STUDY OF SEDIMENTAL CHRONOLOGY, ELEMENTAL AND RADIOLOGICAL CONTAMINATION OF THE SUNDARBAN BY NEUTRON ACTIVATION ANALYSIS AND ALPHA SPECTROSCOPY TECHNIQUES

Abstract

The utilization of nuclear methods has significant potential for human health and sustainable environmental management. This study utilized various nuclear techniques (alpha spectrometry, neutron activation analysis, and gamma-ray spectrometry) to evaluate the sediment accumulation rate, elemental and radiological contamination history, and ecological risks posed by elements in the Sundarban mangrove forest in Bangladesh to develop a sustainable management strategy for the ecosystem. The mass accumulation rates of sediments, determined for the first time in this area by 210Pb dating, varied from 0.068 to 3.20 kg m -2 y -1, with an average of 0.61 kg m -2 y -1. The contamination history of 11 metal(loid)s (Al, Ca, V, Cr, Fe, Ni, Cu, Zn, As, Hg, and Pb) for this ecosystem has also been assessed. Different environmental contamination indices suggested that Sundarban mangrove sediments have been contaminated by As. Considering different sediment quality guidelines, it has been observed that Cr, Ni, and As posed occasional adverse biological effects on marine organisms. This study also evaluated the essential and toxic element contents of three widely used medicinal plant leaves (Acanthus ilicifolius, Avicennia officinalis, and Xylocarpus mekongensis) of the Sundarban mangrove ecosystem and the possible health risks associated with the consumption of these plants for long time. The measured concentrations of the elements have been compared with available WHO permissible limits, which indicated that the average concentration of Zn in A. ilicifolius (41.2 ± 10.1 mg kg-1) is higher than that of the permissible WHO limit (27 mg kg-1). The average concentrations of Cr in A. ilicifolius (3.35 ± 0.37 mg kg-1) and X. mekongensis (4.02 ± 2.30 mg kg-1) are also higher than that of the WHO permissible limit (2 mg kg-1). Nevertheless, the average daily intake of the toxic elements is below the maximum tolerable daily intake (MTDI) values. In addition, the natural radioactivity (226Ra, 232Th, and 40K) concentration levels of some medicinal plants and soil cores of the Sundarban mangrove forest in Bangladesh have been assessed using a gamma-ray spectrometry technique. The average activity concentrations at the four cores of this mangrove system are relatively higher than those of their global mean values. Depth-wise activity concentration variations for most of the cores indicate that 226Ra, 232Th, and 40K activities increase from the deeper layer to the surface soils, whereas for other cores, variations are haphazard or overall showed no change over the studied depths. The average activity concentrations of 226Ra, 232Th, and 40K in the studied VII medicinal plant leaves varied from 9.03 ± 3.01 to 66.9 ± 8.2 Bq kg-1, 25.3 ± 5.0 to 155 ± 16 Bq kg-1, and 68.1 ± 8.3 to 139 ± 12 Bq kg-1, respectively. For the studied medicinal plants, the soil-to-plant transfer factors of 226Ra, 232Th, and 40K ranged from 0.20 to 1.36, 0.27 to 2.08, and 0.036 to 0.108, respectively. Although some radiological hazard parameters for soils are higher than the global average values, excess lifetime cancer risk values were below the acceptable limit. Different statistical analyses have been performed to find out the origin and behavior of the metals and radionuclides in the mangrove system. This study indicated that the calculated radiological risk parameters for medicinal plants are lower than the acceptable limits prescribed by UNSCEAR. Therefore, the consumption of these medicinal plants by an adult is radiologically safe. Multivariate statistical approaches suggested that sources of metal(loid) pollution are a combination of human activities and natural geological processes. The findings of this study will enhance our understanding of metal pollution and the associated ecological and radiological risks to organisms, hence facilitating the development of novel approaches and management strategies to preserve vulnerable mangrove ecosystems.