The Arroyo Hondo ground water study reveals a complex, three-dimensional ground water system with multiple hydrostratigraphic units and aquifers. Distribution of the geologic and hydrostratigraphic units is presented through geologic maps and seven detailed cross sections that depict the distribution of geologic and hydrostratigraphic units, well data, surface water features, water levels, faults, and zones of fracturing and sediment layers in volcanic rocks. Cross sections are constructed both parallel and perpendicular to regional ground water flow and illustrate aquifers in the context of the geologic framework, the Rio Grande and the Rio Hondo, local acequias and other surface water features.
Ground water exists primarily within the Quaternary-Tertiary alluvial fan sediments known as the Blueberry Hill deposit, the Servilleta Formation basalts and interbedded sediments, the lower Tertiary Chamita Formation of the Santa Fe Group, and locally within the Cerro Negro dacite. A shallow alluvial aquifer in the Blueberry Hill alluvial fan deposit is limited to an area south of the Rio Hondo and east of the Airport fault. This aquifer is semi-perched on the Cerro Negro dacite, a massive, crystalline volcanic unit with relatively low regional permeability, and is recharged primarily through local irrigation return flow. A shallow perched aquifer also exists in the alluvial fan deposits north of the Rio Hondo, on the southwest flank of Cerro Negro. Here, perched ground water accumulates on the Cerro Negro dacite, recharges from Acequia de Atalaya, and discharges from the Medina spring, and possibly the lower Rio Hondo springs.
The Cerro Negro dacite contains localized, productive aquifers in fractured and rubble zones but generally appears to behave as a perching bed for shallow alluvial aquifers. Static water levels are highly variable, regionally inconsistent, and significantly lower than those measured in overlying perched or shallow alluvial aquifers. It is likely that fractured aquifers in the Cerro Negro dacite are highly compartmentalized, and have only limited or partial interconnection with adjacent aquifers.
The Servilleta Formation, named the deep volcanic-alluvial aquifer in this study, contains productive zones in thin basalts and sediments west of the Airport fault. A uniform hydraulic gradient between the Airport fault and the Rio Grande suggests that ground water moves easily between and through the interlayered basalts and sediments. This regional potentiometric surface grades westward to the elevation of cool springs that emerge in the Rio Grande gorge. Due to limited data and variable, regionally inconsistent water levels in deep wells east of the Airport fault, it is uncertain whether this deep aquifer system underlies the entire study area or if it includes the Cerro Negro dacite.
A transition zone between shallow and deep aquifers occurs at the Airport fault, which defines the western limits of the Cerro Negro dacite and the shallow alluvial aquifer. A vertical head differential of more than 300 feet exists across the Airport fault, which forms a high-permeability zone with a strong, local downward gradient. At the fault, shallow water levels rapidly merge with those in deep volcanic-alluvial wells on the west side of the fault, implying downward leakage from shallow to deep aquifers. Relatively young recharge ages in springs discharging in the Rio Grande gorge from the deep volcanic-alluvial aquifer suggest that shallow alluvial ground water merging with the deep aquifer at the Airport fault transition zone comprises a significant portion of the ground water discharging at the springs.
The Rio Hondo is part of a stream-connected aquifer system with gaining, neutral and losing reaches controlled by the permeability of contiguous, underlying geologic units. The alluvial deposits along the Rio Hondo were probably above the water table under natural conditions, but now have locally perched or semi-perched aquifers due to the development of irrigation. In the Rio Hondo drainage, accretion of ground water from surface water and irrigation recharge is a critical factor controlling the extent of the shallow alluvial aquifer. Because of interconnections between the Rio Hondo, the Rio Grande and shallow and deep aquifers, depletions of ground water from these aquifers have the very real potential of contributing to surface water depletions in a matter of decades or less. Similarly, reductions in irrigation would have negative effects on ground water levels in the shallow alluvial aquifer.
Results
- Bauer, Paul W.; Johnson, Peggy S.; Felix, Brigitte, 2009, Hydrogeologic investigation of the Arroyo Hondo Area, Taos County, New Mexico, New Mexico Bureau Geology Mineral Resources, Open-file Report, v. 0505, pp. 1-41.