Introduction Groundwater resources are very important for public water supply. Many of the environmental problems are directly or indirectly related to the location of ground- water and its protection from contamination sources of various kinds (Sharma 1997). It is well known that groundwater contamination can easily occur and carry on for a long time, because recovery is slow and difficult. The problems associated with municipal, abandoned or non-controlled landfills are of general concern, especially because of the inter- actions between the hazardous contents of the leachates derived from them and the groundwater. In view of the large number of poorly documented landfills, fast and inexpensive methods to investigate the shallow subsurface are becoming increasingly important. Non-invasive surface geophysical techniques are increasingly used for landfill characterization, particu- larly where intrusive methods are hazardous and pose significant risks to health and safety. Physical properties of many rocks are significantly altered by the presence of water in pores and fissures, the degree of alteration varying with the nature and content of the contaminants present; this makes it possible to locate water-bearing structures and to investigate the quality of water using appropriate geophysical tech- niques (Sharma 1997). E. Martinho F. Almeida 3D behaviour of contamination in landfill sites using 2D resistivity/IP imaging: case studies in Portugal Received: 20 March 2005 Accepted: 31 October 2005 Published online: 14 February 2006 Ó Springer-Verlag 2006 Abstract Aiming at defining a valid spatial contamination model, resis- tivity and induced polarization (IP) measurements were used to investi- gate contamination plumes in the vicinity of two municipal landfills (Ovar and I ´ lhavo). Previous geo- physical surveys and underground water samples confirmed the con- tamination. However 2D resistivity/ IP surveys enabled in obtaining a more accurate spatial model. The Ovar survey consisted of two profiles with nine Wenner soundings each; the I ´ lhavo survey was carried out along two individual lines using a Wenner standard pseudo-section. In both situations, negative IP values were found associated with positive IP values, which can be explained mainly by 2D or 3D geometric ef- fects caused by the presence of the conductive plumes. The data were modelled using a 2D inversion pro- gram (RES2DINV) and the result- ing resistivity and chargeability distributions were displayed as pseudo-sections. The resistivity and chargeability pseudo-sections define the contamination plumes and the sedimentary structure. These case studies illustrate the advantages of 2D resistivity/IP surveys for the mapping of shape and dimension of contamination associated with landfills. Keywords Contamination plume Æ Landfill Æ Resistivity Æ Chargeability Æ 2D inversion Æ Aveiro Æ Portugal Environ Geol (2006) 49: 1071–1078 DOI 10.1007/s00254-005-0151-7 ORIGINAL ARTICLE E. Martinho (&) Centro de Modelizac¸a˜o de Reservato´rios Petrolı´feros, Departamento de Engenharia de Minas, Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal E-mail: emar@mail.ist.utl.pt Tel.: +351218419164 Fax: +351218417111 F. Almeida Departameto de Geocieˆncias, Universidade de Aveiro, 3810-193 Aveiro, Portugal E-mail: falm@geo.ua.pt