Changes in mercury speciation and the release of methyl mercury as a result of marine sediment dredging activities

During the dredging of a moderately contaminated marine waterway Lavaca Bay, Texas, USA., high concentra- tions of total mercury THg, 75]1533 ngrl. and methyl mercury MHg, 5]70 ngrl. were observed in the decant water from an upland dredge spoils settling basin. By comparison, the water column of the bay contains 7]200 ngrl THg and 0.05]0.8 ngrl MHg. The Hg concentrations observed in the decant water were higher than could be accounted for by the release of sediment pore water, which contained elevated MHg 2]200 ngrl. in the surface 3 cm of the sediments, but diminished exponentially with depth. Laboratory mixing experiments showed that approximately 5% of the sediment bound MHg, and less than 1% of the total mercury was released upon shaking with seawater. Although significant MHg releases could occur from the upper 0]5 cm of the sediments, when integrated over the entire dredged volume, this source is insufficient to account for the levels found in the settling basin decant water. The settling basin became very biologically active, containing algal blooms and releasing hydrogen sulfide to the air, suggesting that increased methylation in the disturbed sediment was responsible for the releases. A laboratory mesocosm study was performed to simulate the findings in the settling pond, but no obvious increase in biological activity occurred, and MHg concentrations increased only to 0.6 ngrl. The implications of these observations may be significant for the local region approximately a dredging site, although crude mass balance considerations show that the MHg flux from this source is trivial compared to bay-wide processes