Across Africa, Asia, Central and South America, groundwater levels are dropping as much as 10 feet a year, largely due to intensive irrigation, the U.N. Environment Program said in a report released in London.
As aquifers subside, sea water seeps into the water table in coastal areas, making the water undrinkable, co-author John Chilton told reporters. The problem is causing the ground to sink in major cities, including Mexico City and Bangkok, and will force small farmers out of business, he said.
"What is needed, along with fresh water, is fresh thinking. We need to learn how to value water," said U.N. Secretary General Kofi Annan in a statement ahead of World Environment Day on Thursday.
The report, "Groundwater and its Susceptibility to Degradation," said water tables were falling most rapidly in South Asia, Mexico and in semiarid countries where agriculture relies heavily on irrigation.
"Some two billion people and as much as 40 percent of agriculture is at least partly reliant on these hidden stores. Groundwater also supplements river flow, springs and wetlands vital for rural and urban communities and wildlife," the executive director of UNEP, Klaus Toepfer, said in a statement.
"If a lake, reservoir or river becomes depleted or dries up, the event is highly visible, there is public outcry and often action taken. I hope that this report will serve as a wake up call concerning the human, social and economic consequences of squandering our vital underground water supplies."
Chilton, principal hydrogeologist at the British Geological Survey, said that while groundwater was diminishing across India, for example, it was rising in the capital of Saudi Arabia, Riyadh.
He said water pumped from coastal desalination plants was leaking from pipes and storage tanks and seeping into the water table, causing it to rise by up to 16 inches a year and flooding basements.
The United Nations called on governments to curb the use of groundwater through regulation. Worldwide action was needed to ensure that countries relying on irrigation diversify away from water hungry crops, the report added.