- Email: [email protected]
Risk assessment of Cd polluted paddy soils in the industrial and township areas in Hunan, Southern China
Chemosphere 144 (2016) 346e351
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Risk assessment of Cd polluted paddy soils in the industrial and township areas in Hunan, Southern China Meie Wang, Weiping Chen*, Chi Peng State Key Laboratory of Urban and Regional Ecology, Research Centre for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China
h i g h l i g h t s Extremely higher Plant Uptake Factor was found, ranging from 0.351 to 6.02. Low pH values ranging from 4.98 to 6.02 in paddy soil were found. Ingestion of the 78% of the local rice grain would have adverse health risks end.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2015 Received in revised form 19 August 2015 Accepted 1 September 2015 Available online xxx
Cadmium (Cd) contamination in rice in Youxian, Hunan, China is a major environmental health concern. In order to reveal the Cd contamination in rice and paddy soils and the health risks to the population consuming the local rice grain, field surveys were conducted in eight towns in Youxian, China. The Cd contents of paddy soils averaged 0.228e1.91 mg kg1, 90% exceeding the allowable limit of 0.3 mg kg1 stipulated by the China Soil Environmental Quality Standards. Low average pH values (for air dried oxidized soils) ranging from 4.98 to 6.02 in paddy soil were also found. More than seventy percent (39 of 53) of the grain samples exceeded the maximum safe concentration of Cd, 0.2 mg kg1 on a dry weight basis. Considering the high consumption of local rice (339 g capita1 DW d1) and Cd levels measured, dietary ingestion of 78% of the sampled rice grains would have adverse health risks because the intake exposure of Cd was greater than the JECFA recommended exposures, 0.8 mg Cd BW kg1 day1 or 25 mg Cd BW kg1 month1. © 2015 Published by Elsevier Ltd.
Keywords: Cd contaminated rice Human health risk PUF Southern China
1. Introduction The possible risks derived from the enrichment of Cd in cropland soils, particularly in the paddy soils, are of great concern. Many paddy soils in southern China have been adversely affected by Cd contamination (Fang et al., 2014; Huang et al., 2014; Du et al., 2013; Niu et al., 2013, 2012; Liu et al., 2011; Zhuang et al., 2009; Fu et al., 2008; Li et al.). Du et al. (2013) reported that in one prefecture of Hunan Province, Cd concentrations in 58% of the paddy soils exceeded 0.3 mg Cd kg1 soil, the upper threshold for croplands as stipulated in the Soil Environmental Quality Standards of China (GB 15618-1995) and the Cd concentrations of 60% of the randomly sampled rice grains harvested in this prefecture exceeded 0.2 mg Cd DW kg1 grain, the maximum permissible according to the Hygienic Standard for Grains (GB 2715-2005) of China. The Cd
* Corresponding author. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.chemosphere.2015.09.001 0045-6535/© 2015 Published by Elsevier Ltd.
contaminated paddy soils are legacies of the mining and metalsmelting activities over the past 50 years and presented high risks of ecosystem and human health harms. Investigation in high Cd concentration areas in southwestern China revealed the urinary Cd concentrations (mean at 3.92 mg L1 for male and 4.85 mg L1 for female) of local people in study area were significantly higher than those from the control area (mean at 0.8 mg L1 for male and 0.42 mg L1 for female (Liu et al., 2015). There were six plausible pathways through which human could be exposed to Cd in the polluted soils (US EPA, 1989), namely inadvertent soil ingestion, dermal contact, dust inhalation, secondary contamination of drinking water supply, and ingestion of harvested food. Among them, Cd transfer through the human food chain was by far the most significant pathway and the exposure might account for >99% of the overall Cd ingestion exposure (Liu et al., 2013). The Cd concentrations of rice grain grown on contaminated soils customarily were estimated by the dimensionless Plant Uptake
M. Wang et al. / Chemosphere 144 (2016) 346e351
Factors (PUF) that denoted the ratio of Cd concentration in rice grains (mg DW g1) versus Cd concentration of the respective ricegrowing soils (mg kg1). For soils of the same production region, the PUF might be considered as a constant (although strongly affected by soil and plant properties) and the Cd levels of the harvested grains would be proportional to the Cd levels of the soils. However, the PUF varied widely with rice cultivars, soil properties such as pH and salinity, flooding, and the soil nutrient management, and cultivation practices (Li et al., 2012; Cui et al., 2004). Plants absorb Cd from its labile pool in the soils. In reality, phytoavailable Cd of the soils (and soil properties, rice cultivar) would determine the Cd content of rice grain and reports showed that 0.1 M CaCl2 extractable Cd and soil pH (1:2.5, soil: water) determined on field moist samples accounts for 63.8% of the variability in rice grain Cd (Du et al., 2013; Simmons et al., 2008). The human health risks of Cd contaminated paddy soils needed to take into account the soil's phytoavailable Cd. Amounts of phytoavailable soil Cd would be affected by how the soil-borne Cd bonds with the soil fractions (organic matter, clay, Fe and Mn hydroxyoxides) and the bonding strengths. In this research, we selected Youxian, a prefecture of Hunan Province to study how potential human health could be linked to Cd levels of contaminated soils and influenced by industrial and human activities. Youxian is a top rice production region and through media exposures, was known nationwide as the region of “Cd-laced rice”. Cd in paddy soil and transfer to rice grain at representative areas were determined. Risks due to the soil Cd pollution were evaluated by the US EPA human health risk assessment approach.
2. Methods and materials 2.1. Study area and sampling The prefecture of Youxian (113.32 E, 27.01 N) consisted primarily of townships of rice productions. It has 30 thousand population and occupies approximately 2600 km2 of surface area. The potential sources of Cd emissions included coal mining and combustion in rural scale industries such as metal smelters, cement clinking, and brick and roof tile firing, and in households for food preparations and space heating through which the Cd fallout can spread across the entire region. There were approximate 387 km2 of rice-planting areas, 70% of which were found in eight townships (Table 1). The remainder is composed primarily of hilly mountainous topography. Eight areas presenting paddy soils typical for each of the eight townships were selected for soil samplings through review of official maps and documents, interviews of residents, and field surveys (Table 1) in which HFQ was the coal production area, TS was the coal mineral and smelter industrial area, WL was the cement clinking and brick firing area and LT, YJQ, DTQ, and SYT were
347
intermixed areas of commercial, residential and rice productions and JS was the reference site located in the mountainous area upwind and far away from known pollutant emission sources. Eight (0e100 cm) paddy soil profiles, 0e20 cm, 20e40 cm, 40e60 cm, 60e80 cm, 80e100 cm, were sampled, one for each township in Sept. 2013. One soil sample was collected for each layer of the profile. One “natural” soil profile was also sampled in the site nearby the paddy soil profiles where there was no apparent anthropogenic disturbance. It is hypothesized that the main input pathway of Cd in local soils is atmosphere deposition and the depth of leaching downward is limited. Thus, natural soil profiles were sampled in three layers: 0e10 cm, 10e20 cm and 20e40 cm at the same time. Surface paddy soil (0e10 cm) samples were also collected in June and September, 2013, around point sources (coal mineral sites and factories, etc.) for investigating the impact of contamination sources. Altogether 51 surface paddy soil samples were collected and the sample numbers for each township were shown in Table 1. Rice grain for health risk estimation was sampled in July and September, 2013, and July, 2014. Altogether 53 samples from the eight townships were collected. Samples for PUF calculation were collected specifically in TS, DTQ, WL and a commercial farm XS. These four sites had high detection rate of rice grains exceeding 0.2 mg Cd DW kg1 grain, the maximum permissible according to Chinese Grain Safety Standard. Altogether 32 surface paddy soil and 32 rice grain samples were collected. They belonged to 32 individual fields and were not limited to the area of point contamination sources. In each area, the fields selected for studying were as close to each other as possible. For the 32 rice grain samples, 12 samples were the same rice cultivar (VY8) planted in three areas, TS, DTQ and XS for studying effects of soil environmental conditions on PUF. The remaining 20 grain samples included 6 cultivars.
2.2. Chemical analysis The total Cd concentration of soils was measured, by dissolving the soil-borne Cd in aqua regia and then by determining the Cd recovered in solution phase with the graphite furnace atomic absorption spectroscopy (GFAAS). The Cd concentration of rice grain was obtained by solubilizing the grain-borne Cd in concentrated HNO3eHClO4 mixture and then by measuring the Cd of solution phase with the GFAAS. For quality control and assurance, standard reference materials, GSS-5 for soils and GSB-23 for rice grain, were included in every batch of the analyses. The measured value of GSS-5 in our experiment was 0.473 ± 0.054 mg kg1, which was close to the standard value of 0.45 ± 0.06 mg kg1. The measured value of GSB-23 was 0.194 ± 0.022 mg kg1, which was also close to the standard value 0.19 ± 0.02 mg kg1. Soil pH was determined in 1:2.5 (w/v) soil and water suspension after shaking the mixture for 30 min.
Table 1 Average Cd concentrations and corresponding pH of soils of Youxian. Area
Number of samples
Soil Cd (mg DW kg1) Mean ± S.D.
TS HFQ WL LT DTQ SYT YJQ JS Overall
12 12 11 4 3 3 4 2 51
0.924 0.920 0.631 0.654 0.504 0.440 0.435 0.420 0.719
± ± ± ± ± ± ± ± ±
0.634 0.323 0.200 0.241 0.103 0.072 0.149 0.105 0.406
Soil pH
Description
Range 0.228e1.91 0.372e1.27 0.267e0.960 0.519e1.02 0.401e0.607 0.380e0.520 0.290e0.609 0.346e0.494 0.228e1.91
6.02 5.51 5.78 5.39 4.98 5.81 5.05 5.50 5.60
± ± ± ± ± ± ± ± ±
0.434 0.518 0.442 0.672 0.098 0.807 0.245 0.841 0.527
Coal mine Coal mine and smelter Cement factory Township Township Township Township Clean area in mountainous part of the region
348
M. Wang et al. / Chemosphere 144 (2016) 346e351
2.3. Estimation of health risk The health risk index (HRI) for those residing in the affected areas and consuming exclusively the locally produced Cd contaminated rice was calculated using the following equation (USEPA, 2000):
HRI ¼
ADD RfD
(1)
Where RfD is the reference dosage for Cd, that is 0.8 mg Cd (kg body weight)1 day1 (JECFA, 2010). When the calculated HRI was equal or greater than one, the consumers of the contaminated rice would be exposed to incrementally higher risks in proportion of increase in HRI. When the calculated HRI was less than one, the exposed population would be safe from chronic Cd poisoning. The individual-level average daily dose (ADD) was calculated using the following equation (USEPA, 2000):
ADD ¼
C IR BW
(2)
Where C is the concentration of Cd in rice (mg DW g1); IR is the daily average rice consumption per capita (DW g capita1 d1); BW is the average body weight (kg capita1). A 60 kg BW adult of the rice-consuming areas in south China countryside would consume on average 339 DW g of rice d1 (Ma et al., 2005)
from 0.420 to 0.924 mg kg1 with respective standard deviations varying from 0.072 to 0.634 mg kg1 (Table 1). Areas representing paddy soils at the mining and industrial areas such as TS and HFQ showed the highest soil Cd contents with a mean of 0.924 and 0.920 mg kg1, respectively. Paddy soils at the other industrial area (WL) had a mean Cd contamination level (0.631 mg kg1). At the township area, mean Cd concentration in paddy soils varied from 0.435 to 0.654 mg kg1. Overall, fields at the mining and industrial areas contained an average Cd content of 0.830 mg kg1, which is higher than those further from the mining and industrial areas (mean of 0.514 mg kg1) and the remote mountain site JS. The results suggested that industrial activities had resulted in significant accumulation of Cd in paddy fields. Furthermore, the remote mountain site JS, that was away from any known Cd emission source, registered soil Cd contents of 0.420 ± 0.105 mg kg1 indicative of the pervasiveness of past Cd emissions or geogenic enrichment. The average pH values of the eight areas were all below 7. TS had the highest average pH was 6.02, while the lowest average pH of DTQ was only 4.98. Thus, the potential uptake of Cd by rice was strong due to the acidic paddy soil. In all, the Cd contamination of Youxian's agricultural soils is significant and pervasive. Variation of local emission sources as well as soil background resulted in high spatial heterogeneity of soil Cd concentration, which causes great challenges for risk assessment and management.
3.2. Distribution and accumulation of Cd in soil profile 3. Results and discussions 3.1. Soil Cd pollution Altogether, 51 agricultural fields across the prefecture were sampled. Soil Cd contents in these fields ranged from 0.228 to 1.91 mg kg1, with a mean of 0.737 mg kg1 (Table 1). 90% of the fields (46 of 51) exceeded the soil Cd threshold permissible for crop production, 0.3 mg kg1as stipulated in China's Soil Environmental Quality Standards, suggesting a serious Cd contamination situation in the study area (SEPAC, 1995). There was great spatial heterogeneity of soil Cd concentration. The resulting mean soil Cd contents of the eight townships ranged
The Cd distributions in the 0e100 cm soil profiles are shown in Fig. 1. Except for SYT, the concentrations of Cd in the layer of 0e20 cm were significantly higher compared to the concentrations below 20 cm. Further, Cd concentrations in profiles below 40 cm depth in all sites had small variance. At the HFQ site, Cd enrichment in the surface 20 cm layer was quite serious, reaching 1.67 mg kg1. However, high Cd concentration over 1.0 mg kg1 in the lower soil layers were also observed, suggesting that there was a geological anomaly in this region and the background soil Cd concentration had been significantly elevated. Soils with higher levels of clay and FeeMn oxides hold onto trace elements over millennia. As shown in Table 2, the clay
Fig. 1. The Cd content profiles from 0 to 100 cm in paddy soils.
M. Wang et al. / Chemosphere 144 (2016) 346e351 Table 2 Profile distributions of clay concentration in paddy soil (%). Depth (cm)
0e20 20e40 40e60 60e80 80e100
Location DTQ
HFQ
JS
LT
SYT
TS
WL
YJQ
71.8 83.1 86.3 84.8 79.6
66.6 75.2 84.1 86.7 84.3
73.3 81.2 84.7 88.4 87.5
70.4 91.9 83.1 87.4 84.6
76.1 77.5 88.1 82.9 85.4
62.8 67.0 81.7 90.9 79.8
74.1 87.4 85.6 85.0 88.0
52.4 71.9 68.8 77.8 71.2
concentration in 0e100 cm profiles in YJQ was the lowest compared to other seven townships. The clay concentrations in the surface profile (0e20 cm) were lower than the profiles below (>40 cm) where the concentrations were near to or above 80% for all the eight sites. The distribution of clay contents in paddy soil profiles suggested the downward transportation of Cd from the surface was slow. If we consider the Cd concentration in 80e100 cm depth as the background value, the amount of Cd accumulation in the 0e20 cm depth could be calculated approximately by dividing the concentration of Cd in the 0e20 cm depth by the Cd in the 80e100 cm depth and then, multiplying to the soil bulk density of 0e20 cm profile. The bulk density of paddy soil ranged from 0.88 to 1.16 g cm3. So in this paper the average value 1.10 g cm3 was used. The results are shown in Table 3. The approximate Cd accumulations in the 0e20 cm depth layer were more than 1 kg ha1 in mine areas HFQ and TS, as well as town area YJQ. The accumulation amounts in DTQ and JS were more than 0.6 kg ha1. Industrial area WL had an accumulation amount of about 0.376 kg Cd ha1, while the accumulation of Cd in areas LT and SYT are below 0.1 kg ha1. Accumulation of Cd in the surface layer is the balance of the inputs and outputs. The outputs are mainly dependent on the yield and Cd content in rice grain and straw. Based on data in Table 5, the annual removal of Cd through rice grain harvest could vary from 0.67 to 21.9 g ha1 (based on two crop season and a grain yield of 12,000 kg ha1). If the rice straw was not reincorporated to the field, the Cd removal would be much higher. Therefore, the accumulative inputs (recycle from straw) would be much greater than those given in Table 3. Given that the commercial fertilizers in Chinese market were generally low in Cd, the main input pathways would be through irrigation water and air deposition. Our field investigation showed that at HFQ area, sediment Cd concentration
349
in irrigation channel was quite high, reaching 8.07 mg kg1, while the irrigation water (not filtered, because we suppose the suspended particulates may also transport to the field with the water flow.) contained 1.42 mg Cd L1 (annual input through irrigation water is estimated to be overall 10 g ha1). At TS area, annual air deposition of Cd was 12.0 g ha1, about three times higher than the national average. Given that the environmental management now is much stricter than before, it could be possible that there was significant environmental Cd aerosol pollution in the study areas from coal mining and other industrial activities which released the huge amount of Cd to the environment and then through water or air transport to the paddy fields. 3.3. Soil-plant transfer The transfer of Cd from contaminated soils to harvested rice grain can be depicted by the dimensionless plant uptake factor (PUF). As shown in Table 4, overall, the values of PUF ranged from 0.351 to 6.02. The average PUF was 2.61. Twenty-six samples out of 32 had PUF more than 1, indicating high level of transfer of soil Cd to the grains. The variation of PUF for the cultivar VY8 among different sites ranged from 0.653 to 6.02. Ten out of 12 samples had PUF more than 1. As for the PUF from the same area, large variation was also found among samples with different cultivars. The highest PUF was 4e5 folds of the lowest. And most of the samples had PUF more than 1. It was reported most of the Cd is recovered from the Zn during ore processing (Chaney, 2015). Field investigation in heavy metal contamination area in Palmerton, PA indicated that ratios of Zn to Cd (lower than 100 to 1) increase the Cd content in plants (Chaney, 2015). However, our research found more than 90% of the paddy soil samples had Zn: Cd higher than 100: 1, and the highest is 338:1. The average Zn concentration in paddy soil is 99.3 mg kg1, almost equal to the local background value. It was also reported by field survey that Zn had little or no effect on Cd accumulation by rice (Simmons et al., 2008). Investigation of rice Cd in contaminated area with higher soil pH values (4.1e5.7) and non-contaminated area with low soil pH (4.0e4.7) found higher PUF in noncontaminated area, which might attribute to the higher bioavailability of Cd at a lower soil pH (Takijima et al., 1973). Thus the effects of Zn on the uptake of Cd in rice in Youxian could be ignored. Zhao et al. (2015) indicated acidic nature of soils in many areas of southern China was one of the important reasons caused high
Table 3 Estimated Cd accumulation in 0e20 cm depth of the soil profile. Cd Deposit
Location DTQ
HFQ
JS
LT
SYT
TS
WL
YJQ
0e20 cm Depth profile (mg kg1) 80e100 cm Depth profile (mg kg1) Estimated Cd accumulation (kg ha1)
0.360 0.053 0.675
1.67 1.18 1.08
0.346 0.053 0.645
0.312 0.270 0.092
0.111 0.098 0.029
0.683 0.188 1.09
0.334 0.163 0.376
0.645 0.120 1.16
Table 4 Calculated Cd plant uptake factor (PUF) and the corresponding Cd concentrations in paddy soil and rice grains and pH value in selected areas in Youxian, Hunan. NO. of samples (n)
Overall VY8 cultivar DTQ TS WL XS
32 12 10 6 10 6
PUF Range
Mean
0.351e6.02 0.653e6.02 0.653e3.11 1.18e4.60 0.351e6.02 0.870e4.11
2.61 2.64 1.90 2.61 3.41 2.50
NO. of samples (PUF>1)
Rice Cd (mg DW kg1)
Soil Cd (mg DW kg1)
pH
26 10 7 6 8 5
0.050e1.00 0.117e1.00 0.080e0.611 0.435e0.823 0.050e1.00 0.367e0.968
0.100e0.446 0.139e0.446 0.100e0.287 0.164e0.370 0.114e0.219 0.195e0.446
4.83e5.71 4.83e5.53 4.94e5.45 4.95e5.71 4.84e5.55 4.83e5.67
350
M. Wang et al. / Chemosphere 144 (2016) 346e351
Table 5 Cd in rice in the eight studied areas in Youxian (mg kg1).
n Min Max Mean SD NO. of samples (>0.2 mg kg1)
DTQ
HFQ
JS
LT
SYT
TS
WL
YJQ
18 0.046 0.955 0.412 0.314 10
3 0.103 0.285 0.191 0.091 1
2 0.827 1.10 0.961 0.134 2
6 0.109 0.956 0.569 0.370 5
4 0.438 0.740 0.601 0.144 4
4 0.142 0.742 0.495 0.314 3
12 0.091 1.83 0.892 0.465 11
4 0.056 0.849 0.426 0.399 3
accumulation of Cd in rice. Zhao et al. (2015) indicated acidic nature of soils in many areas of southern China was one of the important reasons causing high accumulation of Cd in rice, and their calculation using the prediction model presented by Rӧmkens et al. (2009), suggested that strong acidity of paddy soil would cause the exceeding of the grain Cd limit in some “uncontaminated” soils. As shown in Table 4, extremely low pH values, ranging from 4.83 to 5.71, were found in studied area, which would be the main reason for high Cd PUFs in Youxian, Hunan. Increasing of Mn in solution would also decrease the uptake of Cd from root to grains (Yang et al., 2014). Our investigation found the average concentration of Mn in paddy soil in Youxian, Hunan was 327 mg kg1, lower than the background value 459 mg kg1. And early field drainage might promote Cd transfer to grain as well (Yang et al., 2009; Chino and Baba, 1981). In any regard, the high variation of PUF from site to site suggests a complicated relationship between soil Cd, soil pH, soil drainage and rice grain Cd, thus a great challenge for risk assessment and management. 3.4. Health risk assessment The maximum permissible concentration of Cd in rice grain, 0.2 mg Cd DW kg1 according to the Hygienic Standard for Grains (GB 2715-2005) of China is different from CODEX level of 0.4 mg Cd FW kg1. But it is reasonable if we take the permit Cd intake dose of 25 mg Cd BW kg1 month1 by JECFA into consideration. An 60 kg weighting adult who is supposed to daily intake 339 g dry weight rice grain will intake about 33.9 mg Cd BW kg1 month1 if the Cd concentration in rice grain is about 0.2 mg Cd DW kg1, which exceeds the permit Cd intake dose by JECFA. The mean Cd concentrations of polished rice that were harvested at the studied areas in Youxian ranged from 0.191 to 1.55 mg DW kg1 (Table 5) and, except for those at HFQ, the average rice Cd exceeded the maximum safe concentration of 0.2 mg Cd DW kg1 grain. From the view of individual rice grain samples, 73.6% (39 of 53) exceed the maximum safe concentration of Cd with a range from 0.046 to 0.956 mg kg1 (Table 6). While the
Table 6 The average daily dose (ADD) of Cd intake from rice grain and health risk index (HRI) in the eight studied areas in Youxian. Area
DTQ HFQ JS LT SYT TS WL YJQ Overall
ADD (mg Cd BW kg1 day1)
HRI
Max
Min
Max
Min
Percentile of high risk (HRI1)
5.40 1.61 5.13 5.40 4.18 3.41 10.3 4.80 10.3
0.261 0.583 0.96 0.616 2.48 0.800 0.516 0.316 0.261
6.75 2.01 6.41 6.75 5.23 5.24 12.9 6.00 12.9
0.326 0.730 1.20 0.77 3.09 1.00 0.645 0.400 0.326
64.7% 66.7% 100 83.3% 100 75% 91.7% 75% 78%
mining and industrial sites TS and HFQ registered the highest two soil Cd concentrations, rice grain harvested from these two sites contained much lower Cd in comparison with the other sites, ranking the lowest and the third lowest among the 8 areas. Similar to the paddy soil Cd contamination, there was a high spatial heterogeneity as indicated by the great range and coefficient of variation from area to area (Table 5). The average daily dose, ADD, and the health risk indices, HRI, of the representative sampling locations were calculated from Cd concentrations of the respective rice grain, according to Equation (2) and the populations. The ADD ranged from 0.261 to 10.3 mg Cd BW kg1 day1 and the HRI ranged from 0.326 to 12.9 overall (Table 6). It was suggest by JECFA that 0.8 mg Cd BW kg1 day1 or 25 mg Cd BW kg1 month1 of the intake exposure would correspond to the breakpoint 5.24 mg of Cd per gram creatinine at a lower bound of the 5th percentile dietary Cd exposure (on a population level). It could be suggested from Table 6 that the maximum of ADD was 10.3 mg Cd BW kg1 day1 among the eight investigated towns, more than 100 folds of the permit exposure amount. If it was estimated by HRI, large percentile of the local rice grain in those eight towns had high risk for ingestion, average 78% overall, to exceed the permit Cd exposure dose, 0.8 mg Cd BW kg1 day1 recommended by the JECFA. 4. Conclusion Youxian in Hunan Province, China, a fertile river basin for rice production, has been experienced soil metal contamination in the past. Emissions from coal mining, metal mining and smelting, and brick and tile kilns over the past fifty years caused the Cd concentration of surface soils to range from 0.228 to 1.91 mg kg1. Ninety percent of the fields (46 of 51) exceeded the soil Cd threshold permissible for crop production, 0.3 mg kg1as stipulated in China's Soil Environmental Quality Standards. Variation of local emission sources as well as soil background resulted in high spatial heterogeneity of soil Cd concentration. Large differences in Cd concentrations among 0e100 soil profiles were found, which might be partly due to high contents of clay (more than 80% below 40 cm layer). However, the limit of 0.3 mg kg1 is lower than other nations and less important than field drainage and soil pH in Cd accumulation in rice. As early as the early 1970, Japanese researchers had found that early drainage caused higher Cd accumulation, and later rains caused lower Cd in rice grain. It is also true in China. The water-saving irrigation measure for rice planting spreading since 1990s requires early and frequent drainage during the whole rice growth stage. Thus, changes in traditional management of flood and pH are in needed. Extremely higher PUF was found, ranging from 0.351 to 6.02. Twenty samples out of 32 had PUF more than 1, indicating high level of accumulation of Cd in rice grain, which might be contributed to early drainage before harvest and low pH values in paddy soils (ranged 4.83e5.71). The mean Cd concentrations of polished rice that were harvested at the studied areas in Youxian ranged from 0.191 to 1.55 mg kg1. More than seventy percentages (39 of
M. Wang et al. / Chemosphere 144 (2016) 346e351
53) of the grain samples exceed the maximum safe concentration of Cd, 0.2 mg DW kg1. Overall, ingestion of the 78% of the local rice grain samples would cause daily Cd intake to exceed the JECFA permit dose of 25 mg Cd BW kg1 month1. Acknowledgement We gratefully acknowledged financial supports provided by the Special Foundation of State Key Lab of Urban and Regional Ecology (SKLURE2013-1-04). References Chaney, R.L., 2015. How does contamination of rice soils with Cd and Zn cause high incidence of human Cd disease in subsistence rice farmers. Curr. Pollu. Rep. 1, 13e22. Chino, M., Baba, A., 1981. The effects of some environmental factors on the partitioning of Znic and Cadmium between roots and tops of rice plants. J. Plan. Nutr. 3, 203e214. Cui, Y., Zhu, Y., Zhai, R., Chen, D., Huang, Y., Qiu, Y., Liang, J., 2004. Transfer of metals from soil to vegetables in an area near a smelter in Nanning, China. Environ. Inter 30, 785e791. Du, Y., Hu, X., Wu, X., Shu, Y., Jiang, Y., Yan, X., 2013. Effects of mining activities on Cd pollution to the paddy soils and rice grain in Hunan province, Central South China. Environ. Mon. Assess. 185, 9843e9856. Fang, Y., Sun, X., Yang, W., Xin, Z., 2014. Concentrations and health risks of lead, cadmium, arsenic, and mercury in rice and edible mushrooms in China. Food Chem. 147, 147e151. Fu, J., Zhou, Q., Liu, J., Liu, W., Wang, T., Zhang, Q., Jiang, G., 2008. High levels of heavy metals in rice (Oryza sativa L.) from a typical E-waste recycling area in southeast China and its potential risk to human health. Chemosphere 71, 1269e1275. Huang, Z., Pan, X., Wu, P., Han, J., Chen, Q., 2014. Heavy metals in vegetables and the health risk to population in Zhejiang, China. Food control. 36, 248e252. JECFA/73/SC. Joint FAO/WHO Expert Committee on Food Additives. Seventy-third Meeting, Geneva, 8e17, June 2010. http://www.fao.org/ag/agn/agns/jecfa_ index_en.asp. Li, Q., Chen, Y., Fu, H., Cui, Z., Shi, L., 2012. Health risk of heavy metals in food crops grown on reclaimed tidal flat soil in the Pearl River Estuary, China. J. Hazard. Mater 227e228, 148e154.
351
Liu, J., Zhang, X., Tran, H., Wang, D., Zhu, Y., 2011. Heavy metal contamination and risk assessment in water, paddy soil, and rice around an electroplating plant. Environ. Sci. Pollut. Res. 18, 1623e1632. Liu, X., Song, Q., Yang, Y., Li, W., Xu, J., Wu, J., 2013. Human health risk assessment of heavy metals in soil-vegetable system: a multi-medium analysis. Sci. Total Environ. 463e464, 530e540. Liu, Y.Z., Xiao, T.F., Baveye, P.C., Zhu, J.M., Ning, Z.P., 2015. Potential health risk in areas with high naturally-occurring cadmium background in southwestern China. Ecotox. Environ. Safe. 112, 122e131. Ma, W.J., Deng, F., Xu, Y.J., 2005. The study on dietary intake and nutritional status of residents in Guangdong. South China J. Prev. Med. 31, 1e7. Niu, L., Yang, F., Xu, C., Yang, H., Liu, W., 2013. Status of metal accumulation in farmland soils across China: from distribution to risk assessment. Environ. Pollut. 176, 55e62. Rӧmkens, P.F.A.M., Gou, H.Y., Chu, C.L., Liu, T.S., Chiang, C.F., Koopmans, G.F., 2009. Prediction of cadmium uptake by brown rice and derivation of soil-plant transfer models to improve soil protection guidelines. Environ. Pollut. 157, 2435e2444. SEPAC (State Environmental Protection Administration of China), 1995. Chinese Environmental Quality Standard for Soils (GB15618-1995). Simmons, R.W., Noble, A.D., Pongsakul, P., Sukreeyapongse, O., Chinabut, N., 2008. Analysis of field emoist Cd contaminated paddy soils during rice grain fill allows reliable prediction of grain Cd levels. Plant Soil 302, 125e137. Takijima, Y., Katsumi, F., Takezawa, K., 1973. Cadmium contamination of soils and rice plants caused by Zinc mining, Ⅱ. Soil conditions of contaminated paddy fields which influence heavy metal contents in rice. Soil Sci. Plant Nutr. 19 (3), 173e182. US EPA, 2000. Handbook for Non-cancer Health Effects Evaluation. Environmental Protection Agency Press, Washington DC. US EPA, 1989. Risk Assessment Guidance for Superfund (RAGS). In: Human Health Evaluation Manual (HHEM)-part a. Baseline Risk Assessment, vol. 1. Office of emergency and remedial response, Washington DC [EPA/540/1e89/002]. Yang, J.C., Huang, D.F., Duan, H., Tan, G.L., Zhang, J.H., 2009. Alternate wetting and moderate soil drying increases grain yield and reduces cadmium accumulation in rice grains. J. Sci. Food Agric. 89, 1728e1736. Yang, M., Zhang, Y.Y., Zhang, L.J., Hu, J.T., Zhang, X., Lu, K., 2014. OsNRAMP5 contributes to manganese translocation and distribution in rice shoots. J. Exp. Bot. 65 (17), 4849e4861. Zhao, F.J., Ma, Y.B., Zhu, Y.G., Tang, Z., McGrath, S.P., 2015. Soil contamination in China: current status and mitigation strategies. Environ. Sci. Tech. 49, 750e759. Zhuang, P., McBride, M.B., Xia, H., Li, N., Li, Z., 2009. Health risk from heavy metals via consumption of food crops in the vicinity of Dabaoshan mine. South China. Sci. Total Environ. 407, 1551e1561.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Risk assessment of Cd polluted paddy soils in the industrial and township areas in Hunan, Southern China Meie Wang, Weiping Chen*, Chi Peng State Key Laboratory of Urban and Regional Ecology, Research Centre for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China
h i g h l i g h t s Extremely higher Plant Uptake Factor was found, ranging from 0.351 to 6.02. Low pH values ranging from 4.98 to 6.02 in paddy soil were found. Ingestion of the 78% of the local rice grain would have adverse health risks end.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2015 Received in revised form 19 August 2015 Accepted 1 September 2015 Available online xxx
Cadmium (Cd) contamination in rice in Youxian, Hunan, China is a major environmental health concern. In order to reveal the Cd contamination in rice and paddy soils and the health risks to the population consuming the local rice grain, field surveys were conducted in eight towns in Youxian, China. The Cd contents of paddy soils averaged 0.228e1.91 mg kg1, 90% exceeding the allowable limit of 0.3 mg kg1 stipulated by the China Soil Environmental Quality Standards. Low average pH values (for air dried oxidized soils) ranging from 4.98 to 6.02 in paddy soil were also found. More than seventy percent (39 of 53) of the grain samples exceeded the maximum safe concentration of Cd, 0.2 mg kg1 on a dry weight basis. Considering the high consumption of local rice (339 g capita1 DW d1) and Cd levels measured, dietary ingestion of 78% of the sampled rice grains would have adverse health risks because the intake exposure of Cd was greater than the JECFA recommended exposures, 0.8 mg Cd BW kg1 day1 or 25 mg Cd BW kg1 month1. © 2015 Published by Elsevier Ltd.
Keywords: Cd contaminated rice Human health risk PUF Southern China
1. Introduction The possible risks derived from the enrichment of Cd in cropland soils, particularly in the paddy soils, are of great concern. Many paddy soils in southern China have been adversely affected by Cd contamination (Fang et al., 2014; Huang et al., 2014; Du et al., 2013; Niu et al., 2013, 2012; Liu et al., 2011; Zhuang et al., 2009; Fu et al., 2008; Li et al.). Du et al. (2013) reported that in one prefecture of Hunan Province, Cd concentrations in 58% of the paddy soils exceeded 0.3 mg Cd kg1 soil, the upper threshold for croplands as stipulated in the Soil Environmental Quality Standards of China (GB 15618-1995) and the Cd concentrations of 60% of the randomly sampled rice grains harvested in this prefecture exceeded 0.2 mg Cd DW kg1 grain, the maximum permissible according to the Hygienic Standard for Grains (GB 2715-2005) of China. The Cd
* Corresponding author. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.chemosphere.2015.09.001 0045-6535/© 2015 Published by Elsevier Ltd.
contaminated paddy soils are legacies of the mining and metalsmelting activities over the past 50 years and presented high risks of ecosystem and human health harms. Investigation in high Cd concentration areas in southwestern China revealed the urinary Cd concentrations (mean at 3.92 mg L1 for male and 4.85 mg L1 for female) of local people in study area were significantly higher than those from the control area (mean at 0.8 mg L1 for male and 0.42 mg L1 for female (Liu et al., 2015). There were six plausible pathways through which human could be exposed to Cd in the polluted soils (US EPA, 1989), namely inadvertent soil ingestion, dermal contact, dust inhalation, secondary contamination of drinking water supply, and ingestion of harvested food. Among them, Cd transfer through the human food chain was by far the most significant pathway and the exposure might account for >99% of the overall Cd ingestion exposure (Liu et al., 2013). The Cd concentrations of rice grain grown on contaminated soils customarily were estimated by the dimensionless Plant Uptake
M. Wang et al. / Chemosphere 144 (2016) 346e351
Factors (PUF) that denoted the ratio of Cd concentration in rice grains (mg DW g1) versus Cd concentration of the respective ricegrowing soils (mg kg1). For soils of the same production region, the PUF might be considered as a constant (although strongly affected by soil and plant properties) and the Cd levels of the harvested grains would be proportional to the Cd levels of the soils. However, the PUF varied widely with rice cultivars, soil properties such as pH and salinity, flooding, and the soil nutrient management, and cultivation practices (Li et al., 2012; Cui et al., 2004). Plants absorb Cd from its labile pool in the soils. In reality, phytoavailable Cd of the soils (and soil properties, rice cultivar) would determine the Cd content of rice grain and reports showed that 0.1 M CaCl2 extractable Cd and soil pH (1:2.5, soil: water) determined on field moist samples accounts for 63.8% of the variability in rice grain Cd (Du et al., 2013; Simmons et al., 2008). The human health risks of Cd contaminated paddy soils needed to take into account the soil's phytoavailable Cd. Amounts of phytoavailable soil Cd would be affected by how the soil-borne Cd bonds with the soil fractions (organic matter, clay, Fe and Mn hydroxyoxides) and the bonding strengths. In this research, we selected Youxian, a prefecture of Hunan Province to study how potential human health could be linked to Cd levels of contaminated soils and influenced by industrial and human activities. Youxian is a top rice production region and through media exposures, was known nationwide as the region of “Cd-laced rice”. Cd in paddy soil and transfer to rice grain at representative areas were determined. Risks due to the soil Cd pollution were evaluated by the US EPA human health risk assessment approach.
2. Methods and materials 2.1. Study area and sampling The prefecture of Youxian (113.32 E, 27.01 N) consisted primarily of townships of rice productions. It has 30 thousand population and occupies approximately 2600 km2 of surface area. The potential sources of Cd emissions included coal mining and combustion in rural scale industries such as metal smelters, cement clinking, and brick and roof tile firing, and in households for food preparations and space heating through which the Cd fallout can spread across the entire region. There were approximate 387 km2 of rice-planting areas, 70% of which were found in eight townships (Table 1). The remainder is composed primarily of hilly mountainous topography. Eight areas presenting paddy soils typical for each of the eight townships were selected for soil samplings through review of official maps and documents, interviews of residents, and field surveys (Table 1) in which HFQ was the coal production area, TS was the coal mineral and smelter industrial area, WL was the cement clinking and brick firing area and LT, YJQ, DTQ, and SYT were
347
intermixed areas of commercial, residential and rice productions and JS was the reference site located in the mountainous area upwind and far away from known pollutant emission sources. Eight (0e100 cm) paddy soil profiles, 0e20 cm, 20e40 cm, 40e60 cm, 60e80 cm, 80e100 cm, were sampled, one for each township in Sept. 2013. One soil sample was collected for each layer of the profile. One “natural” soil profile was also sampled in the site nearby the paddy soil profiles where there was no apparent anthropogenic disturbance. It is hypothesized that the main input pathway of Cd in local soils is atmosphere deposition and the depth of leaching downward is limited. Thus, natural soil profiles were sampled in three layers: 0e10 cm, 10e20 cm and 20e40 cm at the same time. Surface paddy soil (0e10 cm) samples were also collected in June and September, 2013, around point sources (coal mineral sites and factories, etc.) for investigating the impact of contamination sources. Altogether 51 surface paddy soil samples were collected and the sample numbers for each township were shown in Table 1. Rice grain for health risk estimation was sampled in July and September, 2013, and July, 2014. Altogether 53 samples from the eight townships were collected. Samples for PUF calculation were collected specifically in TS, DTQ, WL and a commercial farm XS. These four sites had high detection rate of rice grains exceeding 0.2 mg Cd DW kg1 grain, the maximum permissible according to Chinese Grain Safety Standard. Altogether 32 surface paddy soil and 32 rice grain samples were collected. They belonged to 32 individual fields and were not limited to the area of point contamination sources. In each area, the fields selected for studying were as close to each other as possible. For the 32 rice grain samples, 12 samples were the same rice cultivar (VY8) planted in three areas, TS, DTQ and XS for studying effects of soil environmental conditions on PUF. The remaining 20 grain samples included 6 cultivars.
2.2. Chemical analysis The total Cd concentration of soils was measured, by dissolving the soil-borne Cd in aqua regia and then by determining the Cd recovered in solution phase with the graphite furnace atomic absorption spectroscopy (GFAAS). The Cd concentration of rice grain was obtained by solubilizing the grain-borne Cd in concentrated HNO3eHClO4 mixture and then by measuring the Cd of solution phase with the GFAAS. For quality control and assurance, standard reference materials, GSS-5 for soils and GSB-23 for rice grain, were included in every batch of the analyses. The measured value of GSS-5 in our experiment was 0.473 ± 0.054 mg kg1, which was close to the standard value of 0.45 ± 0.06 mg kg1. The measured value of GSB-23 was 0.194 ± 0.022 mg kg1, which was also close to the standard value 0.19 ± 0.02 mg kg1. Soil pH was determined in 1:2.5 (w/v) soil and water suspension after shaking the mixture for 30 min.
Table 1 Average Cd concentrations and corresponding pH of soils of Youxian. Area
Number of samples
Soil Cd (mg DW kg1) Mean ± S.D.
TS HFQ WL LT DTQ SYT YJQ JS Overall
12 12 11 4 3 3 4 2 51
0.924 0.920 0.631 0.654 0.504 0.440 0.435 0.420 0.719
± ± ± ± ± ± ± ± ±
0.634 0.323 0.200 0.241 0.103 0.072 0.149 0.105 0.406
Soil pH
Description
Range 0.228e1.91 0.372e1.27 0.267e0.960 0.519e1.02 0.401e0.607 0.380e0.520 0.290e0.609 0.346e0.494 0.228e1.91
6.02 5.51 5.78 5.39 4.98 5.81 5.05 5.50 5.60
± ± ± ± ± ± ± ± ±
0.434 0.518 0.442 0.672 0.098 0.807 0.245 0.841 0.527
Coal mine Coal mine and smelter Cement factory Township Township Township Township Clean area in mountainous part of the region
348
M. Wang et al. / Chemosphere 144 (2016) 346e351
2.3. Estimation of health risk The health risk index (HRI) for those residing in the affected areas and consuming exclusively the locally produced Cd contaminated rice was calculated using the following equation (USEPA, 2000):
HRI ¼
ADD RfD
(1)
Where RfD is the reference dosage for Cd, that is 0.8 mg Cd (kg body weight)1 day1 (JECFA, 2010). When the calculated HRI was equal or greater than one, the consumers of the contaminated rice would be exposed to incrementally higher risks in proportion of increase in HRI. When the calculated HRI was less than one, the exposed population would be safe from chronic Cd poisoning. The individual-level average daily dose (ADD) was calculated using the following equation (USEPA, 2000):
ADD ¼
C IR BW
(2)
Where C is the concentration of Cd in rice (mg DW g1); IR is the daily average rice consumption per capita (DW g capita1 d1); BW is the average body weight (kg capita1). A 60 kg BW adult of the rice-consuming areas in south China countryside would consume on average 339 DW g of rice d1 (Ma et al., 2005)
from 0.420 to 0.924 mg kg1 with respective standard deviations varying from 0.072 to 0.634 mg kg1 (Table 1). Areas representing paddy soils at the mining and industrial areas such as TS and HFQ showed the highest soil Cd contents with a mean of 0.924 and 0.920 mg kg1, respectively. Paddy soils at the other industrial area (WL) had a mean Cd contamination level (0.631 mg kg1). At the township area, mean Cd concentration in paddy soils varied from 0.435 to 0.654 mg kg1. Overall, fields at the mining and industrial areas contained an average Cd content of 0.830 mg kg1, which is higher than those further from the mining and industrial areas (mean of 0.514 mg kg1) and the remote mountain site JS. The results suggested that industrial activities had resulted in significant accumulation of Cd in paddy fields. Furthermore, the remote mountain site JS, that was away from any known Cd emission source, registered soil Cd contents of 0.420 ± 0.105 mg kg1 indicative of the pervasiveness of past Cd emissions or geogenic enrichment. The average pH values of the eight areas were all below 7. TS had the highest average pH was 6.02, while the lowest average pH of DTQ was only 4.98. Thus, the potential uptake of Cd by rice was strong due to the acidic paddy soil. In all, the Cd contamination of Youxian's agricultural soils is significant and pervasive. Variation of local emission sources as well as soil background resulted in high spatial heterogeneity of soil Cd concentration, which causes great challenges for risk assessment and management.
3.2. Distribution and accumulation of Cd in soil profile 3. Results and discussions 3.1. Soil Cd pollution Altogether, 51 agricultural fields across the prefecture were sampled. Soil Cd contents in these fields ranged from 0.228 to 1.91 mg kg1, with a mean of 0.737 mg kg1 (Table 1). 90% of the fields (46 of 51) exceeded the soil Cd threshold permissible for crop production, 0.3 mg kg1as stipulated in China's Soil Environmental Quality Standards, suggesting a serious Cd contamination situation in the study area (SEPAC, 1995). There was great spatial heterogeneity of soil Cd concentration. The resulting mean soil Cd contents of the eight townships ranged
The Cd distributions in the 0e100 cm soil profiles are shown in Fig. 1. Except for SYT, the concentrations of Cd in the layer of 0e20 cm were significantly higher compared to the concentrations below 20 cm. Further, Cd concentrations in profiles below 40 cm depth in all sites had small variance. At the HFQ site, Cd enrichment in the surface 20 cm layer was quite serious, reaching 1.67 mg kg1. However, high Cd concentration over 1.0 mg kg1 in the lower soil layers were also observed, suggesting that there was a geological anomaly in this region and the background soil Cd concentration had been significantly elevated. Soils with higher levels of clay and FeeMn oxides hold onto trace elements over millennia. As shown in Table 2, the clay
Fig. 1. The Cd content profiles from 0 to 100 cm in paddy soils.
M. Wang et al. / Chemosphere 144 (2016) 346e351 Table 2 Profile distributions of clay concentration in paddy soil (%). Depth (cm)
0e20 20e40 40e60 60e80 80e100
Location DTQ
HFQ
JS
LT
SYT
TS
WL
YJQ
71.8 83.1 86.3 84.8 79.6
66.6 75.2 84.1 86.7 84.3
73.3 81.2 84.7 88.4 87.5
70.4 91.9 83.1 87.4 84.6
76.1 77.5 88.1 82.9 85.4
62.8 67.0 81.7 90.9 79.8
74.1 87.4 85.6 85.0 88.0
52.4 71.9 68.8 77.8 71.2
concentration in 0e100 cm profiles in YJQ was the lowest compared to other seven townships. The clay concentrations in the surface profile (0e20 cm) were lower than the profiles below (>40 cm) where the concentrations were near to or above 80% for all the eight sites. The distribution of clay contents in paddy soil profiles suggested the downward transportation of Cd from the surface was slow. If we consider the Cd concentration in 80e100 cm depth as the background value, the amount of Cd accumulation in the 0e20 cm depth could be calculated approximately by dividing the concentration of Cd in the 0e20 cm depth by the Cd in the 80e100 cm depth and then, multiplying to the soil bulk density of 0e20 cm profile. The bulk density of paddy soil ranged from 0.88 to 1.16 g cm3. So in this paper the average value 1.10 g cm3 was used. The results are shown in Table 3. The approximate Cd accumulations in the 0e20 cm depth layer were more than 1 kg ha1 in mine areas HFQ and TS, as well as town area YJQ. The accumulation amounts in DTQ and JS were more than 0.6 kg ha1. Industrial area WL had an accumulation amount of about 0.376 kg Cd ha1, while the accumulation of Cd in areas LT and SYT are below 0.1 kg ha1. Accumulation of Cd in the surface layer is the balance of the inputs and outputs. The outputs are mainly dependent on the yield and Cd content in rice grain and straw. Based on data in Table 5, the annual removal of Cd through rice grain harvest could vary from 0.67 to 21.9 g ha1 (based on two crop season and a grain yield of 12,000 kg ha1). If the rice straw was not reincorporated to the field, the Cd removal would be much higher. Therefore, the accumulative inputs (recycle from straw) would be much greater than those given in Table 3. Given that the commercial fertilizers in Chinese market were generally low in Cd, the main input pathways would be through irrigation water and air deposition. Our field investigation showed that at HFQ area, sediment Cd concentration
349
in irrigation channel was quite high, reaching 8.07 mg kg1, while the irrigation water (not filtered, because we suppose the suspended particulates may also transport to the field with the water flow.) contained 1.42 mg Cd L1 (annual input through irrigation water is estimated to be overall 10 g ha1). At TS area, annual air deposition of Cd was 12.0 g ha1, about three times higher than the national average. Given that the environmental management now is much stricter than before, it could be possible that there was significant environmental Cd aerosol pollution in the study areas from coal mining and other industrial activities which released the huge amount of Cd to the environment and then through water or air transport to the paddy fields. 3.3. Soil-plant transfer The transfer of Cd from contaminated soils to harvested rice grain can be depicted by the dimensionless plant uptake factor (PUF). As shown in Table 4, overall, the values of PUF ranged from 0.351 to 6.02. The average PUF was 2.61. Twenty-six samples out of 32 had PUF more than 1, indicating high level of transfer of soil Cd to the grains. The variation of PUF for the cultivar VY8 among different sites ranged from 0.653 to 6.02. Ten out of 12 samples had PUF more than 1. As for the PUF from the same area, large variation was also found among samples with different cultivars. The highest PUF was 4e5 folds of the lowest. And most of the samples had PUF more than 1. It was reported most of the Cd is recovered from the Zn during ore processing (Chaney, 2015). Field investigation in heavy metal contamination area in Palmerton, PA indicated that ratios of Zn to Cd (lower than 100 to 1) increase the Cd content in plants (Chaney, 2015). However, our research found more than 90% of the paddy soil samples had Zn: Cd higher than 100: 1, and the highest is 338:1. The average Zn concentration in paddy soil is 99.3 mg kg1, almost equal to the local background value. It was also reported by field survey that Zn had little or no effect on Cd accumulation by rice (Simmons et al., 2008). Investigation of rice Cd in contaminated area with higher soil pH values (4.1e5.7) and non-contaminated area with low soil pH (4.0e4.7) found higher PUF in noncontaminated area, which might attribute to the higher bioavailability of Cd at a lower soil pH (Takijima et al., 1973). Thus the effects of Zn on the uptake of Cd in rice in Youxian could be ignored. Zhao et al. (2015) indicated acidic nature of soils in many areas of southern China was one of the important reasons caused high
Table 3 Estimated Cd accumulation in 0e20 cm depth of the soil profile. Cd Deposit
Location DTQ
HFQ
JS
LT
SYT
TS
WL
YJQ
0e20 cm Depth profile (mg kg1) 80e100 cm Depth profile (mg kg1) Estimated Cd accumulation (kg ha1)
0.360 0.053 0.675
1.67 1.18 1.08
0.346 0.053 0.645
0.312 0.270 0.092
0.111 0.098 0.029
0.683 0.188 1.09
0.334 0.163 0.376
0.645 0.120 1.16
Table 4 Calculated Cd plant uptake factor (PUF) and the corresponding Cd concentrations in paddy soil and rice grains and pH value in selected areas in Youxian, Hunan. NO. of samples (n)
Overall VY8 cultivar DTQ TS WL XS
32 12 10 6 10 6
PUF Range
Mean
0.351e6.02 0.653e6.02 0.653e3.11 1.18e4.60 0.351e6.02 0.870e4.11
2.61 2.64 1.90 2.61 3.41 2.50
NO. of samples (PUF>1)
Rice Cd (mg DW kg1)
Soil Cd (mg DW kg1)
pH
26 10 7 6 8 5
0.050e1.00 0.117e1.00 0.080e0.611 0.435e0.823 0.050e1.00 0.367e0.968
0.100e0.446 0.139e0.446 0.100e0.287 0.164e0.370 0.114e0.219 0.195e0.446
4.83e5.71 4.83e5.53 4.94e5.45 4.95e5.71 4.84e5.55 4.83e5.67
350
M. Wang et al. / Chemosphere 144 (2016) 346e351
Table 5 Cd in rice in the eight studied areas in Youxian (mg kg1).
n Min Max Mean SD NO. of samples (>0.2 mg kg1)
DTQ
HFQ
JS
LT
SYT
TS
WL
YJQ
18 0.046 0.955 0.412 0.314 10
3 0.103 0.285 0.191 0.091 1
2 0.827 1.10 0.961 0.134 2
6 0.109 0.956 0.569 0.370 5
4 0.438 0.740 0.601 0.144 4
4 0.142 0.742 0.495 0.314 3
12 0.091 1.83 0.892 0.465 11
4 0.056 0.849 0.426 0.399 3
accumulation of Cd in rice. Zhao et al. (2015) indicated acidic nature of soils in many areas of southern China was one of the important reasons causing high accumulation of Cd in rice, and their calculation using the prediction model presented by Rӧmkens et al. (2009), suggested that strong acidity of paddy soil would cause the exceeding of the grain Cd limit in some “uncontaminated” soils. As shown in Table 4, extremely low pH values, ranging from 4.83 to 5.71, were found in studied area, which would be the main reason for high Cd PUFs in Youxian, Hunan. Increasing of Mn in solution would also decrease the uptake of Cd from root to grains (Yang et al., 2014). Our investigation found the average concentration of Mn in paddy soil in Youxian, Hunan was 327 mg kg1, lower than the background value 459 mg kg1. And early field drainage might promote Cd transfer to grain as well (Yang et al., 2009; Chino and Baba, 1981). In any regard, the high variation of PUF from site to site suggests a complicated relationship between soil Cd, soil pH, soil drainage and rice grain Cd, thus a great challenge for risk assessment and management. 3.4. Health risk assessment The maximum permissible concentration of Cd in rice grain, 0.2 mg Cd DW kg1 according to the Hygienic Standard for Grains (GB 2715-2005) of China is different from CODEX level of 0.4 mg Cd FW kg1. But it is reasonable if we take the permit Cd intake dose of 25 mg Cd BW kg1 month1 by JECFA into consideration. An 60 kg weighting adult who is supposed to daily intake 339 g dry weight rice grain will intake about 33.9 mg Cd BW kg1 month1 if the Cd concentration in rice grain is about 0.2 mg Cd DW kg1, which exceeds the permit Cd intake dose by JECFA. The mean Cd concentrations of polished rice that were harvested at the studied areas in Youxian ranged from 0.191 to 1.55 mg DW kg1 (Table 5) and, except for those at HFQ, the average rice Cd exceeded the maximum safe concentration of 0.2 mg Cd DW kg1 grain. From the view of individual rice grain samples, 73.6% (39 of 53) exceed the maximum safe concentration of Cd with a range from 0.046 to 0.956 mg kg1 (Table 6). While the
Table 6 The average daily dose (ADD) of Cd intake from rice grain and health risk index (HRI) in the eight studied areas in Youxian. Area
DTQ HFQ JS LT SYT TS WL YJQ Overall
ADD (mg Cd BW kg1 day1)
HRI
Max
Min
Max
Min
Percentile of high risk (HRI1)
5.40 1.61 5.13 5.40 4.18 3.41 10.3 4.80 10.3
0.261 0.583 0.96 0.616 2.48 0.800 0.516 0.316 0.261
6.75 2.01 6.41 6.75 5.23 5.24 12.9 6.00 12.9
0.326 0.730 1.20 0.77 3.09 1.00 0.645 0.400 0.326
64.7% 66.7% 100 83.3% 100 75% 91.7% 75% 78%
mining and industrial sites TS and HFQ registered the highest two soil Cd concentrations, rice grain harvested from these two sites contained much lower Cd in comparison with the other sites, ranking the lowest and the third lowest among the 8 areas. Similar to the paddy soil Cd contamination, there was a high spatial heterogeneity as indicated by the great range and coefficient of variation from area to area (Table 5). The average daily dose, ADD, and the health risk indices, HRI, of the representative sampling locations were calculated from Cd concentrations of the respective rice grain, according to Equation (2) and the populations. The ADD ranged from 0.261 to 10.3 mg Cd BW kg1 day1 and the HRI ranged from 0.326 to 12.9 overall (Table 6). It was suggest by JECFA that 0.8 mg Cd BW kg1 day1 or 25 mg Cd BW kg1 month1 of the intake exposure would correspond to the breakpoint 5.24 mg of Cd per gram creatinine at a lower bound of the 5th percentile dietary Cd exposure (on a population level). It could be suggested from Table 6 that the maximum of ADD was 10.3 mg Cd BW kg1 day1 among the eight investigated towns, more than 100 folds of the permit exposure amount. If it was estimated by HRI, large percentile of the local rice grain in those eight towns had high risk for ingestion, average 78% overall, to exceed the permit Cd exposure dose, 0.8 mg Cd BW kg1 day1 recommended by the JECFA. 4. Conclusion Youxian in Hunan Province, China, a fertile river basin for rice production, has been experienced soil metal contamination in the past. Emissions from coal mining, metal mining and smelting, and brick and tile kilns over the past fifty years caused the Cd concentration of surface soils to range from 0.228 to 1.91 mg kg1. Ninety percent of the fields (46 of 51) exceeded the soil Cd threshold permissible for crop production, 0.3 mg kg1as stipulated in China's Soil Environmental Quality Standards. Variation of local emission sources as well as soil background resulted in high spatial heterogeneity of soil Cd concentration. Large differences in Cd concentrations among 0e100 soil profiles were found, which might be partly due to high contents of clay (more than 80% below 40 cm layer). However, the limit of 0.3 mg kg1 is lower than other nations and less important than field drainage and soil pH in Cd accumulation in rice. As early as the early 1970, Japanese researchers had found that early drainage caused higher Cd accumulation, and later rains caused lower Cd in rice grain. It is also true in China. The water-saving irrigation measure for rice planting spreading since 1990s requires early and frequent drainage during the whole rice growth stage. Thus, changes in traditional management of flood and pH are in needed. Extremely higher PUF was found, ranging from 0.351 to 6.02. Twenty samples out of 32 had PUF more than 1, indicating high level of accumulation of Cd in rice grain, which might be contributed to early drainage before harvest and low pH values in paddy soils (ranged 4.83e5.71). The mean Cd concentrations of polished rice that were harvested at the studied areas in Youxian ranged from 0.191 to 1.55 mg kg1. More than seventy percentages (39 of
M. Wang et al. / Chemosphere 144 (2016) 346e351
53) of the grain samples exceed the maximum safe concentration of Cd, 0.2 mg DW kg1. Overall, ingestion of the 78% of the local rice grain samples would cause daily Cd intake to exceed the JECFA permit dose of 25 mg Cd BW kg1 month1. Acknowledgement We gratefully acknowledged financial supports provided by the Special Foundation of State Key Lab of Urban and Regional Ecology (SKLURE2013-1-04). References Chaney, R.L., 2015. How does contamination of rice soils with Cd and Zn cause high incidence of human Cd disease in subsistence rice farmers. Curr. Pollu. Rep. 1, 13e22. Chino, M., Baba, A., 1981. The effects of some environmental factors on the partitioning of Znic and Cadmium between roots and tops of rice plants. J. Plan. Nutr. 3, 203e214. Cui, Y., Zhu, Y., Zhai, R., Chen, D., Huang, Y., Qiu, Y., Liang, J., 2004. Transfer of metals from soil to vegetables in an area near a smelter in Nanning, China. Environ. Inter 30, 785e791. Du, Y., Hu, X., Wu, X., Shu, Y., Jiang, Y., Yan, X., 2013. Effects of mining activities on Cd pollution to the paddy soils and rice grain in Hunan province, Central South China. Environ. Mon. Assess. 185, 9843e9856. Fang, Y., Sun, X., Yang, W., Xin, Z., 2014. Concentrations and health risks of lead, cadmium, arsenic, and mercury in rice and edible mushrooms in China. Food Chem. 147, 147e151. Fu, J., Zhou, Q., Liu, J., Liu, W., Wang, T., Zhang, Q., Jiang, G., 2008. High levels of heavy metals in rice (Oryza sativa L.) from a typical E-waste recycling area in southeast China and its potential risk to human health. Chemosphere 71, 1269e1275. Huang, Z., Pan, X., Wu, P., Han, J., Chen, Q., 2014. Heavy metals in vegetables and the health risk to population in Zhejiang, China. Food control. 36, 248e252. JECFA/73/SC. Joint FAO/WHO Expert Committee on Food Additives. Seventy-third Meeting, Geneva, 8e17, June 2010. http://www.fao.org/ag/agn/agns/jecfa_ index_en.asp. Li, Q., Chen, Y., Fu, H., Cui, Z., Shi, L., 2012. Health risk of heavy metals in food crops grown on reclaimed tidal flat soil in the Pearl River Estuary, China. J. Hazard. Mater 227e228, 148e154.
351
Liu, J., Zhang, X., Tran, H., Wang, D., Zhu, Y., 2011. Heavy metal contamination and risk assessment in water, paddy soil, and rice around an electroplating plant. Environ. Sci. Pollut. Res. 18, 1623e1632. Liu, X., Song, Q., Yang, Y., Li, W., Xu, J., Wu, J., 2013. Human health risk assessment of heavy metals in soil-vegetable system: a multi-medium analysis. Sci. Total Environ. 463e464, 530e540. Liu, Y.Z., Xiao, T.F., Baveye, P.C., Zhu, J.M., Ning, Z.P., 2015. Potential health risk in areas with high naturally-occurring cadmium background in southwestern China. Ecotox. Environ. Safe. 112, 122e131. Ma, W.J., Deng, F., Xu, Y.J., 2005. The study on dietary intake and nutritional status of residents in Guangdong. South China J. Prev. Med. 31, 1e7. Niu, L., Yang, F., Xu, C., Yang, H., Liu, W., 2013. Status of metal accumulation in farmland soils across China: from distribution to risk assessment. Environ. Pollut. 176, 55e62. Rӧmkens, P.F.A.M., Gou, H.Y., Chu, C.L., Liu, T.S., Chiang, C.F., Koopmans, G.F., 2009. Prediction of cadmium uptake by brown rice and derivation of soil-plant transfer models to improve soil protection guidelines. Environ. Pollut. 157, 2435e2444. SEPAC (State Environmental Protection Administration of China), 1995. Chinese Environmental Quality Standard for Soils (GB15618-1995). Simmons, R.W., Noble, A.D., Pongsakul, P., Sukreeyapongse, O., Chinabut, N., 2008. Analysis of field emoist Cd contaminated paddy soils during rice grain fill allows reliable prediction of grain Cd levels. Plant Soil 302, 125e137. Takijima, Y., Katsumi, F., Takezawa, K., 1973. Cadmium contamination of soils and rice plants caused by Zinc mining, Ⅱ. Soil conditions of contaminated paddy fields which influence heavy metal contents in rice. Soil Sci. Plant Nutr. 19 (3), 173e182. US EPA, 2000. Handbook for Non-cancer Health Effects Evaluation. Environmental Protection Agency Press, Washington DC. US EPA, 1989. Risk Assessment Guidance for Superfund (RAGS). In: Human Health Evaluation Manual (HHEM)-part a. Baseline Risk Assessment, vol. 1. Office of emergency and remedial response, Washington DC [EPA/540/1e89/002]. Yang, J.C., Huang, D.F., Duan, H., Tan, G.L., Zhang, J.H., 2009. Alternate wetting and moderate soil drying increases grain yield and reduces cadmium accumulation in rice grains. J. Sci. Food Agric. 89, 1728e1736. Yang, M., Zhang, Y.Y., Zhang, L.J., Hu, J.T., Zhang, X., Lu, K., 2014. OsNRAMP5 contributes to manganese translocation and distribution in rice shoots. J. Exp. Bot. 65 (17), 4849e4861. Zhao, F.J., Ma, Y.B., Zhu, Y.G., Tang, Z., McGrath, S.P., 2015. Soil contamination in China: current status and mitigation strategies. Environ. Sci. Tech. 49, 750e759. Zhuang, P., McBride, M.B., Xia, H., Li, N., Li, Z., 2009. Health risk from heavy metals via consumption of food crops in the vicinity of Dabaoshan mine. South China. Sci. Total Environ. 407, 1551e1561.