INTRODUCTION:

There is a greater demand of electricity or energy supply since the onset of industrial revolution. The world’s power demands are expected to rise 60% by 2030, and the IEA estimates that the fossil fuels will account for 85% of the energy market by 2030.Coal-fired thermal power plants are now considered as the foremost global concern, as these are responsible to produce detrimental effects on environment. A typical thermal power plant emits huge amount of SO_X, NO_X, PM, fly-ash ¹ and a number of trace elements are also released to the environment. Again, the effluent from the thermal power plant carries the load of oil and grease, free available chlorine including other inorganic and organic pollutants. Thermal power plants affect the surrounding agricultural soil quality because all the pollutants from a typical thermal power plant are directly or indirectly dumped into the nearby agricultural field. The trace elements like Pb, Cd etc. produce some adverse effects on agro-ecosystem, e.g., the presence of Pb reduces the enzymatic activity of the biota and consequently incompletely decomposed organic material accumulates in the soil. The increased concentration of Cd in soil affects the food grain which is unfit for human consumption².

Agrawal et al (2010) ³ has made an attempt to measure soil contamination around four large coal-based thermal power plants and recorded highest concentration of arsenic followed by lead and nickel and lowest cadmium. The pollutants also have the capability to alter the physical properties like bulk density, particle density, porosity, moisture, water holding capacity, color, and temperature of the soil. The impact of coal-fired power plant emission on certain physical and chemical properties of the soil was studied by Singh et al, 1995⁴. Khan et al (1996) ⁵ observed that the fly-ash concentrations in normal field soil increased the porosity and water holding capacity along with other physico-chemical properties of soil.

Keeping in mind the impact of a coal fired thermal power plant, present study has been undertaken to evaluate the effect of effluent of Durgapur Thermal Power Station, West Bengal, on the nearby agro ecosystem.

MATERIALS AND METHODS:

Durgapur is an industrial city in the state of WB, India, and located at 23°48’ N and 87°32’ E (Fig 1), and it is 160 km away from Kolkata. It is one of the most important urban industrial zones of eastern India. It has an average elevation of 65 meters and the topography is undulating. The climate of this area is humid and tropical. Representative soil samples were collected through random sampling technique from the agro-fields in the vicinity of the Durgapur Thermal Power Station. The upper layer of the soil containing the leaf litters, plant residues etc. were removed, the soil samples were collected following the composite sampling method from 3 to 4 cm depth of the soil surface and were kept in separate plastic bags. The soil samples were collected in each season like summer, monsoon and winter in each year. After collection, these samples were brought into the laboratory. The wetted samples were spread out on separate large sheets of brown paper to become air-dry, the large lumps were broken up into pieces, then the air-dried samples were grinded well in the mortar and pestle to crush the aggregate particles and then were screened through a 2 mm sieve and kept in plastic bags for further physico-chemical analysis. In this work, besides the polluted sites, soil samples were also collected simultaneously following the same method from a control site, which has been selected as Golapbag agro-fields, almost 60 km. away from the Durgapur industrial belt. For the detection and quantification of As, Cd, Cr, Pb, the soil samples were digested first as follows: 1 g air-dried soil was taken in a 250 ml. conical flask, then 4ml of HClO₄ was added to it and then 4ml of HNO₃ was also added, whole mixture was evaporated to dryness, and then 2 ml of HClO₄ was added to it and again evaporated to dryness. Then the mixture was cooled at room temperature and 5ml. H₂O₂ was added to it and again evaporated to dryness, then cooled and 20 ml of 7.5 (N) H₂SO₄ was added to it and boiled for 10 minutes

and cooled. Little amount of distilled water was added to it and then the whole mixture was filtered through Whatman 42 filter paper, the filtrate was collected in 100 ml. volumetric flask and volume was diluted up to the mark with distilled water. By taking this digested soil solution, the concentration of As, Cd, Cr, Pb was analyzed using AAS (Atomic Absorption Spectra, model-GBC Avanta). The physical parameters like bulk density and particle density of soil were measured using specific gravity bottle ⁶.The soil porosity was measured with the help of the data of bulk density and particle density⁷ and soil moisture was measured immediately after collected from the field using evaporating dish, lab oven etc.⁸

Fig1. Study area

RESULTS AND DISCUSSION:

The results of heavy metals like As, Cd, Cr, Pb in agricultural soil along with their seasonal variation are given in the Table 1. It is known from earlier discussion, that the agricultural fields are very adjacent to the DTPS, so, the fields are always exposed to pollution load, which may be in term of effluent, solid deposition, fly -ash deposition or gaseous emission. Fly-ash is disposed off either in wet slurry process or dry disposal process. In both the methods, fly -ash is dumped in open land, which degrades the soil and enhances the air and water pollution and ultimately affects the human health. Fly-ash contains several nutrients as well as toxic metals like Cr, Pb, Hg, Ni, V, As, Cd, Ba etc,⁹which can increase the concentration of these heavy metals in soil and also can changes the physical properties like bulk density, WHC,porosity,texture of soil. The effluent of the DTPS was also analyzed. From the analysis result given in the Table 2, it is found that the concentration of As, Cd, Cr and Pb are very low in the effluent of DTPS. Therefore, the soil of this area may have been affected by dry-deposition of fly-ash, because from the Table 1 it is observed that the Cd, Pb and Cr concentration in soil of agro-fields of that area are much higher than the control site, except As, which showed higher concentration in the control site than surrounding areas of DTPS. The results demonstrated that in 2007 higher concentration of arsenic was noted in C5 during summer, but in same season only C2 and C3 showed higher arsenic level than C1 and C4 where higher level of arsenic was noted during monsoon. If we discuss on the concentration of heavy metals in soil of agro-fields of DTPS site, higher value of Cd was observed during summer in comparison to the monsoon and winter in the year 2007,and maximum concentration was found in P5 during winter, while in 2008,higher concentration of Cd was found during summer in P3.The concentration of Cr was higher during summer season in comparison to winter and monsoon in 2007 and the maximum value was found in P3,but in 2008,maximum concentration of Cr was found in P2 during summer and in the same year, the concentration of Cr was comparatively lower during monsoon than that of summer and winter. In case of Pb, highest concentration of Pb was observed in P4 during winter and lowest concentration was noted in P2 during monsoon in 2007, whereas in 2008, the maximum value of Pb was observed in P1 during summer while lowest value was found in P4 in the same season.

The physical properties like bulk density, particle density, moisture and porosity of soil of both the DTPS area and control sites, during different seasons were also analyzed and the results are shown in the respective graphs (Fig 2: a-h). The soils of DTPS area have slightly lower bulk density and lower particle density, slightly higher moisture and higher porosity content in comparison to the control site. These changes in soil can also affect the seeding emergence, crop establishment, root and shoot growth and crop yields.

Table 1: Concentration of heavy metals in soil near DTPS

SAMPLE AREA	As(mg/kg)
	2007			2008
	S	M	W	S	M	W
P1	0.209±0.122	0.915±0.329	0.814±0.162	0.255±0.157	0.371±0.224	0.584±0.200
P2	0.747±0.254	0.399± 0.231	1.586±0.239	0.861±0.300	0.418±0.148	1.589±0.251
P3	1.098±0.388	1.292±0.081	1.671±0.229	0.440±0.255	0.704±0.145	2.481±0.234
P4	2.197±0.259	0.939±0.295	0.255±0.147	0.219±0.128	0.309±0.187	1.347±0.330
P5	0.267±0.267	0.191±0.112	0.163±0.163	2.001±0.311	0.215±0.125	1.788±0.189
C1	3.442± 0.569	3.814± 0.220	1.782± 0.249	1.516± 0.586	2.843± 0.374	3.978± 0.183
C2	4.967± 0.564	3.631± 0.166	1.881± 0.334	4.869± 0.304	4.476± 0.539	3.031± 0.391
C3	5.364± 0.246	2.978± 0.189	2.745± 0.175	4.248± 0.271	3.143± 0.314	4.869± 0.388
C4	2.355± 0.234	4.959± 0.394	3.693± 0.136	4.813 ± 0.224	2.249± 0.132	4.076± 0.166
C5	3.261± 0.293	4.659± 0.299	5.720± 0.144	3.861± 0.123	4.684± 0.379	2.835 ± 0.268

SAMPLE AREA	Cd(mg/kg)
	2007			2008
	S	M	W	S	M	W
P1	0.575±0.103	0.200±0.041	0.175±0.025	1.125±0.149	0.325±0.075	1.700±0.268
P2	0.925±.095	0.075±0.048	0.925±0.221	1.500±0.178	0.100±0.041	1.825±0.138
P3	0.300±.041	0.450±0.166	0.175±0.103	2.025±0.063	0.150±0.150	1.600±0.212
P4	0.650±0.155	0.650±0.185	0.100±0.041	1.350±0.296	0.825±0.149	1.600±0.158
P5	0.950±0.176	0.125±0.048	1.075±0.075	1.175±0.165	1.400±0.141	1.050±0.194
C1	0.050± 0.029	0.025± 0.025	0.075± 0.048	0.150± 0.096	0.050 ± 0.029	0.050± 0.050
C2	0.125± 0.048	0.050 ± 0.050	0.025± 0.025	0.125± 0.048	0.100± 0.058	0.075± 0.025
C3	0.050± 0.029	0.050± 0.029	0.025± 0.025	0.125± 0.075	0.025± 0.025	0.125± 0.075
C4	0.100± 0.041	0.075± 0.048	0.050± 0.050	0.050± 0.029	0.125 ± 0.048	0.075± 0.048
C5	0.125± 0.075	0.050± 0.029	0.075± 0.075	0.100 ± 0.041	0.050± 0.029	0.050± 0.050

SAMPLE AREA	Cr(mg/kg)
	2007			2008
	S	M	W	S	M	W
P1	29.650±0.793	23.200±2.858	23.775±3.219	23.525±2.145	14.675±1.842	23.375±1.952
P2	28.775±2.819	22.150±2.978	25.750±2.629	33.800±0.977	18.725±1.319	21.025±0.589
P3	38.775±2.547	22.850±2.597	23.775±3.067	25.175±1.031	13.600±0.982	28.250±1.619
P4	28.325±2.141	24.900±3.524	26.500±3.716	22.825±2.054	16.900±1.696	19.050±1.020
P5	26.025±2.191	38.525±1.767	24.900±4.031	22.675±1.735	15.575±1.305	20.625±3.023
C1	5.700± 0.261	1.375± 0.466	1.850± 0.307	4.875 ± 0.325	1.775± 0.189	5.625± 0.384
C2	5.100± 0.178	2.800± 0.129	2.275± 0.315	5.075± 0.357	2.500± 0.212	5.675± 0.328
C3	4.200± 0.204	2.225± 0.221	2.950± 0.259	3.775± 0.103	1.400± 0.492	4.325± 0.649
C4	2.825± 0.138	3.525± 0.214	4.675 ±0.388	2.600± 0.196	5.100± 0.178	3.700± 0.381
C5	2.300 ± 0.129	4.825 ± 0.259	2.850± 0.185	2.500± 0.227	4.275 ± 0.284	3.450± 0.202

SAMPLE AREA	Pb(mg/kg)
	2007			2008
	S	M	W	S	M	W
P1	10.400±0.667	8.400±0.965	17.300±0.652	19.550±1.224	9.800±0.826	12.875±1.825
P2	11.750±1.396	8.200±1.229	10.925±1.246	14.150±1.475	10.300±1.157	12.075±1.115
P3	11.525±0.896	16.400±0.964	9.325±1.020	16.225±0.716	13.325±1.798	15.150±2.429
P4	9.475±0.812	14.07±1.151	18.625±2.157	8.875±0.806	11.775±1.776	10.950±0.542
P5	9.125±0.602	12.125±0.645	13.300±1.586	12.050±1.237	11.225±1.352	11.175±1.352
C1	1.225± 0.338	2.150 ± 0.371	2.975± 0.253	1.825± 0.651	0.750± 0.272	3.225± 0.317
C2	1.350± 0.210	2.075± 0.111	1.575± 0.229	1.275± 0.232	0.450± 0.259	2.350± 0.323
C3	1.650± 0.296	0.975± 0.075	0.750± 0.272	1.030± 0.096	1.475± 0.214	1.975± 0.131
C4	1.125± 0.471	1.175± 0.165	0.650± 0.377	1.225± 0.165	2.475± 0.330	1.250± 0.165
C5	1.075± 0.118	1.025± 0.111	1.625± 0.197	1.200± 0.426	2.200± 0.196	1.825± 0.150

P-DTPS area, C-control area, Value given as (Mean ± S.E); S-summer ,M-monsoon, W-winter

Table 2: Concentration of heavy metals in DTPS effluent

Sample number	Cd(ppm)*
	2007			2008
	S	M	W	S	M	W
S1	0.0004± 0.00006	0.0007± 0.00007	0.0007 ± 0.00009	0.0006± 0.00009	0.0004 ± 0.0002	0.0006± 0.00009
S2	0.0001± 0.00007	0.0002 ± 0.00009	0.0006 ±0.0009	0.0005± 0.00014	0.0003 ± 0.00011	0.0004± 0.00010
S3	0.0002 ± 0.00006	0.0005 ± 0.00007	0.0005± 0.00007	0.0001± 0.00009	0.0001± 0.00007	0.0003± 0.00013
S4	0.0001 ± 0.00005	0.0005± 0.0001	0.0005± 0.0001	0.0002 ± 0.00012	0.0001 ± 0.00005	0.0002 ± 0.00011

Sample number	Cr(ppm)
	2007			2008
	S	M	W	S	M	W
S1	0.0042 ±0.0002	0.0058 ±0.0006	0.0047 ±0.0005	0.0044 ± 0.0005	0.0028 ± 0.0004	0.0050 ±0.0007
S2	0.0036± 0.0003	0.0050± 0.0007	0.0059± 0.0006	0.0031 ± 0.0004	0.0021 ±0.0003	0.0039± 0.0006
S3	0.0036± 0.0004	0.0037 ± 0.0005	0.0049± 0.0006	0.0036 ±0.0004	0.0027 ± 0.0004	0.0045± 0.0007
S4	0.0038 ± 0.0004	0.0062 ± 0.0004	0.0036± 0.0004	0.0029± 0.0003	0.0030± 0.0004	0.0033± 0.0004

Sample number	Pb(ppm)
	2007			2008
	S	M	W	S	M	W
S1	0.0026 ±0.0005	0.0036± 0.0003	0.0026 ±0.0002	0.0036 ±0.0004	0.0027 ±0.0004	0.0035 ±0.0003
S2	0.0026± 0.0003	0.0030± 0.0002	0.0026± 0.0004	0.0029± 0.0006	0.0027± 0.0005	0.0023± 0.0005
S3	0.0031± 0.0005	0.0025 ± 0.0004	0.0023± 0.0004	0.0038± 0.0002	0.0018± 0.0003	0.0027± 0.0004
S4	0.0023± 0.0003	0.0026± 0.0005	0.0029± 0.0006	0.0031± 0.0004	0.0020± 0.0003	0.0024± 0.0004

*Arsenic-Not Detectable; S1, S2, S3, S4-sampling sites; Value given as (Mean ± S.E); S—summer, M—monsoon, W—winter