Delineation of Salinization and Recharge Sources Affecting Groundwater Quality Using Chemical and Isotopic Indices in the Northwest Coast, Egypt

Abstract

Salinization of coastal aquifers is a serious issue affected by climate change and enhanced by overexploitation of groundwater resources. This research aims to explore the hydrogeochemical processes that cause salinization of groundwater in coastal aquifers, such as the area located between Barrani and Baqbaq, on the northwestern coast of Egypt. Various techniques were applied, including Gibbs plots and hydrochemical facies diagrams (HFE-D), ion ratios and stable isotope bivariate plots, statistical analyses, a groundwater quality index for seawater intrusion (GQISWI), and a seawater mixing index (SMI). Based on the total dissolved solids (TDS), groundwater can be classified into four groups: slightly saline (9%), moderately saline (45%), highly saline (43%), and salty water (3%). The geochemical properties were further catergorized on the basis of other parameters and ion ratios, such as Caexcess, Nadeficit, Na/Cl, Cl/HCO3, and Br/Cl, which suggest the influence of cation exchange, seawater, and marine sediment dissolution. Additionally, stable isotopes indicated two groups. One of these has relatively high salinity and low isotopic content and is affected by the leaching and dissolution of marine deposits. The other group is enriched in δ18O and δD content, with much higher salinity due to mixing with seawater and evaporation. The GQISWI categorizes groundwater as saline and mixed (55 and 41%, respectively), followed by saltwater (4%), whereas the SMI calculations indicate that about 10% of the groundwater samples are impacted by seawater. Finally, the areal distribution of GQISWI and SMI identified some patches along the coastline as well as other inland places located about 12.5 km away from the sea that have undergone saltwater intrusion. In conclusion, overexploitation of groundwater should be avoided because the amount of annual rainfall is very limited.