Closed Mine Waste Facilities (MWFs), both tailings and waste rock facilities, represent a potential hazard to the downgradient surface water and groundwater environment. Adaptive, integrated engineering designs at mine sites with short-term life-of-mine resources necessarily mean that closure should be factored into designs and mitigation measures very early in the project lifecycle. Not least, as key decision makers can see the finish line even before the first ore has been shipped. Also, there is emphasis on the importance of having a mechanism for providing a demonstrable analysis, which increases the confidence of the designer/owner, the regulatory authorities, and the public, that the solution is providing environmental protection. The assessment of the long-term risks such facilities pose to the water environment is an important issue for mine closure, particularly when the potential for an impact on the water environment has been identified, necessitating the need to make financial provision for aftercare costs. In addition to the need for financial provision during aftercare, European and international regulatory frameworks require mitigation of potential impacts on the water environment in the long-term, necessitating assessments to account for long-term cover/cap and liner performance as well as changes in climate. This paper presents a case study describing the use of a series of modelling approaches, including probabilistic modelling, to evaluate design performance and risk associated with an Integrated Mine Waste Facility (IMWF) at Dundee Precious Metals Krumovgrad's (DPMK's) proposed mine site in Krumovgrad, Bulgaria. The case study illustrates the application of probabilistic modelling involving the use of probabilistic risk assessment to appraise differing closure and remediation strategies for the IMWF at the design stage based on available environmental data. In the case study presented, probabilistic modelling was successfully applied to quantify the nature of the risk to groundwater and surface water in closure, and the degree of amelioration afforded by differing management techniques (placement of a basal liner, installation of a groundwater capture system for a defined period, and capping of the site in closure). The ultimate decision that the site should move forward with a groundwater capture scheme to provide containment for the site during operation and early post-closure was supported by impact assessments demonstrating that the tailings and waste rock source would not pose a long-term risk to groundwater and surface water in closure.