HYDROGEOLOGIC FRAMEWORK OF TRANSBOUNDARY AQUIFER SYSTEMS IN THE PASO DEL NORTE REGION--TEXAS, NEW MEXICO, AND CHIHUAHUA

The primary transboundary aquifer systems of the Paso del Norte region of western Texas, southwestern New Mexico, and northern Chihuahua comprise intermontane-basin and river-valley deposits of Late Cenozoic Age. The region is in the southern Rio Grande rift (RGr) tectonic province and includes a long segment of the binational Rio Grande Valley and the El Paso-Ciudad Juárez metropolitan complex (population about 2 million). Major advances have recently been made in development of digital hydrogeologic models and related GIS datasets that incorporate large amounts of information on RGr basin-fill (Santa Fe Group) and river-valley aquifer systems, particularly in western Hueco and southern Mesilla Bolson areas connected by El Paso del Norte narrows. The GIS format (e.g. ESRI ArcGIS^®) allows integration of surface and subsurface information that can be used in ongoing groundwater-flow and geochemical modeling. Aquifer hydrogeology is defined in terms of 1) major lithofacies-assemblages (LFAs) that are grouped as informal hydrostratigraphic units (HSUs), and 2) basin-boundary and intra-basin structural controls. Hydrogeologic maps and cross-sections (msl base) completed to date include maps of surficial-features and bedrock topography (structure-contour), and 20 sections.

Tectonic controls on both basin-fill composition, and groundwater flow and chemistry in the study region include half-graben basins and flanking ranges that are linked across zones of structural accommodation. Major aquifer systems are formed by basin-floor LFAs deposited by Late Neogene ancestral fluvial-deltaic distributaries, and Late Quaternary Rio Grande Valley fill. These poorly consolidated sediments are grouped into middle and upper Santa Fe Gp and river-valley HSUs, with horizontal hydraulic conductivities in the 3-30 m/day range. Saturated basin-fill fluvial sequences are as much as 300 m thick and 30 km wide, while river-valley fill is less that 30 m thick and 9 km wide. Water quality is fresh to moderately brackish. Except for eolian-sand facies, subjacent lower to middle Santa Fe Gp basin-floor LFAs (Miocene) and intertonguing piedmont-slope deposits have much lower aquifer potential because of finer matrix, and greater consolidation and cementation.