Groundwater is an essential resource for sustaining civilization, Sampat noted. About 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. 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, Sampat predicts. Aging underground storage tanks can leak gasoline and other contaminants into groundwater Groundwater also plays a key ecological role by replenishing rivers, streams and wetlands. It provides much of the flow for the Mississippi, the Niger, the Yangtze and many other great rivers - some of which would otherwise not run year round. Some of this waste has a half-life of 250,000 years. 15 percent of shallow groundwater sampled below cities and farms in the U.S. in the mid-1990s had nitrate concentrations that exceeded the guideline for drinking water.