Evolution of salinity in the Gaza Strip over the last five decades deduced from field data

Abstract

Gaza coastal aquifer has been exposed to severe overexploitation to meet the human needs for water supply. This exploitation has resulted in lowering of the groundwater level to by more than 15 m below mean sea level in the south and around 5 m in the north. Groundwater level drop has exposed the aquifer to seawater intrusion and inland saline upconing in the central south and south-east of the Gaza Strip.We developed five cross sections based on wells drilled over the past five decades representing aquifer stratigraphy in the five governorates (North, Gaza, Deir El Balah, Khan Younis and Rafah) of the Gaza Strip. Those cross sections have been used to map the chloride concentration over the past five decades (years 1972, 1980, 1990, 2000, 2010, 2017 and 2021).The results show three main sources of aquifer salinity: (1) seawater intrusion, (2) lateral inflow from Eocene rocks in the east and/or from agricultural return flow as a result of using treated wastewater for irrigation in Israel close to the eastern border and (3) upconing due to fossil marine salt water trapped at the bottom of the aquifer. The clay layers which are subdividing the aquifer have caused a difference in sub-aquifer level chloride difference. Before the year 2000, the public water supply wells were placed far away from the sea and no seawater intrusion was noticed especially in the south of Gaza Strip (Khan Younis and Rafah governorates), while afterwards public water supply wells appeared within 2 km from the Mediterranean Sea. From then on, seawater intrusion has been taking place due to heavy groundwater abstraction. Also, lateral inflow with chloride concentrations of more than 1,000 mg/l is well noticed during the different years, related to eastern inflow from Eocene rock formation where the coastal aquifer rests directly on it, 10–15 km from the coast. Inland areas with high chloride contents were observed as a result of upconing from deep saline groundwater. Cumulative Distribution Function (CDF) curves were created for all wells in each cross section and showed a remarkable shrinkage of freshwater percentage in the aquifer.