New Mexico Geological Society Annual Spring Meeting — Abstracts

Understanding the linkages between dissolved redox-sensitive species and solid reactive phases in an aquifer matrix is crucial to the prediction of nutrient pathways, biogeochemical cycling of carbon, and contaminant transport in groundwater systems. Iron redox cycling at Rio Calaveras (Jemez Mts, NM) is strongly influenced by dynamic mixing of surface and groundwater in the hyporeic zone. Iron concentrations can range from 0 ppm to greater than 17 ppm depending on location. At the Rio Calaveras site, two separate stream reaches were instrumented with nests of shallow, medium, and deep piezometers in order to determine flow paths and soluble iron concentrations. Conservative and non-conservative tracers were injected to constrain flow paths, residence times, and hydraulic conductivity. Measurements of DO, Eh, pH and Fe concentration were taken under low flow, steady state conditions. In general, the presence of oxygenated water in the aquifer coincides with low iron concentrations. In areas with oxygenated conditions, thick accumulations of iron oxyhydroxides were observed. Within the piezometer nests and separate stream reaches vertical and horizontal iron heterogeneities exist. While the hydrogeochemical aspects of the system explain some of these heterogeneities, the presence of heterotrophic bacteria playa significant role in iron mobility and water chemistry.