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ARSENIC IN SOILS AND PLANTS 1

Chapter 5 Arsenic in soils and plants: accumulation and bioavailability

In Chapter 4 we discussed the occurrence of high concentrations of arsenic in groundwaters of the deep reducing aquifers of Chianan and Lanyang plains, which are used at present for irrigation and aquaculture, and formerly also for drinking water supply, the last was causing severe health effects in the population. The common features of these two aquifers are that they are from deep aquifers with stagnant groundwater and reducing conditions, whereas in both areas the shallow wells, which tap the phreatic aquifer which has oxidizing conditions and low As concentrations. The known sources of the groundwater arsenic as of 2004 (Drury 2006) are as follows: arsenic-contaminated aquifers, arsenic released by mining activities and arsenic release related to geothermal activities, the last explaining high As concentrations in the area of Guandu plain. In the present chapter we will first overview the accumulation and behavior of As in the Taiwanese soil with emphasizing on the Guandu plain where the arsenic concentration in the paddy soil greatly exceeded the 30 mg kg–1 threshold for concern under Taiwanese regulation. We will then discuss the bioaccumulation of As in plants and crops and compare the As concentrations in soil, rice, crops and vegetables in BFD and non BFD areas as well as in the Guandu plain. Moreover, total

As (Astot) and As species concentrations in rice in Taiwan are compared with different countries to elucidate the potential health risk associated with ingestion of arsenic- contained rice.

5.1 ACCUMULATION AND BEHAVIOR OF ARSENIC IN SOIL In Taiwan, As concentrations in soils are generally low to medium. The average As concentrations in 14 townships of Taiwan surveyed in 1973, at soil depths 0–12, 12–24 and 24–36 cm were 8 (range 1–44), 14 (range 2–34) and 19 mg kg–1 (range 3–177), respectively (Li et al. 1979a). Later, in 1976, the Taiwan Plant Protection Centre surveyed the As concentrations in soils of 45 other townships. The average As concentrations in soil at depths 0–12, 12–24 and 24–36 cm were 7.86 (range 1–22), 10 (range 2–19) and 10 (range 6–19) mg kg–1, respectively (Li et al. 1979a). During 1982–2001, the Taiwan Environmental Protection Administration conducted a four-phase nationwide survey of the concentration of eight heavy metals and metalloids (As, Cd, Cu, Cr, Hg, Ni, Pb and Zn) in soils (Table 5.1). The results of the 2nd phase survey, in which one sample represented the area of 100 ha, indicated that the average As concentration was 7 mg kg–1 (range 3–13 mg kg–1) (Fig. 5.1). In the 4th phase of the detailed survey, 100,000 ha area were surveyed with one sample taken per ha The results showed that only in four of all the 1 ha soil units, did soils exceed the 30 mg kg–1 threshold for concern under Taiwanese regulations. Three of those units were situated in Taipei city with average As concentration of 69 mg kg–1. One unit was in Hsinchu city with average As concentration of 42 mg kg–1. The contamination source of As in the soil of Taipei city is suspected from the natural hot spring and is classified as geogenic, whereas the source of soil contamination with As in the Hsinchu city is 2 BOOK TITLE OR AUTHORS NAMES

mainly the agrochemical manufacturing industry, an anthropogenic As source (CTCI Corporation, 2001). Notably, the warning and control standards of As concentrations in soil, enforced by the Taiwan EPA, are 30 and 60 mg kg–1, respectively (Taiwan EPA 2003).

Table 5.1.

Fig. 5.2. Locations of the sites in the Guandu plain and hot springs where the water was sampled. Superimposed on the satellite images from Google Earth). (a) Surface and groundwater generally flow from the northeastern geothermal spring area to the downstream Kee Lung ARSENIC IN SOILS AND PLANTS 3

river. High As concentrations were accumulated in the surface soil of Guandu plain; (b) Sampling points for As analysis in the upstream geothermal spring area (modified from Taipei EPB 2006). exceeding 60 mg kg–1 at 0–15 cm depth. Over 61,500 kg of As present in the top- and in the subsoils are estimated to originate from external sources. This large quantity of As was mainly from natural sources due to the absence of any anthropogenic As sources in Guandu plain. The possible sources of As in Guandu plain are volcanic eruptions and hot spring water discharge (Shyu et al. 2009). Table 5.2 lists the As concentrations of hot spring water sampled along the creeks in the so called “geothermal spring area” The sampling points themselves are shown in the map of Fig. 5.2b). As concentrations were high in the geothermal spring and decreased sharply downstream due to river water mixing and also very likely, due to precipitation and accumulation in the bottom sediments. Local farmers have used this water with its' high concentration of As for irrigation for hundreds of years. The long term use of water with elevated As concentrations for irrigation has led to As accumulation in the paddy soil. Although rice has been cultivated in the Guandu plain over the last 200 years, there has been no epidemiologic evidence showing As threat to public health through rice ingestion, suggesting low bioavailability of As in the Guandu soils (Su et al. 2007). To investigate the vertical distribution of As concentrations, soil samples were collected at different depths of As-containing paddy land and an adjacent dry-land (up to a depth of 140 cm) (Wu et al. 2009). Chemical extractions were applied to determine the concentrations of Astot and selected elements, such as Fe, Al, Pb, and Ba. As speciation in soils within those two profiles was also characterized using sequential extraction (Wenzel et al. 2001). The results showed that As was predominately associated with Fe oxides and was less recalcitrant in the surface soils of the rice-paddy land (Wu et al. 2009). Nonetheless, the vertical distributions of As showed no indication of enhanced mobility in the rice paddy soil compared with its dry-land counterpart. Thus, Wu et al. (2009) hypothesized that the high concentrations of Fe oxides or hydroxides in those soils may restrict As mobility and availability during flooding seasons. A large portion of As was associated with the fine fractions (< 0.05 mm) of the soils at any depth; on the other hand, As concentration was the highest in the coarsest fractions (> 0.50 mm and 0.50–0.25 mm) of soils and decreased with decreasing particle size. Thus, the high As concentrations of the soils in the Guandu may result from weathering of As-bearing parent materials which is similar to the conclusions drawn by Shyu et al. (2009). The environmental impacts of high As concentrations in soil and the fate and transport of As in the Guandu plain require further study.

Table 5.2. Arsenic concentrations in the hot spring water sampled along two creeks in the geothermal spring area. As is predominately present as As(V), with exceptions of sampling points near the spring where As(III) is the predominant species, and no organic As species were detected.

Sample As(III) As(V) Astot number (μg L–1) (μg L–1) (μg L–1) D0 ND ND ND D1 4785.0 880.2 5665.2 D2 3917.3 1488.1 5405.4 D3 25.3 708.0 733.3 D4 30.9 953.4 984.2 4 BOOK TITLE OR AUTHORS NAMES

D5 ND 919.0 918.8 D6 ND 1153.4 1153.2 D7 ND 717.6 717.4 D8 ND 348.2 348.0 D9 ND 2.6 2.4 D10 ND 1.8 1.8 D11 ND 0.8 0.8

5.2 BIOACCUMULATION OF ARSENIC IN PLANTS AND CROPS The bioavailability, uptake and accumulation of As is dependent on a number of factors that include source and concentration of the element, its chemical form, soil properties such as clay content, pH and redox conditions, other ions, type and amount of organic matter present, as well as numerous plant-dependent factors, including plant species.

As chemical form was reported to be more important than the level of Astot concentration in solution in determining phytotoxicity effect on plants (Marin et al., 1992, Carbonell-Barrachina et al. 1999). In Taiwan, rice is one of the most important crops. Although the daily intake rate of rice has decreased gradually from 366 g per capita in 1972 to 133 g per capita in 2005 (Taiwan COA 2005), the daily intake rate of rice still makes up 54.3% of total cereals consumed. Rice is the staple food of Taiwanese. Irrigation water for rice paddies in Taiwan is mainly provided by local irrigation associations where 85% of water comes from surface river water, 10% from rain water and the remaining 5% from groundwater. The bioaccumulation of As in rice from irrigation water is considered unimportant. However, bioaccumulation of As in plants and crops has received great attention in the hyper-endemic areas of a blackfoot disease (BFD). Lo et al. (1983) conducted an extensive survey of As concentrations in rice and other crops. Several crops and vegetables including rice, corn, potato, water spinach and Chinese white cabbage were collected and analyzed for their As concentrations in BFD areas. Samples of the same crops and vegetables from non-BFD areas were also analyzed for comparison. Additionally, 35 and 45 soil samples from BFD and non- BFD areas, respectively, were collected and analyzed. The average As concentrations of different parts of rice plants including: grain, stem, leaf and husk were 1.5, 3.3, 4.6 and 1.6 mg kg–1, respectively, in BFD areas and 0.5, 0.8, 2.2 and 1.0 mg kg–1 in non- BFD areas. Moreover, the results showed that the average As concentrations in corn, corn leaf, potato, potato leaf, water spinach and Chinese white cabbage were 0.4, 0.9, 0.4, 0.4, 0.7 and 0.6 mg kg–1, respectively, in BFD areas, whereas the average As concentrations in the same plants were 0.2, 0.5, 0.5, 0.3, 0.5 and 0.2 mg kg–1, respectively, in the non-BFD areas. The average As concentrations in soils in the BFD and non-BFD area were 9.9 mg kg–1 (range 2.8–15.7) and 4.6 mg kg–1 (range 1.2–10.4), respectively; thus, the average As concentration in BFD soil was by 2.1-fold higher than that in non-BFD soil (Lo et al. 1983). The detailed results are listed in Table 5.3. Bioaccumulation of As in rice plants, crops and vegetables in the BFD areas were significantly higher than those of in the non-BFD areas.

Table 5.3. Arsenic concentrations in soil, rice, crops and vegetables in BFD and non-BFD areas (mg kg–1) (Lo et al. 1983). BFD area Non-BFD area range mean sample # range mean sample # ARSENIC IN SOILS AND PLANTS 5

Soil 2.76–15.7 9.91 30 1.22–10.4 4.60 45 Rice grain 0.74–2.03 1.54 14 0.09–2.33 0.55 28 Rice stem 0.62–6.80 3.32 14 0.08–3.78 0.84 28 Rice leaf 1.78–8.58 4.57 14 0.37–6.22 2.21 28 Rice husk 1.22–2.14 1.62 14 0.29–2.86 1.04 28 Corn 0.31–0.54 0.45 13 1.09–0.32 0.21 6 Corn leaf 0.38–1.46 0.87 16 0.28–0.94 0.54 12 Potato 0.31–0.57 0.44 2 0.20–1.14 0.47 7 Potato leaf 0.24–0.60 0.42 2 0.11–0.51 0.34 7 Water spinach 0.31–1.12 0.72 2 - 0.54 1 Chinese white cabbage 0.40–0.80 0.60 2 - 0.20 1

The first nation-wide survey of As concentration in rice grains was conducted in 1975. Thereby, 328 samples of rice grains were collected from 86 townships around Taiwan. The results indicated that 95% of the samples contained detectable amounts of As, and the average As concentration was 0.30–0.53 mg kg–1 with a maximum value of 1.74 mg kg–1 (Li et al. 1979b). Moreover, the As concentrations in rice exceeding 1 mg kg–1 were found in the Pingtung, Kaoshiung, Chiayi, Yunlin and Taoyuan counties, and Chiayi had the maximum value of 1.74 mg kg–1 (Fig. 5.1). Since As concentration in rice plant was greatly concerned in the blackfoot disease hyper-endemic areas, rice plants were collected and analyzed in that area (Lo et al. 1983). As concentrations in part of the rice grain samples exceeded the statutory limit of 1.0 mg As kg–1 (dry wt) established in Australia (National Food Authority 1993). Notably, no legal standards for As in food and plants are set in Taiwan. Schoof et al. (1999) conducted a market basket survey of inorganic As (i-As) in food in Taiwan. They analyzed Astot and i-As concentrations in rice grains collected from Taiwan in 1993 and 1995. The Astot concentration and the percentage of i-As in –1 –1 Astot in the polished rice samples were 0.30 mg kg and 59% in 1993 and 0.12 mg kg and 72% in 1995, respectively. Lin et al. (2004) conducted a second nation-wide survey and collected 137 samples from different rice storage houses and 280 samples from market baskets. The average As concentrations from rice storage houses and market baskets were 0.05 mg kg–1 (range 0.01–0.14 mg kg–1) and 0.1 mg kg–1 (range 0.01–0.63 mg kg–1). Williams et al. (2005) reported that As concentrations of three sets of white long grain rice in Taiwan were 0.19, 0.20 and 0.76 mg kg–1, respectively. Table 5.4 shows a compilation of the As concentration in rice from surveys in various countries.. Zavala and Duxbury (2008) estimated the normal (uncontaminated) –1 levels of Astot in rice grain as 0.08–0.20 mg kg based on the results of previous studies. If we adopt these data as a preliminary standard, then it appears that As concentrations in the part of Taiwanese rice grain exceed the normal ranges.

Table 5.4. Survey of As concentration in the rice grain produced in different countries.

Astot in grain Country in mg (kg wt-day)––11 Sample Reference number Min-Max Mean±SD 6 BOOK TITLE OR AUTHORS NAMES

Taiwan 0.10–0.63 0.10±0.08 280 Lin et al. 2004 0.19–0.22 0.20 Schoof et al. 1998 0.19–0.76 0.38 Williams et al. 2005 Thailand 0.06–0.14 0.10±0.01 15 Williams et al. 2006 Philippines 0.00–0.25 0.07±0.02 22 Williams et al. 2006 Australia 0.02–0.04 0.03±0.00 5 Williams et al. 2006 India (basmati rice) 0.03–0.07 0.05±0.00 10 Williams et al. 2006 China (Beijing) 0.07–0.19 0.12±0.01 32 Williams et al. 2006 Bangladesh 0.03–0.28 0.11 17 Williams et al. 2005 West Bengal (India) 0.11–0.40 0.26 7 Roychowdhury et al. 2002 Vietnam 0.03–0.47 0.21 31 Phuong et al. 1999 United States 0.11–0.40 0.26 7 Williams et al. 2005

Particularly, soil contamination in Guandu plain area has received much public attention. The grains of rice grown in As-contaminated soils can potentially accumulate high levels of As. Possible uptake of As by rice plants may play an important role in the transfer of this toxic element into food chains, resulting in potential threats to human health (Meharg and Rahman 2003). The food safety issues raised by consumption of crops produced on As-contaminated soil located at Guandu plain was of great concern to local residents and government. The preliminary results showed that various vegetables, fruits, plants and rice produced in Guandu plain were safe for consumers even though the Astot concentration in soils were very high, in the range 100–500 mg As kg–1, because the concentration of bioavailable As in soils were extremely low (Shyu and Lin 2006). Yau (2008) reported that the average As concentration in rice grown in the As-contaminated Guandu paddy zone was 0.23±0.08 mg kg–1 (mean ± SD; range 0.11–0.49 mg kg–1). As concentrations in rice grains only slightly exceeded the normal range, which confirms low bioavailability of As in the Guandu soils. Su and Chen (2008) investigated As fractionation in soils and its relationships with As concentration in rice grain. Astot concentrations in 13 soil samples ranged widely from 12–535 mg kg–1, but the As concentrations in brown rice were all below 0.35 mg kg–1 and no relationships were found between them. The concentrations of non-specifically-bound As in soil, which represented bioavailable As, were very low

(less than 1% of Astot) which may explain the facts that As concentrations in rice grain cultivated in seriously As-contaminated soils were not significantly enhanced, and no adverse effects were shown on rice growth. Pot experiments conducted by Su et al. (2009) gave similar results, and showed that carrot cultivated in highly As- contaminated soil was safe for consumers, however, the As concentrations in garland chrysanthemum increased with increasing Astot concentrations in soil (Table 5.5). As concentrations in field-collected white mushroom were elevated, which is inconsistent with the results published by Environmental Protection Bureau of Taipei City Government (Taipei EPB 2006). Therefore, it should be concluded that the crops produced in Guandu plain may pose potential risks for human health and establishing a regular monitoring program of crops is suggested. In addition, the agronomic practices should be carefully considered, since the application of lime materials or phosphorous fertilizers may enhance the bioavailability of As in soil (Wenzel et al. 2001). A significant linear relationship was found between specifically-bound As in soil and As concentration in garland chrysanthemum (r2 = 0.830, p < 0.001). This suggests that specifically-bound As in soil may be used as an indicator for assessing the food safety of vegetable crops cultivated in As-contaminated soil of the Guandu plain (Su et al. 2009). ARSENIC IN SOILS AND PLANTS 7

Table 5.5. Arsenic concentrations in soil and crops in the Guandu plain (mg kg–1) (Su et al. 2009). Soil depth Garland Taros Sample point (cm) surface part subsurface part surface part stem root 1 6.33 0.36 40.7 - - - 2 68.8 1.30 154.4 - - - 3 72.8 0.9 92.1 - - - 4 9.81 1.37 109 - - - 5 111 0.78 92.7 - - - 6 221 4.81 ND - - - 7 224 0.38 0.16 15.3

The data on As speciation in the rice grain are limited in Taiwan. Williams et al. (2006) and Schoof et al. (1998) have collected samples and determined the As speciation in rice grain of Taiwan. The results are listed in the Table 5.6. Inorganic As made up 69–83% of Astot in rice grain, whereas, organic As (Asorg) amounted 17–31% of Astot, indicating a low methylation capability of As in rice grain. Williams et al. (2005) reported various amounts of As(V) and As(III) in rice collected from various sites in the world, and the percentage of i-As(V) was 64% for European, 80% for Bangladeshi and 81% for Indian rice. In contrast, DMA (dimethylarsenic acid) was the predominant species of As in rice from USA with only 42% being i-As.

Table 5.6. Average concentrations of arsenic species in Taiwanese rice.

Sample Astot MMA DMA Sum of Sum of i-As species –1 –1 –1 –1 (μg g ) (μg g ) (μg g ) Asorg (μg g ) (% of Astot) species (% of Astot) Rice (grain) (n=1)1) 0.76 0.058 0.046 17 0.506 83 Rice (polished) (n=2)1) 0.20 0.016 0.030 28 0.118 72 Rice (polished) (n=5)1) 0.15 0.013 0.013 19 0.11 81 Rice (polished)2) 0.19 0.03 0.015 27 0.12 73 Rice (polished)2) 0.20 0.03 0.02 31 0.11 69 Rice (polished)2) 0.76 0.05 0.06 18 0.51 82 1)Schoof et al. 1998, 2)Williams et al. 2005.

The average Astot concentrations in rice grain in Taiwan were in the range 0.30– 0.53 mg kg–1 as surveyed by Li et al. (1979b) and 0.11–0.51 mg kg–1 as surveyed by Williams et al. (2005) and Schoof et al. (1998). Those values remain below the statutory limit of 1.0 mg As kg–1 dry weight established for rice grain in Australia (National Food Authority 1993), however, in most cases, i-As exceeded the concentration of 0.15 mg kg–1 which is the China standard for i-As (GB2762-2005 China National Standard). Assessment of the health risk caused by the presence of As in rice has largely been based on i-As concentrations because these species have generally been proved more toxic than Asorg (Fitz and Wenzel 2002). The mean contribution of i-As to Astot in grains was reported by Schoof et al. (1998) and Williams et al. (2005) to be 70.7% These authrs had surveyed As concentrations in the rice grain in Taiwan. Assuming that i-As makes up 75% of Astot in the rice grain in Taiwan, and that Astot concentrations in rice grains are in the range 0.30–0.53 mg kg–1, the i-As health effects of the concentrations in the rice grains will be assessed at the concentration level of: 8 BOOK TITLE OR AUTHORS NAMES

0.23–0.40 mg kg–1 . This exceeds the standard in China also to reduce effects on health, of i-As (0.15 mg kg–1). However, the Taiwanese government has not set up a similar standard yet. Thus, Astot and i-As concentrations in rice need to be surveyed comprehensively before the standard for i-As in crops is established in Taiwan.

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