Transboundary aquifers (TBAs) are shared by different political entities, and their management often requires multilateral efforts. However, despite their strategic importance in sustaining ecosystems and human communities, the level of cooperation over TBAs remains generally low. Lack of awareness and political willingness are often cited for this. This paper further demonstrates the need for and relevance of TBA cooperation through an overview of real cases of cross-border groundwater impacts and joint management interventions across the world. The product of an extensive review of academic and grey literature, this study provides key insights into the types of cross-border groundwater impacts and joint management interventions, as well as the TBA settings where cases have been identified. This allows for important lessons on the scope of TBA cooperation to be drawn. Notably, the evidence-base suggests that in large TBAs, joint management interventions could often be prioritized over the border area. It also shows the need for proactive cooperation mechanisms to develop joint management interventions, not only to mitigate or remediate cross-border groundwater impacts, but also to prevent them.

Attention on the use of transboundary aquifers (TBAs) and their cross-border impacts is growing as countries become increasingly concerned about their long-term water security. Cross-border impacts, in groundwater quality and quantity, tend to concentrate in specific parts of TBAs, as they largely depend on the transboundary flow dynamics where anthropogenic actions operate. Thus, there is a growing consensus that strategies intended to prevent or mitigate such impacts should be implemented in strategic zones rather than in the whole TBA. These transboundary groundwater management zones (TGMZs) are relatively recent but have become a prominent topic in TBA management. However, until now, limited effort has been put into exploring the concept of TGMZs and the methods for their delineation. This research aims to fill these gaps and provide a basis for the delineation of TGMZs, thus helping neighbouring countries meet international responsibilities regarding the right to use and enjoy groundwater in TBAs. By reviewing academic and grey literature accessible from public sources, we present an overview of the concept and terminology of TGMZs, the approaches proposed for their delineation, and current operating examples. Additionally, we build a conceptual framework for assessing cross-border groundwater impacts by identifying their typologies and causal factors. We then apply our framework to evaluate and compare three reported methods which identify and delineate TGMZs from distinct perspectives, thereby gaining insights into their principles, performances, and limitations. Finally, we provide recommendations for further research towards optimising methods for delineating TGMZs.