Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh: an overview

Abstract

Arsenic in the groundwater of Bangladesh is a serious natural calamity and a public health hazard. Most groundwater from the shallow alluvial aquifers (<150 m), particularly in the Holocene plain lands, are vulnerable to As-enrichment. Delta plains and flood plains of the Ganges–Brahmaputra river system are moderately to severely enriched and more than 60% of the tube wells are affected. Shallow aquifers in the Meghna river basin and coastal plains are extremely enriched with more than 80% of the tube wells affected. Aquifers in the Pleistocene uplands and Tertiary hills are low in As. The vertical lithofacies sequence of the sediments from highly enriched areas of the country show two distinct lithofacies associations—a dominantly sandy channel-fill association and a fine-grained over bank association. The sediments can be grouped into 4 distinct lithofacies, viz. clay, silty clay, silty sand and sand. Thin section petrography of the As-enriched aquifer sands shows that the sands are of quartzolithic type and derived from the collision suture and fold thrust belt of the recycled orogen provenance. Groundwater is characterized by circum-neutral pH with a moderate to strong reducing nature. The waters are generally of Ca–Mg–HCO₃ or Ca–Na–HCO₃ type, with HCO₃⁻ as the principal anion. Low SO₄²⁻ and NO₃⁻, and high dissolved organic C (DOC) and NH₄⁺ concentrations are typical chemical characteristics of groundwater. The presence of dissolved sulfides in these groundwaters indicates reduction of SO₄. Total As concentration in the analyzed wells vary between 2.5 and 846 μg l⁻¹ with a dominance of As(III) species (67–99%). Arsenic(III) concentrations were fairly consistent with the DOC and NH₄⁺ contents. The HNO₃ extractable concentrations of As in the sediments (0.5–17.7 mg kg⁻¹), indicate a significant positive correlation with Fe_NO3, Mn_NO3, Al_NO3 and P_NO3. The concentrations of S_NO3 (816–1306 mg kg⁻¹) peaked in the clay sediments with high organic matter (up to 4.5 wt.%). Amounts of oxalate extractable As (As_ox) and Fe (Fe_ox) ranged between 0.1–8.6 mg kg⁻¹and 0.4–5.9 g kg⁻¹, respectively. Arsenic_ox was positively correlated with Fe_ox, Mn_ox and Al_ox in these sediments. Insignificant amounts of opaque minerals (including pyrite/arsenopyrite) and the presence of high As contents in finer sediments suggests that some As is incorporated in the authigenically precipitated sulfides in the reducing sediments. Moreover, the chemical extractions suggest the presence of siderite and vivianite as solid phases, which may control the aqueous chemistry of Fe and PO₄³⁻. Reductive dissolution of Fe oxyhydroxide present as coatings on sand grains as well as altered mica (biotite) is envisaged as the main mechanism for the release of As into groundwater in the sandy aquifer sediments.

Introduction

Arsenic enrichment in Bangladesh groundwater is considered to be one of the greatest current environmental disasters in the world. Since the first detection of As in Bangladesh groundwater, a number of studies have been conducted which provide substantial data and discussion about the extent and distribution of the problem. These data have been acquired by both field test kits (UNICEF/DPHE, BAMWSP, 2002, pers. comm.) and laboratory analyses (NRECA, 1997, SOES/DCH, 2000, BGS and DPHE, 2001, UNICEF and INFS, 2000) (acronyms and other abbreviations used in the text are presented in the box Nomenclature). The largest database on the occurrence of As in groundwater is available from the DPHE/UNICEF field kit survey where 29% of the tested 50,998 wells exceeded the BDWS (50 μg l⁻¹). Complete screening of more than one million wells in 64 upazillas using field kits by BAMWSP, UNICEF and DPHE showed that 53% of the tested wells exceeded the BDWS (BAMWSP, pers. comm.). The higher percentage of wells exceeding the BDWS in this survey can be explained by the fact that known high risk areas were included for this study.

The SOES and DCH hold the largest database of laboratory analyses of more than 34,000 water samples where 37% samples exceed the BDWS and 56% samples exceed the WHO provisional drinking water limit (10 μg l⁻¹) (SOES/DCH, 2000, pers. comm.). BGS and DPHE (2001) conducted the most systematic national survey where 3534 wells from different parts of the country were analyzed. The results indicate that nearly 27% of the shallow wells (<150 m) contain As concentrations above the BDWS, while 46% exceed the WHO provisional drinking water limit. Although the number of deep tubewells (>150 m deep) is relatively small, only 1 and 5% of the analyzed groundwaters exceeded the BDWS and WHO limit, respectively. Analysis of nearly 6000 samples by UNICEF/INFS show that 23% of wells exceed BDWS and 42% samples exceed WHO limit (UNICEF and INFS, 2000). Estimates of major national surveys showed that about one third of the tested wells exceeded the BDWS. Accordingly, it is approximated that nearly 3 million tubewells, placed at depths of between 10 and 50 m, yield groundwater with As concentrations exceeding the BDWS.

In the present paper, the authors present an overview of the nature and extent of As enrichment in groundwater of Bangladesh with reference to the geological and geomorphological characteristics of the Bengal Basin. Emphasis has been laid on discussion of the relationship between As-enriched groundwater with the hydrogeological setup of the basin, hydrogeochemical characteristics and the sedimentological, provenance and geochemical characteristics of the aquifer sediments.