The use of portable X-ray spectrometer in monitoring anthropogenic toxic metals pollution in soils and sediments of urban environment of Dar es Salaam Tanzania

Abstract

The levels of toxic metals in sediments and soils along high traffic Sam Nujoma highway and in the densely populated Sinza area in Dar es Salaam Tanzania were determined using portable X-ray fluorescence spectrometer (pXRF) and atomic absorption spectrometer (AAS) in order to assess the levels of the metals in the sediments and soil and evaluate the use of pXRF in monitoring anthropogenic toxic metal pollution in the urban environment.Sediments from different areas in Sinza, especially those occurring near metal workshops, contained high toxic metal contents (e.g. Co, Zn, Pb and Ti) above the maximum permissible allowable limit by the Tanzania Bureau of Standards (TBS) for soils or sediments in an urban environment. This toxic metal content decreases sharply (~5m) away from workshops, and so reflecting pollution of the metals by workshop activities. Along Sam Nujoma highway, toxic metal contents of sediments and soils were lower than those of samples from Sinza, but also decreases as one moves from the road to the surroundings of the highway. Chromium and Zn contents in parts of Sam Nujoma highway were found, in places, to be above the maximum permissible allowable limit by the TBS for soils or sediments. As the geology of Dar es Salaam is mainly unconsolidated sands and clay and coral limestones, which are not likely natural source of the metals, then results of this study reflect anthropogenic addition of the metals to the Dar es Salaam environment.Similarity in concentration of Pb in sediments determined from previous study in the year 2009 along Sam Nujoma highway and those presented for this study suggests that Pb and most likely other metals as well, are not accumulating along or near the highway over time, but dispersed by water during wet season, when water table is high and most of roads and street are covered by surface runoff. Part of these contaminated surface runoff ends up in water, sediments and soils in downstream in the Msimbazi River, in which most of vegetables are grown and food sources like fish thrive, which highlights a potential risk of exposure of toxic metals in human food chain. Moreover, high levels of toxic metals in soils and sediments near workshops in Sinza pose high exposure risk to children and people leaving around the workshops.Successful application of pXRF in monitoring of anthropogenic toxic metals pollution in urban environment of Dar es Salaam open up possibility for similar applications in fast growing cities or artisanal miners’ sites that are frequently affected by anthropogenic pollution in the country and abroad.