A world of thirst.

For African teenager Abdullah Mahmoud, every day is pretty much the same. "I go to school soem days, but mostly I take the donkey to the well to bring back water for my family," he says.

A citizen of the North African desert nation of Mauritania, Abdullah lives in a village that is bone dry. To collect enough water for his mother and younger siblings, he must journey to a distant well several times a day. It's the only source of clean drinking water around.

Sadly, the Mahmoud family's water troubles are far from unique. More than a billion people, most of them living in the world's least affluent nations, struggle to find the two liters of clean drinking water that their bodies need each day. And worldwide, dehydration kills more people each year than either cancer or AIDS.

Many learn the need for clean drinking water the hard way. A girl in Yemen, for example, once decided not to make the long journey to a clean well. Instead, she brough back water from the nearby watering hole where women wash laundry, children swim, and cattle bathe. Shortly after drinking some of this dirty water, the girl's sister was struck by a near-fatal case of diarrhea.

This situation--the shortages and illnesses--must stop, say health officers and politicians around the world. "Water is a basic human need for health," says United Nations official Nikolai P. Napalkov, "and therefore, it is not an exaggeration to call it one of the basic human rights."

In other words, he says, every government has an obligation to assure that its peopel have enough clean water to drink. Last summer, leaders from more than 129 countries vowed to work together to fulfill this obligation. The key to their success will likely be a sharing of technologies that make the most of Earth's limited water supply.

JUST A DROP

The news that water is scarce may suprise you. After all, we live on the "Blue Planet"; water covers two thirds of Earth's surface.

But 97 percent of this supply is salty-undrinkable--ocean water. And the vast majority of Earth's fresh water is trapped in polar ice caps and glaciers. We humans, all five billion of us, must get by on the remaining fraction of the planet's water supply--less than one percent.

And people aren't the only ones vying for this limited supply. Whereever you find fresh water, you'll find all sorts of organisms--everything from moutain lions to mosquitoes ot microbes, doing everything from drinking to breeding to defecating in it.

Some of the microorganisms living in water and in animal wastes can cause diseases like cholera and dysentery. As a result, the water in many rivers, lakes, and streams is undrinkable.

WORLDS APART

In wealthy countries like the United States, state-of-the-art plumbing systems carry drinkable water many miles from remote reservoirs to thisty towns. In cities, high-tech water treatment plants make river and lake water drinkable by filtering out chemical and biological contaminants. And modern sweage treatment plants prevent disease-causing wastes from mixing with drinking water.

Because most Americans can go to the drinking fountain anytime they like, we don't realize how important these technologies are--until they fail, as happened last spring in Milwaukee, Wisconsin. When a waterborne parasite eluded the water-treatment system there, more than 100,000 people suffered from severe diarrhae. Nine people some of them weakened by prior illness, died as a result. Victims of last summer's floods in the Midwest faced similar problems when their water-treatment systems broke down.

Unfortunately, what spells rare disaster in the U.S. is a situation poor countries face all the time. These countries often can't afford elaborate water-treatment technology, says hydrologist Martin Beyer, who runs the U.N.'s water-relief efforts. the poor nations' best hope: Take advantage of Earth's own plumbing system.

Nearly every nation sits atop large stores of untapped fresh water, Beyer says. In fact, there's more fresh water--thirty times more--in the ground than in all of our rivers and lakes combines.

"Earth's crust acts like a giant sponge," Beyer says. Slowly and steadily, it absorbs much of the rainwater and melted snow that washes across the surface. In the ground, this water collects in porous soil or rocky crevices called aquifers.

HOPE UNDERGROND

In developing nations, where there is little industrial pollution, such groundwater supplies are clean, Beyer says. "The aquifer naturall filters the water." As the water slowly precolates through, the rocks and soil clear out chemical impurities and harmful microorganisms.

One catch: Aquifers are often buried beneath thousands of feet of dry rock. And even the meager equipment needed to tap these resources is expensive to a nation struggling to survive. As a result, poor nations countinue to go thirty. But that is starting to change.

In recent years, the U.N. and other international organizations have started supplying poor countries with modern drilling technology, says Beyer. With such equipment, a well that will satisfy the daily needs of 350 people can be dug in just six hours. That means more wells can be dug closer to the many communities, like Abdullah Mahmoud's, where they are needed.

The results of such programs have been encouraging, say U.N. officials. Since 1980, 1.3 billion people--most living in rural areas in developing nations throughout the world--have gained access to clean drinking water.

FUTURE CHALLENGES

Meeting the challenge of supplying clean water to the cities of developing nations will be a greater struggle. In the world's most impoverished nations, poor people have flocked to cities in search of work, overwhelming local watersupply systems. In Mexico City, the fastest growning urban area in the world, millions of peopel live in slums with no plumbing at all.

Many public officers say the most important step is solving this problem is to get the world's exploding population under control, a controversial suggestion (see "The population predicament," p. 12). "There is enough water on this planet for all of our five billion people," says U.N. spokesperson Rajeswary Iruthayanathan--that is, if all people have the technology to get it. "But there is not enough for 15 billion." If our population reaches that level--perhaps as soon as 2075--will technology alone solve the problem?

Related article: THE POPULATION PREDICAMENT

The world's population is growing by more than 90 million people each year, faster than at any other time in history (see graph, right). Many observers say this growth has to stop. Otherwise, they warn, Earth's limited supply of natural resources, including water, will soon run dry.

But others say it's not right to curtail human reproduction. For on thing, effors to control world population might disproportionately harm the poor. In non-iodustrialized countries, for example, where people rely on agriculture to support themselves, families need children to help grow food and do other work. In addition, children provide "social security" for the elderly in societies where no government-run welfare programs exist.

When you consider that rich nations have used and degraded more of Earth's resources than poor ones, it's especially unfair for poor nations to suffer the "punishment" of population control, some officials of these nations say.

Other people are opposed to population control on religious and moral grounds. And some religious and ethnic groups say it's their right--and in some cases, their duty--to increase their own numbers.

For a biologist's standpoint, that might make perfect sense. After all, natural selection, the driving force of evolution, favors individuals who pass on their genes. In fact, many have argued that this is the main goal of every living thing: to maximize reproductive success. If so, do you think humans could ever--should ever--be convinced to limited theirs?