The growth in global population and economic development has resulted in tremendous pressures on existing sources of fresh water. Human water use over the past three centuries increased by a factor of thirty-five and is growing by four to eight percent annually. Coupled with recurring international disputes over water resources, poor water management, and the realization that water is an indispensable but finite resource, these trends have propelled the use and management of transboundary groundwater resources to the forefront of legal debate.

Until recently, matters relating to groundwater resources were relatively ignored in the context of international law applicable to transboundary water resources. In particular, international water law failed to satisfactorily consider or comprehend the physical interrelationship and interdependence between surface and groundwater resources. As a consequence, most legislators, policymakers, and even international legal scholars continue to regard underground water sources as dissimilar from surface waters with respect to ownership and usage, and omit the resource from the legal regime of international water law.

Part I of this article examines international water law and proposes the application of this legal regime to both surface and underground waters equally and without distinction. This application is founded on the basis that due to the indissociable nature of and interdependency between the two water resources, surface and underground waters cannot be utilized or protected adequately or efficiently unless they are considered simultaneously under the same rubric of management and law.

Section A of Part I discusses the general legal principles that comprise international water law, and that heretofore were applied primarily to surface water resources. Section B follows with a basic scientific understanding of ground water resources and of the nexus typically found between surface and subsurface waters. Thereafter, Section C presents an overview of the international legal regime of transboundary waters, which includes groundwater resources, as defined and codified by international organizations and instruments. Section D offers a summary and closing remarks.

In Part II of this article, groundwater issues are considered in the context of the Gabcikovo-Nagymaros controversy between Hungary and Slovakia. Although the controversy does not focus solely on groundwater issues, the dispute provides fertile ground for the application of international water law, in its fullest sense, to questions of the use and ownership of transboundary groundwater resources.