Engineering, KTH, SE , Stockholm, Sweden 5 Ramboll, Box 17009,SE Stockholm, Sweden. Abstract

Transcription

1 ASSESSMENT OF HYDROGEOLOGICAL CONDITIONS FOR SAFE DRINKING WATER TO UNPRIVILEGED COMMUNITY IN REGIONS WITH HIGH ARSENIC GROUNDWATER AT MATLAB, BANGLADESH M Moklesur Rahman 1, A S M Woobaidullah 2, Kazi Matin Ahmed 2, M. Aziz Hasan 2, M Hossain 3, Prosun Bhattacharya 4, Mattias Von Bromssen 4, 5, Gunnar Jacks 4 1 Department of Petroleum and Mining Engineering, Jessore Science and Technology University, Jessore, Bangladesh. moklesh@gmail.com 2 Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh 3 NGO Forum for public Health 4 KTH-international groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, KTH, SE , Stockholm, Sweden 5 Ramboll, Box 17009,SE Stockholm, Sweden Abstract Bangladesh consists mainly of the Bengal Delta Plain (BDP) created by the Ganges-Brahmaputra-Meghna river system. Geogenic arsenic already has limited the usage of groundwater is primarily associated with the Holocene Alluvial aquifers of the BDP (BGS, 2001; Ravenscroft, 2001). Most of the shallow tubewells (depths 15m -50m) installed in Holocene aquifers is found to be highly As contaminated (Ahmed et al., 2004; van Geen et al., 2002, 2005). Matlab at Chandpur district in middle southeastern part of Bangladesh is one of such arsenic hotspots in Bangladesh. Many unprivileged people in Matlab have been suffering from serious arsenicosic disease. To abate this acute problem, an attempt has been taken to assess hydrogeological conditions for providing safe drinking water as a sustainable option. Therefore, all climatological data and geological data for that area were collected and intensively analyzed. The mean annual rainfall of the area is about 2000mm. Geological borelog data reflects that the subsurface geology of the area is divided in six lithologic units viz. Gray Silty Clay superficial unit extended vertically from 5 meter to 15 meter, Gray fine Sand encountered at different depths the predominant unit, Gray hard Clay more sticky and continuous layer having reasonable thickness (18 to 36 meter), Whitish grey medium to coarse Sand more localized and pinched out unit, Bluish grey medium to coarse Sand and Yellowish brown medium to coarse Sand so called oxidized horizon continuous unit with reasonable thickness. These units have broadly categorized into Aquitard-1 very thin and topmost layer, Aquifer-1 varies in thickness from 3m to 61m providing arsenic contaminated water to people, Aquitard-2 variable thickness (3m to 37m) prohibits vertical percolation of contaminated water and Aquifer-2 with reasonable thickness is considered good reservoir of potable water for sustainable groundwater development and arsenic concentration is within the permissible limit. Groundwater monitoring results shows that no long term water level depletion has occurred rather seasonal fluctuations yet and the hydraulic gradient is towards the Meghna River. Key words: Arsenic, Hydrogeology, Groundwater, Lithology, Aquifer, Aquitard, Matlab 67

2 1. INTRODUCTION: Bangladesh is mothered by rivers therefore water is available wherever and whenever we arrive. Unfortunately, in recent years, the presence of elevated concentration of various organic and inorganic pollutants from natural and anthropogenic sources has limited the availability of safe water. Among drinking water pollutants, arsenic (As) is regarded as the most formidable contaminant on a global scale (ATSDR, 2007). Evidence of wide spread high As groundwater in deltaic environments are documented from many parts of the world (Berg et al., 2001; Smedley and Kinniburgh, 2002; Bhattacharya et al., 2003; Nickson et al., 2005; Charlet and Polya, 2006; Nath et al., 2008a). Bengal Delta Plain comprised most of Bangladesh and parts of West Bengal is the worst As affected deltaic environment in terms of human exposure (~> 40 million) and wide areal coverage (BGS and DPHE, 2001; Bhattacharya et al., 2001, 2006; Mukherjee and Bhattacharya, 2001; Bhattacharya and Mukherjee, 2002; Smedley and Kinniburgh, 2002; Chatterjee et al., 2005; Cheng et al., 2005; Dhar et al., 2008; Hasan, 2008; Nath et al., 2008b,c; Rahman et al., 2009). The Groundwater in this Plain often exceeds WHO (10 μg/l) and National (50 μg/l) guide line values of arsenic concentration (CGWB, 1999; BGS and DPHE, 2001; Nath et al., 2008a-c; Pal and Mukherjee, 2009). Therefore, based on the arsenic stretching and severity Bengal Delta Plain is also considered to be of the largest environmental hazard in the contemporary world (Ahmed et al., 2001; Ahmed, 2005). The Bengal Delta formed by the sedimentation of the rivers Ganges- Brahmaputra-and Meghna along with their numerous tributaries and distributaries is the world s largest fluvio-deltaic basin. Groundwater of Holocene sediments in BDP is reported as highly contaminated with As rather than the sediments deposited in Tertiary Age. Matlab upazila at Chandpur district is covered by a sequence of deltaic deposits of Quaternary age. Taking spatial distribution (BGS and DPHE, 2001) and severity of arsenic contaminant in groundwater into account, Matlab upazila is one of the hotspots in Bangladesh. Studies under Arsenic Mitigation Project (AsMat) in Matlab by International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR, B) and Bangladesh Rural Advancement Committee (BRAC) revealed that approximately 80% of the domestic tube wells (n=13,000) have exceed the arsenic concentrations of Bangladesh drinking water standard of 50 μg/l (BGS and DPHE, 2001; Jakariya et al., 2007). To abate the human exposure to arsenic groundwater, various Governmental and donor Organizations were working through multi-parallel approaches. Like rain water harvesting, pond sand filter, treating of surface water, filtering of arsenic groundwater etc. Unfortunately none of the approaches could reach the goal properly and successfully. It is therefore an urgent need for the people themselves to find practical mitigation options, if the UN Millennium Development Goal (MDG) for providing safe water is to be met by Installation of tubewells into deeper aquifer could be used to tap potable water as an alternative potential source to meet the growing demand for sound lives. Therefore it is urgent to conduct an extensive survey to assess the geological as well as hydrogeological conditions of the area. With the advent of technology, analyzing of borelog data, groundwater level data, meteorological data etc are very much effective means of evaluation of geology and hydrogeology thereby the safe drinking water to community of an area. 2. THE STUDY AREA: Matlab Upazila in Chandpur district covers an area of about 40 sq km located in the southeastern part of Bangladesh near the confluence of the Ganges (Padma) and Meghna Rivers. It is about 60 km south-east to the capital city Dhaka and also well communicated with Dhaka and surrounding cities. The study area encompasses six unions that are entirely crowded with large number of communities. In most communities, several options are being used as sources of potable water, such as sand filtering of water from pond, river and khal, rainwater harvester, dug well, shallow and deep tubewells. Geomorphologically, this is a lowlying area. During monsoon, most of the areas inundates by annual flooding following heavy rain. Consequently most of the water bodies go out of use due to several contaminants. Hence, for potable water, peoples are highly motivated to use groundwater. Presently, more than 90% people of 68

3 this area use tubewell as a source of potable water (Jakariya, 2007) and for irrigation as well. But unfortunately, about 80% of the domestic tubewells (n=13,000) of this area have arsenic levels above the Bangladesh drinking water standard of 50μg/L (Jakariya et al., 2007). 2.1 Physiographic and Hydrogeologic Settings of the Area: The study area encompasses a triangularshaped tract known as Chandina Deltaic Plain bounded by the Meghna River in the west, Lalmai Hills in the east and Old Meghna estuary at its south. The topographic feature is represented by a vast expanse of deltaic plain and old Meghna estuarine flood plain deposits. The Chandina Deltaic Plain is referred to as Tipperah surface by Morgan and McIntyre (1959), Older Alluvial surface by Umitsu (1987, 1993) and Old Meghna Estuarine surface by Brammer (1996). The unit consists of Silt, Silty Loam, Silty Clay and grayish Clay has been named as Chandina Formation. The sediments resembling those of the Recent Meghna Flood Plain are more compacted, decomposed and oxidized. Most parts of Bangladesh are covered with Quaternary sediments deposited by the Ganges Brahmaputra Meghna river systems. The sediments are thick over most of the areas and have a good water-bearing capacity. The study area is included in South-East hydrologic regions as per National Water Management Plant (NWMP 2000) and in zone G (UNDP 1982). This zone consists primarily of old flood plain deposits of the Meghna River. The main aquifer was considered for only deep tube-well development with discharge up to 2 cusec. 3. MATERIALS AND METHODS: 3.1. Borelogs: The hand percussion drilling method is being used for installation of domestic tubewells at the study area. Though the method is obsolete but found to be very effective in Bangladesh due to its simple and friendly functions to installed hand tubewells upto 100 m or more depends on the congenial conditions of subsurface (Horneman at al., 2004; Rahman and Ravescroft, 2003). In the study area forty six (46) tubewells were installed between 50 meter to 100 meter depths during AsMat project period conducted by International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR, B) and Bangladesh Rural Advancement Committee (BRAC) using the hand percussion drilling method. During drilling, disturbed sediment samples were collected for every 3.0 m, or more often if characteristics of the sediment changed. Only washed and disturbed sediments could be sampled as a result of the hand percussion technique (Rahman and Ravescroft, 2003). Washed sediments were collected in a bucket and allowed to settle down before being transferred on a bamboo carpet or simple mat. Later the sediment samples were allowed to drain (but not dry). A professional geologist or trained personnel made the borelogs on a prescribed format up to end of bore hole by describing the characteristics of each of the drained sediment samples according to their texture and color for each of the tubewells (fig. 3.1). Later the sediment samples were putted in to the plastic bags with proper marking of each sample in order to carry them from field to office. Later in the office visual inspection and justification of the samples was carried out and compared with the Munsell standard soil colour chart. At the very beginning of analysis of borelogs once again cross-check of logs were made with preserved sediment samples for every borehole. All sediments from top to bottom of borehole have been categorized based on the identical color and texture. Analyzed borelogs results are used to exhibit the distribution of subsurface lithology thereby aquifers and aquitards of the study area. Subsurface distribution map has been prepared with the help of Rock-ware 2004, Surfer 8.03 and ArcGIS 9.2 also used for mapping the location of the study area and the borelog points. 3.2 Meteorological and Groundwater Level Data: Bangladesh Meteorological Department (BMD) monitors the rainfall, temperature, 69 Figure 3.1. Example of Borelogs

4 evaporation humidity etc all over the Bangladesh. In this work, rainfall data was collected from BMD. There are five rain gauge stations in and around the study area. BMD personnel records rainfall reading once a week round the year that preserve in record files e.g. Microsoft Excel or Access sheet. The collected rainfall data were carefully analyzed and presented as graph using the Microsoft Excel program. Groundwater levels in Bangladesh is monitoring by Bangladesh Water Development 4. RESULTS AND DISCUSSION: 4.1 Geology: Borelog bears the continuous records of the subsurface is a potential means to anticipate the real picture of subsurface of an area. Indeed forty six (46) borelogs for a small area is good enough to decipher the subsurface geology and hydrogeological potentiality. The grain size scale prepared by JICA, Munsell standard soil color chart and empirical experiences has been used as rationale for analyzing and interpreting the borelogs data. Based on the analyzed results, the subsurface lithology of the area is divided into six units viz. Board (BWDB), who maintains a network of dug wells and piezometers. Six such observation piezometers are located in and surrounding the study area. Their personnel also records groundwater level data once a week round the year which also preserve in record files e.g. Microsoft Excel sheet or Access Sheet. Groundwater level data for study area collected from BWDB analyzed sincerely and presented with Arc GIS 9.2 and Surfer 8.0 software. Silty Clay, gray fine Sand, gray hard Clay, whitish gray medium to coarse Sand, bluish gray medium to coarse Sand and yellowish brown medium to coarse Sand. Their lateral and vertical distributions are disharmonic in fashion. Silty Clay the superficial unit covers the whole area and extends downward upto about 15 meters at some locations. Geologic cross-section along AA' (fig. 4.1) reveals the bluish grey medium to coarse Sand is insignificant under the area but the grey hard Clay unit is continuous with considerable thickness and depth (fig.4.2a). It s characterized thickness varies from 18 meter to 36 meter. Grey Figure 4.1. Map of the study area with borelogs locations and section lines 70

5 Figure 4.2 Lithological cross-section a) Along AA and b) Along BB fine Sand over the grey hard Clay is stretched over the area. Very localized whitish grey medium to coarse Sand attributed as pinch-out shape. Yellowish brown medium to coarse Sand is continuous but encountered at different levels although thickness is apparently identical. Bluish grey medium to coarse Sand in figure 4.2b is the major unit underneath the grey hard Clay. It has also been identified from the figure 4.2b that the eastern margin is characterized by mainly the gray hard Clay and gray fine Sand. 3-dimensional distribution model (fig.4.3) representing that the subsurface geology of the area is composed of six distinguished lithologic units. These units are not distributed in order fashion. It is crystal clear that the grey hard Clay is presents everywhere in study area but not vertically distributed in same level. At some places the lower most unit is yellowish brown medium to coarse Sand while whitish grey medium to coarse Sand is most lower elsewhere. 71 Figure Dimentional distribution of lithologic units

6 4.2 Hydrogeology: It is convention that Sandy material serves as groundwater reservoir termed as Aquifer and Silty Clay to Clay materials are restricted to transmit water termed as Aquitard. Delineation of Aquifer and Aquitard geometry is essential to evaluate the groundwater potentiality of an area. Based on analyzed borelogs results the hydrogeological system of the study area is designated as of i) Aquitard-1 ii) Aquifer-1 iii) Aquitard-2 and iv) Aquifer-2. Aquitard-1 composed of top Soil i.e. Silty Clay and Silty Sand characterized by high porosity and low permeability. It is superficial layer that floors the entire study area vertically extended with variable thickness (3 meter to 15 meter) (fig.4.4a). Its physical and hydraulic properties are very important as regulates the vertical recharge to the underlying aquifers. Aquifer-1 composed of Sandy sediments is persistent in the study area with least thickness about 3 meters and maximum thickness of 61 meter (fig. 4.4a). The water table fluctuates within this layer. Most of the shallow tubewells tap water from this aquifer which is severely contaminated with arsenic. Aquitard-2 most concerning unit composed of mostly Silty Clay to Clay separates the well known arsenic free oxidized and arsenic rich reduced horizons. It is continuous but depth of occurrence varies from 15 meter to 61 meter and thickness from 3 meter to 37 meter (fig. 4.4b). It is assumed that Aquitard-2 resists the vertical Figure 4.4. Aquifer and Aquitard distribution a) along AA and b) along BB percolation of arsenic rich groundwater into the safe aquifers. The following unit Aquifer-2 seems apparently the deeper aquifer upto drilled depth. This layer attributed with reasonable thickness is safely guarded by Aquitard-2 for intrusion of arsenic rich groundwater into it. This layer is termed as oxidized zone and regards as most potential reservoir for groundwater development. Nowadays people are being motivated to install tubewells in this aquifer. 4.3 Hydrometeorology: As part of Bangladesh, the study area has a tropical monsoonal climate, with the bulk of rainfall occurring between the month of May and October. It has been found from the analysis of BMD rainfall data from , the mean annual rainfall is about 2000mm (fig. 4.5). It has also been found that he rainfall pattern for last few years is almost alike excluding exceptional cases and most of the annual rainfall (80% to 90%) occurs during the monsoon 72

7 season (May to October) (fig. 4.5). Figure 4.5. The rainfall pattern in Chandpur region during the period from (BMD 2004, station E, N) 4.4 Groundwater flow and hydraulics: Groundwater levels are monitored in Bangladesh by BWDB, who maintains networks of dug wells and piezometers. Six such observation piezometers are located in the Matlab area. In the long-term hydrograph of water level in Matlab area (Fig. 4.6), there is no indication of Figure 4.6. Long term hydrograph of groundwater level (BWDB, station CMO 62) long-term water level depletion rather its periodic fluctuation due to seasonal cyclicity of dry and wet season. It has been observed that the maximum groundwater level elevation is 7.12 m above MSL during the wet season, while the lowest groundwater level elevation is 0.74 m above MSL during dry season over the last fifteen years. The water table begins to rise in response of recharge before the onset of the monsoon (June) because of delayed drainage from the aquitard. The level then rises steeply during the early part of monsoon and reaches their highest level in late July. Groundwater levels are semi-constant between July and October in response to the rapid drainage of surface water and changes in drainage and base levels. The rate of decline is highest in October-November but with the onset of irrigation after January. The general trend of the groundwater flow follows the general trend of the surface gradient of the study area. The regional groundwater flow in the main aquifer is 73

8 towards the Meghna River and thereby Bay of Bengal. 5. CONCLUSIONS: Bangladesh is located at the head of the Bay of Bengal on the Indian sub-continent. The country consists mainly of the Bengal Delta Plain (BDP) created by the Ganges-Brahmaputra-Meghna river system. One or two decades ago, sources of potable water were sufficient enough to meet the demand of millions of country people. Unfortunately potable water is being limited by various organic and inorganic pollutants. Specially, elevated level of geogenic arsenic in groundwater the most formidable environmental crisis in the contemporary world and an estimated million people in Bangladesh are at risk. High As concentrations in groundwater is primarily associated with the Holocene alluvial aquifers of the BDP (BGS, 2001; Ravenscroft, 2001). In most parts of Bangladesh, shallow tubewells (depths 15m -50m) installed in Holocene aquifers is found to be highly As contaminated (Ahmed et al., 2004; van Geen et al., 2002, 2005). Matlab at Chandpur located in the middle southeastern part of Bangladesh is one of such arsenic hotspots in Bangladesh. Outcome of many studies says that almost 80% of the domestic tubewells (n=13,000) at Matlab Area exceeds arsenic level of Bangladesh standard (50 μg/l) for drinking water (BGS and DPHE, 2001; Jakariya et al., 2007). To abate this acute problem, an attempt has been taken to assess hydrogeological conditions for providing safe drinking water as a sustainable option. Therefore, all climatological data and geological data for that area were collected and intensively analyzed. The mean annual rainfall of the area is about 2000mm. Geological borelog data reflects that the subsurface geology of the area is divided in six lithologic units viz. Gray Silty Clay superficial unit extended vertically from 5 meter to 15 meter, Gray fine Sand encountered at different depths the predominant unit, Gray hard Clay more sticky and continuous layer having reasonable thickness (18 to 36 meter), Whitish grey medium to coarse Sand more localized and pinched out unit, Bluish grey medium to coarse Sand and Yellowish brown medium to coarse Sand so called oxidized horizon continuous unit with reasonable thickness. These units have broadly categorized into Aquitard-1 very thin and topmost layer, Aquifer-1 varies in thickness from 3m to 61m providing arsenic contaminated water to people, Aquitard-2 variable thickness (3m to 37m) prohibits vertical percolation of contaminated water and Aquifer-2 with reasonable thickness is considered good reservoir of potable water for sustainable groundwater development and arsenic concentration is within the permissible limit. Groundwater monitoring results shows that no long term water level depletion has occurred rather seasonal fluctuations yet and the hydraulic gradient is towards the Meghna River. REFERENCES: 1. Ahmed, K. M., 2003: Constraints and issues of sustainable groundwater exploitation in Bangladesh. Proceedings of the International Symposium on Safe and Sustainable Exploitation of Soil & Groundwater Resources in Asia, Okayama University, Japan, pp Von Bromssen, M., Jakariya, M., Bhattacharya, P., Ahmed, K.M., Hasan, M.A., Sracek, O., Jonsson, L., Jacks, G., Targeting low-arsenic in aquifer in Matlab Upazila, Southeastern Bangladesh. Science of Total Environment 379 (2-3): Hasan, M.A., Von Bromssen, M., Bhattacharya, P., Ahmed, K. M., Sikder, A.M., Jacks, G., Sracek, O., Geochemistry and mineralogy of shallow alluvial aquifers in Daudkandi upazila in the Meghna flood plain, Bangladesh. Environmental Geology 57: BAPEX, 1981: Geological Report on Dakhin Nhila Anticline, unpublished Report Bangladesh Bureau of Statistics (BBS) Bugg, S. F. and Lloyd, J.W., 1976: A study of fresh water lens configuration in the Cayman Island using resistivity method, Q.J. Eng. Geol

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