20 water and 20 sediment samples were collected (August, 2019) from the Dhaleshwari River at Jhauchor near Savar in Dhaka. The locations were selected as it covers a range of few meters to several kilometers. Water samples were collected using plastic sampling bottles and sediment samples in polythene bags with proper leveling stored at -4oC. The instruments used were pH meter (Model: JENWAY 3510), EC meter (Model: JENWAY 4510), TOC analyzer (Model: TOC-VCPH, Shimadzu, Japan), Basic titration glass apparatus, Incubator and multi parameter instrument (Model: session 156, Hach), AAS (Model: 6800, Shimadzu), Vortex machine (Model: KEBO Lab REAX-2000).
Physicochemical Parameters The pH of the water samples were measured with a pH meter at 25-30ºC. Intense fluctuation of pH threatens the aquatic life whether it can be acidic, basic or neutral.17 Sediment sample was mixed with distilled water at 1:10 into a 50 mL Teflon tube and vortexed for 5 min. Then it was centrifuged and filtered for pH. The EC of water is an index of salt content and it is the ability of water to conduct electricity which was measured with a conductivity meter.18 To determine EC of sediment, it was mixed with distilled water at 1:10 into a 50 mL Teflon tube and vortex for 5 min. Then it was centrifuged and filtered for EC. TOC in water was measured in Centre for Advanced Research in Sciences (CARS), Dhaka University by TOC analyzer. TOC in a sediment sample was determined by Walkley and Black’s Wet oxidation method. The oxidation was done by weighing 2g air dried sample passing through a 75-mesh sieve into a 500 mL conical flask. 10 mL of standard 0.5 M K2Cr2O7 and 20 mL conc. H2SO4 was added and mix thoroughly about 30 min. for completing the reaction. Then 200 mL distilled water was added for dilution. 10 mL of orthophosphoric acid or 0.2gof NaF powder and 2mL of diphenylamine indicator were added. A deep violet color was appeared. Then titration was done with standard FeSO4 solution till the color changes from violet to blue and finally bright green or bottle green and recording the volume of ferrous sulphate needed. A blank titration was carried out (without sediment) in same way. Calculation was done as, Organic carbon% in sediment=(X-Y) ml× 0.003 × 100 ÷ 2×W=Z.
Organic carbon% in sediment= Z×1.3= R, Where, X= blank titration reading, Y= sample reading, W= Weight of sediment used. Assume that organic matter contains 58% carbon, thus the organic matter content of the sediment will be calculated as Organic matter in sediment (%) = R × 1.724
The hardness of water was measured by titrimetric method. DO and BOD of water samples were measured using a BOD incubator and a multi parameter analyzer (Model: Session 156, Hach). BOD implies the presence of biologically active living systems in the water body that use DO and reduce the amount of it in water.19 TSS test is performed by Whatman 934-AH glass microfiber filter (or equivalent) which has pore size of 1.5 mm and a predetermined volume usually 0.1 L is passed through it. The filter is weighed before the sample is filtered, and after the filter is dried to a constant weight at 103-105°C. Certain amount of sediment sample was taken into pre-cleaned, weighed and dried crucible and placed them into the furnace. To determine moister content sample was heated for 4 hours at 105°C, then cooled and weighed and from that we got moisture content. Finally, these samples were heated for another 6 hours at 700 °C to determine ash content in sediment samples.
Metal ions were measured by flame AAS. Samples were aspirated through nebulizer and absorbance was measured with a blank as reference. At first standard solution of different concentration was prepared for individual elements. 10 water samples were analyzed for Cr at the laboratory of Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka, Bangladesh. For analysis of Cr in sediment, samples were dried and then microwave digestion was carried out. The digestion was done by weighing about 0.5 g of sample into each vessel then 10 mL of conc. HNO3 was added followed by 1 mL of H2O2 and stand about 15 min.20 When the initial reaction between the acids and the samples subsided, all vessels were tightly sealed with screw caps and placed in the microwave. For this program, no cooling step was used and microwave digestion was carried out for 30 min. at 180 °C. After digestion, the content in the vessels were poured into clean 50 mL plastic tubes and diluted with MilliQ-water up to 50 mL. In this study, a statistical calculation, called Pearson Correlation matrix, is done to correlate the physicochemical parameters analyzed in sediment and water.