Hydrogeochemical and isotopic signatures for the identification of seawater intrusion in the paleobeach aquifer of Cox's Bazar city and its surrounding area, south-east Bangladesh

In order to better understand the encroachment of seawater into the groundwater system a total 115 groundwater samples were collected in an area of ∼7 km2 in Cox's Bazar paleobeach and its vicinity, from different types of tubewells, which included hand pump fitted bore wells (<50 m depth), moderately deep energized bore wells (50 m–100 m depth) and deep bore wells (>100 m depth) during June 2013. A combined hydro-geochemical indicators, selected trace elements (Sr and B) and in addition to δ18O and δ2H were used in this study. Results show that the high values of EC (202 μS/cm to 6730 μS/cm) and TDS (135 mg/L to 4509 mg/L) are found in groundwater of wells closer to the coast and few wells from the northern and central part of the study area. The cross plot of HCO3/Cl and TDS shows that groundwater along the paleobeach and some isolated areas are saline with TDS > 1000 mg/L, associated with high ratios of Cl/TDS (0.065). The groundwater facies generally evolves from freshwater (Ca-Mg-HCO3) to saline water (Na- Cl) type with an intermediate Ca-Mg-Cl type indicating that the aquifer system interacts with seawater and undergoes cation exchange. Results also reveal that the area affected by marine water intrusion has ionic ratios of Br/Cl (0.0006–0.0021) similar to seawater (0.0015) with some higher values. Groundwaters along the paleobeach and some isolated areas, low ionic ratios of SO4/Cl (0.01–6.53) and Na/Cl (0.20–152.09), relative to marine ratios (0.05 and 0.86 respectively), are also observed. Groundwaters with Seawater Mixing Index >1.0 and TDS >1000 mg/L constitute about 20% of the studied groundwaters and have relatively high δ18O (>- 4.0‰) values and the linear relationships between TDS and most of the ions, including B and Sr, and the chemical signature of the saline plumes (e.g., marine SO4/Cl, Na/Cl and Br/Cl ratios) suggest that mixing processes control the chemical composition of the seawater within the aquifers. However, these geochemical variations also reveal that the quality of potable water has deteriorated to a large extent due to seawater intrusion along the paleobeach, migrating inland toward the heavily groundwater exploited areas from the coast line and may entail various future health hazards.