New Mexico Geological Society Annual Spring Meeting — Abstracts

Lava Tube spring is a subaqueous spring that discharges from the Servilleta Basalt directly beneath the Rio Grande 21 km northwest of Questa, New Mexico. Lava Tube spring is the largest single active spring in New Mexico, with a discharge rate of approximately 22,680 L/min (6000 gal/min). Additional springs discharging within the study area are characterized by low flow rates (<150 L/min, <40 gal/min). Water samples collected upstream, at Lava Tube spring, and downstream along the Rio Grande on July 12, 2012 were analyzed for major ions, trace metals, tritium, and stable isotopes of hydrogen and oxygen. The upstream and downstream stations are 273 m north and 182 m south of Lava Tube spring, respectively. Surface-water flow rates average 1.90 cms (67 cfs) upstream of Lava Tube spring and 2.69 cms (95 cfs) downstream of the spring during sampling. Surface-water flow velocities averaged 0.16 and 0.27 m/sec (0.38 and 0.87 ft/sec) upstream and downstream of Lava Tube spring, respectively. Groundwater discharging from Lava Tube spring mixes with the Rio Grande surface water and constitutes between 32 and 38 percent of chemical tracers measured in the Rio Grande below the spring in the study area. Mixing calculations are based on concentration differences of chloride, sulfate, and uranium, tritium activity, and δ¹⁸O and δ²H values. Lava Tube spring and the Rio Grande are oxidizing with respect to Fe and dissolved oxygen concentrations range from 8.84 to 9.77 mg/L during sampling. Rio Grande surface water and groundwater discharging from Lava Tube spring are characterized by a Na-Ca-HCO₃ composition, with higher concentrations of major ions and selected trace elements (As, B, Mn, Mo, and Sr) and tritium activities measured in surface water. Lava Tube spring δ¹⁸O and δ²H values are more negative than Rio Grande surface water, possibly reflecting recharge at higher elevation. Lava Tube spring and the Rio Grande are undersaturated with respect to calcite and amorphous silica and oversaturated with respect to albite, gibbsite, and chalcedony, based on geochemical calculations using the computer program PHREEQC. Lava Tube spring approaches equilibrium with respect to Na- and K-saponite and analcime most likely produced from hydrolysis reactions with Servilleta basaltic glass. Concentrations of dissolved U(VI) below 10^-8 molal are stable as UO₂(CO₃)₂^2- and UO₂(CO₃)₃^4- based on geochemical modeling simulations. Speciation calculations suggest that concentrations of dissolved As(V) below 10^-7 molal are dominantly stable as HAsO₄^2-.