Mapping saline water intrusion into the coastal aquifer with geophysical and geochemical techniques: the University of Lagos campus case (Nigeria)

Saltwater Intrusion 1nto the coastal aqu1fer. a phenomenon brou9ht by the now of seawater into fre;hwJter aquifers onginally caused by groundwater extract1on near the coast, has long been recogn1~ed as a maJOr concern around the world In this study, we employed geophysical and geochemical techniques to map and provide ev1dences that the coastal aquifers 1n the study area have been Intruded bv saltwater from the adJilCPnt LaCJOS lagoon. The 1eSIStiv1ty dara were acqu1rcd With an electrode spacll)g (a) that va1y between 1.6 ro H m. ;md expans1on factor n of 30. The depth 1nverted ,models obt<Hned from mvers1on of the f1fteen res1St1v1ty dcltd obtJ1110d in the area revealed significant impact of the ragoon water on the aqu1fers 1nd1cated as low res1stiv1ty usually below 7 Om. A combination of four different electrode arrays - Schlumberger. Wenner. D1pole-d1pole and pole-dipole. with at least three deployeci at each site (except for three traverses- traverses 13, 14 and 15), y1e,d beuer horizontal and vertici:ll resolution. hav1ng depth range of 36-226 m with 1.6-8 m electrode spacing used. rhe delineated geoelectric layers were juxtaposed with logs from both boreholes located within the campus. Evidence from geochemical study of borehole and the lagoon water samples corroborateci the ERT resu lt rroqrrssive decrease in total d1ssolved solute (TOS) ancl electncal concJuclivily (LC) from the lagoon to the co.'lst.~l aqt11frr buttresses gradual encroachment o f the mland aquifers by the intrud1ng ldgoon water. In