Sediments in a fast urbanizing catchment in Central Brazil – an analysis of anthropogenic impacts on sediment geochemistry and sediment sources

Field: Environmental Science (Physical Sciences)

Over the last decades, fast urban sprawl and accelerated land use change have drastically increased the pressure on water resources of the capital Brasília and its surrounding area. The water supply of the metropolitan region of Brasília depends largely on surface water collected in reservoirs. There are increasing concerns regarding water shortages due to sediment aggradations, and of water quality due to geochemical modification of sediments from human activities. The complexity of various socio-environmental problems, such as non-point source pollution, soil erosion or silting of water reservoirs within urban catchments evoked the need for more effective and sustainable strategies to use land and water resources. Accurate identification and management of sediment source areas, however, is hampered by the lack of reliable information on the primary sources of sediment and on sediment geochemistry.\n\nThe fingerprinting approach and a multivariate mixing model have been proven to be a valuable sediment source tracing technique across the globe and for various environmental settings. A multi-component methodology, including geochemical and geophysical analyses of representative sediment source and alluvial sediment samples, statistical analyses and a multivariate mixing model, was utilized to obtain the impact of different anthropogenic activities on sediment and water quality and to identify the major sources of sediments within the Lago Paranoá catchment. \n\nHowever, sediment source appointment and geochemical signatures of sediments in urbanized tropical regions, such as the Lago Paranoá catchment in the DF, are hampered by severe challenges; (i) the presence of various types of land use and heavy urbanized areas, (ii) large differences between sub-catchments and (iii) model structural failures in representing the sediment source contribution within urban tropical river basins. The present cumulative thesis addresses the challenges in geochemical analyses of different types of source and alluvial sediments, and in sediment source appointment for the Lago Paranoá catchment and it´s five sub-catchments. The aim of the study was to assess the distribution of chemical elements and geochemical/physical properties of potential sediment sources in the Lago Paranoá catchment. Principal component analysis and hierarchical cluster analysis were used to investigate the influence of different land use types on the geochemistry of sediments. Geochemical fingerprints of anthropogenic activities were developed based on the results of the cluster analysis grouping. The anthropogenic input of land use specific geochemical elements was examined and quantified by the calculation of enrichment factors using the local geological background as reference. \n\nThe existing findings suggest a strong relationship between land use and quantifiable features of sediment geochemistry, and identified the combined effects of specific anthropogenic activities and metal enrichment in source and alluvial sediments. Through comparison of the geochemical signature of potential sediment sources and alluvial sediments of the Lago Paranoá and sub-catchments, the relative contribution of land use specific sediment sources to the sediment deposition of the main water reservoir were estimated. This assessment indicated that urban land use had the greatest responsibility for recent silting in the Lago Paranoá. \n\nIn fact, one of the most challenging issues within the scope of IWRM is to quantify the contribution of sediment sources within fast urbanizing, mixed used, tropical catchments. Therefore, statistically verified composite fingerprints and a modified multivariate mixing model have been used to identify the main land use specific sources of sediment deposited in the silting zones of the Lago Paranoá, Central Brazil. Because of the great variability of urban land use types within the Lago Paranoá sub-catchments, the fingerprinting approach was additionally undertaking for the Riacho Fundo sub-catchment. This sediment source tracing technique provides valuable information on the response of the main sediment sources in a fast growing agglomeration with respect to specific land uses and human activities and allowed to examine the uncertainty in model prediction. The main contributions from individual source types (i.e. surface materials from residential areas, constructions sites, road deposited sediment, cultivated areas, pasture, farm tracks, woodland and natural gullies) varied between the whole catchment and the Riacho Fundo sub-catchment, reflecting the different proportions of land uses. \n\nThe sediments deposited in the silting zones of the Lago Paranoá originate largely from urban sources (85±4%). Areas with (semi-) natural vegetation and natural gullies contribute 10±2% of the sediment yield. Agricultural sites have only a minor sediment contribution of about 5±4 % within the whole catchment. However, there is no mechanism considered to reflect seasonality in the tropics, e.g. phenological change of the vegetation between wet and dry season, or and temporal changes in land use, e.g. construction sites, which influence model estimates. Nevertheless, the study reveals that even 58 % of the land remains in (semi-) natural state, the main sediment source are urban areas. \n\nBeside the analyses of sediments, it was found that metal concentrations in surface water of the main tributaries to the Lago Paranoá are generally low, but show seasonal variability. Terrestrial inputs of metals occur during the rainy season and depend largely on the influence of urban land use. \n\nThe present thesis shows the great influence of anthropogenic activities on sediment generation, and at least to some degree, on sediment associated pollution loads. It depicts region specific challenges, but also provides essential information to guide management responses towards more effective sediment source-reduction strategies.:Contents\nDeclaration of the candidate\t i\nAcknowledgements\t iii\nAbstract\t v\nZusammenfassung\t vii\nContents\t ix\nList of Figures\t xi\nList of Tables\txiii\n1 Introduction\t 1\n1.1 Sediments in urbanized river basins: impact of land use changes on sediment dynamic and geochemistry\t 1\n1.2 Sources of sediments and associated contaminants within urbanized catchments\t 4\n1.3 Sediment issues in the tropics\t 6\n1.4 Research area\t 10\n\n2 Objectives and research questions\t 16\n2.1 Sediment geochemistry and sediment sources\t 16\n2.2 Sediment associated “metal” loads and water quality\t 18\n2.3 List of publications\t 19\n\n3 Material and Methods\t 20\n\n4 Geochemical signature and properties of sediment sources and alluvial sediments within the \n Lago Paranoá catchment, Brasília DF: a study on anthropogenic introduced chemical elements in \n an urban river basin\t 22\n5 Assessment and evaluation of metal contents in sediment and water samples within an urban \n watershed – an analysis of anthropogenic impacts on sediment and water quality\n in Central Brazil\t 45\n\n6 Sediments in urban river basins: Identification of sediment sources within the Lago Paranoá \n catchment, Brasília DF, Brazil - using the fingerprint approach\t 73\n\n7 Summarizing discussion and conclusion\t 95\n7.1 Sediment properties and geochemistry\t 95\n7.2 Relationship between urban land use and metal concentrations\n in surface water samples\t 98\n7.3 Sediment source identification\t 99\n7.3.1 Sediment source discrimination\t 99\n7.3.2 Sediment source appointment\t101\n7.4 Future research questions and tasks\t103\n\nReferences\t106