New Mexico Geological Society Annual Spring Meeting — Abstracts

Fluctuating redox conditions in alluvial aquifers exert fundamental control over contaminant and nutrient cycling, particularly in systems characterized by rapid stage fluctuations and seasonal recharge. Along the Rio Grande, shallow alluvial sediments experience repeated wetting–drying cycles driven by monsoonal recharge and intervening dry periods. These oscillations likely drive coupled carbon oxidation–reduction reactions that regulate iron and manganese phase transformations and, consequently, trace element mobility. However, the spatial distribution of redox fronts and the mechanisms governing element partitioning across the zone of intermittent saturation remain poorly constrained. This research investigates redox-sensitive element cycling in several sediment cores collected from alluvial deposits adjacent to the Rio Grande (New Mexico). We utilized micro- to bulk – scale geochemical approaches to quantify the hydrogeologic controls on trace element mobility. Electron probe microanalysis (EPMA) will be used to generate high-resolution elemental maps and identify microscale associations among Fe, S, Mn, and trace metals. X-ray diffraction (XRD) will constrain iron oxide, manganese, and carbonate mineral phases. A modified sequential extraction protocol will quantify operationally defined redox fractions and estimate sedimentary redox capacity.

We hypothesize that episodic saturation during monsoon recharge promotes reductive dissolution of Fe³⁺ and Mn oxides, releasing associated trace elements to porewaters, particularly uranium and arsenic, whereas subsequent oxic conditions favour re-precipitation and secondary mineral formation. The magnitude and rate of water level change are expected to control the extent of mineral transformation and trace element redistribution. Because the Rio Grande and its underlying aquifers supply drinking and irrigation water to regional communities, characterizing redox-driven element mobilization has direct implications for contaminant transport and groundwater vulnerability. By linking hydrologic variability to mineralogical transformations and trace element partitioning, this work aims to clarify how dynamic redox boundaries regulate contaminant and nutrient cycling in intermittently saturated alluvial systems.