Hydrogeochemical processes identification and groundwater pollution causes analysis in the northern Ordos Cretaceous Basin, China

Hydrodynamic Groundwater Modeling and Hydrochemical Conceptualization of the Closure Mining Area of the WuMa River Watershed of China

The WuMa River (WMR) watershed is located in Renhuai City, Guizhou Province of China, which is a first-class tributary of the Chishui River. The geochemical investigation mainly included the determination of groundwater pH, total hardness, total dissolution solid, major cationic and anionic, and the geochemical groundwater modeling. The principal component analysis (PCA) and Gibbs model were used to analyze the pollution type and geochemical composition. The geochemical investigation results show that the cations of groundwater are dominated by Ca2+ and the anions are dominated by HCO3-; therefore, two main hydrochemical types in the study area are identified as Ca2+-Mg2+-HCO3- and Ca2+-Mg2+-SO42-. The chemical composition of groundwater in this area is mainly controlled by weathering of the carbonate rocks. The ion concentration of groundwater in the study area exhibited significant spatial variability between dry and wet seasons, while temporal changes of cationic and anionic concentrations exhibited irregularities. In PCA and FA analysis, PC1, PC2, and PC3 were extracted, which could explain 51.92, 26.98, and 12.61% of the total information, respectively. F1 explained 67.44% of the total variance, among which Ca2+, Mg2+, K+, SO42-, and Cl- contributed the most among the factors and were the main factors controlling the chemical composition of groundwater. The relative error between the measured water level and the simulated water level is less than 2%, which meets the requirements of simulation accuracy. During the simulation period of the model, a total recharge of 339.05 × 104 m3 was observed in the simulated area, primarily attributed to infiltration from rainfall. The total excretion amounted to 330.78 × 104 m3, primarily through evaporation, with a minor amount of lateral outflow. The migration pathway of pollutants in groundwater primarily follows the direction of groundwater flow while diffusing vertically. The migration range of the pollutant is in accordance with the direction of groundwater flow and extends along the larger hydraulic gradient, demonstrating consistency. The findings of this study serve as a reminder that the closure of coal mines can constitute a significant source of water pollution. Simultaneously, they offer empirical data and theoretical references for the simulation and prediction of groundwater contamination in enclosed coal mines.

Both natural and anthropogenic factors control surface water and groundwater chemistry and quality in the Ningtiaota coalfield of Ordos Basin, Northwestern China

An understanding of the vertical variations in hydrogeochemical processes in various aquifers and quality suitability assessment is crucial for the utilization of groundwater in the Ningtiaota coalfield of Ordos Basin, Northwestern China. Based on 39 water samples collected from surface water (SW), Quaternary pore water (QW), weathered fissure water (WW), and mine water (MW), we conducted self-organizing maps (SOM) algorithm, multivariate statistical analysis (MSA), and classical graphical methods to elucidate the mechanisms controlling the vertical spatial variations in SW and groundwater chemistry and conducted a health risk assessment. The findings indicated that the hydrogeochemical type showed a transition from the HCO₃^--Na⁺ type in SW to the HCO₃^--Ca²⁺ type in QW, then to the SO₄^2--Mg²⁺ type in WW, and back to HCO₃^--Na⁺ type in MW. Water-rock interaction, silicate dissolution, and cation exchange were the main hydrogeochemical processes in the study area. Additionally, groundwater residence time and mining operations were critical external factors that affect water chemistry. Contrary to phreatic aquifers, confined aquifers featured greater circulation depth, water-rock interactions, and external interventions leading to worse quality and higher health risks. Water quality surrounding the coalfield was poor, causing it to be undrinkable, with excessive SO₄^2-, arsenic (As), and F^-, etc. Approximately 61.54% of SW, all of QW, 75% of WW, and 35.71% of MW can be used for irrigation.

Identifying strontium sources of flowback fluid and groundwater pollution using 87Sr/86Sr and geochemical model in Sulige gasfield, China

Hydraulic fracturing technology has made unconventional oil and gas development economically viable; however, it can lead to potential environmental issues such as groundwater pollution. Strontium isotope (⁸⁷Sr/⁸⁶Sr) is considered as a sensitive tracer to indicate potential groundwater contamination. In this study, strontium (Sr) and ⁸⁷Sr/⁸⁶Sr sources of hydraulic fracturing flowback fluid are identified with 87 flowback fluid samples and 5 borehole core samples. High Sr concentrations and ⁸⁷Sr/⁸⁶Sr values were found in fracturing flowback fluid. The hydrogeochemistry evidence shows high Sr and ⁸⁷Sr/⁸⁶Sr in fracturing flowback fluid mainly comes from formation water with high ion concentrations, while Sr and ⁸⁷Sr/⁸⁶Sr of formation water develop in diagenesis and long term water-rock interaction (e.g., feldspar dissolution and clay mineral transformations) under the high temperature and pressure. A complete evaluation system was executed to assess the sensitivity of ⁸⁷Sr/⁸⁶Sr indicating potential pollution on groundwater. The mixing curves which ⁸⁷Sr/⁸⁶Sr combined with Sr and Cl were also established by mixing models to indicate groundwater pollution. The modeling results show mineral dissolution/precipitation and cation exchange have little impact on ⁸⁷Sr/⁸⁶Sr in the mixing process between fracturing flowback fluid and groundwater, which ⁸⁷Sr/⁸⁶Sr can identify contamination when only 0.89% of fracturing flowback fluid mixes with groundwater. Finally, the potential contamination pathways are discussed. It is highly unlikely fracturing flowback fluid contaminates groundwater and soil through upward migration, whereas leakage is a more prevalent pollution pathway.

Investigating sources, driving forces and potential health risks of nitrate and fluoride in groundwater of a typical alluvial fan plain

Groundwater of alluvial fan plains is the foremost water source, especially in arid/semiarid regions. Its contaminants are big issues for water supply and public health concern. To reveal the groundwater chemistry, contaminants sources and health threats in alluvial aquifers, 81 groundwaters were collected from a typical alluvial fan plain of northern China for nitrogen, fluoride and major ions analysis. Statistical analysis and hydrochemical diagrams as well as human health risk assessment were performed. Nitrate is widely distributed and 53% of groundwaters exceed the permissible limit with the maximum concentration up to 326 mg/L. The distributions of nitrite, ammonia and fluoride contaminants are sporadic in spatial, and the concentrations of fluoride in groundwaters are slightly beyond the permissible limit of 1 mg/L. The hydrochemical facies shift from HCO₃-Ca or Mixed HCO₃-Na·Ca type to Mixed Cl-Mg·Ca and ClCa type with the increase of nitrate content. Two factors (Factor-1 and Factor-2) are extracted by factor analysis and account 63% of the total variances. The positive loading of F^- and negative loading of NO₃^- on Factor-2 reveal geogenic and anthropogenic origins, respectively. The significant positive loadings of TDS, TH, SO₄^2-, Cl^-, Ca²⁺, Mg²⁺ on Factor-1 reveal the governing mechanisms on groundwater chemistry by intermixed sources of geogenic origins and anthropogenic inputs. Hydrogeochemical evolution in the study area is driven by both water-rock interaction and anthropogenic forces. Anthropogenic inputs/influences are the dominated forces increasing groundwater nitrate content and salinity in the piedmont zone and the residential and industrial zone of the southeastern lower parts, and would pose potential non-carcinogenic risks to various populations via oral intake pathway. Rational measures should be taken to protect groundwater quality out of the threats of anthropogenic pollution. The geogenic fluoride in groundwater would threat the health of children through oral pathway and should be also concerned. CAPSULE: The driving forces of groundwater chemistry in alluvial fan plains were revealed using integrated approach of factor analysis and geostatistical modelling.

Fluoride and nitrate contamination of groundwater in the Loess Plateau, China: Sources and related human health risks

Fluoride (F^-) and nitrate (NO₃^-) in groundwater have caused serious health problems worldwide. However, in the Chinese Loess Plateau where groundwater is the primary source of drinking water, previous studies have rarely reported the health risks from fluoride and nitrate in groundwater. Therefore, we collected 105 groundwater samples (78 from shallow aquifers and 27 from deep aquifers) from the western district of the Loess Plateau for physicochemical and isotopic analysis to investigate the sources of F^- and NO₃^- in groundwater and associated health risks. Fluoride concentration in 73.1% of shallow groundwater and 22.2% of deep groundwater exceeds 1.5 mg/L, while NO₃^- content in 76.3% of shallow groundwater and 51.9% of deep groundwater surpasses 50 mg/L. High-F^- groundwater is associated with HCO₃-Na, SO₄-Na·Mg and Cl-Na·Mg types water. Fluorine-bearing minerals dissolution, cation exchange, calcite precipitation, evaporation, and anthropogenic activities contribute significantly F^- in groundwater. Mixing with shallow groundwater is an important source of F^- in deep groundwater. The NO₃^- content is highest in Cl type water, followed by SO₄ type and HCO₃ type water. NO₃^- mainly originates from soil organic nitrogen (SON), chemical fertilizers (CF), and manure and sewage (M&S). Nitrification is the dominant transformation process of nitrogen nutrients in groundwater. The hazard index (HI) values for shallow groundwater are 0.203-9.232 for adults, 0.253-11.522 for teenagers, 0.359-16.322 for children, and 0.507-23.043 for infants, while those for deep groundwater are 0.713-5.813 for adults, 0.890-7.254 for teenagers, 1.261-10.277 for children, and 1.780-14.508 for infants. Approximately 96.2% of shallow groundwater poses non-carcinogenic risks to infants and children, followed by 92.3% to teenagers, and 89.7% to adults. All deep groundwater poses non-carcinogenic risks to infants and children, followed by 92.6% to teenagers, and 74.1% to adults. This study is helpful to develop strategies for the integrated management of high fluoride or nitrate groundwater in arid areas.

Groundwater geochemical signatures and implication for sustainable development in a typical endorheic watershed on Tibetan plateau

Groundwater resource is significantly important for sustainable development of the world, especially for arid endorheic watersheds. A total of 28 groundwaters were collected for hydrogeochemical analysis from the arid Chaka watershed on Tibetan plateau to illustrate the hydrochemical evolution, formation mechanisms and feasibility of groundwater in small arid endorheic watersheds where groundwater is much scarcer. The results showed groundwater has a slightly alkaline nature, and varies from soft fresh HCO₃-Ca type to hard brackish/saline Cl-Na type along the groundwater flow path in the watershed with the total hardness in the range of 270-2,127 mg/L and the total dissolved solids in the range of 282-41,770 mg/L. Nitrogen and fluoride in phreatic water are found sporadically exceeding the permissible limits with the maximum value of 118 mg/L for nitrate, 1.2 mg/L for ammonia and 1.2 mg/L for fluoride. Hydrochemistry of phreatic and confined groundwater is naturally governed by water-rock interactions including minerals (halite, gypsum and anhydrite) dissolution, silicate weathering and cation-exchange reaction. The salinity of phreatic water is also dominantly controlled by the strong evaporation. Human activity is one of the important mechanisms influencing the hydrochemical signature of groundwater regardless of the depth. Groundwater has a great hydrogeochemical discrepancy spatially across the watershed and varies from excellent to extremely poor quality in phreatic aquifers. A better water quality that under the good to medium categories was observed in the confined aquifers with 80% of samples having the EWQI value less than 100 and others in the range of 100-150. Phreatic groundwater away from the river and in the downstream area has a relatively poor quality for domestic and agricultural purposes, and should be avoided to direct utilization. This research can improve the understanding of groundwater hydrogeochemical feature, genesis, and its constraints on the availability and feasibility of groundwater resources in small arid watersheds worldwide.

Groundwater quality evaluation for different uses in the lower Ketar Watershed, Ethiopia

Groundwater quality samples from 33 wells were collected in the lower Ketar watershed (Ethiopia) to study its suitability for domestic and irrigation purposes. Samples were evaluated for major ions and physicochemical properties. In 58% of the samples analyzed, Ca²⁺ is the dominant cation and Na⁺ dominates the remaining 42% of the samples. Among the anions found during analyzation, HCO₃^- is the solo dominant ion in all the wells sampled. The order of the concentration of the major ions was Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ for the cations and HCO₃^- > SO₄^2- > Cl > NO₃^- for the anions. AquaChem analysis shows that Ca-HCO₃ and Na-HCO₃ are the major water types in the area. The analyses indicated that the dissolution of fluorite or fluorapatite is the possible source of the high fluoride concentration in the area. And, the interactions between water and rock and cation exchanges mainly determine the water quality. The suitability of the groundwater for use in irrigation was evaluated based on the salinity (EC), SAR, %Na, RSC, PI, KR, and the USSL Salinity diagram. The groundwater from most of the wells can be used for irrigation without any significant restriction except for a few of the wells downstream. Its suitability for domestic use was evaluated by comparing with the WHO standard limits. The parameters limiting the use of this groundwater for drinking purposes are F^- (94%), HCO₃^- (45%), and Ca²⁺ (33%). All the remaining major cations and anions complied with the WHO standard limits for drinking.

Hydrogeochemical Characterization and Assessment of Contamination by Inorganic and Organic Matter in the Groundwater of a Volcano-Sedimentary Aquifer

The chemical composition of groundwater is a product of the evolution and transformation of major ions, which come from natural hydrogeochemical processes or from anthropogenic interference. The objective of this study was to identify the hydrogeochemical processes and the influence of anthropogenic activity on the variation of chemical composition in Toluca Valley groundwater. The type of water in the zone is fundamentally Mg-Ca-HCO₃. Three groups with different evolutionary tendencies were identified: one within a local recharge zone and two others in an intermediate region with anthropic activity. The latter, which show contamination by inorganic matter (fertilizers) and organic matter (urban or industrial wastewater). The content of N-NO₃^- (0.024-0.219 mEq L^-1), N-NH₄⁺ (0-0.022 mEq L^-1), Porg (0.03-1.02 mEq L^-1) and PO₄^3- (0.0-0.28 mEq L^-1) indicated contamination coming from inorganic and organic matter. These chemical compounds were identified by way of a 3D fluorescence technique. The results of this study demonstrate that the main processes that affect and control the chemical composition of the water in the Toluca Valley aquifer are weathering of silicates, the ion exchange and a mixture process generated by a source of anthropic contamination.

Shallow Groundwater Quality and Its Controlling Factors in the Su-Xi-Chang Region, Eastern China

Understanding factors influencing groundwater quality is critical to the development of best management practices at the large watershed scale. In this study, the shallow groundwater (10–20 m depth) in the Su-Xi-Chang region, eastern China, was investigated as part of a monitoring program from 2007 to 2008 to analyze the regional groundwater quality as well as the hydrogeochemical processes and their controlling factors. Conventional physicochemical water parameters (pH, turbidity, electrical conductivity, dissolved oxygen, total phosphorus), major cations (Na⁺, Ca²⁺, Mg²⁺ and NH₄⁺) and anions (Cl⁻, NO₃⁻ and SO₄²⁻) were measured. Hydrochemical methods and multivariate statistical methods were applied to analyze the hydrogeochemical signatures, origins, the similarities among the variables and to identify the main pollution sources in the groundwater. The results showed that (1) the concentrations of TDS (224.89–1086.70 mg/L) and turbidity (0.1–18.60 NTU) were higher than the class II groundwater quality standards in China and the WHO drinking water standards, (2) there were extremely high concentrations of ammonia (0.01–32.90 mg/L), with a mean value of 0.72 mg/L and (3) the nitrate concentrations (average value of 22.07 mg/L) exceeded the class III groundwater quality standards. The study also provided evidence that weathering, dissolution of carbonate, halite and silicate and cation exchange were the possible primary hydrogeochemical control mechanisms in the groundwater. The sources of ammonia, total phosphorus, sulfates and nitrates included rock–water interactions and anthropogenic activities. The groundwater administration of pollution sinks and sources, long-term legal frameworks and economic incentives should be improved to optimize watershed scale management in the context of rapid development in China.

Hydrogeochemical characteristics of a multi-layered coastal aquifer system in the Mekong Delta, Vietnam

Groundwater is a primary freshwater source for various domestic, industrial and agricultural purposes, especially in coastal regions where there are lacking surface water supply. However, groundwater quality in coastal regions is often threatened by seawater intrusion and contamination due to both anthropogenic activities and natural processes. Therefore, insights into groundwater geochemistry and occurrences are necessary for sustainable groundwater management in coastal regions. The main aim of this study is to investigate the hydrogeochemical characteristics and their influencing factors in a coastal area of the Mekong Delta, Vietnam (MD). A total of 286 groundwater samples were taken from shallow and deep aquifers for analyzing major ions and stable isotopes. The results show that deep groundwater is dominated by Ca-HCO[Formula: see text], Ca-Na-HCO[Formula: see text], Ca-Mg-Cl, and Na-HCO[Formula: see text] while shallow groundwater is dominated by the Na-Cl water type. In this region, the main geochemical processes controlling groundwater chemistry are ion exchanges, mineralization and evaporation. Groundwater salinization in coastal aquifers of the Mekong Delta is caused by (1) paleo-seawater intrusion and evaporation occurring in the Holocene and Pleistocene aquifers, (2) dissolution of salt sediment/rock and leakage of saline from upper to lower aquifers due to excessive groundwater exploitation and hydraulic connection. High nitrate concentrations in both shallow and deep aquifers are related to human activities. These results imply that groundwater extraction may exacerbate groundwater quality-related problems and suitable solutions for sustainable groundwater management in the coastal area of the Mekong Delta are needed.

Quality criteria for groundwater use from a rural part of Wanaparthy District, Telangana State, India, through ionic spatial distribution (ISD), entropy water quality index (EWQI) and principal component analysis (PCA)

The present study region comprises granite and granite gneisses aquifer system constituted by Precambrian rocks. Groundwater is the primary source for drinking and other domestic purposes. Many developing regions in the world suffer from lack of safe drinking water. A rural part of Wanaparthy District in Telangana State, India, is one of them. For this reason, the groundwater samples collected from the study region were analyzed for pH, TDS, Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^-, Cl^-, SO₄^2-, NO₃^- and F^- and evaluated groundwater quality criteria, using ionic spatial distribution (ISD), entropy water quality index (EWQI) and principal component analysis (PCA). The ISD maps show that some locations are not suitable for drinking purpose due to exceeding concentrations of TDS, Mg²⁺, Na⁺, K⁺, HCO₃^-, Cl^-, NO₃^-and F^-, compared to those with national drinking water quality standards. According to the EWQI, about 3%, 47%, 43% and 7% of the total area come under the excellent, good, medium and extremely poor water quality types for drinking purpose, respectively. Chadha's diagram classified the area as carbonate hardness (63%), non-carbonate alkali (17%), carbonates alkali (13%) and non-carbonate hardness (7%) zones. The binary diagrams (Na⁺ + K⁺ vs TC, Na⁺ vs Ca²⁺ and HCO₃^- vs TC) indicate that the quality of groundwater is controlled by influences of water-rock interactions, mineral weathering and dissolution, ion exchange and evaporation as well as the impact of anthropogenic sources. The PCA transferred the chemical variables into three principal components accounts for about 81% of the total variance. The high positive loadings of PC1 (Cl^-, TDS, SO₄^2-, Na⁺, NO₃^-, Mg²⁺ and HCO₃^-) stand for processes of silicate weathering and dissolution, ion exchange and evaporation, and the influence of domestic waste waters, irrigation return flows and chemical fertilizers on the groundwater system, the PC2 (F^- and pH) signifies the alkaline nature of groundwater, which causes fluorosis, and the PC3 (K⁺) is a result of potassium fertilizers. The study helps to take remediate measures at a specific site and hence suggests the treatment of water before its drinking and also the recharge of the aquifer artificially to improve the groundwater quality.

Geochemical factors controlling the occurrence of high-fluoride groundwater in the western region of the Ordos basin, northwestern China

Hydrogeochemistry and isotope hydrology were carried out to investigate the spatial distribution of fluoride (F^-) and the mechanisms responsible for its enrichment in the western region of the Ordos basin, northwestern China. Sixty-two groundwater samples from the unconfined aquifer and fifty-six from confined aquifer were collected during the pre-monsoon (June 2016). Over 77% of groundwater samples from the unconfined aquifer (F^- concentration up to 13.30 mg/L) and approximately 66% from confined aquifer (with a maximum F^- concentration of 3.90 mg/L) exhibit F^- concentrations higher than the Chinese safe drinking limit (1.0 mg/L). High-F^- groundwater presents a distinctive hydrochemical characteristic: a high pH value and HCO₃^- concentration with Ca-poor and Na-rich. Mineral dissolution (e.g., feldspar, calcite, dolomite, fluorite), cation exchange and evaporation in the aquifers predominate the formation of groundwater chemistry, which are also important for F^- enrichment in groundwater. Mixing with unconfined groundwater is a significant mechanism resulting in the occurrence of high-F^- groundwater in confined aquifer. These findings indicate that physicochemical processes play crucial roles in driving F^- enrichment and that may be useful for studying F^- occurrence in groundwater in arid and semi-arid areas.