Evaluation of the potential for trace element mobilization resulting from oxygenating sediments from a small Mississippi reservoir

160 Pages·2003·4.2 MB·English

by  Beard, William C

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TLD STATLS DEPARTMLNT OF
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AgriculturalResearch Service
National Sedimentation Laboratory
Oxford, Mississippi 38655
Evaluation of the Potential for Trace Element Mobilization
Resulting from Oxygenating Sediments from a Small
Mississippi Reservoir
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By William C. Beard III, Gregg Davidson, Sean J. Bennett, and
Fred E. Rhoton
Research Report No. 37 March 2003
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ACKNOWLEDGEMENTS
This report comprises the thesis submitted by William C. Beard III as partial
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fulfillment ofhis M.S. Degree in Geology for the University ofMississippi. thank
Dan McChesney, Vincent Campbell, Glenn Gray, and Rob Wells (USDA-ARS) for the
long hours ofhelp in the lab and in the field, and John Hart for his chemical analysis
work.
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ABSTRACT
Sediment cores were collected from a small Mississippi reservoir and analyzed to
determine the potential for mobilization oftrace elements following dredging. Twenty
cores were collected from Hubbard-Murphree Reservoir, near Charleston, MS, in May
and August 2001 Eh values measured at multiple depths varied between -500 and -300
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mV. A general increase in redox potential with depth was observed in cores collected in
May, 2001. Water level measurements from a nest ofpiezometers installed in the lake, in
November 2001, confirm the presence ofupwelling groundwater, a likely cause ofthe
increasing Eh with depth. This influence maybe seasonal.
Two cores were processed forbulk chemistry analysis at a resolution of2 cm.
Previous studies suggest bulk elemental concentrations are strongly correlated to grain
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size in Hubbard-Murphree sediments. comparison ofgrain size and concentration as a
function ofdepth confirms this relationship.
Leaching experiments conducted on splits ofhomogenized sediment samples
under oxidizing and reducing conditions were used to determine how the solubilities of
specific trace elements would be affected by an increase in redox potential. The aqueous
concentrations ofCa, Fe, Mg, Na, Sr, Al, Mn, and K under oxidizing conditions were at
least 20% less than under reducing conditions. Coprecipitation and adsorption with Fe
and Mn oxides likely caused the decrease in mobility following oxygenation. pH was
nominally lower in the oxygenated sediments than in the reducing sediments, likely due
to the relase ofH^ following Fe oxidation and cation exchange. H^ is commonly
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exchanged with other cations on mineral surfaces and are also released during the
oxidation ofFe. Immobilization rather than mobilization would likely occur for the
detected trace elements ifthe sediments were to be dredged.
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