INFLUENCE OF RESERVOIR SEASONALITY ON DOWNSTREAM MANGANESE CONCENTRATIONS

Water quality downstream of hydropower dams is strongly controlled by the geochemical conditions in the impounded reservoirs. During summer-fall reservoir stratification, dissolved oxygen concentrations are often depleted in the reservoir hypolimnion. The formation of anoxic conditions can lead to mobilization of redox-sensitive metals such as iron (Fe) and manganese (Mn). Dam intakes may release the hypolimnetic water directly to the river and act as a point source for metals that would not be present under natural oxygenated river conditions.

In this study, we are examining the influence of seasonality on Mn concentrations in a 180 km reach of the Roanoke River downstream of Leesville Dam. In this reach, historic and current monitoring data indicate that total Mn concentrations in the river can exceed the secondary drinking water standard of 50 ppb. Our monitoring results to date show that Mn concentrations are typically elevated near the dam in summer and fall and decrease as a function of river travel distance to about 80 km downstream. A mass balance of the first 19 km downstream from the dam supports the hypothesis for loss via sedimentation of particulates. Average Mn concentrations reach a minimum around the 80 km mark and begin to increase with distance further downstream. Within this downstream reach, Mn concentrations in the river also correlate with stream discharge and the suspended particle load. Along this reach, tributaries to the river are characterized by higher concentrations of Mn than observed in upstream tributaries which appear to be driving the increasing concentrations in the river beyond 80 km. These results support the implementation of a two-domain reservoir/hydrologic model for interpreting riverine water quality and manganese concentrations at the valley-floor scale.