New Mexico Geological Society Annual Spring Meeting — Abstracts

The regional aquifer located below Pajarito Plateau is the only sustainable water source for municipal use. This aquifer is separated from the ground surface by saturated alluvium within some large canyons, and several hundred feet of unsaturated volcanics, basalts, and sedimentary debris shed from the Jemez volcanic pile and the Cerros del Rio lava flows. In addition, there are numerous intermediate perched water zones located between the alluvial and regional aquifers. Hydraulic communication between units is well documented. The shallow alluvial and intermediate perched aquifers are recharged by infiltrating snowmelt, thunderstorm runoff, and permitted industrial and sanitary effluents. Infiltration below the mesa tops is insignificant. The predominant recharge area for the regional aquifer is located west of the Los Alamos townsite in the Serria de los Valles region east of Valles Caldera. This recharge area extends eastward below the deeply cut canyons on the plateau. Hence, the intermediate perched aquifers located below the saturated alluvial systems act as a supplemental recharge source to the regional aquifer. The observed piezometric surface in the regional aquifer rises westward from the Rio Grande in White Rock Canyon and across the plateau towards Sierra de los Valles. Topographically, the land surface also rises westward from 5,400 ft (1,645 m) at the Rio Grande to 10,400 ft (3,170 m) at Pajarito Mountain over a lateral distance of 14 miles (23 km). Therefore, the piezometric surface for the regional aquifer is a subdued reflection of plateau topography. Water in the regional aquifer is under phreatic conditions in the western and central portions of the plateau, and partially confined to fully artesian conditions near the river. There are numerous springs in White Rock Canyon that emerge from the regional aquifer where an estimated 6,000 ac-ft/yr (740 ha-m/yr) is discharged into the river. Collectively, this picture suggests a classic regional groundwater circulation pattern originating below the topographic highs on the west and moving east-southeast toward the river.

Below Pajarito Plateau, the regional aquifer is primarily located in Santa Fe Group sediments. These ancestral Rio Grande floodplain deposits fill a Miocene-aged trough at the base of the Bandelier Tuff. This Miocene axial depression is a northeast to southwest trending basin extending from Santa Clara Canyon on the north to Ancho Canyon on the south. The Puye Formation is sandwiched between these two units and portions of it (i.e., Totavi Lentil) hydraulically dominate the flow field. The aquifer transmitting characteristics within these sediments vary due to differences in lithology and formation thickness. Within high-yielding zones, lateral flow rates vary from 100-350 ft/yr (30-107 m/yr); however, flow rates within lower yielding zones are only 20-50 ft/yr (6-15 m/yr). Separation of deep groundwater circulation patterns from shallow canyon-bottom infiltration can also be inferred from major-ion water chemistry. These analyses suggest that fast-moving recharge waters that originate near Sierra de los Valles on the west, or that move within the Totavi, have a calcium bicarbonate character. However, slower moving or deeply circulated waters in the lower Santa Fe have a sodium bicarbonate character.