Reza et al.

sources of environmental pollution, with an

Materials and Methods

impact on the content of pollutants in the plants

cultivated on these soils (Gogoi et al., 2003; Hassan

Study area

et al., 2005; Jamrah et al., 2007) . During the last three

The study was carried out near the Digboi refinery

decades, it has become more apparent that the total

area of Tinsukia district, Assam, north-eastern India,

concentrations of heavy metals in soils and plants,

extended between 27°21′048″ to 27°24′50″ N latitude

their chemical forms, mobility and availability to the

and 95°31′27″ to 95°36′42″ E longitude covering an

food chain provide the basis for understanding a

area of 2378 ha (Figure 1). The climate is humid

range of problems in crop, animal and human health.

subtropical. The average annual rainfall ranges

Soils are the ultimate sink for trace elements in the

between 2100 and 2900 mm with maximum rainfall

terrestrial environment and have a great capacity for

during July−September. The climate is moderately

receiving, purifying and decomposing wastes and

warm during summer but cold in winter. Mean

pollutants of different kinds (Boon and Soltanpour,

monthly minimum and maximum temperatures

1992). The effects of industrial effluents on heavy

were 7 °C and 36 °C, respectively.

metals contents soil and plant are well documented

( Karaczun et al., 2007; Owamah, 2013).

Soil sampling and analysis

Risk assessment involves calculation of risk in

A total of 97 surface soil samples were collected

affected areas and provides valuable information

from a depth of 0-25 cm (plough layer) using a

regarding feasible rehabilitation options. The

square 500×500 m grid (Figure 1) covering not only

methodology is mainly based on the principle

the waste disposal site, but also the surrounding

“source – pathway – target”. Risk is better assessed

cultivated areas with the help of a hand-held global

if quantitative techniques are used to account for

positioning system. Soil samples were air-dried and

spatial and temporal variations. A probabilistic

ground to pass through a 2 mm sieve. A combined

assessment takes into account variability of

glass calomel electrode was used to determine the

parameters and uncertainty in measurement (Korre

pH of aqueous suspension (1:2.5 soil:solution ratio).

et al., 2002). Geostatistics is extensively used to

Organic carbon was determined by the Walkley and

assess the level of soil contamination and calculate

Black (1934) method. Digestion of 0.50 g samples was

the risk in contaminated sites, by preserving the

performed with concentrated HNO ₃ , HF and HClO ₄

spatial distribution and uncertainty of the estimates.

in a microwave digester (model Start D, Milestone).

In addition, geostatistics and GIS provide useful

Subsequently, the total concentration of heavy

tools for the study of spatial uncertainty and hazard

metals was determined by a Shimadzu AA6300

assessment (McGrath et al., 2004; Komnitsas and

atomic absorption spectrophotometer.

Modis, 2006; Reza et al., 2013).

Statistical analysis

In the northeastern India, Digboi refinery is theAsia’s

oldest refinery was set up at Digboi in 1901 in upper

The main statistical parameters, including mean,

Assam district of Tinsukia. Digboi is also known as

standard deviation, variance, coefficient of variance,

the birth place of Indian oil industry. The refinery

and extreme maximum and minimum values, which

had an installed capacity 0.50 MMTPA (million

are generally accepted as indicators of the central

metric tonnes per annum). Our study investigate the

tendency and of the data spread, were analyzed.

extent of contamination of heavy metals (Cr, Cd, Ni

The Pearson correlation coefficients were estimated

and Pb) in soil by Digboi refinery using statistics,

for all possible paired combinations of the response

geostatistics and GIS techniques in order to reveal

variables to generate a correlation coefficient matrix.

the spatial distribution patterns and provide a basis

These statistical parameters were calculated with

for hazard assessment.

EXCEL ® 2007 and SPSS 15.0 ® (SPSS Inc., Chicago,

III., USA).

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