BIOGEOCHEMISTRY AND SEDIMENTARY PETROLOGY OF THE SHALLOW ALLUVIAL AQUIFER OF THE MIDDLE RIO GRANDE BOSQUE, NEW MEXICO

The Middle Rio Grande, NM is a reach of river that has been impacted by modern flow regulation. These controls have largely eliminated the flood pulse on which the bosque, a riparian ecosystem, depends. As part of ongoing restoration ecology research, biogeochemical studies of groundwaters beneath the cottonwood riparian forest, coupled with petrographic analyses of aquifer sediments, are in progress. Major ions and nutrients have been measured in water samples collected from shallow monitoring wells since the beginning of the 2000 growing season, in order to compare the effects of flooding and evapotranspiration on the aqueous chemistry of the shallow alluvial aquifer, and the mineralogy of associated sediments.

The shallow aquifer system, located in the Albuquerque basin of the Rio Grande rift, consists of Quaternary river alluvium that overlies unconsolidated, basin-fill sediments of the Cenozoic Santa Fe Group. Aquifer sediments consist of predominately coarse sand with scattered pebbles and cobbles throughout, and minor clay lenses. Sediment samples were collected in conjunction with installation of monitoring wells and were examined for overall texture, and primary and secondary mineralogy. Water samples were collected from monitoring wells screened at various depths, and include samples obtained from discrete depth intervals using a multi-level sampling device. Observations of the solid phase are compared to these vertical water chemistry profiles.

Seasonal and diurnal variations in concentrations of redox sensitive solutes, including Fe, Mn, and S species, are associated with changes in evapotranspiration and the river's hydrologic regime. Solid-phase, precipitation/dissolution reactions are occurring in the region of the upper water table. Occurrences of Fe-oxyhydroxides and sulfide minerals in sediment samples are compared to profiles of dissolved iron and sulfur species in water samples. Additional research focuses on the mineralogical variability of secondary sediments from the unsaturated, intermittently saturated, and saturated zones.