title: Vulnerability assessment of the coastal aquifers in the Cox’s Bazar area, Bangladesh using hydrochemical tools and the GALDIT model
Creators: name: Fatema, Suraiya
Publisher: name: UNSPECIFIED
Year: 2020

description: The Cox’s Bazar region at the south-eastern coast of Bangladesh has gradually been changed from a rural settlement into a densely populated urban area, caused by the rapid growth of tourism. Nearly 2 million visitors visit every year for enjoying its unbroken 120 km long sandy beach during the dry season. Water demand is mainly covered by groundwater, and the hotels and resorts are typically operating their own groundwater wells without metering and regulations. Thus, overexploitation and possible seawater intrusion threatens the groundwater resources and its sustainability in the coastal aquifers. In this study, the temporal and spatial variations in groundwater quality concerning hydrochemistry and hydraulic heads in the Cox’s Bazar area were evaluated over a period of four years, and an analysis of the effects of the groundwater extractions on the status of the groundwater resources has been conducted. At the same time, the hydrochemical data were also combined with the stable isotopes of δ18O, δ2H and δ34S for identification of sources of salinity in the coastal aquifers of Cox's Bazar area. Finally, to assess the vulnerability to seawater intrusion, a qualitative approach of GALDIT model was used to give a visual overview of saltwater intrusion vulnerabilities. Due to the pronounced seasonality of rainfall, the aquifer system was found to be highly dynamic even without human interference, and seawater intrusion into the aquifers from the Bay of Bengal as well as from the Bakkhali river in the north was detected. The groundwater abstraction caused groundwater levels in some touristic centers to be already permanently below sea level, and a trend to a further lowering of hydraulic heads was observed. Such situation coincides with an overall tendency of increasing electrical conductivities in the groundwater. The Cl-/Brmass ratios also showed a clear inclination of seawater mixing in the groundwater as well as surface water. Seawater mixing with groundwater can be explained by the combined effects of overexploitation of groundwater and the tidal influence in the surface water. The isotopic composition of δ18O and δ2H in groundwater samples is scattered along the GMWL and LMWL. Although the apparent contribution (fsea) of seawater in the groundwater samples varies from 1 to 25% and in the surface water 4 to 99% depending on distance and season, it is difficult to identify one reason for the scattered isotopic composition found in shallow wells (depth &lt;50 m), intermediate wells (depth 50-150 m) and deep wells (depth &gt;150 m). This scatter could be due to seawater mixing, evaporation or scatterings of rainwater isotopic composition. In such a case, δ34S isotopes turned out to be a useful tool to identify seawater encroachment in the coastal aquifers of Cox’s Bazar. δ34S in sulphate in groundwater samples located close to the Bay have a similar δ34S isotopic composition as modern sea water. The negative linear correlation of δ34S with SO42-/Cl- mass ratios also confirmed the source of salinity from seawater. Similarly, the results of the vulnerability index modeling depict that the area close to the Bay of Bengal and Bakkhali river is more vulnerable to seawater intrusion than the south-eastern part of the study area. The results of this research also revealed that 9% and 78% of shallow wells are highly and moderately vulnerable to seawater intrusion, respectively, and 13% are potentially at low risk under the present condition. At the same time, considering sea level rise of 0.5 m would result in a substantial increase of the highly and moderately vulnerable areas in this coastal aquifer. In this case, 22% of the shallow wells (&lt;50 m) are highly susceptible to seawater encroachment upon sea level rise. The consistency of the hydrochemical and isotope data with the vulnerability zoning map indicates that for a sound and sustainable development of the Cox’s Bazar region, water management strategies and a regulatory framework for water abstraction and its central distribution must be developed. These findings also suggest that a combined approach using hydrochemical, isotopic, and qualitative indicators like the GALDIT index could be an essential tool for sound decisions in water resources management in coastal aquifers.