Quantification of Submarine Groundwater Discharge in the Gaza Strip

Ashraf Mushtaha,Kristine Walraevens

doi:10.3390/w10121818

Abstract

Gaza Strip has suffered from seawater intrusion during the past three decades due to low rainfall and high abstraction from the groundwater resource. On a yearly basis, more than 170 million m3 of groundwater is abstracted, while the long-term average recharge from rainfall is 24.4 million m3/year. Submarine groundwater discharge (SGD) has never been studied in the Gaza Strip, due to lack of experience in this field, next to the ignorance of this subject due to the seawater intrusion process taking place. Continuous radon measurements were carried out in six sites along the Gaza Strip to quantify the SGD rate. The final result shows SGD to occur in all sampled sites. The range of SGD rates varies from 0.9 to 5.9 cm·day−1. High values of SGD are found in the south (Rafah and Khan Younis governorates). The high values are probably related to the shallow unconfined aquifer, while the lowest values of SGD are found in the middle of Gaza Strip, and they are probably related to the Sabkha formation. In the north of Gaza Strip, SGD values are in the range of 1.0 to 2.0 cm·day−1. Considering that SGD would occur with the measured rates in a strip of 100 m wide along the whole coast line, the results in a quantity of 38 million m3 of groundwater being discharged yearly to the Mediterranean Sea along Gaza coast. Nutrient samples were taken along Gaza Strip coastline, and they were compared to the onshore wells, 600 m away from the Mediterranean Sea. The results show that SGD has higher NO3− + NO2− than nutrient-poor seawater, and that it is close to the onshore results from the wells. This confirms that the source of SGD is groundwater, and not shallow seawater circulation. In a coastal strip of 100 m wide along the Gaza coast, a yearly discharge of over 400 tons of nitrate and 250 tons of ammonium occurs from groundwater to the Mediterranean Sea.

Highlights

Submarine groundwater discharge (SGD) may not play a significant role in the global water balance, there are reasons to believe that the geochemical cycles of some major and minor elements may be strongly influenced either by the direct discharge of fresh groundwater into the sea, or by chemical reactions that occur during the recirculation of seawater through a coastal aquifer system [1,2,3]
Using nutrient analytical results along the shoreline helps us to identify potential sites of submarine groundwater discharge, after eliminating sites where nutrients may be due to other potential sources
The raised nutrient concentrations in shallow groundwater collected near the shoreline, helped us to confirm that SGD is taking place, and that it can be further investigated by the means of 222 Rn measurements

Summary

Introduction

Submarine groundwater discharge (SGD) may not play a significant role in the global water balance, there are reasons to believe that the geochemical cycles of some major and minor elements may be strongly influenced either by the direct discharge of fresh groundwater into the sea, or by chemical reactions that occur during the recirculation of seawater through a coastal aquifer system [1,2,3]. Groundwater contamination, being a wide-spread problem, SGD may bring pollution to coastal seawater [3,4]. Moore [2] proposed the term “subterranean estuary (STE)” for the aquifer zone, where recirculating seawater mixes with fresh groundwater, and where water–rock interaction affects the mobility of constituents, including nutrients, towards the sea [11]. Lebbe [12], Water 2018, 10, 1818; doi:10.3390/w10121818 www.mdpi.com/journal/water

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