Chapter 11 Water Resources
Chapter 11 Outline:
11.1 The Water Cycle and Water Distribution
11.3 Water Supply & Water Shortages – Problems and Possible Solutions
11.4 Introduction to Water Pollution
11.5 Categories of Water Pollution
11.7 Case Study: The Aral Sea – Going, Going, Gone
Learning Outcomes:
After studying this chapter, each student should be able to:
- 11.1 Describe the water cycle, including the processes of evaporation, sublimation, and precipitation.
- 11.2 Describe the human need for water on the planet and the major freshwater sources of this water
- 11.3 List some of the major challenges to human’s supply of water and how those challenges may be met with changes to sustainable use
- 11.4 Define water pollution and describe point and nonpoint sources of contaminants
- 11.5 Describe the different types of organic and non-organic water pollutants and their major sources
- 11.6 Explain how water is treated for safe human consumption
- 11.7 Tell the story of the Aral Sea, and describe why environmental policies are so important for the maintenance of our freshwater resources
- 11.8 Summarize the chapter
11.1 The Water Cycle and Water Distribution
Like the cycling of nutrients, such as carbon, nitrogen, and phosphorus, water also follows a cycle during which water is transported from one location to another and exists in different forms (Figure 1). The water cycle is driven by the Sun’s energy as it warms the oceans and other surface waters. This warming leads to evaporation, the conversion of liquid surface water to gaseous water vapor that exists in the atmosphere. Sublimation is the conversion of ice directly into water vapor, also moving water from the Earth’s surface into the atmosphere. Over time, this atmospheric water vapor undergoes condensation (gas becoming liquid again) and forms clouds, which contain either liquid water or ice and eventually return water to the Earth’s surface as precipitation in the form of rain, hail, sleet, or snow. Precipitation reaching the Earth’s surface may evaporate again, flow over the surface, or percolate into the ground. Most easily observed is surface runoff: the flow of freshwater over land either from rain or melting ice. Runoff can make its way through streams and lakes to the oceans.
Figure 1. The Water Cycle. Arrows depict movement of water to different reservoirs located above, at, and below Earth’s surface. (Credit: United States Geological Survey)
In many natural terrestrial environments rain and other forms of precipitation encounters vegetation before it reaches the soil surface. Some of this water evaporates immediately from the surfaces of plants. Most of the precipitation reaches the soil and begins to move down into the earth. Surface runoff into streams and rivers will occur only if the soil becomes saturated with water in a heavy rainfall, or the soil is dry and hard packed, as in many drier areas of our earth. Water entering the soil can be then taken up by plant roots. The plant will use some of this water for metabolism and some will find its way into the animals (herbivores) that eat the plants. Much of the plant’s water, however, will return to the atmosphere through a process known as transpiration. In this process water enters the vascular system of plants through the roots and evaporates, or transpires, through the leaves, often through small, microscopic structures called stomata (which open for gas exchange, but are also an exit point for evaporating water). Ecologists sometimes combine transpiration and evaporation from plants into a single term: evapotranspiration. Water in the soil that is not taken up by a plant and that does not evaporate may percolate into the subsoil and bedrock where it forms groundwater, and may reach underground areas known as aquifers.
Surface Water Transports Water over Land
Precipitation does not always stay where it initially lands on the Earth’s surface. Surface water flowing from rain and melted snow and ice enters river channels by surface runoff. River discharge describes the volume of water moving through a river channel over time (Figure 2.). Soon after a heavy rainstorm, river discharge increases due to surface runoff. Gravity pulls river water downhill toward the ocean, eroding rock and soil particles and dissolving minerals along the way. Groundwater contributions to streams and rivers can also add dissolved minerals to river water.
Figure 2. River Discharge Colorado River, U.S.. Rivers are part of overland flow in the water cycle and an important surface water resource. (Credit: : Gonzo fan2007 at Wikimedia Commons)
The geographic area drained by a river and its tributaries is called a drainage basin or watershed. The Mississippi River watershed (drainage basin), for example, includes approximately 40% of the U.S. land area (Figure 3). This watershed includes many smaller drainage basins, such as the Ohio and Missouri River watersheds that flow into the Mississippi.
Figure 3. The Mississippi River river basin or watershed includes approximately 40% of the United States, with several other rivers draining into it. (Credit: “Watershed of Mississippi” is in the Public Domain, CC0)
In addition to rivers, lakes contain large amounts of freshwater. They usually receive water from surface runoff and groundwater discharge. They tend to be short-lived on a geological time-scale because they are constantly filling in with sediment supplied by rivers. Lakes form in a variety of ways including glaciation, recent tectonic uplift (e.g., Lake Tanganyika, Africa), and volcanic eruptions (e.g., Crater Lake, Oregon). Humans have also created artificial lakes (reservoirs) by damming rivers. Changes in global climate patterns have resulted in major changes in the size and very existence of many lakes. As Earth was coming out of the last Ice Age about 15,000 years ago, the climate in the western U.S. changed from cool and moist to warm and arid, which caused more than 100 large lakes to disappear. The Great Salt Lake in Utah, for example, is a remnant of a much larger lake.
Groundwater
Groundwater is a significant, subsurface reservoir of freshwater, and exists in the pores between particles in dirt, sand, gravel, or fissures in rocks. Groundwater can flow slowly through these pores and fissures and may eventually find its way back up to an opening in the earth’s surface, adding water to a spring, stream, or lake, or may be drawn up to the surface by wells. Some streams flow not because they are replenished from rainwater directly but because they receive this constant inflow from groundwater.
An aquifer is a kind of groundwater found in a body of porous rock or sediment below the soil surface. Aquifers are saturated with groundwater that may have seeped down through the soil after rains or other precipitation. Other sources of aquifer groundwater include seepage from surface water (lakes, rivers, reservoirs, and swamps), surface water deliberately pumped into the ground by humans, irrigation, and underground wastewater treatment systems (septic tanks). Recharge areas are locations where surface water infiltrates the ground rather than running into rivers or evaporating. Wetlands, for example, are excellent recharge areas. Groundwater discharge is the movement of water out of an area of saturated soil, into springs, lakes, streams or other areas of surface water.
Many aquifers are the major source of drinking water or irrigation water that is drawn up through wells. In many cases these aquifers took millions of years to fill (called fossil water), and persisted for millennia. and may be depleted by groundwater pumping faster than they are being replenished by new water percolating in. In dry areas across the earth these wells may yield less and less water over time and go dry, since the aquifer water is not being replaced as fast as it is being removed.
To learn more about issues related to groundwater in Arizona, watch the following video:
University of Arizona Extension Service – Arizona Project Wet (2022, May 12) Arizona groundwater – episode 2 – groundwater basics [Video – YouTube] https://youtu.be/TQNawfSkBM4?list=PLI7_l38IJ9TKgnT2QgRQDIQGaAfU4bQ_o
Glaciers
A glacier is a large, perennial accumulation of ice, snow, rock, sediment, and some liquid water that accumulates on land. Glaciers moves down slope under the influence of their own weight and gravitational pull. Although glaciers represent the largest reservoir of freshwater, they generally are not used as a water source by humans because the water is frozen and they are located too far away from most people (Figure 4). Melting glaciers in the high mountains of many regions on earth do provide a natural source of river water and groundwater, though. During the last Ice Age there was as much as 50% more water in glaciers than there is today, which caused sea level to be about 100 m lower. Over the past century, sea level has been rising in part due to melting glaciers. If Earth’s climate continues to warm because of human actions, the melting glaciers will cause an additional rise in sea level.
Figure 4. Mountain Glacier in Argentina Glaciers are the largest reservoir of freshwater but they are not used much as a water resource directly by society because of their distance from most people. (Credit: Luca Galuzzi – www.galuzzi.it)
Final Notes on the Distribution of Water
Earth can truly be called “the water planet”. The abundance of liquid water on Earth’s surface distinguishes this planet from other bodies in the solar system. About 70% of Earth’s surface is covered by oceans and approximately half of Earth’s surface is obscured by clouds (also made of water) at any given time. There is a very large volume of water on our planet, about 1.4 billion cubic kilometers (km3) (330 million cubic miles) or about 53 billion gallons per person on Earth. All of Earth’s water could cover the United States to a depth of 145 km (90 miles).
The hydrosphere is the area of Earth where water moves or is stored, either as ice, groundwater (aquifers), surface water, or even water vapor. The storage areas (often referred to as reservoirs) include liquid water on the earth’s surface (rivers, streams, lakes, & oceans), water beneath the surface (groundwater), water stored as ice, (polar ice caps and glaciers), and water in the form of gaseous vapor in the atmosphere. Of these stores of water, 97.5 percent is salt water, primarily in the oceans and seas (Figure 5).
The remaining 2.5% of water on Earth is freshwater, with more than 99 percent of freshwater occurring in the form of groundwater or ice. Thus, less than one percent of freshwater is present in lakes and rivers – forms most easily accessible to humans. Numerous other species are also dependent on this small percentage of total water, a lack of which can have serious negative effects on ecosystems.
Figure 5. Earth’s Water Reservoirs. Bar chart Distribution of Earth’s water including total global water, freshwater, and surface water and other freshwater and Pie chart Water usable by humans and sources of usable water. (Credit: United States Geographical Survey; Igor Skiklomanov’s chapter “World freshwater resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s freshwater Resources)
11.2 Water Use by Humans
The human body needs water to perform metabolic reactions, to eliminate waste products, and to transport materials throughout the body. In fact, humans are made up of about 60-75% water. Physiologically, humans need on average about one gallon of water per day for bodily needs. Of course, this amount varies depending on climate and daily activities. A human can live without food for several weeks, but has to replenish water on a daily basis to survive.
People need much more water, however, to take care of household requirements. A typical U.S. household uses approximately 100 gallons of water per day, which includes activities such as cooking, washing dishes and clothes, flushing the toilet, and bathing. Even more is needed to water lawns and plants. The food we eat also requires a lot of water to produce: one tomato takes three gallons of water, one loaf of bread takes 150 gallons, and just one pound of beef takes 1,600 gallons of water! The water needed to create these foods and others goes beyond just irrigating plants; water is needed for harvesting, cleaning, packaging, and transporting the food to local grocery stores.
Humans also need water to extract energy resources such as fossil fuels from the earth, to meet our needs for mineral resources such as iron and copper, and to meet the needs of countless other human activities. The steel for one small car (1 ton), for example, takes 63,000 gallons to manufacture.
Humans have developed numerous technologies to increase water availability, such as digging wells to harvest groundwater, storing rainwater, and using desalination to obtain drinkable water from the ocean. Although this pursuit of drinkable water has been ongoing throughout human history, the supply of freshwater continues to be a major issue in modern times.
One important environmental goal is to provide clean water to all people. Fortunately, water is a renewable resource and it is difficult to destroy. Evaporation and precipitation combine to replenish our freshwater supply constantly. Unfortunately, the availability of water is complicated by its uneven distribution over the earth. Arid climates and dense populations of humans have combined in some parts of the world to create water shortages, which are projected to worsen in the coming years due to continued population growth and climate change.
Human activities such as water overuse and the pollution of available water sources have significantly compounded the water crisis that exists today. Hundreds of millions of people lack easy access to safe drinking water, and over a billion people lack access to sanitation as simple as a pit latrine. As a result, nearly two million people die every year from diarrheal diseases and 90% of those deaths occur among children under the age of five. Most of these deaths could be prevented with improved sanitation and the availability of clean water.
Why is it so difficult for some people on this water planet known as Earth to get access to clean, freshwater? Remember that over 97% of water is seawater, which is too salty to drink or use for irrigation. The water sources most commonly used by humans are the rivers and lakes, which contain less than 0.01% of the world’s total water! Although most people in the world use these surface waters for their needs, groundwater is actually a much larger reservoir of usable freshwater, containing more than 30 times more water than rivers and lakes combined.
Groundwater is a particularly important resource in arid climates, where surface water may be scarce. In addition, groundwater is the primary water source for most people living in rural areas, providing 98% of that water demand in the United States, for example. Most groundwater originates from rain or snowmelt, which percolates into the ground and moves downward until it reaches the saturated zone (where groundwater completely fills pore spaces in earth materials). As noted previously, some of these groundwater sources were created over long periods of time, and are not currently being replenished for continued use.
The following video explores the various ways that water plays important roles in human lives:
National Science Foundation (2017, Feb 9) Human water cycle: water, food, and energy [Video – YouTube] https://youtu.be/bdnUWRmCMD8
Precipitation Varies Around the World
The location of freshwater is not equally distributed around the world, making certain areas rich in water resources, while other areas have a life threatening lack of freshwater. More precipitation falls near the equator (Figure 6), whereas less precipitation tends to fall in areas 30 degrees north and south of the equator, where the world’s largest deserts are located. These rainfall and climate patterns are related to global wind patterns and the angle of the sun on the earth. Global precipitation and climate patterns are also affected by the size of continents, major ocean currents, and mountains. Climate patterns and precipitation amounts have been altered by deforestation and human caused climate change, so that both record draughts and record flooding has been occurring across the globe.
Figure 6. World Rainfall Map. The map above shows the amount of precipitation that falls around the world, with the darkest blue areas receiving the most and the light green areas receiving the least precipitation. Areas of high rainfall include Central and South America, western Africa, and Southeast Asia. Since these areas receive so much rainfall, they are where most of the world’s rainforests grow. Areas with very little rainfall usually turn into deserts. The desert areas include North Africa, the Middle East, western North America, and Central Asia. (Credit: United States Geological Survey Earth Forum, Houston Museum Natural Science)
11.3 Water Supply & Water Shortages – Problems and Possible Solutions
Global total water use (water withdrawal) is steadily increasing at a rate greater than the growth in world population. During the 20th century the global human population tripled, while water demand increased by a factor of six. This increase in global water demand is due to improved standards of living for many people, with people living in larger homes and consuming more water dependent products. This increase has not been adequately offset by attempts at water conservation. Increased production of goods and energy entails a large increase in water demand. The major global water uses are irrigation (68%), public supply (21%), and industry (11%). For the United States, water use has somewhat leveled off, as the graphs below showing water consumption over time illustrates (Figures 7). In parts of the country, especially near urban areas, agricultural land, which uses a lot of water, is being converted to housing developments and other types of uses, requiring less water per acre.
Figure 7. Trends in total water withdrawals in the U.S. from 1950 to 2015 by water use category, including bars for thermoelectric power, irrigation, public water supply, and rural domestic and livestock. Thin blue line represents total water withdrawals using vertical scale on right. (Credit: United States Geological Survey)
Rivers and lakes are important water resources for irrigation of cropland and for the drinking water needed by many cities around the world. But as this resource becomes scarce (with large population increases), disputes have arisen both within nations and states, and across boundaries. International disputes over the use of their waters include the Colorado River (between Mexico and the United States.), the Nile River (between Egypt, Ethiopia, and Sudan), the Euphrates River (between Iraq, Syria and Turkey), the Ganges River (between Bangladesh and India), and the Jordan River (between Israel, Jordan and Syria).
Problems also exist with the use of groundwater. As water is pumped from water wells, there can be a localized drop in the water table around the well called a cone of depression (Figure 8). When there are a large number of wells that have been pumping water for a long time, the regional water table can drop significantly. This is called groundwater mining, the practice of withdrawing groundwater at rates in excess of natural recharge. This excess of water removal can force the drilling of deeper, more expensive wells that often encounter more saline groundwater.
Figure 8. Formation of a Cone of Depression around a Pumping Water Well (Source: Fayette County Groundwater Conservation District, TX)
Rivers, lakes, and artificial lakes (reservoirs) can also be depleted due to overuse. Some large rivers, such as the Colorado River in the U.S. and the Yellow River in China, run dry some years due to large demand. The case history of the Aral Sea discussed later in this chapter involves depletion of an entire lake. Finally, glaciers are being depleted due to accelerated melting associated with human caused global warming.
A water resource problem associated with groundwater mining is saltwater intrusion. This problem is particularly severe in ocean coastal areas, such as the densely populated coastal areas of Florida, where overpumping of freshwater aquifers near the coastlines causes saltwater to enter the depleting freshwater zones (Figure 9.)
Figure 9. This image shows how removing water from aquifers along ocean coasts can allow saltwater to move inland. The zone of transition is where the saline water is replacing freshwater in groundwater aquifers. (Credit: US Environmental Protection Agency)
Another problem with excess water withdrawal from unconfined aquifers is that this can change the direction of regional groundwater flow, sending nearby polluted groundwater toward a pumping well. Finally, the problem of subsidence (gradual sinking of the land surface over a large area) and sinkholes (rapid sinking of the land surface over a small area) can develop due to a drop in the water table as water is pumped out of underground aquifers (Figure 10).
Figure 10. A massive sinkhole opened up in Harbor, Oregon in January 2016. (Credit: “Sinkhole in Harbor, Oregon 2” by Oregon Department of Transportation is licensed under CC BY 2.0)
Water Supply Crisis – Other Problems and Solutions
The water crisis (sometimes called water stress) refers to a global situation where people in certain areas of our planet lack access to sufficient clean water. In general, water stress is greatest in areas with very low precipitation (major deserts), large population density (e.g., India), or both. Future global warming could worsen the water crisis by shifting precipitation patterns away from certain areas and by completely melting mountain glaciers that have recharged rivers downstream for centuries. Melting glaciers will also contribute to rising sea level, which will worsen saltwater intrusion in freshwater aquifers near ocean coastlines.
Fresh water on Earth is finite. According to a report by the United Nations Development Program (UNDP), climate change is shifting our reliable access to water, upon which civilization depends, and pollution restricts its uses. Today, half of the world population (4 billion people), live with severe water scarcity for at least one month of the year. About half a billion face water scarcity year-round. Approximately 4.2 billion lack safe forms of sanitation, 2.2 billion people lack safe drinking water, and 700 million people could be displaced due to scarcity of water by 2030
Most of those facing water stress live in the Middle East and North Africa, which have arid climates. In the coming years, the UNDP report projects that more than 3 billion people (about 40% of the world’s population) will live in water-stressed areas, with the largest increases coming mainly in China and India, due to rapid population growth. The lack of water will also impact food production and our ability to feed the ever-growing human population. We can expect future global tension and even conflict associated with water shortages and pollution. Historic and future areas of water conflict include the Middle East (Euphrates River and Tigris River conflict among Turkey, Syria, and Iraq; Jordan River conflict among Israel, Lebanon, Jordan, and the Palestinian territories), Africa (Nile River conflict among Egypt, Ethiopia, and Sudan), Central Asia (Aral Sea conflict among Kazakhstan, Uzbekistan, Turkmenistan, Tajikistan, and Kyrgyzstan), and south Asia (Ganges River conflict between India and Pakistan).
The following short video gives an update on the global water crisis, and its many ramifications for human lives:
BBC News (2023, Mar 22) Global water crisis is looming, UN says.[Video- YouTube] https://youtu.be/hfTB5TgJq5w
Sustainable Solutions to the Water Supply Crisis?
The water crisis described above requires multiple approaches to extending our freshwater supply and moving towards sustainability:
Creating dams is one of the longstanding traditional approaches to ensuring a steady water supply – they store water that would normally flow downstream, even out into the ocean. Reservoirs that form behind these dams can collect water during wet times and provide water during dry periods. Dams are used for urban water supplies and flood control, but are also important sources of hydroelectricity and areas of recreation. But dams do have major drawbacks, including serious damage to river ecosystems (riparian areas), flooding of valuable land, and interference with the migration and spawning of fish such as salmon. In addition, dams and canals created in arid regions lose a lot of water to evaporation, and the disruption of habitats that the canals bisect. Today, more dams are being dismantled in the United States than are being built.
Desalinization can actually increase the amount of freshwater on Earth. This involves removing dissolved salt from seawater or saline groundwater to produce freshwater. There are several ways to desalinate seawater including boiling, filtration, and electrodialysis. All of these procedures are moderately to very expensive, however, and require considerable energy input, making the water produced much more expensive than freshwater from conventional sources. In addition, the process creates highly saline wastewater, which must be disposed of without creating significant negative effects on the local environment. Desalination is most common in the Middle East, where energy from oil is abundant but water is scarce.
Conservation is the practice of using water more efficiently, so that less water is needed. Around the home, conservation can involve technological improvements, such as the use of high-efficiency clothes washers and dishwashers and low-flow showers and toilets. Conservation can also come from changes in human behavior, such as growing native plants in landscaping that can be sustained by local weather patterns, to turning off the water while brushing teeth, and fixing leaky faucets and toilets.
Rainwater harvesting involves catching and storing rainwater for later use, usually to water plants. Making changes to use more efficient irrigation can have a large effect on local water use, since agricultural irrigation accounts for the greatest demand for water in most areas. Water conservation strategies in agriculture include growing crops in areas where the natural rainfall can support them, installing more efficient irrigation systems such as drip systems that minimize losses due to evaporation, no-till farming that reduces evaporative losses by covering the soil, and reusing treated wastewater from sewage treatment plants for agricultural purposes. Recycled wastewater has also been used to recharge aquifers, rebuilding this important reservoir of water.
11.4 Introduction to Water Pollution
Water pollution has made the global water crisis worse. For water to be useful for drinking and irrigation, it must not be polluted beyond certain thresholds. Water pollutants include waterborne disease-causing organisms, chemical pollution (from agriculture, industry, or communities), and mining related waste discharge (effluent). Some pollutants have serious and well-known health effects, whereas many others have not been studied enough to determine their long-term health effects. In the U.S. currently more than 40,000 bodies of water fit the definition of “impaired” set by the EPA, which means they could neither support a healthy ecosystem nor meet water quality standards. Globally, improving water safety, sanitation, and hygiene could prevent up to 9% of all disease and 6% of all deaths.
All naturally occurring water contains some amount of dissolved chemicals, some of which are important human nutrients while others can be harmful to human health. The concentration of a water pollutant is commonly given in very small units such as parts per million (ppm) or even parts per billion (ppb). An arsenic concentration of 1 ppm means 1 part of arsenic per million parts of water. This is equivalent to one drop of arsenic in 50 liters of water. Total dissolved solids (TDS) represent the total amount of dissolved material in water, which may include minerals, salt, organic matter, and pollutants. While some dissolved solids may be natural or normal for a particular body of water, other dissolved solids may be an indication of pollution.
Water pollution is described as the contamination of water by an excess amount of a substance that can cause harm to human beings or the ecosystem. The level of water pollution depends on the abundance of the pollutant, the ecological impact of the pollutant, and the use of the water. Pollutants are derived from biological, chemical, or physical processes. Although natural processes such as volcanic eruptions or evaporation sometimes can cause water pollution, most pollution is derived from human, land-based activities. Water pollutants can move through different water reservoirs, as the water carrying them progresses through different stages of the water cycle. Water residence time is the average time that a water molecule spends in a water reservoir. Water in rivers has a relatively short residence time, as it flows elsewhere, so pollution may be present only briefly. Of course, pollution in rivers may simply move to another reservoir, such as the ocean, where it can cause further problems. Groundwater is typically characterized by slow flow and longer residence time, which can make groundwater pollution particularly problematic. Finally, pollution residence time can be much greater than the water residence time because a pollutant may be taken up for a long time within the ecosystem or absorbed into sediment.
Point Sources of Pollution
Pollutants may enter water supplies from point sources, with easily identifiable locations, or from nonpoint sources, which include large and more diffuse areas. Point sources of water pollution include industrial plants, direct discharge by factories, and more recently animal factory farms (Figure 11) that create concentrated waste as large numbers and densities of livestock (especially cows, pigs, and chickens) are raised .
Figure 11. Large animal farms are often referred to as concentrated feeding operations (CFOs). These farms are considered potential point sources of pollution because untreated animal waste may enter nearby water bodies as untreated sewage. (Credit: Environmental Health Perspectives)
Sewage treatment plants also are point sources, but a well run plant should produce little pollution. However, in some cities, storm water is also sent to treatment plants (called a combined sewer system), so that during heavy rain, incoming waters may exceed the treatment plant’s capacity, causing it to release the untreated sewage directly into local surface waters.
Nonpoint Sources of Pollution
Nonpoint sources of pollution are more difficult to control, since they originate from large areas of human altered landscapes. Sources may include agricultural fields, lawns and driveways, roads, both urban and suburban areas, and abandoned mines. Rainfall that runs across these types of lands and through their soil may pick up pollutants such as herbicides, pesticides, road salts, and chemicals, as well as animal wastes and fertilizers, depositing them into local streams, rivers, or lakes. Acidic runoff and a variety of toxic chemicals can also seep for years from both active and abandoned mines. Nonpoint source pollution is the leading cause of water pollution in the U.S. and is usually more difficult and expensive to control than point source pollution because of its multiple and widespread sources, and greater volume of water (Figure 12).
Figure 12. Sources of Water Contamination. Sources of some water pollutants and movement of pollutants into different water reservoirs of the water cycle. (Credit: U.S. Geological Survey)
Plastics in Water
Plastics have become a serious threat to aquatic ecosystems in the past few decades. Experts estimate that over 12 million tons of plastic enters waterways and oceans every year. In addition to the harm caused by the plastics themselves, various pollutants can stick to plastic, sometimes in microscopic amounts, as these plastics move through rivers and streams, and are ultimately discharged into the ocean. If ingested by marine organisms, contaminated plastics can sicken many forms of marine life. Of course, if humans eat contaminated seafood, they too are in peril. Larger pieces of plastic can choke or entangle marine life, including sea birds, sea turtles, and marine mammals of all kinds, even whales.
So much plastic has been discharged into the world’s oceans that scientists have identified five enormous plastic garbage patches floating mid-ocean, in areas of where ocean currents have concentrated these pollutants. Although these plastics may degrade into smaller particles, most are very resilient, and may never degrade completely. As more and more plastics enter the ecosystem, the garbage patches enlarge, endangering any sea life that enters these areas. The largest accumulation of ocean plastic, called the Great Pacific Garbage Patch, covers an area of 1.6 million square kilometers, or twice the size of Texas, and is growing wider each year.
Clean Water Act
During the 1900s, rapid industrialization in the U.S. resulted in widespread water pollution due to free discharge of waste into surface waters. The Cuyahoga River in northeast Ohio caught fire numerous times due to oils and industrial wastes being discharged directly into the river, including a famous fire in 1969 that caught the nation’s attention (Figure 13).
In 1972 Congress passed one of the most important environmental laws in U.S. history, the Federal Water Pollution Control Act, which is more commonly called the Clean Water Act. The purpose of the Clean Water Act and later amendments is to maintain and restore water quality, or in simpler terms to make our water swimmable and fishable. It became illegal to dump pollutants into surface water and U.S. water quality improved significantly as a result. More progress is needed because currently the EPA considers over 40,000 U.S. water bodies as impaired, most commonly due to pathogens, metals, plant nutrients, and oxygen depletion. Another concern is protecting groundwater quality, which is yet to be addressed by federal law.
Figure 13. Cuyahoga River on fire, June 22, 1969. A thick oil slick is what was actually burning on this highly contaminated river, but this event brought attention to the highly polluted state of many of our nation’s rivers and lakes. (Credit: National Oceanic and Atmospheric Administration)
The following video discusses the burning of the Cuyahoga river, as well as addressing other topics related to water pollution.
Bozeman Science (2016, Jan 4) Water Pollution [Video- YouTube] https://youtu.be/GNGKsubYJ9U
11.5 Types of Water Pollutants
Water pollution comes in many different forms including, but not limited to, organic wastes, oil spills, pathogens, and toxic chemicals, including heavy metals.
Organic Wastes
Organic waste in water most commonly refers to human and animal waste products in the form of manure or sewage, or the decomposing remains of dead organisms. This waste ultimately uses up water’s oxygen stores and can be a devastating pollutant to ecosystems. Surface water in contact with the atmosphere has dissolved oxygen, which is needed by aquatic organisms for cellular respiration and decomposition reactions. When bacteria in water sources decompose dead organic matter, they remove dissolved oxygen (O2) from the water according to the following reaction:
Organic Matter + O2 → CO2 + H2O
This is a naturally-occurring and necessary reaction, but problems occur when too much decaying organic matter is present in water, allowing the bacteria to multiply and remove most or all available oxygen, killing other living things, such as fish, crustaceans, mussels, and protists. The amount of oxygen used by aerobic (oxygen requiring) bacterial decomposition of organic matter is called biochemical oxygen demand (BOD).
An example of an unpolluted, low BOD body of water would be a turbulent river flowing through a healthy forest. Turbulence continually brings water in contact with the atmosphere where the O2 content is restored. The dissolved oxygen content in such a river ranges from 10 to 14 ppm O2, BOD is low, and clean-water fish such as trout thrive.
An example of a polluted, high BOD body of water might be a stagnant, deep lake that receives runoff from agricultural fields or a combined sewer system. This system allows for a high input of organic matter and limited chances for water circulation and further contact with the atmosphere. In such a lake, the dissolved O2 content is ≤5 ppm, BOD is high, and low O2 tolerant fish, such as carp and catfish may dominate.
Eutrophication is a process where the rapid growth of aquatic algae (due to organic pollution) leads to rapid bacterial growth, ultimately destroying life in an aquatic environment as oxygen is depleted. Eutrophication is caused by excessive amounts of plant fertilizers or other organic sources entering water, particularly those rich in nitrogen (N) and phosphorus (P), pollutants that may significantly raise BOD. Aquatic plants and algae require about 15 nutrients for growth, most of which are plentiful in water. Nitrogen and Phosphorous are called limiting nutrients, however, because they usually are present in water at low concentrations and therefore restrict the total amount of algal growth. High concentrations of N and P from human sources (mostly agricultural and urban runoff including fertilizers, sewage, and phosphorus-based detergent) can cause the eutrophication described above. Exponential growth of algae (called algal blooms) can create dense algal populations and even thick mats of floating algae, which can be followed by prolific bacterial growth (Figure 14).
Figure 14. Algal blooms occur in Downfall Creek at times when eutrophication is exacerbated by drought and low discharge. (Credit: “Downfall Creek algal bloom Newman Rd Chermside IMGOP9383” by John Robert McPherson is licensed under CC BY-SA 4.0)
When the prolific algal populations die, they can be broken down my bacteria, becoming oxygen-demanding waste, creating very low O2 concentrations in the water (< 2 ppm), a condition called hypoxia. This results in a dead zone where most or all aquatic organisms die for lack of oxygen (Figure 15).
Figure 15. Map showing dissolved oxygen levels in the Gulf of Mexico off the coast of Texas and Louisiana. Areas with low levels of dissolved oxygen are hypoxic dead zones. (Credit: “Dead Zone NASA NOAA” by NASA NOAA is in the Public Domain, CC0)
An estimated 50% of lakes in North America, Europe, and Asia are negatively impacted by eutrophication. In addition, the size and number of marine hypoxic zones have grown dramatically over the past 50 years, including a large dead zone located in the Gulf of Mexico near the Louisiana coastline. Eutrophication and hypoxia are difficult to combat, because they are caused primarily by nonpoint source pollution, which is more difficult to regulate than point sources. Farm fields have to be landscaped to prevent runoff, and wastewater has to be treated more thoroughly to reduce the amount of nitrogen and phosphorus.
Oil Spills
Oil spills are another source of organic water pollution, caused by the accidental discharge of oil onto land or more commonly into water by oil tankers, ships, barges, pipelines, refineries, drilling rigs and storage facilities, and to a lesser extent, recreational boats and marinas. Some of the most dramatic and ecologically damaging oil spills often result from supertanker accidents such as when the Exxon Valdez oil tanker ship ran aground on a reef in 1989. This tanker spilled 10 million gallons of oil into the rich ecosystem of coastal Alaska, killing massive numbers of animals. The largest marine oil spill to date was the Deepwater Horizon disaster, which began with a natural gas explosion at an oil well 65 km offshore of Louisiana (Figure 16). The oil leaked from the damaged oil well for 3 months in 2010, releasing an estimated 200 million gallons of oil. During the Persian Gulf war of 1991, Iraq deliberately dumped approximately 200 million gallons of oil in offshore Kuwait and set more than 700 oil wells on fire, releasing enormous clouds of black smoke and creating acid rain for over nine months.
Figure 16. Deepwater Horizon Explosion Boats fighting the fire from an explosion at the Deepwater Horizon drilling rig in Gulf of Mexico offshore Louisiana on April 20, 2010. (Credit: United States Coast Guard via Wikimedia Commons)
When an oil spill occurs in water, the oil floats to the surface because it is less dense than water, and the lightest hydrocarbons evaporate, decreasing the size of the spill but polluting the air. Then, bacteria begin to decompose the remaining oil, in a process that can take many years. After several months only about 15% of the original volume of spilled oil may remain, but it is in thick asphalt-like lumps; a form that is particularly harmful to birds, fish, and shellfish.
Cleanup operations for this kind of water pollution can include skimmer ships that vacuum oil from the water surface, a method that is most effective for small spills. Controlled burns of oil can work to burn off spilled oil in the early stages of a spill, before the light, highly flammable surface components evaporate. Dispersants are detergents that are released to break up oil into smaller components that decompose more rapidly, but sometimes the dispersants may be toxic. Lastly, microorganisms may be released into the spill which specialize in quickly decomposing oil (a method called bioremediation), but this method may also disrupt the natural ecosystem.
Pathogens
Pathogens are disease-causing microorganisms (e.g., viruses, bacteria, parasitic worms, and protozoa), some of which thrive in freshwater. When these pathogens are found in polluted water sources used by humans, a variety of intestinal diseases can proliferate, such as dysentery, typhoid fever, and cholera. Unfortunately nearly a billion people around the world are exposed to waterborne pathogen pollution daily and around 1.5 million children, mainly in poorer and less developed countries, die every year of waterborne diseases from water borne pathogens. Pathogens enter water primarily from human and animal fecal waste, in areas with inadequate sewage treatment. Untreated or minimally treated human and animal sewage may be discharged directly into local waters. In more developed countries untreated sewage discharge can occur from overflows of a combined sewer system, poorly managed livestock factory farms, or leaky or broken sewage collection and distribution systems. Water with pathogens can be remediated by adding chlorine or ozone, or by boiling, but even performing this simple step is beyond the means of many people (Figure 17).
Figure 17. Water Pollution. Obvious water pollution in the form of floating debris. Sometimes more invisible water pollutants can be much more harmful than visible ones, such as viruses and bacteria. (Credit: Stephen Codrington at Wikimedia Commons)
Toxic Chemicals
Toxic chemicals of many types may make their way into water, from a variety of sources, but they come primarily from industry and mining. Toxic chemicals include persistent organic pollutants (POPs) such as DDT (a pesticide), dioxin (an herbicide by-product), and polychlorinated biphenyls (PCBs), used as a liquid insulator in electric transformers. These POPs are long-lived in the environment, biomagnify (become more concentrated) as they move through the food chain, and can be highly toxic.
A second category of toxic chemicals are radioactive materials such as cesium, iodine, uranium, and radon gas. Significant exposure to acute radiation can lead to radiation sickness and death. Significant, long-term damage to DNA and cellular development can be done if these substances enter the body through food or the air. One notable long-term consequence of radiation exposure is the development of various forms of cancer.
A final group of toxic chemicals are heavy metals such as lead, mercury, arsenic, cadmium, and chromium, which can accumulate through the food chain. Heavy metals are commonly produced by industry and at metallic ore mines. Arsenic (As) has been famous as an agent of death for many centuries. Only recently have scientists recognized that health problems can be caused by ingesting small amounts of arsenic from water over a long period of time. Arsenic enters the water supply from the natural erosion of arsenic-rich minerals in the soil and artificially from human activities such as coal burning and smelting of metallic ores. The worst case of arsenic poisoning occurred in the densely populated impoverished country of Bangladesh. Because of a serious pathogen problem in drinking water, in the 1970s the United Nations provided aid for millions of shallow water wells, which resulted in a dramatic drop in pathogenic diseases. Unfortunately, many of the wells produced water naturally rich in arsenic, which inadvertently exposed over 77 million people (about half of Bangladesh’s population) to toxic levels of arsenic. The World Health Organization has called it the largest mass poisoning of a population in history.
Mercury (Hg), another heavy metal, is used in a variety of electrical products, such as dry cell batteries, fluorescent light bulbs, and switches, as well as in the manufacture of paint, paper, vinyl chloride, and fungicides. Mercury acts on the central nervous system, causing the loss of sight, feeling, and hearing as well as nervousness, shakiness, and death. Like arsenic, mercury enters the water supply naturally from weathering of mercury-rich minerals and from human products and from activities such as coal burning and metal processing. A tragic, large scale mercury poisoning occurred in Minamata, Japan, when methylmercury-rich industrial wastes were discharged from the Chisso Chemical Plant into the local ocean. People in nearby fishing villages, who ate fish up to three times per day, were severely impacted, with thousands of people showing severe neurological damage and over 2200 deaths.
Groundwater Pollution
Groundwater can also be polluted by some of the same sources that pollute surface water, but may also have underground sources of pollutants. For example, leaking underground storage tanks for fuel, septic tanks, agricultural activities, landfills, and fossil fuel extraction can all pollute groundwater. These pollutants include nitrates, pesticides, organic compounds, and petroleum products. Another troublesome feature of groundwater pollution is that small amounts of certain pollutants, e.g., petroleum products and organic solvents, can contaminate large areas. In Denver, Colorado 80 liters of organic solvents contaminated 4.5 trillion liters of groundwater and produced a 5 km long contaminant plume.
A major threat to groundwater quality is from underground fuel storage tanks (Figure 18). Fuel tanks commonly are stored underground at gas stations to reduce explosion hazards. Before 1988 in the U.S. these storage tanks could be made of metal, which can corrode, leak, and quickly contaminate local groundwater. Now, leak detectors are required and the metal storage tanks are supposed to be protected from corrosion or replaced with fiberglass tanks. Currently there are around 600,000 underground fuel storage tanks in the U.S. and over 30% still do not comply with EPA regulations regarding either release prevention or leak detection.
Figure 18. Leaking fuel tanks, underground and often forgotten, can release contaminants which pollute the soil and groundwater. (Credit: California Environmental Protection Agency)
11.6 Water Treatment
Resolution of the global water pollution crisis requires multiple approaches to improve the quality of our freshwater and move towards sustainability. The most deadly form of water pollution, pathogenic microorganisms that cause waterborne diseases, kills almost 2 million people in less developed countries every year.
The best strategy for addressing this problem of pathogens in the water is proper treatment of wastewater, especially sewage containing human wastes. Untreated sewage is the major cause of pathogenic diseases and is also a major source of oxygen-demanding waste (primarily N and P), and other toxins. Wastewater treatment is done at a sewage treatment plant in urban areas and through a septic tank system in rural areas.
The main purpose of wastewater treatment is to remove organic matter (oxygen-demanding waste) and to kill bacteria. The numerous steps at a conventional sewage treatment plant to remove wastes include: pretreatment (screening and removal of sand and gravel), primary treatment (settling or flotation to remove organic solids, fat, and grease), secondary treatment (aerobic bacterial decomposition of organic solids), tertiary treatment (bacterial decomposition of nutrients and filtration), disinfection (treatment with chlorine, ozone, ultraviolet light, or bleach to kill most microbes), and either discharge to surface waters (usually a local river) or reuse for some other purpose, such as irrigation, habitat preservation, or artificial groundwater recharge (Figure 19).
Figure 19. Steps for the treatment of human sewage at a sewage treatment plant. (Credit: Leonard G. via Wikimedia Commons)
The concentrated organic solid produced during primary and secondary treatment is called sludge, which is treated in a variety of ways including landfill disposal, incineration, conversion into fertilizer, and anaerobic (absence of oxygen) bacterial decomposition. Anaerobic decomposition of sludge produces methane gas, which can be stored or piped for use as an energy source. To reduce water pollution problems, having separate sewer systems (where street runoff is shunted to acceptable areas) and only sewage goes to a wastewater treatment plant are much better than combined sewer systems, which can overflow and release untreated sewage into surface waters during heavy rain. Some cities such as Chicago, Illinois have constructed large underground caverns and also use abandoned rock quarries to hold storm sewer overflow. In this system, after the rain stops, the stored water goes to the sewage treatment plant for processing, preventing overflow.
A septic tank system is an individual sewage treatment system for homes, typically found in rural settings where a connection to a sewage pipe is impractical or too costly. The basic components of a septic tank system (Figure 20) include a sewer line from the house, a septic tank (a large container where sludge settles to the bottom and microorganisms decompose the organic solids anaerobically), and the drain field (a network of perforated pipes where the clarified water seeps into the soil and is further purified by bacteria). Water pollution problems occur if the septic tank leaks, the drain field is poorly located, or the soil is not porous enough to allow the clarified water to percolate.
Figure 20. Septic System: Septic tank system used for sewage treatment. (Credit: United States Geological Survey)
For many developing countries, financial aid is necessary to build adequate sewage treatment facilities. The World Health Organization estimates a cost savings of between $3 and $34 for every $1 invested in clean water delivery and sanitation. The cost savings are from health care savings, gains in work and school productivity, and prevented deaths. Simple and inexpensive techniques for treating water at home include chlorination, filters, and solar disinfection. Another alternative is to use constructed wetlands technology (marshes built to treat contaminated water), which is simpler and cheaper than a conventional sewage treatment plant.
The following video explores the topic of water treatment, and how it is effective in restoring water:
American Water (2011, Apr 8) Water and you: the water treatment process [Video-YouTube] https://youtu.be/KMP9-49I1U4
11. 7 Case Study: The Aral Sea – Going, Going, Gone
The Aral Sea is a lake located east of the Caspian Sea between Uzbekistan and Kazakhstan in central Asia. This area is part of the Turkestan desert, which is the fourth largest desert in the world; it is produced from a rain shadow effect by Afghanistan’s high mountains to the south. Due to the arid and seasonally hot climate there is extensive evaporation and limited surface waters in general. Summer temperatures can reach 60°C (140°F)!
The water supply to the Aral Sea is mainly from two rivers, the Amu Darya and Syr Darya, which carry snow melt from mountainous areas. In the early 1960s, the then-Soviet Union diverted the Amu Darya and Syr Darya Rivers for irrigation of one of the driest parts of Asia to produce rice, melons, cereals, and especially cotton. The Soviets wanted cotton or white gold to become a major export. They were successful, and, today Uzbekistan is one of the world’s largest exporters of cotton. Unfortunately, this action essentially eliminated any river inflow to the Aral Sea and caused it to disappear almost completely (Figure 21).
Figure 21. A comparison of the Aral Sea in 1989 (left) and 2014 (right). (Credit: This work is in the Public Domain, CC0 )
In 1960, Aral Sea was the fourth largest inland freshwater body; only the Caspian Sea, Lake Superior, and Lake Victoria were larger. Since then, it has progressively shrunk due to evaporation and lack of recharge by rivers. Before 1965, the Aral Sea received 2060 km3 of freshwater per year from rivers and by the early 1980s it received none. By 2007, the Aral Sea shrank to about 10% of its original size and its salinity increased from about 1% dissolved salt to about 10% dissolved salt, which is 3 times more saline than seawater. These changes caused an enormous environmental impact. A once thriving fishing industry is dead as are the 24 species of fish that used to live there; the fish could not adapt to the more saline waters. The current shoreline is tens of kilometers from former fishing towns and commercial ports. Large fishing boats lie in the dried up lake bed of dust and salt (Figure 22). A frustrating part of the river diversion project is that many of the irrigation canals were poorly built, allowing abundant water to leak or evaporate. An increasing number of dust storms blow salt, pesticides, and herbicides into nearby towns causing a variety of respiratory illnesses including tuberculosis.
Figure 22. This abandoned ship lies in a dried up lake bed that was the Aral Sea near Aral, Kazakhstan (Credit: Staecker at Wikimedia Commons)
The wetlands of the two river deltas and their associated ecosystems have disappeared. The regional climate is drier and has greater temperature extremes due to the absence of moisture and moderating influence from the lake. In 2003, some lake restoration work began on the northern part of the Aral Sea and it provided some relief by raising water levels and reducing salinity. The southern part of the Aral Sea has seen no relief and remains nearly completely dry. The destruction of the Aral Sea is one of the planet’s biggest environmental disasters and it is caused entirely by humans.
The following video explores the environmental catastrophe of the loss of the Aral Sea:
BBC News (2015, Feb 28) Aral sea: the sea that dried up in 40 years. [Video- YouTube] https://youtu.be/5N-_69cWyKo
Lake Chad in Africa is another example of a massive lake that has nearly disappeared for the same reasons as the Aral Sea. Aral Sea and Lake Chad are the most extreme examples of large lakes destroyed by unsustainable diversions of river water. Other lakes that have shrunk significantly due to human diversions of water include the Dead Sea in the Middle East, Lake Manchar in Pakistan, and Owens Lake and Mono Lake, both in California.
The destruction of the Aral Sea is one of the planet’s biggest environmental disasters and it is caused entirely by humans.
1.8 Summary
Precipitation—a major control of freshwater availability—is unevenly distributed around the globe. More precipitation falls near the equator, and landmasses there are characterized by a tropical rainforest climate. Less precipitation tends to fall near 30 degrees north and south latitude, where the world’s largest deserts are located.
The water crisis refers to a global situation where people in many areas lack access to sufficient water or clean water or both. The current and future water crisis requires multiple approaches to extending our freshwater supply and moving towards sustainability. One of the longstanding traditional approaches to providing freshwater is to build dams to hold river water until needed, but this has a negative effect on river ecosystems.
Water pollution is the contamination of water by an excess amount of a substance that can cause harm to human beings and the ecosystem. The level of water pollution depends on the abundance of the pollutant, the ecological impact of the pollutant, and the use of the water. The most deadly form of water pollution, pathogenic microorganisms that cause waterborne diseases, kills almost 2 million people in underdeveloped countries every year.
Resolution of the global water pollution crisis requires multiple approaches to improve the quality of freshwater. A major strategy for improving water quality is proper sewage treatment, since untreated wastewater adds pathogens and nutrients to surface water, causing the spread of waterborne diseases, algal blooms and dead zones.
Attributions
Content in this chapter includes original work created by Lauren Roberts and Paul Bosch as well as from the following sources, with some modifications:
Essentials of Environmental Science by Kamala Doršner is licensed under CC BY 4.0. Modified from the original by Matthew R. Fisher.
Theis, T. & Tomkin, J. (Eds.). (2015). Sustainability: A comprehensive foundation. Retrieved from http://cnx.org/contents/1741effd-9cda-4b2b-a91e-003e6f587263@43.5. Available under Creative Commons Attribution 4.0 International License. (CC BY 4.0). Modified from original.
