Landfills are rarely properly constructed or lined and therefore represent a potential source of contamination.Solid waste in the form of hundreds of unregulated landfills and illegal dumping sites in Croatia, Bosnia-Herzegovina, Montenegro and Albania constitutes one of the main threats to the overall groundwater quality of the Dinaraic Karst Transboundary Aquifer System (DIKTAS). For the vulnerable karst environment of the Dinaric region, which has a very limited auto-purification capacity, this is the most serious current as well as (potential) future problem.Data were collected in the framework of the DIKTAS project. For more information, visit: http://diktas.iwlearn.org/resources/projectdocuments/transboundart-diagniostics-analysis/transboundary-diagnostic-analysis-tda/view