Sediment-water fluxes of mercury in Lavaca Bay, Texas

The aqueous flux of inorganic Hg and monomethyl Hg from sediments to the water column was determined at several sites in Lavaca Bay, an estuary along the Texas Coast, historically impacted by Hg discharges. Diffusive fluxes were calculated at 15 sites using interstitial pore water gradients and compared to direct flux measurements obtained at two sites using benthic flux chambers. The diffusive flux of monomethyl mercury (MMHg), when modeled as a chloride species, varied over 3 orders of magnitude from 0.2 to 1500 ng m^{- 2} day^-1. Diffusive fluxes determined at a single site revealed that MMHg fluxes varied seasonally; maximal fluxes occurred in late winter to early spring. Flux chamber deployments at an impacted site revealed that MMHg was the Hg species entering the water column from sediments and the flux was not in steady-state; there was a strong diurnal signal with most of the MMHg flux occurring during dark periods. The flux of inorganic Hg was smaller and not as easily discernible by this method. The MMHg flux during the dark period (830 ng m^-2 day^-1) was about 6 times greater than the estimated diffusional flux (140 ng m^-2 day^-1) for MMHgCl, suggesting that biological and/or chemical processes near the sediment-water interface were strongly mediating the sediment-water exchange of MMHg.