Study Area: The Ganges-Brahmaputra-Meghna (G-B-M) drainage basin occupies the total Bengal Basin, which is one of the unique basins of the world (Datta and Subramanian, 1998). These three river basins cover about 1.75×106 km2 across five different countries-China, Nepal, India, Bhutan and Bangladesh (Mirza et al., 1998). The Meghna river is the main outlet of the Ganges-Brahmaputra river system collecting water from a vast catchments area of these countries. The total catchment area of the Meghna river is 82000 km2 and 13 million tons of sediment is being transported per year (Rahman, 2013). The precipitation, evaporation and discharge of water at different seasons of the Meghna river. The Meghna Ghat and adjacent area in Narayangonj and Munshigonj districts had been chosen in order to assess the surface water and sediment samples on both banks of the river. A total of eleven sampling sites were selected depending upon the presumed water and sediment quality and extent of pollution by visiting the study area before sample collection.
Sample Collection and Preparation: Eleven water and 11 sediments samples were collected on 6 August, 2014 from the selected sampling sites. Surface water samples were collected from 15-30 cm below the river water surface and at distances of 40- 80 cm from the bank of the river in pre-labeled sample bottles which was washed with 10% HNO3 acid and rinsed repeatedly with distilled water. Before the sample collection, sample bottles were rinsed three times with the river water (Tareq et al., 2013). For measurement of heavy metal concentration, 65% concentrated HNO3 acid was added to each sample immediately to bring the pH blow 2 to minimize precipitation and adsorption onto container walls (APHA, 1998). Sediment samples were taken at a depth of 0-10 cm and then immediately transferred into polyethylene bags. Prior to sampling the polyethylene were washed with 10% HNO3 acid solution and ringed with distilled water (Manoj et al., 2012; Rabee et al., 2011; Ogbeibu et al., 2014). Water and sediments samples were transported using ice box to the laboratory and water samples were properly labeled and preserved in refrigerator at 4°C temperature. Sediment samples were dried in a dry and dust-free place at room temperature, ground into fine powder using pestle and mortar before sieved under 2 mm mesh. The samples were then stored in plastic container (Jumbe and Nandini, 2009).
Physico-chemical Analysis of river Water and Sediment: Eight physico-chemical parameters of water samples were measured by using different instruments and methods. A centigrade thermometer was used for the measurement of temperature. pH, TDS and EC were measured by portable multiparameter meter (Model: Sense Ion, 156; HACH, USA). Turbidity and DO were measured by portable turbidity meter (Model: 93703; Hanna Instruments, Hungary) and digital DO meter (Model: HQ 30 D; HACH, USA), respectively. BOD was measured by BOD trak apparatus (Model: BODTrakIITM; HACH, USA). COD was determined by titrimetric method according to Huq and Didarul-ul-Alam (2005). For sediment’s pH measurement, 20 gm sediment sample was taken into clean glass beaker and added 50 mL distilled water. The mixture was shaken on a shaking plate more than 30 min and then pH measured with portable multiparameter meter (Model: Sense Ion, 156; HACH, USA). Organic Matter (OM) was determined by wet oxidation method of Walkley and Black (1934).
Estimation of Heavy Metals in Water and Sediment: Water samples were digested with concentrated HNO3 acid as described by APHA (1998) and sediment samples were digested with concentrated HNO3 acid and concentrated HClO4 acid according to Huq and Didarul-ul-Alam (2005). Then the digested water and sediment samples were analyzed by Flame Atomic Absorption Spectrophotometer (Model: AA-7000; Shimadzu, Japan) for detection of heavy metals like Cr, Cd, Pb, Ni, Fe, Zn and Mn. The instrument was calibrated with chemical standard solutions in accordance with manufacturer’s instructions.
Sediment Quality Assessment
Geo-Accumulation Index (Igeo): The geo-accumulation index (Igeo) has been widely used in trace metal studies since the late 1960s (Yaqin et al., 2008). It has been successfully applied to the measurement of bottom sediments contamination (Loska et al., 2003). The Igeo enables the assessment of metal contamination in sediments by comparing current concentrations with pre-industrial levels (Qingjie and Jun, 2008). Igeo is calculated using the following formula (Müller, 1969):
Igeo = log( Cn/1.5 XBn) ............. (1)
where, Cn is the measured concentration of the metal (n) in the sediment and Bn is the geochemical background of the metal (n). The factor 1.5 is used for the possible variations of the background data due to lithological variations. Average shale standard for different metals reported by Turekian and Wedepohl (1961) was taken as background concentration throughout the study. Müller (1981) classified Igeo values into seven grades or classes. The Igeo factor is not readily comparable to the other indices of metal enrichment due to the nature of the Igeo calculation, which involves a log function and a background multiplication of 1.5 (Abrahim and Parker, 2008).
Contamination Factor (CF): The Contamination Factor (CF) and Contamination Degree (CD) are used to assess the pollution load of the sediments with respect to heavy metals (Manoj et al., 2012). The CF is the ratio obtained by dividing the concentration of each metal in the sediment by baseline or background value (Varol, 2011). CF for each metal was determined by the following formula (Hakanson, 1980):
CF = (Measured metal concentration/Background concentration of the samemetal).................................(2)
CD for each site was calculated as sum of all contamination factors (Ahdy and Khaled, 2009). Hakanson (1980) has provided four grade ratings of sediments based on CF values.
Pollution Load Index (PLI): Pollution Load Index (PLI) is used to find out the mutual pollution effect at different stations by the different elements in soils and sediments (El-Sammak and Abdul-Kassim, 1999). The PLI gave an assessment of the overall toxicity status of the each sampling site and also it is a result of the contribution of the measured seven metals. PLI for each site was determinedas the nth root of the multiplications of the contents (CFmetals) by the following equation proposed by Tomlinson et al. (1980):
PLI = n√(CF1×CF2×CF3×...×CFn)...............................(3)
where, CF is the contamination factor and n is the number of metals. The PLI value of >1 is polluted, whereas <1 indicates no pollution (Harikumar et al., 2009). This index is quickly understood by unskilled personal in order to compare the pollution status of different places.