Ecological and human health risk evaluation using pollution indices: A case study of the largest mangrove ecosystem of Bangladesh

Wetlands such as mangroves are major centres of biodiversity and the production of ecosystem services from which society gathers goods and benefits. However, these attributes rely on a high environmental quality. That quality is recorded by quantitative indicators which are essential for environmental management. As such, three different salinity domains (Kachikhali, Katka, and Dublarchar) in the Sundarban Mangrove Forest, a world heritage site, were sampled in 2019 and 2020 to determine the heavy metal contamination levels and estimate the probable ecological and human health risks. The study revealed that with the exception of the changed dissolved oxygen (DO) concentration (4.5 to 6.05 mg/L), there was no significant difference between the two years in eight water quality parameters. Heavy metal concentrations followed the sequence in sediment as (mg/kg): Fe (31500) > Mn (281) > Sr (208) > Zn=Cu (77) > Co (6) > Pb (2) and in water as (μg/L): Fe (33.5) > Sr (0.6) > Cu (0.3) > Zn (0.22) > Pb (0.15) > Mn=Co (0.05). Sediment was moderately contaminated by copper as indicated by the degree of contamination (Cd=3.6 to 5.42) and the geo-accumulation index (Igeo=0.07 to 0.33). The human health hazard index (HI) values were 261, 20.6 and 20.6 for children, adult male and adult female, respectively, in Mn due to the inhalation process, indicating elevated health risk. Children can be at high risk due to manganese contamination by skin contact where HI = 2.18 ×10. In contrast, all the Hazard Quotient (HQ) and HI values for other metals were deemed to be safe (<1). This study gives both a comparison for less-contaminated sites and a model for similarly contaminated sites.