Evaluation of riverine suspended particulate matter sampling with sedimentation boxes and flow-through centrifuges for total mercury and methylmercury analysis.

Mercury (Hg) is a highly toxic element. Despite recent efforts in reducing emissions, large amounts of anthropogenic Hg and its different species are still present in riverine systems. A common sampling technique for suspended particulate matter (SPM), the main carrier phase of Hg in river waters, are sedimentation boxes. Yet, their accuracy and representativeness with respect to changing hydrological conditions and preservation of Hg including methylmercury (MeHg) has not been investigated. This study investigates the annual cycle of total Hg, MeHg, and total organic carbon (TOC) contents from monthly SPM samples of the legacy-contaminated Elbe River (Germany). Samples were obtained between June 2022 and September 2023 at four locations with varying Hg concentrations and compared to day-specific SPM samples collected by flow-through centrifugation. Total Hg contents of SPM agreed well between the two sampling systems, but considerably higher MeHg (up to 5-fold) and TOC (up to 4-fold) concentrations were observed in sedimentation box SPM in spring and summer compared to centrifuge SPM. This suggests increased in-situ Hg methylation driven by labile organic matter, elevated water temperature, and anoxic conditions after SPM deposition. Centrifuge SPM showed less pronounced seasonality in MeHg with up to 3-fold higher concentrations and effects of 'bio-dilution' from biomass production in spring and summer. Our data suggests diagenetic processes, which increase Hg methylation, make sedimentation box samples less representative for MeHg loads in riverine SPM. We propose sedimentation boxes may be perceived as a river specific in-situ model to assess the methylation potential of recently deposited SPM.