Assessment of Hydro-chemical Processes Inducing the Groundwater Salinisation in Coastal Regions: Case Study of the Salalah Plain, Sultanate of Oman

Abstract

Salalah plain aquifer, located in the southeast of Sultanate Oman, has been extensively used for agricultural, industrial and municipal purposes since the early 1970s. Over abstraction to satisfy the growing need of freshwater demand has contributed to the groundwater salinisation by seawater intrusion. The aim of this study is to evaluate the impact of this phenomenon on the groundwater quality in the Salalah plain. The occurrence of seawater intrusion in this plain was analysed using a hydrodynamic and hydrochemical approaches. The first approach was based on the analysis of groundwater table depth and salinity data available during the period from 1984 to 2009. The groundwater piezometric heads recorded in the upstream and intermediate regions of the plain showed seasonal fluctuations due to the monsoon. In the coastal region, the piezometric heads showed small fluctuations, which are indicative of a quasi-static regime. The groundwater salinity showed large variations and generally increased in the flow direction from north to south. Near the shoreline, the groundwater salinity has increased by 8420 µS/cm during the period from 1984 to 1994. In the study area, 11 groundwater samples were collected during October 2015 and analysed for EC, pH and major ions. Results indicate that groundwater chemistry was classified as Na–Cl type in one well, Ca–Mg–Cl type in three wells and Ca–Cl type in seven wells. The first two water types indicate the effect of seawater intrusion and comparatively the Ca–Cl water type highlights the effect of ion exchange reactions on the mineralisation of groundwater. The high concentrations of nitrate recorded especially in the Hafah agricultural area confirm the impact of fertilizers application on groundwater quality.