There are today thousands of active and disused landfills around Sweden. New landfills have to meet high standards to prevent spreading of contaminants from the landfill, and the more hazardous the waste is, the higher the requirements. This study investigates how contaminants from the landfill Ullnatippen can spread around the area. Ullnatippen is a landfill that today only is used for inert waste, which is waste that does not release hazardous contaminants. The landfill has been used since the 1950s and there is some information that suggests that more hazardous waste has been left on the landfill in the past. Ullnatippen has a control program, consisting of 6 groundwater pipes where samples have been taken every year for many years. The purpose with the control program is to discover contaminants spreading from the landfill. This study also investigates whether the control program is designed in such a way that contaminants measured in the pipes actually derive from the landfill.  To be able to investigate this a groundwater model was built with the program Modflow. From this model, a particle tracking to see the possible routes for contaminants to spread was performed. The result showed that the groundwater from the landfill is moving towards the lake Ullnasjön. The model also confirmed that possible contaminants from the landfill are measured by the control program, but that there is room for improvement since the pipes are located with such long distances. The study also discusses general uncertainties with groundwater modeling and which improvements that have to be done to establish the result from this study.