Baseline evaluation of freshwater–saltwater interface in coastal aquifers of Badagry, south-western Nigeria (original) (raw)
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IOSR Journals , 2019
Recently, the deterioration of water quality in the coastal areas of Akwa Ibom State, Southeastern, Nigeria due to saltwater infiltration into the freshwater aquifer has become a major concern. In this study, the geophysical method we adopted was the 2-D Electrical Resistivity Tomography (ERT) using resistivity meter and other accessories to obtain the field data. Wenner electrode configuration with electrode spacing ranging from 5 to 200m was used for field measurement and a total of twenty (20) randomly distributed ERT profiles were covered. The modeled field data revealed the resistivities and depth to bottom layer distribution across the study area with predominant lithologies being coastal plain sands. The interpretation showed upper resistivity limit of 232.5 m for medium to fine grained sands; 3,959 m for lateritic sands; 1,580 m for gravelly sands and 185.5 m for sandy/clayey sand and clay soils. The depth of inverse models from the geoelectrical resistivity data obtained in the area revealed significant impact of the saline water in delineated aquifer with very low resistivity values uniquely below 3.24 m. ERT results also shows the lateral invasion and upconing of saline water within the aquifer systems. Both natural and anthropogenic factors are responsible for saline intrusion across the study area. The uniqueness of the techniques and investigations through field data analysis showed the extent a saltwater intrusion into the freshwater aquifer along the interface.
Deterioration of water quality in the coastal areas of Lagos due to saltwater infiltration into the freshwater aquifer has become a major concern. Geophysical techniques involving 2D electrical resistivity tomography (ERT) using Wenner and dipole-dipole arrays were incorporated with geochemical evaluation using ICP-OES and 1C analytic methods to study the extent of saline water intrusion and its environmental impacts. The resistivity data were acquired with minimum electrode spacing of 5m (Traverses 1, 2, and 3) and 6 m (Traverse 4). Data level of 8 (maximum electrode spacing of 40.0 m) was achieved in each of the profiles; dipole length with n-factor of 6 was employed for the dipole-dipole array. The depth of inverse models from the geoelectrical resistivity data obtained in the area revealed significant impact of the saline water on delineated aquifers with very low resistivity values uniquely below 4.0 m . ERT results also show the lateral invasion and upconing of saline water within the aquifer systems. Results of the physiochemical analysis indicate that the water is alkaline with pH values range 8.6-8.9. The salinity is also high with the total dissolved solids TDS values and chloride concentration ranges 10405 – 12005 (mg/L) and 432 – 724 (mg/L) respectively. The water has very high electrical conductivity (EC) values range 2005-3013 μS/cm and very Hard with 121 to 180 mg/L range of total hardness thereby falling below the minimum permissible standard of the Indian standard institution (ISI), European union (EU) and world health organization (WHO) for portable water consumption.
SpringerPlus, 2013
Saltwater intrusion into the coastal aquifer, a phenomenon brought by the flow of seawater into freshwater aquifers originally caused by groundwater extraction near the coast, has long been recognised as a major concern around the world. In this study, we employed geophysical and geochemical techniques to map and provide evidences that the coastal aquifers in the study area have been intruded by saltwater from the adjacent Lagos lagoon. The resistivity data were acquired with an electrode spacing (a) that vary between 1.6 to 8 m, and expansion factor n of 30. The depth inverted models obtained from inversion of the fifteen resistivity data obtained in the area revealed significant impact of the lagoon water on the aquifers indicated as low resistivity usually below 7 Ωm. A combination of four different electrode arrays - Schlumberger, Wenner, Dipole-dipole and pole-dipole, with at least three deployed at each site ( except for three traverses - traverses 13, 14 and 15), yield better...
Scientific African, 2022
Integrated geophysical methods of Electrical resistivity tomography (ERT), Induced Polar- ization (IP), and Well Logging were employed in this study to delineate the lateral and depth extent of saline water intrusion in Oniru area of Lagos State, South-western Nigeria due to reported saltwater contamination of some wells at relatively shallow depths. Ergo, improving the scientific understanding of fresh and saline waters occurrence in the coastal areas of Lagos State. Five traverses, each of variant lengths (100 to 200 m) were established for both ERT and IP as space permitted. For each profile level, Wenner electrode configuration was adopted with a minimum electrode spacing of 3 m and maximum electrode spacing of 50 m. Ten levels were achieved for profiles 1, 2, 4, and 5 while eight levels were accom- plished for profile 3. The ERT and IP data were processed using RES2DINV 2-D resistivity data processing software. The well log data utilized were acquired from two existing litho- logical logs within the study area to validate the results of the ERT and IP. The unconsolidated sandy formation was characterized by high resistivity with cor- responding low to medium chargeability while the clay units were characterized by low resistivity with equivalent high chargeability. The sandy formation saturated by saltwa- ter was characterized by very low resistivity with corresponding low chargeability while that of freshwater was characterized by medium to high resistivity with corresponding low chargeability. The integrated geophysical methods of ERT, IP and borehole logging in the study area identified three lithological sequences. The first layer corresponded to high resistivity values of unconsolidated sandy topsoil, mostly less than 5 m. However, in profiles 3 and 4, it occurred as saturated marshy topsoil. The top layer was underlain by sand/clayey sand formation, which was observed to have pockets of saline water intrusion as evinced from the results of the ERT and IP data. The saline water-bearing zones were observed to intermingle with the freshwater-bearing sand units. The freshwater-saline water transition tends to be conspicuous especially in profiles 3 and 5. The base of profiles 1 and 2 was occupied by clay units which were evident from the results of the borehole log, the ERT, and the IP sections. The integrated geophysical method was able to successfully delineate the fresh and saline water interface and extent of saltwater intrusion within the study area and thus would provide a reliable database for future groundwater development and management within the study area.
Applied Water Science
Saltwater intrusion, erosion, and periodic flooding during severe storms are some of the critical problems bedeviling the precarious coastal environment of Nigeria. A time-lapse study aimed at monitoring the dynamics of saltwater intrusion within the coastland surrounding Ibeno, southeastern Nigeria is presented. Three (3) vertical electrical soundings (VES) and four time-lapse electrical resistivity tomography (ERT) profiles with minimum and maximum electrode spacing of 5 m and 40 m and profile lengths of 120 m and 150 m respectively were acquired in 2016 and 2017 using the Wenner and Schlumberger arrays. Data acquired from the field surveys were modeled with WINRESIST and RES2DINV software to generate 1D and 2D resistivity images of the subsurface. The VES results with a maximum survey length of AB = 300 m revealed a four-layer KH curve. The 2D geo-electrical sections delineated contrast between the high resistivity values obtained for the saturated freshwater zones and the low re...
Journal of Earth and Atmospheric Research, 2020
Saline water intrusion has been reported within Badore–Ajah area of Lagos coastal environment which necessitated electrical resistivity survey involving the use of Vertical Electrical Sounding (VES) and 2-D imaging with the aim of delineating fresh water aquifer in the area. Twenty-eight (28) VES and 2-D data along four (4) traverses were acquired within the study area. The interpretation of the 2-D data was carried out using the DIPPRO software, while the results of the inverted VES data using partial curve matching technique was improved with the aid of WINRESIST software. Results obtained were further refined with output data constrain using wireline log and lithology log. The results of the sounding curves revealed four geoelectric layers comprising of topsoil (mixture of sand and clayey sand) with resistivity values ranging from 61 to 920 Ωm, clayey sand (61-98 Ωm), sandy clay (35-56 Ωm), clay (5-19 Ωm) and sand (123-495 Ωm) respectively. The topsoil has layer thickness values that vary between 0.9 m and 1.4 m. The second stratum consists of sand with some lenses of clayey sand, having layer thickness values that ranges from 2.3 and 20.6 m. The third layer is predominantly sandy clay with some intercalation of sand and clay with layer thickness values ranging from 17.0 to 56.2 m and depth extending from 26.0 to 65.6 m. The fourth horizon is predominantly sand on traverses 4 and 5, and mostly clay on traverse 2, which falls within the swamping area. It was established that sand horizons on traverses 1, 4 and 5 were potential fresh water aquifers from depth of 50 meters and above. The correlation of the geo-sections with the wireline and borehole lithology logs have established the distribution of freshwater aquifer within the study area to vary from 50 to about 130 meters. From this study, VES 1, 3, 23, 27 and 28 locations were recognized to be favourable for groundwater development with fresh water aquifers at depth equal or greater than 50 m.
Groundwater Quality Assessment of a Coastal Aquifer in Niger Delta Using Geo-electrical Techniques
The geo-electrical techniques involving the VES and 2-D resistivity imaging was used in assessing the groundwater quality of a coastal aquifer in Ogu-Bolo Local Government Area of Rivers State. This study used two Vertical Electrical Sounding data and three horizontal profiling data obtained using ABEM SAS 300B Terrameter. A computer modeling was used in processing the Data and partial curve matching was employed in the interpretation of the VES data. Five geoelectric layers of varying thicknesses, depth and resistivities were delineated. The fourth layer is where the best aquifer ranged from 41.70m to 42.90m at the various VES stations. The mean aquifer thickness, depth and resistivity value were calculated to be 23.74m, 42.30m and resistivity value of 3033.50 Ωm. The results of the interpretation of the two VES stations and the 2-D data have offered the opportunity of knowing the possible aquiferious zone within the study area. The results reveal that beyond the depth of 15.00m an aquifer system devoid of salt water intrusion will be possible.