The transboundary nature of water dividing Mexico and the United States (U.S.) transforms the entire border region into an instrument of cooperation, a source of conflict, a national security issue, and an environmental concern. Reasonable data collection and research analysis has been documented for surface waters by joint governmental institutions and non-governmental bodies. However, with the exception of the U.S. Transboundary Assessment Act Program (TAAP) (focusing on the Hueco Bolson, Mesilla Bolson, San Pedro and Santa Cruz aquifers), there is no comparable research, institutional development, or assessment of transboundary groundwater issues on the frontier. Moreover, data collection and methodologies vary between the two countries, there is no broadly accepted definition of the transboundary nature of an aquifer, and available legal and policy frameworks are constrained by non-hydrological considerations. Hence, there is a conceptual and institutional void regarding transboundary groundwater resources between Mexico and the U.S. The purpose of this paper is to bridge this void and characterize transboundary aquifers on the Mexico-US border. It reviews existing international frameworks for identifying hydrological and social criteria that characterize an aquifer as transboundary. It then assesses data from both countries to propose where and which aquifers could be considered transboundary. Finally, the paper proposes an agenda for assessing Mexico-US transboundary aquifers as a means for improving groundwater management in the border region.
- Data collection: Material collected included technical studies and raw data generated by state and federal agencies, non-governmental organizations, and private researchers in both countries. It also included peer reviewed technical studies and publications discussing the governance framework administering transboundary aquifers on the border.
- Data analysis: The study reviews and analyzes the international, binational and national efforts to characterize and classify aquifers in the border region as transboundary. It also assesses the hydrogeologic data related to frontier aquifers and identifies gaps and inconsistencies in the information.
- Data categorization: The study proposes an approach to categorize transboundary aquifers based on available hydrological and institutional data. Three categories were created in relation to levels of confidence of a transboundary relationship: "reasonable," "some" and "none." "Reasonable" level of confidence applies to aquifers for which the technical and related data evidencing a transboundary character was convincing. "Some" level of confidence applies to aquifers for which technical and related evidence was available but not definitive. "None" level of confidence refers to aquifers for which there is no technical data of a transboundary nature, but where other evidence suggest the possibility of a transboundary aquifer. GIS is used to present the study's preliminary results for the entire border region.
- Data analysis: The analysis uses ISARM's boundaries categorization as a reference for identifying aquifer boundaries in the border region. The true/system aquifer boundary refers to the limits of groundwater bodies hydraulically connected with relatively high transmisivity and storage capacity. The basin aquifer boundary relates to the boundaries of the whole hydrological basin, which includes groundwater systems. The geological aquifer boundary delimits the extension of the geological formation, which can potentially include aquifers but not necessarily across the whole unit. Because Mexican and U.S. approaches for aquifer identifications differ (Mexico uses basin aquifer boundary methodology exclusively, while the U.S. uses different approaches for different aquifers) Mexican data was used as the guiding reference for identifying and counting potential transboundary aquifers on the border. Where Mexican and U.S. data for a particular region conflicted, the Mexican data was used.
Results and Discussion Based on the collected data, a total of 36 aquifers and aquifer basins were identified along the Mexico-US border.
The analysis evidenced important differences between the states and countries in defining aquifer boundaries. Texas for example, defines its aquifer boundaries according to geological features. In contrast, Arizona uses true aquifer boundaries and basin boundaries depending on the aquifer. In the case of Mexico, data collection and methods are developed by one centralized agency, which uses a basin aquifer boundary methodology.
Another challenge emerging from the study is that with the exception of aquifers researched through the TAAP, all of the available studies depict border aquifers with a boundary that terminates at the border. This reality is crucial to understand the lack of comprehensive research in the region, the institutional weakness in promoting sustainable management practices, the uncertainty in the condition of transboundary groundwater resources in the region, and the urgent need to revise the research and institutional agenda to address transboundary aquifers.
Table 1 lists the preliminary results of the study. Of the 36 aquifers and aquifer basins identified eighteen were categorized as transboundary with "reasonable" level of confidence, while seven were categorized as transboundary with "some" level of confidence. There was no hydrological, social, or other data available for the remaining eleven aquifers to classify them as potentially transboundary.
The level of knowledge about transboundary groundwater resources between Mexico and the U.S. is alarming. Water quality conditions, availability, future demand, vulnerabilities, safe yield, and many other factors are unknown in about 90 percent of the transboundary aquifers identified in the border region. In addition, joint institutional and management practices are nonexistent or poorly organized throughout the frontier. Except for the four aquifers studied under the TAAP, holistic or binational research on the border's aquifers is practically nonexistent.
The categorization developed by this study offers preliminary results on two important issues: where and how many transboundary aquifers potentially exist on the Mexico-US border. The transboundary nature was identified based on available geological and social data, with priority given to hydrological considerations. However, to develop a governance framework that manages the border-region's aquifers sustainably, there needs to be a common methodology for determining aquifer boundaries. Moreover, the approach for determining the transboundary nature of aquifers requires standardization of criteria used by the riparians to efficiently assess management strategies at the transboundary level.