Stratigraphic evolution of the Ganges-Brahmaputra lower delta plain and its relation to groundwater arsenic distributions

Abstract

Throughout the Holocene, a large sediment load coupled with river avulsions, tectonic activity and eustasy developed the complex stratigraphy of the Ganges-Brahmaputra-Meghna delta. Understanding this development is useful in predicting how the delta will respond to changes in the environmental setting such as climatic shifts and sea-level rise. The complex stratigraphy is also linked to high concentrations of naturally occurring arsenic in the shallow groundwater aquifers of Bangladesh through its controls on hydrogeology and aquifer biogeochemistry. This research investigates differences in the fluvio-deltaic deposits of the Ganges and Brahmaputra rivers, as well as differences in the tectonic setting across the lower delta plain. We investigate how these overarching controls influence stratigraphic architecture, the resulting aquifer systems, and ultimately arsenic distribution within the lower delta plain. A transect of 48 sediment cores spanning the lower delta plain of Bangladesh was drilled to a depth of 90m. Individual samples were analyzed for lithology, provenance and age to determine the stratigraphic architecture, the source of the sediments and the depositional history of the rivers. Delta stratigraphy was related to arsenic distribution by measuring groundwater arsenic near each borehole and combining this data with previously collected groundwater data. This study provides a better understanding of Holocene delta evolution and river behavior, as well as a more complete understanding of the geologic controls on arsenic and the characteristics of contaminant distribution in fluvial settings as they relate to stratigraphic features.