Sediment Transport and Hg Recovery in Lavaca Bay, as Evaluated from Radionuclide and Hg Distributions

Abstract

Mercury was released in the late 1960s from a chlor-alkali facility managed by ALCOA and deposited into sediments of Lavaca Bay, TX. Sediments have recorded this event as a well-defined subsurface concentration maximum. Radionuclide, mercury, X-radiography, and grain size data from sediment cores taken 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 bomb fallout nuclide (137Cs, 239,240Pu) peaks in 1963 and from the steady-state delivery of 210Pb from the atmosphere. Sedimentation rates are highest (∼2 cm/yr) at near-shore sites near the ALCOA facility and generally decrease away from shore. Sedimentation rates in some areas are likely influenced by anthropogenic activities such as dredging. Particle reworking, as assessed from 7Be measurements, is generally restricted to the upper 2−7 cm of sediments. Numerical simulations of Hg profiles using measured sedimentation and mixing parameters indicate that at most sites high remnant mercury concentrations at 15−60 cm depth cannot supply substantial amounts of Hg to surface sediments. Assuming no future Hg supplies, Hg concentrations in surface sediments are predicted to decrease exponentially with a recovery half-time of 4 ± 2 years.