THREE YEARS MONITORING OF NATURALLY OCCURRING RADIOGENIC CONTAMINANTS IN COASTAL PALEOBEACH AQUIFERS IN COX’S BAZAR-TEKNAF, BANGLADESH: POTENTIAL THREAT TO PUBLIC HEALTH

The Cox's Bazar-Teknaf paleobeach (backdune) area is composed of Quaternary unconsolidated sedimentary formations of fine to medium size sands, which form the groundwater aquifers, together with minor patches of silty-clay and alluvium. Unaltered sandy beaches with a shoreline that faced the Bay of Bengal evolved in front of the paleobeach, at a height of 0.46 to 8.84 meters above mean sea level. Significant concentrations of minerals like monazite and zircon that are U and Th-bearing have been found in the paleobeach areas. The aquifers in this region have been over-exploited to meet the demand for not only the local people but also thousands of tourists and millions of Rohingya refugees who reside in the paleobeach areas, despite the possible danger of radiogenic contamination. To assess the radiogenic pollution of the water supply, twenty shallow tubewells (03 to 100 m depth) scattered throughout the area were monitored for geochemical parameters of groundwater in June 2012, June 2013, and June 2016 with the goal of evaluating the radiogenic pollution of the drinking water. The results demonstrated that variations in pH and Eh over a three-year period were correlated with variations in radiogenic pollutants, specifically U and Th concentrations. In comparison to 2012 (0.01-3.40 µg/l for U and 0.002-1.59 µg/l for Th), 2013 concentrations of U (0.09-9.57 µg/l) and Th (0.02-127.09 µg/l) were significantly higher. Eh and pH values were slightly higher in 2013 (0.11-0.61V and 7.52-8.47) than in 2012 (0.26-0.47V and 6.56-8.30), respectively. While the concentrations are still slightly higher than those in 2012, samples from 2016 show lower values of Th (0.01-8.33 g/l) and U (0.11-5.93 g/l) than those from 2013 did. The pH ranged from 6.40 to 7.67, and Eh was also high (0.46-0.68 V) in the 2016 samples. The influence of seawater has also been observed in the substantial association of U and Th with groundwater level, EC (202-6730 µS/cm), alkalinity (1.24-13.12 meq/l), and concentrations of Ca, Mg, Na, K, and Cl. While chemical weathering (dissolution and reprecipitation) of detrital monazite and zircon grains must be the essential mechanism for releasing U and Th into the aquifer(s), changes in the hydrological conditions in soils and aquifers brought on by changes in landuse may promote increasing those levels, which is a significant and growing concern for drinking water in the area.