RADIOGENIC CONTAMINANTS IN COX’S BAZAR-TEKNAF PALEOBEACH AQUIFER, SOUTH-EASTERN BANGLADESH: POTENTIAL THREAT TO PUBLIC HEALTH

Cox’s Bazar-Teknaf paleobeach (backdune area) areas have been over exploited the subsurface aquifers to meet the demand for thousands of tourists (over 1,000,000 tourists visit each year) along with domestic aquaculture and agricultural purposes. A total of 115 groundwater drinking water samples collected from different depths of tubewells and analyzed using ICP-MS. Geochemical and mineralogical investigations were also carried out on the continuous core sediments (up to 18.9m depth).

Results show that shallow tubewells within the depth range of 25m waters are contaminated with radiogenic (U & Th) heavy metals with the maximum concentration of 9.70 µg/L of U and 127.10 µg/L of Th. This enrichment mostly occurred in highly oxic environments (Eh = 0.35–0.5 V). U and Th are correlated with alkalinity (1.24-13.12 meq/l), Ce (0.01- 9.89ppb) and an inversely with Pb (0.03-80.40 ppb). Elevated levels of total U (1.1-33.4mg/kg) and Th (6.3 -202.3mg/kg) are found in core sediments, which are also enriched with monazite (2.36wt.%) and zircon (3.28wt.%) found in organic-rich fine to very fine-grained sand sediments (up to 1.2m depth). In addition, monazite and zircon itself resulted to relatively rich in Th (3395.9-3937.5mg/kg) and U (850.7-990.6mg/kg), and Th (275.5-318.4mg/kg) and U (256.3-290.5mg/kg) respectively.

It appears that 5% of studied tubewells exceed the WHO (1998) guideline 2.0ppb of U, but Th does not have WHO health-based drinking water guidelines for certain tubewells. These geochemical variations suggest 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. However, chemical weathering (dissolution and reprecipitation) of detrital monazite and zircon grains, which releases U and Th into the groundwater, may be a major and growing concern for drinking water in the study area.