When the U.N. General Assembly adopted the Convention on the Non-Navigational Uses of International Watercourses in 1997, it took a decisive step in recognizing the important role that transboundary ground water resources play in human progress and development. In so doing, it also acknowledged the need to establish principles of law governing this "invisible" but valuable natural resource. Transboundary ground water historically has been neglected in treaties, ignored in projects with international implications, and cursorily misunderstood in much of legal discourse.

While the Convention provides substantial clarification on the status of ground water under international law, it also leaves considerable gaps and generates confusion about the types of aquifers that fall within the scope of the treaty. A close review of the scope and definitions of the treaty reveals that the Convention's focus on surface water overshadows its ground water cousin and excludes many of the world's aquifers.

Focusing on the scope and definitions, this article critically examines the treatment of ground water under the Convention from a hydrogeological perspective. Using six science-based aquifer models with transboundary implications (representing the majority of transboundary aquifers presently known in nature), the analysis identifies the types of aquifers that are included within the scope of the Convention and assess the rationale for excluding other aquifer types. The article also considers the special case of non-recharging aquifers.