Coastal and estuarine sediment archives for flood-induced pollution in subtropic / tropic areas – a proof-of-concept study
Physical Geography
Final Report Abstract
The high relevance of flood events as transporting agents for sediment bound pollutants is well known. Extended flooding of usually not overflown areas induces fluvial erosion. However, these areas are not protected against water and, hence, can become a source of pollutants. This mechanism is of particular importance in urban floodplains. The second aspect that leads to intensified pollutant dynamics due to flooding is the resulting increase in flow velocity, which generally intensifies erosion processes. As a result, pollutants stored in fluvial sediments can be eroded and transported. In this context, a deeper understanding on the deposition dynamics of these sediments, which determines the possible risks, is mandatory. Chennai, India, is a metropolis that has grown rapidly in recent decades, with a large number of mostly unregulated sources of pollution. The city's wastewater treatment system is inadequate and the city has large centers of industrial production. A correspondingly high level of pollution of urban waters was expected. In the first phase of the project, the floodplain sediments of the Adyar and Cooum rivers in the metropolis of Chennai were examined. A broad spectrum of pollutants was detected, but in relatively low concentrations and without spatial hotspots of deposition. Based on these findings, the sampling was extended in a second project phase to the coastal and estuary areas of the two rivers. Chennai lies in the area of influence of the winter monsoon with peak river discharge between November and February. The hypothesis of the second project phase assumed that the catchment areas would be washed out by the runoff peaks of the monsoon months. At the same time, the rivers on the Indian coast are influenced by strong coastal currents, which repeatedly lead to the formation of sandbanks that dam the rivers towards the ocean. As a result, it can be assumed that pollutant-laden sediments are temporarily stored in the estuary areas and further along the beaches. In areas which are only flooded by high discharge events sediment archives can form, which can make the pollution history of the catchment area traceable through marker substances. Unexpected but similar to the first phase, only low concentrations of inorganic pollutants were found in the second phase of the project. This is attributed to the special characteristics of estuarine sediments: On the one hand, the sand fraction strongly dominates, while heavy metals are predominantly associated with finer sediments. Secondly, the increased salinity influences the pollutant concentrations, as it can increase the solubility of heavy metals. Also, for the organic pollutants only low to moderate levels of concentrations could be detected to date. Further investigations to reveal major sediment accumulation deposits are deemed necessary. It is further assumed that monsoon floods cause a flush-out of sediments and therefore of particulate-bound pollutants rather than a deposition on floodplains.
Publications
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Proof-of-concept study for investigations on riverine and urban sediment deposits as flood induced pollution archives in subtropic/tropic areas: the case study Chennai city, India. Abstract of the IGCS Workshop ‘Urban Resilience’, Chennai, November 2019
F. Lehmkuhl, J. Schwarzbauer, B. Narasimhan, B.S. Murty, O. Konechnaya & J. Weise
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Contemporary Contamination of Urban Floodplains in Chennai (India). Water, Air, & Soil Pollution, 233(8).
Bellanova, Luisa; Uphoff, Fabienne; Bellanova, Piero; Engels, Nina; P., Prabu; Pulipatti, Yaswanth; Lehmkuhl, Frank; Schulte, Phillip; Reicherter, Klaus & Schwarzbauer, Jan
