New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Periodic synoptic sampling of surface flows in the southern Albuquerque basin during the 2021 calendar year revealed spatial and temporal variations in the proportion and provenance of groundwater contributions to surface flow. Differential flow gauging and analysis of geochemical tracers (major and minor anions and cations, trace metals, field parameters, δ¹⁸O, δ²H, ⁸⁷Sr/⁸⁶Sr) were performed at five riverside irrigation drain locations along a 70-km interval between Bosque Farms and San Acacia, NM, with additional geochemical samples collected from the Rio Grande. Prevailing conceptual models for salinization in the middle Rio Grande focus on the input of deeply sourced brine. However, shallow groundwater with high salinity inherited from interactions with Mesozoic and Paleozoic sediment may be another significant solute source (such as groundwater associated with the Rio Puerco and Abo Arroyo). While these shallow groundwaters are thought to discharge into the river, their influence has not previously been constrained. We consider the chemical, spatial, and temporal variability of groundwater discharge to identify and disaggregate contributions from distinct water sources to understand their individual influence on the riparian system to inform water management decisions, salinization mechanics, and drought resilience. ⁸⁷Sr/⁸⁶Sr data collected from March and May are consistent with three system endmembers: Rio Grande through-flow (⁸⁷Sr/⁸⁶Sr = 0.7096), upwelling sedimentary brine (⁸⁷Sr/⁸⁶Sr ≥ 0.7106), and water influenced by Paleozoic sediment (⁸⁷Sr/⁸⁶Sr = 0.7090). Preliminary end-member mixing analysis indicates one or more additional end-member waters could be present. Strontium isotope mass balance showed that sedimentary brine and Paleozoic-influenced water accounted for as much as 5% and 11% of drain flow respectively. The greatest net increase in the Rio Grande’s salinity throughout the project area was observed in March, where Cl^- concentration increased from 21.2 to 50.5 mg/l and ⁸⁷Sr/⁸⁶Sr increased from 0.70955±0.00016 to 0.70996±0.00003 between Bosque Farms and San Acacia, indicating a final composition of 2% Paleozoic-influenced water, 2% brine, and 96% Rio Grande throughflow. Groundwater flux measured in irrigation drains ranged from -13.1 to 21.0 m³/day/m (4.2 m³/day/m average). Notable trends in water evolution include increases in specific conductance and a transition from a Ca-HCO₃^- towards a Na-HCO₃^- water type. The rate of change increases with proximity to the basin’s terminus. ⁸⁷Sr/⁸⁶Sr in samples collected near sub-basin structural highs showed locally elevated brine fractions, inconsistent with the general trend. Increases in Na⁺/Ca²⁺ did not always correspond to increases in Cl^- , suggesting cation exchange and water mixing have independent influence. These results demonstrate that shallow lateral groundwater flows are a primary source of salinization in addition to upwelling brine.