Assessment of heavy metals in soil irrigated with sewage water of different areas around Bikaner city

Anuradha Aseri, Sushma Jain

P.G. Department of Chemistry, Government Dungar College, Bikaner - 334 003 (India)

*Corresponding Author E-mail: aaanu04@gmail.com

ABSTRACT:

Consumption of food crops contaminated with heavy metals is a major food chain route for human exposure. Heavy metal contamination of soil resulting from sewage irrigation is a cause of serious concern due to the potential health impacts of consuming contaminated products. In this study an assessment made of the impact of sewage irrigation on heavy metals contamination of soil in which many vegetables like Spinach, Cabbage, Beetroot, Reddish, Okra, Tomato and Cucumber is widely cultivated and consumed in Bikaner, particularly by the poor. The objective of this investigation was to determine the effects of untreated sewage water irrigation on accumulation of metals in soil. A field study was conducted at five major sites that were irrigated by untreated sewage water in the suburban areas aronnd Bikaner city, India. For analyzing the amount of heavy metals 12 samples of irrigated soil were collected during the summer season in the month of may 2016 and were analyzed for 5 heavy metals. The monitored heavy metals included Cd, Cr, Ni, As and Pb for their accumulation to provide baseline data regarding environmental safety and the suitability of sewage irrigation in the future. The C/P index was calculated. The C/P index of the studied soils indicated the significance of pollution indices of heavy metals. The study concludes that the use of untreated sewage water for irrigation has increased the contamination of Cd in soil causing health risk in the long term from this practice. The study also points to the fact that the extent of Cd in irrigated soils from all areas ranged from severe contamination to slight pollution.

KEYWORDS:Heavy metals, sewage water, soil, accumulation, health risk.

INTRODUCTION:

Protection of environment is the most vital issue today. Explosive population growth, rapid progress in science and technology, massive industrialization and use of various chemicals in agriculture and most importantly, human activities are the factors threatening the quality of life. Among the different pollutants heavy metals have received escalating attention due to their possible injurious effects to man, animal and plants as these are recorded to be cytotoxic, mutagenic and carcinogenic.

Heavy metals form the major group of toxic pollutants among the other pollutants, as these metals temper the harmony of the ecosystem.¹ Sewage water irrigation is known to contribute significantly to the heavy metal contents of soils. Plant species have a variety of capacities in removing and accumulating heavy metals, so there are reports indicating that some species may accumulate specific heavy metals, causing a serious health risk to human health when plants based food stuff are consumed. Disposal of sewage water and industrial wastes is a great problem. Often it is drained to the agricultural lands where it is used for growing crops including vegetables. These sewage effluents are considered not only a rich source of organic matter and other nutrients but also they elevate the level of heavy metals like Fe, Mn, Cu, Zn, Pb, Cr, Ni, Cd, As and Co in receiving soils. Heavy metals are one of a range of important types of contaminants that can be found on the surface and in the tissue of fresh vegetables. Prolonged human consumption of unsafe concentrations of heavy metals in foodstuffs may lead to the disruption of numerous biological and biochemical processes in the human body.^2-4Irrigation with wastewater may have implications at two differentlevels: alter the physico-chemical properties andmicrobiological content of the soil and/or introduce and contribute to the accumulation of chemical and biologicalcontaminants in soil. The first may affect soil productivityand fertility; the second may pose serious risks to the human and environmental health. Of the two opposing effects of wastewater irrigation, the negative effect of heavy metal toxicity outweighs the positive effects of organic nutrients.^5-7These metals enter the human body primarily through two routes, namely, inhalation and ingestion, with ingestion being the main route of exposure to these elements in the human population.⁸Wastewater irrigation for vegetable production is a highly prevalent practice in Bikaner and a numberof articles have been published on physico-chemical characterstics of sewage water and sewage-irrigated soils. However, to the best of our knowledge, an insight into assessment of heavy metals in sewage water-irrigated soils is non-existent in the city. Long-term effect of sewage water irrigation on the build-up of heavy metals in soils around Bikaner farming sites was evaluated.^9-32 Soils around Bikaner were examined with the following objectives: (1) to measure the concentrations of selected metals (i.e., Cd, Cr, Ni, As and Pb ) in soil; (2) to assess the significances of pollution indices of heavy metals in soil; (3) to identify pollution source of heavy metals in soil; and (4) to compare metal contents of sewage irrigated soils of different irrigation areas around Bikaner city respectively, with the maximum permissible levels set by World Health Organization (WHO). The results of this research are needed for the government to control heavy metal pollution. The results are expected to create awareness among the public on the safety of consuming vegetables grown in such areas. The standard employed for interpreting soil heavy metal contamination/pollution varies from country to country based on chosen factors.

C/P value = Contamination level of heavy metals in sample of sewage irrigated soil / Maximum permissible limit of heavy metal in irrigated soil

A distinction between soil contamination and pollution range was established by means of this contamination/pollution index:

Table 1: Significance of intervals of contamination/pollution index (C/P)

C/P	Significance	Symbols
<0.1	Very slight contamination	Vsl
0.10 – 0.25	Slight contamination	Sl
0.26 – 0.5	Moderate contamination	Mi
0.51 – 0.75	Severe contamination	Stl
0.76 – 1.0	Very severe contamination	Vstl
1.1 – 2.0	Slight pollution	Sp
2.1 – 4.0	Moderate pollution	Mp
4.1 – 8.0	Severe pollution	Stp
8.1 – 16.0	Very severe pollution	Vstp
>16	Excessive pollution	Ep

The maximum allowable limits of heavy metals in soils and vegetables have been established by standard regulatory bodies such as World Health Organization (WHO), Food and Agricultural Organization (FAO) as shown in Table 2 below³³:

Table 2: Maximum Allowable Limits of Heavy Metal in Irrigation Soils (mg/kg)

S. No.	Heavy Metal	Maximum permissible level in soils (mg/kg)
1	Cd	3
2	Cr	100
3	Ni	50
4	As	20
5	Pb	100

MATERIALS AND METHOD:

Study areas:- Over the past few decades, human activities and prolonged use of irrigation water mixed with sewage have resulted in soil pollution with heavy metals of the study area. The study area has hot semi-arid climate, and the annual average temperature for the area is 26.3° C, annual precipitation in the range of 260-440 mm and the annual relative humidity in the range of 25% - 61%.³⁴ Field surveys were carried out around Bikaner city, to collect soil. The experiment was conducted on sandy loam soil for sewage water. For the analysis of heavy metals in soil, 12 samples were collected. Out of these 12 samples 3 samples are of Chandmal baag ( 1, 2 and 3 ), 2 are of Sujandesar ( 4 and 5 ), 2 are of Kadari colony (Shriramsar) ( 6 and 7), 3 are of Kocharo ka bhaththa (Gangashahar-Bhinasar) ( 8, 9 and 10 ) and 2 are of Gharsisar ( 11 and 12 ).

Soil sampling and analysis:- Soil samples were collected at the depths 0-15cm, with the help of a stainless steel tube auger. The representative soil samples were transferred into air tight polythene bags and brought into the laboratory. Soil samples were dried at 40°C for 48 h in hot air oven and crushed to pass through a 2 mm. nylon sieve. A di-acid mixture was used to find out the heavy metals Cd, Cr, Ni, As and Pb in the soil. A known amount (5 g) oven dry soil was weighed and transferred into 100 ml beaker to which 25 ml of concentrated HNO₃ and 5 ml of perchloric acid was added. [25 ml conc. HNO₃ and 5 ml of HClO₄ (5:1)]. Mixture was placed on a hot plate at 105°C for one hour and then temperature was increased up to 140°C until the sample was completely dry. After cooling, sample was mixed with de-ionized water to leach all metals, and then rinsed three times in H₂O and filtered through Whatman No. 42 filter paper into a 50-ml volumetric flask. Digested samples were then analyzed for Cd, Cr, Ni, As and Pb on AAS (Atomic Absorption spectrophotometer).³⁵

RESULT AND DISCUSSION:

The concentrations of the heavy metals in surface soils irrigated with sewage water have been illustrated in Figs. 1 to 5.

Table 3: Significance of pollution indices of Cd in the irrigated soils

Areas	Chandmal Baag	Sujandesar	Kadari colony (Shriramsar)	Kocharo ka bhaththa (Gangashahar-Bhinasar)	Gharsisar
Mean Conc. of Agriculture fields Soil ( mg/kg )	2.85	1.80	4.40	1.85	2.70
C/P index	0.95	0.60	1.47	0.62	0.90
Implication of C/P index	Very severe contamination	Severe contamination	Slight pollution	Severe contamination	Very severe contamination

Table 4: Significance of pollution indices of Cr in the irrigated soils

Areas	Chandmal Baag	Sujandesar	Kadari colony (Shriramsar)	Kocharo ka bhaththa (Gangashahar-Bhinasar)	Gharsisar
Mean Conc. of Agriculture fields Soil ( mg/kg )	3.50	3.50	3.80	3.35	3.60
C/P index	0.0350	0.0350	0.0380	0.0335	0.0360
Implication of C/P index	Very slight contamination	Very slight contamination	Very slight contamination	Very slight contamination	Very slight contamination

Table 5: Significance of pollution indices of Ni in the irrigated soils

Areas	Chandmal Baag	Sujandesar	Kadari colony (Shriramsar)	Kocharo ka bhaththa (Gangashahar-Bhinasar)	Gharsisar
Mean Conc. of Agriculture fields Soil ( mg/kg )	8	9	9	10	8
C/P index	0.16	0.18	0.18	0.20	0.16
Implication of C/P index	Slight contamination	Slight contamination	Slight contamination	Slight contamination	Slight contamination

Table 6: Significance of pollution indices of As in the irrigated soils

Areas	Chandmal Baag	Sujandesar	Kadari colony (Shriramsar)	Kocharo ka bhaththa (Gangashahar-Bhinasar)	Gharsisar
Mean Conc. of Agriculture fields Soil ( mg/kg )	1.05	1.20	1.30	0.90	1.10
C/P index	0.0525	0.0600	0.0650	0.0450	0.0550
Implication of C/P index	Very slight contamination	Very slight contamination	Very slight contamination	Very slight contamination	Very slight contamination

Table 7: Significance of pollution indices of Pb in the irrigated soils

Areas	Chandmal Baag	Sujandesar	Kadari colony (Shriramsar)	Kocharo ka bhaththa (Gangashahar-Bhinasar)	Gharsisar
Mean Conc. of Agriculture fields Soil ( mg/kg )	3.30	3.50	3.10	2.75	2.60
C/P index	0.0330	0.0350	0.0310	0.0275	0.0260
Implication of C/P index	Very slight contamination	Very slight contamination	Very slight contamination	Very slight contamination	Very slight contamination

The results showed that the concentration of Cr, As and Pb in all areas fields soils range very slight contaminated. However, concentrations of Ni in all areas fields soils ranges slight contaminated. The level of contaminations of the soils is that, Cd in Kadari colony (Shriramsar) is slightly polluted whereas Chandmal bag, Gharsisar and Sujandesar, Kocharo ka Bhaththa soils are very severe contaminated and severe contaminated respectively. High contamination of Cd in Kadri colony area is due to dye factories. These factories drain all effluents in nearby sewage naala and this water is used for irrigation. Cadmium and its compounds might travel through the soil, but its mobility depends on several factors, such as pH and the amount of organic matter, which will vary depending on the local environment. Generally, cadmium binds strongly to organic matter, becoming immobile in the soil and is taken up by plant life, eventually entering the food chain. Carrots grown in soils contaminated by Cd have the potential to cause toxicological problems in men, women and young children. High levels of Cd in soil were identified as causing itai-itai disease in Toyama Prefecture, Japan. The chronic effects of Cd consist of lung cancer, pulmonary adenocarcinomas, prostatic proliferative lesions, kidney dysfunction, bone fractures, and hypertension.³⁵

CONCLUSION:

The major environmental concern is an urbanizing India relate to high levels of water pollution due to poor waste disposal, inadequate sewerage and drainage and improper disposal of industrial effluents. The sewage and industrial effluents contain essential nutrients or possess properties which can easily be utilized for irrigating the field crops. But the sewage water of many cities where industrial effluent is mixed in the sewage system contained toxic metals.The heavy metals came mainly from sewage water irrigation. Thereby, the control of pollution in sewage water is vital for the mitigation of toxic metal contamination in study area. Additionally, a well-managed waste disposal system is necessary to controlling over the pollution caused by toxic metals like Cd.These results emphasize the need for further investigations on assessment of other heavy metals of soilds and vegetables and crops grown in sewage irrigated soil from the study sites. Still, health risk exposure of human and children through the consumption of local vegetables should also be investigated due to their high sensitivity to metal exposure. There is a great respite that toxic metals like Cr, Ni, As and Pb have not pose health risk even after long term use of this water for irrigation. However, intermittent monitoring of the metals from irrigation water, in soil and crops may be required to prevent their build-up in the food chain.³⁶

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