New Mexico Geological Society Annual Spring Meeting — Abstracts

Effective water management in the Middle Rio Grande Basin requires a clear understanding of surface-groundwater interactions. These processes remain poorly defined in the San Acacia Reach, where water losses affect the state's ability to meet Rio Grande Compact deliveries to Elephant Butte Reservoir. Within this reach, the Low Flow Conveyance Channel (LFCC), an engineered drain built to support compact compliance, strongly influences surface and groundwater exchanges. Because it lies below the riverbed, it acts as a sink, and captures water from shallow alluvium, deep basin groundwater, and Rio Grande/irrigation return flow. However, their relative contributions are unresolved. This study quantifies surface-water and groundwater inputs to the LFCC across contrasting seasonal hydrologic conditions.

Five drainage and environmental tracer surveys were conducted along the LFCC and Rio Grande during the 2025 irrigation and monsoon cycle (February – December). Water samples were analyzed for major ions, trace metals, and stable isotopes of water (δ18O, δD). Groundwater fluxes to the LFCC were estimated using differential gauging and 222-Rn measurements. A preliminary multi-tracer Bayesian mixing model was developed to resolve the contributions of surface water, shallow alluvium, and deep basin groundwater.

Seasonal chloride and stable-isotope patterns reflect strong irrigation and snowmelt controls on the hydrologic system. δ18O values are depleted during pre-irrigation (February) and become progressively enriched by mid-monsoon and mid-irrigation (July and September), indicating a transition from snowmelt-dominated river inputs to increased contributions from irrigation-influenced shallow groundwater and return flow. At most LFCC sites, chloride concentrations were moderate in February (pre-irrigation) and decreased slightly in May (early irrigation) with the introduction of canal water, which consists of low-salinity surface water diverted from the river. Then, concentrations increased in July and September (mid-monsoon and mid-irrigation). In December (post-irrigation), chloride concentrations returned to their pre-irrigation levels. Sulfate showed a comparable seasonal pattern. This trend suggests an early-dilution signal followed by a shift toward greater influence of saline shallow groundwater, as cumulative recharge elevates the water table and enhances seepage to the LFCC, leading to higher salinity later in the irrigation season. Summer monsoon rainfall can temporarily dilute surface water but sustain elevated groundwater levels.

Major ion chemistry provides constraints on groundwater sources affecting the LFCC. The waters are mixed, but bicarbonate is the dominant anion, and the percentage of sulfate is greater than the percentage of chloride. Pre-, early-, and post-irrigation samples exhibit similar water chemistry, whereas peak and late irrigation seasons are marked by sulfate enrichment associated with irrigation return flow and reflect gypsum dissolution from monsoon runoff east of the Rio Grande. In the upper reach, calcium is the dominant cation, consistent with recharge from the Magdalena Mountains, while the lower reach waters are sodium dominated, reflecting ion exchange within the clays of the principal aquifer system and contributions from deeper basin groundwater. These results show that LFCC water chemistry responds to seasonal changes in irrigation return flow, monsoon recharge, and basin-scale groundwater.