A Multi-Method Approach for Quantification of In- and Exfiltration Rates from the Subterranean Estuary of a High Energy Beach

Abstract

Accurate SGD (submarine groundwater discharge) mass export calculations require detailed knowledge of the spatial and temporal variability in SGD rates. In coastal aquifers, SGD includes a terrestrial freshwater component as well as a saline component originating from circulating seawater. Representative field measurements of SGD rates are difficult to conduct, because SGD is often patchy, diffuse, and temporally variable, especially under tidal influence and high wave activity. In this study, a combination of lysimeters, seepage meters, temperature sensors, pore water radon, and numerical modeling was used to estimate the volumes of infiltrating seawater and exfiltrating groundwater in the intertidal zone of a mesotidal, high energy beach on Spiekeroog Island, northern Germany. Additionally, a 3D-laser scanner was used over short (days) and medium time scales (months) to determine changes in beach topography. The results showed net water infiltration above mean sea level (MSL) and net exfiltration below MSL. Water exchange rates fluctuated between 0.001 and 0.61 m day−1, showing similar ranges within the multiple method approaches. The beach topography was subject to strong fluctuation caused by waves, currents, wind driven erosion and sedimentation, even over short time scales. A comparison of extrapolated in- and exfiltrating water volumes along a beach transect from the mean high water to mean low water line at different times highlights the variability of total in or outflow. The results show that exchange rates depend on beach topography, which in turn changes significantly over time.

Cross-Shore and Depth Zonations in Bacterial Diversity Are Linked to Age and Source of Dissolved Organic Matter across the Intertidal Area of a Sandy Beach

“…This distribution is presumably related to observable changes within geochemical conditions along the flow path of the intertidal recirculation cell, most likely caused by different sources and residence times of porewater [ 19 , 51 , 52 , 53 ]. The degradation state and composition of DOM which influence the presence of cluster II and III at Spiekeroog beach are in turn influenced by the mixing of the terrestrial and marine endmembers.…”

“…Porewater samples that are linked to cluster III were characterized by the highest ammonium, silicate, dissolved iron, and FDOM concentrations, which is in line with their link to the lower beach (ridge, LWL). This can especially be explained by the LWL being an exfiltration area of the subterranean estuary fed by recirculating seawater and fresh groundwater from the island’s freshwater lens [ 51 , 53 ]. On average, the DOM compounds associated with cluster III had the lowest degradation index and DBE but highest ITerr.…”

“…This system is frequently relocated within the intertidal area due to tides, waves, and storm surges. At elevations above the mean sea-level, seawater infiltrates into the sediments, while areas below are net submarine groundwater discharge (SGD) areas of the STE [ 51 ]. Due to the strong beach morphodynamics, in- and exfiltration areas are not static.…”

The Spiekeroog Coastal Observatory: A Scientific Infrastructure at the Land-Sea Transition Zone (Southern North Sea)

“…It has a typical hydrological zonation with an upper saline plume, a freshwater discharge tube, and a saltwater wedge (Beck et al, 2017), whereby the ridge acts as an additional infiltration zone inducing the existence of two, at least temporally occurring, saltwater recirculation cells (Grünenbaum et al, 2020a). With a multi-method approach, Grünenbaum et al (2020b) quantified in-and exfiltration rates in the intertidal zone and showed that infiltration occurs above mean sea level while net exfiltration occurs below, with exchange rates clearly depending on beach topography. With a transient variable density groundwater flow and transport model Greskowiak and Massmann (2021) demonstrate how transient beach morphology and regular storm floods affect subsurface flow and transport pattern (Figure 7).…”

The impact of morphodynamics and storm floods on pore water flow and transport in the subterranean estuary

“…Also, it was done to allow to introducing the effect of dynamic morphology without the need for a step-wise/transient re-gridding of the model domain (see below). Note that the vertical hydraulic gradients in the system are very small compared to the lateral gradients such that the unresolved vertical gradients within the up to 3 m thick saturated zone in the first layer have not much effect on the overall groundwater flow behaviour in the deeper zones.The model accounts for the topography variation over time, loosely based on linear interpolation of available topography scans fromGrünenbaum et al (2020b). A transient Cauchy-Boundary ( ┌ S ) in the intertidal zone(Figure 1 top)reflects the tide-averaged hydraulic heads depending on a time-varying morphology(Figure 1, bottom).…”