New Mexico Geological Society Annual Spring Meeting — Abstracts

Continuous monitoring of springs along the Nacimiento Fault was carried out to provide new insight into changes in the groundwater regime of this complex geological area. The focus has been to highlight the connectivity of springs along faults, assess temporal and spatial variations, recognize signals present in the data and identify relationships between spring parameters.

The parameters monitored are, pressure (a proxy for depth), temperature and conductivity. Multi-parameter sensors were deployed in the study area from December 2012 until May 2015. Resolution was set at 15 and 5-minute intervals.

The springs are situated along the Nacimiento fault both north and south of the Rio Salado. Fluids are carbonic, and have a demonstrably endogenic component evidenced by helium isotopes and key tracers such as lithium. All springs are between ~ 10 - 100 m higher elevation than the proximal Rio Salado, indicating artesian characteristics. The elevation and the co-alignment along the fault indicate a confined or semi-confined aquifer sealed by the Triassic Chinle Formation, with fluid movement along the fault. Three springs are collinear and located along the fault that cores the Tierra Amarilla anticline, the fourth is off axis of the fault. The latter, is noted as being geochemically distinct from the rest, with greater temporal variation. A fifth spring, north of the Rio Salado was monitored for the final 6 months of the study period. Where springs discharge, they form pools within travertine mounds and cisterns whose depth range from 0.2 to 8 m. The sizes of the spring pools vary along their long axis from 0.1 to 5 m. Travertine occurs at all locations, and many inactive springs are represented by dry travertine mounds and vent orifices. U-series age geochronology indicates springs have been active intermittently since 270 ka, at highest elevations, to modern actively forming mounds. Degassing of CO₂ occurs as bubbling at most springs and pool surfaces are often at depth below ground level, in collapsed travertine mounds, up to 5m deep, confirming that water elevations were higher in the past.

Depth variations highlight seasonal as well as event driven fluctuations, however not all springs respond in the same manner, possibly due to the nature of the spring vent, active verses collapsed mound. Water temperature tracks atmospheric temperature with maximum values in the summer. Analysis of conductivity was problematic due to biofouling.

Fluid connectivity between springs is highlighted most prominently by near-synchronous changes in water depth for each spring. Spectral analysis indicates water depth has spectral peaks of 1 and 0.5 days, which coincide with solid Earth tides of ~12 and ~24 hours. Water temperature has spectral peaks at 24 hours, associated with daily maximum air temperature, while specific conductivity also has peaks at 24 hours, which could be associated with maximum daily temperature.

This work has implications to help understand the role of faults as conduits for fluid movement, define the degree of confinement of the aquifer and estimate storage coefficient.