Environmental Engineering Reference
In-Depth Information
Conclusions
The present work has yielded a comprehensive database on the quality of the sur-
face runoff of Guwahati city. The observed pH of the runoff varied from 4.6 to 7.8.
The results indicated that the pH might be controlled by leaching of acidic elements
such as Al and Fe from the soil that replace Ca and Mg. The average pH of the run-
off samples was 6.69, probably due to neutralization. Most of the samples had
pH > 5.6. This may be an indication of alkaline contamination of the rainwater as the
pH of natural rain water is ~5.6. Cadmium and lead are positively correlated with
pH of the runoff for three sites J, N and R, suggesting the entry of the metal into the
runoff from the surrounding topsoil. In contrast, Co, Cu, Mn and Ni are inversely
correlated with pH in most of the locations. Thus, more and more metals leach into
the runoff from the topsoil as pH goes down. The concentration of the trace metals
is found to depend on the traffi c intensities in the sampling sites. Location J, which
has high traffi c density, has the maximum content of the metals, namely Co
(0.895 mg/L), Mn (1.022 mg/L) and Zn (0.408 mg/L). The location N has the maxi-
mum content of Cd (0.024 mg/L) possibly due to contribution from nearby indus-
tries. Considerable amount of Cu (0.614 mg/L) at this site may be due to both
vehicular and industrial inputs. Location Z has the highest content of Ni (0.208 mg/L)
and a large part may have come from anthropogenic activities including vehicular
exhausts and atmospheric deposition. Mn is the most abundant and Cd the least
abundant metals in the surface runoff. The average concentrations of the metals are
in the order of Mn (0.313 mg/L)>Co (0.289 mg/L)>Zn (0.136 mg/L)>Cu
(0.088 mg/L) > Pb (0.050 mg/L) > Ni (0.044 mg/L) > Cd (0.005 mg/L). It is possible
that the runoff is contaminated due to leaching from the top layers of soil which has
served as a receptor for various wastes generated by anthropogenic activities in the
rapidly urbanized city. Increase in traffi c volume, congestion, growing fuel use etc.,
might have contributed signifi cantly to runoff contamination.
References
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