A widely reproduced error in hydrogeologic maps originated from a provisional delineation drawn in the mid-1990s to depict the southern extent of the Mesilla Bolson aquifer along the western margin of the El Paso–Juárez metropolitan area. The delineation was created solely to satisfy a U.S. Environmental Protection Agency contractual milestone during early binational groundwater inventory efforts and was never intended to represent a final hydrogeologic basin limit. Nevertheless, the outline persisted, became institutionalized in reports, models, management documents, and public imagery, and was ultimately labeled the “Mesilla/Conejos–Médanos Basin Transboundary Aquifer,” where it continues to be treated as a valid transboundary aquifer delineation. This paper documents the origin of the provisional delineation and proposes a revised delineation for the Mesilla/Conejos–Médanos Basin Transboundary Aquifer, based on internationally recognized definitions of transboundary aquifers and groundwater basins, including United Nations frameworks, and established scientific criteria. These criteria include basin-scale geologic and structural controls, hydrostratigraphic continuity, groundwater divides, and permeability contrasts at basin interfaces. Results indicate that the newly defined aquifer delineation falls within Mexico’s much larger administrative Acuífero Conejos–Médanos (0823), a unit that represents groundwater management jurisdiction in Mexico rather than solely a hydrogeologic basin. The proposed transboundary aquifer is defined on hydrogeological principles and does not coincide with either the administrative unit of Mexico or the historic provisional outline that has become widely used by multiple binational entities and by different experts in groundwater science.
Hibbs et al. (Mon,) studied this question.