Groundwater travel time distribution in the subsurface of a high energy beach – a multi tracer approach

In beach aquifers two water bodies, relatively old terrestrial freshwater and young oceanic saltwater mix, biogeochemical reactions change the solute composition of the water and groundwater discharge modifies element net fluxes to the ocean. Residence times are baseline information for the biogeochemical interpretation and help to understand groundwater flow and transport regimes. In the present study we used environmental tracers, i.e. apparent tritium-helium (3H/He) ages, temperatures and silica (Si) concentrations to derive groundwater ages and travel times in the subsurface along a cross-shore transect at the high energy beach aquifer on Spiekeroog, a barrier island in North-Western Germany. Recent generic modelling studies suggested that in beach aquifers under high energy conditions, characterized by high waves and tidal amplitudes as well as seasonal storm floods, flow and transport patterns in space and time are highly variable. As a consequence, the typical salinity and age stratification is distorted as compared to the classical stable concept of water bodies in beach aquifers derived from more embayed sites. To advance the understanding of such highly dynamic systems we obtained two sets of apparent 3H/He ages one year apart at three permanently installed multilevel wells each filtered in four depths (6, 12, 18, 24 m bgs), located at the dune base, near the mean high water line and near the mean low water line respectively. At the same locations, data loggers continuously recorded groundwater temperatures and were used to calculate travel times. In addition, Si was measured in samples taken every six weeks over one year. The results show relatively young apparent 3H/He ages in all samples, ranging from weeks to approximately 18 years. The water was youngest in the shallow part and near the high water line and ages increased with depth and towards the low water line and dune base. Interestingly, 3H/He ages vary significantly at some locations in the two data sets. Temperature derived travel times, representing the young water component (from the North Sea), overall agree well with the mixed apparent 3H/He ages. Si accumulating with time shows a similar trend. In the next steps, the results will help to constrain site specific groundwater modelling and support the interpretation of geochemical data and underlying processes in order to finally better understand the functioning  of high energy beach systems.