Speciation and cycling of mercury in Lavaca Bay, Texas, sediments

Sediment depth profiles of total mercury (THg) and monomethylmercury (MMHg) were collected at 15 sites in an anthropogenically contaminated estuarine system (Lavaca Bay, TX). THg in the solid phase increased with depth to a maximum located at 10-30 cm, which corresponds to historic industrial discharges to the bay. MMHg in the solid phase was highest in the upper 0-3 cm of the cores, decreasing rapidly with depth. The MMHg content of the surface sediment was a narrowly constrained fraction of the total (0.65 ± 0.34%) over a range of sediment types, while making up only 0.01-0.05% of THg at depth. Porewater concentrations exhibited trends similar to but more exaggerated than in the solid phase. The distribution coefficients (log K(d)) for inorganic Hg (IHg = THg - MMHg) were similar in most samples, averaging 4.89 ± 0.43. The log K(d) for MMHg averaged 2.70 ± 0.78 over all sites and depths but exhibited a subsurface minimum of 2.29 ± 0.67 at the point of maximum dissolved Fe. A time series showed a maximum in both solid phase and porewater MMHg during the early spring, followed by a decrease throughout the remainder of the year.