LAUSR

Abstract Freshwater lenses below barrier islands are a precious resource for the local water supply and important for coastal ecosystems. The aim of this study was to investigate the hydrochemical evolution of a freshwater lens, using the barrier island Spiekeroog, Germany, as an example. For this purpose, groundwater samples were obtained during several campaigns, and hydrochemical data and 13C/12C isotope ratios of dissolved inorganic carbon were linked to apparent groundwater ages. Results show that apparent groundwater ages increase with depth and range between 4 and 51 years. All groundwater samples were close to equilibrium with respect to calcite and considerably enriched in calcium and bicarbonate, suggesting calcite dissolution in the unsaturated zone of the dune sediments. The estimated average rate of decalcification was ∼13 mm/a, resulting in a decalcification depth of ∼4.6 m for the oldest sediments of an approximate age of 350 years. Moreover, 13C/12C isotope ratios of dissolved inorganic carbon indicated secondary carbonate mineral reactions within the aquifer, such as recrystallization and closed-system calcite dissolution. Cation exchange was primarily observed in older groundwater, i.e. the deeper part of the aquifer, and the calculated time of complete freshening of the aquifer is ∼600 years. Regarding redox reactants, dissolved oxygen and nitrate concentrations decreased rapidly in young groundwater, while dissolved manganese and iron were virtually absent in samples collected below the zones of oxygen and nitrate reduction. Dissolved sulfide species indicate sulfate reduction in older groundwater, and methanogenesis was detected locally. The absence of solute manganese and iron may be explained by the formation of minerals, such as iron sulfides, siderite, and rhodochrosite, that serve as possible sinks for redox-sensitive solutes. In conclusion, this study showed that hydrochemical data can be linked to groundwater ages to describe the hydrochemical evolution of a freshwater lens in a homogeneous sandy aquifer as a function of residence time.