Tidal induced dynamics and geochemical reactions of trace metals (Fe, Mn, and Sr) in the salinity transition zone of an intertidal aquifer

Highlights: • Fe²⁺ production at the fresh groundwater front of the salinity transition zone • Fe removal in a form of FeS mineral in the high salinity zone of an intertidal aquifer • Tidal induced variation of trace metal geochemistry in an intertidal aquifer • Clarify multiple pathways of iron cycling in the intertidal aquifer • Iron cycling in intertidal aquifer affects Fe discharge flux. -- Abstract: Biogeochemical reactions in an intertidal aquifer influences the submarine groundwater discharge (SGD) associated trace metal flux to the ocean. Tidal fluctuation greatly affects the physical mixing, and biogeochemical transformation of trace metals in the intertidal aquifer. This study presents the dynamics of trace metals (Fe, Mn, and Sr) and the production of Fe²⁺ in the salinity transition zone is discovered. The variations of Fe²⁺ are led by the shifts of both physical mixing and biogeochemical reaction during tidal fluctuation. The transformation from amorphous Fe(OH)₃ to FeS is the main reason for the enrichment of Fe²⁺ in the zone with a salinity of 0.5–10. Mn behaves much less active than Fe in the intertidal aquifer due to the very limited Mn in the solid phase and the major driving force of Mn²⁺ variation is the physical mixing rather than geochemical reaction. Sr²⁺ behaves conservatively and shows a synchronous with salinity in the salinity transition zone. This study found that Fe²⁺ precipitates in a form not limited to Fe (hydro)oxides and the FeS minerals is the most possible form of precipitation in reduced aquifers. In that case, only a small part of Fe²⁺ discharges to the sea associated with SGD, but Mn²⁺ has a comparatively conservative property during the transport in the intertidal aquifer and majority of the Mn²⁺ originated from fresh groundwater will discharge with SGD in this study. The biogeochemical transformation pathways of Fe and Mn observed in this study provides insights into the cycles of Fe and Mn in an intertidal aquifer, which is of significance to accurately estimate the SGD derived Fe and Mn fluxes to the ocean.