Mercury associated with colloidal material in an estuarine and an open-ocean environment

We have used traditional filtration and tangential flow ultrafiltration to isolate mercury into 4 size fractions: particulate (> 0.4 μm), high molecular weight (>10 kD, 1kD = 1000 daltons), medium molecular weight (1–10 kD), and low molecular weight (< 1 kD). Samples with varying salinities were collected from the Ochlockonee River/Estuary in January 1993 and 1994. A small set of open-ocean samples was collected and processed during the 1993 Baseline Survey Expedition for Trace Contaminants in the Atlantic Ocean, sponsored by the Intergovernmental Oceanographic Commission (IOC). Concentrations of total dissolved Hg down the estuary ranged from 30 to 3 pM. Total dissolved Hg measurements from the IOC cruise ranged from 0.5 to 2.5 pM. Colloidal Hg (> 1 kD) represented 35–87% of the total dissolved Hg within the estuary and 10–50% of the total dissolved Hg in the North Atlantic. Equilibrium Hg speciation modeling supports speculation that colloidal Hg is bound by thiol-type functional groups associated with the colloidal organic carbon.