The abandoned sulfide ore mines located at Folldal, in central Norway, have been closed since 1993. However, centuries of mining activities have resulted in waste rock and tailings exposed to the natural environment. All the three ingredients are present to generate acid mine drainage (AMD), pyrite, water and oxygen. This has severely impacted the quality of the natural waters and the associated ecosystems at Folldal. The tailings are spread over a large area, and capping is therefore being considered by the Norwegian Directorate of Mining, as a way of reducing the oxidation of sulfide minerals and infiltration of AMD into the groundwater and associated streams and rivers. To test how successful a potential capping would be at Folldal two column experiments have been set up to test two different capillary barrier caps covering reactive tailings. In addition to the two columns with capped reactive material, two reference columns were set up to see how the acid generating tailings and the pre-oxidized material (that made up one of the caps) developed on their own. Previous studies have shown how successful the two capping options were at reducing oxygen diffusion into the tailings. It was clear that the physical properties of the two different caps resulted in different physical properties, even as the effect on the AMD formation was yet to be investigated. The grain size difference between the coarse and fine material of the pre-oxidized cap option was more ideal and succeeded to a greater extent in creating the desired capillary effect. The aim of this Master Project was to analyze and interpret the experimental results obtained, with a special focus on the interpretation of the geochemical composition and changes in the collected leachates. Leachates were collected and chemically analyzed over time to characterize their physico-chemical composition and changes. The geochemical modeling software PHREEQC was used to interpret the chemical data collected and gain insight into the expected future development of the leachate composition in the area surrounding the mines. This thesis works chemical investigations show that the distinct chemistry of the two caps results in different physio-chemical responses. The cap made up of fine-sand had more alkaline minerals to begin with, but the leachate development of the two columns was very similar when it came to their effectiveness in reducing the concentration of heavy metals. Both the reference columns were acid generating and thus the total content of acid generating tailings in the column with the pre-oxidized cap was greater. Thus, when it is taken into account that the pre-oxidized material would be acid generating in itself it seems clear that using the pre-oxidized material in a cover in order to reduce the heavy metal leaching and acid generating potential of the more concentrated reactive tailings is a good purpose to put them to. The physical measurements, the dissolved oxygen, pore water pressure and water saturation measurements suggested that the pre-oxidized capping was better suited than the sand capping because of the more suited physical properties.