Stable isotope evidence for identifying the recharge mechanisms of precipitation, surface water, and groundwater in the Ebinur Lake basin

Evolution of groundwater hydrochemical characteristics and formation mechanism during groundwater recharge: A case study in the Hutuo River alluvial-pluvial fan, North China Plain

A pilot project for groundwater recharge from rivers is currently being carried out in North China Plain. To investigate the influence of river recharge on groundwater hydrochemical characteristics, dynamic monitoring and analysis of groundwater samples were conducted at a typical recharge site in the Hutuo River alluvial-pluvial fan in the North China Plain from 2019 to 2021. Hydrochemical, isotopic, and multivariate statistical analyses were used to systematically reveal the spatiotemporal variation of groundwater chemistry and its driving factors during groundwater recharge process. The results showed that the groundwater hydrochemical types and characteristics in different recharge areas and recharge periods exhibited obvious spatiotemporal differences. The groundwater type varied from HCO3·SO4-Na·Mg to HCO3·SO4-Ca·Mg in an upstream ecological area, while the groundwater type changed from SO4·HCO3-Mg·Ca to HCO3·SO4-Ca·Mg in the downstream impacted by reclaimed water. Changes in the contents of Ca2+, Mg2+ and HCO3- were mostly controlled by the water-rock interactions and mixing-dilution of recharge water, while the increases in Na+, NO3-, Cl-, SO42- and NO3- contents were mainly due to the infiltration of reclaimed water. Nitrogen and oxygen isotope (δ15N and δ18O) tests and the Bayesian isotope mixing model results further demonstrated that nitrate pollution mainly originated from anthropogenic sources, and the major contribution came from manure and sewage, with an average proportion of 64.6 %. Principal component analysis indicated that water-rock interactions, river-groundwater mixing and redox environment alternation were dominant factors controlling groundwater chemical evolution in groundwater recharge process.

Seasonal characteristics of groundwater discharge controlled by precipitation and its environmental effects in a coal mining subsidence lake, eastern China

Many regions have formed subsidence lakes due to underground mining in the world. However, seasonal variations of lacustrine groundwater discharge (LGD) rate and solute fluxes in the coal mining subsidence were rarely reported. In this study, we conducted four seasonal samplings in a coal mining subsidence, during which samples for stable water (δ18O) and radioactive (222Rn) isotopes were collected to quantify the seasonal dynamics of LGD rates. The LGD rates estimated from the 222Rn mass balance model were 10.2 ± 8.7, 5.5 ± 3.2, 11.5 ± 7.8, and 7.8 ± 4.5 mm d-1 in summer, autumn, winter and spring, respectively. According to the 18O mass balance model, the corresponding LGD rates were 15.1, 7.3, 15.6, and 11.3 mm d-1 in summer, autumn, winter and spring, respectively. We found a significant correlation between precipitation and LGD rates, suggesting precipitation was recognized as the main control factor for seasonal variations of LGD rates. Based on this correlation, the extrapolated LGD rates over a year ranged from 3.1 to 12.7 mm d-1 with an average of 8.8 mm d-1. Moreover, the fluxes of dissolved silicon (DSi), iron (Fe), and manganese (Mn) from LGD in autumn were (1.6 ± 0.9) × 105, (1.9 ± 1.1) × 104, and (1.1 ± 0.6) × 104 mol a-1, respectively. Correspondingly, in winter they were (3.5 ± 2.4) × 105, (4.1 ± 2.8) × 103, and (2.8 ± 1.9) × 103 mol a-1, respectively. This study demonstrated significantly seasonal variations of LGD, with precipitation being the main control factor of LGD in the coal mining subsidence lake. The fluxes of dissolved substance (DSi, Fe, Mn) from LGD need to be emphasized because they may have important impacts on the ecological stability in coal mining subsidence lakes.

Tracing spatial patterns of lacustrine groundwater discharge in a closed inland lake using stable isotopes

Tracing lacustrine groundwater discharge (LGD) is essential for understanding the hydrological cycle and water chemistry behaviour of lakes. LGD usually exhibits large spatial variability, but there are few reports on quantitatively revealing the spatial patterns of LGD at the whole lake scale. This study investigated the spatial patterns of LGD in Daihai Lake, a typical closed inland lake in northern China, based on the stable isotopes (δ2H and δ18O) of groundwater, surface water, and sediment pore water (SPW). The results showed that there were significant differences between the δ2H and δ18O values of different water bodies in the Daihai Lake Basin: groundwater < SPW < lake water. The LGD through SPW was found to be an important recharge pathway for the lake. Accordingly, stable isotopes of SPW showed that LGD in the northeastern and northwestern of Daihai Lake was significantly greater both horizontally and vertically than that in the other regions, and the proportions of groundwater in SPW in these two regions were 55.53% and 29.84%, respectively. Additionally, the proportion of groundwater in SPW showed a significant increase with profile depth, and the proportion reached 100% at 50 cm below the sediment surface in the northeastern of the lake where the LGD intensity was strongest. The total LGD to Daihai Lake was 1.47 × 107 m3/a, while the LGD in the northeastern and northwestern of the lake exceeded 1.9 × 106 m3/a. This study provides new insights into assessing the spatial patterns of LGD and water resource management in lakes.

Large-scale surface water-groundwater origins and connectivity in the Ordos Basin, China: Insight from hydrogen and oxygen isotopes

The sustainability of water resources is a major challenge for the Ordos Basin and Loess Plateau of China. The basis of effective water management is an understanding of the water cycle process. This study investigated the surface water-groundwater origins and connectivity using stable isotopes (δD and δ18O) of surface water and groundwater in 11 river basins in the Ordos Basin. It was found that the surface water-groundwater origins and hydraulic connection were characterized by regional differences, mainly induced by climatic characteristics, hydrogeological conditions and human activities. Specifically, the impact of thick loess deposits caused surface water and groundwater to take long time to produce a hydraulic connection. In contrast, areas with thin loess deposits and frequent human activities showed a good connectivity between surface water and groundwater. As for water origins, summer precipitation was a common source of surface water and groundwater in the study area, and groundwater discharge was another source of surface water. However, surface water and groundwater were subjected to different degrees of evaporation during receiving precipitation recharge. Notably, thick loess deposits had an impact on groundwater evaporation because both the recharge of precipitation to groundwater and the discharge of groundwater to surface water took a long time. In addition, it was found that frequent human activities (mining, irrigation and urban construction) could weaken the impact of evaporation. This large-scale analysis provided new insights into the origins and connectivity of surface water and groundwater in areas with thick unsaturated zones for water resources management.

Increasing areas of aquaculture ponds and reservoirs reshape runoff coefficients: evidence from a subtropical catchment, China

Detention pond is a key storm water management measure employed both to attenuate surface runoff and to regulate depression storage, yet the effects of aquaculture ponds and reservoirs on runoff coefficient are not well quantified in a subtropical humid monsoon climate zone, China. Here, a set of six subcatchments ranging in size from 0.7 hm² to 10,000 hm² were evaluated over the 2011-2015 period. (i) The annual average runoff coefficient differed with different subcatchments due to the spatial heterogeneity of landscape patterns, while the event-based runoff coefficient under the same catchment showed a decreasing trend with increasing rainfall intensity. (ii) The annual average and event-based runoff coefficients initially increased and then decreased with an increase in the area ratio of aquaculture ponds and reservoirs. The critical area ratio of aquaculture ponds and reservoirs for the maximum runoff coefficient in annual, light, and moderate rainfall intensity was about 4%; but this value would be transferred forward to the position of < 4% under the intensity of heavy rain, rainstorms, and heavy rainstorms. (iii) All runoff coefficients decreased with increasing forestland but increased with increasing paddy fields, and the decreasing rate was greater than the increasing rate. The trends of runoff coefficient for the annual and event-based rainfall are opposite between river development coefficient and watershed shape coefficient. The results provide underlying insights for decision-makers in aquaculture land-use planning and the sustainable utilization of water resources in the upstream and downstream systems of a catchment.

Water chemistry and stable isotope characteristics of subsidence lakes in coal mining areas, Eastern China

Many subsidence lakes have formed in eastern China as a result of underground coal mining. These coal mining-related subsidence lakes vary in their formation time and connectivity with rivers. These factors may influence the water chemistry and hydrogen and oxygen stable isotope characteristics of the lake water. This study collected and tested subsidence lake water, atmospheric precipitation, river water, and shallow groundwater in the study area. The results showed that the water chemical types of the subsidence lake water and river water are Cl-Na and HCO₃·Cl-Na and that the water chemical types of the shallow groundwater are mainly HCO₃·Cl-Na and HCO₃·Cl-Ca. There are no significant differences in the water chemical characteristics of subsidence lakes with different subsidence ages and types. The major ions in each water body mainly come from evaporite dissolution and silicate weathering, and ion exchange occurs. Reverse ion exchange occurs in some shallow groundwater samples. The stable isotopes of hydrogen and oxygen in the subsidence lake water, river water, and shallow groundwater are distributed along a straight line with a slope less than that of the LMWL, indicating that these water bodies have a common source, namely, precipitation. With increases in the formation time of the subsidence lakes, the heavy isotopes in the lake water gradually become depleted, and the d value gradually increases, mainly driven by precipitation dilution, weakening evaporation, river recharge, and groundwater recharge. The isotopic values of different types of lakes with the same subsidence time differ little. The research results may provide scientific guidance for the rational development and utilization of water resources in coal mining subsidence areas, enrich the study of the hydrological cycle in the area, and are of great significance for the protection of the local water balance and water environment.

Formation mechanism of hydrogeochemical characterization of mineral water in Antu County, Changbai Mountain area

Antu County in the Changbai Mountains is an important source of mineral water, but there is a lack of research on the source of groundwater characteristic components, affecting the protection of water resources. This study obtained hydrochemical and isotopic data (28 groups in total, April and September in 2019) by summarizing research and sampling data in order to identify the formation process of characteristics. The formation mechanism of the characteristic components was revealed using geostatistical, isotopic, and hydrogeochemical inversion simulations. The results show that the metasilicic acid is a common component of groundwater water chemistry in the study area. The water body primarily receives stable recharge from low-mineralized precipitation with ages ranging from 27.7 to 38.4 years and recharge elevations ranging from 1160 to 2393 m, providing ample time for water-rock interaction. The dissolution of olivine, pyroxene, albite, and other siliceous minerals is the source of characteristic components, and deep faults and deep basalt heat flow are the key conditions for the formation of metasilicic acid. When low-mineralized precipitation recharges the underground aquifer, it dissolves the silica-aluminate and silicon-containing minerals in the surrounding rocks through the water-rock action under the effect of CO₂, causing a large amount of metasilic acid to dissolve into the groundwater and forming metasilic acid-type mineral water.

Surface-subsurface hydrological processes of rainwater harvesting project in karst mountainous areas indicated by stable hydrogen and oxygen isotopes

Rainwater harvesting (RWH) projects in a decentralized way are significant measures to deal with the water scarcity dilemma in rural areas of the karst mountains in Southwest China at present. Due to the differences in cistern construction features and geomorphological positions, the water sources of cisterns were characterized by marked spatial variability, and the recharge stability of cisterns was strongly influenced by precipitation seasonality. Nevertheless, in hydrological processes on karst hillsides, the identification of different runoff types of RWH has not been sufficiently studied. The stable isotopes of hydrogen and oxygen of eleven cisterns and epikarst springs in subtropic cockpit karst landforms were monitored from 2020 to 2021 to investigate the runoff characteristics in RWH. Evaporative fractionation in different hydrological cycles is the predominant factor regulating the stable isotopic signature of cistern water. The results indicated that the typical roles that occurred in the recharge process contributed differently to water harvesting, with surface runoff (SR) and subsurface runoff (SSR) contributing much more than rainwater (RW) and epikarst runoff (ER). Three mixing patterns were proposed by end-member analysis in which SR + SSR, ER, and RW were three end members with indicators of isotopic value and the total dissolved solids (TDS). The recharge of SR + SSR was the predominated source, which contributed to 64% of the total water resources collected through RWH in the rainy season. In addition, the influence of various runoffs on the recharge stability of the cistern can be reflected by the multiple statistical analysis of isotopic fluctuation. Poor recharge stability is caused by excessive SR + SSR, whereas a higher percentage of ER and RW leads to better recharge stability. The applied method of hydrological process analysis is significant to the cistern water resources management in rural areas of the karst mountains.

Integration of a Shallow Soda Lake into the Groundwater Flow System by Using Hydraulic Evaluation and Environmental Tracers

Lake Velence is a shallow soda lake whose water level and water quality show a severely deteriorating tendency in recent years. Until recently, the groundwater component in the lake’s water budget has not been taken into consideration. To integrate the lake into the groundwater flow system at the regional scale, methods of “basin hydraulics” were applied. In addition, 17 water samples were collected for δ2H and δ18O, and for ΣU, 226Ra and 222Rn activity measurements to use these parameters as environmental tracers of groundwater contribution. Groundwater mapping revealed that groundwater recharges in Velence Hills and the local elevations south of the lake, whereas discharge occurs by the lake’s shoreline and along surface watercourses. The results indicated that Lake Velence is the discharge point of local groundwater flow systems known to be more sensitive to climate changes and anthropogenic activities (e.g., contamination, overexploitation). Groundwater and lake water have similar uranium activity concentrations serving as another sign of groundwater inflow into the lake. Therefore, it is necessary to consider both the groundwater component in the lake’s water management and its vulnerability regarding local and short-term changes in the catchment area.

Anthropogenic impacts on isotopic and geochemical characteristics of urban streams: a case study in Wuhan, China

Urbanization and human activities have significantly modified the geochemical signatures of urban streams worldwide. However, the geochemical characteristics of urban streams in Wuhan, one of the core cities in the Yangtze River Economic Belt in China, remain largely unstudied. Here, we examined the stable isotopes and geochemistry of urban streams at 73 locations in the central districts in Wuhan during May 2019. Maps of isotopic signatures reflected a non-free-flowing state in part of the urban stream system in Wuhan. A lower DO and a higher EC level were found in urban streams relative to the adjacent Yangtze River. The Na⁺, K⁺, and Cl^- concentrations in urban streams were > 3.0 times as high as those in the Yangtze River, and there was a slight increasing trend between 1.1 and 1.4 times for other major ions. The mildly elevated Fe concentration (1.3 times) and markedly elevated Mn concentration (> 5.0 times) were observed in urban streams. Spearman's correlation analysis indicated strong positive bivariate correlations among Na⁺, K⁺, and Cl^- in urban streams, and an urban geochemical principal component was identified by principal component analysis. Plotting Na/(Na + Ca) versus total dissolved solids (TDS) indicated a potential risk of "urban stream syndrome." These findings can enhance the knowledge of anthropogenic impacts on current urban stream water quality and provide reference for the restoration and improvement of water ecology functions of the urban stream system in Wuhan.

River damming and drought affect water cycle dynamics in an ephemeral river based on stable isotopes: The Dagu River of North China

The flow regime and biogeochemical cycles are greatly affected by river damming and drought, especially in ephemeral rivers. However, the combined effects have been rarely considered. This study, taking the Dagu River in Jiaodong Peninsula of North China as an example, investigated the dynamic changes in water cycle related to river damming and drought using stable water isotopes for the period 2018-2019. The results indicated that river water isotopes significantly varied temporally and spatially. The temporal variations in river water isotopes appeared to be linked with those in precipitation, but the relationship between river water and precipitation isotopes was greatly affected by river damming, river water-groundwater exchange and potential water pollution. Spatially, a single dam exhibited no significant effect on river water isotopes, but the accumulative impacts of cascade dams resulted in the enrichment of heavy isotopes in river water towards the downstream through increasing hydraulic residence time and water evaporation largely. The inter-annual variations in river water isotopes with increased evaporative fractionation were highlighted by their strong response to the drought in 2019. The combined effects of cascade dams and drought greatly changed water cycle dynamics and further exacerbated water shortage, which should thus be fully considered for water resource management, especially for regions with water-limited but heavily-regulated rivers.

Digital Mapping of Soil Organic Carbon Using Sentinel Series Data: A Case Study of the Ebinur Lake Watershed in Xinjiang

As an important evaluation index of soil quality, soil organic carbon (SOC) plays an important role in soil health, ecological security, soil material cycle and global climate cycle. The use of multi-source remote sensing on soil organic carbon distribution has a certain auxiliary effect on the study of soil organic carbon storage and the regional ecological cycle. However, the study on SOC distribution in Ebinur Lake Basin in arid and semi-arid regions is limited to the mapping of measured data, and the soil mapping of SOC using remote sensing data needs to be studied. Whether different machine learning methods can improve prediction accuracy in mapping process is less studied in arid areas. Based on that, combined with the proposed problems, this study selected the typical area of the Ebinur Lake Basin in the arid region as the study area, took the sentinel data as the main data source, and used the Sentinel-1A (radar data), the Sentinel-2A and the Sentinel-3A (multispectral data), combined with 16 kinds of DEM derivatives and climate data (annual average temperature MAT, annual average precipitation MAP) as analysis. The five different types of data are reconstructed by spatial data and divided into four spatial resolutions (10, 100, 300, and 500 m). Seven models are constructed and predicted by machine learning methods RF and Cubist. The results show that the prediction accuracy of RF model is better than that of Cubist model, indicating that RF model is more suitable for small areas in arid areas. Among the three data sources, Sentinel-1A has the highest SOC prediction accuracy of 0.391 at 10 m resolution under the RF model. The results of the importance of environmental variables show that the importance of Flow Accumulation is higher in the RF model and the importance of SLOP in the DEM derivative is higher in the Cubist model. In the prediction results, SOC is mainly distributed in oasis and regions with more human activities, while SOC is less distributed in other regions. This study provides a certain reference value for the prediction of small-scale soil organic carbon spatial distribution by means of remote sensing and environmental factors.

Quality change mechanism and drinking safety of repeatedly-boiled water and prolonged-boil water: a comparative study

Quality, safety and potability of repeatedly-boiled water (RBW) and prolonged-boil water (PBW) lead to concern and even misgivings in the public from time to time, especially in China, and other societies have a habit of drinking boiled water, with improvements of living standards and owing to increasing concerns for human health. This phenomenon is mainly attributed to the fact that the conclusions drawn from existing scientific experiments could not respond well to the concerns. In order to make up for this deficiency, tap water was selected to carry out RBW and PBW experiments independently. The quality changes of RBW and PBW show very similar trends that are not as great as might be imagined, and both are impacted by the tap water quality and the physiochemical effects. The dominating physiochemical effects are the water evaporation and the resulting concentration of unreactive components (most dissolved components), which can be easily explained by the existing evaporation-concentration theory. The results show that tap water will be still safe and potable after being frequently boiled or after having undergone prolonged boiling, as long as it satisfies the sanitary standards of drinking water prior to heating. Therefore, there is no need to worry about drinking RBW or PBW in daily life.

Isotopic characterisation and dating of groundwater recharge mechanisms in crystalline fractured aquifers: example of the semi-arid Banabuiú watershed (Brazil)

Sustainable groundwater management implies a good knowledge of recharge processes, especially in areas with water deficit, like the semi-arid region of Banabuiú watershed (Ceará State, Northeast of Brazil). In this zone, phreatic aquifers consist of Precambrian crystalline fractured reservoirs characterised by a high spatial anisotropy, both in terms of hydrodynamics and water quality. This study implemented a multi-tracer approach (¹⁸O, ²H, ¹⁴C, ³H, CFC, SF₆), combined with hydrodynamic data (i.e. groundwater levels) to identify the groundwater recharge origin and the recharge mechanisms, and to estimate the groundwater residence time. At the basin scale, hydrodynamic data and local observations indicated the high reactivity of aquifers to precipitation and suggested that infiltration processes occur mostly through preferential infiltration zones. Stable isotope data showed a major contribution of evaporated surface water in the recharge process from many artificial or natural ponds. Groundwater residence time determination highlighted the spatio-temporal heterogeneity of flow path organisation within aquifers, with variable contributions between fast vertical flow (present-day end-member; 15-85 %) and a slower horizontal flow (old end-member <1960), underlining the vulnerability of aquifers to present-day environmental stress or pollution.

Influence of Subsoiling on the Effective Precipitation of Farmland Based on a Distributed Hydrological Model

Effective precipitation plays an important role in crop growth, and subsoiling may have an impact on the effective precipitation of farmland. The question how subsoiling influences effective precipitations has prompted this research. The major objective of this study was to quantify the effect of subsoiling on effective precipitation of farmland. The main soil type in the study area is loam. Six scenarios were set with three factors, namely, the thickness of the soil ploughing layer, porosity, and soil permeability. The hydrological process from 2000 to 2015 was simulated with a distributed hydrological model. The results showed that a 10-cm increase in the soil thickness of the plough layer had little effect on the effective precipitation. When soil porosity increased by 0.1, the effective precipitation increased by approximately 19%. When the soil permeability coefficient increased by 0.5 times, the farmland and watershed surface runoff decreased by 24% and 13%, respectively, and the effective precipitation increased by 1.7%. This study proves that subsoiling has a positive effect on the local effective precipitation and confirms previous hypotheses.

Temporal Scaling of Water Level Fluctuations in Shallow Lakes and Its Impacts on the Lake Eco-Environments

Managing lake water levels from an ecological perspective has become an urgent issue in recent years in efforts to protect, conserve, and restore lake eco-environments. In this study, we considered the actual situation of Ebinur Lake basin to develop a lake water balance model using a System Dynamics (SD) method. The objective of this study is based on the lake water balance model to sufficiently understand the variation and relationship between the lake depth–area–volume. We combined field investigations and hydrological data analysis to expose the major factors affecting lake water level fluctuations (WLFs), as well as the impact of WLFs on lake eco-environments. All with the aim of providing a theoretical basis to manage Ebinur Lake ecosystems for conservation and restoration. The main findings of this study include: (I) The model’s calculation results agree with the observation value, as the monthly lake surface area was used to validate the model. (II) The factors influencing the dynamic changes in the water level of the lake are ranked in ascending order (from the lowest to the highest) as follows: Precipitation, groundwater recharge, evaporation, river inflow. (III) Fluctuations in water level play a significant role in lake shoreline displacement variation, and when the lake’s water level drops below 1 m, the surface area of the water body decreases to approximately 106 km2. (IV) The magnitude and frequency of WLFs drive major differences in the ecology of lake littoral zones, influencing not only the structure and functioning of benthic assemblages but also littoral habitat structure. These results established a quantitative linkage between hydrological variables and ecosystem health for the Ebinur Lake wetlands. These findings could be widely used in managing the Ebinur Lake basin as well as other similar water bodies, and could provide a useful tool for managing lake ecosystems for conservation and restoration.

Spatial Variation and Controlling Factors of H and O Isotopes in Lancang River Water, Southwest China

Climate changes and other human activities have substantially altered the hydrological cycle with respect to elevation. In this study, longitudinal patterns in the stable isotopic composition (δ²H and δ¹⁸O) of Lancang River water, originating from the Qinghai–Tibetan Plateau, are presented, and several controlling factors in the wet season are hypothesized. Lancang River water δ²H (−145.2‰ to −60.7‰) and δ¹⁸O (−18.51‰ to −8.49‰) were low but close to those of the Global Meteoric Water Line. In the upper reaches of the river, δ²H decreased longitudinally, potentially due to groundwater inputs and melting ground ice in the headwater zone and to an increasing proportion of glacier meltwater with decreasing elevation. In the middle reaches of the river, δ²H values increased slowly moving downstream, likely due to shifts in precipitation inputs, as evidenced by the isotopic composition of tributaries to the main stream. In the lower reaches of the river, the isotopic composition was relatively invariant, potentially related to the presence of large artificial reservoirs that increase the water resident time. The results reveal different hydrological patterns along an alpine river in central Asia associated with both natural and anthropogenic processes. Understanding the degree and type of human interference with the water cycle in this region could improve water management and water security.