Water supply and options

Despite the enormous volume of fresh water that circulates through the continents annually - easily enough to satisfy the needs of humankind for centuries - many people around the world do not have access to this vital liquid. There are several reasons for this. First, although water is abundant, fresh water only exists in large volumes in small areas of the planet (the lower reaches of rivers, large lakes, and high-yield aquifers). Second, available fresh water is not always fit for human consumption, sometimes because of natural causes, but more often as a result of anthropogenic degradation. Third, not all water sources are renewed at a sufficiently high rate to be suitable for long-term use. Finally, water demand is concentrated in a few densely populated areas, which do not necessarily coincide with the sites of greatest availability.

In brief, good-quality fresh water available in sufficient volumes and in a sustainable manner to meet the needs of populations and productive activities is not easily found. Increasingly, it has become a limiting factor in demographic and economic growth.

Water use and overuse

Water is the most widely used substance on Earth: it is needed in homes for washing, cooking, and drinking; it is used by industries as a raw material, for cooling or washing, to make possible certain processes; it is required for farming and for many other purposes. Farmers are responsible for more than 80% of the world’s water consumption. Of the remaining 20%, about half is consumed domestically and the rest is used by industries and for other activities.

These figures reflect only the water that is actually used, however. Additional large volumes of natural water are only “affected” by human action. Good-quality river or lake water is often rendered unfit for use by the return of untreated or insufficiently treated wastewater into its environment. The volume of natural water that is affected by human activities is enormous and difficult to quantify. In all likelihood, the volume of degraded water is probably on at least the same order of magnitude as all the water used worldwide, and may be substantially greater.

Another anthropogenic cause of water degradation or, at least, decreased availability relates to inappropriate soil management on slopes. Inadequate farming or grazing practices cause soil erosion, and runoff water carries agricultural fertilizers and pesticides. In such “overused” areas, water flow is often concentrated over a short period, causing flooding and making optimum use of the water resource more difficult. Floodwater is usually loaded with suspended particles that not only lower its quality but also clog intake mechanisms at filtration plants, making its treatment more costly and difficult.

Anthropogenic impact on water systems

In ancient times, hydrographic basins evolved naturally at variable paces depending on climatic, geologic, and biologic factors. Since the beginning of history, however, societies have introduced other factors. Agriculture, raising cattle, logging, excavation of quarries, and construction of artificial structures have had an effect on hydrodynamics throughout the planet. The growth of the world’s population, particularly after the industrial revolution, has gradually increased the impact of these factors as widespread modifications were made to the land surface. Anthropogenic effects have been particularly intense since the urban revolution of the 20th century. Overpopulation in many rural areas and the development of large cities with populations in the millions have created a concentrated and growing demand for water.

During this century, the amount of water used for agricultural, domestic, industrial, and other purposes has continued to increase; dams have been built, wells drilled, and water taken from natural sources at an unprecedented rate. “Used water” of lower quality is being resumed to the environment, causing widespread degradation of streams, lakes, and aquifers.

Vulnerability of water resources

The vulnerability of water resources to contamination varies from place to place. Generally, it depends on volume. Large rivers are less vulnerable than smaller rivers. The same rationale applies to lakes, although they are more susceptible than rivers because of their slower rate of renewal. However, surface water sources are relatively easy to clean up once a commitment is made to do so.

Groundwater, on the other hand, is less vulnerable than surface water in the short term. It takes longer for contaminants to find their way into deep aquifers. In some cases, they may even be protected by impermeable layers of soil or rock. However, groundwater reservoirs can be polluted easily by contamination of their recharge areas or inappropriate drilling operations. When this happens, the damage may be difficult and expensive to correct. In most cases, contaminated aquifers cannot be used for a long time and, in some cases, they may never again be suitable for any practical purpose.

Water problems in densely populated areas

The industrial revolution resulted in gradual growth of urban centres, with the populations of London, New York, and Paris exceeding I million by the beginning of the 20th century. Today, as
many as 200 cities have populations over I million and more than 20 have over 10 million people.

In most cases, water resources were abundant when these cities were first established. Many drew the water supplies from nearby rivers or lakes, which were more than sufficient. Where surface fresh water was not available, cities used easily accessible underground aquifers. In fact, in almost all cases, it was the presence of water resources that made the development of the new cities possible.

In cases of both spontaneous and planned development, however, almost without exception, the location of cities was not chosen based on anticipation of the growth that has taken place in many of the largest urban areas of the world. In the 18th and 19th centuries, most of today’s largest cities would now be considered small or medium sized. By 1800, no city in the Americas had a population over 100 thousand.

For these levels of population, only relatively limited water resources were necessary. In the early 19th century, however, even small cities had poorly developed water-supply systems. For this reason, per-capita consumption levels were much lower than they are today.

During the late 19th and 20th centuries, many cities grew to become megalopolises. At the same time, their need for water increased dramatically: in some cases, per-capita consumption increased to 600 litres per year. Large cities consume large volumes of water. Los Angeles, Mexico City, and Tokyo - three of the world’s largest cities - use 50 to 150 cubic metres of water every second. These volumes may seem impressive; however, they are relatively minor compared with the flow of our planet’s largest rivers. The efflux of the Amazon into the Atlantic Ocean is about 150 thousand cubic metres per second, 2 thousand times the consumption rate of the largest megacity of the world. The Congo flows at an average rate of 60 thousand cubic metres per second, and many other rivers, such as the Parana, Yangtze, and Mississippi, deliver over 5 thousand cubic metres of water to the sea or coastal estuaries every second.

This apparent overabundance of water does not reflect reality, however. The Amazon and the Congo are not typical because a significant portion of their basins lies in high rainfall areas. Many other rivers with large basins (such as the Nile and the Niger) have considerably lower flow volumes. On average, much less water is actually available. The numbers given here reflect the flow at the mouth of these rivers, where it is greatest. In other stretches of the rivers and in their tributaries, the flow is much lower in relation to the size of the upstream basin and local rainfall. Also, not many large cities or densely populated areas are located at the mouth or in the lower reaches of the largest rivers or their tributaries where water flow is at its maximum. As a result, the actual surface water resources available for cities and densely populated areas are much smaller than they would be if the cities were ideally located.

Many cities that are at the mouths of large rivers (such as Georgetown in Guyana and Montevideo in Uruguay) cannot use the water directly because of its brackish quality, which is caused by invading seawater during the dry season. Some cities are close to divides, so available water is limited (Sao Paulo and Madrid), or next to relatively small streams (Los Angeles and Lima). Available resources frequently cannot meet the growing needs of neighbouring metropolitan areas.

Despite potential problems, at the beginning of the 20th century, the world’s main urban centres were managing to survive using their nearby water resources without major problems of scarcity. During the 20th century, however, the situation changed radically. Cities that formerly had populations of 50 to 100 thousand have grown to urban areas of 10 to 15 million, housing as many or more people in surrounding areas. At current rates of growth, or even with some stabilization in the near future, by the beginning of the next decade there will be several megalopolises with over 15 million people.

In many of these megacities, local water resources have been exhausted or were degraded many decades ago, and water authorities have been forced to turn to neighbouring hydrographic basins or aquifers. As a result, the cost of water has risen considerably, although in most cases it is somewhat disguised in national budgets. Often, urban water-supply accounts list only operational costs, investments are financed at the national level, and, in some cases, even replacement costs are not fully considered.

When cities do not pay the full price of their water, however, someone else does. In many countries, large cities are being subsidized by the population at large, including taxpayers in small towns and rural people who do not benefit from the waterworks.

Continuing growth of large urban areas will make the problem more acute. New sources of water can only be farther away or deeper; tapping them will require more costly dams, conduction systems, storage structures, distribution networks, and treatment plants. A successful strategy will have to be aimed at redefining management strategies not only to increase supply, but also to reduce demand, unnecessary consumption, and losses. A longer term solution will require reexamination of the “constant growth” development paradigms that are the cause of unsustainability in current systems. A new approach may be required in which water consumption will be related to its distribution and availability and where rational and equitable demand policies are given priority over additional spending and waste.

Power generation

Water overuse is often related to the need to generate power. In Armenia, historical Lake Sevan is gradually being drained by the Razdan River power plants to produce badly needed electricity. The power plants were built in the 1940s and the water levels at that time were 20 metres higher that they are today. The lake is now eutrophic and its area is rapidly decreasing. In addition, because of the recent conflict between Armenia and Azerbaijan, Armenia was cut off from its oil supplies (which used to come from Azerbaijan) and electricity production in the Razdan plants has increased, accelerating the process of degradation (Gray 1993). This is yet another demonstration of how wars and conflicts between nations can destroy the environment.

Use of groundwater

In many areas, the volume of groundwater resources may be much greater than that of surface water; in terms of usable fresh water, the difference may be several orders of magnitude. However, the amount of groundwater available should not be measured by its volume, but in terms of its rate of renewal. When groundwater resources are used faster than they are replaced, water levels in aquifers drop, pumping costs increase, and sooner or later the resource is depleted.

Judging aquifers in terms of their renewability, available volumes of water are about equal to or less than those of surface resources. In addition, groundwater availability and urban populations do not necessarily coincide. Some large aquifers are located in sparsely populated areas or where they are not needed because sufficient surface water exists; at the same time, many large urban areas have very little groundwater in close proximity. Despite these limitations, the use of groundwater offers many advantages

· It is less vulnerable to contamination;

· It usually does not require treatment to make is suitable for drinking;

· It can be exploited using a modular approach with smaller capital investment and local participation;

· It does not require large, sophisticated distribution systems; and

· It does not need expensive storage structures - it is already stored underground.

Although groundwater may be a feasible alternative for providing water to urban areas, particular care must be taken to protect it from degradation by outside sources and from overuse. As mentioned above, although aquifers are less vulnerable to contamination, when they are affected, the damage may be irreversible.

The particular problems of coastal cities

A common limitation to water supplies in coastal areas is the intrusion of brackish water into the lower reaches of rivers. This has forced cities such as London (on the Thames) and Guayaquil (on the Guayas) to relocate their water-supply intake farther upstream.

Coastal cities depending on nearby aquifers for water have also experienced problems with saltwater intrusion as a result of drawing too much water from the underground resource. Seawater enters the aquifer when the piezometric level drops below a certain point.

Many of these cities - such as Recife, Brazil; Calcutta, India; Dakar, Senegal; Georgetown, Guyana; and Maracaibo, Venezuela - have had to pipe water from distant rivers or groundwater sources. Others, on salty rivers or estuaries, have turned to nearby freshwater tributaries. New York, for example, was forced to use groundwater because the Hudson River is brackish. Currently, its water supply comes from upstream reservoirs. Montevideo, Uruguay, cannot depend on the Rio de la Plata, which has an average salinity of 10%0 (per thousand), but gets its water from intakes in the Santa Lucia River, a tributary of the Rio de la Plata, 30 kilometres upstream of its mouth and 15 to 30 kilometres from the city.

Some coastal cities are not close to a river, especially those located on karstic or volcanic sites (for example, Djakarta, Indonesia; Manila, Philippines: Miami. United States: Havana. Cuba: and Merida, Mexico). These cities rely entirely on groundwater for their water supplies.

Other limitations

The growing contamination of surface water - resulting from a lack of wastewater treatment - is gradually becoming a central issue. In many densely populated areas, all types of wastewater find their way into the natural water systems. For example, there is a $3 billion plan to clean up the Tiete River in Sao Paulo. However, it is unlikely to be carried out before the year 2000 and there are indications from international and local environmental NGOs that, because of special interests and constraints, the project may never even approach its final goal.

In summary, there are two basic limitations affecting the water supplies of cities and densely populated areas of the world. One is the inappropriate location of cities in relation to existing natural water resources; the other is the growing degradation of those resources.