Transport of Trace Metals by the Rainwater Runoff in the Urban Catchment of Guwahati, India - Management of Natural Resources in a Changing Environment - page 215

The sources of pollutants in urban runoff may depend considerably on location
and it is important to compare different types of land use in urban areas. The study
area is confi ned to Guwahati which is now the largest city of the entire north-east
India and has undergone rapid expansion with a consequent increase in population.
As per the 2011 census, population of Guwahati is 963,429 with a total area of
340 km 2 . The city is the main corridor for passage to the states of Assam, Meghalaya,
Mizoram, Tripura, Manipur, Nagaland and Arunachal Pradesh and all the roads,
including the two highways, NH31 and NH37, passing through the city, are busy
with vehicular traffi c day and night. The climate is homogenous over the city.
Summer temperatures range from a minimum of 22° to a maximum of 38° C. Winter
temperatures range from a minimum of 10° to a maximum of 25° C. The average
annual rainfall is 180 cm during the months of May to September.
The city of Guwahati is situated between the southern bank of the Brahmaputra
river and the foothills of the Shillong plateau and is located approximately along
26° 10

E longitude. The city is situated on undulating plain
with varying altitudes of 49.5-55.5 m above mean sea level (MSL). The central part
of the city is interspersed with small hillocks like Sarania hill (193 m), Nabagraha
Hill (217 m), Nilachal hill (193 m), and Chunsali hill (293 m). Structurally, this
region is situated on the 50 m thick alluvium of the middle Brahmaputra valley. The
topography of the city is non-uniform with a preponderance of high and low areas.
Guwahati city straddles the valley of the river Bharalu, except where it is discharged
to the Brahmaputra. The Nilachal hill with a maximum height of 303 m in the mid-
dle of the city is said to be the home of Goddess Kamakhya. There is Nabagraha
temple on the top of Chitrachall hill. The Narakasur hill was named after the legend-
ary king of ancient Assam, Fatashil hill and Japarigog hills are having approxi-
mately same height. The western periphery of the city contains the Deepar Beel, a
huge natural water reservoir and the adjoining wetland areas. Three other major
reservoirs viz., Sorusola Beel, Borsola Beel, Silsako Beel also exist in the city.

′ N latitude and 92° 49 ′

Methodology

The rainfall runoff was collected at seven selected locations (Fig. 1 ) along the major road
systems in the city during and immediately after major rain events of April to October at
5-20 min intervals between two adjacent samples. Plastic containers of 1 L capacity,
detergent cleaned, acid rinsed, washed with de-ionized water several times, and dried,
were used for sampling. A description of the sampling details is given in Table 1 .
The water samples were immediately fi ltered through Whatmann 42 micron fi lter
paper, stored and analysed as per standard procedure (APHA 1995 ). The pH of each
sample was measured immediately after collection of the runoff samples with a
digital pH-meter (Elico pH-Meter Model LI 127) using standard buffers of pH 4 and
9 (for calibration purposes).
For analysis of the trace metals, nitric acid digestion technique (APHA 1995 )
was used for the runoff samples. The trace metals, namely, Cd, Co, Cu, Mn, Ni, Pb