and Awoye low resistivity values (1 - 3 ohm-m) were recorded, indicating possible presence saline water within the coastal alluvium, sandy clay/mud-peat and clayey sand layers.

4.2. Aquifer Layers

The aquifer layers across the study area were identified and presented as resistivity and depth maps.

Figure 7. Geoelectric section along N-S direction.

Figure 8. Geoelectric section along N-S direction.

Table 2. Correlation of VES and IPS results.

The resistivity of the first aquifer layer (Figure 9) varies from 0.2 (Obenla) to 1569 ohm-m (Ayadi). In the coastal areas and Agbabu, Ilubirin and part of Ode Aye in the northern part of the study area, the resistivity values fall below 60 ohm-m, suggesting that the shallow aquifers in these locations might contain brackish or saline water. The IP sounding results (Table 2) give low chargeability values within the low resistive layer at Agbabu, but high value at Ode Aye, thereby confirming possible saline water intrusion only in Agbabu. That saline water exists in Agbabu was equally attested to by hydrochemical analysis earlier carried out across the study area [11] [24] . The depth to first aquifer layer ranges from 0.7 (Eruna and Molutehin) to 151.5 m (Itebukunmi). The depth to first aquifer layer map ((Figure 10) shows that depth to first aquifer layer is generally shallow (less than 5 m) in the coastal area and generally water logged. The first aquifer unit in this area is highly susceptible to anthropogenic pollutants due to its shallow nature and possible poor protection by the overlying sandy layers.

Figure 9. First aquifer layer resistivity map.

Figure 10. Depth to first aquifer layer map.

The resistivity of the second aquifer layer (Figure 11) varies from 0.5 (Molutehin) to 904 ohm-m (Apata Ijaw). The low resistivity values along this aquifer extends only to some coastal towns, such as Obe-Rebiminu, Eruna, Ugbo, Gbabijo, Adagbakuja, Abealala, Awoye, Ugbonla, Araromi seaside, Ayetoro, Molutehin and Oretan. It also extends to the north eastern area of Owode, Iyansan, Agadagba, Laworo, Legbogbo, Lokaka, and Irele and including Ode Aye, Oluagbo, Okitipupa and Idepe in the north central part of the area. The IP sounding results (Table 2) again shows high chargeability values at Odeaye, Iyansan road and Agadagba thereby eliminating the possibility of saline water within the second aquifer layer within the northeastern area. The depth to second aquifer across the study area varies from 1.4 (Awoye) to 305.5 m (Owode

Figure 11. Second aquifer layer resistivity map.

road). The map showing depth to second aquifer layer (Figure 12) indicates that depth to this aquifer layer is shallow (less than 23 m) in some part of the coastal areas, such as Obe-Rebiminu, Araromi Seaside, Temidire, Ugbonla, Ayetoro, Awoye, Molutehin and Oretan. Likewise in some places in the mainland the intermediate aquifer also exists at shallower depths. Some of these areas are closer to streams and river tributaries which are directly or indirectly connected to the sea water. This explains the possible brackish/saline water suspected in these areas.

The resistivity of the third aquifer unit (Figure 13) varies from 0.4 (Eruna) to 665 ohm-m (Oriopo). The low resistivity values along this layer extends to Araromi seaside, Obinehin, Gbabijo, Adagbakuja, Ugbonla, Eruna, Ayetoro, Awoye and Molutehin in the coastal area. This is indicative of saline water intrusion in this area. However in the northeastern part of the mainland, low resistivity val-

Figure 12. Depth to second aquifer layer map.

ues were delineated in many places such as: Ayadi, Legbogbo, Irele road, Lokaka, Laworo, Agadagba, Arogbo and Amapere. This probably suggests that aquifers in this area contain brackish to saline water. Again the IP sounding results nullified any suspicion of occurrence of brackish/saline water intrusion in these areas, based on high chargeability values obtained from these area.

The depth to the third aquifer layer (Figure 14) ranges from 12.9 (Awoye) to 452.9 m (Arogbo). The depth to the third aquifer layer is generally significant (about 100 m) in most parts of the coastal towns and mainland with the exceptions of Zion, Temidire, Ogoluwayo, Ebute Ipare and Abealala, in the western and eastern parts of the study area.

Figure 13. Third aquifer layer resistivity map.

4.3. Average Longitudinal Resistivity

Average longitudinal resistivity; a second order geoelectric parameter was calculated from the primary geoelectric parameters and presented as map (Figure 15). The map enabled the delineation of lateral extent of saline water intrusion across the study area based on resistivity values. Low resistivity values (less than 60 ohm-m) were considered to be brackish to saline water intruded zone.

4.4. Saline Water Extent

Saline map showing the extent of saline water into areas (Figure 16) was generated based on the three aquifer layer maps and the average longitudinal resistivity map. The map project possible extents of saline water intrusion across the study area. The map shows that the southeastern part are the largely affected by saline water intrusion, this perhaps due to the fact that there are more tributaries in this area through which sea water can move land ward.

4.5. Depth to Saline Water Predictive Model

Strong correlation was also observed between the depth to the saline water in a given location and the distance from salinity source; Ocean, tributaries, canals, streams or rivers bearing saline water (Figure 17). Where a location is close to the source of saline water, the depth to the saline water will be shallow and conversely where the location is far from salinity source depth to the saline water will be deeper. The correlation curve (Figure 17) shows a strong direct correlation (r² = 0.8564) between distance of location from the saline water source and

Figure 14. Depth to third aquifer layer map.

Figure 15. Average longitudinal resistivity map.

depth to saline water. This can therefore serve as a predictive model to determine depth to saline water at any location within the saline water zone in the study area.

5. Conclusion

This study has revealed the presence and extent of saline water intrusion in the coastal areas and Agbabu in the north central part of the study area, and this probably suggests presence of connate water. Aquifer layers in the coastal area also exist at shallow depths which makes these aquifers highly susceptible to surface pollution in addition to the fact that their proximity to the oceans makes them highly vulnerable to saline water intrusion.

Figure 16. Saline water extent map.

Figure 17. Depth to saline water and distance from salinity source correlation curve.

Acknowledgements

The authors wish to acknowledge the following students of the Department of Applied Geophysics FUTA, who assisted during the field work stage of this work, Olemu Ogheneochuko, Orekoya Abimbolu, Oghene Ortega, Oduwaye Aron, Olaogun Oluwole, Oloriegbe Olatubosun, Ologun, B.J., Amosun Joel, Obamoyegun Niyi, Akinsola Samson, Egbukuyomi Pat, Abiola Adekunle and others.

Cite this paper
Adeyemo, I. , Omosuyi, G. and Adelusi, A. (2017) Geoelectric Soundings for Delineation of Saline Water Intrusion into Aquifers in Part of Eastern Dahomey Basin, Nigeria. Journal of Geoscience and Environment Protection, 5, 213-232. doi: 10.4236/gep.2017.53015.
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