Globally, groundwater is by far the largest store of liquid freshwater, making it a key component of a secure water supply. However, over the past few decades the amount of usable groundwater available around the world has rapidly decreased. This depletion is caused primarily by mismanagement (e.g., overpumping, contamination, and under-regulation), but also by reduced natural recharge due to climate change and urbanization. Management of groundwater resources is particularly challenging for the nearly 600 aquifers that are transboundary, meaning that they extend across international political borders. To understand how governance mechanisms can reduce water insecurity in transboundary groundwater contexts, we review key literature from what we view as the most relevant fields: groundwater management, water security, international water law and international water governance. We then formulate a set of recommendations for improved groundwater governance that can address the specific physical nature of groundwater systems, enhance water security, and apply to transboundary groundwater settings. We argue that groundwater governance in transboundary contexts requires processes that (1) enhance context-specific and flexible international mechanisms (2) address the perpetual need for groundwater data and information (3) prioritize the precautionary principle and pollution prevention, in particular (4) where appropriate, integrate governance of surface and subsurface water and land and (5) expand institutional capacity, especially of binational or multinational actors.
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