Managing Transboundary Groundwater Along the Border: Is Progress Possible?
Next year—August 30, 2023, to be precise—marks the 50th anniversary of the landmark agreement resolving the protracted mid-20th-century U.S.-Mexico dispute over salinity. That agreement, the International Boundary and Water Commission (IBWC)’s Minute 242, consolidated bilateral commitment through the important 1944 Water Treaty, the foundational agreement sustaining longstanding binational cooperation on water-sharing across the international boundary (Treaty 1944). Among Minute 242’s various provisions, however, one remains unsatisfied—the expectation that the two countries would strive to reach a comprehensive agreement on groundwater in shared aquifers (see below).
Pending the conclusion by the Governments of the United States and Mexico of a comprehensive agreement on groundwater in the border areas, each country shall limit pumping of groundwaters in its territory within five miles (eight kilometers) of the Arizona-Sonora boundary near San Luis to 160,000 acre-feet (197,358,000 cubic meters) annually. (IBWC 1973, Article 5; italics added)
Article 5 of Minute 242, as seen above, also limited each nation’s groundwater extraction along the boundary at San Luis Rio Colorado, Sonora, to 160,000 acre-feet annually. No other aquifers along the boundary today are subject to any binational regulation. Article 6 committed each country to inform the other of any groundwater developments in the border area that could impact the other country (IBWC 1973).
The need for greater binational management of the many shared aquifers along the boundary has long been recognized, dating back to the salinity era and subsequent scholarly analysis inspired by Minute 242. Apart from a number of technical studies of select aquifers, binational progress on groundwater has been stymied by intergovernmental proprietary and regulatory differences and, until recently, a perceived lack of urgency by stakeholders in shared transboundary aquifers.
As the sobering reality of protracted drought now pervades the border region, that sense of urgency is rising. Groundwater, always a hedge against droughts and variable surface water stocks, now figures prominently in the water augmentation schemes now flourishing throughout the desert Southwest and the Rio Grande. It is this reality that suggests greater binational progress may be possible in advancing the stewardship of these important water supplies.
This report reviews extant progress and recent dynamics favoring greater binational engagement on groundwater, accenting the constraints as well as the incentives affecting binational interest in greater cooperation. It traces the development of binational concern with transboundary groundwater and the evolution of binational cooperation in this area, and profiles how recent water-sharing agreements may facilitate a groundbreaking formal agreement on groundwater and how this is bound to depart from the comprehensive model for joint management contemplated in the IBWC’s Minute 242. The paper concludes by arguing that rising utilization of groundwater in the border region spotlights both the need for greater binational cooperation on managing transboundary aquifers and the greater authority and experience that may enable the governments to do so, acting through the IBWC.
Transboundary Groundwater: Background and Progress
Numerous aquifers penetrate the international boundary. Hydrologists are now aware of more than 30 such water bodies abutting or straddling the boundary, some better known and more accessible than others (Sanchez, Lopez, and Eckstein 2016; Sanchez and Eckstein 2020). Along the international boundary it is hard to find an urban area or agricultural zone that does not rely on groundwaters to some extent. Some sister cities along the land boundary, such as Ambos Nogales and Ambos Naco, which share the Arizona-Sonora line, are entirely dependent on groundwater while others—El Paso, Texas, and Juarez, Chihuahua, for example—are heavily reliant on the resource.
Historically, the most rapid development of border groundwater occurred during the salinity crisis of 1961–1973, as Mexico ramped up pumping to mitigate damage sustained by draining brackish groundwaters from the soils of Yuma, Arizona, orchards to the Colorado River (Mumme, forthcoming). That problem was eventually resolved by the IBWC’s Minute 242. Less known is that Mexico, as early as 1964, approached the U.S. with a proposal to negotiate a comprehensive groundwater treaty addressing all such shared waters along the boundary (Mumme and Tapia-Villasenor 2022). The U.S. resisted, fearing that even exploratory talks would weaken its diplomatic stance on Colorado River water-sharing. But Mexico’s concern was essentially acknowledged in Minute 242’s Article 5, as seen above.
Moving beyond Minute 242’s provision for protective pumping on the San Luis Mesa has proven difficult, however, if not intractable. Even as 1970s prognostics in some boundary areas foresaw pumping wars and the rapid depletion of local groundwater stocks, the threat was not imminent and policy action was thus easily deferred. Complicating matters, the national legal regimes for groundwater are at odds. Mexico’s constitution defines groundwater as a national resource to be managed by the federal government. The U.S. constitution reserves groundwater (and water resources generally) to states’ governance. Along the border, each U.S. state’s rules governing ownership and utilization of groundwater differs, ranging from a radical “rule of capture” in Texas permitting overlying proprietors to take as much as they desire to rules that designate certain critical aquifers for state regulation, as seen in Arizona (Mumme 1988). Though the IBWC quietly and informally explored the possibility of talks addressing water allocation on the Santa Cruz and San Pedro Rivers that may have included groundwater (Arizona Interstate Stream Commission 1969), Arizona and other U.S. border states and their local border communities expressed little interest in groundwater-sharing.
Two factors affected this impasse after 1980: first, rising concern for groundwater depletion at El Paso and Juarez, the border’s second-largest conurbation, confronted looming water shortages; second, a growing realization that groundwater availability could be and was being degraded by industrial and urban contamination. As early as 1979, the Texas Water Development Board expressed concern that El Paso was on an unsustainable path to exhausting the freshwater resources of the vast Hueco Bolson, the principal aquifer on which both the city and county of El Paso and the neighboring municipality of Juarez rely for 30% to 100% of their water supply (Bredehoeft et al. 2004; El Paso Water Utilities 2022).
Rising awareness of groundwater contamination also gained a modicum of public attention in the 1980s in various locations along the boundary. Chronic sewage spills to the Tijuana River threatened to pollute groundwater wells serving U.S. farms near the Tijuana River estuary (California Department of Water Resources 2006). At the twin cities of Nogales, Sonora, and Nogales, Arizona, rupture and spillage from the sewage connector line, the International Outfall Interceptor (IOI), fed worries about local groundwater quality (Ingram, Laney, and Gillilan 1995, 99). Declining groundwater levels in El Paso and Juarez’ heavily used Hueco Bolson contributed to the comingling of fresh water with brackish water, resulting in degraded extracted water quality made worse by industrial contamination (Estepp 1990; Lloyd 1984; Texas Commission on Environmental Quality 2005). The rapid growth of border cities suggested such problems would worsen absent domestic and binational attention (Eaton and Anderson 1987, 61-62).
Such concerns led the IBWC to cautiously invest in a small number of groundwater studies in the 1990s, well aware that any hint of an interest in actually pursuing binational management would invoke the opposition of U.S. states fearing even the appearance of federal insertion into state management of groundwater. Technical studies focusing on the Hueco Bolson at El Paso and Juarez between 1995 and 1998 identified overextraction and contamination as serious threats to sustainable use of the aquifer (IBWC 1997; 1998; 2001). These binational efforts contributed to the signing of a joint memorandum between El Paso and Juarez authorizing the sharing of technical groundwater data between the two cities’ water authorities (MOU 1999). Acting under the authority of its sanitation mandate, the commission also undertook a joint study in 1997-1998 focusing on Nogales Wash on the Arizona-Sonora boundary, which revealed contamination from industrial solvents and other pollutants threatening the health of residents of Ambos Nogales (EPA 1995; IBWC 2001a).
A further advance in binational cooperation on groundwater came with domestic legislation introduced by New Mexico Senator Jeff Bingaman in 2005. The legislation, the Transboundary Aquifer Assessment Act of 2006, better known as the Transboundary Aquifer Assessment Program (TAAP), authorized the U.S. Geological Survey, assisted by the U.S. IBWC, to conduct an assessment of aquifers at El Paso and along the New Mexico and Arizona boundaries (PL 109-448, 2006). Though not a binational initiative, the measure implicitly required Mexico’s cooperation. The IBWC, acting as binational facilitator, gained Mexico’s guarded assent in 2009 with a carefully worded principal engineers’ agreement (not a formal minute) that stipulated conditions for undertaking, financing, and sharing technical information gleaned from authorized TAAP groundwater studies. Pursuant to this initiative, three studies were undertaken between 2008 and 2016 before TAAP’s 10-year authority lapsed. These studies respectively focused on the Hueco Bolson and Mesilla (Conejos Medranos in Mexico) Bolson on the Texas-New Mexico-Chihuahua boundary, and the San Pedro River and San Cruz River aquifers along the Arizona-Sonora line.
The TAAP technical studies are the most important advance to date toward binational collaboration in understanding the hydrological characteristics of aquifers along the boundary. The value of the studies is obvious, not simply for the data as such but for the fact that these studies are binational products. The procedural aspects, seen from a diplomatic perspective, are possibly more important, establishing as they have some key parameters that are likely to apply to future assessments along the boundary.
Contributing to a more favorable diplomatic context for advancing toward a formal transboundary groundwater agreement and charting a potential path forward are a number of binational water agreements struck since 1973 that modestly strengthen the IBWC’s mandate in this issue area. The expansion and greater priority accorded border sanitation under the authority of the 1944 Water Treaty’s Article 3, which set in with IBWC’s Minute 261 in 1979 and was reinforced by the Environmental Protection Agency (EPA)’s binational La Paz Agreement in 1983, is now well consolidated. The commission’s jurisdiction is now clearly established where transboundary water quality issues are concerned, inclusive of groundwater, should the governments decide to pursue any agreement on groundwater pollution.
Another diplomatic approach the IBWC has utilized with some success over the past 20 years is the deployment of framework agreements that establish technical and policy advisory bodies and parameters for identifying and pursuing subsidiary agreements within an issue area. This is seen in Minute 306, which provides for ecological assessment on the lower Colorado River along and below the international boundary, and Minute 320, which supports further study and development of binational solutions to chronic sedimentation, waste deposition, and pollution problems on the Tijuana River (IBWC 2000; 2015).
The IBWC’s 2019 water summit focused directly on transboundary groundwater, noting its importance in the context of sustained drought and climate change and rising demand driven by demographic growth (IBWC 2019). Its summary conclusions supported further joint aquifer studies along the border. Adding to these advances, the IBWC has for more than a decade cautiously, though not formally, warmed to the idea of examining binational water issues from a watershed perspective. Its border water conferences since 2005 produced reports advancing such a perspective, and its formal agreements on the Colorado and Tijuana Rivers have cast a broader lens on water-use and its related impacts on those rivers (IBWC 2005; 2012; 2015; 2017).
In sum, 50 years since Minute 242 identified the need for a formal agreement on shared aquifers along the boundary, some binational cooperation on groundwater has emerged, if only with respect to a limited number of technical studies. That binational cooperation tied directly or indirectly to groundwater has grown over the past 20 years provides modest ground for optimism that further gains are possible where sustainable management of transboundary groundwater is concerned. Which raises the question: if further progress is to be had, how is it likely to manifest, and what is it likely to look like?
Prospects for a Transboundary Groundwater Agreement
At present, it is accurate to say there is no one hotspot along the boundary where local stakeholders are clamoring for a binational agreement on groundwater. The area of greatest cooperation on the utilization of groundwater is the El Paso-Juarez metropolitan zone, where the 1999 MOU on groundwater data-sharing applies with the declared option of coordinating groundwater development plans and projects to ensure the sustainable use of the Hueco Bolson aquifer (MOU 1999). To date there continues to be some data-sharing for the Hueco Bolson and Mesilla-Conejos Medranos Aquifers, although no other formal agreements on groundwater have been reached between the municipalities’ water agencies. An effort to include the Juarez water agency (Junta Municipal de Aguas) as a participant in the El Paso-Las Cruces Regional Sustainable Water Plan in 2001 was unsuccessful, although the 1999 MOU provides a window for binational sharing of regional water plans (IBWC 2001b).
The El Paso-Juarez memorandum describes one potential avenue for binational cooperation on transboundary groundwater, one based on quasi-formal local cross-border coordination of management activities with little involvement by the federal governments or, by extension, the IBWC. In this case, the IBWC is kept in the policy loop as it must be given Minute 242’s stated obligation that each country must inform the other of groundwater developments that could impact that country (IBWC 1973, Resolution 6). Any formal binational arrangements, even at the local level, would necessarily involve the IBWC. It is conceivable that other such coordinated domestic management based on informal protocols or understandings and a commitment to data-sharing and discussion of aquifer conditions on each side of the boundary could emerge in other closely linked municipal areas along the border, though there is little evidence of this now.
Moving toward more formal agreement is problematic. A comprehensive border-wide groundwater agreement, as originally contemplated in the diplomatic discussions that led to Minute 242, is likely only attainable in the broadest, most general sense. It is clear that the IBWC has sufficient jurisdiction under Minute 242 to pursue a further minute if the governments so desire. It is less likely that the governments would seek to reach a separate treaty on groundwater as originally proposed by Mexico in the 1960s. Border water scholars understand that any initiative to pursue joint management of groundwater will require a very soft touch by the federal governments and, by extension, the IBWC (Megdal et al. 2022). The most likely avenue of diplomatic progress is the pursuit of a framework agreement on transboundary groundwater enabling local transboundary aquifer agreements to emerge along the boundary. In this scenario, the IBWC functions as a facilitator of locally situated binational aquifer agreements, not as the manager or regulatory authority supervising the operational aspects of transboundary agreements.
Such an agreement would likely bear a family resemblance to other recent framework agreements that set parameters and provide facilities for further discussions and agreements within an issue domain. The commission, pursuant its treaty authority, has the power to convene binational task forces or work groups as needed and called for by aquifer stakeholders on both sides of the boundary with the concurrence of applicable local, state, and federal authorities who, most likely, would participate as members of such bodies. These binational working groups would pursue data-driven discussions focused on the needs and uses in play and various pathways for achieving sustainable utilization and protection for their aquifer. Agreements reached would be grounded in applicable state and federal groundwater law at the various levels of government in each country. The scope and degree of management would essentially be a local binational affair with the commission recognizing these arrangements in discrete implementation minutes. The IBWC, partnering with other agencies in both countries, would also facilitate financing and the provision of technical advice required for the implementation of these agreements.
The procedural parameters for such a comprehensive framework agreement on transboundary groundwater would likely resemble those agreed upon by the IBWC’s principal engineers as a condition of Mexico’s participation in TAAP implementation. At least five of TAAP’s six core principles of agreement (see below), principles 1–5, seem applicable to a future framework agreement. Principle 6, eschewing external influence in domestic water management, would need to be modified to accommodate the mutually negotiated management conditions for particular aquifers.
Core Principles for TAAP Implementation
- Activities described under this agreement should be beneficial to both countries.
- Aquifers to be jointly studied, as well as the scope of the studies or activities to be done on each aquifer, should be agreed upon within the framework of the IBWC.
- The activities should respect the legal framework and jurisdiction requirements of each country.
- No provisions set for in this agreement will limit what either country can do independently in its own territory.
- Nothing in this agreement may contravene what has been stipulated in the Boundary and Water Treaties between the two countries.
- The information generated from these projects is solely for the purpose of expanding knowledge of the aquifers and should not be used by one country to require that the other country modify its water management and use. (IBWC 2009; Megdal 2017)
These developments over the past two decades have laid a foundation for future progress toward a binational transboundary groundwater agreement. They converge with growing international commitment to groundwater stewardship, evident in the United Nations’ draft rules on transboundary groundwater management and the United Nations Economic and Social Council’s 2016 statement of principles for sustainably managing transboundary aquifers (UN 2008; UNESCO 2016).
Whether such a comprehensive framework agreement on Mexico-U.S. transboundary groundwater is attainable in the next five to 10 years remains uncertain. Thus far, the advocacy momentum for such development is mainly seen in the scholarly community. Local governments and groundwater stakeholders along the boundary have yet to mount a vigorous effort to persuade the IBWC to push for such an accord even as, in the case of the Santa Cruz aquifer on the Arizona-Sonora boundary, there are now a growing number of domestic and binational stakeholder initiatives addressing local watershed sustainability, almost all of which impact the underlying aquifer’s viability in one manner or another (Albrecht 2019).
And yet, as protracted drought afflicts the boundary from the Gulf of Mexico to the Pacific coast, there is little doubt that groundwater will play an even greater role in the water budgets and water augmentation strategies of border communities than is presently the case. The need to get ahead of this accelerating groundwater utilization curve along the international boundary and sustainably manage transboundary aquifers has never been more urgent. The structural impediments bedeviling U.S.-Mexican cooperation on groundwater to date still remain. Fortunately, as the preceding discussion suggests, today both countries, acting through the IBWC, have more authority and cooperative experience to work with in pursuing this long-elusive binational objective.
 Subsequent analysis, including studies sponsored by the IBWC, suggested that with aggressive conservation the aquifer might be viable through 2070, though greater utilization on both sides of the boundary could shorten that timeline (Bredehoeft et al. 2004). Those concerns, coupled with the known availability of greater quantities of brackish groundwater in the aquifer, drove El Paso Water Utilities (EPWU) to study and develop the Kay Bailey Hutchinson Desalinization Plant, inaugurated in 2007, to augment El Paso’s water supply.
 Industrial contamination of groundwater at the now-decommissioned American Smelting and Refining Corporation (ASARCO) smelter in El Paso was estimated by Texas Commission on Environmental Quality (TCEQ) to require $52,085,186 in remedial cleanup. Contaminated groundwater threatened to degrade surface water quality in the Rio Grande (TCEQ 2005).
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