Evaporite Weathering Research Articles

Application of major ions and Sr isotopes to indicate the evolution of river water and shallow groundwater chemistry in a typical endorheic watershed, northwestern China

Studying the hydrochemical evolution of river water and groundwater is vital for comprehending complex regional hydrological cycles and managing water resources. An investigation of hydrochemical evolution mechanism of river water and groundwater was executed in an arid and semi-arid endorheic watershed located in the eastern edge of Qaidam Basin, northwest China, by analyzing major ions and strontium isotopic compositions with the combination of hydrogeochemical methods, PMF model and Pearson correlation analysis. From upstream to downstream, the major ions, 87Sr/86Sr values and hydrochemical types of river water and groundwater show spatial variations. Major ions and 87Sr/86Sr analysis reveal that the hydrochmical compositions of river water and groundwater are primarily attributed to silicate weathering, carbonate weathering, evaporite weathering, and agricultural activity. The contribution of four factors affecting the hydrochemical evolution show obvious spatial variations along the flow path. The hydrochemical compositions of river water from the upstream to downstream are mainly contributed by carbonate and silicate weathering as well as weak evaporite weathering (<25%), which are affected by agricultural activity with a great contribution (>25%) in the middle and lower reaches. The contribution to the hydrochemical compositions of groundwater is relatively small (<25%) for carbonate, silicate and evaporite weathering, but great (>25%) for agricultural activity. This study provides implications for understanding the formation and evolution of water and better watershed water management in that typical endorheic watershed basin and even at larger scales.

Anthropogenic impacts on the water chemistry of a transboundary river system in Southeast Asia

The Red River originating from Yunnan province, China is the second largest river in Vietnam in terms of length and discharge. Combination of water chemistry monitoring data of 4 years (2018–2022) from different sub-basins of the Red River (the Da, Lo, Thao, Tra Ly, and Day) with historical datasets indicates a decline in pH from 8.1 in 2000 to 7.7 in 2021, greater CO2 concentrations and a shift from waters naturally dominated by carbonate weathering to waters dominated by evaporite weathering. Such changes were most apparent in the delta area where heavy human activities have increased influxes of most dissolved chemicals, except SiO2. Evaporite weathering is particularly enhanced by mining and deforestation occurring in upstream regions of both China and Vietnam. Pyrite oxidation, alongside silicate weathering, is enhanced along the Red River Fault Zone but reduced in tributaries with a higher proportion of hydropower reservoirs. Longer water residence times in these large reservoirs (total volume > 2.7x1010 m3) located in the Da and Lo sub-basins have also increased primary productivity, leading to higher evasion/uptake of CO2 and SiO2, lower total dissolved solids (TDS), and higher pH. The total physical and chemical denudation rates of upstream mountain tributaries ranged between 0.107 ± 0.108 and 0.139 ± 0.137 mm yr−1, mainly due to reservoir implementation and instream aquatic biogeochemistry changes. Our findings demonstrate that anthropogenic activities are profound factors impacting the water chemistry of the Red River system.

Water chemical formation mechanism and water quality analysis of Shichuan River under the influence of human activities

ABSTRACT It is necessary to analyze the characteristics of the water environment and factors influencing the basin in order to determine its current ecological status. This study aims to investigate the chemical properties in the Shichuan River Basin using Gibbs diagram, Piper triangle diagram, correlation analysis, factor analysis, and main ion ratio method. The enhanced Nemerow comprehensive index method and fuzzy comprehensive evaluation method are employed for river water quality assessment. The findings reveal that runoff water has a weak alkaline nature with low salinity. The hydrochemical type of river water includes Ca2++Mg2++Na+-SO4 2-, Ca2++Mg2+-SO4 2-+Cl-+HCO3 -, Ca2++Na+-SO4 2-+Cl-+HCO3 -. Silicite and dolomite weathering are identified as primary influencing factors, while evaporite weathering also contributes to variations in chemical composition within the basin's waters. Ca2+, Mg2+, SO4 2- and HCO3 - originate from carbonate rock and silicate rock weathering processes, whereas Na+, K+ and Cl-, are primarily derived from evaporite weathering influenced by domestic sewage and agricultural activities. Agricultural production practices along with discharge of domestic sewage have an impact on Na+, K+, Cl- and NO3 -concentrations in aquatic systems. Water quality at all three monitoring sections falls under level IV classification. Phosphorus emerges as a key limiting factor for eutrophication within this basin's watersheds.

Hydrogeochemical Characterization of an Intermontane Aquifer Contaminated with Arsenic and Fluoride via Clustering Analysis

The controlling hydrogeochemical processes of an intermontane aquifer in central Mexico were identified through multivariate statistical analysis. Hierarchical cluster (HCA) and k-means clustering analyses were applied to Na+, K+, Ca2+, Mg2+, F−, Cl−, SO42−, NO3−, HCO3−, As, pH and electrical conductivity in 40 groundwater samples collected from shallow and deep wells, where As and F− are contaminants of concern. The effectiveness of each hierarchical and k-means clustering method in explaining solute concentrations within the aquifer and the co-occurrence of arsenic and fluoride was tested by comparing two datasets containing samples from 40 and 36 wells, the former including ionic balance outliers (&gt;10%). When tested without outliers, cluster quality improved by about 5.4% for k-means and 7.3% for HCA, suggesting that HCA is more sensitive to ionic balance outliers. Both algorithms yielded similar clustering solutions in the outlier-free dataset, aligning with the k-means solution for all 40 samples, indicating that k-means was the more robust of the two methods. k-means clustering resolved fluoride and arsenic concentrations into four clusters (K1 to K4) based on variations in Na+, Ca2+, As, and F−. Cluster K2 was a Na-HCO3 water type with high concentrations of As and F. Clusters K1, K3, and K4 exhibited a Ca-HCO3, Na-Ca-HCO3, and Ca-Na-HCO3 water types, respectively, with decreasing As and F concentrations following the order K2 &gt; K3 &gt; K1 &gt; K4. The weathering of evaporites and silicates and Na-Ca ion exchange with clays were the main processes controlling groundwater geochemistry. The dissolution of felsic rocks present in the aquifer fill is a likely source of As and F−, with evaporation acting as an important concentration factor.

Reservoirs and Hydrogeochemical Characterizations of the Yanggao Geothermal Field in Shanxi Province, China

Geothermal water is the product of deep circulation within the crust, and the understanding of its hydrogeochemical process can provide effective information for integrated research on its circulation pattern and formation mechanism. Based on the geothermal geological conditions of the Yanggao geothermal field, this study analyzed water samples from thermal springs and geothermal wells in the geothermal field, ascertaining their hydrochemical components, along with their hydrogen and oxygen isotopes. Using methods like piper diagrams, ionic component ratio characterization, Na–K–Mg equilibrium diagrams, and reverse path simulations, this study elucidated the recharge source of geothermal water in the study area, revealed the water–rock interactions the geothermal water experienced, and evaluated the geothermal reservoir temperatures. The results show that the geothermal water has hydrochemical types of Na–Cl–HCO3 and Na–HCO3–Cl, and is primarily recharged by the atmospheric precipitation in the northern mountainous area. The geothermal water has experienced extended water runoff and deep thermal circulation, and its hydrochemical composition primarily results from the weathering and dissolution of silicate rocks and evaporites. The major hydrogeochemical processes of the geothermal water involve the dissolution of calcite, dolomite, gypsum, and kaolinite. In addition, the canon-exchange also changes the chemical component of the geothermal water. The SiO2 Geothermometer, a multimineral equilibrium diagram, and the silica–enthalpy model reveal the presence of deep and shallow geothermal reservoirs in the study area, which exhibit temperatures of 73 °C and ranging from 125 to 150 °C, respectively. The open geothermal reservoir environment results in the mixing of geothermal water and cold water, with shallow and deep geothermal water mixing with cold water at ratios of 57% and 76%, respectively.

Controlling Factors of Groundwater Salinization and Pollution in the Oasis Zone of the Cherchen River Basin of Xinjiang

Groundwater is a vital resource for local human life and production in the oasis zone of the Cherchen River Basin of Xinjiang. Understanding the controlling factors of groundwater salinization and pollution is of great significance for the sustainable utilization of groundwater resources and protection of the ecological environment in desert oasis areas. In light of this, a total of 36 single structure unconfined groundwater samples and 54 multi-layered structure unconfined and confined groundwater samples were collected from the oasis zone of the Cherchen River Basin and evaluated for the distribution characteristics and pollution status of major ions. Hydrogeochemical methods (e.g., Piper diagram, multivariate statistics, Gibbs model, and relationships among ions) were used to determine the main controlling factors of groundwater salinization and pollution. Differences in hydrogeochemical zonation were found from the single structure unconfined aquifers in sloping plains of piedmont areas to the multi-layered structure unconfined and confined aquifers in alluvial-proluvial plain areas, and Cl-Na (87.8%) was the main hydrochemical type in the groundwater of the study area. The quality of single structure unconfined groundwater was starkly better than that of the multi-layered unconfined and confined groundwater, which was mainly caused by Na+ (mean value of 9 969 mg·L-1), Cl- (13 687 mg·L-1), and SO42- (5 840 mg·L-1). Moreover, the natural hydrogeochemical process was the main reason for the deterioration of groundwater quality. The hydrochemistry was mainly controlled by the water-rock interaction and evaporation processes. The mineral dissolution of silicates and evaporites was an important source of chemical ions in the groundwater. Furthermore, the chemical weathering of evaporites combined with the processes of evaporation and cation exchange had a significant influence on the salinization of multi-layered unconfined and confined groundwater in alluvial-proluvial plain areas. In addition, synthetic fertilizers were the main pollution sources of NO3- in groundwater in the intensive agricultural zones.

The Chemical Weathering of Rocks and Its Carbon Sink Effect in the Naqu River Basin of the Nujiang River Source Area, Southwest China

Carbon plays an important role in global climate change. The mechanisms of carbon sources and carbon sinks have also received wide attention from society, and the physical and chemical characteristics of riverine ions can reflect the chemical weathering of rocks and carbon sink capacity of river basins. Based on the data on river, rainwater, and rock samples from 2019, this study used various methods, such as ion ratio diagrams and ternary diagrams, to analyze the chemical characteristics of water; the chemical weathering and carbon sink effects of rocks were also calculated while assuming three scenarios based on the main sources of ions in the Naqu River. The results showed that for the whole catchment, the main ion sources in the river were: carbonate rock chemical weathering &gt; silicate rock chemical weathering &gt; evaporite dissolution &gt; atmospheric precipitation input. According to the calculations, in the three scenarios, the carbonate weathering rates were 16.84, 11.32, and 14.08 t/km2/yr, and the carbon sink capacities were 66.47, 121.13, and 93.80 mol/km2/yr, respectively; the evaporite weathering rates were 2.20, 9.63, and 5.92 t/km2/yr, respectively. The silicate chemical weathering rate and carbon sink capacity did not change significantly in either scenario, with 6.82 t/km2/yr and 248.6 mol/km2/yr, respectively. This study quantified the ion sources in the Naqu River basin and accurately analyzed their chemical genesis, which helps in understanding the role of the rivers of the Qinghai–Tibet Plateau in the global carbon cycle and global climate change, in addition to providing a reference for the scientific development of the Nujing River.

Characteristics of hydrochemical and stable isotopes in the Upper and Middle Reaches of the Yarlung Tsangpo River

There have been few studies on the spatiotemporal variation of the hydrochemistry and stable isotopes characteristics in the upper and middle reaches of the Yarlung Tsangpo River owing to its high elevation and alpine climate, and the primary controlling mechanisms have yet to be clarified. Statistical analysis, a Piper diagram, Gibbs diagrams, ion correlation analysis, and other methods were integrated to analyze water samples from the main stream and tributaries of the Yarlung Tsangpo River together with rainwater samples from certain areas obtained in May 2020. And both the formation mechanism of the hydrochemistry and the process of chemical weathering were explored. The results revealed that river water in the basin was of HCO3−–SO42−–Ca2+ type (i.e., weakly alkaline), evaporite weathering had little impact on chemical weathering in the upper and middle reaches of the Yarlung Tsangpo River, and chemical weathering in the basin was controlled jointly by weathering of silicate rock and carbonate rock. Overall, the mean value of deuterium excess of the samples from the upper and middle reaches of the Yarlung Tsangpo River was 7.22‰, i.e., lower than the global mean of precipitation (10‰), indicating that the sampled water was subject to evaporation enrichment to some extent. Owing to the unique climatic and geographic conditions of the Qinghai–Tibet Plateau, the upper and middle reaches of the Yarlung Tsangpo River were also affected by water–rock interaction (chemical weathering and physical erosion). In addition, the relationship of stable isotopes between rainwater and riverwater also suggested that precipitation is an important source for river water in the upper and middle reaches of the Yarlung Tsangpo River.

Mobile evaporite enhances the cycle of physical–chemical erosion in badlands

Abstract. Chemical weathering driven by physical erosion is a natural process that strongly affects chemical and solid matter budgets at the Earth's surface. However, the influence of extreme climatic erosion on chemical weathering dynamics is poorly understood. Badland landscapes formed in highly erodible substrates have the potential to respond to individual events on scales that are rapid enough for direct observation. Here, we assess the geochemical and grain-size composition of suspended sediment and riverine chemistry measurements collected from two catchments during the 2017 Nesat and Haitang typhoons in southwestern (SW) Taiwan. During the typhoons, the Na+ concentration covaried with suspended sediment concentration (SSC), which we attributed to sodium-induced deflocculation. Evaporite weathering at peak rainfall is succeeded by peak silicate weathering at maximum discharge. Overall, our observations suggest that initial weathering of near-surface evaporite enhances the physical erosion of silicate rock during extreme rainfall events.

Lithium isotopic fingerprints of sources and processes in surface waters of the Ebro River Basin (Spain)

The Ebro River in north-eastern Spain is among the largest contributors of freshwater to the Mediterranean Sea and ends in the Ebro delta, one of the major wetlands in Europe. The bedrock of the Ebro River basin mainly consists of carbonate rocks and evaporites of Palaeozoic and Mesozoic age, and the river flows through several large cities, and agricultural and industrial areas. The Ebro outlet at Amposta was sampled once a month for a year (2006), and a field campaign in April of the same year sampled the Ebro along its main course as well as its principal tributaries. In the present study, the behaviour of Li and its isotopes was investigated at basin scale, with the objective of elucidating the processes controlling the lithium-isotope signatures of a large river draining mostly sedimentary bedrock. δ7Li values show a narrow range from +17.1 ‰ to +18.3 ‰ along the Ebro main stream, and between +16.3 and +18.9 ‰ at the outlet. In the major tributaries, the δ7Li values ranged from +12.9 ‰ to +20.9 ‰, with bedrock values ranging from +0.5 to +29.3 ‰. Comparing Li concentrations with Cl and SO4 ones, it appears that evaporite weathering plays an important role in controlling Li concentrations, but no anthropogenic agricultural or industrial influence on Li concentrations was detected. The Na/Li, Cl/Li and SO4/Li ratios clearly reflect the role of halite dissolution for some tributaries (Gallego, Ega and Aragon), gypsum dominating others (Guadalope, Matarrana, Huerva and Segre), and little influence of carbonate in all tributaries, the Ebro itself being a mixture of all tributaries. We tentatively applied the simple Rayleigh fractionation model, but most δ7Li values of the Ebro water samples plotted away from the fractionation line, reinforcing the major role of mixing processes in the Ebro basin, rather than fractionation processes during water/rock interactions. A comparison of δ7Li values and 87Sr/86Sr ratios further demonstrates the role of gypsum/anhydrite and limestone in the Ebro and its tributaries. Sr-isotopes show a non-negligible role of carbonate dissolution, generally considered to be weak in the control of the lithium cycle in catchments.

Fluorine-Rich Shallow Groundwater in Weigan River Basin (Xinjiang): Enrichment Factors and Spatial Distribution

Long-term usage of drinking water with excessive fluoride concentration (above 1.5 mg/L) is known to harm human health. Since the fluoride concentration in the shallow groundwater of the Weigan River Basin, China, exceeds this level, this study explored fluoride’s sources and enrichment mechanism for this basin. Based on information retrieval and regional geological environment analyses, 47 sets of shallow groundwater samples in the Weigan River Basin were collected and analyzed. The research results show that the fluoride content in the drainage basin of the research area ranged from 0.2 to 5.46 mg/L, with an average value of 1.14 mg/L. The test results showed that 40.43% of the water sample test results exceeded the national domestic drinking water safety standard (1.0 mg/L). The average pH value of the research area was 8.37, which was slightly basic. Type Na+–Cl−/(SO42−) was the primary hydrochemical type. In the research area, groundwater with a high fluoride concentration contained much sodium and little calcium. The main mechanism of excessive fluoride formation in the groundwater was the continued dissolution of fluorine-containing minerals (fluorite) in the research area, while the main fluorine source was weathering of silicate rocks and evaporites. Evaporation-crystallization and cation exchange were the main factors controlling fluorine enrichment. These findings indicate that physicochemical processes are the major mechanisms for controlling F− enrichment and that may be useful for studying F− occurrence in groundwater in arid and semi-arid areas.

Arsenic and fluorine in groundwater in northern Mexico: spatial distribution and enrichment factors.

North-central Mexico has groundwater contaminated with arsenic (As) and fluoride (F). Based on the dispersion patterns of these solutes, their sources are linked to felsic volcanic rock fragments and secondary minerals (clays, iron oxyhydroxides) within the alluvium fill of the aquifers. However, little is known about the effect of the enrichment factors for F and As in this area. Natural enrichment factors include evaporation, Ca/Na, and competitive adsorption and desorption from solid phases. This study used 1237 groundwater quality data measurements from 305 sampling sites collected between 2012 and 2019 in the state of Durango in north-central Mexico. To determine the contribution of enrichment factors to As and F content, the study area was divided into four sections, two being in the mountainous part of the state and two in the high plateaus. The data were compared among sections and analyzed using Spearman correlation and Piper and Block diagrams. The results indicate that the main solute enrichment mechanisms are evaporation and weathering of silicates and evaporites. Among the four sections, As, pH, and HCO3 seemed not to vary, F varied slightly, and nitrate and total dissolved solids varied the most. The lack of variation in As among sections is associated to its strong adsorption to clay minerals and iron oxyhydroxides, whereas the diminished F content in the eastern sections is likely linked to the adsorption of F to precipitating calcite (since groundwater is saturated with respect to calcite (SIcalcite = 0.43) and undersaturated for fluorite (SIfluorite = - 1.16). These processes shed light on the distribution of F and As in this area, and are likely operating in other states in northern Mexico and in semi-arid areas elsewhere.

The effects of rainfall on groundwater hydrogeochemistry and chemical weathering.

Due to the special hydrogeological conditions in karst areas, groundwater responds quickly to rainfall. The covariation of ion concentrations and spring discharge can help better understand the hydrogeochemical process of groundwater occurring in the heterogeneous karst aquifers. In this study, high-resolution monitoring of groundwater discharge, hydrochemistry, and stable isotopes was conducted at the Qingjiangyuan (QJY), a spring of the Qingjiang watershed in Hubei, China. The purpose is to investigate the changes in hydrogeochemical processes and chemical weathering under the influence of rainfall. The dynamics of spring discharge indicate the presence of pipelines and fissures of different sizes. According to the spring discharge attenuation curves, there are at least three medium types in the aquifer, which account for 45.7%, 34.2%, and 20.1% of the total groundwater. Pearson correlation analysis shows that the main sources of the solute in the QJY are carbonate minerals (mainly calcite and dolomite), evaporites (mainly gypsum and sylvite), celestite, and strontianite. Anthropogenic activities have less impact on groundwater solutes. Although carbonate minerals dominate the hydrochemistry, the changes in hydrogeochemical behavior caused by rainfall may come from gypsum, which is supported by the ion concentrations. At the early rainfall stage, Ca2+ concentration increased from 42.9 to 45.6 mg/L, followed by the SO42- from 15.2 to 16.6 mg/L. When the discharge increased to the maximum (2320 L/s), Ca2+ and SO42- showed opposite trends, decreased to 39.7 mg/L and 10.4 mg/L, respectively. The results also suggest that carbonate rocks and evaporites have important roles in hydrochemistry. The contributions of these three end-members were quantified based on the law of mass conservation. The proportions of carbonate weathering and evaporite weathering were 83.4% (85.2-80.3%) and 11.6% (6.9-18.0%), respectively, and rain was 5.0% (0.1-10.4%). These results were integrated into a hydrogeological conceptual model that explains the hydrogeochemical processes, including rock weathering, piston, and dilution effects caused by rainfall. The proposed conceptual model helps to improve the understanding of hydrogeochemical processes and chemical weathering in karst areas.

Source tracing and chemical weathering implications of strontium in agricultural basin in Thailand during flood season: A combined hydrochemical approach and strontium isotope

87Sr/86Sr of river water are of great significance in constraining oceanic strontium (Sr) record and terrestrial climate change due to the connection of continental weathering and the adjacent ocean. This work presents the geochemical characteristics of dissolved Sr and hydrochemistry, and estimates chemical weathering rate together with elemental Sr flux during the flood season of the Mun River, the largest tributary of Mekong River. Hydrochemistry analysis indicates the dominance of Cl− and HCO3− for major anions with the average of 34.6 and 43.0 mg/L, respectively, and Na+ and Ca2+ together dominated the cationic composition with the average of 22.9 and 10.5 mg/L, respectively. The ion concentrations during flood season were lower than that in dry season, implying tremendous river runoff due to extreme rainfall. The dissolved Sr ranges 6.1–237.5 μg/L with higher contents in the upper Mun. Sr contents in flood season are lower and less fluctuated than that in dry season, whereas the divergence between up and downstream becomes larger. 87Sr/86Sr ranges 0.7100–0.7597, slightly higher than global average. Elemental molar ratio analysis partly corroborates the inference from correlation analysis, but 87Sr/86Sr does not correlate with Na/Ca, indicating additional influence except for the weathering of evaporites and silicates. Comparing to regional wastewater and rainwater, the lower reaches exhibits superimposed impact of agricultural inputs on weathering to dissolved loads, especially in downstream with more tributary convergence. Extreme rainfall during flood season and extensive agricultural production activities may interfere in altering riverine solutes. Silicate weathering rate and CO2 consumption rate are calculated as well as the yearly 87Sr in excess to the Mekong River and finally to the Pacific Ocean with a Sr flux of 1.98 × 103 tons/year, indicating significant influence on seawater strontium isotope evolution in the long run. Together with tropical climate and high-intensity precipitation, the accelerated chemical weathering process seems inevitable. Therefore, the impact of agricultural interference in the pan-Mekong River basin needs more systematic and multi-angle research to provide a comprehensive insight on better watershed management under tropical climatic conditions.

Hydrochemical Characteristics and Controlling Factors of Surface Water and Groundwater in Wuding River Basin

Taking Wuding River as the research object, the study explored the hydrochemical characteristics and discussed the source of solute and control factors of groundwater and surface water in the basin, in order to provide a reference for water quality management. Considering the seasonal differences, water samples were collected during the dry season and the flood season. By comprehensively using graphic methods, correlation analysis, and forward deduction models, we analyzed the temporal and spatial evolution characteristics of water chemistry, explored the formation mechanism of water chemistry, and quantified the contribution rates of different sources to solutes. The results showed that the overall water quality was weakly alkaline in Wuding River basin. HCO3- and Na+ were the main anions and cations in the water, respectively, and the main water chemistry type was HCO3·SO4-Na·Ca. The water quality gradually deteriorated along the river course from west to east, and the sampling points that exceeded the level Ⅲ water were mainly distributed in tributaries during the dry season and downstream during the flood season. The cation exchange effect increased the Na+ and K+ in the water, and NO3- and HCO3- differed significantly in different seasons, which may be affected by seasonal precipitation leaching soil and land use types. Evaporite weathering and silicate weathering were the main sources of solute contribution in the Wuding River basin, which were 35.0% and 46.5% in the dry season and 46.7% and 42.3% in the flood season,respectively.

Natural Water Chemistry Change in the Surface Water of Chengdu and Impact Factors

To investigate the impact of megacities on the chemistry of surface waters, monthly sampling and monitoring were conducted in the Chengdu section of the Minjiang and Tuojiang River basin, corresponding to the upper reaches of the Yangtze River since the spring of 2019, including the influent and effluent water samples from 57 sewage treatment plants in Chengdu. All the samples were analyzed for major ions and other water chemistry parameters, and compared with the historical data of the Minjiang and Tuojiang River. The results showed that the Chengdu surface water still presented a natural chemistry with medium-low total dissolved solids(TDS), and calcium bicarbonate chemistry type, which is the natural consequence of the weathering of carbonate rocks in the basin effected by the weathering of silicates and evaporites. The natural water chemistry of the surface waters in Chengdu presented monthly variation, i.e.,the concentration of major ions and TDS was higher in the dry season compared to the wet season, reflecting the variations of point source. Spatially, the concentration of major ions and TDS downstream of the city was higher than those in the upper reaches, and the concentration in the tributary was higher than that in the mainstream, which may reflect urban influence. Further analyses, such as simulation calculations, indicated that urban activities were the major driving factor for the chemistry change in the surface waters in Chengdu, which is evidenced by the significant contribution of the sewage discharge to the elevated Cl- and Na+ and the ratio of hardness/alkalinity>1 from anthropogenic acid gas emissions. A comparison with the water chemistry of the Minjiang and Tuojiang River in the 1960s indicated that, the current Cl-/Na+ ratio has significantly increased, which has been evidenced by a salinization trend. As a megacity nearest to the source of the Yangtze River, the impact of Chengdu on the natural water chemistry of the Yangtze River system and its environmental effects deserves more attention.

Spatial Hydrochemical Characteristics and Controlling Factors of Surface Water in the Yancheng Area

Surface water resources are crucial to economic development in China's eastern coastal areas. Under the influence of intense human activities, problems such as abnormal water quality and pollution are very prominent. Here, the chemical composition of surface waters and their controlling factors were analyzed in the Yancheng area. The results showed that:① The surface water pH is low and the concentrations of total dissolved solids(TDS) are high in the study area. pH is likely controlled by the acidic organic pollutants discharged by human activities as well as acidic substances formed by anaerobic decomposition in soils. TDS data showed low values in the west and high values in the east, and low values in north and high south of the study region. ② Cation content was dominated by Na+, anions were dominated by HCO3- and Cl-, and, overall, hydrochemical composition was controlled by HCO3-(Ca+Na), although there was significant variability between the different regions. ③ Because the silt layer in the region is mainly composed of clay, subclay, and sandstone, the soluble matter in surface water is mainly derived from the decomposition of Na-feldspar in silicate debris via carbonation, supplemented by carbonation. Due to the low-lying terrain and high groundwater level in the tidal flat area, solutes are significantly affected by evaporation-driven concentration alongside evaporite weathering replenishment. ④ Although the surface waters in different regions were found to be affected by human activities to varying degrees, the impact was always strong. In particular, in areas of tidal wetland expansion, water quality is not only affected by urban sewage and agricultural irrigation but also industrial wastewater discharge.

Hydrogeochemistry, identification of hydrogeochemical evolution mechanisms, and assessment of groundwater quality in the southwestern Ordos Basin, China.

Understanding the evolution process of hydrogeochemistry and groundwater quality is essential for water supply and health in the southwestern Ordos Basin, where groundwater is a vital source for drinking. This study systematically illustrates the hydrogeochemical characteristics and evolution mechanism based on the groundwater samples (n = 67) collected from Loess area by integrating multivariate statistical methods and hydrogeochemical methods. Furthermore, the entropy water quality index (EWQI) and water quality indices combined with spatial analysis were employed to evaluate the suitability of groundwater for drinking and irrigation purposes and analyze the spatial variation of water quality. The hierarchical cluster analysis and principal component analysis classified groundwater dataset into four clusters and four components which were examined using a Piper diagram and Gibbs diagram, representing different hydrogeochemical characteristics and controlling factors. Based on results, the groundwater chemistry was characterized by representative water types: freshwater (cluster 1, cluster 2), low salinity (half of cluster 3), high salinity (half of cluster 3, cluster 4), and the main controlling factors of hydrogeochemistry revealed by Gibbs diagram were evaporation crystallization (cluster 3, cluster 4) and water-rock interactions (cluster 1, cluster 2). Moreover, the Gaillardet diagram, chloro-alkaline indices, binary diagram, and saturation index further comprehensively illustrate that the silicate and evaporite weathering, ion exchange, dissolution of halite, gypsum, and anhydrite are responsible for hydrogeochemical process. Based on EWQI and ArcGIS, the groundwater quality is categorized as excellent (47.0%), good (31.8%), medium (4.5%), poor (6.1%), and extremely poor (10.6%) types, and the quality in the south of the study area is better than north. Additionally, the USSL diagram shows that most of samples belong to C3S1 (high-salinity hazard and low-sodium hazard) and C2S1 (medium-salinity hazard and low-sodium hazard), and Wilcox diagram shows that 77.2% of samples are suitable for irrigation.

Using geochemistry and environmental tracers to study shallow unconfined aquifer recharge and mineralization processes in the Yinchuan Plain, arid Northwest China

Abstract Irrigation water extracted from the Yellow River plays a key role in water resource management in the Yinchuan Plain (YCP), arid Northwest China. Investigating the soluble matters (ion and gas) of groundwater provides information to explain the unconfined shallow aquifer recharge and groundwater mineralization processes after long-term flood irrigation activity. Environmental tracing with the elements, 2H, 18O, 3H, and CFCs, combining geochemistry using major ions and selected trace elements, was conducted for 43 water samples from September to October 2019 in the YCP. Evaporite and silicate weathering dominate the shallow unconfined groundwater geochemical compositions. Water–rock interactions control the mineralization characteristics regularly along the groundwater flow paths from the southwest toward the northeast. Stable isotopes suggest that Yellow River water and precipitation in winter and/or from Helan Mountainous area are the main recharge sources. The shallow unconfined aquifer mixed young (post-1940) and old (pre-1940) water with young water ratios from 53.1 to 73.5% inferred from the CFC concentrations and 3H activities. Water reinfiltrations extracted from the Yellow River and from the old groundwater are confirmed. Lateral flow recharge for the shallow unconfined aquifer is less indistinctive than that from the water re-infiltration in the plain areas.

Variability in groundwater flow and chemistry in the Mekong River alluvial aquifer (Thailand): implications for arsenic and manganese occurrence

Arsenic (As) is widespread in alluvial aquifers along rivers draining the Himalayas but has received little attention along the Mekong River upstream of its delta. This study investigates linkages among groundwater recharge, flow, chemistry, and river stage at two sites along the Mekong River in northeast Thailand. Hydraulic head and chemistry were monitored in January and June 2014. In addition, hydraulic head and electrical conductivity were continuously logged at one site from January 2014 to March 2015. During the dry season, groundwater tends to flow toward the river, and saline water from shallow evaporites can intrude the alluvial aquifer. As stage rises, hydraulic-gradient reversals can result in groundwater-river water mixing, thereby promoting geochemical disequilibrium. Groundwater chemistry reflects silicate, carbonate, and evaporite weathering; multiple redox reactions; and spatial and temporal variability in recharge. Concentrations of As and manganese (Mn) in groundwater exceeded drinking-water guidelines by as much as an order of magnitude. Results suggest As is mobilized in groundwater by reduction of ferric (oxyhydr)oxides, consistent with the findings elsewhere in South and Southeast Asia. Upwelling of saline water can increase the solubility of (oxyhydr)oxide phases that sequester Mn and As, can complex Mn and can mobilize adsorbed As oxyanions. Conversely, influxes of oxic river water could promote (oxyhydr)oxide precipitation.