Hydrogeochemical Characteristics and Groundwater Inrush Source Identification for a Multi‐aquifer System in a Coal Mine

Correct identification of water inrush sources is particularly important to prevent and control mine water disasters. Hydrochemical analysis, Fisher discriminant analysis, and geothermal verification analysis were used to identify and verify the water sources of the multi‐aquifer groundwater system in Gubei coal mine, Anhui Province, North China. Results show that hydrochemical water types of the Cenozoic top aquifer included HCO₃–Na+K–Ca, HCO₃–Na+K–Mg and HCO₃–Na+K, and this aquifer was easily distinguishable from other aquifers because of its low concentration of Na⁺+K⁺ and Cl^–. The Cenozoic middle and bottom aquifers, the Permian fissure aquifer, and the Taiyuan and Ordovician limestone aquifers were mainly characterized by the Cl–Na+K and SO₄–Cl‐Na+K or HCO₃–Cl–Na+K water types, and their hydrogeochemistries were similar. Therefore, water sources could not be identified via hydrochemical analysis. Fisher model was established based on the hydrogeochemical characteristics, and its discrimination rate was 89.19%. Fisher discrimination results were improved by combining them with the geothermal analysis results, and this combination increased the identification rate to 97.3 % and reasonably explained the reasons behind two water samples misjudgments. The methods described herein are also applicable to other mines with similar geological and hydrogeological conditions in North China.     

Geochemical evolution of groundwater in southern Bengal Basin: The example of Rajarhat and adjoining areas,West Bengal,India

Detailed geochemical analysis of groundwater beneath 1223 km² area in southern Bengal Basin along with statistical analysis on the chemical data was attempted, to develop a better understanding of the geochemical processes that control the groundwater evolution in the deltaic aquifer of the region. Groundwater is categorized into three types: ‘excellent’, ‘good’ and ‘poor’ and seven hydrochemical facies are assigned to three broad types: ‘fresh’, ‘mixed’ and ‘brackish’ waters. The ‘fresh’ water type dominated with sodium indicates active flushing of the aquifer, whereas chloride-rich ‘brackish’ groundwater represents freshening of modified connate water. The ‘mixed’ type groundwater has possibly evolved due to hydraulic mixing of ‘fresh’ and ‘brackish’ waters. Enrichment of major ions in groundwater is due to weathering of feldspathic and ferro-magnesian minerals by percolating water. The groundwater of Rajarhat New Town (RNT) and adjacent areas in the north and southeast is contaminated with arsenic. Current-pumping may induce more arsenic to flow into the aquifers of RNT and Kolkata cities. Future large-scale pumping of groundwater beneath RNT can modify the hydrological system, which may transport arsenic and low quality water from adjacent aquifers to presently unpolluted aquifer.     

Carbonate weathering and connate seawater influencing karst groundwaters in the Gevas–Gurpinar–Güzelsu basins,Turkey

There are 59 springs at the Gevas–Gurp?nar–Güzelsu basins, 38 of these springs emerge from the fractured karst aquifers (recrystallized limestone and travertine) and 21 emerge from the Yuksekova ophiolites, K?rkgeçit formation and alluvium. The groundwater samples collected from 38 out of the total of 59 springs, two streams, one lake and 12 wells were analyzed physico-chemically in the year 2002. EC and TDS values of groundwater increased from the marble (high altitude) to the ophiolites and alluvium (toward Lake Van) as a result of carbonate dissolution and connate seawater. Five chemical types of groundwater are identified: Ca–Mg–HCO₃, Mg–Ca–HCO₃, Mg–Na–HCO₃, Na–Ca–HCO₃ and Mg–Ca–Na–HCO₃. The calculations and hydrochemical interpretations show that the high concentrations of Ca²⁺, Mg²⁺ and HCO₃ ^? as predominant ions in the waters are mainly attributed to carbonate rocks and high pCO₂ in soil. Most of the karst springs are oversaturated in calcite, aragonite and dolomite and undersaturated in gypsum, halite and anhydrite. The water–rock interaction processes that singly or in combination influence the chemical composition of each water type include dissolution of carbonate (calcite and dolomite), calcite precipitation, cation exchange and freshening of connate seawater. These processes contribute considerably to the concentration of major ions in the groundwater. Stable isotope contents of the groundwater suggest mainly direct integrative recharge.     

Hydrogeochemical and isotopic evidences of the groundwater regime in Datong Basin, Northern China

Datong Basin is one of the Cenozoic faulted basins in Northern China’s Shanxi province, where groundwater is the major source of water supply. The results of hydrochemical investigation show that along the groundwater flow path, from the margins to the lower-lying central parts of the basin, groundwater generally shows increases in concentrations of TDS, HCO₃ ^?, SO₄ ^2?, Cl^?, Na⁺ and Mg²⁺ (except for Ca²⁺ content). Along the basin margin, groundwater is dominantly of Ca–HCO₃ type; however, in the central parts of the basin it becomes more saline with Na–HCO₃-dominant or mixed-ion type. The medium-deep groundwater has chemical compositions similar to those of shallow groundwater, except for the local area affected by human activity. From the mountain front to the basin area, shallow groundwater concentrations of major ions increase and are commonly higher than those in medium-deep aquifers, due to intense evapotranspiration and anthropogenic contamination. Hydrolysis of aluminosilicate and silicate minerals, cation exchange and evaporation are prevailing geochemical processes occurring in the aquifers at Datong Basin. The isotopic compositions indicate that meteoric water is the main source of groundwater recharge. Evaporation is the major way of discharge of shallow groundwater. The groundwater in medium-deep aquifers may be related to regional recharges of rainwater by infiltrating along the mountain front faults, and of groundwater permeating laterally from bedrocks of the mountain range. However, in areas of groundwater depression cones, groundwater in the deep confined aquifers may be recharged by groundwater from the upper unconfined aquifer through aquitards.     

Groundwater chemistry patterns in the phreatic aquifer of the central Belgian coastal plain

The Holocene geological evolution of the Belgian coastal plain is dominated by a transgression of the North Sea, silting up of the coastal plain and human intervention (impoldering). This has led to a typical pattern in groundwater quality which is discussed here for the central part of the coastal plain. Therefore, a database with available groundwater samples is composed. Water type according to the Stuyfzand classification is determined and different hydrosomes and their hydrochemical facies are identified. Based on this, the origin and evolution of the water types is explained using Piper plots and geochemical calculations with PHREEQC. Before the impoldering, salinising and freshening conditions alternated with a general salinisation of the aquifer after about 3400 BP. This results in a dominance of brackish and salt NaCl subtypes which are still found in the deeper part of the aquifer. The subsequent impoldering resulted in an major freshening of the aquifer leading to NaHCO₃, MgHCO₃ and CaHCO₃ subtypes. Overall, mixing, cation exchange, carbonate mineral dissolution and oxidation of organic matter are identified as the major processes determining the general water quality. The close link between geological evolution, water quality and what is still observable today is illustrated with this example of the Belgian coastal plain.     

Hydrochemistry of groundwater and its assessment for irrigation purpose in coastal Jeffara Aquifer, southeastern Tunisia

Groundwater is of a paramount importance in arid areas, as it represents the main water resource to satisfy the different needs of the various sectors. Nevertheless, coastal aquifers are generally subjected to seawater intrusion and groundwater quality degradation. In this study, the groundwater quality of the coastal Jeffara aquifer (southeastern Tunisia) is evaluated to check its suitability for irrigation purposes. A total of 74 groundwater samples were collected and analyzed for various physical and chemical parameters, such as, electrical conductivity, pH, dissolved solids (TDS), Na, K, Ca, Mg, Cl, HCO₃, and SO₄. Sodium adsorption ratio, magnesium adsorption ratio, Sodium percentage, and permeability index were calculated based on the analytical results. The analytical results obtained show a strong mineralization of the water in the studied aquifer. TDS concentrations range from 3.40 to 18.84 g?L^?1. Groundwater salinity was shown to be mainly controlled by sodium and chloride. The dominant hydrochemical facieses are Na–Cl–Ca–SO₄, mainly as a result of mineral dissolution (halite and gypsum), infiltration of saline surface water, and seawater intrusion. Assessment of the groundwater quality of the different samples by various methods indicated that only 7% of the water, in the northwest of the study area, is considered suitable for irrigation purposes while 93% are characterized by fair to poor quality, and are therefore just suitable or unsuitable for irrigation purposes.     

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.     

Hydrogeochemistry and groundwater quality assessment of the multilayered aquifer in Lower Kelantan Basin,Kelantan, Malaysia

Continual expansion of population density, urbanization, agriculture, and industry in most parts of the world has increased the generation of pollution, which contributes to the deterioration of surface water quality. This causes the dependence on groundwater sources for their daily needs to accumulate day by day, which raises concerns about their quality and hydrogeochemistry. This study was carried out to increase understanding of the geological setup and assess the groundwater hydrogeochemical characteristics of the multilayered aquifers in Lower Kelantan Basin. Based on lithological data correlation of exploration wells, the study area can be divided into three main aquifers: shallow, intermediate and deep aquifers. From these three aquifers, 101 groundwater samples were collected and analyzed for various parameters. The results showed that pH values in the shallow, intermediate and deep aquifers were generally acidic to slightly alkaline. The sequences of major cations and anions were Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and HCO₃^? > Cl^? > SO₄^2? > CO₃^2?, respectively. In the intermediate aquifer, the influence of ancient seawater was the primary factor that contributed to the elevated values of electrical conductivity (EC), Cl^? and total dissolved solids (TDS). The main facies in the shallow aquifer were Ca–HCO₃ and Na–HCO₃ water types. The water types were dominated by Na–Cl and Na–HCO₃ in the intermediate aquifer and by Na–HCO₃ in the deep aquifer. The Gibbs diagram reveals that the majority of groundwater samples belonged to the deep aquifer and fell in the rock dominance zone. Shallow aquifer samples mostly fell in the rainfall zone, suggesting that this aquifer is affected by anthropogenic activities. In contrast, the results suggest that the deep aquifer is heavily influenced by natural processes.     

Application of multivariate statistical procedures on major ions and trace elements in a multilayered coastal aquifer: the case of the south Rhodope coastal aquifer

Multivariate statistical techniques including cluster analysis and principal components analysis were applied on 22 variables consisted of 3 physicochemical parameters, 8 major ions and 11 trace elements. Samples were collected from the south Rhodope multilayered coastal aquifer in north Greece which is facing saltwater intrusion and anthropogenic contamination over the last 35 years. Cluster analysis grouped the variables into five main groups while principal components analysis revealed four distinct hydrochemical processes in the aquifer system, explaining 84.5 % of the total variance between the variables. The identified processes correspond to, saltwater intrusion and subsequent reverse cation exchange, the presence of deep connate groundwater masses, application of fertilizers in shallow wells and anthropogenic contamination with heavy metals nearby an improperly constructed landfill. The wells categorized with the above techniques were grouped and five constituent ratios Na/Cl, (Mg + Ca)/Cl, Ca/(HCO₃ + SO₄), Ca/SO₄ and Ca/Mg were utilized to identify the ones which enable the more accurate distinction between the group cases. The results of stepwise discriminant analysis showed that the calculated classification function can distinguish almost 80 % of groundwater samples with the Na/Cl ratio being the most statistically significant grouping variable. All the aforementioned statistical models managed to successfully identify numerous hydrochemical processes in a complex multilayered aquifer system and to explicitly attribute them for every investigated well, allowing a deeper insight into groundwater chemical characteristics with the use of an optimized smaller number of variables.     

Groundwater geochemistry of a small reservoir catchment in Central Tunisia

Due to the scarcity of water resources in semiarid sedimentary basins, hill reservoirs are often constructed to recharge groundwater and limit runoff induced water loss. The impact of such reservoirs on groundwater chemistry is investigated in the aquifers of the El Gouazine watershed, Central Tunisia. Three groundwater types are recognised, Ca–HCO₃, Na–Cl and Ca–SO₄. The strong similarity between host rock and groundwater chemistries indicates significant rock–water interaction. A flowpath, along which the chemical composition of the groundwater evolves, can be identified using the contrast in stable isotope signature between upstream and downstream groundwater. Shallow upstream groundwater is recharged by the infiltration of rainwater with the rate of recharge strongly linked to the permeability of the host lithology. Calcium and HCO₃ are supplied to an alluvial aquifer from a more rapidly recharged limestone aquifer with the concentration of Ca and HCO₃ ions decreasing by dilution. The alluvial aquifer is also enriched in Ca and SO₄ during the downstream flow of groundwater through gypsiferous materials. There is evidence of mixing between meteoric groundwater and evaporated reservoir water. Below the reservoir and partly responsible for reservoir leakage is a sandy aquifer, formed by weathering and erosion of a sandstone host which also supplies water to the alluvial aquifer.     

Hydrogeophysical investigations on the Pleistocene aquifer,Kom Hamada area,West Nile Delta,Egypt

Hydrogeophysical investigations of the Pleistocene aquifer at the Kom Hamada area, Egypt, have been conducted to determine the characteristics of groundwater. The main water-bearing formations in the study area are composed of Quaternary deposits. Water samples were taken and chemically analyzed at 29 sites. The constructed iso-salinity contour map of the study area showed an increase in salinity from 451.75 mg/l at eastern parts to 1,091.85 mg/l at western parts. The groundwater of the study area showed a hydrochemical evolution from Ca–HCO₃ at the eastern side to Na–Cl at the western side. Some of groundwater constituents have high concentration values exceeding the safe limit for drinking. Eighteen vertical electrical soundings (VES) were conducted in the study area. These soundings were conducted near existing wells to obtain layer parameters of the various penetrated layers and to calculate the petrophysical characteristics of the aquifers. The resistivity of the first water-bearing layer ranges between 34 and 47 Ω m. The thickness of this layer ranges between 26 and 79 m. This layer represents the first aquifer, where it is followed by another water-bearing layer with resistivity ranges between 29 and 62 Ω m and extends downward. The two aquifers are hydraulically connected. Variation of the resistivities of these two water-bearing layers is mainly due to the lithological variation. The resistivity values along with the TDS values of the two water-bearing layers indicate fresh to brackish water types.     

State of knowledge of coastal aquifer management in South America

A comparative analysis of the existing hydrogeological and management information from 15 coastal aquifers in South America was performed in order to obtain insight into common features of the sub-continent coastal zones. Some knowledge from other areas has been incorporated. There is a very variable degree of knowledge and management practice, ranging from almost no data and no action (the most common case), to sound conceptual models about aquifer behaviour and comprehensive management actions such as relocation of abstractions, pumping brackish groundwater, and aquifer vulnerability mapping. Some common features are: intensive groundwater exploitation; lack of characterization studies to support resource planning and management; lack of monitoring networks; and the need for raising awareness within society and its involvement in resource planning and management action programmes. Quality and quantity problems arising in heavily populated areas associated with coastal aquifers in South America point to unsustainable groundwater development. The sustainable use of those aquifers must rely on adequate evaluation of aquifer characteristics and monitoring.     

Geochemical Appraisal of Groundwater Quality in Ottapidaram Taluk,Thoothukudi District,Tamil Nadu using Graphical and Numerical Method

Groundwater qualities of coastal aquifers in the Ottapidaram taluk of Thoothukudi district, Tamil Nadu have been extensively monitored in post monsoon seasons in 2014 to assess its suitability in relation to domestic and drinking uses in four regions (N-S-EW). 34 groundwater samples were analyzed for various physicochemical attributes like pH, electrical conductivity (EC), Total dissolved solid (TDS), Na, K, Ca, Mg, Cl, HCO₃, CO₃, SO₄, NO₃, PO₄. Most of these parameters fall under not permissible limits. The western part of the study area is highly polluted from K, Cl, HCO₃ due to industrial/agriculture activity. The southern part is less polluted compared to other region. Hydrogeochemical processes controlling the water chemistry (Gibbs) indicates that most of groundwater samples fall at rock-weathering supremacy zone. Geochemical processes and temporal variation in the groundwater in this area are influenced by evaporation processes, ion exchange and dissolution of minerals. Major cation and anion ionic interaction indicate that weathering reactions have an inconsequential role in the hydrochemical processes of the shallow groundwater system. As a result of the hydrogeochemical analysis, seawater intrusion, aquifer rock weathering, sewer leakage are the overriding factors that determine the major ionic composition. The appropriate management plan is necessary to preserve precious groundwater resources.     

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.     

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.     

Study of hydrogeochemical processes of the groundwater in Ghatprabha river sub-basin, Bagalkot District, Karnataka, India

The alluvial aquifer of the Ghatprabha River comprises shallow tertiary sediment deposits underlain by peninsular gneissic complex of Archean age, located in the central–eastern part of the Karnataka in southern India. In order to establish the baseline hydrochemical conditions and processes determining the groundwater quality, groundwater samples were collected as part of an integrated investigation that coupled multivariate statistical analysis with hydrochemical methods to identify and interpret the groundwater chemistry of the aquifer system. Three main hydrochemical types (Ca–Mg–Cl, Ca–Mg–HCO₃, and Na–SO₄) were identified. Gibbs plots indicate that the evolution of water chemistry is influenced by water–rock interaction followed by evapotranspiration process. The results of factor analysis indicated the total variance explained by the extracted factor 79.9% and 87.1% for both pre- and post-monsoon, respectively. And other processes such as silicate weathering, ion exchange, and local anthropogenic activities affect the groundwater chemistry.     

Hydrogeological and mixing process of waters in aquifers in arid regions: a case study in San Luis Potosi Valley,Mexico

The climatic conditions of arid regions are characterized by high temperatures, low precipitation and high evapotranspiration rates that can explain the reduced recharge of aquifers. Thus, in these regions, there are some problems related to the groundwater quality and recharge that makes worse the problem of groundwater supply. A model, taking into account ternary mixtures, is presented and applied to a case study: the aquifer of San Luis Potosi valley located in the highlands of the central part of Mexico. In this valley, four hydrochemical facies were identified that correspond to the Ca–Na + K–HCO₃, Na + K–Ca–HCO₃, Ca–HCO₃ and Ca–SO₄ types. From this characterization, it was found out that the recharge area (known as Bledos Graben) is located at the SE of the valley; the deep water flow comes from there (Villa de Reyes and Alvarez Range) to the center of the valley. Mixture fractions were obtained by using chlorides and fluorides as conservative elements, from which it was possible to quantify the contribution of each member to the groundwater quality. According to these results, the contributions to the water extracted from this aquifer are as follows: shallow flows 50%, deep flows from Villa de Reyes 27%, and flows coming from the Alvarez Ranges about 15%.     

江苏南通地下水补给源、水化学特征及形成机理

在地下水的大规模开采条件下,江苏沿海一带,特别是南通许多地区的地下水一度出现咸化趋势,对区域水资源及环境产生了极大的影响,已成为制约生态环境建设和经济社会发展的重要因素.为查明地下水的补给来源、水化学特征和矿化度增高的机理,对南通地区深浅层地下水开展了野外调查取样.通过对各种水化学参数的讨论分析,系统地研究了该区地下水...     

Ionic exchange in groundwater hydrochemical evolution. Study case: the drainage basin of El Pescado creek (Buenos Aires province,Argentina)

The phreatic aquifer beneath the Pampean plain, in eastern central Argentina, constitutes a relevant source of water supply in the area. The objective of this work was to assess the significance of the cation exchange processes in the hydrochemical evolution of this aquifer, based on a study case located in the middle and upper basin of the El Pescado creek. Results indicate that Ca²⁺/Na⁺ exchange is the main process determining the evolution of groundwater from the recharge areas (Ca–HCO₃) towards the local discharge areas (Na–HCO₃), as well as representing a source of Na⁺ contribution to the water in the aquifer. This hydrochemical characteristic is central to the identification of local discharge areas within a plain environment which extends regionally. The ion exchange capacity of these discharge areas has environmental importance, due to its influence on groundwater quality and potential groundwater uses. These results may be applied to any aquifer sharing similar hydrogeological characteristics.