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Water resources are fundamental components of ecological life that must be maintained since they are at the risk of both pollution and depletion. Recently, one of the main factors threatening water resources is the groundwater salinization. In this study, electrical resistivity method via vertical electrical sounding (VES) technique and time-domain induced polarization (IP) method were applied on 34 points to show the electrical characteristic of the coastal area of Dikili district, NW İzmir, Turkey. Possible freshwater/seawater interface and the existence of the geothermal potential were successfully elucidated by a joint interpretation of the results inferred from both methods. Four different types of water resources were classified according to the apparent resistivity and chargeability values, which is freshwater (<10 Ω.m and 10–620 msec.), saline water (<0.6 Ω.m and 600 > msec.), geothermal water (8–150 Ω.m and > 14,000 msec.) and mixed water (0.5–60 Ω.m and 200–1600 msec.). To show the temporal and spatial change of the geo-electrical properties in the subsurface, geoelectrical sections obtained from 1D and 2D inversion of resistivity data were compared with those of an electrical resistivity survey conducted on 119 VES points in 1978 by the General Directorate of State Hydraulic Works (DSI in Turkish acronym). The results of this study highlight significant areas where the resistivity values are lower than <0.6 Ωm as this implies evidence of presumable seawater intrusion (SWI), while previous resistivity values below 10 Ωm denote salt intrusion restricted in narrow areas. A comparison of the two resistivity sections indicates evidence of SWI into the fresh groundwater that spatially has been extended as 20 km2. We suggest streams that are cut by faults and anthropogenic effects, instead of rising sea level, likely govern the salinization in the district of Dikili during a short time period of 40 years. [Display omitted] • 2D resistivity sections show areas interrupted by the seawater intrusion (SWI). • 3D Spatio-Temporal Variation of the SWI is determined by geoelectrical methods. • Outcomes of this study should be considered by decision-makers in coastal areas. • The variation of resistivity values are determined in a time interval of 40-years. • NW-SE oriented two new faults determined on the central part of the study area. [ABSTRACT FROM AUTHOR]