In this study, saltwater intrusion in coastal aquifers and prevention techniques were investigated using a new computer model, the Sea Water Intrusion (SWI) package (Bakker and Schaars, 2005), which simulates three-dimensional regional seawater intrusion in coastal multi-aquifer systems. Prevention methods consisting of injection wells, extraction wells, a combination of injection and extraction wells, and a subsurface barrier were evaluated as means of controlling saltwater intrusion by using SWI to simulate groundwater flow in an idealized rectangular coastal aquifer with a pumping well. For the first case, a line of injection wells was located along the column between the sea and the pumping well. For the second case, a line of extraction wells pumping saltwater was located between the sea and the pumping well. The third case consisted of a line of injection wells and a line of extraction wells. In order to represent the subsurface barrier, the hydraulic conductivity along a column towards the sea was reduced. The results obtained were compared to each other, and the subsurface barrier technique was determined to give the best result, i.e., it resulted in the least amount of upconing beneath the pumped well. © 2007 ASCE.