Porewater geochemistry in a Galician Ria (NW Iberian Peninsula): Implications for benthic fluxes of dissolved trace elements (Co, Cu, Ni, Pb, V, Zn)

The geochemical behaviour of dissolved trace elements in sediment interstitial waters from the Vigo Ria (Galicia, Spain) has been determined. Dissolved concentrations in the pore waters ranged between 1.7–23.2 nM for Co, 1.5–131 nM for Cu, 19.5–159 nM for Ni, 0.02–19.3 nM for Pb, 5.4–260 nM for V and 0.2–241 nM for Zn. Values for Cu, Pb and Zn were highest along the axis of the ria, whereas Co, Ni, V and Zn were more abundant in the area of the industrialised shipyards. The metal profiles were affected by sedimentary redox processes, which were monitored by means of Mn and Fe concentrations and pore water pH. Dissolved Cu and V had relatively high concentrations in the oxic surface layer and their concentrations decreased with depth, although in deeper layers concentrations of V increased in association with the presence of dissolved organic ligands. Cobalt and Ni followed the behaviour of Mn, although they were depleted in surface oxic layers their concentrations increased in the Mn reduction zone. Lead and Zn had highly irregular profiles because of their lower redox-sensitivity and their complexation with organic material is not as pronounced as that for Cu. Dissolved metal concentrations in overlying waters oscillated between 0.7–2.1 nM for Co, 5.5–50.5 nM for Cu, 4.6–10.0 nM for Ni, 1.4–9.1 for Pb, 5.7–28.5 nM for V and 14.6–66.5 nM for Zn. Using Fickʹs first law of diffusion, the benthic fluxes of the elements varied between 0.03–0.24 nmol cm− 2 y− 1 for Co, (− 0.02)–1.11 nmol cm− 2 y− 1 for Cu, 0.16–1.70 nmol cm− 2 y− 1 for Ni, (−0.14)–0.03 nmol cm− 2 y− 1 for Pb, 1.52–4.35 nmol cm− 2 y− 1 for V and (− 0.37)–0.90 nmol cm− 2 y− 1 for Zn. Fluxes were higher in the ria axis for Cu, Ni, V and Zn and in the shipyard area for Co, while Pb fluxes were negligible at all the stations. This work has demonstrated that the redox geochemistry and diagenetic processes of the sediments and pore waters control the benthic fluxes in the Vigo Ria. When the upper layer of the sediment is oxic a high exportation of Cu and V to the overlying waters occurs and during less oxidizing conditions of this layer, Ni and Co exportation from the pore water increases. Dissolved trace metal benthic fluxes to the Vigo Ria were significant for most of the metals (Cu, Ni and V) and were of the same order of magnitude to the riverine inputs.