Sediment transport and Hg recovery in Lavaca Bay, as evaluated from radionuclide and Hg distributions

Mercury was released in the late 1960s from a chloralkali facility managed by ALCOA and deposited into sediments of Lavaca Bay, TX. Sediments have rec orded this event as a well-defined subsurface concentration maximum. Radion uclide, mercury, X-radiography, and grain size data from sediment cores tak en in 1997 at 15 stations in Lavaca Bay were used to assess sediment and Hg movements in the bay. Sediment accumulation rates were calculated from bom b fallout nuclide (Cs-137, Pu-239,Pu-240) peaks in 1963 and from the steady -state delivery of Pb-210 from the atmosphere. Sedimentation rates are high est (similar to 2 cm/yr) at near-shore sites near the ALCOA facility and ge nerally decrease away from shore. Sedimentation rates in some areas are lik ely influenced by anthropogenic activities such as dredging. Particle rewor king, as assessed from Be-7 measurements, is generally restricted to the up per 2-7 cm of sediments. Numerical simulations of Hg profiles using measure d sedimentation and mixing parameters indicate that at mast sites high remn ant mercury concentrations at 15-60 cm depth cannot supply substantial amou nts of Hg to surface sediments, Assuming no future Hg supplies, Hg concentr ations in surface sediments are predicted to decrease exponentially with a recovery halftime of 4 +/- 2 years.