Particulate and dissolved mercury export in streamwater within three mid-Appalachian forested watersheds in the US

Forested uplands retain Hg in soils from atmospheric deposition and are a potential long-term source of Hg to downstream waters. Accurate estimates of dissolved and particulate mercury (HgD and HgP, respectively) streamwater fluxes are needed to track the movement and storage of Hg in these ecosystems. It is well established that frequent sampling during high-flow events, when Hg concentrations can vary by orders of magnitude, is necessary to quantify Hg export in these systems. However, in part due to the difficulties of consistently sampling during these short-duration periods, and lack of alternative surrogate measures, no studies have quantified HgD and HgP export during storm flow relative to the total annual flux. At three mid-Appalachian forested watersheds, we sampled streamwater bi-weekly and hourly during storm events using both manual and automated techniques for 18 months and investigated the feasibility of using turbidity measured with an in situ sonde as a surrogate measure for HgP. We determined turbidity had a much stronger correlation to HgP (r2 = 0.78–0.98) as compared to specific discharge (r2 = 0.36–0.55), making it an effective high-frequency surrogate at each site. For the year-long study, we found that approximately 80% of the total Hg (HgT) flux was exported during the high-flow periods corresponding to approximately 1% of the time. Particulate Hg accounted for the majority of annual HgT fluxes at all three sites (58–85%) as a consequence of being more strongly flow-activated relative to the dissolved form. Despite being associated with relatively low dissolved organic carbon (DOC) concentrations, the HgT fluxes from these forested Appalachian watersheds, which ranged from 1.26 to 3.71 μg m−2 yr−1, were comparable to fluxes reported in other regions of the world