Earth and Planetary Sciences ETDs

Publication Date

12-14-2002

Abstract

The Rio Grande in central New Mexico flows through a semi-arid, historically aggrading Quaternary rift basin. Flow regulation measures include dams, irrigation diversions, levees, and bank stabilization. These have caused severe impairment including incision, lowered water tables, and less overbank flooding; disrupted groundwater - surface water interactions; altered seasonal organic carbon dynamics; and declining native biota. Previously dynamic flowpaths in the shallow alluvial aquifer (hyporheic corridor) have become are less reversible due to parallel drain ditches with lower beds than the river. These ditches impose relatively static hydraulic gradients on the alluvial aquifer that force water to flow from the river to the drains.

A water sampling campaign from May 2001 through April 2002 established seasonal major element and redox chemistry using dialysis multi-level samplers, wells, and surface water sampling. Sediment extractions quantified and characterized authigenic Fe/Mn oxyhydroxides, as well as solid phase P, all of which were more widespread in intermittently wetted sediments near the water table. Filter papers were incubated in the aquifer to grow seasonal precipitates and examined by scanning electron microscopy with chemical characterization by energy-dispersive X-ray spectroscopy.

Oxygen is rapidly depleted from Rio Grande water shortly after it enters hyporheic corridor sediments. A series of terminal electron-accepting processes, including denitrification, manganese reduction, iron reduction, and sulfate reduction, occurs under anoxic conditions as microorganisms metabolize organic carbon. These reactions occur down-flow from the Rio Grande through the alluvial aquifer toward the drainage ditch system. These redox processes depend on changes in hydraulic head driven by diel, seasonal, interannual, and irregular (anthropogenic) variations in river stage. Oxic - anoxic cycling produces Fe/Mn oxyhydroxide and Fe sulfide minerals near the water table. Stalky, helical, and spherical oxide morphologies and cubic sulfide morphologies were observed. These phases grow on the scale of weeks, probably as microbia1I respiration products. Seasonal mineralogy and organic carbon dynamics may affect water quality and the success of potential restoration efforts. Restoration should include geochemical and hydrologic monitoring of groundwater - surface water interactions.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Laura Crossley

Second Committee Member

Adrian Brearley

Third Committee Member

Clifford N. Dahm

Language

English

Document Type

Thesis

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