The dynamic deep subsurface of high-energy beaches 

Abstract

Coastal aquifers are dynamic, shaped by natural and anthropogenic boundary conditions acting on very different time scales from seconds (waves) up to millennia (sea-level rise). In subterranean estuaries (STEs), inland aquifers connect with the sea. With terrestrial freshwater and circulating seawater, chemically different waters mix in STEs and are modified before they discharge into coastal waters. The cooperative interdisciplinary project DynaDeep studies the subsurface of high-energy beaches, which have so far hardly been investigated due to the difficulties associated with working under high-energy conditions. We propose that these systems are particularly dynamic environments, where frequent sediment relocation affects groundwater flow and transport up to depths of tens of meters below the ground surface. This may lead to strong spatiotemporal variability of geochemical conditions, presumably attracting a unique microbial community. The state-of-the-art concept of groundwater flow and transport in STEs with an upper saline plume overtopping a freshwater discharge tube is likely distorted under such conditions, with consequences for the biogeochemical functioning of these STEs. Within DynaDeep, a unique cross-shore research site was established on the northern beach of the barrier island Spiekeroog facing the North Sea. It consists of permanent infrastructure, such as a pole with measuring devices, multi-level groundwater wells and an electrode chain. This forms the base for autonomous measurements, regular repeated sampling and interdisciplinary field campaigns using, for example, direct push techniques supported by modelling and experimental work to understand and quantify the functioning of the biogeochemical reactor. Field results show that morpho- and hydrodynamics are clearly affected by waves, tides and stormfloods. Stormfloods are particularly relevant and can be traced into the subsurface. In the infiltration zone, the groundwater is rather young as shown by age dating and temperature tracing. Oxygen and nirtrate reach deep into the subsurface and a redox transition from oxic to anoxic conditions occurs at 12-15 m depth. In contrast, relatively old and anoxic water discharges near the low water line. Numerical modelling aids in process understanding and hypothesis development, and generic models show that moderate deviations in hydrogeological parameters severely change both salinity as well as hydrogeochemical patterns. The dynamic nature of high-energy STEs, depending on frequencies and amplitudes of change in environmental conditions, as well as the global relevance for high-energy STEs have yet to be explored.