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Seasonal Variations of Arsenic in the Ganges and Brahmaputra River, Bangladesh

S. M. N. Islam*
Department of Environmental Sciences, Jahangirnagar University, Dhaka, Bangladesh-1342

S. H. Rahman
Department of Environmental Sciences, Jahangirnagar University, Dhaka, Bangladesh-1342

D. A. Chowdhury
Analytical Chemistry Division, Atomic Energy Research Establishment Centre, Dhaka, Bangladesh

M. M. Rahman
Department of Environmental Sciences, Jahangirnagar University, Dhaka, Bangladesh-1342

S. M. Tareq
Department of Environmental Sciences, Jahangirnagar University, Dhaka, Bangladesh-1342

Abstract/ Executive Summary:

Dissolved arsenic concentrations in the Ganges, Brahmaputra Rivers and confluence of these two rivers show important seasonal variations and maximum arsenic concentrations are observed during the monsoon season (July–October). These seasonal variations of dissolved arsenic concentrations were closely related to intense river-water discharge during the monsoon season with high arsenic-rich suspended particulate matter (SPM) loads. These arsenic-rich SPM mainly are primarily originated from erosion of agricultural land in upstream region irrigated with arsenic contaminated shallow groundwater and to some extent weathering of bed rocks. Considerable amount of iron and manganese enriched SPM adsorbs arsenic and increased water temperature in the summer accelerates microbiallymediated reduction of arsenic (V) to more soluble arsenic (III). Additionally, dissolution of solid arsenic-bearing mineral phases also attributes to high arsenic concentrations in water and causes seasonal variations. It is realized that the SPM of these two major rivers primarily controls the arsenic inputs into the Ganges-Brahmaputra-Meghna delta system. The cycling of arsenic in this delta is related to the monsoon seasonal dynamics, land use patterns and biogeochemical processes.

Keywords: Arsenic; Ganges; Brahmaputra; Seasonal variation; Bangladesh.

Location: Daulatdia Ghat, Aricha ghat and Paturia ferry ghat

Start Date: 00-01-2008

End Date: 00-12-2008

Program Area: Risk Management in Agriculture

Commodity: Arsenic

Objectives:

This study monitors and reports the seasonal distribution of arsenic in river water, and sediments in the Rivers Ganges and Brahmaputra to understand the mobilization and ecotoxicological potentials of arsenic in the Bengal Basin.

Methodology:

River water samples were collected from three sites namely Daulatdia Ghat (23?46'41"N & 89?47'12"E, Ganges River), Aricha ghat (23?50'16"N & 89?46'46"E, Brahmaputra River) and Paturia ferry ghat (23?47'02"N & 89?48'23"E, confluence of Rivers Ganges and Brahmaputra or G-B confluence). Bangladesh Water Development Board (BWDB) provided the river discharge data for the Rivers Ganges and Brahmaputra and GB confluence for the observation period of January 2008 to December 2008). Water and suspended particulate matter (SPM) were collected each month in 2008. Six liters water samples were filtered immediately and collected in pre-washed polyethylene bottles. Samples were acidified using ultra pure HNO₃ to bring the pH down to ~1 and stored in a dark place at 4°C before analysis. After filtration of six liters sample water, remaining (defined SPM) on filter papers in varying amount (grams to several milligram) were collected and preserved for trace element analyses. Physicochemical parameters of river water were measured in-situ at a monthly frequency. Temperature and conductivity were measured using a mercury thermometer and a conductivity meter (Model No. HI 8033, HANNA instruments) respectively; pH and dissolved oxygen (DO) concentrations were recorded by a HANNA pH meter (Model No. S357056) and Microprocessor Auto Cal Dissolved Oxygen Meter (Model No. HI 9143, HANNA instruments) respectively. Total Dissolved Solids (TDS) were measured using a TDS meter (Model No. HI 8734, HANNA instruments). 2.2. Analyses Representative sub samples (30 mg of dry, powdered and homogenized material) were digested using 225 µl HNO₃ (analytical grade) and 75µl HClO₄ (analytical grade) in open Teflon beaker with the aid of sand bath at 120?C. Care was taken to avoid dryness of the solution, and after achieving complete cooling the digested solution was brought to 25 ml using ultra-pure water. Blank samples were also prepared during digestion for the background correction. Due to very low concentration of dissolved arsenic in river water, it was necessary to pre-concentrate the samples. Initially, we took 500 ml of water in 800 ml clean beaker, added 10 ml concentrated HNO₃ and then covered the beaker with a clean glass and let the water evaporated on a sand bath at 100-120?C until the volume of water in beaker came to approximately 15 ml. We then added to the reduced 15 ml solution another 5 ml HNO3 and 2.5 ml HClO₄ and then evaporated in the similar way using a temperature of almost 200?C until the volume of solution reduced down to 5 ml. Then we filtered the solution into a 25-ml volumetric flask and made to the mark with ultra-pure water. Arsenic and iron concentrations in these digested solutions were measured using an Atomic Absorption Spectrophotometer (AAS, AA-6800, Shimadzu). The analytical methods employed here were checked for quality by repeated three times for each analysis. Furthermore, randomly selected samples (10% of the total sample) were analyzed by the same procedure using AAS for quality control.

Source of Information: J. Sci. Res. 4 (1), 65-75 (2012)

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Our study reveals that dissolved and particulate arsenic concentrations in Ganges, Brahmaputra and G-B confluence are found high during the monsoon period. Maximum concentrations of dissolved arsenic and particulate arsenic in these three sites are 4000 ng l -1 and 11.1 mg kg-1 respectively. High river-water discharges (maximum is 89,105 m3 s -1) along with significant amount of suspended particulate matter (SPM) (maximum concentration of 900 mg l-1 ) originated mainly from the seasonal erosion of agricultural land during monsoon season are responsible for the variation in arsenic concentration. High temperature (maximum is 30?C) during the summer which enhances the biological activity through microbial reduction of As (V) to less particle active As (III) species in water is also attributed to the seasonal variation in arsenic concentrations in river waters. Nevertheless, all dissolved arsenic concentrations measured in Ganges and Brahmaputra Rivers during the one year observation were lower than the permissible limit for drinking water. However, substantial amount of dissolved and particulate arsenic might be deposited in the GBM delta through these river systems and may cause arsenic enrichment in shallow aquifers mainly in southern Bangladesh.

Knowledge Management: Journal