Sediment-water exchange of Mn, Fe, Ni and Zn in Galveston Bay, Texas

In-situ benthic flux studies were conducted at three stations in Upper Galv eston Bay twice during March 1996 to directly measure release rates of diss olved Mn, Fe, Mi and Zn from the sediments. Results showed reproducible inc reases with time in both replicate light and light-dark benthic chambers, r esulting in average fluxes of - 1200 +/- 780, - 17 +/- 12, - 1.6 +/- 0.6 an d -2.4 +/- 0.79 mu mol m(-2) day(-1) for Mn, Fe, Mi and Zn, respectively. S ediment cores collected during 1994-1996 showed that surficial pore water c oncentrations were elevated compared to overlying water column concentratio ns, suggesting diffusive release from the sediments. Diffusive flux estimat es of Mn and Zn agreed in direction with chamber fluxes measured on the sam e date, but only accounted for 5-38% of the measured flux. Diffusive fluxes of Fe agreed with measured fluxes at the near Trinity River station but ov erestimated actual release in the mid and outer Trinity Bay regions, possib ly due to inaccurate determination of the Fe pore water gradients or rapid oxidation processes in the overlying water at these stations. In general, measured fluxes of Mn and Mi were higher in the mid Trinity Bay region and suggested a mechanism for the elevated trace metal concentratio ns previously reported for this region of Galveston Bay. However, the fluxe s of Fe were highest in close proximity to the Trinity River, supporting th e elevated Fe concentrations measured in this region during this and other studies, and decreased towards middle and outer Trinity Bay. Trace metal tu rnover times were between 0.1 and 1.2 days for Mn, between 1.3 and 4.6 days for Fe, and between 27 and 100 days for Mi and 12-20 days Zn, and were con siderably shorter than the average Trinity Bay water residence time (1.5 ye ars) for this period. Comparing area averaged benthic inputs to Trinity Riv er inputs shows the sediments to be a significant source of tract: metals t o Galveston Bay. However, while benthic inputs of trace metals were measure d, water column concentrations remained low despite rapid turnover times fo r Mn and Fe, suggesting removal of these metals from the water column after release from the sediments. (C) 2001 Elsevier Science B.V. All rights rese rved.