A new ranking approach based on Grey Systems Theory using PROMETHEE II (Preference Ranking Organization METHod for Enrichment Evaluation II) is applied to a Multiple Criteria Decision Making (MCDM) problem. The objective is to rank alternative suggestions to decision makers for maintaining source water quantity and quality . The method uses grey sets to represent uncertain information in the PROMETHEE II method. An inherent advantage of this approach is its ability to comprehensively consider quantitative and qualitative factors, making reliable decisions in an environment of uncertainty and subjective information. In this case study, involving the evaluation of source water strategies in the Waterloo Region, located in Ontario, Canada, the linguistic values of alternatives are expressed as grey sets, which are used to aggregate the opinions of experts and stakeholders on the impacts of the strategies according to selected criteria.