Potential contributions of dissolved organic matter to monomethylmercury distributions in temperate reservoirs as revealed by fluorescence spectroscopy

Abstract

The monomethylmercury (MMHg) concentrations, water quality parameters (e.g., pH, suspended particles, total phosphorus, sulfate, and chlorophyll-a), and compositions of dissolved organic matter (DOM) were analyzed to understand how the quality of DOM is related to the MMHg distributions in the surface waters of 14 reservoirs. The excitation-emission matrix (EEM) fluorescence spectroscopy identified six fluorescence peaks, and a parallel factor analysis (PARAFAC) of EEM spectra identified three components of DOM: microbial humic-like (C1), terrestrial humic-like (C2), and protein-like DOM (C3). Using the observation data, the principal component analysis (PCA) were carried out to understand the relative importance of the fluorescence properties of DOM, representing DOM quality, on the MMHg distribution. The loading plot of PCA showed a strong positive correlation between the MMHg and protein regions of the EEM spectra and no correlation between MMHg and the terrestrial humic regions of the EEM, suggesting that autochthonous DOM production is a key factor in increasing MMHg concentration in reservoir water. The preliminary mass flux estimation, which was carried out to identify the major sources of MMHg in Okjeong reservoir, revealed that the major sources are sediment diffusion and water column methylation. Because the studied reservoirs are located remotely from a large-sized river and industrial region, most MMHg in reservoir water is likely diffused from the surface sediment or produced in the water column, and these sources tend to increase in reservoirs enriched with autochthonous DOM. It is suggested that EEM fluorescence can improve our ability to trace the major sources of MMHg in diverse reservoirs.