Transverse Unit Resistance Research Articles

Integrated hydrogeophysical and GIS based demarcation of groundwater potential and vulnerability zones in a hard rock and sedimentary terrain of Southern India

This research has identified the groundwater potential and vulnerability zones in Tiruchirappalli district of Tamil Nadu, India. The Schlumberger electrode array has been used to conduct vertical electrical sounding (VES) at 95 sites with a maximum electrode spacing of 150 m. The study area comprises of hard rock and sedimentary formations. Geographical Information System (GIS) has been used to integrate the geoelectrical data and to prepare spatial variation maps for various parameters. Finally, groundwater potential and vulnerability zones have been demarcated, and these outputs have been validated using water level and nitrate data, respectively. The Dar-Zarrouk parameters such as longitudinal conductance (S), transverse unit resistance (T), and aquifer anisotropy (λ) have been used along with the spatial variation of resistivity and aquifer thickness to find out groundwater potential areas with the support of GIS. The thickness of topsoil, weathered zone and fractured zone are not uniform in the research area. Top soil plus weathered zone acts as a water table (phreatic) aquifer, which extends up to 38 m from the surface. Fractured zone extends up to 45 m, which acts as a kind of confined/semi-confined aquifer. Open and bore wells have been constructed to tap groundwater from the unconfined (water table) and confined/semi-confined aquifers, respectively. High to very high groundwater potential areas are associated with low resistivity, high thickness, low longitudinal conductance, high transverse unit resistance and high aquifer anisotropy areas. Very high groundwater potential areas are mostly confined to flood plain (alluvium) deposits in the central portion of the study area. High potential areas are noticed in the northern part, whereas low potential areas are noticed in the southern part. The areas with high longitudinal conductance indicate low permeable zones with less possibility of external pollution. Since agriculture is an important activity in the study region, this work will be useful to provide water supply for irrigation as well as for domestic needs.

Groundwater Potential Evaluation in Parts of Southwestern Nigeria Using Dar-Zarouk Parameters

Dar-Zarouk parameters namely longitudinal conductance, transverse unit resistance, longitudinal resistivity, traverse resistivity, and coefficient of anisotropy, derived from electrical resistivity soundings, were employed in the western and south western portions of Sheet 223 Ilorin NW, Southwestern Nigeria to evaluate the subsurface water prospect. This is aimed at determining the potentiality and vulnerability of groundwater in the area. The longitudinal conductance values obtained range from 0.027 S at VES 3 in the north, indicating the poorest protective capacity, to 26440.95 S at VES 157 in the south, indicating the highest protectivity. Thus, the protective capacity rating of the study area shows very poorly, weak, moderate, good, and excellent ratings at VES stations 10, 12, 9, 2, and 178 respectively. The total transverse resistance range is 8.6 – 32733.87 Ωm2, with the lowest at VES 206 in the southeastern part, indicating high prospects, and the highest at VES 47 in the northern part of the area, indicating poor prospect. The lowest longitudinal resistivity (0.1951 Ωm), indicating high potential as well, occurs at the southern part and while the highest value of 8095.63 Ωm is obtained at the northern part.Furthermore, the coefficient of anisotropy in the area ranges between 0.029 and 5349.78. The south-western boreholes have values that fall between the standard range of 1.39-1.66, indicating that those areas have boreholes with high productivity. In conclusion, most parts of the study area have excellent protective capacities and high potentiality.

Investigation of groundwater resources using electrical resistivity sounding and Dar Zarrouk parameters for Uthal Balochistan, Pakistan

Uthal is a small town in the Balochistan province of Pakistan. It has an arid to semi-arid climate and groundwater is the primary source of freshwater for domestic and agricultural uses. To evaluate groundwater's nature, an electrical resistivity survey (ERS) was conducted in Uthal. Groundwater investigation for 25-point data was carried out with the PASI Electrical Resistivity Imaging System, using the Schlumberger array. The data was acquired with a maximum current electrode spacing of 300 m. The geoelectrical parameters measured were analyzed by IPI2win software by curve matching, and five geoelectrical layers were identified, and the Dar Zarrouk parameters were calculated. The top thinnest layer of unconsolidated material had a resistivity between 600 and 2600 Ωm. The second layer was of unconsolidated sand and gravel with a resistivity of 50–11,500 Ωm; the third with dry sand and clay had a resistivity of up to 200 Ωm; the fourth showed a thick freshwater zone with a resistivity between 75 and 100 Ωm followed by a thick layer of sand and clay with fresh water had resistivity between 48 Ωm and 90 Ωm. The Dar Zarrouk parameters, longitudinal unit conductance S is from 0.1 to 1.65 mho; transverse unit resistance T greater than 25,000 Ωm2; longitudinal resistance RS is from 50 to 1350 Ωm and transverse resistivity RT from 22 to 1150 Ωm. ERS data and Dar Zarrouk parameters showed that the study area has freshwater resources in the subsurface. Saline water is present in isolated locations in small pockets, resulting from dry surface conditions. The results of ERS data and Dar Zarrouk parameters show no occurrence of seawater intrusion.

Integrated Geophysical and Hydrogeochemical Characterization and Assessment of Groundwater Studies in Adum West Area of Benue State,Nigeria

Integration of geophysical and hydrogeochemical methods has been scientifically proven to be useful in vulnerability study and groundwater characterization. Subsurface geoelectric parameters such as resistivity and thickness obtained from geophysical method (Vertical Electrical Sounding VES) was used to determine aquifers vulnerability, longitudinal resistance (ρL) and transverse unit resistance (Rt). Thirty four water samples were collected from groundwater sources for physicochemical analysis.Estimated results from longitudinal conductance (S), (Rt) and (ρL) showed that the values ranges from 0.03 to 2.5mhos, 103.64 to 1964417.8 Ω/m2 and 215.41 to 65731.68 Ω-m respectively. Result from S suggested that 50 % of groundwater is considered to be vulnerable to contamination from the earth surface, while the remaining 50 % is considered to be slightly vulnerable to surface contamination. Further findings obtained from hydrogeochemical analysis such as Gibb’s and Chadba plots revealed that groundwater is highly influenced by rock water interaction,groundwater is classified to be Na+ + HCO3¯, Ca2+ + Mg2++ HCO3¯ , Na+ + Cl¯ and Ca2+ + Mg2+ + Cl¯ water type. Deduction from Soltan classification suggested that groundwater is classified to be of Na+ ˗ HCO3¯ and Na+ ˗ SO4 2¯ water type. Results obtained from Ec and pH suggested that the values were below WHO permissible limit, while result obtained from TDS showed that at some sampling points TDS values were above WHO limit. Based on pH value obtained groundwater within the study area fell within slightly basic to acidic.

Delineating saline and fresh water aquifers in Tuticorin of southern India by using geophysical techniques

Geo-electrical survey was conducted in a highly industrialized area to delineate the freshwater and saline water aquifers. A total of 30 vertical electrical soundings (VES) were made along the coastal aquifers of Tuticorin, which covers both sedimentary formation and crystalline rocks. By using VES data, GIS was used to create spatial maps for resistivity and thickness at various depths. The results show that Q and H were the dominant curve types in the study area. Q type curves indicate saline water by decreasing resistivity values with respect to depths. In H type curves, low resistivity saline water aquifer was sandwiched between high resistivity topsoil and hard rock at the deeper horizon. The geophysical indicators such as Dar-Zarrouk parameters were calculated using the VES results. The Longitudinal unit conductance (S) values ranged between 0.075 and 7500 S. The transverse unit resistance (T) of the study area varied from 16.06 to 5909.25 Ωm2. The anisotropy (λ) values ranged from 1.00 to 55. Interpretation of data reveals that the saline water intrusion and industrial impact on shallow aquifers were observed in the coastal region of the study area. Few pockets of the freshwater aquifer were also noted in deeper horizons. Since the quality of groundwater was deteriorating, proper management plans will be needed to save this fragile coastal aquifer.

Modeling of groundwater yield by the electrical method case of the Triassic sandstone aquifer (Tataouine South-Eastern Tunisia)

Abstract The method of electrical resistivity has proven very effective in the evaluation of groundwater. This specialized technique uses Dar-Zarrouk (D-Z) parameters in the estimation of longitudinal unit conductance, transverse unit resistance, and longitudinal resistivity to examine the groundwater level, to distinguish the fresh, brackish, and saline water interface, and to assess the storage capacity of groundwater in the Triassic sandstone aquifer system in the Tataouine region (South-Eastern Tunisia). In this context, 23 vertical electrical soundings (VESs) were carried out in the Tataouine region using the Schlumberger configuration with a current electrode with a maximum spacing of the current electrodes (AB) of 500–600 m. The results indicate that the study area consists of three types of aquifers: (i) silt/clay saline water (<20 Ωm), (ii) a mixture of sand and clay freshwater (20–40 Ωm), and (iii) sand freshwater (40–200 Ωm). These sand freshwater aquifers are characterized by low longitudinal unit conductance (0–2.8 S), high values of transverse unit resistance (more than 9,000 Ωm2), and longitudinal resistivity (more than 35 Ωm) and are mainly concentrated in the north, south, and south-west regions of the study area. It should also be noted that the coefficient of anisotropy (λ) overlaps and does not clearly differentiate the characteristics of the aquifers of fresh, brackish, and saline water. An interpretation of VESs can also determine the storage capacity of groundwater by determining yield index values. Groundwater supply for the entire study area was classified as low yield, with a percentage of 13% and a maximum of 31% of the study area and 56% of moderate yield. Lastly, the real data from the drilling confirm all these results presented previously. The findings suggest that D-Z parameters are useful for making a distinction of various aquifer zones.

Application of geographical information system to geoelectrical data for evaluation of the vulnerability of aquifers in parts of Bauchi, Northeastern Nigeria

The poor quality and inconsistent nature of most surface water supply in arid and semi-arid regions of the world has increased the reliance on groundwater resources. However, the susceptibility of groundwater to anthropogenic pollution in most cities and rural communities in Nigeria has become a major source of concern. In this study, the protective capacity of the aquifers in parts of the Bauchi State, Northeastern, Nigeria, from surface contamination was investigated in light of the geoelectrical characteristics of the overburden materials for sustainable management of the resources. This study applied a geographical information system (GIS) technique to secondary geoelectrical parameters (Dar-Zarrouk) derived from 94 vertical electrical soundings (VES) to delineate aquifer vulnerability zones. Five different thematic maps (topsoil longitudinal unit conductance, total overburden longitudinal unit conductance, total transverse unit resistance, coefficient of anisotropy, and total overburden isopach) were developed. Suitable weights were assigned to the thematic maps based on their relative influence on the protection of groundwater systems in the area. Subsequently, the ranks of the subclasses of the themes were reclassified based on their respective suitability before the thematic maps were integrated using ArcGIS 10.3 software to generate the overall aquifer protective capacity map of the study area. To validate the map, water samples were collected from 40 wells and trace elements (e.g., Ni, Cu, Sb, Cr, and As) were analyzed. The study area was categorized into three aquifer vulnerability zones: moderate (32.4%), weak/fair (54.5%), and low/poor (13.1%). Moderate zones with limited portions of fair/weak zones were the predominant zones within the basement setting. This reflects the occurrence of clayey saprolitic units within the basement terrain ( 15,000 Ωm2). Over 60% of the basement settings are characterized by moderate aquifer protective capacity. On the other hand, the sedimentary zones due to weak and poor protective capacity zones are more vulnerable to pollution. The trace elemental analysis shows the predominance of As, Cu, and Sb in the sedimentary zones as compared to the basement setting which corroborates the aquifer protective capacity map produced.

Assessment of Groundwater Resources in Coastal Areas of Pakistan for Sustainable Water Quality Management Using Joint Geophysical and Geochemical Approach: A Case Study

Delineation of fresh/saline groundwater is essential for sustainable water quality management, especially in the coastal areas all around the globe. Seawater intrusion causes substantial degradation in quality of freshwater resources in the coastal areas. The main reason for saltwater intrusion is the changing environment in terms of sea-level rise, climate change, and over-extraction of freshwater resources to meet the growing demands. In this study, an integrated approach of geophysical and geochemical methods was used to assess saltwater intrusion in the coastal areas of Bela Plain, Pakistan. The inverted electrical resistivity computed from 50 vertical electrical sounding (VES) constrained the subsurface into five layers and two aquifers through 3D imaging, such as silty clay and sandy clay containing saline water, and sand, sandy gravel, and gravel containing freshwater. However, the narrow range of resistivity values shows an overlap of saline/fresh groundwater. Such ambiguity in the resistivity interpretation was removed by Dar-Zarrouk (D-Z) parameters. D-Z parameters, namely transverse unit resistance (Tr), longitudinal unit conductance (Sc), and longitudinal resistivity (ρL) estimated from VES, marked a clear distinction between saline and fresh aquifers with a wide range of values. The geochemical method was performed using 20 water samples for the main cations (K+, Ca2+ Na+, and Mg2+), anions (SO42−, HCO3−, Cl−, and NO3−), and other parameters (TDS, EC, and pH). Fresh/saline aquifers revealed by D-Z parameters are in good agreement with those delineated by physicochemical parameters and local hydrogeological conditions. This study delineates seawater intrusion of about 13–42 km from Sonmiani Bay in the Arabian Sea towards the inlands of Bela Plain. Therefore, it is expected that this investigation will be helpful in future planning for the management and exploitation of freshwater resources in the study area. Our study suggests that D-Z parameters can be used as the most inexpensive alternative to the traditional geotechnical and environmental tests for the demarcation of fresh/saline groundwater with a large coverage in any coastal or contaminated area under a homogeneous or heterogeneous setting.

Delineation of Groundwater Potential Zones using Integrated Approach in Semi-Arid Hard Rock Terrain, Kanavi Halla Sub-Basin, Belagavi District, Karnataka

Abstract Water scarcity has become a major problem especially in arid and semi-arid regions due to insufficient rainfall throughout the globe. This has led in serious search for groundwater resources. Kanavi Halla Sub Basin (KHSB) of Ghataprabha River basin drained by Ghataprabha River, Belagavi District of Karnataka state in India has been considered for present study. Electrical resistivity survey was carried out at selected sites using Schlumberger method with four electrode configuration. A total of 22 Vertical Electrical Sounding (VES) were conducted throughout KHSB and representative of different litho units were considered. Data obtained were then interpreted using curve matching technique with IPI2Win software. The interpreted graphs show three to six layers. The various interpreted curve types are of AAK-type, HA-type, A- type, KH-type, HAA-type, KQH-type, AK-type, K-type, HQH-type, H-type and AKQH-type. From the results it shows that the maximum number of curves are associated with A type which refers to hard rock terrain, whereas curve associated with H type indicates water potential site. The expected subsurface strata are interpreted on the basis of observed bore well drilling data. Borewell log reveals soil formation followed by moderately weathered formation, weathered jointed formation, jointed formation, hard formation from top to bottom. Further deep, the formation is devoid of joints and fractures. Therefore, the weathered, jointed formation may be interpreted as water bearing zone. 2-D geo-electric section has been drawn using IPI2WIN over eight selected trends to understand the geometry of the aquifer formed. The investigation was meant to characterize the aquifer in KHSB along with the risk assessment for contaminants through seepage in terms of protective capacity with the help of Dar-Zarrouk (D-Z) parameters such as longitudinal unit conductance (S), longitudinal resitivity (ρl), transverse unit resistance (T), transverse resistivity (ρt), Electrical anisotropy(λ) and root mean square resistivity (ρm). These were evaluated to know the aquifer conditions in order to demarcate the freshwater bearing zones. South eastern part of KHSB might be contaminated and 30% of the area is weak to poor capacity protective zone for contaminants. The work adequately highlights the practical use of geophysical techniques, combination of geo-electrical modelling, D-Z parameters, borehole log in the groundwater resource assessment process. This technique is highly recommended for the area with similar geological setup.

Assessment of Aquifer Vulnerability Using Integrated Geophysical Approach in Weathered Terrains of South China

Abstract Despite being rich in groundwater resources, assessment of hard-rock aquifers in many areas of Asia is difficult given their strong heterogeneity. However, delineation of such aquifers is essential for estimation of the groundwater reserves. In addition, the vulnerability of hard-rock aquifers is controlled by the weathered/fractured zones because it is the place where most of the groundwater reserves are contained. In this work, an integrated approach of the electrical resistivity tomography (ERT), high precision magnetic, X-ray Diffraction (XRD), physicochemical analysis and pumping test data was performed to investigate the hard-rock aquifers occurring in the weathered terrains. This approach reveals seven fractures/faults (F1 to F7) and four discrete layers such as the topsoil cover, highly weathered, partly weathered and unweathered rock. The groundwater resources are estimated as a function of different parameters i.e., aquifer resistivity (ρo), transverse unit resistance (Tr), hydraulic conductivity (K), transmissivity (T), rock formation factor (F) and rock porosity (Φ). These parameters divide the groundwater resources into four aquifer potential zones with specific ranges of ρo, Tr, K, T, F and Φ i.e., high, medium, poor, and negligible potential aquifers. The results suggest that the high potential aquifer reserves are contained within the weathered/fractured and fault zones. The X-ray diffraction (XRD) technique analyzes quartz as the major mineral (>50%). The physicochemical and geophysical analysis suggests good groundwater quality in the investigated area. The integrated results are highly satisfied with the available borehole information. This integrated geophysical approach for the estimation of groundwater resources is not only applicable in the weathered terrains of South China, but also in many other areas of the weathered/fractured aquifer in Asia and beyond.

Mapping Shallow Aquifers Intruded by Saltwater in the Coastal Area of Benin River in Western Niger Delta fromDar-Zarrouk Parameters

Shallow aquifers are the major source of groundwater to the inhabitants of the coastal area of the Niger Delta. Groundwater is harnessed by hand-dug wells because they are easier and inexpensive to construct compared to boreholes. The closeness of groundwater water table to the surface encourages the construction of dug wells. The coastal area under investigation is faced with acute groundwater quality and assessment problems arising from saltwater intrusion. Dar-Zarrouk parameter comprising of longitudinal unit conductance(S) and transverse unit resistance (T) were coupled with resistivity to discriminate freshwater from saltwater zones. S across the area ranged from 0.03 to 2.96 Siemens and T from 1.4 to 790 Ω m2. S values greater and lesser than 0.3Siemens were considered as saltwater and freshwater zones respectively, while T values that ranged from 1.32 to 161.33Ω m2 were regarded as saltwater zones. Resistivity high of 185Ωm to 1511.58Ωm was assigned freshwater zones. The resistivity data considered showed evidence of freshwater at depth of 27.6m and saltwater at 34.3m. The lateral interface between saltwater and freshwater is situated at 565m from the sea. The intrusion revealed by the study does not suggest over-withdrawal of groundwater because of the small population of inhabitants but to natural hydrodynamic forcing connected with aquifer adjacent to the sea. However, the tendency of upconing of saltwater into the freshwater aquifer is probably at the depth of 3.9 to 12.3m by over-withdrawing of groundwater may not be ruled out in the future. Induced polarization is suggested as a follow-up investigation to unravel ambiguity that may be associated with resolution of resistivity data.Key words: Benin River, Niger Delta, Saltwater intrusion, Dar-Zarrouk Parameters, longitudinal unit conductance, transverse unit resistance.

Delineation of Saline-Water Intrusion Using Surface Geoelectrical Method in Jahanian Area, Pakistan

Groundwater is the main supply of fresh water in many parts of the world. The intrusion of saline water into the fresh water is a serious threat to groundwater resources. Delineation of fresh-saline aquifer zones is essential to exploit the potable fresh water. The conventional method to differentiate fresh-saline water interface is to collect and test groundwater samples from boreholes using a number of laboratory tests. However, such techniques are expensive and time consuming. A non-invasive geoelectrical method, in combination with borehole data and physicochemical analysis, is proposed to assess the fresh-saline aquifers. This investigation was conducted in Jahanian area of Pakistan with forty-five vertical electrical soundings (VES) using Schlumberger array, nine bore wells and fifty physicochemical samples. The fresh-saline aquifers are delineated by aquifer resistivity and Dar-Zarrouk parameters namely transverse unit resistance and longitudinal unit conductance. The aquifer potential of fresh-saline water zones is estimated by the aquifer parameters namely transmissivity and hydraulic conductivity. Integration of subsurface resistivity with hydrogeological information reveals the subsurface formation of five layered succession, that is, topsoil having dry strata with resistivity greater than 30 Ωm, clay containing saline water with resistivity less than 15 Ωm, clay-sand with brackish water having resistivity between 15 and 25 Ωm, sand containing fresh water with resistivity ranging from 25 to 45 Ωm and gravel-sand having fresh water with resistivity greater than 45 Ωm. The geoelectrical columns and geological cross-sections constructed by the aquifer resistivity provide effectiveness of the interpretations for the evaluation of fresh-saline aquifers. The results of physicochemical analysis using WHO guideline validate the fresh-saline aquifer zones delineated by the geophysical method. This investigation contributes towards predicting the fresh-saline water interface using inexpensive geoelectrical method.

Geophysical Investigation of Fresh-Saline Water Interface: A Case Study from South Punjab, Pakistan.

The importance of the study of fresh-saline water incursion cannot be over-emphasized. Borehole techniques have been widely used, but they are quite expensive, intrusive, and time consuming. The electrical resistivity method has proved very successful in groundwater assessment. This advanced technique uses the calculation of Dar-Zarrouk (D-Z) parameters, namely longitudinal unit conductance, transverse unit resistance, and longitudinal resistivity has been employed by using 50 vertical electrical sounding points to assess the groundwater and delineate the fresh-saline water interface over 1045 km2 area of Khanewal in Southern Punjab of Pakistan. The x-y plots and maps of D-Z parameters were produced to establish a decipherable vision for the occurrence and distribution of different water-bearing formations of fresh-saline water aquifers through a complicated situation of intermixing of different resistivity ranges for fresh-saline water bodies. This technique is useful to reduce the ambiguity produced by the process of equivalence and suppression which cause intermixing in differentiating fresh, brackish, and saline aquifers during interpretation. The fresh-saline water interface is correlated very well with the previous studies of water quality analysis carried out in Khanewal area. The results suggest that the D-Z parameters are useful for demarcating different aquifer zones. The behavior and pattern of D-Z parameters with respect to occurrence and distribution of different water-bearing formations were effectively identified and delineated in the study area.

Estimation of Hydraulic Characteristics from Electrical Resistivity Data in Coastal Aquifers of Southern India

Abstract: The applicability of geophysical techniques has been examined for evaluating aquifer properties like transmissivity and hydraulic conductivity of coastal aquifers, Tuticorin, Tamil Nadu. The pumping test data of 10 wells are interpreted by using forward modelling to obtain the aquifer characteristics in the study area. The available vertical electrical soundings (VES) data in the vicinity of the sites of pumping test have been interpreted; and true resistivity and thickness are determined at each site in the study area. Empirical relationships are established for estimating the hydraulic parameters from the electrical data. The area under study is divided into three major geological formations viz. Archaean (hard rock), Tertiary and coastal sediments. The empirical relationships have been shown (1) between formation factor and hydraulic conductivity; (2) between transmissivity and transverse resistance and (3) between aquifer resistivity and hydraulic conductivity, in three different geological formations. The result shows a linear relationship between hydraulic conductivity and transverse unit resistance and between hydraulic conductivity and formation factor. These relations are further used to estimate the hydrogeological parameters viz transmissivity and hydraulic conductivity at remaining 18 vertical electrical sounding points, where pumping test data is not available. The hydraulic properties of aquifer at different points are then used to generate the corresponding contour maps. The thematic maps are finally utilized for the selection of suitable well sites in the study area using the electrical and aquifer parameters. The results indicate that the hard rock formation near the coast ofTuticorin is the most potential zone of groundwater for future exploitation.

Dar Zarrouk Parameter as a Tool for Evaluation of Well Locations in Afikpo and Ohaozara, Southeastern Nigeria

The inhabitants of this area depend solely on contact springs as supply source of potable water. However, provision of potable water to meet the needs of the people still remains an unsolved problem. Therefore, this paper attempts to solve this problem by using Dar Zarrouk (D-Z) Parameters; Total Transverse Unit Resistance, T (Ωm2) and Total Longitudinal Unit Conductance, S (Ω-1) to suggest optimal locations for drilling of boreholes in the study area. To attain this purpose, 50 Schlumberger Vertical Electrical Sounding (VES) curves with maximum current electrode spacing of AB/2 = 681 m were interpreted. Thus, the aquifer parameters information estimated from the (VES) curves were used to prepare contour maps of T (Ωm2), S (Ω-1), aquifer thickness h (m), aquifer resistivity ρ (Ωm), and Water Table Depth (WTD). For effective use of these parameters, iso-thickness and iso-resistivity maps were compared with contour map of transverse resistance. The good agreement between these parameters provided the basis for identification of prolific aquiferous zones. It was observed that the Southern part of the study area majorly underlain by the Afikpo Sandstone of Nkporo Formation (Campanian-Maastrichtian), relatively showed higher T (Ωm2), h (m), and ρ (Ωm) values, which implies high yield aquiferous zones. The relatively loose structure of this sandstone unit, coarse grains, and sorting enables it to be porous and permeable. The Northern part of the region which shows low values for T (Ωm2), h (m), and ρ (Ωm) suggests low productivity for the aquiferous zones. The paucity of water in this parts of the study area can be explained to be as a result of the dominant geology. The high S, values at the Uburu and Okposi locations in this region suggests the presence of saline aquifer. This study would be relevant to the development of effective ground water scheme and for future hydrogeological investigations in the area.

Electrical resistivity tomography for assessment of groundwater salinity in west Delhi, India

Indiscriminate use of agro-chemicals for intensive agriculture is mainly responsible for deterioration of groundwater quality in west Delhi of India. Geophysical and hydrochemical investigations were carried out to decipher subsurface geologic formation and assess groundwater salinity. Electrical resistivity tomography surveys revealed the presence of potential groundwater zones at 20–30 m below ground level. The groundwater quality was moderate to poor as evidenced from the low resistivity value of the potential aquifer. Only 20.2 % of the study area showed good quality groundwater potentiality, situated at few pockets in eastern, western, and northern parts of the study area. Low resistivity formation at southern part of the study area was due to infiltration of highly polluted drain water along with fertilizer rich surface water to the aquifer. Hydrochemical investigation also confirmed the result. The average electrical conductivity of groundwater was around 5,950 μS cm−1. Longitudinal unit conductance and transverse unit resistance values were calculated and used for delineation of saline groundwater zones. A direct relationship between longitudinal unit conductance and groundwater salinity made it possible to develop empirical equations by which salinity can be predicted from computed longitudinal unit conductance value. It helps for periodic monitoring of the pollution level by spending less labour and money and benefits immensely the policy makers. The article suggests further study and research work that can lead to sustainable exploitation/use and management of groundwater resources in west Delhi.

Estimation of Aquifer Hydraulic Characteristics from Surface Geoelectrical Methods: Case Study of the Rima Basin, North Western Nigeria

Forty vertical electrical resistivity soundings (VES) were acquired, using a Schlumberger array. Three soundings were additionally made at existing boreholes in the vicinity of the study area for comparison. Since the area is a basement complex terrain, a maximum current electrode spacing of 200 m which is believed to probe down to the fresh basement in the area was used. This is believed to enable fresh basement in the area to be observed. The concept of Dar Zarrouk parameters (transverse unit resistance and longitudinal conductance in porous media) was used to determine aquifer hydraulic characteristics in the sedimentary formation underlying the area. The results of the interpreted VES data using a combination of curve matching technique and computer iterative modeling suggest that the area is underlain by between 3–5 geoelectric layers. The geologic sections derived from the equivalent geoelectrical sections revealed lithologic sequence of mostly five layers. However, there are minor isolated cases of three layers. The geoelectrical sections showed that the resistivity of the aquifer varies between 14.1 and 128.8 Ω m. The thickness of the aquifer ranges from 1.0 to 48 m while its depth from the surface lies between 1.0 and 50 m. Relationships between aquifer characteristics and the geoelectrical parameters revealed the hydraulic conductivity K and transmissivity T variations to be 3.10–28.34 m/day and 10.0–796.6 m2/day, respectively. The maximum K and T occur around VES 4 and 14; thus the maximum yield could be expected around VES 4 and 14.

English

  Hydrogeophysical studies have been undertaken at Irepodun/Ifelodun local government area of Ekiti State, Nigeria. The area is underlain by the Basement Complex rocks. The study involved very low frequency electromagnetic (VLF-EM) and electrical resistivity methods. The results are presented as profiles, sections, curves, charts and maps. VLF-EM anomalies were evaluated using geoelectric soundings. Geoelectric sounding results enabled the determination of overburden thickness, transverse unit resistance, longitudinal unit conductance and coefficient of anisotropy (l). Overburden thickness zones of 45 to 55 m were classified as high, 15 to 45 m as moderate and values less than 15 m as low. Afao, Obo, Iyin, Araromi, Iketun and Igede are of moderate to high groundwater potential, while other towns are of low groundwater potential. Longitudinal unit conductance showed that the area is characterized by sequence of good, moderate and weak groundwater protective capacities. Computed λ values revealed that the northern flank (Esure and Iropora) are underlain by charnockite of low λ (1.0 - 1.1) with other areas of high λ (1.1 - 2.1) underlain by granite and migmatite gneiss. Groundwater development is feasible in Araromi, Afao, Iyin, Obo, Iketun and Igede but marginally feasible in the other towns. Thus, groundwater potential of Irepodun/Ifelodun local government area is generally low.   Key words: Electromagnetic, geoelectric sounding, longitudinal unit conductance, coefficient of anisotropy, groundwater potential.

The estimation and significance of Dar-Zarrouk parameters in the exploration of quality affecting the Gulf of Aqaba coastal aquifer systems

The subsurface resistivity layer parameters of 47 vertical geoelectrical soundings are analyzed for a rectangle shaped area of 70 km2 from the Gulf of Aqaba coast. For this purpose, the Dar-Zarrouk parameters i.e., (i) total longitudinal unit conductance, (ii) total transverse unit resistance and (iii) average longitudinal resistivity are used to determine the resistivity regime of the water bearing formations. The significance of these parameters is to establish a vision about the occurrence and distribution of water bodies, while dealing with complicated situations of resistivity ranges intermixing for saline, brackish and fresh water aquifers. The results obtained from the study area suggest that the Dar-Zarrouk parameters proved useful in providing confident solution to delineate water-bearing formations. The behavior of the Dar-Zarrouk parameters and its patterns in space over large areas with respect to the occurrence of water aquifer systems in the deltaic coastal area has been demonstrated.

Geoelectric Investigation of Aquifer Vulnerability within Afe Babalola University, Ado –Ekiti, Southwestern Nigeria.

Many investigation techniques are commonly employed with the aim of estimating the spatial distribution of transmissivity and protective capacity of groundwater resources. Unfortunately, the conventional methods for the determination of hydraulic parameters such as pumping tests, permeameter measurements and grain size analysis are intrusive and relatively expensive. A non-intrusive and less-expensive geoelectric investigation involving vertical electrical sounding was carried out in some parts of the campus of Afe Babalola University, Ado Ekiti, Ekiti State, Nigeria. A total of fifty-nine (59) vertical electrical sounding (VES) data were acquired using R 50 D.C. resistivity meter within the campus which is underlain by the Precambrian basement rock of southwestern Nigeria. Following the interpretation of the VES data, maps and 2D-sections were generated. The geoelectric sections enabled the subsurface to be characterized into five geoelectric layers namely: Topsoil, clayey/sandy-clay, weathered layer, fractured basement and fresh basement. The assessment and analysis of the materials above the aquifers showed that longitudinal conductance (S) values ranged from 0.08438 to 0.73449 mhos; thus the area is classified into weak (0.1 – 0.19 mhos), moderate (0.2 – 0.69 mhos) and good protective capacity (0.7 mhos and above). The major aquifer delineated is the weathered/fractured basement aquifers. These aquifers are characterized by thick overburden, moderate/good protective capacity, moderate to relatively high value coefficients of anisotropy and low transverse unit resistance. This suggests that the materials above the aquifers act as seal, thus protecting the major aquiferous units. However, the aquifer matrix itself is relatively permeable. Areas with weak protective capacity are therefore vulnerable to infiltration of polluting fluid.