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A significant volume of an aquifer along the coastline in the German Bight is salinized by seawater intrusion. The mean sea-level rise (MSLR) is expected to continue in the future due to global climatic change, subsequently degrading the fresh groundwater resources. To impede further salinization in the future, a solution is proposed based on weir construction in an existing canal hydraulically connected to the aquifer. The effect is twofold: (1) the elevated groundwater level can upgrade present fresh groundwater resources by shifting the saltwater–freshwater interface position further seaward, or by inhibiting its landward movement, and (2) the inland water level can be elevated, expanding surface water ponds. A fully coupled three-dimensional numerical surface-subsurface model (a modified HydroGeoSphere code) was used to simulate the effects of variable weir construction heights under different MSLR rates, and to quantify the gain of aquifer freshwater volume and loss of usable land due to surface ponding. Construction of a higher weir increases the desalinized aquifer volume and decreases the newly salinized aquifer volume under future MSLR. A minimum height of a weir was determined under a certain MSLR rate to maintain the present freshwater resource. Both weir construction and MSLR can cause the loss of land usage. Computed loss-gain ratio curves can be utilized to determine the optimal weir height, meeting the economic requirements of coastal land management under future MSLR. [ABSTRACT FROM AUTHOR]

Application of numerical models is a common method to assess groundwater resources. The versatility of these models allows consideration of different levels of complexity, but the accuracy of the outcomes hinges upon a proper description of the system behaviour. In seawater intrusion assessment, the implementation of the sea-side boundary condition is of particular importance. We evaluate the influence of the slope of the sea-side boundary on the simulation results of seawater intrusion in a freshwater aquifer by employing a series of slope variations together with a sensitivity analysis by varying additional sensitive parameters (freshwater inflow and longitudinal and transverse dispersivities). Model results reveal a multi-dimensional dependence of the investigated variables with an increasing relevance of the sea-side boundary slope for seawater intrusion (decrease of up to 32%), submarine groundwater discharge zone (reduction of up to 55%), and turnover times (increase of up to 730%) with increasing freshwater inflow or dispersivity values. [ABSTRACT FROM AUTHOR]

Climate change is expected to induce sea level rise in the German Bight, which is part of the North Sea, Germany. Climate change may also modify river discharge of the river Weser flowing into the German Bight, which will alter both pressure and salinity distributions in the river Weser estuary. To study the long-term interaction between sea level rise, discharge variations, a storm surge and coastal aquifer flow dynamics, a 3D seawater intrusion model was designed using the fully coupled surface–subsurface numerical model HydroGeoSphere. The model simulates the coastal aquifer as an integral system considering complexities such as variable-density flow, variably saturated flow, irregular boundary conditions, irregular land surface and anthropogenic structures (e.g., dyke, drainage canals, water gates). The simulated steady-state groundwater flow of the year 2009 is calibrated using PEST. In addition, four climate change scenarios are simulated based on the calibrated model: (i) sea level rise of 1 m, (ii) the salinity of the seaside boundary increases by 4 PSU (Practical Salinity Units), (iii) the salinity of the seaside boundary decreases by 12 PSU, and (iv) a storm surge with partial dyke failure. Under scenarios (i) and (iv), the salinized area expands several kilometers further inland during several years. Natural remediation can take up to 20 years. However, sudden short-term salinity changes in the river Weser estuary do not influence the salinized area in the coastal aquifer. The obtained results are useful for coastal engineering practices and drinking water resource management. [ABSTRACT FROM AUTHOR]

A comparison of groundwater velocity in heterogeneous aquifers estimated from hydraulic methods, heat and solute tracers was made using numerical simulations. Aquifer heterogeneity was described by geostatistical properties of the Borden, Cape Cod, North Bay, and MADE aquifers. Both heat and solute tracers displayed little systematic under- or over-estimation in velocity relative to a hydraulic control. The worst cases were under-estimates of 6.63% for solute and 2.13% for the heat tracer. Both under- and over-estimation of velocity from the heat tracer relative to the solute tracer occurred. Differences between the estimates from the tracer methods increased as the mean velocity decreased, owing to differences in rates of molecular diffusion and thermal conduction. The variance in estimated velocity using all methods increased as the variance in log-hydraulic conductivity ( K) and correlation length scales increased. The variance in velocity for each scenario was remarkably small when compared to σ2ln( K) for all methods tested. The largest variability identified was for the solute tracer where 95% of velocity estimates ranged by a factor of 19 in simulations where 95% of the K values varied by almost four orders of magnitude. For the same K-fields, this range was a factor of 11 for the heat tracer. The variance in estimated velocity was always lowest when using heat as a tracer. The study results suggest that a solute tracer will provide more understanding about the variance in velocity caused by aquifer heterogeneity and a heat tracer provides a better approximation of the mean velocity. [ABSTRACT FROM AUTHOR]

This paper evaluates the remediation potential of a salinized coastal aquifer by utilizing a scenario simulation. Therefore, the numerical model OpenGeoSys is first validated against analytical and experimental data to represent transient groundwater level development and variable density saline intrusion. Afterwards, a regional scale model with a three-dimensional, heterogeneous hydrogeology is calibrated for a transient state and used to simulate a best-case scenario. Water balances are evaluated in both the transient calibration and scenario run. Visualization techniques help to assess the complex model output providing valuable insight in the occurring density-driven flow processes. Furthermore, modeling and visualization results give information on the time scale for remediation activities and, due to limitations in data quality and quantity reveal potential for model improvement. [ABSTRACT FROM AUTHOR]

• Cut-off wall can prevent lateral and vertical seawater intrusion. • Cut-off wall does not slow down the post-inundation recovery of freshwater storage in early stage. • Cut-off wall slows down the average recovery in high permeability islands. Freshwater lenses (FWL) underneath islands are often unique freshwater resources. Constructing cut-off walls in coastal islands to increase the FWL size has been proposed and tested effective. Meanwhile, seawater inundation induced by ocean surges on low-lying islands can cause the vertical intrusion of saltwater and salinization of FWL. Constructing cut-off walls can influence the post-inundation recovery once the FWL is salinized during the inundation. This influence has not been investigated and considered when discussing the feasibility of cut-off walls. In this study, we numerically simulated an ocean-surge inundation on a 2D, vertically oriented island constructed with a cut-off wall, using a coupled surface–subsurface approach. We varied the aquifer permeability and the depth of the cut-off wall. The effect of the cut-off wall on FWL was quantitatively assessed in terms of FWL volume and flushing time. The results show that the cut-off wall can significantly increase the volume of FWL, confirming previous studies, especially when it is constructed in high-permeability (>10−10.5 m2) islands and deeper. In addition, the cut-off wall can significantly reduce seawater infiltration by up to 40% during the inundation. Therefore, the recovery tends to be accelerated in the early stage, and in islands of low & intermediate permeability. However, the average recovery in high-permeability islands can be slowed down, because the cut-off wall reclaims larger and deeper space that is harder for flushing. As a first attempt to assess the feasibility of the cut-off wall construction from the perspective of FWL recovery, our study is important for the management of freshwater on islands facing the risk of ocean-surge inundation. [ABSTRACT FROM AUTHOR]

Abstract: Coastal aquifers are complex hydrologic systems because many physical processes interact: (i) variably saturated flow, (ii) spatial–temporal fluid density variations, (iii) tidal fluctuations, (iv) storm surges overtopping dykes, and (v) surface runoff of storm water. The HydroGeoSphere model is used to numerically simulate coastal flow dynamics, assuming a fully coupled surface–subsurface approach, accounting for all processes listed above. The diffusive wave approximation of the St. Venant equation is used to describe surface flow. Surface flow and salt transport are fully coupled with subsurficial variably saturated, variable-density flow and salt transport through mathematical terms that represent exchange of fluid mass and solute mass, respectively. Tides and storm surges induce a time-variant head that is applied to nodes of the surface domain. The approach is applied to real cases of tide and storm surge events. Tide simulation results confirm the existence of a recirculating zone, forming beneath the upper part of the intertidal zone. By monitoring the exchange fluid flux rates through the beach, it was found that the major inflow to the aquifer takes place at the upper part of the intertidal zone, which explains the formation of the recirculating zone. The recirculating zone is forming particularly during rising tide. Results from a storm surge simulation show that plume fingers develop below the flooded land surface. Natural remediation by seaward flowing freshwater is relatively slow, such that reducing the salt concentration in the aquifer down to drinking water standards takes up to 10years. [Copyright &y& Elsevier]

Groundwater is the most unexplored element of the hydrologic cycle on the Tibetan Plateau (TP) due to harsh climate conditions. This study aims at delineating and characterizing the unexplored Zhanongtang–Ganmanong aquifer, situated in the Zhagu subcatchment of the Nam Co catchment, south-central TP. Multiple hydrogeophysical and lithological in situ field and laboratory methods are applied: depth-to-water-table measurements, grain size analysis, hydraulic empirical and field methods to estimate hydraulic conductivity (K), and analysis of electrical resistivity tomography profiles. Integration of these methods revealed the existence of a Quaternary hydrostratigraphic unit that was found to be unconsolidated, laterally heterogeneous and homogeneous over depth. The results revealed consistent K ranges of three K zones, which is in accordance with local lithology. The K ranges are applicable to other locations within the Nam Co catchment with similar lithology as in the study area without further field experiments. Permafrost was found to be absent in the study area ranging from 4730 m a.s.l. to 5200 m a.s.l. altitude. These results provide insight into the hydrogeological conditions of the TP and are useful for conceptual and numerical groundwater flow modeling to predict future changes of water fluxes and water budgets caused by climatic change, especially in remote areas. [ABSTRACT FROM AUTHOR]

Pipe leakage consists of pipe water exfiltration (exchange flux from defect pipes into the variably saturated subsurface) and groundwater infiltration (exchange flux from the saturated subsurface into defect pipes). Pipe water exfiltration may lead to problems such as groundwater contamination. Groundwater infiltration may lead to a reduction of groundwater recharge, land subsidence and increased expenses to water cleaning facilities. Due to inaccessibility of pipe defects, mathematical models are required to quantify leakage and mitigate associated risks. Leakage models in literature can be distinguished into both, inaccurate simplified models with small computational costs which neglect unsaturated-saturated flow (e.g. DeSilva et al., 2007, Vizintin et al., 2009, Karpf & Krebs, 2011), and accurate coupled physically-based models with large computational costs, which include unsaturated-saturated flow (Kidmose et al., 2015; Peche et al., 2017). In the present study, we compare model results from a simplified and a coupled physically-based model. The conceptional model represents storm water pipe leakage in suburban residential areas. Based on model results, we investigate the validity range in which simplified models are sufficiently accurate and introduce a novel criterion, which can be used for choice of leakage model. We show that the use of this model choice criterion may reduce computational costs significantly while maintaining result accuracy.References:DeSilva, D., Burn, S., Moglia, M., Tjandraatmadja, G., Gould, S., and Sadler, P. (2007). Chapter. 2 the models: Network exfiltration and infiltration model neimo. Urban Water Resources Toolbox-Integrating Groundwater into Urban Water Management, pages 34-50.Karpf, C. and Krebs, P. (2011). A new sewage exfiltration model - parameters and calibration. Water Science and Technology, 63(10):2294-2299.Kidmose, J., Troldborg, L., Refsgaard, J., and Bischoff, N. (2015). Coupling of a distributed hydrological model with an urban storm water model for impact analysis of forced infiltration. Journal of Hydrology, 525:506-520.Peche, A., Graf, T., Fuchs, L., & Neuweiler, I. (2017). A coupled approach for the three-dimensional simulation of pipe leakage in variably saturated soil. Journal of Hydrology, 555, 569-585.Vizintin, G., Souvent, P., Veselič, M., and Curk, B. C. (2009). Determination of urban groundwater pollution in alluvial aquifer using linked process models considering urban water cycle. Journal of Hydrology, 377(3-4):261-273. [ABSTRACT FROM AUTHOR]

Despite that fractured coastal aquifers are widespread, the influence of fracture characteristics on seawater intrusion (SWI) has not been explored in previous studies. This research uses numerical modelling in a first step towards understanding the influence of fracture orientation, location and density on the extent of seawater and accompanying patterns of groundwater discharge in an idealised coastal aquifer. Specifically, aquifers containing single fractures or networks of regularly spaced fractures are studied using modified forms of the Henry SWI benchmark problem. The applicability of equivalent porous media (EPM) models for representing simple fracture networks in steady-state simulations of SWI is tested. The results indicate that the influence of fractures on SWI is likely to be mixed, ranging from enhancement to reduction in seawater extent and the width of the mixing zone. For the conceptual models considered here, vertical fractures in contact with the seawater wedge increase the width of the mixing zone, whereas vertical fractures inland of the wedge have minimal impact on the seawater distribution. Horizontal fractures in the lower part of the aquifer force the wedge seaward, whereas horizontal fractures located within the zone of freshwater discharge enhance the wedge. Inclined fractures roughly parallel to the seawater-freshwater interface increase the landward extent of seawater and fractures perpendicular to the interface inhibit the wedge. The results show that EPM models are likely inadequate for inferring salinity distributions in most of the fractured cases, although the EPM approach may be suitable for orthogonal fracture networks if fracture density is high and appropriate dispersivity values can be determined. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) is also called the "Third pole" because its aquifers constitute the origin for several major rivers which are the water supply for millions of people all over Asia. Due to increasing population especially on the Asian continent, water supply is getting increasingly important. As a consequence of climatic change in the past decades, increasing trends of precipitation, melting of glaciers and degradation of permafrost have generally lead to rising water levels in lakes on the TP. To ensure future water supply, aquifer characterisation is therefore an important issue on the TP. However, due to the remote character of the TP, hydrogeological aquifer information is scarce. The aim of this study is therefore to hydrogeologically characterise the Zhagu basin aquifer situated in the Nam Co Lake basin, which is the second largest lake on the TP. This project is part of the International Research Training Group "Geoecosystems in transition on the Tibetan Plateau" (TransTiP), funded by the DFG.During a field work campaign in July 2018, the Zhagu basin aquifer was hydrogeologically characterised by the following methods: (i) borehole profiles were excavated, (ii) material samples considering different drill core layer properties were taken to be analysed for hydrogeological parameters, (iii) piezometers to monitor piezometric heads were installed, and (iv) geophysical methods (electrical resistivity tomography, ERT) was used to identify hydrogeological units and aquifer properties. The initial drill core assessment on one selected borehole shows six different layer properties over a depth of 8 m. Overall, the aquifer consists mainly of sand including silty and gravelly components. In addition, piezometric head measurements reveal groundwater occurrence at 3 m below ground surface, which was confirmed by the ERT analysis. An important result is therefore that appearance of liquid groundwater was shown by independent methods. Furthermore, the ERT enables the extrapolation of the local borehole information to a 2D section to identify the aquifer geometry. The applied methods show consistent results for detecting changes in hydrogeological units and detection of groundwater in the Zhagu basin aquifer as part of the Nam Co Lake basin. Further analysis of samples taken at all boreholes and sieve analyses will be carried out in the future in order to generate a 3D hydrogeological picture of the Zhagu basin aquifer. The measurements, observations, interpretation and conclusion on the initial drill core and aquifer structure will be shown and discussed in the contribution. [ABSTRACT FROM AUTHOR]