Contamination and Ecological Risk Evaluation of Metals in Sediments from a Wetland of International Importance (Ramsar Site)

Abstract

Wetlands are highly productive and diverse ecosystems providing home to thousands of organisms. These ecosystems reduce water pollution, sequester carbon, support livelihoods, and increase food security. However, these ecological functions are being impeded due to increased levels of metals in the environment. Therefore, the primary objective of this study was to evaluate the degree of metal contamination in the surface sediment of a wetland, Tanguar Haor, for the first time. The result demonstrated that the mean concentrations of Fe, Cu, Zn, As, Pb and Hg varied from 13140.39 to 45675, 40.07 to 46.29, 47.60 to 57.15, 18.89 to 35.23, 1.24 to 2.64, and 0.35 to 0.42, respectively. The concentration of As was found to be higher than the average shale value. The concentration of Cu (44.19 μg/g) and Hg (0.38 μ/g) was very close to the shale value (45 μg/g and 0.40 μg/g, respectively), indicating a moderate level of contamination. The contamination level was further evaluated by multi-indices, e.g., the contamination factor (CF), the enrichment factor (EF), and the geo-accumulation index (Igeo). The average EF values for As (115.41), Cu (57.68), and Hg (55.47) were >50, indicating a high degree of contamination (extremely severe enrichment). However, CF values showed varied levels of pollution; for example, the majority of the area was only somewhat contaminated with As, Cu, and Hg, but less contaminated with Fe, Zn, and Pb. According to Igeo, sampling sites were found to be unpolluted or less polluted by heavy metals. Based on potential ecological risk assessment (PERI), the degree of risk from the six heavy metals decreased in the following sequence: As > Cu > Zn > Pb > Cr > Ni. PERI values indicated the study area has been exposed to moderate risk to As and low risk to other metals. This study provides an opportunity for frequent monitoring of heavy metals in this ecologically critical environment, and thus curbing heavy metal pollution.