Erik Romijn and Alexander Zaporozec
8.1 Arsenic contamination in groundwater in Bangladesh Jan Nonner
8 . 1 . 1 P r o b l e m d e f i n i t i o n
In Bangladesh, the contamination of shallow groundwater with arsenic has reached critical levels.
The Government of Bangladesh became first aware of the problem in 1993, and ever since, reports on illnesses and poisoning by drinking of arsenic-containing groundwater have been increasing.
By 1997, the affected areas comprised the western and southwestern part of the country, and alarming reports were also received from the Sylhet area in the northeastern part of Bangladesh.
8 . 1 . 2 H y d r o g e o l o g i c a l s e t t i n g
More than 75 percent of the Bangladesh area is made up of deltaic, alluvial, and marshy deposits (Fig. 8.1.1). Residual deposits associated with old land surfaces (II) and areas of consolidated sedimentary rock (I) are located in the isolated central and northwestern parts of the country, and in the east, respectively. The deltaic (IV) and alluvial (III) deposits contain numerous layers of sand with occasional gravel, which form good aquifers that are exploited on a large scale by municipalities and smaller communities. Groundwater, which is pumped from these deposits using deep and shallow wells, contains arsenic that is, supposedly, derived from the arsenic-containing mineral-rich sediments deposited in the area by the rivers. The Ganges River appears to be one of the main sources of these sediments. The arsenic contained in the mineral-rich sediments is therefore of natural origin, but there has been speculation that it may have been released into groundwater as a result of human interferences in the area including pumping, dewatering, and reservoir construction.
8 . 1 . 3 E x i s t i n g i n v e n t o r y o f t h e a r s e n i c s o u r c e
The inventory of arsenic-affected areas in Bangladesh has been done on a rather unplanned basis.
Water sampling for arsenic has concentrated on wells in communities where a lot of cases of arsenic poisoning caused concern with the local health officers. Understandably, the local and national press has played a role in focusing attention on these communities. The uncoordinated sampling programs have caused the information on arsenic to be only partly available at a large number of organizations including the Department of Public Health Engineering (DPHE), the Universities in Rajshahi and Dhaka, consultancies, and non-governmental organizations (IIIHEE et al., 1997). Some of these organizations have produced maps showing percentages of well samples with elevated arsenic concentrations arranged per district.
8 . 1 . 4 A n e w i n v e n t o r y a p p r o a c h
An additional systematic groundwater sampling program in the affected areas in Bangladesh was carried out in 1998. A total of just over 2,000 shallow and deep wells were sampled on a ‘thana’
(municipality) basis. Using a suitable geographical grid, 5 to 10 wells per thana were system-atically selected, and subsequently visited. The sampling density varied between 30 and 40 km2 per sample. A protocol describing georeferencing procedures, use of water sample collection forms, bottling techniques, etc was developed and used. Samples collected were primarily analysed by the DPHE for arsenic, iron, and hardness (DPHE, 1997). Cross-checking of sample analyses was carried out by the British Geological Survey in Wallingford, England. The storage and processing of hydrogeological and hydrochemical data including arsenic was done with the help of the Visual Foxpro data base code, which was linked to ArcView for the generation of layers of various parameters. With all data being stored on CD-ROM, the procedures that followed formed a sound basis to present and analyse the results in a professional manner.
8 . 1 . 5 P r e s e n t a t i o n o f r e s u l t s
Results of the inventory have been presented as plain text, tables, diagrams, and in particular, maps. Diagrams and maps have been compared with each other to find correlations between the distribution of groundwater rich in arsenic and geological formations, groundwater chemistry, groundwater use, etc. For example, the maps in Fig. 8.1.2 show the comparison between the
Groundwater contamination inventory FIGURE8.1.1 Geological units in Bangladesh (Source:British Geological Survey)
arsenic concentration in groundwater of the deeper geological formations and the concentration in shallower formations. Overlaying these maps against the map shown in Fig. 8.1.1, arsenic concentrations can also be compared with geological formations outcropping at the land surface.
The diagrams shown in Fig. 8.1.3 a) and b) present examples of relationships between arsenic FIGURE8.1.2 Comparison of arsenic levels in groundwater in the deeper and shallower aquifers
(Source:British Geological Survey/Mott MacDonald Ltd., 1999)
concentrations in groundwater and the redox potential (an essential parameter in groundwater chemistry) and arsenic concentrations and groundwater use, respectively. The map and diagram-comparative approach to compare groundwater contamination-related parameters has led to some essential findings for the Bangladesh case.
Groundwater contamination inventory
FIGURE 8.1.3 Relationships between a) arsenic concentration and redox potential and b) arsenic and gross groundwater abstraction (Source:British Geological Survey/Mott MacDonald Ltd., 1999)
8 . 1 . 6 C o n c l u s i o n s f o r t h e B a n g l a d e s h b a s i n
One of the essential conclusions related to the groundwater system in Bangladesh was that the arsenic-rich groundwater is confined to shallow geological formations (especially from 20 to 40 m below the land surface), and in particular, to the recent deltaic and alluvial deposits. Groundwater contaminated with arsenic is also positively correlating with low redox potentials (reducing conditions) (Fig. 8.1.3a), high iron concentrations, and generally low sulfate concentrations. No direct positive relation exists with groundwater use (Fig. 8.1.3b). Based on these comparisons and the results of sediment samples taken at wells in Bangladesh, the origin of the arsenic in groundwater was mainly attributed to the arsenic-containing iron-hydroxide coating on fine sand, which is deposited in the basin by the large Ganges, Brahmaputra, and Sylhet Rivers. Burial of the sediments causing transfer to reducing conditions stimulated the dissolution of the coatings, which released the arsenic into the groundwater.
R e f e r e n c e s
BRITISHGEOLOGICALSURVEY/MOTTMACDONALDLTD(1999). Groundwater Studies for Arsenic Contam-ination in Bangladesh.Phase I: Rapid Investigation Phase.
DEPARTMENT OFPUBLICHEALTHENGINEERING(1997).Working Paper on Arsenic in Drinking Water in Bangladesh and its Intervention.Government of Bangladesh, Dhaka, Bangladesh.
INTERNATIONAL INSTITUTE FOR INFRASTRUCTURAL, HYDRAULIC AND ENVIRONMENTAL ENGINEERING/ BRITISH GEOLOGICAL SURVEY/WORLD BANK(1997). Arsenic Contamination in Groundwater in Bangladesh, Status Review and Proposal for a Rapid investigative Phase.World Bank Fact Finding Mission, World Bank Office, Dhaka, Bangladesh.