Bharalu Tributary originates in the foot-hills of Khasi Hills and
meets the Brahmaputra River flowing northwards through Guwahati city. Trace
metals viz., Pb, Ni, Cr, Cd, Cu and Zn were estimated in the bed sediment of
Bharalu and their concentrations were found to be 39.54 ± 31,
40.62 ± 19, 32.00 ± 18, 0.93 ± 1,
65.45 ± 33 and 435.08 ± 246 μg g
$^{-1}$
respectively. Levels of Cd, Pb and Zn in the bed sediments were 3, 1.5 and 5
times, respectively, higher than the concentrations in shale. Enrichment
factors (EF%) of the metals showed substantial loading of the sediments with
trace metals. Muller's Geoaccumulation index (Igeo) of the metals was
calculated which showed that the river was moderate to strongly polluted.
Pearson's correlation showed stronger associations for the metal pairs – Pb-Cd (r
= 0.90), Ni-Cu (r = 0.68), Ni-Zn (r = 0.61), Cu-Zn (r = 0.87) and Pb-Cu (r =
0.75) – indicating similar sources. A multivariate statistical tool, Principal
Component Analysis (PCA), was used to define the possible input sources of the
trace metals and three main sources viz. industrial discharge, vehicular
contribution and waste dumping were identified.
