Health risks of heavy metals in contaminated soils and food crops irrigated with wastewater in Beijing, China
Introduction
Heavy metals are ubiquitous in the environment, as a result of both natural and anthropogenic activities, and humans are exposed to them through various pathways (Wilson and Pyatt, 2007). Wastewater irrigation, solid waste disposal, sludge applications, vehicular exhaust and industrial activities are the major sources of soil contamination with heavy metals, and an increased metal uptake by food crops grown on such contaminated soils is often observed. In general, wastewater contains substantial amounts of beneficial nutrients and toxic heavy metals, which are creating opportunities and problems for agricultural production, respectively (Chen et al., 2005, Singh et al., 2004).
Excessive accumulation of heavy metals in agricultural soils through wastewater irrigation, may not only result in soil contamination, but also lead to elevated heavy metal uptake by crops, and thus affect food quality and safety (Muchuweti et al., 2006). Heavy metal accumulation in soils and plants is of increasing concern because of the potential human health risks. This food chain contamination is one of the important pathways for the entry of these toxic pollutants into the human body. Heavy metal accumulation in plants depends upon plant species, and the efficiency of different plants in absorbing metals is evaluated by either plant uptake or soil-to-plant transfer factors of the metals (Rattan et al., 2005).
Vegetables cultivated in wastewater-irrigated soils take up heavy metals in large enough quantities to cause potential health risks to the consumers. In order to assess the health risks, it is necessary to identify the potential of a source to introduce risk agents into the environment, estimate the amount of risk agents that come into contact with the human-environment boundaries, and quantify the health consequence of the exposure (Ma et al., 2006). According to the National Research Council (NRC), 1983, this process consists of four steps, hazard identification, exposure assessment, dose/response assessment, and risk characterization. Chronic level intake of toxic metals has adverse impacts on humans and the associated harmful impacts become apparent only after several years of exposure (Bahemuka and Mubofu, 1999, Ikeda et al., 2000). However, the consumption of heavy metal-contaminated food can seriously deplete some essential nutrients in the body that are further responsible for decreasing immunological defenses, intrauterine growth retardation, impaired psycho-social faculties, disabilities associated with malnutrition and high prevalence of upper gastrointestinal cancer rates (Iyengar and Nair, 2000, Türkdogan et al., 2003).
Wastewater irrigation is a widespread practice in the world and recently a number of articles have been published on wastewater-irrigated soils contaminated with heavy metals (Liu et al., 2005, Mapanda et al., 2005, Rattan et al., 2005, Rothenberg et al., 2007). However, an additional insight into metal uptake, accumulation and assessment of human health risks associated with wastewater-irrigated soils is still needed. This study was conducted to investigate the soil pollution load, to understand the appropriateness of wastewater-irrigated soils for vegetable cultivation, and to assess the metal uptake by food crops and the potential health risks associated with human consumption of food crops contaminated with heavy metals.
Section snippets
Study area
The study area is located in southwest of Tongzhou District, Beijing, China. The area has a continental monsoon climate, characterized by a wide seasonal variation in annual rainfall (620 mm), cold and dry winters, and hot and rainy summers. Being the capital of China, Beijing City is generating a huge amount of wastewater from domestic, commercial and industrial sectors. In metropolitan areas, the wastewater is biologically treated and mainly reused in agricultural production during the
Soil contamination
Table 1 summarizes the physicochemical characteristics of all samples, including both wastewater-irrigated and reference soils. Soil pH was not significantly affected by the wastewater irrigation. In the wastewater-irrigated soils, the water soluble organic carbon values were not significantly increased compared with the reference soils. The water soluble organic carbon contents ranged from 70.6 mg kg−1 to 80.8 mg kg−1 in wastewater-irrigated soils, while the corresponding values for reference
Discussion
The application of wastewater has led to changes in some soil physicochemical characteristics and heavy metal uptake by food crops, particularly vegetables. The soil pH changes depend on pH of the wastewater used for irrigation, and the soil pH has a great influence on the mobility and bioavailability of heavy metals (Nigam et al., 2001). The results showed that wastewater application dropped soil pH by 0.1–0.2 units compared the wastewater-irrigated soil to the reference soil. The water
Conclusion
The long term wastewater irrigation has led to contamination of soils and food crops in the study areas. Heavy metals were shown a substantial buildup with a significant increase over reference soils. The pollution load index values indicated that the wastewater-irrigated soils were moderately enriched with Cr, Cu, Ni, Pb, and Zn, and strongly enriched with Cd. Furthermore, it was concluded the wastewater-irrigated grown plants were contaminated with those heavy metals and exceeded the
Acknowledgments
This research work was financially supported by the Ministry of Science and Technology (2007CB407301). Mr. Sardar Khan thanks HEC, Islamabad, Pakistan for awarding him PhD scholarship. We thank the anonymous reviewers for their valuable comments.
References (29)
- et al.
Heavy metals in edible green vegetables grown along the sites of the Sinza and Msimbazi Rivers in Dar es Salaam, Tanzania
Food Chemistry
(1999) - et al.
Environmental contamination of heavy metals from zinc smelting areas in Hezhang County, western Guizhou, China
Environment International
(2006) - et al.
Residues and source identification of persistent organic pollutants in farmland soils irrigated by effluents from biological treatment plants
Environment International
(2005) - et al.
Transfer of metals from soil to vegetables in an area near a smelter in Nanning, China
Environment International
(2004) - et al.
Exposure to metal mixtures and human health impacts in a contaminated area in Nanning, China
Environment International
(2005) - et al.
Urban population exposure to lead and cadmium in east and south-east Asia
Science of the Total Environment
(2000) - et al.
Impacts of sewage irrigation on heavy metals distribution and contamination in Beijing, China
Environment International
(2005) - et al.
The effect of long-term irrigation using wastewater on heavy metal contents of soils under vegetables in Harare, Zimbabwe. Agriculture
Ecosystem and Environment
(2005) - et al.
Heavy metal content of vegetables irrigated with mixture of wastewater and sewage sludge in Zimbabwe: implications for human health. Agriculture
Ecosystem and Environment
(2006) - et al.
Long-term impact of irrigation with sewage effluents on heavy metal content in soils, crops and groundwater-a case study. Agriculture
Ecosystem and Environment
(2005)