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Characterization of the spatial distribution of geomaterials and of the associated attributes is a key step associated with the set up of a hydrogeological model. Geological information are often used as a basis for this purpose. One of the most common sources of geological information is provided by available borehole data. However, geological and hydraulic information are often available at different scale. In most cases hydraulic parameters are only measured at point locations, e.g. based on pumping test, which cannot be directly transferred into 3D large-scale parameter fields. However, in some regions even geological information are scare. In such situations, information about aquifer facies and material groups need to be interpolated and serve then as a base to derive key hydraulic parameters, such as hydraulic conductivity, or transport parameters, such as porosity, diversity or reactive surfaces. Sedimentary descriptions are usually achieved when drilling a borehole. Classification of sediments rests on a well defined procedure and provides a preliminary assessment on particle size distributions of the samples analyzed. Based on sedimentary descriptions of the boreholes we construct synthetic particle size distribution curves. These particle size distribution curves can be used to calculate major local attributes of the system (e.g., hydraulic and some specific transport parameters). Based on these types of readily-available information this study aims at developing a procedure to assist construction of a high resolution geological model suitable to be transferred into a flow and transport model that is then used for water resources management issues. We therefore aim to estimate storage and transmissivity with a high reliability by accounting for the material composition in the interpolated space. We rely on a compositional data analysis framework and represent particle size fractions associated with a given location as a compositional vector. These vectors are then projected onto a computational grid through compositional kriging to characterize the spatial heterogeneity of the system. We compare these results against an approach that is based on clustering the ensuing information to obtain distinct geomaterial classes and then assess their spatial distribution through indicator kriging. After the 3D field of grain size distribution curves is generated, they are transferred into hydraulic parameter. Although the process of clustering and using material classes is inevitably associated with a loss in information the procedure of forming a representative particle size distribution around the compositional clusters attempts to keep this loss of information at a minimum. The benefit of interpolating the compositional data instead of directly interpolating inferred parameters is that the particle size distribution curves contain a huge set of information from hydraulic to transport and reactive parameters, which would be lost using hydraulic conductivity exclusively, while the use of material classes increases the efficiency of the calibration of the groundwater model.

Induced bank filtration is a known method for sustainable drinking water production in regions with limited groundwater resources. However, this method is at risk from surface water contaminations, e.g., by pathogens. Numerical models simulating pathogen fate in groundwater are typically too complex to be used as standard tools by waterworks managers or environmental agencies. To mitigate this problem, we present a methodology for the construction of easy-to-use reduced order models as surrogates for complex numerical reactive transport models for pathogens and pathogen indicators in induced bank filtration.First, a streamlined one-dimensional numerical model was set up for the reactive transport of pathogens and pathogen indicators in induced bank filtration. Processes in the model include advection-dispersion, inactivation, attachment to and detachment from the sediment as well as straining and the presence of a clogging layer. Model parameters are divided into two groups: Group A includes site specific parameters for which values are typically available (with limited uncertainty) for management- and engineer-level users (e.g., grain size, distance of extraction well to the river); Group B includes process parameters which are typically affected by high uncertainty (e.g., inactivation and detachment coefficients).We rely on an extensive dataset for coliforms and somatic coliphages collected over a 16-month period at an active induced bank filtration site. Stochastic inversion is used to assess uncertainty for model parameters of Group B (constrained on the dataset), while those of Group A are set to the values of the specific site. Principal component analysis (PCA) is applied to reduce the dimensionality of model parameter space and correlation amongst the uncertain parameters of Group B. A surrogate model is then constructed through generalized polynomial chaos expansion (gPCE). In this, the value range of Group A parameters is based on typical scenarios for induced bank filtration sites, while the range of the PCA-reduced Group B parameters is set to the uncertainty identified in the stochastic inversion.The surrogate model allows to evaluate, at significantly reduced computational cost, the removal of coliforms and somatic coliphages in induced bank filtration based on user-defined values for parameters of Group A, but also including the uncertainty stemming from parameters of Group B. The surrogate model estimates for removal are in good agreement with observed removals for coliforms and somatic coliphages at the monitored site and with other (albeit limited) datasets from induced bank filtration sites found in the published literature. At this stage, the obtained surrogate model can be considered as a prototype. The assessment of its full potential requires additional extensive validation against other field sites. In general, surrogate models together with the overall methodological framework we propose, can be seen as a promising tool to assist informed decisions about pathogen transport at induced bank filtration sites.

Climate simulations indicate that the Mediterranean region will be severely affected by climate change and is often referred to as the most prominent climate change hotspot (Gao and Giorgi, 2008). This study addresses the combined effects of climate change and three groundwater consumption scenarios on the water availability of the Western Mountain Aquifer (WMA) in Israel and the West Bank. Generally applied methods to quantify recharge and water resources rely on linear regressions or simplified models, such as data-driven approaches (i.e., lumped-parameter and black-box approaches). However, they are unfit to assess climate change impacts because the predictive power of data-driven approaches is low, should the variability of, e.g., precipitation expand beyond historically observed fluctuations, such as expected from climate change effects. Furthermore, they do not honor the physics of flow. Therefore, assessing the impact of climate change requires the application of distributed process-based numerical models that incorporate as many relevant physical flow processes as feasible. For example, when karstified vadose zones measure several hundreds of meters, such as in the case of the WMA, variably saturated flow is a highly relevant flow process controlling vadose storage at large timescales and altering recharge flux at the “control plane” groundwater table.We simulate the complex dynamics of the dual-domain infiltration and partitioning of the precipitation input signal by employing HydroGeoSphere (HGS) for transient variably saturated water flow. Flow in the limestone rock matrix and secondary high porosity system (i.e., conduits and fractures) is modeled by overlapping individual continua based on the bulk effective Richards’ equation with van Genuchten (VG) parameterization. The model input of this study stems from two coherent dynamically downscaled high-resolution regional climate projections (daily, 3km, and 8km resolution) until the year 2070, assuming the IPCC RCP4.5 climate change scenario. The results indicate that long-term average recharge quantities will decrease by circa 10 % compared to the reduction in average precipitation by 30 %. The mitigated impact on recharge is an effect of the pronounced heterogeneity of karstic flow (i.e., preferential recharge along with karst dissolution features) and increased intensity of individual rainfall events, emphasizing the need to apply spatiotemporally resolved climate models with daily precipitation values as input to the recharge assessment. However, despite the comparatively moderate decrease in recharge, the length and severity of consecutive drought years with low recharge values are likely to increase while freshwater demand is believed to increase during these periods, emphasizing the need to adjust the current management practices to climate change. Finally, the model is used to simulate managed aquifer recharge applications to mitigate the effects of more extended drought periods by strategic freshwater reserves.

Riverbank filtration is an established and quantitatively important approach to mine high-quality raw water for drinking water production. Bacterial fecal indicators are routinely used to monitor hygienic raw water quality, however, their applicability in viral contamination has been questioned repeatedly. Additionally, there are concerns that the increasing frequency and intensity of meteorological and hydrological events, i.e., heavy precipitation and droughts leading to high and low river levels, may impair riverbank filtration performance. In this study, we explored the removal of adenovirus compared with several commonly used bacterial and viral water quality indicators during different river levels. In a seasonal study, water from the Rhine River, a series of groundwater monitoring wells, and a production well were regularly collected and analyzed for adenovirus, coliphages, E. coli, C. perfringens, coliform bacteria, the total number of prokaryotic cells (TCC), and the number of virus-like particles (TVPC) using molecular and cultivation-based assays. Additionally, basic physico-chemical parameters, including temperature, pH, dissolved organic carbon, and nutrients, were measured. The highest log10 reduction during the >72 m of riverbank filtration from the river channel to the production well was observed for coliforms (>3.7 log10), followed by E. coli (>3.4 log10), somatic coliphages (>3.1 log10), C. perfringens (>2.5 log10), and F+ coliphages (>2.1 log10) at high river levels. Adenovirus decreased by 1.6–3.1 log units in the first monitoring well (>32 m) and was not detected in further distant wells. The highest removal efficiency of adenovirus and most other viral and bacterial fecal indicators was achieved during high river levels, which were characterized by increased numbers of pathogens and indicators. During low river levels, coliforms and C. perfringens were occasionally present in raw water at the production well. Adenovirus, quantified via droplet digital PCR, correlated with E. coli, somatic coliphages, TCC, TVPC, pH, and DOC at high river levels. At low river levels, adenoviruses correlated with coliforms, TVPC, pH, and water travel time. We conclude that although standard fecal indicators are insufficient for assessing hygienic raw water quality, a combination of E. coli, coliforms and somatic coliphages can assess riverbank filtration performance in adenovirus removal. Furthermore, effects of extreme hydrological events should be studied on an event-to-event basis at high spatial and temporal resolutions. Finally, there is an urgent need for a lower limit of detection for pathogenic viruses in natural waters. Preconcentration of viral particles from larger water volumes (>100 L) constitutes a promising strategy.

Overpumping or overexploitation of groundwater is one of the major threats for aquifer systems in arid and semi-arid areas. Managed aquifer recharge (MAR) has been suggested by many researchers as a sustainable and effective method to alleviate negative impacts of overpumping. Optimizing artificial recharge considers the selection of suitable MAR sites in terms of surface and subsurface characteristics. While surface characteristics at potential MAR sites could be modified (e.g. slope, soil texture, etc.), subsurface characteristics cannot be changed through engineering work. Characteristics of the aquifer, such as depth to groundwater, play an important role in determining the capability of an aquifer to store a specific volume of infiltrated water. Currently, only a limited number of quoted researches are available that consider factors related to aquifer characteristics and the range of these factors to identify optimal MAR sites. In this study, a new approach is presented, that employs numerical groundwater modeling to generate MAR suitability maps considering sub-surface characteristics, such as depth-to-groundwater, aquifer transmissivity and specific yield. Multiple model-runs are conducted to simulate groundwater table response with respect to the volume of infiltrated water. Simulation results are used to calibrate a groundwater mound empirical equation that calculates the groundwater level increase as a function of the transmissivity and infiltrated water volume for a given value of aquifer’ specific yield, range of vertical hydraulic conductivities and a specific design and operation conditions of the MAR system. The empirical equation is employed in GIS to spatially calculate the height of groundwater mound beneath a hypothetical MAR site and to generate, based on that, suitability maps for MAR implementation. Assuming that MAR structures capture the median of monthly surface runoff rates at the respective wadi (catchment area), suitability maps are generated for different configurations/scenarios of aquifer hydraulic conductivity in a parameter study. The results highlight the importance of integrating aquifer characteristics (geometry and hydraulic parameters) and expected magnitudes and fluxes of infiltration water in delineating suitable sites for MAR.

The Azraq basin, Jordan, is characterized by high variability and a heterogeneous spatial distribution of groundwater recharge. Most of the recharge occurs in the north of the basin. The central part of the basin is covered by wetland (Azraq Oasis), characterized by a unique ecosystem and biodiversity. Groundwater abstraction for agricultural and domestic purposes occurs mainly near the oasis and results in a considerable decline of the water table. A groundwater model was developed for the Azraq basin to analyze and predict the groundwater level response to different pumping strategies. The model also enables determination of the safe yield. The extent to which the safe yield and specific pumping strategies are socially and economically acceptable was analyzed. Multi-criteria analysis using an analytic hierarchy process was applied to obtain the abstraction rate that achieves the most suitable trade-off between environmental and socio-economic criteria under two scenarios of future socio-economic development. Based on the calibrated groundwater model the response of the groundwater level until 2045 was simulated, resulting in a 15–25 m decline if current pumping strategies continue. At present, the safe yield of the groundwater resource equals 30% of the overall actual pumping rate, indicating that the aquifer is highly exploited. Results of the multi-criteria analysis show that the ‘safe yield alternative’ is the most suitable for the basin under a scenario of future economic prosperity. For circumstances of poor economy and low social awareness, keeping the current pumping rate was found to be the scenario with the highest priority.

Karst aquifers provided 9.2 % of the world’s population with fresh water in 2016 (Stevanović, 2019), but due to their dual flow behavior they are highly vulnerable to anthropogenic impacts and shifts in climate. In the near future, 52 out of 356 Mediterranean aquifers will be exposed to more extreme climatic conditions, which will enhance their water stress if the water usage is not adapted to available water resources (Nußbaum, 2020). Therefore, accurate and high resolution numerical - and empirical models are essential to calculate the groundwater recharge and water availability in complex karst aquifers that cover ~ 14 % of the earth’s ice free land (Stevanovic, 2019).During the last decades, several empirical equations have been developed to calculate the recharge for Israel´s most important source of freshwater, the Western Mountain Aquifer (WMA). These equations calculate annual groundwater recharge of the entire 1.812 km2 recharge area based on annual or monthly precipitation data. We analyzed the applicability of several new methods, such as Soil & Water Assessment Tool (SWAT), HydroGeoSphere (HGS) and Hydro- / Pedo- Transfer Functions (HPTF) to estimate groundwater recharge with a higher resolution as this is essential to calculate proper water fluxes though the vadose zone of karstic aquifers when precipitation is affected by a high variability in space and time.The hydrologic balance models, e.g. SWAT (Neitsch et al., 2009), calculate the water balance on a daily basis for specified Hydrologic Response Units (HRUs), while generalized HPTFs (Wessolek et al., 2009) use soil-, land cover- and climate data to calculate annual percolation rates on a coarse grid, in our case 500 m grid size. The dual continuum model using the code HGS (Brunner et al., 2011) is able to simulated based on Richards’s flow equation down- and upward water fluxes in the unsaturated zone accounting for both, a rapid flow component though the high permeable conduit and a slow flow component through the rock matrix.The comparison of these empirical and new methods for groundwater recharge estimation show significant differences for hydrological extreme years, while results are similar during years with precipitation rates near the average value. For example, the empirical equation of Guttman & Zukerman (1995) gives highest recharge values of all approaches during wet years, while the equation of Abusaada (2011) and the SWAT-model calculates highest recharge values of all approaches during dry years. Overall, the mean recharge ranges from 120 to 177 mm/a which equals 25 – 37 % of the average precipitation between 1990 – 2018.These recharge rates are calculated based on IMS climate data. However, for recharge values used in water resources management regional climate projections are needed. For Israel a high resolution CORDEX-MENA climate projection (Hochman et al., 2018) is available for RCP4.5, showing an increase in temperature and decrease of precipitation during the winter of 2.5 °C and 40 %, respectively. Based on these climate projections the SWAT-model estimates, that the average groundwater recharge for 2050 – 2070 will be 16 % lower than the reference period between 1980 – 2000.

In fractured aquifers, the permeability of open fractures could change over time due to precipitation effects and hydrothermal mineral growth. These processes could lead to the clogging of individual fractures and to the complete rearrangement of flow and transport pathways. Existing fractured rock characterization techniques often neglect this dynamicity and treat the reconstruction as a static inversion problem. The dynamic changes then later added to the model as an independent forward modeling task. In this research we provide a new data assimilation-based methodology to monitor and predict the dynamic changes of fractured aquifers due to mineralization in a quasi-real-time manner.We formulate the inverse problem as a dynamic ‘hidden Markov process’ where the underlying model dynamicity is just partly known. Data assimilation methods are specifically designed to model such systems with strong uncertainties. A typical example for such problems is weather forecasting, where the combination of nonlinear processes and the partial observations make the forecasting challenging. To handle the strong random behavior, data assimilation approaches use stochastic algorithms. In this study we combine DFN-based stochastic aquifer reconstruction techniques with data assimilation algorithms to provide a dynamic inverse modelling framework for fractured reservoirs. We use the transdimensional DFN inversion of (Somogyvári et al., 2017) to initialize the data assimilation. This method uses a transdimensional MCMC approach to identify the most probable DFN geometries given the observations. Because the method is transdimensional it can adjust the number of model parameters, the number of fractures within the DFN. We developed this idea further by enhancing a particle filter algorithm with transdimensional model updates, allowing us to infer DFN models with changing fracture numbers.We demonstrate the applicability of this new approach on outcrop-based synthetic fractured aquifer models. To create a dynamic DFN example, we simulate solute transport in a 2-D fracture network model using an advection-dispersion algorithm. We simulate fracture sealing in a stochastic way: we define a limit concentration above which the fractures could seal with a predefined probability at any timestep. At the initial timestep, a hydraulic tomography experiment is performed to capture the initial aquifer structure, which is then reconstructed by the transdimensional DFN inversion. At predefined timesteps hydraulic tests are performed at different parts of the aquifer, to obtain information about new state of the synthetic model. These observations are then processed by the data assimilation algorithm, which updates the underlying DFN models to better fit to the observations.

Investigating the structure of conduit networks in karst aquifers is a common challenge when working in these complex hydrogeological environments. The network geometry plays an important role in karst flow dynamics, but highly karstified areas are often difficult to characterize by field measurements. Here, we present a methodology that generates karst conduit network geometries reasonably quick without solving complex flow or dissolution equations, and that uses only little input information. The stochastic approach also enables the investigation of the uncertainty of generated networks in the form of a karst probability map.The “Stochastic Karst Simulator” (SKS) is a stochastic modeling approach developed by Borghi et al. (2012) to generate a 3D karst conduit network by computing a minimum effort path between the given inlet and outlet points. This study uses such a modeling approach to characterize the karst network geometry of the Western Mountain Aquifer (WMA), a highly karstified and exploited carbonate aquifer located in Israel and the West Bank. The SKS simulations are based on a conceptual model of the aquifer’s karst genesis, to identify the position of karst springs and recharge zones over past geological ages.Three different phases of karst formation are identified for the WMA. Phase 1: a paleo-discharge zone exists, located close to the present-day coastline of Israel, phase 2: a period of extreme low sea levels during the Messinian salinity crisis, when paleo-canyons were reactivated along this coastline, and phase 3: the modern-day outlets of the aquifer. The iterative approach of the SKS algorithm accounts for these different phases and creates new conduit pathways by building on ones formed in earlier phases. The algorithm also uses the hydrological model of the study site as soft information, providing knowledge about the internal heterogeneities of the karst formations (e.g. statistical properties of fractures). The resulting karst probability map is compared to the location of the most productive pumping wells in the region, assuming a high yield in groundwater abstraction indicating major karst conduits near the pumped well. We demonstrate the method by showing a reconstruction of the karst conduit networks at the WMA model area, an otherwise not available spatial information. The simulations show that the changes in karst spring and recharge locations have a great impact on the geometry and connectivity of the conduit network. Overarching trends in the conduit orientation of the resulting probability map are in keeping with the proposed karst genesis model, resulting in the evolution of a hierarchical network. High karstification is indicated around modern-day springs, also in agreement with the location of numerous pumping wells in that region.The SKS algorithm is a useful tool to test different hypotheses of karst genesis and to understand the evolution of karst network geometries. The methodology is numerically efficient, and its inputs can be easily adjusted. Soft information on karst development allows for the generation of a sound hydraulic parameter field, which can be implemented in hydrological models to better understand and manage these aquifer systems.

Present methods to quantify recharge in karst aquifers in many cases rely on spatially and temporally aggregated precipitation values, neglecting the highly erratic, non-linear nature of infiltration dynamics that give rise to a dual-domain behavior with a slow diffuse and fast focused recharge component. Here, we demonstrate the applicability of integrated surface-subsurface flow models to simulate diffuse and preferential infiltration within the large scale Western-Mountain-Aquifer (WMA) in Israel and the Palestinian territories. A semi-arid climate region with a highly pronounced seasonality of precipitation and intense short-duration rainfalls, such as the Mediterranean region, emphasizes the importance of understanding and accounting for the complex dynamics of dual-domain infiltration and partitioning of the precipitation input signal via spatially discretized overland flow processes.We apply HydroGeoSphere as a dual-continuum flow simulator for transient variably-saturated water flows, discretizing the rock matrix and secondary porosity (i.e., conduits and fractures) as separate overlapping continua. Flow is respectively computed via the Richards' equation with volume-averaged van Genuchten parameters, assuming that the Richards' equation is valid for both domains. The presented model accounts for surface flow via the two-dimensional Saint-Vénant equations under nonexistent inertial forces. We apply precipitation directly to the overland flow continuum and naturally account for the partitioning into Horton overland flow and percolating water. However, modeling of unsaturated flow through the conduit/fracture continuum with the van Genuchten parameterization is often limited, as the Richards' equation describes flow solely in terms of capillary forces, leading to high matric suction in the matrix continuum as a result of the smaller pore spaces (and hence constant exchange from the fracture continuum to the matrix system). In a natural system, non-linear transfer processes govern the transfer between fracture/conduit and matrix flow, such as inertia-driven infiltration (i.e., droplet, rivulet, and film flow) that initially retains itself from equilibration of capillary pressure heads and avoids instant matrix imbibition. This study demonstrates parametrization strategies to allow for infiltration through the fracture/conduit continuum using small-scale process-based simulations. Further, we offer procedures that help to achieve convergence of complex catchment-scale variably-saturated simulations.

Bank filtration is a sustainable source for drinking water production in urbanized regions but is increasingly at risk by contamination with pathogenic bacteria and viruses from surface water receiving wastewater discharge. While recent advances have improved our process understanding for pathogen transport on laboratory scale, simulations and predictions on field scale under transient conditions, as in bank filtration, are still highly uncertain. To improve our understanding on field scale, we performed a sampling survey over 16 months at an observation well transect in a heterogeneous sand-gravel aquifer of an active bank filtration waterworks at the river Rhine in Germany. Water samples were collected from the river, the production well, and 4 multi-level observation wells. Samples were analysed for main anions/cations, and hygienic indicators (E. coli and coliform bacteria via plate counts, coliphages via plaque assay, and adenoviruses via ddPCR). A two-dimensional reactive transport model was set up using PFLOTRAN to simulate the transport of heat and dissolved species, aerobic respiration, denitrification, and colloid-based transport of bacteria and viruses. For the latter, adsorption to and desorption from the sediment, straining, blocking, and inactivation are considered. Model parameters were estimated from prior knowledge of the site or calibrated with the obtained data using particle swarm optimization.Field observations show a strong seasonal variation of river hydraulics with up to 8 m difference in water level, a prolonged low in the summer/fall and short-termed river level increases in the winter. Aerobic respiration was strongly controlled by the temperature variation (6-24°C in groundwater), leading to an increase in oxygen consumption and limited denitrification during the warm summer/fall. Bacteria and virus concentrations in the groundwater were elevated following a flood in the first winter (up to 500 MPN/100mL coliforms, 2 PFU/100mL coliphages, 1000 copies/100mL adenovirus). Measurable concentrations were still observed during the summer (e.g., up to 10 MPN/100mL coliforms, 0.7 PFU/100mL coliphages, 500 copies/mL adenovirus), but concentrations were below the detection limit for most of the second winter, where no significant flood occurred. In the well closest to the river (40 m distance), the concentration reduction compared to the river varied over time between 1 to ≥4 log-units for coliforms, 1.5 to ≥3 log-units for coliphages, and 0.5 to ≥3 log-units for adenoviruses. The model results suggest the main driving processes for the variation in the bacteria and virus concentrations are (i) the changing groundwater velocity (driven by river level variations and pumping rate), (ii) occurrence of low dissolved oxygen concentrations which lower inactivation, and (iii) transient colmation layer properties (permeability and effective grain size). The colmation layer is affected by reworking of riverbed sediments during floods, bio-clogging during summer, and physical clogging due to constant forced infiltration caused by the bank filtration plant. This is supported by the observation of high bacteria concentrations in the aquifer for a short duration after pumps were reactivated following a 40-day maintenance period. Overall, bacteria and virus attenuation during bank filtration was high, only a strong flood resulted in significantly higher contaminant concentrations in the aquifer.

Approximately 10% of the global population rely on groundwater from karst aquifers. Due to complex karst structures, these aquifers have high infiltration capacities and hydraulic conductivities, which makes them vulnerable to pollution and, as prediction and management are complicated, overexploitation. As populations are growing and demand rises, we assess the current level of groundwater stress in karst aquifers with Mediterranean climates and their vulnerability (defined as the change in groundwater stress) to expected changes in temperature and precipitation on the global scale.Our approach is based on a Groundwater Stress Index (GSI), which is calculated for 356 karst aquifers (as identified in the World Karst Aquifer Map) that have some of their area located in Mediterranean climate zones (Csa, Csb, and Csc after Köppen/Geiger). GSI are calculated from seven indicators: groundwater recharge, storage, and abstractions, surface runoff, climatic water balance, water-intensity of crops, and groundwater-dependent ecosystems. Each indicator is spatially and temporally averaged to describe a recent trend on aquifer level, resulting in one complex attribute table for the 356 aquifers. GSI is calculated as the average of the normalized indicators for each aquifer, ranging from 0 (no water stress) to 1 (extreme water stress).Aquifers are then grouped based on similarities in two classification parameters – degree of karstification (P1) and land cover (P2). Comparison of aquifers with similar classification parameters allows to focus more directly on the relationship between groundwater stress and climate, disregarding relatively constant influences. For each group (e.g., well-developed karst, primarily agriculturally used), we plot calculated GSI values with current temperature and precipitation data. By investigating four Shared Socioeconomic Pathways (SSPs) until 2100, we identify aquifers that mimic future climate conditions for others with similar P1 and P2. We then measure the difference in groundwater stress accompanied by altered climatic factors. This change is interpreted as vulnerability to climate change.This approach, which relies on present-day observed conditions, allows us to predict the effect of a changing climate without the need to develop a complex numerical model, which requires large amounts of data and functional understanding of aquifer behavior. While analysis is currently ongoing, we expect both groundwater stress and vulnerabilities to be high. Predicted climate zone shifts by Beck et al. (2018) indicate that, out of 356 karst aquifers with Mediterranean climates, 52 could move to more extreme arid climate zones by 2100.Results will be visualized in the form of vulnerability maps that may serve as an “early-warning system”. For particularly threatened aquifers, we will derive recommendations for more sustainable management by suggesting strategies to lower groundwater stress. This is done by taking a closer look at the aquifer’s indicator values and identifying factors that currently contribute the most to groundwater stress.

Managed aquifer recharge through bank filtration is an important method to produce sustainable drinking water. Yet, water quality related to transport of pathogens (bacteria and viruses) into groundwater systems from surface waters can be a matter of concern, especially in urbanized regions. Based on a 1-year monitoring campaign, a reactive transport model was developed for bacteria transport at a riverbank filtration site located in Germany. The model allows simulating advective-dispersive transport and relies on the colloid filtration theory to mimic attachment and detachment of bacteria to and from the sediment in addition to inactivation, straining and blocking of bacteria. Due to the complexity of the investigated processes, the reactive transport model is characterized by a high level of parametrization, encompassing parameters driving flow as well as solute and colloid transport. A global sensitivity analysis has been applied to identify the most relevant model parameters with respect to piezometric pressure heads, groundwater temperature, and concentrations of chloride, oxygen, coliforms, and Escherichia coli. The model has been calibrated within a stochastic framework, to provide model parameter estimates and to quantify their uncertainty. Our results suggest that bacteria transport models are highly sensitive to inactivation coefficients, straining coefficients, and bacteria size. Permeability of the colmation layer at the riverbank is a key factor for bacteria transport through its influence on residence times. Seasonal variability of boundary conditions, especially anoxic aquifer conditions in the summer and high groundwater flow velocities during flooding periods, resulted in a reduction of inactivation and increased bacteria concentrations at observation wells.

The identification of reactive hotspots in shallow porous aquifers is challenging, and normally requires extensive geochemical data collection combined with computationally intensive reactive transport modeling. In this study we propose an alternative, practical approach, combining conservative and reactive tracers using a tomographic setup to localize such zones. Tracer tomography combines multiple tracer tests to capture and reconstruct aquifer heterogeneities. These methods generally use “ideal” tracers that are dominantly transported by conservative advection to provide a hydraulically accurate aquifer reconstruction. We compare these conservative tracer tomography results to those obtained by using reactive tracers in a similar tomographic setup. Assuming that the conservative tracer is ideal, the differences between the two tracers will be mainly related to reactive transport. In natural systems, the biogeochemical reactions happen in localized zones, these will be the parts of the reconstructions with the greatest differences. We present the viability of the approach in a simple 2-D example, and on an aquifer analog model-based complex example. With extensive sensitivity analysis, we test the effectiveness of the method under different reaction intensities to define the chemical requirements for an ideal reactive tracer for such investigations.

Karstified aquifers respond rapidly to hydrological events, such as heavy rain storms or draughts. Our ability to predict the response of the aquifer after such events strongly depends on i) temporal and spatial resolution of the available monitoring data and ii) suitable modelling approaches to assess recharge at the respective level of detail. The study catchment, the Western Aquifer Basin (WAB), is Israel´s most important source for freshwater supply. The recharge area of the WAB has an area of 1,812 km2. Recharge is characterized by high spatial variability in topography and a high variability in precipitation and temperature, land use, and vegetation. Precipitation also shows a seasonal variability: while annual precipitation mainly occurs during the winter months accompanied by floods in the otherwise dry wadis (October to March, ca. 90 %), summer periods (April to September) are hot and dry, and precipitation decreases to nearly zero.We employ SWAT to simulate the large-scale hydrological water balance (evapotranspiration, recharge, run-off) in the recharge area of the WAB on a daily and monthly temporal resolution. The SWAT model uses a SRTM DEM from NASA, soil maps from FAO, soil properties of the Harmonized World Soil Database, and land use maps from the ESA CCI project covering the time period from 1992 to 2015. These datasets are merged in SWAT into 361 Hydrologic Response Units with unique characteristics in soil, land use, and slope, respectively. The calibration of soil water balance model with SWAT-CUP employs monthly actual evapotranspiration and daily surface runoff data. Run-off was measured in hydrometric stations between 2004 – 2015. Evapotranspiration with a spatial resolution of 500 m x 500 m is obtained from the MODIS satellite mission and covers a period between 2001 and 2013 with individual time steps of 8 days. Calculated long-term groundwater recharge is compared with spring discharge measured during the period 1990 – 2013. Climate projections have been obtained with the RCM COSMO-CLM at resolution of 8km, under the IPCC RCP4.5 scenario, nested into the MENA-CORDEX domain.The calibrated water balance model allows for scenario analysis for predicted shifts in climate until 2050 to address the impact of climate change on groundwater recharge. In addition to an increase in temperature, fewer but more extreme rainfall events are to be expected. Furthermore, the effect of future land use changes, such as expansion of farm land or urban areas, on recharge depth are analyzed. Finally, simulated high-resolution recharge provides an updated estimate for the currently developed groundwater flow model of the aquifer system. SWAT provides daily recharge for the equivalent porous medium model of the WAB, simulated by MODFLOW. One of our challenges is the calculation of recharge in the hilly region i) characterized by steep slopes and ii) vadose zones of several 100 meters of thickness. Our investigations are expected to provide information on the impact of shifts in climate and global changes on recharge processes and to illustrate the effect of short-term hydrologic events on water resources in large carbonate aquifers under Mediterranean climate.

Integrated surface-subsurface flow models solving the Richards’ equation allow to simulate flow within all compartments (e.g. vadose, phreatic and surface zone) and their reciprocal interaction, and provide a useful tool to investigate the impact of climatic changes on infiltration dynamics. The Mediterranean karst aquifers are in particular prone to shifting climates, as a decrease in mean precipitation with increasing intensity and frequency of short-duration extreme rainfall, may have a significant impact on recharge dynamics and the overall water budget. Here, we use the finite element, distributed, multi-continuum flow simulator HydroGeoSphere (Aquanty, 2015) on a high-performance-computing platform to simulate infiltration and groundwater flow of the Western-Mountain-Aquifer (WMA) considering changing hydrologic conditions. A thin soil cover and abundant exposed bare karstic carbonate rock compose the recharge area, providing efficient pathways for fast direct infiltration along karst features (e.g., sinkholes and dolines). The lowered total annual precipitation may not result in a decrease in recharge since the severity and frequency of individual rainfall storms are projected to increase. To account for the duality of karst flow dynamics, both in the vadose and phreatic zones, with rapid flow through conduits and slow flow through the fractured rock matrix, we apply a double-continuum approach based on the volume-effective Richards’ equation with van Genuchten parameters. A 2-D friction-based overland flow continuum, coupled via a first-order exchange term to the subsurface, accounts for overland flow due to infiltration excess. This allows to represent the partitioning of rainfall into diffuse and rapid direct recharge, e.g., along dry valleys or sinkholes. This modeling approach, therefore, accounts for complex spatially distributed infiltration characteristics of the rock-soil landscape, with focused recharge along karst features and transmission losses of ephemeral streams (wadis) under variable precipitation patterns.To get a better understanding of the complex interaction dynamics of the surface and subsurface domain in a coupled unsaturated single- and dual-continuum model we carried out small-scale process-oriented studies. Two types of synthetic karst features, (1) a dry valley averaged from field data (i.e., Wadi Natuf) and (2) an analytical generalized doline, were investigated. Geometries close to natural systems, such as sub-catchments of the WMA, were implemented. Sensitivity studies reveal complex dependencies of domain properties on linear- and log-scales. However, the exchange parameters controlling the coupling between the subsurface continua are insensitive.

Carbonate aquifers supply freshwater to about one-quarter of the world population. Their particular hydrodynamic behavior is a valuable property for groundwater extraction, on the downside, carbonate aquifers are vulnerable to overexploitation and pollution. Fractures, fissures, and typical karst features, such as conduits and vertical shafts, create high regional hydraulic conductivities and fast response times to hydrological events, complicating numerical modeling and management of carbonate aquifers in general. Here, we develop a new method to assess the vulnerability of Mediterranean karst aquifers concerning shifts in climate. Particularly, we are interested in 1) which types of karst aquifers are most vulnerable and 2) which factors have the highest impact on their climate vulnerability.Our approach is based on a vulnerability index, which is calculated from selected indicators of aquifer behavior that refer to land cover, soil types, wetlands, water demand, current change of groundwater levels, total water volume, run-off, water exploitation index, and freshwater production. First, we calculate vulnerability indices for all karst aquifers – as identified in the World Karst Aquifer Map by the World-wide Hydrogeological Mapping and Assessment Programme (WHYMAP WOKAM v1 database; Chen et al., 2017) – that have at least 90% of their area belonging to Mediterranean climate zones (Csa, Csb, and Csc). Then, we group these aquifers into classes representing different physical behaviors and morphological characteristics (e.g. highly karstified systems in mountainous areas).An evident approach to investigate various aquifers in terms of their vulnerability is the development of numerical flow models. The advantage is that the boundary conditions, such as average annual precipitation and temperature, can be modified to consider different climatic scenarios. Thus, the resulting impact on water volumes and the aquifer response can be simulated accordingly. However, this approach requires large amounts of data and high computational costs.Our method uses selected sets of karst aquifers representing different variations of Mediterranean climates (i.e. that are similar in terms of temperature and precipitation patterns). These aquifers are compared by analyzing and plotting regional climate variables versus previously calculated vulnerability indices. By identifying and comparing climate-vulnerability relations within aquifer sets, we can mimic changes in climate for individual aquifers in line with the RCP4.5 scenario until 2050. This approach, which relies on present-day observed conditions, allows us to predict the effect of a changing climate on the vulnerability of an aquifer class without the need to develop a complex numerical model.The results are visualized in the form of vulnerability maps and used to derive recommendations for the sustainable management of karst aquifers under Mediterranean climates.

Groundwater recharge is an important variable for sustainable groundwater resources management in regions affected by water scarcity. The specifics of the Mediterranean require adapted techniques to also account for climate change implying a higher frequency of extreme events. Appropriate techniques are highly relevant for recharge with low rates. We compare three methods for the Western Mountain Aquifer, a karst in Israel: soil moisture budget calculations at basin scale, empirical functions, and machine learning algorithms. Resulting recharge are compared with measured spring discharge.Neural networks have the advantage of not requiring much knowledge about physical processes or hydrogeological and hydrological conditions, nor about model parameters. This data-driven machine learning algorithms learn the non-linear relationship between precipitation events and spring water discharge given a sufficient amount of training data is available. After training, the neural network could be used as a nonlinear function to model recharge of any predicted precipitation time series. However, this approach does not allow for any quantitative analysis of external forcing, such as land use, or internal parameter, such as soil characteristics, nor does it account for any expected future change in precipitation pattern.Hydro-pedotransfer functions (HPTF) are based on empirical relationships between precipitation and recharge. HPTFs account for potential evapotranspiration, annual precipitation, land cover, and a critical water supply (a threshold when actual evapotranspiration depends only on atmospheric conditions). Resulting percolation rates consider i) vegetation types, ii) precipitation during the vegetation growth period, iii) runoff, iv) plant available soil water, and v) capillary rise. The application of HPTF to a karst aquifer has the advantage that only limited input data are required. However, our results indicate that HPTFs are not able to capture the rapid recharge component observed in karst systems and thus underestimate recharge.The Soil Water Assessment Tool (SWAT) employs a hydrological and soil moisture budget calculations. Objective functions are actual evapotranspiration and surface runoff. Evapotranspiration is obtained from MODIS remote sensing data. Calibration of actual evapotranspiration is especially challenging for summer periods due to the impact of vegetation and irrigation. However, the most relevant parameter determining daily recharge rates are water loss by surface-runoff and surface water storage in wadi beds generating episodic recharge.Impact of shifts in climate is considered by climate projections obtained with the RCM COSMO-CLM at resolution of 3 km, under the IPCC RCP4.5 scenario, nested into the MENA-CORDEX domain. However, we believe that changes in land use from natural vegetation (trees, grass-, and shrublands) to rain-fed agricultural area could possibly shift the water budget from deficit to surplus conditions (recharge dominated). During the period 1992 to 2015 natural vegetation decreased by 8% and urban areas increased by up to 6%, while (rain-fed) agricultural areas remained almost constant. We investigate if land use changes might have (a much) larger impact on percolation rates than the predicted climate change effect. Thus, in future recharge may be controlled and enhanced in regions with water scarcity by better management of land use employing an optimized combination between precipitation, irrigation, and crop type.

Managed aquifer recharge through riverbank filtration is an important method to produce drinking water in densely populated regions. Due to the discharge of wastewater into streams, this type of drinking water production can be affected by organic contaminants originating from surface water inflow. Transport and degradation of anthropogenic contaminants depend on several factors, such as pH, temperature, oxygen content, and redox conditions. One of the key factors that drive the degradation of organic contaminants like x-ray contrast media is the prevailing redox system as many pharmaceuticals and pesticides are transformed under aerobic conditions but are more persistent under anaerobic conditions.We conducted a 1-year monitoring campaign at an active riverbank filtration plant at the Rhine river in Düsseldorf, Germany. Samples were taken every two weeks from the Rhine, a production well, and five observation wells with three different depths along a transect perpendicular to the river and parallel to the main flow direction. Samples were analyzed for main cations and anions, redox-species, and microbiological parameters. Water samples were also screened for 100 organic contaminants, pharmaceuticals, and pesticides.A 2D reactive transport model was set-up using PFLOTRAN to simulate the redox zonation during a hydrological year. It includes aerobic respiration and denitrification with dissolved organic carbon using Monod kinetics and also accounts for temperature-dependency. Our results show that hot spots for biogeochemical processes develop close to the river, and thus most of the inflowing oxygen is already consumed within the first few decimeters. We also found a substantial seasonal variability of reaction rates due to seasonal temperature variations leading to oxygen depletion and limited denitrification in the warmest period (late summer/early fall).Reactive transport is affected by the hydrogeological properties of the aquifer, which are influenced by its geological development. Thus, model results will depend on the reliability and accuracy of the employed conceptual geological model. Based on structural information obtained from grain size sieve analysis, and geophysical investigations such as geoelectric and natural gamma-ray measurements, we created a set of plausible conceptual models with increasing complexity. These models range from a simple homogeneous aquifer, to a multi-layer aquifer system or a cross-bedded aquifer structure. The conceptual models include different representations of the colmation layer at the interface between river and aquifer.Numerical analysis of the different conceptual models indicates that a homogeneous aquifer can represent a single flow path over a hydrological year. However, only more complex aquifer structures were able to reproduce the spatial and seasonal temporal variability of temperature and redox species observations (O2, NO3-). Additionally, proper integration of the colmation layer is the key factor to simulate heat transport as well as the spatial distribution of redox-species and thus redox-zonation during the entire hydrological year, including droughts and flooding periods. Therefore, an accurate and detailed integration of the geological system into the reactive transport model, especially characteristics (e.g., size, type of material) of the colmation layer, are of highest relevance for enhanced predictions of redox zones in highly transient hydrogeological systems and at hydrodynamic interfaces.

Field application of livestock manure introduces colloids and veterinary antibiotics, e.g. sulfonamides (SAs), into farmland. The presence of manure colloids may potentially intensify the SAs-pollution to soils and groundwater by colloid-facilitated transport. Transport of three SAs, sulfadiazine (SDZ), sulfamethoxypyridazine (SMPD), and sulfamoxole (SMOX), was investigated in saturated soil columns with and without manure colloids from sows and farrows, weaners, and fattening pigs. Experimental results showed that colloid-facilitated transport of SMOX was significant in the presence of manure colloids from fattening pigs with low C/N ratio, high SUVA 280 nm and protein C, while manure colloids from sows and farrows and weaners had little effect on SMOX transport. In contrast, only retardation was observed for SDZ and SMPD when manure colloids were present. Breakthrough curves (BTCs) of colloids and SAs were replicated well by a newly developed numerical model that considers colloid-filtration theory, competitive kinetic sorption, and co-transport processes. Model results demonstrate that mobile colloids act as carriers for SMOX, while immobile colloids block SMOX from sorbing onto the soil. The low affinity of SMOX to sorb on immobile colloids prevents aggregation and also promotes SMOX's colloid-facilitated transport. Conversely, the high affinity of SDZ and SMPD to sorb on all types of immobile colloids retarded their transport. Thus, manure properties play a fundamental role in increasing the leaching risk of hydrophobic sulfonamides.

©2018. American Geophysical Union, In regions with intensive agriculture nitrate is one of the most relevant contaminants in groundwater. Denitrification reduces elevated nitrate concentrations in many aquifers, yet the denitrification potential is limited by the concentration of available electron donors. The aim of this work was to study the denitrification potential and its limitation in natural sediments. A column experiment was conducted using sediments with elevated concentrations of organic carbon (total organic carbon 3,247 mg C/kg) and pyrite (chromium reducible sulfur 150 mg/kg). Groundwater with high nitrate concentration (100 mg/L) was injected. Measurements were taken over 160 days at five different depths including N‐ and S‐isotope analysis for selected samples. A reactive transport model was developed, which couples nitrate reduction with the oxidation of organic carbon (heterotrophic denitrification) and pyrite (autolithotrophic denitrification), and considers also transport and growth of denitrifying microbes. The denitrification pathway showed a temporal sequence from initially heterotrophic to autolithotrophic. However, maximum rates were lower for heterotrophic (11 mmol N/(L*a)) than for autolithotrophic denitrification (48 mmol N/(L*a)). The modeling showed that denitrifying microbes initially preferred highly reactive organic carbon as the electron donor for denitrification but were also able to utilize pyrite. The results show that after 160 days nitrate increased again to 50 mg/L. At this time only 0.5% of the total organic carbon and 46% of the available pyrite was oxidized. This indicates that denitrification rates strongly decrease before the electron donors are depleted either by a low reactivity (total organic carbon) or a diminishing reactive surface possibly due to the presence of coatings (pyrite).

Based on small-scale laboratory and field-scale lysimeter experiments, the sorption and biodegradation of sulfonamide sulfadiazine (SDZ) were investigated in unsaturated sandy and silty-clay soils. Sorption and biodegradation were low in the laboratory, while the highest leaching rates were observed when SDZ was mixed with manure. The leaching rate decreased when SDZ was mixed with pure water, and was smallest with the highest SDZ concentrations. In the laboratory, three transformation products (TPs) developed after an initial lag phase. However, the amount of TPs was different for different mixing-scenarios. The TP 2-aminopyrimidine was not observed in the laboratory, but was the most prevalent TP at the field scale. Sorption was within the same range at the laboratory and field scales. However, distinctive differences occurred with respect to biodegradation, which was higher in the field lysimeters than at the laboratory scale. While the silty-clay soil favored sorption of SDZ, the sandy, and thus highly permeable, soil was characterized by short half-lives and thus a quick biodegradation of SDZ. For 2-aminopyrimidine, half-lives of only a few days were observed. Increased field-scale biodegradation in the sandy soil resulted from a higher water and air permeability that enhanced oxygen transport and limited oxygen depletion. Furthermore, low pH was more important than the organic matter and clay content for increasing the biodegradation of SDZ. A numerical analysis of breakthrough curves of bromide, SDZ, and its TPs showed that preferential flow pathways strongly affected the solute transport within shallow parts of the soil profile at the field scale. However, this effect was reduced in deeper parts of the soil profile. Due to high field-scale biodegradation in several layers of both soils, neither SDZ nor 2-aminopyrimidine was detected in the discharge of the lysimeter at a depth of 1m. Synthetic 50 year long simulations, which considered the application of manure with SDZ for general agricultural practices in Germany and humid climate conditions, showed that the concentration of SDZ decreased below 0.1 μg/L in both soils below the depth of 50 cm.

A groundwater model characterized by a lack of field data about hydraulic model parameters and boundary conditions combined with many observation data sets for calibration purpose was investigated concerning model uncertainty. Seven different conceptual models with a stepwise increase from 0 to 30 adjustable parameters were calibrated using PEST. Residuals, sensitivities, the Akaike information criterion (AIC and AICc), Bayesian information criterion (BIC), and Kashyap's information criterion (KIC) were calculated for a set of seven inverse calibrated models with increasing complexity. Finally, the likelihood of each model was computed. Comparing only residuals of the different conceptual models leads to an overparameterization and certainty loss in the conceptual model approach. The model employing only uncalibrated hydraulic parameters, estimated from sedimentological information, obtained the worst AIC, BIC, and KIC values. Using only sedimentological data to derive hydraulic parameters introduces a systematic error into the simulation results and cannot be recommended for generating a valuable model. For numerical investigations with high numbers of calibration data the BIC and KIC select as optimal a simpler model than the AIC. The model with 15 adjusted parameters was evaluated by AIC as the best option and obtained a likelihood of 98%. The AIC disregards the potential model structure error and the selection of the KIC is, therefore, more appropriate. Sensitivities to piezometric heads were highest for the model with only five adjustable parameters and sensitivity coefficients were directly influenced by the changes in extracted groundwater volumes.

A large-scale numerical flow and transport model was developed for the central-eastern arid part of the Arabian Peninsula. The model was applied to a region with freshwater resources dating back to more humid periods of the past, which are faced with overexploitation today. Model inflow was based on infiltration around wadi beds and groundwater recharge. Inflow was balanced by natural outflows, such as evaporation from sabkhas, spring discharge, and discharge to the sea. Two models were developed: (1) a short-term present-day model to estimate effective model parameters, and (2) a long-term model to study the development of the groundwater resources during the Mid- and Late Holocene and the natural response of the groundwater system to changes in climate. Hydraulic model parameters (hydraulic conductivity and specific storage) were assigned with respect to geological structures. Hydraulic parameters were estimated with an inverse PEST model by calibrating against observed depression cones cause by groundwater abstraction. Sensitivity analysis demonstrated that estimated model parameters were associated with a high uncertainty at a certain distance from agricultural areas when calibration data were lacking. A long-term model starting 10,000 years BP was calibrated by spring discharge and palaeo-groundwater levels and validated using measured 14C groundwater ages. The long-model predicted that groundwater levels adapted in response to changes in precipitation. During the Mid-Holocene, which was characterized by an intensification of the monsoon season, groundwater levels increased by 10 m on the mainland within the shallow aquifers and adapted quickly to higher recharge rates. The deeper aquifers were less affected by changes in climate. Along the present-day coastline, the groundwater level rose by about 25 m due to the declined sea level in the Mid-Holocene. During this period, surface run-off was possible as groundwater levels temporarily reached the ground surface. The natural groundwater budget reacted sensitively to changes in climate. Between 10 and 3 ka, groundwater storage occurred. During the Late Holocene, at 3 ka, natural depletion of the groundwater system began, which still prevails today.

The water demand in arid regions is commonly covered by groundwater resources that date back to more humid periods of the Pleistocene and Holocene. Within the investigated arid part of SE Saudi-Arabia information about climate, groundwater levels, and pumping rates are only available for regions where groundwater extractions occur at present-day. For the prediction of the impact of long-term climate changes on groundwater resources an understanding of the hydrogeological and hydrological past and the development of the aquifers is necessary. Therefore, all available information about hydrology and hydrogeology for the past 10,000 years BP were collected and compiled to a conceptual model of the aquifer development on the Arabian Peninsula since the last Ice-Age. The climatic history was displayed by changes in precipitation, temperature and recharge during the mid-S and late Holocene. The hydrogeological development is described by groundwater ages, sea level fluctuations, movement of the coastline, and the development of sabkhas. The most sensitive parameter to describe the development of aquifer system is recharge. Present-day recharge was calculated with the hydrological model system HEC-HMS accounting for current precipitation, temperature, wind, soil types, and geomorphology. With respect to changes in precipitation and temperature over the past 10,000 years the temporal and spatial variability of groundwater recharge was calculated using empirical equations valid for semi-arid and arid settings. Further inflow into the groundwater system results from surface water infiltration in wadi beds, while natural outflow from the groundwater system occurs by discharge to the Gulf, evaporation from sabkhas, and spring discharge. Backward predictions can be verified by sedimentological observations of palaeo-river systems and lakes indicating that groundwater levels reached temporarily the surface under wetter climate conditions and 14C groundwater ages displaying groundwater residence times.

[1] Groundwater and surface water are in many cases closely linked components of the water cycle with respect to both quantity and quality. Bank filtrates may eventually be impacted by the infiltration of wastewater-derived micropollutants from surface waters. Artificial sweeteners such as acesulfame have recently been reported as a novel class of potentially valuable tracers to study the fate of wastewater-derived substances in groundwater and, in particular, to determine the (bio)degradability of micropollutants. In this paper, a model-based analysis of a field experiment within the hyporheic and riparian zone of a highly polluted German stream was performed to assess the physical and chemical behavior of the artificial sweetener acesulfame. In the first part of this study, a reliable flow and transport model was established by jointly using hydraulic heads, temperatures, and acesulfame concentrations as inverse model calibration constraints. The analysis confirmed the conservative behavior of acesulfame and, therefore, its usability as an indicator of sewage flux provenance. However, a comparison of the appropriateness of hydraulic head, temperature, and acesulfame concentrations revealed that the characterization of the surface water-groundwater flux data indicated diurnal temperature fluctuations are the best indicator in terms of characterizing the flow and transport behavior in the groundwater system.

Batch and saturated soil column experiments were conducted to investigate sorption and mobility of two 14C-labeled contaminants, the hydrophobic chlordecone (CLD) and the sulfadiazine (SDZ), in the absence or presence of functionalized multi-walled carbon nanotubes (MWCNTs). The transport behaviors of CLD, SDZ, and MWCNTs were studied at environmentally relevant concentrations (0.1-10 mg L-1) and they were applied in the column studies at different times. The breakthrough curves and retention profiles were simulated using a numerical model that accounted for the advective-dispersive transport of all compounds, attachment/detachment of MWCNTs, equilibrium and kinetic sorption of contaminants, and co-transport of contaminants with MWCNTs. The experimental results indicated that the presence of mobile MWCNTs facilitated remobilization of previously deposited CLD and its co-transport into deeper soil layers, while retained MWCNTs enhanced SDZ deposition in the topsoil layers due to the increased adsorption capacity of the soil. The modeling results then demonstrated that the mobility of engineered nanoparticles (ENPs) in the environment and the high affinity and entrapment of contaminants to ENPs were the main reasons for ENP-facilitated contaminant transport. On the other hand, immobile MWCNTs had a less significant impact on the contaminant transport, even though they were still able to enhance the adsorption capacity of the soil.

In arid regions like the Arabian Peninsula, available water resources are essentially restricted to groundwater, requiring a detailed understanding of the local and regional hydrogeological conditions and water budgets. In the framework of the IWAS initiative, the 1.8 × 106 km2 large sedimentary Upper Mega Aquifer of the Arabian Peninsula was chosen as a model region to develop concepts and methodologies to quantify water fluxes in such an arid environment. Field and laboratory studies were conducted to analyse (i) precipitation patterns, (ii) groundwater recharge, (iii) the hydrochemical evolution of groundwater and (iv) evaporation particularly from Sabkhas in detail. Results were used as input parameters for a 3D groundwater model for the central part of the Peninsula, which was later extended to the full dimension of the Upper Mega Aquifer. It could be shown that in such a region different components of the water cycle have to be quantified with great care and several methods should be applied to reduce data uncertainty. It was not possible to make use out of satellite products to receive reliable actual precipitation patterns for the peninsula. It was observable; recharge estimations based on average annual precipitation are not applicable but should be based on singular precipitation events. A threshold of 6 mm/event was derived, below of which no recharge in sand seas occurs. The loss of water from UMA, due to sabkha evaporation reaches about 40 mm/a under the given recent climatic conditions.

Summary Hydrodynamic exchange between surface-water and groundwater was studied at a river located within the Rhine Valley in Germany. Piezometric pressure heads and environmental tracers such as temperature, stable isotopes, chloride, X-ray contrast media, and artificial sweetener were investigated within the hyporheic zone and river water plume. Vertical profiles of environmental tracers were collected using multi-level wells within the neutral up-gradient zone, beneath the river bed, and within the horizontal proximal and distal down-gradient zone. Infiltration velocities were calculated from pressure heads, temperature fluctuations and gradients. The amount of river water within groundwater was estimated from vertical profiles of chloride, stable isotopes, and persistent pharmaceuticals. Profiles of stable isotopes and chloride reveal the existence of down-welling within the shallow hyporheic zone that is generated by river bed irregularities. Due to down-welling an above-average migration of river water into the hyporheic zone establishes even under upward hydraulic pressure gradients. The investigated environmental tracers could not distinctively display short-time-infiltration velocities representative for flood waves, while average infiltration velocities calculated over several months are uniform displayed. Based on vertical temperature profiles the down-gradient migration of the river water plume could be observed even after long periods of effluent conditions and over a distance of 200 m from the river bank. X-ray contrast media and artificial sweeteners were observed in high concentrations within the proximal zone, but were not detected at a distance of 200 m from the river bank. Using temperature as environmental tracer within the hyporheic zone may result in overestimating the migration of pollutants within the river water plume as the process of natural attenuation will be neglected. Furthermore, temperature was not able to display the effect of down-welling. Stable isotopes and chloride were found to be suitable environmental tracers to forecast the release and fate of organic contaminants within the hyporheic zone.

Anomalous behaviour of specific electrical conductivity (SEC) was observed at a karst spring in Slovenia during 26 high-flow events in an 18-month monitoring period. A conceptual model explaining this anomalous SEC variability is presented and reproduced by numerical modelling, and the practical relevance for source protection zoning is discussed. After storm rainfall, discharge increases rapidly, which is typical for karst springs. SEC displays a first maximum during the rising limb of the spring hydrograph, followed by a minimum indicating the arrival of freshly infiltrated water, often confirmed by increased levels of total organic carbon (TOC). The anomalous behaviour starts after this SEC minimum, when SEC rises again and remains elevated during the entire high-flow period, typically 20–40 µS/cm above the baseflow value. This is explained by variable catchment boundaries: When the water level in the aquifer rises, the catchment expands, incorporating zones of groundwater with higher SEC, caused by higher unsaturated zone thickness and subtle lithologic changes. This conceptual model has been checked by numerical investigations. A generalized finite-difference model including high-conductivity cells representing the conduit network (“discrete-continuum approach”) was set up to simulate the observed behaviour of the karst system. The model reproduces the shifting groundwater divide and the nearly simultaneous increase of discharge and SEC during high-flow periods. The observed behaviour is relevant for groundwater source protection zoning, which requires reliable delineation of catchment areas. Anomalous behaviour of SEC can point to variable catchment boundaries that can be checked by tracer tests during different hydrologic conditions. Copyright © 2011 John Wiley & Sons, Ltd.

Increased interest in geothermal energy has led to the need for more sophisticated analyses of available geothermal resources for heating and power production. The evaluation methods have traditionally used 1D models, but progress in computing power allows us now to perform joint interpretations that include large-scale effects of geological, topographical and hydrogeological structures. This paper describes the steps taken in such an integrated assessment. The evaluation of geothermal potential is actually finalized for one of the most populated areas of Switzerland. The methodology accounts for individual utilization scenarios and is based on various temperature data that have been systematically collected over many years. The state-of-the-art compilation involves comprehensive 3D regional geological and thermal models. An excellent data fit with >20 boreholes in northern Switzerland is achieved using the calculation scheme presented here. Zones of significant convective flow are identified and the flow velocity is quantified. In contrast to the results obtained from earlier geothermal resource assessments, the existing geological, hydrogeological and petrophysical data are included in the full 3D numerical evaluation. The results obtained for a regional scale of a well-documented subsurface area in northern Switzerland are displayed in terms of geothermal productivity and energy. The calculations identify the topmost crystalline basement as a most promising structure for geothermal exploitation with predicted maximum doublet productivities of >100 MWt. The annually extractable energy of 13 PJ km−3 easily covers a broad range of Swiss energy needs. The atlas can be further expanded to cover utilization schemes not treated here, such as ground-coupled heat pump systems.

The permeability and thermal conductivity of bentonite/crushed rock mixtures used as backfill for a nuclear waste repository have an important impact on the maximum radioactive load of the waste canister that can be embedded. Our research pursues useful methods for estimating the permeability, thermal conductivity, and specific heat of various bentonite/crushed rock mixtures for the conditions expected to prevail at the Aspo Hard Rock Laboratory (ASPO HRL). We conducted laboratory experiments and employed inverse modeling techniques to estimate effective thermal and hydraulic parameters suitable for predictive modeling of non-isothermal flow and transport from a nuclear waste repository. Thermal parameters are often calculated based on empirical relationships developed for homogeneous clays, i.e., they are not necessarily valid for mixtures. The applicability of these methods to model thermal-hydraulic processes within the bentonite/crushed rock mixtures in a deep repository needs to be assessed. All experiments were conducted with mixtures containing sodium- (SPV Volclay) or calcium-bentonite (Calcigel) and we used water from Aspo. Hydraulic column experiments were carried out with a specially designed permeameter and Darcy's law was applied to determine the hydraulic conductivity, which followed a lognormal distribution with mean values of 1.64×10 −11 and 4.93×10 −9 m/s for the two bentonite/crushed rock mixtures studied. The thermal laboratory experiments were analyzed using inverse modeling techniques. The simulated temperature distribution matched the measured data very well at all locations along the column and for all times. The inversely estimated thermal conductivity ranged from 1.6 to 2.2 W/mK, and the specific heat from 810 to 1020 J/kg K, both consistent with the predictions of the empirical relationships. However, the calculation of the effective parameters was very sensitive to heat loss through the insulation. The newly developed experimental setup in combination with inverse modeling allows the identification of key parameters governing the hydraulic and thermal processes of bentonite/crushed rock mixtures under repository conditions.

Mixtures of sodium bentonite and crushed rock are being examined as components of the engineered barrier system in a geologic repository of high-level nuclear waste. Laboratory experiments were performed to determine the thermal and unsaturated hydraulic properties of bentonite/crushed diorite mixtures. Water-retention curves were conventionally obtained from pressure cell and evaporation experiments. In addition, transient data from heating and gas injection experiments on laboratory columns were analyzed using inverse modeling techniques. Measured pressures, temperatures, and drained-water volumes were jointly inverted to estimate absolute permeability, thermal conductivity, specific heat, and capillary strength parameters. Simultaneous matching of all available data specifically the gas breakthrough at the top of the column proved difficult, pointing towards aspects of the experimental design and the conceptual model that need to be refined. The analysis of sensitivity coefficients and the correlation structure of the parameters revealed the importance of accurately capturing coupled thermal hydrological processes within the column as well as the details of the experimental apparatus, such as heat losses and storage of water and gas in the measuring burette. The parameters estimated using different experimental and analytical procedures were consistent with one another, providing backfill material properties useful for the simulation of gas- and heat-generating nuclear waste repositories.

The transport and biochemical transformations of the iodinated X-ray contrast medium (ICM) iomeprol were studied at the stream/groundwater interface. During a one-month field experiment piezometric pressure heads, temperatures, and concentrations of redox-sensitive species, iomeprol and 15 of its transformation products (TPs) were collected in stream- and groundwater. The data set was analyzed and transformation processes and rates identified by comparing conservative and reactive transport simulations. ICM and TP transformations were simulated as a cometabolic process during organic carbon degradation. Using iomeprol/TPs ratios as calibration constrain mitigated the uncertainties associated with the high variability of the ICM wastewater discharge into the investigated stream. The study provides evidence that biodegradation of ICM occurs at the field-scale also for predominantly denitrifying conditions. Under these anaerobically dominated field conditions shortest simulated half-life (21 days) was in the same range as previously reported laboratory-determined half-lives for aerobic conditions.

The suitability of acesulfame to trace wastewater-related surface water fluxes from streams into the hyporheic and riparian zones over long-term periods was investigated. The transport behavior of acesulfame was compared with the transport of water stable isotopes (δ(18)O or δ(2)H). A calibrated model based on a joint inversion of temperature, acesulfame, and piezometric pressure heads was employed in a model validation using data sets of acesulfame and water stable isotopes collected over 5months in a stream and groundwater. The spatial distribution of fresh water within the groundwater resulting from surface water infiltration was estimated by computing groundwater ages and compared with the predicted acesulfame plume obtained after 153day simulation time. Both, surface water ratios calculated with a mixing equation from water stable isotopes and simulated acesulfame mass fluxes, were investigated for their ability to estimate the contribution of wastewater-related surface water inflow within groundwater. The results of this study point to limitations for the application of acesulfame to trace surface water-groundwater interactions properly. Acesulfame completely missed the wastewater-related surface water volumes that still remained in the hyporheic zone under stream-gaining conditions. In contrast, under stream-losing conditions, which developed after periods of stagnating hydraulic exchange, acesulfame based predictions lead to an overestimation of the surface water volume of up to 25% in the riparian zone. If slow seepage velocities prevail a proportion of acesulfame might be stored in smaller pores, while when released under fast flowing water conditions it will travel further downstream with the groundwater flow direction. Therefore, under such conditions acesulfame can be a less-ideal tracer in the hyporheic and riparian zones and additional monitoring with other environmental tracers such as water stable isotopes is highly recommended.

A groundwater model characterized by a lack of field data to estimate hydraulic model parameters and boundary conditions combined with many piezometric head observations was investigated concerning model uncertainty. Different conceptual models with a stepwise increase from 0 to 30 adjustable parameters were calibrated using PEST. Residuals, sensitivities, the Akaike Information Criterion (AIC), and the likelihood of each model were computed. As expected, residuals and standard errors decreased with an increasing amount of adjustable model parameters. However, the model with only 15 adjusted parameters was evaluated by AIC as the best option with a likelihood of 98%, while the uncalibrated model obtained the worst AIC value. Computing of the AIC yielded the most important information to assess the model likelihood. Comparing only residuals of different conceptual models was less valuable and would result in an overparameterization of the conceptual model approach. Sensitivities of piezometric heads were highest for the model with five adjustable parameters reflecting also changes of extracted groundwater volumes. With increasing amount of adjustable parameters piezometric heads became less sensitive for the model calibration and changes of pumping rates were no longer displayed by the sensitivity coefficients. Therefore, when too many model parameters were adjusted, these parameters lost their impact on the model results. Additionally, using only sedimentological data to derive hydraulic parameters resulted in a large bias between measured and simulated groundwater level.

In arid regions, groundwater resources are prone to depletion due to excessive water use and little recharge potential. Especially in sand dune areas, groundwater recharge is highly dependent on vadose zone properties and corresponding water fluxes. Nevertheless, vadose zone water fluxes under arid conditions are hard to determine owing to, among other reasons, deep vadose zones with generally low fluxes and only sporadic high infiltration events. In this study, we present an inverse model of infiltration experiments accounting for variable saturated nonisothermal water fluxes to estimate effective hydraulic and thermal parameters of dune sands. A subsequent scenario modeling links the results of the inverse model with projections of a global climate model until 2100. The scenario modeling clearly showed the high dependency of groundwater recharge on precipitation amounts and intensities, whereas temperature increases are only of minor importance for deep infiltration. However, simulated precipitation rates are still affected by high uncertainties in the response to the hydrological input data of the climate model. Thus, higher certainty in the prediction of precipitation pattern is a major future goal for climate modeling to constrain future groundwater management strategies in arid regions.