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THE MISSISSIPPI RIVER occupies a mythic place in the American imagination, in part because it is so huge. At any given moment, on average, approximately 555 trillion gallons of water sweep across its broad bottom. If you were to dive down about 35 feet and lie on the bottom, you might think that the whole river was flowing over you, but you would be wrong. At any point in time, just one percent of the water in the Mississippi River system is running downstream to the Gulf of Mexico. The other 99% lies hidden beneath the bottom, locked in massive strata of rock and sand.

It is natural to think of water as something that flows or evaporates. We see it coming down as rain, coursing in rivers, or drifting around us as fog. Most freshwater is not so easily observed because it lies deep underground in aquifers--geological formations made of porous materials such as sand and gravel, or spaces between subterranean rocks. These formations retain enormous amounts of water. So, the Mississippi is not unique in its ratio of surface to underground water--around 97% of the planet's liquid freshwater is stored in aquifers.

In the last half-century, as global population and food demand have more than doubled and rivers and streams have become more polluted, we have increasingly turned to aquifers to supply drinking and irrigation water. In the process, we have made a sobering discovery. Despite the popular impression that groundwater is shielded from contaminants, scientists are uncovering cases of pollution in aquifers near farms, factories, and cities on every continent. We are learning that the water buried beneath our feet is not only susceptible to pollution, it is in many ways more vulnerable than water above ground.

This is a distinction of enormous consequence. Because it is underground and slow moving, groundwater stores pollutants far longer than, say, rivers or air do. Hence, the very characteristic that makes aquifers ideal reservoirs of freshwater--their ability to accumulate and retain liquid for longer periods of time--enables them to become long-term sinks for contaminants. It is true that some aquifers recycle water back to the environment fairly quickly, but the average length of time groundwater remains in an aquifer is 1,400 years, as opposed to 16 days for river water. Some aquifers contain water that fell as rain as much as 30 millennia ago. So, instead of being flushed out to sea or becoming diluted with constant additions of freshwater--as rivers, lakes, and streams are--aquifers continue to accumulate pollutants, decade after decade, thus steadily diminishing the amount of clean water they can yield for human use.

Many of the contaminants trickling underground are substances that are routinely used and discarded by modern societies: solvents used to make computer chips, nitrates from fertilizer applied on cornfields, or chemicals sprayed to kill insects and weeds on farms and front lawns. Throughout history, civilians have used the subterranean world as a receptacle for waste, a place to bury the dead or landfill trash, for instance.

Prior to the 20th century, these practices did not usually result in serious damage to groundwater. As water percolated down through soil and rocks, bacteria, fungi, and other such biological pollutants were naturally filtered out or diluted. In recent years, though, groundwater's natural defense systems have been vastly overextended. The sheer volume of pollutants sent underground has escalated, and, at the same time, scientists have introduced thousands of new substances not found in nature. Globally, the production of synthetic chemicals vaulted from under 150,000 tons in 1935 to more than 150,000,000 in 1995. Many of these substances not only endure far longer in the environment, they are often more toxic than their predecessors. Pesticide formulations available today are between 10 and 100 times more potent than those sold in 1975.

Scientists are learning that the unique makeup of aquifers makes it possible for persistent substances to endure especially long underground. Aquifers usually contain less in the way of minerals, microbes, dissolved oxygen, and organic matter than soils do, thus making it difficult for chemicals to break down easily. As a result, the herbicide alachlor, for example, has a half-life--the time it takes for 50% of a chemical's mass to decay--of 20 days in soil, but of nearly four years in groundwater.

Taken together, all these factors--the remoteness of groundwater, its slowness to recharge, the enormous volume of contaminants that reach it, and their slowness to break down underground--make groundwater pollution virtually irreversible. Perhaps the most daunting challenge is the sheer size of many aquifers. The Ogallala, for instance, spans portions of eight states in the midwestern U.S. and covers 453,000 square kilometers. (A kilometer equals 0.62 miles.) The volume of water it would take to purge such a system of chemicals is "unimaginably large" notes the U.S. National Research Council. Deeper aquifers are very hard to get to, and the persistence of many contaminants found underground further complicates cleanup.

Besides pollution, the world's groundwater faces a second major onslaught--depletion. Many major aquifers are being drained much more rapidly than their natural rates of recharge, thus shrinking water reserves by an estimated 200,000,000,000 cubic meters a year and effectively spending down precious capital. (A cubic meter equals 1.307 cubic yards.) Removing large amounts of water from an aquifer can magnify the concentration of pollutants in the groundwater that remains. In some cases, polluted surface flow or salty ocean water may pour into the aquifer to replace the depleted groundwater, further shrinking supplies.

Groundwater overuse and pollution have proceeded unchecked in large part because we know so little about the water buried beneath our feet. Very few countries have regular monitoring programs to gauge...

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