Some 97 percent of the planet's liquid freshwater is stored in underground aquifers. Nearly one third of all humanity relies almost exclusively on groundwater for drinking, including the residents of some of the largest cities in the developing world, such as Jakarta, Dhaka, Lima, and Mexico City. Almost 99 percent of the rural US population, and 80 percent of India's villagers, depend on groundwater for drinking. Groundwater irrigates some of the world's most productive cropland. More than half of irrigated farmland in India, and 43 percent in the United States, are watered by groundwater.
Irrigation already accounts for about two-thirds of water use worldwide.
As rivers and lakes are dammed, dried up, or polluted, and as food demand grows in the next 50 years, farmers will become increasingly dependent on groundwater for irrigation. Groundwater also plays a key ecological role by replenishing rivers, streams, and wetlands. In the northern Chinese provinces of Beijing, Tianjin, Hebei, and Shandong, nitrate concentrations in groundwater exceeded the health guideline in more than half of the locations studied in 1995. Water recycles extremely slowly underground, too slowly to flush out or dilute toxic chemicals. Water that enters an aquifer remains there for an average of 1,400 years, compared to only 16 days for rivers.