The primary purpose of this project was to create a digital hydrogeologic-framework model for the Mesilla Basin area, including the Mesilla to Rincon section of the Rio Grande Valley and adjacent parts of the southern Jornada del Muerto (Jornada) Basin (Plates 1 to 7). The binational, tri-state study area is mostly in Doña Ana County, New Mexico; but it also includes parts of the El Paso-Ciudad Juárez metropolitan area in Texas and Chihuahua, Mexico. Scope of work was dictated by requirements of the Lower Rio Grande Water Users Organization and New Mexico Interstate Stream Commission for a state-of-the-art hydrogeologic model that will provide a sound basis for ongoing modifications and updates of the existing groundwater-flow model for the Lower Rio Grande-Mesilla Basin area. Project emphasis is on 1) the hydrogeologic framework of the Rio Grande rift-basin and river-valley fills that collectively form the major aquifer systems; and 2) how basin-fill composition and structural-boundary conditions influence groundwater flow and chemistry.

Geographic Information System (GIS) methodology (ARCINFO® platform) is used to portray and integrate the major framework components of aquifer-system lithology and stratigraphy, basin boundaries, and internal basin structure. The hydrogeologic-framework model “template” is three-dimensional and has a combined hydrogeologic-map—fence-diagram format (Plates 1 to 7). Map-scale is 1:100,000, model base elevation is 1,000 ft asl (above mean sea level), and vertical exaggeration of the 17 cross-sections is 10x.

Baseline information for about 160 “key wells” is used in design of the 17 hydrogeologic sections (Plates 3 to 6). The data base includes 60 digital borehole geophysical logs, driller and drill-cutting logs, and groundwater head and chemical data, as well as interpretations of the major lithologic, stratigraphic and structural elements of basin-bounding bedrock units (Appendix, Tables A1 to A4, Plates A1 to A9). GIS-framework components include area features (polygons), such as those showing the spatial extent of geologic-mapping units, line features such as fault-zones, and point features showing locations of key wells and other sites with detailed information on subsurface geology. This digital template is a significant advance over previous work, because hydrogeologic databases and interpretations were presented in a wide variety of map and section formats with inconsistent scale, spatial coverage, organization, quality, and flow-model utility.

The primary aquifer systems of the study region are the thick (up to 3,000-ft) sedimentary fills of the Mesilla and Jornada Basins that are linked by the valley of the Rio Grande (Plate 1). Alluvial, eolian, and playa-lake sediments of the Santa Fe Group (Late Cenozoic) form most of the basin fill and include an upper sequence of ancestral river deposits. Thin (<80 ft), upper Quaternary alluvium of the inner-river valley is the only other major aquifer. Basin aquifer systems have three hydrogeologic components: lithofacies assemblages (LFAs), hydrostratigraphic units (HSUs), and bedrock and structural-boundary controls. Fluvial and eolian LFAs with excellent aquifer potential are very thick and extensive in Santa Fe Group HSUs of the central Mesilla Basin.

The Rio Grande Valley provides the only inter-basin connection for both surface and shallow-subsurface flow between the Rincon, Mesilla and southern Jornada Basins. Selden Canyon and El Paso del Norte “narrows,” respectively at the north and south ends of the Mesilla valley, form very effective barriers to significant inter-basin groundwater flow from aquifer systems of the Rincon Valley and to the Hueco Bolson. In much of the northern Mesilla Basin, upper to middle parts of the basin- and valley-fill aquifer systems are well connected with respect to the surface and shallow-groundwater flow regimes of the inner-river valley. There is also a substantial (but still unquantified) “paleo-recharge” contribution to groundwater flow in the southernmost Mesilla Basin from the very large basin of “pluvial-Lake” Palomas in northwestern Chihuahua. The ultimate discharge zone for the entire (regional and local) groundwater-flow system is in the lower Mesilla Valley area between Anthony (NM-TX) and El Paso de Norte.

The Jornada Basin aquifer system has a groundwater-flow regime that is isolated from the Rio Grande Valley recharge sources. The very small amount of underflow discharge to the Mesilla Valley is restricted to a few shallow saddles in a partly buried bedrock “high” that separates the Jornada and Mesilla structural basins. The only area with mountain-front recharge sources and potential for long-term, moderate-yield production is limited to the southernmost basin area between the Doña Ana Mountains and Organ-southern San Andres range. Gypsiferous playa-lake sediments and brackish-water conditions characterize Santa Fe HSUs in the basin fill to the north, with minor underflow discharge to the Rincon Valley occurring only in the area between the Rincon Hills and San Diego Mountain.