Transport of Trace Metals by the Rainwater Runoff in the Urban Catchment of Guwahati, India - Management of Natural Resources in a Changing Environment - page 225

Manganese
The runoff had manganese in the range of 0-1.022 mg/L. The sample J1 had the
maximum Mn content. It was observed that 23 samples (26 %) of the fi ve locations
had values below detection level.
For the location J, the values decreased for two events, I and II; increased for
three events III, IV and V; and Mn could not be detected for event VI and lastly no
specifi c trend was seen for the event VII. In the location N, manganese showed
increasing trend for the event I, and no specifi c trend for the events II and III. For
the location K, increasing trend was observed for the event I, decreasing trend for
the event II and no specifi c trend for the event III. In the location M, the values fi rst
decreased and then increased for the two events I and II, and no manganese was
detected for the event III.
In the location R, there was no specifi c trend seen in the event I while event II had
decreasing trend and no distinct trend could be detected in event III. For location U,
the range of Mn was from 0.438 to 0.672 mg/L with a mean 0.558. No distinct trend
could be found in any of the three events. Location Z had Mn content from 0.018 to
0.082 mg/L with a mean 0.048.
Mn concentration showed an increase in the runoff water with decrease in pH for
the locations J, K, M, U and Z. This may be attributed to the fact that at lower pH
the mobility of Mn increases (Heal 2002 ) from soil to the runoff which enhances the
concentration of Mn.
Naturally, manganese oxides are found in various forms of discrete particles,
coatings, nodules, micro-nodular deposits, thin layers on mineral surfaces, or inter-
spersed in clay minerals (Koljonen et al. 1976 ). These oxides tend to be deposited
at the redox front which may occur near the water table and also at places along the
ground-water fl ow path owing to changes in vertical and horizontal permeability.
Under favourable conditions of water circulation pattern and/or chemical composi-
tion, these salts can be redissolved (Lind et al. 1987 ) and leached into the runoff.
Nickel
Its contents of the runoff were in the range of 0-0.208 mg/L with the maximum
value at Z4. For two of the samples (2.2 %) Ni could not be detected. The values
showed different diurnal trends for the events of seven runoff locations.
For the location J, increasing trend was observed for events II, III and V and
event I showed decreasing trend, but such distinct trend was missing in the rest three
events. Also the events VI and VII recorded very high Ni contents. For the location
N, decreasing trend was seen for the event I; no specifi c diurnal trend for the events
II and III could be observed. The same was observed for the location K that increas-
ing trend was observed in event I and no general trend could be seen in the rest two
events. For the location M, decreasing trend was seen for the events I and II, and no
specifi c trend for the event III. For location R, decreasing trend was seen for the
event II and no specifi c trend for the events I and III. For the location U and Z no
specifi c trend could be detected for any of the events.