Ionic ratios as tracers to assess seawater intrusion and to identify salinity sources in Jazan coastal aquifer, Saudi Arabia

Abstract

The impact of seawater intrusion in coastal aquifers is a major groundwater contamination issue worldwide. The shallow coastal aquifer in Jazan, southwestern part of the Red Sea coast of Saudi Arabia, is vulnerable to salinization by seawater intrusion due to overexploitation. The present study helps to understand the impact of seawater intrusion in Jazan aquifer and to assess and determine the most dominant hydrogeochemical processes controlling the groundwater salinity. For these purposes, 70 groundwater samples have been collected and chemically analyzed for major cations and anions. Based on the analytical results, the groundwater in the study area is generally brackish to saline and alkaline in nature. Chadda’s, Piper, and other bivariate diagrams that presented the geochemical facies of the groundwater showed Na-Cl as the dominant water type followed by Ca/Mg-Cl, thus indicating the influence of saltwater intrusion and inverse cation exchange reactions in the aquifer. Salinity values (EC) of groundwater are highly variable rising from 430 μS/cm in the boreholes located further inland to 13,000 μS/cm close to the coastline (avg 3246 μS/cm) with the majority above 2500 μS/cm indicating significant salinization of groundwater. The contribution of seawater in groundwater varies from less than 0.01 % in the inland areas to 17 % in areas close to the coastline. Moreover, a significant correlation observed between EC with Na+, Cl−, and SO4 2− ions suggests the same source and possible incursion of nearby saline water. The combination of ionic ratios and recorded EC values clearly justifies the view that the main mechanism for salinization is seawater intrusion, where the saline groundwater results from mixing with seawater. Three main strategies were proposed including monitoring and management to help in the long run to reduce and control the deterioration of groundwater quality in the study area.