| The presence of saline groundwater has become one of the most acute problems for water resource management worldwide,since it causes deterioration in water quality and endangers future exploitation of groundwater resources.Salinity problem is widely reported in groundwater in different parts of the world,especially from the arid/semi-arid areas in developing countries.It is estimated that the total area of groundwater with high salinity reaches approximately 24 million km2,accounting for about 16%of the total land area on earth.About 1.1 billion people around the world are living in the saline groundwater affected areas.Diverse mechanisms governing groundwater salinization and fluoride enrichment in natural environments have been proposed,including continental salinization and seawater intrusion.More recently,anthropogenic influences,such as agricultural irrigation,have raised increasing concern on the groundwater salinization.In this study,Yuncheng Basin was selected for our case study to understand the genesis of high fluoride saline groundwater.This basin is characterized by the existence of a low-altitude inland Salt Lake.Since 1980s,the Yuncheng Basin has been greatly impacted by large-scale intensive agricultural activities and anthropogenic uses.An integrated approach of hydrogeochemical paramters,multiple isotopes(oxygen isotope?18OH2O,hydrogen isotope?D,sulfur isotope?34SSO4,stable and radio carbon isotopes?13C-14C,and non-traditional isotope?7Li),and geochemical simulation was used to investigate the distribution of high fluoride saline groundwater,to elucidate the sources of salt in groundwater and to identify the major natural and anthropogenic factors controlling groundwater salinization at Yuncheng Basin.The mechanisms controlling fluoride enrichment in groundwater were also interpreted,as a secondary environmental issue?F contamination?of groundwater salinization found in this case study.The groundwater samples in the Yuncheng Basin are classified by Q-mode hierarchical cluster analysis?HCA?into four clusters?C1-C4?according to their chemical and isotopic compositions.C4 has just one sample.According to the significant differences among the groups,the other three clusters can be named as fresh water group?C1,TDS<1,000 mg/L?,brackish-saline water group?C2,1,000 mg/L<TDS<3,000 mg/L?and saline water group?C3,TDS>3,000 mg/L?.Except four samples from C1 that were Ca-Mg-HCO3 water,the samples in this cluster were mostly Na-Ca?Mg?-HCO3?SO4?water.Three deep waters from C2 were Na-Cl-SO4 type.Other samples in C2 and all the samples from C3 and C4 belong to Na-SO4-Cl type,reflecting the potential effect of mixing,dissolution and cation exchange as the major processes contributing to the groundwater chemistry.Saline groundwater is widely distributed in shallow aquifers in the basin;over 60%of the shallow groundwater samples have total dissolved solids?TDS?more than 1000mg/L.And About 50%of deep groundwater samples are saline with TDS>1,000 mg/L.Based on the R-mode factor analysis,the spatial distribution of saline groundwater is significantly distinct in each aquifer.Shallow saline groundwater is mainly distributed in the discharge and flow-through zones in the central basin,whilst deep saline groundwater is located along the faults or the lowland areas.This suggests that the groundwater salinity in the shallow and deep aquifers may be controlled by distinct mechanisms.Large-scale F,As contamination and scattered B contamination in groundwater are distributed in the Yuncheng Basin.Spatial factor analysis indicates that groundwater F,As and B contamination is closely related to groundwater salinization and agricultural activities?i.e.,exploration of geothermal water?.According to the risk of contamination,groundwater in the basin is spatially divided into:shallow groundwater with high risk located in the north side of the salt lake,shallow groundwater with moderate risk and deep groundwater with low risk.Shallow groundwater samples show a clear increase in the concentrations of components,including salinity and ionic composition,along regional flow paths towards the center of the basin where the Salt Lake is located.The results of stable isotopes(?2H,?18O)show that a few shallow groundwater samples at Yuncheng Basin are located near the evaporation line,revealing that groundwater has undergone evaporation and concentration.Especially in the central basin,the intense evaporation aggravates the salt accumulation in groundwater.Based on hydrogeochemical analysis and saturation indices,water-rock interaction,including dissolution of halite,gypsum,and mirabilite and carbonate weathering contribute to the formation of shallow saline groundwater.Salt Lake water intrusion is one of the important mechanisms contributing to shallow groundwater salinity.Over-exploitation of groundwater in the basin results in the occurrence of depression cones,leading to the intrusion of Salt Lake saline waters into shallow groundwater.The results of chloride mass balance show that shallow groundwater salinization as a result of Salt Lake intrusion was limited to the northern shore of the lake where the percentage reaches up to 4.27%.Cation exchange occurs in groundwater at the recharge and flow-through area and the contribution of cation exchange to groundwater salinity?in Na+concentration?are estimated to be 1.23-15.20mmol/L.Depleted stable isotope compositions(?18O-H2O and?D values as low as-11.03‰and-81.7‰,respectively),together with low radiocarbon activities?mostly less than 20pmC?in deep aquifers indicate that deep groundwater comprises palaeo-waters largely recharged in the late Pleistocene and early Holocene?5 to 11 ka B.P.?during a cooler period and the palaeo-temperature is approximately 3°C cooler than the modern temperature.Three types of salt sources for deep groundwater were identified by combining the indices of multi-isotopic and major,minor compositions.Evaporation of palaeo-saline waters is responsible for the locally occurring deep groundwater salinity in the Qiji and Wenxi.Mixing with deep geothermal water explains the elevation of TDS contents in saline groundwater sampled near the faults while gradual vertical leakage of shallow saline groundwater has an impact on deep groundwater salinity near the depression cone.Major ion chemistry and?34S data reflect the dissolution of evaporite salts.Trends in groundwater HCO3 contents and?13C values indicate that carbonate dissolution is a significant source of DIC.Groundwater 87Sr/86Sr ratios are similar to those in both local rainfall and loess carbonate.Despite substantial evidence for carbonate weathering,groundwater is generally Na-rich and Ca-poor due to cation exchange.Genesis of deep freshwater in the E'mei Plateau can be related to bacterial sulfate reduction and carbonate weathering.As a representative of non-traditional isotope tracer,Li isotope can be efficiently applied in the identification of salt sources in groundwater.Results of this research show that groundwater?7Li in the Yuncheng Basin ranges between 10.97‰and31.78‰,with a median value of 22.53‰.The lowest?7Li value is from the deep geothermal waters?10.97‰-14.29‰,median 11.79‰?,followed by other deep groundwater?14.65‰-21.87‰,median 21.87‰?,while shallow groundwater contains the highest?7Li?19.57‰-31.78‰,median 24.81‰?.Significant Li isotopic variations in different end members show that?7Li can be used to trace salinization processes and to delineate multiple salt sources.Anthropogenic activities contribute significantly to groundwater salinization at Yuncheng Basin.An integrated analysis of Cl/Br ratio,?18OH2O,?D,?13C and?7Li shows that shallow groundwater is more vulnerable to pollution.The highest salinity and Cl/Br ratios are found in the shallow groundwater located in the runoff area between Kaolao Lowland and the center of the Yuncheng Basin.The consistency of?18O and?2H values with increasing Cl concentrations and Cl/Br ratios indicate that halite is flushed during infiltration of irrigation return flow.The observed characteristics of high nitrate and low?7Li and medium?13C values suggest that agricultural processes,like wastewater and fertilizer,contribute to the groundwater salinization in the center of the Yuncheng Basin.According to the results of combined use of Cl concentrations and Cl/Br ratios with?7Li values,the shallow groundwater in the northern part of Salt Lake is affected by the Salt Lake intrusion.This is due to the disturbance of the dynamic balance between fresh water and saline water induced by over-exploitation of groundwater.In addition,intensive groundwater abstraction causes vertical leakage of shallow groundwater near the cone of depression,increasing the risks of contamination for deep groundwater.Finally,groundwater salinization occurs due to exploitation of geothermal water and/or anthropogenic geothermal wastewater drainage.The maximum F concentration in groundwater at Yuncheng Basin reported in this study is up to 14.1 mg/L,which is much higher than the WHO provisional drinking water guideline of 1.5 mg/L.Vertically,natural high F groundwater occurs both in shallow,intermediate and deep aquifers.Over 69%of the shallow groundwater?with a F-concentration up to 14.1 mg/L?,44%of groundwater samples from the intermediate and 31%from the deep aquifers have F-concentrations above 1.5 mg/L.Horizontally,high F groundwater mainly occurs in the discharge area in the center of the basin.Groundwater with high F-concentrations displays distinctive major ion chemistry:Na-rich and Ca-poor with a high pH value and high HCO3-content.Hydrochemical diagrams and hydrogen,oxygen isotope compositions indicate that variations in the major ion chemistry and pH are controlled by salinization processes including mineral dissolution,cation exchange and evaporation in the aquifers,which are important for F-mobilization as well.Dissolution of fluorine-bearing minerals and desorption of exchangeable F-in the loess supply most of F-in groundwater.Calcite precipitation removes Ca2+from the solution and thus results in a further dissolution of F-bearing minerals?such as fluorite and fluorapatite?,releasing F-into the groundwater.Cation exchange promotes the release of F-from sediments into groundwater via the conversion of Ca-rich groundwater to Na-rich groundwater and results in release of more F-from the mineral phases into the aqueous solution.Competitive anions,especially HCO3-,enable F-enrichment in groundwater.Evaporation that causes elevation of ion?including F-?concentrations is another factor that controls the occurrence of high-F-shallow groundwater.Our research shows that high fluoride saline groundwater in the Yuncheng Basin is evolved via a combination of natural geological,hydrogeological and anthropogenic processes.It is suggested that effective management of groundwater is required to meet the demands of ever-increasing populations and economic activities without further deterioration of groundwater resources.Long-term plans should be established for the estimation of water supply and demand for each sector.Well production is also needed to be granted by a“permission card”.Local government should intensify surface-water pollution control and implement emission permit system.It is equally important that chemical fertilizers and pesticides should be employed on a sound basis to avoid agriculture non-point pollution.Local government should also adopt measures to ensure the“green”exploitation of geothermal groundwater. |
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