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.
New Mexico Bureau of Geology & Mineral Resources
New Mexico Institute of Mining & Technology
801 Leroy Place
Socorro, NM 87801-4796
information: (575) 835-5420
fax: (575) 835-6333
http://geoinfo.nmt.edu
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