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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.

Two problems currently facing agriculture are drought and the availability of mineable phosphorus minerals used for fertilization. More frequent and longer drought periods are predicted to threaten agricultural yields in the future. The capacity of soils to hold water is a highly important factor controlling drought stress of plants during the growing phase. High phosphorus availability in soils is also necessary for high agricultural yields, however, over application has also led to a range of environmental problems, foremost being the eutrophication of waterways. Amorphous silica (ASi) has been suggested as one solution to mitigate both water and phosphorus availability. In this work we analyzed the effect of ASi on phosphorus mobility and the soil water storage of a sandy soil. In a lysimeter experiment we found that ASi strongly increased the water storage capacity (WSC) of soils (up to 180% by addition of 3 wt.% ASi). Furthermore, the ASi is in direct competition with phosphorus for sorption sites on iron oxides and other soil minerals increasing nutrient mobilization and increasing potential bioavailability for plants. Following calibration to the lysimeter experiment a process based hydrological model was used to extrapolate experimental results to a sandy agricultural soil with and without ASi for 1 year. For the soil with ASi, the water storage capacities for the yearly scenario were up to 40 kg/m2 higher compared to the untreated soil. Our results suggest that ASi enhances the WSC and phosphorus mobility in soil and that this may be one way to mitigate the predicted climate change related drought stress in sandy soils.