Globally, salinity intrusion is a frequent and ubiquitous problem in any developed coastal aquifer system, which is also experienced by the regional aquifer in southwestern Bangladesh. Immediate attention must be paid to quantify the extent of this problem for taking the appropriate countermeasures. Nowadays, numerical modeling technique plays an important role in this regard since traditional analytical approaches are insufficient to replicate the physical system adequately. This book demonstrates a new system of numerical modeling for an integrated river-aquifer system by taking the regional aquifer in southwestern Bangladesh as a case study area. A conceptual model of the existing integrated river-aquifer system is formulated, which is followed by the development of a 3D numerical model for simulating groundwater flow and salinity transport. Groundwater flow characteristics and dynamic interaction can be satisfactorily simulated and extent of salinity intrusion has been quantified using the developed model. This book will be helpful as a reference to the undergraduate and postgraduate students, hydrological researchers, hydrogeologists, consultants, and environmental agencies.