Map of the world's groundwater resources
There is an article in the current issue of The Economist on efforts to protect aquifers that cross national borders. I quote:
Aquifers, like fish stocks, are most at risk when they cross national borders, making property rights weaker. Groundwater provides about a fifth of the planet’s water needs and half its drinking water. In arid countries such as Libya or Saudi Arabia, that figure is close to 100%. Almost 96% of the planet’s freshwater resources are stored as groundwater, half of which straddles borders. UNESCO, a United Nations body, estimates that 273 aquifers are shared by two or more countries.The United Nations passed a resolution in 2008 on creating a legal regime for aquifers (it may become a full convention next year). Now sampling and monitoring have resumed, under the aegis of the International Atomic Energy Agency (which has a sideline in environmental monitoring).
Such scientific work is crucial because aquifers are still poorly understood. Until a UNESCO inventory in 2008, nobody knew even how many transboundary aquifers existed. Experts are still refining the count: the American-Mexico border may include 8, 10, 18 or 20 aquifers, depending on how you measure them. Defining sustainability vexes hydrologists too, particularly with ancient fossil aquifers that will inevitably run dry eventually. Estimates for the life of the Nubian sandstone aquifer range from a century to a millennium.