Freshwater lenses below barrier islands are dynamic systems affected by changes in morphodynamic
patterns, groundwater recharge and discharge. They are also vulnerable to pollution and overabstraction
of groundwater. Basic knowledge on hydrogeological and hydrochemical processes of freshwater lenses
is important to ensure a sustainable water management, especially when taking into account possible
effects of climate change. This is the first study which gives a compact overview on the age distribution,
recharge conditions and hydrochemical evolution of a barrier island freshwater lens in the southern
North Sea (Spiekeroog Island, Eastfrisian Wadden Sea). Two ground- and surface water sampling campaigns
were carried out in May and July 2011, supplemented by monthly precipitation sampling from
July to October. 3H–3He ages, stable oxygen and hydrogen isotopes and major ion concentrations show
that the freshwater lens reaches a depth of 44 mbsl, where an aquitard constrains further expansion in
vertical direction. Groundwater ages are increasing from 4.4 years in 12 mbsl up to >70 years at the freshwater–
saltwater interface. Stable isotope signatures reflect average local precipitation signatures. An
annual recharge rate of 300–400 mm was calculated with 3H–3He data. Freshwater is primarily of
Na–Ca–Mg–HCO3– and Ca–Na–HCO3–Cl type, while lowly mineralized precipitation and saltwater are
of Na–Cl types. A trend towards heavier stable isotope signatures and higher electric conductivities in
the shallower, younger groundwater within the freshwater lens may indicate increasing atmospheric
temperatures in the last 30 years