Groundwater ages, recharge conditions and hydrochemical evolution of a barrier island freshwater lens (Spiekeroog, Northern Germany)

Freshwater lenses below barrier islands are dynamic systems affected by changes in morphodynamic patterns, groundwater recharge and discharge. They are also vulnerable to pollution and overabstraction of groundwater. Basic knowledge on hydrogeological and hydrochemical processes of freshwater lenses is important to ensure a sustainable water management, especially when taking into account possible effects of climate change. This is the first study which gives a compact overview on the age distribution, recharge conditions and hydrochemical evolution of a barrier island freshwater lens in the southern North Sea (Spiekeroog Island, Eastfrisian Wadden Sea). Two ground- and surface water sampling campaigns were carried out in May and July 2011, supplemented by monthly precipitation sampling from July to October. 3H–3He ages, stable oxygen and hydrogen isotopes and major ion concentrations show that the freshwater lens reaches a depth of 44 mbsl, where an aquitard constrains further expansion in vertical direction. Groundwater ages are increasing from 4.4 years in 12 mbsl up to >70 years at the freshwater– saltwater interface. Stable isotope signatures reflect average local precipitation signatures. An annual recharge rate of 300–400 mm was calculated with 3H–3He data. Freshwater is primarily of Na–Ca–Mg–HCO3– and Ca–Na–HCO3–Cl type, while lowly mineralized precipitation and saltwater are of Na–Cl types. A trend towards heavier stable isotope signatures and higher electric conductivities in the shallower, younger groundwater within the freshwater lens may indicate increasing atmospheric temperatures in the last 30 years

The drivers of biogeochemistry in beach ecosystems: A cross-shore transect from the dunes to the low water line

This study addresses key processes in high-energy beach systems using an interdisciplinary approach. We assess spatial variations in subsurface pore water residence times, salinity, organic matter (OM) availability, and redox conditions and their effects on nutrient cycles as well as on microbial community patterns and microphytobenthos growth. At the study site on Spiekeroog Island, southern North Sea, beach hydrology is characterized by the classical zonation with an upper saline plume (USP), a saltwater wedge, and a freshwater discharge tube in between. Sediment and pore water samples were taken along a cross-shore transect from the dunes to the low water line reaching sediment depths down to 5 m below sediment surface. Spatial variations in pore water residence time, salinity, and organic matter availability lead to steep redox and nutrient gradients. Vertical and horizontal differences in the microbial community indicate the influence of these gradients and salinity on the community structure. Modeled seawater flux through the USP and freshwater flux through the tube are on average 2.8 and 0.75 m3 per day and meter of shoreline, respectively. Furthermore, ridge sediments at the lower beach discharge seawater at rates of 0.5 and 1.0 m3 per day and meter of shoreline towards the runnel and seaside, respectively. Applying seawater and freshwater fluxes and representative nutrient concentrations for the discharge zones, nutrient fluxes to adjacent nearshore waters are 117 mmol NH4+, 55 mmol PO43 − and 575 mmol Si(OH)4 per day and meter of shoreline. We propose that this nutrient efflux triggers growth of microphytobenthos on sediment surfaces of the discharge zone. A first comparison of nutrient discharge rates of the beach site with a nearby sandy backbarrier tidal flat margin indicates that the beach system might be of less importance in supplying recycled nutrients to nearshore waters than the backbarrier tidal flat area

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

In this study, we demonstrate by numerical density-dependent groundwater flow and transport modelling how transient beach morphology and regular storm floods that are typical for high-energy beaches change this classical picture of a subterranean estuary. The model results suggest that the variable beach morphology and seasonal storm floods lead to strong spatiotemporal variability of hydrodynamic and transport patterns reaching several 10th of meters into the subsurface, thereby distorting the classical salinity stratification. We believe that these findings are particularly relevant for sandy high-energy beaches which are commonly present at global coastlines

Dispatch of reactive power compensators in transmission grids

The phase-out of conventional power plants reduces the reactive power reserve from synchronous generators in transmission grids. Operational concepts need to be adapted to integrate reactive power compensators such as STATCOMs into voltage control schemes. This paper presents a novel two-stage optimization method for determining reactive power schedules, distinguishing manual and automatized control variables. The first stage optimizes the reactive power dispatch iteratively for the considered period using successive linear programming based on continuous variables due to relaxed integrality conditions. Afterwards, an embedded genetic algorithm re-discretizes integer variables. The second stage determines pareto-optimal reactive power schedules based on grid losses and switching frequency. Therefore, points of time are modeled as nodes and loss energies between two time-coupled points of time are modeled as arcs in a directed acyclic graph. The investigated scenario indicates that a high number of switching actions for passive compensators, such as shunt capacitors or reactors, is not necessary. Furthermore, it becomes apparent that the automated control of flexible devices like STATCOMs enables a reduction of active power losses