Hydrogeochemical and Mineralogical Evaluation of Groundwater Arsenic Contamination in Murshidabad District, West Bengal, India
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HYDROGEOCHEMICAL AND MINERALOGICAL EVALUATION OF GROUNDWATER ARSENIC CONTAMINATION IN MURSHIDABAD DISTRICT, WEST BENGAL, INDIA by ANDREW W. NEAL B.S., GEORGIA COLLEGE AND STATE UNIVERSITY, 2007 A THESIS submitted in partial fulfillment of the requirements for the degree MASTER OF SCIENCE Department of Geology College of Arts and Sciences KANSAS STATE UNIVERSITY Manhattan, Kansas 2010 Approved by: Major Professor Saugata Datta Abstract More than 75 million people in the Bengal Delta of eastern India and Bangladesh are exposed to drinking water with dangerously high arsenic (As) concentrations; the worst case of environmental poisoning in human history. Despite recognition of dangers posed to chronic exposure to drinking water with elevated As, its biogeochemical cycle is inadequately constrained in groundwater flow systems due to its complex redox chemistry and microbially-mediated transformations. Arsenic concentrations in Bengal Delta sediments are comparable to global averages, but its highly heterogeneous spatial distribution (on scales of meters to kilometers) in sediments and groundwaters is poorly understood. Though many research efforts have targeted understanding this heterogeneity in Bangladesh, less work has been done in eastern India. Murshidabad (23°56.355‘N, 88°16.156‘E), an eastern district in West Bengal, India, where groundwaters are highly As-affected (~4000 μg/l), was chosen as our study area. Research objectives were: (1) characterize sediment cores (mineralogically, geochemically) and groundwaters (hydrochemically, isotopically) in areas with contrasting As concentrations—west (low-As) and east (high-As) of river Bhagirathi, a major distributary of Ganges flowing through the heart of Murshidabad; (2) describe and understand the extent of spatial variability, laterally and vertically, of dissolved As concentrations in shallow (< 60 m) aquifers, comparing sediment core chemistry to water chemistry; (3) identify source(s) of aquifer recharge and (4) role(s) of inorganic carbon within the aquifer to understand the bioavailability and mobilization of As from sediments to groundwaters. Mineralogical differences between high-As (grey) and low-As (orange-brown) sediments, were the presence of greater amounts of micas, Fe- and Mg-rich clays, amphiboles, carbonates, and apatite in high-As sediments; these were virtually absent from low-As sediments. In high-As areas, As was associated with amorphous and poorly-crystalline Fe-oxyhydroxide phases and labile (specifically-sorbed) phases, especially where Fe(II):FeT was high in the sediments. High-As groundwaters had high As(III):AsT, iron, bicarbonate, phosphate, and ammonium, and low concentrations of chloride and sulfate. Dry season precipitation was probably the main source of aquifer recharge; lighter values of 13C in dissolved inorganic carbon resulted from oxidation of natural organic matter. This study points to an idea that both microbially-mediated oxidation- reduction and competitive ion-exchange processes occurring in shallow aquifers of Murshidabad drive As mobilization and sequestration by aquifer sediments. Table of Contents List of Figures ........................................................................................................................................ v Acknowledgements .............................................................................................................................. vii Chapter 1 - Introduction ......................................................................................................................... 1 Arsenic Geochemistry ....................................................................................................................... 1 Global Occurrence of Arsenic ........................................................................................................... 3 Arsenic in South and Southeast Asia ................................................................................................. 4 Geology of the Bengal Basin and its Relation to Arsenic Occurrence .............................................. 5 Chapter 2 - Background ......................................................................................................................... 7 Chapter 3 - Objectives and Hypotheses ............................................................................................... 15 Objectives ........................................................................................................................................ 15 Hypotheses ....................................................................................................................................... 15 Chapter 4 - Methods and Materials ...................................................................................................... 17 Study Area Description .................................................................................................................... 17 Sample Collection ............................................................................................................................ 18 Water Sampling ........................................................................................................................... 18 Sediment Sampling ...................................................................................................................... 21 Sample Transport ............................................................................................................................. 24 Analyses ........................................................................................................................................... 26 Field Analyses ............................................................................................................................. 26 Lab Analyses ............................................................................................................................... 31 Chapter 5 - Results ............................................................................................................................... 41 Sediment Characterization ............................................................................................................... 41 Mineralogy and Chemistry of Sediment Cores............................................................................ 41 Water Chemistry .............................................................................................................................. 59 Field Analyses ............................................................................................................................. 59 Laboratory Analyses .................................................................................................................... 62 Chapter 6 - Discussion ......................................................................................................................... 74 Sediment Geochemistry ................................................................................................................... 74 Physical Characterization ............................................................................................................ 74 Petrography and Mineralogy of Sediment Cores ......................................................................... 75 iii Major and Trace Element Geochemistry of Sediment Cores ...................................................... 76 Sequential Extractions of Sediment Cores .................................................................................. 77 Summary of Sediment Characterization ...................................................................................... 79 Water Chemistry .............................................................................................................................. 80 General Water Attributes ............................................................................................................. 80 Speciation and Spatial Variability of Arsenic in Waters in Murshidabad ................................... 81 Ionic Constituents in Waters from Murshidabad ......................................................................... 82 Hydrogen and Oxygen Isotopes as Indicators of Groundwater Recharge ................................... 83 δ13C-Dissolved Inorganic Carbon of Waters in Murshidabad ..................................................... 85 Summary of Water Chemistry ..................................................................................................... 86 Chapter 7 - Conclusions ....................................................................................................................... 87 References ............................................................................................................................................ 89 Appendix A - Water Analyses ........................................................................................................... 103 Appendix B - Sediment Analyses ...................................................................................................... 113 iv List of Figures Figure 1: Distribution of documented problems with As in groundwaters. ........................................... 4 Figure 2: Map showing location of sampling areas within Murshidabad ............................................ 19 Figure 3: Typical hand-pumped tubewell ...........................................................................................