Results and Discussions

The results of the simulations provide insights into the degree of sustainability of the groundwater under the imposed pressures in terms of both quantity and quality. To investigate the sustainability of the quantity, groundwater budget calculated by the model is examined. Recharge and discharge components of the groundwater budget, as well as the changes in these components induced by the simulated pressures are determined for each sub-scenario. Based on these outcomes, capture attributable to pumping is evaluated and compared to the pumping rate of the corresponding scenario. However, it is observed that for each of the sub-scenarios, system attains an equilibrium state by reducing the groundwater discharge into the sea at an amount equal to the rate of groundwater pumped. Even though this situation could be referred as "sustainable" in terms of water quantity, increased pumping rates - despite the compensation by the reduced discharges - causes a gradual upconing which eventually causes pumping of the saltwater. Hence, it is concluded that; such a groundwater budget analysis, discarding the groundwater quality is not sufficient to assess the sustainability of this coastal system. Therefore, rather than the budget analysis, decreases in the volume of the freshwater lens due to the saltwater intrusion caused by decreasing recharge rates are calculated for each of the sub-scenarios. Melloul and Collin [8] refer this volumetric loss from the freshwater reserves caused by the intruding saltwater to as permanent reserve loss, which is very difficult to be replaced. Results indicated that:

• In the base scenario, volume of the freshwater lens decreases in the range 0-6% only in response to the changing pumping rates

• In the second scenario, volume of the freshwater lens decreases in the range 10-16% due to the combined effects of changing pumping rates and decreasing recharge rates

The reserve loss as a consequence of decreasing recharge rate is more significant than that of increasing demand at the end of the 100 years simulation period. Moreover due to the fact that it imposes additional stress to the groundwater resource, it should be considered in determination of the sustainable yield of the system. It is obvious that there has to be a compromise between the groundwater reserve loss and the pumping rate that will supply the increasing demand, which is even more severe under accelerating stresses exerted by the climate change.

Lateral intrusion and upconing of the freshwater-saltwater interface are the two main criteria concerned in determination of the sustainability of groundwater quality. Lateral intrusion of the freshwater-saltwater interface is examined at the end of

Fig. 6.3 Elevation of the freshwater-saltwater interface versus pumping rate

100 years at the model layer corresponding to the bottom elevation of the pumping well. The numerical results, listed below, reveal that the effect of decreasing recharge rate on lateral saltwater intrusion is higher than that of increasing demand:

• In the base scenario, freshwater-saltwater interface laterally intrudes a distance in the range 0-26 m only in response to the changing pumping rates

• In the second scenario, freshwater-saltwater interface laterally intrudes a distance in the range 94-130 m due to the combined effects of changing pumping rates and decreasing recharge rates

The changes in the elevation of freshwater-saltwater interface due to upconing for the two scenarios and their sub-scenarios that correspond to different pumping rates are demonstrated in Fig. 6.3. This figure reflects a very crucial response of the system to the imposed stresses of increasing demand and decreasing recharge rates. Accordingly, for the base scenario, almost 12 L/s of water can be pumped sustain-ably considering water quality, as the freshwater-saltwater interface is below the bottom of the well (at -25 m elevation). This amount, however, reduces by about 30% (to almost 8 L/s) in case of a decrease in recharge by 15%.

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