Groundwater in Eti-Osa and Ibeju-Lekki local government areas of Lagos state were investigated for the effects of beach water influence on adjacent aquifers. Beach water and Borehole samples at distances of 40-50 m and 100-150 m from adjacent beaches in the study areas were taken and analysed for pH, Temperature, Electrical Conductivity, Total Dissolved Solids, Salinity, Ca2+, Mg2+, K+, Na+, HCO3- , NO3-, Cl-SO42-, total hardness and dissolved oxygen. The results were compared with World Health Organisation (WHO) and National Agency for Food and Drug Administration and Control (NAFDAC) standards for water quality. Piper diagram was used to delineate the hydrochemical calfacies of the aquifer and to identify hydrogeochemical changes. The results showed that Cl- dominated in boreholes of both Ibeju-Lekki and Eti-Osa. Sampled boreholes in Eti-Osa showed prevailing dominance of major seawater ions (Na+> Ca2+> K+>: Cl- > SO-) at levels far beyond permissible limits of potable water quality set by WHO and NAFDAC and showed a Ca-SO4 and NaCl water type for the boreholes 40-50 m and 100-115 from the coastline line respectively. Water quality requirements of WHO and NAFDAC are within permissible limits for boreholes in Ibeju-Lekki. Analysed Water samples from Ibeju- Lekkiare characterised as Ca-SO4 water type.Groundwater resources of studied boreholes in Ibeju-Lekki are less influenced by saltwater intrusion from nearby beach, as opposed to Eti-Osa. This is can be attributed to overtly unsustainable exploitation of groundwater resources in Eti-Osa, Which makes groundwater in Eti-Osa unsuitable for domestic purposes.
Key words: Characterization, groundwater, seawater, intrusion, standard.
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