New Mexico Geological Society Annual Spring Meeting — Abstracts

This study differentiates four hydrostratigraphic units (HSUs) in the northwestern Albuquerque basin: the Upper, Middle, and Lower Rio Rancho HSUs underlain by the Zia HSU. This area hosts Rio Rancho (population of 104,000), which relies solely on groundwater for its municipal needs. We mapped these HSUs in the subsurface and assessed spatial trends in permeability and TDS. The study entailed constructing a 3D, geologic model showing the elevations of the bounding surfaces of these HSUs, and this model explicitly includes major faults. The HSUs exhibit layer-cake geometry, thicken towards the southeast, and thin over the Ziana horst in the northern part of Rio Rancho. Hydraulic conductivity data compiled from pumping tests indicate that the Upper Rio Rancho (RR) HSU has higher values compared to the Middle RR HSU: 1.1-6.4 ft/day, median of 2.9, vs. 3.2-21 ft/day, median of 9.3 (xx-xx = 10-90 percentile range). However, the thick, saturated portions of the Upper RR HSU are only found east of the Tamaya fault and in the southeastern part of the study area. The saturated Middle RR HSU is over 1,000 ft thick across most of the study area, including a north-trending prong between the Ziana horst and the Zia fault. Several wells indicate a coarsening upward trend in the Middle RR HSU, so areas where its upper part is saturated are more favorable than where just its lower part is saturated. There are no strong lateral permeability trends across the Middle RR HSU over most of the study area, but hydraulic conductivities from two pumping tests suggest higher permeability values in the southwest part of the study area, within 8 km northwest of the western end of Paseo del Norte. A northward increase in sand proportions also occurs north of the approximate latitude of well RRU-9 (35°20’0”), suggesting an increase in permeability that remains to be confirmed by pumping-test data. TDS values are 205-412 ppm in the Upper RR HSU, (excluding one well at 1,100 ppm) and 190-530 ppm in the Middle RR HSU, with the highest values in the latter occurring over the Ziana horst. The Lower RR HSU is mainly penetrated by wells on the Ziana horst, where it exhibits relatively high TDS values (478-1,400 ppm); the EPA recommends treatment for TDS >500 ppm. Various well-based permeability proxies and a single pumping test, in agreement with field observations, indicates it has relatively high permeability. The Lower RR HSU extends across the northern 2/3 of the study area, but may become finer-grained (corresponding to lower permeability) south of Southern Boulevard. The lower-middle Zia HSU is notably sandy, based on outcrop observations and wildcat oil well data. Its relatively higher elevation on the Ziana horst may possibly provide an accessible deep-water aquifer, but economical methods for water blending or desalinization would need to be formulated. The 3D geologic model along with maps of the potentiometric surface and TDS will be useful for managing groundwater resources in Rio Rancho and for potential groundwater-flow models of the area.