The US-Mexico border divides the communities of Palomas, Chihuahua and Columbus, New Mexico, but they remain intimately linked. Both communities suffer from inadequate social services, poor public infrastructure, high unemployment and high poverty rates. To confront these challenges, Palomas and Columbus work together, sharing resources like hospitals, firefighters, and even schools. Palomas and Columbus also share another vital resource—groundwater. In the parched Chihuahuan desert, the communities depend on this groundwater as their sole water supply source, yet their aquifer is contaminated with arsenic and fluoride. Local governments acknowledged this contamination as early as the 1970s, but it was not until the 2000s that they received the needed reverse osmosis technology and water/wastewater infrastructure to ameliorate household exposure to water contamination. This thesis compares how Columbus and Palomas have addressed water insecurity over a twenty-year period from 1996-2016, using a 1996 study as the baseline (Tanski et al. 1998). New data include a household survey of 152 households, 60 semi-structured interviews, and participant observations of water practices collected during two months of fieldwork in the summer of 2016. The central research questions of this thesis are Q1) What causes household water insecurity on the US-Mexico border? and Q2) How can water policymakers and providers more equitably provide users with access to clean, reliable, and affordable drinking water? From a human development perspective, water security is defined as having an adequate supply of reliable and affordable water for a healthy life. This thesis uses a political ecological lens to more critically examine how water security connects to socio-political processes of water governance and power imbalances. Following Jepson (2014), this thesis argues that water (in)security is produced by problems in water access, water quality, and water affect (or water distress) and unfolds within a complex, hydrosocial landscape. Applying Jepson’s (2014) water security typology to Columbus and Palomas revealed that each local water utility adopted a distinct approach to addressing groundwater contamination, predicated on their financial and social resources, and structured by national and bi-national water policies as well as their institutional parameters. The survey found household water security has improved in terms of water access and reliability. But, centralized water filtration technology increased costs and reduced affordability in Columbus, while decentralized water filtration technology inadequately resolved household water supply contamination in Palomas. Thus, despite the technological improvements, households remain unevenly exposed to water contamination and costs. This raises concern about approaches to water security, which should be more finely attuned to water equity. Water equity means the rights to access clean water are more equitably distributed within the communities, and there is greater recognition/participation of community members in decision making on water management.

Dès le début du 20ème siècle, les eaux souterraines de l’aquifère transfrontalier Milk River (MRA) ont constitué une importante ressource dans le sud de l’Alberta (Canada) et le nord du Montana (USA). L’utilisation intensive de cette ressource sous un climat semi-aride a provoqué une baisse importante des niveaux d’eau localement, si bien que des inquiétudes concernant la pérennité du MRA sont apparues dès les années 60. Les études précédentes du MRA étaient limitées par les frontières nationales, empêchant ainsi une compréhension complète de la dynamique de l’aquifère. L’objectif de cette thèse était de réaliser une étude transfrontalière du MRA afin de caractériser cette ressource régionale d’eau souterraine selon ses limites naturelles. À cette fin, trois modèles transfrontaliers ont été réalisés: un modèle géologique, un modèle conceptuel hydrogéologique et un modèle numérique d’écoulement souterrain. Des travaux de terrain de part et d’autre de la frontière Canada/USA et une revue de littérature exhaustive des études précédentes ont supporté la réalisation de ces modèles. De plus, une première évaluation des niveaux d’exploitation historiques du MRA a été réalisée en Alberta.Le modèle géologique 3D (50 000 km2) représente le MRA (Membre Virgelle de la Formation Milk River/Eagle) et ses unités confinantes continûment à travers la frontière internationale. Le développement de ce modèle a requis une harmonisation des nombreuses nomenclatures stratigraphiques de la région et la délimitation transfrontalière du MRA.Le modèle conceptuel hydrogéologique du MRA a montré deux flux d’écoulement souterrains transfrontaliers, dirigés du Montana vers l’Alberta. La Milk River intercepte la majeure partie du flux souterrain venant du sud, si bien que l’écoulement au nord de la rivière est très faible. Les analyses isotopiques confirment que les eaux du MRA sont principalement fossiles à l’exception de la zone de recharge située le long de la zone d’affleurement du MRA. Les zones du MRA avec une forte conductivité hydraulique et de faibles concentrations en chlorures correspondent à des voies d’écoulement préférentielles. L’émergence des eaux du MRA a lieu via la drainance à travers les unités confinantes notamment le long des vallées enfouies.Le modèle numérique (26 000 km2) d’écoulement souterrain du MRA en régime permanent est une transposition des modèles géologique et conceptuel. Il montre que le modèle conceptuel précédemment développé est hydrauliquement plausible. Le modèle d’écoulement donne une meilleure compréhension du système aquifère en représentant la dynamique de l’écoulement souterrain dans la situation pré-exploitation. Le traçage de particules indique des temps de résidence advectifs de près de 750 000 ans à la limite nord du MRA, ce qui est inférieur aux âges obtenus par les analyses isotopiques (2 Ma).Les bilans en eau des modèles conceptuels et numériques montrent tous les deux que l'extraction de l'eau souterraine dépasse de loin la recharge au nord de la Milk River.Un modèle d'écoulement transitoire serait requis pour définir le volume d'eau provenant de l'emmagasinement dans le MRA et préciser le rôle des aquitards durant le pompage. Par ailleurs, en tant que ressource internationale partagée, une gestion transfrontalière des eaux du MRA serait justifiée dans la région comprise entre la zone de recharge au Montana et le sud de la Milk River en Alberta. Ainsi, ces trois modèles transfrontaliers du MRA forment une base commune internationale de connaissances scientifiques à l'échelle de l'aquifère et pourraient supporter l'évaluation future du meilleur usage possible de cette ressource partagée et limitée. De futurs travaux peuvent inclure l'effet des champs de gaz localisés aux limites du MRA et les conditions paléo-hydrogéologiques liées à l'évolution géochimiqie de l'au souterraine. / Since the beginning of the 20th century, groundwater from the Milk River Aquifer (MRA) has been an important water resource in southern Alberta (Canada) and northern Montana (USA). The intensive use of this resource under a semi-arid climate has locally led to important drops in waters levels, thus raising concerns on the MRA sustainability since the 1960’s. Previous MRA studies were limited by the national boundaries, thus preventing a full understanding of the aquifer dynamics. The objective of this thesis was to carry out a transboundary study of the MRA to define this regional groundwater resource within its natural boundaries. For this purpose, three cross-border models were developed: a geological model, a conceptual hydrogeological model and a numerical groundwater flow model. These developments were supported by focused field work on both sides of the Canada/USA border and a comprehensive review of previous studies. Furthermore, a first assessment of the historical exploitation levels of the MRA was done for southern Alberta.The 3D geological model (50,000 km2) continuously represents the MRA (Virgelle Member of the Milk River/Eagle Formation) and confining units through the international border. Development of this model required harmonization of various stratigraphic nomenclatures in the study area and the transboundary delineation of the MRA extent.The hydrogeological conceptual model of the MRA indicated two transboundary groundwater fluxes from Montana to Alberta. The Milk River intercepts most of the groundwater flux incoming from the south, thus leading to limited groundwater flow north of the Milk River. Isotopic analyses confirm that the MRA contains mostly fossil groundwater, with the exception of the recharge area located along the outcrop area of the aquifer. Areas within the MRA with a high hydraulic conductivity and low chloride concentrations correspond to preferential groundwater flowpaths. Discharge from the aquifer occurs as vertical leakage through the overlying confining units, especially along bedrock valleys.The numerical steady-state groundwater flow model of the aquifer system (26,000 km2) is a transposition of the geological and conceptual models, and it shows that the previously developed conceptual model is hydraulically plausible. The groundwater flow model provides a better understanding of the aquifer system by representing the dynamics of groundwater flow under pre-development conditions. Particle tracking indicates advective residence times of up to 750,000 years in the northern limit of the aquifer, which is lower than ages obtained from isotopic analyses (2 Ma).The area south of the Milk River benefits from all of the transboundary groundwater flux from Montana whereas north of the Milk River, only a portion of the recharge flux coming from the south is transmitted due to the interception of the Milk River.Groundwater budgets from both the conceptual and numerical models show that groundwater extraction far exceeds recharge north of the Milk River. A transient model would be required to determine how much water is derived from storage in the MRA and to assess the role of the confining units during pumping. Furthermore, as an internationally shared resource, a transboundary management of the MRA would be appropriate, especially in the area comprised between the recharge area in Montana and the Canadian reach of the Milk River.Together, these transboundary models of the MRA constitute a common international basis of scientific knowledge at the aquifer scale and could support further assessments of the best possible exploitation of the shared but limited resource.Future works could include the study of the effects of the gas fields located at the limits of the MRA as well as paleo-hydrogeological conditions relative to the geochemical evolution of groundwater.

Since 2002, NASA's GRACE Satellite mission has allowed scientists of various disciplines to analyze and map the changes in Earth's total water storage on a global scale. Although the raw data is available to the public, the process of viewing, manipulating, and analyzing the GRACE data can be difficult for those without strong technological backgrounds in programming or geospatial software. This is particularly true for water managers in developing countries, where GRACE data could be a valuable asset for sustainable water resource management. To address this problem, I have a developed a utility for subsetting GRACE data to particular regions of interest and I have packaged that utility in a web app that allows water managers to quickly and easily visualize GRACE data these regions. Using the GLDAS-Noah Land Surface Model, the total water storage for the regions derived from the raw GRACE data is decomposed into surface water, soil moisture, and groundwater components. The GRACE Groundwater Subsetting Tool is easily deployed, open-source, and provides access to all of the major signal processing solutions available for the total water storage data. The application has been successfully applied to both developed and developing countries in various parts of the world, including the Central Valley region in California, Bangladesh, the La Plata River Basin in South America, and the SERVIR Hindu Kush Himalaya region. The groundwater data in this application has proven capable of monitoring groundwater use based on drought trends as well as agricultural demand in a number of locations and can assist in uniting decision makers and water users in the mission of sustainably managing the world's groundwater resources.