GEOLOGIC AND HYDROSTRATIGRAPHIC MAPPING OF THE SANTA FE REGION, NEW MEXICO: APPLICATIONS FOR ASSESSING AND MANAGING GROUNDWATER RESOURCES

The region around Santa Fe, in north-central New Mexico, faces challenging groundwater supply and management issues due to rapid population growth and declining well-field productivity. Located in the southern Española Basin, this region is underlain by clastic sediment of the Santa Fe Group (upper Oligocene–lower Pleistocene fill of the Rio Grande Rift), which serves as the sole-source aquifer for Santa Fe and surrounding communities. To improve understanding of the regional hydrogeology, geologic mapping of six 7.5-minute quadrangles was recently completed at scales of 1:24,000 and 1:12,000. Mapping emphasized identification of faults as well as study of provenance, paleochannel characteristics, and textural criteria. These criteria were used to subdivide the basin fill into lithostratigraphic units that may have distinctive hydrostratigraphic characteristics. The general structure of the southern Española Basin, inferred from geologic mapping, examination of subsurface data, and available geophysical data, is that of a north-plunging syncline that transitions to a west-facing monocline to the north.

Each quadrangle was digitized into a GIS and then all were compiled into a geologic map of the basin (1:50,000) utilizing a simple joined lookup table that automates correlation of like units between maps with differing nomenclatures and levels of detail. This timesaving technique allows for quick revision of correlation criteria. Holocene alluvial cover was placed on a separate coverage allowing display of a subcrop map that facilitates hydrologic interpretation.

The resulting maps and cross sections provide a framework for identifying and characterizing subsurface hydrostratigraphic units and, when integrated with hydrologic data, lay a critical foundation for addressing hydrogeologic questions such as the influence of faults, sedimentologic trends, and dipping strata on regional groundwater flow and well-field drawdowns. This work will aid effective placement of new production wells and in understanding the geologic controls on aquifer compartmentalization. The maps and the hydrologic interpretations derived from them will generate improvements to groundwater flow models for water-rights administration and resource management.