Santa Fe Group Aquifer Investigations in the Northwestern Albuquerque Basin, New Mexico

In order to better understand and manage their groundwater, the City of Rio Rancho funded a study to: 1) map the 3D distribution of the Santa Fe Group aquifer, including internal hydrostratigraphic units, in the northwestern Albuquerque Basin, and 2) assess permeability differences within and between hydrostratigraphic units in the aquifer. An ancillary goal was to assess spatial trends of TDS and radiocarbon ages. Four stratigraphic units were recognized based on their texture and composition which were inferred to possess unique hydrogeologic properties: the Upper Rio Rancho (URR), Middle Rio Rancho (MRR), Lower Rio Rancho (LRR), and Zia hydrostratigraphic units (HSUs). Contacts separating these units are exposed on the land surface north and west of Rio Rancho. These contacts were projected into the subsurface, guided by subsurface picks in well data (made using geophysical logs and archived cuttings), via drawing of structural contours in ArcMAP. Structural contours were also digitally drawn for dipping fault planes of major faults (with >100 m vertical offset). Gridded raster files were created with the ArcGIS tool called Topo to Raster, where the inputs were the structural contours and the elevations of the picked contacts in wells. Isopach maps for each HSU were made by subtracting the top and base of a given HSU. A visualization of the 3D model was rendered in ArcScene using the gridded raster surfaces and fault structural contours. We also created 2D images that display geologic features of the model in an intuitive format. To further aid visualization of the Santa Fe Group aquifer, we made a 3D block model using two orthogonal cross sections and a Google Earth-derived land surface. All four HSUs thicken to the southeast, and are slightly thicker northward between the Zia and Coronado fault zones. The URR and MRR HSUs thin over a north-trending Ziana horst. Correlating hydraulic pump-test data with HSU (based on screened depths of a given well) indicate that the URR and possibly LRR have higher permeabilities (by a factor of 2-3) compared to the MRR. However, the extent and relatively large saturated thickness of the MRR means it plays an important role in providing groundwater for the City. Available pumping test data suggest that hydraulic conductivities in the MRR are slightly higher in the southwest part of the study area, perhaps due to the proportionally high sand bedload of the Benevidez Member of the Cerro Conejo Formation, but more well data are needed to confirm this apparent lateral trend. The youngest C-14 values in the groundwater are found to the east, near the Rio Grande, in the URR. The highest TDS values are found in the LRR HSU on or immediately adjacent to the Ziana horst, consistent with earlier hydrogeologic studies by Glorieta Geoscience Inc. Fault zones bounding the Ziana horst may be conduits for expelling poor-quality water from fractured bedrock at depth, and the structurally high, relatively permeable LRR may act as a pathway in transmitting this lower-quality water southwards into the study area.