HIGH-RESOLUTION VERTICAL EXAMINATION AT THE WATER TABLE: RELATIONSHIP BETWEEN REACTIVE SOLIDS AND REDOX CHEMISTRY OF SHALLOW GROUNDWATER ALONG THE RIO GRANDE, NEW MEXICO

In conjunction with ongoing UNM research in hydrology and restoration ecology, our work focuses on the groundwater of the forested hyporheic corridor of the Rio Grande near Belen, New Mexico. These recent alluvial sediments host a shallow aquifer, with discharge - recharge events occurring on time scales ranging from hours to months. Using a multi-level sampler with dialysis cells (DCs), we have sampled the upper 1.5 m of the saturated zone at a 10 cm vertical resolution. Using the DC approach, we can passively sample a well over a period of time that permits the cell to equilibrate with its environment. In 2001, sampling encompasses spring snowmelt, the late summer monsoon season, and fall baseflow conditions. Water samples have been analyzed for major ion chemistry as well as redox-sensitive parameters (iron; manganese; dissolved oxygen; sulfur; dissolved organic carbon, including organic acids; and redox potential). Depth-related trends emerge through the DC approach that are not evident from conventional well sampling methodologies. In conjunction with continuously recorded water table data, we can assess redox cycling in terms of groundwater-surface water interactions.

While redox species exhibit systematic depth-related changes during seasonal baseflow, trends are equivocal during major hydrologic events such as flooding. Data for these transitional periods suggest redox disequilbrium or even modest oxidation at depth, indicating the importance of heterogeneity in recharge paths, particle size, microbial respiration, and/or seasonal organic carbon dynamics.

Examination of natural sediments and experimental substrates (including ion exchange resin beads deployed at selected depths) provides information on the relationship between reactive solids and redox-sensitive solutes through characterization of authigenic mineral growth. Methods include optical microscopy, x-ray diffraction, chemical extractions, and scanning electron microscope / energy-dispersive X-ray spectroscopy.