Legacy of Past Mining Activity Affecting the Present Distribution of Dissolved and Particulate Mercury and Methylmercury in an Estuarine Environment (Nalon River, Northern Spain)

At the Nalon River estuary (Asturias, Northern Spain), the occurrence of Hg is due to historical mining activity which has resulted in environmental issues of great concern. Although several studies have investigated the sediment compartment regarding Hg contamination, no information is currently available on the fate of Hg and MeHg in the water column. Considering different hydrodynamic/seasonal conditions, water samples were collected along the estuary to evaluate Hg and MeHg distribution and partitioning behaviour between solid and aqueous phases. The complementary effect of the river discharge and tidal currents contributed to the prevalence of the dissolved (4.02 +/- 1.33 ng L-1) or particulate (8.37 +/- 4.20 ng L-1) Hg under different conditions of discharge in summer and autumn, respectively. Conversely, particulate MeHg prevailed when the river flow was low, especially at the estuary mouth (25.8 +/- 19.1 pg L-1) and most likely due to the resuspension of fine particles promoted by a stronger tidal current. In comparison with the total Hg concentration, extremely low amounts of dissolved and particulate MeHg were observed, and strong interactions between MeHg and organic carbon highlighted a negligible risk of increased mobility and potential bioaccumulation of MeHg.

Automated summary

At the Nalon River estuary in Northern Spain, the distribution and partitioning behavior of Hg and MeHg in the water column were studied, showing the influence of river discharge and tidal currents on the prevalence of dissolved and particulate Hg under different seasonal conditions. Particulate MeHg was found to prevail when river flow was low, especially at the estuary mouth, possibly due to resuspension of fine particles caused by strong tidal currents. Compared to total Hg concentration, extremely low amounts of dissolved and particulate MeHg were observed, with strong interactions between MeHg and organic carbon indicating a minimal risk of increased mobility and potential bioaccumulation of MeHg.