U. S. Geological Survey Middle Rio Grande basin study; proceedings of the Fourth annual workshop, Albuquerque, New Mexico, February 15-16, 2000

This volume contains a collection of short papers that document some of the conclusions and results of on-going investigations related to ground-water resources of the Middle Rio Grande Basin, New Mexico. These results, and additional project reports, were presented at the Fourth Annual Middle Rio Grande Basin workshop convened in Albuquerque during February 2000. The workshop was jointly sponsored by the U.S. Geological Survey and the New Mexico Bureau of Mines and Mineral Resources as a forum for public presentation and discussion of multidisciplinary studies conducted during the previous 4 years. This considerable body of work was undertaken to improve the scientific understanding of factors that control ground-water availability and quality, and to explore land-surface characteristics that have affected the development and use of this valley that currently supports a population in excess of 600,000. The USGS principal science partners that have been involved in the investigations have been the New Mexico Bureau of Mines and Mineral Resources, New Mexico Office of the State Engineer, University of New Mexico, New Mexico Institute of Mining and Technology, University of Texas at El Paso, and the New Mexico Museum. Many agencies have collaborated closely in these studies, including the City of Albuquerque, Middle Rio Grande Council of Governments, Bernalillo County, numerous Indian tribal governments, and additional county and municipal departments, some of whom have provided funding to support phases of the work. This volume is organized under topical headings for Geologic and Hydrologic Framework, Modeling, River-Aquifer Interaction, Recharge, and Related Studies. These papers provide snapshots of on-going investigations, and many are expected to be published in greater detail as the joint study concludes following 2001. Interested readers are encouraged to contact the authors directly for further information. Several papers merit attention for significant new results. The paper by Sanford and others and the paper by Plummer and others describe two interrelated aspects of a joint study of water chemistry and isotopic composition across the entire Middle Rio Grande Basin. The results were preliminary in some details, but several broad conclusions can be summarized. First, the geochemical data define distinct zones that suggest pathways of flow within the basin. Second, the groundwater model, as calibrated with geochemical and water-table data, suggests that recharge from tributary and basin-margin sources may be significantly lower than previous estimates. Independent calculations of external recharge also suggest lower values. Third, calculations based on carbon-14 ages and the ground-water model indicate that tributary and basin-margin recharge has been low for at least the last 10,000 years. Papers on stratigraphy and sedimentology of the Santa Fe Group sediments, the principal aquifers of the basin, support the growing consensus that the lithology and sequence of the Group varies considerably from place to place. These papers also describe evidence that sedimentation was strongly influenced by local faulting and that the make-up of the main aquifer units differs across major sub-basins. Geophysical data have made important contributions to the groundwater studies in locating faults and documenting growth-fault geometries.