Coupled geophysical and geochemical approach to detect the factors affecting the groundwater salinity in coastal aquifer at the area between Ras Sudr and Ras Matarma area, South Sinai, Egypt

Abstract

The integration of geophysical and hydrochemical approaches successfully reflects the hydrogeological vulnerability of the arid coastal aquifer in the study area. Both techniques defined the high and low groundwater potentialities regarding water quality and quantity and delineated the fresh/saline water interface.The groundwater exploration comprises 36 transient electromagnetic (TEM) sounding and 4 Two-dimensional electrical resistivity imaging (2DERI) lines. The inversion of both geophysical methods reveals four consequent subsurface layers; 1) the surficial unsaturated alluvial sand deposits layer has a resistivity range of 50–538 Ωm, and thickness <10 m, 2) the partially saturated alluvial sand, 3) the saturated alluvial sand and silt with fresh/brackish groundwater, and 4) the lower saturated alluvial sand and gravel layer with saline groundwater and clay sheet intercalations. The third water-bearing layer acts as a thin fresh/brackish groundwater lense of a moderate resistivity (0.8–55 Ωm) floating over deeper saline groundwater with a very low resistivity range (0.8–5.8 Ωm).The relatively moderate to higher resistivity of the fresh/brackish groundwater zone recorded in the eastern side at the downstream portion of the study area indicates promising subsurface recharge. The thickness of this zone ranges from 4 to 24 m (Groundwater Group I; 507 ≤ TDS ≤3920 mg/L & Groundwater Group II; 4031 ≤ TDS ≤4717 mg/L). While the lower resistivity values characterize the northern and southern portions of higher groundwater salinity (Group III; 5144 TDS 6771 mg/L & Group IV 7452 TDS 27573 mg/L), suggesting mixing with deeper saline/seawater and intercalations of clay sheets. Regarding water quality, it is recommended to excavate the groundwater only for the freshwater/brackish zone to avoid up conning of deep saline groundwater. Finally, flash floods are the primary source of groundwater in this vulnerable area. With a recent intensive expansion of groundwater excavation, a continual monitoring program or groundwater modelling is highly recommended.