Observations and implications of sediment-water interactions in the Niagara impoundment of southwest Virginia

A program of in situ and laboratory studies was conducted to elucidate the role of the deposited sediments of the Niagara Reservoir in affecting the quality of the flow of the Roanoke River above the headwaters of Smith Mountain Lake. Agronomic analysis techniques were adapted and applied to provide quantitative analysis capability for the sediments.

The superficial sediments were observed to constitute a considerable reservoir of nitrogen and phosphorus, which, during the anoxic bottom conditions occurring during the summer months, caused an enhancement of concentrations in the water column. Conversely, during the winter months, the superficial sediments acted as a phosphorus and nitrogen sink, removing both materials from the water column.

Laboratory batch and microcosm studies made it possible to quantify the release and removal of nitrogen and phosphorus under aerobic and anaerobic conditions, and to elucidate the factors upon which the exchanges depended.

The phosphorus interactions were found to be governed principally by sediment iron and organic matter content. Under aerobic conditions, phosphorus adsorption could be described by the Langmuir Isotherm. The most important nitrogen release mechanism was found to be a cation exchange phenomenon under reducing conditions.

Aithough the effective zone of exchange was found to extend only about 2 mm into the sediment surface, the analysis of cores showed the existing profile to be rich in both phosphorus and nitrogen forms, which could be exposed to the water column with sufficient mixing.

The deposited sediments of Niagara Impoundment showed a capacity to redistribute the impoundment influent nutrient load, causing an enhance~ ment of the quantities released during the summer months, thus magnifying the impact on the waters of Smith Mountain Lake. The maintenance of aerobic benthic conditions could, at a tremendous energy cost, prevent the sediment releases from occurring. In the long term, sediment nutrients deposited in the past, without proper control, could provide a continuing source of N and P to the waters of Smith Mountain Lake, thereby reducing the effectiveness of upstream control of point and diffuse sources.