The Science behind the Stories 4th Edition Withgott/Brennan

2 © 2011 Pearson Education, Inc. This lecture will help you understand: Water’s importance to people and ecosystems Water’s distribution Use and alteration of freshwater systems Problems of water supply and solutions Problems of water quality and solutions How wastewater is treated

3 © 2011 Pearson Education, Inc. Gambling with water in the Colorado River basin 7 states share the Colorado river Droughts and overuse are threatening supplies Las Vegas, Nevada, needs more water than it is allotted Other states will let Las Vegas drill for underground water Drilling threatens the area’s ecology and people This issue will end up in Nevada’s Supreme Court

4 © 2011 Pearson Education, Inc. The Colorado River The Colorado River originates in the Rocky Mountains -Draining into the Gulf of California Its waters chiseled the Grand Canyon -But it has been reduced to a mere trickle Dams provide flood control, recreation, and hydroelectric power -30 million people use the water

5 © 2011 Pearson Education, Inc. Freshwater systems Water may seem abundant, but drinkable water is rare Freshwater = relatively pure, with few dissolved salts -Most is tied up in glaciers, ice caps, and aquifers

6 © 2011 Pearson Education, Inc. Water is renewed and recycled As water is cycled it redistributes heat, erodes mountains, builds river deltas, maintains ecosystems and organisms -It also shapes civilizations and political conflicts Surface water = on Earth’s surface -1% of freshwater Runoff = water that flows over land -Water merges in rivers and ends up in a lake or ocean Tributary = a smaller river slowing into a larger one Watershed (drainage basin) = the area of land drained by a river system (river and its tributaries)

7 © 2011 Pearson Education, Inc. Water is renewed and recycled as it moves through the hydrologic cycle

8 © 2011 Pearson Education, Inc. Water Cycle Scramble

9 © 2011 Pearson Education, Inc. Rivers and streams wind through landscapes Rivers shape the landscape Braided river = an interconnected series of watercourses that run through steep slopes Meandering river = rivers in flatter areas -Water rounding a bend erodes soil from the outer bank -Sediment is deposited on the inside of the bend -Rivers become exaggerated oxbows

10 © 2011 Pearson Education, Inc. Rivers shape the landscape Oxbows = areas where river bends become exaggerated Oxbow lake = erosion cuts off and isolates the oxbow into a U-shaped water body

11 © 2011 Pearson Education, Inc. A river may shift course over time Floodplain = areas nearest to the river’s course that are flooded periodically -Frequent deposition of silt makes floodplain soils fertile -Good areas for agriculture Riparian = riverside areas that are productive and species-rich Rivers and streams hosts diverse ecological communities -Algae, insects, fish, amphibians, birds, etc.

12 © 2011 Pearson Education, Inc. Lakes and ponds are ecologically diverse Lakes and ponds = bodies of open, standing water Littoral zone = region ringing the edge of a water body -Rooted aquatic plants grow in this shallow part Benthic zone = extends along the bottom of the water body -Home to many invertebrates Limnetic zone = open portion of the lake or pond where sunlight allows photosynthesis Profundal zone = water that sunlight does not reach -Supports fewer animals because there is less oxygen

13 © 2011 Pearson Education, Inc. A typical lake

14 © 2011 Pearson Education, Inc. Lakes vary in their nutrients and oxygen Oligotrophic lakes and ponds = have low nutrient and high oxygen conditions Eutrophic lakes and ponds = have high nutrient and low oxygen conditions Eventually, water bodies fill completely in through the process of succession The largest lakes are known as inland seas -Great Lakes, The Caspian Sea

15 © 2011 Pearson Education, Inc. Wetlands include marshes, swamps, bogs, and seasonal pools Wetlands = the soil is saturated with shallow standing water Freshwater marshes = shallow water -Plants grow above the surface Swamps = shallow water in forested areas -Can be made by beavers Bogs = ponds covered in thick floating mats of vegetation -A stage in aquatic succession Species in vernal pools are adapted to seasonal drying

16 © 2011 Pearson Education, Inc. Wetlands are valuable Wetlands are extremely valuable for wildlife -They slow runoff, reduce flooding, recharge aquifers, and filter pollutants People have drained wetlands, mostly for agriculture -Southern Canada and the U.S. have lost over half of their wetlands In 2006 the Supreme Court told the Army Corps of Engineers it must create guidelines to determine when wetlands are valuable enough to protect by law

17 © 2011 Pearson Education, Inc. Groundwater plays a key role Groundwater = water beneath the surface held in pores in soil or rock -20% of the Earth’s freshwater supply Aquifers = porous, sponge-like formations of rock, sand, or gravel that hold water -Zone of aeration = pore spaces are partly filled with water -Zone of saturation = spaces are filled with water -Water table = boundary between the two zones Recharge zone = any area where water infiltrates Earth’s surface and reaches aquifers

18 © 2011 Pearson Education, Inc. A typical aquifer

19 © 2011 Pearson Education, Inc. There are two categories of aquifers Confined (artesian) aquifer = water-bearing, porous rocks are trapped between less permeable substrate (clay) layers -Is under great pressure Unconfined aquifer = no upper layer to confine it -Readily recharged by surface water Groundwater’s average age is 1,400 years -It may be tens of thousands of years old Groundwater becomes surface water through springs or human-drilled wells

20 © 2011 Pearson Education, Inc. The Ogallala Aquifer The world’s largest known aquifer Underlies the Great Plains of the U.S. Its water has allowed farmers to create the most bountiful grain-producing region in the world

21 © 2011 Pearson Education, Inc. Water is unequally distributed across Earth Water is unevenly distributed in space and time -Different areas possess different amounts of water -People erect dams to store water Many densely populated areas are water-poor and face serious water shortages

22 © 2011 Pearson Education, Inc. Climate change may bring shortages Climate change will cause -Altered precipitation patterns -Melting glaciers -Early season runoff -Intensified droughts -Flooding Lake Mead is already hurting from drought

23 © 2011 Pearson Education, Inc. How we use water We have achieved impressive engineering accomplishments to harness fresh water -60% of the world’s largest 227 rivers have been strongly or moderately affected -Dams, dikes, and diversions Consumption of water in most of the world is unsustainable -We are depleting many sources of surface water and groundwater

24 © 2011 Pearson Education, Inc. Water supplies houses, agriculture, and industry Proportions of these three types of use vary dramatically among nations -Arid countries use water for agriculture -Developed countries use water for industry Consumptive use = water is removed from an aquifer or surface water body and is not returned -Irrigation = the provision of water to crops Nonconsumptive use = does not remove, or only temporarily removes, water -Electricity generation at hydroelectric dams

25 © 2011 Pearson Education, Inc. Why does agriculture use so much water? Rapid population growth requires more food and clothes The Green Revolution uses irrigation -We use 70% more irrigation water than 50 years ago Irrigation can double crop yields -18% of land is irrigated but produces 40% of our crops Irrigation is highly inefficient -Water evaporates in “flood and furrow” irrigation Overirrigation leads to waterlogging and salinization -Reducing world farm income by $11 billion

26 © 2011 Pearson Education, Inc. Governments subsidize irrigation Irrigation subsidies promote food self-sufficiency -But irrigation uses up huge amounts of groundwater for little gain Water in the Colorado River Valley is diverted for cotton and other crops grown in the desert Farmers in California’s Imperial Valley pay only 1 penny for 220 gallons of water

27 © 2011 Pearson Education, Inc. We divert surface water for our needs People divert water to farm fields, homes, and cities The once mighty Colorado River has been extensively diverted and used

28 © 2011 Pearson Education, Inc. Water-poor regions take water from others Politically strong, water-poor areas forcibly take water from weaker communities Los Angeles commandeered water from rural areas -Turning the environment into desert, creating dustbowls, and destroying the economy In 1941, L.A. diverted streams that fed Lake Mono -Lake levels fell, salt concentrations doubled Las Vegas wants to import water from sparsely populated eastern Nevada -An ecologically sensitive area

29 © 2011 Pearson Education, Inc. We build dikes and levees to control floods Flooding = a normal, natural process where water spills over a river’s banks -Spreading nutrient-rich sediments over large areas In the short term, floods damage property Dikes and levees (long, raised mounds of earth) along the banks of rivers hold water in channels -Levees make floods worse by forcing water to stay in channels and then overflow Dams prevent flooding and change a river’s nature -Releasing water periodically simulates flooding

30 © 2011 Pearson Education, Inc. Levees increase flooding A major levee along the Mississippi River failed after Hurricane Katrina, allowing parts of New Orleans to be flooded

31 © 2011 Pearson Education, Inc. We have erected thousands of dams Dam = any obstruction placed in a river or stream to block the flow of water to prevent floods, provide drinking water, allow irrigation, and generate electricity -45,000 large dams have been erected in more than 140 nations Only a few major rivers remain undammed -In remote regions of Canada, Alaska, and Russia Dams are great engineering feats -Many stand hundreds of feet tall

32 © 2011 Pearson Education, Inc. A typical dam

33 © 2011 Pearson Education, Inc. China’s Three Gorges Dam The dam, on the Yangtze River, is the largest in the world -186 m (610 feet) high, 2.3 km (1.4 mi) wide -Its reservoir stretches for 616 km (385 mi) -Provides flood control, passage for boats, and electricity

34 © 2011 Pearson Education, Inc. Drawbacks of the Three Gorges Dam Cost $39 billion to build Flooded 22 cities and the homes of 1.24 million people Submerged 10,000-year-old archaeological sites Drowned farmland and wildlife habitat Tidal marshes at the Yangtze’s mouth are eroding Pollutants will be trapped It will cost $5 billion to build sewage treatment plants to treat water

35 © 2011 Pearson Education, Inc. Some dams are being removed Some people feel that the costs of dams outweigh their benefits -They are pushing to dismantle dams The Federal Energy Regulatory Commission (FERC) renews licenses for dams -If dam costs exceed benefits, the license may not be renewed 400 dams have been removed in the U.S. -Property owners who opposed the removal change their minds once they see the healthy river

36 © 2011 Pearson Education, Inc. We are depleting surface water In many places, we are withdrawing water at unsustainable rates -Reduced flow drastically changes the river’s ecology, plant community, and destroys fish and invertebrates The Colorado River often does not reach the Gulf of California

37 © 2011 Pearson Education, Inc. The Aral Sea Once the fourth-largest lake on Earth -It lost 80% of its volume in 45 years The two rivers leading into the Aral Sea were diverted to irrigate cotton fields 60,000 fishing jobs are gone Pesticide-laden dust from the lake bed is blown into the air Cotton cannot save the region’s economy

38 © 2011 Pearson Education, Inc. Can the Aral Sea be saved? People have begun saving the northern part of the Aral Sea

39 © 2011 Pearson Education, Inc. Irrigation wastes water 15–35% of water withdrawals for irrigation are unsustainable Water mining = withdraws water faster than it can be replenished

40 © 2011 Pearson Education, Inc. The world is losing wetlands Wetlands are being lost as we divert and withdraw water -Channelize rivers, build dams, etc. As wetlands disappear, we lose ecosystem services -Filtering pollutants, wildlife habitat, flood control, etc. Many are trying to protect and restore them The Ramsar Convention on Wetlands of International Importance (1971) -Seeks the conservation and wise use of wetlands in the context of sustainable development -1,900 sites covering 185 million ha are protected

41 © 2011 Pearson Education, Inc. We are depleting groundwater Groundwater is easily depleted -Aquifers recharge slowly -Used by one-third of all people As aquifers are mined, water tables drop -Salt water intrudes in coastal areas Sinkholes = areas where ground gives way unexpectedly -Aquifers can’t recharge Wetlands dry up

42 © 2011 Pearson Education, Inc. Can we quench our thirst for bottled water? Groundwater is being withdrawn for bottled water -An average American drinks 29 gallons/year People drink bottled water for portability, convenience -They think it tastes better or is healthier Bottled water is no better than tap water -It is heavily packaged and travels long distances using fossil fuels -Bottles are not recycled -Corporations move in, deplete water, and move away

43 © 2011 Pearson Education, Inc. Bottled water is popular but problematic Bottled water is popular but it has several problems Energy costs of bottled water are 1,000–2,000 times greater than those of tap water

44 © 2011 Pearson Education, Inc. Will we see a future of water wars? Freshwater depletion leads to shortages, which can lead to conflict -261 major rivers cross national borders Water is a key element in hostilities among Israel, Palestinians, and neighboring countries Many nations have cooperated with neighbors to resolve disputes -They sign water-sharing treaties

45 © 2011 Pearson Education, Inc. Solutions can address supply or demand We can either increase supply or reduce demand Increasing supply through intensive extraction -Diversions increase supply in one area but decrease it elsewhere Reducing demand is harder politically in the short term -International aid agencies are funding demand-based solutions over supply-based solutions -Offers better economic returns -Causes less ecological and social damage

46 © 2011 Pearson Education, Inc. Desalinization “makes” more water Desalination (desalinization) = the removal of salt from seawater or other water of marginal quality -Distilling = evaporates and condenses ocean water -Reverse osmosis = forces water through membranes to filter out salts Desalinization facilities operate mostly in the arid Middle East -It is expensive, requires fossil fuels, kills aquatic life, and produces salty waste

47 © 2011 Pearson Education, Inc. The world’s largest reverse osmosis plant Near Yuma, Arizona Intended to remove salt from irrigation runoff Too expensive to operate and closed after 8 months -Engineers are trying to re-open it in a cost- effective way

48 © 2011 Pearson Education, Inc. Agricultural demand can be reduced Line irrigation canals Level fields to reduce runoff Use efficient irrigation methods -Low-pressure spray irrigation sprays water downward -Drip irrigation systems target individual plants Match crops to land and climate Eliminate water subsidies Selective breeding and genetic modification to produce crops that require less water Eat less meat

49 © 2011 Pearson Education, Inc. Residential demand can be reduced Install low-flow faucets, showerheads, washing machines, and toilets Rainwater harvesting = capturing rain from roofs Gray water = wastewater from showers and sinks Water lawns at night We can save hundreds or thousands of gallons/day Xeriscaping uses plants adapted to arid conditions

50 © 2011 Pearson Education, Inc. Industrial demand can be reduced Shift to processes that use less water -Wastewater recycling Use excess surface water runoff to recharge aquifers Patch leaky pipes and retrofit homes with efficient plumbing Audit industries Promote conservation/education

51 © 2011 Pearson Education, Inc. Market-based approaches to conservation End government subsidies of inefficient practices -Let the price of water reflect its true cost of extraction -But since industrial uses are more profitable than agricultural uses, poorer, less developed countries suffer Privatize water supplies: construction, maintenance, management, and ownership -May improve efficiency -There is little incentive to provide access to the poor Decentralization of water control may conserve water -Shift control to the local level

52 © 2011 Pearson Education, Inc. Freshwater pollution and its control Water for human consumption and other organisms needs to be: -Disease-free -Nontoxic Half of the world’s major rivers are seriously depleted and polluted -They poison surrounding ecosystems -Threatening the health and livelihood of people The invisible pollution of groundwater has been called a “covert crisis”

53 © 2011 Pearson Education, Inc. Point and nonpoint sources of pollution Pollution = the release of matter or energy that causes undesirable impacts on the health and well-being of humans or other organisms Point sources = discrete locations of water pollution -Factories, sewer pipes -Addressed by the U.S. Clean Water Act Nonpoint sources = pollution arises from multiple inputs over larger areas (farms, city streets, neighborhoods) -The major source of U.S. water pollution

54 © 2011 Pearson Education, Inc. Freshwater pollution sources

55 © 2011 Pearson Education, Inc. Nutrient pollution Nutrient pollution from fertilizers, farms, sewage, lawns, golf courses leads to eutrophication -Fertilizers add phosphorus to water, which boosts algal and aquatic plant growth -Spreading algae cover the surface, decreasing sunlight -Bacteria eat dead algae, reducing dissolved oxygen -Fish and shellfish die Solutions include treating wastewater -Reducing fertilizer application -Using phosphate-free detergents -Planting vegetation to increase nutrient uptake

56 © 2011 Pearson Education, Inc. Eutrophication is a natural process, but… Human activities dramatically increase the rate at which it occurs

57 © 2011 Pearson Education, Inc. Pathogens and waterborne diseases Enter water supplies through inadequately treated human waste and animal waste from feedlots Fecal coliform bacteria indicate fecal contamination -They are not pathogenic organisms -But the water may also hold other disease-causing pathogens (e.g., giardiasis, typhoid, hepatitis A) Bacterial pollution causes more human health problems than any other type of water pollution -Conditions are improving -86% of people now have safe water

58 © 2011 Pearson Education, Inc. Pathogens cause human health problems 1 billion are still without safe water 2.6 billion have inadequate sewer or sanitary facilities -Mostly rural Asians and Africans Health impacts kill 5 million people per year Solutions: -Disinfect drinking water -Treat sewage -Public education to encourage personal hygiene -Government enforcement of regulations protecting food

59 © 2011 Pearson Education, Inc. Toxic chemicals Pesticides, petroleum products, synthetic chemicals -Arsenic, lead, mercury, acid rain, acid drainage from mines Effects include poisoned animals and plants, altered aquatic ecosystems, and decreased human health Solutions: -Issue and enforce more stringent regulations of industry -Modify industrial processes -Modify our purchasing decisions

60 © 2011 Pearson Education, Inc. Sediment pollution Sediment in rivers can impair aquatic ecosystems Clear-cutting, mining, clearing land for housing, and cultivating farm fields expose soil to erosion It dramatically changes aquatic habitats -Fish may not survive Solutions: -Better management of farms and forests -Avoid large-scale disturbance of vegetation

61 © 2011 Pearson Education, Inc. Thermal pollution Water that is too warm causes problems -Warmer water holds less oxygen -Dissolved oxygen decreases as temperature increases -Industrial cooling heats water -Removing streamside cover raises water temperature Water that is too cold also causes problems -Water at the bottom of reservoirs behind dams is colder -When water is released, downstream water temperatures drop suddenly, killing aquatic organisms

62 © 2011 Pearson Education, Inc. Indicators of water quality Scientists measure properties of water to characterize its quality -Biological indicators: presence of fecal coliform bacteria, disease-causing organisms, algae, etc. -Chemical indicators: nutrient concentrations, pH, taste, odor, hardness, dissolved oxygen -Physical indicators: color, temperature, turbidity

63 © 2011 Pearson Education, Inc. Groundwater pollution is a difficult problem Groundwater is increasingly contaminated -But is hidden from view and difficult to monitor -“Out of sight, out of mind” Groundwater pollution is hard to address -It retains contaminants for decades and longer -It takes longer for contaminants to break down because of lower sunlight, microbes, and dissolved oxygen

64 © 2011 Pearson Education, Inc. Sources of groundwater pollution Some toxic chemicals occur naturally -Aluminum, fluoride, sulfates Pollution from human causes wastes leach through soils -Pathogens enter through improperly designed wells -Leaking underground storage and septic tanks So far, the EPA has cleaned up 388,000 leaking tanks

65 © 2011 Pearson Education, Inc. Agriculture and industry pollute groundwater Agricultural pollution comes from several sources -Pesticides are in most of the shallow aquifers tested -Nitrates from fertilizers have caused cancer, miscarriages, and “blue-baby” syndrome -Pathogens like Escherichia coli (E. coli) Manufacturing industries and military sites have been heavy polluters -By-products seep into water from miles around -Radioactive wastes will contaminate water for 750,000 years

66 © 2011 Pearson Education, Inc. It is best to prevent pollution It is far better to prevent pollution than use “end-of-pipe” treatment and cleanup Other options are not as good: -Removing just one herbicide from water in the U.S. Midwest costs $400 million/year -Pumping, treating, and re-injecting it takes too long Consumers can purchase sustainably made products -Become involved in local “riverwatch” projects -Urge government to pursue policies to fight pollution

67 © 2011 Pearson Education, Inc. Legislative efforts reduce pollution Water pollution was worse decades ago -Citizen activism and government response resulted in legislation during the 1960s and 1970s -The situation is much better now The Federal Water Pollution Control Act (1972) -Renamed the Clean Water Act in It is illegal to discharge pollution without a permit -Sets standards for industrial wastewater -Funded sewage treatment plants

68 © 2011 Pearson Education, Inc. Enforcement of water quality is decreasing Underfunded and understaffed state and federal regulatory agencies were pressured by industries and politicians Violations of the Clean Water Act have risen to over 100,000 documented violations/year -10% of Americans are unknowingly exposed to unsafe drinking water -The new EPA administrator has promised to improve Citizens pushed politicians to improve the Great Lakes -The water quality of the lakes has dramatically improved

69 © 2011 Pearson Education, Inc. We treat our drinking water Technology and government regulation have improved our pollution control -Treated drinking water is widespread and successful in developed nations Before water reaches the user, it is chemically treated, filtered, and disinfected The EPA sets standards for over 90 drinking water contaminants -Local governments and private water suppliers must meet these standards

70 © 2011 Pearson Education, Inc. We treat our wastewater Wastewater = water people have used in some way -Household, manufacturing, stormwater runoff, etc. -It is treated before being released into the environment Septic systems = the most popular method of wastewater disposal in rural areas -Underground septic tanks separate solids and oils from wastewater -The water drains into a drain field, where microbes decompose the pollutants -Solid waste is periodically pumped out and landfilled

71 © 2011 Pearson Education, Inc. Municipal sewer systems In populated areas, sewer systems carry wastewater to treatment locations Primary treatment = physically removes contaminants in settling tanks (clarifiers) Secondary treatment = water is stirred and aerated -Aerobic bacteria degrade organic pollutants -Water treated with chlorine (and/or ultraviolet light) is piped into rivers or the ocean Reclaimed water is used for lawns, irrigation, or industry

72 © 2011 Pearson Education, Inc. A typical wastewater treatment facility Sludge = solid material resulting from treatment -Is decomposed microbially -Then landfilled, incinerated, or used as fertilizer on cropland Methane-rich gas created by decomposition can be burned to generate electricity

73 © 2011 Pearson Education, Inc. Artificial wetlands clean wastewater After primary treatment at a conventional facility -Water is pumped into the wetland -Microbes decompose the remaining pollutants Cleansed water is released into waterways -Or percolates underground They are havens for wildlife and areas for human recreation The U.S. has over 500 artificially constructed or restored wetlands

74 © 2011 Pearson Education, Inc. Conclusion Obtaining future supplies of freshwater requires citizen action, legislation and regulation, technology, economic incentives, and education With expanding population and increasing water usage, we are approaching conditions of widespread scarcity Water pollution is already harming health, economies, and societies of both rich and poor nations Better regulation has improved water quality in the U.S. and other developed nations

75 © 2011 Pearson Education, Inc. QUESTION: Review The picture shows a(n): a) Braided river b) Meandering river c) Oxbow d) River delta

76 © 2011 Pearson Education, Inc. QUESTION: Review The area of a lake that rings the edge and contains rooted plants is called the _______ zone. a)Littoral b)Benthic c)Limnetic d)Profundal

77 © 2011 Pearson Education, Inc. QUESTION: Review An unconfined aquifer is defined as: a)An aquifer that traps porous rocks between layers of less permeable substrate b)An aquifer that traps porous rocks under one layer of less permeable substrate c)An aquifer with porous rocks resting on bedrock d)An aquifer with no upper layer

78 © 2011 Pearson Education, Inc. QUESTION: Review Why do governments subsidize irrigation? a)It promotes food self-sufficiency. b)Governments have to or food could not be grown. c)Governments want to lower water tables. d)Governments do not subsidize irrigation.

79 © 2011 Pearson Education, Inc. QUESTION: Review Which of the following statements is NOT true about dams? a)They change habitat. b)They generate electrical power. c)They have created more farmland upstream. d)Pollutants are trapped in reservoirs.

80 © 2011 Pearson Education, Inc. QUESTION: Review Which of the following is a point source of water pollution? a)A factory b)Roads c)Agricultural fields d)All are point sources

81 © 2011 Pearson Education, Inc. QUESTION: Review Which of the following type of water pollution causes the most severe human health problems? a)Nutrient pollution b)Pathogens c)Toxic chemicals d)Sediment pollution

82 © 2011 Pearson Education, Inc. QUESTION: Review Which statement is NOT correct regarding using artificial wetlands to treat wastewater? a)Water first undergoes primary treatment. b)Microbes decompose pollutants. c)Cleansed water cannot be released into waterways. d)They are good areas for wildlife.

83 © 2011 Pearson Education, Inc. QUESTION: Interpreting Graphs and Data In this figure of a wastewater treatment facility, what is the first step in treatment? a)Physical screening to remove large debris b)Aeration in basins c)Secondary clarification d)Piping the treated water into a river

84 © 2011 Pearson Education, Inc. QUESTION: Viewpoints During times of drought, conflicts erupt between farmers (who need water for irrigation) and ecologists (who want water left in rivers to protect wildlife). Who should have the highest priority? a)Farmers - they need the water for their crops. b)Wildlife - animals will die without water. c)Farmers should be paid subsidies to withdraw water from other places. d)Farmers should be paid to plant different crops that do not require so much water.

85 © 2011 Pearson Education, Inc. QUESTION: Weighing the Issues Should cities in dry areas, such as Las Vegas, be allowed to increase their populations, so that they will require more water? a)Yes; it’s un-American to limit what cities can do. b)Yes; but make the people pay the true cost of water. c)Yes; but only if the people are required to use drastic conservation measures. d)No; enough is enough, and cities in arid environments simply cannot continue growing.

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