バングラデシュのガンジス・ブラマプトラ・メグナ川下流域における汚染陸水の地球化学的研究

概要

Inland waters (e.g. surface and ground) in Bangladesh are more prone to be polluted. Bangladesh is a developing country with increasing industrial activities. The surface water includes rivers, lakes, ponds and other water bodies. Bangladesh is primarily a flat delta that is traversed by numerous rivers and their tributaries. There are a ton of more bodies of water all around the country, including lakes, canals, and streams. Geographically, Bangladesh is a part of the Bengal Basin; the Ganges, Brahmaputra, and Meghna rivers, collectively known as the Ganges-Brahmaputra-Meghna (GBM) river system. River water and groundwater are Bangladesh's primary sources of freshwater. Nearly 75 percent of its freshwater comes from outside sources that have significantly impacted the country's development and daily social and economic life.

Water resources are plentiful in Bangladesh, but they are constantly being polluted. Both sources of surface water and groundwater are contaminated with various pollutants such coliforms, hazardous trace metals, and other organic and inorganic pollutants. Bangladesh is one of the densely populated countries where majority of people rely on these water sources, particularly the country's groundwater supplies, which have elevated levels of arsenic, drinking water carries a very high risk of health problems. Bangladesh has a high rate of water-borne illness deaths, especially among youngsters. Water contamination is mostly caused by anthropogenic sources, including untreated industrial effluents, inappropriate home waste disposal, and agricultural runoff. Chemical contamination of inland water resources is primarily caused by industries. Large-scale wastewater-producing industries have the potential to contaminate the nearby aquifer's groundwater or surface water, which has an impact on the quality of water pumped for drinking. The quality of surface water may be impacted by the rise in water pollution from industrial operations. The groundwater table is also lowered, which may have an impact on agriculture if it is over-pumped.

The primary supplies of irrigation and drinking water in some regions of Bangladesh are the groundwaters, which are highly arsenic contaminated, particularly in the middle and southwest. There are trace amounts of arsenic (As) in all rocks, soil, dust, water, and air. Arsenic concentrations in sedimentary and recent sediments can reach 4000 mg/kg. The mineral that contains the greatest arsenic is called arseno-pyrite. The first detection of arsenic poisoning in groundwater in Bangladesh's southwest was made in 1993 by the Department of Public Health Engineering (DPHE), Bangladesh. The concentration of As is not uniformly distributed over Bangladesh in terms of space. Groundwater with a high As content is restricted to Holocene-era sedimentary aquifers. Alluvial aquifer water is naturally contaminated with As from the Ganges, Brahmaputra, and Meghna (GBM) floodplain's alluvium deposits from the Holocene. Arsenic sources and release mechanisms have been studied in the last decades. There, are still debate in arsenic sources and release mechanisms in Bangladesh. Pyrite oxidation is the oldest hypothesis, silicate mineral decomposition is the recent hypothesis followed by FeOOH reductions. A new dimension has been explored in the present study for arsenic mobilizations. The sources of major and minor elements were examined to understand the geochemical processes in high arsenic groundwater. Groundwaters with high levels of arsenic have their major and minor elements and their sources examined. The highest cation and anion, respectively, are calcium dissolved in solution and alkalinity. Nearly half of the calcite Ca/alkalinity ratio is represented by the ratio of 0.24. Other processes, such as the dissolving of silicate minerals (such as mica and amphibole), in addition to the concurrent dissolutions of carbonate, may be used to explain this ratio (mainly calcite). Some groundwaters exhibit relatively high Na and Cl contents, which indicate the mixing of seawater. Relatively high Fe concentrations indicated that the anoxia of organic matter decomposition was reducing Fe-oxyhydroxides. Alkalinity and the dissolved Fe have just a weak relationship. The coprecipitation of iron (Fe) with sulfide (Fe-sulfides) and/or carbonate (siderite) minerals may be the cause of the weak correlation. Most likely, the reducing and oxidizing conditions in the aquifers control the amount of dissolved Fe. Some samples with relatively high S04 concentrations and high Na and Cl concentrations indicate that S04 originated in seawater. Arsenic that has been dissolved displays a correlation plot with a belt-shaped figure. One possible explanation for this link between As and Fe is precipitation. The weak correlation may be explained by a number of other factors, including the presence of high organic matter, which reduces the ability of Fe-oxyhydroxides to adsorb As, the occurrence of As release from Fe-oxyhydroxides without complete dissolution, and the occurrence of As release into groundwater during the weathering of As-bearing detrital mica. Other As-bearing detrital ~inerals should also be taken into consideration.as sources of As in groundwater because of the warm, humid environment and seasonal variations in groundwater table, which encourage weathering of minerals in shallow aquifers by allowing atmospheric air into the subsurface.

Holocene alluvial aquifers in Bangladesh contain the highest levels of arsenic in groundwater. The Tertiary and Pleistocene aquifers are not contaminated with arsenic. The Ganges-Brahmaputra river basin areas are where the majority of the Holocene aquifers are found. Because the chemical features and properties of groundwaters vary, the diverse geochemical processes of three different river basin systems are evaluated. The Ganges, Brahmaputra, and Meghna (GBM) river basins are trans-boundary river basins that span several nations. They have a combined size of just over 1. 7 million km2, with 119,000 km2 belonging to Bangladesh. The GBM river system is classified. as a single trans-boundary river basin even though it exhibits a variety of characteristics and passes through distinct geological fom辺 tionsalong the majority of its length. Bangladesh is home to the Ganges, Brahmaputra, and Meghna (GBM) river systems'combined delta. Three separate river basins in Bangladesh were used to study the hydrogeochemical and groundwater geochemistry processes. Data were gathered from the BGS website. To learn more about the various groundwater geochemistry and hydrogeochemical processes, 113 groundwater samples (10-274 m depth) collected from various pfaces in Bangladesh's Ganges-Brahmaputra-Meghna (GBM) river basins were analyzed. The investigations took into account the concentrations of groundwater ca2+, Mg2+, Na+, K+, Zn2+, ci-, HCOふ andso/-. The rank of major cations of The Ganges, Brahmaputra, and Meghna basins are Ca2+>Mg2+>Na+>Zn2+>K+, Ca2+>Na+>Mg2+>K+, and K+>Mg2+>Zn2+>Na+>Ca2+, respectively. The rank of major anions of The Ganges, Brahmaputra, and Meghna basins are Ct>SOわ HC03―,Ct>HCO戸 so/-,HCO戸 Ct>So/-, respectively. Factor analysis and bivariate plot interpretation were used in the experiments. The primary processes in the Ganges basin, with the exception of carbonate weathering, are silicate weathering, ion exchange, and seawater intrusion. In the Brahmaputra and Meghna basins, carbonate weathering is the predominant process. The aquifers in the Brahmaputra and Meghna basins also exhibit some silicate and gypsum weathering. For all three basins, the c1-over So/-ratio suggested anthropogenic groundwater impacts.

Additionally, anthropogenic activities continuou'sly contaminate the surface water (such as rivers and lakes) in the GBM basin. Rivers and lakes near the city accumulate more pollution than other bodies of surface water. In order to comprehend how much human activity contributes to lake water contamination, the qualities of the lakes in Dhaka city have been evaluated. Surface water makes up around 10-15% of the total land area of Dhaka metropolis. The city's significant surface water features are Hatirjheel, Gulshan Lake, Banani Lake, Ramna Lake, and Dhanmondi Lake; nevertheless, they are polluted by nearby slums, sewage, and enterprises and factories. Human activity is the primary source of pollution in city lake water. The turbidity of highly polluted lake water was examined to identify any changes that occurred during COVID-19 lockdown. For the purpose of observing the qualitative change in lake water turbidity before, during, and after the lockdown period, a total of eight satellite images from Sentinel-2's Multispectral Instrument (MSI) and Landsat-S's Operational Land Imager (OLI) are employed. For change detection, the imageries from January to June, as well as September and October, were examined. Imageries without clouds have been chosen for study.

The Normalized Difference Water Index(NDWI) and the Normalized Difference Turbidity Index (NDTI) measure the water pixels and turbidity, respectively. With the exception of May, the blue, red, and NIR band reflectance data shows that much of the lake's water becomes clearer than it was before the lockdown period. After the lockdown, there is little pollution, according to the turbidity index. From January to June, high turbidity was seen all over the place. High turbidity was only present in the top portion of the lake in September and October. This most likely implies the lake's floating contaminants. The COVID-19 lockout appears to improve the water quality in city fakes, according to a qualitative evaluation of lake water turbidity utilizing a remote sensing technique.

The groundwater in Bangladesh is naturally contaminated with arsenic. By putting up deep tubewells, the governI?ent of Bangladesh attempts to solve the arsenic problem. However, such an effort only temporarily solves the issue as the introduction of fresh water into the deep aquifer may cause the release of arsenic into the groundwater. To lessen the arsenic problem, it could be beneficial to utilize less groundwater while using more treated surface water. The development of filters with the ability to extract arsenic from groundwater may be the key to effective arsenic removal techniques. By promoting awareness, the anthropogenic contamination of surface water may be decreased: In addition, enforcement of environmental protection laws could lessen polhition from industrial sources. Although Bangladesh has strong environmental protection laws, they should be enforced more strictly to prevent dangerous effluents from entering surface water bodies. Without conducting any laboratory tests, the surface water quality might be determined using remote sensing techniques. This method is also highly effective and complex, making it particularly suitable for managing water quality.