Hydrogeological assessment of a major spring discharging from a calcarenitic aquifer with implications on resilience to climate change

Groundwater is a vital source of freshwater, serving ecological, environmental, and societal needs. In regions with springs as a predominant source, such as the Northern Apennines (Italy), resilience of these springs to climate-induced recharge changes is crucial for water supply and ecosystem preservation. In this study, Nadìa Spring in the Northern Apennines is examined through an unprecedented array of multidisciplinary analyses to understand its resilience and unique characteristics. The Nadìa Spring's exceptional response, characterized by a sustained base flow even in the face of drought, is attributed to a combination of factors including a substantial groundwater reservoir, a complex network of faults/fractures, slope instabilities, and karst dissolution. The investigation reveals a dual porosity system in the aquifer, consisting of fast-flow conduits and a diffuse fracture network. While fast-flow conduits contribute to rapid responses during high-flow conditions, the diffuse system becomes predominant during low-flow periods. This dual porosity structure helps the spring maintain a consistent base flow in the face of climate-induced recharge fluctuations. The study shows that Nadìa Spring exhibits remarkable resilience to year-to-year variations in recharge, as evidenced by stable minimum discharge values. While the spring has undergone a decline in discharge over the past century due to long-term climate change, it is becoming more resilient over interdecadal timescales due to transition to a diffuse drainage system that mitigates the impact of reduced recharge. The availability of a century-long spring discharge monitoring was a crucial piece of information for understanding the spring's discharge response and drawing conclusions about its long-term resilience to recharge fluctuations. Continuing long-term monitoring and research in the future will be essential to validate and expand upon these findings in the context of changing climatic conditions. This research serves as a model for assessing strategic groundwater discharge points in geological settings similar to the Northern Apennines.

A Fractional-order dual-continuum model to capture non-Fickian solute transport in a regional-scale fractured aquifer

Contaminant transport in fractured media exhibits complex dynamics, including multiple peaks in breakthrough curves (BTCs) and non-Fickian diffusion, thereby posing significant challenges to the application of traditional transport models. Here we undertook a detailed study of a natural-gradient tracer test conducted in a regional-scale fractured carbonate aquifer situated in southwestern Germany, where the observed BTCs contained both dual peaks and positive skewness. These BTCs were used to optimize parameters and interpret their physical meanings for several transport models, including the dual-continuum model (DCM) and the fractional derivative equation (FDE) model. Tracer concentration distributions were simulated in both single- and dual-continuum media employing the DCM and FDE models. Our results demonstrated that while the DCM model could reasonably replicate the bimodal BTC, the FDE (which accounts for solute retention) outperformed in capturing the heavy-tailed BTC. This was attributed to the limitations of grid-based numerical models that assume Fickian diffusion and fail to map small-scale medium heterogeneity exhaustively. In contrast, a parsimonious model like the FDE, with upscaled parameters, was found to be more effective in capturing regional-scale non-Fickian transport. To further characterize the multiple BTC peaks the standard FDE missed, we proposed a fractional derivative dual-continuum model (fDCM). This model was found to be adept at capturing both the multi-peak and late-time heavy tail in the BTC. Our study thus opens an alternate pathway for modeling solute transport in regional-scale fractured to partially karstified aquifers.

A DFN-based framework for probabilistic assessment of groundwater contamination in fractured aquifers

It is challenging to conduct groundwater contamination risk assessment in fractured aquifers containing a large number of complex fractures, especially in a situation where the uncertainty of massive fractures and fluid-rock interactions is inevitable. In this study, a novel probabilistic assessment framework based on discrete fracture network (DFN) modeling is proposed to assess the uncertainty of groundwater contamination in fractured aquifers. The Monte Carlo simulation technique is employed to quantify the uncertainty of fracture geometry, and the environmental and health risks of the contaminated site are probabilistically analyzed in conjunction with the water quality index (WQI) and hazard index (HI). The results show that the contaminant transport behavior in fractured aquifers can be strongly affected by the distribution of the fracture network. The proposed framework of groundwater contamination risk assessment is capable of practically accounting for the uncertainties involved in the mass transport process and effectively assessing the contamination risk of fractured aquifers.

Low antimicrobial resistance in Escherichia coli isolates from two large Austrian alpine karstic spring catchments

The increased occurrence of antibiotic resistant bacteria poses a threat to global public health. Clinically relevant resistances also spread through the environment. Aquatic ecosystems in particular represent important dispersal pathways. In the past, pristine water resources have not been a study focus, although ingestion of resistant bacteria through water consumption constitutes a potentially important transmission route. This study assessed the antibiotic resistances in Escherichia coli populations in two large well-protected and well-managed Austrian karstic spring catchments representing essential groundwater resources for water supply. E. coli were detected seasonally only during the summer period. By screening a representative number of 551 E. coli isolates from 13 sites in two catchments, it could be shown that the prevalence of antibiotic resistance in this study area is low. 3.4 % of the isolates showed resistances to one or two antibiotic classes, 0.5 % were resistant to three antibiotic classes. No resistances to critical and last-line antibiotics were detected. By integrating fecal pollution assessment and microbial source tracking, we could infer that ruminants were the main hosts for antibiotic resistant bacteria in the studied catchment areas. A comparison with other studies on antibiotic resistances in karstic or mountainous springs highlighted the low contamination status of the model catchments studied here, most likely due to the high protection and careful management while other, less pristine catchments showed much higher antibiotic resistances. We demonstrate that studying easily accessible karstic springs allows a holistic view on large catchments concerning the extent and origin of fecal pollution as well as antibiotic resistance. This representative monitoring approach is also in line with the proposed update of the EU Groundwater Directive (GWD).

Precise application of grouting technology in underground coal mining: water inrush risk of floor elimination

Many coal mines in China suffer from the Ordovician limestone (OL) and Carboniferous limestone (CL) containing water inrush frequently due to the complex geology condition; water inrush has become a serious disaster in coal mining. To prevent water-bursting disaster from the floor of mine and cutting down deaths and economic loss, a grouting method for limestone fracture aquifer has been widely adopted. In this paper, a time-dependent empirical grouting model considering (1) time-dependent behavior of the injected slurry, (2) response of the rock, and (3) interaction between the slurry and rock was proposed to describe the slurry diffusion mechanism in rock fracture. Aiming at the problem of water disaster prevention of a coal seam floor in 16,104 working face of Luxi coal mine, that was, the grouting failed to seal the water flow channel of aquifer, the causes of the failure of grouting for water sealing were analyzed. According to the aquifer parameters and model calculation results, a grouting scheme with a slurry diffusion radius of 20 m and grouting pressure of 8 MPa was proposed; 2 rounds of grouting were implemented, following the split spacing principle, to ensure that finer fractures can be sufficiently sealed. Finally, with the comparative analysis through the network parallel electrical method (NPEM) and the drilling exploration method for water-rich areas before and after grouting, it was verified that the grouting effect for floor limestone aquifer was good, and the grouting scheme proposed and the grouting pressure designed were reasonable. Results may provide reference to a karst fractured aquifer grouting project, as well as reduced the losing because of blindness and experience.

Groundwater resources and recharge processes in the Western Andean Front of Central Chile

In Central Chile, the increment of withdrawals together with drought conditions has exposed the poor understanding of the regional hydrogeological system. In this study, we addressed the Western Andean Front hydrogeology by hydrogeochemical and water stable isotope analyses of 23 springs, 10 boreholes, 5 rain-collectors and 5 leaching-rocks samples at Aconcagua Basin. From the upstream to the downstream parts of the Western Andean Front, most groundwater is HCO₃-Ca and results from the dissolution of anorthite, labradorite and other silicate minerals. The Hierarchical Cluster Analysis groups the samples according to its position along the Western Andean Front and supports a clear correlation between the increasing groundwater mineralization (31-1188 μS/cm) and residence time. Through Factorial Analysis, we point that Cl, NO₃, Sr and Ba concentrations are related to agriculture practices in the Central Depression. After defining the regional meteoric water line at 33°S in Chile, water isotopes demonstrate the role of rain and snowmelt above ~2000 m asl in the recharge of groundwater. Finally, we propose an original conceptual model applicable to the entire Central Chile. During dry periods, water releases from high-elevation areas infiltrate in mid-mountain gullies feeding groundwater circulation in the fractured rocks of Western Andean Front. To the downstream, mountain-block and -front processes recharge the alluvial aquifers. Irrigation canals, conducting water from Principal Cordillera, play a significant role in the recharge of Central Depression aquifers. While groundwater in the Western Andean Front has a high-quality according to different water uses, intensive agriculture practices in the Central Depression cause an increment of hazardous elements for human-health in groundwater.

Strong release of viruses in fracture flow in response to a perturbation in ionic strength: Filtration/retention tests and modeling

Effluents derived from a municipal wastewater treatment plant were used for virus filtration/retention experiments by using a horizontal laboratory filter. Filtration tests were performed to examine how soil geochemical heterogeneity and fracture patterns affected the transport of viruses in groundwater in order to model the influence of reductive perturbations in ionic strength (IS) during wastewater filtration. Although perturbations of IS and velocity are known to result in resuspension of colloids, we found that the effect of soil geochemical heterogeneity can produce strong and instantaneous virus releases in fractured aquifers, likely an internal additional source of viruses. Sixteen limestone slabs were packed in a PVC box filter at the Bari Laboratory (South Italy) to replicate wastewater filtration throughout a fractured medium similar to the Bari carbonate aquifer. Terra rossa, which is an aggregate of sand, silt and clay, was unevenly spread on the surface of each limestone slab within the filter. Since the mineralogical composition of terra rossa includes iron (hematite, magnetite, and goethite) oxides, the soil exhibited localized unfavorable colloid/collector interactions for attachment. In contrast, soil-free parts of the fracture surfaces maintained favorable colloid/collector interactions. We found in our experiments that the lowering of IS due to the reduction of water salt content, which could occur during runoff injections after rainfall, might be sufficient to cause strong detachment of viruses from fracture surfaces, allowing further migration into the groundwater. The model in this work can predict the count and pathways of released viruses in groundwater fractures under soil geochemical heterogeneity and originated by reductions of IS, by using analytical solutions.

Assessment of the impact of sea-level rise due to climate change on coastal groundwater discharge

An assessment of sea intrusion into coastal aquifers as a consequence of local sea-level rise (LSLR) due to climate change was carried out at Murgia and Salento in southern Italy. The interpolation of sea-level measurements at three tide-gauge stations was performed during the period of 2000 to 2014. The best fit of measurements shows an increasing rate of LSLR ranging from 4.4mm/y to 8.8mm/y, which will result in a maximum LSLR of approximately 2m during the 22nd century. The local rate of sea-level rise matches recent 21st and 22nd century projections of mean global sea-level rise determined by other researchers, which include increased melting rates of the Greenland and Antarctic ice sheets, the effect of ocean thermal expansion, the melting of glaciers and ice caps, and changes in the quantity of stored land water. Subsequently, Ghyben-Herzberg's equation for the freshwater/saltwater interface was rewritten in order to determine the decrease in groundwater discharge due to the maximum LSLR. Groundwater flow simulations and ArcGIS elaborations of digital elevation models of the coast provided input data for the Ghyben-Herzberg calculation under the assumption of head-controlled systems. The progression of seawater intrusion due to LSLR suggests an impressive depletion of available groundwater discharge during the 22nd century, perhaps as much as 16.1% of current groundwater pumping for potable water in Salento.

Contribution of the finite volume point dilution method for measurement of groundwater fluxes in a fractured aquifer

Measurement of groundwater fluxes is the basis of all hydrogeological study, from hydraulic characterization to the most advanced reactive transport modeling. Usual groundwater flux estimation with Darcy's law may lead to cumulated errors on spatial variability, especially in fractured aquifers where local direct measurement of groundwater fluxes becomes necessary. In the present study, both classical point dilution method (PDM) and finite volume point dilution method (FVPDM) are compared on the fractured crystalline aquifer of Ploemeur, France. The manipulation includes the first use of the FVPDM in a fractured aquifer using a double packer. This configuration limits the vertical extent of the tested zone to target a precise fracture zone of the aquifer. The result of this experiment is a continuous monitoring of groundwater fluxes that lasted for more than 4 days. Measurements of groundwater flow rate in the fracture (Q(t)) by PDM provide good estimates only if the mixing volume (V(w)) (volume of water in which the tracer is mixed) is precisely known. Conversely, the FVPDM allows for an independent estimation of V(w) and Q(t), leading to better precision in case of complex experimental setup such as the one used. The precision of a PDM does not rely on the duration of the experiment while a FVPDM may require long experimental duration to guarantees a good precision. Classical PDM should then be used for rapid estimation of groundwater flux using simple experimental setup. On the other hand, the FVPDM is a more precise method that has a great potential for development but may require longer duration experiment to achieve a good precision if the groundwater fluxes investigated are low and/or the mixing volume is large.

Use of electrical tomography methods to determinate the extension and main migration routes of uncontrolled landfill leachates in fractured areas

This study focuses on the uses of the electrical tomography and its relationship with hydrochemical data in order to characterize contaminated groundwater flows in fractured aquifers. The studied area is contaminated with different hazardous substances like lyndanes, organochlorinated compounds and benzenes coming from the old non-controlled Sardas landfill. The enormous volumes of wastes filling the landfill have generated a convoluted mixture of leachates. Due to the lack of a landfill liner, the leachates have migrated through the fractured Eocene marls towards the Gallego River. The striking correlation between high concentrations of polluted groundwater and low electrical resistivity of the subsurface (<8Ω·m) allows defining the principal contaminant migration route thanks to the distribution of these conductive anomalies. This mapping verifies that there is intense tectonical-structural control of the leachate migration, because the deep migration presents the same direction as the geological axis fold.

Particle Swarm Optimization for inverse modeling of solute transport in fractured gneiss aquifer

Particle Swarm Optimization (PSO) has received considerable attention as a global optimization technique from scientists of different disciplines around the world. In this article, we illustrate how to use PSO for inverse modeling of a coupled flow and transport groundwater model (MODFLOW2005-MT3DMS) in a fractured gneiss aquifer. In particular, the hydroPSO R package is used as optimization engine, because it has been specifically designed to calibrate environmental, hydrological and hydrogeological models. In addition, hydroPSO implements the latest Standard Particle Swarm Optimization algorithm (SPSO-2011), with an adaptive random topology and rotational invariance constituting the main advancements over previous PSO versions. A tracer test conducted in the experimental field at TU Bergakademie Freiberg (Germany) is used as case study. A double-porosity approach is used to simulate the solute transport in the fractured Gneiss aquifer. Tracer concentrations obtained with hydroPSO were in good agreement with its corresponding observations, as measured by a high value of the coefficient of determination and a low sum of squared residuals. Several graphical outputs automatically generated by hydroPSO provided useful insights to assess the quality of the calibration results. It was found that hydroPSO required a small number of model runs to reach the region of the global optimum, and it proved to be both an effective and efficient optimization technique to calibrate the movement of solute transport over time in a fractured aquifer. In addition, the parallel feature of hydroPSO allowed to reduce the total computation time used in the inverse modeling process up to an eighth of the total time required without using that feature. This work provides a first attempt to demonstrate the capability and versatility of hydroPSO to work as an optimizer of a coupled flow and transport model for contaminant migration.

Management of aquifer recharge in Lebanon by removing seawater intrusion from coastal aquifers

This study investigates the feasibility of management of aquifer recharge (MAR) in Lebanon by designing well barriers to remove seawater intrusion from the fractured carbonate aquifers. Groundwater flow and saltwater/freshwater 50% sharp interface have been modeled along the coastal area using the Ghyben-Herzberg theory. The groundwater flow simulations have been supported by field transmissivity estimations and depth measurements carried out on 44 wells during 2003. Results have shown the seawater intrusion in coastal aquifers at Jieh and Damour regions. Three well-injection barriers have been proposed. The water volumes for recharge and the barrier positions have been defined by means of groundwater flow simulations. MAR can provide a valuable contribution to colloid (even pathogen) removal from injectant water, although during water infiltration in subsoil the reduction of aquifer permeability causes clogging. A simple new model for estimating the soil-rock permeability reduction due to the well clogging has been presented. The MAR, including the soil aquifer treatment at Damour and Jieh regions, has been studied by considering aquifer transmissivity (and soil porosity) reduction caused by clogging. Furthermore, the appropriate mixing of the injectant water by using reclaimed water, groundwater and surface water can be simulated using the proposed models. The time required to achieve 5% of rock permeability reduction at the proposed well barriers ranged from 71 to 935 d, by changing water quality and flow rate for recharge. This study can assist regional governments with water management in areas affected by scarcity of freshwater by implementing appropriate well-barrier projects.