<p>Freshwater lenses are an important water resource in coastal areas as well as on oceanic islands, and understanding the dynamic forces acting upon this resource is vital for their sustainable management. A key water-management objective is to understand and manage these freshwater lenses, which requires accurate estimates of drawdown and groundwater recharge. Groundwater levels in such systems, however, are dominated by multiple dynamic factors, such as tidally and meteorologically forced ocean level fluctuations, coastal morphology, aquifer properties, recharge, and groundwater extraction. Unfortunately, tidal influences often dominate groundwater levels in these systems, which confounds the quantification of aquifer recharge and extraction.</p> <p>This work uses regression deconvolution to quantify oceanic influences on groundwater levels by generating an “Ocean Response Function” (ORF) that is used to reveal groundwater recharge and extraction, once influences have been removed. We use groundwater levels from an unconfined and unconsolidated (mostly fine sand) aquifer on the island of Norderney located in the North Sea in Northwest Germany. Confounding tidal influences are removed from observed groundwater levels to reveal underlying processes. Most prominently, seasonal recharge patterns are now clearly visible, along with responses to daily groundwater extraction from a nearby water-supply well. The obtained ORF also constrains the aquifer hydraulic diffusivity, in that higher diffusivities induce faster responses. Overall, this work demonstrates how regression deconvolution leads to improved insight into groundwater processes and properties when applied to coastal and island groundwater observations.</p>

Abstract. The sustainability of limited freshwater resources in coastal settings requires an understanding of the processes that affect them. This is especially relevant for freshwater lenses of oceanic islands. Yet, these processes are often obscured by dynamic oceanic water levels that change over a range of time scales. We use regression deconvolution to estimate an Oceanic Response Function (ORF) that accounts for how sea-level fluctuations affect measured groundwater levels, thus providing a clearer understanding of recharge and withdrawal processes. The method is demonstrated using sea-level and groundwater- level measurements on the island of Norderney in the North Sea (Northwest Germany). We expect that the method is suitable for any coastal groundwater system where it is important to understand processes that affect freshwater lenses or other coastal freshwater resources.

Geological information is required to parameterize hydrogeological properties in groundwater flow models. Our aim was to provide a hydrogeological model for the island of Norderney, Northwest Germany and the surrounding Wadden Sea for this purpose. The model focuses on Holocene, Pleistocene and Pliocene deposits which are the most relevant to groundwater flow in and around the island's freshwater lens. For these geological units, borehole data was available that allowed us to distinguish between sediments acting as aquifers and aquitards. Conceptual units were derived that comprise the most common stratigraphic and petrographic features into discrete entities. The borehole data was supplemented by maps of the pre‐Holocene surface as well as data from an existing stratigraphic model for deeper geological units. The model was developed and created using the open‐source geological modelling software GemPy. The resulting model contains major hydrogeological units that can be assumed continuous over a larger extent of the model area based on the available data. From the deeper geology, a possible range of locations of the aquifer base below Norderney was extracted. By integrating borehole data, existing geological models and geological interpretations available in the literature, this dataset complements the so far mainly cross‐sectional and partial descriptions of the hydrogeology below Norderney.