Direct Recharge Research Articles

Spatiotemporal variation of water level in wetlands based on multi-source remote sensing data and responses to changing environments

Under changing environmental conditions, water level is a crucial indicator for assessing the wetland hydrological cycle. However, due to some wetlands being located in remote and widely dispersed areas, acquiring data on wetland water level changes presents significant challenges, making wetland water level monitoring exceptionally difficult. Wetlands are extensively distributed in western Jilin Province, China, and are experiencing significant degradation due to various factors including natural conditions, agricultural activities, and social development. To address this challenge, this study proposes a monitoring method that combines Sentinel-3 radar altimetry satellites with optical remote sensing images to obtain wetland water level data. Additionally, the study takes into account the Chinese government's Interconnected River System Network Project (IRSNP), classifying wetlands in western Jilin Province into three different water recharge scenarios: direct recharge through main and branch canals, indirect recharge through ditches, and no recharge to isolated wetlands. This study analyses the relationship between wetland water level changes and climatic factors, and assesses how IRSNP can mitigate the negative impacts of climate factors on wetland water levels across different recharge scenarios. The results show that: (1) the wetland water level monitoring method, has high accuracy and feasibility. The average difference between the in-situ measured and satellite-monitored water levels was 0.254 m. (2) There was an overall increasing trend in wetland water levels directly influenced by IRSNP, an insignificant decreasing trend in wetland water levels indirectly influenced by IRSNP. (3) Increased precipitation and decreased evaporation are the predominant climatic factors contributing to rising wetland water levels. Conversely, lower relative humidity and higher temperatures primarily lead to declining water levels. The construction of IRSNP can mitigate the impact of climate change on water levels. Thus, under changing environmental conditions, the implementation of IRSNP has positively impacted wetland protection and provides valuable insights for understanding wetland water level changes and managing water resources effectively.

Analysis on Spatial Variability of Hydrochemical Characteristics and Control Factors of Jinan Baotu Spring Area

To understand the karst groundwater hydrochemical characteristics and ion sources of the Jinan Baotu Spring area, this study focused on the three functional zones as an indirect recharge area, direct recharge area, and discharge area. Through water sample collection and testing, the spatial variability of groundwater chemical characteristics in different functional zones and the formation mechanism were analyzed using hydrochemistry parameter statistics, multivariate statistics, self-organizing map, hydrochemistry graphical analysis, ion ratios, and other methods, guided by the theory of groundwater flow system and combined with regional physical geography and hydrogeological conditions. The results showed that： the groundwater of each functional zone was alkaline as is typical in the dissolution of carbonate minerals. Owing to the different groundwater runoff pathways, the variability of water chemistry parameters in different functional areas was obvious. The groundwater of the discharge area was recharged by both the direct recharge area and the indirect recharge area. The hydrochemistry type changed from HCO3-Ca type to HCO3·SO4-Ca type through the indirect recharge area to the discharge area. The presence of a small amount of gypsum dissolution within the aquifer generated Ca2+ and SO42-. The ions in groundwater mainly came from the dissolution and filtration of aquifer rock minerals, and at the same time, they were affected by cation exchange, mineral dissolution equilibrium, and the combined effects of human activities. The groundwater in the Baotu Spring area was greatly influenced by human activities, which to some extent affected the evolution of groundwater hydrochemistry in the spring area.

How much rainwater contributes to a spring discharge in the Guarani Aquifer System: insights from stable isotopes and a mass balance model

ABSTRACT Outcrops play an important role in groundwater recharge. Understanding groundwater origins, dynamics and its correlation with different water sources is essential for effective water resources management and planning in terms of quantity and quality. In the case of the Guarani Aquifer System (GAS) outcrop areas are particularly vulnerable to groundwater pollution due to direct recharge processes. This study focuses on the Alto Jacaré-Pepira sub-basin, a watershed near Brotas, a city in the central region of the state of São Paulo, Brazil, where groundwater is vital for supporting tourism, agriculture, urban water supply, creeks, river and wetlands. The area has a humid tropical climate with periods of both intense rainfall and drought, and the rivers remain perennial throughout the year. Therefore, the aim of this study is to investigate the interconnections between a spring and its potential sources of contribution, namely rain and groundwater, in order to elucidate the relationships between the different water sources. To achieve this, on-site monitoring of groundwater depth, rainfall amount, and stable isotope ratios (deuterium (2H) and oxygen-18 (18O)) from rain, spring discharge, and a monitoring well was carried out from 2013 to 2021. The results indicate that the mean and standard deviations for δ18O in rainwater exhibit higher variability, resulting in −4.49 ± 3.18 ‰ VSMOW, while δ18O values from the well show minor variations, similar to those of the spring, recording −7.25 ± 0.32 ‰ and −6.94 ± 0.28 ‰ VSMOW, respectively. The mixing model’s outcomes reveal seasonal variations in water sources contribution and indicate that groundwater accounts for approximately 80 % of spring discharge throughout the year. Incorporating stable isotopes into hydrological monitoring provides valuable data for complementing watershed analysis. The values obtained support the significance of the aquifer as a primary source, thereby offering critical insights into stream dynamics of the region.

Evaluation of augmented infiltration based LIDs for low lying urbanizing coastal catchments: a case study of Chennai city, India

ABSTRACT The study assessed the effectiveness of infiltration-based Low Impact Development (LID) techniques and augmented infiltration LIDs (such as recharge shaft) in a coastal urban catchment in India using a semi-coupled approach between numerical models VS2DI and Storm Water Management Model (SWMM). Site specific VS2DI modelling indicated that the infiltration LIDs resulted in more saturation excess runoff due to the soil’s insufficient hydraulic conductivity and increased lateral flow. Catchment-scale modeling indicated that LIDs significantly improved water retention (p ≪ 0.05), with a 35% reduction in total runoff volume. The recharge shafts were shown to be more efficient in reducing runoff volume and peak runoff, particularly for high runoff rates (more than 22 mm/hr). The study suggests that direct recharge to the aquifer through recharge shafts could be a suitable solution to manage runoff and meet the demand for domestic water supply during times of water scarcity in coastal urban catchments.

Investigating the enigma of an irregular groundwater age pattern in a confined, presumed “fossil” complex aquifer through mixing cell flow modeling

Significant fluctuations in the groundwater (GW) age along the eastern flow path of the Nubian Sandstone Aquifer's (NSA), as derived from Krypton-81 groundwater dating, have suggested that this aquifer, located in Israel's Negev Desert and previously presumed to be a fossil, is not entirely isolated but mixes with younger and even recent water. The intermittent rejuvenation and drastic increases in the GW age across short distances most likely imply hydraulic connectivity with the surrounding aquifers, which contribute both younger and more ancient water to the NSA. The current study aims at modeling the GW flow system to locate and quantify its water sources despite the aquifer's hydrogeological complexity and the scarcity of hydrological data. We implemented the Mixing Cell Modeling (MCM) approach, understanding that the alternating rejuvenations and increases in the GW age downstream of the NSA's eastern flow trajectory reflect the mixing of the NSA's groundwater with young and old GW bodies, respectively. Thus, prompted by the 81Kr water age distribution, yet independent of the Kr radioisotope data, a multi-tracer mixing cell flow model was adopted based on a set of balance equations of water, dissolved minerals, and stable environmental isotopes. The findings indicate that (1) there is a small, yet substantial, intrusion of old brackish GW from a deep-seated, highly pressurized aquifer into the NSA in the northeastern Negev; (2) the rejuvenation of GW in the NSA is due to significant mixing with water from nearby overlying carbonate and chert aquifers, and (3) the NSA is substantially replenished through the Nubian Sandstone (NS) outcrops along the Negev Desert anticlines. Most GW intrusions into the NSA occur near the intersections of the eastern flow path with some of the Negev's major faults and synclines, such as the Paran and Ramon Fault zones and the Zin Syncline. In light of the relatively young GW age at the end of the NSA's western flow path in the northern Negev, and based on the similarities in the hydrogeological structures in the Negev and northern Sinai Deserts, we propose that similar mixing processes with GW from the overlying carbonate aquifers and direct GW recharge through the NS outcrops also occur in the northern Sinai Peninsula. The approach presented in this study might apply to examining recharge processes and hydraulic connectivity in other aquifers that were formerly classified as “fossil,” such as the immense NSA found in the Arabian (Jordan & Saudi Arabia) and the Western (Egypt) Deserts.

Hydrochemical and isotopic studies providing a new functional model for the coastal aquifers in Douala Coastal Sedimentary Basin (DCSB)/Cameroon

A new conceptual model of the hydrogeological systems in Cameroon's Douala Coastal Sedimentary Basin (DCSB) was constructed. The model is based upon the basin's known geology, plus data from recent field campaigns that allowed the collection of rainwater and groundwater samples for analyses of stable isotopes (δ2H, δ18O, δ13C), radiogenic isotopes (3H, 14C), and water chemistry. Aquifer characteristics that were thereby deciphered include recharge, isotopic distributions, residence times, and mixing processes. Rainfall samples (mean δ18O = −2.0 ‰; mean δ2H = −6.80 ‰; weighted mean = −2.4 ‰ δ18O, −9.85 ‰ δ2H) scatter along two distinct lines, thus indicating that local rainfall events undergo processes during convective events, variability in humidity, amount effects, and seasonal variations. Stable isotope values of river water samples are close to the weighted mean of local precipitation, with some downstream enrichment. The Quaternary/Mio-Pliocene superficial aquifer system (depth < 70 m) and the intermediate Oligocene/Upper Eocene aquifer system (depth: 70 to 200 m) exhibit evidence of similar fractionation processes through an enrichment gradient of δ-values. The enrichment is more pronounced at the top of the superficial aquifer, which is very exposed to direct rainfall water infiltration, evaporation, and amount effects. The depth profiles of δ-values coupled to water chemistry and tritium contents, evidence leakage between (i) the superficial system's Quaternary alluvium sands and Mio-Pliocene sands; and (ii) the superficial and intermediate systems. Thus, the aquifers that contain modern, post nuclear groundwater are characterized by flow exchanges and direct recharge from rainfall events. In contrast, the Upper Eocene system has depleted δ-values and lower bicarbonate contents, suggesting not only that this system was recharged by rapid infiltration (with limited effect of evaporation), but that this recharge occurred during a cooler time in the past. The residence times (computed from 14C dates) indicate uncorrected ages ranging from hundreds to thousands of years.

Developing a conceptual model of groundwater – Surface water interactions in a drought sensitive lowland catchment using multi-proxy data

Frequencies and intensities of droughts have increased due to climate change, providing an urgent impetus to improve understanding of ecohydrological fluxes in drought-sensitive areas where water and food security are threatened. We used a multi-proxy approach for assessing the effects of landuse and catchment properties on recharge and groundwater – surface water interactions during a prolonged drought starting in summer 2018 in a 66 km2 lowland, mixed landuse catchment (Demnitzer Mill Creek, Germany). We used water table monitoring, water stable isotopes, tritium isotopes, hydrogeochemical tracers, streamflow observations and geophysical investigations to characterise the spatial and temporal patterns of groundwater recharge in a shallow, unconfined aquifer system. Long-term groundwater levels showed a declining trend since 2011, which accelerated after 2018 resulting in increasingly intermittent streamflow. Geophysical surveys and groundwater monitoring indicated that shallow water tables (typically < 3 m deep) in low to moderate permeability surficial deposits are generally recharged during winter, leading to higher groundwater – surface water connectivity in riparian alluvial aquifers, which is the first order control on streamflow generation. This was supported by similar geochemical characteristics of groundwater and streamflow. Water stable isotopes indicated a high damping in groundwater with a bias towards winter precipitation and direct recharge. Groundwater age estimates using tritium showed high uncertainty but indicated that shallow groundwater was < 10 years old and generally similar to streamflow. Such multi-proxy approaches help understand changes in groundwater recharge and stream-aquifer interactions during droughts and contribute to the development of sustainable land and water management strategies for groundwater systems that are sensitive to climate change.

Optimized Irrigated Water Management Using Numerical Flow Modeling Coupled with Finite Element Model: A Case Study of Rechna Doab, Pakistan

The fate of agriculture in Pakistan is predominantly concerned with excessive water mining threats to the subsurface water resources. The current study integrates the Visual MODFLOW-2000 application to estimate the water balance of an aquifer bounded by the Chenab River in the West and the Ravi River in the East, which covers an area of about 2.98 million hectares. An assimilated method of groundwater flow is employed to characterize the flow dynamics of the Rechna Doab aquifer. The Digital Elevation Model (DEM) produced by the Shuttle Radar Topography Mission (SRTM) and a mesh of discretized cell size (2500 m) were incorporated into the model design. The conceptual model of the alluvial aquifer involves trifold vertical boundaries (an initial fold thickness set up to 150 m). The model input parameters are precipitation, seepage through irrigation, return flow, recharge, hydraulic conductivity and evapotranspiration. Empirical relations are established (at the basin scale) for the discharge input of irrigation canals. Model results confirm that groundwater flow follows the topographic configuration of the study area (i.e., northeast to southwest), and the seepage from irrigating canals and rainfall appeared to be the main source of groundwater recharge among various resources. The zone budget study under steady state simulation showed that the total direct recharge to the aquifer is calculated as 522,910 acre foot. The simulated water balance of the studied aquifer reflects more fluctuations in river leakage. The predictive optimized model reflects an adaptation of canal lining and installation of additional tube wells that will minimize canal seepage by 70% and lead to the reclamation of 37,000 acres of water-logged land for normal cropping.

Insights into karst groundwater hydrogeochemical characteristics and spatial evolution in the Jinan karst aquifer system, northern China

Abstract Karst aquifers are strategically important as they supply domestic water and are a resource for irrigation and industry in northern China. The heterogeneity of the karst aquifer medium makes it vulnerable to external influences. Here, samples of surface water and groundwater in a typical karst groundwater system of Jinan (China) were collected in December 2021, and the sample data were analyzed to further elucidate the hydrogeochemical processes. Results showed that the predominant water chemical type is HCO3-Ca, with a lesser proportion of type HCO3·SO4-Ca and SO4-Ca. The dominant water–rock interactions comprise the dissolution of carbonate minerals, gypsum, and halite and ion exchange. Dissolution/precipitation of calcite, dolomite, and gypsum determines the concentrations of Ca2+, Mg2+, HCO3-, and SO42-. In terms of spatial distribution, the indirect and direct recharge areas are dominated by calcite dissolution, followed by dolomite dissolution, and are prone to ion exchange. The hydrogeochemical formation mechanism of the discharge area is more complicated by other hydrochemical processes and anthropogenic activities. These results provide guidance for global karst groundwater resource management and pollution prevention.

Snowpack and groundwater recharge in the Atlas mountains: New evidence and key drivers

Study regionThe Atlas Mountains of Morocco, specifically the High Oum Er-rbiaa (HOER) and Ourika catchments. Study focusto identify the recharge processes within the semi-arid watersheds, in the Atlas Mountains, through monthly monitoring of snow, rainfall, surface water, and groundwater isotope signal, but also the usage of remote sensing data. New hydrological insights for the regionThe spatial-temporal analysis of groundwater and precipitation isotopes reveals significant spatial heterogeneity, primarily influenced by the geological variations in each aquifer. Temporal variations indicate that direct recharge occurs in response to winter precipitation, whereas a delayed response is observed during the summer when snow replenishes groundwater towards the end of the melting season. The findings are further supported by the "Gravity Recovery and Climate Experiment" (GRACE) dataset, which demonstrates that high values of Total Water Storage (TWS) align with groundwater isotopes. This highlights the substantial groundwater abstraction rate between March and June to compensate for the lack of precipitation during this period. The analysis of isotope data indicates that 50% of groundwater recharge in the upstream Jurassic aquifer and 80% in the downstream Triassic-Paleozoic aquifers in the HOER catchment is sourced from snowmelt. Similarly, in the Ourika catchment, snowmelt contributes 30% and 50% of groundwater recharge in the upstream and downstream portions of the catchments respectively. This disparity is due to different melting rates across altitudinal ranges and variations in the lithology of each catchment.

Effects of recharge process on groundwater nitrate concentration in an oasis of Tengger Desert hinterland, China.

Groundwater nitrate concentrations cannot be effectively diluted in an oasis of desert hinterland without direct recharge from external rivers. Therefore, there is an urgent need to understand the relationship between groundwater nitrate concentration and the groundwater recharge process. Using hydrochemicals, stable isotopes, LUCC, and combining these with the MixSIAR model, the distributions of groundwater nitrate concentration in the Dengmaying Basin (DMYB) in 2006 and 2020 were obtained. The contributions of groundwater recharge and nitrate sources were also quantified. With the development of agriculture in the DMYB, groundwater irrigation leakage has gradually become a crucial recharge source of groundwater, with a recharge proportion reaching 30.3%. From 2006 to 2020, under the influence of well irrigation and agricultural fertilization, the groundwater nitrate concentration in the DMYB increased significantly, with an increased range of 1.3 ~ 14.3mg L-1. Moreover, the δ15N-NO3- and δ18O-NO3- values of nitrate in cultivated soil water were similar to those in groundwater, which also proves the process of carrying nitrate from the vadose zone into groundwater by irrigation water. The contribution of anthropogenic sources (54.9%) to groundwater nitrate exceeded that of natural sources (45.1%) to groundwater nitrate in the DMYB. These results indicate that the potential for nitrate pollution in groundwater must be considered, even in desert oases.

Physico-chemical properties and stable isotope composition of springwater in Leuwisari subdistrict and surrounding, Tasikmalaya, West Java, Indonesia

Springs are a crucial source of water supply in quarternary volcanic environments, such as Galunggung Volcano in Tasikmalaya Regency, yet their quality and relationships remain poorly defined. Physicochemical properties and stable isotopes of hydrogen (δ2H) and oxygen (δ18O) were used to examine springwater’s eligibility as drinking water sources, assess the connectivity among springs and surface water, and evaluate the origin of groundwater resources. Compared with WHO standards, the springwater samples met the criteria for drinking water eligibility. Eight springs assessed are expected to originate from an unconfined aquifer actively recharging. The isotopic data indicate little or no influence of direct recharge from the rivers on this aquifer. These isotope data, integrated with available information on physical hydrogeology and water chemistry, will be used to identify and define the spring recharge and protection zones.

Stable Isotopic Evaluation of Recharge into a Karst Aquifer in a Glaciated Agricultural Region of Northeastern Wisconsin, USA

Ground water contamination from septic systems and the application of dairy cattle manure has been a long-standing problem in rural northeastern Wisconsin, especially in areas with thin soils over karstified Silurian dolostone bedrock, where as many as 60% of the wells show evidence of fecal contamination. We present the results of a citizen science supported water-isotope study in Kewaunee County, Wisconsin to evaluate aquifer recharge processes in the critical zone and to demonstrate the viability of time-series stable isotope data as a supplement to traditional water quality indicators in a contamination-prone aquifer. A meteoric water line was also constructed for Green Bay, Wisconsin, providing reasonable isotopic ranges for aquifer recharge events. Volunteer homeowners collected water samples from their domestic wells for a period of ~14 months to provide a measure of long-term isotopic variation in produced water and to determine whether event-driven responses could be identified using δ18O and δ2H isotopic values. Three shallower wells with a prior history of contamination exhibited significant seasonal variation, while the deepest well with the greatest soil thickness (above bedrock) showed less variation. For moderate precipitation events, the shallowest well showed as much as 5–13% of produced water coming from direct recharge, with smaller contributions for deeper wells. Our case study provides a clear example of how citizen science can collect useful time-series isotopic data to support groundwater recharge studies.

Large-scale quantification of groundwater recharge threshold conditions using machine learning classifications: An attempt over the Australian continent

The large-scale quantification of groundwater recharge threshold conditions is a bottleneck in the field of research, considering that groundwater recharge processes can be quite complex and affected by different factors in different regions. Traditional statistical approaches have either focused on local to regional scales or are limited to a single factor such as rainfall, resulting in a lack of a comprehensive understanding of groundwater recharge mechanisms on large scales. Considering that finding a threshold to instantly determine whether the groundwater is in the state of recharge or discharge is a classification task involving multiple factors, machine learning techniques with self-learning capabilities, efficient processing of big data, and mining relationships between multiple factors is, thus, applied to this study. In this study, three traditional machine learning classification techniques are explored; Classification and Regression Tree (CART), Random Forest (RF), and Logistical Regression (LR), to quantify specific threshold values for each condition (including rainfall, evaporation, soil moisture, runoff and vegetation) since they are more interpretable compared to advanced machine learning techniques such as neural networks. Using the Australian continent as an example, CART is the fastest method and provides an average classification accuracy (76%) at almost the same levels compared to RF (77–78%) and LR (79–80%). Meanwhile, CART is the only method to rank the importance of conditions while providing a specific threshold for each condition. When inferring the groundwater recharge mechanism through threshold conditions, the CART method suggests that a primary (the most important) threshold condition is sufficient for most parts of the Australian continent, e.g., rainfall as the primary threshold condition in northern Australia indicates a direct recharge mechanism. Only the coastal areas of southeastern and southwestern Australia are dominated by multiple threshold conditions, showing the complex groundwater recharge mechanism. Overall, the significance of this work is to provide the spatial maps of threshold conditions that mainly control groundwater recharge and their specific threshold values, which are crucial for managers to carry out a good plan over a large area. More importantly, this study demonstrates the potential of traditional machine learning methods to reveal groundwater recharge mechanisms, which may help further refine the development of groundwater models in the future.

Environmental isotope constraints and hydrogeochemical evolution of groundwater in the semi-arid national capital environs of Delhi, India

Changes in hydrological regimes due to rapid urbanization and increasing anthropogenic stress on hydro-geological system creating an alarming condition for the management of ground water resources. Isotope and hydro-geochemical integrated approach has been used to determine the processes governing chemical composition of groundwater and the mechanism influencing the chemical composition of groundwater in National Capital Region (NCR), Delhi. The chemical evolution of groundwater has studied by using flow direction, hydro-chemical facies, bivariate plots of ionic ratios and saturation index. Stable isotopes and tritium are used to understand the recharge mechanism. Pollution index of groundwater (PIG) is also used to check the level of contamination in the groundwater. In total, 57 water samples were collected for water chemistry, 36 water samples for environmental isotopes and 13 samples for tritium measurement. The dominant water type is Ca-Mg-HCO3 and mixed water type. The dominant cation are Ca2+ > Na+ > Mg2+ > K+ and dominant anions are HCO3− > Cl− > SO42− > NO3−. The bicarbonate type water is dominant in quartzite, implies rapid and direct recharge through fracture and joints. Ionic ratios and saturation indices show that major processes controlling groundwater evolution are silicate hydrolysis and reverse cation exchange in the quartzite. Average values of δ2H and δ18O are −43.66 ‰ and −6.28‰. Stable isotopic ratio of groundwater of alluvium area falling on evaporation line, due to the prevailing semi-arid and arid climatic conditions, evaporation is the predominant factor affecting the chemistry of groundwater. The stable isotope shows that the groundwater in the quartzite is mostly meteoric in origin and acting as a recharge area for the nearby alluvium aquifer. Tritium results reveal that groundwater occurring in quartzite and alluvium aquifer in western parts are older than that of groundwater occurring in alluvium aquifer in the eastern parts.13% of groundwater samples are falling in very high pollution zone mainly occurring in alluvium aquifer that lies in the Eastern parts. It is inferred that alluvium aquifer (low lying) is having greater impact of anthropogenic activity than quartzite aquifer (high land).

Hydrogeochemical characterization of the alluvial aquifer of Catania Plain, Sicily (South Italy)

A hydrogeochemical study was carried out on the shallow Catania Plain alluvial aquifer, in eastern Sicily to reconstruct its hydrogeological structure, the meteoric recharge and to assess the influence of human activities on groundwater. To characterize the geochemistry of the shallow aquifer, two sampling campaigns were carried out, August–October 2004 and April–May 2005 in 47 sites distributed throughout the plain. The samples were collected and analyzed for physical–chemical parameters and major ions, as well as stable isotopes (δ18O and δ2H). Alluvial deposits with heterogeneous grain sizes constitute the aquifer. Varying conditions of vertical and horizontal permeability lead to the presence of a multilayered aquifer with different conditions of confinement and partial interconnection among layers. The sampled waters were separated into four groups of different compositions due to the water–rock interaction with the different lithologies present in and around the study area. Maps of electrical conductivity and sulfate show a systematic control of land use, in correspondence with the biggest farms. High sulfate concentration is due to both the natural interaction between local meteoric waters and Etna’s plume and the mixing with groundwater coming from the area where evaporitic rocks of the Gessoso Solfifera formation are present. In addition, anthropogenic contamination cannot be ruled out. A rain gauge network, consisting of 3 sites located at different altitudes, was installed to collect rain waters to determine isotopic data (δ2H and δ18O) and to measure the monthly rainfall amount. Based on the isotopic composition of sampled waters, it has been established that beyond the direct meteoric recharge, the recharging areas are in the North (Mt. Etna) and the South (Hyblean Plateau).

Where does the runoff come from in dry season in the source region of the Yangtze River?

The present study focuses on the recharge source of the runoff of dry season based on the runoff change, stable isotopes and hydrochemical characteristics evidence in the source region of Yangtze River. The results showed that the runoff showed a significant increasing trend from 1956 to 2018 and the runoff increased by 21.5 m3/s·10a (Zhimenda station (ZMD)) and 40.0 m3/s·10a (Tuotuohe station (TTH)), especially in dry season. Moreover, the hydrogen and oxygen isotopes in dry season was more positive than that in wet season in the source region of Yangtze River. The main source of runoff in dry season is mainly affected by precipitation, supra-permafrost water and soil water in summer. Meanwhile, atmospheric precipitation, supra-permafrost water and soil water had a great impact on the recharge and the stable isotope characteristics of runoff of dry season. Furthermore, the hydrochemical type of runoff of dry season was similar as that for supra-permafrost water. Whether from the stable isotope or hydrochemical characteristics, it provides us with an idea that the main source of runoff of dry season is supra-permafrost water. Through the further analysis of recharge mode and recharge ratio of runoff of dry season, the major recharge sources of runoff of dry season is precipitation in autumn (TTH:58%, ZMD:61%). Moreover, the recharge patterns of runoff of dry season were direct recharge and indirect recharge. The subsequent findings could provide an overall understanding of the variational characteristics, causes, and sources of runoff of dry season and its hydrological effects in cold regions.

Enhancement of Groundwater Recharge from Wadi Al Bih Dam, UAE

Groundwater and harvested rainwater represent the only conventional freshwater resources in the United Arab Emirates (UAE). Groundwater resources in Wadi Al Bih, UAE, are sustainable due to the low exploitation rate for domestic and agricultural purposes. Thus, the groundwater depletion in this area is far less than in other parts of the country. The Wadi Al Bih area is very important for achieving water security in UAE. Therefore, the possible measures of increasing groundwater recharge (e.g., managed aquifer recharge (MAR) methods) are investigated in this paper. The available water resource data were collected, reviewed, validated, and stored in a GIS database. Then, a GIS-based water budget model (WBM) was developed to evaluate the available groundwater resources in Wadi Al Bih and recharge sources. The analyses showed that only 49% of the accumulated rainwater behind the dam is recharging the underlying aquifer. Due to the absence of any direct recharge techniques, the remaining 51% is lost by direct evaporation (15%), and as soil moisture increases in the unsaturated zone (36%), it will subsequently evaporate or percolate depending on the precipitation pattern and air temperature. The results of the WBM indicated that the freshwater resources were decreasing at an alarming rate of approximately thirty-five million cubic meters (MCM) per year until 2019. The groundwater storage and salinity were governed by the rates and patterns of precipitation. For example, the recharge resulting from the two consecutive maximum monthly precipitation events in December 2019 and January 2020 has significantly increased the fresh groundwater reserve and slightly retreated the saline/brackish water toward the shoreline. Moreover, a Mann–Kendall trend analysis was conducted to assess the influence of precipitation, temperature, and evaporation on groundwater recharge. The outcomes suggested that climate variables had a significant effect on groundwater supplies. The mitigation measures include revising groundwater withdrawal rates based on the annual recharge and enhancing recharge using different MAR techniques and dam operation plans.

Combined use of statistical Bayesian model and strontium isotopes deciphering the high complexity groundwater flow in the Guarani Aquifer System (GAS)

The Guarani Aquifer System (GAS) is one of the most important groundwater reservoirs in Latin America. Even though its geological aspects and hydraulic behavior are deeply studied, questions regarding hydraulic connections between the GAS and the overlying (SGAS) and underlying (PRE-GAS) aquifers within the Paraná Sedimentary Basin geological framework are recently being pursued. This paper presents a new tracer approach within the GAS, incorporating strontium isotopes in a statistical Bayesian model. The multi-pronged approach is used to quantify mixing proportions and identify different processes related to water-rock interaction and hydrochemical data. Strontium isotopic ratio ranged from enriched values (>0.711) associated with higher GAS groundwater contribution, to lower values associated with contribution from the SGAS (∼0.707) and PRE-GAS (∼0.709) units, as well as isotopic fingerprint associated with direct rainwater recharge. As groundwater flows through the GAS, the mixing proportions between the end-members changes along with the hydrogeochemical evolution. Near recharge zones, GAS represents the dominant source (38% ± 10%), followed by PRE-GAS (28% ± 16%), SGAS (19% ± 11%) and rainwater (14% ± 10%). In transition waters between outcrop and confined areas, the GAS contribution remains dominant (48% ± 9%), followed by PRE-GAS (37% ± 14%) and SGAS (14% ± 8%). In the GAS confined zone, where contributions from rainwater and SGAS is absent, 87Sr/86Sr ratio is more associated with waters from PRE-GAS (53% ± 16%) than from GAS (46% ± 16%). This behavior indicates that groundwater flow in the GAS represent a more complex and vulnerable groundwater mixing system, closely related to the regional geological context of a closed intracratonic basin, than previously understood. The use of strontium isotopes as a hydrogeochemical tracer applied in a statistical mixing model, contributes to a more refined understanding of regional groundwater flow and water origin in the GAS.

A Resilient and Nature-Based Drinking Water Supply Source for Saline and Arsenic Prone Coastal Aquifers of the Bengal Delta

Salinity causes a hostile environmental impact throughout the year in the coastal region of Bangladesh, and its severity increases day by day. Because of upstream freshwater flow reduction and massive groundwater extraction, salinity has increased substantially over the last three decades. Moreover, arsenic contamination in shallow groundwater makes the groundwater unsuitable for potable use. Consequently, the coastal area suffers from acute storage of safe water supply. Salinity also negatively impacts human activities, livelihood, agricultural production, and the aquatic ecosystem. Though the shallow aquifer contains high salinity and a small amount of Arsenic (As), the very shallow aquifer (within 3m to 8m) contains fresh water in many areas in the rainy season due to the direct recharge of rainwater. However, rainfall recharge varies significantly depending on the geological and hydrogeological settings. Specifically, up to 50% of annual rainfall is stored in shallow aquifers of Quaternary sands through direct infiltration. The research’s principal objective is to identify the safe and sustainable drinking water source in the arsenic and saline-prone coastal region. Groundwater samples were collected from the different locations of the study area during both dry and wet seasons and examined seasonal variations in groundwater table and salinity levels. The chemical analyses and Physico-chemical parameters indicate that the groundwater samples are suitable for drinking. Except for some groundwater samples from the wet season, the salinity of all samples was under the allowable limit for Bangladesh (&lt;2000 µS/cm), and the targeted aquifer was almost arsenic (50 µg/l) free. Therefore, a comprehensive analysis has been made to accomplish the study goals. Particularly, the groundwater’s electrical conductivity (EC) values of most samples were measured within the limit of fresh or brackish water (&lt;2000 μS/cm). Overall, the results indicate the prospect of a very shallow aquifer as a source of freshwater for drinking purposes throughout the year, considering both arsenic and salinity, which effectively solve the freshwater shortage, especially in the saline-arsenic prone area.