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Status of cadmium accumulation in agricultural soils across China (1975–2016): From temporal and spatial variations to risk assessment
Chemosphere 230 (2019) 136e143
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Status of cadmium accumulation in agricultural soils across China (1975e2016): From temporal and spatial variations to risk assessment Taoran Shi a, 1, Yunyun Zhang a, b, 1, Yiwei Gong a, Jin Ma a, *, Haiying Wei b, Xiao Wu a, Long Zhao a, Hong Hou a a b
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 100012, China College of Environmental and Resource Sciences, Shanxi University, Taiyuan, 030006, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cd pollution in agricultural soil of China (1981e2016) were systematically analyzed. Soil Cd concentrations accumulated gradually from 1981 to 2016. Fertilizers were important Cd sources for agricultural soils in China. Soil Cd concentrations varied greatly in different regions.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2018 Received in revised form 5 March 2019 Accepted 28 April 2019 Available online 10 May 2019
Based on 1186 published studies, the first national-scale assessment of cadmium (Cd) contamination in agricultural soils across China was conducted. Cd concentrations, temporal and spatial variations, and ecological and health risks resulted from Cd exposure were analyzed. A small part of sampling sites with Cd concentration surpass the screening value and the control value (GB15618-2018), respectively. Soil Cd concentrations in South China were higher than other regions. Ecological risks resulting from Cd contamination were low. Soil Cd concentrations accumulated gradually from 1981 to 2016. Cd mainly came from anthropogenic activities, such as mining, smelting, sewage irrigation, and fertilization. Linear correlations were observed between application amounts of fertilizers and Cd concentrations in soil, indicating that the application of nitrogen, phosphorus, potassium, and compound fertilizers is an important contributor of Cd in soils. This study details the overall Cd contamination status of agricultural soils in China, thus can provide insights for policymakers regarding contamination prevention measures. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Patryk Oleszczuk Keywords: Cadmium Agricultural soil Temporal and spatial variations Ecological and health risks China
1. Introduction Cadmium (Cd) is a highly toxic heavy metal that exists widely in environment and causes extensive threats to humans, animals, and
* Corresponding author. E-mail address: [email protected] (J. Ma). 1 Taoran Shi and Yunyun Zhang contributed equally to this work. https://doi.org/10.1016/j.chemosphere.2019.04.208 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
plants (Huang et al., 2011; Hechmi et al., 2014). Anthropogenic activities, such as mineral resource exploitation, metal processing and smelting, chemical production, industrial emission, and sewage irrigation are the primary sources of Cd contamination in soil (Liu et al., 2005; Chen et al., 2014; Zhang et al., 2015). Over the past six decades, more than 125.893 tons of Cd have been released into the environment by sewage irrigation, most of which have accumulated in soil and thus caused serious Cd contamination in
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China (http://www.stats.gov.cn/tjsj/ndsj/). According to the first National Soil Pollution Investigation of China from 2005 to 2013, Cd ranked the first in the percentage of soil samples (7.0%) exceeding the Ministry of Environmental Protection limit (Zhao et al., 2015). Cd enters the human body through different exposure pathways and can induce numerous adverse health effects (WHO , 1992), including noncarcinogenic and carcinogenic risks (Johri et al., 2010). The serious potential health consequences of Cd contamination have been well known since the 1960s outbreak of Itai-itai disease in Japan. Soil ingestion is a potentially important source of exposure to environmental contaminants for both children and adults (Wang et al., 2018). Hence, health risks resulted from exposure to Cd-polluted soil in China have attracted attention recently. Generally, most previous researches focused on measuring Cd concentrations and assessing the contamination levels and associated risks in a typical selected area. Yang et al. (2017) identified the sources of soil Cd concentration increment in Wuhan, China. Some studies focused on industrial and agricultural regions in China, such as mines in Shaanxi (Xiao et al., 2017), Hunan (Fan et al., 2017), and other provinces in China (Qu et al., 2012; Liu et al., 2016), agricultural areas in southeast and northwest China (Deng et al., 2017; Wang et al., 2017; Wu et al., 2017). However, systematical investigations on Cd contamination in agricultural soils of China are very limited. An overall macro-evaluation on a national scale is urgently needed. Therefore, this study presents an overview of the contamination status, pollution sources and risks assessment of Cd in agricultural soils in China. The objectives of this study were 1) to evaluate the Cd contamination levels in agricultural soils in China, 2) to estimate the potential ecological and health risks posed by Cdcontaminated soils, and 3) to propose policy recommendations to relevant agencies.
2. Materials and methods 2.1. Data collection and analysis A total of 1186 papers published between 1975 and 2016 were collected from Web of Science, Elsevier Science Direct, Science Online, Chinese Periodical Full-text, and China National Knowledge Infrastructure databases, corresponding to the search terms “Cd, agricultural soil, China”. The sampling and processing methods used in the collected studies are all widely accepted by the scientific community. Soil samples were digested with mixed acid (HNO3, HF, and HClO4) and treated by a variety of analytical methods. There were lots of analytical methods of soil Cd concentration, among of them, atomic fluorescence spectrophotometry and inductively coupled plasma atomic emission spectrometry were used widely. This study collected the data of Cd concentrations from 2193 sampling sites in agricultural regions in a host of papers (Fig. 1). Each of these studies investigated only one or a few sites that were suspected of being polluted.
2.2. Ecological risk assessment Muller (1969) developed geoaccumulation index (Igeo), which has been widely applied in heavy metal studies (Anbuselvan et al., 2018; Wu et al., 2018). It enables the assessment of environment contamination by comparing differences between current and preindustrial concentrations. To calculate the Igeo of the soils at the examined sites, the following equation is used:
Igeo ¼ log2
Cn 1:5Bn
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(1)
where Cn is the measured concentration of Cd in the soil (mg/kg), and Bn is the geochemical background value of Cd in the soils of different provinces (mg/kg), as shown in Table S1. A constant of 1.5 is the background matrix correction factor due to lithospheric effects (Islam et al., 2018). The ranges of Igeo and the corresponding contamination levels were given in Table S2. 2.3. Health risk assessment China is a big agricultural country, farmers account for 42.65% of the total population (CSY, 2017). Hence, health risks resulted from exposure to Cd in agricultural soils could not be negligible. Health risk assessment model developed by the US EPA was widely used to assess the non-carcinogenic and carcinogenic effects on humans by heavy metals (NRC, 1983). Due to behavioral and physiological differences, in this study people was divided into three groups, namely, adult females, adult males and children. The detailed assessment method is described in the supplementary material, and the parameters are listed in Tables S3 and S4. 2.4. Statistical analysis Normality test was conducted before making comparisons of Cd concentrations in different regions. Cd concentrations and logarithms transformation in five regions did not comply with the normal distribution. So there were no difference analysis among five regions just a statistics. All statistical analyses were performed using SPSS 16.0. The correlation coefficient was used at significance levels of P < 0.05 and P < 0.01. 3. Results and discussion 3.1. Pollution status of Cd in agricultural soils in China The collected 2193 examined sites are distributed in 33 provinces throughout China, including 22 provinces, 5 autonomous regions, 4 municipalities, and 2 special administrative regions. As shown in Fig. 1, the examined sites are densely distributed in the east, south, and north China, but obviously sparse in some regions, such as Qinghai, Tibet, Ningxia, and Jilin. We found that economically developed regions, such as the Beijing-Tianjin-Hebei region, Yangtze River Delta, and Pearl River Delta, have accumulated more literature and data. However, there were no correlations between the density of the sampling points and contamination degrees (Fig. 3). The distribution of Cd concentrations in agricultural soils across China was present in Fig. 1. Cd concentrations in Hunan, Yunnan, Guangxi, Gansu, and Liaoning Province were relatively high from the database collected. Previous studies suggested that Cd concentrations in agricultural soil in these provinces greatly exceeded the screening value (0.3 mg/kg). For example, Wang et al. (2008) found that Cd concentrations in 68.5% sites in Xiangjiang River, Hunan Province exceeded the maximum allowable heavy metal content. Wu et al. (2015) found that Cd concentrations in agricultural soil in Du'an County, Guangxi were 70.6% higher than the screening value. Considering regional differences, we classified the agricultural soil in China into five regions (the South China Sea and Macao were not evaluated due to the limited data) (Fig. 2). The average Cd concentrations in the five major regions were shown in Fig. 3. As shown in Fig. 3, the median Cd concentrations
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Fig. 1. Sampling sites distribution in China.
varied greatly in five regions, and all were higher than the national background soil value in China (0.097 mg/kg) (Chen et al., 1991), indicating that Cd was likely introduced into agricultural soils from exterior sources associated with anthropogenic activities. The number of the sampling sites in different period based on original literature was not uniform which may influence the geographical distribution of Cd concentrations. According to Yang et al. (2018), Cd concentrations in 36.7% of soil samples surpass the Grade II value (0.3 mg/kg) of environmental quality standard for soils in China (GB15618-1995). In this study, we used the screening value (0.6 mg/kg) and control value (4.0 mg/kg) as references to assess the contamination level. It was found that there were a small part of sampling sites with Cd concentration surpass the screening value and the control value, respectively. In this study, high Cd concentrations in agricultural regions were distributed mainly in the areas of Guangxi, Hubei, Anhui, Hunan, and Jiangxi Province. Guangxi and Hunan Province are both rich in mineral resources, known as the homes of non-ferrous metals (Huang et al., 2016). Daye mine in Hubei Province is one of the mines with the earliest exploitation history in China, and one of the six major copper mines and ten major iron ore production bases in China, and also an important building materials production base (Sun et al., 2013a). Huainan in Anhui Province is a coal mining city with a history of over 100 years (Chen et al., 2011). Guixi smelter in Jiangxi Province is the first large-scale copper flash smelter in China (Zhou et al., 2018). It was reported that the concentrations of Cd in most vegetables grown in mine-affected soils exceeded the maximum allowable level in Tonglushan mine in
Hubei (Cai et al., 2015). Overall, high Cd concentrations were distributed in the South China, such as the Yunnan-Guizhou Plateau, and the southern part of the Yangtze River. There are rich mineral resources, such as Pb, Zn, Mn, Sn, and Sb in these regions. For example, Panzhihua V-Ti magnetite in Sichuan, Shizhuyuan polymetallic mine in Hunan, and Dachang Sb-polymetallic mine in Guangxi are all located in highly contaminated provinces. 3.2. Temporal trends of Cd in agriculture soil in China As shown in Fig. 4, the Cd concentrations increased gradually from 1981 to 2016, and the temporal trend could be divided into two stages: an upward trend with a relatively high speed from 1981 to 2001 and a slow upward trend from 2002 to 2016. Heavy metals in contaminated soil could increase the potential risks of cancers (Lu et al., 2015). It was reported that cancer villages in China emerged in the 1980s, grew rapidly in the 1990s, and continued to grow after 2000 year (Yu and Zhang, 2009), which was in consist with the temporal trends of Cd concentrations in agricultural soil in China. Within the biogeochemical cycle of heavy metals, the element concentrations in soil are in a state of dynamic balance. Based on the mass balance theory, the concentration of heavy metal in soil is enriched when the input exceeds the output, and depleted when the output exceeds the input (Shi et al., 2018). Chinese government has made great efforts by implementing a series of measures to cut off the sources of pollution and avoid further pollution in recent
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Fig. 2. Geographical classification of agricultural soil in China in this study.
Fig. 3. Cd concentrations in agricultural soils of different regions of China.
years. For example, policies were enacted to prohibit the use of some fertilizers and pesticides since 2002 (Luo et al., 2009). So it would have added little input of Cd in agricultural soil. Generally, heavy metals persist in soils for a long time after their introduction. So the outputs of heavy metals is relatively stable. The decrease in inputs with constant outputs could partially explain the slow upward trend from 2002 to 2016. It is worth noting that there will still be a number of limitations.
Fig. 4. Temporal trend of Cd concentration in agricultural soils of China from 1981 to 2016.
The temporal trend of Cd concentration was extrapolated based on arithmetic mean, being neither median nor geometric mean. The temporal trend might be somewhat different based on different statistic data. Moreover, the number of the data from collected papers was not uniform in different time period. In addition, most studies have tended to be conducted in more polluted sites. Thus,
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assessments based on the data set compiled in this study may overestimate Cd concentrations in agricultural soils in China. 3.3. Fertilizer contribution to Cd contamination Based on the data of fertilizers (nitrogen, phosphorus, potassium, and compound fertilizers) application amounts collected from the National Bureau of Statistics website, Cd concentrations in agricultural soils were significantly correlated with fertilizer application amounts (P < 0.05) (Fig. 5). This result was in consist with previous studies. Wei and Yang (2010) reported that Cd was derived mainly from anthropogenic activities, such as sewage irrigation and fertilizer application, caused significantly contamination to agricultural soils. Lu et al. (2015) found that the usage of fertilizers was positively correlated with Cd concentrations in grain (including rice, wheat and corn). Hajar et al. (2012) found that there was significant relationship between the amount of potash fertilizer and Cd concentration in the soil (P < 0.05). Huang et al. (2018) found that the application of phosphate fertilizers containing Cd was a main source of Cd accumulation in paddy soils. In fact, soil Cd is often regarded as a marker of agronomic activities involving chemical fertilizer usage (Sun et al., 2013b). Previous studies showed that different application fertilizer strategies, amounts, and types can affect not just Cd accumulations in soil but also on the Cd concentration in crops (Zheng et al., 2015; Grüter et al., 2017). During the growth and development of rice, the application of fertilizers not only supplies nutrients for crops growth but also contains some toxic and harmful substances, such as Cd.
Many studies have shown that nitrogen, phosphorus, and potassium fertilizer application may increase Cd accumulation in plants (Oliver et al., 1993; Grant and Baily, 1997; Basta et al., 1998). Soil Cd can be taken up by roots of agricultural crops and transported to the above ground tissues (including grains), and then transferred through food chain to accumulate in human beings. Therefore, the application of fertilizers should be treated with great caution in the future. 3.4. Ecological risk assessment Generally, heavy metals never undergo microbial or chemical degradation and thus persist in soil for a long time after their introduction. Considering the uneven distribution of data collected from literature and the long degradation cycle of heavy metals, we evaluated the ecological risk in four periods, namely, 1975e2000, 2001e2005, 2006e2010, and 2011e2016. Fig. 6 shows the class distribution of Cd concentration in agricultural soil from different regions in China during four periods (Table S5). There were few sampling sites posed ecological risk above class 2 of geoaccumulation index in four periods. This result is consistent with the previous study (Wei and Yang, 2010). In four time periods, namely, 1975e2000, 2001e2005, 2006e2010, 2011e2016, the proportion of sampling sites from Class 0 to Class 5 declined gradually, but the decrease trend was not observed from Class 5 to Class 6. The proportion of sampling sites in Class 6 obviously decreased from 1975 to 2016. However, the proportions in Class 1 to Class 5 showed an increasing trend. It can be found that the highest ecological health risk values
Fig. 5. Relationships between Cd concentrations in agricultural soil and chemical fertilizer (N, P, K, Compound fertilizer) application amount.
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histories of mining for these areas with large-scale mining activities. For example, Baiyin in Gansu Province has a long history of sewage irrigation due to the lack of water resources. Wastewater from battery production industry has been used for irrigation for a long time in Xinxiang, Henan Province. This indicates that large amounts of exogenous inputs have the most direct effects on soil heavy metal accumulation. 3.5. Health risk assessment
Fig. 6. Class distribution of Igeo of Cd in agricultural soils of China in different periods.
were in mines or sewage irrigation areas, such as Gejiu Tin mine, Yunnan Province, and molybdenum ore area in Yang valley, Liaoning Province. Sewage irrigation areas included Xinxiang, Henan Province and Baiyin, Gansu Province. There are long
3.5.1. Non-carcinogenic risk assessment (HQ) To evaluate the human exposure to Cd quantitatively, exposure scenarios via ingestion and dermal contact were established for three age groups, namely adult male, adult female and children. In this study, there were very few sampling sites with hazard quotient (HQ) values above 1 for adult males, adult females, and children, respectively. Children suffered greater non-carcinogenic risk than adults. One possible reason is that children are likely to have higher rates of heavy metal absorption owing to their unique physiological characteristics, such as hand/finger sucking-a critical exposure pathway of soil heavy metals for children (White and Marcus, 1998). Similar findings have been reported in previous studies by Li et al. (2014), Chen et al. (2015). As shown in Fig. 7, nearly all of the examined sites with HQ > 1 were located in mining
Fig. 7. Non-carcinogenic risk of children through exposure to Cd in agricultural soils of China in different periods.
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areas, industrial areas and sewage irrigation areas. For example, the non-carcinogenic risk values were high in the agricultural soils around Baiyin, Gansu; copper-zinc-lead mine in central Tibet; leadzinc mine in Daxin, Guangxi; Zhuzhou smelter in Hunan; industrial zone in Henan; and sewage irrigation areas in Changchun, Jilin. Mining, industrial production and sewage irrigation resulted in serious Cd contamination in soil, indicating that Cd in agricultural soils was possible to pose non-carcinogenic risks to the surrounding population. The non-carcinogenic risk values surpassed 10 in the regions of Gansu, Hunan, Henan, and Guangxi, which should be controlled with priority. As shown in Fig. 7, the level of non-carcinogenic risk varied in different periods. The number of samples with HQ value in the range of 0e1 increased from 1975 to 2016. The number of samples with HQ in the range of 1e3 showed a basic increasing trend from 1975 to 2016. The sample points with HQ > 3 did not appear in 1975e2000, but increased obviously in the following three periods. On the whole, there was an increasing trend of areas suffered non-carcinogenic risks over time. Note that increased data over time is sure to raise the possibility of more cases of HQ > 1, which could have biased the results. 3.5.2. Carcinogenic risk assessment (LCR) Carcinogenic risks surpassing 1 104 are viewed as unacceptable, risks below 1 106 are not considered to pose significant health effects, and risks lying between 1 104 and 1 106 are generally considered an acceptable range by the USEPA (2001). In this study, very few part of Cd carcinogenic risks values were over 1 104, indicating potential carcinogenic risks resulted by exposure to Cd in soils. According to parameters for carcinogenic risk assessment, body weight and exposed skin surface area maybe the main factors of difference between the carcinogenic risks of adult females and adult males. The carcinogenic risk for children was lower than that for adults, it may be due to the shorter exposure duration of children (Li et al., 2015a). Similarly, carcinogenic risk level distributions were mapped with adult female carcinogenic risk values (i.e., the severest carcinogenic risk among the three population groups). It was found that some regions showed high carcinogenic risks in four periods. For example, values of LCR in sewage irrigation area in Changchun, Jilin Province (Li et al., 2015b), molybdenum mining area in Huludao, and lead-zinc mining area in Yunnan Province (Wang et al., 2017) were higher than the acceptable risk. So sewage irrigation areas, smelters and mining areas above-mentioned need urgent attention. The control and remediation measures are urgently needed to reduce Cd accumulation in agricultural soil in these areas. Due to limited data in 1975e2000 period, the levels of carcinogenic risk in some regions can hardly been assessed accurately, such as Tibet, Xinjiang, Qinghai and Inner Mongolia. However, it is worth noting that there were sporadic samples with carcinogenic risk in Xinjiang and Tibet after 2005. Long-term mining activities have brought serious heavy metal contamination in soil, which are possibly carcinogenic to the surrounding population (Jiang et al., 2017). The studies of Tianjin, Beijing, Hebei and coastal areas increased greatly in 2001e2005 period. Carcinogenic risks in most of these areas were at the acceptable levels of carcinogenic risks. With the increase of studies during 2006e2016, the sampling sites with unacceptable carcinogenic risks distributed densely in southern China, especially in Yunnan, Hunan and Guangxi Province. These areas are the priority control areas in China. Note that increased data over time is sure to raise the possibility of more cases of LCR>1 104, which could have biased the results. In addition, other potential factors (e.g., soil properties and heavy metal species, etc.) may affect heavy metals absorption, thus bringing some uncertainty to the health risk assessment results (Lu
et al., 2015). The sampling sites with carcinogenic risk values in the range assumed to have health effects were concentrated in Yunnan, Guizhou, and Guangxi provinces, most likely due to mining and smelting activities. 4. Conclusion This study gives a description of the overall contamination levels of Cd in agricultural soils of China and their ecological and health risks. Soil Cd concentrations accumulated gradually from 1981 to 2016 in China. Mining, sewage irrigation, and fertilizer application significantly contributed to Cd accumulation in agricultural soils. Cd concentrations were relatively high in agricultural soils from south regions of China. Ecological and health risks resulting from soil Cd contamination were in low levels. Acknowledgments This work was supported by Central Level, Scientific Reseach Institutes for Basic R&D Special Fund Business (2019YSKY006) and National Key Research and Development Program of China (2016YFD0800302). The authors would like to express our sincere gratitude to the co-authors who have put considerable time and effect into data collection, extraction and analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.04.208. References Anbuselvan, N., Senthil, N., Sridharan, M., 2018. Heavy metal assessment in surface sediments off coromandel coast of India: implication on marine pollution. Mar. Pollut. Bull. 131, 712e726. Basta, N.T., Raun, W.R., Gavi, F., 1998. Wheat grain cadmium under long-term fertilization and continuous winter wheat production. Better Crops 82, 14e15. Cai, L.M., Xu, Z.C., Qi, J.Y., Feng, Z.Z., Xiang, T.S., 2015. Assessment of exposure to heavy metals and health risks among residents near tonglushan mine in hubei, China. Chemosphere 127, 127e135. Chen, H., Teng, Y., Lu, S., Wang, Y., Wang, J., 2015. Contamination features and health risk of soil heavy metals in China. Sci. Total Environ. 512e513, 143e153. Chen, J., Liu, G.J., Jiang, M.M., Chou, C.L., Li, H., Wu, B., Zheng, L.G., Jiang, D.D., 2011. Geochemistry of environmentally sensitive trace elements in Permian coals from the Huainan coalfield, Anhui, China. Int. J. Coal Geol. 88, 41e54. Chen, J., Wei, F., Zheng, C., Wu, Y., Adriano, D.C., 1991. Background concentrations of elements in soils of China. Water Air Soil Pollut. 57e58, 699e712. Chen, K., Huang, L., Yan, B., Li, H., Sun, H., Bi, J., 2014. Effect of lead pollution control on environmental and childhood blood lead level in Nantong, China: an interventional study. Environ. Sci. Technol. 48, 129e130. CSY, 2017. China Statistical Yearbook. National Bureau of Statistics of China Edition, China Statistics Press, China. Deng, W., Li, X., An, Z., Yang, L., Hou, K., Zhang, Y., 2017. Identification of sources of metal in the agricultural soils of the Guanzhong Plain, northwest China. Environ. Toxicol. Chem. 36, 1510e1516. Fan, Y., Zhu, T., Li, M., He, J., Huang, R., 2017. Heavy metal contamination in soil and brown rice and human health risk assessment near three mining areas in central China. J. Healthc. Eng. 1e9. Grant, C.A., Baily, L.D., 1997. Nitrogen, phosphorus and zinc management effects on grain yield and cadmium concentration in two cultivars of durum wheat. Can. J. Plant Sci. 78, 63e70. Grüter, R., Costerousse, B., Bertoni, A., Mayer, J., Thonar, C., Frossard, E., Schulin, R., Tandy, S., 2017. Green manure and long-term fertilization effects on soil zinc and cadmium availability and uptake by wheat (Triticum aestivum L.) at different growth stages. Sci. Total Environ. 599e600, 1330e1343. Hajar, B., Masoud, Y., Amir, H.M., Mahmood, A.M., Mohammad, H.D., Shahrokh, N., 2012. Cadmium, lead and arsenic concentration in soil and underground water and its relationship with chemical fertilizer in paddy soil. J. Mazand. Univ. Med. Sci. 22, 20e28. Hechmi, N., Aissa, N.B., Abdenaceur, H., Jedidi, N., 2014. Evaluating the phytoremediation potential of Phragmites australis grown in pentachlorophenol and cadmium co-contaminated soils. Environ. Sci. Pollut. Res. 21, 1304e1313. Huang, H.G., Yu, N., Wang, L.J., Gupta, D., He, Z., Wang, K., Zhu, Z.Q., Yan, X.C., Li, T.Q., Yang, X., 2011. The phytoremediation potential of bioenergy crop Ricinus
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Status of cadmium accumulation in agricultural soils across China (1975e2016): From temporal and spatial variations to risk assessment Taoran Shi a, 1, Yunyun Zhang a, b, 1, Yiwei Gong a, Jin Ma a, *, Haiying Wei b, Xiao Wu a, Long Zhao a, Hong Hou a a b
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 100012, China College of Environmental and Resource Sciences, Shanxi University, Taiyuan, 030006, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cd pollution in agricultural soil of China (1981e2016) were systematically analyzed. Soil Cd concentrations accumulated gradually from 1981 to 2016. Fertilizers were important Cd sources for agricultural soils in China. Soil Cd concentrations varied greatly in different regions.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2018 Received in revised form 5 March 2019 Accepted 28 April 2019 Available online 10 May 2019
Based on 1186 published studies, the first national-scale assessment of cadmium (Cd) contamination in agricultural soils across China was conducted. Cd concentrations, temporal and spatial variations, and ecological and health risks resulted from Cd exposure were analyzed. A small part of sampling sites with Cd concentration surpass the screening value and the control value (GB15618-2018), respectively. Soil Cd concentrations in South China were higher than other regions. Ecological risks resulting from Cd contamination were low. Soil Cd concentrations accumulated gradually from 1981 to 2016. Cd mainly came from anthropogenic activities, such as mining, smelting, sewage irrigation, and fertilization. Linear correlations were observed between application amounts of fertilizers and Cd concentrations in soil, indicating that the application of nitrogen, phosphorus, potassium, and compound fertilizers is an important contributor of Cd in soils. This study details the overall Cd contamination status of agricultural soils in China, thus can provide insights for policymakers regarding contamination prevention measures. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Patryk Oleszczuk Keywords: Cadmium Agricultural soil Temporal and spatial variations Ecological and health risks China
1. Introduction Cadmium (Cd) is a highly toxic heavy metal that exists widely in environment and causes extensive threats to humans, animals, and
* Corresponding author. E-mail address: [email protected] (J. Ma). 1 Taoran Shi and Yunyun Zhang contributed equally to this work. https://doi.org/10.1016/j.chemosphere.2019.04.208 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
plants (Huang et al., 2011; Hechmi et al., 2014). Anthropogenic activities, such as mineral resource exploitation, metal processing and smelting, chemical production, industrial emission, and sewage irrigation are the primary sources of Cd contamination in soil (Liu et al., 2005; Chen et al., 2014; Zhang et al., 2015). Over the past six decades, more than 125.893 tons of Cd have been released into the environment by sewage irrigation, most of which have accumulated in soil and thus caused serious Cd contamination in
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China (http://www.stats.gov.cn/tjsj/ndsj/). According to the first National Soil Pollution Investigation of China from 2005 to 2013, Cd ranked the first in the percentage of soil samples (7.0%) exceeding the Ministry of Environmental Protection limit (Zhao et al., 2015). Cd enters the human body through different exposure pathways and can induce numerous adverse health effects (WHO , 1992), including noncarcinogenic and carcinogenic risks (Johri et al., 2010). The serious potential health consequences of Cd contamination have been well known since the 1960s outbreak of Itai-itai disease in Japan. Soil ingestion is a potentially important source of exposure to environmental contaminants for both children and adults (Wang et al., 2018). Hence, health risks resulted from exposure to Cd-polluted soil in China have attracted attention recently. Generally, most previous researches focused on measuring Cd concentrations and assessing the contamination levels and associated risks in a typical selected area. Yang et al. (2017) identified the sources of soil Cd concentration increment in Wuhan, China. Some studies focused on industrial and agricultural regions in China, such as mines in Shaanxi (Xiao et al., 2017), Hunan (Fan et al., 2017), and other provinces in China (Qu et al., 2012; Liu et al., 2016), agricultural areas in southeast and northwest China (Deng et al., 2017; Wang et al., 2017; Wu et al., 2017). However, systematical investigations on Cd contamination in agricultural soils of China are very limited. An overall macro-evaluation on a national scale is urgently needed. Therefore, this study presents an overview of the contamination status, pollution sources and risks assessment of Cd in agricultural soils in China. The objectives of this study were 1) to evaluate the Cd contamination levels in agricultural soils in China, 2) to estimate the potential ecological and health risks posed by Cdcontaminated soils, and 3) to propose policy recommendations to relevant agencies.
2. Materials and methods 2.1. Data collection and analysis A total of 1186 papers published between 1975 and 2016 were collected from Web of Science, Elsevier Science Direct, Science Online, Chinese Periodical Full-text, and China National Knowledge Infrastructure databases, corresponding to the search terms “Cd, agricultural soil, China”. The sampling and processing methods used in the collected studies are all widely accepted by the scientific community. Soil samples were digested with mixed acid (HNO3, HF, and HClO4) and treated by a variety of analytical methods. There were lots of analytical methods of soil Cd concentration, among of them, atomic fluorescence spectrophotometry and inductively coupled plasma atomic emission spectrometry were used widely. This study collected the data of Cd concentrations from 2193 sampling sites in agricultural regions in a host of papers (Fig. 1). Each of these studies investigated only one or a few sites that were suspected of being polluted.
2.2. Ecological risk assessment Muller (1969) developed geoaccumulation index (Igeo), which has been widely applied in heavy metal studies (Anbuselvan et al., 2018; Wu et al., 2018). It enables the assessment of environment contamination by comparing differences between current and preindustrial concentrations. To calculate the Igeo of the soils at the examined sites, the following equation is used:
Igeo ¼ log2
Cn 1:5Bn
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(1)
where Cn is the measured concentration of Cd in the soil (mg/kg), and Bn is the geochemical background value of Cd in the soils of different provinces (mg/kg), as shown in Table S1. A constant of 1.5 is the background matrix correction factor due to lithospheric effects (Islam et al., 2018). The ranges of Igeo and the corresponding contamination levels were given in Table S2. 2.3. Health risk assessment China is a big agricultural country, farmers account for 42.65% of the total population (CSY, 2017). Hence, health risks resulted from exposure to Cd in agricultural soils could not be negligible. Health risk assessment model developed by the US EPA was widely used to assess the non-carcinogenic and carcinogenic effects on humans by heavy metals (NRC, 1983). Due to behavioral and physiological differences, in this study people was divided into three groups, namely, adult females, adult males and children. The detailed assessment method is described in the supplementary material, and the parameters are listed in Tables S3 and S4. 2.4. Statistical analysis Normality test was conducted before making comparisons of Cd concentrations in different regions. Cd concentrations and logarithms transformation in five regions did not comply with the normal distribution. So there were no difference analysis among five regions just a statistics. All statistical analyses were performed using SPSS 16.0. The correlation coefficient was used at significance levels of P < 0.05 and P < 0.01. 3. Results and discussion 3.1. Pollution status of Cd in agricultural soils in China The collected 2193 examined sites are distributed in 33 provinces throughout China, including 22 provinces, 5 autonomous regions, 4 municipalities, and 2 special administrative regions. As shown in Fig. 1, the examined sites are densely distributed in the east, south, and north China, but obviously sparse in some regions, such as Qinghai, Tibet, Ningxia, and Jilin. We found that economically developed regions, such as the Beijing-Tianjin-Hebei region, Yangtze River Delta, and Pearl River Delta, have accumulated more literature and data. However, there were no correlations between the density of the sampling points and contamination degrees (Fig. 3). The distribution of Cd concentrations in agricultural soils across China was present in Fig. 1. Cd concentrations in Hunan, Yunnan, Guangxi, Gansu, and Liaoning Province were relatively high from the database collected. Previous studies suggested that Cd concentrations in agricultural soil in these provinces greatly exceeded the screening value (0.3 mg/kg). For example, Wang et al. (2008) found that Cd concentrations in 68.5% sites in Xiangjiang River, Hunan Province exceeded the maximum allowable heavy metal content. Wu et al. (2015) found that Cd concentrations in agricultural soil in Du'an County, Guangxi were 70.6% higher than the screening value. Considering regional differences, we classified the agricultural soil in China into five regions (the South China Sea and Macao were not evaluated due to the limited data) (Fig. 2). The average Cd concentrations in the five major regions were shown in Fig. 3. As shown in Fig. 3, the median Cd concentrations
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Fig. 1. Sampling sites distribution in China.
varied greatly in five regions, and all were higher than the national background soil value in China (0.097 mg/kg) (Chen et al., 1991), indicating that Cd was likely introduced into agricultural soils from exterior sources associated with anthropogenic activities. The number of the sampling sites in different period based on original literature was not uniform which may influence the geographical distribution of Cd concentrations. According to Yang et al. (2018), Cd concentrations in 36.7% of soil samples surpass the Grade II value (0.3 mg/kg) of environmental quality standard for soils in China (GB15618-1995). In this study, we used the screening value (0.6 mg/kg) and control value (4.0 mg/kg) as references to assess the contamination level. It was found that there were a small part of sampling sites with Cd concentration surpass the screening value and the control value, respectively. In this study, high Cd concentrations in agricultural regions were distributed mainly in the areas of Guangxi, Hubei, Anhui, Hunan, and Jiangxi Province. Guangxi and Hunan Province are both rich in mineral resources, known as the homes of non-ferrous metals (Huang et al., 2016). Daye mine in Hubei Province is one of the mines with the earliest exploitation history in China, and one of the six major copper mines and ten major iron ore production bases in China, and also an important building materials production base (Sun et al., 2013a). Huainan in Anhui Province is a coal mining city with a history of over 100 years (Chen et al., 2011). Guixi smelter in Jiangxi Province is the first large-scale copper flash smelter in China (Zhou et al., 2018). It was reported that the concentrations of Cd in most vegetables grown in mine-affected soils exceeded the maximum allowable level in Tonglushan mine in
Hubei (Cai et al., 2015). Overall, high Cd concentrations were distributed in the South China, such as the Yunnan-Guizhou Plateau, and the southern part of the Yangtze River. There are rich mineral resources, such as Pb, Zn, Mn, Sn, and Sb in these regions. For example, Panzhihua V-Ti magnetite in Sichuan, Shizhuyuan polymetallic mine in Hunan, and Dachang Sb-polymetallic mine in Guangxi are all located in highly contaminated provinces. 3.2. Temporal trends of Cd in agriculture soil in China As shown in Fig. 4, the Cd concentrations increased gradually from 1981 to 2016, and the temporal trend could be divided into two stages: an upward trend with a relatively high speed from 1981 to 2001 and a slow upward trend from 2002 to 2016. Heavy metals in contaminated soil could increase the potential risks of cancers (Lu et al., 2015). It was reported that cancer villages in China emerged in the 1980s, grew rapidly in the 1990s, and continued to grow after 2000 year (Yu and Zhang, 2009), which was in consist with the temporal trends of Cd concentrations in agricultural soil in China. Within the biogeochemical cycle of heavy metals, the element concentrations in soil are in a state of dynamic balance. Based on the mass balance theory, the concentration of heavy metal in soil is enriched when the input exceeds the output, and depleted when the output exceeds the input (Shi et al., 2018). Chinese government has made great efforts by implementing a series of measures to cut off the sources of pollution and avoid further pollution in recent
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Fig. 2. Geographical classification of agricultural soil in China in this study.
Fig. 3. Cd concentrations in agricultural soils of different regions of China.
years. For example, policies were enacted to prohibit the use of some fertilizers and pesticides since 2002 (Luo et al., 2009). So it would have added little input of Cd in agricultural soil. Generally, heavy metals persist in soils for a long time after their introduction. So the outputs of heavy metals is relatively stable. The decrease in inputs with constant outputs could partially explain the slow upward trend from 2002 to 2016. It is worth noting that there will still be a number of limitations.
Fig. 4. Temporal trend of Cd concentration in agricultural soils of China from 1981 to 2016.
The temporal trend of Cd concentration was extrapolated based on arithmetic mean, being neither median nor geometric mean. The temporal trend might be somewhat different based on different statistic data. Moreover, the number of the data from collected papers was not uniform in different time period. In addition, most studies have tended to be conducted in more polluted sites. Thus,
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assessments based on the data set compiled in this study may overestimate Cd concentrations in agricultural soils in China. 3.3. Fertilizer contribution to Cd contamination Based on the data of fertilizers (nitrogen, phosphorus, potassium, and compound fertilizers) application amounts collected from the National Bureau of Statistics website, Cd concentrations in agricultural soils were significantly correlated with fertilizer application amounts (P < 0.05) (Fig. 5). This result was in consist with previous studies. Wei and Yang (2010) reported that Cd was derived mainly from anthropogenic activities, such as sewage irrigation and fertilizer application, caused significantly contamination to agricultural soils. Lu et al. (2015) found that the usage of fertilizers was positively correlated with Cd concentrations in grain (including rice, wheat and corn). Hajar et al. (2012) found that there was significant relationship between the amount of potash fertilizer and Cd concentration in the soil (P < 0.05). Huang et al. (2018) found that the application of phosphate fertilizers containing Cd was a main source of Cd accumulation in paddy soils. In fact, soil Cd is often regarded as a marker of agronomic activities involving chemical fertilizer usage (Sun et al., 2013b). Previous studies showed that different application fertilizer strategies, amounts, and types can affect not just Cd accumulations in soil but also on the Cd concentration in crops (Zheng et al., 2015; Grüter et al., 2017). During the growth and development of rice, the application of fertilizers not only supplies nutrients for crops growth but also contains some toxic and harmful substances, such as Cd.
Many studies have shown that nitrogen, phosphorus, and potassium fertilizer application may increase Cd accumulation in plants (Oliver et al., 1993; Grant and Baily, 1997; Basta et al., 1998). Soil Cd can be taken up by roots of agricultural crops and transported to the above ground tissues (including grains), and then transferred through food chain to accumulate in human beings. Therefore, the application of fertilizers should be treated with great caution in the future. 3.4. Ecological risk assessment Generally, heavy metals never undergo microbial or chemical degradation and thus persist in soil for a long time after their introduction. Considering the uneven distribution of data collected from literature and the long degradation cycle of heavy metals, we evaluated the ecological risk in four periods, namely, 1975e2000, 2001e2005, 2006e2010, and 2011e2016. Fig. 6 shows the class distribution of Cd concentration in agricultural soil from different regions in China during four periods (Table S5). There were few sampling sites posed ecological risk above class 2 of geoaccumulation index in four periods. This result is consistent with the previous study (Wei and Yang, 2010). In four time periods, namely, 1975e2000, 2001e2005, 2006e2010, 2011e2016, the proportion of sampling sites from Class 0 to Class 5 declined gradually, but the decrease trend was not observed from Class 5 to Class 6. The proportion of sampling sites in Class 6 obviously decreased from 1975 to 2016. However, the proportions in Class 1 to Class 5 showed an increasing trend. It can be found that the highest ecological health risk values
Fig. 5. Relationships between Cd concentrations in agricultural soil and chemical fertilizer (N, P, K, Compound fertilizer) application amount.
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histories of mining for these areas with large-scale mining activities. For example, Baiyin in Gansu Province has a long history of sewage irrigation due to the lack of water resources. Wastewater from battery production industry has been used for irrigation for a long time in Xinxiang, Henan Province. This indicates that large amounts of exogenous inputs have the most direct effects on soil heavy metal accumulation. 3.5. Health risk assessment
Fig. 6. Class distribution of Igeo of Cd in agricultural soils of China in different periods.
were in mines or sewage irrigation areas, such as Gejiu Tin mine, Yunnan Province, and molybdenum ore area in Yang valley, Liaoning Province. Sewage irrigation areas included Xinxiang, Henan Province and Baiyin, Gansu Province. There are long
3.5.1. Non-carcinogenic risk assessment (HQ) To evaluate the human exposure to Cd quantitatively, exposure scenarios via ingestion and dermal contact were established for three age groups, namely adult male, adult female and children. In this study, there were very few sampling sites with hazard quotient (HQ) values above 1 for adult males, adult females, and children, respectively. Children suffered greater non-carcinogenic risk than adults. One possible reason is that children are likely to have higher rates of heavy metal absorption owing to their unique physiological characteristics, such as hand/finger sucking-a critical exposure pathway of soil heavy metals for children (White and Marcus, 1998). Similar findings have been reported in previous studies by Li et al. (2014), Chen et al. (2015). As shown in Fig. 7, nearly all of the examined sites with HQ > 1 were located in mining
Fig. 7. Non-carcinogenic risk of children through exposure to Cd in agricultural soils of China in different periods.
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areas, industrial areas and sewage irrigation areas. For example, the non-carcinogenic risk values were high in the agricultural soils around Baiyin, Gansu; copper-zinc-lead mine in central Tibet; leadzinc mine in Daxin, Guangxi; Zhuzhou smelter in Hunan; industrial zone in Henan; and sewage irrigation areas in Changchun, Jilin. Mining, industrial production and sewage irrigation resulted in serious Cd contamination in soil, indicating that Cd in agricultural soils was possible to pose non-carcinogenic risks to the surrounding population. The non-carcinogenic risk values surpassed 10 in the regions of Gansu, Hunan, Henan, and Guangxi, which should be controlled with priority. As shown in Fig. 7, the level of non-carcinogenic risk varied in different periods. The number of samples with HQ value in the range of 0e1 increased from 1975 to 2016. The number of samples with HQ in the range of 1e3 showed a basic increasing trend from 1975 to 2016. The sample points with HQ > 3 did not appear in 1975e2000, but increased obviously in the following three periods. On the whole, there was an increasing trend of areas suffered non-carcinogenic risks over time. Note that increased data over time is sure to raise the possibility of more cases of HQ > 1, which could have biased the results. 3.5.2. Carcinogenic risk assessment (LCR) Carcinogenic risks surpassing 1 104 are viewed as unacceptable, risks below 1 106 are not considered to pose significant health effects, and risks lying between 1 104 and 1 106 are generally considered an acceptable range by the USEPA (2001). In this study, very few part of Cd carcinogenic risks values were over 1 104, indicating potential carcinogenic risks resulted by exposure to Cd in soils. According to parameters for carcinogenic risk assessment, body weight and exposed skin surface area maybe the main factors of difference between the carcinogenic risks of adult females and adult males. The carcinogenic risk for children was lower than that for adults, it may be due to the shorter exposure duration of children (Li et al., 2015a). Similarly, carcinogenic risk level distributions were mapped with adult female carcinogenic risk values (i.e., the severest carcinogenic risk among the three population groups). It was found that some regions showed high carcinogenic risks in four periods. For example, values of LCR in sewage irrigation area in Changchun, Jilin Province (Li et al., 2015b), molybdenum mining area in Huludao, and lead-zinc mining area in Yunnan Province (Wang et al., 2017) were higher than the acceptable risk. So sewage irrigation areas, smelters and mining areas above-mentioned need urgent attention. The control and remediation measures are urgently needed to reduce Cd accumulation in agricultural soil in these areas. Due to limited data in 1975e2000 period, the levels of carcinogenic risk in some regions can hardly been assessed accurately, such as Tibet, Xinjiang, Qinghai and Inner Mongolia. However, it is worth noting that there were sporadic samples with carcinogenic risk in Xinjiang and Tibet after 2005. Long-term mining activities have brought serious heavy metal contamination in soil, which are possibly carcinogenic to the surrounding population (Jiang et al., 2017). The studies of Tianjin, Beijing, Hebei and coastal areas increased greatly in 2001e2005 period. Carcinogenic risks in most of these areas were at the acceptable levels of carcinogenic risks. With the increase of studies during 2006e2016, the sampling sites with unacceptable carcinogenic risks distributed densely in southern China, especially in Yunnan, Hunan and Guangxi Province. These areas are the priority control areas in China. Note that increased data over time is sure to raise the possibility of more cases of LCR>1 104, which could have biased the results. In addition, other potential factors (e.g., soil properties and heavy metal species, etc.) may affect heavy metals absorption, thus bringing some uncertainty to the health risk assessment results (Lu
et al., 2015). The sampling sites with carcinogenic risk values in the range assumed to have health effects were concentrated in Yunnan, Guizhou, and Guangxi provinces, most likely due to mining and smelting activities. 4. Conclusion This study gives a description of the overall contamination levels of Cd in agricultural soils of China and their ecological and health risks. Soil Cd concentrations accumulated gradually from 1981 to 2016 in China. Mining, sewage irrigation, and fertilizer application significantly contributed to Cd accumulation in agricultural soils. Cd concentrations were relatively high in agricultural soils from south regions of China. Ecological and health risks resulting from soil Cd contamination were in low levels. Acknowledgments This work was supported by Central Level, Scientific Reseach Institutes for Basic R&D Special Fund Business (2019YSKY006) and National Key Research and Development Program of China (2016YFD0800302). The authors would like to express our sincere gratitude to the co-authors who have put considerable time and effect into data collection, extraction and analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.04.208. References Anbuselvan, N., Senthil, N., Sridharan, M., 2018. Heavy metal assessment in surface sediments off coromandel coast of India: implication on marine pollution. Mar. Pollut. Bull. 131, 712e726. Basta, N.T., Raun, W.R., Gavi, F., 1998. Wheat grain cadmium under long-term fertilization and continuous winter wheat production. Better Crops 82, 14e15. Cai, L.M., Xu, Z.C., Qi, J.Y., Feng, Z.Z., Xiang, T.S., 2015. Assessment of exposure to heavy metals and health risks among residents near tonglushan mine in hubei, China. Chemosphere 127, 127e135. Chen, H., Teng, Y., Lu, S., Wang, Y., Wang, J., 2015. Contamination features and health risk of soil heavy metals in China. Sci. Total Environ. 512e513, 143e153. Chen, J., Liu, G.J., Jiang, M.M., Chou, C.L., Li, H., Wu, B., Zheng, L.G., Jiang, D.D., 2011. Geochemistry of environmentally sensitive trace elements in Permian coals from the Huainan coalfield, Anhui, China. Int. J. Coal Geol. 88, 41e54. Chen, J., Wei, F., Zheng, C., Wu, Y., Adriano, D.C., 1991. Background concentrations of elements in soils of China. Water Air Soil Pollut. 57e58, 699e712. Chen, K., Huang, L., Yan, B., Li, H., Sun, H., Bi, J., 2014. Effect of lead pollution control on environmental and childhood blood lead level in Nantong, China: an interventional study. Environ. Sci. Technol. 48, 129e130. CSY, 2017. China Statistical Yearbook. National Bureau of Statistics of China Edition, China Statistics Press, China. Deng, W., Li, X., An, Z., Yang, L., Hou, K., Zhang, Y., 2017. Identification of sources of metal in the agricultural soils of the Guanzhong Plain, northwest China. Environ. Toxicol. Chem. 36, 1510e1516. Fan, Y., Zhu, T., Li, M., He, J., Huang, R., 2017. Heavy metal contamination in soil and brown rice and human health risk assessment near three mining areas in central China. J. Healthc. Eng. 1e9. Grant, C.A., Baily, L.D., 1997. Nitrogen, phosphorus and zinc management effects on grain yield and cadmium concentration in two cultivars of durum wheat. Can. J. Plant Sci. 78, 63e70. Grüter, R., Costerousse, B., Bertoni, A., Mayer, J., Thonar, C., Frossard, E., Schulin, R., Tandy, S., 2017. Green manure and long-term fertilization effects on soil zinc and cadmium availability and uptake by wheat (Triticum aestivum L.) at different growth stages. Sci. Total Environ. 599e600, 1330e1343. Hajar, B., Masoud, Y., Amir, H.M., Mahmood, A.M., Mohammad, H.D., Shahrokh, N., 2012. Cadmium, lead and arsenic concentration in soil and underground water and its relationship with chemical fertilizer in paddy soil. J. Mazand. Univ. Med. Sci. 22, 20e28. Hechmi, N., Aissa, N.B., Abdenaceur, H., Jedidi, N., 2014. Evaluating the phytoremediation potential of Phragmites australis grown in pentachlorophenol and cadmium co-contaminated soils. Environ. Sci. Pollut. Res. 21, 1304e1313. Huang, H.G., Yu, N., Wang, L.J., Gupta, D., He, Z., Wang, K., Zhu, Z.Q., Yan, X.C., Li, T.Q., Yang, X., 2011. The phytoremediation potential of bioenergy crop Ricinus
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