Arsenic is a very toxic metalloid. It has a serious impact on the human health. Several million people are at risk from arsenic (As) contaminated drinking water in India (Chatterjee et al. 1995), Bangladesh (Bhattacharya et al. 2002) and China (Wang et al. 2003). The problem of As contamination groundwater of Bangladesh has been considered as dreadful proportion as nearly 75 million people are now at high risk (Chatterjee et al. 1995). The ground water As concentration is recorded from <2.5 to 846 μg L−1 in Bangladesh. The survey of Frisbie et al. (2002) revealed unsafe levels of manganese (Mn), lead (Pb), nickel (Ni), and chromium (Cr) in drinking water of Bangladesh. Norra et al. (2005) reported the average concentration of As, zinc (Zn), copper (Cu), lead (Pb), and organic carbon (OC) in rice field (irrigated with As rich groundwater) as 101 mg kg−1, 38.2 mg kg−1, 48.3 mg kg−1, 22.8 mg kg−1, and 1.4%, respectively in West Bengal, India. The concentrations of Cd, Cu, Pb, and Zn were found in the range of 0.01–0.12 mg kg−1, 0.10–2.90 mg kg−1, 0.02–0.23 mg kg−1, and 0.51–3.35 mg kg−1, respectively in soil of Lagos, Nigeria (Awofolu 2005). Soil Cd range was recorded from 0.46 to 1.04 mg kg−1 in some parts of China (Dong et al. 2001). Arsenic and trace metal rich ground waters have been using for irrigation in Bangladesh for several decades. So, the top soil of Bangladesh may be at high risk of contamination from arsenic.

The inorganic form of As is toxic to human being. Arsenic and trace metal can inter into the plants from soil and hence in food chain from contaminated soil and may infect human being as well. The terrestrial plants can accumulate a large amount of arsenic (inorganic form) from soils and transfer it to the aboveground biomass (Zhang et al. 2002). On the other hand, marine plants and animals have arsenic detoxification system and for this reason a large amount of arsenic in marine organisms is found in organic forms, such as arsenosugars in algae, and arsenobetaine and arsenocholine in fish, mollusks, and crustaceans (Francesconi et al. 1994). However, terrestrial plants do not have arsenic detoxification system and this is perhaps the reason why inorganic arsenics are predominant in terrestrial plants (Mattusch et al. 2000). A few studies have been conducted to find out the distribution of As and other trace metals in floodplain agricultural soil of Bangladesh. So, the current study has been taken to get an idea about the magnitude of soil contamination with As, Cu, Pb, Mn, Fe, Zn, Co, Se, Hg, Cd, Cr, and Ni in floodplains agricultural soil of two regions [Faridpur (FD) and Dhamrai (DM)] of Bangladesh.

Materials and Methods

The surface soils samples were collected from floodplain agricultural fields of Faridpur (l23°10′ to 23°40′ N and 89°35′ to 90°10′ E) and Dhamrai (23°50′ to 24°05′ N and 90°00′ to 90°20′ E) during dry season (15–29th December, 2006). Ground water has been using for irrigation in dry season in these areas.

Composite soil samples (about 500 g each for different analysis) were collected at a depth of 0–3 cm level. The area of each sampling field was between 0.5 and 5 ha. Samples were transferred into zip locked polythene bags for analysis with proper labeling. Soils samples were dried in room temperature and sieved with a sieve of <2 mm fraction (ISO/DIS 11464). After that, the samples were transferred into zip locked polythene bags. All the samples were then transferred to University of Cadiz, Spain for chemical analyses.

About 0.5 g dried sample was transferred into HP 500 vessel for total digestion. After that 10 mL of deionised water (milliQ water) was added. A mixture of 5 mL of HNO3, 4 mL of HF, and 1 mL of HCL was also added. The vessels were then inserted into a microwave (CEM MARS 5) and heated at 210°C for 20 min at 175 PSI pressure. After cooling the vessels at room temperature, the caps of the vessels were opened slowly and 30 mL of boric acid solution were added in each vessel. Then the samples were again heated at 210°C for 5 min at 175 PSI pressure. After cooling the digested samples were transferred into a volumetric flask and made up to the final volume (50 mL) with deionised water. The samples were then transferred into plastic containers and kept in refrigerator at about 4°C prior to analysis. All reagents were analytical grade or Suprapur quality (Merck, Germany). Milli-Q water (Millipore, Bedford, MA, USA) was used in all experiments. Cleaning of plastics and glassware was carried out by soaking in 14% (v/v) HNO3 for 24 h and then rinsing with water.

All digested samples were analyzed for As, Hg, Pb, Co, and Se by ICP-MS and Fe, Mn, Cr, Cu, Zn, and Ni by ICP-AES. Cadmium was determined by GF-AAS. Reactive blanks and reference material (CRM 478 BCR) were used to ensure the quality control. The concentration was calculated on a dry weight basis. The organic carbon concentration was determined by using El Rayis (1985) method.

Statistical analyses were performed on a personal computer using Microsoft Excel programme. A probability level of p < 0.05 was considered for statistically significance.

Results and Discussion

The present study showed that in the soil of Faridpur (FD) and Dhamrai (DM) the average As concentration were 33.15 mg kg−1 and 6.10 mg kg−1, respectively (Tables 1 and 2). The positive correlations of As with trace metals (Cu, Pb, Mn, Fe, Zn, Co, Se, Hg, Cd, Cr, and Ni) were found both in Faridpur and Dhamrai. The average As concentration in Faridpur was more than three times higher than the world standard (10 mg kg−1) for soil and 100% of the samples exceeded the world’s guideline. This value is also much higher than the average As (13.3 mg kg−1) of Samta village one of the As rich areas of Bangladesh (Alam et al. 2003).

Table 1 Arsenic and trace metals in soil of Faridpur (FD), Bangladesh with comparison to Dutch and World limit
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Table 2 Arsenic and trace metals in soil of Dhamrai (DM), Bangladesh with comparison to Dutch and World limit
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The ground-water of Faridpur is highly contaminated with As (Frisbie et al. 2002) and the contaminated water has been using for irrigation for long time. The present study indicated that Faridpur soils contained five times more As than Dhamrai soil. This may be occurred due to use of As rich groundwater for irrigation of rice. The mean concentration of Zn was estimated 97.24 mg kg−1 in soil of Faridpur, whereas Zn concentration was recorded 98.85 mg kg−1 in soil of Dhamrai (Tables 1 and 2). However, the average Zn was higher than the world standard for Zn (90 mg kg−1) and 87% of the samples exceeded that level. Arsenic and Zn correlation (r = 0.0136) was relatively poor in Faridpur soil, whereas that correlation (r = 0.84) was significant in Dhamrai (Tables 3 and 4).

Table 3 Correlation coefficient matrix for soil parameters of Faridpur (FD), Bangladesh
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Table 4 Correlation coefficient matrix for soil parameters of Dhamrai (DM), Bangladesh
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This finding indicates that Zn concentration may effect the As fixation in the soil. The mean concentrations of Zn both in Faridpur and Dhamrai soils were lower than the Dutch standard for Zn (140 mg kg−1). This value is also lower than the Zn concentration in soil Nigeria (Awofolu 2005). Zinc is an essential element for human health.

The present study found the average Mn concentrations were 449.68 and 553.75 mg kg−1 in the soil of Faridpur and Dhamrai, respectively (Tables 1 and 2). Average Mn is lower than the limit of the world standard in both areas. However, the average Mn of these two areas is higher than the Mn (386 mg kg−1) in paddy soil of China (Wang et al. 2003).

Manganese correlated significantly with As (r = 0.86) in Dhamrai soil, whereas that correlation (r = 0.0261) was insignificant in Faridpur soil (Tables 3 and 4). The poor correlation of Mn with As in Faridpur may play a subordinate role in the fixation of As in soil because oxidized Mn phases are well known for their ability to adsorb high amounts of As (Chiu and Hering 2000).Selenium concentrations in Faridpur and Dhamrai soils were 1.04 and 1.01 mg kg−1, respectively (Tables 1 and 2) and 100% of samples of the both the areas were under the world standard of Se (7 mg kg−1). Selenium and arsenic correlations of FD (r = 0.0420) were very poor and insignificant in comparison to that of DM (r = 0.94) (Tables 3 and 4). The average Se in soil of Faridpur and Dhamrai were also very lesser than the average Se (0.25 mg kg−1) of China (Wang et al. 2003). Selenium is an important element of the human body which prevents the cytotoxic effect of As. The average Se contents (0.003 μg L−1) in Bangladesh groundwater is very low in comparison to WHO limits (0.010 μg L−1) (Frisbie et al. 2002).This general deficiency of Se both in soil and water can decreases the Se level in human body and hence increases the intensity of arsenic related diseases.

The mean concentrations of Fe in soil of Faridpur and Dhamrai were found 48.37 and 37.00 mg g−1, respectively (Tables 1 and 2). A positive insignificant correlation between As and Fe were obtained both in Faridpur (r = 0.36) and Dhamrai (r = 0.39). Norra et al. (2005) reports that Fe- oxides/hydroxides may play a significant role in fixation of As but the present study does not show this tendency. However, soil texture, crystallinity, pH, conductivity and total alkalinity also play significance role in this regard. The average Ni concentrations were 48.85 and 46.09 mg kg−1 in Faridpur and Dhamrai, respectively (Tables 1 and 2). Nickel concentration (both FD and DM) exceeded the Dutch standard (35 mg kg−1) but did not exceed the world limit (50 mg kg−1). The average Ni in Faridpur and Dhamrai were lower than Ni (25.5 mg kg−1) of paddy soil in China (Wang et al. 2003). Relatively strong positive significant correlations of Ni with As (r = 0.79) were obtained in Dhamrai, whereas the correlation of Ni and As (r = 0.0003) was insignificant in FD (Tables 3 and 4).

Copper concentrations in Faridpur and Dhamrai were 48.42 and 31.84 mg kg−1, respectively which exceeded the world limit for Cu (30 mg kg−1) in soil (Tables 1 and 2). These values are also higher than the average Cu concentration (30 mg kg−1) of Samta Village, Bangladesh (Alam et al. 2003). However, the average Cu of Faridpur agricultural soil is nearly similar to the Cu concentration (48.3 mg kg−1) of rice field soil of neighbouring West Bengal, India (Norra et al. 2005). The correlation coefficient of Cu with As was insignificant (r = 0.3140 and 0.74) both in Faridpur and Dhamrai. On the other hand, the average Cd in Faridpur (0.153 mg kg−1) and Dhamrai (0.155 mg kg−1) were nearly same and did not exceed the Dutch standard (0.8 mg kg−1) and world limit (0.35 mg kg−1) as well (Tables 1 and 2). The average Pb (26.82 mg kg−1) and Hg (0.081 mg kg−1) of Faridpur soil did not exceed the limit of Dutch standard for good soil and world limit as well (Table 1). Lead of Dhamrai floodplain soil did not exceed the Dutch standard and world limit but Hg (0.78 mg kg−1) was found higher than Dutch regulation (Table 2). The average Pb concentration in both Faridpur and Dhamrai did not exceeded the average Pb (28 mg kg−1) of Samta village, Bangladesh (Alam et al. 2003). However, Pb in Faridpur and Dhamrai soil is slightly higher than Pb (22.8 mg kg−1) in agricultural soil (paddy field) of West Bengal, India (Norra et al. 2005).

The present study also reported that Cr (85.95 mg kg−1) concentration in Faridpur soil was higher than the world limit (70 mg kg−1) and 100% samples exceeded that limit (Table 1), whereas Cr level in Dhamrai (76.08 mg kg−1) was lower than Dutch regulation but higher than the world standard. Average Co was nearly same both Faridpur (17.40 mg kg−1) and Dhamrai (16.41 mg kg−1). Copper concentration has been exceeded the world limit in both areas as well (Table 2). The concentrations of soil Cr and Co in DM and FD were also higher than the concentration of Cr and Co in rice field of China (Wang et al. 2003).

So, the most important finding of the present study is that the soil of Faridpur contains nearly four times higher As than the world limit and more than five times higher As than the soil of Dhamrai. This study also predicts that the area which has As contaminated ground water may also contain high level of As in the agricultural soil due to irrigation with ground-water. However, further studies are essential to draw the final conclusion in this regard. More studies are also required to determine the concentration of As and trace metals in agricultural soil of other areas of Bangladesh. It is also important to find-out the mechanisms of arsenic and trace metals uptake, translocation and transformation in rice and vegetable plants.