Middle Aquifer Research Articles

Coupled redox cycling of arsenic and sulfur regulates thioarsenate enrichment in groundwater

High‑arsenic groundwater is influenced by a combination of processes: reductive dissolution of iron minerals and formation of secondary minerals, metal complexation and redox reactions of organic matter (OM), and formation of more migratory thioarsenate, which together can lead to significant increases in arsenic concentration in groundwater. This study was conducted in a typical sulfur- and arsenic-rich groundwater site within the Datong Basin to explore the conditions of thioarsenate formation and its influence on arsenic enrichment in groundwater using HPLC-ICPMS, hydrogeochemical modeling, and fluorescence spectroscopy. The shallow aquifer exhibited a highly reducing environment, marked by elevated sulfide levels, low concentrations of Fe(II), and the highest proportion of thioarsenate. In the middle aquifer, an optimal ∑S/∑As led to the presence of significant quantities of thioarsenate. In contrast, the deep aquifer exhibited low sulfide and high Fe(II) concentration, with arsenic primarily originating from dissolved iron minerals. Redox fluctuations in the sediment driven by sulfur‑iron minerals generated reduced sulfur, thereby facilitating thioarsenate formation. OM played a crucial role as an electron donor for microbial activities, promoting iron and sulfate reduction processes and creating conditions conducive to thioarsenate formation in reduced and high‑sulfur environments. Understanding the process of thioarsenate formation and the influencing factors is of paramount importance for comprehending the migration and redistribution of arsenic in groundwater systems.

Hydrogeochemical evolution and processes based on multivariable statistical and inverse simulation modeling: A coal mine in the Northern Coalfield, China

ABSTRACT Considering groundwater from the aquifers overlying the bedrock is an important water source for drinking purposes. As such, the investigation of its property is essential. Based on the spatial structure of aquifers in the study area, the aquifers in the Cenozoic strata are divided into three groups. The multivariate statistical approaches are employed to identify the hydrogeochemical processes and hydro-chemical types, and hydrogeochemical inverse modeling is applied to further validate and elucidate the hydrogeochemical process and water–rock interaction. The results are as follows: (1) The hydro-chemical type of the upper aquifer is dominated by the K + Na-HCO3 type, while others have similar water quality types, which are dominated by the K + Na-Cl type and the K + Na-SO4 type. (2) The saturation index of anhydrite, gypsum, halite, and CO2(g) is below zero in three aquifers, indicating that they are unsaturated. While aragonite, calcite, and dolomite in the middle aquifer remain in the unsaturated–saturated state. (3) The cation exchange process accelerating the reduction of Ca2+ concentration and the increase of SO42− concentration occurs in three aquifers, and the dissolution of calcite and dolomite minerals occurs in most cases. This study supports the fundamental evidence for the hydrogeochemical processes and water resource utilization and has a certain practical significance.

Effects of the groundwater flowing and redox conditions on arsenic mobilization in aquifers of the Datong Basin, Northern China

A multi-level field site was built at the As-affected area of the Datong basin. The one-year monitoring results showed that the three aquifers exhibit different redox conditions and hydrogeochemical behaviors. In the shallow aquifer, groundwater As concentration is up to 220.5 μg/L, which is related to the dissimilatory reduction of poor-crystalline Fe minerals. In the middle aquifer, groundwater sulfate reduction shows two sides effects on As mobilization that the reduced sulfide on the one hand promotes the release of sediment As via abiotic reductive dissolution of Fe minerals and on the other hand immobilizes groundwater As via the adsorption on the newly formed Fe(II)-sulfide minerals. In the deep aquifer, the reductive dissolution of crystalline Fe minerals was the dominant process causing the release of sediment As into groundwater with the highest As concentration up to 341.1 μg/L. Based on the one-year monitored data, a coupled flow and reactive transport model was developed. The model results showed that only considering the groundwater flow underestimated the concentration of groundwater As. When adding the hydro-biogeochemical processes, the good agreement was achieved between the observed and simulated groundwater As. In the shallow and deep aquifers, microbially mediated reduction of poor-crystalline and crystalline Fe minerals contributed approximately 85 % of groundwater As. In the middle aquifer, abiotic reduction of Fe minerals driven is responsible for around 35 % of the release of sediment As, and around 17 % of released As was re-adsorbed or co-precipitated onto the neo-formed Fe(II) minerals. Under different redox conditions, the hydrogeochemical behavior of As was dominantly controlled by the Fe-S-As cycling in the groundwater system.

Time series study for groundwater level evaluation in monitoring well

Climate and hydrogeological conditions of the Brazilian semi-arid demand sustainable and efficient water solutions. Groundwater monitoring programs are tools to subsidize the decision-making in this sense. In Ceará state, the monitoring of Araripe sedimentary basin aquifers is important for the development of the region. In this scenario, the present work aimed to study the groundwater level through an exploratory analysis of time series. The study area covered the eastern portion of the Araripe sedimentary basin, in the municipality of Milagres, in Ceará state. As the object of this study, it was obtained the time series of monthly average groundwater levels in a monitoring well of RIMAS/CPRM and installed in the Middle Aquifer System. Graphical and numerical methods were applied for the identification and description of time series main characteristics. Precipitation data in the study area were used to evaluate the system recharge. Results were discussed according to the environmental aspects of the study area. As a result, it was possible the identification and description of time series patterns such as trend and seasonality through the applied methods. It is also highlighted the sharp drawdown of groundwater levels in long term in the time series, reflecting the quantitative state of the aquifer system, as well as the groundwater recharge during the rainy season of the region, evidenced by the study of time series seasonality together with the precipitation data..

Critical Tectonic Limits for Geothermal Aquifer Use: Case Study from the East Slovakian Basin Rim

The Pannonian basin is a major geothermal heat system in Central Europe. Its peripheral basin, the East Slovakian basin, is an example of a geothermal structure with a linear, directed heat flow ranging from 90 to 100 mW/m2 from west to east. However, the use of the geothermal source is limited by several critical tectono-geologic factors: (a) Tectonics, and the associated disintegration of the aquifer block by multiple deformations during the pre-Paleogene, mainly Miocene, period. The main discontinuities of NW-SE and N-S direction negatively affect the permeability of the aquifer environment. For utilization, minor NE-SW dilatation open fractures are important, which have been developed by sinistral transtension on N–S faults and accelerated normal movements to the southeast. (b) Hydrogeologically, the geothermal structure is accommodated by three water types, namely, Na-HCO3 with 10.9 g·L−1 mineralization (in the north), the Ca-Mg-HCO3 with 0.5–4.5 g·L−1 mineralization (in the west), and Na-Cl water type containing 26.8–33.4 g·L−1 mineralization (in the southwest). The chemical composition of the water is influenced by the Middle Triassic dolomite aquifer, as well as by infiltration of saline solutions and meteoric waters along with open fractures/faults. (c) Geothermally anomalous heat flow of 123–129 °C with 170 L/s total flow near the Slanské vchy volcanic chain seems to be the perspective for heat production.

A Case Study of the Water Abundance Evaluation of Roof Aquifer Based on the Development Height of Water-Conducting Fracture Zone

In the eastern mining area of Ningxia, China, multi-layered sandstone aquifers are widely distributed in the underground. However, the water abundance of these aquifers is not clear, which brings great challenges to the prevention and control of mine water disaster. In this paper, five mining areas in eastern Ningxia were taken as the study area, and the distribution characteristics of aquifer and the mine water filling source were analyzed through the analysis of geological data and numerical simulation. Finally, the lithology influencing index (LII) was constructed, and the water abundance of the aquifer was evaluated. The results show that the sandstone aquifer III in the lower part of the Jurassic Zhiluo formation is the main water-source aquifer of the #2 coal seam in the study area, while the middle aquifer II and the upper aquifer I are indirect water-source aquifers; the areas with extremely strong and strong water abundance are Maiduoshan, Hongliu, the south of Shicaocun, southeast and southwest of Meihuajing, and the central and southern areas of Shuangma mining areas; when the depth of water drainage boreholes in the roof reaches the development height of the water-conducting fracture zone, the pre-drainage measures can effectively control the mining inrush water of the working face and ensure the safety production. This research is of great significance to the prevention and control of mine water disaster.

Estimates of groundwater depletion under extreme drought in the Brazilian semi-arid region using GRACE satellite data: application for a small-scale aquifer

The temporal and spatial monitoring of groundwater levels is among the most widely used techniques for understanding groundwater reserves, which is essential for the management of regions with drought-related issues. Between 2010 and 2017, the Brazilian semi-arid region suffered a severe drought, presenting intensity and societal impacts undetected in decades. This research aimed to understand how Gravity Recovery and Climate Experiment (GRACE) data can be used as a tool for monitoring groundwater reserves in one of the most important aquifers in the Araripe Sedimentary Basin (Middle Aquifer System), located in a developing region with scarce amounts of data, and where 84,000,000 m3 of groundwater is abstracted annually through pumping. Groundwater storage (GWS) in-situ data were related to GWS estimates based on a combination of GRACE-based terrestrial water storage (TWS with both mascon and spherical harmonic solutions) and Global Land Data Assimilation System (GLDAS) soil moisture (CLM, MOS, NOAH and VIC models were evaluated). Results were analyzed with Nash-Sutcliffe (NS) and Pearson correlation coefficient metrics, and showed that the GWS GRACE-based estimate using the Community Land Model (CLM) land-surface model was more suitable for representing aquifer storage variations. Seven wells (58%) demonstrated a NS > 0.50 for both GWS GRACE-based solutions. In conclusion, GWS GRACE-based methodology has potential for monitoring the 1,394-km2 outcrop area of the Middle Aquifer System.

Age and forming conditions of groundwater in the middle Pliocene aquifer in the Mekong Delta, Vietnam, based on the results of carbon radioactive isotope study

The results of studying and assessment of the impact of climate change on the groundwater in the Mekong delta have indicated that the exploitation of quaternary aquifers is not enough to solve the task of water supply in the region. Therefore, in the exploitation plan of groundwater in the Mekong Delta, the greatest prospects are bound with Pliocene aquifers. So, by using of carbon radioactive isotope to determine the age and forming conditions of groundwater in the middle Pliocene aquifer in the Mekong Delta, the important information for planning exploitation and zoning of areas subject to restrained exploitation of groundwater is retrieved. The determination of the age with the use of carbon radioactive isotope study in the complex with the geostatistical interpolation techniques allowed creation age map of middle-Pliocene aquifer with high accuracy nad determination the recharge area on the north-east of the Mekong delta (South-Eastern area). From the recharge area, groundwater flows in the south-west direction at a flow velocity of 8 m/year and discharges into the sea (Siam Golf and Eastern sea). It is the first time by using geostatistical interpolation «cokriging» of two parameters (age - key parameter and depth of sampling - extra parameter). The tasks to improve the accuracy of specialized maps in Vietnam have been solved. The age map has been made with the correlation parameter 0,9, that justifies the high efficiency of the studies. Furthermore, results of the correlation between age and concentration chlorine of groundwater in the middle-Pliocene aquifer (n22) allowed making a suggestion, that formation of mineralized water is a result of mixing of meteoric water with the sedimentation waters of initial sea genesis, which were formed during Flandrian transgression which began about 21000 years ago and ended 4200 years ago.

Predicting aquifer characteristics to enhance hydrogeophysical model in fractured/karstified rocks of Dammam Formation at Bahr Al-Najaf Basin, Iraq

Groundwater represents a major water source in Bahr Al-Najaf basin. So, the knowledge of the aquifer parameters is an essential goal to assess and manage groundwater resources. A comprehensive view of the fractured and karstified aquifers within Dammam Formation at Bahr Al-Najaf basin is very important, because the karstified environments are very complex. This study is an attempt to extract mathematically empirical relationships between the hydraulic parameters and the geoelectrical properties, to improve the construction of a hydrogeophysical model, instead of using the conventional procedures of pumping tests or drill new well which need great effort and high cost. The enhanced hydrogeophysical model shows that the main aquifer has a uniform lithology of carbonate rocks, good water saturation and belongs to M. Dammam member. The correlation between hydraulic conductivity (K) m/day and aquifer resistivity ( $${\rho _0}$$ ) Ωm gave a logical relationship in such kind of aquifer, which has correlation coefficient (R2) equals to 0.737. In addition, the correlation between transmissivity (T m2/day) and both of transverse resistance (Tr) Ωm2 and longitudinal conductance (SL) Ω−1 revealed the similarity in lithological characteristics of middle Dammam aquifer due to the approximately equal values of strong R2 = 0.862 and 0.868, respectively. This may be due to the nature of the studied area as a discharge zone, which has a good porosity ( $$\varphi$$ ) of 22% and a high water saturation (Sw) equals to 1 approximately. The extracted relationship of direct proportionality between T (m2/day) and SL (Ω−1) is an important one. Therefore, this relationship can be used directly to calculate T (m2/day) mathematically. In the present study, the relations between the hydraulic parameters and the geoelectrical properties are expressed as linear equations instead of nonlinear, because we believe that it is mathematically more acceptable and reliable for hard rock aquifers that the M. Dammam aquifer is confined with a fully saturated thickness. In addition, the comparison between R2 of the linear and nonlinear relation of T (m2/day) with Tr (Ωm2) and T (m2/day) with SL (Ω−1) appeared to have little divergence of R2 equal to 0.028 and 0.001, respectively.

Aquifer structure identification through geostatistical integration of geological parameters: case of the Triassic sandstone aquifer system (SE Tunisia)

This paper presents an integrated modeling approach for constructing a geometric model of an Early to Middle Triassic sandstone aquifer system, commonly referred to as the Sahel El Ababsa aquifer, in southeastern Tunisia. The approach combines thorough interpretations derived from geological data along with various geostatistical methods to create accurate representations of the faulted bounding surfaces. The resulting architectural model was used to define the main aquifer system compartments and explain the nature of the hydraulic relations and communications within and beyond the aquifer system. A database was constructed based on multiple and various data sources, including water boreholes, exploration-petroleum wells, and seismic sections. These features were georeferenced and linked to the database used to build a stratigraphic framework of the area. The geological correlations based on wells information allowed a valuable observation on the structural trends and fault network as well as the lateral facies variation of the aquifer system. The derived interpreted faults were displayed on a georeferenced map which was later converted into a vertex database needed for the next modeling steps. A proper architectural modeling approach of the Early to Middle Triassic sandstone aquifer system was then performed using geostatistical methods that provide a variety of calculation techniques to model the reservoir bounding surfaces, through kriging methods including ordinary kriging and kriging with inequalities. The latter method was used to deal with the lack of data as it can retrieve information from the end of the boreholes that do not reach the surface to be calculated and integrate them in the modeling procedures. This enabled us to build the first architectural model with an accurate description of the bounding surfaces of the Early to Middle Triassic rocks. In fact, although it has been the center of geological and hydrogeological studies, the Upper Permian surface has not previously been the focus of an integrated geomodeling study. The generated models were analyzed, verified through kriging variance maps, then compared with the outcomes of the geological study. The results depict a new structural scheme of the Sahel El Ababsa area and provide an in-depth characterization of the complex compartmentalization affecting the aquifer system that allowed the definition and grouping of the hydrostratigraphic units. Therefore, this work is the first attempt through which an integrated architectural modeling approach is used to characterize and identify the Sahel El Ababsa aquifer system.

FORMATION CONDITIONS OF GROUNDWATER IN THE MEKONG DELTA (VIETNAM) BASED ON ISOTOPES RESEARCH OF OXYGEN AND HYDROGEN IN WATER

The origin and formation of the groundwater in the Mekong Delta are the complex problem, which has not one solution nowadays. So the exploitation scheme still has many limitations. In the aquifers of the Mekong Delta there are both fresh water and mineralized water, which are very complex and heterogeneous in the distribution. The mineralized water has been considered to have sedimentagenous genesis (buried seawater), and freshwater has been believed to originate from infiltration of meteoric water. Studying of the stable isotopes of oxygen and hydrogen of the groundwater in the Mekong Delta has shown that the groundwater originates mainly from the infiltration of the meteoric water. In addition, a significant factor in the formation of groundwater in the Middle, Lower Pliocene and Miocene aquifer is the mixing of the meteoric and sea waters. Increasing in values of stable isotopes with growing depth of groundwater is related with that the recharging areas of Paleogene (deep) aquifers are distributed higher by absolute depths and farther from the coastline than recharging areas of Quaternary (shallow) aquifers. The results of the research can be used to optimize the scheme for the exploitation of the fresh groundwater, limiting the intrusion of sea water in the exploited groundwater in the Mekong Delta.

HYDROGEOLOGICAL WINDOWS IN THE UPPER AQUIFER OF THE CHAD FORMATION IN THE CHAD BASIN, NORTH-EASTERN NIGERIA

Groundwater is the major source of drinking water in both urban and rural communities around Chad Basin in Northeastern Nigeria. Besides, it is an important source of water for the agricultural and the industrial sector. In this work, hydrogeological windows of the Upper aquifer subzones of the Chad Basin around Bornu sub-basin have been studied. The research involves collection of borehole logs from Borno State Water Board in which the position of each borehole located on a topographic map at scale of 1:500,000. The thicknesses of Upper aquifer subzones at different locations were screened with the top and bottom bed values subtracted from their elevations. Contours of each corresponding values were drawn and contours of recharge windows were demarcated. The results showed that the upper aquifer is divided into ‘a’, ‘b’ and ‘c’ subzones. It also revealed detailed structural behavior of hydrogeological openings that enable the penetration of water from the surface to the three subzones of the Upper aquifer through hydrogeological windows and vice versa. The upper a-subzone is connected to lower upper c-subzone in the upper aquifer that subsequently permit flow of water to the Middle aquifer. These hydrogeological windows are mostly located around Bama, Maiduguri, Damaturu, Monguno, Damasack, Ngala and Kala-Balge. Conclusively, these windows are conduits of recharge into to the upper zones and infiltrate to the Upper b-subzone of the middle aquifer and contribute to balance abstracted water in middle aquifer for economic purposes.

Determination of the origins and recharge rates of the Sfax aquifer system (southeastern Tunisia) using isotope tracers

The origin of groundwater in the Sfax aquifer system was studied using environmental isotopic tracers (δ18O, δ2H, and 3H). In total, 164 water samples were analyzed for stable isotopes: 73 collected from the Sfax shallow aquifer, 63 from the Sfax middle aquifer, and 28 from the Sfax deep aquifer. Recent recharge of the groundwater in the Sfax aquifer was identified using tritium concentrations in 82 groundwater samples from different depths. The isotopic ratios of the shallow aquifer range from −5.55 to −1.59 ‰ for δ18O and from −38.38 to −14.19 ‰ for δ2H, and the isotopic ratios in the middle aquifer range from −6.86 to −2.97 ‰ for δ18O and from −44.18 to −22.38 ‰ for δ2H. The deep aquifer exhibited markedly lower isotopic values, ranging from −6.70 to −5.70 ‰ for δ18O and from −42.40 to −38.89 ‰ for δ2H. The mixing proportions inferred from stable isotopic mass balance calculations suggest that the deep aquifer contributes significantly to the middle aquifer through geologic structures and may reach 100 % in the Menzel Chaker region. The isotopic mass balance model also indicates that the middle groundwater aquifer may contribute up to 100 % of the shallow Plio-Quaternary aquifer, particularly in the western and northeastern parts of the study area, between Bir Ali ben Khalifa and Djebeniana. The tritium data support the existence of recent recharge. The tritium and stable isotope data clearly indicate the presence of mixing processes, especially in the northwestern and coastal portions of the study area. A conceptual model is established, explaining the pressure differences that generate vertical leakage, which is a reasonable mechanism for flow between the aquifers.

Hydrogeochemical analysis of multiple aquifers in a coal mine based on nonlinear PCA and GIS

Hydrogeochemical characteristic analysis of multiple categorical aquifers, including the delineation of hydrogeochemical spatial distributions, can provide a basic understanding of the groundwater flow system necessary for the prevention and control of groundwater damage associated with coal mines. Nonlinear principal component analysis (PCA) was used to analyse the groundwater chemical characteristics of water samples from five aquifers in Gubei Coal Mine, Anhui Province, China. In particular, Ca2+, Mg2+, Na+ + K+, HCO3−, Cl−, SO42−, and pH were analysed. To analyse the spatial distribution of groundwater chemistry in this Permian fractured aquifer, interpolation of principal component 1 (PC1) scores obtained from the nonlinear PCA was performed using the inverse distance weighting (IDW) method. Additionally, concentrations of the hydrogeochemical variables were plotted against depth. The relationships between water samples, variables, and water samples with variables were simultaneously analysed by nonlinear PCA. The results show: (1) nonlinear PCA explains more variability than standard PCA in the first two principal components (PCs); (2) the Cenozoic top aquifer has low concentrations of Na+ + K+ and Cl−; the Cenozoic middle aquifer and Cenozoic bottom aquifer are characterised by high concentrations of Ca2+ and Mg2+ and low concentrations of HCO3−; the shallow groundwater of the Permian fractured aquifer is recharged by the Cenozoic bottom aquifer, while other parts have high concentrations of HCO3− and low concentrations of Ca2+ and Mg2+. The Taiyuan limestone aquifer has similar groundwater chemistry to that of the Cenozoic bottom aquifer, and thus it may be recharged by the Cenozoic bottom aquifer in outcropping Taiyuan Formation strata; (3) the wide south-western and north-western areas of the Permian fractured aquifer may be recharged by the Cenozoic bottom aquifer; and (4) concentrations of Ca2+ and Mg2+ in the Permian fractured aquifer are negatively correlated with sampling depth, while the concentrations of Na+ + K+ and HCO3− are positively correlated with sampling depth, which verifies that the shallow groundwater of this Permian fractured aquifer is recharged by the Cenozoic bottom aquifer. The findings demonstrate that combining nonlinear PCA with GIS can be a powerful approach for conducting spatial nonlinear analysis of hydrogeochemical characteristics.

Noble gases reveal the complex groundwater mixing pattern and origin of salinization in the Azraq Oasis, Jordan

Azraq Oasis in the eastern Jordanian desert is an important freshwater resource of the country. Shallow groundwater reserves are heavily exploited since the 1980s and in consequence the groundwater table dropped significantly. Furthermore, some wells of the major well field drilled into the shallow aquifer show an increasing mineralization over the past 20 years. A previous study using conventional tracers did not result in a satisfactory explanation, from where the salt originates and why only a few wells are affected. In this study, the application of dissolved noble gases in combination with other tracer methods reveals a complex mixing pattern leading to the very localized salinization within the well field. It is found that primarily the wells affected by salinization 1) contain distinctly more radiogenic 4He than the other wells, indicating higher groundwater age, and 2) exhibit 3He/4He ratios that argue for an imprint of deep fluids from the Earth's mantle.However, the saline middle aquifer below is virtually free of mantle helium, which infers an upstream from an even deeper source through a nearby conductive fault. The local restriction of the salinization process is explained by the wide range of permeabilities of the involved geologic units. As the wells abstract water from the whole depth profile, they initially pump water mainly from the well conductive top rock layer. As the groundwater table dropped, this layer fell progressively dry and, depending on the local conductivity profile, some wells began to incorporate more water from the deeper part of the shallow aquifer into the discharge. These are the wells affected by salinization, because according to the presented scheme the deep part of the shallow aquifer is enriched in both salt and mantle fluids.

Estimativa da vulnerabilidade natural à contaminação do aquífero no município de São Sepé - RS

Em consequência ao aumento significativo da urbanização e do incremento de serviços necessários ao suprimento da população, cresce a utilização de águas subterrâneas para suprir diversos usos. A partir disso, os estudos de vulnerabilidade do aquífero a contaminação tornam-se uma importante ferramenta. O presente estudo tem por objetivo principal mapear a vulnerabilidade das águas subterrâneas do município de São Sepé-Rio Grande do Sul, localizado no sul do Brasil, situado no escudo Uruguaio Sul Rio-Grandense, aquífero cristalino fissural. Para a espacialização foi utilizado o método GOD, adaptado para as condições brasileiras. Para tanto elaborou-se um banco de dados hidrogeológicos relativos a 8 captações que continham as informações necessárias à aplicação do método (grau de confinamento, meio aquífero e profundidade do nível da água). O cartograma da vulnerabilidade apontou que a classe predominante foi a baixa, equivalente a 44,56 % do total da área, seguida da classe média representada por 39,33% e da classe alta com 12,91% mapeado. As classes extrema e insignificante representaram 2,78% e 0,42% respectivamente. Esses resultados apontam a necessidade de índices de vulnerabilidade a serem considerados em licenciamento de empreendimentos potencialmente poluidores e na implantação de planos diretores.

Hydrogeochemical and isotopic studies of groundwater in the middle voltaian aquifers of the Gushegu district of the Northern region

This work is to establish the hydrochemistry and origin of groundwater in some parts of the Gushegu district of the Northern Region of Ghana. Hydrochemical data from 19 groundwater and 7 rock samples have been used to evaluate water quality, water types, and sources of various ions as well as origin of the groundwater. The study results show that the quality of groundwater from the area is generally not good due to their fluoride (F−), bicarbonate (HCO3−) and electrical conductivity (EC) concentrations. The F− contents of the groundwater have values as high as 1.97 mg/L, with 53 % of the groundwater having concentrations of F− exceeding the WHO recommended allowable limits. These high F− values have the potential of causing serious health problems such as kidney failure, dental and skeletal fluorosis, reproductive problem and reduction in intelligent quotient of consumers. A plot of Gibbs diagram reveals that rock weathering and precipitation are the major hydrogeochemical processes regulating the water chemistry of the study area. Petrographic thin-section analyses of rock samples identify minerals present to be muscovite, plagioclase feldspars, quartz, sericite and iron oxide. Stable isotope (18O and 2H) composition of the waters reveals that most of the groundwater is likely to be recharged from local precipitation, indicating their meteoric origin. Some samples, however, showed considerable evaporation.

Hydrogeochemical and Biophysical Characterization of Groundwater in Eastern Nigeria: A Case Study of Onitsha and Environs

Hydro-geological studies revealed that Onitsha and environs tapped their groundwater from the middle aquifer (> 40 m Zn > Mn > Ba > Cu. The results for the major cations analyzed shows a mean value of Na + (3.32 mg l -1 ), K + (0.8mg l- 1 ) Ca 2+ (8.62 mg l -1 ), Mg 2+ (2.57mg l -1 ), Temperature, pH, Total Dissolved Solid (TDS) and Electrical Conductivity (EC) where measured in situ, major anions where also analyzed and the results shows that the mean value for HCO 3 - (61mg l -1 ), PO 4 3- (61.5 mg l -l ), NO 3 - (21.6 mg l -1 ) , SO 4 2- (99 mg l -1 ), while Cl - (450 mg l -1 ), Correlation at P < 0.05 using micro soft excel 2007shows that metals assessed were from similar source. The ultimate result when compared with World Health Organization shows that the groundwater of the middle aquiferous horizon in Onitsha and environ in Eastern Nigeria is polluted in some areas covered in this research and not fit for consumption except when treated appropriately. http://dx.doi.org/10.4314/njt.v34i4.29

Mathematical Modeling of Bara Groundwater System Central Sudan

Groundwater has proved to be a major resource in Northern Kordofan State (NKS) in the development plans of water supply, irrigation as well as industrial sectors. Drinking water supply in the state relies on groundwater for more than 65% of the total consumption. In Bara Basin, the Cretaceous sediments extend underlying the Umm Ruwaba sediments, which are tapped by Abu Tenetin and Umm Sayala boreholes at the northern periphery of the Bara Basin (RRI, 1990). The main purpose of the modeling exercise was to assess the potential of the aquifer to satisfy the Bara well field needs. This designed model has predicted the possible drawdown for 25 years due to planned groundwater development in the project area. Other specific objectives were achieved such as: water balance calculation of the Bara basin (i.e. recharge from rainfall, leakage between upper, middle and lower aquifers, flow across the boundaries of the aquifers, water released from storage in the main aquifer due to heavy pumping), in addition to the prediction of the reactions of the hydraulic system due to groundwater withdrawal, and final determination of the safe yield of the basin.

Processes affecting geochemistry and contaminant movement in the middle Claiborne aquifer of the Mississippi embayment aquifer system

Groundwater chemistry and tracer-based age data were used to assess contaminant movement and geochemical processes in the middle Claiborne aquifer (MCA) of the Mississippi embayment aquifer system. Water samples were collected from 30 drinking-water wells (mostly domestic and public supply) and analyzed for nutrients, major ions, pesticides, volatile organic compounds (VOCs), and transient age tracers (chlorofluorocarbons, tritium and helium-3, and sulfur hexafluoride). Redox conditions are highly variable throughout the MCA. However, mostly oxic groundwater with low dissolved solids is more vulnerable to nitrate contamination in the outcrop areas east of the Mississippi River in Mississippi and west Tennessee than in mostly anoxic groundwater in downgradient areas in western parts of the study area. Groundwater in the outcrop area was relatively young (apparent age of less than 40 years) with significantly (p 50 m depth) indicated contaminant movement from shallow parts of the aquifer into deeper oxic zones. Given the persistence of nitrate in young oxic groundwater that was recharged several decades ago, and the lack of a confining unit, the downward movement of young contaminated water may result in higher nitrate concentrations over time in deeper parts of the aquifer containing older oxic water.