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

Introduction

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 point is shown in Figure 1. Wheat samples were collected from the 48 selected wheat areas, with 24 from sewage-irrigated areas and 24 from clean-water-irrigated areas. Six wheat plants were randomly selected in each area from which to obtain wheat seeds.

All soil samples were air dried, ground, and passed through a 0.149-mm mesh (100 mesh) sieve in a timely manner in the laboratory before the samples were measured and analyzed. Those samples were then sealed in Kraft paper envelopes until analysis. After the wheat seed samples were dried and decorticated indoors, the wheat grains were subjected to an ultrasonic cleaning; then they were washed with deionized water three times, roasted in an oven at 80[degrees]C to a constant weight, and smashed after cooling down. The smashed samples were preserved in Kraft paper bags for further analysis.

Sample Determination and Analysis

Potentiometric technique was used to determine the soil pH by a pH meter. The potassium dichromate oxidation method was used to determine the content of the organic matter in the soil. Quantitative potassium dichromate-sulfuric acid solution was added to oxidize the organic carbon in the soil under heat. The remaining potassium dichromate was titrated with ferrous sulfate standard solution. Meanwhile, silica was subjected to the same steps to be used as an experimental blank for comparison. The organic matter in the soil could be calculated from the difference in the mass of the oxidant before and after oxidation.

A 0.10-g sample of dried soil that was filtered through 100 mesh was weighed precisely. This sample was subsequently digested according to the USEPA-3050B acid digestion recommendations from the U.S. Environmental Protection Agency (U.S. EPA, 1996). Eventually, the volume of the soil solution was adjusted to a certain value (10 mL). In addition to preparing the sample plant solutions, 0.20 g of a wheat test sample were accurately weighed and put into a polytetrafluoroethylene crucible; then 3 mL of nitric acid and 1 mL of perchloric acid were added successively. After adding acid, crucibles were placed into cans, which were sealed to prevent volatilization and placed into an oven at 150[degrees]C for four hours. Again, 1 mL of nitric acid was added, and the volume was adjusted to 10 mL with deionized water. The quantitative analyses of the heavy metals (copper, chromium, lead, and zinc) in the solutions were performed using ICP-AES.

All the sample determinations were performed in triplicate to minimize the risk of error, and the arithmetic mean value was the final result. The blank reagent and standard reference soil and plant materials (from the National Research Center for Standards in China) were included in each sample batch to verify the accuracy and precision of the digestion procedure and subsequent analyses.

Results and Discussion

Physicochemical Properties of Soils

Our results revealed that the mean pH value (water-soil ratio is 5:1) of the topsoil in sewage-irrigated soils was 8.30 with a standard deviation of approximately 4%. The mean pH value of the topsoil in clean-water-irrigated soils was 7.30 with a standard deviation of approximately 7%. The mean value of the organic matter mass ratio in sewage-irrigated soils was 1.590 g/kg and the standard deviation was approximately 43%. In clean-water-irrigated soils, the value was 1.40 g/kg and the standard deviation was approximately 34%. In sewage-irrigated soils, a decrease in the pH of 1.00 was accompanied by an increase in the organic matter of 13.57% compared to clean-water-irrigated soils, which suggested that the soil properties were significantly affected by sewage irrigation.

Heavy Metal Concentrations in Soils

The data for four heavy metals found in sewage-irrigated soils are presented in Table 1, and the data for four heavy metals found in clean-water-irrigated soils are shown in Table 2. The average mass ratio of chromium in the topsoil of the sewage-irrigated area was obviously higher than the background value (57.3-73.9 mg/kg) (Environmental Monitoring Station, 1990; State Environmental Protection Administration of China, 1994); this value was also remarkably higher than the global background value (55.0 mg/ kg) in soils that were developed from loess and silt sediment (Kabata-Pendias, 1985). Nearly 15 of the 24 sampling points in the sewage-irrigated areas had a mass ratio of chromium in the topsoil that exceeded 75.0 mg/kg. The mass ratio of copper was found to be equivalent to the background value of copper (20.7-27.3 mg/kg) in the study area and to the global background value (25.0 mg/kg) in soils developed from loess and silt sediment. Seven out of the 24 sampling points in the sewage-irrigated areas, however, had a mass ratio of copper in the topsoil that exceeded 27.5 mg/kg. The average mass ratio of zinc in the topsoil of the sewage-irrigated areas was remarkably higher than the global background value (58.5 mg/ kg) in soils developed from loess and silt sediment (Kabata-Pendias, 1985). Nearly 20 of the 24 sampling points in the sewage-irrigated areas had a mass ratio of zinc in the topsoil that exceeded 88.5 mg/kg.

All of the results were in agreement with the heavy metal contents in the soils from the Liangfeng-irrigated area that were measured by Yang, Zheng, and co-authors (2005). Further statistical analysis showed that the organic matter content in the soil had a significant positive correlation with the heavy metal concentrations of chromium, copper, and zinc (p < .05). This result may be attributable to the adsorption of heavy metals onto organic matter, with most of the heavy metals being delivered to the soil through sewage irrigation.

Figure 2 also clearly shows that the average concentrations of copper, chromium, lead, and zinc in the sewage-irrigated soil were higher than the background values of the soil in clean-water-irrigated regions; the concentration of chromium showed the biggest difference between sewage-irrigated soils and clean-water-irrigated soils, and copper showed the smallest difference. In the sewage-irrigated soils, the average concentrations of copper, chromium, lead, and zinc exceeded those of the clean-water-irrigated areas by 1.03-, 1.29-, 1.20-, and 1.18-fold, respectively, indicating the accumulation of these heavy metals in the soil of the sewage-irrigated fields. Furthermore, the concentrations of copper, chromium, lead, and zinc in the sewage-irrigated fields can be ranked as follows: chromium (101.29 mg/kg) > zinc (85.59 mg/kg) > lead (28.04 mg/kg) > copper (26.51 mg/kg). Compared with the values measured in 1985, a large accumulation of heavy metals occurred in sewage-irrigated fields. But when compared with the national secondary standard values, the concentrations of heavy metals were all under the minimal threshold value, indicating that the study area can temporarily guarantee agricultural production and human health.

The concentrations of copper, chromium, lead, and zinc in the clean-water-irrigated fields can be ranked in a similar order: chromium (78.5 mg/kg) > zinc (72.29 mg/kg) > copper (25.66 mg/kg) > lead (23.34 mg/kg).

The results indicated that sewage irrigation has increased the concentration of heavy metals in the soil, especially the concentrations of lead and zinc. Similar results were found in previous studies (Khan, Cao, Zheng, Huang, & Zhu, 2008; Liu, Zhao, Ouyang, Soderlund, & Liu, 2005). The distribution of metals in the farmland at each site was primarily affected by the location of the farmland and the duration of irrigation time. Farmland close to the main channel that was irrigated with sewage for many years showed a higher level of contamination. This observation indicates that continuous sewage irrigation may result in heavy metal contamination in the soil.

Heavy Metal Concentrations of the Wheat Samples

As indicated in Table 3, the concentrations of zinc and copper in the dry matter of wheat seeds grown in sewage-irrigated soil were highest among the four heavy metals. Correlation analysis revealed that the zinc concentration in the wheat seeds was significantly correlated with that of the soil in which the wheat seeds were grown (p < .05). Chromium and lead are unnecessary elements for growth in plants and were absorbed primarily through passive absorption. The data in Table 3 show that the concentration of lead ranged between 0 and 0.63 mg/kg. The mean value was 0.17 mg/kg, which is below the tolerance limit for Chinese standards. The mean concentration of chromium in wheat seeds was 4.62 mg/kg, which is 4.6 times higher than the tolerance limit for Chinese standards. The proportion of samples that exceeded this tolerance limit was as high as 93%; thus, chromium contamination of wheat seeds demands increased attention.

Large variations were apparent between sewage-irrigated and clean-water-irrigated regions (Figure 2). The average concentrations of copper, chromium, and zinc in seeds from sewage-irrigated soil were higher than those from clean-water-irrigated areas by 1.24-, 9.63-, and 1.81-fold, respectively, while the concentrations of lead in the two types of seeds were almost equal. Combined with the data presented above, this result indicates that the concentrations of some heavy metals exceeded the national secondary standard; thus, continuous sewage irrigation can cause heavy metal contamination in the edible seeds of crops grown in this soil.

In addition, analysis of variance revealed that the concentrations of chromium and lead were not distributed uniformly throughout the studied areas. On the one hand, heavy metals in the soil originally demonstrated some spatial differences. On the other hand, however, factories and an airport were near the study area, which would have affected the concentrations of heavy metals. Therefore, it is necessary to pay attention to uncertainty in this study region.

Heavy Metal Transfer From Soil to Edible Seeds

The bioconcentration coefficient (BCF) of a heavy metal in the soil can be used to measure how difficult it is for that heavy metal to be absorbed by plants from the soil. In terms of agriculture, the absorbed dose (concentration) of the edible portion of a food crop (kernel) is generally used as an assessment of a heavy metal's effectiveness. The computational formula used is as follows:

[BCF.sup.Wheat.sub.heavymetal] = [C.sup.seed.sub.heavymetal]/ [C.sup.soil.sub.heavymetal]

[C.sup.seed.sub.heavymetal] is the concentration of heavy metals in the wheat kernel and [C.sup.soil.sub.heavymetal] is the concentration of heavy metals in the soil. The BCFs of seeds collected from sewage-irrigated soil and clean-water-irrigated soil are shown in Table 3 and Table 4, respectively.

The BCF trend of the seeds grown in sewage-irrigated soil was as follows: zinc (0.61) > copper (0.23) > chromium (0.046) > lead (0.006). The higher BCF values for zinc and copper might be correlated with the physiological characteristics of the plants and the heavy metals. When comparing the BCFs of sewage-irrigated seeds with those of clean-water-irrigated seeds, it is clear that irrigation with sewage could lead to increased BCF values in the seeds. Among the heavy metals, the BCFs of zinc and copper were 1.53- and 1.21-fold higher, respectively.

Conclusion

The average concentrations of copper, chromium, lead, and zinc in the sewage-irrigated soil were higher than the concentrations in the clean-water-irrigated soil, indicating the accumulation of these heavy metals in the soil. Compared with the national secondary standard, however, the concentrations of heavy metals were all under a certain value. Therefore, the study area can temporarily guarantee agricultural production and human health, but if sewage irrigation continues, the risk for heavy metal contamination in the soil will significantly increase in the future.

In "soil-wheat" systems in sewage-irrigated areas, the average concentrations of copper, chromium, and zinc in seeds mean that crops grown in sewage-irrigated soil were contaminated with heavy metals, some of which exceeded the permissible limits. Also, irrigation with wastewater can lead to increased BCF values. Therefore, issues of food contamination attributable to sewage irrigation with regard to the quality of agricultural products and human health deserve more attention.

Aside from wheat, many other foods are possible avenues for human ingestion of heavy metals. No other foods or intake methods besides wheat were considered in our study. Thus, further research into heavy metal health risks attributable to other foods (e.g., vegetables, meats, eggs, milk, and so on), drinking water, breathing outdoors, and contact with the outside world should be conducted to determine the ratios of influence from various sources and provide a theoretical basis for public health.

Acknowledgements: This work was financially supported by the Ministry of Land and Resources' Special Funds for Scientific Research on Public Benefit (No. 2010110061) and the National Key Basic Research and Development (973) Program of China (Grant No. 2007CB407302). We are grateful to Dr. Ke Sun for her comments. In addition, we thank the participating institutes, as well as the anonymous reviewers for their kind and helpful comments regarding the manuscript.

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Ye Zhao, PhD

Sha-sha Han, MS

State Key Laboratory of Water

Environment Simulation

School of Environment, Beijing

Zhi-fan Chen, PhD

College of Environment and Planning

Henan University

Jing Liu, MS

Hong-xia Hu, MS

State Key Laboratory of Water

Environment Simulation

School of Environment, Beijing

Corresponding Author: Ye Zhao, Professor, School of Environment, Beijing Normal University, Beijing, 100875, China. E-mail: zhaoye@bnu.edu.cn.

TABLE 1
Basic Characteristics and Concentrations of Heavy Metals in
Sewage-Irrigated Topsoil in Suburban Beijing (mg/kg)

Element       Range        Mean     SD     National
                                           Secondary
                                           Standard

Copper     13.31-60.04    26.51    10.34      100
Chromium   64.55-166.83   101.29   22.05      300
Lead        19.9-42.83    28.04    5.98       300
Zinc       61.59-142.52   85.59    18.74      250

Element       Soil      Concentration      Ratio of
           Background      in 1985        Exceeding
            Value in                    Standard Value
            Beijing
Copper        23.6          10.07             0
Chromium      68.1          52.66             0
Lead          25.4           13               0
Zinc         102.6          42.32             0

TABLE 2
Basic Characteristics and Concentrations of Heavy Metals in
Clean-Water-Irrigated Topsoil in Suburban Beijing (mg/kg)

Element       Range       Mean     SD     National    Ratio of
                                          Secondary   Exceeding
                                          Standard    Standard
                                                        Value

Copper     13.54-48.12    25.66   5.97       100          0
Chromium   55.89-138.82   78.5    12.04      300          0
Lead       15.85-27.85    23.34   2.66       300          0
Zinc       41.63-98.39    72.29   10.31      250          0

TABLE 3
Concentration of Heavy Metals in the Wheat Seed of Sewage-Irrigated
Areas (mg/kg)

Element       Range      Mean     SD     National    Concentration
                                         Secondary      in 1985
                                         Standard

Copper     4.21-12.34    6.09    1.57       10            6.3
Chromium    0.22-8.95    4.62    2.08        1           0.147
Lead         0-0.63      0.17    0.21       0.2          0.046
Zinc       30.70-95.72   52.38   13.05      50           17.7

Element    Bioconcentration      Ratio of
             Coefficient        Exceeding
                              Standard Value

Copper           0.23              0.04
Chromium        0.046              0.93
Lead            0.006              0.37
Zinc             0.61              0.52

TABLE 4
Concentration of Heavy Metals in the Wheat Seed of Clean-WaterIrrigated
Areas (mg/kg)

Element       Range      Mean     SD    National
                                        Secondary
                                        Standard

Copper      2.6-8.56     4.91    1.69      10
Chromium    0.12-0.95    0.48    0.25       1
Lead        0.09-0.31    0.16    0.06      0.2
Zinc       17.03-47.03   28.88   9.24      50

Element    Bioconcentration      Ratio of
             Coefficient        Exceeding
                              Standard Value

Copper           0.19               0
Chromium        0.006               0
Lead            0.006              0.12
Zinc             0.4                0
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Title Annotation:INTERNATIONAL PERSPECTIVES
Author:Zhao, Ye; Han, Sha-sha; Chen, Zhi-fan; Liu, Jing; Hu, Hong-xia
Publication:Journal of Environmental Health
Article Type:Report
Geographic Code:9CHIN
Date:Jan 1, 2015
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