Iron sequestration in lake sediments from artificial hypolimnetic oxygenation: Richard B. Russell Reservoir

The Upper Savannah River watershed has numerous impoundments, and the three largest hydroelectric reservoirs, from north to south, are Hartwell, Richard B. Russell, and J. Strom Thurmond Lakes. During the summer months, these reservoirs undergo thermal and chemical stratification, which results in the formation of cool, hypoxic/anoxic hypolimnia and warm, oxic epilimnion. To maintain fisheries habitat, the United States Army Corps of Engineers operates a hypolimnetic oxygenation system in the forebay of Richard B. Russell Lake. The purpose of this system is to improve the water quality of the releases from Richard B. Russell Dam by maintaining a dissolved oxygen concentration of 5 to 6 ppm.;Under anoxic conditions in the hypolimnion, some metals and nutrients become soluble, and as water is released from the reservoir during hydroelectric generation, these dissolved nutrients can be transported downstream. During fall mixing, nutrients that have been dissolved in hypolimnion and not released during generation may be cycled back to the surface. The oxygenation system, operated when the hypolimnion would be under anoxic conditions, facilitates the oxidation and sedimentation of certain reduced chemicals, and their sedimentation effectively removes them from the water column. In this manner they may be sequestered in the sediments of Richard B. Russell Lake.;Of particular interest was iron (Fe), chosen because of its ability to react with oxygen. In addition, under certain conditions, phosphorus can bond to iron or can adsorb to flocculated iron to form ferric hydroxo-phophate precipitates. Manganese exhibits many of the same chemical characteristics as iron with regard to redox activity and phosphorus interactions. However, under identical conditions, manganese oxidation proceeds more slowly than iron. If the artificial oxygenation system facilitates these reactions, then Richard B. Russell Lake may be a sink for iron or manganese, and potentially phosphorus, for the Savannah River system.;Sediment samples were taken from the forebay of Hartwell Lake and from the forebay of Richard B. Russell Reservoir upstream from and near the hypolimnetic oxygenation system. Sediment samples were also collected along a longitudinal gradient through J. Strom Thurmond Lake as well as in the J. Strom Thurmond forebay to detect evidence of sequestration upstream. Based upon the results of chemical analyses, I discovered that Richard B. Russell Lake had significantly more iron in its forebay sediments at the oxygenation system than at upstream locations within the lake or in Hartwell Lake's forebay. Hartwell Lake's forebay sediments had significantly more iron than J. Strom Thurmond Lake's forebay. Phosphorous had a similar distribution; with all three lake forebays being significantly different from one another. Manganese was greater in the Richard B. Russell forebay sediments at the oxygenation system than at an upstream location or in the forebay of Hartwell Lake. However, the forebay of J. Strom Thurmond Lake had approximately ten times the amount of manganese in the sediments as Lake Hartwell, and almost five times as much as the forebay sediments in Lake Richard B. Russell. These observations are consistent with a hypothesis of enhanced sequestration of materials into lake sediments under the influence of hypolimnetic oxygenation.