Association of hydrogeological factors in temporal variations of fluoride concentration in a crystalline aquifer in India

The major part of groundwater in India is found in granitic aquifers. Fluoride in groundwater from a crystalline aquifer in a semi-arid region of granitic rocks in India, known as Maheshwaram watershed, was analyzed for spatial and temporal variability during 1999–2002 to assess the effect of hydrogeological factors on fluoride concentration. Samples were collected from 32 representative wells in the area for the pre- and post-monsoon seasons and analyzed for F content. The CHESS computer program was used to calculate ionic activities of aqueous species and the mineral saturation index (SI) for calcite and fluorite. The GARDENIA computer program was used to calculate the recharge values in the study area. The influences of dissolution kinetics of fluoride minerals and recharge from rainfall on fluoride concentration were of interest and results clearly indicate that fluoride content in groundwater depends on the interaction period of groundwater with host rock. Results could also be utilized for designing remedial measures particularly with dilution method in an optimal way.     

Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania

Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer overexploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na⁺ and Cl^? near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na–Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca–HCO₃ and Ca–Mg–HCO₃ groundwater types. Further along flow paths, Ca²⁺ and Na⁺ ion exchange causes groundwater evolution to Na–HCO₃ type. From the PHREEQC simulation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system.     

天津市宁河北奥陶系灰岩水源地的水文地球化学模拟

天津市宁河北水源地属埋藏型地下水源地,其开采层奥陶系灰岩含水层呈北东向的向斜展布,隐伏于石炭系-二叠系砂页岩之下,并被新近系覆盖,仅在东北部与第四系含水层有水力联系,地下水的补给主要来自浅层第四系含水层的越流补给。应用水文地球化学模拟方法,研究从第四系含水层到奥陶系含水层地下水经历的水-岩作用,包括方解石、白云石、萤石和硬石膏的溶解或沉淀、二氧化碳气体的溶解或逸出等,地下水硬度、矿化度总体上有下降趋势,这是由于含Ca²⁺、Mg²⁺和HCO^-₃的矿物相发生溶解,迁入溶液的比率要小于发生沉淀迁出溶液的比率,具体表现在从第四系含水层到奥陶系含水层Ca²⁺、和HCO^-₃浓度的降低幅度较大。通过模拟计算,可以定量确定从补给区到研究区沿水流路径上的水-岩相互作用及质量交换,有助于揭示该区地下水化学环境的演化机制。     

Using non-conservative tracers to characterise karstification processes in the Merinos-Colorado-Carrasco carbonate aquifer system (southern Spain)

The systematic sampling of the chemical composition of the groundwater from five karst springs (including an overflow spring) and one outflowing borehole have permitted to determine distinctive chemical changes in the waters that reflect the geochemical processes occurring in a carbonate aquifer system from southern Spain. The analysis of the dissolution parameters revealed that geochemical evolution of the karst waters basically depends on the availability of the minerals forming aquifer rocks and the residence time within the aquifers. In the three proposed scenarios in the aquifers, which include the preferential flow routines, the more important geochemical processes taking place during the groundwater flow from the recharge to the discharge zones are: CO₂ dissolution and exsolution (outgassing), calcite net dissolution, calcite and dolomite sequential dissolution, gypsum/anhydrite and halite dissolution, de-dolomitization and calcite precipitation. A detailed analysis of the hydrochemical data set, saturation indices of the minerals and partial pressure of CO₂ in the waters joined to the application of geochemical modelling methods allowed the elaboration of a hydrogeochemical model of the studied aquifers. The developed approach contributes to a better understanding of the karstification processes and the hydrogeological functioning of carbonate aquifers, the latter being a crucial aspect for the suitable management of the water resources.     

滇东北矿集区茂租铅锌矿床地球化学特征

茂租铅锌矿床位于扬子地台西南缘,是滇东北矿集区内的一个大型矿床,矿体主要呈似层状产于震旦系灯影组白云岩中;矿石矿物以闪锌矿为主,次为方铅矿;脉石矿物主要为白云石、方解石和萤石。本文对该矿床中与铅锌矿密切共生的团块状白云石、方解石和萤石以及围岩灯影组白云岩的REE地球化学特征和C、O、Sr同位素进行了对比研究。结果表明:团块状白云石和方解石的稀土配分模式、C同位素和Sr同位素比值与围岩灯影组白云岩比较接近,表明形成团块状白云石和方解石的成矿流体主要来源于围岩灯影组白云岩的溶解;但这两种矿物的稀土总量ΣREE高于灯影组白云岩,说明成矿流体除了主要由围岩提供REE外,还有部分其他富含REE流体的加入。萤石则具有LREE亏损和分配曲线相对平缓的稀土配分模式特征,表明萤石形成于成矿的晚阶段,有更多的外部流体的加入。团块状白云石、方解石和萤石表现出明显的Eu正异常,且团块状白云石和方解石的O同位素低于灯影组白云岩,反映了存在较高温度的流体活动,这3种脉石矿物是由高温热液流体形成的。灯影组白云岩和3种脉石矿物都具有明显的Ce负异常,说明成矿流体可能主要来源于地层循环水,继承了围岩的Ce负异常特征。方解石和萤石的Sr同位素比值高于围岩震旦系灯影组白云岩和峨眉山玄武岩,但小于基底岩石昆阳群和会理群,说明成矿流体主要由赋矿围岩等沉积地层中的循环流体与流经了基底岩石的深部流体混合形成。     

Fluoride dynamics in the granitic aquifer of the Wailapally watershed,Nalgonda District,India

High concentrations of fluoride (up to 7.6 mg/L) are a recognized feature of the Wailapally granitic aquifer of Nalgonda District, Andhra Pradesh, India. The basement rocks provide abundant sources of F in the form of amphibole, biotite, fluorite and apatite. The whole-rock concentrations of F in the aquifer are in the range 240–990 mg/kg. Calcretes from the shallow weathered horizons also contain comparably high concentrations of F (635–950 mg/kg). The concentrations of water-soluble F in the granitic rocks and the calcretes are usually low (1% of the total or less) but broadly correlate with the concentrations observed in groundwaters in the local vicinity. The water-soluble fraction of fluoride is relatively high in weathered calcretes compared to fresh calcretes.Groundwater major-ion composition shows a well-defined trend with flow downgradient in the Wailapally aquifer, from Na–Ca–HCO₃-dominated waters in the recharge area at the upper part of the catchment, through to Na–Mg–HCO₃ and ultimately to Na–HCO₃ and Na–HCO₃–Cl types in the discharge area in the lowest part. The evolution occurs over a reach spanning some 17 km. Groundwater chemistry evolves by silicate weathering reactions, although groundwaters rapidly reach equilibrium with carbonate minerals, favouring precipitation of calcite, and ultimately dolomite in the lower parts of the watershed. This precipitation is also aided by evapotranspiration. Decreasing Ca activity downgradient leads to a dominance of fluorite-undersaturated conditions and consequently to mobilisation of F. Despite the clear downgradient evolution of major-ion chemistry, concentrations of F remain relatively uniform in the fluorite-undersaturated groundwaters, most being in the range 3.0–7.6 mg/L. The rather narrow range is attributed to a mechanism of co-precipitation with and/or adsorption to calcrete in the lower sections of the aquifer. The model may find application in other high-F groundwaters from granitic aquifers of semi-arid regions.     

Fluoride hydrogeochemistry and bioavailability in groundwater and soil of an endemic fluorosis belt,central Iran

Presence of fluoride in groundwater is a public health problem in the so-called endemic fluorosis belt of the central Iran, where the groundwater is the major source of drinking water in most urban and rural areas. Therefore, an attempt has been made to determine the hydrogeochemical factors controlling fluoride enrichment in the groundwater resources at this belt. Fluoride concentrations ranged from 0.20 to 1.99 mg/L (1.02 ± 0.47) in groundwater samples. The presence of different F-bearing minerals and also clay minerals in the soils and aquifer materials was confirmed using XRD analysis. To identify probable sources of dissolved F^? and investigate groundwater quality, multivariate statistical analyses were carried out. Geochemical modeling indicated that all samples were undersaturated with respect to fluorite, halite, gypsum and anhydrite and mostly oversaturated with respect to calcite and dolomite. Contrary to most high-fluoride regions in the World, the high F^? content was dominated by Na–Cl- and Ca–SO₄-type groundwater in the study area. Besides, fluoride showed negative relationship with pH and HCO₃ ^? in groundwater. In order to assess the bioavailability of fluoride in soils, a two-step chemical fractionation method was applied. The results showed that fluoride in soils mostly accompanied with the residual and water-soluble fractions and was poorly associated with soil’s bonding sites. Calculated aqueous migration coefficient demonstrated that fluoride in the studied soils was mobile to easily leachable to the groundwater. Finally, the results demonstrated that combination of water–rock interaction and influence of clay minerals is geochemical mechanism responsible for controlling fluoride enrichment in groundwater.     

基于反向地球化学模拟的地下水形成作用: 以安徽省泗县为例

在了解安徽省泗县水文地质条件基础上,分析了区域水文地球化学特征及类型,探讨了其空间分布特征。根据开采条件下的地下水动力场条件,选择了3条模拟路径,采用PHREEQC软件进行了水文地球化学模拟研究,定量分析了地下水的形成机理及演化。结果表明,路径Ⅰ发生了岩盐、石膏以及伊利石的溶解,高岭土、石英、白云石、萤石发生了沉淀;钙蒙脱石、方解石不参与反应,NaX解吸,CaX_2被吸附;路径Ⅱ发生了岩盐、石膏、伊利石、石英等的溶解以及钙蒙脱石、方解石的沉淀,NaX解吸,CaX_2被吸附;路径Ⅲ发生的反应基本与路径Ⅰ相同,不同之处在该路径上的白云石发生了溶解,其原因可能是地下水在径流过程中溶解CO_2,使其继续溶解白云石以及受沉淀滞后的影响。研究结果表明地下水开采条件下,泗县地下水化学组分主要受到了岩盐和石膏等矿物的溶解作用、钙钠离子交换作用以及钙蒙脱石、方解石沉淀作用的控制。     

菱镁矿开采引起的Dubravsk"非汉字符号" Massif水文地质结构中的地下水化学成分变化

Dubravsky Massif菱镁矿体的开采产生了部分局限在石炭纪低裂缝渗透变质岩中的喀斯特型裂缝含水层。大量的开矿过程对含水层中的地下水循环和化学成分都造成了可观的变化。本文研究了在这种复杂条件下地下水化学成分变化成因模型。根据化学配位模型,计算出所选矿物的饱和指数,从而指示所有喀斯特裂缝含水层饱和带中的地下水对菱镁矿、白云石、方解石的过饱和度和石膏的不饱和度。假设地下水未受人类活动的污染,而白云石中黄铁矿的氧化对成矿影响甚微,则水化学数据的统计描述代表了含水层中的水文地球化学背景。矿山中矿石焙烧释放的富氮气体凝聚产生的醋酸,能促进菱镁矿和白云石的溶解。这样产生的地下水相对背景值来说则富集NO3,Mg,和TIC。因此,由于地下开矿,岩溶作用估计将是原来的1.5倍。尽管有污染,这些所测参数并没有超过斯洛伐克卫生保健部颁发的No.29／2002 Z.z.饮用水标准临界值。因此采矿后含水层的开采也是可行的。     

人类活动影响下娘子关岩溶水系统地球化学演化

娘子关泉是我国北方最大的岩溶泉之一,也是阳泉市工农业生产和人民生活的重要供水水源。地下水地球化学演化分析表明,在地下水由补给区向排泄区运移过程中,除固有的水岩相互作用外,由于受采矿活动和地表水入渗补给的影响,岩溶水由低离子含量的HCO3-SO4或HCO3型水逐渐成为SO4型、SO4-HCO3型和SO4-HCO3-Cl型水。在泉群集中排泄区,区域流动系统与局部流动系统的地下水发生混合作用,最终形成了水质相对良好的HCO3-SO4型或SO4-HCO3型岩溶泉水。在此过程中,地下水对方解石和白云石也由最初的溶解作用演变为沉淀再结晶。尽管石膏呈持续溶解现象,但在采煤活动严重影响区域,石膏的沉淀也可能出现。地球化学模拟表明,在岩溶含水层中,地下水首先以方解石（白云石）的溶解为主;随着石膏溶解数量的增加,方解石（白云石）的溶解开始受到抑制,进而发生沉淀,石膏的溶解成为控制地下水水化学的主导过程。当矿坑水混入时,地下水相对石膏过饱和,地下水对碳酸盐岩含水介质的溶蚀能力得到增强。随着水岩反应的演进,铁氢氧化物大量沉淀,通过共沉淀和吸附作用去除了地下水中的重金属类污染物。      

山东大汶河流域中上游古近系含水岩组水文地质特征

利用古近系大汶口组及朱家沟组含水岩组5眼施工钻孔及55眼井孔调查资料,通过岩心、地下水常规离子组分及氢氧同位素样品测试结果,研究大汶河流域中上游莱芜盆地、大汶口盆地古近系含水岩组分布特征、物性及富水性、地下水化学及补给循环特征。结果表明:富水较好的大汶口组上段位于盆地腹部偏北,含水层岩性为半固结砂砾岩,水化学类型HCO_3-Ca·Mg型为主;朱家沟组含水岩组位于盆地边界断裂下降盘,含水层岩性为溶蚀发育的灰质砾岩,水化学类型复杂多变。现代大气降水作为地下水主要补给来源,蒸发浓缩作用,方解石、白云石和石膏溶解及人类污染活动参与了地下水化学组分的形成。归纳总结3种古近系不同类型含水岩组地下水富集规律:构造裂隙控水模式、半固结含水岩组裂隙孔隙水富集模式和灰质砾岩裂隙岩溶水富集模式。     

Geochemical processes,evidence and thermodynamic behavior of dissolved and precipitated carbonate minerals in a modern seawater/freshwater mixing zone of a small tropical island

The geochemical processes and thermodynamic behavior of dissolved and precipitated carbonate minerals controlling the hydrochemistry of an aquifer in the seawater/freshwater mixing zone of a small island are identified. Field and laboratory analyses, geochemical modeling (PHREEQC) and multivariate statistical analysis (MSA) provide a quantitative interpretation for the geochemistry of the carbonate-dominated aquifer. Geochemical analyses and modeling results show that dissolution and re-precipitation of CaCO₃ are the prevalent processes governing geochemical reactions in the mixing zone. Furthermore, this was confirmed by coherent statistical output that incorporates Principle Component Analysis (PCA) and k-means Cluster Analysis (k-CA). Generally, the composition of the lowland sandy soil was rather homogeneous and was primarily composed of quartz, aragonite, calcite and Mg-calcite. Thermodynamic model calculations indicate that the carbonate minerals calcite, aragonite and dolomite are supersaturated in the mixing zone. Nevertheless, Powder X-ray Diffraction (PXRD) and Scanning Electron Microscope (SEM) examination verified the occurrence of low-Mg-calcite (LMC) and the absence of dolomite, attributed to thermodynamic/kinetic hindrance, cation disorder and the presence of dolomite crystal growth rate inhibitors (such as SO₄). The results suggest that dissolution of aragonite and precipitation of LMC drives the solid phase geochemistry in the small tropical island aquifer.     

Groundwater flow and residence time in a karst aquifer using ion and isotope characterization

In order to identify the origin of the main processes that affect the composition of groundwater in a karstic aquifer, a hydrogeochemical and isotopic study was carried out of water from numerous observation wells located in Sierra de Gador, a semiarid region in SE Spain. Several natural and anthropogenic tracers were used to calculate groundwater residence time within this complex aquifer system. Analysis of major ions enabled the principal geochemical processes occurring in the aquifer to be established, and the samples were classified into four distinctive solute groups according to this criterion. Dissolution of carbonate rocks determines the chemical composition of less mineralized water. In another group, the concurrent dissolution of dolomite and precipitation of calcite in gypsum-bearing carbonate aquifer, where the dissolution of relatively soluble gypsum controls the reaction, are the dominant processes. Marine intrusion results in highly mineralized waters and leads to base exchange reactions. The groundwater enrichment of minor and trace elements allowed classification of the samples into two classes that are linked to different flow patterns. One of these classes is influenced by a slow and/or deep regional flow, where the temperature is generally elevated. The influence of sulphate reduces by up to 40 % the barium concentration due to the barite precipitation. Isotope data (T, ¹⁴C) confirm the existence of recent local flows, and regional flow system, and ages of ground water may reach 8000 years. The importance of gypsum dissolution in this aquifer is proved by the δ³⁴S content.     

菱镁矿开采引起的Dubravsk(y) Massif水文地质结构中的地下水化学成分变化

Dubravsky Massif菱镁矿体的开采产生了部分局限在石炭纪低裂缝渗透变质岩中的喀斯特型裂缝含水层.大量的开矿过程对含水层中的地下水循环和化学成分都造成了可观的变化.本文研究了在这种复杂条件下地下水化学成分变化成因模型.根据化学配位模型,计算出所选矿物的饱和指数,从而指示所有喀斯特裂缝含水层饱和带中的地下水对菱镁矿、白云石、方解石的过饱和度和石膏的不饱和度.假设地下水未受人类活动的污染,而白云石中黄铁矿的氧化对成矿影响甚微,则水化学数据的统计描述代表了含水层中的水文地球化学背景.矿山中矿石焙烧释放的富氮气体凝聚产生的醋酸,能促进菱镁矿和白云石的溶解.这样产生的地下水相对背景值来说则富集NO3,Mg,和TIC.因此,由于地下开矿,岩溶作用估计将是原来的1.5倍.尽管有污染,这些所测参数并没有超过斯洛伐克卫生保健部颁发的No.29/2002 Z.z.饮用水标准临界值.因此采矿后含水层的开采也是可行的.     

GIS assisted inverse geochemical modeling for plausible phase transfers in aquifers

A geographical information system (GIS) assisted approach that couples a groundwater flow model and an inverse geochemical model is presented to quantify the phase mole transfers between two points on the flow path within a groundwater system. It is used to investigate the plausible phase transfers in the unconfined aquifer of Mehsana district of Gujarat State, India. X-ray diffraction analysis of soil samples is carried out for mineral characterization. The groundwater flow field is simulated using MODFLOW and flow paths used for inverse geochemical modeling are traced using PMPATH, a particle tracking algorithm. The plausible phase mole transfers in the flow path are quantified using PHREEQC geochemical code. The different scenarios generated by inverse modeling routine are used as input to the geochemical model and simulation runs are taken as forward models. The obtained results are compared to the target solution chemistry by using the square of the Pearson product moment correlation coefficient. Results reveal that the groundwater is undersaturated with anhydrite, carbon dioxide (gas), fluorite, gypsum, halite, jarosite-K, and siderite. It is oversaturated with aragonite, calcite, dolomite, ferrihydrite, goethite, and hematite. Results further reveal that calcite is precipitating, while dolomite, gypsum, carbon dioxide, and fluorite are dissolving together with ion exchange in the flow path.     

Inverse geochemical modeling of groundwater salinization in Azraq Basin,Jordan

The aim is to define the mechanism of chemical reactions that are responsible for the salinization of the Azraq basin along groundwater flow path, using inverse modeling technique by PHREEQC Interactive 2.8 for Windows. The chemical analysis of representative groundwater samples was used to predict the causes of salinization of groundwater. In addition, the saturation indices analysis was used to characterize the geochemical processes that led to the dissolution of mineral constituents within the groundwater aquifer system. According to the modeling results, it was noted that the groundwater at the recharge area was undersaturated with respect to calcite, dolomite, gypsum, anhydrite, and halite. Thus, the water dissolved these minerals during water rock interaction, and therefore, the concentration of Ca, Mg, Na, and SO₄ increased along the groundwater flow path. Furthermore, the groundwater at the discharge area was oversaturated with respect to calcite and dolomite. This meant that the water would precipitate these minerals along the flow path, while the water was undersaturated with respect to gypsum and halite throughout the simulated path; this showed the dissolution processes that take place during water-rock interaction. Therefore, the salinity of the groundwater increased significantly along the groundwater flow paths.     

Fluoride contamination in Gharbar Village of Dhanbad District,Jharkhand, India: source identification and management

This paper reports occurrence of high fluoride concentration in groundwater of Gharbar Village, Dhanbad District, Jharkhand, India. The concentration of fluoride varied from 0.3 to 14.9 mg/L in 27 groundwater samples. The correlation studies demonstrate that fluoride has strong positive correlation with pH and sodium and negative correlation with calcium. It indicates dominance of ion exchange activity and rock water interaction. Thermodynamic consideration shows that all the samples were oversaturated with calcite and most of the samples were under saturated with fluorite. The results indicate that occurrence of high concentration of fluoride is leading by geochemical composition of rocks, alkaline environmental condition, weathering of rocks and ion exchange processes.     

Multivariate statistical analyses of the groundwater system in Darb El-Arbaein, Southwestern Desert, Egypt

In Darb El-Arbaein, the groundwater is the only water resource. The aquifer system starts from Paleozoic–Mesozoic to Upper Cretaceous sandstone rocks. They overlay the basement rocks and the aquifer is confined. In the present research, the performance of the statistical analyses to classify groundwater samples depending on their chemical characters has been tested. The hydrogeological and hydrogeochemical data of 92 groundwater samples were obtained from the General Authority for Rehabilitation Projects and Agricultural Development authority in northern, central, and southern Darb El-Arbaein. A robust classification scheme for partitioning groundwater chemistry into homogeneous groups was an important tool for the characterization of Nubian sandstone aquifer. We test the performance of the many available graphical and statistical methodologies used to classify water samples. R-mode, Q-mode, correlation analysis, and principal component analysis were investigated. All the methods were discussed and compared as to their ability to cluster, ease of use, and ease of interpretation. The correlation investigation clarifies the relationship among the lithologic, hydrogeologic, and anthropogenic factors. Factor investigation revealed three factors, namely, the evaporation process–agricultural impact–lithogenic dissolution, the hydrogeological characteristics of the aquifer system, and the surface meteoric water that recharge the aquifer system. Two main clusters that subdivided into four subclusters were identified in the groundwater system based on hydrogeological and hydrogeochemical data. They reflect the impact of geomedia, hydrogeology, geographic position, and agricultural wastewater. The groundwater is undersaturated with respect to most selected minerals. The groundwater was supersaturated with respect to iron minerals in northern and southern Darb El-Arbaein. The partial pressure of the groundwater versus saturation index of calcite shows the gradual change in PCO₂ from atmospheric to the present aquifer pressures.     

化隆—循化盆地不同类型含水层组高氟地下水的分布及形成过程

天然成因的高氟地下水是世界范围内备受关注的环境问题和饮用水安全问题。前人对高氟地下水的形成过程已开展了大量研究,但是对于高原盆地复杂水文地质条件下不同类型含水层组(第四系松散层含水层、基岩裂隙或岩溶含水层以及新生代古近纪以来的碎屑岩含水层)高氟地下水的分布和形成过程尚不明确。本文以化隆—循化盆地为研究区,通过采集、测试研究区内的各类地下水样品,分析研究区内不同类型含水层中地下水的化学特征及同位素特征。结果表明,高氟地下水(1.007.73 mg/L)主要分布在沿黄河的河谷区域和巴燕低山丘陵区域的泉水和潜水中以及深部的承压水中,在垂向上高氟地下水无明显分布规律。接受黄河水入渗补给的河谷潜水中氟离子浓度较低,补给黄河的河谷潜水中氟离子浓度较高。贫钙富钠的弱碱性苏打型水有利于地下水中氟的富集。泉水和潜水中氟主要来源于萤石的溶解,而承压水中氟除了来源于萤石外,还来源于其他含氟矿物。对于潜水和第四系松散层泉水,蒸发浓缩作用促进了地下水中氟的富集。另外,阴离子竞争吸附作用、阳离子交换吸附作用是泉水(第四系松散层泉水和基岩裂隙泉水)和潜水中氟元素富集的主要原因,而承压水中氟离子浓度受竞争吸附作用影响较大,阳离子交换吸附作用影响较小。研究成果可为化隆—循化盆地低氟地下水的勘查和开发提供科学依据。     

Geochemical processes controlling fluoride-bearing groundwater in the granitic aquifer of a semi-arid region

The aim of the present study is to identify the geochemical processes responsible for higher fluoride (F^–) content in the groundwater of the Yellareddigudem watershed located in Nalgonda district, Andhra Pradesh. The basement rocks in the study area comprise mainly of granites (pink and grey varieties), which contain F-bearing minerals (fluorite, biotite and hornblende). The results of the study area suggest that the groundwater is characterized by Na⁺: HCO facies. The F^– content varies from 0.42 to 7.50 mg/L. In about 68% of the collected groundwater samples, the concentration of F^– exceeds the national drinking water quality limit of 1.5 mg/L. The weathering of the granitic rocks causes the release of Na⁺ and HCO ions, which increase the solubility of ions. Ion exchange between Na+ and Ca²⁺, and precipitation of CaCO₃ reduce the activity of Ca²⁺. This favours dissolution of CaF₂ from the F-bearing minerals present in the host rocks, leading to a higher concentration of F^– in the groundwater. The study further suggests that the spatial variation in the F^– content appears to be caused by difference in the relative occurrence of F-bearing minerals, the degree of rockweathering and fracturing, the residence time of water in the aquifer materials and the associated geochemical processes. The study emphasizes the need for appropriate management measures to mitigate the effect of higher F groundwater on human health.