Hydrological cycle and an Overview of Global Water Pollution R. Nagendran Centre for Environmental studies Anna University, Chennai 600025 rnagendran@gmail.com Introduction The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above and below the surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water cycle. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go. Where is all the Earth’s water? Water is the most widespread substance to be found in the natural environment and it is the source of all life on earth. Water covers 70% of the earth’s surface but it is difficult to comprehend the total amount of water when we only see a small portion of it. The distribution of water throughout the earth is not uniform. Some places have far more rainfall than others. There are many reasons for this, such as where the mountains are and where the prevailing winds blow. This rainfall distribution partly explains the differences in vegetation and why some areas are deserts and some are rainforests. Water exists in three states: liquid, solid and invisible vapour. It forms the oceans, seas, lakes, rivers and the underground waters found in the top layers of the earth’s crust and soil cover. In a solid state, it exists as ice and snow cover in polar and alpine regions. A certain amount of water is contained in the air as water vapour, water droplets and ice crystals, as well as in the biosphere. Huge amounts of water are bound up in the composition of the different minerals of the earth’s crust and core. To assess the total water storage on the earth reliably is a complicated problem because water is so very dynamic. It is in permanent motion, constantly changing from liquid to solid or gaseous phase, and back again. The quantity of water found in the hydrosphere is the usual way of estimating the earth’s water. This is all the free water existing in liquid, solid or gaseous state in the atmosphere, on the Earth’s surface and in the crust down to a depth of 2000 metres. Current estimates are that the earth’s hydrosphere contains a huge amount of water - about 1386 million cubic kilometres. However, 97.5% of this amount exists as saline waters and only 2.5% as fresh water. The greatest portion of the fresh water (68.7%) is in the form of ice and permanent snow cover in the Antarctic, the Arctic and in the mountainous regions. 29.9% exists as fresh groundwaters. Only 0.26% of the total amount of fresh water on the earth is concentrated in lakes, reservoirs and river system, where it is most easily accessible for our economic needs and absolutely vital for water ecosystems. Different Processes connected with water cycle Precipitation Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes snow, hail, fog drip, and sleet. Approximately 505,000 km3 (121,000 cu mi) of water fall as precipitation each year, 398,000 km3 (95,000 cu mi) of it over the oceans. Canopy interception The precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground. Snowmelt The runoff produced by melting snow. Runoff The variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may infiltrate into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses. Infiltration The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater. Subsurface Flow The flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (eg. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years. Evaporation The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Total annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans. Sublimation The state change directly from solid water (snow or ice) to water vapor. Advection The movement of water — in solid, liquid, or vapor states — through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land. Condensation The transformation of water vapor to liquid water droplets in the air, producing clouds and fog. Transpiration The release of water vapor from plants into the air. Water vapor is a gas that cannot be seen. What is the Hydrological Cycle? The values for stored water given above are for natural, static, water storage in the hydrosphere. It is the amount of water contained simultaneously, on average, over a long period of time, – in water bodies, aquifers and the atmosphere. For shorter time intervals such as a single year, a couple of seasons or a few months, the volume of water stored in the hydrosphere will vary as water exchanges take place between the oceans, land and the atmosphere. The total amount of water on the earth and in its atmosphere does not change but show that rain and flowing rivers must be a motion that transfers water in a neverending cycle. Oceans, rivers, clouds and rain, all of which contain water, are in a frequent state of change. This circulation and conservation of earth’s water as it circulates from the land to the sky and back again is called the ‘hydrological cycle’ or ‘water cycle’. How does the Hydrological Cycle work? The stages of the cycle are: Evaporation Transport Condensation Precipitation Groundwater Run-off Evaporation Water is transferred from the surface to the atmosphere through evaporation, the process by which water changes from a liquid to a gas. The sun’s heat provides energy to evaporate water from the earth’s surface. Land, lakes, rivers and oceans send up a steady stream of water vapour and plants also lose water to the air (transpiration). Transport The movement of water through the atmosphere, specifically from over the oceans to over land, is called transport. Some of the earth’s moisture transport is visible as clouds, which themselves consist of ice crystals and/or tiny water droplets. Clouds are propelled from one place to another by the jet stream, surface-based circulations like land and sea breezes or other mechanisms. However, a typical cloud 1 km thick contains only enough water for a millimetre of rainfall, whereas the amount of moisture in the atmosphere is usually 10-50 times greater than this. Condensation The transported water vapour eventually condenses, forming tiny droplets in clouds. Precipitation The primary mechanism for transporting water from the atmosphere to the surface of the earth is precipitation. When the clouds meet cool air over land, precipitation, in the form of rain, sleet or snow, is triggered and water returns to the land (or sea). A proportion of atmospheric precipitation evaporates. Groundwater Some of the precipitation soaks into the ground and this is the main source of the formation of the waters found on land - rivers, lakes, groundwater and glaciers. Some of the underground water is trapped between rock or clay layers - this is called groundwater. Water that infiltrates the soil flows downward until it encounters impermeable rock and then travels laterally. The locations where water moves laterally are called ‘aquifers’. Groundwater returns to the surface through these aquifers, which empty into lakes, rivers and the oceans. Under special circumstances, groundwater can even flow upward in artesian wells. The flow of groundwater is much slower than run-off with speeds usually measured in centimetres per day, metres per year or even centimetres per year. Run-off Most of the water which returns to land flows downhill as run-off. Some of it penetrates and charges groundwater while the rest, as river flow, returns to the oceans where it evaporates. As the amount of groundwater increases or decreases, the water table rises or falls accordingly. When the entire area below the ground is saturated, flooding occurs because all subsequent precipitation is forced to remain on the surface. Different surfaces hold different amounts of water and absorb water at different rates. As a surface becomes less permeable, an increasing amount of water remains on the surface, creating a greater potential for flooding. Flooding is very common during winter and early spring because frozen ground has no permeability, causing most rainwater and melt water to become run-off. This entire process repeats as illustrated in Figure 1. Figure 1 Stages of Hydrological cycle Water Balance A considerable portion of river flow does not reach the ocean, having evaporated those areas with no natural surface run-off channels. On the other hand, some groundwater bypasses river systems altogether and goes directly to the ocean or evaporates. Every year, the turnover of water on Earth involves 577,000 km3 of water. This is water that evaporates from the ocean surface (502,800 km3) and from land (74,200 km3). The same amount of water falls as atmospheric precipitation, 458,000 km3 on the ocean and 119,000 km3 on land. The difference between precipitation and evaporation from the land surface (119,000 – 74,200 = 44,800 km3/year) represents the total run-off of the Earth’s rivers (42,700 km3/year) and direct groundwater runoff. How does Water Supply and Sewage Disposal fit into the Hydrological Cycle? We have seen that water flows into rivers, lakes and into groundwater storage. Most importantly, for our daily water needs, it also flows into our homes and taps. A network of underground pipes, pumping stations and treatment works ensures that clean, fresh drinking water is delivered by our local water utility to our homes every day of the week. After water has been used, the water utility collects, transports and then cleans this dirty water and returns it safely back into rivers where it can continue its journey downstream to the sea. The water utility’s responsibility begins at the precipitation stage of the hydrological cycle. Utilities in some water-scarce countries encourage the collection of rainwater from rooftops (rainwater harvesting) but in most of Europe the hydrological cycle begins with surface waters. Figure 2 illustrates the water utility’s role in the hydrological cycle. Figure 2 Water utility’s role in the hydrological cycle Surface Waters Rain water searches for the quickest route to the sea and flows into rivers, streams, lakes and underground stores. The water in the surface waters is clean enough to support a variety of wildlife. Although it is clean, it is not safe to drink and needs to be treated in a water treatment works to remove any substances that may be harmful. Water Treatment Works Water is abstracted from underground sources via boreholes or alternatively is pumped from rivers and stored in reservoirs before being passed through sand filter beds which trap any dirt and organisms. It is then treated using the most up to date advanced water treatment (AWT) technology such as ozonation and carbon filtration (granular activated carbon) which remove the substances that we cannot see. Water Distribution Clean, fresh drinking water is pumped into an underground network of pipes and storage reservoirs and is not seen again until it reaches your tap. This guarantees that the water you drink remains clean and fresh. Water Use On average, in European countries, each person uses around 55,000 litres of water every year. Baths, showers, washing up, washing clothes and toilet flushing all use large amounts of water. These activities transform clean tap water into dirty wastewater. The water utility not only supplies clean drinking water but also collects, transports and disposes of the dirty water after it has been used. Sewerage Dirty water or sewage is collected firstly in drains and then in underground sewers and is transported via a sewerage system (a network of pipes and tunnels) to a sewage treatment works. Sewage Treatment Works These works use natural micro-organisms to remove harmful substances from dirty water. The solid material (sludge) is separated from the liquid (effluent) and both are treated to produce clean effluent that can be released back to the river and bio-solids that can be used in agriculture as a fertilizer or incinerated to produce energy. Completing the Cycle The river continues its journey back to the sea where the cycle starts again. Water evaporates to form clouds, condenses to droplets and eventually falls as rain on to the ground. Water Pollution Although we as humans recognize the fact that water is essential for everything on our planet to grow and prosper, we disregard it by polluting our rivers, lakes, and oceans. Subsequently, we are slowly but surely harming our planet to the point where organisms are dying at a very alarming rate. In addition to innocent organisms dying off, our drinking water has become greatly affected as is our ability to use water for recreational purposes. In order to combat water pollution, we must understand the problems and become part of the solution. Point and nonpoint sources of water pollution According to the American College Dictionary, pollution is defined as: “to make foul or unclean; dirty”. Water pollution occurs when a body of water is adversely affected due to the addition of large amounts of materials to the water. When it is unfit for its intended use, water is considered polluted. Two types of water pollutants exist; point source and nonpoint source. Point sources of pollution occur when harmful substances are emitted directly into a body of water. The major oil spills (E.g., Exxon Valdez oil spill) best illustrates point source water pollution. A nonpoint source delivers pollutants indirectly through environmental changes. An example of this type of water pollution is when fertilizer from a field is carried into a stream by rain, in the form of run-off which in turn affects aquatic life. The technology exists for point sources of pollution to be monitored and regulated, although political factors may complicate matters. Nonpoint sources are much more difficult to control. Pollution arising from nonpoint sources accounts for a majority of the contaminants in streams and lakes. Causes of water pollution Many causes of pollution including sewage and fertilizers contain nutrients such as nitrates and phosphates. In excess levels, nutrients over stimulate the growth of aquatic plants and algae. Excessive growth of these types of organisms consequently clogs our waterways, use up dissolved oxygen as they decompose, and block light to deeper waters. This, in turn, proves very harmful to aquatic organisms as it affects the respiration ability or fish and other invertebrates that reside in water. Pollution is also caused when silt and other suspended solids, such as soil, wash off plowed fields, construction and logging sites, urban areas, and eroded river banks when it rains. Under natural conditions, lakes, rivers, and other water bodies undergo Eutrophication, an aging process that slowly fills in the water body with sediment and organic matter. When these sediments enter various bodies of water, fish respiration becomes impaired, plant productivity and water depth become reduced, and aquatic organisms and their environments become suffocated. Pollution in the form of organic material enters waterways in many different forms as sewage, as leaves and grass clippings, or as runoff from livestock feedlots and pastures. When natural bacteria and protozoan in the water break down this organic material, they begin to use up the oxygen dissolved in the water. Many types of fish and bottom-dwelling animals cannot survive when levels of dissolved oxygen drop below two to five parts per million. When this occurs, it kills aquatic organisms in large numbers which leads to disruptions in the food chain. Pathogens are another type of pollution that proves very harmful. They can cause many illnesses that range from typhoid and dysentery to minor respiratory and skin diseases. Pathogens include such organisms as bacteria, viruses, and protozoan. These pollutants enter waterways through untreated sewage, storm drains, septic tanks, runoff from farms, and particularly boats that dump sewage. Though microscopic, these pollutants have a tremendous effect evidenced by their ability to cause sickness. The pathways of water contamination are indicated in Figure 3. Figure 3 The pathways of water contamination Additional forms of water pollution Three last forms of water pollution exist in the forms of petroleum, radioactive substances, and heat. Petroleum often pollutes water bodies in the form of oil, resulting from oil spills. The previously mentioned Exxon Valdez is an example of this type of water pollution. These large-scale accidental discharges of petroleum are an important cause of pollution along shore lines. Besides the supertankers, off-shore drilling operations contribute a large share of pollution. One estimate is that one ton of oil is spilled for every million tons of oil transported. This is equal to about 0.0001 percent. Radioactive substances are produced in the form of waste from nuclear power plants, and from the industrial, medical, and scientific use of radioactive materials. Specific forms of waste are uranium and thorium mining and refining. The last form of water pollution is heat. Heat is a pollutant because increased temperatures result in the deaths of many aquatic organisms. These decreases in temperatures are caused when a discharge of cooling water by factories and power plants occurs. Classifying water pollution The major sources of water pollution can be classified as municipal, industrial, and agricultural. Municipal water pollution consists of waste water from homes and commercial establishments. For many years, the main goal of treating municipal wastewater was simply to reduce its content of suspended solids, oxygen-demanding materials, dissolved inorganic compounds, and harmful bacteria. In recent years, however, more stress has been placed on improving means of disposal of the solid residues from the municipal treatment processes. The basic methods of treating municipal wastewater fall into three stages: primary treatment, including grit removal, screening, grinding, and sedimentation; secondary treatment, which entails oxidation of dissolved organic matter by means of using biologically active sludge, which is then filtered off; and tertiary treatment, in which advanced biological methods of nitrogen removal and chemical and physical methods such as granular filtration and activated carbon absorption are employed. The handling and disposal of solid residues can account for 25 to 50 percent of the capital and operational costs of a treatment plant (Figure 4). Figure 4 A schematic of wastewater treatment The characteristics of industrial waste waters can differ considerably both within and among industries. The impact of industrial discharges depends not only on their collective characteristics, such as biochemical oxygen demand and the amount of suspended solids, but also on their content of specific inorganic and organic substances. Three options are available in controlling industrial wastewater. Control can take place at the point of generation in the plant; wastewater can be pretreated for discharge to municipal treatment sources; or wastewater can be treated completely at the plant and either reused or discharged directly into receiving waters. Agriculture, including commercial livestock and poultry farming, is the source of many organic and inorganic pollutants in surface waters and groundwater. These contaminants include both sediment from erosion cropland and compounds of phosphorus and nitrogen that partly originate in animal wastes and commercial fertilizers. Animal wastes are high in oxygen demanding material, nitrogen and phosphorus, and they often harbor pathogenic organisms. Wastes from commercial feeders are contained and disposed of on land; their main threat to natural waters, therefore, is from runoff and leaching. Control may involve settling basins for liquids, limited biological treatment in aerobic or anaerobic lagoons and a variety of other methods. Global water pollution Estimates suggest that nearly 1.5 billion people lack safe drinking water and that at least 5 million deaths per year can be attributed to waterborne diseases. With over 70 percent of the planet covered by oceans, people have long acted as if these very bodies of water could serve as a limitless dumping ground for wastes. Raw sewage, garbage, and oil spills have begun to overwhelm the diluting capabilities of the oceans, and most coastal waters are now polluted. Beaches around the world are closed regularly, often because of high amounts of bacteria from sewage disposal, and marine wildlife is beginning to suffer. Perhaps the biggest reason for developing a worldwide effort to monitor and restrict global pollution is the fact that most forms of pollution do not respect national boundaries. The first major international conference on environmental issues was held in Stockholm, Sweden, in 1972 and was sponsored by the United Nations (UN). This meeting, at which the United States took a leading role, was controversial because many developing countries were fearful that a focus on environmental protection was a means for the developed world to keep the undeveloped world in an economically subservient position. The most important outcome of the conference was the creation of the United Nations Environmental Program (UNEP). Water quality Water quality is closely linked to water use and to the state of economic development. In industrialized countries, bacterial contamination of surface water caused serious health problems in major cities throughout the mid 1800s. By the turn of the century, cities in Europe and North America began building sewer networks to route domestic wastes downstream of water intakes. Development of these sewage networks and waste treatment facilities in urban areas has expanded tremendously in the past two decades. However, the rapid growth of the urban population (especially in Latin America and Asia) has outpaced the ability of governments to expand sewage and water infrastructure. While waterborne diseases have been eliminated in the developed world, outbreaks of cholera and other similar diseases still occur with alarming frequency in the developing countries. Since World War II and the birth of the “chemical age”, water quality has been heavily impacted worldwide by industrial and agricultural chemicals. Eutrophication of surface waters from human and agricultural wastes and nitrification of groundwater from agricultural practices has greatly affected large parts of the world. Acidification of surface waters by air pollution is a recent phenomenon and threatens aquatic life in many area of the world. In developed countries, these general types of pollution have occurred sequentially with the result that most developed countries have successfully dealt with major surface water pollution. In contrast, however, newly industrialized countries such as China, India, Thailand, Brazil, and Mexico are now facing all these issues simultaneously. Conclusion Clearly, the problems associated with water pollution have the capabilities to disrupt life on our planet to a great extent. Congress has passed laws to try to combat water pollution thus acknowledging the fact that water pollution is, indeed, a serious issue. But the government alone cannot solve the entire problem. It is ultimately up to us, to be informed, responsible and involved when it comes to the problems we face with our water. We must become familiar with our local water resources and learn about ways for disposing harmful household wastes so they don’t end up in sewage treatment plants that can’t handle them or landfills not designed to receive hazardous materials. In our yards, we must determine whether additional nutrients are needed before fertilizers are applied, and look for alternatives where fertilizers might run off into surface waters. We have to preserve existing trees and plant new trees and shrubs to help prevent soil erosion and promote infiltration of water into the soil. Around our houses, we must keep litter, pet waste, leaves, and grass clippings out of gutters and storm drains. These are just a few of the many ways in which we, as humans, have the ability to combat water pollution. As we head into the 21st century, awareness and education will most assuredly continue to be the two most important ways to prevent water pollution. If these measures are not taken and water pollution continues, life on earth will suffer severely. Global environmental collapse is not inevitable. But the developed world must work with the developing world to ensure that new industrialized economies do not add to the world's environmental problems. Politicians must think of sustainable development rather than economic expansion. Conservation strategies have to become more widely accepted, and people must learn that energy use can be dramatically diminished without sacrificing comfort. In short, with the technology that currently exists, the years of global environmental mistreatment can begin to be reversed. Acknowledgement This article is based the information sourced from the publications of several experts in the field. I wish to acknowledge all the authors.