THE EFFECTS OF HYPOLIMNETIC OXYGENATION ON METAL RELEASE FROM SEDIMENTS AND REMOVAL FROM THE WATER COLUMN IN A SMALL, EUTROPHIC DRINKING WATER RESERVOIR

Manganese (Mn) and iron (Fe) are released from reservoir bed sediments and enter the water column when anoxic, reducing conditions develop at the sediment-water interface (SWI). Accumulation of these metals in drinking water reservoirs degrades the aesthetic quality of water and increases treatment costs. In situ oxygenation of the deep, hypolimnetic zone in the water column has been used to prevent the accumulation of Mn and Fe by increasing dissolved oxygen (DO) concentrations. However, the effect of oxygenation on reservoir metal budgets is poorly understood. We designed a mass balance to quantify the inflows, outflows and change in metal mass in an oxygenated reservoir to examine how operation of the oxygenation system influences internal metal loads, removal from the water column and sequestration in sediments.

Our study was conducted in a seasonally stratified drinking water reservoir in southwest Virginia. Metal (total and soluble) and DO concentrations were measured weekly in the reservoir, surface water inflow, and shallow groundwater wells during the stratified period. The oxygenation system was deactivated for two intervals during the stratified period to develop hypoxic conditions (<2 mg/L DO) near the sediment-water interface. Before and after each interval, the system was activated to restore well oxygenated conditions (>6 mg/L DO) in the hypolimnion.

Our results show that Mn and Fe concentrations were highest near the SWI throughout the stratified period, consistent with release of metals from the sediment during periods of activation and deactivation. Particulate metal concentrations increased when DO in the hypolimnion was greater than ~6 mg/L. The particulate Fe fraction was greater than Mn under these conditions, suggesting that Fe precipitates more rapidly than Mn. Oxygenation did not prevent the release of metals, but increased the rate of metal oxidation to insoluble forms that could settle through the water column and return to the sediments.