Micropollutant abatement in granular activated carbon (GAC) filters is governed by both sorption and biodegradation processes. Yet a comprehensive understanding of which micropollutants are biodegraded, how much biodegradation contributes to GAC's performance, and how GAC's properties affect biodegradation, remains limited. This study addresses these knowledge gaps by combining reactive transport modeling with pilot-scale experiments, investigating the fate of 45 micropollutants in parallel GAC and sand filters operated at contact times of up to 100 min. Tracer tests and modeling showed that GAC's high intragrain porosity (40%) prolongs solute-biomass contact times by a factor of 1.7 compared to sand. Higher respiration activity in GAC was attributed to prolonged contact times for biodegradation of both micropollutants and other dissolved organics. Significantly enhanced abatement at contact times exceeding 25 min suggested biodegradation for 25 of 45 micropollutants. The formation of biotransformation products further confirmed biodegradation of diclofenac and the highly sorptive compounds venlafaxine and hydrochlorothiazide exclusively in GAC. Overall, these findings demonstrate that (i) GAC enables biodegradation of more micropollutants than sand (24 vs. 16), regardless of sorption affinity; (ii) intragrain porosity prolongs solute-biomass contact times, being relevant for degradable compounds featuring little sorption; and (iii) GAC hosts functionally distinct microbial biomass capable of unique biotransformation compared to sand.

Abstract Tidal influences are rarely considered in parameter estimation procedures for coastal groundwater flow models. Yet, considering tides adds to the already high computational burden associated with such models due to the presence of density‐dependent flow. Our objective was to present a parameter estimation procedure that includes tidal overheight in coastal groundwater flow models and to analyze the appropriateness of the chosen model, process setup, and its simplifications. As an example, the procedure was applied to a model for the barrier island Norderney (Germany). The focus of the model was to replicate seasonal groundwater recharge dynamics and the influence of storm surges on the freshwater lens as well as the freshwater/saltwater interface. We applied an iterative‐ensemble smoother (PESTPP‐IES) for history matching with a model chain of a constant‐density freshwater‐lens model and a density‐dependent submodel around the island's water works. An adequate match between measured and simulated observations was obtained from history matching. Model verification was performed using a density‐dependent model of the entire island. Additional verification runs focusing on the transient dynamics of a single boundary condition each (i.e., the sea level or recharge) were compared to deconvoluted signal components of monitored, high‐resolution hydraulic head time series. This revealed that seasonal recharge patterns and the magnitude of storm surge influences were replicated well. However, the simulated hydraulic heads and interface depth underestimated observed data. We attribute this to the scarce information available regarding salinity observations and a possibly too pronounced focus on an accurate representation of tidal influences in the modeling setup.

Tide- and wave-induced recirculation in sandy beach aquifers contributes to submarine groundwater discharge (SGD), which is important for the element budget of the global ocean. Estimates of tide- and wave-induced recirculation fluxes are rare and information on its uncertainty does not yet exist. Model-based global tideand wave-induced recirculation flux and uncertainty assessments are hindered by numerical constrains of time consuming tide-resolved simulations and lack of data on relevant system parameters. Numerical constrains can be overcome by speeding up computation times using analytical and/or semi-analytical approximations. In the present work a semi-analytical approximation was developed to calculate the tide- and wave-induced recirculation in sandy beach aquifers, based on 8 critical system parameters. The solution agrees very well with corresponding tide-resolved numerical simulations including wave setup for a total of 1937 parameter combinations. In a Monte-Carlo approach the new solution was used to calculate the possible range of combined tide- and wave-induced recirculation fluxes based on existing parameter distributions from previously compiled global datasets. It was found that the median, mean and 95% quantile of these fluxes were 1.9 m2/d (m3/d per m shoreline), 8.3 m2/d, and 33 m2/d, respectively. These results agree very well with previously reported estimates of tide-induced recirculation fluxes. Under the assumption to be representative for the global coastline, they are of a similar magnitude to that of other previously calculated global SGD components, namely the fresh groundwater discharge, and the SGD flux associated with the density-driven saltwater wedge. It was also found that the tide-induced recirculation flux was most sensitive to the tidal amplitude and the horizontal hydraulic conductivity.

Coastal freshwater resources, particularly freshwater lenses, are increasingly vulnerable to saltwater intrusion due to overexploitation and climate change. Managed Aquifer Recharge (MAR) using desalinated water offers a promising countermeasure to mitigate seawater intrusion by sustaining freshwater heads and restoring natural hydraulic gradients. However, high costs and nutrient removal restrict its sustainability. A novel approach applies capacitive deionization with monovalent-selective membranes to produce monovalent-partial desalinated water (mPDW), reducing mainly Na+ and Cl-while retaining divalent cations. This study evaluates geochemical interactions during infiltration of synthetic mPDW into soils from the barrier island Langeoog (Germany) to assess the applicability of MAR with mPDW. Column experiments with beach sand and soils of the grey and brown dunes monitored major ions, pH and electric conductivity to assess hydrochemical changes linked to soil development. Across all soils, cation exchange dominated with temporary adsorption of Ca2+ and Mg2+ and desorption of Na+. Carbonate dissolution occurred in beach sand and grey dune soil as evidenced by elevated Ca2+ and HCO3-(up to 6.5 mmol/L). Conversely, brown dune soils, enriched in organic matter and Fe oxides, showed complete retention of Ca2+ and Mg2+, and reductive dissolution of Fe oxides (Fe2+ up to 2.2 mmol/L). The infiltration of mPDW slightly increased the effective cation exchange capacity in brown dune soils by 50 %. Overall, the soil type strongly controlled the hydrogeochemical response to mPDW infiltration and the grey dune is the most suitable site. These results highlight the feasibility of MAR with mPDW from geochemical and hydrological perspectives.

Micropollutant abatement in granular activated carbon (GAC) filters is governed by both sorption and biodegradation processes. Yet a comprehensive understanding of which micropollutants are biodegraded, how much biodegradation contributes to GAC's performance, and how GAC's properties affect biodegradation, remains limited. This study addresses these knowledge gaps by combining reactive transport modeling with pilot-scale experiments, investigating the fate of 45 micropollutants in parallel GAC and sand filters operated at contact times of up to 100 min. Tracer tests and modeling showed that GAC's high intragrain porosity (40%) prolongs solute-biomass contact times by a factor of 1.7 compared to sand. Higher respiration activity in GAC was attributed to prolonged contact times for biodegradation of both micropollutants and other dissolved organics. Significantly enhanced abatement at contact times exceeding 25 min suggested biodegradation for 25 of 45 micropollutants. The formation of biotransformation products further confirmed biodegradation of diclofenac and the highly sorptive compounds venlafaxine and hydrochlorothiazide exclusively in GAC. Overall, these findings demonstrate that (i) GAC enables biodegradation of more micropollutants than sand (24 vs. 16), regardless of sorption affinity; (ii) intragrain porosity prolongs solute-biomass contact times, being relevant for degradable compounds featuring little sorption; and (iii) GAC hosts functionally distinct microbial biomass capable of unique biotransformation compared to sand.

Abstract Subterranean estuaries (STEs) under high‐energy conditions function as effective biogeochemical reactors, where waterbodies of varying physico‐chemical properties mix. Chemical reactions within the STE modulate the solute composition of groundwater discharging to the sea providing for example nutrients for plankton growth to coastal ecosystems. Groundwater residence times are fundamental to understanding groundwater flow and transport regimes, and thus to interpreting (bio) geochemical data. The present study aims to exemplarily show for the barrier island Spiekeroog exposed to high energy conditions, how morphological changes of the beach surface imprint on subsurface flow, groundwater age and mixing patterns. To this end, we have combined multiple environmental tracers, including electrical conductivity, apparent tritium‐helium groundwater ages, temperature (T) and dissolved silica (Si), obtained over a period of 1.5 years from different depths and locations along a 200 m long cross‐shore transect. The results of apparent groundwater ages, travel times and seawater mixing fractions allowed delineating zones where the different endmembers were prevailing. Recirculating seawater of the upper saline plume (USP), infiltrating near the high water line, was youngest with travel times of days to weeks. Near the dune base, freshwater entering the beach from the islands interior was several years old, subsequently forming the freshwater discharge tube below the USP, and directed toward the discharge zone where decade old brackish groundwater was encountered at depths below 18 m. The results support the interpretation of geochemical data as well as the underlying processes and advance the understanding of the functioning of the biogeochemical reactor in high energy beach systems.

Although groundwater flow directions are influenced by hydraulic gradients and density gradients, density gradients are often assumed negligible outside coastal zones and saline lake environments. Density gradients are rarely considered in mining studies, but can alter groundwater flow patterns, and cause outflow of water from pit lakes despite inward hydraulic gradients. Open mine pits often intersect regional water tables, requiring dewatering during mining operations. At the end of mine life, groundwater abstraction ceases, frequently leading to the development of pit lakes. Due to prolonged water residence time, and evaporation, pit lake water quality may deteriorate with salinization being a common problem. While salinity differences between pit lake water and groundwater are small initially, they increase with time, inducing density contrasts. Consequently, dense pit lake water may move along density gradient towards less dense groundwater. This changes the flow patterns around the pit lake and impact on surrounding aquifer water quality. In this study, we advance process understanding of how density effects alter flow and salinity patterns in pit lake environments post-mining using numerical modeling. We show the impact of ambient groundwater salinity, regional hydraulic gradients, evaporation rates, and hydraulic conductivities on the interaction between a pit lake and the surrounding aquifer. We demonstrate how density effects can substantially increase lake water outflow and decrease pit lake water salinities. Pit lakes can turn from terminal sinks into throughflow systems purely due to variable-density flow. Understanding the hydraulic and salinity evolution of pit lakes is crucial for planning post mining rehabilitation.

Managed aquifer recharge (MAR) can improve water availability by enhancing storage, and water quality through biodegradation and filtration processes. Impaired water sources such as WWTP effluent often contain trace organic chemicals (TOrCs) which may have adverse effects on the environment and human health. Removing TOrCs using activated carbon or ozonation is costly and energy intensive. Instead, biodegradation of TOrCs in the aquifer can be enhanced by changing the environmental conditions e.g. via sequential managed aquifer recharge technology (SMART). SMART consists of an initial infiltration step (e.g., bank filtration) followed by an aeration step and subsequent infiltration step under oxic and carbon limited conditions. This study aims to implement SMART in a heterogeneous aquifer and demonstrate the attenuation of TOrCs. SMART was implemented on a demonstration scale at a former waterworks site in Berlin to produce raw water which could potentially be used for later drinking water production. Impaired bank filtrate is aerated and iron and manganese are removed. Then, the water is infiltrated into a 25 m long, 1 m wide, 7 m deep infiltration trench filled with gravel. Two production wells located 25 m away from the trench establish a controlled, homogeneous flow field. Another production well 60 m away hydraulically shields the system. The pumping regime extracts more water (approx. 24 m³/h) than is infiltrated (10 m³/h) to comply with permitting requirements. Hydraulic retention time confirmed by tracer tests is approximately seven days from the trench to the first two wells. Groundwater monitoring wells provide online monitoring data at different depths (groundwater level, electrical conductivity, temperature, and dissolved oxygen (DO)). Due to operational constraints, drinking water was infiltrated for approximately one year, facilitating the establishment of plug-flow conditions and an oxic zone (>1 mg/L of DO) in the subsurface. Pre-treated bank filtrate has been infiltrated since February 2024. Additionally, hydraulic and reactive transport models of the site were created to understand and confirm subsurface processes.Trace organic contaminants were removed in the flow field and can be categorized into different groups. Persistent compounds such as candesartan showed limited biodegradation potential (removal of less than 30 %). A group of easily degradable and volatile substances was already removed during pre-treatment under oxic conditions. The third group of redox-sensitive compounds such as diclofenac were better removed under the oxic and carbon-limited conditions of SMART with removal between 30 % and up to 80 % from the influent to the monitoring well located 20 m downstream of the trench.This strong attenuation demonstrates the potential of enhancing biodegradation in a heterogeneous aquifer by actively managing the in-situ redox regime to achieve sustainable TOrCs biodegradation. The demo-scale system is currently in operation and open research questions are being investigated, such as how the microbiome adapts and what is needed for a transfer to new locations. Depending on the local conditions, SMART needs to be combined with further advanced drinking water treatment steps to ensure sufficient TOrCs removal. Overall, this approach is a promising solution to implement water reuse locally.

High-energy beaches mark a highly variable land-ocean transition where matter fluxes are modulated by dynamic subsurface biogeochemical reactions. At the beach face, seawater infiltration into the saline recirculation cell of the intertidal beach aquifer creates a high input of electron acceptors and organic matter. Microorganisms rapidly degrade fresh organic matter in the upper sandy beach layer under advective flow conditions. Filtration of particulate organic matter and constant supply of oxygen (O2) in the shallow sand body result in much of this turnover taking place under predominantly oxic conditions. In temperate regions, this filter effect combined with seasonal seawater inputs results in a strong seasonality of reaction rates as well a seasonally heterogeneous distribution of rates. Additionally, subsurface transport dynamics of seawater containing biogeochemical reactants highly depends on the physical forcings such as tides, waves and the beach morphology, adding complexity to the system. We assume that the variable O2-consuming degradation processes in the upper layer in combination with dynamic physical forcing regimes lead to a fluctuating oxycline in the beach aquifer. Therefore, the aim of our study was to investigate the impact of seasonally variable oxygen demand under different physical forcing regimes on redox zonation in the beach subsurface. We used O2 consumption rates from the beach face at Spiekeroog Beach (Germany), measured down to 1m depth and over a year-long sampling campaign within the project DynaDeep, to develop a numerical reactive transport model at field scale. The results from the field data showed a strong seasonal depth dependency of O2 consumption rates. Lowest rates were found in winter and increased substantially in summer, with the strongest increase in rates in the upper decimeters.Modelling case studies for a summer and a winter situation were carried out to simulate both, quasi-stationary and dynamic conditions. Model results show that the oxic zone is significantly larger in winter than in summer, aligning with the general O2 distribution measured in the field. We found that in summer, dynamic tidal conditions lead to greater variations in O2 concentrations than in winter. In addition, the model shows that during tidal inundation, O2 can overcome the high consumption rates in the upper decimeters in summer, thereby increasing the oxic zone. Finally, the model will be used to explore the impact of additional physical forcings in order to better constrain the oxycline as a variable redox boundary for subsequent anoxic processes.

High-energy beaches receive high inputs of organic matter from seawater infiltration, fueling intensive oxygen (O2) consumption rates in the upper sand layer, which depend on seasonal variations in temperature and organic matter supply and can exceed 10 times the average subtidal rates during the growing season. Despite the intensive rates, recent studies have found deep O2 penetration underneath, extending down several meters. To investigate this anomaly, we applied a reactive transport model to simulate O2 supply and consumption for winter and summer conditions. Well-known conditions at a high-energy beach on Spiekeroog Island, Germany, were used to define topography, hydraulic conductivity, tide, and wave amplitudes, and the model was built using measured O2 distribution and O2 consumption rates. The tide-resolving model was capable of simulating the periodic tidal desaturation of the surface layer. We found that the aeration with atmospheric O2 during desaturation is a significant O2 source, contributing up to 30-60% of total O2 consumption. Meanwhile, seawater O2 quickly bypasses the upper reactive layer, extending the oxycline to depths of 11-16 m in the summer and winter, respectively. This mechanism mitigates the seasonal imprint into deeper layers yet promotes intensive aerobic OC remineralization of up to 0.7 gC m-2 d-1.

Subterranean Estuaries (STEs) are important biogeochemical reactors at the land-ocean interface. They transform dissolved species prior to discharge, thereby influencing chemical fluxes from land to sea. The coupling between physical flow and biogeochemical reactions in the STE is complex, and a deeper process understanding demands the application of reactive transport modeling (RTM). Most previous RTM studies focused on idealized STEs, investigating the impacts of relevant oceanic forcings, such as tides and waves. The aim of this study is to investigate the presently unknown interplay between STE biogeochemistry and beach morphodynamics, storm floods as well as seasonal groundwater recharge. 2-D cross-sectional RTMs for a sandy beach aquifer were developed for this purpose, assessing the effects of the three individual as well as all combined dynamic coastal forcings, respectively. We find that beach morphodynamics enhance the transience of aerobic-to-suboxic zones in near-surface groundwater, whereas storm floods cause temporal concentration changes at greater depth. The impact of seasonal groundwater recharge is less pronounced. The concentrations of dissolved species are further impacted by precipitation/dissolution of the minerals calcite, goethite, siderite, iron sulfide and hydroxyapatite as well as complexation at goethite surfaces. Our study contributes to an advanced understanding of the interplay between STE biogeochemistry and the dynamics of relevant coastal forcings encountered at high-energy beaches. However, further field-based investigations are needed to verify conclusions of our generic RTM study.

In beach aquifers two water bodies, relatively old terrestrial freshwater and young oceanic saltwater mix, biogeochemical reactions change the solute composition of the water and groundwater discharge modifies element net fluxes to the ocean. Residence times are baseline information for the biogeochemical interpretation and help to understand groundwater flow and transport regimes. In the present study we used environmental tracers, i.e. apparent tritium-helium (3H/He) ages, temperatures and silica (Si) concentrations to derive groundwater ages and travel times in the subsurface along a cross-shore transect at the high energy beach aquifer on Spiekeroog, a barrier island in North-Western Germany. Recent generic modelling studies suggested that in beach aquifers under high energy conditions, characterized by high waves and tidal amplitudes as well as seasonal storm floods, flow and transport patterns in space and time are highly variable. As a consequence, the typical salinity and age stratification is distorted as compared to the classical stable concept of water bodies in beach aquifers derived from more embayed sites. To advance the understanding of such highly dynamic systems we obtained two sets of apparent 3H/He ages one year apart at three permanently installed multilevel wells each filtered in four depths (6, 12, 18, 24 m bgs), located at the dune base, near the mean high water line and near the mean low water line respectively. At the same locations, data loggers continuously recorded groundwater temperatures and were used to calculate travel times. In addition, Si was measured in samples taken every six weeks over one year. The results show relatively young apparent 3H/He ages in all samples, ranging from weeks to approximately 18 years. The water was youngest in the shallow part and near the high water line and ages increased with depth and towards the low water line and dune base. Interestingly, 3H/He ages vary significantly at some locations in the two data sets. Temperature derived travel times, representing the young water component (from the North Sea), overall agree well with the mixed apparent 3H/He ages. Si accumulating with time shows a similar trend. In the next steps, the results will help to constrain site specific groundwater modelling and support the interpretation of geochemical data and underlying processes in order to finally better understand the functioning of high energy beach systems.

Subterranean estuaries (STEs) are biogeochemical reactors modifying the chemistry of salt- and freshwater as they flow through the subsurface sediments. Boundary conditions such as tides, waves, beach morphology, seasonal meteoric groundwater recharge and storm events control endmember mixing and residence time distributions within STEs. These in turn affect biogeochemical reactions and thus elemental fluxes discharging to the ocean via submarine groundwater discharge (SGD). Especially at high-energy beaches exposed to high tidal ranges and high wave energy, boundary conditions are very dynamic and likely imprint on groundwater flow and reactive transport within the STEs. A quantitative understanding of mixing processes and residence time distributions is necessary in order to adequately describe biogeochemical processes and can be achieved with the help of numerical modelling. Yet, transient field-scale modelling approaches calibrated to comprehensive observational data sets are still lacking, in particular for real-world high-energy STEs. In the present study, for the first time a density-dependent groundwater flow and transport model was developed and calibrated for a high-energy beach. The north beach of the barrier island Spiekeroog, northern Germany, thereby served as an example field site exposed to high-energy characteristic boundary conditions. The model was calibrated to a 1.5-year extensive dataset of groundwater heads, salinities, temperatures and 3H / He groundwater ages at various shore-perpendicular locations along the beach at depths down to 24 m below ground surface. The calibrated model is able to replicate the principal behaviour of the highly transient system and enabled the identification of hot spots of high temporal variability in the investigated state-variables. The dynamics in salinity are most intense at the in- and exfiltration locations of the tide-induced recirculating seawater. The groundwater age variability was largest seawards of the low tide mark as well as below the deep recirculating seawater cell at around 20-30 m depth near the dunes, where very old freshwater from the islands' freshwater lens mixes with young brackish water from the upper beach. Temperature variations were seasonal and confined to the upper 5-10 m below the beach. Computed saline SGD water fluxes varied considerable on daily and spring-neap time scales, as well as on the longer term, i.e., monthly to yearly time scales. The rather gradual, longer-term changes in flux appear to be mainly controlled by changes in spatial variability of the beach slope. The simulated groundwater age of the fresh SGD component varied between 4 and 25 years, and predominantly depended on the magnitude of saline SGD flux. Overall, the model provided important insights into the dynamics of the flow and transport processes.

Study region: Barrier island Langeoog at the German North Sea coast Study focus: Due to climate change it is predicted that extreme weather events such as droughts and floods will occur more often in future. Since such events strongly influence the groundwater recharge, they will likely affect the freshwater volume on many small barrier islands. This is critical where freshwater lenses are the only source of drinking water. Groundwater abstraction for drinking water supply also influences the freshwater volume. To investigate the potential near-future climate change impacts, the influence of consecutive dry or wet years on the freshwater lens combined with several pumping scenarios is calculated using a density-dependent groundwater flow and transport model. Chosen sub-annual recharge and pumping rates are based on values from the past and, therefore, considered realistic for the island. New hydrological insights for the region: Five extremely dry years would shrink the volume of the freshwater lens by 20 %. Extremely wet years cause flooding of large areas in the island interior. As the water table adjusts fast to new recharge conditions, flooding within the second winter is almost as severe as in the fifth winter. Tides and storm surges at the mesotidal coast have a great impact on the freshwater lens by constraining the area where fresh groundwater develops and affecting the water table in the interior of the island.

In high-energy beach aquifers fresh groundwater mixes with recirculating saltwater and biogeochemical reactions modify the composition of groundwater discharging to the sea. Changing beach morphology, hydrodynamic forces, and hydrogeological properties control density-driven groundwater flow and transport processes that affect the distribution of chemical reactants. In the present study, density-driven flow and transport modelling of a generic 2-D cross-shore transect was conducted. Boundary conditions and aquifer parameters were varied in a systematic manner in a suite of 24 cases. The objective was to investigate the individual effects of boundary conditions and hydrogeological parameters on flow regime, salt distribution, and potential for mixing-controlled chemical reactions in a system with a temporally variable beach morphology. Our results show that a changing beach morphology causes the migration of infiltration and exfiltration locations along the beach transect, leading to transient flow and salt transport patterns in the subsurface, thereby enhancing mixing-controlled reactions. The shape and extent of the zone where mixing-controlled reactions potentially take place, as well as the spatiotemporal variability of the freshwater–saltwater interfaces, are most sensitive to variable beach morphology, storm floods, hydraulic conductivity, and dispersivity. The present study advances the understanding of subsurface flow, transport, and mixing processes that are dynamic beneath high-energy beaches. These processes control biogeochemical reactions that regulate nutrient fluxes to coastal ecosystems.

Coastal aquifers build the transition zone of freshwater and saltwater. Hence, large salinity gradients are encountered in the subsurface below beaches and it is important to assess the salinity in a high resolution in order to understand coastal groundwater flow dynamics and consequently geochemical and microbial processes in subterranean estuaries. Within the project DynaDeep, we used both geophysical and hydrogeological methods to determine the bulk and fluid electrical conductivities (bulk/fluid EC) with the aim to convert the EC to salinity to monitor its temporal and spatial changes. This was done at a high-energy beach on the North Sea Island of Spiekeroog.Numerous EC techniques have been used to acquire a unique dataset since 2022, covering a 2D transect from the dune base to the low water line. The site was subject to strong topographic changes over the seasons. Among the methods applied, we used electrical resistivity tomography (ERT) to get access to 2D distributions every six weeks. Additionally, continuous monitoring was carried out using a saltwater monitoring system (SAMOS) with a vertical electrode chain down to a depth of 20 meters located at the high water line. Direct push (DP) data at various locations as well as fluid EC values from water samples gathered via DP give access to high resolution information. In three multilevel wells (four levels each at 6, 12, 18, and 24 meter depth below ground) we logged the fluid EC and temperature and took water samples on a regular basis.For an especially dense dataset between January and March 2023 we compared in detail the applied EC methods and found a general agreement in all of the gathered data after suitable calibration and temperature correction. We furthermore derived a formation factor model for the conversion to salinity.Finally, we a combined inversion of the ERT data with the additional data aiming for fluid EC directly under the assumption of this temporally fixed formation factor model. In contrast to standard inversion techniques, this allowed for a naturally occurring smooth transition of salinities over the different geological units, which was critical when analyzing the spatial and temporal changes.

Aquifers beneath sandy beaches act as land-ocean conduits for groundwater and are active biogeochemical reactors modifying chemical fluxes across the land-sea interface. Subterranean estuaries of high-energy beaches with large tidal and wave amplitudes could be particularly reactive due to the exchange of large seawater volumes and transport of marine derived constituents deep into the subsurface. In this study, we first present a new classification for coastal energy regimes as a function of mean tidal range and mean significant wave height and define the term “high-energy”. We establish a global distribution map of coastal energy regimes and classify porewater study sites in sandy beach aquifers related to their prevalent energy regime. Despite their extensive contribution to the global shoreline, the porewater biogeochemistry of high-energy environments is largely unknown. Through a summary of the few existing porewater studies at high-energy beaches we reveal patterns in morphology, hydrology, and biogeochemistry, describe promising research strategies, and highlight future research avenues in these challenging environments.

Parameter estimation for coastal aquifers generally is a time-consuming and computationally expensive task. It requires compromises on the number of parameters to estimate as well as how and if to incorporate variable-density driven flow and transport. While locations in coastal aquifers where certain data types (e.g., hydraulic head or salinity data) are most informative for inverse modeling have been established, data at such locations may not be routinely collected. This adds difficulty to the process of estimating a unique parameter set for a coastal aquifer.A further challenge at coastal sites influenced by ocean tides and episodic sea-level variations (e.g., caused by storm events) is the consideration of tidal overheight, which can elevate the groundwater table well above mean sea level. Due to the computational expenses of simulating tidal influences in regional-scale groundwater flow models, tidal overheight is often neglected in such models. During a parameter estimation procedure, the neglected tidal overheight would be erroneously compensated for by lower hydraulic conductivities to match observed groundwater levels.Our objective was to include the effects of tidal overheight in a parameter estimation procedure to characterize the hydraulic properties of the island aquifer below Norderney (Germany). For this purpose, a phase-averaged tidal boundary condition and routinely collected groundwater observation data were used. The model was implemented in MODFLOW-2005 and depicts the freshwater lens of the island as a steady-state groundwater flow model. The freshwater/saltwater interface, estimated using the Ghyben-Herzberg relation, is assumed a no-flow boundary due to a lack of salinity data describing the transition zone between fresh- and saltwater. Observed data were hydraulic heads averaged over a time frame of 10 years (2006-2015) and respective vertical differences in hydraulic heads at multi-level observation wells. Observation weights were defined based on measurement uncertainty and standard error of the mean.Parameter estimation was performed using PESTPP-GLM with Tikhonov regularization and first-order second-moment (FOSM) uncertainty analysis. Estimated parameters were: horizontal hydraulic conductivities and anisotropy factors for different zones based on a hydrogeological structural model for the island; conductances for river and drainage boundary conditions, which describe surface waterbodies and drainage channels present on the island; water levels for the river boundary condition; a scaling factor for production well skin sediment.Results suggest that simulated heads match observed heads reasonably well, while prior parameter uncertainties were only reduced for horizontal hydraulic conductivities and vertical anisotropy factors of certain zones. The observed head data show pronounced variability on a smaller scale likely originating from locally present confining clay lenses and areas of lower permeability, which are known to exist from borehole data. For validation, transient simulations were performed with MODFLOW-2005 and the saltwater intrusion package (SWI2) to simulate the salt-/freshwater interface.

Subterranean estuaries below high-energy beaches are understudied, despite being potential powerful biogeochemical reactors at the land/sea transition zone affecting the quality of coastal waters. Highly transient hydro(geo)logical boundary conditions and density-effects lead to dynamic subsurface flow and transport patterns which are difficult to understand and hard to replicate by models. A comprehensive and unique 1-year dataset of hydraulic heads, salinity and temperature data in combination with apparent 3H/He ages was obtained at a beach research site on Spiekeroog Island in North Germany. The site includes 3 multilevel groundwater monitoring wells and a vertical electrode chain with 10 temperature sensors, all positioned on a transect aligned along the principal cross-shore flow direction and all reaching down to 24 m depth below ground surface. The data-set was used to set up and calibrate a site-specific groundwater flow and transport model, aiming to approximate the highly dynamic groundwater flow patterns on that transect. The simulation time needed to be 20 years because of the long model spin-up. Due to the complex and nonlinear nature of the system, model calibration was carried out via particle swarm optimization, which is superior to gradient-based optimization techniques with respect to finding a global minimum of the objective function. The calibration results were reasonable. The dynamics of hydraulic head data were well captured, however, simulated values were constantly higher than those observed. The observed salinities were best captured for the multilevel wells near the mean high water and low water line. At the highest multilevel well located at the upper beach right at the dune base, simulations matched observations less well. Similarly, groundwater temperatures and ages were best replicated at the location in the infiltration zone near the high-water line. Groundwater ages and their temporal dynamics at the dune base and mean low water line could only be replicated down to 12 m depth. Deviations between simulations and observations are likely due to 3D flow effects in longshore direction, which could not be captured with the 2D vertical cross-sectional model approach. However, long model run times hindered calibration of a full-blown 3D density-dependent, 20-year long-term groundwater flow and transport model. The next step is to estimating the importance of longshore hydraulic gradients. Finally, the model will be extended for hydrobiogeochemical reactions to assist in the analysis and understanding of the observed hydrochemical data at this site.