Statistical categorization geochemical modeling of groundwater in Ain Azel plain (Algeria)

Water analysis data of 54 groundwater samples from 18 uniformly distributed wells were collected during three campaigns (June, September and December 2004). Q-mode hierarchical cluster analysis (HCA) was employed for partitioning the water samples into hydrochemical facies. Interpretation of analytical data showed that the abundance of major ions was identified as follows: Ca ? Mg > Na > K and HCO₃ ? Cl > SO₄. Three major water facies are suggested by the HCA analysis. The samples from the area were classified as recharge area waters (Ca–Mg–HCO₃ water), transition area waters (Mg–Ca–HCO₃–Cl water), and discharge area waters (Mg–Ca–Cl–HCO₃ water). Inverse geochemical modeling suggests that relatively few phases are required to derive the water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into two categories: (1) evaporite weathering reactions and (2) precipitation of carbonate minerals.     

Hydrogeochemical assessment of groundwater in Neyveli Basin, Cuddalore District, South India

In the light of progressive depletion of groundwater reservoir and water quality deterioration of the Neyveli basin, an investigation on dissolved major constituents in 25 groundwater samples was performed. The main objective was detection of processes for the geochemical assessment throughout the area. Neyveli aquifer is intensively inhabited during the last decenniums, leading to expansion of the residential and agricultural area. Besides semi-aridity, rapid social and economic development stimulates greater demand for water, which is gradually fulfilled by groundwater extraction. Groundwaters of the study area are characterized by the dominance of Na?+?K over Ca?+?Mg. HCO₃ was found to be the dominant anion followed by Cl and SO₄. High positive correlation was obtained among the following ions: Ca–Mg, Cl–Ca,Mg, Na–K, HCO₃–H₄SiO₄, and F–K. The hydrochemical types in the area can be divided into two major groups: the first group includes mixed Ca–Mg–Cl and Ca–Cl types. The second group comprises mixed Ca–Na–HCO₃ and Ca–HCO₃ types. Most of the groundwater samples are within the permissible limit of WHO standard. Interpretation of data suggests that weathering, ion exchange reactions, and evaporation to some extent are the dominant factors that determine the major ionic composition in the study area.     

Assessment of natural and anthropogenic influences on regional groundwater chemistry in a highly industrialized and urbanized region: a case study of the Vaal River Basin,South Africa

The Vaal River Basin is an economically significant area situated in the interior of South Africa (SA), where mining, industrial, domestic and agricultural activities are very intense. The purpose of the study was to assess the influence of geology and anthropogenic activities on groundwater chemistry, and identify the predominant hydrochemical processes in the basin. Data from seventy groundwater sites were retrieved from the national database, and attention was paid to fifteen water quality parameters. Groundwater samples were clustered into seven hydrochemically distinct groups using Hierarchical Cluster Analysis (HCA), and three samples treated independently. A Piper plot revealed two major water types, Ca–Mg–HCO₃ and Ca–Mg–SO₄-Cl, which were linked to dissolution of the underlying geology and mine pollution. The Ca?+?Mg vs HCO₃?+?SO₄ plot indicated that reverse ion exchange is an active process than cation exchange in the area. Principal component analysis (PCA) was used to identify the main natural and anthropogenic processes causing variation in groundwater chemistry. Four principal components were extracted using PCA that explains 82% of the total variance in the chemical parameters. The PCA results can be categorized by four components: (1) evaporites and silicates weathering enrichment of Na, K, Cl, SO₄ and F, and anthropogenic Cl; (2) dissolution of dolomite, limestone and gypsum; (3) agricultural fertilizers (4) wastewater treatment. This study reveals that both natural and anthropogenic activities are the cause of groundwater variation in the basin.     

Shallow and deep aquifer hydrochemistry and chemical variability of water quality at Shariatpur district, Bangladesh

Water quality and hydrochemistry of Shariatpur district were evaluated in terms of hydrochemical composition and some important physico-chemical parameters. The groundwater of the study area is good for drinking, domestic as well as for irrigation purposes. Among the major ions, shallow tube well waters give higher concentration of Ca²⁺ which ranges from 24 to 260 mg/L. The deep tubewell waters show higher concentration of Na⁺ which varies from 74 to 582 mg/L during dry season. Among the trace elements most of the shallow aquifer samples show higher concentration of Fe²⁺, Mn²⁺ and As. Concentration of Fe²⁺ varies from 0.655 to 18.8 mg/L, and Mn²⁺ from trace to 0.868 mg/L during dry period. Hydrochemical analyses reveal significant seasonal variation in water quality of shallow aquifer. Both the shallow aquifer and the surface water of the study area are predominantly of Ca–Mg–HCO₃ type, while the deep aquifer water is mainly of Na–K–Cl–SO₄ type with slight inclination to Ca–Mg–HCO₃ type. The study area is suitable for groundwater development if comprehensive and holistic approaches towards water resource management are taken into consideration.     

Chemical evolution of a gas-capped deep aquifer,southwest of Iran

The Kangan Aquifer (KA) is located below a gas reservoir in the crest of the Kangan Anticline, southwest of Iran. This aquifer is composed of Permo-Triassic limestone, dolomite, sandstone, anhydrite and shale. It is characterized by a total dissolved solid of about 332,000 mg/L and Na–Ca–Cl-type water. A previous study showed that the source of the KA waters is evaporated seawater. Chemical evolution of the KA is the main objective of this study. The major, minor and trace element concentrations of the KA waters were measured. The chemical evolution of KA waters occurred by three different processes: evaporation of seawater, water–rock and water–gas interactions. Due to the seawater evaporation process, the concentration of all ions in the KA waters increased up to saturation levels. In comparison to the evaporated seawater, the higher concentrations of Ca, Li, Sr, I, Mn and B and lower concentrations of Mg, SO₄ and Na and no changes in concentrations of Cl and K ions are observed in the KA waters. Based on the chemical evolution after seawater evaporation, the KA waters are classified into four groups: (1) no evolution (Cl, K ions), (2) water–rock interaction (Na, Ca, Mg, Li and Sr ions), (3) water–gas interaction (SO₄ and I ions) and (4) both water–rock and water–gas interactions (Mn and B ions). The chemical evolution processes of the KA waters include dolomitization, precipitation, ion exchange and recrystallization in water–rock interaction. Bacterial reduction and diagenesis of organic material in water–gas interaction also occur. A new type of chart, Ca_excess versus Mg_deficit, is proposed to evaluate the dolomitization process.     

Assessment of natural and anthropogenic impacts in groundwater,utilizing multivariate statistical analysis and inverse distance weighted interpolation modeling: the case of a Scopia basin (Central Greece)

In Scopia basin, central Greece, a hydrochemical investigation was completed. Groundwater samples from 41 sites were used to assess the natural and anthropogenic impacts in groundwater, utilizing the principal component analysis (PCA) involved with the inverse distance weighted (IDW) interpolation modeling and hierarchical cluster analysis (HCA). Best fit model to explain the spatial distribution of both hydrochemical parameters and PCA was chosen by optimizing the IDW interpolator’s parameters. Precision of the model was picked based on less root-mean-squared prediction error (RMSPE) amongst predicted and actual values measured at the same locations. Groundwater exhibit Ca–Mg–HCO₃ as the dominant hydrochemical type and their greater part are mixed waters with non-dominant ion. Interpolation models demonstrate high estimations of nitrates in zones with agricultural activities and high estimations of nickel and chromium in regions with the strong presence of ultrabasic rocks. Dominant part of the groundwater samples surpasses in many cases the European Community (EC) drinking water permissible limits. Thus, they are unsuitable for human consumption. PCA illustrated four factors, which clarified 80.62% of the aggregate variance of data and HCA classified two statistically significant clusters of sampling sites. Results show natural procedures ascribed to the weathering of the minerals contained in the ultrabasic rocks and anthropogenic influences related to the use of fertilizers and wastewater leak. In light of FAO standards and Richards’s classification, the groundwaters are reasonable for irrigation purposes, featuring waters with low sodium hazard and moderate salinity hazard.     

Preliminary groundwater quality assessment in the central region of Ghana

Insufficient knowledge of the hydrogeochemistry of aquifers in the Central Region of Ghana has necessitated a preliminary water quality assessment in some parts of the region. Major and minor ions, and trace metal compositions of groundwater have been studied with the aim of evaluating hydrogeochemical processes that are likely to impair the quality of water in the study area. The results show that groundwater in the area is weakly acidic with mean acidity being 5.83 pH units. The dominant cation in the area is Na, followed by K, Ca, and Mg, and the dominant anion is Cl^?, followed by HCO₃ ^? and SO₄ ^2?. Two major hydrochemical facies have been identified as Na–Cl and Na–HCO_3, water types. Multivariate statistical techniques such as cluster analysis (CA) and factor analysis/principal component analysis (PCA), in R mode, were employed to examine the chemical compositions of groundwater and to identify factors that influenced each. Q-mode CA analysis resulted in two distinct water types as established by the hydrochemical facies. Cluster 1 waters contain predominantly Na–Cl. Cluster 2 waters contain Na–HCO₃ and Na–Cl. Cluster 2 waters are fresher and of good quality than cluster 1. Factor analysis yielded five significant factors, explaining 86.56% of the total variance. PC1 explains 41.95% of the variance and is contributed by temperature, electrical conductivity, TDS, turbidity, SO₄ ^2?, Cl^?, Na, K, Ca, Mg, and Mn and influenced by geochemical processes such as weathering, mineral dissolution, cation exchange, and oxidation–reduction reactions. PC2 explains 16.43% of the total variance and is characterized by high positive loadings of pH and HCO₃ ^?. This results from biogenic activities taking place to generate gaseous carbon dioxide that reacts with infiltrating water to generate HCO₃ ^?, which intend affect the pH. PC3 explains 11.17% of the total variance and is negatively loaded on PO₄ ^3? and NO₃ ^? indicating anthropogenic influence. The R-mode PCA, supported by R-mode CA, have revealed hydrogeochemical processes as the major sources of ions in the groundwater. Factor score plot revealed a possible flow direction from the northern sections of the study area, marked by higher topography, to the south. Compositional relations confirmed the predominant geochemical process responsible for the various ions in the groundwater as mineral dissolution and thus agree with the multivariate analysis.     

Evolution of saline waters and brines in the Benue-Trough,Nigeria

Hydrogeochemical assessment of 40 saline waters and brines from 20 locations within the lower (southern) and middle regions of the Benue-Trough, Nigeria are presented and discussed in terms of genesis of the primary salinity and subsequent hydrochemical evolution. The total dissolved ions range from 5263 to 88,800 mg/L and 5148 to 47,145 mg/L in the lower and middle region, respectively.The saline waters and brines are characteristically Na–Cl type enriched in Ca and Sr on the one hand and depleted in Mg and SO₄ on the other, relative to the seawater evaporation trend. Ionic ratios, Na–Cl–Br systematic and divalent cations suggest two likely sources of primary salinity: a fossil seawater source and dissolution of halite. However, water–rock interaction involving Mg uptake by clay minerals and possibly dolomitization during diagenesis appear to be responsible for further modification of the primary chemistry. A conceptualized hydrogeological/flow model for the brines is presented.     

Investigation of hydrochemical characteristics of groundwater from the Cretaceous-Eocene limestone aquifer in southern Ghana and southern Togo using hierarchical cluster analysis

The most relevant controls on the water quality within the Cretaceous-Eocene limestone aquifer of the Keta Basin, Ghana, and the coastal sedimentary basin of Togo were assessed using Q-mode hierarchical cluster analysis (HCA) and mass-balance modelling. The pattern recognition technique of HCA was employed for partitioning hydrochemical data from a total of 65 surface and borehole samples from the study area into water groups. A spatial plot of the water groups consisting of samples from the limestone aquifer shows that the vast majority of samples belonging to the same group are located in close proximity to one another, suggesting the same processes and/or flow paths in the limestone aquifer system. Geochemical reaction models of selected water groups were constructed using PHREEQC-2. The hydrochemical compositions of the water groups and the mass-balance calculations indicate that the dominant processes and reactions responsible for the hydrochemical evolution in the system are: (1) carbonate equilibria, (2) silicate weathering reactions, (3) limited mixing with saline water, and (4) ion exchange. The combined use of HCA and mass-balance modelling has shown to be a useful approach in interpreting groundwater hydrochemistry in an area where large uncertainties exist in the understanding of the groundwater flow system.     

Multivariate statistical analysis of chemical and stable isotopic data as indicative of groundwater evolution with reduced exploitation

Groundwater resources in the North China Plain (NCP) are undergoing tremendous changes in response to the operation of groundwater exploitation reduction (GWER) project. To identify groundwater evolution in this complex context, hierarchical cluster analysis (HCA) and principal component analysis (PCA) were combined to interpret an integrated dataset of stable isotopes and chemical data from four sampling campaigns in a pilot area of groundwater control. We proposed a novel HCA approach integrating stable isotopes and chemical signals, which successfully partitioned the groundwater samples into the unconfined and the confined water samples. Stable isotopic evidence showed that the lateral inflow and the surface water may contribute more to groundwater recharge in this region than local modern precipitation. The unconfined water’s main hydrochemical types were Na type with mixed anions, and Na–Cl–SO₄ type, while the confined water was mainly Na–Cl and Na–SO₄ types. Geochemical processes mainly involved the dissolution/precipitation of halite, gypsum, Glauber's salt, feldspar, calcite and dolomite, as well as the cation exchange. PCA results showed that water–rock interaction (i.e., salinity-based and alkalinity-based processes) predominated the hydrochemical evolution, along with local nitrate contamination resulting from fertilizers and domestic sewage. The GWER project regulated the natural evolution of unconfined water chemistry, and significantly reduced the unconfined water’s salinity (mainly Na⁺, Mg²⁺, SO₄^2?). This may be attributed to upward leakage from low-salinity confined water at some parts of the aquifer. Additionally, insignificant changes in the confined water’s salinity reflected that the impact of GWER on the confined aquifer was negligible. This study facilitates the groundwater classification effectively in the areas lack of geological data, and enhances the knowledge of groundwater chemical evolution in such a region where groundwater restoration is in progress, with important implications for groundwater sustainable management in similar basins worldwide.     

An appraisal of groundwater quality in Seymour and Blaine aquifers in a major agro-ecological region in Texas,USA

Aquifer-based groundwater quality assessment offers critical insight into the major hydrochemical processes, and aids in making groundwater resources management decisions. The Texas Rolling Plains (TRP), spanning over 22 counties, is a major agro-ecological region in Texas from where highest groundwater nitrate (NO₃ ^?) levels in the state have been reported. In this study, we present a comparative assessment of major hydrochemical facies pertaining to NO₃ ^? contamination and a host of species such as sulfate (SO₄ ^2?), chloride (Cl^?), and total dissolved solids (TDS) in different water use classes in the Seymour and Blaine aquifers, underlying the TRP. Aquifer-stratified groundwater quality information from 1990 to 2010 was obtained from the Texas Water Development Board and aggregated over decadal scale. High groundwater salinization was found in the municipal water use class in the Blaine aquifer with about 100, 87 and 50 % of observations exceeding the secondary maximum contaminant level for TDS, SO₄ ^2?, and Cl^?, respectively in the 2000s (2000–2010). The NO₃-contamination was more alarming in the Seymour aquifer with 82 and 61 % of observations, respectively, exceeding the maximum contaminant level (MCL) in the irrigation and municipal water use classes in the 2000s. Salinization was more influenced by SO₄ ^2? and Cl^? in the Blaine aquifer and by NO₃ ^? in the Seymour aquifer. High NO₃ ^? (>MCL) observations in the Seymour aquifer occurred in the Ca–HCO₃ and Ca–Mg–HCO₃ facies, the domains of fresh water recharge and anthropogenic influences (e.g., agricultural activities, waste disposal). High SO₄ ^2?, Cl^? and TDS observations in the Blaine aquifer dominated the Ca–Cl, Na–Cl, and mixed Ca(Mg)–SO₄(Cl) facies indicating evaporite dissolution, mixing and solute exchange, and lack of fresh recharge.     

Hydrogeochemistry of groundwater and anthropogenic control over dolomitization reactions in alluvial sediments of the Deoria district: Ganga plain,India

Groundwater is a critical resource in Deoria district, as it is the main source of drinking water and irrigation. The aquifer has deteriorated to a high degree, during the last two to three decades, in quality and quantity due to high population growth and environmental pollution. More than 90% of the population get their drinking water from subsurface waters. Fifteen wells were sampled in June 2006 to probe the hydrogeochemical components that influence the water quality. The results show that groundwater have EC, TDS, Na⁺, Mg²⁺, HCO₃⁻ and TH higher than the WHO, 1997 maximum desirable limits. A hydrogeochemical numerical model for carbonate minerals was constructed using the PHREEQC package. The regression analysis shows that there are three groups of elements which are significantly and positively correlated. The main hydrochemical facies of the aquifer (Ca + Mg–HCO₃) represents 33.33% of the total wells. The geochemical modeling demonstrated that the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of salts, (2) precipitation of dolomite, (3) ion exchange. Solubility of dolomite, calcite, aragonite and gypsum were assessed in terms of the saturation index. The thermodynamic prerequisites for dolomite supersaturation reactions are satisfied by subsurface waters, since they are supersaturated with respect to dolomite, undersaturated (or in equilibrium) with respect to calcite, and undersaturated with respect to gypsum. The Ca²⁺ versus SO₄²⁻ and Mg²⁺ versus SO₄²⁻ trends are also compatible with homologous trends resulting from dolomite supersaturation.     

Identifying the Sources of Solutes in Karst Groundwater in Chongqing,China: a Combined Sulfate and Strontium Isotope Approach

Groundwater from karst subterranean streams is among the world’s most important sources of drinking water supplies, and the hydrochemical characteristics of karst water are impacted by both natural environment and people. Therefore, the study of hydrochemistry and its solutes’ sources is very important to ensure the normal function of life support systems. In this paper, thirty?five representative karst groundwater samples were collected from different aquifers (limestone and dolomite) and various land use types in Chongqing to trace the sources of solutes and relative hydrochemical processes. Hydrogeochemical types of karst groundwater in Chongqing were mainly of the Ca?HCO3 type or Ca (Mg)?HCO3 type. However, some hydrochemical types of karst groundwater were the K+Na+Ca?SO4 type (G25 site) or Ca?HCO3+SO4 type (G26 and G14 site), indicating that the hydrochemistry of these sites might be strongly influenced by anthropogenic activities or unique geological characteristics. The dissolved Sr concentrations of the studied groundwater ranged from 0.57 to 15.06 μmmol/L, and the 87Sr/86Sr varied from 0.70751 to 0.71627. The δ34S?SO42? fell into a range of ?6.8‰?21.5‰, with a mean value of 5.6‰. The variations of both 87Sr/86Sr and Sr values of the groundwater samples indicated that the Sr element was controlled by the weathering of limestone, dolomite and silicate rock. However, the figure of 87Sr/86Sr vs. Sr2+/[K++Na+] showed that the anthropogenic inputs also obviously contributed to the Sr contents. For tracing the detailed anthropogenic effects, we traced the sources of solutes collected karst groundwater samples in Chongqing according to the δ34S value of potential sulfate sources. The variations of both δ34S and 1/SO42? values of the groundwater samples indicated that the atmospheric acid deposition (AAD), dissolution of gypsum (GD), oxidation of sul?de mineral (OS) or anthropogenic inputs (SF: sewage or fertilizer) have contributed to solutes in karst groundwater. The influence of oxidation of sul?de mineral, atmospheric acid deposit and anthropogenic inputs to groundwater in Chongqing karst areas was much widespread.     

Application of mass-balance and flow simulation calculations to interpretation of mixing at Äspö, Sweden

The hydrogeology of a vertical fracture zone at 70 m depth at the access tunnel to the Äspö Hard Rock Laboratory was monitored over 3 a for hydrochemical changes that could be effected by construction of a deep repository for high-level nuclear waste. Tunnel construction dramatically disturbed the hydrogeological system, but this provided an opportunity to integrate hydrogeochemical and hydrological evaluation of the zone. The objective of this study was to evaluate hydrogeochemical evolution, groundwater flow and surface water intrusion during the experiment using an integrated approach of geochemical mass-balance calculations and numerical flow simulations.The dilution of major ions was the dominant hydrochemical trend. However, HCO₃ and SO₄ showed significant enrichment. Increasing activity of ¹⁴C suggested that oxidation of organic C was the likely source of HCO₃. Any mineral source dissolving during the experiment seemed insufficient to account for changes in SO₄ and current intrusion of sea water was excluded according to the data. Cation exchange as well as minor calcite reactions in fractures were assumed probable in such temporary chemical conditions. Conservative two end-member mixing models with shallow groundwater in the zone and initial groundwater at tunnel level also assumed remarkable mass transfer (several mmol/l). Therefore a third SO₄-rich end-member, a regional shallow groundwater type which may mix by lateral flow in the system, was tested. This was also expected from hydraulic measurements and preliminary flow simulations assuming homogeneity.Three end-member mixing calculations using Cl and SO₄ as conservative tracers give a constant proportion of lateral water in all boreholes after 300 days, which is consistent with the steady state character of the flow field in the late part of the experiment. To predict reactions on plausible levels needs significant adjustments of initial and final waters, indicating uncertainties in the hydrochemical information of the fracture zone. In the flow simulations the transmissivities were selected so that the chemical mixing proportions would match simulated portions of flow as closely as possible. The simulated total recoveries (drawdowns) differ from the measurements mainly due to overly simple parametrisation of the transmissivity in the fracture zone. However, integrating hydrochemistry in flow modelling is considered encouraging in producing additional information of the heterogeneity of a flow structure.     

Hydrogeochemical evolution of groundwater in a Mediterranean coastal aquifer, Mersin-Erdemli basin (Turkey)

In this study, hydrogeologic and hydrochemical information from the Mersin-Erdemli groundwater system were integrated and used to determine the main factors and mechanisms controlling the chemistry of groundwaters in the area and anthropogenic factors presently affecting them. The PHREEQC geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into four categories: (1) silicate weathering reactions; (2) dissolution of salts; (3) precipitation of calcite, amorphous silica and kaolinite; (4) ion exchange. As determined by multivariate statistical analysis, anthropogenic factors show seasonality in the area where most contaminated waters related to fertilizer and fungicide applications that occur during early summer season.     

Assessment of chemical properties and hydro-geochemical coefficients at the Qareh Sou Basin,Golestan Province,Iran

This paper focuses on the Qareh Sou Basin in Golestan Province, Iran. Golestan Province is the third largest cereal producer in Iran and water scarcity and salinity are major problems in this area. This study attempts to facilitate the comprehension of system behavior with respect to water quality issues and hydro-geochemical coefficients within the Qareh Sou Basin. This study was carried out during the year 2010. Various parameters, such as pH, EC, chloride, sulfate, bicarbonate, sodium, potassium, calcium and magnesium have been determined for evaluation purposes. Then, Ca/Mg, Na/Cl, Mg/(Ca + Mg), Ca/HCO₃, (Ca + Mg)–(HCO₃ + SO₄), (Na + K)–Cl, (Ca + Mg + Na + K)–Cl, HCO₃ + SO₄, Ca + Mg and chloro-alkaline indices (CAI) were calculated. Results show that cation exchange probably is an important factor in the hydrochemistry and silicate mineral weathering. Also, CAI-1 plot against CAI-2 demonstrates that most of samples have positive values which suggest normal ion exchange in the system. The carbonic acid is the main agent of calcite, limestone and dolomite weathering which occurs in some stations. According to Chadha’s diagram, the type of water is determined as Ca–Mg–HCO₃.     

Hydrochemical characterization and groundwater quality in Delta Tokar alluvial plain, Red Sea coast—Sudan

The leaching processes along the flow path and over abstraction of the alluvial aquifer, the principal aquifer in delta Tokar, by the agricultural and domestic sectors and natural factors, have led to its salinization which may be due to interaction between geological formations and adjacent brackish and saline water bodies as well as seawater transgression. The main objectives of this study are to assess the hydrochemical characteristics of the groundwater and to delineate the locations and the sources of aquifer salinization. Water samples in the project area were chemically analyzed for major cations and anions at the laboratory by the standard analytical procedures. Chemical data and water level measurements were manipulated using GIS techniques for hydro chemical and flow direction maps and piper diagram for chemical facies and SPSS software for statistical analyses such as basic statistics (mean and standard deviation) and Spearman’s correlation matrix. The general flow direction of the groundwater is from Southwest towards East and Northeast. The hydraulic gradient is relatively steeper at the apex of the delta (0.06) and amounts to 0.005 at the distal part of the delta. The average transmissivity value of the water bearing formations was found to be about 4.5?×?10³ m²/s, whereas, the storage coefficient was about 0.28. A hydrochemical study identified the locations and the sources of aquifer salinization and delineated their areas of influence. The investigation indicates that the aquifer water quality is significantly modified as groundwater flows from the southwestern parts of the study area, where the aquifer receives its water by lateral underflow from Khor Baraka flood plain, to the central and northeastern parts, with few exceptions of scattered anomalous concentration pockets in the deltaic plain. Significant correlation between TDS and/or EC with the major components of Na⁺, Cl^?, and SO ₄ ^?2 ions is an indication of seawater influence on the groundwater salinity. Moreover, Cl^?, SO ₄ ^2? , and Na⁺ are predominant ions followed by Ca²⁺ and HCO ₃ ^? . Hence, four types of groundwater can be chemically distinguished: Na–Ca–SO₄–Cl– facies, Na–Cl–SO₄–HCO₃– facies, Na–Ca–Mg–SO₄–Cl–HCO₃ facies, and Na–Ca–Mg–Cl–SO₄ facies. The processes that govern changes in groundwater composition as revealed by chemical and statistical analyses are mainly associated with over-abstraction, biodegradation, marine intrusions, and carbonate saturation.     

安徽马鞍山市当涂地区地下水水化学特征及演化机制

【研究目的】 了解长江中下游平原地区地下水流系统并深入分析其地下水水化学特征及其演化机制。【研究方法】 综合马鞍山市当涂地区的水文地质条件、水动力场等,基于研究区水化学基本特征,运用多元统计分析、水化学图件、离子比值和反向水文地球化学模拟等方法对该地区浅层地下水水化学演化进行分析。【研究结果】 结果表明：（1）研究区地下水主要为低矿化度偏碱性水,地下水组分中阳离子以Ca²⁺和Mg²⁺为主,阴离子以HCO₃^-和SO₄^2-为主。（2）研究区地下水水化学类型主要可分为7类,其中松散岩类孔隙含水岩组和碎屑岩类孔隙裂隙含水岩组的水化学类型主要为HCO₃^-Ca型、HCO₃^-Ca·Na型、HCO₃·Cl-Ca·Na型以及HCO₃^-Ca·Mg型;基岩类裂隙含水岩组的化学类型主要为HCO₃·SO₄-Ca·Mg型和SO₄·HCO₃^-Ca·Mg型。（3）研究区浅层地下水水样超标率为46%,总体水质较差,超标率较高的组分依次为Mn、高锰酸盐指数（COD_Mn）、硝酸盐（以N计）、Fe、As、氨氮（以N计）等。（4）研究区地下水的化学组分主要受到岩石风化作用的控制;此外,还存在Na-Ca的正向阳离子交替吸附作用。反向水文地球化学模拟结果进一步定量论证了水岩相互作用对本区浅层地下水组分的形成和演化起着主导作用。【结论】 研究区地下水主要为低矿化度偏碱性,主要可分为松散岩类孔隙水、碎屑岩类孔隙裂隙水和基岩类裂隙水。主要离子比例和反向水文地球化学模拟揭示了本区浅层地下水化学组分主要是地下水溶滤方解石、白云石等碳酸盐矿物、石英、长石等硅酸盐矿物,高岭土等黏土矿物以及岩盐、石膏等达到过饱和之后形成的。     

Origin of components in Chilean thermal waters

Thermal waters of northern (18°–27°S) and southern (37°–45°S) Chile occur in two very different climatic, geologic and hydrologic environments: arid closed basins with abundant evaporites in the north; humid climate and well drained valleys in the south. The origin and behavior of the main components of the two groups of waters are examined and compared to each other. The modeling of the alteration of volcanic rocks leads to water compositions very different from those observed both in the north and south. In addition to hydrothermal alteration and deep emanations, the Cl/Br ratio reveals a major contribution of saline waters to the two groups: infiltrating brines from salt lakes in the north; seawater in the south.In the north, concentrations of Cl, Br, Na, K, Ca, SO₄, Li, B, Si result from the mixing of alteration waters with recycled brines. Hydrothermal alteration is obscured by this massive saline input, except for Mg. δ³⁴S values are consistent with an origin of sulfate from salar brines, which are themselves derived from deep Tertiary gypsum. In the south, two processes account for the composition of thermal waters: mixing of alteration waters with seawater and deep magmatic contribution. The mixing process controls the concentration of Cl, Br, Na, Alk, Si, K, Ca, Mg. Magmatic inputs are detectable for SO₄, Li and B. δ³⁴S suggests that sulfate stems from the mixing of alteration waters with either marine SO₄ in coastal waters or with deep SO₂ in inland waters. In both the north and south, the Mg concentration is drastically lowered (<1 μmol/L) by the probable formation of a chlorite-type mineral. In the south, very small amounts of seawater (<1% in volume) are sufficient to imprint a clear signature on thermal waters. Not only coastal springs are affected by seawater mixing, but also remote inland springs, as far as 150 km from the sea. Subduction of marine sediments in the accretive margin could be the source of the marine imprint in thermal waters of southern Chile. Seawater may be expelled from the subducted lithosphere and incorporated into the mantle source.