BIOGEOCHEMISTRY OF THE MIDDLE RIO GRANDE BOSQUE, NM: LINKS AMONG SURFACE WATER, GROUNDWATER, AND SEDIMENTS

The forested riparian ecosystem (bosque) along the Middle Rio Grande in New Mexico is a currently affected by both flood control regulations and the water needs of the Southwest. These controls have largely eliminated the natural flood pulse. As part of multidisciplinary restoration ecology research, we have conducted biogeochemical studies of groundwater beneath the cottonwood riparian forest and petrographic analyses of aquifer sediments, as well as chemical examination of the Rio Grande and its floodwaters. Sampling techniques include groundwater withdrawal from wells installed at 3 m, 5 m, and 13 m depths adjacent to the river channel and the use of high resolution multi-level samplers to examine the vertical chemistry of the aquifer. Petrographic examination and Fe and Mn extractions of Quaternary alluvial sediments have been performed to further understand the relationship between the bosque groundwater and its alluvial aquifer. Understanding the linkage between dissolved redox-sensitive species and solid reactive phases (organic and inorganic) in an aquifer matrix is crucial to understanding nutrient pathways, biogeochemical cycling of carbon, and contaminant transport in groundwater systems.

Examination of the bosque groundwaters from the Albuquerque South Valley and Belen show that these waters are influenced by microbial activity and mineral precipitation/ dissolution reactions, as well as changes in concentration of the Rio Grande surface waters. Noteworthy are the trends of dissolved oxygen (increasing) and the related redox chemistry with depth. These phenomena may be linked to concentrations of organic matter or variations in hydraulic conductivity (affecting groundwater residence time) of the sediments. Except for nitrate concentrations, floodwater is higher in major element concentrations than river water and more closely resembles the groundwater. The increases in reactive Fe and Mn oxides in the zone of intermittent saturation suggest that the changing water table drives the biogeochemical reactions taking place. Investigation of the sediments by optical microscopy has documented the formation of Fe-oxides/hydroxides within this zone, but no clear relationship exists between gross sediment mineralogy and depth over the range examined.