Sources of U and Th in groundwater of the paleobeach aquifer at Cox's Bazar, Southeast Bangladesh

To explore the influence of radioactive heavy minerals on thorium (Th) and uranium (U) contamination in groundwater, we performed geochemical and mineralogical investigations of core samples (from the ground surface to 18.9 m depth) from a paleobeach area in Cox's Bazar, southeastern Bangladesh. Geochemical data document elevated levels of total Th (6.3–202.3 mg/kg, average 32.9 mg/kg) and U (1.1–33.4 mg/kg, average 5.2 mg/kg), and lithological observations document monazite concentrations as great as 2.36 wt% and zircon concentrations as great as 3.82 wt%. These minerals were found in organic-rich fine to very fine sand in the uppermost meter of the sediment column (down to 1.2 m depth) rather than in coarser sandy sediment. Energy-dispersive spectroscopy of several monazite and zircon grains indicated that alteration was greater in oxic sediment (down to 3 m depth) than reduced sediment (deeper than 3 m). Monazite and zircon had relatively high concentrations of Th (3395.9–3937.5 mg/kg and 275.5–318.4 mg/kg, respectively) and U (850.7–990.6 mg/kg and 256.3–290.5 mg/kg, respectively). 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. Shallow tubewells (<20 m depth) waters were enriched in U (0.09–9.57 μg/L) and Th (0.02–127.09 µg/L) and that the enrichment mostly occurred in highly oxic environments (Eh = 0.35–0.5 V). We suggest that hydrological and biogeochemical conditions in the aquifer that follow land-use changes influence the release of Th and U into the groundwater of the study area.