Nitrate is found in groundwater due to natural and anthropic processes. Nitrate content in groundwater is associated with factors such as human activities, soil type, climate, geology and chemistry of groundwater. Some of these factors (climate and geology) coincide with those that determine the type of groundwater flow system (local, intermediate or regional) present in an area which, in turn, is influenced by climate, stratigraphy, and type of subsoil and surface rocks; therefore, it is expected that the concentration of nitrate is related to the type of groundwater flow. The relationship between the concentration of nitrate in groundwater samples and the type of flow was analyzed in an aquifer system located in the Trans-Mexican Volcanic Arc, within the Michoacán-Guanajuato volcanic complex. The system is composed of two hydrogeological units, one volcanic and the other sedimentary, with the presence of geological faults, in a context where there is agricultural activity and deficient domestic wastewater management. To improve understanding of the overall aquifer system, 34 groundwater samples (28 wells, 6 springs) were analyzed. The results indicate that each flow system presents characteristic patterns of nitrate concentration and groundwater chemical composition. A high nitrate concentration was found in local and local-intermediate flow systems. Nitrate concentration decreased from local to intermediate and regional flows. The nitrate concentration decreased depending on groundwater flow direction, so it is possible to use nitrate as a parameter to differentiate groundwater flow systems.

Crystalline aquifers are among the most complex groundwater systems, requiring adequate methods for realistic characterization and suitable techniques for improving the long-term management of groundwater resources. A tool is needed that can assess the aquifer hydrodynamic parameters cost-effectively. A model is presented, based on a groundwater-budget equation and water-table fluctuation method, which combines the upscaling and the regionalization of aquifer parameters, in particular specific yield (Sy) in three dimensions (3D) and the recharge in two dimensions (2D) from rainfall at watershed scale. The tool was tested and validated on the 53-km² Maheshwaram watershed, southern India, at a 685 m × 685 m cell scale, and was calibrated on seasonal groundwater levels from 2011 to 2016. Comparison between computed and observed water levels shows an absolute residual mean and a root mean square error of 1.17 and 1.8 m, respectively, showing the robustness of the model. Sy ranges from 0.3 to 5% (mean 1.4%), which is in good agreement with previous studies. The annual recharge from rainfall is also in good agreement with earlier studies and, despite its strong annual variability (16–199 mm/year) at watershed scale, it shows that spatial recharge is clearly controlled by spatial structure, from one year to another. Groundwater levels were also forecasted from 2020 to 2039 based on the climate and groundwater abstraction scenarios. The results show severe water-level depletion around 2024–2026 but it would be more stable in the future (after 2030) because of a lower frequency of low-rainfall monsoons.

The quantification of vertical groundwater fluxes across semi-confining layers is fundamental to evaluate groundwater recharge and discharge rates to and from aquifer systems. Methods to estimate vertical groundwater fluxes from temperature–depth profiles have been available since the 1960s. While some methodologies assume steady-state conditions, changes in land-surface temperatures as well as hydrogeological conditions can lead to transient heat flow conditions. Indeed, many studies have indicated that transient temperatures in deeper confined aquifers are widespread. A study is presented that uses transient-temperature–depth curves obtained from groundwater observation wells in the Chianan coastal plain in southern Taiwan. In this area, sedimentary aquifer systems consist of a stack of alternating sand and mud layers, over several hundred meters in thickness. Groundwater has been abstracted from these aquifers for decades, resulting in large hydraulic gradients between the shallow and deeper aquifers. Hence, vertical groundwater flow is likely enhanced across finer-grained, semi-confining units. A set of temperature–depth profiles is available from this area. Constrained by these profiles, numerical models of one-dimensional transient heat transfer were used to infer vertical fluxes of 3.3 × 10⁻⁸ to 3.9 × 10⁻⁸ m/s using thermal data from 2013 to 2016. An analytical solution was also employed that assumes steady-state conditions. Calculated fluxes using the latter approach were lower, at approximately 1.1 × 10⁻⁸ to 1.6 × 10⁻⁸ m/s. The study suggests that vertical fluxes derived from using Bredehoeft and Papadopulos’s analytical solutions result in underestimates of actual vertical seepage rates across aquitards.

This study presents a three-dimensional reactive transport model to simulate upward and lateral migration of CO₂ plumes under different scenarios through a leaky section of a plugged and abandoned well at a hypothetical CO₂ storage site into the Texas Gulf Coast Aquifer (TGCA). The TGCA is the most active region in research and operation of carbon capture, utilization, and storage, with the largest technically accessible resource of CO₂ storage, in the United States. The results suggest that dissolved inorganic carbon (DIC) concentration and pH, better than Cl concentration (or total dissolved solids), can be indicators for leakage detection in the TGCA; DIC has earlier detection time than pH. The modeling results show that detection of CO₂ leakage in the shallow aquifers may take hundreds of years because of the confining unit in the TGCA, suggesting that (1) monitoring wells should be placed as deep as possible and (2) characterization of confining units in the overlying aquifer system is critical. Regional hydraulic gradient and groundwater pumping in the TGCA are important factors for monitoring well placement. While this study was conducted in the TGCA, the results provide valuable information for groundwater monitoring at other geological carbon sequestration sites.

Groundwater management decisions require robust methods that allow accurate predictive modeling of pollutant occurrences. In this study, random forest regression (RFR) was used for modeling groundwater nitrate contamination at the African continent scale. When compared to more conventional techniques, key advantages of RFR include its nonparametric nature, its high predictive accuracy, and its capability to determine variable importance. The latter can be used to better understand the individual role and the combined effect of explanatory variables in a predictive model. In the absence of a systematic groundwater monitoring program at the African continent scale, the study used the groundwater nitrate contamination database for the continent obtained from a meta-analysis to test the modeling approach; 250 groundwater nitrate pollution studies from the African continent were compiled using the literature data. A geographic information system database of 13 spatial attributes was collected, related to land use, soil type, hydrogeology, topography, climatology, type of region, and nitrogen fertilizer application rate, and these were assigned as predictors. The RFR performance was evaluated in comparison to the multiple linear regression (MLR) methods. By using RFR, it was possible to establish which explanatory variables influence the occurrence of nitrate pollution in groundwater (population density, rainfall, recharge, etc.). Both the RFR and MLR techniques identified population density as the most important variable explaining reported nitrate contamination. However, RFR has a much higher predictive power (R² = 0.97) than a traditional linear regression model (R² = 0.64). RFR is therefore considered a very promising technique for large-scale modeling of groundwater nitrate pollution.

The Kasserine Aquifer System (KAS) is a transboundary aquifer, located in an arid region in central Tunisia and extending into northeastern Algeria. The system consists of four compartments: Oum Ali-Thelepte, Feriana-Skhirat, and the Plateau and the Plaine of Kasserine. The challenge of this study was to evaluate the influence of regional faults on groundwater flow in the different compartments of the KAS and to estimate the regional impact of current and future groundwater use. A three-dimensional saturated regional groundwater flow model for the steady state and transient conditions (1980–2015) was created and calibrated. This work was achieved using numerical flow modelling, coupled with geological modelling, using FEFLOW and GeoModeller software. The significance of regional faults as potential barriers or conduits to groundwater flow in the different aquifer compartments was evaluated by considering the different recharge rates. Two connectivity hypotheses were proposed at each major fault, and the general hydraulic relationship of units that are juxtaposed by each fault were considered. This study contributes rigorous estimates for the diffuse and concentrated recharge in the arid study region, and evaluates the groundwater behavior that shows a gradual decline in the water table over time, using a regional model. Different predicted outcomes for the KAS based on variable potential groundwater extraction scenarios for the period 2015–2050 have been developed. The results of numerical simulation provide useful information regarding the behavior of the KAS aquifers, and contribute significant knowledge to guide sustainable practice for present and future groundwater management.

The assessment of groundwater vulnerability/sensitivity to pollution in karstic aquifers usually concentrates on recognition of fast-flow (conduit flow) and slow-flow (diffuse flow) components or intermediate regimes and their ratio in the total discharged volume. Analysis of master recession curves and correlation between physical characteristics of springs and temporal variations in spring water chemistry were applied to two major karst springs of Albania: Selita Spring (mean discharge 510 L s⁻¹), exploited for Tirana water supply, and Blue Eye Spring (mean discharge 18,182 L s⁻¹), used for electric power generation. These springs are recharged by precipitation in two very different karst areas with respect to their karstification degree, which influences also groundwater circulation patterns within karstic aquifers. Different regional groundwater flow types are subsequently reflected in the different spring hydrographs and in the temporal hydrochemical variations. Based on the spring master recession curves, Selita Spring is characterised as a conduit spring where the fast-flow component represents the majority of groundwater flow, and its catchment area should be linked with a high degree of sensitivity to pollution. On the other hand, in the discharge regime of Blue Eye Spring, the slow-flow component dominates, and although having a discharge of one order of magnitude bigger, this is a diffuse-flow spring and its catchment area should have lower sensitivity to potential pollution. The same results were also confirmed by statistical treatment of the temporal variations in spring water chemistry and evidence of surface karst phenomena in their recharge areas.

The isotopic compositions (D and ¹⁸O) of 177 precipitation samples collected at seven observation stations in Toyama Prefecture and one station in Gifu Prefecture in the northern part of central Japan were determined. The source and characteristics of the isotopes were clarified and their contribution to the groundwater flow systems of three major alluvial fans in the same area were investigated. The δD and δ¹⁸O values ranged from −113.3 to −26.7‰ and − 16.4 to −4.2‰, respectively. Precipitation samples collected from May to September (summer) and November to March (winter) plotted along two meteoric water lines, with d-excess = 10 and 30, respectively. Conversely, precipitation samples collected in April and October, and some samples in November to March, plotted between the two meteoric water lines. The contribution of precipitation to the groundwater systems was modelled based on the assumption that groundwater is a mixture of major river water and precipitation. According to the observed δ¹⁸O values for the precipitation, river water, and groundwater samples, the contribution of local precipitation to groundwater reservoirs ranged from 5 to 100%. Groundwater flows near the rivers did not always originate from 100% river runoff; however, the contribution of river runoff to groundwater decreased with increasing distance from the rivers, and groundwater flows far from the river were generated mainly by precipitation. The possibility of using groundwater for a ground-source heat pump system, for air conditioning in houses and to melt the snow on roads, is also discussed.