Biogeochemical processes in a clay formation in situ experiment: Part D - Microbial analyses - Synthesis of results

The purpose of the Porewater Chemistry (PC) experiment at the Mont Terri (MT) Underground Rock Laboratory (URL) was to measure geochemical parameters, such as pH, Eh and pCO₂, in the porewater of the Opalinus Clay formation. Although the PC experiment was designed and implemented carefully from a geochemical perspective, conditions were not sterile and some microbial and nutrient contamination likely occurred. Microbial activity in the added synthetic porewater in the borehole was apparent shortly after initiation of the experiment and affected the geochemical parameters observed in the porewater. This paper summarizes the results from microbial analyses of post-termination PC water and overcore clay samples, conducted to attempt to elucidate the role of microbial activity in the evolution of the geochemical conditions in the PC experiment. Microbial analyses of the PC borehole water, and of clay overcore samples from around the borehole, were carried out at three laboratories and included both molecular biology and culturing methods.Results indicated the presence of heterotrophic aerobic and anaerobic organisms that resulted likely from the initial, non-sterile conditions, sustained by suspected contamination with organic matter (glycerol, acetone). The results also indicated the presence of NO₃-reducers, Fe-reducers, SO₄-reducers and methanogens (i.e., Bacteria as well as Archaea), suggesting a reducing environment with Fe(III)- and SO₄ reduction, and methanogenesis occurring in the PC water and adjacent clay. A black precipitate containing pyrite (identified by XRD and SEM) and a strong H₂S smell in the porewater confirmed the occurrence of SO₄ reduction. Microorganisms identified in the porewater included Pseudomonas stutzeri, Bacillus licheniformis, Desulfosporosinus spp. and Hyphomonas spp. Species identified in enrichment cultures from the overcore samples included Pseudomonas stutzeri, three species of Trichococcus spp., Caldanaerocella colombiensis, Geosporobacter subterrenus and Desulfosporosinus lacus. Overall the results indicated a thriving microbial community in the PC water and adjacent clay in contrast to “undisturbed” Opalinus Clay for which limited evidence for a small viable microbial community has been given in a previous study.     

Biogeochemical processes in a clay formation in situ experiment: Part A - Overview, experimental design and water data of an experiment in the Opalinus Clay at the Mont Terri Underground Research Laboratory, Switzerland

An in situ test in the Opalinus Clay formation, termed porewater chemistry (PC) experiment, was carried out for a period of 5 years. It was based on the concept of diffusive equilibration whereby a traced water with a composition close to that expected in the formation was continuously circulated and monitored in a packed-off borehole. The main original focus was to obtain reliable data on the pH/pCO₂ conditions of the porewater, but because of unexpected microbiologically-induced redox reactions, the objective was extended to elucidate the biogeochemical processes occurring in the borehole and to understand their impact on pH/pCO₂ and porewater chemistry in the low permeability clay formation.The behaviour of the conservative tracers ²H and Br⁻ could be explained by diffusive dilution in the clay and moreover the results showed that diffusive equilibration between the borehole water and the formation occurred within about 3 year’s time. However, the composition and pH/pCO₂ conditions differed considerably from those of the in situ porewater. Thus, pH was lower and pCO₂ was higher than indicated by complementary laboratory investigations. The noted differences are explained by microbiologically-induced redox reactions occurring in the borehole and in the interfacial wall area which were caused by an organic source released from the equipment material. The degradation of this source was accompanied by sulfate reduction and - to a lesser extent - by methane generation, which induced a high rate of acetogenic reactions corresponding to very high acetate concentrations for the first 600 days. Concomitantly with the anaerobic degradation of an organic source, carbonate dissolution occurred and these processes resulted in high pCO₂ and alkalinities as well as drop in pH. Afterwards, the microbial regime changed and, in parallel to ongoing sulfate reduction, acetate was consumed, leading to a strong decrease in TOC which reached background levels after about 1200 days. In spite of the depletion of this organic perturbation in the circuit water, sulfate reduction and methanogenesis continued to occur at a constant rate leading to near-to-constant concentrations of sulfate and bicarbonate as well as pH/pCO₂ conditions until the end of the experiment. The main sink for sulphur was iron sulfide, which precipitated as FeS (am) and FeS₂.The chemical and isotopic composition was affected by the complex interplay of diffusion, carbon degradation rates, mineral equilibria and dissolution rates, iron sulfide precipitation rates, and clay exchange reactions. The ¹³C signals measured for different carbon species showed significant variations which could only be partly explained. The main cations, such as Na, Ca and Mg remained remarkably constant during the experiment, thus indicating the strong buffering of the formation via cation and proton exchange as well as carbonate dissolution/precipitation reactions.     

The effect of high pH alkaline solutions on the mineral stability of the Boom Clay – Batch experiments at 60 °C

Boom Clay is currently viewed as a reference host formation for studies on deep geological disposal of radioactive waste in Belgium. The interactions between bulk rock Boom Clay and 0.1 M KOH, 0.1 M NaOH, 0.1 M Ca(OH)₂, young cement water and evolved cement water solutions, ranging in pH from 12.5 to 13.2, were examined as static batch experiments at 60 °C to simulate alkaline plume perturbations, which are expected to occur in the repository due to the presence of concrete. Both liquids and solids were investigated at specific times between 90 and 510 days in order to control the elemental budget and to search for potential mineralogical alterations. Also, the clay fraction was separated from the whole-rock Boom Clay at the end of each run and characterized for its mineralogical composition. Thereby, the importance of the mineral matrix to buffer the alkaline attack and the role of organic matter to protect clay minerals were also addressed. The results indicate that the degree of geochemical perturbation in Boom Clay is dependent on the initial pH of the applied solution together with the nature of the major cation in the reactant fluids. The higher the initial pH of the media, the stronger its interaction with Boom Clay. No major non-clay mineralogical alteration of the Boom Clay was detected, but dissolution of kaolinite, smectite and illite occurred within the studied experimental conditions. The dissolution of clays is accompanied by the decrease in the layer charge, followed by a decrease in the cation-exchange capacity. The highest TOC values coincide with the highest total elemental concentrations in the leachates, and correspondingly, the highest dissolution degree. However, no quantitative link could be established between the degree of organic matter decomposition and clay dissolution.     

Stable isotope composition of CO2 outgassed from cores of argillites: a simple method to constrain δ18O of porewater and δ13C of dissolved carbon in mudrocks

Stable isotope characterization of porewater, and dissolved species, in mudrocks and argillaceous rocks is notoriously difficult. Techniques based on physical or chemical extraction of porewater can generate significant analytical artefacts. The authors report a novel, simple approach to determine the δ¹⁸O of porewater and δ¹³C of dissolved C in argillites. The method uses core samples placed in specifically-designed outgassing cells, sealed shortly after drilling and stored in well-controlled conditions. After 1–2 months, CO₂ naturally outgassed by argillite porewater is collected, purified and analyzed for C and O isotopes. Porewater δ¹⁸O and dissolved C δ¹³C are calculated from CO₂ isotope data using appropriate fractionation factors. This methodology was successfully applied to the Callovo-Oxfordian argillites from Bure (eastern Paris Basin, France) and the Opalinus Clay formation from Mont Terri (Switzerland). In both clay formations, results indicate that porewater is meteoric and dissolved C is of marine origin. The main advantage of the approach is that it does not induce any major physical or chemical disturbance to the clay–water system investigated. Further testing on argillaceous rocks of variable composition and organic content is needed to assess extent of applicability.     

Simulation of thermal stress influence on the Boom Clay kerogen (Oligocene,Belgium) in relation to long-term storage of high activity nuclear waste: I. Study of generated soluble compounds

Closed pyrolyses were performed on the Boom Clay kerogen to simulate the weak thermal stress applied during the in situ CERBERUS heating experiment (80 °C for 5 a). Two stronger thermal stresses, encompassing the range generally considered for the long-term disposal of high-activity nuclear waste (80 °C for 1 ka and 120 °C for 3 ka), were also simulated. Quantitative and qualitative studies were carried out on the products thus generated with a focus on the C₁₂₊ fraction, especially on its polar components. It thus appeared that the soluble C₁₂₊ fractions generated during these simulation experiments comprise a wide variety of polar O- and/or N-containing compounds, including carboxylic acids and phenols. The nature and/or the relative abundance of these polar compounds exhibit strong variations, with the extent of the thermal stress, reflecting the primary cracking of different types of structures with different thermal stability and the occurrence of secondary degradation reactions. These observations support the idea that the compounds, generated upon exposure of the Boom Clay kerogen to a low to moderate thermal stress, may affect the effectiveness of the geological barrier upon long-term storage of high-activity nuclear waste.     

Iron geochemical zonation in a tidally inundated acid sulfate soil wetland

Tidal inundation is a new technique for remediating coastal acid sulfate soils (CASS). Here, we examine the effects of this technique on the geochemical zonation and cycling of Fe across a tidally inundated CASS toposequence, by investigating toposequence hydrology, in situ porewater geochemistry, solid-phase Fe fractions and Fe mineralogy. Interactions between topography and tides exerted a fundamental hydrological control on the geochemical zonation, redistribution and subsequent mineralogical transformations of Fe within the landscape. Reductive dissolution of Fe(III) minerals, including jarosite (KFe₃(SO₄)₂(OH)₆), resulted in elevated concentrations of porewater Fe²⁺ (> 30 mmol L^?1) in former sulfuric horizons in the upper-intertidal zone. Tidal forcing generated oscillating hydraulic gradients, driving upward advection of this Fe²⁺-enriched porewater along the intertidal slope. Subsequent oxidation of Fe²⁺ led to substantial accumulation of reactive Fe(III) fractions (up to 8000 μmol g^?1) in redox-interfacial, tidal zone sediments. These Fe(III)-precipitates were poorly crystalline and displayed a distinct mineralisation sequence related to tidal zonation. Schwertmannite (Fe₈O₈(OH)₆SO₄) was the dominant Fe mineral phase in the upper-intertidal zone at mainly low pH (3–4). This was followed by increasing lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) at circumneutral pH within lower-intertidal and subtidal zones. Relationships were evident between Fe fractions and topography. There was increasing precipitation of Fe-sulfide minerals and non-sulfidic solid-phase Fe(II) in the lower intertidal and subtidal zones. Precipitation of Fe-sulfide minerals was spatially co-incident with decreases in porewater Fe²⁺. A conceptual model is presented to explain the observed landscape-scale patterns of Fe mineralisation and hydro-geochemical zonation. This study provides valuable insights into the hydro-geochemical processes caused by saline tidal inundation of low lying CASS landscapes, regardless of whether inundation is an intentional strategy or due to sea-level rise.     

Geochemical and mineralogical impacts of H2SO4 on clays between pH 5.0 and −3.0

Natural and constructed clay liners are routinely used to contain waste and wastewater. The impact of acidic solutions on the geochemistry and mineralogy of clays has been widely investigated in relation to acid mine drainage systems at pH > 1.0. The impact of H₂SO₄ leachate characterized by pH < 1.0 and potentially negative pH values on the geochemistry and mineralogy of clays is, however, not clear. Thus, laboratory batch experiments were conducted on three natural clay samples with different mass ratios of smectite, illite and kaolinite to investigate the impact of H₂SO₄ on the geochemistry and mineralogy of aluminosilicates from pH 5.0 to −3.0. Batch testing was conducted at seven pH treatments (5.0, 3.0, 1.0, 0.0, −1.0, −2.0 and −3.0) using standardized H₂SO₄ solutions for four exposure periods (14, 90, 180, and 365 d). Aqueous geochemical and XRD analyses showed: increased dissolution of aluminosilicates with decreasing pH and increasing exposure period, that smectite was more susceptible to dissolution than illite and kaolinite, precipitation of an amorphous silica phase occurred at pH ? 0.0, and anhydrite precipitated in Ca-rich clays at pH ? −1.0. In addition, global dissolution rates were calculated for the clays and showed good agreement to literature smectite, illite and kaolinite dissolution rates, which suggests global dissolution rates for complex clays could be determined from monomineralic studies. A stepwise conceptual model of the impact of H₂SO₄ on aluminosilicate geochemistry and mineralogy between pH 5.0 and −3.0 is proposed.     

Pyrite oxidation in the tertiary sands of the London Basin aquifer

Possible groundwater quality changes related to pyrite oxidation during artificial groundwater recharge and its storage in the Tertiary sands of the London Basin are investigated. Pyrite textures in the Tertiary sands are examined by scanning electron microscopy while an experimental approach is used to study mechanisms of pyrite oxidation and of some associated chemical reactions. In the Tertiary sands of the London Basin aquifer, pyrite occurs as aggregates made of discrete individual crystals 0.5–5 μm in size or, in a cryptocrystalline form, often as pseudomorphs of biogenic debris. It can expose a very large specific surface area to porefluids. Although ferric iron, which can be an oxidising agent of pyrite, is abundant in the solid phase of the Tertiary sands, it does not appear to take a significant part in this case. Pyrite oxidation seems to rely on a supply of oxygen. Leaching experiments using a 0.001 M H₂SO₄ solution were carried out to examine interactions between mildly acidic groundwater resulting from pyrite oxidation at a moderate rate and the host-sediment. In the presence of CaCO₃ in the solid phase, H⁺ is rapidly buffered by CaCO₃ dissolution. Oscillations of this reaction around equilibrium appear to trigger cation-exchange reactions on clay mineral surfaces, resulting in the release of major cations (e.g. K and Mg) into solution. In the absence of CaCO₃ in the solid phase, H⁺ buffering occurs less efficiently solely through exchange of cations for H⁺ on clay minerals surfaces. If the rate of pyrite oxidation in the Tertiary sands becomes high enough for the buffering capacity of the system to be exceeded, the groundwater pH begins to fall. Interactions between low pH (2) groundwaters and the host sediments were examined by leaching solid material in 0.01 M and 0.1 M H₂SO₄ solutions. Concentrations of Fe, Mg and K increase in solution throughout the experiment, indicating partial dissolution of clay minerals. The composition of the porefluid thus depends on the geochemical composition and surface area of the different clay minerals present.     

A robust model for pore-water chemistry of clayrock

The chemistry of pore water is an important property of clayrocks being considered as host rocks for long-term storage of radioactive waste. It may be difficult, if not impossible, to obtain water samples for chemical analysis from such rocks because of their low hydraulic conductivity. This paper presents an approach for calculating the pore-water compositions of clayrocks from laboratory-measured properties of core samples, including their leachable Cl and SO₄ concentrations and analysed exchangeable cations, and from mineral and cation exchange equilibria based on the formation mineralogy. New core sampling and analysis procedures are presented that reduce or quantify side reactions such as sample oxidation (e.g. pyrite) and soluble mineral dissolution (celestite, SrSO₄) that affect measured SO₄ concentrations and exchangeable cation distributions. The model considers phase equilibria only with minerals that are observed in the formation including the principal clay phases. The model has been used to calculate the composition of mobile pore water in the Callovo-Oxfordian clayrock and validated against measurements of water chemistry made in an underground research laboratory in that formation. The model reproduces the measured, in situ pore-water composition without any estimated parameters. All required parameters can be obtained from core sample analysis. We highlight the need to consider only those mineral phases which can be shown to be in equilibrium with contacting pore water. The consequence of this is that some conceptual models available in the literature appear not to be appropriate for modelling clayrocks, particularly those considering high temperature and/or high pressure detrital phases as chemical buffers of pore water. The robustness of our model with respect to uncertainties in the log K values of clay phases is also demonstrated. Large uncertainties in log K values for clay minerals have relatively small effects on modelled pore-water compositions.     

Groundwater composition,hydrochemical evolution and mass transfer in a regional detrital aquifer (Baza basin,southern Spain)

The physical–chemical characteristics of the groundwater in the Baza–Caniles detrital aquifer system indicate that a wide diversity of hydrochemical conditions exists in this semiarid region, defining geochemical zones with distinct groundwater types. The least mineralized water is found closest to the main recharge zones, and the salinity of the water increases significantly with depth towards the center of the basin. Geochemical reaction models have been constructed using water chemistry data along flow paths that characterize the different sectors of the aquifer system, namely: Quaternary aquifer, unconfined sector and shallow and deep confined sectors of the Mio–Pliocene aquifer. Geochemical mass–balance calculations indicate that the dominant groundwater reaction throughout the detrital system is dedolomitisation (dolomite dissolution and calcite precipitation driven by gypsum dissolution); this process is highly developed in the central part of the basin due to the abundance of evaporites. Apart from this process, there are others which influence the geochemical zoning of the system. In the Quaternary aquifer, which behaves as a system open to gases and which receives inputs of CO₂ gas derived from the intensive farming in the area, the interaction of the CO₂ with the carbonate matrix of the aquifer produces an increase in the alkalinity of the water. In the shallow confined sector of the Mio–Pliocene aquifer, the process of dedolomitisation evolves in a system closed to CO₂ gas. Ca²⁺/Na⁺ cation exchange and halite dissolution processes are locally important, which gives rise to a relatively saline water. Finally, in the deep confined sector, a strongly reducing environment exists, in which the presence of H₂S and NH⁺₄ in the highly mineralized groundwater can be detected. In this geochemical zone, the groundwater system is considered to be closed to CO₂ gas proceeding from external sources, but open to CO₂ from oxidation of organic matter. The geochemical modeling indicates that the chemical characteristics of this saline water are mainly due to SO₄ dissolution, dedolomitisation and SO₄ reduction, coupled with microbial degradation of lignite.     

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.     

ATLAS III in situ heating test in boom clay: Field data,observation and interpretation

The ATLAS III small scale in situ heating test aimed at assessing the thermo-hydro-mechanical (THM) effects on the Boom clay of the significant temperature gradients generated in this host rock as a consequence of the geological disposal of radioactive, heat-emitting wastes. This paper presents data on temperature, pore water pressure and total stress measured during the experiment and highlights several interesting observations regarding the thermal anisotropy and THM coupling in the Boom clay. The test has a simple geometry and well defined boundary conditions, which facilitates the comparison between measurement and numerical modeling studies. These studies included three dimensional coupled THM modeling of the test. The good agreement between measurement and numerical modeling of temperature and pore water pressure yields a set of THM parameters and confirms the thermo-mechanical anisotropy of the Boom clay.     

In situ and laboratory investigation of the alteration of Boom Clay (Oligocene) at the air–geological barrier interface within the Mol underground facility (Belgium): Consequences on kerogen and bitumen compositions

The Boom Clay formation (Oligocene) is studied as a reference host rock for methodological studies on deep geological disposal of radioactive waste. During excavation of galleries within the Clay formation (HADES underground research facility, Mol, Belgium), the physico-chemical conditions are significantly modified as an air–clay interface is created. In order to study the long-term impact of the air–clay contact on the organic matter contained in the Boom Clay, two types of samples were studied: (1) a reference series of clay samples having been in contact with the atmosphere of the HADES gallery for increasing times up to several years and (2) unaltered clay samples submitted to artificial oxidation in a ventilated oven at 80 °C. The evolution of geochemical data of the two series was compared using Rock-Eval pyrolysis, GC–MS and size exclusion chromatography. The organic matter of the unaltered clays sampled in the HADES galleries is dominated by type III kerogen (terrestrial) with some contribution of type II (marine) and is thermally immature. The evolution of geochemical parameters during air alteration for the two series are very similar. They show progressive oxidation of kerogen accompanied by the release of bitumen enriched in low molecular weight constituents. Molecular analysis evidences the presence of a complex mixture of aliphatic and aromatic O-bearing compounds, inherited from the degradation of kerogen as well as from the clay catalyzed oxidation of the bitumen. These results show that (1) air oxidation is a major process in the in situ alteration of the organic matter of Boom Clay within the HADES galleries, (2) laboratory oxidation experiments at 80 °C yield similar results as in situ air alteration of Boom Clay and (3) artificial air oxidation may be used to assess the long term exposure of the organic matter to air.     

Geochemical evolution and quality assessment of water resources in the Sarcheshmeh copper mine area (Iran) using multivariate statistical techniques

Hydrochemical data were gathered throughout the last 12 years from 57 sampling stations in the drainage basin of the Sarcheshmeh copper mine, Kerman Province, Iran. The mean values of these data for each sampling station were used to evaluate water quality and to determine processes that control water chemistry. Principal component analyses specified the oxidation of sulfide minerals, dissolution of carbonate and sulfate minerals and weathering of silicate minerals as the principal processes responsible for the chemical composition of water in the study area. Q-mode cluster analysis revealed three main water groups. The first group had a Ca-HCO₃–SO₄ composition whereas the second and third groups had Ca–SO₄ and Ca–Mg–SO₄ composition, respectively. The results of this study clearly indicated the role of sulfide minerals oxidation and the buffering processes in the geochemical evolution of water in the Sarcheshmeh area. Due to these processes, extensive changes occurred in the chemical composition of water by passage through the mining area or waste and low-grade dumps, so that the fresh water of the peripheral area of the pit evolved to an acid water rich in sulfate and heavy metals at the outlet of the pit and in the seepages of waste and low-grade dumps.     

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.     

Genetic geochemical model for mining affected groundwaters of the Lusatian post-mining district

Using groundwater quality data from the Lusatian post-mining district a hydrogeochemical model is derived for the evolution of mining affected groundwaters in pyrite-rich dumps which consist mainly of silicates and variable amounts of calcite. Pyrite oxidation paralleled by buffer processes leads to gypsum saturation in a significant portion of the water. Gypsum precipitation controls SO₄ and Ca concentrations in groundwaters above an ionic strength (I) of 60 mM. It has been found that there is always a clear relationship between I, SO₄ and Ca concentrations. In particular, there is a tendency that Ca concentrations decrease with increase in ionic strength above I = 60 mM and a striking rareness of samples with SO₄ concentrations between 20 and 30 mM above an ionic strength of 100 mM. These observations are explained by a genetic model. This model also explains the observed relationship between the c(Fe)/c(SO₄)-ratio, the ionic strength, and the observed pH-values. Based on the field data and supported by geochemical equilibrium calculations, it is shown that silicate weathering along with calcite dissolution must be a significant buffering process at least in some areas.     

Statistical and geochemical assessment of groundwater quality in Teboursouk area (Northwestern Tunisian Atlas)

Teboursouk region, Northwestern Tunisia, is characterized by the diversity of its natural resources (petroleum, groundwater and minerals). It constitutes a particular site widely studied, especially from a tectonic stand point as it exhibits a complex architecture dominated by multi-scale synclinals and Triassic extrusions. It has typical karst landform that constitutes important water resources devoted for human consumption and agriculture activities, besides to the exploitation of the Mio-Plio-Quaternary aquifer (MPQ). Thus, hydrogeological investigations play a significant role in the assessment of groundwater mineralization and the evaluation of the used water quality for different purposes. Hence, the current study based on a combined geochemical–statistical investigation of 50 groundwater samples from the multilayered aquifer system in the study area give crucial information about the principal factors and processes influencing groundwater chemistry. The chemical analysis of the water samples showed that Teboursouk groundwater is dominantly of Ca–Mg–Cl–SO₄ water type with little contribution of Ca–Mg–HCO₃, Na–K–Cl–SO₄ and Na–K–HCO₃. The total dissolved solids (TDS) values range from 0.37 to 3.58 g/l. The highest values are located near the Triassic outcrops. Furthermore, the hydrogeochemistry of the studied system was linked with various processes such as carbonates weathering, evaporites dissolution of Triassic outcrops and anthropogenic activities (nitrate contamination). Additionally, the main processes controlling Teboursouk water system were examined by means of multivariate statistical analysis (PCA and HCA) applied in this study based on 10 physicochemical parameters (TDS, pH, SO₄, HCO₃, pCO₂, Ca, Mg, Na, K, Cl and NO₃). Two principal components were extracted from PCA accounting 61% of total variance and revealing that the chemical characteristics of groundwater in the region were acquired through carbonates and evaporite dissolution besides to nitrate contamination. Similarly, according to Cluster analysis using Ward’s method and squared Euclidean distance, groundwater from the studied basin belongs to five different groups suggesting that the geochemical evolution of Teboursouk groundwater is controlled by dissolution of carbonates minerals, chemical weathering of Triassic evaporite outcrops, cation exchange and anthropogenic activities (nitrate contamination).