Mercury distribution in the water column of the stratified Krka river estuary (Croatia): importance of natural organic matter and of strong winds

Estuarine water samples were collected at the vertical profile on one selected station in the Krka river estuary (Croatia) during the period from September 1995 to July 1998. The concentrations of reactive and total mercury (in the UV-irradiated samples) were measured in unfiltered and acidified samples by atomic absorption and show non-conservative behavior. The location of the mercury maximum found in the region, 2−12.5 ngl−1, changes and is not always at the freshwater/seawater interface (FSI) as suggested in earlier estuary studies. The proportion of reactive to total mercury, two operational species, also changed, depending on salinity, seasonal and weather changes not only in fresh water and at the FSI but in some cases in seawater down to the bottom. Maximal apparent mercury complexing capacity was determined as 2.7 ngHgl−1. Information about surfactant activity in the same vertical profile was obtained using a hanging mercury drop electrode (HMDE), by a.c. out of phase voltammetry at two fixed potentials (−0.2 and −0.6 V). Continuous variation of hydrophylic and hydrophobic substances was observed. No correlation with the mercury maximum was found. In samples showing the total mercury maximum, hydrophobic heterodispersed substances (>0.7 μm) were found, giving evidence that mercury is partly bound to particulate organic carbon having hydrophobic properties. Submicron colloids and aggregates (10 nm–1 μm) were directly proved using electronmicroscopy. Characteristic estimated time for the coagulation/sedimentation process in the Krka river estuary (about 8 days) can be influenced by the strong northerly Bura wind. Removal of mercury from the water column was evidenced in winter, when extremely low (less than 1 ngl−1) mercury concentrations were found in the water column after the very strong Bura wind. During the Jugo wind the outflow of brackish water decreases. The current going from sediment to FSI accumulates mercury at FSI (12.5 ngl−1), predominantly in inorganic form.