New Mexico Geological Society Annual Spring Meeting — Abstracts

Several springs flow into San Pedro Creek providing intermittent flow downstream of the springs. San Pedro Creek lies on the east side of the Sandia Mountains in New Mexico and generally flows north from the Sandia Park area towards the Hagan Basin connecting with tributaries to the Rio Grande. The intermittent flows create perennial reaches that tend to disappear into the creek bed along bedrock outcrops.

Springs investigated during this study primarily include San Pedro, Rock, and Cottonwood Springs, and their locations are controlled by local geologic contacts and structures. The San Pedro synclinorium, a plunging fold package dominated by a broad syncline dips towards the south creating bands of outcrops of the major hydrostratigraphic units. Springs tend to discharge along these outcrops at geologic contacts where local aquifers are truncated at the surface by erosion. Springs flow from the primary hydrostratigraphic units such as the combined San Andres Limestone and Glorieta Sandstone aquifer and the Abo Formation aquifer. Water quality data indicates that water emanating from the springs is a mixture of older groundwater and younger recharge. 

The Sandia Mountains have a complex geologic history that is reflected in the many rock types and geologic structures that are encountered in the area. The area near San Pedro Creek is dominated by a geologic structure named the San Pedro Synclinorium, which is constrained by the Tijeras and San Antonito faults and contains rocks that have been folded and faulted. Similar geologic structures adjacent to this area are the Tijeras graben and Monte Largo horst. All of the rocks in the study area are highly fractured and jointed due to the local tectonic history.

Rocks from Proterozoic to Quaternary ages are present in the area. Subsurface exploration for water supply development under New Mexico Office of the State Engineer (NMOSE) permit #S-2618 penetrated almost the entire stratigraphic sequence, with drilling terminated at 3,694 feet below ground surface (ft bgs) when intrusive rocks within the Sandia Formation were encountered. Geologic and geochemical characteristics of the major units were analyzed during the exploratory drilling program, and this data was used to help understand the hydrogeology of the springs.

Geochemical data was collected from test zones during exploratory drilling and from springs along San Pedro Creek. Concentrations of major ions and total dissolved solids (TDS) were analyzed along with stable isotopes of oxygen and hydrogen, carbon-14 and tritium. Major ion ratios tend to be similar between spring and groundwater quality samples. Spring water quality tends to be more dilute relative to the groundwater, and tritium data suggests that a younger source of water is mixing with the older groundwater discharging into San Pedro Creek at the springs.