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We conducted laboratory experiments and numerical modeling to investigate the instability of the freshwater-seawater interface and associated submarine groundwater discharge locations under tidal forcing. Contrary to today's common view of the hydraulic conditions in the intertidal region, i.e., the formation of a stable upper saline plume concurrent with the existence of a so-called "freshwater discharge tube" below, which pinches out at the beach surface close to the low tide mark, is actually extremely unstable under certain conditions. We found that for a gentle beach slope of 1 : 12, a transient upper freshwater-seawater interface develops between the high and low water line characterized by several migrating saltwater fingers that intrude into the aquifer. Groundwater discharge occurs between these saltwater fingers at various locations in the intertidal region and is not limited to the low tide mark. Numerous scenarios are possible between the stable and the unstable cases depending on the ambient groundwater flux, tidal amplitude, beach slope, permeability of the sediment, heterogeneity, and anisotropy.

Ground-based handheld thermal infrared imagery was used for the detection of small-scale groundwater springs at the northwestern beach of Spiekeroog Island (northwest Germany). The surveys and in situ measurements of electric conductivity were carried out from shortly before to shortly after low tide along the low water line. Several brackish groundwater discharge springs with a diameter of 1-2 cm were observed along the beach at a distance of 2-3 m above the low water line. The high fresh water portion in the discharging water derives from the fresh water lens in the center of the island. During cold weather, the springs were identified by a significantly increased temperature (3-5 °C higher) and a lower electric conductivity (<10 mS/cm) in contrast to the surrounding sea water (1-2 °C, >30 mS/cm). During warmer weather conditions, an inverse temperature contrast was observed. The measurements confirm the applicability of thermal imagery for the detection of small-scale groundwater discharge locations as an extension to the established method of aerial thermal scans and prove the existence of submarine groundwater seeps in porous systems. A ground-based handheld thermal infrared imagery survey enables a precise installation of sampling devices as, for example, seepage meters.

Simple closed-form approximations are presented for calculating the steady-state groundwater age distribution in two-dimensional vertical cross sections of idealized fresh water lenses overlying salt water, for aquifers that are vertically semi-infinite and of finite thickness. The approximations are developed on the basis of existing one-dimensional analytical solutions for travel-time calculation in fresh water lenses and approximate streamline formulations. The two-dimensional age distributions based on the closed-form solutions match convincingly with numerical simulations. As expected, notable deviations from the numerical solution are encountered at the groundwater flow divide and when submarine groundwater discharge occurs. Ratios of recharge over hydraulic conductivities are varied to explore how the magnitude of the deviations changes, and it is found that the approximate closed-form solutions perform well over a range of conditions found in natural systems.