Radiocarbon has been used in a number of studies to establish the age of intruded seawater, yet there appears to be a paucity of investigations that has attempted to quantify the processes that influence radiocarbon along the flow path of actively intruding seawater. The present study contributes to filling this research gap using radiocarbon and hydrochemical data of intruding North Sea water along two shore-perpendicular transects across the coastal dune belt of the western Netherlands. The objective is to establish how well radiocarbon can be used to determine the intruding seawater's age, considering the corrections that are required to account for geochemical reactions and mixing. The effect of geochemical reactions was quantified for each of the 18 samples of intruded seawater based on a chemical mass balance calculated with a new Excel based code (R + SWi). It considers 20 quality parameters (including delta C-13(DIC) and C-14(DIC)) and 15 reactions and utilizes Excel's Solver routine to calibrate the model parameters. The reactions along the flow path are initially dominated by O-2 and NO3 reduction by sediment organic matter in seafloor sediment, with little CaCO3 dissolution. Next, during passage of a Holocene tidal aquitard, Fe(OH)(3) and some SO4 are reduced, pyrite and CaCO3 precipitate, and opaline SiO2 dissolves. In deeper, Pleistocene aquitards and further downgradient in aquifers, cations are exchanged, some CaCO3 precipitates due to Ca exchange, and siderite dissolves. Correcting radiocarbon concentrations for the calculated sedimentary carbon contributions yields ages between 0.37 and 6 ka. Sensitivity analysis reveals that the corrected C-14 age is most sensitive to the assumed C-14 activity of the carbonate and organic matter of the Holocene seabed sediments. The intruded seawater's age appears to be bimodal: Old seawater (3-6 ka) intruded when the coastline was located much further to the east than at present. Younger seawater (<3 ka) started to intrude after the freshwater lens developed when the coastline had reached its present-day position. Groundwater extraction and especially the reclamation of the Haarlemmermeer lake accelerated intrusion rates massively. The results of a 3.5 ky numerical model simulation of freshwater lens formation and seawater intrusion are consistent with the radiocarbon age pattern inferred from the samples. The spatial distribution of seawater ages indicates a higher vulnerability to salinization in the northern part of the study area, highlighting the added value of radiocarbon data of saline groundwater for water resource management purposes.

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.

Saltwater intrusion is the greatest risk to coastal community water supplies where they are dependent on fresh groundwater as the main source of supply. For small, fractured bedrock island aquifers, the fresh groundwater lens dynamics and transition zone geometry are complex. This study investigated the impacts of projected increases in groundwater pumping on a fresh groundwater lens to evaluate changes to the lens geometry and localized up-coning from the deeper, more saline aquifers beneath a small bedrock island in the tropics. A combination of traditional hydrogeological datasets and an airborne electromagnetic survey were used to develop a three-dimensional density-dependent groundwater flow and solute transport model using the SEAWAT code. This investigation represents one of very few studies that have taken such an approach. The model was calibrated using observed groundwater hydraulic heads and chloride concentration data, and calculated chloride values based on bulk conductivity measurements determined from inverted geophysical data. A staged calibration approach was adopted, firstly assessing the time-average lens extent and geometry, and secondly considering the seasonal groundwater level response. In the calibration, the geophysical data helped constrain the lens geometry in the absence of hydraulic head and chloride data. The calibrated model was used to test scenarios where groundwater pumping rates were increased above the current demand of 452 m3 d-1, showing that the lens is likely to be stable, i.e., its available storage is not expected to contract excessively, for extraction rates of up to 3,000 m3 d-1. The combined use of geophysical data and a numerical modeling approach was advantageous in investigating the lens characteristics. It also demonstrated how these techniques can be used together to evaluate coastal water resources and to manage water supply risks for coastal communities. The study demonstrated that the freshwater lens can likely support the freshwater demands of the remote community, and is a preferred option compared with high-cost and more complicated options such as seawater desalination and managed aquifer recharge.

This study aimed to identify ocean- and land-based sources of nutrients to the coral reef communities surrounding the Southern Caribbean islands Aruba, Bonaire, and Curaçao (ABC islands). The composition of water masses around these islands were assessed to depths up to 300 m and three distinct overlying water masses were identified, separated by mixing zones. A fluctuating pycnocline separating surface from deeper (>∼50 m) water indicated the presence of internal waves. Nutrient profiles were typical of tropical waters with oligotrophic waters occurring above the pycnocline and a deep chlorophyll-a maximum (DCM) just below it (∼65 m). Concentrations of dissolved nutrients differed among islands. Inorganic nitrogen (DIN) and phosphate concentrations were respectively lowest around Bonaire and Curaçao. The spatial distribution of chlorophyll-a (indicative of phytoplankton biomass), rather than nutrient concentrations, suggested the presence of higher-than-average nutrient concentrations in islands with higher population densities and near urbanized/industrial areas.

Subterranean estuaries (STE) are hotspots of biogeochemical reactions. Here, dissolved constituents in waters of terrestrial and marine origin are transformed before they discharge to the coastal oceans. The involved biogeochemical reactions are complex and non-linear, calling for the application of numerical reactive transport modeling (RTM) to improve the process understanding. The aim of this study was to assess the roles of organic matter degradation and coupled secondary mineral reactions for the fate of dissolved species in STEs of sandy beaches. A comprehensive RTM approach was applied for this purpose, accounting for the effects of ion activities, pH, pe, redox reactions, mineral equilibria (calcite, goethite, siderite, iron sulfide, hydroxyapatite and vivianite) as well as surface complexation. Results show that the STE biogeochemistry and associated species fluxes are very sensitive to the assumed reaction network. For example, inorganic carbon and pH were largely controlled by calcite and siderite dynamics, and dissolved Fe2+ and HS- were precipitated as goethite, siderite and/or iron sulfides. Moreover, PO43- concentrations were affected by both the formation of vivianite or hydroxyapatite as well as surface complexation. This work helped to establish the relative importance of some of the major biogeochemical processes in the STE. However, further field studies are needed to understand which processes play a role in real-world STEs, including an exploration of the deep subsurface of STEs. Such field-based observations will improve our conceptual process understanding, which is key to developing well-constrained RTMs.

The travel time of groundwater plays a major role in the understanding of hydrogeological systems; however, large data sets necessary for regional studies of groundwater age are rare. In this study, a unique large data set of groundwater samples analysed for tritium and helium isotopes collected over the last 20 years from Cenozoic aquifers of the North German Plain is explored. Hereby, the variety of natural and technical influences on the tritium-helium age, including screen depth and length, groundwater recharge rate and climatic effects, are investigated. To a sampling depth of ~40 m below ground level, the median tritium-helium age increases almost linearly with depth, reaching a maximum of 40 years. Below, the portion of older, tritium-free water rises. The tritium-helium ages of the tritium-bearing portion increase only slightly to a maximum of about 46 years. The depth distribution of the tritium-helium age shows a dependency on groundwater recharge rates. Considering the same depth level, younger ages are related to higher groundwater recharge rates as compared to groundwater that infiltrated in areas with lower recharge rates. This is especially observed for shallow depths. Tritium-helium ages younger than 40 years are reflected well in the atmospheric tritium input curves, while deviations from it can be related to anthropogenic influences such as input from nuclear power plants and irrigation with deep, tritium-poor groundwater. The regional distribution for shallow wells indicates increasing tritium-helium ages from west to east, corresponding to decreasing groundwater recharge rates due to the more continental climate in the east.

Subterranean estuaries are connective zones between inland aquifers and the open sea where terrestrial freshwater and circulating seawater mix and undergo major biogeochemical changes. They are biogeochemical reactors that modify groundwater chemistry prior to discharge into the sea. We propose that subterranean estuaries of high-energy beaches are particularly dynamic environments, where the effect of the dynamic boundary conditions propagates tens of meters into the subsurface, leading to strong spatio-temporal variability of geochemical conditions. We hypothesize that they form a unique habitat with an adapted microbial community unlike other typically more stable subsurface environments. So far, however, studies concerning subterranean estuaries of high-energy beaches have been rare and therefore their functioning, and their importance for coastal ecosystems, as well as for carbon, nutrient and trace element cycling, is little understood. We are addressing this knowledge gap within the interdisciplinary research project DynaDeep by studying the combined effect of surface (hydro- and morphodynamics) on subsurface processes (groundwater flow and transport, biogeochemical reactions, microbiology). A unique subterranean estuary observatory was established on the northern beach of the island of Spiekeroog facing the North Sea, serving as an exemplary high-energy research site and model system. It consists of fixed and 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, interdisciplinary field campaigns and experimental work, all of which are integrated via mathematical modelling to understand and quantify the functioning of the biogeochemical reactor. First results show that the DynaDeep observatory is collecting the intended spatially and temporally resolved morphological, sedimentological and biogeochemical data. Samples and data are further processed ex-situ and combined with experiments and modelling. Ultimately, DynaDeep aims at elucidating the global relevance of these common but overlooked environments.

The new isotope module in HYDRUS‐1D can be used to infer the origin of root water uptake (RWU), a suitable dynamic indicator for agriculture and forest water management. However, evidence shows that the equilibrium fractionation between liquid water and water vapor within the soil is affected not only by soil temperature but also by soil tension. How soil tension affects isotope transport modeling and interpretations of the RWU origin is still unknown. In this study, we evaluated three fractionation scenarios on model performance for a field data set from Langeoog Island: (a) no fractionation (Non_Frac), (b) the soil temperature control on equilibrium fractionation as described by the standard Craig‐Gordon equation (CG_Frac), and (c) CG_Frac plus the soil tension control on equilibrium fractionation (CGT_Frac). The model simulations showed that CGT_Frac led to more depleted isotopic compositions of surface soil water than CG_Frac. The vertical origin of RWU was estimated using the water balance (WB) calculations and the Bayesian mixing model (SIAR). While the former directly used water flow outputs, the latter used as input simulated isotopic compositions (using different fractionation scenarios) of RWU and soil water. Both methods provided similar variation trends with time and depth in different soil layers' contributions to RWU. The contributions of all soil layers interpreted by the CGT_Frac scenario were always between Non_Frac and CG_Frac. The temporal origin of RWU was deduced from particle tracking (PT, releasing one hypothetical particle for individual precipitation event and tracking its movement based on the water balance between particles) and a virtual tracer experiment (VTE, assigning a known isotope composition to individual precipitation event and tracking its movement based on the cumulative isotope flux). Both methods revealed similar variation trends with time in drainage and root zone (RZ) travel times. The interpreted drainage and RZ travel times were generally ranked as Non_Frac > CGT_Frac > CG_Frac. Overall, the factors considered in the standard CG equation dominated isotope fractionation, transport, and interpretations of the RWU origin. Isotope transport‐based methods (SIAR, VTE) were more computationally demanding than water flow‐based methods (WB, PT).

The travel time of groundwater plays a major role in the understanding of hydrogeological systems; however, large data sets necessary for regional studies of groundwater age are rare. In this study, a unique large data set of groundwater samples analysed for tritium and helium isotopes collected over the last 20 years from Cenozoic aquifers of the North German Plain is explored. Hereby, the variety of natural and technical influences on the tritium-helium age, including screen depth and length, groundwater recharge rate and climatic effects, are investigated. To a sampling depth of ~40 m below ground level, the median tritium-helium age increases almost linearly with depth, reaching a maximum of 40 years. Below, the portion of older, tritium-free water rises. The tritium-helium ages of the tritium-bearing portion increase only slightly to a maximum of about 46 years. The depth distribution of the tritium-helium age shows a dependency on groundwater recharge rates. Considering the same depth level, younger ages are related to higher groundwater recharge rates as compared to groundwater that infiltrated in areas with lower recharge rates. This is especially observed for shallow depths. Tritium-helium ages younger than 40 years are reflected well in the atmospheric tritium input curves, while deviations from it can be related to anthropogenic influences such as input from nuclear power plants and irrigation with deep, tritium-poor groundwater. The regional distribution for shallow wells indicates increasing tritium-helium ages from west to east, corresponding to decreasing groundwater recharge rates due to the more continental climate in the east.

Subterranean estuaries (STE) below beaches are biogeochemical reactors that modify the composition of fresh meteoric groundwater and recirculating seawater before they enter the ocean via submarine groundwater discharge (SGD), which can affect coastal ecosystems. Thereby, prevailing redox conditions have a major impact on the concentrations and mass fluxes of water constituents, e.g., nutrients, metals and organic molecules within the STE. Due to the transient nature of the flow and transport within STEs as well as the variable hydrogeochemical boundary conditions, redox zoning in the STE is likely highly dynamic. Elucidating the factors that affect redox zoning and its dynamics is essential for the interpretation and understanding of hydrogeochemical data and the prediction of coastal solute fluxes. In the present study we investigated the individual and combined effects of storm floods, seasonal changes of temperatures and groundwater recharge rates, as well as beach morphodynamics on the redox behavior, i.e., redox zoning in the STE in a generic modelling approach. A 2D cross-shore density-dependent flow and reactive transport model was set up for this purpose, mimicking a beach aquifer exposed to high-energy conditions due to high tides, waves and storm floods. The results of this study show that redox dynamics can occur well down to a depth of 20 m. Morphodynamics were shown to be the most important factor for redox zoning in the STE. For cases where morphodynamics are less pronounced, e.g., at low-energy sites, storm floods and the seasonal temperature changes may be dominating. Seasonal changes in meteoric groundwater recharge rates seem to be least relevant for the redox dynamics in STEs. The results of the present study increase the understanding of STEs as biogeochemical reactors.

Redox conditions are a major control for the concentrations and mass fluxes of water constituents, e.g., nutrients, metals and organic molecules within subterranean estuaries (STE). Due to the transient flow and transport processes in STEs as well as the variable temperatures and input of redox reactants, redox zoning in the STE is believed to be highly dynamic. In the present study we analyzed the individual and combined effects of storm floods, seasonal changes of temperatures and groundwater recharge rates, as well as beach morphodynamics on the redox zoning in the STE. For this purpose, a 2D cross-shore density-dependent flow and reactive transport model was set up representing a beach aquifer exposed to high-mesotidal and medium-to high-energy wave conditions, as well as to storm floods. The simulation results show that under the given conditions, redox dynamics may occur down to a depth of 20 m. Morphodynamics appear to be the most important factor for the transience of redox zones compared to the other factors. Seasonal changes in meteoric groundwater recharge rates appear to be least relevant for the redox dynamics in STEs.

Coastal forcing including waves lead to complex and dynamic mixing between discharging groundwater and recirculating coastal water in permeable unconfined nearshore aquifers. This mixing can drive biogeochemical transformations that affect the fate of pollutants in nearshore aquifers and their ultimate discharge to coastal waters. The objective of this study was to examine the effect of time-varying wave conditions on the fate of nitrogen in a freshwater unconfined nearshore aquifer. This was achieved by conducting numerical groundwater flow and reactive transport simulations using MODFLOW-NWT 2005 with a new modified wave-PBC package together with PHT3D v2.10. The simulations demonstrate that time-varying wave conditions enhance mixing in the nearshore aquifer compared to simulations that only consider constant wave conditions, and in doing so impact nitrogen transformations. For the conditions and reaction network simulated, time-varying wave conditions resulted in up to 99% of nitrogen removal in the nearshore aquifer due to the intensified mixing and increased delivery of organic matter into the mixing zone. Sensitivity analysis shows that the abundance and reactivity of coastal-derived organic matter plays a key role in nitrogen removal in aquifers along wave-influenced shorelines. The simulations highlight the importance of considering time-varying wave conditions when designing field investigations and analysing field data. The findings have implications for prediction and estimation of chemical fluxes to the coastal waters and also suggest that increasingly frequent storm events and higher wave conditions as the climate changes may modify pollutant fluxes from nearshore aquifers to coastal waters.

The creation of artificial dunes for coastal protection may have important consequences for freshwater lenses in coastal aquifers. The objective of this study was to compare the recharge processes below such a young dune with scant vegetation to an older dune covered by grass and herbaceous vegetation. To this aim, soil and water samples were collected from the unsaturated zone at two sites on Langeoog Island in northern Germany, and the soil water was analysed for stable water isotopes and chloride. Recharge rates were calculated by using a new version of HYDRUS-1D, which was modified to simulate isotope fractionation during evaporation. Both the model outcomes and the data highlight the importance of fractionation, which is slightly more pronounced at the older, more vegetated dune. At the newly constructed dune, vegetation dieback seemingly reduces the importance of transpiration during summer. Recharge occurs year-round, albeit predominantly during the winter months. Calculated recharge rates are consistent with lysimeter measurements, but are significantly higher than previously reported rates based on groundwater age data, which is primarily attributed to the absence of dune shrub at the sites investigated here. More data are needed to establish the importance of soil-water repellency and overland flow. Based on the results, it is proposed that repeated isotope sampling can yield important insights into the dynamics of recharge processes, including their response to climate change.

This book provides a detailed description of how Python can be used to give insight into the flow of groundwater based on analytic solutions. Starting with simple problems to illustrate the basic principles, complexity is added step by step to show how one-dimensional and two-dimensional models of one or two aquifers can be implemented. Steady and transient flow problems are discussed in confined, semi-confined, and unconfined aquifers that may include wells, rivers, and areal recharge. Special consideration is given to coastal aquifers, including the effect of tides and the simulation of interface flow. Application of Python allows for compact and readable code, and quick visualization of the solutions. Python scripts are provided to reproduce all results. The scripts are also available online so that they can be altered to meet site-specific conditions. This book is intended both as training material for the next generation of university students and as a useful resource for practitioners. A primer is included for those who are new to Python or as a refresher for existing users.

We have better maps of the surfaces of Venus, Mars, and the Moon than of the Earth’s seafloor. There is even less information available about the geologic structure below the seafloor. In particular, the transition zone deep beneath and crossing the coastline is a very poorly studied frontier resulting from limitations of technology and logistical barriers. Here, we point out the significance of this region for understanding fundamental geologic processes, geohazards, and especially coastal aquifers. One prominent example is the increasing awareness of the importance of groundwater exchange between land and sea. This Perspective defines the region beneath the coastal transition zone, or coastal white ribbon as an underexplored frontier, and highlights the need for characterization of this critical region to depths of tens of km. We discuss available geophysical methods and their limitations with coastal groundwater used as the primary illustration. Advances in geophysical and drilling technology, coupled with numerical modeling, are needed to enable better accounting of this poorly understood component of the geosphere.