This paper presents results of the numerical modeling of pollution transport and groundwater quality monitoring for an old sanitary landfill where an advanced protection system has been introduced. The protection system consists of a vertical bentonite barrier and peripheral leachate drainage. The 3-D FEMWATER numerical program has been used for the groundwater flow and transport simulation. The local monitoring program includes chemical analyses of surface and ground water leachate concentration as well as groundwater level observations. The modeling and monitoring results were aimed at the assessment of the vertical barrier effectiveness as a groundwater protection system in landfill surroundings. These analyses were performed for the Radiowo landfill located near Warsaw. This site was established in 1962, and the permission for its exploitation was extended to the end of 2010. The landfill remedial works have been conducted since 1998 and include: installation of the vertical bentonite barrier, the leachate drainage system, the leachate recirculation system, the mineral cover, the degassing system, and the regulation of water relations in surroundings. The influence of remedial works on groundwater quality on landfill surroundings also has been analyzed in the paper.