Abstract

Water quality deterioration is a key challenge for sustainability in river basins under high anthropogenic pressures. This study evaluates the evolution of the Water Quality Index (WQI) in the Biobío River Basin (Chile) between 1994 and 2023 in relation to major environmental regulatory milestones, including Law No. 19,300, Decreto Supremo No. 90, the establishment of the Environmental Superintendency (SMA), and the implementation of the Secondary Environmental Quality Standard (NSCA). A temporal analysis of the WQI was conducted using data from stations along the main river course and its tributaries, complemented by a causal loop conceptual model to explore the interactions between regulation, compliance, and water quality. The results indicate initial improvements in WQI values following regulatory milestones, followed in some cases by stabilization or decline associated with reduced enforcement. Case studies, such as the closure of the Inforsa pulp mill in 2013, illustrate differentiated responses to regulatory change. The conceptual model reveals feedback loops linking enforcement perception and compliance behavior. These findings underscore the importance of sustained implementation and institutional capacity to achieve long-term improvements in water quality. © 2025 by the authors.