Bioaccessibility and health risk assessment of arsenic in arsenic-enriched soils, Central India

Ecotoxicology and Environmental Safety 92 (2013) 252–257

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Bioaccessibility and health risk assessment of arsenic in arsenic-enriched soils, Central India Suvendu Das n, Jiin-Shuh Jean n, Sandeep Kar Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 November 2012 Received in revised form 6 February 2013 Accepted 25 February 2013 Available online 21 March 2013

Incidental soil ingestion is expected to be a significant exposure route to arsenic for children because of the potentially high arsenic contents found in certain soils. Therefore, it is prudent to get information on oral bioaccessibility of arsenic following incidental soil ingestion and its relevance in health risk assessment for future remediation strategies. Soil samples were collected from eight villages of Ambagarh Chauki block, Chhattisgarh, Central India. The soils from seven villages had total arsenic content more than the background level of 10 mg kg  1 (ranged from 16 to 417 mg kg  1), whereas the total arsenic content of soil from Hauditola was 7 mg kg  1. Bioaccessible arsenic assessed by the simplified bioaccessibility extraction test (SBET) ranged from 5.7 to 46.3%. Arsenic bioaccessibility was significantly influenced by clay content (R2 ¼ 0.53, p o 0.05, n ¼ 8), TOC (R2 ¼ 0.50, p o0.05, n ¼8), Fe content (R2 ¼ 0.47, po 0.05, n ¼ 8) and soil pH (R2 ¼0.75, po 0.01, n ¼ 8). Risk assessment of the study sites showed that hazard index of arsenic under incidental soil ingestion was below 1 in all the study sites, except Kaudikasa. However, carcinogenic risk probability for arsenic to children from the villages Meregaon, Thailitola, Joratarai and Kaudikasa was below acceptable level (o 1  10  4), suggesting potential health risk for children from these sites could not be overlooked. With high carcinogenic risk value (3.8E  05) and HI index (4 1) for arsenic in soils of Kaudikasa, attention should be paid for development of remediation measure. & 2013 Elsevier Inc. All rights reserved.

Keywords: Bioaccessibility SBET Carcinogenic risk Hazard index

1. Introduction The metalloid arsenic (As) is ranked number one in the list of priority pollutants harmful to human health by ATSDR (2007). Exposure to As has been shown to result in the potential development of human carcinogens as well as numerous other health disorders (Bradham et al., 2011). Hence, cancer risk associated with oral ingestion of As-contaminated soils often drives risk assessments for human exposure to metal toxicity at contaminated sites. In an assessment of human health risk from ingestion of As-contaminated soils, in vivo tests using animals (e.g., swine, rats, monkeys, rabbits) have been used to estimate the amount of As arriving in the circulatory system from the gastrointestinal tract (bioavailability) (Koch et al., 2007; Ono et al., 2012). However, the in vitro methods that overcome the time and expense limitations of in vivo studies are currently recognized as fast screening tools in assessing relative bioaccessibility (i.e., the fraction of a contaminant that is soluble and potentially available to be absorbed) of contaminants at metal

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Corresponding authors. Fax: þ 886 6 274 0285. E-mail addresses: [email protected], [email protected] (S. Das), [email protected] (J.-S. Jean). 0147-6513/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ecoenv.2013.02.016

polluted sites (Juhasz et al., 2007a; Girouard and Zagury, 2009). Several in vitro digestion models have been developed to assess the human bioaccessibility (Hu et al., 2011). For example, a twostep physiologically based extraction test (PBET) was proposed to estimate the successive solubility of metals under stomach and intestinal tract conditions (Ruby et al., 1996), and a simplified bioaccessible extraction test (SBET) simulating gastric conditions has been used extensively (Smith et al., 2008; Juhasz et al., 2009; Poggio et al., 2009; Hu et al., 2011). SBET is a simplified form of PBET designed to be fast, easy, and reproducible (U.S. EPA, 2007a; Hu et al., 2011). SBET applied to various contaminated soils has been successfully validated for As with in vivo tests using juvenile swine (Juhasz et al., 2007a). Soil guideline values for human health and ecological risk assessment are not yet well established for As, with suggested values ranging according to the country (DEFRA and EA, 2002; Ono et al., 2012). Hence, a realistic assessment of actual health risks associated with As toxicity in contaminated soils requires evaluation of bioaccessible fractions. Moreover, As bioaccessibility and its relevance in health risk assessment are controlled by soil mineralogy, particle size of soils, pH and Fe/Mn (hydr)oxides (Goldberg, 2002; Yang et al., 2002; Juhasz et al., 2007b). Yang et al. (2002) reported that As bioaccessibility was closely related to the iron (Fe) oxide content and soil pH in arsenate spiked soils

S. Das et al. / Ecotoxicology and Environmental Safety 92 (2013) 252–257

and that these parameters could be used to predict As bioaccessibility in soils. However, the influence of soil properties on As bioaccessibility has not been investigated in tropical soils. The situation of As content in Ambagarh Chauki block, Chhattisgarh, Central India is alarming (Acharyya et al., 2005; Patel et al., 2005; Shukla et al., 2010). It is reported that the concentrations of As in groundwater, soil and sediment of this region are as high as in West Bengal and Bangladesh (Acharyya et al., 2005; Patel et al., 2005). Many studies on As enrichment in this region have focused on metal contents, particle-size and spatial distribution, and source identification (Acharyya et al., 2005; Patel et al., 2005; Shukla et al., 2010). However, the study on the oral bioaccessibility and the exposure risk of As in As-enriched soils of Central India have not yet been conducted, which will have an impact on future remediation policies. The objectives of this study were (i) to evaluate As bioaccessibility in As-enriched soils of Ambagarh Chauki block, Chhattisgarh, India, (ii) to investigate the effect of different soil properties on the magnitude of soil As bioaccessibility, and (iii) to assess the exposure and characterize the risk to children exposed to As-enriched soil of these localities.

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heterogeneity of the As distribution. Selections of sampling sites were based on the previous studies related to As contents of soils of Ambagarh Chauki block (data not presented). An additional representative sample taken from Hauditola (Fig. 1), where total As content of soil was lower than the background level of 10 mg kg  1 (O’Neil, 1995), was our control area. Individual soil cores (2 cm diameter, 15 cm depth) were taken with a sample probe from five different places within each Asenriched site. Collected samples have been air dried in shade at room temperature, sieved (o 2 mm), and then refrigerated at 4 1C until analysis. All analyses on samples were completed within a few days after sampling. Clay, silt and sand percentages of soil were performed by Bouyoucos hydrometer method. Soil pH was measured at soil:water ratio of 1:1.25 using a portable pH meter (Philips model PW 9424) with a combined calomel glass electrode assembly (Das and Adhya, 2012). The total organic carbon (TOC) content of the soil samples was determined in a TOC analyzer (Micro N/C model HT 1300, Analytic Jena, Germany). Total As, Fe and Mn in soils (o 2 mm) were determined on filtered (o0.45 mm) liquid samples with inductive couple plasma-mass spectrophotometer (ICP-MS; VG Elemental, Winsford, UK) after chemical solubilization of metals via microwave digestion for 30 min (Berghof Speedwave MWS 3 þ, Eningen, Germany), based on EPA method 3052 by adding 9 mL of HNO3 and 3 mL of HF to 0.5 g of soil sample (U.S. EPA, 1996). A certified reference material (CRM 025-50) was included in the analysis to ensure internal quality assurance/quality control (QA/QC) practices. All the metal concentrations obtained were consistent with the reference values (within 95% prediction interval).

2.3. Bioaccessible As 2. Materials and methods 2.1. Geography and geology of study area The As-enriched area, Ambagarh Chauki block (area E2000 km2) is located in Chhattisgarh State of India (2013300 –2015100 N and 8013300 –8014700 E at altitudes of Z 430 m from mean sea level). The soils of the study area are rich in silica, iron and aluminium. The rocks are of the early proterozoic age, and exposed acid and basic volcanics, intrusive co-magmatic and contemporaneous Dongargarh granite cover the whole study area (Acharyya et al., 2005). The mechanism of release of As from the rock to the groundwater is expected as reported from Bangladesh and West Bengal (Patel et al., 2005; Shukla et al., 2010).

2.2. Soil sampling and analysis Soil samples (0–10 cm depth) from residential areas of Ambagarh Chauki block (i.e., Bandha bazar, Maldongri, Manzhi tola, Meregaon, Thailitola, Joratarai and Kaudikasa) (Fig. 1) were collected in summer to minimize the error due to

Arsenic bioaccessibility was expressed as the percentage of As content extracted by SBET with respect to its total content in soils. SBET extractable As was performed following the procedure of Juhasz et al. (2007b). Juhasz et al. (2007a) demonstrated that in vivo relative As bioavailability could be accurately predicted using the gastric phase of the SBET in vitro assay. In brief, soil samples were sieved to o250 mm fraction since it represents the fraction more likely to adhere to children’s hands (Ruby et al., 1996; U.S. EPA, 2008). One gram (o250 mm) of soil, mixed with 100 mL of gastric solution (30.03 g L  1 glycine adjusted to pH 1.5 with concentrated HCl) was incubated at 37 1C in a shaking incubator at 40 rev min  1 for 1 h. After determination of pH, gastric phase samples (10 mL) were collected, filtered through 0.2 mm filters and analyzed by ICP-MS. Arsenic bioaccessibility was determined in triplicate soil samples. For quality control and quality assurance, a standard reference soil material (SRM 2711) was analyzed and the experimental values showed good agreement (accuracy: 76% and precision: 7 5%) with the certified values. Arsenic bioaccessibility was calculated by the following equation:  As bioaccessibility ð%Þ ¼

 SBET As  100 Total As

Fig. 1. Map showing sampling sites from Ambagarh Chawki block of Rajnandgaon district of Chhattisgarh, India.

ð1Þ

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2.4. Exposure assessment and risk characterization Since relative oral bioavailability (estimated using SBET method) of As is useful for more accurate exposure assessment, adjustments to dose (chemical daily intake¼ CDI) values were performed using Eq. (2) (Basta et al., 2001; Guney et al., 2010). CDIadjusted ¼ CDImetal  As Bioaccessibilityð%Þ

ð2Þ 1

1

where CDIadjusted ¼ adjusted metal daily intake (mg kg body weight d ); CDImetal ¼metal daily intake, (mg kg  1 body weight d  1). For exposure assessment, CDI of As from incidental ingestion of soil was calculated with Eq. (3) (U.S. EPA, 1989, 2007b): CDImetal ¼

EPC  SIR  EF  ED  CF BW  AT

ð3Þ

where EPC is the exposure point concentration (mg kg  1) determined in the 250 mm fraction; SIR is the soil ingestion rate (mg d  1) at 200 mg soil d  1 for children 2–6 years old (under the greatest risk due to their common soil eating behavior) (U.S. EPA, 2002; Lu et al., 2011); EF is the exposure frequency of 0.5 (182 d  1 yr  1) (Hemond and Solo-Gabriele, 2004; Lu et al., 2011); ED is exposure duration and was taken as 5 years for the children of age group from age 2–6 years (U.S. EPA, 2008; Guney et al., 2010); BW is average body weight as 15 kg for the children from the age group 2–6 years (U.S. EPA, 1989; Hu et al., 2011), which fit into the average body weight of Indian children from age group of 2–6 years; AT is the averaging time (for non-carcinogens, AT¼ ED  365 days; for carcinogens, AT ¼70  365¼ 25,550 days); CF is the unit conversion factor of 10  3. The potential carcinogenic and non-carcinogenic risks for individual metals were calculated using Eqs. (4) and (5), respectively (U.S. EPA, 2007b; Guney et al., 2010). Carcinogenic Risk ¼ CDIadjusted  SF HI ¼

ð4Þ

CDIadjusted RfD

ð5Þ

where ‘carcinogenic risk’ is the probability of carcinogenic effect (unitless); SF is cancer slope factor and is 1.5 mg kg  1 d  1 for As; HI is the hazard index (also stated as hazard quotient). For non-carcinogenic risk, reference dose value (RfD) was taken as 0.3 mg kg  1 d  1 for As (U.S. EPA, 2007b). 2.5. Statistical analysis Data were subjected to statistical analysis by SPSS statistical software version 13.0 (SPSS Inc.). The mean difference comparison between different sampling sites was analyzed by analysis of variance (ANOVA) and subsequently by Duncan’s multiple range test (DMRT) at p o0.05. Linear regression analysis was performed to show the relationship of bioaccessible As with different soil physiochemical parameters.

3. Results and discussion 3.1. Soil physiochemical properties The soil samples from Ambagarh Chauki cover a narrow range of pH (6.7–7.3) and most of the samples had near neutral pH values, generally from 7.1 to 7.3. The pH value did not differ significantly within the sampling sites (Table 1). TOC and clay content of soil from the study sites ranged from 0.68 to 0.82(%) and 20.9 to 37.6(%), respectively and were highest in Hauditola and lowest in Kaudikasa. The sand and silt content ranged from

41.5 to 57.5% and 20.9 to 22.5%, respectively (Table 1). TOC contents were generally lower in sandy soils and the high values were observed in soils with higher clay content. The Fe and Mn content of the soils varied from 24,918 to 46,179 mg kg  1 and 331 to 1029 mg kg  1, respectively (Table 1). The total As content in the soils of Ambagarh Chauki followed the order of Hauditola (7.0 mg kg  1)oBandha bazar (16 mg kg  1)oMaldongri (17.0 mg kg  1)oManzhi tola (19.0 mg kg  1) oMeregaon (48 mg kg  1)oThailitola (108 mg kg  1)oJoratarai (183 mg kg  1)o Kaudikasa (417 mg kg  1) with mean and median values of 101.9 and 33.5 mg kg  1, respectively (Table 2). The values were higher than the background level of 10 mg kg  1 (O’Neil, 1995) except in Hauditola. High standard deviation of the total As content in soils ( 7141.0 mg kg  1) could be due to large variations in the land use and the chemical composition of soil. A high concentration of As (ranged from 15 to 825 mg L  1) in groundwater has been reported from Ambagarh Chauki (Patel et al., 2005; Shukla et al., 2010). Shukla et al. (2010), with their petrographic studies of the granitic host rock and X-ray diffraction, revealed that realgar (a-As4S4), para realgar (AsS), and/or tennantite (Cu12As4S13) are the main mineral that contain As. The groundwater becomes highly As-enriched due to leaching of minerals during the weathering and water–rock interactions (Acharyya et al., 2005; Shukla et al., 2010). It is reported that water for domestic consumption and irrigation in Ambagarh Chauki is derived from As-enriched groundwater, which eventually make the soil contaminated (Acharyya et al., 2005; Patel et al., 2005). The SBET-As, varied from 0.4 to 193 mg kg  1 and was 5.7 to 46.3% of total As of the study sites. The highest value was recorded in Kaudikasa, whereas the lowest value in Hauditola (Table 2). Smith et al. (2008) reported that SBET-As from contaminated (anthropogenic and geogenic) soils ranged from 6 to 48%. However, as per our knowledge there is no report on SBET-As content from tropical soils.

Table 2 Total As and bioaccessible As of As-enriched soils of Ambagarh Chauki block, Central India. Sampling sites

Total As (mg kg  1) SBET-As (mg kg  1)

As bioaccessibility (%)

Hauditola Bandha bazar Maldongri Manzhi tola Meregaon Thailitola Joratarai Kaudikasa

7 72 c 16 74 c 17 77 c 19 76 c 48 712 b 108 726 b 183 7 72 b 417 7 112 a

5.7 21.3 27.6 29.5 24.0 35.2 34.4 46.3

0.47 0.1 e 3.4 71.4 d 4.7 71.6 d 5.6 71.6 d 11.5 72.2 c 38.0 711.0 b 63.0 714.6 b 193.0 734.0 a

Mean of five replicate observations. In a column means followed by a common letter are not significantly different (po 0.05).

Table 1 Physiochemical properties of the As-enriched soils of Ambagarh Chauki block, Central India. Sampling sites

Sand (%)

Silt (%)

Clay (%)

Fe (mg kg  1)

Mn (mg kg  1)

pH

Hauditola Bandha bazar Maldongri Manzhi tola Meregaon Thailitola Joratarai Kaudikasa

41.5 46.5 53.6 47.6 54.2 47.3 46.6 57.5

20.9 20.9 21.0 21.3 21.2 22.5 22.2 21.6

37.6 32.6 25.4 31.1 24.6 30.2 31.2 20.9

46179 42372 26632 27378 37902 24918 28923 39487

922 b 573 c 1029 a 588 c 331 e 411 d 423 d 987 a

6.77 0.8 6.87 0.8 7.17 0.6 6.87 0.5 6.87 0.8 7.17 0.7 7.27 0.6 7.37 0.8

a a c c b c c b

Mean of five replicate observations. In a column means followed by a common letter are not significantly different (p o0.05).

TOC (%) a a a a a a a a

0.82 7 0.08 0.76 7 0.07 0.71 7 0.12 0.81 7 0.10 0.71 7 0.08 0.73 7 0.08 0.73 7 0.10 0.68 7 0.06

a a a a a a a a

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3.2. Soil physicochemical properties related to As bioaccessibility Arsenic bioaccessibility (%) of the samples ranged from 5.7 to 46.3% with an average bioaccessible fraction of 28.0% and followed the order of Hauditola (5.7%) oBandha bazar (21.3%)o Meregaon (24.0%) oMaldongri (27.6%)o Manzhi tolao(29.5%)o Jorataraio(34.4%)oThailitola (35.2%)oKaudikasa (46.3%) (Table 2). In all the soils, As bioaccessibility was less than 50%, indicating that significant proportion of the total As contents may not be available for absorption in the gastrointestinal tract by incidental soil ingestion. This was in agreement with earlier reports as Juhasz et al. (2007a, 2007b), Smith et al. (2008), Meunier et al. (2010). To the best of the authors’ knowledge, no previous research on As bioaccessibility has been reported in As-contaminated (anthropogenic and geogenic) soils of tropical origin, so that data from this study could be compared. However, data from research in contaminated sites worldwide have shown varying average values of bioaccessible As (Juhasz et al., 2007a, 2007b; Guney et al., 2010; Meunier et al., 2010), that was most likely related to the differences in mineralogy and sorbing capacity of the soils/sediments being evaluated. As bioaccessibility ranged from 5 to 36% with an average bioaccessible fraction of 34% in As-contaminated soils of anthropogenic and geogenic origin (Juhasz et al., 2007b). Meunier et al. (2010) reported that the average percent As bioaccessibility was relatively low at 13% (range from 0.1 to 49%, median 8.4%) in gold mining areas of Canada. Gastrointestinal average bioaccessibility values (determined on the o250 mm fraction) of As for CCA-contaminated sandy soils with moderate to high organic content, were reported to be 45.8% with bioaccessible As ranged from 30.9 to 51.2% (Guney et al., 2010). As the first measure of the effect of soil properties on soil As bioaccessibility, we correlated total soil As contents with bioaccessible As extracted with SBET method (Fig. 2). Total As was positively and significantly (p o0.05) correlated with bioaccessible As. The result was consistent with Juhasz et al. (2007b), Sarkar et al. (2007) and Meunier et al. (2010), but contradicted with Pouschat and Zagury (2006), Girouard and Zagury (2009) and Ono et al. (2012). Bioaccessible As ranged from 5.7 to 46.3% of total As (Table 2), with a clear increase in bioaccessible As with increasing total As content in the soils. This trend may be due to the decreasing sorption capacity at higher contaminant levels, where progressively weaker sorption binding sites are accessed as the As loading increases (Meunier et al., 2010). Despite the relatively low percent (average 28.0%) of bioaccessibility, bioaccessible As levels range from 0.4 to 193 mg kg  1 (Table 2). The higher end of this range needed remedial measures to address the potential human health risks from soil exposure. Selection of other soil physiochemical properties i.e., clay content, TOC, soil pH, Fe and Mn contents of soil was based on numerous studies that have been reported to be important soil parameters in controlling As bioaccessibility in contaminated soils (Yang et al., 2002; Juhasz et al., 2007b; Sarkar et al., 2007;

Fig. 2. Linear regression analysis between total As content and As bioaccessibility of As-enriched soils from Ambagarh Chauki block, Chhattisgarh, India.

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Meunier et al., 2010). In the present study, statistically significant negative correlations were observed between As bioaccessibility and soil properties (clay content, TOC and Fe contents of soil) (Fig. 3). Clay-sized particles such as hydrous Fe, Mn, and Al oxides are usually characterized by large specific surface area and internal porosity that may act as a potential contaminant immobilizer in the internal pore network, thereby reducing As bioaccessibility (Sarkar et al., 2007). The inverse relation of As bioaccessibility with TOC was not consistent (Sarkar et al., 2007; Meunier et al., 2010). However, organic carbon content of

Fig. 3. Linear regression analysis between As bioaccessibility and (a) Clay content, (b) TOC, (c) soil pH, (d) Fe content and (e) Mn content of As-enriched soils from Ambagarh Chauki block, Chhattisgarh, India.

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the soil is a major contributor to the overall negative charge in soils and thus is an important sorbent for heavy metal cations (Sparks, 1995). Soil organic carbon has the ability to form strong bonds with As with the metal not being readily released, thereby reducing its bioaccessibility. Soil pH was another important factor governing As bioaccessibility, yielding generally lower bioaccessibility at lower pH (Yang et al., 2002). Positive significant (po0.05) correlation was observed between bioaccessible As and soil pH (Fig. 3), which was in accordance with Yang et al. (2002). In contrast, Juhasz et al. (2007b) observed non significant correlation between bioaccessible As and soil pH. A general relationship between As bioaccessibility, pH and Fe content is not surprising as several studies reporting the importance of these soil parameters in controlling As sorption in soils are available (Yang et al., 2002; Juhasz et al., 2007b; Meunier et al., 2010). The soil pH alters surface charge of colloidal material such as Fe oxides (Smedley and Kinniburgh, 2002). In general, a low soil pH favors the adsorption of As onto Fe oxides due to their high positive surface charge. A rise in pH increases the negative potential on the adsorption surface of Fe oxides, impacting As adsorption and bioaccessibility (Yang et al., 2002). In our study, negative significant (po0.05) correlation was observed between As bioaccessibility and Fe content of soil (Fig. 3). The inverse relationship between bioaccessibility and Fe content may be attributed to the transformation of labile As species to lesssoluble mineral phases by complexation with Fe oxyhydroxides. The finding of Acharyya et al. (2005) that As sorbed in hydrated iron oxide (HFO) that preferably occurs in acidleachable fraction and possibly as coatings on kaolinite, illite and goethite in soil of Ambagarh block, supported the hypothesis. Yang et al. (2002) reported that the pH and Fe oxide content can explain nearly 75% of the variability in As(V) bioaccessibility. Juhasz et al. (2007b) reported that total As contents, as well as total and free Fe contents, were the factors best describing As bioaccessibility in soil. 3.3. Chemical daily intake of As and risk characterization Assessment of exposure risk to incidental soil ingestion can be based on bioaccessibility test (Guney et al., 2010; Lu et al., 2011). In our study, the CDI from incidental soil ingestion was calculated based on SBET method. The CDI for children exposed to Asenriched soil for assessment of carcinogenic risk ranged from 4.4E  04 to 2.5E  02 mg As kg  1 body weight d  1 (mean value of 5.9E  03 mg As kg  1 body weight d  1) assuming an average time of 70  365 days (entire life-span). However for non-carcinogenic risk assessment, CDI ranged from 6.2E  03 to 3.5E 01 mg As >kg  1 body weight d  1 (mean value of 8.3E 02 mg As kg  1 body weight d  1), assuming an average time of 5  365 days (total exposure duration) (Table 3). The results were more comparable with Guney et al. (2010) who reported CDI for carcinogenic risk and non-carcinogenic risk assessment for As ranged from 4.57E  04 to 8.99E  03 mg As kg  1 body weight d  1 and 6.40E 03 to 1.26E 01 mg As kg  1 body weight d  1, respectively for children exposed to contaminated soils from playground, park and picnic area of Turkey. It is interesting to note that soil with high total As content had more CDI value, which is also conformed from Eq. (3)as EPC is directly proportional to CDI. Risk characterization has been done on chronic basis by calculating carcinogenic risk and HI values for As using a slope factor of 1.5 mg kg  1 d  1 and reference dose values of 0.3 mg kg  1 d  1 according to U.S. EPA (2007b). The estimated value of carcinogenic risk is the probability of an individual developing any type of cancer from lifetime exposure to carcinogenic hazards. The acceptable or tolerable risk for regulatory purposes is in the range of 1  10  6–1  10  4 (Hu et al., 2011).

Table 3 Exposure assessment and risk characterization for As in As-enriched sites for children 2–6 years old. Sampling sites

Hauditola Bandha bazar Maldongri Manzhi tola Meregaon Thailitola Joratarai Kaudikasa

Exposure assessment

Risk characterization

Chemical daily intake (mg kg  1 body weight d  1)

Carcinogenic risk

Hazard index

Carcinogenic

Non-carcinogenic

5.2E  05 4.4E  04

7.3E  04 6.2E  03

7.8E 08 6.7E 07

0.002 0.021

6.1E  04 7.3E  04

8.6E  03 1.0E 02

9.2E 07 1.1E 06

0.029 0.034

1.5E  03 5.0E  03 8.2E  03 2.5E  02

2.1E  02 6.9E  02 1.2E  01 3.5E  01

2.3E 06 7.4E 06 1.2E 05 3.8E 05

0.070 0.231 0.384 1.175

The carcinogenic risk levels of As for children (2–6 years age group) of Ambagarh block ranged from 7.8E 08 to 3.8E 05 (average 5.9E 06) (Table 3). The risk values of Hauditola, Bandha bazaar and Maldongri were higher than 1  10  6, indicating carcinogenic risks of these localities cannot be acceptable, whereas the carcinogenic risks of Manzhi tola, Meregaon, Thailitola, Joratarai and Kaudikasa were lower than 1  10  6, indicating that carcinogenic risk of As due to incidental soil ingestion can be acceptable in those areas and the potential health risk for children could not be overlooked. It has been reported that around 10% of the population of the village Kaudikasa are suffering from As-borne disease (Shukla et al., 2010). The carcinogenic risk value stated in the present study was found lower than the cancer risk calculated by U.S. EPA (2008) where mean risk was 4.2  10  5 for children playing on play sets and decks in warm climate and 2.0  10  5 for cold climate. However, risk values of this study were higher than Dube et al. (2004) who reported a risk of 4.9  10  7 for children (2–6 years old) exposed to CCA-treated wood, using a reasonable maximum exposure value for ingestion, dermal and inhalation exposure pathways. Our results could be more comparable with Guney et al. (2010) who reported carcinogenic risk levels of As for children from 2 to 6 years age group ranged from 6.9  10  7 to 1.4  10  5 with an average of 4.6  10  6, following soil ingestion by children exposed to playground, park and picnic areas. For the non-carcinogenic risk, HI values for As, under incidental soil ingestion scenarios exceeded 1.0 in Kaudikasa area only, while other As-enriched sites of Ambagarh block were less than 1.0 (Table 3), indicating that the toxic risk for As was well below the dangerous level. However, keeping in mind that background exposure for As from other sources is 0.3 mg kg  1 d  1, which is equal to reference dose used in HI calculations (Guney et al., 2010), additional As uptake from contaminated soils would lead to a combined HI value more closer to 1.0. In general, if the value of HI is more than 1, it is believed that there is a chance that noncarcinogenic effects may occur, with a probability which tends to increase as the value of HI increases (U.S. EPA, 2001). Factors such as soil ingestion rate, body weight, exposure frequency and bioaccessibility greatly influences the carcinogenic and non-carcinogenic risk assessment in contaminated sites. However, the soil ingestion rate is the factor with the highest uncertainty due to the significant limitations in methodologies used in ingestion studies such as bias in sample selection, poor representation of target populations in terms of race, ethnicity and socioeconomic situation, low reproducibility and limited or absent quality control and assurance (U.S. EPA, 2008; Guney et al., 2010). Therefore, more accurate values for soil ingestion rates are needed for better risk assessment. Body weight is another important factor which fluctuates depending on many factors.

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For example, females tend to weigh less than males and their corresponding average (U.S. EPA, 2008; Guney et al., 2010). Likewise it is evident that setting limits for maximum allowed As values in soils considering only the total As content may overestimate the HI on a risk assessment and generate additional costs in remediation strategies of the contaminated sites. Therefore, bioaccessible values instead of total values should be used in risk assessments for soil ingestion, the main route of exposure for children.

4. Conclusion This study demonstrated the importance of considering sitespecific bioaccessible As data rather than total As data for risk calculations. Soil properties like TOC, clay content, pH and Fe content of soil had a significant impact on As bioaccessibility. The elevated bioaccessible As contents reported for soils in the present study highlight potential risks to children under incidental soil ingestion scenario, and even the low percent As bioaccessibilities may reflect carcinogenic risks where total As is present in high concentrations. The combination of oral bioaccessibility and the health risks assessment will help us to get more reasonable information for risk management of As in tropical contaminated (anthropogenic and geogenic) soils. However, further studies including a wider variety of soil conditions and soil types are needed to validate and generalize a model. The introduction of human bioaccessible concentrations into risk estimations can give more realistic indications for the health risk assessment, leading to better applicable guidelines for soil remediation.

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