Lakes and wetlands play pivotal roles in providing nutrients to living organisms. Their bottom sediments, therefore, are sensitive indicators to determine the pollution loadings as they act as both sources and sinks for the contaminants in an aquatic environment (Varol, 2011, Yin et al., 2011). This necessitates their continuous monitoring and assessment.

Surficial sediment contamination as a consequence of various anthropogenic activities has been a cause of serious concern in recent times. Various aspects covering the pollution of sediments have been studied in the recent past. These include spatial distribution of the contaminants, health risk assessment and pollution source identification through various statistical tools such as factor analysis, cluster analysis, correlation analysis and geostatistical analysis employing GIS.

A study was conducted of Deepor Beel recently to assess sediment pollution loadings and their ecological risk. Deepor Beel sits in the heart of the Indian city of Guwahati as a southern channel of the river Brahmaputra. Initially covering a vast area of 40 km², rapid urbanization and urban sprawl coupled with an inefficient watershed management system by the competing authorities has caused the wetland to shrink magnificently to a meagre 4.1 km² stretch of water-land.

Despite being a Ramsar site (No. 1207) under the Convention on the wetland of international importance for the protection of the global biological diversity, no proper measures for its safeguarding have been incorporated.

Given these perspectives, the primary objectives of the present study published in the journal Ecological Indicators include

1. assessment of the pollution loading of the sediment column of Deepor Beel;
2. evaluating the potential ecological risk (PER) through risk index and
3. estimating the metal speciation to determine the forms in which the heavy metals are present in the sediment column of Deepor Beel.

The study was carried out for three seasons; pre-monsoon (January–March), monsoon (April–September) and post-monsoon (October–December). The study will help understand the current pollution levels of the sediment column, thus assist the concerned authorities related to wetland conservation and administration to carry out necessary steps for planning proper management of resources.

The results of the study indicated that although the anthropogenic interventions to the wetland have not been significant till yet, as far as the sediment contamination is concerned, it would be too late if swift and appropriate measures are not taken for restricting future probable contaminations.

The water quality and eventually the sediments have been continuously subjected to contamination through various outlets spread across the entire wetland; for example, the presence of landfill in the eastern part, numerous small and large-scale industries in the western region, and forest reserve from the south.

Effluents from the industries, as well as the leachate discharge from the landfill, contribute significantly to heavy metals contamination in the wetland (Dash et al., 2019). These heavy metals then get precipitated from the water column and eventually get deposited in the sediment column. Hence, the need for a proper monitoring program for assessing the pollution loadings and ecological risk.

The land-use and land-cover (LULC) map of the watershed of Deepor Beel indicates that the majority of the area surrounding the wetland is the urban sprawl except for the southern portion which is the forest reserve area.

The prime areas of concern surrounding the pollution of Deepor Beel lie in three regions; firstly the eastern part of the wetland, where the presence of the landfill site too close to the wetland has caused serious issues; secondly the discharge of the entire city’s domestic wastewater into the Bharalu river which eventually ends up in the wetland (near the landfill region); and finally the upcoming small and large scale industries to the western part of the wetland discharging large concentrations of trace metals into the water body.

Conclusion

• Hierarchical clustering of the sampling locations statistically categorized them into three distinct clusters; clusters 1 (nearby the Boragaon landfill), 2 (sites in the industrial complex) and, 3 (central part of the wetland). These corresponded to the sites of high, moderate, and low contamination respectively. Principal Component Analysis (PCA) was then employed on the sediment dataset, which showed a varying response to different seasons, thereby indicating significant temporal changes in the pollution sources in Deepor Beel.
• Various indexing approaches were exercised to determine the level of contamination attributed to various heavy metals, both spatially and temporally. The contribution of the seven heavy metals individually across the 23 sampling locations and three seasons showed that the post-monsoon season had the most significant impact on sediment contamination.
• This is mainly because the precipitation of the heavy metals from the water column is maximum in the sediment column in this season due to lesser turbulence and higher metal concentrations as a result of leaching and surface runoffs.

Spatial analysis of the various indices also revealed that the sites close to the landfill are most affected, followed by the sites in the industrial complex and the central portion, respectively. Furthermore, the monsoon season had the least effect on the contamination of the sediment column. The significant rise in the water depth was found to be the primary reason for the same.

• The potential ecological risk assessment suggests that the post-monsoon season has a relatively higher risk as compared to other seasons. All the sites were found to have lower risk during the monsoon, while maximum sites fell under the moderate risk category in the post-monsoon period.
• Furthermore, Cadmium was found to be the major contributor to the ecological risk, with its values exceeding 40 during the post-monsoon season. Lastly, the risk index values displayed low risk for all the sites and the three seasons.
• The chemical speciation of the heavy metals was carried out to analyse the different forms in which these contaminants exist in the sediment column of the wetland to assess their impacts on the aquatic ecosystem. Cr, Mn and Mg were found to have a profoundly negative impact on the aquatic ecology as they are available in the sediment column as exchangeable (F1) form.

Finally, although all the indices suggested that the entire wetland has not been subjected to extreme levels of pollution yet (with the values of contamination factor, pollution load index, enrichment factor and geo-accumulation index lying in the moderate pollution range and the ecological risk indices also provided shreds of evidence of moderate risk to the ecology of the wetland), the absence of swift and immediate appropriate measures to contain the anthropogenic contamination of the wetland can have a detrimental and irreversible impact on the pollution levels. Thus, necessary steps must be taken for the preservation of the wetland and its ecology.