METAL FLUXES ACROSS THE SEDIMENT-WATER INTERFACE IN A DRINKING WATER RESERVOIR

Elevated concentrations of metals, including iron (Fe) and manganese (Mn) degrade drinking water quality by affecting taste, odor and color. Consumption of some metals, including Mn, can also have adverse human health impacts. Many utilities have installed in situ oxygenation systems in drinking water reservoirs to control metal concentrations. Designing successful oxygenation systems benefits from accurate measurement of metal fluxes into the water column. In this study, we conducted in-situ flux chamber experiments to quantify Fe and Mn fluxes across the sediment-water interface of an oxygenated drinking water reservoir (Falling Creek Reservoir, Vinton, VA). We measured Fe and Mn concentrations under changing oxygen conditions over two 10-day intervals during the 2018 summer thermal stratification period to calculate fluxes. In the experiments, we monitored dissolved oxygen (DO), oxidation-reduction potential (ORP), temperature, and pH. In addition to the flux chamber experiments, we also estimated metal fluxes using a mass balance method, which relies on measurements of metal loads into and from the water column.

Our results showed that metal fluxes are highly variable during the stratification period, with some periods having positive fluxes (release of metals from sediment to the water column) and some with negative fluxes (return of metals from the water column to sediment). Metal fluxes are highly sensitive to redox conditions in the water column and at the sediment-water interface. Chamber-measured fluxes are 91-105% higher than those estimated using the mass balance method. This difference supports previous work suggesting that the in-situ flux chamber method likely provides maximum values of fluxes as the isolated chamber water does not allow for mixing with the water column. In contrast, because the mass balance results are affected by mixing and biogeochemical reactions that can remove metals from the water column, flux estimates using this method likely reflect minimum values. However, when combined, these two methods provide a useful tool for constraining metal fluxes under different redox conditions. Results of this study can be used by water utilities to improve the effectiveness of oxygenation systems and water quality management practices related to metals.