Comparative study of heavy metals in "soil-wheat" systems between sewage-irrigated areas and clean-water-irrigated areas in suburban Beijing.

Author:Zhao, Ye


Using urban sewage that has been primarily treated to irrigate farmland is an effective method to address the water shortage problem in agriculture, improve soil fertility, and dispose of municipal wastewater (Wang & Zhou, 2004; Zhu, 2001). Experimental research on sewage irrigation has been performed in China since 1957, and the research has shown that people who consume crops irrigated with sew age effluent have a greater potential for contracting infectious diseases (Bouwer, 2000; O'Hara & Rubin, 2005). Research has also shown that if untreated sewage were regularly used for irrigation, heavy metal elements in the wastewater would accumulate in the soil and lead to heavy metal pollution (Ahmad, Hayat, & Pichtel, 2005; Solis et al., 2005; van der Perk, 2006). Once the heavy metal content exceeds a certain threshold, these metals enter into the crops through a process known as enrichment; this process subsequently reduces the crop yield and quality and harms human health (Li et al., 2010).

Heavy metals can enter the human body in three main ways: respiration, dermal exposure, and dietary intake. Compared with respiration and dermal exposure, dietary intake is the most common method because diets are often large and complex and because of the varying heavy metal content in food (Grasmuck & Scholz, 2005; Jarup, 2003). Of those food items containing heavy metals, cereals have been paid little attention even though cereals are consumed almost daily around the world (Chary, Kamala, & Raj, 2008; Nadal, Schuhmacher, & Domingo, 2004). Thus, performing risk assessments of grain crops has a practical significance.

The farmland near the river in Beijing's eastern suburbs has been irrigated by wastewater for more than 50 years. The accumulation and distribution of heavy metals in the soil have become controversial social issues (Yang, Chen, et al., 2005). Based on earlier studies from scholars abroad and at home, our study focused on determining and analyzing concentrations of copper, chromium, lead, and zinc in soil and wheat seeds through field investigations and sample analysis in a sewage-irrigated area and a clean-water-irrigated area located in a suburb of Beijing. Twenty-four regions with wheat production in sewage-irrigated areas and 24 regions in clean-water-irrigated areas were selected, and soil and wheat samples were collected. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES), which is a rapid, convenient, and precise technique (Wang, Ma, & Itoh, 2005), was used to determine the concentration of heavy metals (copper, chromium, lead, and zinc) in the soil and wheat samples. Our goal was to provide information about soil pollution control and recovery as well as food health risk analysis.

Materials and Methods

Study Sites

The study area is located at the junction of the Tongzhou District in Beijing and Xianghe County in Hebei Province. The site is in a warm, temperate continental climate zone with an average annual temperature of 10.5[degrees]C. The annual precipitation, which is concentrated in summer and fall, is approximately 620 mm.

On the surface of the field, one can find tawny or yellowish-brown sandy loam and silty clay loam carried by the Chaobai River and developing loamy aquatic soil, sandy aquatic soil, and sticky aquatic soil. The background values for all of those soils are relatively uniform (Environmental Monitoring Station, 1990; State Environmental Protection Administration of China, 1994).

In terms of water sources, our research area included parallel regions of the Lianghui River, the Fenggangjian River, and the Beiyun River. The quality of the surface water is significantly different in the regions corresponding to each of these rivers. Areas along the Chaobai River, together with the north plain, have good water quality, while the Beiyun River has poor water quality due to large deposits of sewage from downtown Beijing. The Fenggangjian River's water quality is the worst of the three rivers, primarily because it was originally an artificial river built to receive waste.

As shown in Figure 1, the Fenggangjian River and the western side of the Beiyun River are sewage-irrigated areas; mixed-irrigated areas fall between the Beiyun River and the Chaobai River; and the northeastern coast of the Chaobai River is a clean-water-irrigated area. The study area is dominated by irriga tion agriculture, with crops of winter wheat, summer maize, cotton, and vegetables. In our study, "soil-wheat" systems in sewage-irrigated areas and clean-water-irrigated areas were the main focus.

Sample Collection and Handling

A field investigation of the study area was conducted using remote sensing images and topographic maps (Rejith, Jeeva, Vijith, Sowmya, & Hatha, 2009). Soil samples were collected in 2010 from a concentrated farming region away from the highway. Twenty-four of the selected sample regions were located in sewage-irrigated areas along the Fenggangjian River and the Beiyun River, and 24 of the sample regions were located in clean-water-irrigated areas northeast of the Chaobai River. In each sample region, topsoil (0-20 cm) was collected using the plumb point method. Each sample...

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