New Mexico Geological Society Annual Spring Meeting — Abstracts

Chemical analyses of water samples from the northern end of the Socorro Basin demonstrate that deep groundwater upwelling there has a considerable influence on water quality and microbial community structure. Samples associated with upwelling are characterized by elevated temperature, conductivity, DOC, HCO₃ ^- , Cl^- , Br^- , Cl^- /Br^- mass ratio, SO₄^2-, Mg²⁺, Ca²⁺, K⁺ , Na⁺ , Li⁺ , Sr²⁺, and dissolved Fe and Mn. Furthermore, the pH and δ¹³C of DIC are lower in comparison to the other waters sampled in this study.

We collected 6 surface water and 23 shallow groundwater samples from New Mexico Tech Rio Grande Project transects that cross the Rio Grande floodplain at San Acacia (SAC), Escondida (ESC), and Brown Arroyo (BRN) during February 16 - 22, 2006. SAC lies just south of the terminus of the Albuquerque Basin and has previously been interpreted to host deep groundwater upwelling. The sites further south, ESC and BRN, show no signs of deep groundwater upwelling.

Groundwater west of the low-flow conveyance channel (LFCC) at SAC was affected the most. The temperature of the water there ranges from 16.1 to 20.8°C compared to 15.7 to 17.4°C at ESC and 14.2 to 17.6°C at BRN. The average conductivity of the water west of the LFCC at SAC ranged from 907 to 6050 µS compared to 1099 to 1246 µS at ESC and 636 to 1246 µS at BRN. Cl/Br mass ratios west of the LFCC at SAC range from 332 to 1262 compared to 454 to 655 at ESC and 297 to 726 at BRN.

In contrast to the groundwater west of the LFCC, the characteristics of groundwater between the Rio Grande and the LFCC at each site varied little and are similar to Rio Grande water. For all 3 sites, the temperature of the water ranged from 12.1 to 16.3°C, the conductivity ranged from 445 to 597 µS, and the Cl/Br mass ratio ranged from 200 to 367.

The conservative components of all of the surface water and groundwater samples fall approximately along a simple binary mixing line. Some samples deviate from this line, however, suggesting that evaporation and other end-member waters may be important. Furthermore, the lower DIC δ¹³C and elevated DOC, HCO₃^- , and dissolved Fe and Mn of samples collected west of the LFCC at SAC compared to all other samples suggest that the upwelling waters have enhanced reducing conditions in the shallow aquifer by increasing the availability of energy sources associated with degradation of organic matter.