Behaviour and speciation of mercury in the Scheldt estuary (water, sediments and benthic organisms)

Abstract

Surface waters, sediments and the polychaete Nereis diversicolor were sampled in the Scheldt Estuary between 1990 and 1994. In surface waters particulate Hg (HgP) concentrations ranged from 350‐1610 ng g 1 . They are essentially controlled by physical mixing of polluted fl uvial particulates with relatively unpolluted marine particulates, but unaffected by seasonal changes. Dissolved Hg species, on the other hand, show large seasonal variations essentially controlled by the redox conditions in the estuary, as well as by bacterial and phytoplankton activity. Total dissolved Hg (HgTD) concentrations ranged from 0.5 to 3.0 ng l 1 with 10 to 90% as reactive Hg. High concentrations of HgTD are found in the upper estuary in the winter and decrease rapidly with increasing salinity. In summer HgTD concentrations are low in the anoxic upper estuary and increase as oxygen is restored in the estuary. Signifi cant variations were observed in dissolved Monomethyl Hg (MMHg) concentrations with concentrations ranging from 0.01 to 0.120 ng l 1 in the winter and 0.08 to 0.6 ng l 1 in summer and autumn. Particulate MMHg ranged from 2t o6n g g 1 in winter and from 4 to 10 ng g 1 in summer and accounted for 20 to 80% of the total MMHg. Hg concentrations ranged from 0.02 to 0.130 ng l 1 and are higher in summer than in winter. In the lower estuary a positive correlation between Hg and phytoplankton pigments was observed. Sediments and the polychaete N. diversicolor were sampled on the intertidal fl at Groot Buitenschoor. HgT concentrations in surface sediments ranged from 144 to 1890 ng g 1 an dM M H gf r o m 0 . 8 t o 6n gg 1 accounting for 0.4 to 0.8% of the total mercury present. Both total Hg (HgT) and MMHg concentrations increased with increased organic matter content and anoxic conditions. On the other hand, accumulation of HgT and MMHg was higher in N. diversicolor living in coarse grain sandy sediments than in muddy sediments. MMHg concentrations in N. diversicolor ranged from 2.2 to 20.9 ng g 1 accounting for an average of 18% of the HgT. Seasonal variations significantly affected Hg speciation in sediments and N. diversicolor. Higher HgT concentrations were found in the sediments in autumn and winter, whereas MMHg concentrations increased in spring and summer. Likewise, higher MMHg concentrations were also observed in N. diversicolor in spring and summer.