Geochemical study of arsenic concentrations in groundwater of the Meghna River Delta, Bangladesh

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Abstract

The geochemistry of groundwaters from the Meghna River Delta of Bangladesh was studied to determine the distribution of arsenic, and to evaluate the potential mobility of As from the host sediments to the groundwater. Groundwaters from tube wells are characterized by pH of 6.5 to 7.5, oxidation reduction potential (ORP) of + 52 to − 2 mV, Ca contents of 7 to 25 mg/l, and Fe contents of 0.01 to 22.2 mg/l. Arsenic concentrations ranged from 6 to 934 μg/l, with an average value of 347 μg/l. A moderate positive correlation exists between As and Fe (r2 = 0.514; p < 0.0001) concentrations in groundwater. Leaching tests under both acidic (1 M HCl) and alkaline (0.5M NaOH) conditions were carried out on core samples collected from the Meghna Plain, Brahmaputra Plain and Madhupur Tract. The results show that As concentrations in acid leachates from silt, clays and peat were 0.021 to 0.212 mg/kg, compared to 0.009 to 0.140 mg/kg from sands. Extracts from alkali treatment contained 0.015 to 0.458 mg/kg and 0.007 to 0.054 mg/kg, respectively. Average amount of As leached under acidic and alkaline conditions were 0.79% and 0.64% from silt, clays and peat, and 0.72% and 0.45% from sands, respectively. Greater amounts of solid phase Fe and Ca were extracted under acidic conditions, probably due to dissolution of oxide coatings on sediment particles and of carbonate phases. Significant amounts of As were extracted by both extraction media, but Fe and Ca concentrations in the alkaline extracts were low. When alkaline conditions prevail, As can be released from organic fractions in sediments even if dissolution of Fe and Ca is low.

Introduction

High concentrations of naturally-occurring arsenic have been found in groundwater in many Asian countries, including Bangladesh, India, Nepal, Cambodia, Thailand, Vietnam, China, and Taiwan (Acharyya et al., 1999, Smedley and Kinniburgh, 2002, Bhattacharyya et al., 2003, Ahmed et al., 2004). Arsenic contamination of groundwater is now a serious hazard among the many environmental issues in Bangladesh. There has been increasing concern over the safety of groundwater from shallow aquifers in the Ganges, Brahmaputra and Meghna (GBM) Delta, where As levels exceed the World Health Organization (WHO, 2004) guideline of 10 μg/l and the 50 μg/l limit adopted in Bangladesh. Even higher concentrations of As (up to 1460 μg/l; Tareq et al., 2003) have been reported from groundwater in the Meghna Floodplain of southeast Bangladesh (e.g. Breit et al., 2001, Anawar et al., 2003, Bibi et al., 2006).

Rural dwellers comprised 97% of the population of Bangladesh in the early 1970s. The rural population used to drink water from hand-dug wells and ponds, and nearly a quarter of a million children died each year from water borne diseases (World Bank Group, 1999). By 1993, more than 4.5 million shallow and deep tube wells had been drilled in an effort to provide safe drinking water for the people. However, following the first discovery of As contamination in the Chapai Nawabganj district of northwest Bangladesh in 1993 (Hoque et al., 2000), there has been increased awareness of high As concentrations in groundwater.

The incidence of As contamination in the Chandpur district is among the greatest in Bangladesh, as waters from 90% of the tube wells in the area exceed the Bangladesh As groundwater standard of 50 μg/l (BGS and DPHE, 2001). According to Islam (2003), about 100,000 tube wells in the counties of the Chandpur district (Chandpur Sadar, Matlab, Haziganj, Kachua, Haimchar, Shahrasti, and Faridganj) are highly contaminated with As, and over three thousand patients affected by arsenic were identified in the Chandpur district alone. Considering this present situation, our study was conducted in this district. Groundwater and core sediment samples were collected from the Matlab, Chandpur Sadar and Haziganj counties in Chandpur district. Core sediment samples were also collected from Brahmaputra Plain and Madhupur Tract in Bangladesh for a comparative study. The objectives of this study are to determine the chemical compositions of groundwater, with emphasis on concentrations of As in the groundwater of the aquifers and their significance, and to clarify the characteristics of solid phase As, Fe, and Ca extracted from the sediment matrices.

Section snippets

Location

Bangladesh is a low-lying country at the head of the Bay of Bengal, and most lies within an elevation of 20 m above sea level. Most of Bangladesh consists of low alluvial, deltaic and coastal plains. Our study sites are in the Chandpur district of southeastern Bangladesh (Fig. 1). The Chandpur district lies between 23°N to 23°30′N and 90°32′E to 91°2′E, about 65 km southeast of the capital, Dhaka. Total area of the district is about 1671 km2. The Meghna River flows through the district from

Physico-chemical properties of the water samples

The average groundwater temperature was around 26.5 °C, and pH values were approximately neutral, within a range of 6.5 to 7.5. EC ranged from 441 to 3650 μS/cm with an average of 1341 μS/cm, and higher concentrations of arsenic occurred in samples with conductivities between 1359 and 3650 μS/cm (Table 1). COD values ranged between 8 and 50 mg/l, but most fell in the range 15 to 45 mg/l. ORP values varied from + 25 to − 2 mV and 20 to 52 mV in the shallow and deep aquifers, respectively. These

Characteristics of As release in groundwater

Regression analyses were carried out between arsenic and other water parameters. Results are shown in Fig. 4. The relationship between As in groundwater and ORP is clearly inverse (r2 = −0.6; p < 0.0001; Fig. 4a). Arsenic was therefore released only when ORP values decreased, suggesting that reducing conditions of the environment are conducive to As mobilization. Akai et al. (2003) also reported that arsenic was dissolved into water when reducing conditions were achieved by microbial activity.

Conclusions

The results show that groundwaters from the shallow aquifers in the Chandpur region are characterized by neutral pH, low ORP, high COD, and high concentrations of As and Fe. Arsenic concentrations in groundwaters are highly variable (6–934 μg/l), and many exceed the maximum permissible limits of the WHO (10 μg/l) and Bangladesh (50 μg/l) for drinking water. The most contaminated aquifers in the Meghna River Delta are confined to a depth of around 15 to 50 m. Elevated As concentrations are

Acknowledgements

We thank Prof. Y. Sampei and Prof. Y. Sawada of Shimane University for access to the AAS and XRF facilities, respectively. Dr. Barry Roser of Shimane University is acknowledged for his constructive review and helpful comments, which improved the manuscript considerably. Thanks are also due to Prof. M. A. Sattar of Bangladesh Agricultural University, Mymensingh, Bangladesh for his cooperation in sampling. This study formed part of an M.S. program in Shimane University, and was financially

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