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J. Bangladesh Acad. Sci., Vol. 42, No. 1, 25-40, 2018 HYDROGEOLOGIC CHARACTERISTICS AND GROUNDWATER POTENTIALITY OF LOWER AQUIFER OF SINGAIR UPAZILA, MANIKGANJ DISTRICT, BANGLADESH MAZEDA ISLAM*, KHAIRUL BASHAR, NASIR AHMED1, MD. GOLAM RASUL1, SAHADAT HOSSAIN1 AND MD. MIZANUR RAHMAN SARKER Department of Geological Sciences, Jahangirnagar University, Savar, Dhaka ABSTRACT The study area is located in the central part of Bangladesh. The aims of this study were to explain the hydrogeological characteristics of aquifers and evaluate the groundwater potentiality. The borehole logs revealed three types of zones: upper aquitard (low permeability clays, silty clays & silts), upper aquifer (very fine sand to fine sand) and lower aquifer (medium to coarse sands and gravels). Both aquifer waters are mainly Ca-Mg-HCO3 type. Except arsenic (As), most of the ions of both aquifer waters are within the limit of drinking standard of WHO (2004) and Bangladesh (DoE, 1997). Arsenic concentrations of upper aquifer exceed both the WHO (10 µg/l) and Bangladesh standard (50 µg/l) and for lower aquifer exceed only WHO standard. Waters of both aquifers in north western and central part of the study area show high arsenic concentration due to lack of continuous impermeable layer between them as revealed from borelog data. Water quality index map also indicates that north western and central part is not suitable for groundwater development because of inferior quality. The average δ18O values for upper and lower aquifer waters are isotopically enriched compared to river water (10.08‰). Similar isotopic composition of upper (4.77‰) and lower aquifer waters (5.50‰) indicates both waters were mixed in the past and mixing may be continued in the future. The mixing may be preferentially from the upper aquifer to lower aquifer because of water abstraction, lack of impermeable layer and high permeability of the upper aquifer etc. Therefore, the potentiality of the lower aquifer may not be suitable for large-scale groundwater development project without any mitigation measure. Key words: Upper aquifer, Lower aquifer, Hydrochemistry, Stable isotope and WQI INTRODUCTION Water is abundant due to heavy rainfall for contaminated with microbial pathogens and several months each year in the Bengal Basin anthropogenic pollutants (Michael et al. of Bangladesh and India (Michael et al. 2009). To avoid contaminated water, 80 deep 2009). The landscape of the study is tube wells and 3070 shallow tube wells are in dominated by the surface water bodies. operation for domestic and agricultural Because of poor sanitation, use of fertilizers, purposes (BADC 2004). Water borne chemicals for cultivation and inadequate diseases and infant mortality rate has reduced waste management practices, the surface dramatically because of the use of water bodies of the study area are nearly groundwater instead of surface water in * Corresponding author: <[email protected]>. 1 Isotope Hydrology Division, Institute of Nuclear Science & Technology, AERE, Savar, Dhaka. 26 ISLAM et al. Bangladesh (Rahman and Ravenscroft 2003). found within the upper 15 m of silty clay Large proportion of wells have elevated sediments. The high concentrations of levels of dissolved arsenic undiscovered until arsenic in groundwater of Bangladesh are 1990s and around 46 million people are at occurring naturally and the dominating the risk of adverse health effects (Yu et al. process is the mobilization of arsenic from 2003) in Bangladesh. According Ahmed et arsenic enriched minerals (Lowers et al. al. (2006) more than half million of those 2007; Halim et al. 2010). A good number of exposed to arsenic are continuously drinking recent studies (Acharyya et al. 1999; water with arsenic concentrations higher than Nickson et al. 2000; Mc Arthur et al. 2001) worldwide drinking water standards. The suggested that Arsenic in Bengal Basin was released mechanism of arsenic from the derived from the Himalayan rocks sediments is widely variable as shown by weathering and subsequently the released As study of arsenic concentrations in water was adsorbed by stream sediments and wells on both large (BGS and DPHE 2001; oxides under aerobic conditions during their NAMIC and BAMWSP 2003) and small transportation by surface water. The World spatial scale (van Geen et al. 2003; van Geen Health Organization (WHO) has set a et al. 2006). But the dissolved arsenic guideline value 10 µg/L for arsenic in concentration is remarkably consistent drinking water because of its adverse health vertically (Michael et al. 2009). In general, effect (WHO 2004). Due to water availability highest concentration of dissolved arsenic most of the water wells for drinking water occur in the shallow aquifer sediments of are abstracting water from the shallow Holocene age, grey and chemically reducing aquifers and the water from the shallow compared to groundwater in older age, aquifer is contaminated with arsenic. People brown or orange oxidized sediments at depth of the study area are thinking to drill more having low arsenic concentration (BGS and deep wells to avoid the arsenic DPHE 2001; Ahmed et al. 2004). A depth of contamination. However, the consequence of approximately 100 m is mostly arsenic water abstraction from the deep aquifer of contaminated in most of the arsenic the study area is very poorly studied. This contaminated areas although this depth varies study will simply try to determine the flow and contact is not clear. Elevated pattern and mobilization of arsenic with the concentration of arsenic is mainly occurred help of stable isotope study. Determination within the shallow sediments and below the of the usability of lower aquifer as a source detection limit at greater depth from the of fresh water with acceptable arsenic results of geological and chemical content is made in the present study. stratification (BGS and DPHE 2001; Ravenscroft et al. 2001; Harvey et al. 2002; MATERIALS AND METHODS van Geen et al. 2003). A study by Halim et Description of the Study Area al. (2014) in the Singair Upazila of Manikganj district revealed that high The studied area, “Singair Upazila” of concentration of As (23 mg/kg) with elevated Manikganj district, is in the central part of concentrations of FeO, MnO and TOC were Bangladesh (90º03´E to 90º16´E and HYDROGEOLOGIC CHARACTERISTICS AND GROUNDWATER POTENTIALITY 27 23º42´N to 23º52.3´N), about 20 km away Department (BMD). Observation well data from Dhaka (Fig. 1). Main rivers are the for ten piezometers in the study area were Kaliganga River and Dhaleshwari River. The collected from BWDB. The maximum Fig. 1. Sample location, borelog, monitoring well and surface geology map of the study area. Kaliganga River in South and Dhaleshwari elevation of the groundwater level in the River bounded in the Eastern and Northern month of September 2006 range from 3.5 to portion and overflow during the wet season. 6.5 mPWD for shallow monitoring well and The Dhaleswari is the main river, which is range from 3.6 to 7.5 mPWD for deep bounded with the north and east portion of monitoring well. In March 2006, the the study area and Kaliganga River flows in minimum elevation of groundwater table in the southern part. The study area lies in the the Singair area range from zero or less than tropical monsoon climatic zone, which is zero to 2 mPWD for shallow monitoring well subjected to tropical climate and is and range from zero or less than zero to 3 characterized by the moderate to high mPWD for deep monitoring well. Water temperature (34.5°C), heavy rainfall and table of minimum elevation indicates that often with excessive humidity. Maximum flow direction from northwest to southeast is rainfall occurs during May to October in towards the present Kaliganga and summer. Geological, hydrogeological and Dhaleswari confluence. Water table of meteorological data of different kinds were maximum elevation indicates that the collected from Bangladesh Water direction from southeast to northwest Development Board (BWDB), Department direction in both shallow and deep of Public Health Engineering (DPHE), monitoring well respectively. From short- Bangladesh Agricultural Development Board term hydrograph, it is found that the upper (BADC) and Bangladesh Meteorological and lower aquifer are hydraulically 28 ISLAM et al. connected and recharged from the same from 39.65 m to 85.41 m. Lower aquifer is source. The cross plot of rainfall, composed of medium to coarse sands and groundwater level and river stage indicates gravels. The thickness of this zone ranges that the rainfall is directly involved in the from 129.2 m to 173.8 m. Most of the deep recharge of groundwater level and river tube wells either for irrigation or for stage. domestic and industrial purposes have been installed in this zone. Absence of lower Geology and Hydrogeology aquitard indicates that the area is susceptible Topographically, the study area is flat. The to contamination. geology of the study area can be attributed to Quaternary alluvial sequence, which is a part Hydrochemistry Data of Ganges – Jamuna flood plain. The north Groundwater samples were collected from and western part of the area is covered by the Pleistocene sediments, which are known as active wells and different types of observation Madhupur clay. In general, the stratigraphic wells installed at different aquifers down to sequence is fining upward sequence i.e. from the maximum 247 m below ground level (bgl) gravels to cobbles, pebbles, sand and finally (Fig. 1). The depth range of upper aquifer silts and clays; micas are abundance in whole water varies from 12 to 70 mbgl and 70 to 247 sequence in Pliocene- Pleistocene-Holocene mbgl for the lower aquifer water. USGS time. From the analysis of the individual log sampling protocols have been followed during and cross-section sections, the aquifer system the collection of groundwater and surface of the study area can be divided into three water samples (Claassen 1982).

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