Geologic origin of arsenic groundwater contamination in Maner block, Bihar, India

Abstract

Arsenic contamination of the groundwater and as a consequence, of the drinking water, has developed to a major problem for many regions globally during the last decades (Chile, China, United States, Argentina, and Mexico). The issue of Arsenic contamination has already been studied extensively in West Bengal, India and Bangladesh, which are some of the affected areas. The importance of studying arsenic groundwater contamination lies in the fact that it has impacts on human health, which indicate that proper measures need to be taken towards a safe drinking water solution. Despite the fact that many researchers around the world have tried to determine arsenic release and mobilization mechanisms, it appears that this is a procedure that is controlled by rather regional parameters and therefore, there is no standard mechanism that can be taken for granted. Each region has different geological and stratigraphic characteristics and since arsenic is naturally formed and released in groundwater for most of the contaminated cases, each region needs to be carefully studied separately. The state of Bihar in India and more specifically the region close to the conjunction of the Ganga and Sone Rivers was chosen as a research location, because, contrary to other affected regions, no geologically related research regarding Arsenic has been carried out there until now. The initial target of this research project was to build a 3D sedimentary architecture model of the area using PETREL software. This model could later be used in flow modeling. The study was then focused on determining the various depositional environments of fluvial sediments and in finally correlating them with the presence or absence of Arsenic in the groundwater. In addition, this study aims to provide a suggestion regarding the possible source of Arsenic in the groundwater for the region of Maner in Bihar. There are two basic hypotheses that will be investigated during this research. One is that Arsenic is confined within the Newer Alluvium of the Ganga River (Holocene sediments) and especially within the point bar alluvial depositions. The second is that Arsenic is already present within the sediments and released in the groundwater due to the presence of Iron oxides in Iron rich minerals (biotite and micas in general) or Iron coatings on Quartz and clay grains. The following methodologies were applied in order to achieve the research purpose. First, literature study of existing research in the area and other Arsenic affected regions was carried out. Then the study area was visited and fieldwork was carried out, which included drilling three approximately 50 m deep boreholes by hand sludging and logging the sediment cores that were obtained as accurately as possible. Hand sludging was used as the drilling technique, even if it is suboptimal for the initial research purpose, because of financial and time constraints. Therefore, it was not possible to obtain undisturbed soil samples while drilling. As a result, core logging, which is an important input for a 3D model, was not always successful. Then, Ultra Violet Spectrophotometry testing was carried out in order to measure the Arsenic content in soil and Scanning Electron Microscopy and X-ray Diffraction in order to determine the mineralogy and chemical composition of the soil samples. In addition, existing measurements of Arsenic levels in drinking water were registered and the coordinates of the handpumps were recorded, using a Global Positioning System device (GARMIN). The purpose of this research is to combine existing knowledge regarding the source of arsenic and specific sedimentary settings in order to correlate local geological characteristics with the presence or absence of arsenic in the groundwater and to suggest a possible source of arsenic for the region of Maner. The results support the hypothesis that it is essential to consider the origin of the sediments to localize regions of high arsenic content.