Sediment water exchange of trace metals and nutrients in Galveston Bay, Texas

Abstract

The benthic fluxes of several trace metals and nutrients were determined for three stations along the salinity gradient in the Trinity Bay region of Galveston Bay, Texas, using whole core squeezing techniques in conjunction with direct in-situ measurements. The stations were visited in consecutive late summers and late winters from 1994 to 1996. For the four sampling trips, calculated diffusive fluxes estimated from porewater concentration gradients, for NH4' ranged from-70 to-427 mmoles/m2/day with an average value of-2391︢37. Diffusive fluxes of P04 3-ranged from 0 to-28.6 gmoles/@/day and averaged-99︢. The diffusive fluxes of N03-1 Si(OH)41 Cl-and S04 2-changed in magnitude and direction in response to reduced river flow and increased coastal inputs into Trinity Bay during this period. Diffusive fluxes of Mn ranged from-20 to-367 gmoles/m2/day with an average value of-1931︢15, whereas Fe fluxes were less, in the range of-7 to-94 with an average value of-542︢4 gmoles/m2/day. Ni fluxes ranged from-0. 12 to-0.91 mmoles/m2/day with an average value of-0.510︢.32, while Zn fluxes ranged from-0.18 to-1.18 mmoles/m2/day with an average value of-0.560︢.32. The calculated diffusive fluxes were in the negative direction implicating the sediments as a source of trace metals to Trinity Bay. To determine the extent of non-diffusive processes, sediment incubation studies using benthic flux chambers were conducted at each of the stations during the 03/13/96 trip. Results show that approximately 5-49% of the net flux from the sediments to the water column can be attributed to diffusion. Diffusive estimates greatly overestimated the flux of Fe when compared to results from the benthic chamber experiments. It was found that 14-18% of the flux of NH4 , 0-10% Of PO-3 4 10-17% of Si(OH)4, 5-31 % of Mn, 39-49% of Ni and 25-38% of Zn could be attributed to diffusion. To gain a further understanding of the distribution of trace metals between solid sediment and porewater, sequential sediment leaching experiments were conducted. Results showed total sediment-porewater partition coefficients of 2.00︢.3, 4.30︢.5, 3.20︢.3 and 3.90︢.3 for Mn, Fe, Ni and Zn respectively.