Integrated geophysical and hydrochemical investigations for seawater intrusion: a case study in southwestern Saudi Arabia

The objective of the present study was to assess the deterioration in groundwater quality due to saline water intrusion in the Ad-Darb region of southwestern Saudi Arabia. An integrated approach using geochemical and geophysical methods was applied to assess the extent of saline water intrusion. Geochemical methods involved the determination of main groundwater facies present in the region using Piper and extended Durov plots. The geophysical methods used in the study included seismic refraction (SR), seismic refraction tomography (SRT), vertical electrical soundings (VES), and electrical resistivity tomography (ERT). Two SR and SRT surveys, three ERT surveys, and nine VES surveys were carried out. Piper plot shows that the water mainly belongs to the SO4-Cl type of anionic facies. The Durov plot indicates base ion exchange, linear mixing, and saline water intrusion as the main factors influencing the groundwater chemistry of the area. The TDS increases towards the coast, with values reaching as high as 9000 mg/l. These results confirm the interpretation of the vertical electrical sounding (VES) and electrical resistivity tomography (ERT) sections which indicates the presence of saline water intrusion, with the thickness of the intruded zone increasing towards the sea. The geoelectric resistivity results indicate four geoelectric resistivity layers. The average thickness of upper layer is 2.5 m and the resistivity ranges from 22.5 to 280 m. The P wave velocity of this layer varies from 488 to 787 m/s. The second geoelectric resistivity zone consists of relatively disconnected layers with 10 to 33 m thickness and 52 to 120 m resistivity. The third zone is a low resistivity zone, increasing in thickness westwards towards the sea and with resistivities of 0.3 to 19.5 m. This is the main zone (consisting of unconsolidated Quaternary sediments) affected by saline water intrusion. This zone is intercalated with muddy marine intercalations, as indicated by its low resistivity (as low as 0.3 m) The second and third geoelectric resistivity layer is represented by a single seismic zone with P wave seismic velocities ranging from 1231 to 1524 m/s, indicating the presence of water. The resistivity of the lowest layer ranges from 56 to 822 m and corresponds to the basement. This bedrock layer has seismic velocities varying between 2315 and 3164 m/s.