Effect of a moderate-size reservoir on transport of trace elements in a watershed

Abstract

ABSTRACTWildman RA, Forde NA. 2016. Effect of a moderate-size reservoir on transport of trace elements in a watershed. Lake Reserve Manage. 32:353–365.We assessed the extent to which Grand Lake, Oklahoma (>30 m deep, >80 km long), retains Fe, Mn, P, As, Zn, Pb, and Cd. Filtered water samples and suspended sediment samples were collected upstream of, within, and downstream of the reservoir. We then estimated instantaneous, seasonal, elemental fluxes. In winter and spring, when storms brought high flows to the reservoir, Grand Lake modified flood water minimally. During these seasons, trace element distributions were determined by the passage of storm inflows through the reservoir. In summer, Fe, Mn, P, and As were enriched in anoxic bottom water and exported through the dam, which draws water from below the surface mixed layer. Concentrations of aqueous elements in the water column were lower following autumn overturn, perhaps due to precipitation of metal oxides and settling. Unlike Fe, Zn was retained in Grand Lake during all seasons. Concentrations of Cd and Pb in filtered samples were often below our detection limit. Logistic regression indicated that Zn predicted detectable Cd, and so Grand Lake probably sequesters Cd. Sequestration of Pb was unclear because detectable Pb was predicted by both Zn and Fe. This study shows that watershed hydrology determines the transport of trace elements through a reservoir during times of high flow but that vertical circulation and biogeochemistry dominate during summertime and autumn low flows. Understanding these mechanisms can aid reservoir managers who seek to reduce downstream loads of trace elements.