Lebanon’s groundwater is contaminated by myriad sources, but none rivals that of seawater intrusion when it comes to the coastal aquifers of Beirut. Additionally, raw sewage compounds the problem in the heavily populated cities of Beirut, Tripoli and Sidon [1] with an estimated well density exceeding 375 per square kilometer [2] . This concurrent deterioration of groundwater has gone unchecked for decades on end. Although contamination differs from pollution in that the latter constitutes an additional health threat [3] , in this study they will be used interchangeably.

Normally, seawater intrusion occurs in unconfined and confined coastal aquifers whenever the water table or piezometric head is inadvertently lowered through excessive pumping. These coastal aquifers usually support large city populations where withdrawals from these aquifers often exceed recharge during the rainy season as has become the case for Beirut and its southern suburbs [4] . This can cause lateral intrusion (from the ocean due to excessive water withdrawals from coastal aquifers) as well as upcoming (from deeper seawater to pumping wells). The encroaching seawater will encounter an area known as the zone of dispersion, where the freshwater and seawater mix and form a broad interface. This interface moves back and forth naturally with the passing of seasons.

In recent years, an unhindered influx of Syrian refugees, topped with recurrent periods of drought has placed tremendous stress on already dwindling groundwater resources. These concurrent tensors have led to seawater replacing fresh groundwater in many wells of Beirut.

In light of any permanent and comprehensive monitoring network for groundwater, aquifers of Lebanon still require a great deal of insight. It is imperative to start with the basics and that would be to delineate clearly the hydraulic parameters of all aquifers through meticulous aquifer tests. Only then, can water experts begin to estimate sustainable yield in order to determine the allowable groundwater abstraction rates without incurring any long term damage to the quantity or quality of aquifer water as well as the hydraulically connected overlying ecosystems.

Some water experts continue to confuse safe with sustainable yield when addressing the maximum amount of water that can be safely withdrawn (or abstracted) from aquifers.

Safe yield is defined as the amount of water abstracted from an aquifer without exceeding its recharge, sustainable yield however takes into account the overlying basin or watershed by equating sustainable yield with a sliding fraction of the safe yield, often regarded in the whereabouts of 10% of an aquifer’s recharge [5] .

Not unlike other countries afflicted with seawater intrusion, a comprehensive monitoring program should be emplaced along the coastal cities of Lebanon to continuously map said encroachment and deal with aquifers afflicted with ever rising TDS values.

To that end, monitoring wells and piezometers further inland should also be emplaced to sample groundwater for other pollutants as well, namely from unregulated and untreated wastewater discharge [6] .

Initial sampling and testing of groundwater began in 2004 (Figure 1) then again during the summer of 2014 (Figure 2) in Greater Beirut. Nearly a third of Lebanon’s total population of about six million currently live in Beirut which has a

surface area of approximately 130 km² [7] , circumscribed by the Kalb River to the north, Damour River to the South, and the Lebanon Mountain range to the east.

Tests were carried out in accordance with ASTM, 2001 guidelines, D4449-01 Standard Guide for Sampling Groundwater Monitoring Wells. The test results of the two summer campaigns are summarized on page nine.

Water wells were analyzed on site for pH and Total Dissolved Solids (TDS), thus minimizing any errors imparted by transporting samples to laboratories. TDS is used interchangeably with salinity in this article for the sake of simplicity. The various parameters were analyzed using standard procedures in accordance to “The Standard Methods for the Examination of Water and Wastewater” [8] . TDS and pH were measured using individually calibrated portable testers (LaMotte, USA).

Over twenty wells were monitored monthly in Beirut in 2004 (Figure 1), then again during the summer of 2014 (Figure 2). The sampled wells were first purged before taking in-situ TDS measurements usually within 15 minutes. In one instance however, well #20 was allowed to run for a few hours. In the first half hour, a TDS reading of 22,000 mg/L was recorded, then upon termination, the TDS had risen to over 30,000 mg/L, nearing that of seawater composition.

The results were evaluated in accordance with the Lebanese Bottled Water Guidelines [9] ; and the World Health Organization (WHO) Guidelines for Drinking Water Quality [10] . ESRI’s ArcGIS 9.3 was used to generate the geological figures in the study area. Note that every square in the figure is equivalent to one square kilometre. All the wells that were sampled in Figure 1, are alpha numerically numbered either with the letter P (for private wells) or G (for government wells).

To understand the aquifers and aquitards in and around Beirut, the hydrostratigraphic units of are briefly summarized in Table 1. The terms, aquifer and aquitards are relativistic terms and preferred to the terms aquiclude and aquifuge which have fallen out of use [11] .

The aquifers of Beirut most vulnerable to seawater intrusion include the coastal Sannine (C₄) limestone formation as well as the Quaternary (q) alluvial deposits.

Seawater would render groundwater in these aquifers unsuitable for public supply with only a 2% contamination [2] . Normally, a 1% mixing would triple groundwater salinity or TDS, while 5% mixing would increase salinity to roughly 4500 mg/L [12] . Hence once freshwater resources are degraded by salt contamination, it will take a very long time for aquifers to recover, and if positive groundwater recharge conditions are not re-established, they may never recover [13] .

The TDS measurements, especially of Well #20 clearly indicate a trend steadily increasing over the years reaching concentrations equivalent to that of

seawater in an increasing number of wells. The question however, whether the rise in groundwater salinity is due to seawater or other sources of salts has been satisfactorily addressed in numerous studies, by Saadeh in 2008 [14] , then again Jurdi in 2010 [15] . It could be suggested that in the span of nearly 10 years, there is nearly a 23 fold increase in groundwater salinity (TDS) in the densely populated southern suburbs of Beirut (Figure 3).

Although the World Health Organization guidelines for drinking water [11] do not express any health hazard from increased TDS concentrations, the economic impact of saline groundwater on Beirut’s infrastructure is incalculable.

The wells most severely affected by seawater intrusion are unsurprisingly located in the densely populated southern suburbs of Beirut. Most notable of the wells sampled in the summer of 2014, is #13 (Table 2) where TDS is equivalent to that of the Mediterranean Sea composition of 37,500 mg/L. Well #18, also in the southern suburbs of Beirut, further attests to the severity of seawater intrusion.

Other severely affected wells include #19 and #20 located in what is referred to as Ras Beirut. Their values of 23,000 and 24,200 mg/L respectively indicate nearly a six-fold increase in groundwater salinity in just under ten years between 2004 and 2014.

Once seawater has encroached so severely upon aquifers, not unlike those of Beirut, reversing the effects even through what is often referred to as Aquifer Storage Recovery (ASR), may take decades to undo, according to many studies including Bear in 1999 [12] , then again Barlow in 2003 [2] . Even though, Aquifer

Storage Recovery or ASR is often employed for coastal aquifers around the world, only potable water standards should provide a reference point for recharge water quality [16] .

The quality of recharge water is in fact a major hindrance to current ASR projects in Beirut, since often enough untreated surface runoff becomes a poor substitute for clean freshwater, only compounding the problem of aquifer contamination.

The results of the initial study in 2004, then again in 2014, reflect the deleterious effect that seawater intrusion has claimed on the coastal aquifers of Beirut and suburbs. Hence, this study emphasizes the need for an immediate implementation of a comprehensive integrated water resources management plan (IWRM).

Water management strategies are by no means a one size fits all approach, and each has to be tuned to its unique set of inherent goals, but the aforementioned is at the very least a step in the right direction.

Tony Allan warns that “wherever we irrigate, society always runs out of water” [17] . Lebanon, and the Middle East continues with its heavy reliance on irrigation consuming 70% of its renewable fresh water resources [6] .

If an improvement of only 10% in agricultural efficiency were to be achieved, that could potentially double the resources available for public water supply [18] .

This study took several years of effort and countless long hours to bring to fruition, which would not have been possible without the unwavering help of my friends and colleagues, Mr. Elie Wakim and Mr. Ali Koumaiha.

Saadeh, M. and Wakim, E. (2017) Deterioration of Groundwater in Beirut Due to Seawater Intrusion. Journal of Geoscience and Environment Protection, 5, 149-159. https://doi.org/10.4236/gep.2017.511011