Prevalence of tide-induced transport over other metal sources in a geologically enriched temperate estuarine zone (NW Iberian Peninsula)

• Importance of tidal flooding over intertidal sediments in ria systems was tested.
• Metal contributions due to diffusion and advection in Atlantic rias were quantified.
• Geological complexes contribute with metal enrichment to the coastal environment.
• Ortegal Complex causes an enrichment of Cr–Ni in shelf sediments of Galician rias.

Tide-induced transport, diffuse fluxes and river inputs of arsenic, cobalt, chromium, manganese and nickel were studied in an estuarine zone located at the Ria of Ortigueira in the Galician coast of NW Spain to evaluate comparatively the magnitude of trace-element inputs in the estuarine ecosystem. Short-sediment cores and flooding water were collected at the intertidal area of La-Caleira Inlet in spring and summer 2008 during the first 50 min of tidal inundation. High concentrations of dissolved chromium (up to 23 nM) and nickel (256 nM) were found in waters of the Lourido River during 2008. Chromium (up to 795 mg·kg− 1) and nickel (up to 533 mg·kg− 1) in surface sediments of the study area were also high, pointing to natural geological sources (Cape Ortegal Complex). Fluxes associated with molecular diffusion and tide-induced transport across the sediment–water interface were quantified and compared to the river contribution. Mean fluxes from the Lourido River were 0.03 μmol·m− 2·d− 1 for As, 0.02 μmol·m− 2·d− 1 for Co, 0.43 μmol·m− 2·d− 1 for Cr, 0.72 μmol·m− 2·d− 1 for Mn and 5.98 μmol·m− 2·d− 1 for Ni. Diffusive fluxes were negative which indicates the flow of trace elements towards the sediment − 0.47 ± 0.12 μmol·m− 2·d− 1 for As; − 0.007 ± 0.001 μmol·m− 2·d− 1 for Co; − 0.003 ± 0.009 μmol·m− 2·d− 1 for Cr; − 5.44 ± 1.65 μmol·m− 2·d− 1 for Mn; and − 0.02 ± 0.05 μmol·m− 2·d− 1 for Ni. Tide-induced transport of trace metals was up to four orders of magnitude greater than diffusive fluxes. The contributions of trace elements to the estuarine zone from fluvial and benthic origin were quantified, emphasizing the importance of tide-induced transport in estuarine systems.