Distribution, threats and protection of selected karst groundwater-dependent ecosystems in the Mediterranean region

Karst groundwater-dependent ecosystems (KGDEs) in the Mediterranean region are important in terms of ecosystem services and biodiversity but are increasingly under anthropogenic pressures and climate-change constraints. For this study, the ecohydrological characteristics, threats, and protection status of 112 selected KGDEs around the Mediterranean Sea, including caves, springs, rivers and wetlands, were evaluated, based on local expert knowledge and scientific literature. Results demonstrate that KGDEs contribute considerably to regional biodiversity. The diversity of karst landscapes, combined with the groundwater emergence at springs, leads to exceptional habitat diversity, particularly in arid climates, where KGDEs serve as a refuge for species that could not thrive in the surrounding environment. The most common threats identified among the selected sites are direct human disturbances, such as mass tourism or overfishing, water-quality deterioration and water shortage from aquifer overdraft and/or climate change. Although most of the selected sites are under protection, conservation measures are frequently insufficient. Such shortcomings are often caused by poor data availability, little knowledge on conservation needs of invertebrate species, and conflicts of interest with the local population. For this purpose, it is necessary to raise environmental awareness and promote interdisciplinary research, in order to monitor water quality and quantity in addition to the status of the biocenoses

Calibration of a semi-distributed lumped karst system model and application to the Qachqouch karst spring (Lebanon) under climate change conditions.

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Assessment of Key Transport Parameters in a Karst System Under Different Dynamic Conditions Based on Tracer Experiments: the Jeita Karst System, Lebanon

Artificial tracer experiments were conducted in the mature karst system of Jeita (Lebanon) under various flow conditions using surface and subsurface tracer injection points, to determine the variation of transport parameters (attenuation of peak concentration, velocity, transit times, dispersivity, and proportion of immobile and mobile regions) along fast and slow flow pathways. Tracer breakthrough curves (TBCs) observed at the karst spring were interpreted using a two-region nonequilibrium approach (2RNEM) to account for the skewness in the TBCs’ long tailings. The conduit test results revealed a discharge threshold in the system dynamics, beyond which the transport parameters vary significantly. The polynomial relationship between transport velocity and discharge can be related to the variation of the conduit’s cross-sectional area. Longitudinal dispersivity in the conduit system is not a constant value (α = 7–10 m) and decreases linearly with increasing flow rate because of dilution effects. Additionally, the proportion of immobile regions (arising from conduit irregularities) increases with decreasing water level in the conduit system. From tracer tests with injection at the surface, longitudinal dispersivity values are found to be large (8–27 m). The tailing observed in some TBCs is generated in the unsaturated zone before the tracer actually arrives at the major subsurface conduit draining the system. This work allows the estimation and prediction of the key transport parameters in karst aquifers. It shows that these parameters vary with time and flow dynamics, and they reflect the geometry of the flow pathway and the origin of infiltrating (potentially contaminated) recharge

Carbamazepine breakthrough as indicator for specific vulnerability of karst springs: Application on the Jeita spring, Lebanon

The pharmaceutical drug carbamazepine is considered an effective wastewater marker. The varying concentration of this drug was analyzed in a mature karst spring following a precipitation event. The results show that carbamazepine is an indicator of wastewater entering the system through a fast flow pathway, leading to an increase of the drug concentrations in spring water shortly after a strong rainfall event. The analysis of the breakthrough curve of carbamazepine along with the electrical conductivity signal and major ions chemograph allowed the development of a conceptual model for precipitation event-based flow and transport in the investigated karst system. Furthermore the amount of newly recharged water and the mass of carbamazepine reaching the aquifer system during the event could be estimated using a simple mixing approach. The distance between the karst spring and the potential carbamazepine source was estimated by the combination of results from artificial tracer tests and the carbamazepine breakthrough curve. The assessment of spring responses to precipitation event using persistent drugs like carbamazepine helps assess the effect of waste water contamination at a spring and gives therefore insights to the specific vulnerability of a karst spring

A New Normalized Groundwater Age-Based Index for Quantitative Evaluation of the Vulnerability to Seawater Intrusion in Coastal Aquifers: Implications for Management and Risk Assessments

International audienceThe vulnerability of coastal aquifers to seawater intrusion has been largely relying on data-driven indexing approaches despite their shortcomings to depict the complex processes of groundwater flow and mass transport under variable velocity conditions. This paper introduces a modelling-based alternative technique relying on a normalized saltwater age vulnerability index post-processed from results of a variable density flow simulation. This distributed index is obtained from the steady-state distribution of the salinity and a restriction of the mean groundwater age to a mean saltwater age distribution. This approach provides a novel way to shift from the concentration space into a vulnerability assessment space to evaluate the threats to coastal aquifers. The method requires only a sequential numerical solution of two steady state sets of equations. Several variants of the hypothetical Henry problem and a case study in Lebanon are selected for demonstration. Results highlight this approach ability to rank, compare, and validate different scenarios for coastal water resources management. A novel concept of zero-vulnerability line/surface delineating the coastal area threatened by seawater intrusion has shown to be relevant for optimal management of coastal aquifers and risk assessments. Hence, this work provides a new tool to sustainably manage and protect coastal groundwater resources

Modeling variable-density flow in saturated-unsaturated porous media: An advanced numerical model

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An advanced discrete fracture model for variably saturated flow in fractured porous media

International audienceAccurate modeling of variably saturated flow (VSF) in fractured porous media with the discrete fracture-matrix (DFM) model is a computationally challenging problem. The applicability of DFM model to VSF in real field studies at large space and time scales is often limited, not only because it requires detailed fracture characterization, but also as it involves excessive computational efforts. We develop an efficient numerical scheme to solve the Richards equation in discretely fractured porous media. This scheme combines the mixed hybrid finite element method for space discretization with the method of lines for time integration. The fractures are modeled as lower-dimensional interfaces (1D), within the 2D porous matrix. We develop a new mass-lumping (ML) technique for the fractures to eliminate unphysical oscillations and convergence issues in the solution, which significantly improves efficiency, enabling larger field applications. The proposed new scheme is validated against a commercial simulator for problems involving water table recharge at the laboratory scale. The computational efficiency of the developed scheme is examined on a challenging problem for water infiltration in fractured dry soil, and compared with standard numerical techniques. We show that the ML technique is crucial to improve robustness and efficiency, which outperforms the commonly used methods that we tested. The applicability of our method is then demonstrated in a study concerning the effect of climate change on groundwater resources in a karst aquifer/spring system in El Assal, Lebanon. Simulations, including recharge predictions under climate change scenarios, are carried out for about 80 years, up to 2099. This study demonstrates the applicability of our proposed scheme to deal with real field cases involving large time and space scales with high variable recharge. Our results indicate that the water-table level is sensitive to the presence of fractures, where neglecting fractures leads to an overestimation of the available groundwater amount. The proposed numerical approach is generic for DFM model and can be extended to different 2D and 3D finite-element frameworks

Effect of Flow‐Direction‐Dependent Dispersivity on Seawater Intrusion in Coastal Aquifers

International audienceAbstract Flow‐direction‐dependent (FDD) dispersivity in coastal aquifers (CAs) may strongly affect the inland extend of seawater intrusion (SWI) and the accompanying vertical salinity distribution. FDD dispersivity may predict greater inland intrusion of the saltwater wedge, but less vertical spreading of salinity than does the classical flow‐direction‐independent (FDI) dispersivity, the standard currently employed in most numerical CA models. Dispersion processes play a key role in the SWI process and directly affect CA pumped water quality. Constant FDI dispersivities may be inappropriate in representing mixing processes due to large differences between depth and horizontal salinity transport scales, and due to typical structured heterogeneities in aquifer fabrics. Comparison of FDI and FDD model forecasts for the classical Henry problem (HP) steady‐state SWI, based on a new HP semianalytical solution with FDD and on a numerical FDI model modified to additionally represent FDD, highlights the theoretical types of differences implied by these alternative dispersivity assumptions and exactly how each parameter affects the solution. Large differences between FDI and FDD dispersivity forecasts of time‐dependent SWI in large scale heterogeneous aquifers occur in a typical CA (Akkar CA, Lebanon). The FDD model forecasts that future salinities in pumping wells will exceed the potable water limit, whereas the FDI model greatly underestimates the historic inland intrusion of the saltwater wedge and forecasts no impact on future Akkar CA potable water supply. These results indicate the importance of employing the appropriate dispersion process representation when creating model‐based SWI forecasts, especially for developing effective CA management strategies