Spatial and temporal trends of anthropogenic pollutants in a tropical, urban environment in East Africa

Abstract

A range of chemicals which are potentially harmful to human health and the environment are used in industrial and consumer products including electronics. Some of these chemicals have physicochemical properties which may make them environmentally persistent, bioaccumulative, toxic, and/or have the potential for long-range environmental transport. This includes mercury (Hg), persistent organic pollutants (POPs) e.g., polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs), and chemicals of emerging concern, e.g., alternative brominated flame retardants (aBFRs), chlorinated paraffins (CPs), and dechloranes. Many of these chemicals were produced and more extensively used in the Global North, though it has been hypothesized there may be an ongoing shift in global source regions for POPs to the Global South. This may also be the case for some chemicals of emerging concern and Hg. Reasons include global trade in products, used products, and waste including electronic waste (e-waste) which contain these chemicals, as well as shortcomings in environmentally sound waste handling and in the ability to enforce environmental regulation. However, there are still large knowledge and data gaps concerning environmental occurrence, emissions, and fate of these chemicals in the Global South. The original contribution of this thesis is to increase the understanding of occurrence, spatial and temporal distribution, sources, and environmental fate of the above-mentioned chemicals in the Global South. To achieve this, a sediment core was collected in a river system in urban Dar es Salaam, Tanzania. Layers were dated, and analysed for PCBs, PBDEs, aBFRs, CPs and dechloranes. Except for PCBs, this is the first report of long-term temporal trends of these chemicals derived from sediment cores from the African continent. Samples were also collected from the atmosphere, using passive sampling techniques, and from soil along two transects in and around Dar es Salaam. These samples were analysed for CPs, dechloranes, and Hg. This is the first reported data on CPs in air and dechloranes in soil from the African continent. In addition, it is the first spatially resolved data of gaseous elementary mercury (GEM) across an urban area in the Southern Hemisphere. The results show that concentrations of PCBs and PBDEs increased in sediment layers corresponding to the late 2010s. Also, the chemicals of emerging concern aBFRs, some dechloranes, and especially the CPs showed increasing concentrations. These increases can partly be explained by regional increases in population, consumption, and waste generation. CPs were ubiquitous in the Dar es Salaam region, with higher concentrations in air and soil in more urbanized areas and near municipal waste and e-waste handling facilities. Dechlorane concentrations in air and soil were lower compared to CPs, with the highest concentrations close to e-waste handling sites, although spatial trends differed for the analysed dechlorane compounds. Some high levels of GEM in air were observed in the urban environment, while concentrations of total Hg in soil were generally low. The increasing trends of PCBs and PBDEs in sediments may suggest that regulatory efforts have been less effective in lowering environmental burdens of these POPs in Dar es Salaam compared to the Global North. Strongly increasing trends of the studied chemicals of emerging concern, and significant sources for GEM in the region also merits further attention. Establishing new- and/or strengthening existing environmental monitoring programs for these chemicals locally and elsewhere in the Global South is therefore recommended to assess the effectiveness of existing regulation and to document needs for further regional and international regulatory efforts.