Groundwater sampling and geophysical methods determined a serious contamination problem associated with refilled exploitation sites at the Cal Dimoni area, Llobregat delta, Barcelona, Spain. To characterise this process, hydrogeochemical analyses were performed and showed the following modifications to groundwater chemical composition: increasing pH values, changing redox conditions, significant increases in total organic carbon (TOC) and certain trace elements, and high groundwater conductivity values. Major ion content accumulations were found under the refilled area. In contrast, elements involved in the oxidation-reduction processes, such as iron, manganese and nitrates, dearly diminished. Electromagnetic prospecting methods were also performed and delineated the contamination plume extent. These methods also showed separate sources of contamination, one clearly related to the groundwater-refilled zone leachate interaction, another as a consequence of the manure-accumulation surface site. Geochemical and geophysical methods have shown similar results for locating groundwater contamination sources, and for determining leachate generation mechanisms and flow paths.