Hydrogeochemistry of near-surface groundwater on a developing barrier island (Spiekeroog, Germany): The role of inundation, season and vegetation

Abstract

Low-lying barrier islands present ideal locations to investigate hydrogeochemical processes in sandy coastal aquifers, as they (i) host typical geographic features of the coast, including beaches, dunes, as well as salt marshes, (ii) are susceptible to the impacts of a global climate change, because of their low surface elevations, and (iii) are ubiquitous (>2,100) amongst the global coastlines. The aim of this study was to investigate the evolution of hydrogeochemical patterns in near-surface groundwater on developing barrier islands, using the Ostplate on Spiekeroog Island, Germany, as an example. The impact of inundation, season and vegetation on the hydrogeochemical processes were of particular interest. Six groundwater monitoring wells were installed for this purpose, covering main geographic features of the Ostplate, i.e., supratidal beaches, dunes and salt marshes. Groundwater samples were collected bimonthly from April 2016 to March 2017. Measured parameters included major ions, nutrients, dissolved organic carbon/matter (DOC/DOM), δ13C signatures of dissolved inorganic carbon, as well as δ34S signatures of dissolved sulfate and sulfide. We found major ion compositions to be a function of the annual flooding frequency, although cation concentrations were further impacted by calcite dissolution and cation exchange. The redox chemistry varied at the different sites and through seasons. Aerobic respiration was the dominant pathway for carbon oxidation at the dune locations, whereas organic matter mineralization in the salt marsh was also mediated by anaerobic processes. Regarding electron donors, the presence of labile DOM was linked to salinity, and terrestrial DOM signatures as well as gross inputs of organic carbon agreed with vegetation patterns. A physico-chemical interpretation of the pore water compositions revealed a potential for the authigenic formation of carbonate and phosphate minerals in the investigated sediments, triggered by the redox processes.