Assessing risk of heavy metals from consuming food grown on sewage irrigated soils and food chain transfer
Introduction
Land spreading of untreated sewage effluents is practiced all over the world because of the economic advantage it offers over effluent treatment. Heavy metals can also accumulate in the soil at toxic levels as a result of long-term application of untreated wastewaters. Soils irrigated by wastewater accumulate heavy metals such as Cr, Zn, Pb, Cd, Ni, etc in surface soil. When the capacity of the soil to retain heavy metals is reduced due to repeated application of wastewater, heavy metals leach into ground water or soil solution available for plant uptake. For the metals derived from anthropogenic sources, this can strongly influence their speciation and hence bioavailability. However, the heavy metal content in plants can also be affected by other factors such as the application of fertilizers, sewage sludge or irrigation with wastewater (Devkota and Schmidt, 2000; Frost and Ketchum, 2000; Mangwayana, 1995). Studies have shown that heavy metals are potentially toxic to crops, animals and humans when contaminated soils are used for crop production (Xian, 1989). Heavy metals may enter the human body through inhalation of dust, consumption of contaminated drinking water, direct ingestion of soil and consumption of food plants grown in metal-contaminated soil (Cambra et al., 1999; Dudka and Miller, 1999). Vegetables constitute an important part of the human diet since they contain carbohydrates, proteins, as well as vitamins, minerals, and trace elements. It is known that serious systemic health problems can develop as a result of excessive accumulation of dietary heavy metals such as Cd, Cr, and Pb in the human body (Oliver, 1997). One important dietary uptake pathway of metals could be through crops irrigated with contaminated wastewater.
In suburban parts of India, the use of industrial or municipal wastewater for irrigation purpose is a common practice (Singh et al., 2004). To face the growing demand for irrigation water, non-conventional resources are often used. Important sources of heavy metals in wastewater are urban and industrial effluents and deterioration of sewerage pipelines. Irrigation with wastewater is known to contribute significantly to the heavy metal content of soils (Mapanda et al., 2005; Nan et al., 2002; Singh et al., 2004). In India, total wastewater generated per annum from 200 cities is about 2600 Mm3 (Kaul et al., 1989) and also the use of sewage effluents for irrigating agricultural lands is on the rise especially in the peri-urban areas. Although the concentration of heavy metals in sewage effluents are low, in many cases these are contaminated with industrial wastes and long-term use of these wastewaters often results in the build-up of the elevated levels of these metals in soils (Rattan et al., 2002). Since food chain contamination is one of the major routes for entry of metals into the animal system, monitoring the bioavailable pools of metals in contaminated soils has generated a lot of interest (Datta et al., 2000; Yadav et al., 2002).
The Musi River flows into Hyderabad, capital of Andhra Pradesh, India as a clean resource until it reaches the T-main sewer where 25 million liters per day (mld) of untreated sewage is released. It is from this point that the river serves as little more than a sewer drain. Even during monsoon, runoff inflows are very low in comparison to the quantity of sewage discharged into the river. During monsoon the Musi River catchment area receives 700–800 mm of rainfall, but because of the large inputs of domestic sewage and industrial wastewaters, self-purification within the river and dilution are minimum (EPTRI, 1997; Sircar, 2000). Due to the rapid unplanned development of the city of Hyderabad, the quantity of sewage generated is many times more than the design capacity of the Amberpet Sewage Treatment Plant (STP). This results in a high percentage of under-treated and untreated sewage entering the river. The capacity of the Amberpet STP is 115 mld, but the plant typically receives 350 mld, which includes industrial effluents (Sircar, 2000; Musi River, 2001; CPCB, 2002). This drainage water which is from both domestic and industrial sources, is channeled to several contiguous plots of land or some times supplemented by water pumped from the river for irrigation.
Organic pollutants in the river are partially eliminated by self-purification and accessible dilution. The inorganic pollutants (heavy metals) are the fraction of greatest concern due to their persistence in sewage sludge, which later becomes a potential source of risk to the nearby soils and vegetation. Episodes of heavy metal pollution of the Musi River and its surroundings have been reported (Chandra Sekhar et al., 2005; Kumari et al., 1991; Venkateshwarulu and Sampath Kumar, 1982; Bansal, 1998). Along the banks of the Musi intensive cultivation of fodder grass (para grass) and food crops occurs in the sewage sludge, and the concentrations of heavy metals are reported to be very high (Kumari et al., 1991; Venkateshwarulu and Sampath Kumar, 1982; Bansal, 1998; Anjaneyulu, 2001; Chandra Sekhar et al., 2005). Grass raised along the river is the fodder for most of the cattle in and around Hyderabad (Buechler et al., 2002; Sircar, 2000). A variety of vegetable crops are also cultivated on the banks of the river and sold in the nearby market.
Earlier studies by authors on river Musi reported concentrations of metals in soils and corresponding accumulation in nearby vegetation (Chandra Sekhar et al., 2005). These studies can only be regarded as base line data since there exists no explanation regarding the metal accumulation status and also possible mobility into nearby plants of the Musi River stretch. Further there are no reported studies on the assessment of possible health risk linked with consumption of these contaminated foods (milk, vegetables). Thereby a study was conducted along the stretch of Musi River to assess the concentration of heavy metals in soils, resulted uptake by the plant and elevated transfer to human food chain that helps in assessing the related health hazard associated with it. The metal concentrations in soil and vegetables are compared with established permissible limits. Also a dietary intake of vegetables by human were calculated and compared with the recommended dietary intakes.
Section snippets
Soil
Soil sampling was carried out at 12 different locations along and across 8 km stretch of the Musi River, including 2 km on either side of the river. Soils were mainly red sandy loam and brownish sandy soil with black clay subsoil. Geologically the Musi basin is covered by granites of Archean age and intercalated with quartz veins here and there and is at an altitude of 500 m above mean sea level (M1) and 470 MSL (M12). The location map of the Musi River stretch along with the 12 sampling sites
Soil
The soil consisted of mainly red sandy loam/brown sandy soil with clay subsoil. pH of the soil ranged from near neutral to moderately acidic (5.9–7.3). The soils reported moderate cation exchange capacity (CEC) which ranged between 22.7 and 29.2 c mol kg−1 indicating average metal retention capabilities. The % organic carbon was found to be in high due to constant sewage flows and ranged between 4.9 and 6.2%. As the subsoils were clayey the organic carbon was found to be in high percent. Metal
Discussion and conclusion
Heavy metals accumulation increases with time in the soils when irrigation is carried out using wastewaters. In this scenario the present article gains significance indicating the need for proper disposal of sewage and further abatement of metal pollution and associated risk due to the consumption of foods grown on sewage wastewaters. There are few such studies conducted in India on the metal contamination of soils resulting from sewage irrigation (Agrawal, 1999; Singh et al., 2004; Rattan et
Acknowledgments
All the authors would like to thank the Council for Scientific and Industrial Research (CSIR), Ministry of Human Resources development, Government of India, for providing research fellowship.
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