The sediments in stormwater runoff are recognised as the major sink of the heavy metals and affect the soil quality in the catchment. The runoff sediments are also important in the management of contaminant transport to receiving water bodies. In the present work, stormwater during several major rain events was collected from nine principal locations of Guwahati, India. The solid phase was separated from the liquid phase and was investigated for the total contents of Cd, Co, Cr, Cu, Mn and Zn as well as their distribution among the prominent chemical phases. Sequential extraction procedure was used for the chemical fractionation of the metals that contains five steps. The total metal concentration showed the trend, Cd < Co < Cu < Cr < Zn < Mn. The relative distribution of the metals showed that Cd was available mostly in the exchangeable and the carbonate bound fractions, which were the most mobile and high-risk fractions. Co with medium mobility was also found to be in the high-risk category. On the other hand, the mobilities of Cu and Zn were relatively low and these were, therefore, the least bioavailable metals in the runoff sediments falling in medium-risk category.
Rainwater runoff has been identified as a significant source of contaminants having tremendous impact on the receiving aquatic environment. In the present study, trace element transport by the surface runoff in the predominantly urban catchment of Guwahati city, India was monitored with a view to determine the chemical denudation rates of the land surface. A number of trace metals, namely Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn were measured in the runoff after 70 major rain events within the city. Cadmium was found to be the least abundant metal and Iron was the most abundant metal in the runoff. The results are interpreted on the basis of temporal and spatial variations in their concentrations. These variations are quite large in some of the events and reflect changes in the local environmental setting, differences in water utilization, variations in runoff volume, gradient and quality.
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