Digital Hydrogeologic-Framework Model of the San Francisco River Basin, West-Central New Mexico and East-Central Arizona

The San Francisco River (SFR) is the major Gila River tributary in the upper Gila basin of west-central New Mexico and east-central Arizona (Figure 1-1). This study and related hydrologic-hydrographic investigations are part of ongoing efforts by the New Mexico Interstate Stream Commission (NMISC) to improve geohydrologic models used in management of both surface-water and groundwater resources of the Gila River subdivision of the “Lower Colorado River (Compact administrative) Basin.” The study area includes parts of Catron and Grant Counties in New Mexico as well as most of Greenlee County and a small part of Alpine County, Arizona (Figure 1-1). It comprises not only the SFR basin portion of the Gila-San Francisco “ground-water basin” (as declared by the New Mexico State Engineer-NMOSE-6/30/1991), but also the southern edge of the “Lower Colorado Basin” and the western Plains of San Agustín subbasin of the “Rio Grande Basin” east of the Continental Divide. The SFR basin is characterized by large topographic relief (3,280-10,760 ft; 1,000-3,279 m), semiarid to humid climatic conditions (including extreme seasonal precipitation events), and complex distribution patterns of basin-fill and bedrock aquifer systems. These factors combine to produce high variability in surface-water/groundwater discharge and availability. Time and budget constraints did not permit site-specific assessment of aquifer potential; and we could only evaluate the essential elements of the hydrogeologic-framework (stratigraphy-lithology-structure) at a drainage basin and subbasin scale in the New Mexico part of the study area. While primarily based on published hydrogeologic work (Trauger 1972; and Basabilvazo 1997), our study also incorporates a large amount of basic geologic information that has only been available since 1994. As a result, this is the first synoptic integration of a large geologic database specifically designed for characterization of SFR basin aquifer systems. Our main achievement has been development of a GIS-based, digital hydrogeologic model using ARC-GIS® and Adobe Illustrator®, respectively, for map and cross-section compilation. From a flow-modeling perspective, hydrogeologic databases and conceptual-framework models have, heretofore, only been available in formats with a wide range of interpretive quality and clarity. Plate 1 is a plan (map) view of the basin-scale hydrogeologic framework. It schematically illustrates surface-distribution patterns of major bedrock and basin-fill mapping units as well as large-scale tectonic and volcanic features. The map was compiled from a variety of mid-scale GIS sources in New Mexico (1:100,000 to 1:500,000 scale) that were merged with a much less detailed Arizona database (1:500,000 to 1:1,000,000). In addition, unit boundaries and definitions were adjusted in many places to reflect more-detailed quadrangle mapping. The subsurface dimension is illustrated by five schematic cross sections (Plate 2a-e) that were created specifically for this study at a map-scale of 1:100,000, base elevation of mean sea level, and 5x vertical exaggeration. Detailed definitions of hydrostratigraphic mapping units (HSUs) and component lithofacies-assemblages (LFAs) are provided in Tables 3-1 to 3 and 4-1. Table A1 in the Appendix contains selected data from published records of 277 wells and springs in the New Mexico part of the study area, and include preliminary interpretations of the hydrostratigraphy and producing aquifer(s) at each site. Approximate well/spring locations are shown on Plate 3. While all geology-based models tend to be “works in progress,” we believe that our digital model and supporting database represents a significant advance over previous work.