Models for managing the deep aquifer in Bangladesh

In southern Bangladesh excessive levels of As in shallow groundwater have led to deeper groundwater becoming the main alternative source of As-free potable water. Hydrogeological configuration indicates that tube-wells pumping from these depths may be vulnerable to As breakthrough from shallow levels. The thesis explores a range of methods of representing lithological heterogeneity of the Bengal Aquifer System (BAS) in models of groundwater flow and travel time. The aim is to support models of arsenic (As) flux to the deep groundwater flow-system of BAS, and hence to aid assessment of the vulnerability of deep groundwater to invasion by As. The research uses an array of geological information including geophysical logs (n=12), hydrocarbon exploration data (n=11), and drillers' logs (n=589) from a 5000 km2 area to characterise the aquifer heterogeneity as a basis for alternative representations of hydrogeological structure in groundwater flow modelling. Groundwater samples from southern Bangladesh were analysed for 14C in order to determine groundwater age (n=23) and for hydrochemical (n=75) and isotopic (n=50) characterisation. A new hypothesis `SiHA (Silt-clay layers influence Hierarchical groundwater flow systems and Arsenic progression in aquifer)' is presented which integrates sedimentological heterogeneities, groundwater flow, and geochemical processes to explain the distribution and geological evolution of groundwater As in the aquifer. The hypothesis explains the spatio-vertical variability of groundwater As concentration by 'groundwater flow systems and differential flushing' in the aquifer. Groundwater flow models based on eight different yet plausible aquifer representations provide adequate simulations of hydraulic head, but contrasting implications for well catchments and travel times. The better representations are judged by comparing model outcomes of travel time with groundwater age determination using 14C. Comparisons demonstrate the importance of incorporating hydrostratigraphy and spatial heterogeneity in order to optimise model representations, and implications for the security of As-free deep groundwater in the BAS.