Spatial distribution and risk assessment of potentially toxic elements in coastal mangrove floodplain Top-Soils of Bangladesh

Abstract

Soil pollution caused by human activity is becoming an increasingly urgent global concern. Floodplain soils are particularly susceptible to contamination due to their role as temporary sinks for sediments, nutrients, and pollutants transported by the river systems. Mangrove floodplains possess significant ecological value, being unique ecosystems that are especially vulnerable to pollution, particularly from potentially toxic elements (PTEs). This study provides a comprehensive analysis of the concentration and spatial distribution of PTEs in the soils of Koyra Upazila, Bangladesh, a region characterized by a coastal mangrove ecosystem. A total of 60 soil samples were collected from three subdivisions to assess pollution levels. The samples were subjected to acid digestion (65 % HNO3 and HClO4 in a 4:1 ratio) and were analyzed using Atomic Absorption Spectroscopy (AAS) with a varian system (Varian AA240 FS). The investigated PTEs, ranked by decreasing concentration, were Zn > Cr > Pb > Ni > Cd > Hg. Zn exhibited the highest average concentration (67.15 ± 12.24 mg/kg), whereas Mercury had the lowest (0.02 ± 0.04 mg/kg). Distribution patterns, illustrated by Q-Q plots, revealed that Pb and Zn data closely followed a normal distribution, while Cd, Cr, and Hg displayed significant deviations, suggesting skewed distributions influenced by both natural variability and anthropogenic activities. Assessments using the Geo-accumulation Index (Igeo), Pollution Load Index (PLI), and Nemerow Integrated Pollution Index (NIPI) indicated that the soils were generally uncontaminated, although moderate pollution levels of Pb and Zn observed in specific locations. Potential Ecological Risk Index (PERI) analyses suggested minimal ecological risks, despite the moderate pollution levels revealed by PLI. Health risk assessments highlighted elevated non-carcinogenic and carcinogenic risks for children, particularly in the central and southern regions of the study area. Furthermore, a comparative analysis against international soil quality standards from Canada, the Netherlands, and Australia confirmed that the PTEs concentrations in the study area remained within permissible limits, indicating predominantly natural origins with minimal anthropogenic influence. The study establishes a critical baseline for PTEs concentration in the mangrove floodplain soils of Koyra Upazila, revealing localized risks, particularly in urban areas like Koyra union. While most PTE levels are within permissible limits, the findings highlight the need for periodic monitoring and targeted mitigation measures to protect vulnerable ecosystems and public health.