Evaluation of toxic metal pollution in freshwater surficial sediments using environmental indices and multivariate statistical approaches – A systematic review

Surface sediments form an integral component of freshwater ecosystems and they are a major sink-and-source for toxic pollutants, providing a reliable indication of a water body's integrity. Distinct freshwater sediment investigations have informed the conclusions made about the ecological and pollution status of aquatic systems worldwide, but a widespread evaluation of the global status of freshwater sediments is lacking. From our perspective, an extensive environmental analysis of the available published data can address this need and improve our wholesome understanding of toxic metal impacts on global freshwater systems. Thus, surface sediment metal data collected from 149 freshwater sites in 32 countries were systematically analyzed using standard environmental indices (e.g. geoaccumulation index, modified hazard quotient, enrichment factor, etc.) and multivariate statistical methods (MSA). Average concentrations of all the metals except cobalt and zinc exceeded the recommended limits. Arsenic, cadmium, and mercury registered the highest frequency of severe pollution impacts on 29 – 69 % of the sites. 4 to 31 % of the studied sites recorded considerable to severe aquatic biota risk majorly from arsenic, cadmium, chromium, and nickel while 65 % of the sites recorded severe ecological risk (CSI > 5, RI ∼ 600). A high linear correlation with low ordination stress (R² = 0.93, Stress = 0.023) from non-metric multidimensional scaling agreed with the Pearson correlation analysis results, while principal component analysis revealed four major components that explained 89 % of the data variance. Source enrichment investigation indicates that pollution is a result of geogenic and anthropogenic contributions. The common anthropogenic sources among study sites include industrial and municipal wastewater and sewage, agriculture, surface runoff, fossil fuel emissions, and mining activities. The study can serve as a reference for future pollution studies, create extensive awareness of the dire ecological status of freshwater systems, and ultimately elicit site-specific remediation and mitigation action plans from policymakers.