Integrating hydrogeological and second-order geo-electric indices in groundwater vulnerability mapping: A case study of alluvial environments

AVI (Aquifer vulnerability index), GOD (groundwater occurrence, overlying lithology and depth to the aquifer), GLSI (geo-electric layer susceptibility indexing) and S (longitudinal unit conductance) models were used to assess economically exploitable groundwater resource in the coastal environment of Akwa Ibom State, southern Nigeria. The models were employed in order to delineate groundwater into its category of vulnerability to contamination sources using the first- and second-order geo-electric indices as well as hydrogeological inputs. Vertical electrical sounding technique employing Schlumberger electrode configuration was carried out in 16 locations, close to logged boreholes with known aquifer core samples. Primary or first-order geo-electric indices (resistivity, thickness and depth) measured were used to determine S. The estimated aquifer hydraulic conductivity, K, calculated from grain size diameter and water resistivity values were used to calculate hydraulic resistance (C) used to estimate AVI. With the indices assigned to geo-electric parameters on the basis of their influences, GOD and FSLI were calculated using appropriate equations. The geologic sequence in the study area consists of geo-electric layers ranging from motley topsoil, argillites (clayey to fine sands) and arenites (medium to gravelly sands). Geo-electric parametric indices of aquifer overlying layers across the survey area were utilized to weigh the vulnerability of the underlying water-bearing resource to the contaminations from surface and near-surface, using vulnerability maps created. Geo-electrically derived model maps reflecting AVI, BOD, FLSI and S were compared to assess their conformity to the degree of predictability of groundwater vulnerability. The AVI model map shows range of values of log C ( −3.46—0.07) generally less than unity and hence indicating high vulnerability. GOD model tomographic map displays a range of 0.1–0.3, indicating that the aquifer with depth range of 20.5 to 113.1 m or mean depth of 72. 3 m is lowly susceptible to surface and near-surface impurities. Again, the FLSI map displays a range of FLSI index of 1.25 to 2.75, alluding that the aquifer underlying the protective layer has a low to moderate vulnerability. The S model has values ranging from 0.013 to 0.991S. As the map indicates, a fractional portion of the aquifer at the western (Ikot Abasi) part of the study area has moderate to good protection (moderate vulnerability) while weak to poor aquifer protection (high vulnerability) has poor protection. The S model in this analysis seems to overstate the degree of susceptibility to contamination than the AVI, GOD and GLSI models. From the models, the categorization of severity of aquifer vulnerability to contaminations is relatively location-dependent and can be assessed through the model tomographic maps generated.

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

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.

Estimation of Aquifer Protective Capacity, Soil Corrosivity and Dar-Zarrouk Parameters in Kaura Area of Kaduna State, Nigeria

This study presents an estimation of aquifer protective capacity, soil corrosivity and Dar-Zarrouk parameters for Kaura area of Kaduna State in northern Nigeria. Electrical resistivity sounding data and borehole pumping test data obtained from 20 locations within the LGA were obtained and used for this study. The geoelectric data exhibited curve types generally consisting of H, HA, KH or K-A-H types from which five-layer lithology were delineated across the entire study area. Well yield varies from 16 – 400 litres/min, pumping rate ranges from 16 to 140 l/min, drawdown varies from 1 – 22m, while specific capacity ranged from 1 – 95 litres/min. The aquifer protective capacity characterization was based on values of longitudinal unit conductance of the overburden, and 35% of the locations showed good protective capacity, while the remaining 65% exhibited moderate protective capacity. From the soil corrosivity evaluation, the upper soil layers were classified as moderately corrosive at one location, four locations were marked as slightly corrosive, while the remaining locations were found to be practically non-corrosive. Based on thicknesses and resistivities of the overburden layers, Dar-Zarrouk parameters were determined. The reflection coefficient ranged from 0.29 to 0.92, resistivity contrast occurred between 0.35 to 25.38, while the coefficient of anisotropy ranged from 0.70 to 3.84 with mean value of 1.57. Values above 1.0 are generally considered high and they occurred more toward the western part of the area than the middle and eastern parts. Both the longitudinal unit conductance map and coefficient of anisotropy map were generated for the area.

Assessment of Hydrogeological Potential and Aquifer Protective Capacity of Odeda, Southwestern Nigeria

Hydrogeological assessment of groundwater resources was carried out with a view to evaluate the potential of the aquifers to provide portable water supply and access the distribution of electrical parameters of hydrogeologic units in some areas in Odeda, Ogun State, Nigeria. A geophysical survey using vertical electrical sounding (VES) with the Schlumberger electrode array, with half-current electrode spacing (AB/2) varying from 1 to 132 m was carried out at 30 different stations in the study area. The VES data were interpreted qualitatively and quantitatively. Three-to-five sub-surface layers consisting of topsoil, weathered layer consisting of clay, sandy clay, clayey sand and sand layers, and fractured/fresh basement were delineated. Layer resistivities and thicknesses obtained on the curves within the study area showed one main aquifer type, which is the fractured basement. The longitudinal unit conductance (ranging from 0.049720 to 1.4520000 mhos) of the study area aided the protective capacity to be rated into good, moderate and weak. About 33% of the study area falls within the weak protective capacity, 57% falls within the moderate protective capacity and 10% falls within the good protective capacity.

Investigation of the Aquifer Protective Capacity and Groundwater Quality around Some Open Dumpsites in Sapele Delta State, Nigeria

Aquifer protective capacity and groundwater quality investigations around some open dumpsites were conducted along New Road Sapele, Delta State. Nine Schlumberger arrangements of vertical electrical soundings (VES) were carried out with a maximum electrode separation of 500 m, and obtained data were interpreted by partial curve matching and computer iteration using Win Resist software. 2D survey using nine dipole-dipole stations was also utilized to give resistivity map of the dumpsites, while standard laboratory methods were used to analyze the groundwater. The study shows four to five geoelectric sections. The aquifer is within the third, fourth, and fifth layers at a depth of 30 m. The overburden protective capacity from the total longitudinal unit conductance indicates that VES 1 and 5 are adequately protected with protective capacity of 0.7 to 0.9 mhos, VES 2, 3, 6, and 7 are moderately protected with conductance values of 0.2 to 0.69 mhos, VES 4 is weakly protected with values of 0.12 mhos, and VES 8 and 9 are poorly protected with values of 0.003 to 0.004 mhos. In the 2D imaging, VES 4, 8, and 9 show contaminant presence to the depth of 20 m while VES 3, 6, and 7 show contamination to a depth of 50 m. However, the aquifer shows good transmissivity, an indication that if it is contaminated, the contaminants will circulate the aquifer at a high rate. The groundwater flows in the northeast (NE) direction, thereby recharging river Ethiope. The study also shows the presence of lead (0.01 mg/l), nickel (0.02 mg/l), and cadmium (0.03 mg/l), which made it unsafe for drinking and use in other life-related activities. Groundwater should hence be sourced from a depth of about 45–50 m in order to tap from the uncontaminated aquifer.

Correlation of vertical electrical sounding and borehole-log data for delineation of saltwater and freshwater aquifers

In a case history from the Mahanadi basin (India), we demonstrate the use of resistivity data from electric-log soundings and from borehole logs to discriminate between saltwater and freshwater aquifers. We use interpreted data from eight surface-based vertical electrical soundings (VES) and electric well logs from three boreholes in this study. We establish a quantitative relation among longitudinal unit conductance S (obtained from VES), water resistivity [Formula: see text], and layer thickness h. We show that ambiguities in resistivity data interpretation limit its ability to distinguish between freshwater and saltwater aquifers. Electric well-log data interpretation is much more accurate but requires boreholes, which are not cost effective when exploring for groundwater. Integrating well-log-based estimates of [Formula: see text] into resistivity interpretation of surface-based soundings improves its ability to discriminate freshwater aquifers while maintaining cost-effective exploration.