Characteristics of aquifer system deformation in the Southern Yangtse Delta, China

Excessive groundwater pumpage has caused regional cones of depression and severe land subsidence in the Southern Yangtse Delta, China. The characteristics of aquifer system compaction are complex because of the difference in the types, compositions and structures of the soils that the hydrostratigraphic units are composed of, and in the histories of groundwater level change the hydrostratigraphic units have experienced. Based on the data measured from extensometer groups and observation wells, the characteristics of deformation for individual hydrostratigraphic units are analyzed. The results show that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics, such as elasticity, elasto-plasticity, and visco-elasto-plasticity, at different sites of the cone of depression or in different periods. If the groundwater level rises continuously and remains constant later, the aquifers and the aquitard units that consist of hard clay primarily exhibit elastic behaviour, but the aquitard units that consist of soft clay deform plastically and by creep and exhibit visco-elasto-plastic behaviour. If the groundwater level falls but is much higher than the previous lowest value, aquifers and aquitards consisting of hard clay exhibit elasto-plastic behaviour. If the groundwater level falls below the previous lowest value, aquifers and aquitards consisting of hard clay deform plastically and by creep and exhibit visco-elasto-plastic behaviour.     

Groundwater occurrence and aquifer vulnerability assessment of Magodo Area,Lagos, Southwestern Nigeria

A combination of vertical electrical soundings (VES), 2D electrical resistivity imaging (ERI) surveys and borehole logs were conducted at Magodo, Government Reserve Area (GRA) Phase 1, Isheri, Southwestern Nigeria, with the aim of delineating the different aquifers present and assessing the groundwater safety in the area. The Schlumberger electrode array was adopted for the VES and dipole-dipole array was used for the 2D imaging. The maximum current electrode spread (AB) was 800 m and the 2D traverse range between 280 and 350 m in the east-west direction. The thickness of impermeable layer overlying the confined aquifer was used for the vulnerability ratings of the study area. Five lithological units were delineated: the topsoil, clayey sand, unconsolidated sand which is the first aquifer, a clay stratum and the sand layer that constitutes the confined aquifer horizon. The topsoil thickness varies from 0.6 to 2.6 m, while its resistivity values vary between 55.4 and 510.6 Ω/m. The clayey sand layers have resistivity values ranging from 104.2 to 143.9 Ω/m with thickness varying between 0.6 and 14.7 m. The resistivity values of the upper sandy layer range from 120.7 to 2195.2 Ω/m and thickness varies from 3.3 to 94.0 m. The resistivity of the clay layer varies from 11.3 to 96.1 Ω/m and the thickness ranges from 29.6 to 76.1 m. The resistivity value of the confined aquifer ranges between 223 and 1197.4 Ω/m. The longitudinal conductance (0.0017–0.02 mhos) assessment of the topsoil shows that the topsoil within the study area has poor overburden protective capacity, and the compacted impermeable clay layer shows that the underlying confined aquifer is well protected from contamination and can be utilized as a source of portable groundwater in the study area. This study therefore enabled the delineation of shallow aquifers, the variation of their thicknesses and presented a basis for safety assessment of groundwater potential zones in the study area.     

Geological and hydrogeological environment in Tianjin with potential geohazards and groundwater control during excavation

This paper discusses the geological and hydrogeological features of Quaternary deposits in Tianjin as well as the geohazards related to groundwater hydrology in this region. The soft soil deposits, comprising silt, sand, silty clay and clay, are composed of four aquifer groups. In the first aquifer group, one phreatic aquifer and two confined aquifers have relationships with underground construction in the urban area. These three aquifers are separated by two aquitards and collectively form a multi-aquifer system. During geotechnical construction, potential geohazards present are related to the groundwater, which include water-in-rushing, quicksand and piping hazards. To prevent the aforementioned geohazards, dewatering is conducted; however, groundwater pumping may result in large settlements of the surrounding ground. To reduce pumping-induced settlement, the dewatering–waterproofing system has been adopted. According to the characteristics of the subsoil, excavation depth and the surrounding environment, the dewatering system can be divided into five patterns. In the first four patterns, when pumping is conducted in the excavation pit, the groundwater head in the adjacent aquifers outside the pit decreases due to the leakage effect of the aquitards located between the aquifers. In the fifth pattern, waterproof curtain has cut off the aquifers completely and dewatering in the pit cannot result in settlement around excavation pit. To avoid geohazards related to groundwater hydrology, countermeasures recommended include construction of an effective waterproof curtain, selection of a reasonable excavation dewatering pattern and withdrawal of required groundwater.     

Mapping groundwater conditions in different geological environments in the northern area of UAE using 2D earth resistivity imaging survey

A water resources database was developed and used to characterize the geological and hydrogeological settings of groundwater aquifers in the northern part of the United Arab Emirates. An intensive 2D Earth Resistivity Imaging (ERI) survey was conducted in selected areas to assess the available groundwater resources and delineate the salt-water intrusion. Drilling information of the existing monitoring wells as well as their records of water table elevations and groundwater salinity were used to measure the horizontal and vertical variations in lithology, degree of saturation, and groundwater salinity and thus to improve the interpretation of ERI data. The results of the chemical analyses of water samples collected from the wells along with the 2D ERI survey profiles were used to obtain an empirical relationship between the inferred earth resistivity and the amount of total dissolved solids. This relationship was used along with the true resistivity sections resulting from the inversion of 2D ERI data to identify and map three zones of water-bearing formation (fresh, brackish, and salt-water zones) in the coastal areas. The results indicated that the depth to the fresh-brackish interface exceeded 50 m at the upstream of the wadies and was in the order of 10 m or less in the vicinity of shoreline. Because of the high resistivity contrast between dry and water saturated fractured rocks; this method was very successful in detecting water-filled fractures and cavities in the carbonate aquifer. The application of this method was unsuccessful in clayey aquifers as it was not possible to isolate lenses of gravel and/or saturated with saline or brackish water from the surrounding clayey materials.     

Hydrogeophysical investigations for assessing the groundwater potential in part of the Peshawar basin,Pakistan

A large number of valleys and basin systems are present in the northwestern part of the Himalayas in Pakistan which form significant aquifers in the region. Hydrogeophysical investigations in the western part of Nowshera District, a part of the intermontane Peshawar basin, were undertaken to help to determine the availability of groundwater resources in the region. Thirty vertical electrical resistivity soundings (VES) were acquired using a Schlumberger expanding array configuration with a maximum current electrode spacing (AB/2) of 150 m in delineating the groundwater potential in the study area. The results of the interpreted VES data using a combination of curve matching technique and computer iterative modeling methods suggest that the area is underlain by 3 to 5 geo-electric layers. The interpretation results showed that the geo-electrical succession consists of alluvium comprising of alternating layers of clay, silty clay, fine to coarse sands, sand with gravels and gravels of variable thickness. High subsurface resistivity values are correlated with gravel–sand units and low resistivity values with the presence of clays and silts. The modeled VES results were correlated with the pumping tests results and lithological logs of the existing wells. The pumping test suggests the transmissivity of the aquifer sediments is variable corresponding to different sediments within the area. The gravel–sand intervals having high resistivity value show high transmissivity values, whereas clay–silt sediments show low transmissivities. It is concluded that majority of the high resistive gravel–sand sediments belong to an alluvial fan environment. These gravel–sand zones are promising zones for groundwater abstraction which are concentrated in the central part of the study area.     

Geoelectric model and hydrochemistry of salinity affected lower Atrai floodplain aquifer,NW Bangladesh: An approach for irrigation management

In the salinity affected lower Atrai floodplain aquifer in the NW Bangladesh, geoelectric resistivity survey and hydrochemical analysis are carried out with an aim to identify fresh and saline groundwater zones; investigate the status of salinity; evaluate hydrochemical processes involved and suggest management approaches for irrigation. Here a two-fold aquifer system, inter-layered by silt, clay and silty-clay aquitard and aquiclude is classified as: upper aquifer — spatially affected by salinity of varying degrees; and lower aquifer — generally characterized by high salinity. The aquifer with resistivity values greater than 69 Ωm is safe for irrigation use. Concentrations of major ions vary as: Ca²⁺>Na⁺>Mg²⁺>K⁺ and HCO₃>Cl>NO₃>SO₄ ^2?. Groundwater is dominated by Na-Ca to Ca-Na, HCO₃-Cl-SO₄, Cl-SO₄-HCO₃ and Cl-SO₄ ^2? facies where Ca²⁺, Mg²⁺, SO₄ ^2?, HCO₃ ^?, Cl^? and NO₃ ^2? ion concentrations are statistically dominant and water is of Ca-Mg, HCO₃-SO₄-Cl and NO₃ types. Geochemically, groundwater is hard and saline to fresh water type. Salinity increases with depth, but spatially towards the southern part. Groundwater quality is a product of water-rock interaction, direct mixing and marine spraying, or fall-out of airborne marine salts, where silicate weathering is the primary source of bivalent cations. Sediment provenance of alkaline earth silicates and higher concentrations of alkalis are derived from sources other than precipitation. In general partially or fully salinity affected upper and lower aquifers in the area except in its eastern part are not suitable for tubewell irrigation. As groundwater demand for irrigation is increasing, the saline water has progressively invaded relatively fresher parts of the aquifer by upconning. So, special salinity control management approaches can be adopted through engineering techniques such as groundwater abstraction optimization, as also through scientific behavioral approaches like groundwater demand management, salt tolerant crops production. In this context, surface water conservation and rain water harvesting for domestic and irrigational uses are recommended in the salinity affected area.     

Delineation of saltwater intrusion into the freshwater aquifer of Lekki Peninsula,Lagos, Nigeria

Recently, the deterioration of water quality in the coastal zones of Lekki Peninsula area of Lagos due to saltwater infiltration into the freshwater aquifer has become a major concern. With the aim of providing valuable information on the hydrogeologic system of the aquifers, the subsurface lithology and delineating the groundwater salinity, vertical electrical resistivity (VES) sounding survey was carried out utilizing surface Schlumberger electrode arrays, and electrode spacing varying between 1 and 150 m. The DC resistivity surveys revealed significant variations in subsurface resistivity. Also, the VES resistivity curves showed a dominant trend of decreasing resistivity with depth (thus increasing salinity). In general, the presence of four distinct resistivity zones were delineated viz.: the unconsolidated dry sand (A) having resistivity values ranging between 125 and 1,028 Ωm represent the first layer; the fresh water-saturated soil (zone B) having resistivity values which correspond to 32–256 Ωm is the second layer; the third layer (zone C) is interpreted as the mixing (transition) zone of fresh with brackish groundwater. The resistivity of this layer ranges from 4 to 32 Ωm; while layer four (zone D) is characterized with resistivities values generally below 4 Ωm reflecting an aquifer possibly containing brine. The rock matrix, salinity and water saturation are the major factors controlling the resistivity of the formation. Moreover, this investigation shows that saline water intrusion into the aquifers can be accurately mapped using surface DC resistivity method.     

Groundwater resources evaluation in calcareous limestone using geoelectrical and VLF-EM surveys (El Salloum Basin,Egypt)

Understanding and developing groundwater resources in arid regions such as El Salloum basin, along the northwestern coast of Egypt, remains a challenging issue. One-dimensional (1D) electrical sounding (ES), two-dimensional (2D) electrical resistivity imaging (ERI), and very low frequency electromagnetic (VLF-EM) measurements were used to investigate the hydrogeological framework of El Salloum basin with the aim of determining the potential for extraction of potable water. 1D resistivity sounding models were used to delineate geoelectric sections and water-bearing layers. 2D ERI highlighted decreases in resistivity with depth, attributed to clay-rich limestone combined with seawater intrusion towards the coast. A depth of investigation (DOI) index was used to constrain the information content of the images at depths up to 100 m. The VLF-EM survey identified likely faults/fractured zones across the study area. A combined analysis of the datasets of the 1D ES, 2D ERI, and VLF-EM methods identified potential zones of groundwater, the extent of seawater intrusion, and major hydrogeological structures (fracture zones) in El Salloum basin. The equivalent geologic layers suggest that the main aquifer in the basin is the fractured chalky limestone middle Miocene) south of the coastal plain of the study area. Sites likely to provide significant volumes of potable water were identified based on relatively high resistivity and thickness of laterally extensive layers. The most promising locations for drilling productive wells are in the south and southeastern parts of the region, where the potential for potable groundwater increases substantially.

     

Hydrochemical and statistical studies of the groundwater salinization in Mediterranean arid zones: case of the Jerba coastal aquifer in southeast Tunisia

Coastal aquifers are considered as major sources for freshwater supply worldwide, especially in arid zones. The weak rainfall as well as the intensive extraction of groundwater from coastal aquifers reduce freshwater budget and create local water aquifer depression, causing both seawater intrusion and a threat to groundwater. This phenomenon was observed in the Jerba Island which is located in southeast Tunisia. Jerba??s unconfined aquifer shows high values of groundwater salinity reaching, locally, 17?g/l and a strong contrast between some zones of the aquifer. High pumping rates and weak recharge disturb the natural equilibrium between fresh and saline water causing water salinization in most areas of the island. This study aims at establishing the salinity map of the aquifer and identifying the origin of groundwater salinization. The salinity map shows that zones characterized by low groundwater salinity are located in the center of the study area. High groundwater salinities are observed near the coast and in some parts having low topographic and piezometric levels. Groundwater geochemical characterization, and Br/Cl and Na/Cl ratios suggest that the origin of abnormal salinity is seawater intrusion. Considering groundwater salinity values and Br concentrations, a seawater intrusion map is established. It shows that many areas of the unconfined aquifer are contaminated by mixed groundwater and seawater. The statistical analysis demonstrates that high mineralization of the groundwater is due to gypsum and carbonate dissolution coupled with the mixed groundwater and seawater in many areas.     

Modelling an inland Delta aquifer system to evolve pre-development management schemes: a case study in Upper Thamalakane River valley, Botswana, southern Africa

 Groundwater modelling studies have been found to be a potential tool in planning the pre-development management of groundwater resources in newly developing aquifer systems. One such study was attempted in Upper Thamalakane River valley, Okavango Delta, Botswana (southern Africa). There are three major aquifers separated by two aquitards in the valley portion. The top two aquifers are freshwater bearing zones and the bottom one is saline. The hydrological set-up of the basin is complex, as the groundwater flow directions are opposite in the upper-unconfined and in the lower-confined aquifers. A preliminary multilayer model was developed for this aquifer system by making use of only available data. The hydrodynamic behavior was then studied under two prediction scenarios to evolve appropriate management decisions for locating the well field (large diameter wells) in the upper aquifer by making use of induced river infiltration during the flood season. The aquifer response for variable river-flow conditions was studied and the induced river infiltration was quantified. Received: 27 August 1998 · Accepted: 8 March 1999     

Assessing aquifer vulnerability to pollutants by electrical resistivity tomography (ERT) at a nitrate vulnerable zone in NE Spain

There is an increasing demand for groundwater vulnerability maps which illustrate the exposure of aquifers against pollution. These maps show areas of greatest potential for groundwater contamination on the basis of local subsurface conditions. Parameters affecting vulnerability are mainly permeability and thickness of each protective layer. For unconsolidated sediments, the permeability is strongly related to the clay content, which can be deduced from indirect resistivity methods, like electrical-imaging. Such geophysical methods can be of great help in groundwater vulnerability studies because they disturb neither the structure nor the dynamics of the soil. Sensibility analysis was performed of the electrical resistivity tomography method for accurately mapping soil media. Managers and public administrators may effectively use this method for assessing the potential risk of groundwater contamination. In the studied zone, electrical resistivity exhibits a wide range of variability that can be easily correlated to soil parameters, such as clay content and hydraulic conductivity. A numerical index of protection has been assessed from the geophysical information derived from 2D electrical resistivity tomography. This work represents a preliminary approach on the natural vulnerability evaluation of shallow aquifers at the Empordà basin (NE Spain) that is highly affected by diffuse pollution by nitrates.     

Geophysical,geochemical and hydrological analyses of water-resource vulnerability to salinization: case of the Uburu-Okposi salt lakes and environs,southeast Nigeria

Until this study, the location and depth of the saline units in Uburu-Okposi salt lake areas and environs have been unknown. This study aimed at delineating the saline lithofacies and dispersal configurations to water bodies, using electrical geophysical methods such as constant separation traversing (CST) and vertical electrical sounding (VES). Results showed weathered zones that represent aquifers mostly at the fourth geoelectric layer: between upper layered aquitards and underlying aquitards at depths 30–140 m. Lateral distribution of resistivity variance was defined by the CST, whereas the VES tool, targeted at low-resistivity zones, detected isolated saline units with less than 10 ohm-m at depths generally >78 m. The saline lithofacies were suspected to link freshwater zones via shear zones, which steer saline water towards the salt lakes and influence the vulnerability of groundwater to salinization. The level of salinization was verified by water sampling and analysis, and results showed general alkaline water type with a mean pH of 7.66. Water pollution was indicated: mean total dissolved solids (TDS) 550 mg/l, electrical conductivity (EC) 510 μS/cm, salinity 1.1‰, Cl^? 200 mg/l, N0₃ ^?35.5 mg/l, Na⁺ 19.6 mg/l and Ca²⁺ 79.3 mg/l. The salinity is controlled by NaCl salt, as deduced from correlation analysis using the software package Statistical Product for Service Solutions (SPSS). Generally, concentrations of dissolved ions in the water of the area are enhanced via mechanisms such as evaporation, dissociation of salts, precipitation run off and leaching of dissolved rock minerals.     

Deep resistivity sounding studies for probing deep fresh aquifers in the coastal area of Orissa, India

Exploration and exploitation of groundwater in sedimentary areas are reasonably simple. However, the problem of salinity in coastal areas makes the job very difficult, especially when the freshwater aquifers are not extensive and are entrapped between saline aquifers. States along the eastern coast of India, particularly Orissa with respect to the Mahanadi basin, have acute problems with groundwater salinity. It has been possible to locate horizons of fresh groundwater entrapped between deep saline aquifers in the southwestern part of Mahanadi delta, with the help of deep resistivity soundings along the Delang-Puri profile. This finding has been validated through boreholes and checked with electrical logs of this region. Three freshwater aquifers have been detected: one at shallow depth between 20 and 60?m, the second in the depth range of 90??60?m, and the third in the fractured/weathered basement. The second freshwater aquifer has the most potential; it has a thickness range of 20??0?m and it could be exploited to overcome problematic salinity issues. In general, the depth to basement is variable and it increases seaward.     

The relationship between the water table and the surface flow of a losing stream, lower Medano Creek, Great Sand Dunes National Monument, Colorado

 Proposed groundwater withdrawals in the San Luis Valley of Colorado may lower the water table in Great Sand Dunes National Monument. In response, the National Park Service initiated a study that has produced a generalized conceptual model of the hydrologic system in order to assess whether a lowering of the water table might decrease the surface flow of lower Medano Creek. Based upon information obtained during the drilling of several boreholes, there appear to be five important hydrostratigraphic units underlying lower Medano Creek within the upper 30 m of the ground surface: 1. a perched aquifer overlying an aquitard located between about 5 and 6 m below the ground surface; 2. the aquitard itself; 3. an unconfined aquifer located between the upper and lower aquitards; 4. an aquitard located between about 27 and 29 m below the ground surface; and 5. a confined underlying the lower aquitard. Because the areal extent of the aquitards cannot be determined from the borehole data, a detailed conceptual model of the hydrogeologic system underlying lower Medano Creek cannot be developed. However, a generalized conceptual model can be envisioned that consists of a complex system of interlayered aquifers and leaky aquitards, with each aquifer having a unique hydraulic head. Water levels in the perched aquifer rise rapidly to their annual maximum levels in response to the arrival of the flow terminus of Medano Creek during the spring runoff event, and the location of the flow terminus is directly dependent upon the discharge of the creek. Water levels in the deeper, non-perched aquifers do not appear to fluctuate significantly in response to the arrival of the flow terminus, demonstrating that it is unlikely that the proposed groundwater withdrawals will decrease the surface flow of lower Medano Creek. Received: 27 December 1995 · Accepted: 20 February 1996     

Application of a revised hydrostratigraphical classification and nomenclature to the Mesozoic and Cenozoic succession of Saudi Arabia

At the dawn of the 21st century many parts of the world are suffering a shortage of water resources. Arid and semi-arid areas in particular are facing challenges and increasing pressure is being put on their groundwater management plans. Such a situation created an urgent need to put forward a design for classification and nomenclature that can help to differentiate highly productive zones from small locally exploited productive zones. A previously proposed hydrostratigraphical classification and nomenclature scheme is revised, modified, and applied to the Mesozoic-Cenozoic sedimentary succession of Saudi Arabia. The scheme utilised in the present paper is modified to include, in addition, productivity, water quality criteria, usability, and recharge as criteria reflecting the hydrogeological importance of the productive zones.The Triassic Sudair Mega-aquitard forms the lower hydrostratigraphic boundary for the Ad-Dahna'a Aquasystem. The Ad-Dahna'a Aquasystem comprises two aquagroups: the Riyadh and Rub'Al-Khali Aquagroups. Each aquagroup is in turn subdivided into superaquifers, aquifers and possibly subaquifers, separated by aquitards of different ranks. The physical and hydrogeological characteristics of each unit are also discussed.     

GIS-based subsurface databases and 3-D geological modeling as a tool for the set up of hydrogeological framework: Nabeul–Hammamet coastal aquifer case study (Northeast Tunisia)

The subsurface data are a basic requirement for the set up of hydrogeological framework. Geographic information systems (GIS) tools have proved their usefulness in hydrogeology over the years which allow for management, synthesis, and analysis of a great variety of subsurface data. However, standard multi-layered systems are quite limited for modeling, visualizing, and editing subsurface data and geologic objects and their attributes. This paper presents a methodology to support the implementation of hydrogeological framework of the multi-layered aquifer system in Nabeul–Hammamet (NH) coastal region (NE, Tunisia). The methodology consists of (1) the development of a complete and generally accepted hydrogeological classification system for NH aquifer system (2) the development of relational databases and subsequent GIS-based on geological, geophysical and hydrogeological data, and (3) the development of meaningful three-dimensional geological and aquifer models, using GIS subsurface software, RockWorks 2002. The generated 3-D geological models define the lithostratigraphy and the geometry of each depositional formation of the region and delineate major aquifers and aquitards. Where results of the lithologic model revealed that there is a wide range of hydraulic conductivities in the modeled area, which vary spatially and control the groundwater flow regime. As well, 17 texturally distinct stratigraphic units were identified and visualized in the stratigraphic model, while the developed aquifer model indicates that the NH aquifer system is composed of multi-reservoir aquifers subdivided in aquifers units and separated by sandy clay aquitards. Finally, this study provides information on the storing, management and modeling of subsurface spatial database. GIS has become a useful tool for hydrogeological conceptualization and groundwater management purposes and will provide necessary input databases within different groundwater numerical models.     

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.     

Origin,lithology and weathering characteristics of Upper Tertiary ‐ Quaternary clay aquitard units on the Lower Murrumbidgee alluvial fan

The Lower Murrumbidgee alluvial fan at the eastern edge of the Murray Basin is comprised of high‐yielding coarse‐grained aquifers and interlayered fine‐grained deposits that exert an important control on recharge and vertical leakage of contaminants such as salt. Concerns over increasingly saline shallow groundwater, particularly in irrigation areas, has focused investigations on the depositional origin and spatial distribution of these fine‐grained deposits (aquitard units), which may constitute both a source of leachable salt and a barrier to leakage. Detailed laboratory analysis of a minimally disturbed core to 83m depth, obtained from a drillhole adjacent to an irrigation bore, was augmented with geophysical investigations from this and other boreholes near the apex of the alluvial fan. Previously mapped clay units (aquitards) are redefined as clayey silts based on clay content variation between 10% and 30%. Mineralogical and lithostratigraphic evidence for three clayey silt units is presented: a lower unit (75–83m), a middle unit (45–64m) and an upper unit (0–16m). Electrical image surveys indicate that the upper unit is discontinuous, interrupted by large palaeodrainage features probably containing sands and gravels. These palaeodrainage channels are buried beneath a veneer of clay and significantly increase recharge and leakage. Some evidence suggests an aeolian component near the surface and within the middle clayey silt unit. However, mixing with fluvial deposits and subsequent weathering has also occurred. The clayey silt units are extensively weathered and oxidised, with the degree of oxidation increasing towards the surface and adjacent to aquifers saturated with oxygenated groundwater. Post‐depositional weathering of the middle and lower units may also have been associated with leaching of salts. No salt remains in the middle and lower units, but 10.2kg/m² is stored within 15m of the surface at the Tubbo site. The upper clayey silt unit is a significant source of leachable salt, which is associated with increasing shallow groundwater salinity at some sites.     

Salinity mapping of coastal groundwater aquifers using hydrogeochemical and geophysical methods: a case study from north Kelantan,Malaysia

Integrated hydrogeochemical and geophysical methods were used to study the salinity of groundwater aquifers along the coastal area of north Kelantan. For the hydrogeochemical investigation, analysis of major ion contents of the groundwater was conducted, and other chemical parameters such as pH and total dissolved solids were also determined. For the geophysical study, both geoelectrical resistivity soundings and reflection seismic surveys were conducted to determine the characteristics of the subsurface and groundwater contained within the aquifers. The pH values range from 6.2 to 6.8, indicating that the groundwater in the study area is slightly acidic. Low content of chloride suggests that the groundwater in the first aquifer is fresh, with an average concentration of about 15.8 mg/l and high geoelectrical resistivity (>45 ohm m). On the other hand, the groundwater in the second aquifer is brackish, with chloride concentration ranging from 500 mg/l to 3,600 mg/l and very low geoelectrical resistivity (<45 ohm m) as well as high concentration of total dissolved solids (>1,000 mg/l). The groundwater in the third aquifer is fresh, with chloride concentrations generally ranging from 2 mg/l to 210 mg/l and geoelectrical resistivity of greater than 45 ohm m. Fresh and saltwater interface in the first aquifer is generally located directly in the area of the coast, but, for the second aquifer, both hydrogeochemical and geoelectrical resistivity results indicate that the fresh water and saltwater interface is located as far as 6 km from the beach. The considerable chloride ion content initially suggests that the salinity of the groundwater in the second aquifer is probably caused by the intrusion of seawater. However, continuous monitoring of the chloride content of the second aquifer indicated no significant changes with time, from which it can be inferred that the salinity of the groundwater is not affected by seasonal seawater intrusion. Schoeller diagrams illustrate that sulphate concentrations of the groundwater of the second aquifer are relatively low compared to those of the recent seawater. Therefore, this result suggests that the brackish water in the second aquifer is probably from ancient seawater that was trapped within the sediments for a long period of time, rather than due to direct seawater intrusion.     

The interference of a deep thermal system with a shallow aquifer: the case of Sodere and Gergedi thermal springs, Main Ethiopian Rift, Ethiopia

An integrated survey program involving geological, hydrogeological and geophysical techniques has been employed to characterize the aquifer geometry, recharge and circulation dynamics of thermal springs within a shallow aquifer system in Ethiopia. The selected springs for the case study are Sodere and Gergedi, which are situated within the tectonically active Main Ethiopian Rift (MER). Geologically, the studied springs are located on Plio-Quaternary volcanic rocks. The geophysical results indicate the presence of subsurface weak zones represented by extensional tectonics and weathering zones which are responsible for thermal water circulation and facilitate recharge from the adjacent surface-water bodies. The structures inferred by the resistivity survey, both sounding and electrical tomography, present contrasts in rock resistivity response. The anomalous zones in the magnetic data are in good agreement with the zones that are revealed by geological mapping and surface manifestation of the thermal water discharge zones. The shallow aquifer of the central MER is under the influence of thermal water, which increases the groundwater temperature and mineral content.