Temporospatial Patterns of Manganese Concentrations in a Hydropower Dam Watershed

Abstract:

Concentrations of manganese (Mn) downstream of a hydropower dam are strongly influenced by seasonal reservoir dynamics. During warm months reservoirs experience stratification that results in the depletion of dissolved oxygen at depth and the concurrent release of dissolved Mn from the sediment into the water column. After being released from the dam these Mn rich waters are mixed with oxygenated river water and the Mn may become oxidized and removed from the water column over time. Previous studies have largely focused on patterns of Mn distribution within the first 20 km or less downstream of a dam. However, water quality issues at our field site extend much further downstream. The primary goal of our work was to investigate how seasonal dynamics in the reservoir couple with heterogeneity at the watershed scale to drive temporal and spatial patterns across large distances. Mn concentrations were monitored downstream of a hydropower dam in southwest Virginia over a two year period that covered the Roanoke River between Leesville Dam and a water treatment plant 150 km downstream. Data that we have collected over the past two years show that Mn concentrations are highest near the dam during reservoir stratification in late summer and fall and decrease with distance downstream. A mass balance of the first 19 km downstream from the dam suggests loss of Mn via sedimentation and filtering of particulates. Mn concentrations appear to reach a consistent minimum at 50 km downstream. However, after the 50 km inflection, Mn concentrations increase with distance downstream. Along the downstream reach (>50 km), Mn concentrations in the river also exhibit a better correlation with stream discharge and the suspended particle load. Tributaries to this reach are also characterized by higher concentrations of Mn than observed in the first 50 km which appear to be driving these increases in concentration. These results provide new insights into the temporospatial patterns of Mn concentrations across these greater distances. Coupling seasonal reservoir dynamics with hydrologic conditions and tributary inputs provides a multi-faceted approach that is more effective for predicting the distribution of Mn in reservoir sourced rivers.