MANGANESE ATTENUATION DOWNSTREAM OF A PUMPED-STORAGE HYDROELECTRIC FACILITY IN WESTERN VIRGINIA

Hydroelectric dams have the potential to act as a point source for manganese (Mn) and other trace elements by releasing water directly from the oxygen-depleted hypolimnion of stratified reservoirs. This study looks at the manganese in a watershed influenced by a pumped-storage hydroelectric facility in western Virginia. The field site consists of a 2.8x10⁹ m³ main reservoir, a 1.2x10⁸ m³ storage reservoir, and 81 miles of river channel. The main reservoir and storage reservoirs are both impounded by hydroelectric dams. Water is released from the main reservoir to generate electricity during peak use hours and is pumped back from the storage reservoir during off hours. Water is released from the storage reservoir to maintain flow in the downstream river channel. During lake stratification from May through November, elevated concentrations (>50 ppb) of dissolved Mn have been measured in the hypolimnion of the main reservoir. The generator intakes in the main dam are located at three elevations, two of which are located below the main reservoir’s thermocline, establishing the conduit for hypolimnetic waters to be directly released into the tailrace. Historical data in the downstream reaches of the river show that total Mn concentrations have exceeded the state’s secondary drinking water standard (50 ppb total Mn). Thirteen monitoring sites were established in 2012 along a 113-mile reach that includes the storage reservoir and 81 miles of river downstream. Quarterly sampling events have shown that total Mn concentrations are greatest near the main dam and decrease with distance. Unlike observations at many single-dam facilities, Mn observed downstream of the storage dam is dominantly in particulate form. The shallow, storage reservoir likely plays a vital role in transforming reduced, dissolved Mn(II) from the main reservoir into the oxidized, particulate form (Mn(IV)) before entering the free-flowing river channel. A Mn mass balance of the first 12 miles downstream from the storage dam supports the hypothesis for loss via sedimentation. This suggests that hydrologic conditions in the river may be the dominant variable in predicting Mn attenuation downstream of this pumped-storage hydroelectric facility.