Elaboración de matriz de prioridad de indicadores

In future scenarios, two main factors might influence the risk of seawater intrusion in this area, (i) increasing water consumption by the growing tourist industry and a connected growing local population with increased water demand and (ii) sea level rise due to the climate change, especially considering the flat topography of the study area.

Fig. 31 shows the distribution of pumping wells which were active during the study period. These are 332 wells including the sampling wells used in this study and identified hotels’ wells. Other private wells were not considered. The highest density of pumping wells is in the western and the north-eastern part of Cox’s Bazar city (Fig. 31), representing the areas with the largest volume of water extraction. According to Trabelsi et al. (2016) areas with 7 to 12 wells per km2 _{are highly hazarded zone compared}

to zones with less than 2 wells per km2_{. In the study area, the density of the wells is more intense as}

shown in Fig. 31. The area of interest for this modelling belongs to the Jhilwanja union of Cox’s Bazar Sadar upazila. Considering the total area of 29 km2 _{of Jhilwanji union, an average density of the wells}

is 11 per km2_{. In addition, distribution of the wells varies between 3 to 114 per km}2 _{only considering the}

areas along the shoreline and old city center of Cox’s Bazar. Although the wells are not evenly distributed it already shows the area is highly hazarded zone to saltwater intrusion. Total households of the Jhilwanji union are 7406 (BBS 2013) if all the households consist of one well; the intensity of vulnerability would be more intense in this coastal aquifer.

Fig. 31: Distribution of the pumping wells in the study area.

More importantly, if the distribution of the pumping wells is compared with the GALDIT vulnerability index (Fig. 30), the zones of vulnerability correspond well with the pumping wells distribution (Fig. 31) in the area. Both the vulnerability index map and pumping well density map indicating that the area along the shore and the old city center are high-risk zones compared to the other parts of the coastal aquifer. This confirms that the main reason for seawater intrusion in this reason due to the over- extraction of wells for the increasing tourists and population water demand. Likewise, it is mentioned in the previous section, that the calculated recharge and the water consumption based on the tourist and local population demand do not balance for the long run. Therefore, the further lowering of water level in this tourist area enhanced the seawater intrusion if more attention is not taken from now on the vulnerable zones especially zone I and II respectively.

5.5.2.2. Scenario 2: Computation of vulnerability index in the context of sea level rise

Sea level rise in principal has the same effect as lowering the water level in the aquifer due to pumping, as gradients are influenced. As Bangladesh considered one of the more vulnerable countries to sea level

rise, a sea level rise of 0.5 m (based on IPCC 5th _{assessment report, sea level rise in Bay of Bengal 20-90}

cm by 2100) was considered in the GALDIT model (Fig. 32).

Fig. 32: Computed GALDIT vulnerability Index (GVI) of the Cox’s Bazar coastal aquifer for the scenario of sea level

rises 0.5 m.

In this case, the groundwater level above MSL was changed based on the sea level rise (SLR) scenario of 0.5 m. So, SLR of 0.5 m scenario resulted in a 0.5 m reduction in groundwater level above mean sea level (MSL). This evaluation shows that the wells which are not currently affected by seawater (Fig. 30), sea level rise of 0.5 m has intensified the extent of saltwater in the aquifer (Fig. 32) and could be more intense than the GALDIT approach predicted in Cox’s Bazar coastal aquifer. Fig. 32 also illustrates that the area of highest risk of seawater intrusion remained same with regards to current water level, i.e. area close to Bay of Bengal and eastern part of the Bay; however, with greater risk. The GALDIT

The simulation results also show that about 22% of the shallow wells (<50 m) are highly vulnerable to saltwater where it was only about 9% based on the current sea level. The extent of saltwater vulnerabilities could be more intense in this coastal aquifer if GALDIT index model would consider sea level rise of 1 m and 1.5 m for simulation of saltwater intrusion vulnerabilities. As the sea level increased, the areas of highest saltwater intrusion vulnerability expanded and intensified. However, this figure is also important for considering in GALDIT model for the future risk assessment with pumping factor on coastal groundwater.

Additionally, some zones require basic recommendations due to the change in the natural system. This characteristic of the study area needs to be considered for sustainable groundwater management. Consequently, special attention is required in the western part as well as north-eastern part of the Cox’s Bazar which is characterized moderate to highly vulnerable based on GALDIT vulnerability index model. Additional risk activities should not be allowed to protect coastal resources and the economic advantages of the tourism sector. The high distance of pumping wells installation should be maintained both in the hotels and domestic sector around highly vulnerable areas. Meanwhile, excessive exploitation of groundwater and installation of wells in the low vulnerable zone should not be considered for the shake of conservation and protecting the coastal resources.

Although, there is a lack of well distribution of sampling wells and lithological information that restricts GALDIT method to calculate the area of the aquifer affected by seawater intrusion. This might be a drawback of the method in such a case of Cox’s Bazar area. However, the visual overview of seawater intrusion vulnerabilities of GALDIT model can be a qualitative indicator of rational decision making for the water resources management of coastal aquifers like Bangladesh.

6. Conclusion and outlook