<p> <?tk 4?>Water assessment plays a vital role in maintaining the balance between human activities and conservation of environment, ensuring that water resources remain safe, healthy, and available for future generations. Through consistent assessment of water quality and accessibility, water pollution can be controlled by enhancing the protection of public health and preservation of ecosystems. The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) is a Multi-Criteria Decision Making (MCDM) approach that provides an efficient and reliable method for evaluating the quality of water resources using several criteria. In real life, the data are vague. To handle this vagueness, the Intuitionistic fuzzy set is chosen as a decision-making tool with Interval-valued Intuitionistic Fuzzy Numbers (IIFNs) taken into account in which the values are in intervals rather than crisp numbers. Interval-valued Intuitionistic Fuzzy TOPSIS (IIF-TOPSIS) is the generalization of the fuzzy TOPSIS approach in which the criteria values are in interval numbers. Furthermore, a new approach is developed by using new interval arithmetic operations. Therefore, an analysis is conducted to compare IIF-TOPSIS with the new approach. This study provides a thorough evaluation and analysis of the water quality in the Bagalkot region, located in Karnataka, from 2011 to 2018 as a case study. Based on the ranking sequence, the water quality level corresponding to the maximum degree of membership and minimum degree of non-membership is the final assessment level.</p>