A study in eastern Taiwan evaluated the importance of montane water contribution (MC) to adjacent valley-plain groundwater (VPG) in a tectonic suture zone. The evaluation used a ternary natural-tracer-based end-member mixing analysis (EMMA). With this purpose, VPG and three end-member water samples of plain precipitation (PP), mountain-front recharge (MFR), and mountain-block recharge (MBR) were collected and analyzed for stable isotopic compositions (δ²H and δ¹⁸O) and chemical concentrations (electrical conductivity (EC) and Cl⁻). After evaluation, Cl⁻ is deemed unsuitable for EMMA in this study, and the contribution fractions of respective end members derived by the δ¹⁸O–EC pair are similar to those derived by the δ²H–EC pair. EMMA results indicate that the MC, including MFR and MBR, contributes at least 70% (679 × 10⁶ m³ water volume) of the VPG, significantly greater than the approximately 30% of PP contribution, and greater than the 20–50% in equivalent humid regions worldwide. The large MC is attributable to highly fractured strata and the steep topography of studied catchments caused by active tectonism. Furthermore, the contribution fractions derived by EMMA reflect the unique hydrogeological conditions in the respective study sub-regions. A region with a large MBR fraction is indicative of active lateral groundwater flow as a result of highly fractured strata in montane catchments. On the other hand, a region characterized by a large MFR fraction may possess high-permeability stream beds or high stream gradients. Those hydrogeological implications are helpful for water resource management and protection authorities of the studied regions.

Geothermal resources are practical and competitive clean-energy alternatives to fossil fuels, and study on the recharge sources of geothermal water supports its sustainable exploitation. In order to provide evidence on the recharge source of water and circulation dynamics of the Tangshan Geothermal System (TGS) near Nanjing (China), a comprehensive investigation was carried out using multiple chemical and isotopic tracers (δ²H, δ¹⁸O, δ³⁴S, ⁸⁷Sr/⁸⁶Sr, δ¹³C, ¹⁴C and ³H). The results confirm that a local (rather than regional) recharge source feeds the system from the exposed Cambrian and Ordovician carbonate rocks area on the upper part of Tangshan Mountain. The reservoir temperature up to 87 °C, obtained using empirical as well as theoretical chemical geothermometers, requires a groundwater circulation depth of around 2.5 km. The temperature of the geothermal water is lowered during upwelling as a consequence of mixing with shallow cold water up to a 63% dilution. The corrected ¹⁴C age shows that the geothermal water travels at a very slow pace (millennial scale) and has a low circulation rate, allowing sufficient time for the water to become heated in the system. This study has provided key information on the genesis of TGS and the results are instructive to the effective management of the geothermal resources. Further confirmation and even prediction associated with the sustainability of the system could be achieved through continuous monitoring and modeling of the responses of the karstic geothermal reservoir to hot-water mining.

The use of snowmelt as an inexpensive multi-component tracer solution for active aquifer characterization is investigated, creating a valid alternative to existing artificial water isotope labelling using enriched deuterium oxide (²H₂O) and water-¹⁸O (H₂¹⁸O). The approach directly takes advantage of natural differences between groundwater and precipitation. It is shown, at laboratory-scale and small field-scale, that a direct injection of snowmelt into a porous medium allows for the tracing of water flow and, therefore, for the determination of transport parameters based on the stable isotope signatures (δ²H and δ¹⁸O) and on the sum parameter electrical conductivity (EC). The differences in the isotope signature between the snowmelt and groundwater applied in this study were significant, with ∆(δ²H) = 61.0‰ and ∆(δ¹⁸O) = 8.2‰, while the EC difference was ~0.5 mS/cm. Stable isotope breakthrough was observed to be almost congruent to sodium chloride (laboratory tracer experiment) and to uranine (field-scale push-drift-pull test), clearly supporting the assumption of conservative transport. A crosscheck of the isotope data in δ²H-δ¹⁸O plots revealed no significant biases in the tests. On the other hand, the snowmelt’s EC breakthrough suffered from a slight retardation due to ion exchange and mineral reactions.

Past discussions around water-resources management and development in the River Nile basin disregard groundwater resources from the equation. There is an increasing interest around factoring the groundwater resources as an integral part of the Nile Basin water resources. This is hampered by knowledge gap regarding the groundwater resources dynamics (recharge, storage, flow, quality, surface-water/groundwater interaction) at basin scale. This report provides a comprehensive analysis of the state of surface-water/groundwater interaction from the headwater to the Nile Delta region. Piezometric and isotopic (δ¹⁸O, δ²H) evidence reveal that the Nile changes from a gaining stream in the headwater regions to mostly a loosing stream in the arid lowlands of Sudan and Egypt. Specific zones of Nile water leakage to the adjacent aquifers is mapped using the two sources of evidence. Up to 50% of the surface-water flow in the equatorial region of the Nile comes from groundwater as base flow. The evidence also shows that the natural direction and rate of surface-water/groundwater interaction is largely perturbed by human activities (diversion, dam construction) particularly downstream of the Aswan High Dam in Egypt. The decrease in discharge of the Nile River along its course is attributed to leakage to the aquifers as well as to evaporative water loss from the river channel. The surface-water/groundwater interaction occurring along the Nile River and its sensitivity to infrastructure development calls for management strategies that account groundwater as an integral part of the Nile Basin resources.

Stable isotopic composition (δ²H, δ¹⁸O) of river water, groundwater, and paddy water in the Mun River catchment, northeast Thailand, were determined to investigate the hydrological processes and the impacts of natural and anthropogenic activities on the water cycle. Quantities of δ²H (−93.9 to −25.4‰) and δ¹⁸O (−12.24 to −2.22‰) in river water in the wet season follow the trend: upper reaches > middle reaches ≈ lower reaches. Trends for δ²H (−52.3 to −22.0‰) and δ¹⁸O (−6.37 to −1.36‰) in the dry season are: upper reaches ≈ middle reaches > lower reaches. In the dry season, groundwater (δ²H: −57.5 to −34.6‰, δ¹⁸O: −8.24 to −4.40‰) shows a lighter isotopic composition, and paddy water (δ²H: −18.2‰, δ¹⁸O: −0.72‰) shows the highest isotopic composition. Spatial variation of δ¹⁸O and deuterium excess suggests that groundwater exchanges with surface water frequently. Rainfall and river water recharge groundwater in the wet season, and groundwater flows back to the river in the dry season, especially in the middle reaches. This process is most likely related to impoundment of the rivers by large dams. On the other hand, the lowest values of stable isotopes of river water are coincident with the extreme flooding that was produced by Tropical Storm Sonca in July 2017. This study contributes to a better understanding of hydrological processes in the Mun River catchment and provides a perspective on the application of stable isotopes to other large tropical monsoon catchments around the world.

Understanding groundwater salinity sources in the Gulf Coast Aquifer System (GCAS) is a critical issue due to depletion of fresh groundwater and concerns for potential seawater intrusion. The study objective was to assess sources of groundwater salinity in the GCAS using ∼1,400 chemical analyses and ∼90 isotopic analyses along nine well transects in the Texas Gulf Coast, USA. Salinity increases from northeast (median total dissolved solids (TDS) 340 mg/L) to southwest (median TDS 1,160 mg/L), which inversely correlates with the precipitation distribution pattern (1,370– 600 mm/yr, respectively). Molar Cl/Br ratios (median 540–600), depleted δ²H and δ¹⁸O (−24.7‰, −4.5‰) relative to seawater (Cl/Br ∼655 and δ²H, δ¹⁸O 0‰, 0‰, respectively), and elevated ³⁶Cl/Cl ratios (∼100), suggest precipitation enriched with marine aerosols as the dominant salinity source. Mass balance estimates suggest that marine aerosols could adequately explain salt loading over the large expanse of the GCAS. Evapotranspiration enrichment to the southwest is supported by elevated chloride concentrations in soil profiles and higher δ¹⁸O. Secondary salinity sources include dissolution of salt domes or upwelling brines from geopressured zones along growth faults, mainly near the coast in the northeast. The regional extent and large quantities of brackish water have the potential to support moderate-sized desalination plants in this location. These results have important implications for groundwater management, suggesting a current lack of regional seawater intrusion and a suitable source of relatively low TDS water for desalination.

Groundwater age has been used to map renewability of water resources within four groups: strong, partial, and rare renewability, and non-renewable. The Baiyang alluvial fan in NW China is a representative area for examining groundwater recharge from river infiltration and for mapping groundwater renewability, and it has been investigated using multiple isotopes and water chemistry. Systematic sampling included 52 samples for ²H and ¹⁸O analysis and 32 samples for ³H, ¹³C and ¹⁴C analysis. The δ¹³C compositions remain nearly constant throughout the basin (median −12.7‰) and indicate that carbonate dissolution does not alter ¹⁴C age. The initial ¹⁴C activity of 80 pmC, obtained by plotting ³H and ¹⁴C activity, was used to correct groundwater ¹⁴C age. The results show that areas closer to the river consist of younger groundwater ages; this suggests that river infiltration is the main recharge source to the shallow groundwater system. However, at distances far away from the river, groundwater ages become older, i.e., from modern water (less than 60 year) to pre-modern water (from 60 to 1,000 years) and paleowater (more than 1,000 yeas). The four classifications of groundwater renewability have been associated with different age ranges. The area of shallow groundwater with strong renewability accounts for 74% of the total study area. Because recharge condition (river infiltration) controls overall renewability, a groundwater renewability map is of significant importance to the management of groundwater exploitation of this area as well as other arid groundwater basins.

Deuterium, oxygen-18 and chloride were analyzed for 84 samples from deep and shallow wells, precipitation and the river White Nile to investigate groundwater recharge/discharge relations in the semi-arid central Sudan. Spatial and vertical variation in isotopic signature and chloride concentration in the groundwater show similar patterns and indicate local recharge and evaporative discharge. Progressive decrease in isotopic composition along the regional groundwater flow path demonstrates aquifer continuity down the NW–SE recharge-discharge path. Isotope-heavy recharged water progressively mixes with lighter older groundwater formed during cooler and humid conditions in the late Pleistocene. However, evaporative fractionation in the flow path’s final reach in the southeast re-enriches the isotopic composition and suggests evaporative loss of groundwater as the plausible discharge mechanism. Chloride concentration increases down the gradient from the recharge area and reaches its peak in the discharge zones indicating: lack of recharge from direct infiltration down the gradient, evaporation and prolonged rock/water interaction. Head differences and increased isotopic concentration in the vicinity of the White Nile suggest recharge from the river from subsurface flow. Reduced chloride content and relatively heavier isotopic composition in the deep groundwater beneath the wadi of Khor Abu Habil indicate recharge from the streambed into the deep aquifer.

Assessing groundwater recharge characteristics (recharge rate, history, mechanisms (piston and preferential flow)) and groundwater age in arid and semi-arid environments remains a difficult but important research frontier. Such assessments are particularly important when the unsaturated zone (UZ) is thick and the recharge rate is limited. This study combined evaluations of the thick UZ with those of the saturated zone and used multiple tracers, such as Cl, NO₃, Br, ²H, ¹⁸O, ¹³C, ³H and ¹⁴C, to study groundwater recharge characteristics in an integrated loess tableland in the Loess Plateau, China, where precipitation infiltration is the only recharge source for shallow groundwater. The results indicate that diffuse recharge beneath crops, as the main land use of the study area, is 55–71 mm yr⁻¹ based on the chloride mass balance of soil profiles. The length of time required for annual precipitation to reach the water table is 160–400 yrs. The groundwater is all pre-modern water and paleowater, with corrected ¹⁴C age ranging from 136 to 23,412 yrs. Most of the water that eventually becomes recharge originally infiltrated in July–September. The Cl and NO₃ contents in the upper UZ are considerably higher than those in the deep UZ and shallow groundwater because of recent human activities. The shallow groundwater has not been in hydraulic equilibrium with present near-surface boundary conditions. The homogeneous material of the UZ and relatively old groundwater age imply that piston flow is the dominant recharge mechanism for the shallow groundwater in the tableland.

Groundwater recharge and evolution in the Shule River basin, Northwest China, was investigated by a combination of hydrogeochemical tracers, stable isotopes, and radiocarbon methods. Results showed the general chemistry of the groundwater is of SO₄ ²⁻ type. Water–rock reactions of halite, Glauber’s salt, gypsum and celestite, and reverse ionic exchange dictated the groundwater chemistry evolution, increasing concentrations of Cl⁻, Na⁺, SO₄ ²⁻, Ca²⁺, Mg²⁺ and Sr²⁺ in the groundwater. The δ¹⁸O and δ²H values of groundwater ranged from −10.8 to −7.7 and −74.4 to −53.1 ‰, respectively. Modern groundwater was identified in the proluvial fan and the shallow aquifer of the fine soil plain, likely as a result of direct infiltration of rivers and irrigation returns. Deep groundwater was depleted in heavy isotopes with ¹⁴C ages ranging from 3,000 to 26,000 years, suggesting palaeowater that was recharged during the late Pleistocene and middle Holocene epochs under a cold climate. These results have important implications for groundwater management in the Shule River basin, since large amounts of groundwater are effectively being mined and a water-use strategy is urgently needed.