POTENTIAL FOR MOBILIZATION OF TRACE ELEMENTS FROM RESERVOIR SEDIMENTS: HUBBARD–MURPHREE RESERVOIR, MISSISSIPPI

Sediment cores were collected from a small Mississippi reservoir and analyzed to determine the potential for mobilization of trace 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 mV. A general increase in redox potential with depth was observed in cores collected in May, 2001. Water level measurements from a nest of piezometers installed in the lake in November, 2001, confirm the presence of upwelling groundwater, a likely cause of the increasing Eh with depth. This influence may be seasonal.

Two cores were processed for bulk chemistry analysis at a resolution of 2 cm. Previous studies suggest bulk elemental concentrations are strongly correlated to grain size in Hubbard-Murphree sediments. A comparison of grain size and concentration as a function of depth confirms this relationship.

Leaching experiments conducted on splits of 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 of Ca, 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 release of H⁺ ions following Fe²⁺ oxidation and cation exchange. H⁺ ions are commonly exchanged with other cations on mineral surfaces and are also released during the oxidation of Fe²⁺. Detected trace elements in the sediments of this reservoir would likely be immobilized rather than mobilized if the sediments were to be dredged.