Tidal variations in groundwater storage and associated discharge from an intertidal coastal aquifer

We applied a hydrogeological numerical simulation to investigate spatial and temporal variations in groundwater flow and discharge at a well-studied tidal flat margin in the German Wadden Sea. The simulation was forced by high-resolution sea level boundary conditions. Accuracy of the simulation was checked by comparing simulated with measured pressure heads obtained from direct burial pressure sensor measurements. Overall discharge calculated from the simulation for a time period of almost 12 month averages 0.97 m(3) per meter of margin length per tide but may vary significantly, most pronounced with the spring-neap tide cycle. The simulation predicts that the maximum hydraulic gradient at ebb tide controls the magnitude of discharge, whereas duration of ebb tide is less important. Additionally, simulated and measured pressure gradients can be used to investigate the influence of varying lithology on the groundwater flow regime. The presence of a low permeable Holocene silt layer, ubiquitous in the study area, creates a layered unconfined/semiconfined aquifer system. Both parts of the aquifer are hydraulically connected to the deeply incising tidal creek but show a different response to the tidal forcing. Groundwater flow in the upper (unconfined) margin sediments can be described as a circulation of seawater, where inflow and outflow are coupled to changes in saturation. Sediments below the confining layer release groundwater from storage during falling and ebb tide and recharge during rising and high tide. We can show that tidally driven water exchange between semiconfined aquifers and coastal waters may have a major impact on total groundwater discharge.