When it comes to delineating the boundaries of an aquifer located between two or more countries, science, and its methods engage in a complex negotiation process, which is why I coined the term/metric: transboundariness metric.

The first time I thought of the term transboundariness was during a conversation with a colleague, trying to explain to him how the boundaries of an aquifer take on a different value, dimension, and scale when they are cross-borders. 

In that conversation, I tried to find a term, a concept, or an approach that could measure why and how the treatment and attention to these transboundary aquifers vary according to multiple factors: social, political, cultural, and economic, that constantly interact.

These variables can affect how we identify an aquifer and how we define its limits. In addition, they help to recognize it as a transboundary aquifer and prioritize it over others.

The transboundariness approach includes these variables and attempts to measure them, which turns a supposedly simple technical task (defining the boundary of an aquifer) into a blurry and undefined process.

In co-authorship with my colleague Gabriel Eckstein, we published the concept of transboundariness in 2017. Since then it has been used to refer to the strategic value that an aquifer acquires by being located on the border between two or more countries, regardless of its physical condition.

One water basin leads to another

Transboundariness is a concept with a fresh approach to identifying and measuring the waters shared between countries, considering hydrogeological variables, but also others, such as informational, scientific, pollution, population factors, and the level of vulnerability of the settlement. So, when I talk about shared waters between countries and their conflicts, that is transboundariness.

Going a little deeper into the subject, let’s see the level of transboundariness an aquifer has beyond its physical limit. Countries have borders that are clearly demarcated and water, which has its own will, passes freely through that border.

What should we consider? On the border between Mexico and the United States there are many aquifers, but not all of them are recognized as transboundary. Why? Because there is a level of transboundariness that distinguishes them. How do you differentiate them? With those variables included in this term.

Why is the Allende-Piedras Negras Aquifer, which is important for the region, not recognized as binational? Because today it does not have any manifested problem at the transboundary level, so its level of transboundariness is lower.

This contrasts with the El Paso Aquifer (Hueco-Bolson), whose level of transboundariness is very high -and it is among the highest- because its water quality is worrying, it has a dependent population of more than three million people, it is an internationally recognized aquifer, it is the only one that has a mathematical model, because it has a history of collaboration agreements, among other variables. The strategic weight of a border aquifer determines its level of transboundariness.

On the US-Mexico border, there are many shared underground water sources. Little is known about these waters or how the two countries manage them, the transboundariness metric helps to better manage this resource.