Use of radiometric (Cs-137, Pb-210), geomorphic,and stratigraphic techniques to date recent oxbow sediments in the rio puerco drainage Grants uranium region,New Mexico |
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Authors: | Carl J Popp John W Hawley David W Love Michael Dehn |
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Institution: | (1) Department of Chemistry, New Mexico Institute of Mining and Technology, 87801 Socorro, New Mexico;(2) New Mexico Bureau of Mines and Mineral Resources, 87801 Socorro, New Mexico;(3) Department of Chemistry, New Mexico Institute of Mining and Technology, 87801 Socorro, New Mexico;(4) Present address: Metallurgy Department, University of Kentucky, Lexington, Kentucky |
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Abstract: | In the absence of historic geochemical baseline data for the Grants uranium region, environmental changes resulting from uranium
mine-mill activities can be determined only by indirect methods. A methodology for determining the age of recent sediments
in streams draining the region has been established based on combined geomorphic, stratigraphic, and radiometric dating techniques.
Because clayrich sediments retain possible radionuclides and heavy metals derived from mineralization and mined sources, sample
sites which contain fine-grained deposits that both predate and postdate mine-mill activity were located in abandoned-channel
segments (oxbows) of major streams draining the eastern Grants uranium region. Aerial photographs (and derivative maps) taken
between 1935 and 1971 provided the historical and geomorphic documentation of approximate dates of oxbow formation and ages
of alluvial fills in the abandoned-channel segments. Pits were dug at these oxbow sites to determine stratigraphy and composition
of the deposits. Samples collected from pit walls and auger holes below the pits were subjected to radiometric analysis by
gamma ray spectrometry for the artificial radionuclide Cs-137 and the natural radionuclide Pb-210 as well as other U-238 and
Th-232 daughters. Because of the dynamic nature of the system, absolute dating with Cs-137 was not possible but samples could
be dated as either pre-or post-1950. The 1950 date is important because it marked the beginning of the uranium exploitation
in the region. The Pb-210 dating was not possible because background Pb-210 was very high relative to fallout Pb-210. |
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