Many of the 15 million inhabitants along the United States-Mexico border derive fresh water from transboundary aquifers straddling and extending far beyond the political boundary separating the two countries. The previous lack of a large-scale cooperative and structured data collection effort and groundwater management strategy for the region has left border communities with little information on current and future groundwater supplies. In 2006, the U.S. Federal Government enacted the United States – Mexico Transboundary Aquifer Assessment Act (Public Law 109–448) to address this issue.