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 for drinking and irrigation water—and in the process, we have made a sobering discovery. Despite the popular impression that groundwater is shielded beneath our feet is not only susceptible to pollution, it is in many ways more vulnerable than water above ground.

Because water moves through the earth with glacial slowness, aquifers become sinks for pollutants, decade after decade. Some aquifers recharge fairly quickly, while others, like the Chalk, store their water for millennia. But the average residence time for groundwater is 1,400 years, as opposed to just 16 days for river water. So instead of being flushed out to the sea or becoming diluted with constant additions of fresh water, its pollutants accumulate. And in these sources, unlike rivers, the pollution is generally irreversible.

For most of human history, groundwater was tapped mainly in arid regions where surface water was in short supply.  In the second half of the twentieth century, the soaring demand for water turned the dowsers' modern-day counterparts into a major industry. Today, massive aquifers are tapped on every continent, and ground-water is the primary source of drinking water for 1.5-2 billion people worldwide.  The aquifer that lies beneath the Huang-Huai-Hai plain in eastern China alone supplies drinking water to nearly 160 million people. Some of the largest cities in the developing world— including Dhaka, Jakarta, Lima, and Mexico City—depend on aquifers for almost all their water. And in rural areas, where centralized supply systems are undeveloped, groundwater is typically the sole source of water. Almost 99 percent of the rural U.S. population and 80 percent of rural Indians depend on groundwater for drinking.

A principal reason for the explosive rise in groundwater use since 1950 has been a dramatic expansion in irrigated agriculture. In fact, irrigation accounts for about two thirds of the fresh water drawn from rivers and wells each year. In India, the leading country in total irrigated area and the world's third largest grain producer, the number of shallow tubewells used to draw groundwater surged from 3,000 in 1950 to 6 million in 1990. Today aquifers supply water to more than half of India's irrigated land. About 40 percent of India's agricultural output comes from areas irrigated with groundwater.

Some major threats to 'drinking quality' grandwater