Evaluation of hydrogeochemical processes in the Pleistocene aquifers of Middle Ganga Plain, Uttar Pradesh, India

Evaluation of major ion chemistry and solute acquisition process controlling water chemical composition were studied by collecting a total of fifty-one groundwater samples in shallow (<25 m) and deep aquifer (>25 m) in the Varanasi area. Hydrochemical facies, Mg-HCO₃ dominated in the largest part of shallow groundwater followed by Na-HCO₃ and Ca-HCO₃ whereas Ca-HCO₃ is dominated in deep groundwater followed by Mg-HCO₃ and Na-HCO₃. High As concentration (>50 μg/l) is found in some of the villages situated in northeastern parts (i.e. adjacent to the concave part of the meandering Ganga river) of the Varanasi area. Arsenic contamination is confined mostly in tube wells (hand pump) within the Holocene newer alluvium deposits, whereas older alluvial aquifers are having arsenic free groundwater. Geochemical modeling using WATEQ4F enabled prediction of saturation state of minerals and indicated dissolution and precipitation reactions occurring in groundwater. Majority of shallow and deep groundwater samples of the study area are oversaturated with carbonate bearing minerals and under-saturated with respect to sulfur and amorphous silica bearing minerals. Sluggish hydraulic conductivity in shallow aquifer results in higher mineralization of groundwater than in deep aquifer. But the major processes in deep aquifer are leakage of shallow aquifer followed by dominant ion-exchange and weathering of silicate minerals.     

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.     

Controls on groundwater flow in the Bengal Basin of India and Bangladesh: regional modeling analysis

Groundwater for domestic and irrigation purposes is produced primarily from shallow parts of the Bengal Basin aquifer system (India and Bangladesh), which contains high concentrations of dissolved arsenic (exceeding worldwide drinking water standards), though deeper groundwater is generally low in arsenic. An essential first step for determining sustainable management of the deep groundwater resource is identification of hydrogeologic controls on flow and quantification of basin-scale groundwater flow patterns. Results from groundwater modeling, in which the Bengal Basin aquifer system is represented as a single aquifer with higher horizontal than vertical hydraulic conductivity, indicate that this anisotropy is the primary hydrogeologic control on the natural flowpath lengths. Despite extremely low hydraulic gradients due to minimal topographic relief, anisotropy implies large-scale (tens to hundreds of kilometers) flow at depth. Other hydrogeologic factors, including lateral and vertical changes in hydraulic conductivity, have minor effects on overall flow patterns. However, because natural hydraulic gradients are low, the impact of pumping on groundwater flow is overwhelming; modeling indicates that pumping has substantially changed the shallow groundwater budget and flowpaths from predevelopment conditions.     

Interaction between deep and shallow groundwater systems in areas affected by Quaternary tectonics (Central Italy): a geochemical and isotope approach

Hydro- and isotope geochemistry are used to refine groundwater conceptual models in two areas of central Italy (Acque Albule Basin and Velino River Valley) affected by extensional Quaternary tectonics, where deep and shallow groundwater flow systems are interacting. The role of geology, of recent deposits filling the plains and of main tectonic features controlling groundwater flowpaths and deep-seated fluids emergences are investigated and discussed. Environmental isotopes (²H and ¹⁸O) confirm recharge in the surrounding carbonate aquifers, and meteoric origin of both shallow and deep groundwater. Major ion chemistry indicates a mixing between shallow Ca-HCO₃ groundwater from carbonate aquifers and deep Ca-HCO₃-SO₄ groundwater, characterised by higher salinity and temperature and high concentration in sulphates. Isotopic composition of dissolved sulphates (δ ³⁴S and δ ¹⁸O) and dissolved inorganic carbon (δ ¹³C), henceforth indicated as DIC, are used to verify the presence of different sources of groundwater, and to validate the mixing model suggested by the major ion analyses. Sulphate isotope composition suggests a marine origin for the groundwater characterised by elevated sulphate concentration, whose source is present in the deep buried sequences. Carbon isotope composition confirms the role of a DIC source associated to CO₂ degassing of a deep reservoir. Groundwater conceptual models are improved underlining the importance of Plio-Pleistocene sequences filling the tectonic depression. In the Acque Albule area, the travertine plateau represents a mixing stratified aquifer, where deep groundwater contribution is spread into the shallow aquifer. The alluvial–clastic–lacustrine leaky aquifer of Velino Valley enables a complete mixing of shallow and deep groundwater allowing spot-located discharge of deep groundwater along tectonic patterns and facilitating sulphate reduction in the lacustrine sediments, explaining locally the presence of H₂S.     

The hydraulic conductivity field and groundwater flow in the unconfined aquifer system of the Keta Strip,Ghana

This study investigates the hydraulic conductivity field and the groundwater flow pattern as predicted by a calibrated steady state groundwater flow model for the Keta Strip, southeastern Ghana. The hydraulic conductivity field is an important parameter in evaluating aquifer properties in space, and in general basin-wide groundwater resources evaluation and management. This study finds that the general hydraulic conductivity of the unconsolidated unconfined aquifer system of the Keta Strip ranges between 2 m/d and 20 m/d, with an average of 15 m/d. The spatial variation in horizontal hydraulic conductivity appears to take the trend in the variations in the nature of the material in space. Calibrated groundwater recharge suggests that 6.9–34% of annual precipitation recharges the shallow aquifer system. This amount of recharge is significant and suggests high fortunes in terms of groundwater resources development for agriculture and industrial activities in the area. A spatial distribution of groundwater recharge from precipitation is presented in this study. The spatial pattern appears to take the form of the distribution in horizontal hydraulic conductivity, and suggests that the vertical hydraulic conductivity takes the same pattern of spatial variation as the horizontal hydraulic conductivity. This is consistent with observations in other areas. The resulting groundwater flow is dominated by local flow systems as the unconfined system is quite shallow. A general northeast – southwest flow pattern has been observed in the study area.     

Hydrogeologic and hydraulic characterization of aquifer and nonaquifer layers in a lateritic terrain (West Bengal,India)

The hydrogeologic and hydraulic characteristics of a lateritic terrain in West Bengal, India, were investigated. Test drilling was conducted at ten sites and grain-size distribution curves (GSDCs) were prepared for 275 geologic samples. Performance evaluation of eight grain-size-analysis (GSA) methods was carried out to estimate the hydraulic conductivity (K) of subsurface formations. Finally, the GSA results were validated against pumping-test data. The GSDCs indicated that shallow aquifer layers are coarser than the deeper aquifer layers (uniformity coefficient 0.19–11.4). Stratigraphy analysis revealed that both shallow and deep aquifers of varying thickness exist at depths 9–40 and 40–79 m, respectively. The mean K estimates by the GSA methods are 3.62–292.86 m/day for shallow aquifer layers and 0.97–209.93 m/day for the deeper aquifer layers, suggesting significant aquifer heterogeneity. Pumping-test data indicated that the deeper aquifers are leaky confined with transmissivity 122.69–693.79 m²/day, storage coefficient 1.01?×?10^?7–2.13?×?10^?4 and leakance 2.01?×?10^?7–34.56?×?10^?2 day^?1. Although the K values yielded by the GSA methods are generally larger than those obtained from the pumping tests, the Slichter, Harleman and US Bureau Reclamation (USBR) GSA methods yielded reasonable values at most of the sites (1–3 times higher than K estimates by the pumping-test method). In conclusion, more reliable aquifers exist at deeper depths that can be tapped for dependable water supply. GSA methods such as Slichter, Harleman and USBR can be used for the preliminary assessment of K in lateritic terrains in the absence of reliable field methods.     

Groundwater modeling in semiarid Central Sudan: adequacy and long-term abstraction

The present study assesses groundwater resources in the semiarid central Sudan, where 20 deep productive wells were installed to supply a major city in the region, El Obeid. The wells, which has an average 20 L/s discharge each, are taping a deep semiconfined to confined aquifer of fluvial silisiclastics deposited in the Tertiary–Pleistocene. Groundwater modeling was used as a technique to interpret the hydrologic system in arid to semiarid central Sudan and to simulate the future influence of the project on the hydrogeologic system. The simulation confirmed that steady-state flow conditions have been currently reached as indicated by consistency of computed heads. It also calibrated the values of the conductivity and recharge and ensured the sustainability of the El Obeid water supply project. A total of 3.5 × 10⁷ m³/year can be continually extracted from the deep aquifer to supply El Obeid city without endangering the groundwater resources in the region. The decline in water level will not exceed 25 m during the first 10 years, while indefinite continuous pumping will affect only the vicinity of the wells in a circle of 30 km diameter. Therefore, aquifer storage capacity and hydraulic properties encourage further groundwater exploitation. The present use of groundwater is extremely lower than the present demand, and it can potentially cover future demands without introducing significant changes to the system. The increase of pumping cost due to the decline in head subsequent to project operation was found to be minimal and of local effect.     

Dissolved noble gases and stable isotopes as tracers of preferential fluid flow along faults in the Lower Rhine Embayment,Germany

Groundwater in shallow unconsolidated sedimentary aquifers close to the Bornheim fault in the Lower Rhine Embayment (LRE), Germany, has relatively low δ²H and δ¹⁸O values in comparison to regional modern groundwater recharge, and ⁴He concentrations up to 1.7?×?10^?4 cm³ (STP) g^–1?±?2.2 % which is approximately four orders of magnitude higher than expected due to solubility equilibrium with the atmosphere. Groundwater age dating based on estimated in situ production and terrigenic flux of helium provides a groundwater residence time of ～10⁷ years. Although fluid exchange between the deep basal aquifer system and the upper aquifer layers is generally impeded by confining clay layers and lignite, this study’s geochemical data suggest, for the first time, that deep circulating fluids penetrate shallow aquifers in the locality of fault zones, implying  that sub-vertical fluid flow occurs along faults in the LRE. However, large hydraulic-head gradients observed across many faults suggest that they act as barriers to lateral groundwater flow. Therefore, the geochemical data reported here also substantiate a conduit-barrier model of fault-zone hydrogeology in unconsolidated sedimentary deposits, as well as corroborating the concept that faults in unconsolidated aquifer systems can act as loci for hydraulic connectivity between deep and shallow aquifers. The implications of fluid flow along faults in sedimentary basins worldwide are far reaching and of particular concern for carbon capture and storage (CCS) programmes, impacts of deep shale gas recovery for shallow groundwater aquifers, and nuclear waste storage sites where fault zones could act as potential leakage pathways for hazardous fluids.     

Estimation of regional-scale groundwater flow properties in the Bengal Basin of India and Bangladesh

Quantitative evaluation of management strategies for long-term supply of safe groundwater for drinking from the Bengal Basin aquifer (India and Bangladesh) requires estimation of the large-scale hydrogeologic properties that control flow. The Basin consists of a stratified, heterogeneous sequence of sediments with aquitards that may separate aquifers locally, but evidence does not support existence of regional confining units. Considered at a large scale, the Basin may be aptly described as a single aquifer with higher horizontal than vertical hydraulic conductivity. Though data are sparse, estimation of regional-scale aquifer properties is possible from three existing data types: hydraulic heads, ¹⁴C concentrations, and driller logs. Estimation is carried out with inverse groundwater modeling using measured heads, by model calibration using estimated water ages based on ¹⁴C, and by statistical analysis of driller logs. Similar estimates of hydraulic conductivities result from all three data types; a resulting typical value of vertical anisotropy (ratio of horizontal to vertical conductivity) is 10⁴. The vertical anisotropy estimate is supported by simulation of flow through geostatistical fields consistent with driller log data. The high estimated value of vertical anisotropy in hydraulic conductivity indicates that even disconnected aquitards, if numerous, can strongly control the equivalent hydraulic parameters of an aquifer system.     

Large-scale hydraulic conductivity distribution in an unconfined carbonate aquifer using the ocean tidal propagation

The hydraulic conductivity of an unconfined carbonate aquifer at the uplifted atoll of Minami-Daito, Japan, was evaluated by a combination of cross-spectral analysis, analytical solution, and density-dependent groundwater modeling based on observed groundwater levels in 15 wells and at sea level. The island area was divided into 10 subregions based on island morphology and on inland propagation of ocean tides. The hydraulic conductivity was obtained for each subregion using analytical solutions based on phase lags of M₂ constituents of ocean tides at each well by assuming two aquifer thicknesses (300 and 1,800 m) and two effective porosities (0.1 and 0.3). The density-dependent groundwater model evaluated the hydraulic conductivity of the subregions by reproducing observed groundwater levels. The hydraulic conductivity in the subregions was estimated as 3.46?×?10^?3 to 6.35?×?10^?2 m/s for aquifer thickness of 300 m and effective porosity of 0.1, and as 1.73?×?10^?3 to 3.17?×?10^?2 m/s for aquifer thickness of 1,800 m and the effective porosity of 0.3. It was higher in southern and northern areas, and higher in interior lowland than in the western and eastern areas. Fissures and dolomite distributions on the island control differences of the omnidirectional ocean tidal propagation and cause these differences in hydraulic conductivity. The method used for this study may also be applicable to other small islands that have few or no data for hydraulic conductivity.

     

Vertical movement of water in a High Plains Aquifer induced by a pumping well

Field observation and numerical simulations were carried out to evaluate the hydraulic relationship between the shallow and deep aquifer of a High Plains Aquifer system, in which shallow and deep aquifers are separated by an aquitard. Pumping from the lower aquifer resulted in a small drawdown in the upper aquifer and a larger drawdown in the aquitard; pumping from the shallow aquifer caused a small drawdown in the aquitard and the deep aquifer. Analysis of pumping test data gives the values of the hydraulic conductivity of the aquitard and the deep aquifer. Long-term observation of groundwater levels in the shallow and deep aquifers showed that a strong downward hydraulic gradient was maintained during an irrigation season. Numerical simulations were used to calculate the induced leakage of water from the shallow to the deep aquifer. Water budget analyses suggested that after pumping continues for a couple of days, the leakage from the overlying layers begins to supply the majority of the withdrawal from the deep aquifer. However, the induced leakage from the upper shallow aquifer can travel only a few meters into the aquitard, and it can not reach the lower aquifer during a 90 day pumping period. The major portion of the induced leakage occurred during the pumping period, but a small leakage can continue as a residual effect after the pumping period. The vertical hydraulic conductivity of the aquitard plays a major role in partitioning the ratio of the induced leakage for the pumping and after-pumping periods.     

Hydro-geomechanical characterisation of a coastal urban aquifer using multiscalar time and frequency domain groundwater-level responses

Hydraulic properties of coastal, urban aquifers vary spatially and temporally with the complex dynamics of their hydrogeology and the heterogeneity of ocean-influenced hydraulic processes. Traditional aquifer characterisation methods are expensive, time-consuming and represent a snapshot in time. Tidal subsurface analysis (TSA) can passively characterise subsurface processes and establish hydro-geomechanical properties from groundwater head time-series but is typically applied to individual wells inland. Presented here, TSA is applied to a network of 116 groundwater boreholes to spatially characterise confinement and specific storage across a coastal aquifer at city-scale in Cardiff (UK) using a 23-year high-frequency time-series dataset. The dataset comprises Earth, atmospheric and oceanic signals, with the analysis conducted in the time domain, by calculating barometric response functions (BRFs), and in the frequency domain (TSA). By examining the damping and attenuation of groundwater response to ocean tides (OT) with distance from the coast/rivers, a multi-borehole comparison of TSA with BRF shows this combination of analyses facilitates disentangling the influence of tidal signals and estimation of spatially distributed aquifer properties for non-OT-influenced boreholes. The time-series analysed covers a period pre- and post-impoundment of Cardiff’s rivers by a barrage, revealing the consequent reduction in subsurface OT signal propagation post-construction. The results indicate that a much higher degree of confined conditions exist across the aquifer than previously thought (specific storage?=?2.3 × 10^?6 to 7.9 × 10^?5 m^?1), with implications for understanding aquifer recharge, and informing the best strategies for utilising groundwater and shallow geothermal resources.

     

Numerical groundwater-flow modeling to evaluate potential effects of pumping and recharge: implications for sustainable groundwater management in the Mahanadi delta region,India

Process-based groundwater models are useful to understand complex aquifer systems and make predictions about their response to hydrological changes. A conceptual model for evaluating responses to environmental changes is presented, considering the hydrogeologic framework, flow processes, aquifer hydraulic properties, boundary conditions, and sources and sinks of the groundwater system. Based on this conceptual model, a quasi-three-dimensional transient groundwater flow model was designed using MODFLOW to simulate the groundwater system of Mahanadi River delta, eastern India. The model was constructed in the context of an upper unconfined aquifer and lower confined aquifer, separated by an aquitard. Hydraulic heads of 13 shallow wells and 11 deep wells were used to calibrate transient groundwater conditions during 1997–2006, followed by validation (2007–2011). The aquifer and aquitard hydraulic properties were obtained by pumping tests and were calibrated along with the rainfall recharge. The statistical and graphical performance indicators suggested a reasonably good simulation of groundwater flow over the study area. Sensitivity analysis revealed that groundwater level is most sensitive to the hydraulic conductivities of both the aquifers, followed by vertical hydraulic conductivity of the confining layer. The calibrated model was then employed to explore groundwater-flow dynamics in response to changes in pumping and recharge conditions. The simulation results indicate that pumping has a substantial effect on the confined aquifer flow regime as compared to the unconfined aquifer. The results and insights from this study have important implications for other regional groundwater modeling studies, especially in multi-layered aquifer systems.     

Effects of solution mining on groundwater quality in the Kazan trona field,Ankara-Turkey: model predictions

Three aquifer systems as deep, middle, and shallow were identified in the Kazan trona ore deposit area. The flow conditions and the interaction between various layers were conceptualized into a site hydrogeological model. Each aquifer system was hydraulically and chemically characterized and represented in a numerical groundwater model. The resulting model has been calibrated under steady-state and transient conditions using available data. The flow model was used in conjunction with a three-dimensional solute transport model to assess the impacts of the pilot well solution mining of the trona deposit on groundwater resources during operation and post-operation periods. The results of operation period indicate that, even under the worse conditions (50 times increase in vertical hydraulic conductivity due to subsidence), ion contribution from the mine area with 118,000 mg/l maximum concentration would be about 58 mg/l into the deep aquifer system. This contribution is about 1.45% of the existing concentration (4,000 mg/l) in the deep aquifer. After 1,000 years of post-operation period, ion contribution from the mine area with maximum 119,000 mg/l concentration would be about 205 mg/l into the deep aquifer under extremely worse conditions. This contribution is about 5–20% of present concentrations in the deep aquifer. Retardation factors, which were not considered during model simulations would decrease the predicted concentrations. It is concluded that pilot well solution mining of the trona deposit would not have significant impact on the quality of groundwater resources in the overlying aquifers.     

Evaluation of the importance of clay confining units on groundwater flow in alluvial basins using solute and isotope tracers: the case of Middle San Pedro Basin in southeastern Arizona (USA)

As groundwater becomes an increasingly important water resource worldwide, it is essential to understand how local geology affects groundwater quality, flowpaths and residence times. This study utilized multiple tracers to improve conceptual and numerical models of groundwater flow in the Middle San Pedro Basin in southeastern Arizona (USA) by determining recharge areas, compartmentalization of water sources, flowpaths and residence times. Ninety-five groundwater and surface-water samples were analyzed for major ion chemistry (water type and Ca/Sr ratios) and stable (¹⁸O, ²H, ¹³C) and radiogenic (³H, ¹⁴C) isotopes, and resulting data were used in conjunction with hydrogeologic information (e.g. hydraulic head and hydrostratigraphy). Results show that recent recharge (<60 years) has occurred within mountain systems along the basin margins and in shallow floodplain aquifers adjacent to the San Pedro River. Groundwater in the lower basin fill aquifer (semi confined) was recharged at high elevation in the fractured bedrock and has been extensively modified by water-rock reactions (increasing F and Sr, decreasing ¹⁴C) over long timescales (up to 35,000 years BP). Distinct solute and isotope geochemistries between the lower and upper basin fill aquifers show the importance of a clay confining unit on groundwater flow in the basin, which minimizes vertical groundwater movement.     

Groundwater flow modeling of Quaternary aquifer Ras Sudr,Egypt

Recently, Ras Sudr (the delta of Wadi Sudr) area received a great amount of attention due to different development expansion activities directed towards this area. Although Quaternary aquifer is the most prospective aquifer in Ras Sudr area, it has not yet completely evaluated. The present work deals with the simulation of the Quaternary groundwater system using a three-dimensional groundwater flow model. MODFLOW code was applied for designing the model of the Ras Sudr area. This is to recognize the groundwater potential as well as exploitation plan of the most prospective aquifer in the area. The objectives were to determine the hydraulic parameters of the Quaternary aquifer, to estimate the recharge amount to the aquifer, and to determine the hydrochemistry of groundwater in the aquifer. During this work, available data has been collected and some field investigation has been carried out. Groundwater flow model has been simulated using pilot points’ method. SEAWAT has been also applied to simulate the variable-density flow and sea water intrusion from the west. It can be concluded that: (1) the direction of groundwater flow is from the east to the west, (2) the aquifer system attains a small range of log-transformed hydraulic conductivity. It ranges between 3.05 and 3.35 m/day, (3) groundwater would be exploited by about 6.4 × 10⁶ m³/year, (4) the estimated recharge accounts for 3 × 10⁶ m³/year, (5) an estimated subsurface flow from the east accounts for 2.7 × 10⁶ m³/year, (6) the increase of total dissolved solids (TDS) most likely due to dilution of salts along the movement way of groundwater from recharge area to discharge area in addition to a contribution of sea water intrusion from the west. Moreover, it is worth to note that a part of TDS increase might be through up coning from underlying more saline Miocene sediments. It is recommended that: (1) any plan for increasing groundwater abstraction is unaffordable, (2) reliable estimates of groundwater abstraction should be done and (3) automatic well control system should be made.     

Hydrogeochemical characteristics and its role in controlling arsenic mobilization in a shallow aquifer

The relevance of groundwater hydrogeochemistry to explain the occurrence and distribution of arsenic in groundwater is of great interest. The insightful discussions on the control of shallow groundwater (< 50 m) hydrogeochemistry in arsenic mobilization are known to be a viable tool to explain the arsenic menace in shallow groundwater. The present investigation emphasizes the hydrogeochemical driver and/or control over the reductive dissolution of Fe-bearing host minerals and thereby releasing arsenic into the shallow groundwater of the study area. The study suggests that hydrogeochemical evolution is mainly governed by carbonate minerals dissolution, silicate weathering, and competitive ion-exchange processes in the shallow aquifers (< 50 m). The present study also indicates the prevalence of carbonate minerals dissolution over silicate weathering. The emergence of Cl⁻ concentration in the shallow groundwater founds the possibilities of anthropogenic inputs into the shallow aquifers (< 50 m). The reducing environment in shallow aquifers (< 50 m) of the study area is evident in the reductive dissolution of Fe- bearing shallow aquifer minerals which absorb arsenic in the solid phase and mobilize arsenic onto shallow groundwater. The study opted for many statistical approaches to delineate the correlation among major and minor ionic constituents of the groundwater which are very helpful to understand the comprehensive mechanism of arsenic mobilization into shallow groundwater.

     

Geochemical and isotopic investigation of the aquifer system in the Djerid-Nefzaoua basin, southern Tunisia

In the Djerid-Nefzaoua region, southern Tunisia, about 80% of agricultural and domestic water supply is provided by the complex terminal (CT) aquifer. However, 20% of this demand is provided by other hydraulically connected aquifers, namely the continental intercalaire (CI) and the Plio-Quaternary (PQ). Overexploitation of the CT aquifer for agricultural practices has contributed to the loss of the artesian condition and the decline of groundwater level which largely increased the downward leakage from the shallow PQ aquifer. Excess irrigation water concentrates at different rates in the irrigation channels and in the PQ aquifer itself. Then, it returns to the CT aquifer and mixes with water from the regional flow system, which contributes to the salinization of the CT groundwater. A geochemical and isotopic study had been undertaken over a 2-years period in order to investigate the origin of waters pumped from the CT aquifer with an emphasis on its hydraulic relationships with the underlying and the overlying CI and PQ aquifers. Geochemistry indicates that groundwater samples collected from different wells show an evolution of the water types from Na-Cl to Ca-SO₄-Cl. Dissolution of halite, gypsum and anhydrite-bearing rocks is the main mechanism that leads to the salinization of the groundwater. Isotopic data indicate the old origin of all groundwater in the aquifer system. Mixing and evaporation effects characterizing the CT and the PQ aquifers were identified using δ²H and δ¹⁸O relationship and confirmed by the conjunction of δ²H with chloride concentration.     

Groundwater flow dynamics in the complex aquifer system of Gidabo River Basin (Ethiopian Rift): a multi-proxy approach

Hydrochemical and isotope data in conjunction with hydraulic head and spring discharge observations were used to characterize the regional groundwater flow dynamics and the role of the tectonic setting in the Gidabo River Basin, Ethiopian Rift. Both groundwater levels and hydrochemical and isotopic data indicate groundwater flow from the major recharge area in the highland and escarpment into deep rift floor aquifers, suggesting a deep regional flow system can be distinguished from the shallow local aquifers. The δ¹⁸O and δ²H values of deep thermal (≥30 °C) groundwater are depleted relative to the shallow (<60 m below ground level) groundwater in the rift floor. Based on the δ¹⁸O values, the thermal groundwater is found to be recharged in the highland around 2,600 m a.s.l. and on average mixed with a proportion of 30 % shallow groundwater. While most groundwater samples display diluted solutions, δ¹³C data of dissolved inorganic carbon reveal that locally the thermal groundwater near fault zones is loaded with mantle CO₂, which enhances silicate weathering and leads to anomalously high total dissolved solids (2,000–2,320 mg/l) and fluoride concentrations (6–15 mg/l) exceeding the recommended guideline value. The faults are generally found to act as complex conduit leaky barrier systems favoring vertical mixing processes. Normal faults dipping to the west appear to facilitate movement of groundwater into deeper aquifers and towards the rift floor, whereas those dipping to the east tend to act as leaky barriers perpendicular to the fault but enable preferential flow parallel to the fault plane.